Fabrication of an ozone jet generator system and its application to the formation of ultra-thin silicon dioxide film was discussed. Superior features of using ozone and/or atomic oxygen in an oxidation process is highlighted through the comparison of reactivity among various oxidizing reagent, and also through the survey of their applications to SiO₂ formation. Then the characteristics of the new ozone jet generator, which can supply high purity ozone flux and was specially designed to overcome the current problems in handling high purity ozone, was described in detail. Finally, the oxidation mechanism of Si with ozone, which was investigated by XPS and SHG methods, was discussed focusing on the role of atomic oxygen generated by the dissociation of ozone on silicon surface.