An 18O Study of Cooperative Diffusion and Chemical Reaction during Thermal Treatments of Silica Films in Water Vapor

Abstract

The mechanism of isotopic exchange of oxygen between silicon oxide films and water vapor was studied using18O as the tracer. layers of natural isotopic composition, obtained by anodic or thermal oxidation, were treated in18O enriched water vapor at 13 Torr and at temperatures ranging from 400° to 930°C. 18O profiling was carried out using the 629 keV narrow resonance in the nuclear reaction 18O(p, α) 15N. The results have been interpreted by a mechanism involving the diffusion of a mobile carrier (probably a free water molecule) reacting with the network. Two simple limit cases have been observed: one corresponds to a strong divergence between the 18O isotopic labeling of the network and that of the carrier, and the other to an isotopic quasi‐equilibrium. We have been able to deduce two parameters characteristic of the network reactivity and of the diffusivity . The values of these parameters depend on the type of oxide (thermal or anodic) and on the kind of annealing. The major effect is on the reactivity (it can be about 20 times smaller for a technological preannealed thermal oxide than for the other ones). The diffusion coefficient of the carrier, showed an activation energy of about 16–18 kcal/mol, which is close to that found for diffusion of OH in bulk silica. For the reactivity, no determination of activation energy, was yet possible; however it appears that it should be very much higher than that for .