Selective Etching of Silicon in Preference to Germanium and Si0.5Ge0.5.

Abstract

The selective etching characteristics of silicon, germanium and Si0.5Ge0.5 subjected to a downstream H2/CF4/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 may be etched in preference to Ge and Si0.5Ge0.5 with an essentially infinite Si:Ge etch rate ratio (ERR), while for Si:Si0.5Ge0.5, the ERR is also infinite at 22 °C and 760 mTorr. XPS data shows the selectivity is due to differential suppression of etching by a ~2 ML thick CxHyFz layer formed by the H2/CF4/Ar plasma on Si and Ge or Si0.5Ge0.5. The data is consistent with the less exothermic reaction of fluorine radicals with Ge or Si0.5Ge0.5 being strongly suppressed by the CxHyFz layer, while on Si the CxHyFz layer is not sufficient to completely suppress etching. Replacing H2 with D2 in the feed gas results in an inverse kinetic isotope effect (IKIE) where the Si and Si0.5Ge0.5 etch rates are increased by ~30x with retention of significant etch selectivity. The use of D2/CF4/Ar instead of H2/CF4/Ar results in less total carbon deposition on Si and Si0.5Ge0.5 and gives less Ge enrichment of Si0.5Ge0.5. This is consistent with the selectivity being due to the differential suppression of etching by an angstrom scale carbon layer.