Vaccum ultraviolet irradiation sheds new light on radiation induced MOS device failure

Charging effects observed in a variety of MOS structures which have been exposed to ionizing radiation, such as that from sputtering plasmas or e-gun deposition, suggest that Vacuum Ultraviolet (VUV) or soft X-radiation is important in produciag these effects. Our experiments show that VUV irradiation of metal-SiO2-Si structures under positive gate bias produces large positive charging effects for hω ≳ 9 eV, the threshold for electron-hole pair creation in the SiO2. This charging seems to be accompanied by changes in interface state density. Etch-off experiments indicate that most of the positive charge resides near the two interfaces. Under negative gate bias, much smaller positive charging effects are observed for photon energies only near the SiO2absorption edge. The ability to control the absorption depth using VUV irradiation has enabled us to show that a model involving hole transport and trapping is consistent with the experimental observation. Therefore, while the hole mobility in SiO2may be quite small, it is nevertheless nonzero. Experimental results and interpretations including gate voltage, photon energy, and dose dependences will be presented.