Effects of Metallic Doping on Ionization Damage in MOS FETS

Detailed studies of radiation damage in metaloxide-semiconductor (MOS) transistors show that the damage due to ionizing radiation is substantially different if chromium is added to the gate oxide. Devices with chromium consistently show smaller gate threshold shifts (due to positive charge storage in the gate oxide) when irradiated with zero or negative gate bias; they also show smaller increases in interface state density. The generation of interface states by radiation is also bias dependent in chromium MOS devices. Another difference between devices with conventional and chromium-doped oxides is the lower rate of thermal annealing when chromium is used. An extended anneal at 350°C is required to remove radiation damage from these devices. A considerable degree of control over ionization damage is thus obtained by chromium doping, and it appears this control can be used to improve radiation hardness of MOS transistors.