Four-beam Interference Optical System for Laser

Four-beam Interference Optical System for Laser
Journal of the Optical Society of Korea
Vol. 13, No. 1, March 2009, pp. 75-79
DOI: 10.3807/JOSK.2009.13.1.075
Four-beam Interference Optical System for Laser
Micro- structuring Using Picosecond Laser
Jiwhan Noh*, Jaehoon Lee, Dongsig Shin, Hyonkee Sohn, Jeong Suh, and Jeongseok Oh
KIMM(Korea Institute of machinery &materials) 104 Sinseongno,
Yuseong-gu, Daejeon 305-343, Korea
(Received November 8, 2008 : revised January 5, 2008 : accepted January 6, 2008)
A four beam interference optical system for laser micro structuring using a pulse laser was demonstrated. The four beam interference optical system using a pulse laser(picosecond laser) can fabricate
micro structure on mold material(NAK80) directly. Micro structure on the polymer can be reproduced
economically by injection molding of the micro structure on the mold material. The four beam
interference optical system was composed by the DOE(Diffractive Optical Element) and two lenses.
The laser intensity distribution of four beam interference was explained by an interference optics
point of view and by the image optics point of view. We revealed that both views showed the
same result. The laser power distribution of a 1 μm peak pattern was made by the four beam
interference optical system and measured by the objective lens and CCD. A 1 μm pitch dot pattern
on the mold material was fabricated and measured by SEM(Scanning Electron Microscopy).
Keywords : Four beam interference, Laser ablation process, DOE(Diffractive Optical Element),
Mold surface micro pattern
OCIS codes : (320.0320) Ultrafast optics; (320.5390) Picosecond phenomena
I. INTRODUCTION
The laser ablation process which is a micro-scale
ablation process using the high intensity energy produced
by focusing a pulse laser beam has been the subject of
many studies recently.[1-4] The laser ablation process
is applicable to various materials, environment-friendly,
and simpler than a photolithography process. Because
the laser ablation process is non-contacting, it is less
influenced by thermal impact and produces less thermal
and mechanical deformation. It also enables micro-processing without a mask by using a laser direct writing
method. However, due to the characteristics of the
laser micro-machining where surface is processed with
a focused laser beam, the fabrication width cannot be
smaller than the diffraction limited focus spot diameter.
[5] When focusing a laser beam using a focusing lens,
the size of the focused beam cannot be smaller than
the limit in diffraction. To reduce the size of the focused
beam, the size of the incident beam to the focusing lens
can be enlarged, the wavelength of the incident laser
*Corresponding author: [email protected]
can be reduced, or a focusing lens having shorter focal
length can be used. The size of the incident beam can
be enlarged using a beam expander, however, there is
a limit in expanding the incident beam size. The method
of shortening the wavelength of the incident laser beam
is limited by the price of the laser source. Using a shortfocal length lens can reduce the focused-spot size, but
the ‘depth of focus’ also is reduced resulting in difficulty
in processing. [6] The problem of irregular processing
caused by insufficient depth of focus may be prevented
by using an auto focusing unit, however, the auto focusing
unit adds complexity to the laser process machine. The
short-focal length lens may be damaged by the particles
generated in ablation.
In order to cope with the diffraction limit, optical
systems using an interference effect are studied. A CW
(continuous wave) laser having good coherence can be
used when exposing photo resistive material. But pulse
lasers are required for an ablation process which requires
higher fluence. As the temporal and spatial coherences
of pulse lasers are not as good as those of CW lasers,
which leads to poor contrast of interference power distribution, pulse lasers are disadvantageous to micro-proc-
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76
Journal of the Optical Society of Korea, Vol. 13, No. 1, March 2009
essing using interference effects. But the most important
reason that interference patterns should be processed
using pulse laser is that micro patterns can be processed
on mold material in a single process. While the photo
resistive material exposing method requires chemical
etching and electroforming processes to produce micropatterns on mold material, the laser ablation process
can produce micro-patterns on mold material in a single
process.
In an optical system using interference effect, the
laser beam is divided into 2 split beams using a beam
splitter, then irradiates the specimen to produce an interference pattern and to perform processing. The interference pattern of the two split laser beams are a line
pattern. However, to produce a dot pattern instead of
a line pattern using interference effects, four beams instead
of two beams have to be combined. It is very difficult
for an optical system using a beam splitter to make four
beams interfere together. To simplify such complexity
of the optical system, studies are being conducted on
the systems using DOE (Diffractive Optical Element)
and lenses. [7-11]
In this study, using a pulse laser with 12 picoseconds
of pulse duration in an interference optical system implemented with DOE and lens, micro patterns were directly
processed on NAK80 mold material. To the best of my
knowledge, there is no report about the micro structuring
on the mold material with four beam interference using
a picosecond laser. The performance of the interference
optical system was evaluated by measuring the light
distribution of the interfering light with objective lens
and CCD. Using the interfered light distribution, micro
dot pattern whose pattern size was 1 μm or less was
processed.
II. EXPERIMENTAL CONDITION
The picosecond laser used in the experiments was a
diode-pumped mode-locked Nd:YVO4 laser with a
pulse duration of 12 ps. The fundamental wavelength
is 1064 nm. The laser is equipped with second harmonic
generators to make laser wavelengths of 532 nm. In the
experiments, we used the laser wavelength of 532 nm.
The diameter of the laser beam was 1.5mm. A half wave
plate and a polarizer were used to control the laser power.
Instead of a mechanical shutter, an external TTL signal
served as a shutter for laser pulses. This prevents extra
irradiation of laser pulses onto the specimen caused by
the time delay of a mechanical shutter at every end point
of laser beam path.
Fig. 1(b) presents the configuration of the interference
optical system in this study. The system comprised
a DOE which splits the laser beam into 4, lens 1 which
makes the split beams parallel with each other, and the
lens 2 which converges the 4 parallel beams. The focal
(a)
(b)
FIG. 1. The schematics of interference optical system
(a) Two beam interference system using beam splitter: BS;
Beam splitter, M; Mirror, SF; Spatial Filter, S; Specimen,
(b) Four beam interference system using DOE(Diffractive
Optical Element) : L1; Lens1, L2;Lens2, S; Specimen
lengths of the lens 1 and lens 2 were 150 mm and 75
mm, respectively. To make the 4 beams split by the
DOE to propagate parallel to each other, the distances
between the DOE and lens 1 was set to 150mm. The
distance between the lens 1 and lens 2 was set to 225
mm.
In this study, the processing material was NAK80,
a mold steel with a uniform hardness of approximately
40HRc throughout: which does not require stress relieving,
even after heavy machining. It has uniform grain structure with no pin holes, inclusions or hard spots. This
material can be machined to obtain a mirror-surface.
Due to these features, NAK80 is widely used for lens
molds. If patterns can be made on NAK80, they can
be replicated on a plastic surface, which means that
mass production is possible at a very low cost. The surface
morphology was investigated by scanning electron microscopy (SEM) with JEOL JSM-6300.
Four-beam Interference Optical System for … - Jiwhan Noh et al.
77
III. EXPERIMENT RESULTS
AND DISCUSSION
Fig. 1 shows the schematics of a two beam interference
optical system using a beam splitter and a four beam
interference system using DOE. In a two beam interference system, the laser beam is split using a beam splitter
and combined on the specimen again to cause interference. It is difficult in this method to implement high
laser fluence on the specimen. This is because, to realize
high laser fluence, the split beams have to be irradiated
on the specimen using a focusing lens, however, this
optical system is not easy to implement. Furthermore,
to make 4 split beams interfere together, more optical
elements are involved and their optical alignment is
very difficult. On the other hand, since the four beam
interference system using DOE splits laser beam into
4 using a simple DOE element and focuses the beams
using a lens system, this system is very useful for the
implementation of high laser fluence by interfering 4
beams together. The first lens makes the 4 beams parallel
with each other and the second lens focuses them, giving
higher laser fluence.
The Equation (1) below expresses the 4 split beams;

  
    
∙ 
 

  
    
∙ 
 

  
    
∙ 
 
(1)

  
    
∙ 
 
 is the 2πλ , λ is the wavelength,  is the
Where 
phase constant and n=1,2,3,4
When these 4 beams interfere together, the light distribution can be expressed with the Equation (2) below;

 

 
 
 

(2)
The intensity of the light having the electric field as
above equation can be expressed with the Equation (3)
below;
   
 
 
 
∙  
 
 
 





              




           

(3)
           
     (i, j = 1~4)
  
 ∙ 
Where Ci-j ≡ COS  

 
Here, the period of the light distribution can be
expressed with the Equation (4) below;
(a)
(b)
FIG. 2. The measurement result of laser power
distribution of 1 μm peak pattern: (a) three dimensional
laser power distribution, (b) two dimensional laser power
distribution

     ∙ 
  
    ×   
  
 ∙  
(4)
Therefore, the period of the light distribution can be
expressed with the Equation (5) below;


    
   
  
(5)
Where is Period of micro pattern, k0 is 2π n / λ , λ
is laser wavelength, θ is incident angle and n is refraction
index. In this paper λ is 532 nm, θ is 15°, n is 1 and
x can be calculated into 1 μm.
While the Fig. 2b can be interpreted in the aspect
of interference, however, it also can be interpreted as
the image of the DOE element projected on the specimen.
That is, the image created from the DOE element pattern
through the two lenses is transcribed onto the specimen.
In this interpretation, the interval of the image pattern
gaps is determined by the Equation (6);
P=
f2
PDOE
2 f 1o
(6)
where f1 is the focal length of the first lens, f2 is the
focal length of the second lens, PDOE is the pitch of DOE
pattern, and


 
is the magnification factor of lens. In
this paper, f1 is 150 mm, f2 is 75 mm, PDOE is 4 μm and
P can be calculated into 1 μm. The results show that
the analysis in the interference aspect and projection
aspect are identical with each other.
Fig. 2 shows the laser power distribution at the interfering part. The Fig. 2(a) shows three dimensional laser
power distribution, and the Fig. 2(b) shows two dimensional laser power distribution. Red color represents
higher fluence and purple represents lower fluence. As
shown in the Equation (1), the laser beam profiler using
conventional CCD is difficult to measure 1 μm -class
78
Journal of the Optical Society of Korea, Vol. 13, No. 1, March 2009
FIG. 3. SEM image of 1 μm pitch pattern on the mold
material: (a) 500 pulses shot, (b) 1000 pulses shot,
(c)3000 pulses shot
laser power distribution, because the pixel size of conventional CCD is 5 μm or larger. It is not possible to
measure 1 μm-scale laser power distribution using a CCD
whose pixel size is 5 μm or larger. Therefore, 1 μm-class
laser power distribution needs to be magnified. In this
study, an objective lens (Nikon Plan Fluor 10x working
distance 16 mm) was used to enlarge 1 μm-class laser
power distribution, and the enlarged image was measured
with CCD (Ophir Beamstar-fx50), while the laser power
was reduced to minimum using polarizer and wave plate
to protect the CCD. In the measurement result, the
contrast of the 1 μm peak pattern was not satisfactory,
because the contrast of the actual interference laser
power distribution itself was not good enough. It is because
both the temporal and spatial coherences of the pulse
laser were not good compared with CW laser. In addition,
it was thought that the measurement values in the Fig. 2
contained the error of the measurement instrument
itself, including the aberration of the objective lens and
the measurement error based on the resolution of the
CCD.
Fig. 3 shows the results of 1 μm-class micro-patterning
on mold material (NAK80) by four beam interference
using DOE and picosecond laser. The average laser power
at the focus of the 4 beams was 2W, the focused spot
size was 60 μm, the repetition rate was 50kHz, and the
laser fluence was 1.4 J/cm2. The laser ablation process
was conducted in ambient condition, without using blowing gas. The Fig. 3a is the case of 500 pulses, Fig. 3b
is the case of 1,000 pulses, and the Fig. 3c is the case
of 3,000 pulses. It can be seen that the increase in the
number of pulse results in deeper patterns. However, as
the number of pulse increases, the boundary between
the processed and unprocessed parts becomes unclear,
due to the heat affection zone generated in the laser
ablation process. Even when the contrast of the light
pattern is very high, larger HAZ(heat affection zone)
is formed when more energy is used in the process. The
HAZ can be reduced by using ultrashort pulse laser whose
pulse duration is between 10 femtosecond to 20 picoseconds.
[12] The energy of the laser irradiating the material is
used to excite the electrons in the initial phase, and then
the excited electrons transmit energy to the solid lattice.
In this process, complicated reactions including electron
emission, melting, vaporization, phase explosion, pallation,
and electrostatic ablation, occur and the material begins
to be removed. The detailed ablation mechanism is still
under investigation. The ablation mechanism differs
according to laser parameters (fluence, wavelength,
pulse duration, et cetera) and material. In an ordinary
pulse laser, the pulse duration is some tens of nanoseconds.
Such a pulse laser induces large heat affection zones in
the material removal process because during the tens
of nanoseconds of irradiation, energy is transferred to
the lattice. On the other hand, when using ultrashort
laser beam, the electron energy is not transferred to the
lattice because the laser irradiation is cut-off after exciting
the electrons and no more energy is transferred to the
lattice of the material. This is because it takes normally
some tens of picoseconds for the electron energy to be
transferred to the lattice of the material, though the
transferring time from electron to lattice differs by
material. This enables clean ablation, that is, processing
with reduced heat affection zone. [13] However, the
interactive mechanism between ultrashort pulse laser
and material has yet to be studied.
On the other hand, the laser coherence becomes worse
as the laser pulse duration shorten in order to reduce
the heat affection zone. In particular, the pulse duration
has to be reduced to reduce heat affection zone, but
be increased to improve the peak contrast of interference
laser power distribution. To this end, the pulse duration
must be optimized. In this study, we chose the laser
source of with pulse duration 12 picosecond. Though
it would be impractical to process micro-structures having
large aspect ratios (deep processing with narrow width)
using interference optical system of pulse laser, the results
of the experiment showed that micro-patterns can be
easily produced on thin film using the interference of
a pulse laser.
Another advantage of interference processing is the
fast processing speed. In the process using focusing
lenses, each pattern has to be processed one by one.
However, in an interfering process, all the patterns within
the spot size can be processed at the same time. Therefore, the processing speed is much faster than that using
a focusing lens. For example, in the Fig. 3b where the
material was processed using 1,000 pulses, 3,000 microdots of 1 μm size can be processed in 20 ms when using
50 kHz. Higher repetition rate than 50 kHz and the high
speed of stage can be used to improve process speed.
IV. CONCLUSION
In this study, about 3,000 micro-dots whose size was
1 μm or less were produced on NAK80 mold material
in the process time of 20 ms, using a four beam interference optical system utilizing DOE and a picoseconds
laser. In order to make the 4 beams interfere together
Four-beam Interference Optical System for … - Jiwhan Noh et al.
using simple optics, the four beam interference optical
system utilizing DOE was used and the picosecond laser
was used to reduce the heat affection zone of the produced
micro-structure. It was found that the pulse duration
of the process laser should be determined to take the
peak contrast of the interference light distribution and
the heat affection zone into consideration. The light
distribution of the four beam interference was analyzed
in the aspect of image projection as well as in the interference aspect. The light distribution of the interfering
beams was measured using an objective lens and CCD
to evaluate the performance of the interference optical
system.
The micro-patterns developed in this study can be
applied in surfaces for reduced friction, such as car engines,
surfaces for adhesion control or for increased absorbability.
Especially, the micro-spike arrays produced by injection
molding will provide super-hydrophobic surfaces at very
cheap cost. [14-16]
5.
6.
7.
8.
9.
10.
ACKNOWLEDGMENT
This work was supported in part by the Korean Ministry of Commerce, Industry and Energy as part of the
‘Development of precision-machining technology using
advanced lasers’ and ‘Technology Development of Plastic
Injection Molding for Superhydrophobic Surface by Nanoon-Micro Patterns’ project
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