FinFETs and methods for forming the same

1. A method comprising:

removing a dummy gate stack in an inter-layer dielectric to form a trench, wherein a semiconductor fin is exposed to the trench;

doping germanium on the semiconductor fin through the trench to form a germanium channel doping layer; and

forming a gate dielectric and a gate electrode in the trench and over the germanium channel doping layer, wherein the gate dielectric comprises a portion overlapping the germanium channel doping layer.

2. The method of claim 1 , wherein the doping germanium comprises implanting germanium into the semiconductor fin through tilt implantation.

3. The method of claim 1 , wherein the doping germanium comprises:

depositing a germanium layer on surfaces of the semiconductor fin; and

performing a thermal process to diffuse the germanium into the semiconductor fin.

4. The method of claim 1 , wherein the doping germanium comprises:

depositing a silicon germanium layer on surfaces of the semiconductor fin; and

performing an oxidation process to concentrate germanium in the silicon germanium layer into the semiconductor fin.

5. The method of claim 4 , wherein a silicon oxide layer is formed by the oxidation process, and the method further comprises, before the gate dielectric is formed, removing the silicon oxide layer.

6. The method of claim 1 further comprising:

before the dummy gate stack is formed, performing a doping process to dope the semiconductor fin with germanium.

7. The method of claim 6 further comprising:

forming a source region and a drain region, wherein the dummy gate stack is between the source region and the drain region, and in the doping process, germanium is doped into the semiconductor fin that is between the source region and the drain region.

8. The method of claim 1 , wherein the semiconductor fin comprises silicon, and the germanium is doped into surface portions of the semiconductor fin, with an inner portion of the semiconductor fin undoped with the germanium.

9. A method comprising:

forming a dummy gate stack on a top surface and sidewalls of a silicon fin;

forming an inter-layer dielectric comprising portions on opposite sides of the dummy gate stack;

removing the dummy gate stack to leave a trench in the inter-layer dielectric;

performing a first doping process through the trench to dope germanium into an outer portion of the silicon fin, wherein a germanium-doped layer is formed on a top surface and sidewall surfaces of an inner region of the silicon fin, and wherein the inner region of the silicon fin is undoped with germanium when germanium is doped into the outer portion;

forming a gate dielectric over the germanium-doped layer;

forming a gate electrode over the gate dielectric; and

forming a source region and a drain region connecting to opposing ends of the silicon fin.

10. The method of claim 9 , wherein the source region and the drain region are formed before the first doping process.

11. The method of claim 9 further comprising:

before the forming the dummy gate stack, performing a second doping process to dope germanium into the silicon fin.

12. The method of claim 11 , wherein the source region and the drain region are formed at a time between the first doping process and the second doping process.

13. The method of claim 9 , wherein the inner region of the silicon fin comprises silicon and carbon, and is substantially free from germanium.

14. The method of claim 9 , wherein the first doping process comprises:

depositing a germanium layer on a top surface and sidewalls of the silicon fin; and

performing a thermal anneal process to diffuse germanium in the germanium layer into the outer portion of the silicon fin, wherein the germanium does not diffuse into the inner region of the silicon fin.

15. The method of claim 14 , wherein the thermal anneal process is performed using hydrogen (H 2 ) or an inert gas as a process gas.

16. The method of claim 9 , wherein the first doping process comprises:

depositing a silicon germanium layer on a top surface and sidewalls of the silicon fin;

performing an oxidation process, so that germanium diffuse into the silicon fin to form the germanium-doped layer, wherein a silicon oxide layer is generated on the germanium-doped layer; and

removing the silicon oxide layer.

17. A method comprising:

recessing isolation regions, so that a portion of a semiconductor strip between the isolation regions protrudes higher than top surfaces of the recessed isolation regions to form a semiconductor fin;

depositing a silicon germanium layer on a top surface and sidewalls of a first portion of the semiconductor fin, wherein when the silicon germanium layer is deposited, a second and a third portion of the semiconductor fin on opposite sides of the first portion are covered by dielectric materials;

performing a thermal oxidation process on the silicon germanium layer to form a germanium-doped layer;

removing a silicon oxide layer generated by the thermal oxidation process to reveal the germanium-doped layer; and

forming a gate dielectric and a gate electrode over the germanium-doped layer.

18. The method of claim 17 , wherein the germanium-doped layer is deposited on a top surface and sidewalls of the semiconductor fin, and the semiconductor fin comprises silicon.

19. The method of claim 17 , wherein the thermal oxidation process is performed in an oxygen (O 2 ) containing environment.

20. The method of claim 17 further comprising:

forming a source/drain region based on a portion of the semiconductor fin;

forming a dummy gate on the semiconductor fin; and

removing the dummy gate, wherein the silicon germanium layer is deposited after the dummy gate is removed.