Ge/ZnS-Based Micromachined Fabry–Perot Filters for Optical MEMS in the Longwave Infrared

This paper reports on the successful demonstration of Ge/ZnS-based Fabry-Perot filters operating in the longwave infrared (LWIR). The suitability of thermally deposited Ge and ZnS as thin-film mirror materials for micromachined LWIR Fabry-Perot filters has been fully investigated, and it is shown that a film growth temperature higher than 150 °C is key to depositing durable ZnS films. The optical constants of Ge and ZnS films in the LWIR band reveal that the material pair possesses high refractive index contrast and excellent LWIR transparency. Fixed-cavity LWIR Fabry-Perot filters with a 150-μm circular single-layer Ge top mirror and a four-layer Ge/ZnS/Ge/ZnS bottom mirror were fabricated. Curvature in the suspended top mirror was corrected using a thin SiNx stress-compensation layer. After curvature correction, a mirror flatness of 550 nm was achieved, and the filter demonstrated a 60% peak transmission with a full-width at half-maximum of 700 nm as well as a out-of-band rejection of 24:1.