Transformation of silicon nitride in oxygen plasma

Abstract

The oxygen plasma treatment of chemical-vapour-deposited Si 3N 4 thin films at floating potential and low T (less than 600 °C) leads to the growth of a silicon oxide surface layer and to a decrease of the nitrogen content of the samples. Nuclear reaction analysis was used to study the composition of the films, the oxide growth kinetics and the loss of nitrogen atoms. After an initial transient oxidation regime, parabolic growth laws were found, indicating that the rate-limiting step in the oxide formation is a diffusion process and not a surface reaction. In the low temperature regime the oxide growth rate and the activation energy were found to be similar to those obtained during the plasma oxidation of bare silicon substrates. 18O-enriched plasmas were used to study the nitride transformation, and the 18O depth distribution was determined by resonance depth profiling. The 18O atoms were found at the surface of the sample, forming a pure SiO 2 layer on the underlying Si 3N 4. The absence of a large mixing at the oxide-nitride interface indicates that the oxide growth occurs by a layer-by-layer oxidation process and not by the formation of an oxynitride compound of varying composition with depth.