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.

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