Mechanisms of Charge Buildup in MOS Insulators

The radiation sensitivity of MOS devices has been recognized to be the result of a charge buildup caused by the sweepout of electrons and the trapping of holes following hole-electron pair production by ionizing radiation. Holes have been shown to have a finite, although small, mobility in thermally grown SiO2. Trapping of holes takes place near the Si-SiO2 interface, possibly by oxygen vacancies in the oxide. Normal thermally grown SiO2 possesses only small concentrations of electron traps. Electron traps have been shown to be generated, however, in the oxide by ion implantation, by irradiation with nonpenetrating electrons, and by exposure of the surface of the oxide to negative ions from a corona discharge. Although Na+ and Li+ ions have been shown to be mobile at room temperature in SiO2, contamination can be kept to levels where ionic charge buildup is negligible. The role of contaminants in the formation of hole traps, however, remains to be determined.