Fabrication of semiconductor devices having planar recessed oxide isolation region

In the fabrication of semiconductor integrated circuits which include recessed oxide isolation regions (29), formation of the undesired "bird's head" and "bird's beak" is avoided by reducing the rate of oxide growth from the sidewalls of isotropically etched recesses (22) while oxide is being grown from the bottoms of the recess regions. A silicon nitride mask (24) formed selectively on each of the sidewalls which has previously been coated with a thin silicon dioxide layer (23) reduces the rate of oxide growth therefrom, so that the oxidized recess regions have substantially planar surfaces after termination of the oxide growth.