Method for forming metal gate and MOS transistor

A method for forming a MOS transistor, comprising:

• providing a substrate;
• forming a sacrificial oxide layer and a polysilicon gate respectively on the substrate in sequence;
• forming a silicon oxide layer on sidewalls of the sacrificial oxide layer and the polysilicon gate;
• forming a stop layer that covers the substrate, the silicon oxide layer and the polysilicon gate, and having a first part parallel to the substrate, said first part being on the silicon oxide layer and the polysilicon gate, and a second part on the sidewalls of the silicone oxide layer, the second part of the stop layer and the silicon oxide layer constituting spacers;
• forming a source in the substrate at one of the spacers and a drain in the substrate at another of the spacers;
• forming a first interlayer dielectric layer on the stop layer for covering the spacers and the polysilicon gate;
• polishing the first interlayer dielectric layer for exposing the stop layer;
• removing the first part of the stop layer on the spacers and the polysilicon gate;
• removing a predetermined thickness of the second part of the stop layer in the spacers, the predetermined thickness corresponding to thickness of the sacrificial oxide layer;
• forming a second interlayer dielectric layer that covers the first interlayer dielectric layer, the spacers and the polysilicon gate;
• polishing the second interlayer dielectric layer to expose the spacers and the polysilicon gate so as to form a third interlayer dielectric layer which comprises the first interlayer dielectric layer and the residual second interlayer dielectric layer, the third interlayer dielectric layer being substantially flush with the silicon oxide layer and the polysilicon gate;
• removing the polysilicon gate to form a trench;
• removing the sacrificial oxide layer in the trench, a portion of the third interlayer dielectric layer and a portion of the silicon oxide layer so that the residual third interlayer dielectric layer and the residual silicon oxide layer are substantially flush with the stop layer in the spacers;
• and
• forming a metal gate in the trench.