Si O x F y passivation layer in silicon cryoetching

Abstract

The SiOxFy passivation layer created on structure sidewalls during silicon cryoetching is investigated. This SiOxFy passivation layer formation strongly depends on O2 content, temperature and bias. It is a fragile layer, which mostly disappears when the wafer is warmed up to ambient temperature. A mass spectrometer was used to analyze the desorbed species during the warm-up and using this instrument allowed us to find a large signal increase in SiF3+ between −80°C and −50°C. SiF4 etching products can participate in the formation of the passivation layer as it is shown by a series of test experiments. SiF4∕O2 plasmas are used to form a thin SiOxFy layer on a cooled silicon wafer. Thickness and optical index of this thin film can be determined by in situ spectroscopic ellipsometry. It is shown that the passivation layer spontaneously desorbs when the silicon wafer temperature increases in good agreement with the mass spectrometry analysis. Two physical mechanisms are proposed to explain the SiOxFy passivation layer buildup involving either the etching products or the SiFx sites created during etching. In both cases, oxygen radicals react at the surface to form the SiOxFy layer.