Method for forming a semiconductor structure

A method for forming a semiconductor structure, comprising:

• forming a gate structure on a substrate, wherein the gate structure comprises:
• an interfacial layer on the substrate;
• a high-k dielectric layer on the interfacial layer;
• a polysilicon layer on the high-k dielectric layer;
• and
• a hard mask layer on the polysilicon layer;
• performing a deposition process to form a nitride layer to cover the substrate and the gate structure, wherein a process temperature of the deposition process is between 600° C. and 650° C.;
• performing an in-situ annealing process to the nitride layer, wherein a process temperature of the in-situ annealing process is higher than 700° C.;
• and
• performing an anisotropic etching process to the nitride layer after the in-situ annealing process to form a spacer on the substrate and covering sidewalls of the interfacial layer, the high-k dielectric layer, the polysilicon layer and the hard mask layer of the gate structure, wherein a bottom width of a portion of the spacer on the sidewall of the interfacial layer is 1.1 times of a middle width of another portion of the spacer on the sidewall of the polysilicon layer.