Method to improve reliability of replacement gate device

1. A method of fabricating a gate stack for a semiconductor device, said method comprising steps of:

after removal of a dummy gate, providing a replacement gate structure by performing steps of:

forming a dielectric layer over an area vacated by the dummy gate;

depositing a metal layer over the dielectric layer, the metal layer comprising a thermally stable metal alloy selected from the group consisting of TiN, TaC, TaN, TaC and combinations thereof;

depositing a sacrificial layer over the metal layer;

performing a first thermal anneal;

removing the sacrificial layer from the metal layer to expose a portion of the thin metal layer directly over a channel region of the semiconductor device;

depositing a work function metal on the metal layer comprising the thermally stable metal alloy; and

depositing a metal layer of low resistivity metal on the work function metal layer in the gate stack.

2. The method of claim 1 , further comprising:

performing a second rapid thermal anneal after annealing the structure.

3. The method of claim 2 , performing the second rapid thermal anneal comprises performing a millisecond anneal with a laser.

4. The method of claim 2 , wherein performing the second rapid thermal anneal comprises performing a millisecond anneal with a flash lamp.

5. The method of claim 1 , wherein performing the first rapid thermal anneal comprises annealing at a temperature between 800° C. and 1100° C.

6. The method of claim 5 , wherein performing the first rapid thermal anneal further comprises spiking the temperature for a period of time up to five seconds.

7. The method of claim 6 , wherein performing the first rapid thermal anneal comprises annealing the structure in ambient nitrogen.

8. The method of claim 1 , wherein depositing the sacrificial layer comprises depositing a layer of polycrystalline silicon.

9. The method of claim 1 , wherein depositing the sacrificial layer comprises depositing a layer of amorphous silicon.

10. A method of fabricating a gate stack for a semiconductor device, said method comprising steps of:

after removal of a dummy gate, providing a replacement gate structure by performing steps of:

growing a dielectric layer over an area vacated by the dummy gate;

depositing a thermally metal layer over the dielectric layer;

depositing a sacrificial layer over the thermally stable metal layer;

performing a first anneal;

depositing a work function metal on the thermally stable metal layer, the work function metal having a composition selected from the group consisting of TiN, TiAl and combinations thereof;

depositing a metal layer of low resistivity metal directly on the work function metal layer in the gate stack; and

performing a second anneal.

11. The method of claim 10 , performing the second anneal comprises performing a millisecond anneal with a laser.

12. The method of claim 11 , wherein performing the second anneal comprises performing a millisecond anneal with a flash lamp.

13. The method of claim 10 , wherein performing the first anneal comprises annealing at a temperature between 800° C. and 1100° C.

14. The method of claim 13 , wherein performing the first anneal further comprises spiking the temperature for a period of time up to five seconds.

15. The method of claim 14 , wherein performing the first anneal comprises annealing the structure in ambient nitrogen.

16. The method of claim 10 , wherein depositing the sacrificial layer comprises depositing a layer of polycrystalline silicon.

17. The method of claim 10 , wherein depositing the sacrificial layer comprises depositing a layer of amorphous silicon.

18. The method of claim 10 , wherein removing the dummy gate comprises a selective etch.