Patent ID: 8003476

Claim:
A manufacturing method of a semiconductor device having more than three kinds of wells in a single substrate, comprising the steps of: (A) forming a silicon nitride film on said substrate; (B) forming a first resist pattern by a photolithography to define a first well area, removing a part of said silicon nitride film corresponding to an opening of said first resist pattern by etching, introducing first impurity ions into said first well area of said substrate by ion implantation so as to form said first well area, and removing said first resist pattern; (C) forming a second resist pattern by a photolithography so as to form a second well area, removing a part of said silicon nitride film corresponding to an opening of said second resist pattern by etching so as to define said second well area, introducing second impurity ions into said second well area of said substrate by ion implantation so as to form the second well, and removing said second resist pattern; (D) applying a heat treatment to said substrate within an oxidizing atmosphere so as to form a thermal oxide film on an area of a surface of said substrate that is not covered by said silicon nitride film and simultaneously diffuse the first and second impurity ions introduced into said substrate to form the first and second wells; (E) removing said silicon nitride film, and introducing third impurity ions into said substrate by using said thermal oxide film as a mask so as to form a third well area in a self-alignment manner; and (F) applying a heat treatment to said substrate in a non-oxidizing atmosphere so as to diffuse the third impurity ions to form the third well.