total_thesis_ver11.

Single-Grain Silicon TFTs on a
Plastic Substrate by Doctor Blade
Coating of Cyclopentasilane
Miki Trifunovic
Master of Science Thesis
in
Electrical Engineering, Microelectronics
Thesis Committee Members:
Prof. R. Ishihara (Supervisor)
Prof. C.I.M. Beenakker
Prof. A. Bossche
Ir. J. Zhang
Responsible Professor: Prof. P.M. Sarro
Faculty of Electrical Engineering,
Mathematics and Computer Science,
Delft University of Technology,
The Netherlands
June 5, 2012
i
Preface
Master
of Science Degree in Microelectronics at the faculty of Electrical Engineering,
Mathematics and Computer Science, at Delft University of Technology.
This thesis is written as a partial satisfaction for the requirements of the
The work is essentially combining the inexpensiveness of solution processing
and the quality of crystalline silicon transistors. The contribution of this work
will lead to the fabrication of quality transistors on exible substrates using the
new liquid silicon material that has only recently been discovered for electrical
applications.
In this process, diculties have arisen in some aspects and it was the help
of many people that have brought this thesis to a success. I would therefore like
to thank the following people:
ˆ
Prof. P.M. Sarro as the responsible professor for enabling my thesis work.
ˆ
Prof. R. Ishihara for his daily supervision and guidance during the course
of my project. Also, for the many discussions during meetings and helping
me publish some of my work.
ˆ
Prof.
R. van Swaaij and Prof.
A. Bossche for their support as Master
Coordinators.
ˆ
Prof. C.I.M. Beenakker and Prof. A. Bossche for their participation in
the thesis defense committee.
ˆ
Jin Zhang and Michiel van der Zwan for their processing support and
discussions.
ˆ
To all members of the TFT group that have helped me enjoy my time
in this research group including: Sten Vollebregt, Aslihan Arslan, Daniel
Tajari Mofrad, Pengfei Sun.
ˆ
To my peers, fellow master students, that have experienced the same curriculum.
ˆ
My family for supporting me always.
ˆ
Finally, I would like to thank anyone that have helped me in any way
during my
dents, etc.
Miki Trifunovic
05/28/12
Masters, including DIMES technicians, teachers, bachelor stu-
ii
Abstract
Liquid silicon is found as the material combining both the advantages of high
quality silicon devices and the low cost solution processing method.
Single-
Grain Thin-Film Transistors can be produced by Excimer Laser Annealing of
the resulting lm and grain location control by the
µ-Czochralski process.
Other
works have used spin-coating and inkjet printing for liquid silicon based devices,
however both processes are not roll-to-roll process compatible.
In addition a
high thermal annealing step (650°C), incompatible to plastics, is required for
the reduction of hydrogen content before laser crystallization.
In this work, both issues are focused on. A precursor of the gravure printing
process, doctor blade coating, is used to imitate a roll-to-roll compatible solution
process and is optimized to produce uniform lms of liquid silicon.
Excimer
Laser Annealing is used as a low temperature pre-annealing method to decrease
the hydrogen content for crystallization.
Pure cyclopentasilane has been used as the liquid silicon material. Silicon
dioxide surface modication by 0.55%HF dip results in a better wetting of the
liquid together with an elevated temperature of 70°C. Higher temperatures lead
to even better wetting properties, but more liquid silicon will evaporate.
After UV polymerization of the CPS for 20 minutes and thermal annealing
at 350°C for 1 hour, an a-Si layer has been formed. Excimer Laser pre-annealing
of many low energy shots removes hydrogen without signicant deterioration of
the lm.
A maximum grain size of 5µm has been produced by using a long
pulse congured laser recipe that decreases the number of shots linearly while
increasing the laser energy density by 50mJ/cm².
SG-TFTs on polyimide have been manufactured at the maximum processing
temperature of 350°C. The mobility of the NMOS was ..., and the mobility of
the PMOS was .... [to be obtained by mid June].
Finally, a next step towards gravure printing has been taken, by advancing
the doctor blade coating method to the removal of the excess layer while keeping
the cavity patterns in the lm lled.
manual blading.
Blade elasticity is a dominant factor in
An elastic blade can remove more excess than a rigid blade
since the exibility allows adjustment on the surface, but will also remove the
liquid from inside the patterns. A combination of a rigid blade and the careful
excess removal by the elastic blade gives the best results.
This work shows the potential of liquid silicon, and brings us closer to the
mass production on exible substrates using this new material.
iii
iv
Contents
Preface
i
Abstract
iii
List of Figures
vii
List of Tables
xii
Abbreviations
xv
1 Introduction
1
2 Solution TFT process and liquid Silicon
5
2.1
Solution Processing . . . . . . . . . . . . . . . . . . . . . . . . . .
6
2.2
Semiconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10
2.2.1
Organic Semiconductors . . . . . . . . . . . . . . . . . . .
11
2.2.2
Solid Silicon . . . . . . . . . . . . . . . . . . . . . . . . . .
16
2.2.3
Liquid Silicon . . . . . . . . . . . . . . . . . . . . . . . . .
19
3 Doctor Blade Coating of Liquid Silicon
3.1
3.2
3.3
3.4
25
Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
26
3.1.1
Doctor blade coating . . . . . . . . . . . . . . . . . . . . .
26
3.1.2
Surface Free Energy
27
3.1.3
a-Si lm formation from liquid silicon
Experimental
. . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . .
30
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
32
3.2.1
Equipment
. . . . . . . . . . . . . . . . . . . . . . . . . .
32
3.2.2
Working procedure . . . . . . . . . . . . . . . . . . . . . .
37
3.2.3
Boundary Conditions
38
. . . . . . . . . . . . . . . . . . . .
Characterization Experiments . . . . . . . . . . . . . . . . . . . .
38
3.3.1
Film breaking . . . . . . . . . . . . . . . . . . . . . . . . .
40
3.3.2
Liquid Silicon . . . . . . . . . . . . . . . . . . . . . . . . .
41
3.3.3
Surface Free Energy
. . . . . . . . . . . . . . . . . . . . .
42
3.3.4
Blade types . . . . . . . . . . . . . . . . . . . . . . . . . .
45
3.3.5
Post-deposition variations . . . . . . . . . . . . . . . . . .
Film spreading recipe
. . . . . . . . . . . . . . . . . . . . . . . .
v
46
46
vi
CONTENTS
3.5
3.4.1
Spin-Coating
3.4.2
Blading
. . . . . . . . . . . . . . . . . . . . . . . . .
48
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
49
3.4.3
Combination
. . . . . . . . . . . . . . . . . . . . . . . . .
52
Conclusions and Recommendations . . . . . . . . . . . . . . . . .
52
4 Low Temperature Annealing and Crystallization
55
4.1
Pre-anneal eects on Hydrogen concentration . . . . . . . . . . .
56
4.2
Experimental
58
4.3
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.1
Excimer Laser setup . . . . . . . . . . . . . . . . . . . . .
58
4.2.2
Approach
. . . . . . . . . . . . . . . . . . . . . . . . . . .
59
4.2.3
Results
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
60
Conclusions and recommendations
. . . . . . . . . . . . . . . . .
5 l-Si SG-TFT on Polyimide
5.1
Transistor structure
64
65
. . . . . . . . . . . . . . . . . . . . . . . . .
65
5.1.1
Fabrication procedure
. . . . . . . . . . . . . . . . . . . .
66
5.1.2
Polyimide . . . . . . . . . . . . . . . . . . . . . . . . . . .
68
5.2
TFT characteristics . . . . . . . . . . . . . . . . . . . . . . . . . .
68
5.3
Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
70
5.4
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
70
6 Excess Liquid Silicon Removal for Gravure Printing
6.1
6.2
71
Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . .
71
6.1.1
Excess removal . . . . . . . . . . . . . . . . . . . . . . . .
72
6.1.2
Pattern deformation
. . . . . . . . . . . . . . . . . . . . .
73
6.1.3
Time Dependency
. . . . . . . . . . . . . . . . . . . . . .
75
6.1.4
Liquid silicon . . . . . . . . . . . . . . . . . . . . . . . . .
76
6.1.5
Surface modication . . . . . . . . . . . . . . . . . . . . .
78
Conclusions and Recommendations . . . . . . . . . . . . . . . . .
79
7 Conclusions and Recommendations
83
7.1
Doctor Blade coating Liquid Silicon
. . . . . . . . . . . . . . . .
83
7.2
Low Temperature Annealing . . . . . . . . . . . . . . . . . . . . .
84
7.3
Liquid silicon devices . . . . . . . . . . . . . . . . . . . . . . . . .
85
7.4
Excess removal using doctor blade
7.5
Recommendations
. . . . . . . . . . . . . . . . .
85
. . . . . . . . . . . . . . . . . . . . . . . . . .
86
A Market Analysis
89
A.1
Radical innovation
. . . . . . . . . . . . . . . . . . . . . . . . . .
A.2
Associated costs
89
. . . . . . . . . . . . . . . . . . . . . . . . . . .
90
A.3
Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
91
B Printer types for electronics fabrication
B.1
B.2
93
Impact Printers . . . . . . . . . . . . . . . . . . . . . . . . . . . .
93
B.1.1
Gravure Printing . . . . . . . . . . . . . . . . . . . . . . .
94
B.1.2
Other impact printers
97
Non-Impact printers
. . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . 100
CONTENTS
B.3
vii
B.2.1
Inkjet
B.2.2
Electrophotography
Conclusion
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
. . . . . . . . . . . . . . . . . . . . . 103
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
C Thin-Film Transistor
107
C.1
OTFT Characteristics
. . . . . . . . . . . . . . . . . . . . . . . . 107
C.2
Transistor Conguration . . . . . . . . . . . . . . . . . . . . . . . 108
D SFE Results
111
E Excimer Laser Crystallization
113
E.1
Crystallization process . . . . . . . . . . . . . . . . . . . . . . . . 113
E.2
Crystallization problems . . . . . . . . . . . . . . . . . . . . . . . 114
Bibliography
117
Abbreviations
123
viii
CONTENTS
List of Figures
1.1
Flexible display (a) and exible solar panel (b)
1.2
Super E-paper
1.3
Flowchart of the SG-TFT fabrication process from the liquid silicon
4
2.1
Overview of dierent types of impact and non-impact printers [13]
9
2.2
. . . . . . . . . .
1
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
Schematic of the dierence between doctor blade coating and
gravure printing.
2.3
and
2.4
. . . . . . . . . . . . . . . . . . . . . . . . . . .
Bonding representation of two carbon molecules with
π-bonds
(a), and its band representation (b).[21] . . . . . . .
Bonding representation of benzene with
14
Bonding representation of an arbitrary polymer (a), and its band
representation (b).[21]
2.6
13
σv-bonds and π-bonds (a),
and its band representation (b).[21] . . . . . . . . . . . . . . . . .
2.5
10
σv-bonds
. . . . . . . . . . . . . . . . . . . . . . . .
15
Performance of the p-type organic material pentacene (a), and ntype F16 CuPc (b), both fabricated in a CMOS design on a plastic
substrate. [24] . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.7
SG-TFT fabrication process using the
μ-Czochralski method.
16
The
plastic substrate in (a), followed by deposition of a SiO2 layer (b).
A grain lter is etched in this layer (c). Subsequent a-Si deposition lls the cavity (d). Excimer laser melts the top layer and
leaves a seed of a-Si at the bottom of the cavity which grows to
become a crystalline island on the surface (e). Within this island
a TFT is created (f ). [28]
2.8
2.9
. . . . . . . . . . . . . . . . . . . . . .
18
CPS synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21
Three main structures of CPS (a).
Infrared spectra displaying
vibrational frequencies associated to the CPS, red lines indicating
the twist and envelope structures. The green dotted lines indicate
intermediate structures after the formation of the Si-H-Si bridge
bond (b). [4]
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22
2.10 TFTs constructed with liquid silicon using inkjet printing, spincoating, and chemical vapor deposition. Transfer characteristics
in (a), output characteristics in (b), image of the TFT using SEM
(c), and the schematic of the TFT structure (d). [1]
ix
. . . . . . .
23
x
LIST OF FIGURES
2.11 Single-Grain TFTs constructed with liquid silicon using spincoating.
NMOS and PMOS transfer characteristics in (a) and
(b) respectively, SEM image of the SG-TFT in (c), and the SGTFT schematic in (d). [2]
3.1
. . . . . . . . . . . . . . . . . . . . . .
Schematic of a system of two parallel plates applying shear force
on a medium present in between the plates. . . . . . . . . . . . .
3.2
23
26
Schematic of the top view of doctor blade coating, showing trail
formation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27
3.3
Surface energies denition schematic . . . . . . . . . . . . . . . .
28
3.4
Si 2p XPS spectra of a-Si lm for dierent UV exposure times
of CPS: a. 3, b. 5, and c. 15 minutes (a) [7]. Gel permeation
chromatogram of liquid silicon (b).
CPS in toluene in a.
and
UV-irradiated CPS in toluene in b. [1] The broad peak indicates
3.5
3.6
the polysilanes of various molecular weights. . . . . . . . . . . . .
32
MBRAUN Glovebox [39] . . . . . . . . . . . . . . . . . . . . . . .
33
UV AHAND 250GS wavelength over wavelength in (a) and intensity over distance in (b).[39]
3.7
. . . . . . . . . . . . . . . . . . .
34
DekTak graphs with proles from various surfaces on which liquid
silicon has been transformed into amorphous silicon.
The area
where the amorphous silicon has been removed is where the layer
was broken. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.8
types of oxide, using pure CPS. . . . . . . . . . . . . . . . . . . .
3.9
41
Contact angle graphs, dierent surface modications on dierent
43
Surface Free energy gures using 3.15. Extracting theγS (a), and
the
γL
. . . . . . . .
44
3.10 Surface energies calculated using Neumann's method . . . . . . .
(b) by sweeping the respective parameters.
45
3.11 Blade type results of silicon only (a), silicon and rubber (b), and
rubber with additional applied force (c)
. . . . . . . . . . . . . .
46
3.12 RAMAN spectroscopy result of a thin a-Si lm (a) and a thick
a-Si lm (b), both annealed at 350°C for 1 hour.
. . . . . . . . .
−1
, for three
47
. . . . . . . . . . . . . . . . . . . . . . .
47
3.13 FTIR graph of absorption peak integrals at 640cm
UV exposure conditions
3.14 Spin-coating experiment results. Double coating of CPS and UV
pre-exposed CPS (a), 20 minutes UV pre-exposed CPS only (b),
20 minutes UV pre-exposed CPS only with 0.2μm lter (c).
. . .
48
3.15 Proles of a-Si layers deposited on the wafer with 1 by 1 mm 250
nm deep square patterns. Fully covered layer in (a), and bulged
square coverage in (b). . . . . . . . . . . . . . . . . . . . . . . . .
50
3.16 SEM images of liquid silicon covering a pattern instead of lling
it (a), and the lling of the grain lter (b) . . . . . . . . . . . . .
51
3.17 Some blading experiment results for the formation of a lm. Silicon blade spreading and rubber scraping with partly hard rubber scraping (a), silicon blade spreading with partly mild rubber
scraping (b), edge formation and cracking in 500nm square wafer
with a small part showing a uniform layer (c). . . . . . . . . . . .
52
LIST OF FIGURES
xi
3.18 Results of the combined coating methods blading and subsequent
spin-coating at 500RPM (a), 1000RPM (b), 1000RPM on a polyimide substrate (c). . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1
53
Maximum shootable energy before which the silicon lm starts
cracking as a function of the number of Excimer Laser shots.
Pretreatment at 300°C (a) and untreated a-Si lm (b) [49] . . . .
4.2
57
Demonstration of laser pre-annealing benets. Single shot without pre-annealing at 500 mJ/cm² (a), 90 shots at 100 mJ/cm²
(b), and a pre-annealed sample with maximum laser energy density of 550 mJ/cm² (c) . . . . . . . . . . . . . . . . . . . . . . . .
57
4.3
Exitech M8000V Excimer Laser system schematic [48]
58
4.4
Visual representation of the recipe types ramped single shot (a),
linear decrease (b), and exponential decrease (c).
4.5
. . . . . .
. . . . . . . . .
59
Laser energies from which the lm starts to show signs of defects
for every laser recipe type(a). Maximum grain size obtained for
the particular recipe type (b). . . . . . . . . . . . . . . . . . . . .
4.6
Excimer laser irradiation results short pulse.
60
Single shot (a)
against an exponentially decreased number of shot with increasing shot densities of 50 mJ/cm² starting at 150 mJ/cm²(b), start-
ing at 200 mJ/cm² (c) and at 250 mJ/cm² (d), with a maximum
of 500 mJ/cm² for all cases. . . . . . . . . . . . . . . . . . . . . .
4.7
Excimer laser irradiation results long pulse.
62
Linear recipe for
which the biggest grain sizes have been obtained. 4 and 3 micron
pitch image (a), and 3 and 2 micron pitch image (b). . . . . . . .
63
4.8
ERD setup schematic[50]
. . . . . . . . . . . . . . . . . . . . . .
63
4.9
RBS setup schematic[50] . . . . . . . . . . . . . . . . . . . . . . .
64
5.1
SG-TFT fabrication process both with (b steps) and without (a
steps) an additional polyimide layer.
The polyimide layer has
been omitted in this schematic after step 1, however step 1 shows
its designated position. . . . . . . . . . . . . . . . . . . . . . . . .
5.2
Chemical structure of the Polyamic Acid Durimide (a)[32], and
the Imide monomer . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1
66
68
Films within patterns getting pulled out by the excess layer connected to the lm inside.
Optical microscope view (a), a SEM
image of such a pattern (b), and a SEM image of a bigger pattern
(c). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
73
6.2
Bubble bursting of CPS due to excess CPS on top of a lled pattern. 73
6.3
Dierent ways of pattern lling. . . . . . . . . . . . . . . . . . . .
74
6.4
Deformation of supposedly dewetted patterns. . . . . . . . . . . .
74
6.5
RAMAN spectroscopy measurements of lled and dewetted patterns.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
75
xii
LIST OF FIGURES
6.6
Dewetting against deformation schematic when properly lled (a)
and when poorly lled (b). The proof of a thin layer within the
250nm deep pattern (c). . . . . . . . . . . . . . . . . . . . . . . .
76
6.7
Eect of pattern depth on liquid silicon
76
6.8
Results of time dependency experiments, good adhesion in the
. . . . . . . . . . . . . .
initial thick layer area (a), area outside this initial layer after (b),
and the transition from initial layer to the bladed area outside (c).
6.9
77
Various UV exposure times. No UV exposure before blading (a).
10 minute UV exposure before blading on top of a wafer (b), but
many intermediate exposures during blading (c).
6.10 Blading results on plasma oxidized surface.
. . . . . . . . .
77
. . . . . . . . . . . .
78
6.11 Dierence in blading of the excess on regular surface and plasma
oxidized surface.
. . . . . . . . . . . . . . . . . . . . . . . . . . .
6.12 Blading results on HF dipped surface.
. . . . . . . . . . . . . . .
79
79
6.13 Recommended setup mainly based on high adhesion within the
pattern, and poor adhesion outside, with a poor adhesive blade. .
81
B.1
The master plates for four main impact printers [13]
. . . . . . .
94
B.2
Gravure printing schematic [13] . . . . . . . . . . . . . . . . . . .
95
B.3
Letterpress schematic diagram [13] . . . . . . . . . . . . . . . . .
98
B.4
Lithography/Oset printing schematic diagram [13] . . . . . . . .
99
B.5
Screen printing schematic [13] . . . . . . . . . . . . . . . . . . . . 100
B.6
Inkjet printing schematic [13]
B.7
Electrophotography schematic [13]
C.1
Various TFT structures [12] . . . . . . . . . . . . . . . . . . . . . 109
. . . . . . . . . . . . . . . . . . . . 102
. . . . . . . . . . . . . . . . . 103
List of Tables
2.1
Comparing conventional and solution processing
. . . . . . . . .
7
2.2
List solution processing types . . . . . . . . . . . . . . . . . . . .
8
2.3
Printing advantages in electronics . . . . . . . . . . . . . . . . . .
9
2.4
Types of materials used as inks for printing dielectrics and their
properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5
10
Types of materials used as inks for printing conductors and their
properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11
2.6
General information Cyclopentasilane
20
3.1
Processing varieties from liquid silicon to amorphous silicon lm
39
B.1
Typical values for gravure printing processes [13]
97
B.2
Comparative analysis of the various printer types . . . . . . . . . 105
xiii
. . . . . . . . . . . . . . .
. . . . . . . . .
xiv
LIST OF TABLES
Abbreviations
AMOLED
Active-Matrix Organic Lighte-Emitting-Diode
a-Si
amorphous silicon
a-Si:H
hydrogenated amorphous silicon
BJT
Bipolar Junction Transistor
CMOS
Complementary Metal-Oxide-Semiconductor
CPS
Cyclopentasilane
c-Si
single-crystalline silicon
ERD
Elastic Recoil Detection
FTIR
Fourier Transform Infrared Spectroscopy
GF
Grain Filter
GPC
Gel Permeation Chromatography
HF
Hydrouoric acid
HOMO
Highest Occupied Molecular Orbital
IC
Integrated Circuit
ICP
Inductively Coupled Plasma
IPA
Isopropanol
LCD
Liquid-Crystal display
LUMO
Lowest Unoccupied Molecular Orbital
MOSFET
Metal-Oxide-Semiconductor Field-Eect Transistor
NIP
Non-impact Printers
OLED
Organic Light-Emitting-Diode
OPRA
Overlay Printing Registration Accuracy
OTFT
Organic Thin-Film Transistor
PCB
Printed Circuit Board
PECVD
Plasma-Enhanced Chemical Vapor Deposition
RBS
Rutherford Backscattering Spectroscopy
RFID
Radio-frequency Identication
RIE
Reactive-Ion Etching
RPM
Rounds per minute
SFE
Surface Free Energy
SG-TFT
Single-Grain Thin-Film Transistor
TEOS
Tetraethylorthosilicate (Si{OCH2 CH3 }4 )
TFT
Thin-Film Transistor
UV
Ultraviolet light
XPS
X-ray Photoelectron Spectroscopy
xv
xvi
LIST OF TABLES
Chapter 1
Introduction
As electronic chips make up an increasing part of our daily lives, there is a
big desire for the optimization in production methods.
One interpretation of
Moore's Law denes a period of 18 months in which chip performance doubles,
and as the physical limitations in chips are being approached, other ways of
improving chip fabrication is being investigated.
Conventional methods for the production of electronic integrated circuits
are complex and require high material compatibility. Large area electronics for
display applications are based on Thin-Film Transistors that can be deposited
on top of a supporting substrate, which is conventionally glass. The fabrication
processes require expensive machinery such as the photolithography stepper, a
vacuum atmosphere that requires energy and limits the producible area, and
are based on batch processing which leads to a low throughput.
In addition,
the process wastes a lot of its materials due to its subtractive processing nature.
The methods are based on batch processing and mass production is because of
all this not possible. Mass producing chips will bring the electronics fabrication
industry to a whole new level, making common chips as inexpensive as printing
on plain paper.
(a)
(b)
Figure 1.1: Flexible display (a) and exible solar panel (b)
1
2
CHAPTER 1.
INTRODUCTION
With the help of low-temperature processing, plastic substrates may be used
for the production of exible, light-weight, inexpensive displays as well as solar
panels, as shown in Fig.
1.1.
In addition, conventional vacuum processing
limits the maximum obtainable sizes of these displays due to the limiting size
of the vacuum chamber. Therefore, a process which does not require a vacuum
ambient will not only lead to large scale manufacturing of displays, but allows
roll-to-roll production.
Solution processing is an alternative way of processing electronics which has
a potential to be used for mass production ends. Although still at its infancy,
a lot of research is focusing on nding the right types of materials that can
be incorporated as inks into mass producing printing machinery. Most of the
research has focused on organic materials due to their ease in conversion to
solutions that can be used as inks.
However, these materials still experience
poor material properties limiting their electrical performance at the level of
hydrogenized amorphous silicon.
This makes them only suitable for limited
applications. In addition, the materials are quite unstable.
Problem
Solution processing at this point has the bottleneck of using the type of inks that
have a low performance as well as stability. It is important to nd a material
that has electrical properties approaching the current high quality crystalline
silicon level, yet still is solution processable as well as highly stable. This will
have a huge impact in the chip manufacturing industry, as well as the display
industry, driving the cost of electronic devices to a minimum while keeping the
quality of these devices high. New application areas will emerge for which costs
would be far too high when constructed by conventional processing methods
such as exible tablet devices as the super e-paper (displayed in Fig.1.2 ), or
food packages RFID and sensors.
Figure 1.2: Super E-paper
3
Liquid silicon has been introduced in other works by Shimoda et al.
among others.
[1]
They have shown the possibility of transforming a precursor
into amorphous silicon by a combination of photopolymerization and thermal
annealing. Also Zhang et al. [2] have reported the fabrication of Single-Grain
Thin-Film transistors. Both use a thermal annealing temperature which is too
high to be used on plastic substrates. They also used spin-coating and inkjet
printing for the deposition of the material, however, neither can be used in
roll-to-roll applications.
Approach
In this work, the following three approaches were taken to solve the problems:
1. A doctor blade coating method is introduced for depositing the liquid
silicon material over a surface which is compatible to roll-to-roll processing.
This method may in the future be extended into slot-die coating and nally
gravure printing, a widely accepted printing method that is known for
high-speed mass production.
2. The second issue of a thermal annealing temperature, incompatible to
plastic substrates is dealt with by using Excimer Laser annealing that
allows dehydrogenation and melting of thin layers without the penetration
of the heat to more sensitive underlying layers such as exible substrates.
3. A SG-TFT device will be fabricated on a polyimide substrate by the doctor
blade coating of liquid silicon.
For the rst time, high quality silicon
devices will be produced on a plastic substrate, bringing high electrical
performance and solution processing together.
Working method
A general method of producing an amorphous silicon lm from the liquid silicon
has been adapted using many dierent variables aimed to optimize the method
of fabricating a uniform lm by doctor blade coating.
Fig.
1.3 shows the
owchart from liquid silicon to the SG-TFT. The focus in this work was in
the liquid and target preparation step, coating step, and the dehydrogenation
step.
The coating has been performed on substrates with grain lters that
allow location controlled crystallization during the subsequent Excimer Laser
irradiation. This irradiation process uses various recipes but is generally based
on shooting at low energy densities to remove hydrogen content from the lm
that cause deterioration of the lm when exposed to crystallizing energy levels,
followed by shooting at increasing energy levels. Finally, the single crystals are
used to fabricate the SG-TFTs.
Substrates with a polyimide layer deposited
on top of the single-crystalline substrate before oxide deposition are used. The
substrate allows peeling o of the polyimide layer to obtain a fully exible result.
4
CHAPTER 1.
INTRODUCTION
Figure 1.3: Flowchart of the SG-TFT fabrication process from the liquid silicon
Goal of this study
The focus of this study is aimed at achieving four specic goals:
1. Uniform layer formation of liquid silicon by doctor blade coating
2. Low temperature (<350°C) formation of a-Si
3. Low temperature single grain formation from the coated a-Si layer
4. SG-TFT production on a plastic substrate
Structure
This thesis starts by explaining some of the conventional processing methods
and explain the benets from the methods and materials used in this work in
Chapter 2.
This chapter is followed by the experiments that are conducted
regarding: doctor blade coating of liquid silicon in Chapter 3, low temperature
annealing by Excimer Laser in Chapter 4, and the measurement results of the
subsequently produced liquid silicon SG-TFTs in Chapter 5. In addition, the
next step towards gravure printing has been investigated in Chapter 6. Finally,
conclusions and recommendations are presented regarding all experiments in
Chapter 7
Chapter 2
Solution TFT process and
liquid Silicon
Many of the electronic devices nowadays, are built using silicon, and the process
has been optimized up to a certain extent for this material. Alternative materials
are being researched now that physical limitations start to play a role. Yet an
alternative way of processing may lead to inexpensive device fabrication formerly
considered as impossible for silicon devices.
Solution processing has gained immense popularity, however pure silicon cannot be used in this process due to its high melting point of 1414°C incompatible
with many other materials. Organic materials are attractive due to their ease
in dissolving without losing their electrical properties.
The organic electrical
characteristics are quite poor as well as unstable, but they seem to be acceptable for some applications such as Organic Light-Emitting-Diodes (OLEDs) but
cannot be integrated in more complex circuitry. Organic material properties are
currently being optimized and have recently surpassed the level of hydrogenized
amorphous silicon (a-Si:H), conventionally used for display applications.
The liquid silicon precursor used in this work can be solution processed similar to organic materials, however, they can be crystallized into polycrystalline
silicon that already have two orders of magnitude higher mobility than the current best organic mobility.
In addition, when the location of these crystals
are controlled, the channel of the transistor can be positioned within a crystal
grain, which is one of the great advantages of Single-Grain Thin-Film Transistor
(SG-TFT).
In this chapter a description will be given on solution processing, which will
be compared to conventional processing methods. This section is followed by
the characteristics of organic semiconductors, silicon as it is used nowadays, and
a full description of the liquid silicon material used in this work.
5
6
CHAPTER 2.
SOLUTION TFT PROCESS AND LIQUID SILICON
2.1 Solution Processing
A type of processing that uses solutions or liquids for the fabrication of electronic
devices is called solution processing. This type of processing has attracted a lot
of attention recently due to the many advantages over conventional processing.
Two dierent types of conventional processing need to be discussed.
The rst one is the use of Metal-Oxide-Semiconductor Field-Eect Transistors (MOSFETs) or Bipolar Junction Transistors (BJTs) that use the bulk of
the silicon material as the channel and are produced by traditional processing
methods that start with the wafers produced by the
ing Zone
Czochralski
and/or
Float-
methods. These wafers are further processed by means of subtractive
processing using photolithography and masking layers. High quality devices are
produced in this way.
A second type of process focuses on the production of devices such as ThinFilm Transistors (TFTs) on top of a supporting substrate, such as the case for
displays. In general this is done on top of glass, however, when suciently low
temperatures are reached, deposition on exible substrates may be possible as
well.
Current technologies use Plasma-Enhanced Chemical-Vapor Deposition
(PECVD) for the deposition of the various layers and use a-Si:H as the semiconductor layer. For display applications, the quality of the devices do not have
to be high.
In both cases, a highly controlled atmosphere is required and many dierent
steps are necessary to produce various patterns on the substrate as well as
doping of the layers within the bulk of the material. Material wastage and time
consumption is a central issue in both cases. It is therefore necessary to nd
a way to produce high quality devices in a less stringent ambient with high
throughput at low temperatures.
As an alternative to the conventional processing, solution processing gained
popularity due to their ability to overcome most of the issues of current processes. Table 2.1 compares solution processing to conventional processing. In
essence, the solution process itself, is the various uses of solutions or liquids
to create electronic devices such as TFTs.
Some of the most commonly used
solution process, that is also used in conventional processing for instance, is
spin-coating.
Table 2.2 gives a list of all solution processing types currently
used and researched [6, 11].
The eects of these dierent types of processes on the thin lm quality of
the deposited layer are mainly dependent on the concentration of semiconductor
material in the solution and its solubility, evaporation rate of the solvent, and
properties of the target surface[9].
Printing is the process of reproducing by means of applying ink to a substrate
into a certain arrangement, and is an advanced solution processing technique.
Today's wildly used applications include the printing of mass produced newspapers, magazines, books, packages, etc. Printers known for their mass production
qualities would be a useful tool for constructing cheap electronic devices allowing to deposit already patterned layers onto various substrates. This makes the
manufacturing process of electronic devices a lot simpler and less expensive.
2.1.
SOLUTION PROCESSING
7
Table 2.1: Comparing conventional and solution processing
Conventional Processing
+
High quality devices can be
Solution Processing
-
the usage of organic
of external factors [3], as well as
semiconductors. These materials
the quality of silicon
+
Quality of the devices is low due to
fabricated due to the high control
Device density is high due to the
are also quite unstable
-
Device density is relatively low
when considering printing only, due
high resolution of this type of
to the current resolution of printers
process [3]
-
Process is quite complex [4, 5]
+
-
Vacuum ambient is required and
+
Simple and fast process [4, 7, 8]
No vacuum ambient is required, so
can be produced by chambers that
no chambers that limit the
limit the size of the substrates[2, 6]
maximum processable substrate
size or pumping down time needed
[4, 9, 10]
-
Dicult handling of the gaseous
±
Easier handling of liquid materials,
although solution issues such as
materials [6]
bleed-out needs to be avoided in
printing systems. [10]
-
Use of a lot of materials due to
±
subtractive processing [2, 3, 4, 5, 6]
Subtractive processing in most
coating methods since patterns are
not directly transferred. Additive
processing only for printer types
-
High cost [4, 5]
+
Lower cost per unit
area[4, 7, 8, 9, 10]
-
Low throughput due to batch
+
processing
-
Substrates are limited to wafers or
High throughput when considering
roll-to-roll processability
+
Flexible, plastic, substrates can be
glass due to high temperature
used due to the low temperature
processing.
processing when using organic
solutions. [10]
Special materials are used for inks that can be used in printers for solution processing of layers with electrical properties. Using silicon itself as ink would be
unrealistic due to its high melting point of over 1400°C making it incompatible
to printers as well as substrates. A lot of research has gone into organic semiconductors due to their ease in creating a solution from the material at room
temperatures. In this way the material can be printed onto exible substrates
that are incapable of handling high temperatures. Furthermore, the size of these
substrates is not limited to the limited size of the conventional machinery, and
therefore large areas of electronics can be printed inexpensively. The price of
these devices can be driven to the level of conventional printing, and many new
applications of electronics can be made feasible. Some of the applications due
to this cost minimizing process take the advantages of printing directly to the
8
CHAPTER 2.
SOLUTION TFT PROCESS AND LIQUID SILICON
Table 2.2: List solution processing types
Spin-coating
Process in which the solution is positioned at the center of
a certain substrate after which the substrate is rotated at a
certain speed. Due to centrifugal force the solution is
spread across the surface. By varying the speed, a varying
thickness of the resulting layer can be formed.
Drop-casting
Deposition of a drop of liquid on a specic surface after
which it is spread due to external forces until solvent is
dried.
Doctor blade
A liquid is spread across a surface by means of a doctor
coating
blade, pushing the liquid in desired areas.
Slot die coating
A liquid is forced out of a chamber through a slot by
pressure onto a moving substrate.
Roll coating
Commonly referred to as roll-to-roll printing processes
which are in most cases impact printer types. Cylinders
are used to transport the solution from one surface to the
other by adhesive forces after applying a certain pressure.
Gravure printing, direct lithography, and
letterpress/exography printing fall into this category.
Oset printing
An intermediate carrier is used to transport a certain
pattern arrangement of the solution from one roll to
another. Generally used in oset lithography setup to
decrease water transport to the target substrate.
Curtain coating
A constant stream of uninterrupted solution is dropped on
a surface which is placed on a conveyer belt and is traveling
in a direction with a certain speed. This constant motion
will result in a uniform layer on top of the substrate.
Dip coating
The substrate as a whole is dipped into a solution. Upon
raising the substrate out of the solution, adhesive forces
will cause the solution to stay on the substrate.
Spray coating
A hose is used to spray a solution on a certain substrate.
Anodization
An anode and a cathode are placed within a solution and a
certain electric eld is applied. This eld will cause some
of the material within the solution to react with one of the
electrodes on which a metallic target may be placed.
Commonly used for anodic oxidation.
Screen printing
Solution patterns are pressed through a mask onto a
certain substrate.
Inkjet printing
A solution is propelled toward a surface in the form of
droplets by thermal, piezo-electric, or acoustic forces
applied at the nozzle.
2.1.
SOLUTION PROCESSING
9
market [12] these are listed in Table 2.3 .
Table 2.3: Printing advantages in electronics
Printing advantage
Electronic application
Inexpensive as paper
Integration in any product (e. g. packages)
Large area production
Billboards, solar cells, displays
Flexible substrate usage
Clothing, anti-vandalism
High throughput[10]
Cheaper electronics overall
Figure 2.1: Overview of dierent types of impact and non-impact printers [13]
There are many dierent types of printers. Each printer has its own significant advantages and disadvantages and can be divided into two main groups:
Impact printers
and
Non-Impact Printers. Impact Printers
are also known as
printers with masters that can be reused to produce highly reproducible inexpensive print runs, whereas the
Non-Impact Printers
do not use a xed master
but allow digital processing and highly variable print runs. Within these groups
a further subdivision can be made.
Fig.
2.1 shows an overview of the main
types of printers that are currently in use. For an overview of the most popular printer types for the fabrication of electronics, the reader is referred to
Appendix B, where the advantages, disadvantages, basic working and current
research regarding the printer type is presented, after which a nal conclusion
10
CHAPTER 2.
SOLUTION TFT PROCESS AND LIQUID SILICON
is given for best printer type for manufacturing electronics.
This thesis studies the doctor blade coating of liquids on a surface, which is
by itself not a printing type. It is however a precursor of gravure printing as
displayed in Fig. 2.2. Initial tests have to be run for doctor blade coating and
accurate excess removal before delving into the gravure printing process.
Figure 2.2: Schematic of the dierence between doctor blade coating and gravure
printing.
The reason why gravure printing is chosen is because of the great advantages
of this type of printer when considering mass production. Highly uniform and
quality print runs are realized which can potentially dominate the printing industry of electronics. Doctor blade coating used in this work itself can already
be used in a roll-to-roll process, and is therefore a good rst step towards mass
production of liquid silicon.
2.2 Semiconductors
Many advantages exist in printing solution processable materials compared to
conventional processing. However, compatibility is required for the conversion of
the right types of materials into inks that can be used in the printing process. It
has been shown that every part of the transistor can be constructed by printing
techniques. Metal wiring by means of nanoparticles for contacts, and dielectric
deposition are, among others, important aspects in the construction of the eldeect transistors. Table 2.4 and 2.5 lists the dierent solution processable types
of materials used for dielectrics and conductors respectively [12, 14].
Table 2.4:
Types of materials used as inks for printing dielectrics and their
properties
Polymers (most frequently used)
+
Easy solution process
+
Good uniformity in thickness
+
Low surface roughness
+
Flexible
+
Compatible to organic semiconductors
The key ingredient in the transistor is the path between one contact to
the other, the channel region, for which a semiconductor is required. The or-
2.2.
SEMICONDUCTORS
11
Table 2.5: Types of materials used as inks for printing conductors and their
properties
Conducting polymers
Metal Flakes
-
Dicult to dissolve
-
Degrades easily
+
Low temperature process
+
Organic semiconductor compatible
+
Low cost
-
Only as good as the smallest
particle
+
High conductivity
+
Low temperature process
+
Metal Nanoparticles
Metal-organic or metal-salt precursors
Wide material variety and
availability
-
Colloidal stability
-
Printability
-
Film roughness
-
Purity of the nal lm
-
Electromigration
+
Low cost
+
Easy to prepare
+
Much better stability due to lack of
discrete particles
+
Solution processable, so no
clogging of the inkjet nozzle
-
Low mass loading that can lead to
-
Purity of nal lm
poor mechanical lms
ganic semiconductor, commonly used as the semiconducting inks will rst be
discussed. Their basic working, application areas, but mainly the drawbacks of
these semiconductors will be made apparent, after which the conventional silicon
is explained, and nally the liquid silicon used in this work will be introduced.
2.2.1 Organic Semiconductors
Organic electronics, are electrical devices that make use of materials that are
based on carbon molecules. In the past, one did not associate organic molecules
to conducting electricity; it was rather found to be a good insulator. Although in
nature it has been noticed that the rey emits light through bio-luminescence
caused by enzymes that excite organic molecules[15]. Inorganic semiconductors
such as silicon or germanium have mainly governed the semiconductor industry
and after the accidental discovery of the usefulness of organic semiconductors in
electronics by Dr. Shirakawa in 1977[16], research of these materials have found
their way in the market due to their[15, 16]:
12
CHAPTER 2.
ˆ
SOLUTION TFT PROCESS AND LIQUID SILICON
Signicant fabrication cost reduction due to solution processability of the
materials,
ˆ
Low temperature processing that allow these materials to be deposited on
inexpensive exible substrates,
ˆ
Molecular tunability, allowing the materials to change properties such as
volatility, solubility, and wetting characteristics.
Unlike high quality single-crystalline silicon materials, these organics are built
in TFT processes much like a-Si:H. Research today has advanced the mobility
of organic semiconductors to surpass the level of a-Si:H (1 cm²/Vs) [17, 18, 19].
Although these are still nowhere near the mobility of single-crystalline silicon,
which is three orders of magnitude higher [16], they show their potential in
rather inexpensive applications.
The basis for the conduction of organic materials are slightly dierent from
their inorganic counterparts. Bonds play a major role in these devices, and this
section will be devoted to the understanding of their operating principles.
Conduction in general
A discussion about charge transport is directly related to the bonding properties
of various materials.
In both inorganic and organic semiconductors, covalent
bonds are the main bond type that keep the molecule together.
This type
of bond is based on the attraction of two atoms due to its desire to ll the
electron shells around the atoms.
Sharing of electrons among the two atoms
realizes this desire. Both silicon as well as carbon for example can share four
electrons with neighboring atoms. Within this type of bonding, the electrons
become a pair and orbit around the pair of atoms; they cannot move around
freely and therefore do not conduct electricity unlike metals that are bound
by a cloud of valence electrons that move around freely.
Covalent bonds in
inorganic semiconductors are relatively weaker so that some of the electrons
may be thermally activated. These materials can conduct to a certain extent
and are hence called semiconductors [20].
The interaction of two atoms results in splitting of the quantized energy
level into two discrete energies.
When many atoms interact with each other,
the energy will be split into a band of discrete energy levels. In some materials
there are energy levels for which the electron can not occupy. These are called
forbidden energies, or the bandgap [20].
Various models try to describe this
band theory further through the use of Schrödinger's equation (e.
g.
The
Kronig-Penny Model, the Ziman model, and the Feynman model). The reader
is referred to [15] for a more thorough understanding of the band theory.
Out of the many energy bands that exist due to the interaction of multiple
atoms, the two highest bands are of particular interest: the valence band and
the conduction band.
In insulators the valence band is lled with electrons,
and the conduction band is completely empty. The bands are separated by a
bandgap, and an application of an electric eld will not cause the electrons to
2.2.
SEMICONDUCTORS
13
change energy levels when the bandgap is suciently large for the electrons to
cross. No current will ow through the material since no electrons are activated.
A partially empty band results in the possibility of electrons increasing their
energy level due to an applied electric eld and move through the material. The
net ow of these electrons will dene the current. This situation happens for
semiconductors in which thermally activated electrons can cross the bandgap
and jump into the conduction band allowing the material to conduct current to
a certain extent since the bands are not completely lled or empty. In metals,
two situations may exist regarding the band lling.
The rst case is a fully
occupied valence band, and a partially lled conduction band that allow these
electrons to move freely when an electric eld is applied. The second possibility
is that valence and conduction bands overlap so that electrons do not have to
bridge a bandgap and can easily occupy other energy states [20].
Organic conduction
Compared to inorganic semiconductors, organic semiconductors that use covalent bonding interaction have quite a large bandgap that prevents the material
from conducting electrons. So the electrons inside this material need to be assisted in order for them to raise their energy and be able to move around. Two
carbon atoms interacting with each other are joined by one covalent bond due
to the pairing of one of the dangling bonds of each atom. This eect can be seen
as the pairing of s orbitals and is therefore also known as
σv-bonds.
Each carbon
atom has 4 dangling bonds and can form covalent bonds with four other atoms.
It can also however form a pair with the already interacting carbon atom and
create weaker bonds perpendicular to the
σv-bond
plane. This weaker bond is
similar to the structure of two p-orbitals and is therefore known as
π-bonds.
The electron conguration of Carbon: 1s², 2s², 2p², the sigma bonds lie closer
to the atoms and are therefore stronger [15] The Fig. 2.3 shows this situation.
(a)
(b)
Figure 2.3: Bonding representation of two carbon molecules with
π-bonds
(a), and its band representation (b).[21]
σv-bonds
and
14
CHAPTER 2.
SOLUTION TFT PROCESS AND LIQUID SILICON
(a)
(b)
Figure 2.4: Bonding representation of benzene with
σv-bonds
and
π-bonds
(a),
and its band representation (b).[21]
These
π-bonds
cause a further splitting of the energy level.
A lower level
is known as the bonding state, and the higher energy level is known as the
anti-bonding state.
Similar to insulators, under normal conditions the lower
bonding states are occupied and the higher anti-bonding states are empty. When
multiple double bonds are formed in an atom, such as the case for the benzene
ring, multiple energy level splitting will occur for every
from Fig.
π-bonds
π-bond
as can be seen
2.4 For large molecule systems such as polymers, this number of
can signicantly increase to form whole bands of bonding and anti-
bonding states.
These states are separated by a bandgap.
The energy state
closest to the bandgap is called HOMO (highest occupied molecular orbital)
for the bonding states, and LUMO (lowest unoccupied molecular orbital) for
the anti-bonding states.
when no
π-bonds
This bandgap however is signicantly smaller than
are present. This results in the possibility of charge transfer
to the higher unoccupied energy states due to external excitation, much like a
semiconductor. In this way the molecule will be able to transfer charge through
the material, only by using the highly polarizable
π-bonds.
Fig.
2.5 shows
the band diagram of conjugated polymers [15]. Doping in these materials will
enable the production of n- or p-type TFTs. Impurity atoms replace some of
the hydrogen atoms in this case and form the source of electrons or holes [15].
Although solution processable, signicant disadvantages exist when using
organic semiconductors:
-
In many cases the organic semiconductors are p-type since stable n-type semiconductors are hard to nd [16]. This makes the production of complementary circuitry such as CMOS hard to realize with organic materials
only.
-
These devices are known to give mobilities that are limited to the level of
a-Si:H
-
Organic materials are highly reactive to water and oxygen and are therefore
prone to reliability issues
2.2.
SEMICONDUCTORS
15
(a)
(b)
Figure 2.5: Bonding representation of an arbitrary polymer (a), and its band
representation (b).[21]
Polymers and Small molecule organic systems
Long polymer chains with alternating double bonds or
π-bonds
are being used
as organic semiconductors. Polymer materials generally are hard to purify since
they have a large variation in molecular size.
They may also have structural
defects due to monomer mislinkage that can induce traps against charge carriers.
Large polymer chains are however not the only way for organic materials to
behave as semiconductors. Small-molecule organic semiconductors can behave
in a similar fashion and form the alternative to polymer organics. Due to their
small size they can assemble more easily into regular geometric arrangements.
This tendency towards the formation of molecular crystals lead to larger carrier
mobilities than compared to disordered chains of polymers. A few dierences
between the two types should however be noted [16, 17, 22]:
ˆ
Small-molecule organic semiconductors are commonly deposited from the
vapor phase whilst polymeric semiconductors are deposited from the solution. Solution processability is an important factor for printed electronics.
Polymeric solvents are however known to be quite toxic since they are either chlorinated or aromatic.
ˆ
Small-molecule systems, when deposited on low roughness surfaces can
form a highly ordered crystalline region leading to ecient
π-stacking and
therefore higher mobilities than polymers.
ˆ
Small-molecules are more easily puried than polymers since their structure is well dened.
Research so far
A lot of research is being conducted in organic electronics. Organic devices are
constructed with higher mobilities and with higher stability [17, 18, 22, 23], and
have already come to the level of a-Si:H (1cm²/Vs). However, it is still a long
16
CHAPTER 2.
SOLUTION TFT PROCESS AND LIQUID SILICON
way before reaching the high quality crystalline silicon level that have mobilities
of a few orders of magnitudes higher.
The organic n-type counterpart of common p-type organic semiconductors
are being researched to be able to make CMOS designs for high speed, energy
ecient circuitry. Currently, many n-type materials are either too unstable or
have a lot lower mobilities. Fig. 2.6 shows both a p-type and an n-type device
characteristics, that have been designed for CMOS purposes.
Figure 2.6: Performance of the p-type organic material pentacene (a), and ntype F16 CuPc (b), both fabricated in a CMOS design on a plastic substrate.
[24]
The organic devices in general have also gained popularity in the consumer
market for displays based on OLED or AMOLED, since these applications do
not require high quality devices.
They can achieve a high brightness, high
resolution, large viewing angle, and are thinner, light weight, and less expensive
than conventional displays [25]. Organic solar cells are also increasingly popular
for their inexpensive and exible properties.
The advantage of inexpensive printing onto large area exible substrates have
not only found improved uses of current applications, but also new applications
such as pressure sensors used for articial skin [26, 27]
2.2.2 Solid Silicon
Organic materials are used for their solution processability.
Their electrical
quality, although close to the a-Si:H level, is still nowhere near polycrystalline
silicon. Silicon as it is used conventionally, can obtain mobilities of three orders
of magnitude higher in today's integrated circuits. An alternative to this is given
by their production in large areas by using chemical vapor deposition technique
that deposits a-Si:H over large areas at the cost of electrical performance.
2.2.
SEMICONDUCTORS
17
Amorphous and polycrystalline silicon
Silicon is the most common known and widely used semiconductor in the electronics industry. Various types of semiconductors are dened by their crystal
structure.
Amorphous silicon is a cluster of silicon atoms with random orientations
throughout the material.
It has relatively low electrical performance and is
currently mainly used in its hydrogenated form as solar cells or display TFTs.
Polycrystalline silicon is a material formed as a combination of multiple
crystals.
It can be directly deposited on a substrate, or formed from an al-
ready deposited a-Si lm. Excimer laser can be used on such a lm to create
a polysilicon layer that can achieve higher mobilities. These mobilities however
are limited to approximately 50 to 100 cm²/Vs.
This is due to crystal grain
boundaries that are formed from randomly produced grains during laser crystallisation. Removal of these grain boundaries can lead to devices approaching
the single-crystalline silicon level, however careful control of grain formation is
important.
Single Grain Thin Film Transistor
The Thin-Film Transistor (TFT) is a device that is similar to the MOSFET,
except that the device is produced on top of a supporting substrate rather than
within the bulk of the substrate material. The reader is referred to Appendix C
for a more detailed description about the characteristics of this type of device.
The main application for this type of device was the display industry where
large area displays are produced by manufacturing TFTs on top of a glass substrate.
This is usually done by PECVD of hydrogenated amorphous silicon
(a-Si:H) as the semiconductor, although organic semiconductors have in some instances been used as well, such as Active-Matrix Organic Light-Emitting Diodes
(AMOLEDs). Producing devices on a large area was the main aim of this type
of device. The solution processable organic materials allowed even lower temperature processing, so that other substrates such as plastics could be used that
are much thinner, cheaper and exible.
Many of the TFT types such as organic semiconductor based ones or amorphous silicon based ones still suer from low mobilities. High mobility TFTs are
desirable to be able to integrate more complicated devices on plastic substrates.
Currently many of the exible displays require external drivers that prevent the
nal product from being fully exible.
The quality of the a-Si semiconductor can be increased by Excimer Laser
crystallization of the lm into polycrystalline silicon. The random grain boundaries limit the electrical performance of the device. When the location of these
grain boundaries and therefore single grains are controlled, high quality devices
may be produced within a single grain.
Single-Grain TFTs are TFTs that are constructed within a single grain from
a polysilicon lm. The location of grains produced during laser crystallisation
are carefully controlled by grain lter cavities.
The
μ-Czochralski
method is
18
CHAPTER 2.
SOLUTION TFT PROCESS AND LIQUID SILICON
the process of creating the location controlled single grains.
step-by-step the fabrication process.
Fig.
5.1 shows
In essence an amorphous silicon layer is
deposited on a substrate with grain lter cavities. A subsequent Excimer laser
crystallization step allows a single crystal seed at the bottom of the cavity,
to grow into the molten layer on top.
This single crystal grain can be used
to produce a TFT from which the channel region contains this high quality
semiconductor lm.
Figure 2.7: SG-TFT fabrication process using the
μ-Czochralski
method. The
plastic substrate in (a), followed by deposition of a SiO2 layer (b).
A grain
lter is etched in this layer (c). Subsequent a-Si deposition lls the cavity (d).
Excimer laser melts the top layer and leaves a seed of a-Si at the bottom of the
cavity which grows to become a crystalline island on the surface (e). Within
this island a TFT is created (f ). [28]
Using the Excimer laser gives four important considerations. The rst one
requires a low hydrogen content of the silicon lm. During Excimer-laser irradiation, any hydrogen atom left inside the silicon lm may destroy the lm while
it is evaporated. Lowering the hydrogen content can either be done by thermal
annealing or laser annealing. Both cases should take the fragility of the plastic
substrate into account.
The second issue is the heat produced by the laser. The amorphous silicon
layer may absorb the laser energy but heat could diuse and destroy the underlying layers. Therefore a buer layer of silicon dioxide is used. The thickness
of this buer layer should be at least 0.5μm since the heat will diuse no more
than 300nm.
Thirdly, an excess layer of amorphous silicon should be available on top of
the cavity. This layer forms the main protection of underlying layers that are
harmed by the laser irradiation itself.
The nal challenge lies in the energy density that the Excimer laser should
and can use. A too high energy density may melt all of the amorphous silicon
and may damage underlying layers. It could also cause the limited amount of
2.2.
SEMICONDUCTORS
19
hydrogen atoms to destroy the silicon lm more easily. A high energy density
leads however to larger grains. On the other hand, a lower energy density will
result in smaller grain sizes. A bigger thickness of amorphous silicon layer could
increase the possibility to enhance the energy density of the laser.
A thicker
layer may however have more hydrogen atoms that can result in ablation. [2, 29]
2.2.3 Liquid Silicon
Organic semiconductors suer from poor stability, and relatively poor electrical
performance and are therefore still not vastly implemented in today's more complex products, despite the fact that they are compatible to solution processing.
Inorganic materials are more stable, can get higher mobilities but are dicult
to use into solution processing. One example is the use of metal chalcogenide
semiconductors achieving a mobility of 10cm²/Vs.
Mobilities of solution pro-
cessed devices approaching the crystalline silicon level are desired and would
open up many possibilities.
Silicon in conventional fabrication techniques has a melting temperature of
1414°C [30].
This means that for printing the silicon as a liquid, not only is
there a lot of energy needed, the printer itself as well as the substrate, need
to be compatible to this high temperature. Flexible substrates such as plastics
have no chance of surviving when processed in this way. Lowering the fabrication
temperature is of essence when considering material compatibility and machine
and energy costs.
This is the area where organic devices have an advantage
of low temperature solution processing, with the lack the quality and stability.
Being able to process silicon at lower temperatures, and yet producing a high
quality and stable device is essential. Using silicon in its purest form is out of
the question.
A compound that can transform into high quality silicon at low temperatures
may be the solution to this problem. This compound has been found in 1973 [31]
but has only recently found to be useful for microelectronic applications. Dr.
Shimoda et al. describe the compound used, and its transformation to silicon
lms [4, 5].
They have successfully fabricated poly-crystalline silicon devices
with the formation of an a-Si layer at a maximum processing temperature of
430°C.
Cyclopentasilane for liquid silicon
To create a high quality silicon layer, the required compound should be carbonand oxygen free [1, 7, 8]. Dangling bonds should then be avoided by hydrogenation.
The choice of the compound is then limited to a composition of silicon
and hydrogen atoms as either a straight molecule (Sin H2n+2 ), a cyclic compound
(Sin H2n ), or a multicycled compound (Sin H2n−2 ) [6]. In order to transform from
these compounds to a high quality silicon layer, the compound needs to polymerize. A cyclic structure is preferred due to its high reactivity with light so
that photopolymerization can occur eciently, these will undergo ring-opening
20
CHAPTER 2.
SOLUTION TFT PROCESS AND LIQUID SILICON
as they are exposed to UV light [6]. A single cyclic structure is preferred over
the multicycled structure due to its ease of synthesis and purication.
The number of silicon elements in the single cyclic structure should be carefully chosen. According to [1, 8], when n is more than or equal to three, the
compound is liquid at room temperature and will transform into a-Si when a
temperature of 300°C is reached. For n smaller than ten, the boiling point of the
compound will be less than 300°C, which would mean that the compound evap-
orates before it decomposes to amorphous silicon. This problem can however be
solved by the photopolymerizing the compound before thermal annealing. The
compound Cyclopentasilane, Si5 H10 has been chosen for its high photo reactivity on UV light and its relative stability[8]. In this work, the term liquid silicon
is used for any compound used in solution processing that can be transformed
into an amorphous silicon lm. This includes: pure CPS, UV irradiated CPS,
CPS mixed with an organic solution, and the combination of UV irradiation of
the CPS mixture.
The compound is also highly reactive to oxygen and should therefore be
processed in an inert gas ambient. When it is exposed to air, the material will
inammate.
Adding oxygen however after the photopolymerization step and
during thermal treatment can create silicon dioxide lms instead of a-Si. Table
2.6 lists the general information of CPS. For more detailed information about
the compound, the reader is referred to the datasheets [32].
Table 2.6: General information Cyclopentasilane
Appearance
Shipping name
Colorless liquid (at room temperature)
Pyrophoric liquid, organic, n. o. s.
(Cyclopentasilan)
Molecular Structure
Freezing point
Boiling point
-10.5°C
194 °C
g/mol
3
g/cm
Molecular weight
150.5
Density
0.963
CAS number
289-22-5
Other Information
Spontaneously Combustible
Insoluble in Water
CPS synthesis
Synthesis of CPS is found to be quite complicated. This is one of the reasons
why research in this eld has been lacking. Although there are multiple ways
to produce this compound, its quality is essential for the conversion to a high
purity silicon lm for good electrical performance of the nal devices. One of
the ways of synthesis will be described in this section.
2.2.
SEMICONDUCTORS
21
Decaphenylcyclopentasilane is rst prepared from a Wurtz-type coupling
(formation of carbon-carbon bond by the reaction of an alkyl-halide with sodium)
of dichlorodiphenylsilane and fused with lithium metal. All phenyl groups are
then substituted with bromine by a cyclosilane reaction with HBr (anhydrous
HBr has been used in a bomb tube at room temperature). This reaction will result in a benzene solution of decabromocyclopentasilane. It is a colorless crystal
that is extremely sensitive to moisture and melts at 195°C. The active breathing
vibration of this ring as well as the Si-Br bonds are at 510cm-1. By exposing
this solution to very pure ethereal (LiAlH4), the material gets hydrogenated.
Slowly it gets added to a benzene solution of bromocyclosilane. All Br groups
are substituted with hydrogen. After removing the solvent and isolation from
residue under reduced pressure, the nal product of cyclopentasilane will result
[7, 31]. Fig. 2.8 shows this process.
Figure 2.8: CPS synthesis
Structure of CPS
When considering the compound CPS, various structural formations need to be
analyzed.
In principle there are three main structures: Envelope (Cs), Twist
(C2), and Planar (D5h) [4, 5, 33]. Fig. 2.9a shows the dierences between these
three structures. [4, 5] has found that from these three structures the twist (C2)
structure is the most stable structure. Both the Twist and Envelope structures
have similar energies that dier less than 0.03meV, but the planar (D5h) structure has 50meV less than the other two structures and are therefore considered
to be less stable. Due to the low energy dierences between the twist and envelope structures, only slight distortions are required for the transformation from
one of the two to the other. The energy barrier is found to be less than 0.1meV.
Vibrational frequencies are a way to identify the dierent structures within
the CPS compound or solution.
The twist and envelope structures have the
−1
respectively. These frequencies
lowest vibrational frequencies at 2.6 and 3.8cm
correspond to the transformations between the two structures. Since the planar
structure is only a second-order stationary point which is much higher in energy,
it is less stable. Other vibrational frequencies correspond to parts of the CPS
−1
structure. The Si-H bond has a frequency from 2100 to 2200cm
. The HSi-H bond can move in three dierent ways corresponding to three dierent
−1
vibrational frequencies: Scissoring occurs from 850 to 950 cm
, rocking from
−1
−1
300 to 400cm
, and wagging at 725cm
. The breathing frequency of the
22
CHAPTER 2.
SOLUTION TFT PROCESS AND LIQUID SILICON
(a)
Figure 2.9:
(b)
Three main structures of CPS (a).
Infrared spectra displaying
vibrational frequencies associated to the CPS, red lines indicating the twist and
envelope structures.
The green dotted lines indicate intermediate structures
after the formation of the Si-H-Si bridge bond (b). [4]
pentagonal ring itself is at 344.8cm
−1
. For pure CPS, no vibrational frequency
−1
. Vibrational frequencies
should be found in the region from 1000 to 2100cm
are displayed in Fig. 2.9b.
Solvents
Although the CPS can be used unaltered during the solution deposition on a
specic substrate, in some cases it may be desirable to modify some of the uid
properties. As mentioned before, exposing the compound to UV photopolymerizes some of the CPS molecules to polysilane chains which are in turn dissolved
in the unconverted CPS. The amount of exposure relates to the amount of
polysilane chains produced that make the resulting liquid change some of its
material properties.
Another option is to use an organic solvent that can be used together with
CPS to form a less viscous and more wettable uid. Some requirements of the
solvent should be met such as:
The solvent should not react with the CPS
during any phase of the processing, and vapor pressure should be in between
0.001mmHg and 200mmHg.
A pressure lower than 0.001mmHg will result in
too slow drying which would increase the possibility of the solvent remaining
in the nal lm decreasing the silicon lm quality.
A pressure higher than
200mmHg leads to a fast evaporate which makes a uniform coating of the lm
very dicult.
Finally, the boiling point of the solvent needs to be less than
300°C so that for the resulting amorphous lm production during its annealing
step, no solvent is remaining that can disturb the quality of the silicon lm. [6]
For more information about the solvents and the reasoning behind the choices,
based on agglomeration energies, the reader is referred to [34].
2.2.
SEMICONDUCTORS
23
Research so far
Various research has been done to characterize the properties of CPS. There is
still a lot to nd out about the compound. Limited research has been conducted
in the fabrication of solution processed liquid silicon devices. Fig. 2.10 shows
the characteristics of chemical vapor deposition, spin-coated and inkjet printed
TFTs from liquid silicon by [1]. The mobilities are a lot higher than those of
2
most organic TFTs that have a mobility close to a-Si:H which is 1cm /Vs. The
2
lower inkjet mobility of 6.5 cm /Vs was claimed to be due to poor crystallinitiy
and rough surface. The spin-coated liquid silicon TFT had a mobility of 108
2
cm /Vs. Still, this mobility is limited by random grain boundaries found in the
channel region that greatly deteriorate transistor performance.
Figure 2.10: TFTs constructed with liquid silicon using inkjet printing, spincoating, and chemical vapor deposition. Transfer characteristics in (a), output
characteristics in (b), image of the TFT using SEM (c), and the schematic of
the TFT structure (d). [1]
Figure 2.11:
Single-Grain TFTs constructed with liquid silicon using spin-
coating. NMOS and PMOS transfer characteristics in (a) and (b) respectively,
SEM image of the SG-TFT in (c), and the SG-TFT schematic in (d). [2]
[2] shows the location control of these random grains, and created SG-TFTs
2
with spin-coated liquid silicon. The achieved mobilities were 391 cm /Vs for
2
electrons and 111 cm /Vs for holes. Fig. 2.11 shows the transfer characteristics
of both PMOS and NMOS.
In both works, solution deposition methods that are incompatible with rollto-roll processing have been used. In addition, a second thermal annealing step
had been conducted in order to remove sucient hydrogen atoms for an error
24
CHAPTER 2.
SOLUTION TFT PROCESS AND LIQUID SILICON
free laser crystallization. This second thermal annealing step makes the whole
process still incompatible to plastic substrates.
Therefore, in this work, the liquid silicon material has been spread by the
precursor of gravure printing: doctor blade coating. This method is compatible
with roll-to-roll fabrication. In addition, the second thermal annealing step has
been replaced by an Excimer Laser pre-annealing step which is known to unalter
the properties of underlying layers such as the plastic substrate.
Chapter 3
Doctor Blade Coating of
Liquid Silicon
In a gravure printing system the blade is known as the soul of the printer. It is
a tool used to remove excess ink that has been covering the surface of a certain
patterned substrate, and dene the patterned areas containing ink and the nonpatterned areas that are clean as a result of the blading. The excess ink may
be reused and the patterns on the roll or substrate are transferred to a target
surface.
Due to its direct inuence to the printing result, any defects on the
blade will signicantly inuence the nal result. In electronics this may be the
dierence between working and failing of an IC.
In this work, the blade has not been used as a scraping tool but as a spreading
tool. In this way, an excess layer is left on top of the substrates while the patterns
are lled. For the production of Single-Grain Thin-Film Transistors (SG-TFTs),
the position of single crystal grains are controlled by grain lter cavities that
need to be lled with the liquid silicon. The excess liquid silicon lm formed on
top of the patterned layer is needed for the grains to grow into during Excimer
Laser crystallization as well as the protection of underlying layers. The thickness
of this layer partly determines the maximum shootable laser energy density and
therefore the maximum obtainable grain size. A layer thickness of at least 100
nm is desired to be able to shoot energies similar to [2].
In this chapter, the main goal was to use the doctor blade coating method
for the deposition of a uniform amorphous silicon layer using the liquid silicon
material. First the theory enabling this uniformity is discussed. This section is
followed by the experimental part, introducing a list of equipment that has been
used as well as the general production method in Section 3.2.
Followed by a
section explaining the initial experiments for evaluating some of the liquid silicon
characteristics. Next, the main experiments for the formation of the amorphous
silicon layer is presented, and nally conclusions and recommendations are given.
25
26
CHAPTER 3.
DOCTOR BLADE COATING OF LIQUID SILICON
3.1 Theory
3.1.1 Doctor blade coating
Forces applied on uids such as the case for doctor blade coating are strongly
related to the viscosity of the liquid. The viscosity is dened as the measure of
resistance of a uid that is undergoing shear or tensile stress. For uids, viscosity
is commonly referred to as the thickness of a liquid due to the diculty of liquid
motion.
Fluid properties can be classied in a system where a material is placed in
between two large parallel plates.
The material is assumed to adhere to the
plates. The bottom plate is xed and a force is applied on the top plate in the
parallel direction of both plates. The way the material responds to a shear force
applied on the top plate can be used to classify properties of the uid.
The
situation is sketched in Fig. 3.1
Figure 3.1: Schematic of a system of two parallel plates applying shear force on
a medium present in between the plates.
A ow within the medium material is a result of shear stress. This ow is
a combination of dierent layers that move at dierent velocities as a result of
shear stress between the layers. The opposing force to this shear stress is dened
as the viscosity.
In this way, a velocity gradient exists from the top plate to
the bottom plate. This shear ow and velocity gradient denes the uid. The
relation between the applied force and the velocity gradient is described as:
−
−
→
u→
x
Fx = µA
y
In which
−
→
Fx
is the applied force on the top plate,
is the displacement,
y
(3.1)
A
is the area of the plate,
is the distance between the plates, and
µ
−
u→
x
is the propor-
tionality factor also known as the dynamic viscosity. In terms of shear stress
→
−
τx :
−
→
∂u
→
−
τx = µ
∂y
(3.2)
3.1.
THEORY
27
−−→
−
→
∂ux
u
x
y is equal to the shear velocity ∂y and is also
−
→
known as the shear strain or relative displacement γx .
The rate of shear deformation
Dierent types of uids have dierent proportionality constants or viscosities. A uid in which a relative rate of movement is proportional to the applied
force is known as a Newtonian uid. In this system
µ
is a constant. This type
includes most common uids such as water, air, glycerin, oils, etc.
A non-Newtonian uid is one where the shear stress and shear rate are not
proportional but are related.
of either shear stress
→
−
τx
µ in this system is not a constant and is a function
−
γ→
x . More complex structured uids are
or shear strain
included for this uid type such as polymers or solutions, suspensions, emulsions,
etc. Liquid silicon is assumed to be a non-Newtonian uid. [35]
The viscosity of liquid silicon can be increased by exposing the base liquid
CPS to UV. The UV produces polysilane chains that will lead to a more viscous
liquid. The increase in viscosity will result in diculty in doctor blade coating
the material.
In doctor blade coating, the situation is slightly dierent; The
top moving plate is replaced with a plate perpendicular to the bottom plate.
The plate also digs into the liquid layer allowing the liquid to be transported
in this method. A high velocity will result in a low shear velocity or relative
displacement as shown by Eq. 3.2. A movement of the blade in one direction
forces the liquid to be pushed in this same direction, however, the liquid has
a low relative displacement, which means that instead of being pushed in the
bladed direction, it will pile up and escape through the edges of the blade. This
will cause bigger tracks than in a situation where the viscosity is low, where the
liquid ows easily in the same direction as the blade. The situation is illustrated
in Fig. 3.2
Figure 3.2: Schematic of the top view of doctor blade coating, showing trail
formation.
3.1.2 Surface Free Energy
In this section, the theory behind the surface free energies will be given and is
based on [36] and [37]. The contact angle (θ) is a way to measure the surface
energy interactions between a liquid that has been deposited on a solid surface.
Fig. 3.3 shows the schematic of the energies present in such a situation.
28
CHAPTER 3.
DOCTOR BLADE COATING OF LIQUID SILICON
Figure 3.3: Surface energies denition schematic
The more than 200 years old Young equation denes the way these forces
interact with each other by:
γS = γSL + γL cos θ
(3.3)
rearranging gives:
cos θ =
in which
θ
is the contact angle,
γS
γS − γSL
γL
(3.4)
is the surface free energy of the solid,
is the surface free energy of the solid-liquid interface, and
energy of the liquid.
γL
γSL
is the surface free
Using this denition one can comment on the way the
contact angle changes with respect to a change in the various surface energies.
When perfect wetting occurs, the liquid is spread over the solid surface
without being able to form an observable contact angle. In this case the contact
angle is equal to 0°. An increase in the contact angle will result in a decrease
in the term
cos θ,
which will mean that either the surface free energy of the
liquid increases or the surface free energy of the solid decreases when keeping
the surface energy of the interface relatively constant. The opposite is needed
for a decrease in contact angle.
In this case, the force of attraction between
the molecules in the liquid and the molecules of the solid become larger than
the attraction between the liquid molecules themselves, therefore the liquid will
spread over the surface.
Using Eq. 3.3, the variables can easily be obtained and computed as long
as
γSL
is known. Various studies have been focusing on an additional relation
between
γSL , γS ,
and
γL ,
so that
γSL
can be made a function of
γS
and
γL ,
also known as the equation of state.
Berthelot was the rst to study this relation by relating the adhesion work
of the interface (WSL ), to the cohesion work of a solid and the cohesion work
of the liquid (WSS and
WLL ):
0.5
WSL = (WSS WLL )
(3.5)
using the Dupre equation on the work of adhesion, and two other relations with
the surface free energies:
WSL = γS + γL − γSL , WSS = 2γS , WLL = 2γL
(3.6)
3.1.
THEORY
a denition of
γSL
29
in terms of
γS
and
γL
had been constructed which is also
known as the Berthelot hypothesis:
0.5
γSL = γS + γL − 2 (γS γL )
(3.7)
which forms the base of following theories attempting to accurately relate
to
γS
and
γSL
γL .
Grifalco and Good introduce the parameter
φ in which the type of interfacial
interactions is further dened:
0.5
γSL = γS + γL − 2φ (γS γL )
Neumann et al. derive three equations dening
γSL
(3.8)
, the rst one was based
on thermodynamics, and the other two were based on the Berthelot hypothesis:
n
o n
o
0.5
0.5
0.5
γSL = (γS ) − (γL )
/ 1 − 0.015 (γS γL )
(3.9)
n
o
2
exp −β1 (γL − γS )
(3.10)
n
o
2
1 − β2 (γL − γS )
(3.11)
0.5
γSL = γS + γL − 2 (γS γL )
0.5
γSL = γS + γL − 2 (γS γL )
Where
β1 = 0.0001247
and
β2 = 0.0001057,
both are experimentally deter-
mined.
Partition to SFE components
Fowkes initiated the idea of a partitioning of the SFE into components due to
various interfacial interactions.
In this way the SFE of the solid is a sum of
various interactive components:
γS = γSd + γSp + γSh + γSi + γSab + γSo
the
(3.12)
d, p, h, i, ab, and o, stood for the dispersion, polar, hydrogen bond, induc-
tion, acid-base, and remaining interactions respectively.
The individual com-
ponents can be computed in various ways such as the Fowkes method, OwensWendt method, and the Van Oss-Chaudhury-Good method. They all require
the measurements of multiple liquids. A second requirement is that one of the
liquids is a dispersion liquid such as diiodomethane.
In this work we have measured multiple liquids: CPS, UV-exposed CPS,
and CPS with cyclooctane solution, however, none of these are full dispersion
liquids and can therefore not be used to compute the individual components of
the SFEs.
30
CHAPTER 3.
DOCTOR BLADE COATING OF LIQUID SILICON
3.1.3 a-Si lm formation from liquid silicon
In the experiments, liquid silicon material is used to produce an amorphous
silicon lm out of a liquid. The term liquid silicon in this context diers from
the liquid silicon used in [1] and [2], and applies to any liquid that has been
used in this work to form the desired amorphous silicon lm. This includes pure
CPS as well as a mixture of CPS and an organic solvent, as well as a either of
the liquids, exposed to UV. The description of the lm formation is given using
CPS since this forms the base material.
The compound CPS can be used to form an amorphous silicon lm after
various processing steps. The compound is liquid at room temperatures, transforms at plastic compatible temperatures and may be used to crystallize into a
higher quality silicon. Due to this property it can be seen as bringing quality
to solution processing.
The compound reacts strongly to oxygen and water. Processing can therefore
not occur in oxygen and water rich ambient for a high quality a-Si lm formation.
Although vacuum is not required, as is the case in conventional processing, an
ambient of an inert gas such as nitrogen should be used [6].
When the temperature of the compound is increased to above 300°C, the
hydrogen bonds break and leave the material, and as a result, pure silicon is
left [4, 5]. One specic issue in this scenario is the fact that the boiling point of
CPS is much lower than this decomposition temperature. Simply heating the
compound to the high temperature will therefore evaporate most of the CPS so
that almost no a-Si layer can be formed. In order to decrease the volatility of
the material, ring-opening polymerization is required [30, 7].
Photopolymerization
The compound can be photopolymerized by exposing
it to UV light of a certain range of wavelengths that can break the Si-Si bonds
(53kcal/mol). The CPS ring structure opens and can transform into (SiH2 )5 radicals. Wavelengths in between 360nm and 420nm give the best results [6, 11].
Wavelengths shorter than 300nm can result in the formation of components
that are insoluble and in addition cause diculty in forming a high quality
amorphous lm. Wavelengths above 420nm polymerize the compound slowly.
Using wavelengths of the specied range will also prevent breaking the chemical
bond of an organic solvent that may be optionally used together with CPS. This
prevents impurity carbon atoms from getting mixed into the silicon network [34].
According to Dr. Shimoda, a wavelength of 365nm gives the best results.
The UV light will structurally cause a deviation in the envelope construction
of CPS, in which the Si atom outside the Si plane deviates at a vibrational
frequency of 73.5cm-1 [4]. The UV exposure opens these rings and the resulting
opened structures can subsequently join other opened CPS structures end-toend to form long chains called polysilanes that are non-volatile.
More Si-Si
bonds can however be broken by the UV exposure, making the availability of
single long polysilane chains low. The silicon radicals that are produced by the
breaking of the Si-Si bonds can react to oxygen at the surface forming Si2 O3 after
5 minutes of UV exposure, and Si(SiO2 ) after 15 minutes of UV exposure[7].
3.1.
THEORY
31
The resulting silane compound radicals are insoluble in most common organic solvents, however it has proven to be soluble in the original low order
silane compound of CPS, as well as a mixture of CPS with a common organic
solvent [7, 8]. In this way, during UV exposure the created polysilane chains
are immediately dissolved in the precursor. UV exposure of CPS can make the
liquid more viscous at rst. After sucient UV exposure, the liquid transforms
into a white solid. This happens when the average chain length of the polysilane chains go up to 400 monomer elements [34]. The length of the polysilane
chains due to this UV polymerization have a great eect on the viscosity, wettability, melting point, boiling point, and adhesion to the substrate, and will
increase these values as the chain length increases. The reactivity with oxygen
will become lower. [34]
The UV exposure of the compound may also break Si-H bonds (76 kcal/mol).
This will cause complications in the production of polysilane chains, and will
lead to non-linearities in the chain structure, especially in the beginning of the
exposure process.
More bonds break with increasing exposure times.
It has
been shown that after 5 minutes of UV exposure already most of the Si atoms
have lost one hydrogen atom.
After 15 minutes of exposure time, hydrogen
atoms cannot be detected anymore. This eect is shown by x-ray photoelectron
spectroscopy (XPS) of the Si 2p spectrum in Fig.
3.4a.
The basis of this
experiment lies in the fact that when a hydrogen atom is attached to a silicon
atom, the binding energy will increase by 0.3 eV for every binding. The peak of
99.68 eV is accounted to the neutral silicon. The peak at 99.08 eV corresponds
to silicon among other silicon atoms, so the removal of most of the hydrogen
atoms. The other peak at 102.48 eV relates to the generation of unsaturated
surface states due to UV exposure. After 15 minutes of UV exposure the surface
is fully oxidized to Si(SiO2 ) and a electron binding energy of 103.15 eV results,
which also conrms the lack of hydrogen in the lm. [7]. Fig. 3.4b. shows the
results of gel permeation chromatography (GPC) where the size of the produced
polymers are visualized as the broad peak formation.
Thermal Annealing
Although most hydrogen atoms are removed during
the UV exposure process, there is still a need to transform the polysilanes to
a three dimensional amorphous network.
For this an elevated temperature is
required that is higher in energy than the binding energies of Si-Si and Si-H.
Temperatures less than 300°C are insucient to decompose the polysilanes, and
it will be impossible to construct a quality silicon lm. The upper limit of the
temperature is dened by the substrate. Although for the construction of polycrystalline silicon, an annealing temperature of higher than 550°C is required,
exposing the amorphous lm to Excimer laser is an alternative way to transform the amorphous material into polycrystalline silicon, resulting in higher
electrical properties without having to expose the substrate to such high temperatures [2, 6, 11]. Previous works [1, 2] have constructed an amorphous silicon
layer from liquid silicon at an anealing temperature of 430°C. A second thermal
anneal step was used for dehydrogenation before Excimer Laser crystallization
32
CHAPTER 3.
DOCTOR BLADE COATING OF LIQUID SILICON
(a)
(b)
Figure 3.4: Si 2p XPS spectra of a-Si lm for dierent UV exposure times of
CPS: a. 3, b. 5, and c. 15 minutes (a) [7]. Gel permeation chromatogram of
liquid silicon (b). CPS in toluene in a. and UV-irradiated CPS in toluene in b.
[1] The broad peak indicates the polysilanes of various molecular weights.
at 650°C. Both of these temperature are aimed to be reduced to a maximum
processing temperature of 350°C.
3.2 Experimental
An overview of the equipment used for the process of this work is introduced.
After that, the general processing steps are explained and the variables and
boundary conditions are listed. The cyclopentasilane material is highly sensitive
and needed to be treated with care.
For the production the Glovebox, UV
lamp, Hot-plate and spin-coater are used. For the measurements the RAMAN
spectroscope, Optical microscope, SEM, FTIR and DekTak prolometer were
used.
3.2.1 Equipment
MBRAUN GmbH Glovebox with Gas purication platform MB20/MB200
The
Glovebox
is a sealed box that can limit the levels of oxygen and water. In
this way, materials sensitive to these components, such as CPS, can be processed
within an inert atmosphere such as nitrogen. The box has a window on one side
3.2.
EXPERIMENTAL
33
with gloves incorporated in it for the operation of tools within the box by the
user, while the chamber keeps its controlled atmosphere.
Oxygen and water
levels can be monitored and are controlled by recirculators and the pumping of
the inert gasses.
Key features of the
Glovebox
include a gas purication system (MB 20-G)
with control panel, the antechamber, and a pressure gauge. The gas purication
system removes the oxygen and water content by continuous circulation using
catalysts. A sealed chamber (antechamber) allows the transportation of tools
and materials in and out of the
Glovebox
without changing the atmosphere
inside. A pressure gauge adjusts the pressure inside the
changes from using the gloves.
Glovebox as the pressure
Typically a positive pressure is employed by
default since air will be pushed out of the box at all times. [38, 39] A schematic
image of the
Glovebox
is displayed in Fig. 3.5.
[6, 11] note that the production of the amorphous silicon lm with CPS
can be done for oxygen levels under 10ppm.
In most of our experiments the
level of oxygen has been limited to under 0.1ppm. Only after fabrication of the
amorphous silicon lm are the substrates transported out of the
further processing and measurements.
Figure 3.5: MBRAUN Glovebox [39]
Glovebox
for
34
CHAPTER 3.
DOCTOR BLADE COATING OF LIQUID SILICON
UV AHAND 250GS
UV light has been used to photopolymerize the CPS. This photopolymerization
makes the liquid deposited on the substrate surface less volatile, and therefore
prevents total evaporation of the liquid during nal annealing for the formation
of the amorphous silicon lm. Two lters are available of which the black light
lter was used.
Fig.
3.6a.
The intensity of the produced wavelengths are displayed in
According to the T. Shimoda group, the CPS reacts best to a
UV wavelength of 365 nm which is within the range of the UV light used our
experiments. Two precautions have to be taken when using this UV light source:
1. The UV requires some warm-up time, during this time the UV does not
have its maximum intensity so exposing the liquid for a bit more time may
be useful.
2. The distance from the liquid to the UV is quite important as the intensity
of the light drops exponentially. Fig. 3.6b displays this exponential drop
(a)
(b)
Figure 3.6: UV AHAND 250GS wavelength over wavelength in (a) and intensity
over distance in (b).[39]
Hot plate
For temperature treatment of the wafer, for example for thermal annealing, a
hot plate has been used since it is small enough to be placed inside the glovebox.
3.2.
EXPERIMENTAL
35
It is able to reach high appropriate processing temperatures while the user is
allowed to observe changes of the heated sample. Overshoot of approximately 10
°C
is a problem with this device even for temperature increases of 10°C. Since
the release of hydrogen atoms during annealing produce silicon radicals and
dangling bonds, the liquid silicon material is prone to contamination even in a
controlled atmosphere with limited levels of oxygen and water. A quartz lid has
therefore been used when annealing the substrate to the high temperature so
that no air can enter the amorphous lm during heating and release of hydrogen
atoms cause a slight overpressure inside the lid.
Optical Microscope
An optical microscope has been used as a preliminary analysis of the wafer. Images could be taken by a mounted camera on top of the microscope. From these
images a rst impression can be made from the resulting lm after annealing,
but also from laser crystallisation for the next chapter.
SEM
The scanning-electron microscope gives a more thorough analysis of the way
the patterns on the substrate are lled by the solution. It is a high resolution
microscope that can produce images in the nanometer range. The basic working
is the collection of various signals that are formed when a focused beam of high
energy electrons are incidented to a sample. The large amount of kinetic energy
of the electron is during absorption of the sample transformed into: secondary
electrons, backscattered electrons, diracted backscattered electrons, photons,
visible light and heat. Secondary electrons and backscattered electrons are the
most important elements for the imaging of the sample. A cross-section of the
wafer is viewed by carefully breaking the wafer with the resulting amorphous
lm from the liquid silicon compound. This not only shows the lling of the basic
patterns, also grain lters are analyzed in this way during the rst experiments.
The downside of this experiment is that the broken wafer cannot be used for
further processing. [40]
Renishaw's inVia Raman Microscope
The optical microscope only gives away the position of the liquid silicon by a
change in color. To be sure that this change in color is accounted to the deposited solution, RAMAN spectroscopy is used. The basic working of this tool
is identication of a vibrational frequency shift of reected light from a sample,
compared to the original laser source incidented on a sample. This frequency
shift is also known as the RAMAN eect and is based on the molecular deformation of the sample into oscillating dipoles due to the electromagnetic wave
from the excitation source.
The periodic deformation results in the material
specic vibration of the molecules. By far, most of the re-emitted photons are
subject to
Rayleigh scattering
that has the exact same frequency as the original
36
CHAPTER 3.
DOCTOR BLADE COATING OF LIQUID SILICON
light source and is of no use for measurement purposes.
reected light is the result of
Stokes eect
Only 0.001% of the
and needs to be ltered. [41]
With this method, the dierence between amorphous silicon and crystalline
silicon can be visualized.
Crystalline silicon will re-emit a narrow peak at a
−1
due to a high uniformity of bond angles and
vibrational frequency of 521cm
bond lengths in the material.
Due to a wide array of bond angles, energies
and lengths as well as dangling bonds, amorphous has a wider peak in the
−1
vibrational spectra positioned at 480cm
. Since CPS is transformed to an
amorphous silicon lm on top of a crystalline silicon wafer the two peaks can be
distinguished and it can therefore be determined if there has been an amorphous
silicon formation.
The light source used for this type of measurement may
crystallize the amorphous silicon itself so a low intensity of 125
µW
has been
used for the measurement. [42]
Veeco Dektak 150 Prolometer
The two-dimensional, prole of any surface can be visualized by using the Dektak 150. It uses a high quality, low force stylus (Low-Inertia Sensor, LIS 3) that
runs in a straight line across a surface and plots the changes in height of the
stylus while it encounters various patterns on the target surface. A resolution of
1 Å can be achieved, at the lowest range setting of 6.5
μm, which was commonly
used in this work since pattern depths were less than 500nm. Due to the limited
range setting, accuracy issues were inevitable. The force of the stylus was set
at 1mg. A video camera allowed the manual positioning of the stylus.[43]
FTIR Spectrometer
Fourier Transform Infrared spectroscopy is a tool that can measure the amount
of certain atomic bonds present inside a lm.
In this work, the bounded hy-
drogen content is measured within the a-Si lm formed from liquid silicon. A
source of infrared radiation is sent to through a sample which absorbs some of
the radiation.
This molecular absorption and transmission is measured.
The
absorption peaks are related to the vibrational frequencies of certain bonding
types (vibrations with a transition dipole moment).
In this way, the bonded
hydrogen can be measured by sensing the distinctive wagging and stretching
vibrations of the monohydride (Si-H), the dihydride (SiH2 ) and the trihydride
(SiH3 ). The wagging mode of the congurations (e.g. the degenerated rocking
and wagging of Si-H and SiH3 , and the pure rocking for SiH2 ) are all located
−1
at 640cm
. The amount of bonded hydrogen present inside the lm (NH ) is
related to the integrated absorption of the predened absorption peak. In case
of the wagging mode:
I640 =
ˆ a
dω
ω
NH640 = A640 · I640
(3.13)
(3.14)
3.2.
I640
EXPERIMENTAL
is the integrated absorption of the 640cm
37
−1
sorption coecient found from the measurement,
and
A640
absorption peak,
ω
α
is the ab-
is the vibrational frequency,
is the proportionality factor which related to the strength of oscil-
lation. This proportionality factor is experimentally determined for which the
9
−2
constant 1.6·10¹ cm
is commonly used for a-Si:H. In this work the I640 of
dierent UV exposure times and thermal annealing procedures are evaluated.
Note that only bound Hydrogen atoms are obtained in this way. [44, 45]
3.2.2 Working procedure
Initial experiments were conducted using a template procedure from which many
variations have been tested for the optimization of the lm formation process.
This template is based on [2]:
1. A crystalline silicon wafer is prepared on top of which TEOS is deposited
using PECVD and is patterned.
2. On top of the substrate a predetermined number of drops of 100%CPS
will be deposited by means of a pipette.
3. The drop will be spread by a doctor blade that will lead to a lling of any
patterns that are present on the surface.
4. After having spread the CPS across the full wafer, the excess is left on top
of the wafer to form a protective layer for the Excimer Laser process.
5. The substrate will then be exposed to UV lighting for 10 minutes, during which the CPS on top of the substrate polymerizes to polysilanes,
preventing the lm from evaporating during the annealing step.
6. The wafer is placed on the hot-plate, and is covered with a quartz lid to
protect the lm from oxidizing during this annealing step while allowing
the evaporated hydrogen to escape. This hot plate is heated to 200°C.
7. After 1 minute of thermal treatment of 200°C, the temperature of the
hotplate is increased to 430°C in 10 minutes.
8. After 1 hour of 430°C thermal annealing, the wafer is slowly cooled down
to room temperature.
Changes to this basic procedure have been applied to understand how the uid
behaves and which procedure gives the best results. Besides the 100% pure CPS
that has been used in many of our experiments we can use other variations on
the liquid silicon material for the creation of the amorphous lm.
The variations to the liquid silicon that we have used include pure CPS
exposed to UV for dierent numbers of minutes, as well as the CPS mixed with
the organic solvent cyclooctane with 20%wt. of CPS against the solvent. UV
exposure of this mixture has also been used for a similar liquid silicon as used
38
CHAPTER 3.
DOCTOR BLADE COATING OF LIQUID SILICON
in [2]. The time of the UV exposure is varied but it is ensured that the resulting
uid does not solidify as it is the case for long UV exposure times.
Many dierent changes can be made within this template procedure. Table
3.1 lists these varieties.
The initial approach was to try various combinations on a substrate to see
what combination gives what type of eect.
This was followed by a number
of repetitions of the best results and attempts to optimize the best obtained
results.
3.2.3 Boundary Conditions
The experimental methods we used had a number of limitations.
These lim-
itations slowed down the optimization process and made repeating processes
inevitable as well as inconsistent.
Blading
Many dierent variables existed during the blading of the liquid silicon
on the substrate.
This was primarily due to the physical inconsistency
since blading was not automated but done by hand. Speed, force, angle of
blade, blading directions, vibrations from the hand were all variables that
could not be kept as a constant and should always be accounted for.
Impurity
Wafers as the target substrates transported into the glovebox from
the open air have some oxygen or water molecules sticking on the surface.
This would inuence the way the liquid silicon is spread on the wafer as
well as the purity of the deposited material. Another important aspect was
the way the 100% CPS was stored for usage. This was in a brown jar on
top of a shelf inside the glovebox. The glovebox although limited in water
and oxygen content, could cause degrading of the material over time due
to not only the molecules in the atmosphere, but also temperature changes
from the hot-plate placed inside the box. Transportation of tools inside
the glovebox increases the oxygen and water content to 15ppm at most.
Equipment
Inaccuracies during measurement can be misleading for the judg-
ment of the nal result. Primary judgment in many cases come from the
optical microscope.
Roughness of the surface although visible optically
through changes in colors, was not detectable using the prolometer or
other tools that we have used.
3.3 Characterization Experiments
Before attempting to get the best amorphous silicon lms, some basic characteristics of the liquid silicon material should be identied. In this section, some
important aspects of the liquid are presented: Thickness of the lm, the eect of
dierent types of liquid silicon, the surface free energies of various surfaces that
we have used, the RAMAN proof that the liquid indeed transforms into amorphous silicon, and FTIR results for the eect of the variations to the standard
procedure to the bounded hydrogen content.
3.3.
CHARACTERIZATION EXPERIMENTS
39
Table 3.1: Processing varieties from liquid silicon to amorphous silicon lm
Material usage
The type of liquid silicon material. Either pure CPS or a certain dilution of
CPS with cyclooctane before deposition on the wafer. Also the UV
pre-exposure times can be varied to change the viscosity of this base material
or other properties of the uid.
The type of material used for the blade can change the way the uid is spread
on the substrate. A hydrophillic blade will cause the uid to stick more to the
surface of the blade whereas a hydrophobic blade will push the uid away from
the blade. The mechanical stiness of the blade will also change the spreading
characteristics
The surface of the substrate can be modied to increase or decrease adhesion
of the uid, or a combination of both. Some treatment can modify the
roughness of the surface, others simply result in a change in surface tension of
the source uid.
Temperature
The time the substrate is annealed at the nal temperature can be extended
for a better quality amorphous silicon lm, although it is desirable to decrease
this time for production throughput reasons.
The ramp up and cooldown of the wafer during annealing can be speeded up
or slowed down. Speeding the temperature change up will give a negative
inuence on the lm due to the sudden shock of temperature dierence that
the lm is exposed to. It will however decrease processing time.
The maximum annealing temperature can be increased to increase the quality
of the silicon lm, or decreased to make it possible to produce the lm on top
of a plastic substrate such as polyimide.
Pretreatment of the substrate surface can change the adhesion characteristics
due to the molecules that are by default sticking to the surface.
Heating the substrate during deposition and spreading to a slightly elevated
temperature will change the adhesion properties.
UV exposure
Exposing the uid to UV will photopolymerize CPS into polysilanes that
dissolve in the non-transformed CPS. This will make the uid less volatile and
therefore prevent the lm from evaporating during annealing. A longer
exposure time will lead to more CPS transforming into polysilanes, as
explained in Section 3.2.2 .
Not only is UV exposure a way to make the liquid silicon less volatile, the
uidic properties will change as well. Exposure will make the liquid more
viscous which will give a dierent eect when spread on the substrate after
UV exposure of the CPS.
Blading
Speed of blading
Force applied on the substrate during blading
Angle of blade to the surface
Direction of blading with respect to the patterns
Moment of blading after drop deposition on the substrate
40
CHAPTER 3.
DOCTOR BLADE COATING OF LIQUID SILICON
3.3.1 Film breaking
The very rst of the observations was one that focuses on the thickness of the
deposited liquid silicon material. Some problems with the lm occurred for areas
on the substrate with a lot of liquid silicon. This was not necessarily a direct
cause of the thickness of the layer, but the gradient of the layer seemed to have
quite some impact on the lm.
This was observed when pattern depths had
increased leading to a bigger change in step height of the resulting amorphous
silicon lm.
The deeper patterns lead to observable pattern edges after lm
deposition. These visible edges were the rst parts that cracked during baking.
On the same wafer a smooth area was observed that did not break during
annealing.
Areas where the gradient exceeded a certain level resulted in the
cracking of the layer. This was the case for four situations:
1. Excess liquid silicon that has been pushed to the edge of the wafer have a
bigger thickness and are more prone to thickness variations and therefore
breaking in many cases initiate at these excess areas.
2. Trails of the liquid silicon material caused by the blade are locally thicker
and will break rst during thermal annealing.
3. Patterns on the substrate that are relatively deep and cause a deformation
of the deposited lm in step-height dierences will break at these step
heights.
4. Edges of the wafer or areas dening a covered and an empty area may
cause breaking of the lm at the junction, this however is not a strong
source of these cracks.
DekTak images such as Fig.
3.7 prove that the thickness of the lm is not
particularly a problem, but the thickness variations are the cause of the breaking
of the layer.
The layer leading towards the empty space, which is where the
crack formed, is getting thicker, while the thickness of the surrounding layer
itself is in all four cases dierent. The broken particles had been removed using
Isopropanol (IPA) and left an area without liquid silicon which was useful as a
reference for the DekTak.
The amorphous silicon layer had various colors that indicate to some extent
the roughness of the layer.
Cracked areas on the other hand created many
small silicon particles that changed its color visibly during thermal annealing,
without using the optical microscope. These colors do not indicate the surface
roughness, but the quality of the silicon layer as more polysilane structures
break and form cross connections with neighboring polysilanes to form an a-Si
network. The transition went from colorless to white at around 200°C, yellow
from approximately 200° to 300°C , maroon from around 330°C, and nally
silver after some time at higher temperatures. The exact temperature on which
the transition occurred was undened since it is also dependent on the duration
of heating.
3.3.
CHARACTERIZATION EXPERIMENTS
Figure 3.7:
41
DekTak graphs with proles from various surfaces on which liq-
uid silicon has been transformed into amorphous silicon. The area where the
amorphous silicon has been removed is where the layer was broken.
3.3.2 Liquid Silicon
The term liquid silicon in this work is used for a liquid material that can be
converted into a solid amorphous silicon lm using the procedure as described
in Sec. 3.2.2. This means that there are many variations that can be used to
the base 100%CPS material. The liquid used in [1] and [2], are both solutions
of 100%CPS with cyclooctane that has been UV irradiated before being spincoated. In this work, experiments have been conducted on the UV irradiated
100%CPS and the 100%CPS mixture with cyclooctane solution and pure CPS.
The advantage of using the cyclooctane solution is the increasing wettability
characteristic of the liquid silicon. While UV exposure makes the liquid more
viscous and thicker, the organic solution can make the liquid less viscous. Using a CPS-cyclooctane mixture, during thermal annealing the cyclooctane will
evaporate leaving only the CPS that has been converted to polysilanes during
photopolymerization. A number of issues exist when using the organic solution:
1. The evaporation of cyclooctane means that a lot more liquid needs to be
42
CHAPTER 3.
DOCTOR BLADE COATING OF LIQUID SILICON
deposited to get a similar amorphous lm thickness as when using pure
CPS. When only small amounts are used, as possible for blading, the
solution quickly dries out.
2. The mixture introduces carbon atoms in the lm that degrade its quality,
although the boiling point of cyclooctane (149°C) is slightly lower than
that of CPS (194°C).
3. When using only a little amount of the liquid silicon, which is possible
for doctor blade coating, the organic solution dries rapidly.
For spin-
coating this problem does not exist, as large amount of the liquid silicon
is deposited and spread across the wafer in a matter of seconds. In doctor
blade coating however, drying will occur during every movement of the
blade causing uniformity issues.
4. Grain lter cavities lled with the mixture of CPS and cyclooctane result
in amorphous silicon that has shrunk inside the lter. As the cyclooctane
evaporates, part of the amorphous silicon is already solidied and cannot
rell the grain lter.
Smaller cavities inside the grain lter will cause
issues during laser crystallization.
Due to these issues, this work does not use any organic solution for the production of the liquid silicon SG-TFT.
Exposing the CPS to UV, will make the liquid more viscous.
A longer
exposure time results in more polysilane production while at the same time
being dissolved into the source CPS material. Varying this exposure time can
change the properties of the liquid silicon such as viscosity, volatility as well
as wettability. Increasing the viscosity through UV exposure has lead to track
formation when doctor blade coating which is undesirable. Omission of this step
results in the evaporation of CPS before the decomposition to a silicon lm.
3.3.3 Surface Free Energy
The way a liquid reacts to a solid surface is dependent on the surface energies
associated to both materials. These surface energies can be obtained by measuring the way a droplet is formed on the solid surface, i.e. the contact angle.
A liquid can in this way obtain controlled spreading as is also used in oset
lithography printing.
Three surface modication techniques as well as a number of dierent substrates have been tested. The modication techniques are: 0.55%HF dip for 4
minute which is a common way to remove native oxide from a silicon wafer at
a rate of 15nm/min, O2 plasma and Argon plasma that bombard the surface,
making it rougher. The HF dip does not signicantly deform the patterns on
the substrate, and Argon is a heavier alternative to Oxygen. Fig. 3.8 shows the
dierent contact angles of pure CPS. The two thermal oxide surfaces indicate
dierent sessions. It is important to note however that in most cases the contact
angle reduced over time indicating a dependence on the time that the liquid is in
3.3.
CHARACTERIZATION EXPERIMENTS
43
Figure 3.8: Contact angle graphs, dierent surface modications on dierent
types of oxide, using pure CPS.
contact with the surface. The experimental data is based on the contact angles
approximately 1 to 2 seconds after droplet deposition.
The main conclusions
that could be derived from these were:
Temperature
An elevated temperature of the substrate has a big eect on the
wettability of the liquid silicon. This is both because it is more dicult
for the molecules within a droplet to stay together due to thermal motion,
and because the liquid reacts in some way with the oxide surface. Both
eects allow the liquid to spread over time.
Surface treatment
0.55%HF dip for 4 minutes gave the best wettability prop-
erties compared to most of the plasma surface modication types. Adding
the elevated temperature of 75°C shows that the HF treatment gives better
wettability than high pressure, 550W Argon, that shows a lower contact
angle at room temperatures. The plasma treatment, bombards the surface
with either Oxygen or the heavier Argon molecules which make the surface
more rough allowing more of the liquid silicon molecules to react to the
surface. The HF however makes the surface more smooth which allows an
easier spreading. Varying the plasma conditions for Oxygen plasma gave
only limited variations to the nal contact angle.
Liquid Silicon
The variations of the liquid silicon material from the base CPS
compound has dierent eects on the contact angle. A 20wt.% of CPS in
cyclooctane reduces the contact angle on all surface to zero. UV exposing
the base CPS material makes the liquid more viscous, which lead to a
higher contact angle on all surfaces.
Material
The surface material, although for SG-TFT a TEOS surface is re-
quired, for the nal gravure printing cylinder, other material types may be
used. Aluminum showed the highest wettability of liquid silicon, whereas
silicon nitride gave the highest contact angle of 33.4°.
44
CHAPTER 3.
DOCTOR BLADE COATING OF LIQUID SILICON
From the contact angles, the surface energies of the various surfaces can be
computed since the surface energy of the liquid (CPS) is calculated in [46] to
be 32.5 mJ/m².
In this work 3.10 has been used to determine the surface energy by rearrangement in 3.3:
0.5
(γS /γL )
By obtaining
γL
n
o
2
exp −β1 (γL − γS ) = 0.5 (1 + cos θ)
(3.15)
of CPS from [46] (32.5 mJ/m²), and by measuring the contact
angle of CPS on various surfaces, the surface free energies of these surface can
be calculated using Matlab.
(a)
(b)
Figure 3.9: Surface Free energy gures using 3.15. Extracting theγS (a), and
the
γL
(b) by sweeping the respective parameters.
Sweeping the function for various values of
of
0.5(1 + cos θ), two solutions can be obtained.
γS
and equating this to the line
The rst solution is found to be
correct by comparing an the eect of an increasing contact angle which should
decrease the surface free energy of the surface, which was only the case for the
rst of the two solutions. A graph of the plot is shown in Fig. 3.9a. matlab
graph needed. The nal results are presented in a table in Appendix D, while
the graph is presented in Sec. 3.3.3. In the same way, after nding the various
γS
parameters, an unknown
γL
can be computed, by using the same surface
modications for a dierent liquid. Sweeping the liquid in this case gives the
correct result. In this work, CPS that had been exposed to 2 minutes of UV
was found to have an average SFE of 33.5 mJ/m². The graph obtained from
sweeping
Fig.
γL
is shown in Fig. 3.9b
3.10 shows the results of computing the surface free energies for all
surfaces used in our experiments. Notice that the surface free energy increases
when the contact angle decreases.
The contact angle for aluminum was 0°,
however using Eq. 3.10, a maximum equal to the surface energy of CPS was
obtained. The actual SFE of aluminum is slightly higher than this number.
Using the same equation (Eq. 3.10), the SFE of another liquid that has been
tested on the same surface can be calculated. A second liquid has been used
3.3.
CHARACTERIZATION EXPERIMENTS
45
Figure 3.10: Surface energies calculated using Neumann's method
for one series of contact angle experiments: CPS exposed to 2 minutes of UV.
From the results an average SFE of 33.5 mJ/m² has been obtained.
3.3.4 Blade types
Although SFEs show some important eects of the liquid reacting to various
surfaces, in the spreading process it dominantly comes down to using either a
exible blade or a rigid blade. The exible rubber blade, when used with little
force, allows some spreading of the liquid, however, as soon as a minor force
is applied, it digs into patterns and removes some of the liquid from inside the
patterns, allowing only little liquid to remain inside the patterns. Bigger and
shallow patterns in this way are more prone to lose their liquid silicon than
compared to smaller and deeper patterns. Rigid blades such as silicon, glass,
titanium nitride, silicon nitride, did not show this removing property when
similar forces were used as the rubber blade process. It is however dicult to
remove excess when using these types of blades. Automated blading may give
accurate excess removing results as is currently used in gravure printers.
Fig. 3.11 shows the dierence between silicon blading, the combination of
silicon and rubber blading, and rubber blading with force.
This shows that
using the force applied on the rubber blade has a big impact on the nal result.
SFE energies have limited inuence on the blade type since none of the
blade types have a large contact angle, and since the liquid sticks onto the
surface of the blade during every spreading movement. Over time the contact
angle decreases, and more and more liquid silicon adheres to the blade making
the type of the blade insignicant. The elasticity of the blade proves to be the
dominant factor in this manual spreading process.
46
CHAPTER 3.
DOCTOR BLADE COATING OF LIQUID SILICON
(a)
(b)
(c)
Figure 3.11: Blade type results of silicon only (a), silicon and rubber (b), and
rubber with additional applied force (c)
3.3.5 Post-deposition variations
Variations to the procedure after the liquid silicon has been spread, are the steps
of photopolymerization under UV and thermal annealing. Variations in both the
duration of UV, the duration of the maximum temperature during annealing,
and the maximum temperature itself can inuence the amount of hydrogen
that is contained inside the nal silicon lm. A reduced maximum temperature
however should ensure that the lm still transforms into amorphous silicon.
Although [2] used 430°C, [7] uses a lower maximum annealing temperature of
350°C. At this temperature, some plastics such as polyimide may survive, and
can therefore be incorporated in this fabrication procedure.
RAMAN spectroscopy conrms that the lm was successfully transformed
into amorphous silicon after using the reduced temperature. All following experiments were therefore conducted at the lower temperature. Fig. 3.12 shows
the results of RAMAN spectroscopy, where the broad peak indicates the a-Si
−1
and the narrow peak at 521 cm
indicates the crystalline silicon. Both peaks
are visible in the rst gure since the light can go through the thin a-Si layer.
The FTIR is used to measure the hydrogen content of the lm produced with
various UV exposure times. Using MatLab, the area of the peak has been used
−1
to extract the integrated absorption at the wagging frequency of 640cm
. The
proportionality factor for an a-Si lm produced from liquid silicon is unknown,
−1
therefore the areas under the 640cm
peak of the samples are compared. Fig.
3.13 shows the content of the dierent process variations.
An increased UV
exposure time results in a lower integrated absorption which is proportional
to the hydrogen content.
20 Minutes of UV exposure has been used in most
experiments.
3.4 Film spreading recipe
Two types of wafers were used in the experiments. Although both wafers have
the same deposited layers of TEOS on top of a crystalline silicon substrate, and
3.4.
FILM SPREADING RECIPE
(a)
47
(b)
Figure 3.12: RAMAN spectroscopy result of a thin a-Si lm (a) and a thick a-Si
lm (b), both annealed at 350°C for 1 hour.
Figure 3.13: FTIR graph of absorption peak integrals at 640cm
−1
, for three UV
exposure conditions
although both wafer types have matrices of grain lters spread across the surface,
one of the two wafers is at and the other has 250nm deep 1 by 1 mm square
patterns inside which the grain lters are positioned. This has been done since
blading on a at surface could lead to a layer that is too thin since essentially
the blade is scraping the liquid over the surface. Therefore, the squares could
ensure that the layers on top of the grain lter cavities is somewhat thicker.
The laser could be aimed at these squares.
A proper lm production using doctor blade coating is desired. The results
from blading are compared to the spin-coating method used in other works
[1, 2]. Also the combination of blading and spin-coating, although incompatible
with roll-to-roll processes, have been experimented with. A thickness of at least
100nm on top of the surface is desired to be able to produce grains similar to
[2] since this forms the protective lms against the laser for underlying layers,
as well as a source for the grain seed to grow into large grains. A thick uniform
48
CHAPTER 3.
DOCTOR BLADE COATING OF LIQUID SILICON
layer is therefore desired during the optimization of this process.
3.4.1 Spin-Coating
Few spin-coating tests were conducted on the available wafers, since spin-coating
in general costs quite some amount of material. Flat wafers lled with matrices
of grain lters have been used for uniformity reasons. A spin-coated wafer by Dr.
Shimoda's group resulted in a layer of approximately 100nm thick. Although we
a higher thickness was desired, when repeating their experiment with the same
RPM we obtained a much lower thickness. The liquid silicon material was pure
CPS dissolved in cyclooctane (20%wt.) and exposing this to a few minutes of
UV. This had been spin-coated at 2000RPM, and a nal lm of approximately
50nm was found. This is assumed to be due to the little time of UV exposure.
The alternatives of pure CPS and UV pre-exposed CPS were used in subsequent
experiments for the reasons stipulated in Sec. 3.3.2, as well as to improve the
thickness of the layer.
(a)
(b)
(c)
Figure 3.14: Spin-coating experiment results. Double coating of CPS and UV
pre-exposed CPS (a), 20 minutes UV pre-exposed CPS only (b), 20 minutes UV
pre-exposed CPS only with 0.2μm lter (c).
First, to increase the thickness, multiple spinning was considered. The rst
layer of pure CPS was spun at 2000RPM, followed by a second, much thicker
layer of UV pre-exposed CPS spun at 1000RPM. This second layer, although
it attached better to the surface due to the already existing thin CPS layer,
resulted in a less uniform layer, and did not spread out across the full wafer. No
cracks were formed on the wafer, so it was hard to measure the thickness. During
the Excimer Laser ablation test, where energies have slowly been increased in
order to nd the energy at which the lm ablates, we found a level that was
similar to the level of ablation of one of our bladed wafers with a thickness of
just 30nm.
For the next experiment, one single spin-coating event with a thicker, 20
minute, UV pre-exposed CPS layer, spun at the limited speed of 1000RPM has
been conducted. This decrease in RPM, although it would lead to a less uniform
3.4.
FILM SPREADING RECIPE
49
spread of the liquid, the irregularities caused by the rst spinning event in the
previous experiment caused the limited spread of the liquid silicon material.
This decrease in spin speed will result in a thicker layer and even if it would not
cover the whole wafer, we wished for it to cover at least some limited number
of dies.
The second experiment, as expected, did not cover the wafer well even though
an increased amount of UV pre-exposed CPS had been used compared to the
previous experiment. The covered area broke completely due to the thickness
variations combined with the large amount of dierent sized polysilanes that
cause the nal layer to lack uniformity and spreadability. Although the thickness
of the cracks was approximately 400 to 450nm as observed from the DekTak
prolometer, the test of ltering out dierent lengths of polysilanes had not
been executed before. Some parts of lms from this experiment had a thickness
of 50 to 100nm.
The third spin-coating experiment had been conducted by using the exact
same UV pre-exposure time from the second experiment of 20 minutes, and
ltering various sized polysilanes out with a 0.2μm lter. Only little liquid had
been deposited due to the lter which reduced the total amount.
This little
liquid rst of all resulted in a worse wafer coverage, and also created a layer
that had cracked all over.
From these experiments the CPS with pre-exposed UV lead to many errors
in the resulting lm. Using CPS only for spin-coating will result in a thickness
which is too low.
A combination of a pre-existing layer such as used in the
rst of the listed experiments, should help cover a bigger area with the liquid
silicon material.
This is due to the time-dependency of the liquid with the
TEOS as found in the SFE experiments in Sec. 3.3.3. But to prevent using a
lot of CPS material for this initial layer, the layer can be bladed with limited
number of droplets to at least cover the whole wafer once. The three spin-coating
experiment results are displayed in Fig. 3.14.
3.4.2 Blading
Blade types have been investigated in Sec. 3.3.4 and it was found that the rigid
blade type was a good way to spread the liquid silicon and ll the patterns,
whereas a exible blade type could be used to remove excess liquid.
A com-
bination of the two lead to the 1 by 1 mm squares losing their contents due
to the size of these squares and the rubber blade. The resulting thickness was
approximately 30nm. The combination had also been tested with less force on
the rubber blade and omission of the rubber blade. Cracks in the latter process
were only observed in limited parts, and revealed a thickness ranging from 200
to 300nm which was our desired thickness. It was also found that the resulting
layer did not ll the squares but simply formed a layer with a similar step height.
Grain lters however did get completely lled displayed in Fig. 3.16.
Care has to be taken when a step height in any part of the wafer is present
since this in many cases leads to a crack in the resulting a-Si layer. Although the
surface modication techniques on the blade would have limited eect, the sur-
50
CHAPTER 3.
DOCTOR BLADE COATING OF LIQUID SILICON
(a)
(b)
Figure 3.15: Proles of a-Si layers deposited on the wafer with 1 by 1 mm 250
nm deep square patterns. Fully covered layer in (a), and bulged square coverage
in (b).
face of the substrate was treated with 0.55% HF for four minutes, that showed
together with the spreading at an elevated temperature to give lowest contact angle and therefore the highest wettability. This means that layers prefer
to smoothen out over the surface rather than forming a droplet, avoiding the
thickness gradient that results in the lm cracking.
Increasing the number of drops may help against step heights from patterns,
however, the thicker layer also induce trails from the blade, as well as thicker
excess around the edges when these are not removed o the substrate.
Both
eects lead to breaking of the lm, and therefore, the amount deposited on the
wafer should be limited.
Wafers have been exposed to a 100°C before liquid silicon deposition since
this step would remove any water molecules attached to the wafer that would
inuence the quality of the nal lm.
Square patterned wafer
The wafer with square patterns are prone to cracking at the edges of the squares.
Using this type of wafer however, resulted in a thicker layer when compared to
at wafers. Also, the liquid adhered slightly better to this patterned surface.
Limited wetting, square patterned wafer, may result in the lling of these
squares without forming a continuous lm over the wafer surface. Optically this
seems to be a good property, however, these areas break very easily since they
are only local lms with a bulging behavior towards the center of the square as
shown in Fig. 3.15b, and should therefore be avoided. Blading multiple times
helps form a continuous lm, however makes the lm also prone to blading trails.
Experiments where the spreading temperature was set at 75°C show a good
nal result with a nal thickness ranging from 100 to 300nm.
An issue in
3.4.
FILM SPREADING RECIPE
(a)
51
(b)
Figure 3.16: SEM images of liquid silicon covering a pattern instead of lling it
(a), and the lling of the grain lter (b)
the process was that spreading the liquid across the surface and sustaining the
covered surface was still dicult although easier than when spreading at room
temperature.
Additional tests were conducted with increased spreading temperatures to
90°C and 110°C. As the temperature is increased, the liquid deposited on the
wafer slowly evaporates.
An increased temperature also greatly reduces the
surface energy of the substrate and the liquid silicon material shows very good
wetting.
Care has to be taken however that only limited number of blading
strokes should be used since the lm becomes thinner as it is spread, and trails
start to form on the surface when this layer is thin enough, which would lead
to a poor lm quality.
The faster evaporation at 110°C caused the edges of
the square wafers to appear, therefore the slightly lower temperature of 90°C
showed good, reproducible results although thickness of the layer was limited to
approximately 100nm. Some of the experiments on this wafer type are displayed
in Fig. 3.17.
Flat wafer
When using the at wafer for the doctor blade coating of liquid silicon without
spreading modications, it was found that the resulting thickness was quite low.
In addition, the smooth surface made it more dicult for the liquid to sustain
a continuous lm, only after multiple blading attempts did the liquid adhere
to the surface, forming a continuous layer.
Blading should be done carefully
on this wafer type, especially when spreading the liquid at room temperature.
An elevated temperature however, leads to a better wetting of the liquid on the
surface. Cracks on the at wafer can now only be formed by the excess liquid
and trails from the blading due to the lack of patterns on the surface.
This
also allowed a slightly higher spreading temperature of 110°C. The increased
wettability allowed minimal blading for full surface coverage. A relatively thin
52
CHAPTER 3.
DOCTOR BLADE COATING OF LIQUID SILICON
(a)
(b)
(c)
Figure 3.17: Some blading experiment results for the formation of a lm. Silicon blade spreading and rubber scraping with partly hard rubber scraping (a),
silicon blade spreading with partly mild rubber scraping (b), edge formation
and cracking in 500nm square wafer with a small part showing a uniform layer
(c).
layer of approximately 100nm resulted, with good reproducibility.
Due to the lack of patterns, an additional spin-coating step could be used to
create a thicker layer on top of the surface.
3.4.3 Combination
The thickness of the layer when only spin-coating is quite low, whereas the
thickness of the layer when using blading on a at surface is also relatively low.
A combination of the two can help increase this thickness. Initially, when the
substrate is bladed, the lm covers the surface.
A second, spin-coating step
allows a better adhesion of the additional CPS droplets on the wafer so that
more of the wafer surface is covered during this subsequent step. Trails from
blading should be minimized to ensure a better wafer coverage when spinning.
This additional step resulted in a lm thickness of approximately 250nm. Some
of the results are displayed in Fig. 3.18.
3.5 Conclusions and Recommendations
Conclusions
A general procedure has been followed in the formation of an a-Si lm from liquid
silicon. Some initial experiments had been carried out for the characterization
of the lm, before optimizing the layer formation.
1. Creating a layer of liquid silicon with excess for a buer layer gives some
diculties in uniformity issues and cracking of the layer during thermal
annealing.
The cracking is in many cases a result of layer step-height
dierences, as observed from excess layer bladed to the edge of the wafer,
3.5.
CONCLUSIONS AND RECOMMENDATIONS
(a)
(b)
53
(c)
Figure 3.18: Results of the combined coating methods blading and subsequent
spin-coating at 500RPM (a), 1000RPM (b), 1000RPM on a polyimide substrate
(c).
the edge of the lm in general, trails of thick CPS from the blading, and
lm deformation due to substrate patterns.
2. Liquid silicon is a material using the base compound CPS, and may include
the irradiation of UV, the mixture with the organic solvent cyclooctane,
or a combination of the two. In this work, it has been found that using the
organic solvent can cause various issues such as: evaporation and drying
issues, carbon contamination, blading uniformity issues, and shrinking in
the grain lter cavities. UV exposed CPS has been found to increase the
surface energy of the liquid and makes it more easier for the blade to create
tracks. Pure CPS has been used for most of the experiments.
3. Contact angle measurements have shown that the Surface Free Energy
(SFE) for the surface modication of 0.55% HF dip for four minutes gives
the highest wettability combined with an elevated temperature
4. Blade types, although the liquid has various eects on dierent materials,
is dominantly dependent on its elasticity, especially because the doctor
blade coating is done manually. An elastic blade can drag out the liquid
from inside the patterns, and a rigid blade allows a proper spreading of
the liquid across the wafer but has diculty in removing the excess.
5. RAMAN shows that thermal annealing the liquid silicon at 350°C already
gives the desired amorphous silicon result and is therefore used in many
of the experiments.
As single spin-coating resulted in limited lm thicknesses, and multiple spincoating to uniformity issues as well as CPS spillage, using this method only,
was found to be unsuitable for the creation of a quality lm. Blading proved to
give reasonable results, and reproducible results in some process congurations.
A combination of rst blading followed by spin-coating to improve uniformity
and thickness gave a good result due to the time-dependency of the CPS on the
substrate surface and the improved adhesion before spin-coating.
54
CHAPTER 3.
DOCTOR BLADE COATING OF LIQUID SILICON
An increased number of droplets improve the coverage of the liquid across
the wafer. A continuous layer is more quickly formed, however, thick tracks can
be produced by blading this large amount of liquid that can break. Using less
liquid silicon, may cause diculty in covering the full wafer so multiple blade
strokes may be necessary but will in its turn cause tracks on the lm that are
prone to breaking or create a low quality lm. Elevated temperatures will lead
to a thinner lm and make it easier to leave tracks.
A solution to all this is
by increasing the spreading temperature to approximately 100°C so that the
wetting and creation of a continuous layer is possible with limited blade strokes.
This increase in temperature can however cause slight evaporation of the lm
but is not a big issue.
The best results were obtained with:
ˆ
250nm deep 1 by 1 mm square wafer, 4 minute 0.55% HF dip, 100°C
pre-annealing, 70°C blading of 45µl CPS
Thicknesses was approximately 200nm. Reproducibility however was poor. A
reproducible result could be obtained with:
ˆ
ˆ
Flat grain-lter-only wafer, 4 minute 0.55% HF dip, 100°C pre-annealing,
110°C blading of 45µl CPS
250nm deep 1 by 1 mm square wafer, 4 minute 0.55% HF dip, 100°C
pre-annealing, 90°C blading of 45µl CPS
The resulting thickness in both cases is approximately 100nm. Although thicker
layers were desired, these layers are good enough for the SG-TFT processing
and does not use a spin-coating step that wastes a lot of material.
Recommendations
For the layer formation, it is very important to use fresh CPS. Oxidized CPS
results in a high possibility of cracks due to oxygen molecules present inside the
amorphous silicon layer that disturb the lm integrity during annealing.
Any sign of trails by the blade that appear locally thicker on the lm may be
removed since this layer will crack undoubtedly and may degrade a neighboring
silicon layer.
A further investigation is required in thermal annealing of the lm. Cracking
is a result of a dierence in thermal expansion coecients. Slowing down the
heating process may prevent cracking altogether. A way of removing local thick
areas has however a higher priority.
Automated blading may help the issues of tracks formed by the blade since
a small blade is currently being used to be able to be held by hand but which
has only the size of a fourth of a wafer. The blade should currently therefore be
used at least in four strokes. A machine that could uniformly spread a blade of
the size of a full wafer should be able to remove the trail formation issues.
Chapter 4
Low Temperature Annealing
and Crystallization
Excimer Laser Crystallization (ELC) is a liquid phase crystallization method
and is a way to crystallize an a-Si lm into polysilicon. Another crystallization
method is the Solid Phase Crystallization, however, the intragrain defect density
is higher and therefore mobilities of the nal devices would be lower than when
using ELC.
The basic working of the ELC process is the use of a high power short wavelength pulsed light source. The shortness of the wavelength ensures a shallow
absorption depth of the laser in the a-Si lm (approximately 6.6nm associated
6
−1
to the absorption coecient of about 1.51 · 10 cm
) . In addition, there is
a low spacial coherence of the light source, which therefore makes the process
fast and suitable for the crystallization of thin lms without having a thermal
impact on underlying layers that may have thermal constraints such as in the
case for plastic substrates or for 3D-IC purposes.
Within the Excimer laser electrons collide with a rare gas such as Kr or
Xe which in turn get into an excited state or even become positively ionized.
These excited atoms will form dimers with halogen molecules such as F or Cl.
Within several nanoseconds the excited molecule will fall back to its ground
state and will in the process emit UV light. The molecule in its ground state
will immediately dissociate and the whole process will repeat itself.
Excimer
comes from the "excited dimer" that is produced in this process and is the
cause of the UV light emission.[47, 48]
Crystallization occurs when the molten a-Si layer cools down since the melting point of c-Si is much lower. Crystallization of the a-Si produces heat that
enables subsequent crystallization of amorphous layers.
A crystal seed is left a the bottom of the cavity as the layer on top gets
molten by the laser. This allows a single crystal to grow from inside the grain
lter to the surface area. The thicker the area, the more the grains can grow
when high laser energies are used. The a-Si layer serves also as a protection of
55
56CHAPTER 4. LOW TEMPERATURE ANNEALING AND CRYSTALLIZATION
underlying layers.
This amorphous layer however should not contain a lot of
hydrogen due to the following errors that can occur:
Hydrogen eusion
Any hydrogen present within the amorphous silicon lm
can euse due to the laser irradiation and may destroy some parts of the
lm.
Since the liquid silicon material is produced from a hydrogenated
silicon compound, and due to limited annealing temperature, a signicant
amount of hydrogen atoms can be left in the lm that can cause defects
by this out-diusion of the hydrogen.
Film agglomeration
Partial dewetting may occur and lead to lm decomposi-
tion into beads which is known as agglomeration. The main source of this
dewetting are the uctuations of the silicon lm that are severe enough to
reach the underlying oxide layer. These uctuations are inuenced by the
pulsed-laser annealing caused by non-uniformities in the spatial prole of
the laser pulse and intensity uctuations from the homogenizer. Also the
interference of the incident beam with laterally scattered beams as well as
the surface tension gradient have an impact on this defect.
Ablation
is related to excessive agglomeration and is known as the explosive
release of hydrogen.
A major issue in this work is indeed the hydrogen
content of the amorphous silicon lm produced from the liquid silicon
material.
These errors can destroy the nal lm and decrease the single grain quality of
the nal transistor. When a polyimide substrate is used, it can have disastrous
eects due to subsequent TFT processing steps that will signicantly harm the
polyimide layer through the broken a-Si lm. Using the Excimer Laser, the right
laser recipe is needed to be constructed to reduce the hydrogen content. It was
shown in [2] that the hydrogen content of the amorphous silicon lm formed from
liquid silicon is still a signicant amount and will lead to ablation issues and poor
crystallization results. In the process of [2], a second thermal anneal was used to
decrease this hydrogen content, however due to thermal constraints when using
plastic substrates it is important to reduce this annealing temperature.
In this chapter, the pre-anneal theory is rst discussed. This is followed by
the experimental process which includes: the equipment, the approach, and the
results. Finally conclusions and recommendations are given.
4.1 Pre-anneal eects on Hydrogen concentration
It was believed that multiple shots with low energy densities may improve the
lm quality by decreasing the hydrogen content without ablating and result in
large grain sizes. In [49] this pre-irradiation had been tested on a 100nm PECVD
amorphous silicon lm. Indeed ramped shots, with a lower energy density than
compared to the single pulse lm deteriorating energy density, improved the
4.1.
PRE-ANNEAL EFFECTS ON HYDROGEN CONCENTRATION
maximum shootable energy density.
57
One shot for every energy density level
had been used, and the results are shown in Fig. 4.1. The results show that as
the lm is exposed to more shots at lower energy densities, that the maximum
shootable energy before signicant lm deterioration increases.
Figure 4.1:
Maximum shootable energy before which the silicon lm starts
cracking as a function of the number of Excimer Laser shots. Pretreatment at
300°C (a) and untreated a-Si lm (b) [49]
The eect of laser pre-annealing is visualized in Fig.
4.2 where a sample
without pre-annealing is compared to one with pre-annealing.
of 500 mJ/cm² has been used on the rst sample.
A single shot
This is a relatively high
energy and destroys most of the lm due to the many Hydrogen atoms that
euse and break loose from the a-Si lm.
The second sample is one that is
has been irradiated at lower energy densities for multiple shots (90 shots at 100
mJ/cm²), showing the lm becoming slightly darker and rougher due to the
minor hydrogen eusion. The third sample shows a pre-annealed sample with a
maximum laser energy density of 550 mJ/cm², clearly less dark and crystallized
due to the pre-annealing eect.
(a)
(b)
(c)
Figure 4.2: Demonstration of laser pre-annealing benets. Single shot without
pre-annealing at 500 mJ/cm² (a), 90 shots at 100 mJ/cm² (b), and a preannealed sample with maximum laser energy density of 550 mJ/cm² (c)
58CHAPTER 4. LOW TEMPERATURE ANNEALING AND CRYSTALLIZATION
4.2 Experimental
4.2.1 Excimer Laser setup
Figure 4.3: Exitech M8000V Excimer Laser system schematic [48]
The schematic setup of the Exitech M8000V System for Excimer Laser Annealing is shown in Fig. 4.3. The gasses used for the laser is a mixture of Xe and
Cl2 , that will form the dimer of XeCl for the irradiation of a 308nm wavelength
beam. In this setup, two Lambda Physik LPX 210 laser sources are used and
are combined by mirror M3 in the gure. Attenuation of the energy density of
either beam occurs before their combination. After their combination the beam
can either go through a pulse duration extender, or continue to lenses LS1 and
LS2.
Although the total setup would produce a beam with a pulse duration of
approximately 25ns, the duration can be extended by timing of the two laser
sources or by adding a pulse duration extender shown in the image by mirrors M4
to M7. The idea behind this setup is that mirror M4 and M6 sends the combined
beam to the pulse extender that consists of numerous semi-transparant mirrors.
Each mirror deecting part of the beam and transmitting another part to a next
semi-transparant mirror. This would result in a chain of deected beams with
slight delays that will in total produce a beam that has a longer total pulse
duration of 250ns.
Lenses LS1 and LS2 are used to guide the beam to the homogenizer. The
homogenizer is a tool to produce a beam with a uniform spatial prole. This
4.2.
EXPERIMENTAL
59
beam continues to the Field lens that sends the beam through a mask for the
alignment of light paths, to a projection lens for the nal exposure on the wafer
surface.
The wafer is placed on an X-Y-Z stage for accurate wafer position
control, and is covered by a quartz plate to protect the projection lens from
ablation of the sample lm.
This setup still may produce some uniformity issues that result in a problem
in repeatability of the exposed results. This makes it all the more dicult when
the lm is lacking uniformity.
Small energy density dierences of 10mJ/cm²
have therefore been avoided and replaced by steps of 50mJ/cm².
4.2.2 Approach
In this work, a similar process to [49] is used, however, many more shots at
lower energy density levels were believed to help reduce the hydrogen content
before ablation. Some issues in this process was the non-uniformity of the lm,
and the non-uniformity of the laser.
An initial test had been conducted to recognize the energy density at which
the lm deteriorates signicantly when a single shot is used.
A few tens of
mJ/cm² lower than this maximum energy, a large number of shots have been
red to the substrate after which the energy density has been ramped up and
the number of shots had been decreased. The optimum type of recipe can be
obtained from these results.
These dierent recipe types include: single shot
recipe, ramped single shot recipe, linear decrease recipe, exponential decrease
recipe, and variations in step size for the exponential shot recipe.
A visual
representation of three of the recipe types are indicated in Fig. 4.4 .
(a)
(b)
(c)
Figure 4.4: Visual representation of the recipe types ramped single shot (a),
linear decrease (b), and exponential decrease (c).
By far, most a large number of recipes have been tested on one particular
wafer on which the CPS had been doctor blade coated and spin-coated,. This
wafer was primarily used to obtain conclusions from identical a-Si lm thicknesses. Although the absolute results cannot be identical to wafers with other
liquid silicon thicknesses, the relative results are still relevant.
60CHAPTER 4. LOW TEMPERATURE ANNEALING AND CRYSTALLIZATION
4.2.3 Results
Optical Microscope Results
Fig. 4.5, shows the nal conclusions derived from optical measurements. The
bars show at which energy density the lm shows the rst signs of deterioration,
where the dark blue bars indicate the long pulse results, and the red bars show
the short pulse results. The red and blue markers indicate the maximum obtained grain size using the specic type of recipe for short pulse and long pulse
respectively.
(a)
(b)
Figure 4.5: Laser energies from which the lm starts to show signs of defects
for every laser recipe type(a). Maximum grain size obtained for the particular
recipe type (b).
Pulse duration
For short pulse duration (25ns), it has been observed that the energy at which
the layer signicantly deteriorates is at approximately
E1,max = 260-300 mJ/cm²
when pre-annealing has been omitted. For the long pulse duration (250ns) this
limit is extended to approximately 420-440 mJ/cm². Extending the pulse duration will decrease the cooling rate of the molten silicon which would result
in bigger grains. It will also help against lm deterioration as observed by the
extended shot density limit.
Step size
In the laser recipes, an increasing energy density was used for the crystallization of the lm.
The step size between two energy density levels should be
approximately 50 mJ/cm², since smaller step sizes are inaccurate due to the
4.2.
EXPERIMENTAL
61
inconsistency of the laser energy. Bigger step sizes usually deteriorate the lm
earlier and may approach the level of a single shot recipe. A nal jump from a
series of 50 mJ/cm² should not exceed 150 mJ/cm².
Recipe types
Using this rst deterioration energy with the single shot tests, various recipes
can be tested.
[49] obtained good results with ramped single shots, however,
increasing the number of shots at lower energy densities could lead to better
removal of the hydrogen atoms while maintaining the quality of the lm. Single
shots, ramped single shots, exponential decrease shots, and linear decrease shots,
have been compared. Single shots gave the lowest maximum shootable energy
and in many cases could not form grains before lm deterioration.
Ramped
single shots gave reasonable results with maximum grain sizes in the short pulse
conguration similar to the two remaining recipe types, however, the deterioration of the lm occurs earlier. The exponential decrease of shots was compared
to the linear decrease of shots, and although similar results were obtained for
the short pulse conguration, the long pulse conguration showed better results
for the linear decrease recipe. This is mainly due to the starting energy of the
exponential decrease recipe, which was too low. This resulted in a fast decrease
of shots to 1, so that the nature of the recipe approached the level of ramped
single shot recipes.
Starting energy density
The starting energy for all recipes should be below the deterioration energy,
E1,max ,
obtained for the single shot experiments. Comparing the starting en-
ergies, an energy density at about 100 mJ/cm² lower than
E1,max
would lead
to similar results compared to energies starting a lot lower than this. Starting
energy density of only a few tens of mJ/cm² lower than
E1,max
results in bigger
grain sizes, however, deterioration of the lm occurs at an earlier stage. Fig.
4.6 illustrates that when
E1,max
is found to be 260 mJ/cm², using the rst pre-
annealing energy density at 200 or 150 mJ/cm² has only little inuence to both
the deterioration energy and the grain size. Using the slightly lower energy of
250 mJ/cm² however will increase the maximum grain size as well as an earlier
initial deterioration energy. This has also been found for linear decrease of the
shot number, as well as in the long pulse series.
The results of the long pulse linear recipe that gave the biggest grains are
shown in Fig. 4.7. The recipe starts at 350 mJ/cm², so only slightly lower than
the
E1,max
with 70 shots, that have been decreased with 10 shots every time
and increased energy densities at steps of 50 mJ/cm² until 800 mJ/cm². Finally
a jump has been made to 950 mJ/cm².
62CHAPTER 4. LOW TEMPERATURE ANNEALING AND CRYSTALLIZATION
(a)
(b)
(c)
(d)
Figure 4.6: Excimer laser irradiation results short pulse. Single shot (a) against
an exponentially decreased number of shot with increasing shot densities of 50
mJ/cm² starting at 150 mJ/cm²(b), starting at 200 mJ/cm² (c) and at 250
mJ/cm² (d), with a maximum of 500 mJ/cm² for all cases.
Elastic Recoil Detection (ERD)
The hydrogen content can be quantitatively measured using Elastic Recoil Detection (ERD). It is an ion beam analysis technique that determines the absolute
concentration values of light elements within a thin lm. An ion beam of several
MeV energy is irradiated on a sample, in this case the a-Si lm. The light elements that are present in such a lm are recoiled in forward direction and can
be detected. The energy spectrum of the ejected atoms can be used to obtain
the concentration depth prole of the sample.
The scattered ions that move
in the same direction from the incident beam are blocked by using a foil that
allows the recoils to pass through. [50, 51]The ion beam does not go through
the substrate, and has a probing depth of 400nm in silicon using the equipment
of [51]. Fig.4.8 shows the schematic of the ERD setup.
Six samples are prepared for this analysis, at an
E1,max
of 500 mJ/cm²: 100
shots of 100 mJ/cm², 100 shots of 300 mJ/cm², 100 shots of 450 mJ/cm², linear
4.2.
EXPERIMENTAL
(a)
63
(b)
Figure 4.7: Excimer laser irradiation results long pulse. Linear recipe for which
the biggest grain sizes have been obtained. 4 and 3 micron pitch image (a), and
3 and 2 micron pitch image (b).
Figure 4.8: ERD setup schematic[50]
decrease from the 100 shots of 300 mJ/cm² to 450 mJ/cm², 650 mJ/cm² and
850 mJ/cm². [results will be obtained by beginning of June]
Rutherford Backscattering Spectroscopy (RBS)
A similar ion beam analysis technique is the Rutherford Backscattering Spectroscopy, a tool used to measure atoms ranging the periodic system. A beam
of He2+ ions at an energy of 1 to 2 MeV is incidented on a sample placed in a
predetermined angle. The backscattering energies of the ions can be detected
from which the concentration depth proles can be obtained. The backscattering energies are directly related to the mass of the particle and the depth of
collision [50, 52]. A probing depth of approximately 2 micron is possible using
the setup from [50]. Fig. 4.9 shows the schematic of the RBS setup.
The system is used to detect atoms heavier than hydrogen in the amorphous
silicon samples. Carbon concentration and oxygen concentration are measured
for three dierent samples: a reference sample annealed at 350°C and UV cured
for 20 minutes, a sample with only 10 minutes UV curing, and a sample with an
increased annealing temperature of 430°C. [results will be obtained by beginning
of June]
64CHAPTER 4. LOW TEMPERATURE ANNEALING AND CRYSTALLIZATION
Figure 4.9: RBS setup schematic[50]
4.3 Conclusions and recommendations
Conclusions
Excimer laser pre-annealing has proven to increase the maximum shootable
energy density. Various laser recipes had been tested which showed generally
that a long pulse duration signicantly helps to increase the maximum shootable
energy as well as the maximum grain size due to the heating rate and cooling
rate respectively. The recipe showing the best results were those that were shot
with a large number of shots at low energies, and a linear decrease in the number
of shots as the energy densities were increased by 50 mJ/cm².
Recommendations
For Excimer Laser Crystallization it is important to consider as little lm deterioration of the lm as possible when working with an underlying polyimide
layer within the substrate, even if this would result in smaller nal grain sizes.
The laser recipe type of exponential decrease should be further investigated for
long pulse congurations since it may still produce quality grains.
Other methods for increasing the grain size should be tested, such as the heating
of the layer during irradiation to reduce the cooling rate.
Chapter 5
l-Si SG-TFT on Polyimide
As the liquid silicon layer lls the grain lter cavities, and forms amorphous silicon during annealing. The samples are exposed to Excimer laser for hydrogen
removal and crystallization. The position of the single crystal grains are accurately controlled by the grain lters, and within these grains, monocrystalline
silicon channels can be formed for high quality devices.
So far reports have been made of liquid silicon devices that have been spincoated and inkjetted without grain location control in [1] and spin-coated with
grain location control in [2]. The latter have produced TFTs at temperatures
incompatible with plastics such as polyimide due to the annealing of the liquid silicon layer for transformation into an amorphous silicon network and the
removal of hydrogen atoms that can have detrimental eects during laser crystallization.
In this work, the annealing temperature of the transformation from liquid
silicon to an amorphous silicon network has been reduced from 430°C to 350°C,
and the hydrogen removal has been conducted with a laser pre-annealing treatment similar to [49]. With this decrease of temperature, the compatibility to
polyimide has been realized and devices both on polyimide as well as devices
without the additional polyimide layer have been constructed and measured.
5.1 Transistor structure
A schematic of the device fabrication process is given in Fig. 5.1. The process
is similar to [2], and uses a
µ-Czochralski
process for the controlled growth of
the crystalline silicon grains. Notice that in this schematic the polyimide layer
has been left out after step 1, although both processes, with and without this
layer, have been used for our nal devices. Step 7a is used when the polyimide
layer has been omitted, whereas step 7b is used for when a polyimide layer had
been implemented in the process.
For the protection of this layer, additional
aluminum has been used to protect the underlying polyimide layer during the
activation of implanted dopants through Excimer laser.
65
66
CHAPTER 5.
L-SI SG-TFT ON POLYIMIDE
5.1.1 Fabrication procedure
Figure 5.1:
SG-TFT fabrication process both with (b steps) and without (a
steps) an additional polyimide layer. The polyimide layer has been omitted in
this schematic after step 1, however step 1 shows its designated position.
1. Using a crystalline silicon wafer as the supporting substrate material, a
polyimide layer may be spin coated on top of this wafer. Using plasmaenhanced chemical-vapor deposition (PECVD) two oxide layers have been
deposited on top of this base layer in the machine Novellus Concept One
using tetraethylorthosilicate (Si{OCH2 CH3 }4 or TEOS) at 350°C. The
rst 750nm thick 1µm wide oxide layer has been patterned to form big,
controllable cavity sizes by using anisotropic reactive-ion etching (RIE) in
a C2 F6 -CHF3 plasma. The second 800nm oxide layer has been deposited
5.1.
TRANSISTOR STRUCTURE
67
by the same PECVD process to decrease the cavity size to a grain lter
level. The grain lters have a depth of 700nm and a diameter of 100nm.
2. According to the process described in Chapter 3, an amorphous lm is
manufactured from the bladed liquid silicon material. A thickness of approximately 200nm is desired using pure CPS, 20 minutes of UV exposure,
and thermal annealing at 350°C for 1 hour in an nitrogen ambient with
oxygen and water levels in below 0.1ppm.
3. Using the Exitech M8000V System for Excimer Laser Irradiation of the
amorphous silicon layer, the top amorphous silicon layer is molten until
a crystal seed is left at the bottom of the grain lter. The laser induces
crystallization as explained in Appendix E
4. Crystal growth in every grain lter induced by the Excimer Laser will
continue at high energy densities, until either a defect of the lm occurs,
or the crystal growth collides with a crystal grown from a neighboring
grain lter. This collision causes a halt to the grain lter size increase and
will form grain boundaries at those positions.
5. Grain islands are formed with RIE. These islands are positioned in a way
that the channel region exists in the single crystalline area produced from
one single grain lter.
6. For the production of a gate oxide layer, to types of oxide are formed on
top of the crystalline silicon area. Using inductively coupled plasma (ICP),
high quality oxide is grown at a mere 250°C with less plasma damage on
the oxide than when using PECVD TEOS. This made out the rst 14nm
of the gate oxide layer for a good semiconductor-oxide interface.
After
this rst layer, PECVD TEOS has been used for another 22nm oxide at
350°C to speed up the process.
7. The aluminum gate has been sputtered on top of the gate oxide by using
the Trikon Technology Sigma 201 cluster tool at room temperature. The
aluminum has a 0.1% silicon content to prevent spiking due to diusion,
and has a thickness of 675nm.
For the polyimide version (7b) the alu-
minum used to protect the underlying polyimide layer should not contain
any silicon for accurate removal of the layer after dopant activation.
8. The aluminum gate pattern has been used as the mask for self-alignment
6
of the ion implantation of the source and drain regions. For PMOS, 1·10¹
6
ions/cm² boron atoms at 20keV were used, and for NMOS TFTs, 1·10¹
ions/cm² phosphorus atoms at 70keV were implanted. Both dopant types
were activated using Excimer laser with an energy density of 300mJ/cm²
at room temperature.
9. The total device structure is passivated again using PECVD TEOS, for
protection of devices and insulation between conductors.
68
CHAPTER 5.
L-SI SG-TFT ON POLYIMIDE
10. Finally, contact holes are etched, followed by aluminum via deposition for
local contacts. The patterning has been done by using photolithography
and Al sputtering.
5.1.2 Polyimide
In this work the polyimide layer is produced using a Polyamic Acid Durimide
150A, that is transformed into a fully stable polyimide after thermal curing.
The structure of the Durimide is shown in Fig. 5.2a.[53]
Polyimide is a polymer of imide monomers, the structure of an imide is shown
in Fig. 5.2b. It is the combination of two acyl groups bound to nitrogen. In the
durimide structure, both nitrogen atoms will replace the nearby OH group while
losing one hydrogen atom to form the imide structure during thermal curing.
An aromatic heterocyclic polyimide is the result. [54]
(a)
(b)
Figure 5.2: Chemical structure of the Polyamic Acid Durimide (a)[32], and the
Imide monomer
The polymeric structure is known for its excellent mechanical properties,
thermal stability, and chemical resistance. This is due to the strong intermolecular forces between the polymer chains.
The polyimide used in this work has a glass transition temperature of 371°C,
and a thermal decomposition temperature of 597°C. The reader is referred to
[53] for more information on the material.
5.2 TFT characteristics
When measuring the nal devices, it is important to look for transistor characteristics that determine the quality of the fabricated devices.
The devices
are based on MOSFETs in which a current is controlled by means of a voltage
dierence at the gate. In this section, parameters such as: eld eect mobility,
subthreshold swing, threshold voltage, and o current are determined from the
transfer characteristics,
I D V G,
and output characteristics,
I D V D graphs. I D
V G is the voltage
is dened as the current that is passing through the channel,
5.2.
TFT CHARACTERISTICS
69
applied at the gate while the voltage dierence across the channel is kept constant, and
VD
is dened as the voltage applied at the drain contact assuming
that the source contact is set at 0V (ground). [47, 48]
Field Eect Mobility
Mobility is related to the carrier transport through a material as a result of
an induced electric eld, and is one of the most important parameters of the
transistor.
The current that runs through the device has various operating
modes and is dened by Eq. 5.1.
ID =
h
(
µF E,n Cox W
L (VGS − Vth ) VDS −
Where
µF E,n Cox W
2
L
ID
(VGS − Vth )
VGS
W
and
i
2
Linear region
(5.1)
Saturation region
µn is the
L are the
is the drain current;
is the oxide capacitance;
respectively;
2
VDS
2
electron eld-eect mobility;
C ox
width and length of the channel
is the gate to source voltage;
Vth
is the threshold voltage
andVDS is the drain to source voltage.
From the saturated region, the mobility can be extracted by measuring the
slope of the
IDV G
curve while keeping the drain voltage at a constant saturated
level:
√
∂ ID
∂VGS
µF E,n =
r
!2 


2L

Cox W
(5.2)
VDS =level of saturation
Subthreshold Swing
The subthreshold swing (S) is a parameter that denes the quality of the turnon characteristics of the device.
regime, which occurs at
VGS
<
It can be obtained from the weak inverting
Vth .
In this regime the swing is dened as the
gate voltage required to increase the drain current by an order of magnitude.
This parameter determines the semiconductor/dielectric interface trap density
and is dened as:
−1
∂ ln (IDS )
∂VGS
kT
Cdepl + Cit
ln 10 1 +
S=
q
Cox
S=
(5.4)
k is the Boltzmann's constant; T the temperature in Kelvin; q is the
Cdepl is the capacitance of the depletion region; Cox is the capacitance
gate-oxide, and Cit is the capacitance of the interface states which is in
In which
charge;
of the
(5.3)
parallel to the depletion capacitance.
70
CHAPTER 5.
L-SI SG-TFT ON POLYIMIDE
Threshold Voltage
The property that denes the gate voltage on which the device turns on is the
threshold voltage.
This voltage needs to be suciently low for the device to
be operational with limited supplying energy.
It is physically dened as the
formation of an inversion layer on the semiconductor-oxide interface that allows
charge carriers to move from the source and drain regions.
O-current
The o-current is the current owing through the channel while no voltage is
applied at the gate. This current should be as low as possible to obtain a proper
switch-like function.
5.3 Results
[Results will be obtained by mid June]
5.4 Conclusions
[Results will be obtained by mid June]
Chapter 6
Excess Liquid Silicon Removal
for Gravure Printing
Chapter 3 has discussed the doctor blade coating of liquid silicon onto a patterned substrate: a process that can be regarded as a precursor of the gravure
printing system. The process allows the liquid to be spread across the patterned
surface without the need of removing any liquid that is not lling a pattern.
Excess liquid silicon removal is another precursor of gravure printing which
requires any excess liquid to be scraped o the patterned substrate. The result
is that the liquid will only ll patterned cavities lled with the specic liquid.
The nal transition towards gravure printing would require this substrate with
the liquid to be pressed against a target surface.
The focus in this chapter was on spreading the liquid silicon while lling
patterns and removing excess liquid in non-patterned areas.
6.1 Experimental Results
The substrates used in this chapter are dierent from the ones used in Chapter 3
in the cavity patterns produced in the TEOS layer. The wafers do not have any
grain lters but various pattern sizes ranging from a few tens of micrometers to
a square of 1 by 1 micrometer. The shape of most of the patterns are like the
letter "H" which is a common structure used for the production gates or the
channel region. The depth of these patterns have been varied from 0 to 1000
nm. Many of the experiments however, were conducted using a depth of 250nm
which is a typical thickness for a semiconductor layer. Grain lters are omitted
since the aim of this work looks at producing a transistor using the lled cavity,
or by gravure printing the pattern on a target substrate.
In both cases, the
grain lters are not required.
The desired result is either a completely lled pattern, a pattern that is only
lled in the corners, or a bulging/dewetted pattern.
Although the latter two
can not be used directly for gravure printing ends, these results allow further
71
72CHAPTER 6. EXCESS LIQUID SILICON REMOVAL FOR GRAVURE PRINTING
modication while using the surrounding structure for self-alignment purposes
for example.
The general procedure used for these experiments for the formation of a-Si
from liquid silicon is the same as the one described in 3.2.2. Again numerous
variations have been tested for obtaining the optimum result.
The charac-
terization results obtained in 3.3 are a good starting point for the following
experiments.
6.1.1 Excess removal
Excess layers, or thick layers, have shown to be problematic in doctor blade
coating experiments when they result in cracks during thermal annealing.
In
this work, additional precautions have to be taken in order to get a desired
result of lled patterns and a clean surface in non-patterned areas. The excess
has to be removed in some way using the blade.
The surface free energy of the blade has negligible eect on the blading result
for the available blade type varieties. A big dierence exist however in blades
of dierent elasticity. The rubber blade allows a good manual removal of the
excess layer in non-patterned areas when a sucient force is applied, since the
exibility of the blade allows it to adapt to the surface. It however, also leads
to the removal of the liquid in some of the lled patterns. A rigid blade on the
other hand is problematic for the manual scraping of the excess due to the direct
eect of any inaccurate hand motion. The patterns however do not lose most
of their liquid and therefore this second type of blade is good for spreading,
whereas the rst type is good for removing excess. A combination of the two
blade types will give the desired result, where the rubber blade is carefully used
for scraping after the rigid blade spreads the liquid.
The blade angle dierences have no observable eect, and the position of
the blade determines partly where the liquid from within the pattern is pushed
towards on a non-patterned area. It also determines the direction of the trails
that may lead to cracking. The blade angle is set approximately vertical for all
experiments and the blade direction is also set in one direction imitating the
conventional gravure printing system.
Another problem exists for the lm within a pattern when the excess layer is
not removed. The excess connected to the lm within the pattern, pulls the lm
out during thermal annealing causing a strong deformation of the lm inside the
pattern, as can be observed from Fig. 6.1 . This is the result of a dierence
thermal expansion coecient. The eect is not as strong in bigger and shallower
patterns as well as in grain lter sized patterns.
Since the excess is removed in many of areas on top of the wafer, small
droplets that are still left may combine to form a droplet that cracks during
thermal annealing. This eect is visible in Fig. 6.2, and can only be helped by
accurate removal of the excess.
Deformation may also occur inside a pattern without excess. This eect will
be discussed in Sect. 6.1.2.
6.1.
EXPERIMENTAL RESULTS
(a) trails covering lled patterns (b) medium sized structures
73
(c) large area structures
Figure 6.1: Films within patterns getting pulled out by the excess layer connected to the lm inside. Optical microscope view (a), a SEM image of such a
pattern (b), and a SEM image of a bigger pattern (c).
Figure 6.2: Bubble bursting of CPS due to excess CPS on top of a lled pattern.
6.1.2 Pattern deformation
The change in surface energies between the liquid and the substrate can lead
to the liquid to change its shape into either more spherical or atter shape.
The change to a more spherical shape is called dewetting, due to commonly
known wettability of liquids to surfaces for which a low wettability is associated
to a more spherical shape. This change in shape of the liquid can be used in
advantageous scenarios in processing transistors. It can for instance be used for
self-alignment. During the formation of an amorphous silicon network through
annealing of the UV exposed liquid silicon, this dewetting may occur in a similar
fashion as shown in Fig. 6.3. The opposite of dewetting may be regarded as
wetting, so the liquid spreading out over a surface with minimum thickness. We
use the term reverse dewetting for the situation in which a pattern once lled
with liquid silicon has formed a hole in the middle, while the silicon has spread
to the edges or corners of the pattern. This eect can also be observed in Fig.
6.3 and can also be used for self alignment purposes.
Although some of the images from Fig. 6.3 appear to be dewetting, this is
in fact not dewetting but simply a change in shape due to thermal expansion
mismatch of the amorphous silicon material and the silicon dioxide. This explains the darker areas formed around the spherical shape. The corners are the
74CHAPTER 6. EXCESS LIQUID SILICON REMOVAL FOR GRAVURE PRINTING
(a) Regular lling
(b) Dewetting appearance
(c) Reverse dewetting
(d) wetting combination 1
(e) wetting combination 2
(f) wetting combination 3
Figure 6.3: Dierent ways of pattern lling.
most prone to be lifted up followed by the edges of the pattern due to the way
the patterns are etched. This eect can be seen from Fig. 6.4. As the gure
shows, many of the dark areas are related to the deforming behavior. RAMAN
spectroscopy conrms the darker areas to be amorphous silicon as well, as can
be observed from Fig 6.5.
(a)
(b)
(c)
Figure 6.4: Deformation of supposedly dewetted patterns.
This gave the idea that the holes are not lled as much as we had hoped
for. The situation that we were aiming for as well as the situation we have at
present are visualized in Fig 6.6a.
It is unlikely for a situation in which the
pattern is completely lled to be deforming to the extent of Fig. 6.4, the layers
that appeared to have lled the patterns are therefore relatively thin.
By using the DekTak we have conrmed that the thickness of the amorphous
lm inside the pattern was limited to a few tens of nanometers as can be seen
6.1.
EXPERIMENTAL RESULTS
(a) non-patterned surface
(b) Pattern sphere
75
(c) Pattern outside sphere
Figure 6.5: RAMAN spectroscopy measurements of lled and dewetted patterns.
from Fig. 6.6c.
Deeper patterns have a higher possibility of deformation due to the shape
of the pattern. The more shallow patterns are more prone to lose their CPS by
blading. Fig. 6.7 shows the eect of various pattern depths.
Not only the depth of the patterns may lead to this eect, but also the area
of the patterns. Large area patterns are relatively easier to lose their CPS since
the exibility of the rubber blade may scoop more of the CPS out of such a
large area pattern. Fig. 6.7a shows this eect.
6.1.3 Time Dependency
Films that are spread after deposition of a number of droplets show that there is
a dierence in properties in the droplet area and the liquid spread area. A time
dependency was observed in a lm of liquid silicon that is left for some time,
and a neighboring lm that has just been spread. This has also been observed
in the contact angle tests, where the contact angle decreased over time.
This time dependency resulted in a better adhesion of the liquid silicon to
the surface, making it more dicult to remove the excess layer as can be seen
from Fig. 6.8a and b. Deformation of the patterns become also less likely as
adhesion increases over time, as shown in Fig. 6.8 c. This adhesion is the result
of a reaction between the liquid material and the oxide surface of the substrate.
It is desirable for the layer of liquid silicon material inside patterns, but not
desirable for the excess layer in non-patterned areas. An experiment where a
drop of liquid silicon was moved across the surface by inclining the substrate,
also conrmed this; at higher speeds the liquid leaves a track of small droplets
while at lower speeds it leaves a thick uniform lm.
76CHAPTER 6. EXCESS LIQUID SILICON REMOVAL FOR GRAVURE PRINTING
(c)
Figure 6.6: Dewetting against deformation schematic when properly lled (a)
and when poorly lled (b). The proof of a thin layer within the 250nm deep
pattern (c).
(a) 270nm depth
(b) 800nm depth
(c) 1100nm depth
Figure 6.7: Eect of pattern depth on liquid silicon
6.1.4 Liquid silicon
Mixing cyclooctane with CPS increases its wettability to the surface. Removal
of the excess is problematic, and the evaporation of the organic solvent results
for the same amount of liquid only a thin layer of a-Si. A higher concentration of
CPS or more liquid will help create a thicker layer, however, the cyclooctane also
introduces carbon atoms inside the nal a-Si layer and is therefore avoided. In
addition, drying of the mixture during blading introduces dicult in spreading
of the liquid across the wafer.
UV increases the viscosity of the liquid silicon due to the photopolymerization process that produces polysilane chains. At a certain point the liquid
solidies into a white substance. Solidifying a liquid silicon layer deposited on
top of a patterned surface will not make it easier to remove the excess by a rigid
blade, breaking this excess layer o from the layer inside the patterns.
The
lm within the patterns get dragged out of in a relatively solid state, as can be
observed from Fig. 6.9b.
6.1.
EXPERIMENTAL RESULTS
(a)
77
(b)
(c)
Figure 6.8: Results of time dependency experiments, good adhesion in the initial
thick layer area (a), area outside this initial layer after (b), and the transition
from initial layer to the bladed area outside (c).
It has been observed that wafers on which UV exposed CPS had been deposited had overall more cracks throughout the area due to the increase in
viscosity of the liquid that lead to more tracks and deformations in the resulting lm as the liquid is thicker. In general, the UV exposed CPS is more dicult
to spread which may be due to the increase in viscosity and surface energy, but
it also may be due to the evaporation of the liquid during exposure of UV light
that at the same time generates some heat.
(a)
(b)
(c)
Figure 6.9: Various UV exposure times. No UV exposure before blading (a). 10
minute UV exposure before blading on top of a wafer (b), but many intermediate
exposures during blading (c).
Finally, exposing the wafer with the liquid silicon, multiple times with UV,
and scraping the excess o after every exposure, some of the patterns were lled
in a strange way, which is assumed to be due to more and more of the UV
exposed CPS lling the patterns. Although this method may improve the way
the patterns are lled, it was also observed that after every UV exposure, it did
not became easier to remove the excess liquid.
Also, the physics behind this
method lies in the solidication of the CPS inside the pattern, while more and
more new CPS material lls the pattern at the same time with intermediate
solidication steps. This may also cause integrity problems. The pattern lling
for this situation is depicted in Fig.6.9c.
78CHAPTER 6. EXCESS LIQUID SILICON REMOVAL FOR GRAVURE PRINTING
6.1.5 Surface modication
Two types of surface modications had been used, both relying on very dierent
types of etching.
The rst one is plasma oxidation in which the substrate is
placed into an oxygen plasma that bombards oxygen atoms onto the surface of
the wafer which makes the surface rougher. The other type is wet etching by
using a low concentration of HF for a short period of time, a commonly used
technique to remove native oxide of a silicon wafer. This wet etching type will
cause the surface to allow the liquid silicon to ow and therefore increase the
surface energy of the substrate.
Oxygen plasma settings were set at 500W for 8 minutes. The plasma resulted
in the adhesion of excess outside the patterns. These excess layers outside the
patterns however were segmented instead of combined.
This was also visible
from some of the droplets that were visible on the surface.
These droplets
formed a cluster of smaller droplets around the main droplet.
This property
did not help with removing the excess by blading since parts of the excess could
break o during scraping rather than forming a bigger droplet that follows the
blade. Fig. 6.10 gives the results of some of plasma oxidized experiments and
Fig.
6.11 elaborates the blading scenario compared to the regular substrate
surface.
Silicon dioxide surfaces is considered to give a good adhesion of to
amorphous silicon lms, an increase in roughness increases the adhesion of the
silicon atoms to the oxygen lm.
(a)
(b)
(c)
Figure 6.10: Blading results on plasma oxidized surface.
For the wet etching setup, a concentration of 0.55% HF has been used for
4 minutes after which the wafers had been rinsed for approximately 5 minutes
and have been spin dried.
The low concentration and relatively low time is
used to ensure that the patterns will not signicantly change their shape. This
setup is also used for the removal of native oxide on a c-Si wafer at an etch
rate of 15 nm/minute [55]. Segmentation of the droplets similar to the plasma
oxidized experiments were not found. Fig. 6.12 shows some of the results that
we have obtained from the experiments.
As the gure suggests, the liquid is
better spread across the surface, however a conclusions about the excess removal
is still premature. The wettability of the liquid silicon increases also in this case.
6.2.
CONCLUSIONS AND RECOMMENDATIONS
79
Figure 6.11: Dierence in blading of the excess on regular surface and plasma
oxidized surface.
(a)
(b)
(c)
Figure 6.12: Blading results on HF dipped surface.
6.2 Conclusions and Recommendations
Conclusions
The results from the experiments in this chapter has helped gain understanding
of the way the liquid silicon behaves to the lling of various patterns, while
removing the excess layer.
1. The excess lm can cause errors such as cracks, caused by liquid silicon
trails from the blade or lm uniformity issues.
In addition, the excess
lm can pull the solidied structures from within the patterns out. Grain
lter patterns and large area patterns are not prone to this pull-out eect,
caused by the dierence in thermal expansion coecient of the silicon lm
and oxide surface.
2. The blade is the tool for spreading the liquid silicon as well as removal
of the excess layer. Blading direction has an eect on the way the excess
layer is spread, however, multiple blading will lead to a lower amount of
liquid within the patterns.
Furthermore, blade types were investigated
80CHAPTER 6. EXCESS LIQUID SILICON REMOVAL FOR GRAVURE PRINTING
for various contact angles, however, in this process it seemed that the
stiness of the blade plays a dominant role.
A more rigid blade allows
the liquid to spread without removing it from inside the patterns.
The
excess is harder to remove in this case. A exible blade can remove the
excess more accurately, however it will also remove liquid inside patterns.
Bigger patterns are in this case more prone to losing their liquid.
A
careful combination of the two could lead to the optimum results. Since
the blading process has been done by hand, it is believed that a more
accurate automated process could lead to better results for the rigid blade.
3. Deformation of patterns occur when the liquid inside the pattern is too
thin and has a poor adhesion to the surface.
The lm can easily be
deformed due to the thermal expansion dierences. The pattern geometry
can strongly inuence the way that it is lled. Shallow patterns are more
likely to lose the liquid silicon that was lling the pattern. Patterns with
bigger area are also more likely to lose their liquid during blading. Deeper
patterns are more prone to deformations due to the cone shaped structure
of the pattern.When even less liquid is inside a pattern the liquid can
spread to the edges and corners of the pattern forming a ring of silicon
that can also be used for self-aligning ends.
4. The liquid silicon has some form of time dependency on the surface of
the substrate. Over time the adhesion increases making it harder to remove the excess liquid material, but also preventing deformation within
patterns.
5. The liquid silicon used could come in various forms by using pure CPS
and varying UV exposure, dissolving it in cyclooctane, or a combination
of the two. From the results a preference in usage of pure CPS had been
grown due to the fast evaporation times, carbon introduction, and low aSi thickness of the cyclooctane solution, and the thickness and spreading
diculty of UV pre-exposed CPS.
6. The target substrate surface can be modied to change the wetting and
adhesion properties. Two types of etching have been used for this. The
rst one is a dry etching process, plasma oxidation that appeared to have
a strong adhesion that results in segmenting of the droplets. The second
type is wet etching by using a low concentration of HF. This resulted in
a higher wettability and easier spreading of the liquid silicon. This latter
case shows an indenite result for the removal of the excess.
Optimization of the excess removal process with lled patterns, lead to a thin
lm inside the pattern that is prone to deformation. At this point the optimized
procedure is: silicon blade coating of pure CPS and a subsequent careful rubber
blade scraping on patterns with a depth of 250nm. After scraping, leaving the
liquid for a few minutes inside the patterns for a better adhesion. HF may be
used to improve the spreading behavior, but will not lead to a benet in excess
removal.
6.2.
CONCLUSIONS AND RECOMMENDATIONS
81
Recommendations
An automated blading machine could provide a consistent way of scraping o excess with a rigid blade type, as it is being used in conventional gravure systems.
Current manual blading introduces many variables that cannot be controlled
and are not present in automated systems.
The main issue at this moment is actually the poor lling of the patterns,
and their deformation. Filling can be improved by creating a situation in which
a liquid that has poor wettability in non-patterned areas, but can easily enter
the patterns and have a strong adhesion to the insides of the patterns. Surfacematerial wise, this would mean that the surface may be made of silicon nitride or
another material with very low surface energy, while the patterns inside can have
a surface of HF etched or a modication that leads to high surface adhesion and
wettability. It must however be ensured that the adhesion to the non-patterned
surface is minimal which is many cases related to a surface with a very low
surface energy. The blade itself can also be made from a silicon nitride material
to minimize the adhesion of the liquid to the blade.
Either CPS or UV pre-exposed CPS need to be used in this scenario since
it is assumed that UV pre-exposed CPS will not be dicult to remove from
the surface anymore due to the increase in hydrophobicity of the silicon nitride
surface.
Figure 6.13: Recommended setup mainly based on high adhesion within the
pattern, and poor adhesion outside, with a poor adhesive blade.
To prevent pattern deformation, patterns that are once lled need to rest
for some time for the liquid silicon to react to the surface within the pattern
for a stronger adhesion. When transistors are made from this lled patterns,
maximum adhesion is desired and some resting time is therefore required. In
the blading process, relatively deep patterns with small areas are preferred. The
total situation is illustrated in Fig. 6.13.
82CHAPTER 6. EXCESS LIQUID SILICON REMOVAL FOR GRAVURE PRINTING
Chapter 7
Conclusions and
Recommendations
Liquid Silicon is used as a way of taking the advantages of a cheap, low temperature, solution fabrication process, and the high electrical property of silicon transistors.
Doctor blade coating is a rst step towards gravure printing
systems allowing roll-to-roll mass production of electronics. Using location controlled grain lters and Excimer Laser pre-annealing, high quality Single-Grain
Thin-Film Transistors can be manufactured on plastic substrates.
7.1 Doctor Blade coating Liquid Silicon
Some basic characteristics of liquid silicon have been investigated, and a method
for obtaining a good a-Si layer is achieved.
The obtained characteristics are
listed:
Excess liquid
Excess liquid can induce large stepheights that break during
thermal annealing due to thermal expansion.
Instances at which these
large lm gradient can occur are besides excess layers from the large
amount of liquid, trails caused by blading, and substrate patterns.
Pure CPS
is the preferred material to work with as liquid silicon.
Manip-
ulations of this base material can increase its viscosity, or decrease its
surface free energy. UV light causes the production of polysilanes within
the material that thicken the liquid. These result in a higher possibility of
blading trails as well as a poorer wettability. Mixing cyclooctane increases
wettability however a number of issues exist: carbon is introduced in the
resulting a-Si lm, the resulting lm evaporates leaving only a thin layer
of polysilane content on the surface, a-Si in grain lters shrink due to this,
and the solution dries quickly leading to uniformity issues when blading.
Surface Free Energy
High wettability of the liquid is desired and can be
83
84
CHAPTER 7.
CONCLUSIONS AND RECOMMENDATIONS
obtained by surface modication techniques.
Increasing the number of
droplets will only create thicker tracks when doctor blade coating which
can crack.
0.55% HF treatment for four minutes together with an el-
evated temperature gives the highest surface free energy (SFE) of the
surface resulting in high wettability. Wettability increased over time due
to reactions of the material to the surface. Care has to be taken for the
evaporation of liquid silicon when spread at elevated temperatures.
Blade types
The SFE of the blade is not a dominant factor when doctor blade
coating due to the errors that may result from manual blading. Elasticity
however gives a signicant eect. Flexible blades such as rubber adjust to
the surface even if the blade is not positioned accurately. This exibility
enables an ease in removing the liquid, and may also remove liquid from
inside the patterns. A more rigid blade allows a better spreading of the
liquid across the surface.
Using these characteristics the liquid silicon has been either spin-coated, bladed,
or the two have been combined. Spin-coating gives poor adhesion and a lot of
wasted liquid silicon. Blading gives good results when a patterned wafer with 1
by 1 mm square patterns are used instead of a at wafer. For the at wafer case,
blading will result in a thin layer of approximately 30nm. Adding a spin-coating
step to this layer improves liquid silicon adhesion and leads to a good layer with
a thickness of approximately 250nm.
The best results are obtained by using
0.55%HF dip for four minutes on a wafer with 1 by 1 mm square patterns, while
spreading 6 drops (total 45µl) at an elevated temperature of 70°C for both types
of wafers. A more reproducible result with a thickness of approximately 100nm
was obtained when spreading at an elevated temperature of 90°C for the square
patterned wafer, and 110°C for the at wafer. The wafers have been exposed to
20 minutes of UV for photopolymerization, and have been annealed at 350°C
for 1 hour.
7.2 Low Temperature Annealing
Similar works of liquid silicon [2] have obtained a SG-TFT using the same material, however a second thermal annealing step of 650°C was used that made the
process incompatible with plastic substrates. This second annealing step was
necessary for reducing the oxygen content for the subsequent Excimer Laser
Crystallization process. Low temperature annealing by using the same Excimer
Laser will lead to a maximum processing temperature of 350°C for which polyimide substrates can be used.
Excimer Laser pre-annealing has been investigated by [49] for the removal
of hydrogen without lm deterioration, and has been further explored in this
work. Several conclusions have been made:
Long pulse
A longer pulse duration (250ns) lead to higher maximum shootable
energies than short pulse duration (25ns). As a result, bigger grains were
7.3.
LIQUID SILICON DEVICES
produced in this conguration.
85
The reason for this is the decrease in
cooling rate.
Step size
Laser recipes with a step size of smaller than 50mJ/cm² are prone
to laser uniformity and inconsistency issues. Step sizes a lot bigger than
50mJ/cm², takes away the eect of laser pre-annealing.
Starting energy
When the starting energy of the laser recipe is a lot lower
than the level of rst deterioration, the lm is less disturbed.
A laser
energy just below the level of rst deterioration will produce more lm
disturbances, however will result in bigger grain sizes.
Recipe type
A large number of energy shots have been shot at the beginning
to remove more hydrogen at these low energy levels.
The decrease in
number of shots have been done linearly and exponentially.
For short
pulse conguration, the results were similar. For long pulse conguration,
the linear decrease recipe resulted in bigger grain sizes due to the fast
decay of shot number in the exponential setup.
The recipe that has achieved the biggest grain size of 5µm grains was: a starting
energy of 350 mJ/cm² for 70 shots, with steps of 50 mJ/cm² and decreasing shot
count by 10 until 1, and a nal jump of 150 mJ/cm² at 800 mJ/cm². A lower
starting energy density is desired when the polyimide substrate is used to ensure
its survival.
7.3 Liquid silicon devices
[Results will be obtained by mid June]
7.4 Excess removal using doctor blade
The next step towards a gravure printing system is the process of using the
doctor blade to remove excess liquid in non-patterned areas, after the liquid
has been spread across a substrate. The patterns are cavities of various shapes,
and these require to be lled completely for the gravure printing process. Other
characteristics of the liquid silicon has been analyzed for this type of solution
processing.
Excess liquid
The excess in this process is slightly dierent from the excess
in the doctor-blade coating section.
The excess needs to be completely
removed in this process. Any leftovers can produce cracks similar to the
previous process. A smooth layer however, which was acceptable in the
other process, may pull out some of the middle sized patterns in this process due to thermal expansion.
Blade types are preferred to be rubber
when excess needs to be removed, however, this blade type may also remove the liquid from inside the patterns, and is generally not a good way
86
CHAPTER 7.
CONCLUSIONS AND RECOMMENDATIONS
for spreading the liquid. Using a silicon blade for spreading and careful
rubber blading for excess removal will give the best results.
Deformation
Many patterns were lled with a very thin layer.
This thin
layer, when adhesion lacks, may easily deform during the annealing step.
A deeper pattern is more prone to deformation due to the shape of the
pattern. A too shallow pattern on the other hand may lose all of its liquid
silicon content.
Time dependency
Liquid silicon reacts to the TEOS surface as has also been
observed during the SFE experiments. This enables an improvement in
adhesion that can prevent deformation during the annealing step. Excess
liquid should be fully removed before pursuing a strategy using this time
dependency.
Other types of liquid silicon gave similar issues as with the doctor blade coating
case. Surface modications of HF dip and O2 plasma were used, however none of
them showed a signicant improvement in lling properties and excess removal.
O2 plasma even improved adhesion of the liquid to the surface. The best results
of lling were obtained when pure CPS was spread by silicon blade after which
the excess is carefully removed by the rubber blade. These results lead to a thin
layer within the patterns, which still needs to be optimized for the production
of a completely lled pattern.
7.5 Recommendations
Doctor blade coating
Oxygen contamination of the CPS gives disastrous eects on the formation of
the a-Si lm. Cracking occurs earlier in the annealing process, and in general a
bigger part of the area breaks. The CPS should be refreshed once every two to
three weeks.
When blading the liquid silicon, trails from the blade are in some cases
visible.
These are thick and narrow lines that are the rst to break during
thermal annealing. The trails may be carefully removed before UV exposure by
a tissue to ensure that the cracked trail does not drag neighboring a-Si layers.
Breaking of the layer is the result of thermal expansion. This may be helped
by slowing down the annealing temperature process over many hours. This of
course is not desirable when considering a roll-to-roll process.
Automated blading can help remove many variables induced by manual blading, and may spread a liquid silicon layer in a consistent way. It is much more
accurate, and a large blade should be mounted so that multiple blading is not
required in order to spread the liquid over the whole wafer.
7.5.
RECOMMENDATIONS
87
Low temperature annealing
Additional precautions have to be taken when handling a wafer with a polyimide
substrate. Any damage through Excimer Laser crystallization or annealing can
signicantly impact the polyimide layer in subsequent processing steps. A lower
initial energy density is desired at the cost of the grain size.
The linear decrease came out to be give the best results in the long pulse
conguration. The exponential recipe decreased to a shot count of 1 very rapidly
in this conguration, as the lm could handle much higher energies. This reduction in shot count made the results of the exponential recipe similar to the one
with single ramped shots. A much higher initial energy, or higher shot count
is required to be able to remove hydrogen in this recipe type at higher energy
levels.
Other methods for increasing the grain size should be tested. Heating of the
wafer for instance, during laser irradiation, could reduce the cooling rate which
leads to the formation of bigger grains.
Excess removal
Similar to doctor blade coating, automated blading will have many benets in
this process. Due to the increased accuracy and stability of the blade, a rigid
blade type may be chosen to remove the excess layers.
Main issue in this work during this process was the lack of lling of the patterns, while excess is in many cases not completely removed. Fundamentally, a
decrease in wettability in the non-patterned areas is desired, while wettability
and adhesion should increase within the patterns. Again the automated rigid
blading will lead to less liquid being dragged out of the pattern. Surface modication techniques may however allow even better lling of the patterns in this
sense. Using silicon nitride or another material with a lower SFE will lead to
better excess removal, whereas HF dip or an alternative with better adhesion
would lead to a better lling of the patterns.
Time dependency of the liquid silicon should be explored further. As adhesion increases, lm deformation may decreases, which may relate to the cracking
of some of the layers.
88
CHAPTER 7.
CONCLUSIONS AND RECOMMENDATIONS
Appendix A
Market Analysis
A.1 Radical innovation
In the business world, two types of innovation exist within the research department.
The rst type is called the incremental innovation, where research is
based on building upon current existing products. For example, Intel's Pentium
III to the Pentium IV processor is an innovation of this type.
The products
are fundamentally similar; however the latter is simply a better version of the
former.
This type of innovation is usually safe, and causes a steady increase
in product quality. The only risk is that a competitor brings a product on the
market that is radically dierent.
That brings us to the second type of innovation, which is the radical innovation.
An innovative product of this type is fundamentally dierent from
its predecessors. It may cause a complete change in the competing basis of a
certain industrial sector. An example would be the change from cellular phones,
to smart phones, to smart phones with touch screens, every innovative change
brings the competing basis to a fundamentally dierent level. This type of innovation is in most cases hard to achieve and is therefore not a reliable source
of increase in product quality
In many cases a company has the choice to invest in many dierent innovative
projects. It is common however to spend the bigger part of the innovative budget
in the incremental type of innovation. However, should a company be prepared
for future changes in the competing race, a signicant share of the budget should
go to radical innovation. By doing so, a company is up-to-date and can keep up
with its competitors or even become a pioneer in a new revolutionizing product.
[56]
That is exactly what the project described in this paper is aiming for. Liquid
silicon is still at its infancy, and many researchers either still focus on improving
current displays by making them sharper, or faster. Other research is also based
on radical innovation, but more in the sector of exible displays using organic
semiconductors.
89
90
APPENDIX A.
MARKET ANALYSIS
By exploiting the potential of liquid silicon, the best of both the high quality
displays as well as the cheap and exible organic displays can be achieved.
The whole basis of competition within the display sector, or even in any chip
fabricating sector, will change. One important claim to consider is the claim of
solution processing beating the costs of conventional processing.
A.2 Associated costs
Current organic solution processing has been reviewed. In this case it is often
claimed that the main advantage of this type of processing lies in the cheap
fabrication method.
A careful analysis is required to value this claim.
High
quality transistors are still hard to achieve with this type of processing, especially
when using organic semiconductors. Printing a complete device will have to be
compared to manufacturing in the conventional method. In this sense, although
printing of organic TFT's loses ground in the quality of transistors, it has a much
lower cost per unit area of a substrate. Several important points are discussed
[3]:
ˆ
Although it is said that solution processing is useful for its lower cost
in processing, it will not achieve a lower cost for linewidths of over 1
micrometer. This is because in this regime there are many lithographic
tools available that have been highly depreciated.
Creating lines with
a width of more than a micrometer is therefore cheaper in the case of
lithography tooling.
ˆ
Although theoretically it is believed that printing can bring an eventual
lower cost for the fabrication of devices, the actual process machinery
still needs to be developed and made suitable for mass IC production.
Although the general idea of the technology is available for other sectors,
a machine for accurately printing full devices on a mass scale still needs
to be developed.
On the other hand, conventional processing of silicon
is based on many decades of optimization, and have settled a good solid
bases to improve upon. It would be hard to convince these machine owners
to move on to the new era of printing chips.
ˆ
Process complexity in any case would be reduced in solution processing
methods which will decrease the overall costs and increase the throughput
of devices fabricated. This is due to its principal idea of additive processing rather than subtractive processing where lithography is used to use
masks, develop the masking layer, baking the masking layer, etching, and
removing the masking layer.
It will lead to a reduction of overall step
count, raw material costs and tooling costs. In this sense costs are greatly
reduced.
ˆ
Low-cost substrates can be handled and potentially, roll-to-roll processing
can be used to increase the throughput to a level of mass production.
Tools for high registration accuracy is required when multiple layers need
A.3.
APPLICATIONS
91
to be deposited on the same respective devices. This again is accounted
for the development of the IC printing devices.
ˆ
Although cost per substrate area may be much lower when considering
conventional processing methods, the cost per transistor, or the cost per
function is much higher due to the worse resolution for current printing
methods as well as the electronic quality limitations of the organic transistor.
ˆ
Cost advantages are depending on specic process ows used
A.3 Applications
Printed electronics make it economically attractive in area-constrained applications rather than requiring functional density[3, 57]
+
displays, sensors (functional density of sensors dominant on size and formfactor of sensing element), RFID tags (operating at relatively low frequencies such that the size of the antenna and passive components dominate
overall size of the tag
Easy integration of mutually incompatible and diverse materials on the same
substrate
+
various sensor types and tags with multiple functions
Relatively poor performance due to low temperature process, lack of self alignment, poor lm quality, large linewidth, low performance materials are used.
Frequency of operation should therefore be less than 1MHz and the device should
be relatively large.
+ displays, simple sensing elements
RFID focus on lower cost (eliminate
expensive chip attach), however, these
will have a limited performance
Printed array sensors
performance trade-os, integration of disparate mate-
rials on the same substrate
Displays
costs per unit area, before only low resolution displays were pro-
duced, for high resolution, the electronics should be of higher quality or a
compatible back plane technology required.
92
APPENDIX A.
MARKET ANALYSIS
Appendix B
Printer types for electronics
fabrication
B.1 Impact Printers
Impact printers are the oldest type of printers known to man. The many types of
impact printers are fundamentally based on a print master which is coated with
ink and transferred to a substrate upon contact.
makes the resulting images highly reproducible.
This full contact property
The fact that the process is
based on bringing ink and substrate into contact by a specic force will lead to
the danger of wear of the master. Nevertheless the process is used nowadays
in areas where mass production of the prints is needed due to the speed and
reproductive quality of this type. The master therefore needs to be highly stable
and carefully optimized. Printers of this type include: letterpress, lithography,
screen, and gravure printers [13]. Their main properties are:
+
High speeds can be achieved due to cylinder to cylinder, or roll to roll,
printing [13]
+
The process can be converted into a web-fed process, rather than sheet press
process. This makes high speeds possible, however, they have longer setup times, more start-up waste, and it makes it dicult to print on varying
formats or substrates.
They have a great advantage however in longer
print runs. [13]
+
-
High repetitive quality [13, 10]
They have defectivity challenges due to the direct contact of the master with
the substrate. [3]
-
Ink splitting should be taken into account. It is a process where, the ink to be
transported is split when the master is released from the target substrate,
causing a thinner printed layer than what might be hoped for.
93
94 APPENDIX B.
PRINTER TYPES FOR ELECTRONICS FABRICATION
Figure B.1: The master plates for four main impact printers [13]
The dierences between the dierent impact printers is primarily in the way
that the master plates are constructed. Fig. B.1 shows dierent masters that are
associated to the dierent types of impact printers. Letterpress is probably the
easiest to understand and is the most well-known type. Patterns that are needed
to be transferred to the substrate are on an elevated level on the print master.
These raised elements bring the ink into contact with the substrate. The second
master in the gure is the lithography printer, where the printing elements are on
the same level as the non-printing levels. The surface is modied so that the ink
will only adhere only to the printing elements. The third type is screen printing,
where the master is patterned by way of making openings through which the
ink is pushed onto the substrate according to the shape of the openings. Finally,
gravure printing may be considered as the inverse of letterpress printing. The
master is patterned by means of small indents lled with ink that can later be
transferred on the substrate by means of uid adhesion. This last type of impact
printer will be important for this thesis and a separate section will be dedicated
for it.
B.1.1 Gravure Printing
Gravure printing has been known for its outstanding reproductive quality although an expensive master is needed.
Today's market share for this type is
limited to 10 to 15 percent, and found to be fruitful for very long print runs
(>1000000). Gravure printing is typically implemented for high-quality, highcirculation printed packages as well as products which include: magazines, catalogs, plastic lms, metal foils, transparent lms, carrier bags, security papers,
stamps, bank notes. In today's commonly used applications, there are various
sizes and speeds in use. [13]
In this master the printing elements are formed in the inverse way of letterpress printing. The elements are recessed in a master with various depths and
sizes for an optimum print quality and prevention of pattern deformation upon
contact, common in letterpress printing. The entire plate or cylinder is ooded
with low viscosity ink.
A cylinder is preferably used for a better throughput
of the process.While this ink lls the holes of the patterns and forms a lm on
B.1.
IMPACT PRINTERS
95
top of the patterns as well as non-patterned areas, a high quality blade removes
any excess ink. The excess ink can be reused and the ink that is left in the cells
of the cylinder or plate is pressed against the substrate under a high pressure.
Depending on the type of ink, the force that has been used or the speed of the
process lateral shear forces can cause a pick-out eect where the ink can be
pulled out of the desired printing area. Optimization of this process with specic ink, force and speed is necessary. [3] A schematic of this process is shown
in Fig. B.2 .
Figure B.2: Gravure printing schematic [13]
The blade, also known as the soul of gravure printing is one of the most
important elements of the whole process. It is a thin, wear-resistant, strip that
ensures the removal of excess ink, and proper lling of the cells and is usually
made of steel. Since the whole cylinder will be pressed against the substrate,
any defects on the blade will have a direct impact on the nal result.
It is
mounted slightly angled depending on the type of blade or the type of ink that
is used. Due to the constant scraping of the blade and its importance, it must
be changed regularly to avoid the eects of wear.
When aiming at high manufacturing speeds, using an engraved cylinder as
the master is the better way when comparing to a plate master. This cylinder
has recesses for the printing elements, however, these elements can have either
variable depth, variable area, or both for an optimum print quality. For every
dierent color, whole cylinders are used to maximize throughput. The impression roller is used as a counter force from the other side of the target substrate.
These rollers should be as small as possible to ensure a narrow printing nip. The
roller needs to be capable of withstanding high pressures without deformation
or deection. Hydraulic cylinders can oer a solution to this problem. Cooling
is also an issue in these structures. [13]
The inks that can be used have a relatively low viscosity for a high speed
lling of the printing cavities.
Although bleed-out can also be a problem for
gravure printing, it is not as critical as it is for screen printing.
No binders
are therefore required that thicken the ink, unlike with screen printing. There
is a large range of useful inks that can easily have their viscosity decreased by
96 APPENDIX B.
PRINTER TYPES FOR ELECTRONICS FABRICATION
creating a solution out of them. Toluene and Xylene are typically used solvents
since they are transparent, can dissolve many types of inks and dry fairly quickly.
The nal ink is kept in an ink pan in which the gravure cylinder is inserted.
The cylinder must not form foams or splash inks to maintain a high quality ink
lling of the cells. When the ink is transferred to the nal substrate, it will not
be able to be transferred completely which is also the case for the other impact
printers.
The ink will split depending on variables such as: thickness of the
ink layer, period of contact, contact pressure, rheological ink properties such
as viscosity and wetting properties, temperature ratios, surface properties and
absorption properties. In the case of gravure printing, two additional variables
can inuence this ink splitting, such as the shape of the cells and the lling level
of the cells.[13]
To summarize, there are a few key aspects of the gravure printing process
that make them suitable for the mass production of electronics:
+
Very high image quality [12, 13]
+
Very high speed [3]
+
Very good image reproduction [13, 10]
+
+
Good uniformity [3]
Low line-edge roughness [3]
+
Good compatibility with many materials [12]
+
Wide range of thicknesses possible due to the cell structure [3]
+
Good scalability up to a linewidth below 20µm [3]
-
Expensive Cylinder [12, 13]
-
Limited by overlay printing registration accuracy (OPRA) [58]
-
Separate cells on the cylinder prevent this type of printer to create smooth
straight lines. [59]
For the implementation in electronics fabrication, it is important to know the
operating regime of the process currently in use. Table B.1 summarizes some of
the typical values associated to gravure printing.
Research has been conducted on the printability of electronics using this
type of printing[58, 60]. The benets of gravure printing has attracted many
researchers. Already many electronic devices have been proven to be producible
with this type of printing such as OLEDs[61, 62, 63] and basic circuit elements
[12, 64, 65].
B.1.
IMPACT PRINTERS
97
Table B.1: Typical values for gravure printing processes [13]
Web width
Web speed
Gravure Cylinder
2.4m - 3.6m
(publication)
1.2m - 1.4m
(packaging)
10m/s - 15m/s
(publication)
5m/s - 6.5m/s
(packaging)
800mm - 1600mm
(publication)
circumference
Screen ruling
40 lines/cm - 140 lines/cm (typically
60 lines/cm - 70 lines/cm)
Cell geometry
Width
30
Depth
10
μm - 230 μm
μm - 30 μm (max
μm - 5 μm
Cell wall width
3
50
μm)
B.1.2 Other impact printers
Letterpress Printing
The oldest type of impact printer and is the least complex type of impact
printing.
Typical non-electrical applications that are used today by letter-
press/exographic printing include small-format jobs, business cards, form printing, packaging printwork, labels, carriers and bags.
The print master used to transfer the ink to the substrate is modied in a
way that the print elements, so the patterns that needed to be transferred, are
raised. The master plate will subsequently be inked and the ink is transferred
by applying pressure to the master, onto the substrate. The raised elements on
which the ink has adhered will be transferred by force. The schematic is shown
in Fig. B.3 [13]
An upgraded version of this type of printing is exography which uses a relief
exible plate to transfer the ink. This type can be used on many dierent types
of target substrates. [59]
This method has proven to have a fundamental advantage but also a significant disadvantage:
+
The manufacturing method is quite easy and straightforward. [12]
+
Continuous lines can be printed unlike other pixelated printer types such as
inkjet, gravure or screen printing. This prevents this type of printer from
pinholes, cell blocking, and missing dots. [66]
-
The master is prone to mechanical deformation due to the excessive force that
is applied on the raised elements. Minor deformations of these elements
can lead directly to a change in the shape of the resulting pattern. [12]
-
On the edges of the printed areas, a certain pattern is visible due to the
squashing behavior of the plate to the substrate. The non-uniformity at
the edges may cause issues in electrical systems.[59]
98 APPENDIX B.
PRINTER TYPES FOR ELECTRONICS FABRICATION
Figure B.3: Letterpress schematic diagram [13]
Flexography has been shows in some instances to be suitable for the printing
of continuous lines in electronics in [66], and has helped the development in
OLED displays in [67].
Within the electronics industry however, alternative
printer types based on letterpress have emerged such as microcontact printing
and nanoimprint. Both show promising results in research, and can achieve high
resolutions.
(Oset) Lithography Printing
Oset lithography is one of the most commonly used type of printer. Typical
applications to this type of printing method include newspapers, magazines,
brochures, books, and packaging.
In lithography there is no problem with deformation of printing elements,
since no elements are raised. Instead, all elements are on the same physical level,
but have dierent material properties that can either adhere or repel ink. The
master plates are usually made of dierent materials with dierent chemical and
surface properties. An aluminum based surface is covered with a photopolymer
which is patterned as the area to which ink will adhere. The whole surface is
then covered with a dampening solution which spreads on the aluminum surface
due to its high surface energy, but will stick poorly to the photopolymer areas
with low surface energy. The surface is subsequently covered with ink that will
only spread on the image areas. This surface is then brought into contact with
the desired substrate. [13, 59]
Oset printing is related to this type of printing by setting an intermediate
carrier for the transfer of ink. The ink is then rst pressed against the intermediate carrier, which subsequently brings the ink to the nal substrate for the
reduction of water brought to this substrate. [13, 59] The schematic is shown
in Fig. B.4 .
Although there are some advantages to this method of printing, there is one
fundamental limitation [12]:
+
No deformation due to impact
B.1.
IMPACT PRINTERS
99
Figure B.4: Lithography/Oset printing schematic diagram [13]
+ The printing quality is high
+ The resolution can be made high
+ plates are easy to make and relatively cheap [59]
- The useable ink is quite limited since it needs to be compatible with various
materials and have the right properties.
-
High viscosity inks are required
Some limitations for the printing of electronics include: issues with the water
based dampening solution that can aect the ink which is could have detrimental electronic properties to the material.
Waterless oset lithography can be
used which replaces the dampening solution with a silicone layer. The second
limitation is that the thickness of the transferred ink is relatively low (1 to 2
µm).
Multiple passes of the printing plate may be required. Finally, inks with
high viscosity are required.
Screen Printing
For printed electronics, this type of printing is the most mature.
The most
commonly known application in electronics are printed circuit boards, that have
been manufactured in this way for decades.
printing is used in:
Typically, besides PCBs, screen
textiles, t-shirts, toys, equipment, packaging, and large-
format advertising posters. [13, 3]
The working principal is based on squeezing ink all the way through a master
plate.
The master plate is constructed from a ne mesh.
The non-printing
100APPENDIX B. PRINTER TYPES FOR ELECTRONICS FABRICATION
elements are coated by a photosensitive screen coating. A squeegee is used to
squeeze the ink through the open meshes, while the relatively viscous ink is
pushed through the mesh and transferred onto the substrate. Fig. B.5 shows a
schematic of the process. [13]
Figure B.5: Screen printing schematic [13]
Although, again no elements are raised that are prone to deformation during
impact, there is a signicant disadvantage of using screen printing and it lies in
the type of ink that can be used as well as its resolution [3]:
+
Can deposit thick lms which may be useful in applications such as contact
line production
-
Resolution is typically worse than 50µm in commercial use
Research has indicated a resolution of less than 10µm to be achievable
The viscosity of the ink should be relatively high (>1000cP). This is because
excessive spreading and bleed-out needs to be avoided.
A low viscosity
ink will cause disastrous eects due to these eects. Binders are added to
increase the viscosity of the ink. This is generally used in graphic arts,
however, for electronics; these binders can destroy semiconductor properties. They can cause excessive leakage currents, dissipation in dielectrics
or drop the conductivity of conductors.
Due to the nal con, screen printing is applied only in electronic applications
where addition of the binders does not lead to unacceptable loss of performance.[3]
The main electronic application for this printer type still lies in the production
of PCBs, however the production of OLEDs [68, 69] and even OLED displays
[70] have also been shown to be possible with this printer type.
B.2 Non-Impact printers
Non-impact printers (NIP) are popular due to their property of printing variable
prints easily. This type is not limited to a stable, physically xed master that
should be used many times before being economically feasible.
Every print
run can be produced in a dierent way, so we would have a print-on-demand
B.2.
NON-IMPACT PRINTERS
101
system. Prints can easily be adjusted digitally allowing the variation of prints
per run. This is a huge advantage when researching new materials. There are
however some negative sides to be considered.
Due to its resetting property,
it can produce a greater variation when two of the same prints are produced.
The paper is usually held by electrostatic forces instead of grippers which gives
limitations to the overall accuracy. Finally, this type is slower than cylindrically
printed, web-fed, impact printers. Each print requires a fresh imaging; it will
however be unlikely to produce large scale benets for mass production. From
this type of printers, Inkjet is most commonly used for research purposes, and
electrophotography is the second most commonly used commercial NIP type.
To summarize, the main properties of Non-Impact Printers include:
+
They are not limited to the stability of the physically xed image carrier.
The patterns are digitally preprocessed.
+
They can imprint a dierent page per print, making optimal use of variability
in print runs
-
They have however a low repetitive quality, since every print run is regarded
as a new print.
-
They have the problem of achieving high speeds.
B.2.1 Inkjet
Printing active electronic circuits today is most commonly done by inkjet printing [12, 3] . It oers a quick and easily variable process and therefore a good
way to research new materials. In this type of printing, low viscosity inks are
used (1-20cP), so again no binders are required as was the case for the screen
printing process.
No masks are needed for the NIP printers, and by digital
manipulation, prototypes can easily be manufactured [10].
Inkjet, unlike electrophotography, does not require an intermediate carrier
for transferring the image information. The ink is shot directly onto the substrate either continuously or by drop-on-demand. In the continuous case, the ink
is constantly shot towards a substrate as a continuous stream of small droplets.
These droplets are charged and directed towards the image by means of an electric eld.
When an area does not require ink, the droplets are deected for
re-usage.
In the case of drop-on-demand, a droplet is only produced when it
is required. The ink is shot either thermally by inducing a gas bubble through
evaporation and sending the droplet in front of the bubble towards the substrate
when the bubble pops. A second way is to induce a bubble piezo-electrically by
bulging the material itself for the ink shot. [printer book paddy French] [13] .
The schematic is shown in Fig. B.6. The summarized properties are:
+
+
It uses a digital input for on the y design changes [3, 10]
It is less prone to wear problems compared to impact printer types
102APPENDIX B. PRINTER TYPES FOR ELECTRONICS FABRICATION
Figure B.6: Inkjet printing schematic [13]
+ It is compatible with many dierent materials [3]
+ It deposits in small volumes [10]
- Since the technique depends on droplets, pixelation
related issues are in-
evitable.
-
Due to propelled droplets rather than full contact of patterns in impact printing methods, there is a statistical variation in the nal position of the
droplets.
-
Due to the complex drying phenomena of droplets it can produce widely variant printed patterns. Pinholes and sharp edges are the result of this. The
non-uniformity of this drying will also lead to non-homogeneous transistor performance. [3] This issue has been solved for a case in [71] which
resulted in a high mobility of 16.4 cm²/Vs
-
Distance between deposited drops can change the linestyles of the nal result.
Small distance between drops will cause bulging, a bigger distance will
lead to scallops. One possible solution to this is ash drying which is the
rapid drying of drops upon substrate contact. [3]
-
Slower than the web-fed cylindrical impact printer types. A higher throughput
can only be achieved when multiple nozzles are used in parallel. Misring
is an issue in these systems [3, 10]
B.2.
NON-IMPACT PRINTERS
103
Inkjet is currently widely used for the research of organic semiconductor devices
due to the ease and exibility of the solution process due to its non-impact
printing advantages. In the case of these electronic devices, a thermal nozzle
can not be used since the heat produced in this process could destroy the properties of the organic semiconductor. A piezoelectric or even an acoustic inkjet
printer is used therefore used. Many works have already shown the production
of electronic devices using this printer type [27, 72, 71, 73], in some instances all
parts of the TFT[74, 75]. Today, subfemtoliter accurate inkjet printers are being used at Someya's Organic Transistor Lab in Tokyo University for extremely
high accuracy [72].
B.2.2 Electrophotography
Electrophotography is based on the transfer of ink to a substrate through electrostatic forces. A drum with a specic type of surface can be used on which a
controlled light source can create patterns. The light source will induce charged
colored particles called toner to be attracted to the specic patterned parts on
the surface of the drum. This toner is subsequently xed on the substrate, after
which the drum is cleaned. The individual process steps are visualized in Fig.
B.7
Figure B.7: Electrophotography schematic [13]
Electrophotography is the second most widely used Non-Impact Printer type,
and is used today mainly as commercial automatic copiers.
Some of the properties of electrophotographic printing are [13] :
+
-
Can produce higher quality prints compared to inkjet
Slower than inkjet and more expensive, although [76] reviews this and claims
that this dierence is quite small
104APPENDIX B. PRINTER TYPES FOR ELECTRONICS FABRICATION
+
Similar to Inkjet printing, there is no xed master, and a dierent charge
image can be produced after every drum revolution.
-
When repetition in prints are required the drum still needs to be recharded
after every rotation.
Fluctuations in two identical print setups are in-
evitable due to both image generation with charges, and the transfer of
toner to the substrate due to electrostatic forces.
ˆ
The type of toner used determines for a large part the quality of the print:
particle size, shape, and chemical structure. Both liquid as well as powder
typed toner may be used. Conventionally, toners with particle sizes of 6
to 8 um are used.
Although electrophotographic print quality is superior to the quality of inkjet
printing, the former is not being used for printing electronics for two main
reasons [59]:
1. The toner used for transfer of materials to the substrate needs to be
charged and go through an electrostatic eld.
Conductors and semi-
conductors will be inuenced by this charging.
2. The transferred toner needs to be xed onto the substrate. This xation
may cause problems in some applications.
B.3 Conclusion
Dierent types of printers have been introduced in this chapter. Some are more
suitable for electronic applications than others. For mass production ends it is
important to achieve high quality at under high production speeds. Table B.2
qualies the various printers according to important aspects from the electronic
device manufacturing point of view. Notice that electrophotography has been
left out of the table due to lack of electronic device applications.
When comparing impact printers to NIPs the biggest dierence is the digital
input with the NIPs. Because of this property, they are excellent for highly variable printing processes, and therefore quite useful for research purposes. They
lack however in throughput, and therefore although very useful for research,
may not be the best option for mass production.
Impact printers on the other hand are aimed for mass producing runs, using
roll-to-roll, web-fed processes. Screen printing achieving limited resolution may
not be useful for electronic ends although contacts in PCBs are still manufactured in this way and there may be a certain reliability to this quality. Oset
lithography is also a viable option, however, there are limited works that have
investigated the fabrication of electronics using this type of printing, this may
be due to the poor compatibility of the inks that can be used as wettability
is a prime focus. Flexography has the issue of pattern deformation, although
the masters are relatively easy to make, the main advantage lies in the printing
of continuous lines, unlike gravure, inkjet or screen printing. Finally, gravure
B.3.
CONCLUSION
105
Table B.2: Comparative analysis of the various printer types
Typical
Ink
Main
Main
Resolution
Viscosity
advantage
disadvantage
+
low
Excellent
expensive
reproducibility
master
Gravure
and delity
Flexography
Oset
±
+
low
high
Lithogra-
continuous line
pattern
printability
deformation
fast and
limited ink
accurate
compatibility
phy
Screen
-
high
reliable
low resolution
Inkjet
+
low
digital input
low throughput
printing has gained quite some attention due to its high reproducibility and delity over long print runs. In addition, the throughput is high and many types
of inks may be used. The cost of creating the master is relatively high, however,
print runs exceeding over 1,000,000 runs, compensate for the costs and make
this type of printing rather inexpensive.
For electronics, many dierent printers may be used but are suitable for
perhaps various parts of the transistor. A combination of printing types may be
the most ecient way to create low cost large area electronics. One particular
printer type, gravure printing, has proven to be the most interesting for mass
production ends.
For research however, it is more useful to produce easily
manipulable devices, and therefore the non-impact printer type inkjet printing,
is the most suitable for the early stages of printing electronics due to its digital
input and high resolution.
106APPENDIX B. PRINTER TYPES FOR ELECTRONICS FABRICATION
Appendix C
Thin-Film Transistor
Thin-lm transistors are dierent from conventional MOSFET's and BJT's in
one essential aspect: they can be built on top of a certain substrate.
Today
many of the applications of these devices are especially large area displays. In
these products the devices are manufactured on top of a glass substrate and
operate by directly inuencing the emission of light through the display. In this
example, it is not essential to create the devices at lower temperatures due to
the temperature stability of the glass substrate. Solution processing in this case
is still however desirable for larger area displays, exible displays, and cheaper
displays.
In essence, thin-lm transistors are a type of MOSFET's built on top of a
substrate. Today, many of the displays are created using hydrogenated amorphous silicon (a-Si:H) as the semiconductor. It is important to notice that most
of the devices in this Appendix are focused on organic semiconductors since
these are the main type of solution processable semiconductors and therefore
the direct competitor of the liquid silicon material.
Organic semiconductors
were preferred due to their solubility with many common solvents without losing their original semiconducting function. These organic semiconductors have
fundamental limitations to their stability, and therefore the impact of dierent
device congurations and processing methods to the nal device is quite big,
whereas this would not be the case for conventionally processed TFT's. Solution processed devices which is necessary for the printing of devices is mainly
focused from an organic TFT point of view.
C.1 OTFT Characteristics
For the organic transistor, current ow is limited as well as their reliability [2, 8].
The main charge carrier transport occurs in the rst 50 angström of the organic
semiconductor layer, away from the semiconductor-dielectric interface [9]. This
interface is therefore essential for the quality of the device.
The thickness of
the semiconductor layer ranges typically from 200-500 angström. Thinner lms
107
108
APPENDIX C.
THIN-FILM TRANSISTOR
could fail due to incomplete surface coverage [9].
The deposition of the semiconductor layer in this type of transistor becomes
crucial. There are three important points about deposition conditions [9]. The
rst one is the deposition rate; it can directly aect the crystallinity and morphology of the resulting layer. A high deposition rate leads to high nucleation
densities. These nucleation densities are related to the grain sizes within a lm.
A high nucleation density corresponds to a high concentration of small grains. A
highly crystalline lm that consists of larger grains, gives better characteristics
of the nal device, as is the case for the dierent crystal structures of silicon
based devices. Bigger grain sizes are therefore desirable, since grain boundaries
deteriorate the current ow of the device. Additional energy is required for the
charge carriers to move from one grain to the other. High deposition rates are
in this case not desired; however it could also induce the formation of a dierent
packing order which may improve the device performance. The surface of the
substrate and substrate temperature also has a great impact on the formation
of the lm.
Substrates with grooves in a particular direction may induce an
ordering of the organic molecules in a way that improves the charge carrier
transport. Finally, post-deposition treatments can also help improve the device
performance.
C.2 Transistor Conguration
The basic construction of TFT's can be divided into four groups depending
on the position of the gate, semiconductor and contacts.
Each has its own
advantages as well as disadvantages and will briey be described below. Again
it is important to consider the organic variant of these devices. These devices
are shown in Fig. C.1.
ˆ
Top-Gate-Bottom-Contact
ˆ
Bottom-Gate-Top-Contact
ˆ
Bottom-Gate-Bottom-Contact
ˆ
Substrate-Gate-Bottom-Contact
Notice that the combination of Top-Gate-Top-Contact has been left out.
This is primarily due to the poor reliability of the organic semiconductor. In
most cases it is undesirable to build upon an organic semiconductor as thermal
sensitivity and contamination may impact the semiconductor characteristics and
therefore the properties of the nal device.
The dierent gate positions relate to some important device characteristics.
A top-gate (staggered) conguration compared to the bottom-gate (invertedstaggered) conguration gives:
+
The spacing between source and drain and therefore the essential channel
length can be determined early on the design, as the contacts are deposited
at an earlier stage.
C.2.
TRANSISTOR CONFIGURATION
109
Figure C.1: Various TFT structures [12]
+
Since the gate layer is deposited later than the semiconductor layer, the gate
dielectric will cover the semiconductor layer. This will give the advantage
of encapsulation of the sensitive semiconductor layer.
+
Due to its inverted staggered conguration, the contact resistance to the gate
is much lower.
A bottom-gate conguration will bring some other advantages on the other
hand:
+
The semiconductor layer will be deposited later on, which helps for the thermal sensitivity of this layer. There are also less processing restrictions in
this way for avoiding contamination of the semiconductor.
+
The choice of dielectric and deposition is much less critical in this case to
avoid changing the semiconductor characteristics of the already deposited
semiconductor layer.
+
The interface between semiconductor and dielectric layers are not determined
by the roughness of the semiconductor layer which is generally relatively
rough.
+
Higher mobility devices can be achieved in this conguration.
The position of the contacts also diers in the performance of the nal device.
It may be confusing to dene characteristics based solely on the position of the
contacts regarding the semiconductor lm.
A better way to characterize the
contact position is with respect to the semiconductor-dielectric interface. Some
general statements to the contact positions are listed below.
ˆ
For devices with the contacts placed close to the semiconductor-dielectric
interface. The charge carriers don't have to travel a long path to reach
channel on the semiconductor-dielectric interface.
therefore lower in this conguration. [72]
Contact resistance is
110
ˆ
APPENDIX C.
THIN-FILM TRANSISTOR
It is undesirable to have the semiconductor placed before the contacts
are placed due to the instability of the organic semiconductor that may
therefore be inuenced by the subsequent contact deposition.
Although some of these characteristics are only limited to the processing of organic transistors, it is important to know what the alternative for liquid silicon
solution processed devices can provide. Some of the information in this section
may also apply to liquid silicon devices, although it is relatively more stable. In
the nal design of the device, the exact worst conguration for organic transistors will be used due to the way liquid silicon Single-Grain Thin-Film Transistors
are constructed.
Appendix D
SFE Results
Thermal Oxide
Surface modication/type
Contact angle
Untreated
21°
HF
O2 (8min500W)
UV exposed
IPA
Thermal Oxide2
O2 (8min500W)
HF
Al
Al(Marangoni)
Polyimide
PR(SPR3012)
PR(SPR3017m)
SiN
Etched back TEOS
Untreated
HF
O2 (10min500W)
O2 (30min300W)
Non-Etched back TEOS
Untreated
HF
O2 (10min500W)
12.5°
16.9°
16.6°
18.4°
19°
14.9°
0°
0°
14°
19°
21°
33.4°
26.6°
19.33°
21.8°
22.6°
17°
16.1°
17.2°
O2 (30min300W)
15.7°
HF + 100° pre-anneal
14.2°
100° pre-anneal
HMDS
Ar (0.5p350W)
Ar(0.5p50W)
Ar(lp350W)
Ar(lp50W)
Untreated 75°
111
25.7°
12.8°
34.9°
10°
22.6°
25°
12.5°
SFE (mJ/m²)
30.4
31.7
31.1
31.2
30.9
30.8
31.4
32.5
32.5
31.6
30.8
30.4
27.5
29.2
30.7
30.3
30.1
31.1
31.3
31.1
31.3
29.4
31.5
31.7
27.1
32.0
30.1
29.5
31.7
112
APPENDIX D.
SFE RESULTS
Surface modication/type
Contact angle
HF 75°
3.92°
O2 (30min300W) 75°
16.08°
HF + 100° pre-anneal 75°
10.4°
O2 (10min500W) 75°
100° pre-anneal 75°
HMDS 75°
Ar(0.5p350W) 75°
Ar(0.5p50W) 75°
Ar(lp350W) 75°
Ar(lp50W) 75°
14.8°
15.6°
11.8°
18.4°
15.4°
20.2°
19.4°
SFE (mJ/m²)
32.4
31.4
31.3
31.3
32.0
31.8
30.9
31.4
30.6
30.7
Appendix E
Excimer Laser Crystallization
E.1 Crystallization process
Two types of crystallisation occur in the ELC process. [47, 48]
Explosive Crystallization
The melting point of a-Si is a few hundred degrees lower than crystalline silicon
(1414°C). This dierence in melting point lies at the core of the explosive crystallization process. When the a-Si is molten, it is rapidly cooled with respect to
the crystalline silicon, which will cause crystallization of the liquid into polysilicon areas (only at very high cooling rates and lack of crystalline fractions will
the liquid solidify back into an amorphous solid).
The heat produced by the
crystallization of the silicon liquid will further melt the remaining a-Si which
again proceeds to solidify due to the supercooled surrounding. In this way the
molten layer moves deeper into the total lm leaving a trail of ne polysilicon
grains behind. The whole process is self-sustaining and is continued until there
is a sucient loss of heat that is required for the further melting of a-Si.
Melting and Solidication
When a certain threshold is reached by the laser, the ne polysilicon grains
that were previously formed by explosive crystallization, remelts at the surface.
This remelting of the polysilicon is not as severely supercooled as is the case for
explosive crystallization. Therefore the solidication rate is much lower, which
results in a reduced number of granular planar defects, and an increased grain
size.
At this second remelting energy a dierence between complete melt and
incomplete melt can be recognized. In the event of a complete melt, there are
no crystalline seeds left for regrowth. Therefore, before solidication may occur,
homogeneous nucleation is required.
These nucleï will stabilize under certain
thermal conditions. After the formation of nucleï, the lm will solidify rapidly.
113
114
APPENDIX E.
EXCIMER LASER CRYSTALLIZATION
The nucleation rate is strongly dependent on temperature, and will result in
a nal lm in which small grains coexist with larger ones. In a near complete
melt condition, some ne polycrystalline grains from the explosive crystallization
survive. Solidication will initiate at the remaining polycrystalline grains due
to heterogenous nucleation rather than the homogenous nucleation. Grain size
will increase signicantly and will grow beyond the lm thickness. This process
is called superlateral growth.
Keeping these processes in mind, grain sizes can be enlarged by increasing
the lateral growth interval. This can be done by reducing the cooling rate of
the molten silicon. Heating of the substrate, extension of the pulse duration, or
simultaneous irradiation of the sample from the front and backside are some of
the possible options. Shooting multiple pulses at a sample can also increase the
grain size due to the growth of preferential orientation of the grains.
E.2 Crystallization problems
Some issues when producing the crystalline lm may also occur:
Random grain boundaries
Grains that are grown from dierent seeds can
collide. During crystallization the silicon will expand due to the density
dierence between solid and liquid phase silicon. The higher density solid
silicon has little eect in the vertical growth direction, however, horizontally the liquid will be pushed by the crystallizing solid silicon.
At the
interface between two growing grains the liquid will built and prevent
further growth. This results in high defect density grain boundaries and
should be avoided when constructing polycrystalline silicon TFTs in this
way.
Breakdown growth
The rate of growth of the crystalline silicon may be too
high which may lead to misplacement of atoms that result in stacking
faults and twins and may even lead to complete breakdown. Orientation
of the crystal highly inuences this type of error.
Thermal stress
Rapid cooling may induce thermal stresses that will result in
planar defects during lateral growth. Tensile elastic strain in the silicon
builds up until yielding of the material occurs.
Again crystallographic
orientation is important for this type of error.
Film cracks
These are formed as a release of thermal stress due to steps in
the lm that have been lithographically produced. The dierence in thermal expansion rates of silicon and silicon dioxide lie at the source of this
problem.
Hydrogen eusion
Any hydrogen present within the amorphous silicon lm
can be euse due to the laser irradiation and may destroy some parts of
the lm. Since the liquid silicon material is produced from a hydrogenated
E.2.
CRYSTALLIZATION PROBLEMS
115
silicon compound, and due to limited annealing temperature, a signicant
amount of hydrogen atoms can be left in the lm that can cause defects
by this out-diusion of the hydrogen.
Film agglomeration
Partial dewetting may occur and lead to lm decomposi-
tion into beads which is known as agglomeration. The main source of this
dewetting are the uctuations of the silicon lm that are severe enough to
reach the underlying oxide layer. These uctuations are inuenced by the
pulsed-laser annealing caused by non-uniformities in the spatial prole of
the laser pulse and intensity uctuations from the homogenizer. Also the
interference of the incident beam with laterally scattered beams as well as
the surface tension gradient have an impact on this defect.
Ablation
is related to excessive agglomeration and is known as the explosive
release of hydrogen.
A major issue in this work is indeed the hydrogen
content of the amorphous silicon lm produced from the liquid silicon
material.
116
APPENDIX E.
EXCIMER LASER CRYSTALLIZATION
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List of Publications
M. Trifunovic, T. Yokota, Y. Kato, T. Tokuhara, I. Hirata, I. Osaka, K. Takimiya,
T. Sekitani, T. Someya, R. Ishihara, "OTFT with PNDT3BT-20 dispersed solution by drop casting method", International Workshop on Active-Matrix Flatpanel Displays and Devices (AMFPD12), Kyoto, Japan, 2012
R. Ishihara, J. Zhang, M. Trifunovic, M. van der Zwan, H. Takagishi, R.
Kawajiri, T. Shimoda and C.I.M. Beenakker, "Single-grain Si TFTs fabricated
by liquid-Si and long-pulse excimer-laser", The Electrochemical Society's 222nd
Meeting, Thin lm transistor technologies 11 (ECS TFT-11), (Abstract Accepted)
To be submitted
"Single-Grain Si TFTs on plastic" IEEE International Electron Device Meeting
(IEDM)
123
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