Formula omitted] thin Low Gain Avalanche Detectors (LGAD) for timing applications

Abstract

LGAD detectors on 300μm thick high resistivity p-type substrates were proposed for the first time by IMB-CNM-CSIC. They are customized Avalanche Photodiodes (APD) to obtain a high electric field region confined close to the reversed junction. Therefore, only electrons generated by an incident particle passing through the detector and drifting to the n+ contact, start the impact ionization process. Thus, the collected charge is multiplied. The basic difference between APDs and LGADs is the gain. LGADs have a moderate gain in order to avoid the inherent problems due to high multiplication: cross talk and high noise. In that way, the detector signal can be kept high without increasing the noise.These devices have been successfully fabricated and extensively characterized, before and after irradiation. Unfortunately, neutron and proton radiation cause the degradation of the gain and the creation of bulk traps, degrading the timing resolution. One way to reduce the radiation induced degradation is to minimize the substrate thickness, thus improving the timing resolution of LGAD detectors.Two technology approaches have been contemplated: the use of SOI (Silicon on insulator) substrates and Silicon to Silicon bonding substrates, both with a very thin active silicon layer of 50μm. As a consequence, drifting distances of generated electrons and holes are significantly reduced, resulting in a decrease in the number of electrons and holes trapped by radiation induced bulk defects.A new family of thin detectors, produced in 2×2 arrays prototypes, for the ATLAS experiment High Granularity Timing Detector (HGTD) is proposed. These detectors are suitable for timing applications with time resolution in the range of 30 ps at 20 °C. Optimization of the LGAD structures for the HGTD experiment and the detector experimental performances are presented and discussed.