Design and optimisation of a MEMS-based tunable Fabry-Pérot infrared filter

This study presents the theoretical design and analysis of a new surface micro-machined tuneable Fabry-Perot (FP) infrared filter for adaptive infrared photon detectors. The proposed structure uses broad spectral range, high reflectivity distributed Bragg reflector (DBR) mirrors based on amorphous Si/air layers, resulting in very high finesse filters. The device utilises three sacrificial layers to define the air-gap in the DBR mirrors, the resonant cavity spacer, and the separation of the top mirror from the supporting flexures. The flexures are fabricated from low-temperature plasma-enhanced chemical vapour deposited (PECVD) silicon nitride (SiN_x) and Au thin films. Separation of the top mirror from the supporting flexures allows for improved fill-factors (up to 79%), as well as increased tuning range. Optical and finite element modelling of the device has shown large wavelength tuning ranges are achievable (1.5-2.5 mum) at low actuation voltages (<15 V), whilst still maintaining high cavity finesse.