Fin field-effect transistor and method of forming the same

1. A method for manufacturing a semiconductor device, comprising:

forming a gate trench over a semiconductor fin, the gate trench including an upper portion and a lower portion, wherein the upper portion is wider than the lower portion;

sequentially forming one or more work function layers, a capping layer, and a glue layer to line the gate trench, wherein the glue layer comprises a first sub-layer and a second sub-layer that have respective different etching rates with respect to an etching solution; and

removing, using the etching solution, the second sub-layer, while leaving a first portion of the first sub-layer fully filled in the lower portion of the gate trench and having at least the capping layer remain in the upper portion.

2. The method of claim 1 , wherein the upper portion of the gate trench is surrounded by first gate spacers, and the lower portion of the gate trench is surrounded by second gate spacers and the first gate spacers.

3. The method of claim 1 , wherein in the step of sequentially forming one or more work function layers, a capping layer, and a glue layer over the gate trench, at least the first portion of the first sub-layer fills the lower portion of the gate trench so as to cause the second sub-layer to be disposed only in the upper portion of the gate trench.

4. The method of claim 1 , wherein the first sub-layer includes tantalum nitride, and the second sub-layer includes titanium nitride.

5. The method of claim 1 , wherein the etching solution includes a mixture of ammonium hydroxide (NH 4 OH) and hydrogen peroxide (H 2 O 2 ).

6. The method of claim 1 , wherein the etching rate of the second sub-layer is greater than the etching rate of the first sub-layer.

7. The method of claim 1 , wherein the step of removing the second sub-layer further comprises removing a second portion of the first-sub layer that is disposed in the upper portion of the gate trench, thereby forming a surface shared at least by the first portion of the first sub-layer and the capping layer.

8. The method of claim 1 , further comprising:

removing a portion of the capping layer in the upper portion of the gate trench; and

removing a portion of the one or more work function layers in the upper portion of the gate trench to form a metal gate in the lower portion of the gate trench.

9. The method of claim 8 , further comprising:

depositing a metal material in the gate trench to form a gate electrode overlaying the metal gate in the lower portion of the gate trench.

10. The method of claim 5 , wherein a volume ratio between the NH 4 OH and the H 2 O 2 is between about 1:1 and about 1:10.

11. A method for manufacturing a semiconductor device, comprising:

forming a dummy gate structure that overlays a portion of a semiconductor fin;

removing the dummy gate structure to form a gate trench; and

forming a metal gate over the portion of the semiconductor fin through a plurality of sequential etching processes,

wherein the metal gate comprises a first portion of a glue layer that remains from a first one of the plurality of etching processes, wherein the first etching process removes a second portion of the glue layer while leaving a capping layer and one or more work function layers upwardly extending above the first portion of the glue layer and fully filling a lower portion of the gate trench with the first portion, and wherein the first portion has a first material and the second portion has a second material different from the first material.

12. The method of claim 11 , wherein the first material includes tantalum nitride, and the second material includes titanium nitride.

13. The method of claim 11 , wherein the first etching process includes applying a solution over the glue layer.

14. The method of claim 13 , wherein the solution includes a mixture of ammonium hydroxide (NH 4 OH) and hydrogen peroxide (H 2 O 2 ).

15. The method of claim 14 , wherein a volume ratio between the NH 4 OH and the H 2 O 2 is between about 1:1 and about 1:10.

16. The method of claim 13 , wherein the first material has a first etching rate with respect to the solution and the second materials has a second etching rate with respect to the solution, and wherein the first etching rate is lower than the second etching rate.

17. The method of claim 11 , wherein the metal gate includes a portion of the capping layer that remains from a second one of the plurality of etching processes, and respective portions of the one or more work function layers that remain from a third one of the plurality of etching processes.

18. The method of claim 11 , further comprising:

depositing a metal material in the gate trench to form a gate electrode that electrically connects to the metal gate.

19. The method of claim 11 , wherein during the first etching process, the first portion is overlaid by the second portion.

20. A method for manufacturing a semiconductor device, comprising:

forming a gate trench exposing a portion of a semiconductor fin, the gate trench including an upper portion and a lower portion, wherein the upper portion is wider than the lower portion; and

forming, through a plurality of sequential etching processes, a metal gate over the portion of the semiconductor fin in the lower portion of the gate trench;

wherein the metal gate comprises: (a) a first portion of a glue layer that remains from a first one of the plurality of etching processes; (b) a first portion of a capping layer that remains from a second one of the plurality of etching processes; (c) and respective first portions of one or more work function layers that remain from a third one of the plurality of etching processes;

wherein the first etching process removes a second portion of the glue layer while leaving a second portion of the capping layer and respective second portions of the one or more work function layers in the upper portion and leaving the first portion of the glue layer fully filled in the lower portion of the gate trench; and

wherein the first portion has a first material and the second portion has a second material different from the first material.