HYDROGEN-FLUORIDE VAPOR ETCHING FOR PRE-EPI SILICON SURFACE PREPARATION

Hydrogen fluoride vapor etching of silicon dioxide was studied for its applicability to the preparation of silicon surfaces for low temperature epitaxy. The etching behavior of hydrogen fluoride vapor with and without added water vapor was studied qualitatively by observing changes in hydrophobicity, and quantitatively using ellipsometry. Epi films grown on vapor-treated surfaces were evaluated using transmission electron microscopy and by fabricating polysilicon-emitter diodes. It was found that complete removal of the silicon dioxide monolayer closest to the wafer surface proceeded much more slowly than etching of a bulk oxide layer. A mechanism for the reaction of HF and SiO2 is proposed, in which the etching of a bulk film takes place in a thin aqueous layer on the oxide surface, while the removal of the final monolayer of oxide takes place as a gas-solid reaction.