A high fill-factor uncooled infrared detector with thermomechanical bimaterial structure

By adopting new capacitance reading scheme, a capacitive type uncooled infrared detector structure with high fill-factor and effectively controllable thermal conductance is proposed. Instead of conventional MEMS capacitor structure (i.e. an insulating gap between top and bottom electrodes), a capacitor with a floating electrode and two bottom electrodes has been applied to the infrared detector. Infrared absorber which also acts as the floating electrode of the capacitor is connected to the substrate via two bimaterial legs. These legs consist of two materials having large difference in thermal expansion coefficient (Al: 25ppm/K and SiO2: 0.35ppm/K), so that the legs are deflected according to the certain temperature change due to the infrared absorption. This leg's movement results in the displacement of the top electrode of the capacitor, and infrared is sensed by measuring the capacitance change. However, the one end tip of the bimaterial leg does not contain Al and consist of SiO2, solely. This leg design enables the absorber to be separated from the substrate thermally as well as electrically, because insulators usually have low thermal conductivity than metals more than an order. The capacitance change by the result of infrared absorption is read only through two bottom electrodes which are placed right under the absorber, and also perform as infrared reflectors. The design has advantages of enlarging fill-factor of the infrared detector, effective thermal conductance controlling and high sensitivity to IR. With only small dimensions of SiO2 (10 mu m x 2 mu m x 0.2 mu m), the device can have low thermal conductance of 1.3x 10(-7) W/K, so that the portion of the legs can be reduced in a pixel area. The device has fill-factor of 0.77 and 14%/K of sensitivity to infrared rays concerning 1 similar to 2K of temperature difference between the structure and the substrate.