PURPOSE: To prevent the crystal defect development due to the distortion by adhesion of an oxidation-resistance mask with the heating disposition at elevated temperatures on the occasion of performing an activating treatment after an ion implantation by carrying out a process where ion implantation is selectively performed to form a well region at a substrate by employing an alignment means as a standard and a process where a selective oxidation is performed after carrying out an anneal treatment activating it.

CONSTITUTION: A recessed part 24 having upright walls is formed by performing an anisotropic etching at the surface of a substrate 21 that is exposed outside. And an ion implantation mask 25 composed of photoresist is selectively formed standard for alignment and then, a well region 26 is formed after performing the ion implantation of boron B constituting of p-type impurities through the ion implantation using the above mask 25 and an activating treatment, that is, an anneal treatment by heating at elevated temperatures of around 1100°C is conducted. After that, in the same manner, an alignment means, that is alignment by employing the recessed part 24 as the standard is performed and an oxidation-resistant film 27 such as Si3N4 film is formed. As a result, using the above film as a mask, a thick oxidation film 28 is formed at a field part by thermal oxidation.