Total-Ionizing Dose Effects on Charge Transfer Efficiency and Image Lag in Pinned Photodiode CMOS Image Sensors

Abstract

The total ionizing dose effects on image lag in pinned photodiode CMOS image sensors are investigated thanks to various device variants in order to isolate the major radiation induced effects on the charge transfer. It is shown that the main cause of the charge transfer degradation is the radiation induced defects generation in the pre-metal dielectric and in the transfer gate spacer vicinity which modifies the potential diagram at the photodiode/transfer gate interface by the creation of a potential pocket retaining the electrons that are not transferred. For 0.1 kGy(SiO2) < TID < 5 kGy(SiO2) the potential pocket degrades the pixel which exhibits very good lag performances, whereas it improves the high lag pixels by creating a speed-up implant enhancing the electron transfer or by reducing the spill-back. For TID > 5 kGy(SiO2) the defects generated in the pre-metal dielectric influence the whole photodiode potential inducing a pinning voltage increase and degrading the charge transfer by enlarging the potential pocket effect which becomes the main image lag source. The reported results clarify the impact of ionizing radiation on the charge transfer suggesting radiation hardened by design solutions for future space or nuclear applications.