Study of Proton-Induced Defects in 40-nm CMOS SPADs

Defects induced by 62 MeV protons in single-photon avalanche diodes (SPADs) are studied. A study of the dark count rate (DCR) variation after irradiations for different biasing conditions underlines the predominance of field enhancement effects such as Poole–Frenkel (PF) effect and phonon-assisted tunneling (PAT) over band-to-band tunneling (BTBT). Activation energy measurements below mid-gap value confirm the important contribution of these field effects. However, unexpected behaviors in these energies are seen: very low activation energies are extracted for SPADs with a DCR induced around 103 counts per second (cps). We attribute them to the contribution of different groups of defects, with different energy-level distribution. This analysis is completed with an annealing study between 100 °C and 300 °C. Results for high DCR SPADs correlate well with the presence of defect clusters. Moreover, histogram of the annealing temperature at which the DCR starts to drop displays two peaks at 170 °C and 250 °C. The first one is attributed to phosphorus vacancies and complex cluster of defects with an energy distribution inside the Si bandgap centered above mid-gap. The second one is attributed to clusters of divacancies that also anneal at temperatures around 250 °C.