Radiation-Induced Increase of Mobile Sodium in MOS Capacitors

MOS capacitors fabricated from commercially-grown gate and field oxides were examined for sodium ion concentrations before and after radiation using the high-temperature voltage ramp technique. The gate oxides were found to have much lower sodium ion concentrations than the field oxides. Mobile sodium levels at 295°C in both gate and field oxides increased as a result of exposure to 1 MRad (SiO2) Co60 gamma radiation. During prolonged exposure to the high-temperature ramp, a further increase in the amount of mobile sodium was detected. A temperature-bias stress experiment on irradiated and unirradiated type 4007 integrated circuits showed much larger numbers of failures in irradiated devices which had received temperature-bias stress compared with those which received stress only or radiation only.