Method for preparing semiconductor device

The invention provides a method for preparing a semiconductor device. The method comprises the steps of providing a semiconductor substrate on which a dummy gate structure is formed, wherein a hard mask layer is formed at the top of the dummy gate structure; successively forming an oxide layer and a silicon nitride layer according to an atomic layer deposition process, thereby covering the semiconductor substrate, the dummy gate structure and the hard mask layer; etching the silicon nitride layer for forming an offset sidewall; and forming an embedded epitaxial layer in the part to form a source/drain region on the semiconductor substrate. According to the method of the invention, after the embedded epitaxial layer is formed, great height reduction of the offset sidewall can be prevented, and furthermore a preset requirement for the height of the dummy grid structure can be satisfied, thereby ensuring the performance of a subsequently formed high-k metal gate.