High Resolution Photomasks with Ion‐Bombarded Polymethyl Methacrylate Masking Medium

Abstract

A new method for fabricating high resolution, hard‐surface photomasks is proposed and investigated experimentally. The masks are prepared by first patterning a PMMA film deposited on a clear photomask plate by electron beam or deep u.v. exposure and then implanting 28Si+ ions until the PMMA film is converted to an opaque and hard masking medium. Results show that resolution is limited by the patterning process. Submicrometer‐size features are resolved in the masks and they were used to transfer micrometer‐size patterns to an layer on a silicon wafer. Optical properties are suitable for both near u.v. (300–400 nm) and deep u.v. (200–250 nm) exposures. Mechanically, the films are well adhered to the substrate and the masks are more abrasion resistant than chromium masks. They resist chemical attack by acids, bases, and organic solvents, but are readily removed in an oxygen plasma. The ion‐bombarded PMMA film composition is mainly carbon in a highly cross‐linked structure.