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

Abstract

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 (BaF 2 ) 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-BaF 2 -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 BaF 2 material. Ge-Air-Ge 3-layer air-gap DBRs, in which air rather than BaF 2 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.

Large-area narrowband Fabry–Pérot interferometers for long-wavelength infrared spectral sensing

“…A detailed study of the optical and mechanical parameters of Ge and BaF 2 thin films was presented in our previous work. 34,35 The optical cavity length between the two DBRs was defined by a 5-μm-thick polyimide PI2611 sacrificial layer, which determines the wavelength of the transmission peak. The square top DBR was perforated with an array of etch holes to facilitate the release process as well as diagonally arranged notches in the four corners to minimize the impact of excessive stress arising from lateral orthogonal directions.…”

“…The deposition process and related internal stress of stand-alone Ge and BaF 2 thin films have been presented in a previous study. 34…”

“…The optical and mechanical properties of these materials have already been presented in our previous studies. 34,35 This paper reviews the modeling framework for determining the optical properties of an ideal FPI, followed by a study of the effect of fabrication-induced imperfections. Subsequently, a reliable and reproducible fabrication process is presented for surface micromachined fixed cavity LWIR filters based on a free-standing top DBR utilizing a three-layer structure of Ge∕BaF 2 ∕Ge optical thin films.…”