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:

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

depositing a thin metal layer over the high-k dielectric layer;

depositing a sacrificial layer over the thin metal layer;

performing a first rapid thermal anneal of the replacement gate structure at a high temperature of not less than 800° C.;

removing both the thin metal layer and the sacrificial layer;

performing a second rapid thermal anneal at a temperature range between 400° C. and 800° C., inclusive;

re-depositing a thin metal layer over the high-k dielectric layer after performing the second rapid thermal anneal; and

depositing a second metal layer.

2. The method of claim 1 wherein performing the first and second rapid thermal anneals comprises annealing the structure in ambient nitrogen.

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

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

5. The method of claim 1 wherein depositing the second metal layer comprises:

depositing a work function metal; and

depositing a gap fill metal of low resistivity.