Selective etching of silicon in preference to germanium and Si0.5Ge0.5

At 50 °C and 760 mTorr, Si may be etched in preference to Ge and Si _0.5 Ge _0.5 with an essentially infinite Si:Ge etch rate ratio (ERR), while for Si:Si _0.5 Ge _0.5 , 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 C _x H _y F _z layer formed by the H ₂ /CF ₄ /Ar plasma on Si and Ge or Si _0.5 Ge _0.5 . The data is consistent with the less exothermic reaction of fluorine radicals with Ge or Si _0.5 Ge _0.5 being strongly suppressed by the C _x H _y F _z layer, while on Si the C _x H _y F _z layer is not sufficient to completely suppress etching. Replacing H ₂ with D ₂ in the feed gas results in an inverse kinetic isotope effect (IKIE) where the Si and Si _0.5 Ge _0.5 etch rates are increased by ~42× with retention of significant etch selectivity. The use of D ₂ /CF ₄ /Ar instead of H ₂ /CF ₄ /Ar results in less total carbon deposition and eliminates fluorocarbon deposition on Si and Si _0.5 Ge _0.5 , and gives less Ge enrichment of Si _0.5 Ge _0.5 . This is consistent with the selectivity being due to the differential suppression of etching by an angstrom scale carbon layer.