20 October 2006 Adding grayscale layer to chrome photomasks
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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 <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.
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David K. Poon, James M Dykes, Chinheng Choo, Jimmy T. K. Tsui, Jun Wang, Glenn H. Chapman, Yuqiang Tu, Patrick Reynolds, Andrew Zanzal, "Adding grayscale layer to chrome photomasks", Proc. SPIE 6349, Photomask Technology 2006, 634931 (20 October 2006); doi: 10.1117/12.686599; https://doi.org/10.1117/12.686599
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