When bimetallic thin films of Bi/In and Sn/In are laser exposed, they oxidize and become variably transparent. By
controlling the writing laser power, binary and grayscale photomasks can be produced with the mask's transparency
(optical density, OD), ranging between ~3.0 (unexposed) to <0.22 OD (fully exposed). Targeting the production of
grayscale masks with 256 levels, the mask-writing system when combined with photodiode sensors obtains real-time OD
and laser power measurements and uses them to adjust the laser's writing power during the patterning process. For a
single-line stepped pattern, laser writing without OD feedback control demonstrates an average absolute error of 4.2 gray
levels, while with OD feedback control and the appropriate parameters, the same pattern is produced with an average
absolute error of 0.3 gray levels. The control parameters are shown to influence the characteristics of the resulting mask
pattern, particularly the overshoot and rise-time of the pixel transitions. With multi-line mask patterns being rasterscanned
written, the overlap of the lines combined with the laser's Gaussian profile creates variations in the mask, and
measurement problems for the OD feedback control. An interlaced raster-scan approach is proposed where a first pass
patterns non-overlapping lines using an ideal set of control parameters. A second and third pass then patterns the lines inbetween
and at the pixel boundaries using another set parameters designed to account for the overlap. The technique
allows feedback to be used for the entire mask writing process.
Under laser exposure, bimetallic thin films of Bi/In and Sn/In oxidize becoming transparent. By controlling the
power, direct-write binary and grayscale photomasks have been produced with the mask's transparency (optical density,
OD), ranging between ~3.0 (unexposed) to <0.22 (fully exposed). Precise 3D micro-optics require both high vertical
accuracy, gray levels over large OD changes, and precise lateral pattern creation. To achieve this result, an OD
measurement system has been developed that provides real-time measurements while the masks are being written.
Using stationary exposures of Bi/In and Sn/In films with varying laser powers, the reduction in OD of the films is
measured with respect to time. Using 1-minute exposures, the films reach a 'saturated' level within a second at 180 mW
while at 50 mW their OD gradually reduces. The influence of film's thickness is examined with thicker films requiring a
longer exposure time in order to reach a similar OD level. For mask-writing, the optimal line spacing is dependent upon
the laser beam's power distribution profile. Using a line-spacing 3-5 times smaller than the effective spot-size,
variations in the patterned mask caused by a Gaussian-distributed beam can be minimized at the cost of increasing the
writing time of the mask by the same factor. The Gaussian-distributed beam at different laser powers is also found to
create shifts in the OD measurements that are problematic for a closed-loop mask-writing system. The influence of the
beam's power distribution is discussed along with solutions to eliminate the problems.
Bimetallic thin films of Bi/In and Sn/In oxidize becoming transparent under laser exposure. By controlling the laser
power, direct-write binary and grayscale photomasks can be produced with the mask's transparency, or optical density
(OD), ranging between ~3.0 (unexposed) to <0.22 OD (fully exposed). An OD measurement system has been developed
that provides real time OD and laser exposure power measurements while the masks are being written. Measurements
are obtained for each combination of films, characterizing their response when patterned with a raster-scanned v-groove
mask. The characterization is performed by writing v-groove step patterns and modifying the mask's writing parameters
such as velocity, line spacing and step width. Stationary results demonstrate Sn/In takes longer to expose compared to
Bi/In. With a moving beam, the oxidation of Sn/In also occurs over a wider power range suggesting film materials with
delayed or slower oxidations may offer power ranges that are better suited for grayscale masks. A narrow power range
is less desirable for grayscale as more control is required over the writing laser. The stationary exposures also
demonstrate both films can produce >64 distinct OD levels provided there is sufficient control over the laser power and
exposure duration. The physical characteristics of the films are also examined to determine a more accurate method of
verifying each film's composition. Combining weight, area, and thickness measurements allows for better
characterization of the films as the thickness for bi-layer films are found to differ significantly from the sum of the
individual layers.
Bimetallic thin-films of Bi/In act as negative thermal resists when laser exposure pulse (7mJ/sq. cm for 4 nsec)
converts the film into a transparent eutectic metallic oxide alloy. Resist transparency varies with exposed laser power,
changing from <0.1% (3.0 OD) unexposed to >60% (0.22 OD) exposed. This generates direct-write gray scale
photomasks, and adding a feedback system where the transparency is measured and adjusts the writing process to
account for local variations in the film, achieves >64 gray level control. These resists are also wavelength invariant,
operating from visible to EUV with a resolution >42nm after development using a diluted RCA-2 solution
(HCl:H2O2:H20 @ 1:1:48) with a gamma of 2-18. Longer duration exposures with lower instantaneous intensities result
in lower gammas, while shorter exposures with higher energies give higher gammas. One limitation on these resists is
that the exposure energy must be delivered in a single pulse. This limitation puts pulse energy requirements into the mJ
per pulse range: greater than desired for EUV exposure systems. Bimetallic thermal resists remain almost unaffected
during a sub-threshold exposure that does not reach the activation energy. It has been shown that the resist and substrate
can be heated below the threshold energy, to temperatures of at least 90°C, without creating any exposure of the resist.
In this research, Bi/In resists are heated through a range of substrate temperatures, measured for their optical exposure
requirements and gammas under these conditions, and used to determine if substrate heating can improve the film's
sensitivity.
Novel grayscale photomasks are being developed consisting of bimetallic thin-films of Bismuth on Indium (Bi/In)
and Tin on Indium (Sn/In) with optical densities (OD) ranging from ~3.0 OD to <0.22 OD. To create precise threedimensional
(3D) microstructures such as microlenses, the mask's transparency must be finely controlled for accurate
gray level steps. To improve the quality of our direct-write masks, the design of a feedback system is presented where
the mask's transparency is measured and used to adjust the mask-patterning process while making the mask. The
feedback would account for local variations in the bimetallic film and enhance the control over the mask's transparency
such that >64 gray level photomasks become possible. A particular application of the feedback system is towards the
production of beam-shaping masks. When placed in the unfocussed path for the photomask-patterning system, they can
improve the consistency of the grayscale patterns by altering the laser to have a more uniform "top-hat" power
distribution. The feedback system aids the production of beam-shaping masks since the processes of patterning,
verifying, and using the mask are all performed using the same wavelength. In developing the feedback system, two
methods were examined for verifying grayscale patterns. The first utilizes the mask-patterning system's focused beam
along with two photodiode sensors; the second utilizes image analysis techniques on lower resolution microscope
images. The completed feedback design would also account for drifts in the laser power used to pattern the bimetallic
thin-film photomasks.
Double exposure/patterning is considered the best candidate for extending 195nm optical lithography below 40nm
resolution. However, double exposure techniques require a resist where the exposures do not add linearly to produce the
final result. A class of negative thermal resists that show this effect are bimetallic thin-films consisting of Bi/In or Sn/In.
The films are bi-layered structured until sufficiently heated by a laser exposure pulse (7 mJ/sq. cm for 4 nsec).
Experiments with interference lithography at 266nm in air demonstrated that Bi/In resists have a resolution limit <42nm,
the exposure system limit. As a first investigation into the resist's potential for immersion lithography, the response of
bimetallic resists to immersion lithography was examined. The Sn/In film used demonstrated successful development as
thermal resist for immersion exposures and the power level required to convert the film was only slightly higher than the
level required for exposing the film in air.
Bimetallic films have demonstrated transmittances <0.1% when unexposed and >60% when highly exposed to an
Argon laser, enabling their application as grayscale photomasks. However, direct laser-writing of the photomasks causes
fine variations in their transparency due to the laser beam's Gaussian power profile. To correct this problem, a beam-shaping
mask was designed to manipulate the power profile of the laser. To help measure mask transparency at a
resolution suitable for characterizing a photomask, two photodiode sensors were added to the writing system. The
profiling ability offered by the modified system allows the use of test structures 100x smaller then previously required.
Bimetallic thin-films offer the ability of producing analog grayscale photomasks with OD ranging from ~3.0OD (unexposed) to <0.22OD (fully exposed). Recent developments have yielded the ability to deposit and pattern bimetallic thin-films on pre-patterned binary Chrome masks. Care is taken to ensure that when writing the grayscale pattern that the underlying Chrome layer is not affected. Through this technique, the advantages of analog grayscale can be added to the high resolution capabilities currently available with Chrome masks. Currently the optical characteristics of bimetallic thin-films limit their effectiveness in high resolution applications. Techniques designed to minimize defects in the uniformity of thin-films after laser exposure are investigated along with different methods of performing the raster-scanning of the photomask patterns. Also discussed is a new application of bimetallic thin-films as a beam-shaping mask. Characterizing the laser beam profile for our writing system, a grayscale mask is designed and tested in an attempt to modify the Gaussian beam profile of the laser into a more uniform flat-top profile. Obtaining a flatter laser power distribution for the writing laser would assist in improving the optical characteristics of the bimetallic thin-films since the primary cause for the photomask's gray level non-uniformities is the Gaussian nature of the laser beam's power distribution causing lines on the photomasks. A flatter profile is shown to eliminate these lines and allow for more uniform gray levels on the laser-exposed bimetallic thin-films.
Two methods were investigated for the creation of encapsulated micro-fluidic channels and bridges in negative tone
SU-8 photoresist. The first uses two exposures at different wavelengths to create the channel sidewalls and microchannel
encapsulation layer; the other method creates both using a single I-line (365 nm) exposure and a grayscale
photomask. These methods can define structures with vertical dimensions ranging to hundreds of microns and
introduces very little extra processing complexity. For the dual wavelength method, an I-line light source is used to
define the channel walls while a non-collimated deep-UV (254 nm) light source provides a large energy dose to the top
surface of the SU-8 to produce a membrane over all the channels. Using the dual wavelength method allows SU-8 to be
used as the material for the channels and the encapsulation method is self-limiting avoiding the requirement for precise
control over the exposure dose. The rate of UV dose and the post-exposure baking parameters are critical to the quality
and strength of the micro-channels. Properly designed channels have been successfully developed in lengths up to 1 cm.
Alternatively using a grayscale Zn/Al bimetallic photomask and a single I-line exposure, 3D bridge micro-structures
were successfully made on SU-8. The use of grayscale masks for both techniques also provides the possibility of
shaping the channel. With the ability to create micro-bridges, further research will be performed to investigate how well
the single exposure technique can be used to produce micro-channels of various sizes and dimensions.
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 <0.1% when unexposed and >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.
Bimetallic films have been found to be promising direct write binary and grayscale photomask materials, as they turn transparent after laser exposure. Current structural analysis shows that the laser exposure is an oxidation process. The amount of the oxidized metal created during the laser writing process is related to the laser power, which in turn, determines the gray level (OD) of the exposed film. New exposure conditions have greatly increased the transparency of exposed films (down to 0.18 OD at 365 nm). Furthermore, this extended to deeper UV (300 nm). As the transparency of exposed area changes with the laser exposure power, grayscale photomasks can be created with the bimetallic films, and 3D structures can be produced in the substrate. Interference lithography has been used to investigate the bimetallic films resolution limit, which can generate much finer structures. Lines of 100-180 nm wide were successfully created on silicon and silicon dioxide. Aluminum thin films were found to turn transparent (0.28 OD) after laser exposure with high power, indicating that Al can also be a potential direct-write photomask material.
Previous research demonstrated Sn/In and Bi/In bimetallic thermal resists are promising new materials for direct-write analogue grayscale photomask processes. These materials turn transparent with increased laser exposure power and their optical density changes smoothly from 3 OD when unexposed to less than 0.22 OD when fully exposed. The transparency is the result of an oxidation process that is controllable with exposure to generate the grayscale levels in the photomask. In order to produce precise 3D structures in regular photoresists, the steps involved in microlithography must be quantified and examined. The lithographic process includes drawing 8-bit grayscale bitmap patterns, computer-aided laser writing photomasks on bimetallic films, and regular photoresist exposure using a mask-aligner. Compensation during the mask-writing process was necessary since the relationship between the optical density of the exposed bimetallic films and the laser writing power was not completely linear. In addition, the response of the photoresists to the mask exposure time was also a non-linear relationship. To investigate the resolution limit for Bi/In and Sn/In bimetallic thermal resists as a masking material, we used a modified form of interference lithography to expose and develop structures in Bi/In resists with widths that are less than 200 nm. As a result of the lithography, we were able to create structures in the Bi/In films that are up to 20 times smaller than previously obtained using the direct-write method.
Bimetallic thin films were previously shown to create laser direct write binary and analog gray scale photomasks. DC-sputtered Sn/In (5at.% Sn, 80 nm) oxidize under laser exposure, modifying the optical density at 365 nm from >3OD to <0.22OD. Bimetallic Sn/In thin film grayscale photomasks have been successfully used to create concave and convex 3D structures using mask aligners with Shipley photoresists. To produce precise 3D structures in the organic photoresists, every mask making step was studied. Compensations during the mask making process were necessary because that the relationship between the optical density of the exposed bimetallic films and the laser writing power is not accurately linear, and also that the response of the photoresists is not linear to the exposure. V-grooves with straight slope profile were produced with calibrations taken into account. X-ray diffraction analysis indicates that structure of laser exposed Sn/In bimetallic films is similar to that of ITO films, suggesting new directions for improvement of bimetallic film optical properties, and that the theoretical maximum transmission should approach pure ITO’s ~0.05OD in the visible wavelength.
New types of analog gray-scale laser direct-write masks have been created using bimetallic thermal resists and a direct- write laser process. Bimetallic resists consist of two layers of thin films, eg. Bi over In or Sn over In, which react to form a low temperature alloy when a laser raises the films above the eutectic temperature. Depending on the exposure energy, resulting alloyed layers appear to become oxides, causing a change of absorption at 365nm from >3OD to <0.3OD. The thermal resists show near wavelength invariance from IR to UV. The Sn/In films, each layer ~40 nm thick, were DC-sputtered onto glass slides or quartz substrates. To make gray-scale photomasks the samples were placed on a computer-controlled high accuracy X-Y table. A bitmap gray-scale pattern was raster-scanned with a CW Argon laser (514 nm) beam. An optical shutter controlled the actual laser power applied onto the thermal resist film according to the gray-scale value. When exposed to a laser beam greater than 0.6 W, the Sn/In film became nearly transparent (0.22OD) at I-line (365nm) wavelength. Sn/In and Bi/In photomasks have been used together with a standard mask aligner to successfully pattern Shipley SPR2FX-1.3 photoresist. CF4/O2 plasma etching has been used to transfer the three-dimensional pattern to SiO2 and Si substrates. Also a 160 beam laser diode thermal imaging tool was used to create BiIn direct-write binary masks.
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