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, AI/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 365 nm 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 16x16, 20 mu m 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 266 nm 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 50 nm.