Electron beam irradiation effects in thick-oxide MOS capacitors

The capacitors were irradiated with an electron beam at doses of 10¹¹ to 10¹⁷ electrons/cm² and energies of 5 kV and 15 kV. C-V measurements were made to characterise the effects of irradiation. The C-V curves revealed a positive fixed charge accumulation in the oxide at dose levels above 10¹³ electrons/cm². In the thicker oxide samples at 5 kV there was evidence of negative charge trapping at the lower dose levels changing to positive at the highest dose. In all cases, there was a reversal in the sense of the hysteresis effects in the C-V curves above 10¹⁵ electrons/cm², implying slow surface states which could follow the d.c. bias voltage but not the a.c. test signal. There was also a marked increase in the magnitude of the turn-on voltage for strong inversion implying an increase in fast surface states or lateral ionic charge nonuniformities.