Polishing method and method for forming a gate

A method for forming a gate, the method comprising:

• forming a dummy gate on a semiconductor substrate, wherein the dummy gate comprises a sacrificial oxide layer and a polysilicon layer overlying the sacrificial oxide layer;
• forming spacers around the dummy gate;
• successively forming a silicon nitride layer and a dielectric layer overlying the silicon nitride layer, wherein the silicon nitride layer covers the polysilicon layer, the spacers, and the semiconductor substrate;
• polishing the dielectric layer until the silicon nitride layer is exposed;
• polishing the silicon nitride layer by a fixed abrasive pad until the polysilicon layer is exposed, wherein a polishing slurry used in the polishing process is featured with a PH value ranging from 10.5 to 11 and comprising an anionic surfactant or a zwitterionic surfactant, wherein the dielectric layer comprising silicon dioxide is polished using the polishing slurry, and wherein a removal rate of the polysilicon layer by the polishing slurry is less than about 50 Å/min;
• forming an opening after removing the dummy gate;
• and
• forming a gate in the opening.