A high fill-factor uncooled infrared detector with thermo-mechanical bimaterial structure

Abstract

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 SiO 2 : 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 SiO 2 , 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 SiO 2 (10µm × 2µm × 0.2µm), the device can have low thermal conductance of 1.3×10 -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 1~2K of temperature difference between the structure and the substrate.

Dual-Level Capacitive Micromachined Uncooled Thermal Detector

“…A focal plane array (FPA) with a high fill-factor is desired to achieve maximum utilization of the inbound infrared power. A method for enhancing the fill-factor of capacitive microcantilever-based thermal detectors was reported in [25], but despite their relative success in achieving better fill-factors than typically reported structures in the literature, the authors' approach was still limited by the previously explained trade-off, because their design was single-level based.…”

“…The higher the TEC mismatch, the greater the deflection will be. A typical choice of materials has been a silicon-based layer such as SiN, SiO 2 [25], or SiC [13] due to their low TEC and good mechanical properties, and a metal layer such as Gold (Au) [13] or Aluminum (Al) for their high TEC. Designing the thickness of the layers is not straightforward since it cannot be deduced intuitively from (4) due to its interlacing relationship with the material's Young's modulus.…”

MEMS thermal imager with optical readout

“…The position of the pixel can be read out either electronically (as capacitance) or optically. There are currently several groups working on such systems, most notably Oak Ridge [8,10], Multispectral Imaging (MII) [5,6], Agiltron [9], and various university groups in the USA [14][15][16][17][18][19] and Asia [20,21]. MII has focused on electrical readout, while Oak Ridge and Agiltron have focused on optical readout.…”