Method for reducing nitride residue in a locos isolation area

Abstract

A method of forming an isolation structure includes the steps of: providing a silicon substrate; forming an upper pad oxide layer superjacent a top surface of the substrate, and a lower pad oxide layer subjacent a bottom surface of the substrate; forming a nitride masking layer superjacent the upper pad oxide layer, and a lower pad silicon nitride layer subjacent the lower pad oxide layer; patterning and etching the nitride masking layer to expose a portion of the upper pad oxide layer; applying a first etching solution to the exposed portion of the upper pad oxide layer to expose a portion of the substrate substantially defining the boundaries of an active area, and simultaneously forming an undercut cavity by removing a portion of the upper pad oxide layer under the exposed edges of the nitride masking layer surrounding the exposed portion of the substrate; performing an oxidation process to form an etching stop layer over the exposed portion of the substrate and in the undercut cavity, the oxidation process also forming an upper thin oxide layer over the nitride masking layer, and a lower thin oxide layer subjacent the lower pad silicon nitride layer; and applying a second etching solution to substantially remove the upper thin oxide layer and the lower thin oxide layer for the purpose of eliminating the occurrence of silicon nitride residual after the performance of subsequent processing steps.