Fluoride gas etching of silicon with improved selectivity

In a method for selectively etching a silicon portion relative to a non-silicon portion of a sample containing both silicon and non-silicon portions, said non-silicon portion consisting of a member selected from the group consisting of a non-silicon metal, a compound of a non-silicon metal, and a silicon-containing compound in which silicon is bonded to a non-silicon element, by exposing both said silicon portion and said non-silicon portion to an etchant gas selected from the group consisting of noble gas fluorides and halogen fluorides, the improvement in which said etchant gas is utilized in the form of a gas mixture in which said etchant gas is mixed with a non-etchant gaseous additive, the partial pressure of said etchant gas in said gas mixture being at least about 0.1 mbar, and the molar ratio of said non-etchant gaseous additive to said etchant gas being from about 1:1 to about 500:1, such that said gas mixture achieves greater etching selectivity toward said silicon portion than would be achieved with said etchant gas alone.