Degradation of dielectric breakdown field of thermal SiO2 films due to structural defects in Czochralski silicon substrates

Abstract

We used heat treatment to intentionally introduce various structural defects in Czochralski silicon substrates. The type, size, and number density of the induced defects were surveyed with transmission electron microscopy, and the defects were then incorporated into SiO2 films (10–50 nm thick) during thermal oxidation in dry O2. The effect of the defects on dielectric strength of the SiO2 films was examined with a time zero dielectric breakdown method. Larger platelet oxygen precipitates caused greater decreases of the breakdown field, and precipitates smaller than the SiO2 film thickness did not appreciably reduce the breakdown field. Every large platelet oxygen precipitate incorporated in the SiO2 film caused a degradation. Octahedral oxygen precipitates caused little degradation. The breakdown field was higher than 7 MV/cm and did not depend much on the SiO2 film thickness and precipitate size. We discussed possible mechanisms for the degradation due to both kinds of precipitates. Oxidation-induced stacking faults formed by a surface oxidation did not markedly reduce the breakdown field when only segments of dislocations and stacking faults were incorporated in the SiO2 film. Another serious degradation was caused by pits that were formed by dissolving octahedral oxygen precipitates in a HF solution. The breakdown field was lower for thicker oxide films, and it recovered as the pit shape became smoother during chemical etching. We proposed that this degradation was caused by a local thinning of SiO2 film due to stress generated in the oxidation of pits. These results suggest that voids rather than the other reported grown-in defects play the most important role in the degradation observed for as-grown silicon.