Method for forming dielectric isolation combining dielectric deposition

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. A layer of a material capable of blocking the oxidation of silicon and having a greater etchability than silicon nitride is then deposited in said recesses and covering said undercut portions of said silicon nitride masks. At this point the structure is blanket etched to remove said blocking material from the portions of the recesses not under said silicon nitride and to, thereby, expose the silicon in these portions. Finally, the structure is thermally oxidized so that the exposed silicon in the recesses oxidizes to form recessed regions of silicon dioxide substantially coplanar with the unrecessed regions of the silicon substrate. Because of the undercutting and the deposition in the undercut portions of the recesses of the blocking material, the 'bird's beak' effect is minimized.