Optics and Photonics Journal, 2013, 3, 227-231
doi:10.4236/opj.2013.32B053 Published Online June 2013 (http://www.scirp.org/journal/opj)
Maskless Microscopic Lithography through Shaping
Ultraviolet Laser with Digital Micro-mirror Device
Xiang-Yu Ding1, Yu-Xuan Ren2,3, Rong-De Lu1
1Department of Physics, University of Science and Technology of China, Hefei, China
2Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences, CAS, Shanghai, China
3National Center for Protein Sciences (Shanghai), Shanghai Institutes for Biological Sciences, CAS, Shanghai, China
Email: yxren@ustc.edu.cn, lrd@ustc.edu.cn
Received 2013
Laser shaping was introduced to maskless projection soft lithography by using digital micro-mirror device (DMD). The
predesigned intensity pattern was imprinted onto the DMD and the input laser beam with a Gaussian or quasi-Gaussian
distribution will carry the pattern on DMD to etch the resin. It provides a method of precise control of laser beam
shapes and photon-induced curing behavior of resin. This technology provides an accurate micro-fabrication of micro-
structures used for micro-systems. As a virtual mask generator and a binary-amplitude spatial light modulator, DMD is
equivalent to the masks in the conventional exposure system. As the virtual masks and shaped laser beam can be
achieved flexibly, it is a good method of precision soft lithography for 2D/3D microstructures.
Keywords: Digital Micro-mirror Device (DMD); Laser Shaping; Maskless Projection; Soft Lithography
1. Introduction
Micro-fabrication is essential to modern science and
technology. As the most successful and main manufac-
turing technology in micro-fabrication, lithography plays
a great role since its invention in 1959 [1]. Although li-
thography brings the rapid development of semiconduc-
tor industry, there are rare applications in non-semicon-
ductor areas, such as biology, chemistry, medicine, which
require non-flat surfaces to fabricate on. Different from
lithography, soft lithography is another micro-fabrication
technology arose in 1990s [2], and it has wider applica-
tions. It is a kind of micro-graphics replication technol-
ogy. Just like a seal, it can copy micro-models to any
surfaces without considering the flatness requirement.
Physical masks and laser beam are two important ele-
ments for both the conventional lithography and soft li-
thography passing images to resin. Masks carry the images
needed for carving. Laser beams transmit the images to
resin. Although they work well together in the last dec-
ades, physical masks require a long production cycle and
can’ t be changed flexibly. Meanwhile the intensity of input
laser beams used for resin exposure mainly has a Gaus-
sian or quasi-Gaussian distribution which will causes the
photon-induced curing behavior of resin with inverse
Gaussian profile. In order to improve the performance,
some new technology needed to be developed to over-
come this obstacle.
Digital micro-mirror device (DMD) is invented by
Texas Instruments (TI)[3-5]. Since its introduction into
market in 1996, it has become one of the most important
devices in projection display field. DMD chips are adopted
as the dynamic or virtual masks generator [6] with the
help of aluminum micro-mirrors and it projects the vir-
tual masks to resin or photo-resist. The lithography with
DMD is maskless. Meanwhile, DMD can be treated as a
kind of spatial light modulator in Digital Light Process-
ing. It can be employed as a switch of light in a special
direction and a laser shaper for laser beams.
This paper introduces the laser shaping into maskless
projection soft lithography by using DMD, error-diffu-
sion algorithm and iterative refinement [7]. After laser
shaping, the input laser beam with a Gaussian or quasi-
Gaussian distribution will become a flat-top. By using
the same method, many other laser intensity distributions
can be achieved, such as tilted beam, dark hollow beam
[8-10]. It provides a method of precise control of laser
beam and photon-induced curing behavior of resin. Sec-
tion 2 illustrates the experimental setup for soft lithogra-
phy utilizing DMD. Section 3 introduces the experimen-
tal process for resin coating and beam shaping. Section4
presents some of the preliminary experimental results
and makes some discusses.
2. Experimental Setup
The experimental setup is schematically shown in Figure
1. An LED (Thorlabs, LED365L2, F# 0.8, typical output
Copyright © 2013 SciRes. OPJ
power 360 mW) is employed as the laser source with
dominant wavelength 365 nm. LED-illumination has the
advantages of being low-cost, ease of use, and free of
speckle-noise. The half divergence angle of the LED can
be estimated from the F# to be 32°, therefore an anti-
reflection coated aspheric condenser lens L1(ACL5040-
A, Thorlabs) is introduced to collimate and expand the
laser beam. The effective focal length of L1 is 40 mm
and after expansion the diameter of the beam is approxi-
mately 37 mm. The expanded beam illuminates the DMD
with an incident angle of 24°.
The DMD has a resolution of 1024 × 768 (Texas In-
struments, width of mirror 13.68 μm). In order to control
the DMD statuses, custom software written in Visual
C++ 6.0 has been utilized to control the individual mi-
cro-mirror. Lens L2 (f = 100 mm) collects the ultraviolet
laser modulated by DMD and passes it through the pin-
hole (tunable diameter) placed near the focus of Lens L2.
The pinhole placed in here acts as a spatial filter for the
purpose of enabling a good beam quality passing to the
next section of the optical train. Lens L3 (f = 100 mm)
and L4 (f = 100 mm) form a telescope system. The tele-
scope relays the light field in the back focal plane (BFP)
of lens L2 to the BFP of microscopic objective.
The microscopic objective (LMU-10X-NUV, Thorlabs)
utilized in our experimental setup is optimized for 325-
500 nm wavelength with a numerical aperture of 0.25.
The magnification of the objective is 10X. The resin is
placed on the focal plane of the microscopic objective.
This plane is conjugated with the DMD surface.
The etching optical path with DMD is employed as the
pattern generator, and another counter propagating beam
from a second LED with different wavelength is em-
ployed to illuminate the sample stage and visualize the
etched pattern. The wavelength of the second LED must
have no influence on the resin in the irradiation process.
A Dichroic mirror placed between Len L4 and the object-
tive reflects the illumination light and a tube lens L5 im-
ages the resin onto a USB CCD camera (DCU223M,
1024 × 768 pixels with pixel size of 4.65 × 4.65 μm2,
Thorlabs Inc.,USA) which is also employed to capture the
output profile of laser beam. The sample stage is a man-
ual XY and Z-axis motorized translation stage (TSMW-
XYZT-1, Zolix, China) that can be moved in 5 dimen-
sions with minimum step of 100 μm and maximum angle
of ±5o. Automated data collection, DMD patterns genera-
tion, and motorized stage movement are implemented by
custom software programmed in Visual C++ 6.0.
3. Experimental Procedure
SU-8 (Microchem, 3050, negative photoresist) is chosen
as the resin used in our experiment. High transmittance
optical glass plates (50 × 60 mm) are employed as the
sub-strate. Before resin preparation, it is necessary to
clean the glass by acetone and methanol solution respec-
tively. Wash the glass substrate with deionized water and
dry the glass with nitrogen air. For safety, the whole
process must be performed in the fume hood. The glass
also needs to be heated on 200℃ through heating plate
after cleaning to enhance the adhesion between the
photoresist and the glass.
As the substrate prepared already, it is time to handle
the resin. The whole process of resin treatment includes
resin coating, baking, exposure and development shown
as in Figure 2. Firstly, the resin should be coated on the
Figure 1. Experimental setup. The beam with dominate
wavelength 365 nm is collimated and illuminated onto the
DMD with incident angle of 2. The predesigned pattern
is loaded onto the DMD through a computer connected to
the DMD electronic board. Lens L3 collects the imprinted
la ser light and a consecuti ve pinhole enables the beam w ith a
good quality through spatial filtering. A telescope formed
by Lens L4 and L5 relay the BFP of L3 to BFP of the mi-
croscopic objective. The glass slide with photosensitive re-
sist if place onto a motorized translation stage. Custom
software can be used to move the stage automatically
through user- friendly software. A tube lens L5 images the
chamber onto a CCD camera.
Figure 2. (Color online) Resin preparation process. The whole
preparation process of the resin is demonstrated on the left
side. The picture on the right side shows typical experimental
results or tools utilized in each step corresponding to the
first column. The resin should be coated on the substrate by
photoresist coating m achine with rotation speed of 2000 rpm
for 30 s. The temperature and time recommended for resin
baking is 6for 6min and then 9for 20 min. By utiliz-
ing the shaped beam through DMD, the photo-resistive
resin will be photo-cured. After development, the projected
image will be imprinted on the resin. All the four processes
should be performed in cleanroom without irradiance of
UV light.
Copyright © 2013 SciRes. OPJ
X. Y. DING ET AL. 229
substrate by photoresist coating machine (KW-4 A, In-
stitute of Microelectronics, Chinese Academy of Sci-
ences) with rotation speed of 2000 rpm for 30 s. The
thickness of the resin film is estimated at around 70 μm.
Secondly, put the glass coated with resin on heating plate
for baking. The temperature and time recommended is
65° for 6 min and then 95° for 20 min. Cooling to the
room temperature. Thirdly, the shaped beam will be ex-
posed the resin on the substrate. The recommended ex-
posure time is 28 s. Finally, use the developer to finish
the resin development in 1min. Isopropanol and Deion-
ized water will be needed for the cleaning of the sub-
strate. All the four processes should be performed in
cleanroom without UV existence.
Compared with traditional well-established UV etching
methods, our experimental setup has two distinct features.
Firstly, DMD enables maskless lithography and the pat-
tern can be easily renewed; secondly, the DMD can fur-
ther improve the laser beam quality and even upgrade the
distribution onto the substrate. These novel features will
minimize the edge effect caused by light diffraction.
The intensity of a laser is commonly with a Gaussian
or quasi-Gaussian profile. This restricts the application in
many research areas, for example, in ultra-cold atom
experiment, flat-top intensity distribution is required.
Therefore, we have to shape the laser beam to accom-
modate the specific application. In principle, the beam
intensities reflected from the DMD are produced by
pulse width modulation of the mirror elements over the
refresh time [11]. By changing the duty ratios of “on”
and “off” state of the micro-mirrors, the reflection func-
tion of DMD can be regulated. Accordingly, projection
of the predesigned images onto the DMD will enable
shaping of the laser beam profile used to illuminate the
photosensitive resin.
Suppose all the micro-mirrors in DMD chip are turned
on and they are all situated on the “on” state. The input
laser beam reflected from DMD will pass through the
pinhole which acts as a spatial-frequency low pass filter
in the back focal plane of L2. According to the input pro-
file, it is easy to get the target function that suits the con-
dition of our experiment. In this paper, a flat-top beam is
taken as an example. Here Gaussian and eighth-order
super-Lorentzian are employed following Liang et al. [8]
as a demonstration.
The reflective function will be achieved by the “sub-
traction” between processed reflect function and input
beam profile which is shown in Figure 3. The reflective
function will be processed by error-diffusion algorithm
which is commonly used in image processing [7]. The
DMD pattern will be designed under the processed re-
flect function with the principal of changing pulse width.
As being shaped by DMD pattern, the output beam pro-
file will be close to the target profile. Lots of algorithms
can be utilized to derive the reflection pattern projected
through DMD to minimize the mean square error of the
output beam profile with respect to the ideal target beam.
More advanced process will adopt online feedback con-
trol to optimize the reflection function and it takes com-
puter time to do the optimization and calculation. As a
demonstration, we here present the ideal of the beam
shaping method that will be adopted in our microscopic
lithography experiment.
If the laser beam is well shaped, the resin can be placed
on the focal plane of the microscopic objective. Through
the magnification ability of the microscopic objective, it
is easy to pass a big image to a small area in the focal
plane. As the beam passing through the objective has al-
ready been shaped, it is able to cure the resin with pre-
designed intensity distribution. As the intensity distribu-
tion is custom designed, the photo-induced curing be-
havior will depend much on the intensity distribution
precisely controlled through computer programs.
4. Experimental Results
DMD acts like an amplitude type video display. It adopts
an impulse modulation technique to load the gray scale
image data. Typically the output power of laser intensity
contributed from an individual DMD mirror is propor-
tional to the duty ratio of that mirror during the data col-
lection period. Actually, for most of the display, includ-
ing phase type spatial light modulator, the output light
intensity is not exactly proportionally increasing with
input signal. This effect will degrade the performance of
those devices. In order to avoid this effect, a correction
algorithm on the Gamma curve has to be derived to
minimize the non-linear effect.
Figure 4 is a typical experimental result demonstrating
the gamma curve correction process. A series of gray
1.0 Inp u t G au ssian pr ofile
p rofile
T arget p rofile
Figure 3. Schematic diagram of achieving reflection func-
tion. The black curve indicates th e un-modulated laser profile
shining on the DMD surface. The red curve demonstrates
one-dimensional i deal beam shape near the focus of Lens L2.
The bi-peak blue line illustrates the typical reflection func-
tion projected onto the DMD converting the Gaussian beam
profile into flat-top beam target intensit y di stribution.
Copyright © 2013 SciRes. OPJ
scale images with gray level from 0-255 are incorporated
into the powerpoint file and projected onto the DMD.
The scattered light from the DMD are collected by lens
L2 and collected through a power meter (PM100D, Thor-
labs). The typical laser intensity of the LED is 360mW.
During data collection, the laser power was chosen to be
80mW for safety. The uncorrected gamma curve is shown
in red diamond in Figure 4. Follow the same procedure,
we correct all the images and repeat the gamma curve
measurement. The corrected gamma curve is shown in
blue circles in Figure 4. Blue and magenta lines are fit-
ting results for corrected and uncorrected gamma curve
Without beam shaping, the input laser beam has a Gaus-
sian or quasi-Gaussian distribution. It will carve the resin
only in an inverse Gaussian profile shown in Figure 5.
The stronger the intensity of beam is, the deeper the
carved depth, just like a valley with a sharp inverse peak.
The edge of cured pattern is tilting along the outside of
inverse Gaussian profile [12]. After shaping, the beam
used to carving the resin has a flat-top intensity distribu-
tion. The photo-induced curing behavior is different from
the common one. In this new situation, the middle region
has quasi-equal intensity which will cause the same depth
as the curing behavior going on. The edge is almost per-
pendicular to the resin surface shown in Figure 4(a). We
have also designed other laser shapes which will poten-
tially achieve many different kinds of laser intensity dis-
tribution, such as valley in middle front, flat-waist and
front with a slope beam etching profile as shown in Fig-
ure 4. (b)-(d).
050100 150 200 25
Gray scale g
Normalized laser intensity
Figure 4. Gamma Curve before (red diamond) and after
correction(blue circle). Magenta and blue curve are fitting
results of the experimentally measured uncorrected and
corrected gamma curve respectively.
Figure 5. Schemati c diagram of some photo-induced curing
behavior with different beam profile (a) Cured with a Gaus-
sian beam; (b) Cured with a flat-top beam; (c) Cured with a
dark hollow beam; (d) Cured with a beam bearing a flat-
waist; (e) Cured with a tilted laser beam.
5. Conclusions
In conclusion, we presented a scheme for microscopic
UV lithography by using digital micro-mirror device
(DMD). Detailed instrument realization and typical ap-
plication are demonstrated in this paper. The benefit of
our method is that the intensity pattern on the
photo-sensitive resin can be customized to be any pattern.
The most interesting feature is that one can optimize the
intensity pattern and compensate on the artifacts due to
optical diffraction, such as edge effect. It provides a
means for precise control of photo-induced curing be-
havior of resin. These new features enable our litho-
graphic instrument based on DMD to be robust and fan-
tastic tool in micro-fabrication of microstructures. We
are confident our instrument will perspectively be util-
ized extensively in micro-fabrication and microfluidics
6. Acknowledgements
The authors gratefully acknowledge the financial support
from National Natural Science Foundation of China un-
der Grant No. 60974038.
Copyright © 2013 SciRes. OPJ
Copyright © 2013 SciRes. OPJ
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