PURPOSE: To obtain element isolation, wherein substrate surface is flat and internal stress is small and also lateral expansion of element isolation width is small, excellently in reproducibility by growing polysilicon so as to bury a trench formed on a semiconductor substrate, and then removing the polysilicon from the surface of the substrate by polishing, and successively oxidizing the substrate thermally.
CONSTITUTION: A trench formed at the surface of a semiconductor substrate 1 is buried by growing polysilicon, and after growth of polysilicon, the polysilicon 5a is removed from the surface of the semiconductor substrate by polishing, and then the semiconductor substrate 1 is thermally oxidizing. That is, since the polishing method is used, even if there is a step at the surface of the polysilicon 5a before polishing, it does not matter, so the film thickness of polysilicon for embedding the trench can be thinned more than before, consequently it becomes possible to shorten the growth time of polysilicon. Further, since the surface is flat, the thickness of an oxide film 4b at the surface of the polysilicon 5 inside the trench can also be thinned. Hereby, it becomes enough with the oxidation at low temperature and in a short time, and the decrease of stress inside the trench and the lateral expansion of element isolation width attendant upon oxidation become possible, and the element isolation, where a trench is used, can be obtained excellently in reproducibility.