A method of forming a trench element separation structure involves (a) forming a non-single crystal silicon film (5) above a first thermal oxide film (2) on the substrate (1); (b) forming a trench (13) extending from the silicon film (5) surface into the substrate (1); (c) thermally oxidising to form second and third oxide films (10) on the trench interior surface including the trench wall and the lateral sidewall (12) of the silicon film (5); and (d) removing the silicon film (5) but not the third thermal oxide film (10) such that the latter remains on the lateral side of the embedded oxide film (11) which projects above the substrate surface. Also claimed are (i) a trench element separation structure formed by the above method; and a DRAM cell structure employing the above trench element separation structure.
