Suspended Large-Area MEMS-Based Optical Filters for Multispectral Shortwave Infrared Imaging Applications

We present the design, fabrication, and optical and mechanical characterization of silicon-/silicon-oxide-based optical filters and distributed Bragg reflectors in sizes ranging from 500 μm × 500 μm to 5 mm × 5 mm. They are designed to be used in conjunction with either single-element photodetectors or large-area focal plane arrays to realize tunable multispectral sensors or adaptive focal plane arrays in the shortwave infrared wavelength range. Surface optical profile measurements indicate a flatness of the order of 30 nm in the fabricated structures across several millimeters. Single-point spectral measurements on devices show an excellent agreement with simulated optical models, and demonstrate Si-SiO_x-Si fixed optical filters with a 94% transmission at 1940 nm with a full-width at half-maximum of 250 nm. Distributed Bragg reflectors demonstrate 90% reflectance across the 1560-2050-nm wavelength range, making them suitable as broadband reflectors. The optical spatial uniformity across a 3-mm × 3-mm device shows only a 3% variation across the entire optically active area. Finally, the mechanical resonance characteristic of a 1-mm x 1-mm fabricated device shows the lowest resonant frequency of the suspended structure to be 39 kHz, indicating excellent immunity to extraneous low-frequency vibrations.