PURPOSE: To prevent a semiconductor integrated circuit from local reduction of thickness of an oxide film at the lower part of the side of a conductive polycrystalline silicon layer, and to contrive to enhance dielectric breakdown strength of the oxide film and yield by a method wherein a cavity at the periphery of the pattern of the conductive polycrystalline silicon layer is buried wholly with the oxide film at oxide film formation time.

CONSTITUTION: A first oxide film 2 is formed at 100∼200 thickness according to thermal oxidation on a semiconductor substrate 1, then a non-oxidizable material 3 is adhered at 200∼ 400 thickness according to vapor phase growth, then a conductive polycrystalline silicon layer 4 is adhered at 4,000∼6,000 thickness according to vapor phase growth, and photosensitive resin is left in the prescribed pattern according to normal photoetching technique. Then, the conductive polycrystalline silicon layer 4 is etched using the patterned photosensitive resin as a mask, and after the photosensitive resin is removed, the non-oxidizable material 3 is over- etched by a phosphoric acid solution by 2∼3 times of film thickness thereof using the patterned conductive polycrystalline silicon layer 4 as a mask, and the first oxide film 2 is over- etched in succession by the amount of over-etching of the non-oxidizable material 3 according to a hydrofluoric acid solution to obtain the prescribed pattern. Then, the semiconductor substrate 1 is thermally oxidized at 500∼600 thickness to form a second oxide film 6.