The UV-excited chlorine radical treatment effectively removed trace metal contaminants from Si ans SiO2 surfaces. We have found that this is predomonantly attributed to reactions between metal and chlorine, and metal oxides and silicon chloride, respectively. The former process was subjected to identify a breakdown spot on a thin gate dielectric film. After dielectric breakdown, a silicon crystallized filament as small as a few nanometers in diameter was formed and was selectively etched with chlorine radicals revealing a successively-etched region of the Si substrate. Nonuniformities of native exides on silicon were also detected using the same method. Gettering and etching wi:h a poly Si layer showed a possibility to make a gate dielectric film damage free and contamination free. Densification of native oxides formed after conventional wet cleaning of silicon was made with UV-excited oxygen radicals. Electrical characteristics of gate dielectrics treated with UV-excited chlorine and oxygen radicals showed high reliability.