Patent ID: 7300834

Claim:
A method of forming a well in a semiconductor substrate comprising: forming an STI trench in a surface of a substrate to define a field area; forming a first ion implantation mask over the substrate and the trench such that a first active area and a first part of the trench are exposed; forming a first conductive type well in the first active area of the substrate and under the first part of the trench so that an end point of the first conductive type well under the first part of the trench is deeper than a bottom of the first conductive type well in the first active area; forming a second ion implantation mask over the first active area and the first part of the trench; forming a second conductive type well in a second active area of the substrate and under a second part of the trench so that an end point of the second conductive type well under the second part of the trench is deeper than a bottom of the second conductive type well in the second active area, wherein the first and second conductive type wells meet under the trench and the end points of the first and second conductive type wells where the first and second conductive type wells meet are deeper than bottoms of the first and second conductive type wells in the first and second active areas; and filling the trench with a dielectric after forming the first and second conductive type wells.