Metal gate structures for field effect transistors

1. A method, comprising:

forming an isolation layer on a bottom portion of a fin structure disposed on a substrate;

forming a dielectric stack on a portion of the fin structure not covered by the isolation layer;

depositing a stack of metal-based layers on the dielectric stack;

depositing a first titanium-aluminum (TiAl) layer on and in physical contact with the stack of metal-based layers, wherein the first TiAl layer comprises a first Al/Ti ratio with a non-zero value; and

depositing a second TiAl layer on the first TiAl layer, wherein the second TiAl layer comprises a second Al/Ti ratio, greater than the first Al/Ti ratio.

2. The method of claim 1 , wherein depositing the first and second TiAl layers comprises depositing the first TiAl layer with the first Al/Ti ratio equal to or less than about 80% of the second Al/Ti ratio.

3. The method of claim 1 , wherein depositing the first TiAl layer comprises depositing the first TiAl layer with a varying Al/Ti ratio between a top surface and a bottom surface of the first TiAl layer.

4. The method of claim 1 , wherein depositing the first TiAl layer comprises depositing the first TiAl layer with an increased Al/Ti ratio at an interface between the first TiAl layer and the second TiAl layer.

5. The method of claim 1 , wherein depositing the first and second TiAl layers comprises depositing the first and second TiAl layers with a dual-precursor source to independently introduce a titanium and an aluminum concentration in the first and second TiAl layers.

6. The method of claim 5 , wherein the dual-precursor source comprises tetrachloride (TiCl 4 ) and trimethylaluminum (Al 2 (CH 3 ) 6 ) or titanium tetrachloride (TiCl 4 ) and aluminumtriethyl ((C 2 H 5 ) 3 Al).

7. The method of claim 1 , wherein depositing the first and second TiAl layers comprises depositing the first and second TiAl layers at a temperature between about 250° C. and about 600° C.

8. A method, comprising:

depositing a dielectric stack over a fin structure formed on a substrate;

depositing a capping layer on the dielectric stack;

depositing a first titanium-aluminum (TiAl) layer on the capping layer, wherein the first TiAl layer comprises a first Al/Ti ratio;

depositing a second TiAl layer comprising a second Al/Ti ratio greater than the first Al/Ti ratio; and

depositing a third TiAl layer on the second TiAl layer and comprising a third Al/Ti ratio less than the second Al/Ti ratio.

9. The method of claim 8 , wherein depositing the first and third TiAl layers comprises forming the first and third TiAl layers with the first and third Al/Ti ratios being less than about 80% of the second Al/Ti ratio.

10. The method of claim 8 , wherein depositing the first TiAl layer comprises varying the first Al/Ti ratio between a top surface and a bottom surface of the first TiAl layer.

11. The method of claim 8 , wherein depositing the first TiAl layer comprises increasing the first Al/Ti ratio at an interface between the first TiAl layer and the second TiAl layer.

12. The method of claim 8 , wherein depositing the third TiAl layer comprises varying the third Al/Ti ratio between a top surface and a bottom surface of the third TiAl layer.

13. The method of claim 8 , wherein depositing the third TiAl layer comprises increasing the third Al/Ti ratio at an interface between the second TiAl layer and the third TiAl layer.

14. The method of claim 8 , wherein depositing the first and third TiAl layers comprises:

varying the first Al/Ti ratio between a top surface and a bottom surface of the first TiAl layer; and

varying the third Al/Ti ratio between a top surface and a bottom surface of the third TiAl layer.

15. The method of claim 8 , wherein depositing the first and third TiAl layers comprises:

increasing the first Al/Ti ratio at an interface between the first and second TiAl layers; and

increasing the third Al/Ti ratio at an interface between the second and third TiAl layers.

16. A method, comprising:

forming a dielectric stack on a fin structure disposed on a substrate;

depositing a capping layer on the dielectric stack;

depositing a barrier layer on the capping layer;

depositing a titanium-aluminum (TiAl) stack on the barrier layer, wherein the TiAl stack comprises a plurality of TiAl layers, and wherein an Al/Ti ratio at a top surface of the TiAl stack is higher than an Al/Ti ratio at a bottom surface of the TiAl stack in contact with the barrier layer; and

depositing a metal fill on the TiAl stack.

17. The method of claim 16 , wherein depositing the TiAl stack comprises depositing a TiAl layer of the plurality of TiAl layers with a varying Al/Ti ratio between a top surface and a bottom surface of the TiAl layer.

18. The method of claim 16 , wherein depositing the TiAl stack comprises depositing a first TiAl layer of the plurality of TiAl layers with a higher Al/Ti ratio than a second TiAl layer of the plurality of TiAl layers.

19. The method of claim 16 , wherein depositing the TiAl stack comprises:

depositing a first TiAl layer with a first Al/Ti ratio; and

depositing, on the first TiAl layer, a second TiAl layer with a second Al/Ti ratio greater than the first Al/Ti ratio.

20. The method of claim 16 , wherein depositing the TiAl stack comprises:

depositing a first TiAl layer with a first Al/Ti ratio;

depositing, on the first TiAl layer, a second TiAl layer with a second Al/Ti ratio; and

depositing, on the second TiAl layer, a third TiAl layer with a third Al/Ti ratio, wherein the second Al/Ti ratio is greater than the first and third Al/Ti ratios.