Water diffusion in silica film during silicon wet oxidation

Water diffusion in silica glass is explained in terms of the migration of molecular water and its reaction with the silica network to form immobile hydroxyl water. It is assumed that the reaction is fast at high temperatures, and a local reaction equilibrium is maintained during water diffusion. While this mechanism appears to explain water diffusion in silica glasses determined from hydroxyl profiles reasonably well, we found that water diffusion-promoted phenomena, such as the oxidation of silicon, require a small quantity of molecular water diffusing into silica glasses ahead of the hydroxyl diffusion front, unrestricted by the local equilibrium. This was demonstrated by studying oxidation kinetics of silicon with a thick (∼15 μm) oxide layer and measuring the delay time before the steady-state oxidation.