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 proces