Multi-layer work function metal replacement gate

1. A field-effect transistor (FET) replacement gate apparatus, comprising:

a channel structure including a base and side walls defining a trench;

a high-dielectric constant (high-k) layer formed on the base and side walls of the trench and having an upper surface conforming to a shape of the trench;

a first layer formed on the high-k layer and conforming to the shape of the trench, the first layer comprising an aluminum-free metal nitride;

a second layer formed on the first layer and conforming to the shape of the trench, the second layer comprising aluminum and at least one other metal, a ratio of aluminum to the at least one other metal is a gradient having a higher ratio at a center portion of the second layer and a lower ratio at ends of the second layer; and

a third layer formed on the second layer and conforming to the shape of the trench, the third layer comprising an aluminum-free metal nitride.

2. The apparatus of claim 1 , wherein the channel structure comprises at least one of a substrate and an insulator formed on the substrate.

3. The apparatus of claim 1 , wherein the first and third layers comprise one of titanium nitride (TiN) and tantalum nitride (TaN).

4. The apparatus of claim 1 , wherein the second layer includes titanium and aluminum (TiAl).

5. The apparatus of claim 1 , wherein the second layer does not include nitrogen.

6. The apparatus of claim 1 , wherein the aluminum and the at least one other metal comprise atomic layers of the aluminum and the at least one other metal, and

the atomic layers of the aluminum exist in greater concentrations in the center portion of the second layer relative to the ends of the second layer.

7. The apparatus of claim 1 , further comprising a metal gap fill layer formed on the third layer to fill a gap defined by the trench via the high-k layer, and the first, second, and third layers.

8. The apparatus of claim 1 , wherein the FET is a finFET and the side walls are at least one of source and drain structures and channels extending between the source and drain structures.

9. The apparatus of claim 1 , wherein the first layer includes no oxygen.

10. A field-effect transistor replacement gate apparatus, comprising:

a substrate and side walls extending from the substrate to form a trench;

a high dielectric constant (high-k) layer formed on at least the substrate;

a first layer formed on the high-k layer, the first layer comprising an aluminum-free metal nitride;

a second layer formed on the first layer, the second layer comprising aluminum and at least one other metal, a ratio of the aluminum to the at least one other metal being a gradient with a peak located in a center region of the second layer and troughs located at ends of the second layer; and

a third layer formed on the second layer, the third layer comprising an aluminum-free metal nitride.

11. The apparatus of claim 10 , wherein the high-k layer includes hafnium.

12. The apparatus of claim 10 , wherein the first and third layers comprise one of titanium nitride (TiN) and tantalum nitride (TaN).

13. The apparatus of claim 10 , wherein the second layer includes titanium and aluminum (TiAl).

14. The apparatus of claim 10 , wherein the second layer does not include nitrogen.

15. The apparatus of claim 10 , wherein layers of aluminum occur with an increased frequency in the center region of the second layer relative to the ends of the second layer.

16. The apparatus of claim 10 , further comprising a metal gap fill layer formed on the third layer to fill a gap defined by the trench via the high-k layer, and the first, second, and third layers.

17. The apparatus of claim 10 , wherein the FET is a finFET and the side walls are at least one of source and drain structures and channels extending between the source and drain structures.

18. The apparatus of claim 10 , wherein the first layer includes no oxygen.