Patent ID: 7504313

Claim:
A method for forming a semiconductor device comprising at least two kinds of wells on a single semiconductor substrate, comprising the steps of: (a) providing a semiconductor substrate including thereon an alignment mark forming region, a first well forming region, and a second well forming region, said first well forming region being between said alignment mark forming region and said second well forming region; a selective etching film on said semiconductor substrate; further thereon a first resist film, said first resist film having an opening overlying said alignment mark forming region and said first well forming region and masking said second well forming region; and removing, with said first resist film serving as a mask, portions of said selective etching film overlying said alignment mark forming region and said first well forming region; (b) implanting said alignment mark forming region and said first well forming region between said alignment mark forming region and said second well forming region with a first dopant prior to formation of an alignment mark in said alignment mark forming region, said first resist film serving as the mask over said second well forming region; (c) removing said first resist film; and forming a second resist film masking said first well forming region on said selective etching film, said second resist film having an opening overlying said alignment mark forming region larger than the opening of said selective etching film overlying a same region; (d) removing, with said second resist film and said selective etching film serving as a mask, a portion of said silicon substrate to form said alignment mark; and then removing said second resist film and said selective etching film; (e) forming a third resist film having an opening overlying said second well forming region, said third resist film masking said first well forming region; (f) implanting, with said third resist film serving as a mask, said second well forming region with a second dopant; and removing said third resist film; and (g) performing a thermal diffusion process to form a first well and a second well on said first well forming region and said second well forming region, respectively.