Abstract: An improved method for performing a local oxidation of silicon (LOCOS) capable of forming a sufficient thickness of a field oxide film even in narrow isolation regions. After defining the isolation region, a first field oxide film is formed in the isolation region by means of a first field oxidation. A film formed of HTO, LTO or SOG, or a pre-oxide film formed of polysilicon is formed on the resultant product. Then, the film, oxide film or the pre-oxide film is removed by anisotropically etching with a dry etching process or a chemical mechanical process so as to be left only in the isolation region, which after a second field oxidation forms a second field oxide film.

Abstract: In an element isolation method of a semiconductor device which can form an element isolation region having a flat surface without regard to the width of the element isolation region, and whose width is below the resolution limit, an insulating film having an aperture in order to define the element isolation region is formed on the semiconductor wafer, wherein an oxidizable material layer is deposited and then first spacers are formed on the sidewalls of the aperture. Then, a thermal oxide film is formed over the entire semiconductor wafer, excluding a first-spacer-formed region, and the first spacer is removed. The wafer surface is exposed to the lower part of the removed first spacer region, and then the portion of the semiconductor wafer below the exposed region is etched to thereby form a trench. After that, an element isolation region is formed by filling up the trench and removing the insulating film around tile trench.