A method for forming a dielectric film containing a Si—O bond a trench formed in an upper surface of a substrate, includes: designing a topology of a final dielectric film containing a Si—O bond formed in the trench by preselecting a target portion to be selectively removed relative to a non-target portion of an initial dielectric film resulting in the final dielectric film; conformally depositing the initial dielectric film on the upper surface and in the trench; and relatively increasing an amount of impurities contained in the target portion of the initial dielectric film relative to an amount of impurities contained in the non-target portion of the initial dielectric film to obtain a treated dielectric film, thereby giving the target portion and the non-target portion different chemical resistance properties when subjected to etching.