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

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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
Page 1 and 2: Adding Grayscale Layer to Chrome Ph
Page 3 and 4: 3. FILM DEPOSITION The first step i
Page 5 and 6: underlying chrome patterns, and wri
Page 7 and 8: Figure 11. Test square structures w
Page 9: Figure 19. Transient time response

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