Enhanced oxygen exchange near the oxide/silicon interface during silicon thermal oxidation

The effect of the SiO2/Si interface on oxygen exchange diffusion during oxidation was investigated using oxygen isotopes. A 40-nm thick Si18O2 layer was first grown in 18O2 and then the sample was reoxidized in 16O2 at 1100 deg. C. The 18O diffusion in Si16O2 during the 16O2 oxidation was investigated by secondary ion mass spectrometry measurements. A significant broadening of the 18O profile toward the newly grown Si16O2 was observed. The average oxygen diffusivity was initially about one order of magnitude larger than the reported thermal diffusivity in the SiO2 network. In addition, the 18O diffusion became slower with oxidation time and hence with increasing distance between 18O diffusion region and the interface. This distance-dependent 18O self-diffusion was simulated taking into account the effect of Si16O molecules generated at the interface upon oxidation and diffusing into SiO2 to enhance the oxygen exchange. The simulation fits the SIMS profiles and shows that the SiO diffusion is greatly retarded by the oxidation with O2 from the oxygen-containing atmosphere. Therefore, the Si16O concentration becomes smaller as the interface leaves the 18O region and the oxygen exchange becomes slower with time