Online Dark Count Rate Measurements in 150 nm CMOS SPADs Exposed to Low Neutron Fluxes

Arrays of single photon avalanche diodes (SPADs) fabricated in a 150 nm CMOS technology have been exposed to neutrons up to fluences of about 4.3 × 10^10 1 MeV neutron equivalent cm^−2, with fluxes around 3 × 10^6 1 MeV neutron equivalent cm^−2 s^−1. Dark count rate (DCR) was monitored during irradiation and for some time, from 5 to 23 min, depending on the irradiation step, at the end of the irradiation interval to investigate the dynamics of defect formation and short term annealing. Measurements were performed both on single and on dual-layer devices, where SPAD arrays are face to face bonded and read out in coincidence. A range of different DCR behaviors were detected after single neutron interaction with the device substrate, including in particular partial performance recovery following a logarithmic relaxation process, but also damped oscillation phenomena, sudden step-shaped changes, and the emergence of RTS-like fluctuations, pointing to different defect reordering dynamics.