Method for forming n and p wells in a semiconductor substrate using a single masking step

Abstract

A method for forming both n and p wells in a semiconductor substrate using a single photolithography masking step, a non-conformal oxide layer and a chemical-mechanical polish step. A screen oxide layer is formed on a semiconductor substrate. A barrier layer is formed on the screen oxide layer. The barrier layer is patterned to form a first opening in the barrier layer over regions of the substrate where first wells will be formed. We implant impurities of a first conductivity type into the substrate to form first wells. In a key step, a non-conformal oxide layer is formed over the first well regions and the barrier layer. It is critical that the non-conformal oxide layer formed using a HDPCVD process. The non-conformal oxide layer is chemical-mechanical polished stopping at the barrier layer. The barrier layer is removed using a selective etch, to form second openings in the remaining non-conformal oxide layer over areas where second well will be formed in the substrate. Using the remaining non-conformal oxide as a mask, we implant impurities of the second conductivity type through the second openings to form second wells. The remaining non-conformal oxide layer and the screen oxide layer are removed.