Response of MNOS Capacitors to Ionizing Radiation at 80°k

MNOS capacitors with oxide thicknesses 85Å-600Å and silicon nitride thicknesses 200-2000Å have been irradiated with 2 MeV electrons at 80°K. Measured flatband shifts are found to depend on both polarity and magnitude of the applied field, oxide thickness, nitride thickness, and variations in device processing. For negative gate bias and effective applied fields 1-2x106 V/Cm, ΔVFB is independent of device processing and magnitude of the applied field. For these bias conditions, it is shown that flatband shifts in all MNOS samples may be explained by considering only generation and trapping of holes in the oxide. The holes travel a mean free path of 125± 25Å in the oxide before being trapped. For positive gate bias, electrons generated in the oxide are trapped at the oxide-nitride interface and/or in the bulk of the nitride, compensating the effect of the positively charged trapped holes in the oxide, and producing a relatively smaller ΔVFB for positive bias. The electron trapping process is considerably processing dependent. For high effective applied fields exceeding ± 2×106 V/cm, a strongly field-dependent mechanism of charge generation in the gate insulator is observed.