Amorphous Silicon Microbolometer Technology
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Amorphous Silicon Microbolometer Technology A. J. Syllaios, T. R. Schimert, R. W. Gooch, W. L. McCardel, B. A. Ritchey, J. H. Tregilgas, Raytheon Electronic Systems Company, Dallas, TX 75266 ABSTRACT Highly sensitive hydrogenated amorphous silicon (a-Si:H) microbolometer arrays have been developed that take advantage of the high temperature coefficient of resistance (TCR) of aSi:H and its relatively high optical absorption coefficient. TCR is an important design parameter and depends on material properties such as doping concentration. Ultra-thin (~2000 Å) aSiNx:H/a-Si:H/ a-SiNx:H membranes with low thermal mass suspended over silicon readout integrated circuits are built using RF plasma enhanced chemical vapor deposition (PECVD) and surface micromachining techniques. The IR absorptance of the bolometer detectors is enhanced by using quarter-wave resonant cavity structures and thin-film metal absorber layers. To ensure high thermal isolation the microbolometer arrays are vacuum packaged using wafer level vacuum packaging. Imaging applications include a 120x160 a-Si:H bolometer pixel array IR camera operating at ambient temperature. Non-imaging applications are multi-channel detectors for gas sensing systems. INTRODUCTION In this paper we discuss the current status of a-Si:H for microbolometer infrared detector array technology. Amorphous silicon is an ideal material for detection of optical radiation from the UV-VIS to the IR spectral range. The operation of a-Si:H imagers with spectral sensitivity in the ultraviolet [1], visible [2] and near infrared [3] is based on radiation induced band to band or band to localized states electronic transitions. Detection is extended to long wavelength infrared by using a-Si thin films in microbolometer or thermal detector structures [4,5,6,7]. Amorphous silicon microbolometer detectors are being developed for low cost, IR imaging and sensing applications. The cost is greatly reduced both because they are manufactured using silicon fabrication compatible processes, and they operate at ambient temperature without active cooling. Recent developments with a-Si bolometers have demonstrated imager operation with no active temperature stabilization and without need of mechanical chopping for background correction [8]. Another recent development impacting the cost of a-Si:H infrared detector technology as well as other microelectromechanical systems (MEMS) technologies is the ability to use wafer level vacuum packaging [9]. This technology uses patterned solder sealing rings on both the device wafer and lid wafer for device encapsulation. This approach will significantly reduce packaging costs and has been applied to aSi bolometers. Collectively, the aforementioned developments will make a-Si bolometer technology an extremely competitive low cost IR sensing technology. a-Si MICROBOLOMETER STRUCTURE AND PROCESS An amorphous silicon microbolometer detector is an ultra-thin, low thermal mass suspended temperature-dependent resistive membrane structure supported by long thermal isolation le
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