Abstract: In the fabrication of integrated circuits, a method is provided for forming recessed silicon dioxide isolation in which the "bird's beak" problem associated with conventional silicon dioxide-silicon nitride composite masking structures is minimized. A conventional composite mask comprising a bottom layer of silicon dioxide and an upper layer of silicon nitride having a plurality of openings defining the regions in the silicon substrate which are to be thermally oxidized is formed on the substrate. Recesses are then etched in the silicon substrate in registration with the openings in the composite mask. The silicon dioxide layer should be, in effect, over-etched to extend the openings in the silicon dioxide to greater lateral dimensions than the openings in the silicon nitride layer whereby the silicon nitride layer at the periphery of the openings is undercut.