Semiconductor device and method for fabricating the same

1. A method for fabricating semiconductor device, comprising:

providing a substrate;

removing part of the substrate to form a first trench and a second trench in the substrate;

forming a first work function metal layer in the first trench and the second trench;

forming a patterned mask to cover the second trench;

removing the first work function metal layer from the first trench;

forming a second work function metal layer in the first trench and the second trench; and

forming a conductive layer in the first trench and the second trench to form a first gate structure and a second gate structure.

2. The method of claim 1 , further comprising:

forming a gate dielectric layer in the first trench and the second trench; and

forming the first work function metal layer on the gate dielectric layer.

3. The method of claim 1 , wherein the patterned mask comprises an organic layer.

4. The method of claim 1 , further comprising:

removing the patterned mask after removing the first work function metal layer; and

forming the second work function metal layer in the first trench and the second trench.

5. The method of claim 1 , wherein the top surfaces of the first gate structure and the second gate structure are coplanar.

6. The method of claim 1 , further comprising forming a shallow trench isolation (STI) in the first trench before forming the first work function metal layer.

7. The method of claim 6 , wherein a top surface of the STI is lower than a bottom surface of the second trench.

8. The method of claim 1 , further comprising forming a hard mask on each of the first gate structure and the second gate structure after forming the conductive layer.

9. The method of claim 8 , wherein the top surfaces of the hard mask and the substrate are coplanar.

10. The method of claim 8 , wherein the hard mask comprises silicon nitride.