Amorphous Silicon Based Particle Detectors
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Amorphous Silicon Based Particle Detectors N. Wyrsch1, A. Franco1, Y. Riesen1, M. Despeisse1, S. Dunand1, F. Powolny2, P. Jarron2 and C. Ballif1 1 Ecole Polytechnique Fédérale de Lausanne (EPFL), Institute of Microengineering (IMT), 2 Photovoltaics and thin film electronics laboratory, Breguet 2, 2000 Neuchâtel, Switzerland. CERN, CERN Meyrin, 1211 Genève 23, Switzerland. ABSTRACT Radiation hard monolithic particle sensors can be fabricated by a vertical integration of amorphous silicon particle sensors on top of CMOS readout chip. Two types of such particle sensors are presented here using either thick diodes or microchannel plates. The first type based on amorphous silicon diodes exhibits high spatial resolution due to the short lateral carrier collection. Combination of an amorphous silicon thick diode with microstrip detector geometries permits to achieve micrometer spatial resolution beneficial for high accuracy beam positioning. Microchannel plates based on amorphous silicon were successfully fabricated and multiplication of electrons was observed. This material may solve some of the problems related to conventional microchannel devices. Issues, potential and limits of these detectors are presented and discussed. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) exhibits two advantages for applications as particle sensors: it is one of the most radiation hard semiconductors [1] and can be deposited as thin layers on various types of substrates over large areas. For these reasons this material has attracted much attention for direct and indirect (using a scintillating layer) particle detection [2]. Using a direct detection scheme, various types of particles such as protons, neutron, electrons or X-ray were successfully detected using thick a-Si:H diode [3,4,5]. Due to the low interaction of minimum ionizing particle (MIP) with silicon (due to the low atomic mass of this material) very thick diodes are necessary. Given the high voltage needed to fully deplete the device, such thick diodes are challenging in terms of the fabrication process and of the quality the material. While a-Si:H has been successfully implemented in indirect particle detectors for X-ray radiography [6], a Si:H detector has never been practically used for direct particle detection in a physics experiment or in a commercial product. Insufficient performance, failure to detect single MIP, metastability of a-Si:H and fabrication issues are some of the reasons for this lack of success. Nevertheless, as demonstrated here, a-Si:H thick diodes can be improved for MIP detection and selected applications could greatly benefit from present a-Si:H based detectors. Vertical integration of a thick a Si:H diode array on top of a readout ASIC (so-called “thin-film on ASIC” or “TFA” technology) (see Fig. 1) enables the fabrication of very radiation hard detector and may help achieving single MIP detection [7]. Using microstrip pixel geometries, very high spatial resolution can be obtained that are comparable or superior to the ones of stateof-the-art c-S
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