Method of forming nitride sidewalls having spacer feet in a locos process

Abstract

The invention provides a method of forming field oxide regions between active regions in a semiconductor substrate. The invention forms nitride feet on the sidewalls of a nitride oxidation mask to prevent formation of the bird's beak on the field oxide and reduce stress in the active areas. The invention begins by forming a first oxide layer and a masking block, over the active regions. A second nitride layer is deposited over the masking block and the substrate surface. The second nitride layer is anisotropically etched with a customized etch forming nitride spacers on the sidewalls of the masking block, and nitride spacer feet on the surface of the first oxide layer. The customized etch of the invention optimizes the microloading effects to properly form the nitride feet. The substrate is oxidized, using the masking block nitride spacers and the nitride spacer feet as an oxidation mask to form field oxide regions. The nitride feet eliminate bird's beak problem. In another embodiment, a trench is formed in the substrate and the trench is oxidized to form a field oxide region which is level with the substrate surface. Lastly, the masking block, the nitride spacer, and nitride spacer feet are removed.