Vacuum Ultraviolet Radiation Effects in SiO2
Charging effects observed in MOS structures which have been exposed to sputtering plasmas or electron beam deposition suggest that Vacuum Ultraviolet (VUV) or soft X-radiation is important in producing these effects. Our experiments show that under positive gate bias VUV irradiation produces large positive charging effects for photon energies above 8.8 eV, the threshold for electron-hole pair creation in SiO2. This charging appears to be accompanied by an increase in interface state density. VUV radiation proves to be more useful than higher energy quanta or particles in studying radiation charging. This is true because one can control the depth of radiation absorption into the oxide. Etching experiments show that positive charge is induced near the Si-SiO2 interface even when radiation is absorbed near the gate electrode. This result is strong evidence in support of the hole transport and trapping model. We present evidence that under irradiation with positive bias, positive space charge is formed near both interfaces. We also show how a large positive space charge can be introduced into the oxide without a gate electrode.