Radiation Effects in N-Channel MNOS CCDs at Low Temperature

This paper describes the effects of ionizing radiation on charge-coupled-devices (CCDs) at 80°K. The CCDs were fabricated with an oxide/nitride (MNOS) dual dielectric gate insulator used for its excellent radiation tolerance at cryogenic temperatures. A novel hydrogen implantation process was employed to obtain improved CCD electrical performance through an extremely low density of interface states at the oxidesilicon interface. N-surface-channel and N-buried-channel MNOS CCDs have been fabricated and irradiated at 80°K. Both CCD types operate following 106 Rad(Si) exposure. Radiation-induced degradation in CCD charge transfer efficiency is much worse in the surface-channel CCD compared to the buried-channel CCD. A model involving formation of potential energy wells due to charge trapping in the insulators between polysilicon gates is proposed to explain this effect. Measurements of the interface state density in the CCDs and test capacitors show that generation of interface states by ionizing radiation is almost completely suppressed at 80°K.