PURPOSE: To enable etching in the state of suppressing the contamination and damage of an Si object to be treated as far as possible and to clean the contaminated and damaged surface in the dry etching stage of said object, by using a gaseous mixture composed of SiF4 and H2.

CONSTITUTION: Resist 1 is deposited on an SiO2 film 2; thereafter, the film 2 is selectively etched by reactive ion etching in the case of, for example, cleaning the surface of an Si substrate 3. Damages are formed on said surface in this stage. Thereupon, the surface is dry etched with a gaseous mixture composed of SiF4 and H2 and is then subjected to an O2 plasma treatment to remove a polymer layer and the resist 1; further the substrate is subjected to a soln. treatment. An SiO2 film 5 is selectively deposited on an Si substrate 4 and is etched with a gaseous mixture composed of SiF4 and H2 in selective etching of an Si type.