MANUFACTURE OF SEMICONDUCTOR DEVICE

PURPOSE: To provide a highly integrated semiconductor device by minimizing the side diffusion of a field oxide. CONSTITUTION: After the successive formation of a pad oxide film 32 and a nitride film on the whole surface of a semiconductor substrate 30, an aperture part 37 is formed to form an element isolation region while in order to form a field oxide film by high temperature oxidation, the nitride film and the pad oxide film 32 are corroded to suppress the bird's beak by side diffusion of oxygen between the pad oxide film 32 and the nitride film so that a nitride film spacer may be formed on the corroded sidewall to cut off the diffusion path. Besides, after the formation of the nitride film spacer leaving a part of the pad oxide film 32 intact on the substrate and undercutting the lower edge of the spacer to be filled up with an oxidizable material for blocking the side diffusion of oxygen as a buffer layer in the oxidizing step. Through these procedures, a thin field oxide film having almost no bird's beak can be formed.