Infrared Vision Using an Uncooled Thermo-opto-mechanical Camera; Design, Microfabrication, and Performance

An uncooled infrared (IR) camera that is based on thermomechanical sensing and visible optical readout has been developed. The system contains a focal plane array (FPA) consisting of bimaterial cantilever beams made of silicon nitride (SiN_x) and gold (Au) in each pixel. Absorption of incident IR radiation in the 8-14 μm wavelength range by SiN_x in each cantilever beam raises its temperature, resulting in proportional deflection due to mismatch in thermal expansion of the two cantilever materials. To maximize the thermal performance, the conductance of each pixel was reduced to about five times of the radiation conductance. Based on thermomechanical analysis, the geometrical shape of the pixels were designed to maximize the cantilever sensitivity within the constraints of the pixel size and layout. Microfabrication of stress-balanced bimaterial cantilevers was achieved by varying the silicon concentration along the thickness of the SiN_x films in order to balance the residual tensile stress in the Au film and the Cr adhesion layer between Au and SiN_x. The optical design of each pixel was based on IR properties of the cantilever materials, IR absorption enhancement due to resonance cavity formation, as well as visible optics of deformable diffraction gratings. The latter formed the foundation for two different optical readout techniques that were both used for IR imaging. The results suggest that objects at temperatures as low as 30'C can be imaged with the best noise-equivalent temperature difference (NETD) in the range of 2-5 K. It is estimated that further improvements that are currently being pursued can improve NETD to about 10 mK.