A mechanism for the nitridation of Fe-contaminated silicon

Abstract

The influence of iron impurity on both the oxidation and nitridation of high purity silicon has been investigated. It is shown that iron is effective in rapidly removing the protective silica film which normally covers silicon. Experimental evidence suggests that the removal is achieved by iron-induced devitrification and disruption of the silica, thus allowing the SiO (g) generated by the Si/SiO2 interface reaction to escape. During the nitridation of iron-contaminated silicon powder compacts it is found that iron significantly enhances the extent of reaction for contamination levels of <1000 p.p.m. Fe (by weight). Above this level there is a decrease in the rate of formation of extra nitride. At all levels of contamination the percentage of silicon converted to β-Si3N4 was observed to be directly proportional to the iron concentration, and it is shown that this β-growth occurs within an FeSix liquid phase. The possible implications of the findings for the optimization of strength of reaction-bonded silicon nitride are briefly discussed.