MEMS based tunable infrared sensors

A low temperature MEMS process integrated with an infrared detector technology has been developed. The integrated microsystem is capable of electrically selecting narrow wavelength bands in the range from 1.6 to 2.5 mu m within the short-wavelength infrared (SWIR) region of the electromagnetic spectrum. The integrated fabrication process is compatible with two-dimensional infrared focal plane array technology. The demonstration prototypes consist of both HgCdTe SWIR photoconductive as well as high density vertically integrated photodiode (HDVIP (R)) detectors, two distributed Bragg mirrors formed of Ge-SiO-Ge, an air-gap optical cavity, and a silicon nitride membrane for structural support. The tuning spectrum from fabricated MEMS filters on photoconductive detectors indicates a wide tuning range and high percentage transmission. Tuning is achieved with a voltage of only 7.5 V, and the FWHM ranged from 95105 nm over a tuning range of 2.2 mu m to 1.85 mu m. The same MEMS filters, though unreleased, and with the sacrificial layer within the optical cavity, have been fabricated on planarised SWIR HDVIP (R) photodiodes with FWHM of less than 60 nm centred at a wavelength of approximately 1.8 mu m. Finite element modelling of various geometries for the silicon nitride membrane will also be presented. The modelling is used to optimize the filter geometry in terms of fill factor, mirror displacement versus applied voltage, and membrane bowing.