Source/drains in semiconductor devices and methods of forming thereof

1. A method comprising:

forming a gate structure over a silicon on insulator (SOI) substrate, the SOI substrate comprising:

a base semiconductor layer;

an insulator layer over the base semiconductor layer; and

a top semiconductor layer over the insulator layer;

depositing a gate spacer layer over a top surface and along a sidewall of the gate structure;

etching the gate spacer layer to define a gate spacer on the sidewall of the gate structure, wherein etching the gate spacer layer comprises a first etch process, wherein the first etch process damages an exposed top surface of the top semiconductor layer;

after etching the gate spacer layer, etching a recess into the top semiconductor layer using a second etch process, wherein the second etch process reduces a roughness of the damaged top surface of the top semiconductor layer, wherein after the second etch process, a portion of the top semiconductor layer remains directly under the recess;

after the second etch process, extending the recess further into the top semiconductor layer using a third etch process, the second etch process is different from the third etch process; and

after the third etch process, forming a source/drain region in the recess, wherein the first etch process and the second etch process are each dry etch processes, and the first etch process is performed at a higher RF power than the second etch process.

2. The method of claim 1 , wherein after the second etch process and before the third etch process, the recess extends a depth of less than 6{acute over (Å)} into the top semiconductor layer.

3. The method of claim 1 , wherein after the third etch process, the recess extends a depth of 10{acute over (Å)} to 100{acute over (Å)} into the top semiconductor layer.

4. The method of claim 1 , wherein after the third etch process, a thickness of the top semiconductor layer directly under the recess is in a range of 10{acute over (Å)} to 50{acute over (Å)}.

5. The method of claim 1 , wherein after the third etch process, a thickness of the top semiconductor layer under the recess is in a range of 20{acute over (Å)} to 30{acute over (Å)}.

6. The method of claim 1 , wherein the third etch process is a fluorine-ozone mixture (FOM) process.

7. The method of claim 6 , wherein the FOM process is a cyclical process, and wherein each cycle of the FOM process comprises:

oxidizing portions of the top semiconductor layer exposed by the recess using ozone to form an oxide region in the top semiconductor layer; and

etching the oxide region using diluted hydrogen fluoride (dHF).

8. The method of claim 1 , wherein the second etch process is a dry etch process using a chlorine-comprising gas.

9. The method of claim 1 , wherein the third etch process is a wet etch process using H3PO4.

10. A method comprising:

forming a gate stack on a silicon on insulator (SOI) substrate, the SOI substrate comprising a top semiconductor layer on an insulator layer;

forming a gate spacer on a sidewall of the gate stack;

after forming the gate spacer, etching the top semiconductor layer to define a recess in the top semiconductor layer;

extending the recess further into the top semiconductor layer using a fluorine-ozone mixture (FOM) process, wherein each cycle of the FOM process comprises:

forming an oxide region in the top semiconductor layer, the oxide region being on sidewalls and a bottom surface of the recess; and

etching away the oxide region; and

after the FOM process, growing a source/drain region in the recess.

11. The method of claim 10 , wherein the source/drain region extends a distance of 10{acute over (Å)} to 100{acute over (Å)} into the top semiconductor layer.

12. The method of claim 10 , wherein a portion of the top semiconductor layer is under the source/drain region and separates the source/drain region from the insulator layer, a thickness of the portion of the top semiconductor layer is in a range of 10{acute over (Å)} to 50{acute over (Å)}.

13. The method of claim 12 , wherein the thickness of the portion of the top semiconductor layer is in a range of 20{acute over (Å)} to 30{acute over (Å)}.

14. A method comprising:

providing a silicon on insulator (SOI) substrate, the SOI substrate comprising:

a base semiconductor layer;

an insulator layer over the base semiconductor layer; and

a top semiconductor layer over the insulator layer;

forming a source/drain recess in the SOI substrate, wherein forming the source/drain recess comprises a first etch process and a second etch process, the second etch process being different from the first etch process, wherein the source/drain recess extends a first distance into the top semiconductor layer, the first distance is in a range of 10{acute over (Å)} to 100{acute over (Å)}, a portion of the top semiconductor layer is directly under the source/drain recess and separates the source/drain recess from the insulator layer, wherein a thickness of the portion of the top semiconductor layer is in a range of 10{acute over (Å)} to 50{acute over (Å)}, wherein the second etch process is a fluorine-ozone mixture (FOM) process, wherein each cycle of the FOM process comprises:

forming an oxide region in the top semiconductor layer, the oxide region being disposed on sidewalls and a bottom surface of the source/drain recess, wherein forming the oxide region comprises exposing top surfaces and sidewalls of the top semiconductor layer to a process gas comprising ozone; and

etching away the oxide region;

growing a source/drain region in the source/drain recess, the portion of the top semiconductor layer remaining directly under the source/drain region and separating the source/drain region from the insulator layer; and

forming a gate structure on a top surface of the SOI substrate and adjacent to the source/drain region.

15. The method of claim 14 , wherein the first etch process comprises etching the source/drain recess in the top semiconductor layer of the SOI substrate, and the second etch process further comprises extending the source/drain recess further into the top semiconductor layer.

16. The method of claim 14 , wherein the thickness of the portion of the top semiconductor layer is in a range of 20{acute over (Å)} to 30{acute over (Å)}.

17. The method of claim 14 , wherein the first distance is in a range of 20{acute over (Å)} to 30{acute over (Å)}.

18. The method of claim 14 , wherein the first distance is in a range of 17{acute over (Å)} to 25{acute over (Å)}.

19. The method of claim 1 , wherein the first etch process and the second etch process are performed in a same processing chamber.

20. The method of claim 14 , wherein the first etch process is a plasma process that is performed using a chlorine-based etchant.