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
$4.3 \times 10^{10}~1$
MeV neutron equivalent cm
$^{-2}$
, with fluxes around
$3 \times 10^{6}~1$
MeV neutron equivalent cm
$^{-2}\text{s}^{-1}$
. 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.