Method for forming integrated circuit regions defined by recessed dielectric isolation

In the fabrication of integrated circuits, a method for forming dielectrically isolated regions in a silicon substrate comprising

• introducing conductivity-determining impurities into the substrate to form at least one region of one-type conductivity at the surface of said substrate
• forming over at least a part of said surface of said region, a composite mask comprising
• a lower layer of silicon dioxide in contact with said surface, and
• an upper layer of silicon nitride on said lower layer,
• said mask having a plurality of apertures extending therethrough,
• etching recesses in the portions of said substrate exposed in said apertures,
• thermally oxidizing the recessed portions of said substrate to an extent sufficient to form regions of recessed silicon dioxide abutting and laterally defining said at least one region of one-type conductivity, said mask acting as a barrier to prevent oxidation of the unrecessed surface of said substrate,
• providing a layer of electrically insulative material over at least a portion of the surface of said substrate by a processing step comprising at least the removal of said silicon nitride layer,
• forming at least one contact opening through said electrically insulative layer to expose at least a portion of said at least one region of one-type conductivity, said contact opening exposing a portion of a region of recessed silicon dioxide abutting said one-type conductivity region and
• depositing metal in said contact opening to form a contact to said region of one-type conductivity, said contact over-lapping said recessed silicon dioxide.