Publication
The proposed next generation of particle physics experiments utilizing ring imaging Cherenkov detectors (RICH) will require photodetectors with 100 ps timing, mm2 granularity, large active area and the ability to operate in increasingly harsh environments. Due to increased luminosity, any detector will be required to demonstrate a neutron radiation hardness of order 1013 neq/cm2 by the end of the experimental run time. Digital silicon photomultipliers (dSiPMs), based on single-photon avalanche diodes (SPADs), have emerged as an attractive proposition for RICH detectors due to their high timing precision and architectural flexibility. Digital implementations in standard CMOS allow the introduction of advanced functionalities directly on chip, including gating, pixel masking and precise timestamping. Neutron radiation hardness, however, remains an ongoing challenge, as irradiation results in an increased dark count rate (DCR), proportional to the fluence. Reduction of the SPAD active volume lowers the increase in DCR for a given dose, so the addition of microlens structures to recover fill factor on small-volume SPADs promises an encouraging route towards radiation-hard dSiPMs. In this work we present a study of multiple SPAD designs, in a variety of technology nodes, to determine the optimal configuration for neutron radiation hardness. We report on testing of optical microlens structures in multiple materials, to determine the effects of neutron radiation up to 1014 neq/cm2