David K. Poon

Creation of embedded structures in SU-8

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.

Creating precise 3D microstructures using laser direct-write bimetallic thermal resist grayscale photomasks

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.

Expanding grayscale capability of direct-write grayscale photomask by using modified Bi/In compositions

Bimetallic thin films have been proven to be effective in creating analog direct write grayscale photomasks. DC-sputtered Bi/In or Sn/In oxidizes under laser writing exposure. The optical density decreases from >3OD as deposited to a transparency of <0.22OD at 365 nm with increasing laser power. The bimetallic film has a response curve that is nearly linear for much of the curve, but non-linear at maximum absorption and transmission. In order to create more accurate gray levels, a more gradual OD change versus laser writing power is desired. In this research a new reactive sputtered, oxygenated Bi/In film was created that has an 8-bits grayscale level sensitivity of 1.1 gray levels/mV, compared with the previous Bi/In of 3.2 gray levels/mV and Sn/In of 2.8 gray levels/mV. This modified Bi/In film provides more than twice the laser writing power range for controlling the same OD range, as compared to our original Bi/In or Sn/In films. This wider power range provides easier and more accurate laser power-to-grayscale calibration, because each grayscale can now be spaced more evenly over the increased laser writing power range. In addition, the surface of modified Bi/In is found to be much smoother than the original Bi/In and Sn/In films, thus increasing the overall quality of grayscale photomask. Finally grayscale uniformity of the laser writing process has been investigated and techniques such as laser beam shaping and defocusing have been used successfully to eliminate the variations.

Adding grayscale layer to chrome photomasks

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.

Laser-induced oxidation of Zn and Zn alloy films for direct-write grayscale photomasks

Previous research showed that bimetallic Bi/In and Sn/In films exhibit good grayscale levels after laser exposure due to controlled film oxidation. While giving a large alteration in optical density (OD) from 3.0OD to 0.22OD at 365 nm, Bi/In and Sn/In films show a very nonlinear OD change with laser power, making fine control of grayscale writing difficult at some gray levels. This paper studies Zn and Zn alloy films as possible candidates for improved direct-write grayscale photomask applications. Zn and Zn alloys laser oxidation have been reported previously, but without grayscale optical measurements and applications. In this paper Zn films (50 nm ~ 240 nm), Sn/Zn (100 nm), Al/Zn (100 nm), Bi/Zn (100 nm) and In/Zn (100 nm) were DC- and RF-sputtered onto glass slides and then were scanned by argon ion CW laser (488 nm). Among these films, the highest OD change, 3OD (from 3.2OD before exposure to 0.2OD after laser exposure) at 365 nm, was found in the In/Zn (25/75 nm or 84at% Zn) film. The characterization of grayscale level to laser power modulation in Zn and Zn alloy films with various thickness or composition ratios were investigated. The Zn OD change versus laser power curve is more linear than those of Sn/In and Bi/In films. In/Zn films have better characterization of grayscale level versus laser writing power than pure Zn film. Among these four Zn alloy films, Zn/Al shows most linear relation of OD at 365 nm to laser power modulation.

Enhanced inorganic bimetallic thermal resists transparency and resolution for photomask fabrication

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.

Real-time optical characterization of laser oxidation process in bimetallic direct write gray scale photomasks

DC-sputtered Sn/In and Bi/In bimetallic thin films oxidize and turn transparent under laser exposure. The films transparency, or optical density (OD), changes smoothly with increasing laser power, from ~3.0OD (unexposed) to <0.22OD (fully exposed). Laser-induced oxidation of bimetallic films can be used to produce direct-write binary and analogue grayscale photomasks. In order to create high quality grayscale photomask and to improve our current laser writing process, requires real time measurement of OD values, and a greater understanding of the laser-induced oxidation process of the bimetallic thin films. An OD measurement system has been developed capable of providing real time optical density and exposure power changes for the bimetallic thin films. Three silicon-based PIN photodiodes were used to monitor the incident and transmitted beams powers, allowing us to measure the OD change as the film oxidizes, giving us a real time measure of the optical density changes of the bimetallic thin film. With this OD measurement system, real time OD data can be used to adjust the laser power to compensate any variations in laser output power, film characteristics and other variations in the laser writing system. Furthermore, with the incident beam focused, we can precisely measure OD level in areas as small as the laser spot size. Exposed Bi/In/O films show an immediate rapid -2OD/ms change in the first 0.5 ms of exposure. However, In/Sn and Zn films show an initial time delay before OD begins to change, and then a less rapid change of -0.56OD/ms or -0.32OD/ms respectively.

Bimetallic Thin Film Grayscale Photomasks for Complex 3D Microstructure Creation in SU-8

Metallic thin films can be fully oxidized by focused laser beams, resulting in their optical density (OD) changing from highly absorbing to very transparent. Previous research found the laser-induced partial oxidation process allows the creation of grayscale photomasks. This work investigates Sn/Zn, Zn/Al and In/Zn thin films, which are DC/RF-sputter deposited and then scanned by an argon ion CW laser. The resulting transmittance for the mask varies from 0.06% (3.2 OD) to 63% (0.2 OD), offering a greater range of transparency at I-line (365 nm) than existing commercial grayscale masks such as chrome halftone binary and analog HEBS photomasks. Having 8-bit gray level precision, bimetallic films are capable of producing complex 3D microstructures using only a single exposure. Using SU-8, a thick negative photoresist, along with standard photolithography, microfeatures with height variations up to 100 mum were successfully generated.

Improving the optical characteristics of bimetallic grayscale photomasks

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 photomasks gray level non-uniformities is the Gaussian nature of the laser beams 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.

Laser-induced oxidation of metallic thin films as a method for creating grayscale photomasks

Bimetallic Bi/In films demonstrate grayscale levels after exposed with different laser powers due to controlled film oxidation. Although large optical density (OD) change from 3.0 OD to 0.22 OD at 365 nm was observed, these films show a rapid and nonlinear OD change with laser power, which is not desirable for fine control of grayscale levels. This paper aims to explore and evaluate some new metal films as possible candidates for direct-write grayscale photomask applications. Sn/In, Al/Zn, Bi/In/O and Al/In films were DC-sputtered onto glass slides and then were raster-scanned by argon CW laser. Among these films, the highest OD change at 365nm was found in Sn/In film, Al/Zn shows the most linear relation of OD to laser power modulation, and Bi/In/O has the best over-all performance as a potential grayscale mask material. A grayscale test photomask of 16×16, 20μm squares over the full OD range was made using Bi/In/O and a test exposure created squares of different heights on regular photoresist. Interference lithography using 266nm DUV has been utilized to investigate the resolution limit of these bimetallic films, which can generate much finer structures. The true resolution limit of Bi/In should be at least less than 50nm.