Structure and method for metal gates with roughened barrier layer

1. A semiconductor structure, comprising:

a gate stack over a substrate; and

source and drain features adjacent to the gate stack,

wherein the gate stack includes:

a gate dielectric layer,

a barrier layer over the gate dielectric layer, the barrier layer having a roughened surface, and

a metal layer over the roughened surface.

2. The semiconductor structure of claim 1 , wherein the roughened surface has an average surface roughness of 3 to 10 angstroms.

3. The semiconductor structure of claim 1 , further comprising a capping layer between the gate dielectric layer and the barrier layer.

4. The semiconductor structure of claim 3 , wherein each of the metal layer and the barrier layer includes aluminum, and wherein the gate stack also includes aluminum between the barrier layer and the capping layer.

5. The semiconductor structure of claim 1 , wherein:

the metal layer includes aluminum and titanium; and

the barrier layer includes aluminum and one of titanium nitride and tantalum nitride.

6. The semiconductor structure of claim 5 , wherein the gate stack further includes aluminum between the barrier layer and the gate dielectric layer.

7. A semiconductor structure, comprising:

a substrate; and

a first gate stack and a second gate stack over the substrate,

wherein the first gate stack includes

a first gate dielectric layer,

a first barrier layer over the first gate dielectric layer, the first barrier layer having a roughened surface, and

an n-type work function metal layer over the roughened surface, and wherein the second gate stack includes

a second gate dielectric layer,

a second barrier layer over the second gate dielectric layer, the second barrier layer having a substantially uniform thickness, and

a p-type work function metal layer over the second barrier layer.

8. The semiconductor structure of claim 7 , wherein the roughened surface has an average surface roughness of 3 to 10 angstroms.

9. The semiconductor structure of claim 8 , further comprising a capping layer between the first gate dielectric layer and the first barrier layer.

10. The semiconductor structure of claim 9 , wherein the first gate stack includes aluminum between the first barrier layer and the capping layer.

11. The semiconductor structure of claim 7 , wherein each of the n-type work function metal layer and the first barrier layer includes aluminum.

12. The semiconductor structure of claim 7 , wherein the second gate stack further includes the n-type work function metal layer over the p-type work function metal layer.

13. The semiconductor structure of claim 7 , wherein the first barrier layer is thinner than the second barrier layer.

14. The semiconductor structure of claim 7 , wherein aluminum concentration in a lower portion of the n-type work function metal layer is greater than aluminum concentration in the first barrier layer.

15. A semiconductor structure, comprising:

a substrate; and

a gate stack over the substrate,

wherein the gate stack includes

a gate dielectric layer,

a capping layer over the gate dielectric layer;

a barrier layer over the capping layer, the barrier layer having a roughened surface, and

a metal layer over the barrier layer, wherein each of the metal layer, the barrier layer, and the capping layer includes aluminum.

16. The semiconductor structure of claim 15 , wherein a first aluminum concentration in a lower portion of the metal layer is greater than a second aluminum concentration in the barrier layer.

17. The semiconductor structure of claim 16 , wherein the second aluminum concentration is greater than a third aluminum concentration in the capping layer.

18. The semiconductor structure of claim 15 , wherein the roughened surface has an average surface roughness of 3 to 10 angstroms.

19. The semiconductor structure of claim 15 , wherein the barrier layer includes one of titanium nitride and tantalum nitride.

20. The semiconductor structure of claim 15 , wherein the metal layer is in direct contact with the barrier layer.