Bimetallic Thin Film Grayscale Photomasks for Complex 3D Microstructure Creation in SU-8

Metallic thin films can be fully oxidized by focused laser beams, resulting in their optical density (OD) changing from highly absorbing to very transparent. Previous research found the laser-induced partial oxidation process allows the creation of grayscale photomasks. This work investigates Sn/Zn, Zn/Al and In/Zn thin films, which are DC/RF-sputter deposited and then scanned by an argon ion CW laser. The resulting transmittance for the mask varies from 0.06% (3.2 OD) to 63% (0.2 OD), offering a greater range of transparency at I-line (365 nm) than existing commercial grayscale masks such as chrome halftone binary and analog HEBS photomasks. Having 8-bit gray level precision, bimetallic films are capable of producing complex 3D microstructures using only a single exposure. Using SU-8, a thick negative photoresist, along with standard photolithography, microfeatures with height variations up to 100 mum were successfully generated.