Dual metal via for contact resistance reduction

1. A semiconductor device, comprising:

an active region over a substrate;

a first cobalt-containing feature disposed over the active region;

a conductive cap disposed over and in physical contact with the first cobalt-containing feature;

a second cobalt-containing feature disposed over and in physical contact with the conductive cap;

a first barrier layer disposed over sidewalls of the second cobalt-containing feature and the conductive cap, wherein the first barrier layer is in physical contact with the second cobalt-containing feature; and

a second barrier layer disposed over the first barrier layer.

2. The semiconductor device of claim 1 , wherein the first barrier layer includes titanium nitride and the second barrier layer includes silicon nitride.

3. The semiconductor device of claim 2 , further comprising:

a contact etch stop layer disposed over a lower portion of sidewalls of the second barrier layer; and

an interlayer dielectric layer disposed over the contact etch stop layer and over an upper portion of the sidewalls of the second barrier layer.

4. The semiconductor device of claim 3 , further comprising:

a conductive feature embedded in the contact etch stop layer; and

a ruthenium-containing feature disposed over and in electrical contact with the conductive feature.

5. The semiconductor device of claim 4 , further comprising:

a third barrier layer over sidewalls of the ruthenium-containing feature, wherein the third barrier layer includes TaN or TiN, wherein the first and second barrier layers are further disposed on sidewalls of the third barrier layer.

6. The semiconductor device of claim 2 , wherein a total thickness of the first and second barrier layers is greater than a depth of diffusion by cobalt elements from the second cobalt-containing feature into the first and second barrier layers.

7. The semiconductor device of claim 2 , wherein the first and second barrier layers have about a same thickness and a total thickness of the first and second barrier layers is about 2 nm to about 3 nm.

8. The semiconductor device of claim 1 , wherein the conductive cap includes tungsten or cobalt silicide (CoSi x ).

9. The semiconductor device of claim 1 , wherein more than 60% of cobalt grains in the second cobalt-containing feature have a grain size about 11 nm to about 13 nm.

10. The semiconductor device of claim 9 , wherein other cobalt grains in the second cobalt-containing feature have a grain size below 10 nm.

11. The semiconductor device of claim 1 , wherein the first cobalt-containing feature is electrically connected to a transistor source/drain feature or a transistor gate feature.

12. A semiconductor device, comprising:

a first cobalt-containing plug disposed over a substrate;

a conductive cap disposed over and in physical contact with the first cobalt-containing plug;

a second cobalt-containing plug disposed over and in physical contact with the conductive cap, wherein a bottom surface of the second cobalt-containing plug is spaced apart from a top surface of the first cobalt-containing plug;

a first barrier layer over sidewalls of the second cobalt-containing plug and the conductive cap;

a second barrier layer over sidewalls of the first barrier layer; and

one or more dielectric layers surrounding the second barrier layer.

13. The semiconductor device of claim 12 , wherein the first barrier layer includes titanium nitride, the second barrier layer includes silicon nitride, and the one or more dielectric layers include silicon oxide.

14. The semiconductor device of claim 12 , further comprising:

a titanium nitride layer embedded in the one or more dielectric layers; and

a ruthenium-containing plug disposed over and in electrical connection with the titanium nitride layer.

15. The semiconductor device of claim 14 , further comprising:

a third barrier layer surrounding the ruthenium-containing plug, wherein the first and second barrier layers are also disposed between the third barrier layer and the one or more dielectric layers.

16. A method, comprising:

providing a structure having a substrate, one or more first dielectric layers over the substrate, a first cobalt-containing plug embedded in the one or more first dielectric layers, and one or more second dielectric layers over the one or more first dielectric layers and the first cobalt-containing plug;

etching a via hole into the one or more second dielectric layers to expose the first cobalt-containing plug;

depositing a first barrier layer having silicon nitride into the via hole;

depositing a second barrier layer having titanium nitride or tantalum nitride into the via hole and over the first barrier layer;

etching the first and second barrier layers in the via hole to expose the first cobalt-containing plug;

forming a conductive cap over the first cobalt-containing plug that is exposed in the via hole; and

selectively growing cobalt over the conductive cap.

17. The method of claim 16 , wherein the forming of the conductive cap includes selectively growing tungsten over the first cobalt-containing plug using WF 6 and H 2 as reaction gas.

18. The method of claim 16 , wherein the selectively growing cobalt is performed using C 5 H 5 (CO) 2 Co and H 2 as reactive gas.

19. The method of claim 16 , further comprising:

cleaning an exposed surface of the first cobalt-containing plug using H 2 plasma before the forming of the conductive cap.

20. The method of claim 16 , wherein the structure further includes a conductive feature embedded in the one or more second dielectric layers, further comprising:

etching a second via hole into the one or more second dielectric layers to expose the conductive feature;

depositing the first barrier layer in the second via hole;

depositing the second barrier layer in the second via hole;

etching the first and second barrier layers in the second via hole to expose the conductive feature;

depositing a third barrier layer having tantalum nitride or titanium nitride in the second via hole; and

depositing a ruthenium-containing plug in the second via hole and over the third barrier layer.