Selective Etching of Silicon in Preference to Germanium and Si_0.5Ge_0.5

The selective etching characteristics of silicon, germanium, and Si_0.5Ge_0.5 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 can be etched in preference to Ge and Si_0.5Ge_0.5, with an essentially infinite Si/Ge etch-rate ratio (ERR), whereas for Si/Si_0.5Ge_0.5, 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 C_xH_yF_z layer formed by the H₂/CF₄/Ar plasma on Si, Ge, and Si_0.5Ge_0.5. The data are consistent with the less exothermic reaction of fluorine radicals with Ge or Si_0.5Ge_0.5 being strongly suppressed by the C_xH_yF_z layer, whereas, on Si, the C_xH_yF_z layer is not sufficient to completely suppress etching. Replacing H₂ with D₂ in the feed gas resulted in an inverse kinetic isotope effect (IKIE) where the Si and Si_0.5Ge_0.5 etch rates were increased by ∼30 times with retention of significant etch selectivity. The use of D₂/CF₄/Ar instead of H₂/CF₄/Ar resulted in less total carbon deposition on Si and Si_0.5Ge_0.5 and gave less Ge enrichment of Si_0.5Ge_0.5. These results are consistent with the selectivity being due to the differential suppression of etching by an angstrom-scale carbon layer.