On the Oxidation Kinetics of Silicon: The Role of Water

Abstract

The theory of the kinetics of the thermal oxidation of silicon is extended after critical examination of the solubility and transport behavior of water in silica. From the solubility data for water in silica reported in the literature, the dissolution appears to be a two‐stage process at temperatures up to 1200°C. In the first stage silanol groups are formed at a relatively slow rate. In the second stage these silanol groups react with water forming hydronium ions false(H3O+false) and silicate ions false(=SiO−false) fixed to the silica network. The second stage is relatively fast compared to the first stage. Consistent with this the transport of water appears to occur by the ambipolar diffusion of hydronium false(H3O+false) and hydroxyl ions (OH−). The dissolution and transport of water predict a linear‐parabolic rate law, which differs slightly from the one obtained by Deal and Grove. The catalytic role of water in mixed ambients is explained by a simple interaction between O2 and H2 formed by the oxidation of Si by H2O . An expression for oxide growth accounting for this interaction is derived and properly fits the experimental data reported in the literature. It predicts correctly the linear dependence of the parabolic rate constant and the square root dependence of the linear rate constant on water partial pressure. It predicts also the initial growth regime.

On the oxidation of silicon

“…varies linearly with distance x from the interface Si/SiO,, that the silanol is in equilibrium with these species, and therefore that the concentration of silanol in the film will vary as (1 -X / X ) ' '~, where X is the thickness of the film and x the distance from the Si surface (Breed and Doremus '1976). Wolters (1980), however, examining the data of Burkhardt, comes to the conclusion that the silanol concentration is observed to vary as (1 -x / X ) * '~ and deduces that the oxidizing agent is not H,O but the ion pair H 3 0 + and OH-. The argument in terms of detailed balancing is as follows.…”

Effect of Water Vapor on the Oxidation Behavior of the Eutectic High‐Temperature Alloy Mo‐20Si‐52.8Ti

“…[26,27] Wolters, for example, who analyzed interactions of O 2 ─H 2 O mixtures with silica, proposed that solubility of water proceeds in two steps at temperatures up to 1100 C: 1) the slow formation of SiOH groups (the mobility of which can be neglected) and 2) the fast hydrolyzation and the formation of H 3 O þ and SiOH À . [27] Finally, it was shown that water migrates through silica by H 3 O þ and OH À . [27] It was, thus, concluded that water may play a significant role in the oxidation rate even at a dilute level.…”

SiSiO2 interfacial atomic scale roughness caused by inhomogeneous thermal oxidation

“…Such a law underestimates the oxide thickness when the linear-parabolic regime is reached, and hence, the mechanism responsible for the oxygen diffusion enhancement in thicker oxides should be found. One plausible mechanism has already been proposed by Wolters et al [45,46], who considered a water-enhanced oxygen diffusion in the thick-oxide growth regime, but other mechanisms should also be tested. However, since the presentation of an oxidation model is out of the scope of this paper, we will consider that the SiO, growth is described by a purely parabolic law, t = Ad2 in the initial oxidation regime, and we will smoothly link this law to a linear-parabolic one that has been found to be valid for the rest of the oxidation process.…”