Ge/BaF₂ thin-films for surface micromachined mid-wave and long-wave infrared reflectors

High performance distributed Bragg reflectors (DBRs) are key elements to achieving high finesse MEMS-based Fabry–Pérot interferometers (FPIs). Suitable mechanical parameters combined with high contrast between the refractive indices of the constituent optical materials are the main requirements. In this paper, Germanium (Ge) and barium fluoride (BaF₂) optical thin-films have been investigated for mid-wave infrared (MWIR) and long-wave infrared (LWIR) filter applications. Thin-film deposition and fabrication processes were optimised to achieve mechanical and optical properties that provide flat suspended structures with uniform thickness and maximum reflectivity. Ge-BaF₂-Ge 3-layer solid-material DBRs have been fabricated that matched the predicted simulation performance, although a degradation in performance was observed for wavelengths beyond 10  μm that is associated with optical absorption in the BaF₂ material. Ge-Air-Ge 3-layer air-gap DBRs, in which air rather than BaF₂ served as the low refractive index layer, were realized to exhibit layer flatness at the level of 10 to 20 nm across lateral DBR dimensions of several hundred micrometers. Measured DBR reflectance was found to be ≳90  %   over the entire wavelength range of the MWIR band and for the LWIR band up to a wavelength of 11  μm. Simulations based on the measured DBR reflectance indicates that MEMS-based FPIs are able to achieve a peak transmission of ≳90 %   over the entire MWIR band and up to 10  μm in the LWIR band, with a corresponding spectral passband of ≲50  nm in the MWIR and <80  nm in the LWIR.