According to the invention there is provided a method of dry gas phase chemically etching a structure comprising the steps of: positioning the structure in an etch chamber, the structure comprising a first material and a second material, wherein the first material is selected from silicon, molybdenum, germanium, SiGe and tungsten, the second material is silicon dioxide or silicon nitride, and at least one surface of the first material is exposed so as to be contactable by a gas phase chemical etchant; etching the first material with a noble gas fluoride or halogen fluoride gas phase chemical etchant; and exposing the etch chamber to water vapour so that the step of etching the first material is performed in the presence of water vapour.