Multiple well device and process of manufacture

Claim 1

A method of fabricating a semiconductor device on a substrate, the method comprising: forming a silicon oxide layer on said substrate with said silicon oxide layer having a top surface; ion implanting a first kind of N-type impurities through said silicon oxide layer into said substrate for N-well regions; forming and patterning a silicon nitride layer on said top surface of said silicon oxide layer with openings therethrough leaving exposed portions of said top surface; ion implanting N-type impurities of a second, heavier kind through said openings through said silicon nitride layer and through exposed portions of said top surface of said silicon oxide layer for doping the well regions to form field implant regions adjacent to said silicon nitride layer; forming a patterned mask over portions of said patterned silicon nitride layer and over portions of said top surface of said silicon oxide layer, said patterned mask having an opening leaving portions of said top surface and said patterned silicon nitride layer exposed which are to be a P-well; ion implanting P-type impurities through said openings in said patterned mask and exposed portions of said patterned silicon nitride layer and said silicon oxide layer for first P-well regions; performing a field oxidation process on portions of said substrate unprotected by said silicon nitride layer so that field oxide regions are formed overlying said field implant regions; stripping said silicon nitride layer; and forming a P-well mask over an N-well region and ion implanting P-type impurities forming second P-well regions in said substrate.

Claim 2

A method of fabricating a semiconductor device on a substrate, the method comprising: forming a silicon oxide layer on said substrate with said silicon oxide layer having a top surface; ion implanting a first kind of N-type impurities through said silicon oxide layer into said substrate for N-well regions; forming and patterning a silicon nitride layer on said top surface of said silicon oxide layer with openings therethrough leaving exposed portions of said top surface; ion implanting N-type impurities of a second, heavier kind through said openings through said silicon nitride layer and through exposed portions of said top surface of said silicon oxide layer for doping the well regions to form field implant regions adjacent to said silicon nitride layer; forming a patterned mask over portions of said patterned silicon nitride layer and over portions of said top surface of said silicon oxide layer, said patterned mask having an opening leaving portions of said top surface and said patterned silicon nitride layer exposed which are to be a P-well; ion implanting P-type impurities through said openings in said patterned mask and exposed portions of said patterned silicon nitride layer and said silicon oxide layer for first P-well regions; performing a field oxidation process on portions of said substrate unprotected by said silicon nitride layer so that field oxide regions are formed overlying said field implant regions; stripping said silicon nitride layer; and forming a P-well mask over an N-well region and ion implanting P-type impurities forming second P-well regions in said substrate.