Gamma and vacuum ultraviolet irradiations of ion implanted SiO2 for MOS dielectrics

Electrical measurements are described of grown silicon dioxide thin films irradiated by gamma rays, X-rays and broadband VUV light. The dependence of radiation-sensitivity on processing variations, including annealing temperature, is compared for unimplanted SiO2 and SiO2 implanted with aluminium, argon, xenon and cerium. The flatband voltage shift is suppressed under positive irradiation bias by each of these chemically disparate ions. This finding suggests that the implantation of ions at energies of several kilovolts, with fluences of 10¹⁴ to 10¹⁵ cm⁻², improves hardness by the production of similar distributions of electron traps with densities of the order of 10¹² traps cm⁻² by lattice damage and not by the direct combination of the ion with intrinsic or radiation-induced hole traps. This is supported by observations that the effect anneals out gradually at temperatures between 300°C and 1000°C and that the same traps can be neutralised by injection of photoelectrons into the oxide. It is thus confirmed that ion implantation into the dielectric is a versatile method for producing radiation-tolerance under positive bias in silicon dioxide. A further finding is that broadband VUV light from a H2 discharge lamp can conveniently and closely simulate the radiation response caused by γ and X-rays in a wide range of dielectrics for MOS devices.