Electron beam irradiation effects in thick-oxide MOS capacitors

MOS capacitors were fabricated on n-type 〈111〉 silicon wafers with aluminum contacts. The oxide thickness ranged from 1000Å to 12,000Å. These capacitors were irradiated with an electron beam at doses of 10
¹¹
to 10
¹⁷
electrons/cm
²
and energies of 5 kV and 15 kV. C-V measurements were made to characterize the effects of irradiation. In the thin oxide samples and in the thick oxide samples at 15 kV, the C-V curves revealed a positive fixed charge accumulation in the oxide at dose levels above 1013 electrons/cm
²
. 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 10
¹⁵
electrons/cm
²
, implying slow surface states which could follow the dc bias voltage but not the ac 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.