A method of forming an opening in an insulating layer covering a semiconductor region including germanium, successively including: the forming of a first masking layer on the insulating layer; the forming on the first masking layer of a second masking layer including an opening; the etching of an opening in the first masking layer, in line with the opening of the second masking layer; the removal of the second masking layer by oxygen-based etching; and the forming of the opening of said insulating layer in line with the opening of the first masking layer, by fluorine-based etching.