Effects of X-Ray and γ-Ray Irradiations on 2-D-Planar and 3-D-Stacked CMOS SPADs

X-ray and
$\gamma $
-ray irradiations are performed on frontside-illuminated (FSI) 2-D-planar and backside-illuminated (BSI) 3-D-stacked CMOS single-photon avalanche diodes (SPADs). Different degradations are observed for nearly the same deposited total ionizing dose (TID). The 2-D SPADs are only slightly degraded, with a dark count rate (DCR), which increases after 320 krad(SiO2) with X-rays, while the DCR spectrum after 300 krad(SiO2) with
$\gamma $
-rays is spread toward high DCR (3 to 4 decade increase). As these SPADs are less sensitive to ionizing radiation effects, this suggests a strong domination of the displacement damage contribution of
$\gamma $
-rays due to secondary electrons induced by Compton scattering, leading to point defects inside the silicon bulk. A large DCR tail in the distribution is also observed with 3-D-stacked SPAD pixels, but the histogram peak is close to the one caused by X-rays. This suggests that 3-D-stacked SPADs are more sensitive to ionizing radiation due to a larger Si/SiO2 interface area that can be degraded and a designed electric field imposed by the BSI configuration that maximizes charge collections in the avalanche region. Activation energy measurements confirm this result, by comparing data with a 62-MeV proton irradiation. The DCR alteration dependence with the SPAD voltage during X-ray irradiations is also studied, with an increasing degradation with the SPAD bias. Then, the random telegraph signal (RTS) behavior of the DCR is studied: while X-rays do not induced DCR fluctuations, different RTS characteristics are observed between the two technologies, suggesting a major role of point defects. Comparisons with proton irradiations and an annealing study strengthen this assumption.