XPS analysis of ultrathin SiO₂ film growth on Si by ozone

We investigated the initial oxidation of Si(100) with ozone by comparison with oxidation by oxygen molecules using a specially fabricated ozone generator that could supply concentrated (typically 30 at.%) ozone gas at 1 atm pressure. We measured the thickness of the SiO₂ film on the Si by XPS, using a thermal oxide film with a known thickness as a reference. We observed the growth of an SiO₂ film on Si by ozone even at a sample temperature of 200 °C, verifying the strong oxidation power of ozone. The SiO₂ film was approximately 2.3 nm thick for 100 min of exposure. The kinetics of oxidation with ozone showed a power law dependence relative to exposure time, with almost the same power value for the ozone gas at 1 atm as at 2000 Pa. However, the growth of the SiO₂ film was saturated at the level of one monolayer when the ozone gas was supplied at 10^-4 Pa, which suggests to us a rapid decrease of the sticking (or reaction) probability of ozone on monolayer SiO₂.