A process for forming a thin film on a surface of a semiconductor device. The process involves formation of a silicon dioxide film by plasma enhanced thermal oxidation, employing a mixture of ozone and oxygen which are generated separately from the reactor chamber in a volume ratio of about 1-10/1, preferably about 5-7/1, at a temperature generally below 440.degree. C., preferably about 350.degree.-400.degree. C. The process is used to form sidewall oxide spacers on polysilicon gates for field effect transistors. A relatively fast oxidation rate is achieved at a temperature significantly below that employed in conventional oxidation processes, and this serves to reduce dopant diffusion from the polysilicon. In addition, the resulting film demonstrates low stress with good conformal step coverage of the polysilicon gates. Another use of the process is to grow thin gate oxides and oxide-nitride-oxide with a thickness of less than 100 .ANG.. An oxide film of uniform thickness is formed by controlling the temperature, RF power, exposure time and oxygen/ozone ratio for thin gate oxide (