Vertically Integrated Amorphous Silicon Particle Sensors
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Vertically Integrated Amorphous Silicon Particle Sensors N. Wyrsch1, C. Miazza1, S. Dunand1, A. Shah1, D. Moraes2, G. Anelli2, M. Despeisse2, P. Jarron2, G. Dissertori3, G. Viertel3, 1 Institut de Microtechnique, Université de Neuchâtel, Breguet 2, CH-2000 Neuchâtel, Switzerland, 2 CERN, CH-1211 Genève 23, Switzerland, 3 Institute for Particle Physics, ETH-Zurich, CH-8093 Zurich, Switzerland. ABSTRACT Vertically integrated particle sensors have been developed using thin-film on ASIC technology. Hydrogenated amorphous silicon n-i-p diodes have been optimized for particle detection. These devices were first deposited on glass substrates to optimize the material properties and the dark current of very thick diodes (with thickness up to 50 µm). Corresponding diodes were later directly deposited on two types of CMOS readout chips. These vertically integrated particle sensors were tested in beta particle beam from 63Ni and 90Sr sources. Detection of single low- and high- energy beta particle was achieved. INTRODUCTION Large-area X-ray imagers based on hydrogenated amorphous silicon (a-Si:H) are gaining considerable attention for medical applications [1]. In these imagers, a-Si:H is used for both the photodiode array detecting the light generated by the scintillating layer (used for the conversion of the X-ray photons into visible light), and for the thin-film transistors addressing the individual photodiodes. The possibility of depositing a-Si:H on large areas (up to several square meters) in a cost-effective way and the relatively high band gap of a-Si:H are, in this context, interesting material characteristics. As a-Si:H is also known to be a radiation hard material [2, 3], it has attracted significant attention in high-energy physics, and several groups have indeed demonstrated that particle detection can be realized with thick a-Si:H diodes [4, 5, 6]. However, single-particle detection at the minimum ionizing energy (MIP) has not yet been fully demonstrated. This lack of success is mainly due to the fact that very thick diodes are necessary in order to generate enough electron-hole pairs. The fabrication of such thick diodes able to sustain high enough electrical field in order to keep a fully depleted state condition over the entire device thickness is a technological challenge. Active pixel sensors in CMOS technology are widely used for many imaging applications. However, the die surface used for the pixel electronics restricts the area devoted to the photodiode and, thus limits the overall sensitivity of the imager. In order to solve this intrinsic limitation, a vertical integration of the photodiode array on the CMOS readout electronics has been proposed. This method is called thin-film on ASIC (TFA) technology (the terms “thin-film on CMOS” (TFC) or “above IC” technology are also used) and it offers an interesting potential for high-sensitivity, low-level and high-dynamic imaging [7, 8]. In high-energy physics and for some medical applications (for example chest radiography), large-area pixel detectors
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