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 °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.