Development of hydrogenated amorphous silicon thin film transistor pixelized SU-8 micro-well detectors for gamma ray and x-ray telescope

This thesis reports the integration of hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFTs) with photolithographically defined micro-wells on polymeric substrates. The goal of the integration is a pixelized micro-well gamma ray detector, but it also serves as an example for low-temperature and MEMS-like processing on nontraditional substrates.; a-Si:H TFTs fabricated on glass substrates are widely used in large area electronic application such as flat panel displays and imagers. In this work a standard tri-layer a-Si:H TFT process was modified to allow the use of flexible polyimide substrates.; Micro-wells are attractive as detectors for a range of high-energy particles and passive micro-well arrays fabricated using laser ablation have been reported. To provide improved compatibility with underlying thin film electronics a photolithographic micro well process was developed. SU-8, an epoxy-based photoresist, and optical lithography were used to fabricate deep (>100 micron) micro-wells and a dry-film photoresist “tent” process was developed to allow electrode patterning on three-dimensional structures. By carefully controlling the post-exposure bake to minimize feature displacement a process compatible with dense micro-well arrays was developed.; Because the micro-wells use very large electric fields for charge multiplication by ionization in a gas ambient, it is necessary to provide electric field isolation for the underlying TFT. Simply placing the TFT under the pixel micro-well anode provides a first level of isolation, but anode voltage changes (which bear the pixel detection information) can cause charge leakage when the anode acts as the gate electrode of a parasitic TFT. Several approaches to reduce or eliminate this effect were developed, including a thick dielectric approach, a double-gate TFT, and detector operation modifications.; Finally, routes to alternative substrates for new pixelized micro-well detectors were investigated. The use of silicon nitride membranes for a high-pixel-density detector was considered. Such detectors may be of particular interest for X-ray polarimeters and can provide very low substrate scattering. Ultra-low temperature a-Si:H processing, suitable for low glass transition temperature polymeric substrates was also studied. TFTs fabricated with a-Si:H deposited at 130°C have good performance but gate dielectric deposition at temperature below 200°C is more problematic.; In summary, a polymeric substrate compatible pixelized micro-well detector process was developed and routes to alternative detectors were studied. Although the research focused on micro-well detector applications, the processes developed will also be useful for a range of polymeric substrate TFT and MEMS-like applications.