Design and simulation of an uncooled double-cantilever microbolometer with the potential for ∼mK NETD

Abstract

A new concept for uncooled infrared (IR) imaging with a double-cantilever microbolometer structure has been proposed. The approach has the potential of reaching a noise equivalent temperature difference (NETD) in the millikelvin range. The proposed microbolometer architecture comprises a thermally sensitive bimaterial element that controls the position of two capacitor plates coupled to the input of a low noise MOS amplifier. The top and the bottom plates of the sensing capacitor are composed of two overlapped free-standing bimaterial cantilever beams. The bimaterial element with maximum difference in thermal expansion coefficients will convert heat into mechanical movement. The orientation of the bimaterials for the top and the bottom cantilever beams is such that the metal layer on the bottom cantilever beam faces the metal layer on the top cantilever beam. IR irradiation will result in a deflection of the double-cantilever beams in opposite directions, enabling a large change in the sensing capacitor. Residual stress and curvature in each layer can be modified by an ion implantation technique. The advantages of the pixel design include extremely high sensitivity and low noise. In addition, the absorption of incident infrared irradiation can be enhanced dramatically by the aid of a resonant cavity configuration between the top and bottom plates. The design and manufacture of double-cantilever microbolometers are very flexible since the resonant cavity and the sensing capacitor gap are realized using two separate sacrificial layers. The effects of sensing capacitor gap distance, leg length, and bimaterial thickness on the microbolometer performance have also been investigated.