Study On MEMS Integrated Room-temperature Infrared Detectors

In recent years, thermal imaging techniques based on micromachined room-temperature infrared detectors have been paid much attention in t1he fields of MEMS and optical imaging. In this dissertation,preparation and thermoelectric characterization of conventional silicon-based materials and devices have been studied, including amorphous silicon (a-Si) films, polycrystalline silicon germanium (polySiGe) films and amorphous silicon thin-film-transistors (a-Si TFT). A novel MEMS-IC integration method based on porous silicon micromachining has been developed, which is used to fabricate resistive bolometers with a-Si and polySiGe films, and a-Si TFT-based room-temperature infrared detector pixels and 8×8 arrays with primary capability of thermal imaging have been reported for the first time.Firstly, high-quality a-Si films are prepared using PECVD. Resistivity of a-Si film is well controlled by gas doping. PolySiGe films are deposited using UHVCVD with optimized thermal treatments. a-Si TFTs with good electrical characteristics are formed. To provide thermosensitive elements of high-performance room-temperature infrared detectors, resistivity-temperature characteristics of a-Si and polySiGe films and channel current-temperature characteristics of a-Si TFT are tested and analyzed.Secondly, to selectively prepare porous silicon in middle resistivity silicon substrates, growth methods of porous silicon are systematically studied. Integration approach based on porous silicon sacrificial layer techniques is developed to integrate MEMS with ICs. Protection methods for pre-formed porous silicon are proposed with a composite membrane consisting of Si3N4 and SiO2 films. Effects of key process steps in MOS fabrication on characteristics of porous silicon are investigated.Finally, based on theoretical analysis and structure designs of a-Si and polySiGe bolometers and TFT-based room-temperature infrared detectors, layouts and process-flow designs are performed. The a-Si and polySiGe bolometers with various structures and dimensions are fabricated using the MEMS-IC integration processes proposed in this dissertation. For the first time, the a-Si TFT-based room-temperature infrared detectors and arrays are demonstrated. Several fabrication runs are conducted to increase the yield. To characterize the devices with figure-of-merits of voltage sensitivity, thermal response time constant, power spectrum density of noise, detectivity and 2-dimensional array uniformity, platforms for measuring blackbody radiation response and thermal imaging are set up. The results show that the maximum detectivities of a-Si and polySiGe bolometers are 2.23×108 and 3.75×108 cmHz1/2W-1 respectively, and TFT-based room-temperature infrared detectors have a maximum detectivity of 1.02×109cmHz1/2W-1, which reaches the same highest level in performances in the world. Primary thermal imaging tests of 8×8 TFT-based arrays are carried out, indicating the array has a capability to construct thermal images.