Abstract: An NF.sub.3 /H.sub.2 mixture as a feed gas for an etchant for etching an SiO.sub.2 film on an silicon wafer is used with a 1 : 160 NF.sub.3 /H.sub.2 mixed ratio. The mixture is made into plasma, and activated species of fluorine, hydrogen and nitrogen are supplied downstream to allow the species to be adsorbed in and on the SiO.sub.2 film. The NF.sub.3 /H.sub.2 mixed ratio of the mixture is so set as not to effect the etching of the SiO.sub.2 film under a chemical action. Then the adsorbed activated species are irradiated with Ar low energy ions so that the activated species are excited and etch the SiO.sub.2 film. During etching, the semiconductor wafer is maintained to about -100.degree. C. Less damage is caused to the silicon wafer and etching can be made in a high selection ratio.