A semiconductor is prepared by continuously etching at least two types of silicon compound membranes such as silicon dioxide (SiO2), silicon nitride (Si3N4) or a polycrystalline silicon membrane which are formed on a silicon substrate. A freon gas plasma is used for etching so that the two types of silicon compound membranes are continuously etched in a sloped form without any undercutting, as occurs in conventional chemical solution etching.
