Towards MEMS-based long-wavelength infrared tuneable Fabry-Perot filters

This paper describes the realization of LWIR (8-12 μm) Fabry-Perot filters on basis of our previously demonstrated SWIR (1.6-2.5 μm) or MWIR (3-5 μm) filter technique. An innovative filter design employing a single layer of Ge as the top mirror is proposed and optical modelling shows that the filter can potentially achieve spectral characteristics required by the LWIR spectral imaging applications. As a proof of concept, filters with fixed air cavity were fabricated and bowing in the suspended mirror was corrected by depositing a thin SiN
_x
stress compensation layer underneath the mirror. Transmission measurement was carried out on the filter exhibiting the least bowing, despite some spectral degradation compared to the theoretical model of the filer, showing not optimal but adequate transmission and bandwidth for multispectral imaging applications.