Semiconductor device producing method requiring only two masks for completion of element isolation regions and P- and N-wells

Claim 1

A semiconductor device producing method comprising the steps of: a) forming element isolation regions of an oxide on a surface of a semiconductor substrate, active regions of the surface of said semiconductor substrate other than said element isolation regions being formed with an oxide film; b) forming a first oxidization prevention layer, a semiconductor layer, and a second oxidization prevention layer in that order on said semiconductor substrate formed with said element isolation regions and oxide film; c) forming a resist pattern on said second oxidization prevention layer, said resist pattern having a hole in a first conductive type channel MOS transistor formation region wherein a first conductive type channel MOS transistor is to be formed; d) removing a portion of said second oxidization prevention layer that exists in said first conductive type channel MOS transistor formation region of said semiconductor substrate, whereby a portion of said semiconductor layer that exists in said first conductive type channel MOS transistor formation region is exposed; e) implanting a second conductive type impurity into said semiconductor substrate in said first conductive type channel MOS transistor formation region using said resist pattern as a mask; f) removing said resist pattern; g) thermally treating said substrate in the presence of an oxidizer substance to transform at least an upper portion of said exposed portion of said semiconductor layer into an oxidized semiconductor layer and at the same time to diffuse the implanted impurity in the substrate to thereby form a second conductive type well; h) removing the remaining second oxidization prevention layer and said semiconductor layer located under the remaining second oxidization prevention layer, said remaining second oxidization prevention layer and semiconductor layer being located in a second conductive type channel MOS transistor formation region wherein a second conductive type channel MOS transistor is to be formed; i) implanting a first conductive type impurity into said second conductive type channel MOS transistor formation region of the semiconductor substrate, using said oxidized semiconductor layer as a mask, to thereby form a first conductive type well; and j) forming the first and second conductive type channel MOS transistors in said second and first conductive type wells, respectively.