PURPOSE: To form a well diffused layer of small substrate effect by reducing impurity concentration and thus contrive to increase the operating speed by ion- implanting an impurity through an oxidation resistant film in addition to an oxide film.
CONSTITUTION: An Si oxide film 22 is formed by thermally oxidizing the surface of an n type Si substrate 21, and then an Si nitride film pattern 23' covering an n type active region scheduled part and a p type one is formed. The substrate surface at the part for forming a field oxide film is exposed by forming an SiO2 film 25 and a resist pattern 26 and then etching them, boron is ion-implanted, and thermal oxidation treatment is performed, and accordingly a p-well 27 is formed. Boron concentration is low and diffusion depth is shallow in the p type active region scheduled part covered with said pattern 23', and a field oxide film forming scheduled part in the periphery is formed with a high boron concentration and a deep diffusion depth. Since said concentration in the active region is small, the substrate effect of an n-MOSFET formed in the p-well 27 can be reduced.
KONDOU TAKEO
ADACHI TAKAO
