Network oxygen exchange during water diffusion in SiO2

Abstract

In an investigation of the mechanism responsible for the transport of water through SiO2 thin films, we have performed tracer diffusion measurements involving network 18O demonstrating the importance of oxygen exchange between the SiO2 network and molecularly dissolved water. We have found that in the presence of water, bound network oxygen diffuses through SiO2 as a constituent of molecularly dissolved water. Employing methods common to state-of-the-art semiconductor technology, the central region within a thermal oxide layer grown on silicon was enriched with immobile 18O by ion implantation. After heating in atmospheres with different water contents, the extent of 18O diffusion was determined by observing changes in the concentration profile of implanted 19O by means of nuclear resonance profiling making use of the 629-keV resonance of the 18O( p,a) 15N reaction. Diffusions conducted in steam at 1 atm (at temperatures as low as 250 C) showed an activation energy of about 17 kcal/mol, which is close to that originally measured for water permeation in SiO2. Diffusions in both air and dry nitrogen showed a similar activation energy, with respective pre-exponential factors two and three orders of magnitude below the steam value. Diffusion in low-pressure water vapor showed a clearly linear dependence on gas phase water concentration down to 80 ppm. These results are consistent with a model of water diffusion in SiO2 in which the diffusion mechanism is the interstitial transport of dissolved molecular water accompanied by a reversible reaction with silicon-oxygen bonds in the network.