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Citation for the original published paper (version of record):
Braun, S., Sandström, N., Stemme, G., van der Wijngaart, W. (2009)
Wafer-Scale Manufacturing of Bulk Shape-Memory-Alloy Microactuators Based on Adhesive
Bonding of Titanium-Nickel Sheets to Structured Silicon Wafers.
Journal of microelectromechanical systems, 18(6): 1309-1317
http://dx.doi.org/10.1109/jmems.2009.2035368
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1
Wafer-Scale Manufacturing of Bulk
Shape-Memory-Alloy Microactuators Based on
Adhesive Bonding of Titanium-Nickel
Sheets to Structured Silicon Wafers
Stefan Braun, Niklas Sandström, Göran Stemme Fellow, IEEE and Wouter van der Wijngaart Member, IEEE
Abstract—This paper presents a concept for the wafer-scale
manufacturing of microactuators based on the adhesive bonding
of bulk shape memory alloy (SMA) sheets to silicon microstructures. Wafer-scale integration of a cold-state deformation mechanism is provided by the deposition of stressed films onto the
SMA sheet. A concept for heating of the SMA by Joule heating
through a resistive heater layer is presented. Critical fabrication
issues were investigated, including the cold-state deformation,
the bonding scheme and related stresses and the TitaniumNickel (TiNi) sheet patterning. Novel methods for the transfer
stamping of adhesive and for the handling of the thin TiNi
sheets were developed, based on the use of standard dicing blue
tape. First demonstrator TiNi cantilevers, wafer-level adhesively
bonded on a microstructured silicon substrate, were successfully
fabricated and evaluated. Intrinsically stressed silicon dioxide and
silicon nitride were deposited using plasma enhanced chemical
vapor deposition to deform the cantilevers in the cold state.
Tip deflections for 2.5 mm long cantilevers in cold/hot-state of
250/70 µm and 125/28 µm were obtained using silicon dioxide
and silicon nitride, respectively. The bond strength proved to be
stronger than the force created by the 2.5 mm long TiNi cantilever
and showed no degradation after more than 700 temperature
cycles. The shape memory behavior of the TiNi is maintained
during the integration process.
Index Terms—Adhesive bonding, blue tape, contact printing,
microactuator, microelectromechanical systems (MEMS), nitinol,
shape memory alloy (SMA), stress layers, titanium-nickel (TiNi),
integration, transfer stamping, wafer-scale integration, wet etching.
I. I NTRODUCTION
ICROELECTROMECHANICAL systems (MEMS)
technology offers a large range of microactuators
for specific applications, based on different actuation
principles such as electrostatic, piezoelectric, thermal and
magnetic actuation [1], [2]. In a comparison of the different
microactuators, the shape memory alloy (SMA) actuators
offer the highest work density, which exceeds that of other
actuation principles by at least an order of magnitude [3].
M
Manuscript received April 3, 2009; revised July 28, 2009. This work was
supported by the European Commission through the sixth framework program.
Subject Editor Y. B. Gianchandani.
The authors are with the Microsystem Technology Laboratory, School of
Electrical Engineering, The Royal Institute of Technology, 100 44 Stockholm, Sweden (e-mail: [email protected]; [email protected];
[email protected]; [email protected]).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JMEMS.2009.2035368
Examples of SMA microactuation can be found in [4], [5],
[6], [7].
The SMA microactuators deploy the shape memory effect,
which is the ability of certain materials to ’remember’ their
initial shape when they are deformed. At low temperatures the
SMA material is in the martensite phase (hereafter referred
to as cold state). The SMA cantilever will easily deform
under stress and will remain deformed after the stress has
been removed. Upon heating, the SMA material will change
to its austenite phase (hereafter referred to as hot state) and
the cantilever will rapidly and completely recover its initial
shape. After subsequent cooling, the cantilever will keep its
shape until it is deformed again.
Even though there are a number of materials showing the
shape memory effect [8], [9], [10], most of the SMA microactuators are based on alloys of Titanium and Nickel (hereafter
referred to as TiNi) because of several advantages over other
materials. TiNi alloys allow for adjusting the transformation
temperatures over a temperature range of typically - 50 °C
to 110 °C, only by changing the Ni over Ti ratio of the
alloy. Furthermore, TiNi alloys can be fabricated with standard
metalworking techniques, they exhibit better shape memory
strain performance than other known alloys and they consist
only of the affordable elements Nickel and Titanium [11],
[12]. The work presented in this paper is based on bulk, coldrolled TiNi films. However, many of the presented methods
are generic and can be applied to other SMA materials.
Despite the advantages, SMA materials are not yet a standard MEMS material, partially due to the lack of proper
methods for the wafer-scale and batch-compatible manufacturing and integration of SMA materials with MEMS structures.
Previous work on the integration of SMA material in MEMS
devices can be summarized in two different approaches. One
approach, the hybrid integration, is to fabricate the SMA
actuator element and the MEMS structure separately. The
bias spring is provided by a mechanical obstruction on the
MEMS structure, which deforms the SMA element during the
assembly of the SMA element and the MEMS structure [13].
This approach features several advantages such as the use
of bulk SMA, which is commercially available in a wide
thickness range and therefore allows for adjustable mechanical
robustness and reduced material cost. Furthermore, using
bending motions generates both large displacement and forces
and for SMA thicknesses above 30 µm bending motions in
2
the substrate-plane can be realized [3]. However, the required
per-component assembly is not batch compatible and results
in unacceptable high costs. Batch processing compatibility is
provided by another method, the sputtering or evaporation of
thin SMA films [4] on the MEMS structure. In this approach,
the monolithic integration, the bias spring is provided by the
built-in film stress. However, sputtering of SMA is complicated and an annealing of the material at high temperatures is
necessary, which implies unwanted interdiffusion processes of
the SMA with the substrate. Furthermore, the thicknesses of
the sputtered TiNi-films are limited to approximately 20 µm [3]
and a recent report states that TiNi-based film sputtering
is mostly feasible for thicknesses up to 10 µm only [14].
For completeness it should be mentioned that in a recent
report [15] a 30 µm thick film of a ternary TiNi based alloy was
achieved by flash-evaporating TiNiCu onto copper substrates,
however, the issues of complicated processing and high postdeposition annealing temperatures are still not overcome.
To the authors’ knowledge, there are only few reports on
wafer-scale integration of bulk SMA sheets with silicon based
microstructures. In a previous work [16], a SMA sheet was
patterned on wafer-scale and the elements were selectively
transferred to single plastic microvalves. However, the coldstate reset was provided by a spacer between the microvalve
and the SMA element, which requires pick and place assembly
and furthermore the electrical contacting of the SMA was
performed using a complicated gap welding process. Another
report [17] introduces the wafer-scale integration of prestrained SMA wires using adhesive bonding and utilizing
single-crystalline silicon as cold-state reset. This approach
looks promising, however, it addresses the integration of SMA
wires instead of SMA sheets.
The present paper introduces a novel concept for both
the wafer-scale manufacturing and the integration of robust
trimorph bulk SMA microactuators on silicon wafers, circumventing the limitations of the previous methods. The work addresses three key aspects for wafer-scale and batch compatible
manufacturing of SMA microactuators based on bulk TiNi.
The first is to provide both a batch manufacturing compatible
and wafer-scale cold-state reset mechanism and a concept for
thermal energy supply. The second aspect is the wafer-scale
integration of bulk TiNi with microstructured silicon wafers.
The third aspect is the patterning of the TiNi using the very
aggressive HF/HNO3 based etchant. The experimental part of
this work focuses mainly on the integration of the TiNi and its
cold state reset. Preliminary results are reported in [18]. The
technology for the transfer of TiNi sheets to silicon described
in this paper is generic and can be used for other metal films.
II. C ONCEPT
M ANUFACTURING
B ULK SMA MICROACTUATORS
A. Wafer-scale and batch-compatible cold-state reset and
thermal energy supply mechanism
The proposed actuator design contains three functional
layers (Figure 1a). The first functional layer consists of the
bulk SMA sheet.
The second functional layer is a stressed film which deforms the SMA in the cold state and, in contrast to previous
OF
FOR THE WAFER - SCALE
a) three separate functions necessary for the SMA actuator
bulk SMA
cold-state reset
heater
two configurations to build microactuator
cold-state reset: thick intrinsically
stressed dielectric
thick combined cold-state reset &
heater (bifunctional layer):
thermally stressed conductor
thin heater, e.g. thin metal
thin dielectric
bulk SMA
bulk SMA
- cross section -
- cross section -
bulk SMA
bulk SMA
cold-state reset
thick combined
cold-state reset
& heater
thin heater
- top view -
(b)
- top view -
(c)
Figure 1. Figure showing (a) an illustration of the functional layers, (b) and
(c) the two different proposed configurations of the functional layers.
bulk SMA microactuators, eliminates the need for pick-andplace integration. Besides the batch-compatibility of depositing stressed thin films, this method also allows for tuning
the actuator characteristics by the choice of layer thicknesses,
material deposition technology and deposition conditions. Depending on these parameters, the stress in the deposited film is
either compressive or tensile. Compressive or tensile stresses
arise from thermal and/or intrinsic stresses. Thermal stresses
result from a mismatch of the coefficient of thermal expansion
(CTE) of the deposited layer towards the substrate, whereas
intrinsic stresses result from non-uniformities during the film
nucleation. When depositing films on a substrate, the intrinsic
stresses are often combined with thermal stresses because of
the different CTE of the two materials, however, in most cases
the intrinsic stress component is much larger than the thermal
stress component [19].
The third functional layer is a heating layer to supply
thermal energy to the actuator via an indirect heating scheme.
The heating layer is a resistive heater, which potentially allows
for straightforward electrical contacting and low actuation
current. The heater-pattern can be optimized to reduce both
thermal gradients along the beam and power consumption.
Furthermore, an indirect heater eliminates the need to pattern
the bulk SMA for direct heating and thereby eliminates the
potential mechanical weakening of the SMA material.
This paper suggests two different arrangements of the coldstate reset and heater layer on top of the bulk SMA layer,
differing in the materials and their thicknesses (Figure 1).
The first configuration (Fig. 1b) contains a thick and stressed
3
dielectric layer deposited on top of the SMA and providing
the cold-state reset. Examples for a dielectric cold-state reset
layer are intrinsically stressed silicon dioxide (SiO2 ) or silicon
nitride (Si3 N4 ) layers deposited by plasma enhanced chemical
vapor deposition (PECVD). On top of the dielectric layer a thin
electrically conductive layer, typically a metal, is deposited and
patterned to form a resistive heater.
The second configuration (Fig. 1c) consists of a thick
electrically conductive layer, which combines the functions
of cold-state reset and thermal energy supply. This bifunctional layer must be patterned to form a resistive heater. An
example of a bifunctional layer is aluminum, sputtered at
elevated temperatures and forming a standard thermal bimorph
scheme [20]. However, to prevent leakage currents from the
bifunctional layer into the SMA the two layers must be
electrically isolated by a thin intermediate dielectric layer such
as SiO2 or Si3 N4 .
Whereas a dielectric stress layer allows for tuning the
type of stress (tensile or compressive), it typically requires
higher deposition temperatures than the metal stress layer.
The preferred configuration will depend largely on its process
compatibility with the rest of the fabrication sequence of the
entire microcomponent in focus.
The presented concept is similar to a standard thermal
bimorph actuator using two passive materials with different
CTE, such as aluminum on a silicon cantilever. The main differences are that in the presented concept the cantilever itself is
the core actuation layer, consisting of the relatively thick TiNi
and upon heating it provides the highest work density among
the MEMS actuators. In the cold state, the TiNi has a very low
yield strength and deforms easily, thus it allows for a high
deflection using a relatively thin stressing layer only. Also,
due to the SMA effect, the actuators’ temperature-deflection
curve shows typical hysteresis behavior, as compared to the
proportional behavior of a standard thermal bimorph.
B. Wafer-scale integration of SMA sheets
This section describes two concepts for integrating SMA
onto structured silicon wafers using adhesive bonding [21]
(Figure 2).
The first method (Figure 2a) is the adhesive bonding of
a wafer-sized, non-patterned TiNi sheet onto the structured
silicon wafer, followed by patterning the SMA sheet to form
the actuator structures. We successfully demonstrated this
method for the integration of postal stamp sized SMA sheets
to silicon [22], but not on 4” wafer scale. Bonding prior to
patterning provides the advantage that the two substrates must
not be precisely aligned during the bonding, simplifying the
bonding step. Furthermore, since the SMA sheet is not patterned it is mechanically robust which simplifies the handling.
For testing this approach, a 5 µm thick Benzocyclobutene
(BCB) layer was spun on a 30 µm thick TiNi sheet, which
was then bonded with the BCB side to a structured silicon
wafer. Next, the BCB was hardcured by placing the stack in a
wafer bonder for 1h at a temperature of 250 °C and applying a
load pressure of 0.8 bar. After the bonding, the stack showed a
bowing and the TiNi sheet partially delaminated at the edges,
structured silicon wafer
TiNi sheet
two integration methods
flexure
interconnections
pattern the TiNi sheet
adhesive
apply adhesive on Si wafer and
bond, no alignment necessary
adhesive
apply adhesive on Si wafer,
align wafer and TiNi sheet and
bond
pattern the TiNi sheet
(a) pattern TiNi after bonding
(b) pattern TiNi before bonding
Figure 2. Illustration of the two different methods for integrating TiNi sheets
with structured Si wafers, based on a) adhesive bonding prior to the patterning
of the TiNi sheet and b) adhesive bonding after pre-patterning the TiNi sheet.
which indicated that the induced thermal stresses upon cooling
were too large for the bond interface. Furthermore, when
etching the TiNi sheet to create the actuator structures, both
the BCB as well as underlying silicon areas were attacked
by the aggressive TiNi etchant. The silicon turned black and
the BCB swelled heavily, which further decreased the bond
quality. Finally, during dicing of the stack the TiNi structures
completely delaminated.
Hence, it was concluded that the TiNi sheet must be
patterned prior to bonding, which is the second integration
method (Figure 2b). This method requires a temporary carrier
substrate for the patterning and handling of the SMA sheet
as well as an alignment of the sheet and the silicon wafer
during the bonding. However, this method offers two major
benefits. First, since the SMA sheet is patterned prior to the
bonding, the SMA etchant cannot attack any sensitive adhesive
layer, silicon or metal structures. Secondly, removing bulk
TiNi material allows to address problems related to excessive
stress in the adhesive interface after the bonding. Stress in
bonded structures is a complex phenomenon [23] and in this
work, we did not perform an entire stress analysis. However,
we made some first hand assumptions by considering two
of the stress components. The first component is the thermal
stresses that stem from the different CTE of the two materials.
The second component is the shearing stresses caused by the
bending moments induced by the thermal bimorph deformation. The bending of the bimorph and thus the shear stress is
4
reduced when parts of the bulk TiNi material are removed
due to mechanically weakening of the TiNi sheet. Yet, to
ensure the mechanical integrity of the SMA sheet, the single
actuator structures in the pattern must remain mechanically
interconnected. To allow the material to release the strain
induced by the bimorph bending, the mechanical interconnections between the actuator structures were patterned as flexure
interconnections, which deform and thereby absorb parts of the
strain (see Fig. 2b). The interconnections can be placed such
that they are removed during the final dicing of the wafer.
Several standard MEMS materials were tested as coldstate reset layer. The tested dielectric layers (Figure 1b) were
silicon dioxide (SiO2 ) and silicon nitride (Si3 N4 ), deposited
by PECVD at 300 °C. PECVD allows to deposit dielectric
films at moderate temperatures and the deposited films are
known to be intrinsically stressed. The tested bifunctional
layer (Figure 1c) is aluminum, sputtered onto the TiNi test
cantilevers at a temperature of 120 °C. Aluminum is well
suited as a thermal bimorph material, using the thermal
mismatch between the aluminum and the substrate to induce
thermal stresses deflecting the actuator. All the layers were
1 µm thick and deposited on 30 µm thick TiNi sheets. The
SMA sheets were then diced into cantilevers with a width and
length of 2 mm and 10 mm, respectively.
Then the radius of curvature, R, was measured in both the
cold and the hot-state. Figure 3 plots the measured curvatures
1
κ= R
of the cantilevers. The aluminum features tensile stress
and both SiO2 and Si3 N4 feature compressive stress, resulting
in deformations as illustrated in the insets in Fig. 3. The results
show that all stress layers deformed the cantilevers in the coldstate. When heated on a hotplate, the TiNi counteracts the
cold-state reset and the cantilevers nearly flatten. For Al and
Si3 N4 , the hot state deformation was too small to measure. In
contrast, the TiNi cantilever cannot fully counteract the SiO2
cold-state reset layer, which results in a measurable hot-state
deformation of the cantilever. However, the large stresses in
the SiO2 layer resulted in a much larger cold-state deformation
as compared to the Si3 N4 and the Al film.
To compare with later test components, the correlated
theoretical tip deflections for single clamped 2.5 mm long
cantilevers were calculated and are displayed in the right-hand
y-axis of the graph in Figure 3. The values indicate a cold-state
tip deflection in the range of 155 µm and 80 µm for Al and
Si3 N4 , respectively. In the hot-state, these cantilevers would
be flat. For SiO2 , a cold-state tip deflection of approximately
180 µm and a hot-state tip deflection of approximately 45 µm
is predicted.
For completeness it should be mentioned that the stresses
in PECVD Si3 N4 layers, in contrast to PECVD SiO2 layers,
can be tuned over a wide range by tuning the processing
parameters [24]. Furthermore, the intrinsic stresses in Si3 N4
are more thermally stable as compared to SiO2 [25], [26], [27].
deposited film
cold state
155
0.05
curvature κ=1/R in mm-1
A. Cold-state reset layer investigation
310
tensile stress
hot state
0
hot state
hot state
- 0.05
cold state
- 155
compressive
stress
Ttheoretical tip deflection for a 2.5 mm long cantilever in µm
III. E XPERIMENTS
0.1
1 µm
Aluminum
deposited film
cold state
- 0.1
- 310
1 µm
SiO2
1 µm
Si3N4
Figure 3. Measurement of the deflection of ten SMA-Al and SMA-PECVD
SiO2 and nine SMA-PECVD Si3 N4 test bimorph structures in the hot state
(90 °C on a hot plate) and in the cold state (at room temperature). The left1
hand y-axis displays the measured curvature κ = R
of the TiNi cantilevers on
which the layers were deposited. The right-hand y-axis displays the calculated
tip deflection if the cold-state reset layers were deposited on 2.5 mm long TiNi
cantilevers with the same thickness and material properties. The Al features
tensile stress and both SiO2 and Si3 N4 feature compressive stress, resulting
in different deformation directions.
B. Investigation of bulk TiNi wet etch concentration and
masking materials
In most MEMS devices, including those in the current work,
the TiNi is wet etched using an aggressive mixture of nitric
and hydrofluoric acids in water (HNO3 /HF/H2 O) with varying
concentrations to control the etching rate [28]. Such acidnitric acid-acetic acid (HNA) etch systems have been well
characterized to isotropically etch silicon [29], but they also
attack many metals and some photoresists [30]. HNO3 is a
powerful oxidizing agent in which the silicon or metal is
oxidized. The addition of HF to the solution etches the formed
oxide films. However, since the HNA etchant attacks so many
materials, a compatible masking material with an acceptable
selectivity must be found. Also, the exposure to HF introduces
the risk for embrittlement of the TiNi. Previous work indicates,
that the selectivity of photoresist allows to etch thin TiNi films
if the resist is thick enough [28].
In the present work five materials were tested as masking
5
Table I
T ESTING OF DIFFERENT MASKING MATERIALS FOR THE ETCHING OF A T I N I SHEET AT DIFFERENT HNA CONCENTRATIONS .
Tested masking materials and their selectivity (mask etch/NiTi etch)
SMA etch mixture
TiNi
Vol %
HF:HNO3 :H2 O
etch rate
µm/min
pos. resist(a)
1.2 µm
neg. resist(b)
2.8 µm
5 : 20 : 75
5 : 30 : 65
10 : 20 : 70
10 : 30 : 60
5
10
20
25
delaminates
delaminates
delaminates
delaminates
deteriorates
deteriorates
delaminates
delaminates
(a)
Megaposit SPR 700-1.2;
(b)
Microresist ma-N 420;
- side view -
- top-view 7 µm
7 µm
30 µm
TiNi cantilever
Figure 4. SEM pictures of the corner of a TiNi cantilever. The etching with
5%:30%:65% HF:HNO3 :H2 O of the 30 µm thick TiNi sheet resulted in a
7 µm undercut (left inset) and a conical side profile (right inset).
material to etch through 50 µm thick bulk TiNi material.
Furthermore, for each of the materials four different HNA
concentrations were tested to identify the most suitable concentration in terms of etch speed and quality.
The tested masking materials were photoresists, sputtered
gold and PECVD deposited SiO2 and Si3 N4 . The tested
photoresist were both a positive and a negative resist, with a
thickness of 1.2 µm and 2.8 µm, respectively. A hexamethyldisilane (HMDS) adhesion promoter was used and the resists
were heat treated according to the manufacturers processing
guidelines. The gold was 200 nm thick and sputtered onto the
SMA after sputtering a 50 nm thick TiW adhesion layer, which
is necessary to avoid delamination of gold from the SMA [28].
The PECVD layers were tested since they potentially allow to
provide both a hardmask for the TiNi etching as well as the
cold-state reset layer.
Table I overviews the tested materials, HNA concentrations
and the obtained results. The experiments show that both
photoresists were heavily attacked by the HNA etch solution.
The positive resist completely delaminated for all concentrations of HNA while the negative resist proved some durability
for low concentrations of HF, however, it was still found to
be unusable as a masking material. The PECVD deposited
SiO2 and Si3 N4 films were etched with an unacceptably low
selectivity towards the bulk TiNi sheet. The gold was not
(c)
TiW/Au(c)
50/200 nm
not
not
not
not
SiO2 (d)
3 µm
Si3 N4 (d)
2 µm
~1/3
~1/3
~1/3
~1/3
~1/15
~1/15
~1/15
~1/15
measurable
measurable
measurable
measurable
sputtered metal layers;
(d)
PECVD at 300 °C
etched at all and therefore identified as the best suited masking
material, which is in agreement with previous work [28], [30].
Furthermore, the HNA concentration 5%:30%:65%
HF:HNO3 :H2 O was found to be optimal in terms of limited
HF exposure, controllable etch rate (approximately 10 µm/min)
and acceptable conical etch profile with 7 µm undercut for a
30 µm deep etch (Figure 4).
C. Handling of patterned TiNi sheets
Patterning the TiNi sheet prior to bonding requires a temporary carrier substrate for the handling of the thin TiNi sheet
during lithography and wet etching. The carrier substrate can
be any substrate that withstands the TiNi etchant and allows for
the necessary process steps such as resist spinning, lithography
and etching. In this work, the carrier substrate was a thermally
oxidized silicon wafer. However, the main challenge is to find
a method for the temporary bonding of the TiNi sheet to
the carrier substrate. The temporary bond must release the
patterned TiNi sheet with limited mechanical stresses, since
the thin sheet is fragile.
In this work, several methods were tested to temporarily
bond the TiNi sheet to the carrier wafer. The tested bonding layers were positive photoresist, thermal release bonding
sheets (Nitto Denko Revalpha) and dicing blue tape (Nitto
Denko SWT 10+).
For testing the photoresist, the TiNi sheet was bonded
to a 4 µm thick, softbaked (90 °C for 30 s) resist layer
and the stack was then further baked (90 °C for 1 min]
before patterning the TiNi sheet using lithography and HNA
wet etching. However, during the HNA etching, the exposed
bonding resist delaminated and contaminated the etchant. After
the etching, the resist did not fully dissolve in Acetone and
the patterned sheet had to be pulled off from the bonding
layer, destroying the thin structures. Hence, photoresist was
considered unsuitable for the process.
The thermal release sheets feature an adhesive layer on both
sides, allowing to bond the TiNi sheet to the carrier wafer.
After bonding and wet etching the TiNi, heating the stack
above 90 °C should release the TiNi sheet from the thermal
release sheet. However, the TiNi sheet was still softly bonded
after heating and had to be pulled off, which destroyed the
fragile TiNi structures. Hence, thermal release sheets were
judged to be not suitable for the desired process.
Finally, standard 65 µm thick dicing blue tape was tested
as illustrated in Figure 5a. The TiNi sheet was applied on the
adhesive side of the blue-tape. Then, the TiNi/blue-tape stack
6
a) SMA sheet preparation
TiW/Au
Sputter-depositing and patterning of
50/200 nm TiW/Au on 30 µm thick
TiNi sheet.
TiNi sheet
c) Wafer-scale bonding
blue tape
adhesive side
top clamp wafer
non-stick sheet
Bond the TiNi sheet to handle
wafer using blue tape as
intermediate bonding adhesive.
handle wafer
bottom clamp wafer
90 °C
Wet etch the TiNi sheet with TiW/
Au as hardmask.
blue tape
Strip Au and remove blue-tape
carrier with Aceton.
Bonding of the stack, using two
clamp wafers and a non-stiction foil
to avoid adhesion of the BCB to the
top clamp wafer. The bonding is
performed using a standard wafer
bonder at 250 ºC for 1h and 0.8 bar
applied load pressure.
flexure
interconnections
b) Silicon substrate preparation
DRIE etching through a 525 µm thick
Si-wafer with a SiO2 hardmask
d) Cold-state deformation
dicing clamp ring
Stamping of ~4 µm BCB on the
surface of the wafer: The BCB is
spun onto the adhesive side of
bluetape and stamped on the
surface of the silicon structures.
The BCB is transferred to the
silicon structures by heating the
stack on a hotplate, then the
bluetape is removed again.
Figure 5.
BCB
blue tape
PECVD nitride
After bonding: depositing cold-state
reset layer (here exemplarily
PECVD Si3N4 at 300 °C) and
dicing into single cantilevers,
thereby removing the flexures.
90 °C
BCB
Outline of the process flow for the first demonstrators.
was applied onto the handle wafer with the non-adhesive side
of the blue-tape facing the handle wafer. The complete stack
was then placed for 5 s on a hotplate at 90 °C, causing a local
melting of the blue tape at the interface to the Si wafer and
adhesion of the blue-tape to the handle wafer. Next, the TiNi
was patterned using lithography and wet etching. The bluetape was found to withstand the TiNi etchant and to provide
an excellent etch-stop. After the etching, the stack was placed
in an Acetone bath to release the TiNi sheet from the bluetape. Acetone causes swelling and softening of the blue-tape
and the patterned TiNi sheet released from the stack without
mechanical support. The TiNi sheet could be lifted off from
the surface of the Acetone. Hence, blue-tape was judged to
be a suitable temporary bonding layer for the etching of TiNi
sheets prior to the adhesive bonding.
D. Optimization of transfer stamping adhesive on patterned
silicon wafer
For the adhesive bonding of the TiNi sheet to a structured
silicon wafer, BCB (Cyclotene 3022-46, DOW, USA) was
chosen as the preferred adhesive because of its curing process
that does not involve catalysts and thus no detectable outgasing
of the polymer occurs after evaporating the solvents. However,
since the wafer is patterned, the BCB cannot be spun onto the
wafer. Spray-coating of the wafer with BCB is not applicable
since this technique covers the sidewalls of etched features,
which can destroy or block moving elements in the wafer. To
transfer the adhesive onto the top surface of the silicon wafer
only, the BCB was applied using an adapted contact printing
(stamping) process, where the BCB is spun on an intermediate
carrier substrate and then stamped onto the patterned target
silicon wafer. In previous work [31], a thin layer of liquid
BCB was transferred by stamping from a stiff auxiliary wafer
substrate onto a patterned and hard-cured thick layer of BCB
on the target wafer, followed by a separation through inserting
7
1 mm
flexure
flexures
interconnections
100 µm wide
bond
area
actuators
1.3 mm
1 cm
Figure 6. Photograph of the patterned TiNi sheet bonded to a structured
silicon wafer. The blow-ups show the single actuator structures and the flexure
interconnections as well as the outer dimensions of a single cantilever with
its bond area.
Cold state, Tip
deflection ~ 250 µm
30 µm TiNi
BCB
1 µm SiO2
AND EVALUATION OF FIRST TEST
silicon
substrate
STRUCTURES
This section presents the fabrication and first evaluation of
test actuator structures fabricated using wafer-scale integration
of bulk TiNi material. The fabrication starts with the separate
preparation of the TiNi sheet (Figure 5a) and the microstructured silicon wafer (Figure 5b).
A 30 µm thick TiNi sheet (Johnson-Matthey, USA,
Ti 44.62 / Ni 55.37 weight %, Af = 46.8 °C) was flattened
by heating on a hotplate at 90 °C after which a 50/200 nm
thick TiW/Au layer was sputter-deposited. The TiNi sheet was
bonded to a temporary silicon handle wafer using blue-tape,
as described above. Subsequently, the TiW/Au hardmask was
patterned using standard positive lithography and wet etched
to mask the outline of 2.5 mm long and 1 mm wide cantilever
structures and their bond pads, interconnected with 100 µm
wide flexure interconnections. Then, the exposed TiNi surface
was wet etched. The Au was stripped off using iodine based
etchant [30], leaving the TiW and the TiNi intact. Finally, the
stack was put in Acetone to strip the photoresist and to release
the TiNi sheet from the carrier substrate.
In parallel, a 525 µm thick silicon wafer was thermally
oxidized and the oxide was patterned using lithography and
wet etching in buffered hydrofluoric acid (BHF). Then, to
allow the SMA microactuators to deflect freely during later
operation, through holes were etched using deep reactive ion
etching (DRIE) with oxide as etch mask. Subsequently, the
etched TiNi sheet with
actuators
with actuators
and flexure
and
interconnections
flexures
0.9 mm
IV. FABRICATION
structured
silicon wafer
2.5 mm
a razor blade inbetween the two wafers.
However, for the current work, the use of a stiff stamping
wafer was found not suitable, mainly for two reasons. First,
the area which should be covered with BCB was much larger
as compared to the previous work, which made it very difficult
to separate the two wafers after the stamping using the razor
blade method. Instead, the wafers could only be separated
by sliding, which smeared the BCB on sidewall features and
resulted in a very inhomogeneous layer thickness. Secondly,
as opposed to the BCB coated target in the previous work, the
target here is a silicon wafer. Because the BCB adheres equally
well to the stamp and the target, a repeatable and reliable layer
transfer is difficult to achieve.
We circumvented these problems by using a flexible stamp
instead. First, a polydimethylsiloxane (PDMS) film as intermediate substrate was tested, however, this process resulted
in an incomplete BCB transfer with very inhomogeneous
thicknesses. Then, standard 65 µm thick dicing blue tape was
successfully tested as auxiliary substrate (Figure 5b). The blue
tape was clamped in a standard 6 inch dicing ring and placed
with the non-adhesive side on a 4 inch chuck of a standard
resist-spinner. The BCB was spun onto the adhesive side of the
blue-tape. Then the blue tape was placed with the BCB side on
the target wafer and the whole stack was heated on a hotplate.
The heat triggered a reflow of the BCB, which transferred fully
to the target wafer, and released the blue tape, which could be
lifted off simply from the stack. Upon inspection we verified
that the BCB was transferred to all top parts of the silicon
structures and no sidewall coverage could be observed.
Figure 7.
SEM-picture of a SMA microactuator structure in the cold-state.
BCB was stamped onto the top surface of the silicon wafer
using the stamping technique described above.
Figure 5c illustrates the integration of the patterned TiNi
sheet with the structured silicon wafer. The TiNi sheet was
manually aligned to the structures of the silicon wafer and
placed on the adhesive layer. To apply pressure while hardcuring the BCB, the stack was placed between two clamping
wafers, with a teflon based non-stick sheet between the TiNi
sheet and the top clamp wafer to avoid a bonding between
the two. Then the stack was hard-cured in a wafer bonder
during 1h at 250 °C and an applied load pressure of 0.8 bar.
Figure 6 shows a photograph of the wafer/TiNi stack after
bonding, including a blow-up of the cantilever structures and
the flexure interconnections.
The final fabrication steps (Figure 5d) included the deposition of the cold-state reset layer and the dicing into single
actuator structures, as described below.
After the fabrication, one wafer was diced in two halves
using photoresist protection. On one half, a 1 µm thick
compressively stressed PECVD Si3 N4 layer was deposited at
Tip deflection in µm
8
350
V. C ONCLUSIONS
300
This paper presents a method for the wafer-scale integration of bulk TiNi based microactuators on silicon. The first
bulk TiNi sheet based cantilevers, wafer-scale integrated on
structured silicon wafers and with a cold state deformation
provided by stressed layers, were shown. After an investigation
of key process parameters, an optimized fabrication process
was selected and demonstrated. 2.5 mm long cantilever test
structures were evaluated and showed strokes of up to 230 µm,
which is in agreement with the previous investigations. The
bond strength proved to be stronger than the force created by
the 2.5 mm long TiNi cantilever and showed no degradation
after more than 700 temperature cycles. Moreover, the shape
memory behavior of the TiNi is maintained during the integration process.
250
200
heating
stroke
~ 230 µm
cooling
150
100
50
0
10
20
30
40
Temperature in °C
50
60
70
Figure 8. Tip deflection of a 2.5 mm long cantilever with 3 µm PECVD
SiO2 at the backside versus hotplate temperature, displaying the SMA typical
hysteresis behavior. In the cold/hot state, the tip deflection is approximately
315/85 µm, respectively, resulting in a stroke of 230 µm.
300 °C on the top side (Fig. 5d), resulting in a downward
cantilever deflection of 125 µm in the cold-state and 28 µm
in the hot-state, i.e. with a stroke of 98 µm. These results are
corresponding to the theoretical tip deflections estimated in
Figure 3.
The second half of the wafer was diced into single actuator
structures, again using photoresist protection. All the structures
survived the dicing and the correlated handling, including the
post dicing Acetone stripping of the resist. Figure 7 shows
a SEM picture of a structure with 1 µm PECVD SiO2 ,
deposited at 300 °C on the backside of the TiNi cantilever and
providing an upward deflection. The cold-state and hot-state
deflections were measured to 250 µm and 70 µm, respectively,
resulting in a stroke of 180 µm. Figure 8 shows a temperature
cycle measurement of another cantilever with 3 µm SiO2 on
the backside. The measurement shows that in the cold state
the martensitic TiNi has a very low yield strength and is
therefore easily deformed by the dominating thin stressing
layer, resulting in a cold state tip deflection of 315 µm at 14 °C.
However, in the hot state the TiNi is in the stiff austenitic state,
dominating the deflection behavior by easily overcoming the
bias spring force of the thin stressing layer and resulting in a
nearly temperature-independent hot state deflection of 85 µm
at 62 °C.
The measured deflections are corresponding to the estimations for the theoretical tip deflections in Figure 3. The stroke
and the hysteresis show that the shape memory behavior of
the TiNi is maintained during the integration process.
In another experiment the structure was placed on a hotplate
and the cantilever tip was pushed downwards using an external
manipulator. The temperature was cycled between temperatures below (30 °C) and above (70 °C) the transformation
temperatures of the TiNi. By preventing the flat shape recovery
of the cantilever, its leverage stresses the bond. The presented
measurement proved that the bond is stronger than the forces
created by the cantilever and the bond strength showed no
degradation after more than 700 temperature cycles.
ACKNOWLEDGMENT
This work was part of the Q2M project. The authors would
like to thank Johnson-Matthey, U.S., for providing the TiNisheets.
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Stefan Braun was born in Germany in 1980. He
received the Dipl.-Ing. (FH) degree in microsystem
technology from the University of Applied Sciences,
Zweibrücken, Germany, in 2003. He is currently
working toward the Ph.D. degree in the Microsystem Technology Laboratory, School of Electrical
Engineering, KTH - Royal Institute of Technology,
Stockholm, Sweden. He was with the Automotive
Sensor Centrum, Robert Bosch GmbH, Reutlingen,
Germany, where he worked with packaging aspects
of inertial sensors and with ZF, Friedrichshafen,
Germany, where he worked on finite-element simulations. His research
interests include microelectromechanical systems switch arrays for automated
telecommunication networks and the wafer-level heterogeneous integration of
shape memory alloy actuator material.
Niklas Sandström was born in Sweden in 1981.
He received the M.Sc. degree in engineering biology
from Linköping University, Linköping, Sweden, in
2007. He is currently working towards the Ph.D.
degree in the Microsystem Technology Laboratory,
School of Electrical Engineering, KTH - Royal Institute of Technology, Stockholm, Sweden.
Göran Stemme (M’98-SM’04-F’06) was born in
Stockholm, Sweden, in 1958. He received the M.Sc.
degree in electrical engineering and the Ph.D. degree
in solid-state electronics from Chalmers University
of Technology, Gothenburg, Sweden, in 1981 and
1987, respectively.
In 1981, he was with the Department of Solid
State Electronics, Chalmers University of Technology, where, in 1990, he was an Associate Professor
(Docent), heading the silicon sensor research group.
Since 1991, he has been with the Royal Institute of
Technology, Stockholm, where he was a Professor and where he currently
heads the Microsystem Technology Group, School of Electrical Engineering.
His research is devoted to microsystem technology based on micromachining
of silicon. His work spans a broad range of technological and application
fields, such as medical technology, biochemistry, biotechnology, microfluidics,
optical applications, wafer-level packaging, and device integration. Some of
the results have successfully been commercialized. He has published more
than 260 research journal and conference papers and has more than 22 patent
proposals or granted patents. He has been a member of the Editorial Board of
the Journal of Microelectromechanical Systems since 1997 and was a Member
of the Editorial Board of the Royal Society of Chemistry journal Lab on a
Chip from 2000 to 2005.
Dr. Stemme was a member of the International Steering Committee of the
Conference series IEEE Microelectromechanical Systems between 1995 and
2001, and he was a General Cochair of that conference in 1998. In 2001, he
was, together with two colleagues, the recipient of the final of the Innovation
Cup in Sweden. He is a member of the Royal Swedish Academy of Sciences.
Wouter van der Wijngaart (M’06) received the
M. Sc. degree in electrotechnical engineering, the
degree of philosophic academy and the mathematics
education degree, all from the Katholieke Universiteit Leuven, Belgium, in 1996 and the Ph. D. degree in Microsystem Technology from KTH - Royal
Institute of Technology in Stockholm, Sweden, in
2002.
In 2005 he became Associate Professor and in 2007
Senior Lecturer with KTH Microsystem Technology
Lab. His research focus is bio-, micro- and nanofluidics with a focus on lab-on-chip diagnostics and MEMS microactuators. He
has over 70 scientific publications in the field. He is the co-founder of the
company Easypark and at the base of the company MyFC, developing micro
fuel cells for mobile applications.
Prof. van der Wijngaart was the ricipient of the Cap Gemini Innovation Award
at the 1999 European Business Plan of the Year Competition in Paris, France,
together with Fredric Ankarcrona, and the Swedish Innovation Cup 2001,
together with Helene Andersson.
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