Patent ID: 7169670

Claim:
A method of forming a semiconductor device comprises steps of: providing a semiconductor substrate having a low voltage region and a high voltage region; forming a pad oxide layer and a pad nitride layer in sequence on the semiconductor substrate; removing the pad nitride layer, the pad oxide layer, and a portion of the semiconductor substrate of the high voltage region so that a surface of the semiconductor substrate of the high voltage region is exposed and recessed; forming a sacrificial layer on the recessed portion of the semiconductor substrate of the high voltage region; removing the sacrificial layer; forming a first gate oxide layer on the surface of the semiconductor substrate of the high voltage region; removing the pad oxide layer and the pad nitride layer left on the semiconductor substrate of the low voltage region, wherein a surface of the semiconductor substrate of the low voltage region is exposed and recessed; and forming a second gate oxide layer on the first gate oxide layer and the surface of the semiconductor substrate of the low voltage region.