Process for forming semiconductor device isolation regions

A process for forming a semiconductor device isolation region which comprises: a) forming on a silicon substrate at least a first thin silicon oxide film and a first silicon nitride film thereon,b) etching the substrate using a resist pattern to form a trench for providing an isolation region,c) forming a second silicon oxide film and a second silicon nitride film on the side walls and bottom wall of the trench,d) subsequently forming a first polycrystalline silicon film on the substrate including the trench, leaving the first polycrystalline silicon film only on the side walls of the trench by anisotropic etching and thereafter oxidizing the remaining first polycrystalline silicon film to form an oxide film on the side walls of the trench, ande) further forming a second polycrystalline silicon film over the semiconductor substrate including the trench, leaving the second polycrystalline silicon film only between the oxide film portions on the side walls of the trench by anisotropic etching and thereafter oxidizing the remaining second polycrystalline silicon film to thereby form an oxide film.