Adding Grayscale Layer to Chrome Photomasks - Professor Glenn ...

Adding Grayscale Layer to Chrome Photomasks
David K. Poon, James M. Dykes, Chinheng Choo, Jimmy T. K. Tsui,
Jun Wang and Glenn H. Chapman 1
School of Engineering Science, Simon Fraser University, Burnaby, BC V5A 1S6, Canada
Yuqiang Tu, Patrick Reynolds, and Andrew Zanzal
Benchmark Technologies, 7E Kimball Lane, Lynnfield, MA, 01940, USA
ABSTRACT
Recent work has shown that bimetallic films, such as Bi/In and Sn/In, can create laser direct-write grayscale
photomasks. Using a laser-induced oxidation process; bimetallic films turn transparent with variations in optical
transparency that are a function of the laser power. The films exhibit transmittances 60%
when full laser exposed. A novel grayscale photolithography technique is presented that utilizes conventional chrome
photomasks as the high resolution pattern-defining layer with a bimetallic thin film layer deposited on top as the
grayscale-defining layer. Having the grayscale layer on top of the chrome, grayscale patterns can be aligned to the
underlying chrome patterns. Laser power and bimetallic thin film thickness are carefully calibrated such that no chrome
ablation or conversion occurs. The calibration ensures that during laser scanning, the bottom chrome layer defines the
fine features of the underlying patterns and remains unchanged, while the bimetallic thin film layer is converted to
provide grayscale tones. To further investigate the optical density (OD) properties of this type of mask, we measured the
transient time response for pure chrome mask and Bi/In coated chrome mask to help fine tune the laser writing
parameters. Using bimetallic Bi/In/Cr photomasks, we have successfully created continuous tone 3D structures with
superimposed binary structures in SU-8 photoresist. By introducing this novel combined chrome-bimetallic mask, the
fine detail features found in binary lithography may be combined with smoothly-varying 3D microstructures best suited
to grayscale methods.
Key words: Grayscale photomask, chrome photomask, analog grayscale photomask, direct-write photomask, 3D MEMS
1. INTRODUCTION
In micro-optics and Micro-Electro-Mechanical Systems (MEMS) fabrication, there is often the need to create 3D
structures that are few microns in dimensions but with smoothly varying slopes and vertical patterns. Grayscale masks
have the ability to produce the smooth 3D shapes but are difficult to fabricate, expensive, and often unable to
manufacture high resolution structures. For example, binary chrome halftone photomasks, which use variable chrome
dot size and out-of-focus exposure technique to create gray levels, have limited gray resolution (typically 16-32 levels)
and require a high exposure dosage. The required out-of-focus exposure technique yields low resolution results and the
high exposure energy often leads to degradation for the mask itself. Direct-write HEBS glass is attractive because the
grayscale level can be controlled by the electron beam dosage and acceleration voltage. However, HEBS glass has a
poor OD range going from ~0.4 to 1.5 OD at 365 nm. In addition, the writing process takes an extremely long time and
expensive e-beam systems are used to expose the glass [5-6]. Previously we have shown Bi/In bimetallic grayscale
photomask provides analogue laser direct-write grayscale capability with a large OD range of >3 OD when unexposed
and

Chrome binary masks have the ability to produce high resolution structures but not the smoothly varying slopes. By
combining the structural resolution of chrome photomasks with the smooth variance of our Bi/In DC-sputtered layer as
an add-on grayscale layer, we would have the benefits of being able to create high resolution, grayscale structures.
I-line (365 nm) lithography systems are often used in the manufacturing of Micro-Electro-Mechanical Systems
(MEMS) because of the relatively large structures being made. Typically, chrome photomasks with chrome thicknesses
of around 100 nm have high optical densities (>3 OD) at relatively short wavelengths (~365 nm), making chrome an
excellent light blocking material to incident exposure light during resist exposure and well suited for use with I-line
systems. In addition, chrome is durable and therefore resistant to scratches and the everyday wear and tear of photomask
usage. Chrome photomasks are typically patterned using e-beam or laser patterning allowing for the production of submicron
structures easily. Chrome masks are binary which can only produce photomasks with maximum (no chrome
blocking) or minimum (chrome blocking) light exposures without the halftone dot process. Further advances in the
photomask creation process, such as phase-shift-mask (PSM) technology and 195 nm wavelengths, brings the resolution
limit of chrome photomasks down even further to 3 OD or 0.1% transmittance for unexposed and

3. FILM DEPOSITION
The first step in creating the combined mask is to create the binary patterns on a chrome photomask by a
conventional commercially available process. The mask was planned to contain a range of structures down to 1 µm
mask size, and up to large clear areas, for a complete test of the chrome/grayscale mask. Once the binary patterns are
defined the masks were shipped in sealed containers to Simon Fraser University for the grayscale bimetallic layer to be
deposited on top of the chrome photomask. Typically the bimetallic layer is composed of either Bi/In of Sn/In. Because
Bi/In has the advantages of smoother surfaces, lower laser conversion power for grayscale levels and is able to produce
more gray levels, Bi/In was chosen as the top grayscale layer for the chrome/grayscale photomask.
To create the Bi/In coated chrome photomask, bismuth and indium film of approximately 40 nm in thickness are
sequentially DC-sputtered using a Corona sputtering system in Simon Fraser University’s Microfabrication Laboratory.
Both metal layers are sputtered onto the chrome photomask without a vacuum break. The vacuum chamber is normally
pumped down to approximately 6 x 10 -7 torr and during deposition argon gas is introduced into the chamber at 10 sccm,
bringing the sputterer chamber pressure to about 4 mTorr. Typical sputter rates for Bi and In are listed in Table 1. Note
that the sputter deposition tool was not designed for 6” masks. Film thickness control was good within the central 4”
circle but was much poorer at the mask edges. Fortunately the outer edges of the mask were not used in this design.
Table 1: Bi and In Sputter Rate
Metal argon Pressure DC bias Current Watt⋅Min Deposit rate Å/W⋅min
Bi 4mTorr 470V 0.23A 2500 12
In 3mTorr 450V 0.23A 2500 4
Once Bi/In deposition is completed, the Bi/In coated chrome mask is ready for grayscale patterning step. Next we
discuss the mask creation process for creating grayscale patterns in this Bi/In coated chrome photomask.
4. GRAYSCALE MASK WRITING PROCESS
Having added the bimetallic film layer on top of the chrome masks, the next step is grayscale patterning the
bimetallic layer. As shown in Figure 1, the combined bimetallic-chrome mask is placed on an X-Y-Z table and
positioned such that the grayscale pattern to be made would be aligned with clear alignment structures of the chrome.
For alignment, the blocking and non-blocking chrome have a slight difference in height which can be seen visually on
the mask through the Bi/In film. The ability to see the underlying chrome layer simplifies the process of aligning the
chrome layer and the bimetallic layer to just orientating the mask correctly to match the orientation of the X-Y-Z table.
A bitmap file containing the grayscale pattern is supplied and converted by a function generator into voltages that
are used to control an electro-optic shutter. The shutter is used to modulate the laser beam from a 488/514 nm CW argon
laser as the X-Y-Z table moves the photomask in a raster-scan motion. As a result of this motion combined with the
laser beam’s modulation, the bitmap file is patterned line-by-line onto the bimetallic film.
Figure 1. Grayscale mask patterning: A bitmap file is read line-by-line (1) and converted into a voltage waveform (2) that is used to
modulate the writing laser beam via an electro-optic shutter (3). The photomask moves in a raster-scan pattern as the focussed beam
patterns the mask (4).

The laser itself is kept at a constant power to obtain maximum power stability, while an electro-optic shutter with
sub-microsecond response is modulated by function generator under computer control using the bit map pattern. The
relationship between the function generator voltage and the output laser power from the electro-optic shutter is nonlinear.
Thus a calibration table translates the designed bitmap values to the function generator’s voltage waveform.
Previous research showed that after laser scanning, the bimetallic layer is converted into a transparent area thru an
oxidation process [1-2, 5-9]. The level of transparency is directly related to the incident laser energy such that at higher
laser exposure powers the written area becomes very transparent, while at lower powers it is unchanged. For the 40 nm
Bi/In or Sn/In films, the unexposed OD is approximately 2 OD while for a fully exposed/oxidized area it is 3 OD
while the non-chrome blocking area with the top bimetallic layer fully exposed has

underlying chrome patterns, and writing different shapes and patterns with grayscale. In the original chrome photomask
shown in Figure 3, there are various binary patterns used for grayscale and resolution tests. On the top part of the mask,
there are ten cleared squares of 5000 µm by 5000 µm, each will eventually be patterned with one of ten distinct
grayscale levels on the bimetallic layer. On the left-bottom part of the mask, there are several line test structures with
chrome lines of different sizes ranging from 1 µm to 10 µm wide, and with each line being 50,000 µm long. On the right
side, there are a series of small cleared squares with sizes from 1 – 50 µm 2 that are used for resolution tests. In the
middle of the mask, there is a large cleared area that is used for writing test grayscale patterns. Lastly, at the four
corners just outside the center cleared area are the alignment cross marks that will be used to test the alignment accuracy
of the grayscale patterns in the Bi/In bimetallic layer with respect to the chrome layer underneath.
In Figure 4, the final grayscale photomask design is shown for Bi/In layer DC-sputtered on top of the chrome mask
and the grayscale patterns written. In the top part of the grayscale mask, there are the ten 5000 µm by 5000 µm squares
with the bimetallic layer for each square being written with a different grayscale level. Ten grayscale levels from 0.1%
to 100% maximum transmission (with equal steps) were set with one level in each of the ten squares. Right below the
ten squares, but within the large cleared chrome area, we also created additional grayscale squares for calibration
purposes (as this was the first mask of this type). The top row was written with the same grayscale levels as the larger
ten grayscale squares along the top part of the mask. The middle grayscale squares were written with 75% of the argon
power used by the top row, while the bottom row with 50% of the maximum argon power of the top row. After writing
OD tests were done on these to confirm the desired gray levels. On the left and bottom part of the mask, the two line test
structures are scanned with a 1 cm wide by 5 cm long 64 grayscale level pattern. Therefore, the spaces in those two test
structures should appear as continuous, grayscales varying lines with 64 gray levels in each of the spaces. On the right
side of the mask, there is a 1 cm by 1 cm square at 100% argon power over the squares test structure making the
grayscale layer is completely transparent and fully revealing the original binary pattern of the underlying chrome layer.
Grayscale
Squares
Test Squares
Clearing Bi/In To
Show Patterns
Below
Grayscale
Patterns
Grayscale Lines
& Spaces
Figure 3. Original chrome photomask patterns. White
areas define the chrome blocking area, gray and black
colored areas define the clear areas of the chrome.
Figure 4. Chrome photomask with grayscale patterns.
Bi/In grayscale layer is DC-sputtered on top of the chrome
mask.
5.1 Chrome Pattern Alignment Tests
To test our system’s ability to align a grayscale pattern with the underlying chrome pattern, patterns originally
present on the chrome layer were traced over and replicated onto the Bi/In layer. In addition, to provide a better visual
measure of our alignment, the grayscale writing system in some cases also placed smaller patterns on the Bi/In layer
within alignment crosshairs present on the underlying chrome. Particularly for this mask, at the four corners of the
photomask there are two types of cross-alignment mark structures shown in Figure 5 and Figure 6. Each corner contains
one 60 µm by 60 µm and one 200 µm by 200 µm crosshair, with widths of 10 µm and 20 µm, respectively. All of the
60 µm by 60 µm crosshairs were laser scanned and the Bi/In converted to maximum transmission. The 200 µm by
200 µm crosshairs were also laser scanned except for a 10 µm by 10 µm square aligned to the center where the Bi/In was
opaque. The 10 µm square in the crosshair measures the accuracy and resolution of our Bi/In laser writing system.
Figure 7 and Figure 8 show the 200 µm by 200 µm and 60 µm by 60 µm crosshair at 20X, respectively.

Figure 5. A 200 by 200 µm crosshair structure at 5X
magnification. The crosshair is 20 µm wide. The
crosshair is laser scanned to transparency by the argon
laser. In the middle of the crosshair, a 10 by 10 µm square
of unexposed Bi/In area remains to show the accuracy of
Bi/In grayscale writing alignment process.
Figure 6. A 60 by 60 µm crosshair structure at 5X
magnification. The width of crosshair is 10 µm wide. The
crosshair is laser scanned by the argon laser.
Figure 7. 200 by 200 µm crosshair at 20X magnification.
Figure 8. A 60 by 60 µm crosshair at 20X magnification.
5.2 Grayscale on Chrome Lines and Spaces
The left-bottom part of the chrome mask contains chrome lines and spaces of 1, 1.2, 1.4, 1.6, 1.8, 2, 2.2, 2.4, 2.6,
2.8, 3, 4, 5, 6, 7, 8, 9 and 10 µm. After Bi/In grayscale material is DC-sputtered onto the chrome mask, the argon laser
created a grayscale slope from maximum to minimum along the direction of the lines, with clear areas at the ends. There
are 64 different gray levels equally spaced along the 5 cm long test structures. The left sides of the line test structures at
the bottom part of the chrome mask are shown rotated 90° counter-clockwise in Figure 9. The numbers along the bottom
side of the figure indicate the width of the lines. In Figure 10, there are the same line test structures but farther down
along the lines and they are written at a lower laser power than the test structures shown in Figure 9. Figure 10 also
shows the effect of lower laser power writing as it produces darker areas indicating lower light transmission.
Figure 9. The left side of the line test patterns with the
following line widths: 1, 1.2, 1.4, 1.6, 1.8, 2, 2.2, 2.4, 2.6,
2.8, 3, 4, 5, 6, 7, 8, 9 and 10 µm.
Figure 10. The middle part of the line test patterns.
5.3 Clearing Bi/In To Show Chrome Pattern Below
As for the test square structures on the right side of the chrome mask, there are columns of test squares with sizes of
1, 2, 3, 5, 10 and 50 µm 2 . The area of the square structures was laser scanned by the argon laser at 0.1W which has
sufficient power to convert the Bi/In grayscale layer to maximum transparency revealing the underlying, original binary
test structure on the chrome layer as shown in Figure 11.

Figure 11. Test square structures with sizes of 1, 2, 3, 5, 10 and 50 µm 2 . The entire area of the test square
structures was laser scanned by the argon laser at 0.1W (maximum) power. Laser scanning makes the
Bi/In layer transparent, thus revealing the original binary structures on the chrome layer.
5.4 Grayscale Test Patterns
Directly above the line test structures at the bottom of the cleared chrome area are the grayscale test structures of
different shapes. The bitmap image used for these test structures is shown in Figure 12. An important setting that
affects how the bitmap is written on to the photomask is the pixel size. The pixel size describes the photomask
dimensions for each pixel in the bitmap. In this example, Figure 12 is a bitmap image with 300 by 3500 pixels. Using a
1 um/pixel pixel size, the resulting pattern written on the mask is 300 µm by 3500 µm. Using a 2 µm/pixel pixel size,
the resulting pattern is 600 µm by 7000 µm. In our experiment, we used a total of three different pixel sizes: 1, 2, and
4 µm/pixel. Figure 13 - Figure 18 show the resulting grayscale structures on the bimetallic-chrome photomask with two
different laser spot size (1 µm/pixel and 2 µm/pixel) and three different minimum to maximum argon laser power ranges
(0.01W to 0.065W, 0.01W to 0.06W and 0.01W to 0.045W). Examining the grayscale bar at the top of each figure, the
effect the voltage can be seen changing the position of where the grayscale shifts from white to black.
Figure 12. Bitmap image for the grayscale test structures.
Figure 13. Grayscale pattern using a minimum to
maximum argon laser power range from 0.01W to 0.065W
and a 1 µm laser spot size. The size of the entire pattern is
300 µm wide by 3500 µm tall.
Figure 14. Grayscale pattern using a minimum to
maximum argon laser power range from 0.01W to
0.065W and a 2 µm laser spot size. The size of the entire
pattern is 300 µm wide by 3500 µm tall.
Figure 15. Grayscale pattern using a minimum to
maximum argon laser power range from 0.01W to 0.06W
and a 1 µm laser spot size. The size of the entire pattern
is 300 µm wide by 3500 µm tall.
Figure 16. Grayscale pattern using a minimum to
maximum argon laser power range from 0.01W to 0.06W
and a 2 µm laser spot size. The size of the entire pattern is
300 µm wide by 3500 µm tall.
Figure 17. Grayscale pattern using a minimum to
maximum argon laser power range from 0.01W to
0.045W and a 1 µm laser spot size. The size of the entire
pattern is 300 µm wide by 3500 µm tall.
Figure 18. Grayscale pattern using a minimum to
maximum argon laser power range from 0.01W to 0.045W
and a 2 µm laser spot size. The size of the entire pattern is
300 µm wide by 3500 µm tall.
5.5 Grayscale Test Squares
Table 2 summarizes the argon laser power used to scan the ten squares at the top of the chrome mask and their
measured optical densities.

Table 2: Laser power and OD measurements for ten squares at top of chrome photomask
Square # Laser Power Used Optical Density
1 6.43X10 -2 W 0.0823
2 5.98X10 -2 W 0.0780
3 4.59X10 -2 W 0.121
4 2.40X10 -2 W 0.311
5 2.07X10 -2 W 0.542
6 1.74X10 -2 W 0.655
7 1.40X10 -2 W 1.101
8 1.07X10 -2 W 1.233
9 7.372X10 -3 W 1.240
10 2.60X10 -3 W 1.291
The ten grayscale squares were written with the input argon laser power ranging from 2.6 mW to 64.3 mW resulting
in optical densities from 0.0823 OD to 1.291 OD, respectively.
The final Bi/In/Cr grayscale photomask has been sent to Benchmark Technologies for exposure and alignment
testing. The grayscale mask will be tested in a commercial I-line 4X stepper system. The photomask will also be used
to produce 3D structures on several photoresists that are typically used in the industry today. Analysis of the resulting
structures using SEM will determine the feasibility of the bimetallic-chrome photomasks for commercial use.
6. OD CHANGE DURING LASER INDUCED OXIDATION OF BIMETALLIC GRAYSCALE
CHROME PHOTOMASK
In this section, we describe our investigation on how the differences in threshold power between chrome and Bi/In
layers affect the film’s OD behaviour. The investigation consisted of measuring the change in OD with time for both
chrome and bimetallic-chrome films as the laser light power is increased beyond the chrome’s threshold level. Another
goal of the investigation was to see if there was a difference in rate of oxidation between chrome and Bi/In. If the rate of
oxidation in chrome was found to be slower, OD changes in Bi/In could maybe be produced without affecting the
chrome layer by scanning the Bi/In/Cr film very fast. We had previously measured changes in OD with respect to time
in our investigations into the oxidation behaviour of Bi/In/O, Sn/In and Zn films [9]. Knowing how the OD of the
Bi/In/Cr mask changes with time should shed light on the underlying oxidation processes taking place in the different
layers of the mask.
The OD transient response measurement system is shown in Figure 19. There are two identical photodetectors used
in this setup: the Texas Instrument Monolithic Photodiode and Single-Supply Transimpedance Amplifier, OPT101P-ND.
Due to the wide range of optical densities measured, the transmitted laser power, after the thin film, may vary from a few
microwatts to about a hundred milliwatts. Previously, we had used a photovoltaic diode to measure low power laser
light and a photoconductive diode to perform measurements in the high power range. In this paper, only one
OPT101P-ND photodetector was used to measure light power for the entire range. To enable the measurement of this
wide range of laser powers, the photodetector’s response was made to behave logarithmically with external diode
circuits.
To divert a portion of the laser beam for measuring purposes, a 60/40 non-polarizing beam splitter cube (Type
BS010 from Melles Griot) splits the main laser beam into a sample beam and a reference beam. The sample beam goes
to the mask and produces OD changes in the film. One of the photodetectors is placed beneath the mask to measure the
remaining laser power after the beam passes through. The reference beam goes directly to one of OPT1010P-ND
detectors allowing us to track changes to the main laser beam’s power. Taking the negative logarithm of the transmitted
sample beam’s laser power over the reference beam’s laser power allows us to calculate the OD of the mask. Both the
sample and reference photodiodes are reverse-biased externally at 30V. A Tektronix TDS2018 digital oscilloscope and a
computer system connected to the oscilloscope’s communication module captures the outputs from the two
photodetectors as the thin bimetallic film is exposed. The electro-optical shutter, which modulates the argon laser
beam’s power when writing grayscale masks, is placed before the beam splitter; in this case, this acts to turn the writing
beam on and off. A laser power versus photodiode voltage calibration is also performed on the system so that laser
power information can be obtained from the photodetector voltages.

Figure 19. Transient time response measurement experimental system set up.
The results for the transient response between chrome and Bi/In coated chrome films are shown in Figure 20. In all
cases the laser beam was focused by the 50x objective to the 2 µm spot used in the mask writing. For the chrome film,
there is a rapid decrease in the OD from 3 to about 1.5 in the first 10 ms of direct exposure to a 0.65 W beam and a
gradual linear decrease to about 1 OD at 100 ms after exposure. We had previously suggested that this rapid change may
be due to a combination of reduction in reflectivity and increased oxidation due to conversion to a liquid state. At 0.6W
laser power, the Bi/In/Cr film exhibits a similar rapid decrease from about 4.4 OD to about 1.58 OD in the first 10 ms
and decreases further to about 1.55 at around 20 ms after exposure. The OD then remained at this value unchanged up to
and beyond 100 ms. At 0.65W laser power, the Bi/In/Cr film shows a different behavior, the OD initially shows a rapid
decrease from 4.1 to 1.2 within the first 10 ms of exposure and then decreases linearly at a low rate to about 0.85 at
100 ms after exposure. These results follow very closely the behavior of chrome indicating that at 0.65W laser power
the chrome layer’s OD is changing together with that of the Bi/In film. In contrast, at 0.6W, as soon as the film reaches
its saturated OD of about 1.55 in about 20 ms no further change in OD is observed. The low rate drop at 0.65W is
probably due to a significant amount of laser light penetrating the Bi/In layer and oxidizing the chrome. Whereas at
0.6W, the amount of laser power penetrating the Bi/In layer is low enough so that oxidation in the chrome does not
continue after 20 ms. The lack of OD change after 20 ms means a much lower amount of chrome is converted at this
power level. The result also implies that at low enough laser power changes in the OD of the Bi/In layer can be made
without damaging the underlying chrome layer.
5
Optical Density (OD)
4
3
2
1
0
0 0.02 0.04 0.06 0.08 0.1
Time (second)
Bi/In/Cr
0.6W
Cr 0.6W
Bi/In/Cr
0.65W
Cr 0.6W Bi/In/Cr 0.6W Bi/In/Cr 0.65W
Figure 20. Optical density transient time response for conventional chrome and Bi/In coated chrome photomasks.

The transient response results also show no time lag between OD changes in chrome and Bi/In/Cr films, indicating
that chrome does not oxidize faster or slower than Bi/In. For all the films tested, the OD changed rapidly within the first
10 ms of exposure, achieving about 75% to 90% of its total OD conversion within this period.
The results from our direct OD measurements in the Bi/In/Cr film show that presently we can only minimize the
damage to the chrome layer via lower laser writing power and that varying the writing speed should not have any affect
on the OD change process.
7. BI/IN CHROME MASK 3D STRUCTURES CREATED IN SU-8
The 6” by 6” grayscale chrome photomask that we created will be use in an industrial stepper to measure the
resulting photoresist 3D structures. With a 4X reduction in size of the stepper, we should be able to test the resolution of
this mask and smallest size 3D structure that our grayscale mask can fabricate. This work is currently proceeding. In
this section will use a 5” by 5” chrome/grayscale photomask in a mask aligner to test the viability of creating 3D
structures. The photomask used has a pre-patterned binary dark-field chrome mask coated with a Bi/In bimetallic layer.
Again grayscale patterns were then written on the bimetallic layer of the mask, resulting in a super-positioning of the
binary and grayscale patterns. To prevent damaging the chrome layer during laser writing, a thin 20nm/20nm Bi/In layer
was deposited on top of the chrome. Furthermore, the laser writing power was limited to a maximum of 0.1W so as to
minimize damage to the chrome layer. The maximum OD variation in the grayscale region is only about 0.8. The
chrome layer used was relatively thin (less than 100 nm), resulting in an OD of about 3. A mask aligner was used to
transfer the pattern to the photoresist.
Continuously varying 3D structures have previously been created in thick photoresists like AZ4620 and AZ1592
using half-tone grayscale mask or continuous tone HEBS grayscale mask [1-7]. However the high gamma level of many
resists make fabrication of 3D structures difficult. We have found that creating continuous tone 3D structures on thick
SU-8 using Bi/In coated chrome mask is a good way to test the grayscale mask behavior.
SU-8, produced by MicroChem, is a negative, epoxy type, near-UV sensitive photoresist that produces transparent
structures. It has a high viscosity and can be used to make high aspect-ratio (around 20) straight walled structures with
thicknesses up to 2 mm. SU-8 is sensitive to near UV light and has been successfully applied in the standard
microlithographic exposure system for fabrication of low-cost ultra-thick high-aspect ratio microfluidic features and
MEMS devices.
Conventional MEMS structures created in SU-8 using binary masks have very straight walls, smooth surfaces and
high-aspect ratios. However, many problems and challenges appear when SU-8 is applied to create microstructures with
continuously-varying thicknesses using grayscale masks. These problems mainly result from the uneven absorption of
SU-8 to light with various wavelengths and dosage. SU-8 is highly transparent at 365 nm but its absorption coefficient
increases rapidly below 350 nm. Due to the significant amount of shorter wavelength light present in our exposure
source, we get relatively short penetration depths at low exposure dosage. Low exposure typically occurs in the darker
regions of the grayscale mask. As shown in Figure 21, with the penetration depths varying with the exposure strength,
exposing the film from the SU-8 side creates a uniformly exposed top surface, or “skin”, in the SU-8 with areas at the
film/substrate interface that are unexposed. The end result after development is an inverted 3D structure. To produce
typical upright 3D structures with our system, we have to expose the SU-8 film from the substrate side. However,
exposing from the substrate side can result in the pattern resolution of the SU-8 being severely deteriorated if the
substrates are too thick. Typical glass substrates, which can be as thick as a few millimeters, would not be useful for
patterning 3D structures. For this reason, very thin glass substrates with thicknesses of about 200 µm were used in our
SU-8 exposure. These substrates are RCA-1 cleaned prior to being spin-coated with SU-8.
A normal SU-8 process includes the following lithographic steps: spin-coat, pre-exposure soft bake, exposure, postexposure
bake and development. The SU-8 photoresist is spin-coated onto the glass substrate at 2000 rpm for about a
minute. The resulting film thickness is about 50 to 80 µm. The pre-exposure bake is then performed for 2 min on a
programmable hotplate set to an initial baking temperature of 50 °C. The pre-exposure bake facilitates maximum
solvent evaporation. The temperature on the hotplate is then ramped up to 95 °C at a rate of 300 °C/Hr ensuring that the
stress between the SU-8 and glass substrate is at a minimum. Upon reaching 95 °C, the film is further baked for another
2 min, after which the hotplate is turned off and the film cooled back to room temperature. The film then sits at room
temperature overnight before being exposed.

Figure 21. Illustration of UV photolithographic exposure processes for SU-8 photoresist and their resulting
3D structures. (a) Exposure from SU-8 side. (b) Inverted 3D structure. (c) Exposure from substrate side.
(d) Typical 3D structure.
Using the combined bimetallic-chrome photomask, SU-8 is exposed in a mask aligner with the substrate side in
contact with the protected chrome of our grayscale mask, as shown in Figure 21(c). The exposure dose for the mask
aligner is 1 min at 12 mJ. Upon exposure, photoacid is produced in the SU-8 which promotes initial cross-linking in the
polymer. A post-exposure bake process is then performed immediately afterwards to further maximize the cross-linking.
Using the hotplate, the post-bake procedure is the same as that for the pre-exposure bake, but in this case, the film does
not sit overnight after cooling. Development is performed immediately after the exposed film has cooled to room
temperature. With the cross-linking in the exposed region completed, the unexposed sections are removed by placing
the patterned SU-8 film into a SU-8 developer and agitated for 2 min. Regions in the SU-8 that are highly cross-linked
will have a very slow development rate and will not be removed. After development, the film is blown dry with nitrogen
gas without rinsing and a thermally stable, very transparent region remains.
Grayscale
Grayscale
Figure 22. Backlight Bi/In/Cr mask with binary pattern
superimposed on grayscale pattern.
Figure 23. A SEM image of 3D structures created from
the photomask pattern shown in Figure 22.
The Bi/In grayscale pattern superimposed on a binary chrome pattern layer is shown in Figure 22. Because the
chrome pattern is dark-field, the binary pattern appears brighter than the surrounding grayscale pattern. The
corresponding 3D structures created from the mask pattern in Figure 22 are shown in Figure 23. A slope created from a
continuous horizontal grayscale variation can be seen to be interspersed with rectangular structures created from the
binary pattern in the chrome mask layer. The developed film thickness measures about 60 µm and the size of the
squares on the slope is about 50 µm in width. Note also the sharp edges of the chrome defined openings, including the

pillars on top of the grayscale slope. The results illustrated in Figure 23 show that we can create 3D structures from both
the grayscale pattern of the Bi/In layer and the binary pattern of the chrome layer. With the appropriate binary pattern
from the chrome mask, we should be able to create very small continuous tone 3D structures.
8. CONCLUSION
A novel grayscale technique using a conventional chrome mask and combining it with a bimetallic thin-film layer
has been developed and tested. The combined mask provides the benefits and commercial value of binary chrome masks
with the advantages of grayscale photomasks. Using this novel grayscale technique, new or existing chrome binary
photomasks can be upgraded with the added grayscale capability easily by using a simple Bi/In DC-sputtering process.
By scanning the Bi/In grayscale layer with an argon laser at carefully calibrated power, grayscale patterns are added with
their transmission intensity varied by the power of the patterning argon laser. Calibration is required to ensure that the
patterning process does not disturb the existing chrome layer and that opaque areas in the chrome mask remain opaque
even after the Bi/In grayscale layer is directly scanned by the argon laser. In our experiments, we have successfully
converted a 6” by 6” chrome binary photomask to a Bi/In/Cr grayscale photomask by performing a 20 nm bismuth and a
20 nm indium DC-sputtering process. Furthermore, grayscale patterns were added to the Bi/In grayscale layer over the
existing chrome binary patterns layer. We have successfully created up to 64 gray levels with a minimum of 0.0780 OD
to a maximum of 1.291 OD within the Bi/In grayscale layer.
In addition, using the real-time OD measurement system, we have characterized the time-transient response of
Bi/In/Cr photomask during laser-induced conversion. Comparing between standard chrome photomask and Bi/In/Cr
photomask, we found that at high laser power, the oxidizing behaviour of both photomasks are similar. However, at
lower laser writing power, the OD saturates earlier, indicating less oxidization occurring in the chrome layer. This
suggests that at low enough laser power, we can induce OD changes in the Bi/In layer with minimum or no change in the
chrome’s OD, allowing us to create grayscale pattern with very high contrast edges defined by the chrome layer.
Therefore, it is advantageous for us to use Bi/In/Cr grayscale photomask over Bi/In grayscale photomask for better
control over the gray levels. Using Bi/In/Cr photomask with combined binary and grayscale patterns, we had
successfully created smoothly varying 3D structures intercede with binary structures in SU-8.
To determine the feasibility of the bimetallic-chrome photomasks for commercial use, a final Bi/In/Cr grayscale
photomask will be exposed in a I line stepper system. Measurements of the combined chrome/grayscale patterns and the
alignment of grayscale to chrome layers will be tested. Further research will also be conducted using bimetallic-chrome
photomasks to produce 3D structures on several photoresists that are typically used in the industry today, including
SU-8.
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