Selective Etching of Silicon in Preference to Germanium and SiGe.

The selective etching characteristics of silicon, germanium, and SiGe subjected to a downstream H/CF/Ar plasma have been studied using a pair of in situ quartz crystal microbalances (QCMs) and X-ray photoelectron spectroscopy (XPS). At 50 °C and 760 mTorr, Si can be etched in preference to Ge and SiGe, with an essentially infinite Si/Ge etch-rate ratio (ERR), whereas for Si/SiGe, the ERR is infinite at 22 °C and 760 mTorr. XPS data showed that the selectivity is due to the differential suppression of etching by a ∼2 ML thick CHF layer formed by the H/CF/Ar plasma on Si, Ge, and SiGe. The data are consistent with the less exothermic reaction of fluorine radicals with Ge or SiGe being strongly suppressed by the CHF layer, whereas, on Si, the CHF layer is not sufficient to completely suppress etching. Replacing H with D in the feed gas resulted in an inverse kinetic isotope effect (IKIE) where the Si and SiGe etch rates were increased by ∼30 times with retention of significant etch selectivity. The use of D/CF/Ar instead of H/CF/Ar resulted in less total carbon deposition on Si and SiGe and gave less Ge enrichment of SiGe. These results are consistent with the selectivity being due to the differential suppression of etching by an angstrom-scale carbon layer.