Radiation Effects in Modified Oxide Insulators in MOS Structures

Radiation-induced permanent shifts in gate threshold voltages were measured for MOS structures with variations in the structure of the oxide insulator and compared to the shifts for pure unaltered silicon dioxide. The variations include phosphosilicate glass coating of the oxide surface, aluminum ion-implantation into the surface, and a silicon nitride layer over the oxide. In all cases, the modified form became relatively insensitive to radiation under positive gate voltages and extremely sensitive to radiation under negative gate voltages in contradistinction to the pure oxide, which is much less sensitive at negative than at positive gate voltages. Only the nitride modification led eventually to hardened devices for both polarities. Analysis leads to the conclusion that the models for positive space charge buildup in the oxide must include photoemission of electrons into the oxide from either the silicon or the metal gate, depending on the gate voltage, and also the possibility of mobile holes.