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29 March 2006 Laser-induced oxidation of metallic thin films as a method for creating grayscale photomasks
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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.
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Glenn H. Chapman, Yuqiang Tu, Chinheng Choo, Jun Wang, David K. Poon, and Marian Chang "Laser-induced oxidation of metallic thin films as a method for creating grayscale photomasks", Proc. SPIE 6153, Advances in Resist Technology and Processing XXIII, 61534G (29 March 2006);

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