(A) capacitive thermo-mechanical infrared detector using high thermal isolatable bimaterial legs

Abstract

In this thesis, a capacitive infrared detector using bimaterial effect has been investigated. Comparing with the conventional microbolometers, it has superior characteristics of low noise, and high sensitivity, resulting in very low NETD. The detector is consisted of three parts: infrared detector (sensitive layer), bimaterial legs and bottom electrodes. Absorber has insulator-metal-insulator structure $(SiO_2-Ti-SiO_2)$, and Ti acts as a top electrode of a variable capacitor composed of two bottom electrodes and top electrode (infrared absorber) which is separated electrically and mechanically from the substrate. Bimaterial legs are composed of $SiO_2$ and AL, and post the absorber to the substrate. They are designed to have electrical insulation, low thermal conductance and temperature dependent deformation characteristics. Heat energy from the absorbed infrared rays is transferred to the bimaterial legs, resulting in leg bending. At the same time, top floating electorode (Ti) is also lifted like the legs, and capacitance change of the variable capacitor will be read to detect infrared. The device is designed to have bimaterial leg bending rate of 0.187μm/K, and capacitance change rate of 15.4%/K (with in $5^\circ C$ of temperature change). This capacitance change rate (TCC) is 3 to 6 times greater than the TCR of amorphous silicon or vanadium oxide microbolometers. Calculated NETD of 21.2mK indicates the superior performance of the device. Fabricated device showed the bimaterial leg bending rate of 0.22μm/K, and capacitance change rate of 11.2%/K which are quite closed to the values we expected.