Design of Bi-material Pixels of an Uncooled Optic Readout IR FPA

Abstract

The analysis and optimized design of an uncooled optic reado ut infrared FPA pixel composed of infrared absorption layer SiNx and bi-material beam surface layer Au (also as visible light reflection layer) based on MEMS technology are presented in the paper. By modeling and simulation, the thickness of SiNx, Au and adhesive metal layer, and the thickness matching relationship of these materials are verified, and an optimized bi-material beam of Au\Cr\SiNx(0.29μm 5nm\0.4μm, 104μm in equivalent beam length) in the pixel has thermo-mechanical response as high as 2.573×10 -3 rad/K, and the visible light reflector in it remains flat enough. A thermal image system using an uncooled infrared focal plane array (IR FPA) as detector is a fast developing field, due to its small size, low power consumption, and low cost. The IR FPA based on Micro-Electro-Mechanical Systems (MEMS) technology is a key element of the system, directly affecting the thermal image quality. Although there are differ ent types of the IR FPAs used for uncooled optic readout (UOR) thermal image systems, the pixels of the various UOR IR FPAs generally consist of bi-material reflectors reflecting visible light and bi-material beams driving the reflectors to rotate when the pixels are radiated by IR. Bi-material structures based on MEMS technology usually composed of a layer of silicon-based thin film with high IR absorptivity, low coefficient of thermal expansion (CTE) and stress, formed by chemical vapour deposition and a layer of metal thin film with high CTE and reflectivity of visible light, by physical vapour deposition. If the two-layer films are not able to form a firm structure, an adhesive layer is needed. While the pixel operates, it is hoped that reflectors remain flatter and the bi-material beams have higher thermomechanical response. In the paper, the optimized design of an UOR IR FPA pixel composed of SiNx as infrared absorption layer, Au as bi-material beams surface layer and visible light reflection layer, and an adhesive metal layer between SiNx and Au are presented, with the help of the analysis of the relationship between the thickness of materials used for bi-material beam and the thermo-mechanical response of the pixel, the thickness of IR radiation absorption layer and the visible light reflection layer. 2. Films thickness and thermo-mechanical response