FORMING METHOD FOR FIELD INSULATING FILM

PURPOSE:To prevent breakdown of a conductor film and an insulating film, by performing oxidation under the state a silicon nitride film is made to remain on the side wall of a recess part, thereby forming the insulating film without a protrusion. CONSTITUTION:A P-type silicon substrate 11 undergoes thermal oxidation, and a silicon oxide film 12, a polycrystalline silicon film 13 and a first silicon nitride film 14 are deposited. A second silicon nitride film 16 is formed on the entire surface, in which a recess part is formed by etching. Anisotropic etching is performed so that the first silicon nitride film 14 and the second silicon nitride film 16 on the side surface of the recess part are made to remain. Ions are implanted in the silicon substrate 11, and a P-type high concentration region 18 for preventing inversion is formed. Wet oxidation is performed, and a field oxide film 19is formed. Then, a silicon nitride film 17 and the polycrystalline silicon film 13 are removed, and the field oxide film 19 is formed. Thus a field insulating film without a protruded part is obtained, and the breakdown of the conductor film and the insulating film can be prevented.