Patent ID: 7608518

Claim:
A method for fabricating a semiconductor device, comprising: forming a pad oxide layer on a semiconductor substrate; forming a pad nitride layer on the pad oxide layer; forming a capping layer on the pad nitride layer; patterning the capping layer, the pad nitride layer, and the pad oxide layer by a photolithography method to expose portions of the semiconductor substrate; forming a field oxidation layer having bird's beaks, the bird's beaks being formed under the pad nitride layer; forming trenches in the semiconductor substrate by anisotropically etching the field oxide layer and the semiconductor substrate using the pad nitride layer as a mask; removing the capping layer, the pad nitride layer, the pad oxide layer, and the bird's beaks; and forming an isolation region in the trenches after removing the pad nitride layer; wherein the field oxide layer is formed to penetrate into sides of the pad oxide layer for about 300 Å to about 700 Å beneath the pad nitride layer, thereby forming the bird's beaks.