Semiconductor device and method

1. A method comprising:

forming a multi-layer stack over a semiconductor substrate, the multi-layer stack comprising a first sacrificial layer over a semiconductor substrate, a first channel layer over the first sacrificial layer, a second sacrificial layer over the first channel layer, and a second channel layer over the second sacrificial layer, the first sacrificial layer having a first atomic concentration of a first semiconductor element, the second sacrificial layer having a second atomic concentration of the first semiconductor element, the second atomic concentration being less than the first atomic concentration;

patterning the multi-layer stack and the semiconductor substrate to form a first trench;

forming an isolation region in the first trench;

forming a first gate stack over the patterned multi-layer stack and isolation region;

etching the patterned multi-layer stack to form a first recess adjacent the first gate stack, the etching comprising an isotropic etching process;

epitaxially growing a first source/drain region in the first recess; and

replacing the first gate stack and the first and second sacrificial layers of the patterned and etched multi-layer stack with a second gate stack, the second gate stack surrounding each of the etched first channel layer and the etched second channel layer.

2. The method of claim 1 , wherein the first semiconductor element is germanium.

3. The method of claim 2 , wherein the first sacrificial layer comprises silicon germanium.

4. The method of claim 1 , wherein etching the patterned multi-layer stack to form the first recess etches the first sacrificial layer at a first etch rate and etches the second sacrificial layer at a second etch rate, the second etch rate being less than the first etch rate.

5. The method of claim 1 , wherein the first sacrificial layer has the first atomic concentration of the first semiconductor element throughout the entirety of the first sacrificial layer.

6. The method of claim 1 , wherein forming the first multi-layer stack over the semiconductor substrate further comprises epitaxially growing each of the first sacrificial layer, the first channel layer, the second sacrificial layer, and the second channel layer.

7. The method of claim 1 , wherein etching the patterned multi-layer stack to form the first recess adjacent the first gate stack further comprises:

anisotropically etching the patterned multi-layer stack and the semiconductor substrate; and

after anisotropically etching, isotropically etching the patterned multi-layer stack and the semiconductor substrate, the isotropically etching step recessing sidewalls of the first and second sacrificial layers of the patterned multi-layer stack.

8. The method of claim 7 further comprising:

forming an inner spacer on the recessed sidewalls of the first and second sacrificial layers, wherein after the second gate stack is formed, the inner spacer is between the second gate stack and the first source/drain region.

9. The method of claim 8 , wherein the inner spacer comprises multiple spacer layers having different material compositions.

10. A method comprising:

forming a multi-layer fin structure over a semiconductor substrate, forming the multi-layer fin structure comprising:

epitaxially growing a first sacrificial layer over a semiconductor substrate, the first sacrificial layer having a first portion and a second portion, the first portion having a first atomic concentration of a first semiconductor element, the second portion having a second atomic concentration of the first semiconductor element, the second atomic concentration being different than the first atomic concentration;

epitaxially growing a first channel layer from the first sacrificial layer;

epitaxially growing a second sacrificial layer from the first channel layer, the second sacrificial layer having a first portion and a second portion, the first portion having a third atomic concentration of the first semiconductor element, the second portion having a fourth atomic concentration of the first semiconductor element, the fourth atomic concentration being different than the third atomic concentration;

epitaxially growing a second channel layer from the second sacrificial layer; and

patterning the first sacrificial layer, the first channel layer, the second sacrificial layer, the second channel layer, and the semiconductor substrate to form the multi-layer fin structure;

forming a dummy gate stack over the multi-layer fin structure;

etching the multi-layer fin structure to form a first recess adjacent the dummy gate stack, the etching comprising an isotropic etching process;

epitaxially growing a first source/drain region in the first recess; and

replacing the dummy gate stack and the first and second sacrificial layers of the etched multi-layer fin structure with an active gate stack, the active gate stack surrounding the etched first channel layer and the etched second channel layer.

11. The method of claim 10 , wherein after etching the multi-layer fin structure, the etched first and second sacrificial layers of the etched multi-layer fin structure have planar sidewalls.

12. The method of claim 10 , wherein after etching the multi-layer fin structure, the etched first and second sacrificial layers of the etched multi-layer fin structure have notched sidewalls.

13. The method of claim 10 , wherein after etching the multi-layer fin structure, the etched first and second sacrificial layers of the etched multi-layer fin structure have tapered sidewalls.

14. The method of claim 10 , wherein the first semiconductor element is germanium.

15. The method of claim 10 , wherein the first portion of the first sacrificial layer is a top portion of the first sacrificial layer, the second portion of the first sacrificial layer being a middle portion of the first sacrificial layer, the top portion and a bottom portion of the first sacrificial layer having a higher atomic concentration of the first semiconductor element than the middle portion of the first sacrificial layer, the middle portion being between the top and bottom portions.

16. A method comprising:

forming a first sacrificial layer over a semiconductor substrate;

forming a first channel region over the first sacrificial layer, the first sacrificial layer having a first atomic concentration of a first semiconductor element;

forming a second sacrificial layer over the first channel region;

forming a second channel region over the second sacrificial layer, the second sacrificial layer having a second atomic concentration of the first semiconductor element, the second atomic concentration being less than the first atomic concentration;

forming a first gate stack over the semiconductor substrate, the first channel region, the first sacrificial layer, the second sacrificial layer, and the second channel region;

forming a first inner spacer extending from the first channel region to the second channel region and along a sidewall of the first gate stack;

forming a second inner spacer extending from the first channel region to the second channel region and along a sidewall of the first inner spacer, the second inner spacer having a different material composition than the first inner spacer, the first inner spacer and the second inner spacer each physically contacting a top surface of the first channel region and a bottom surface of the second channel region;

epitaxially growing a first source/drain region adjacent the first channel region, the second channel region, and the second inner spacer, the first source/drain region physically contacting the first channel region; and

replacing the first gate stack and the first and second sacrificial layers with a second gate stack, the second gate stack surrounding each of the first channel region and the second channel region, the first and second inner spacers being between the second gate stack and the first source/drain region.

17. The method of claim 16 , wherein the first inner spacer physically contacts the second gate stack, and wherein the second inner spacer physically contacts the first source/drain region.

18. The method of claim 17 , wherein the first inner spacer physically contacts the second gate stack at a concave surface of the second gate stack.

19. The method of claim 16 , wherein the first inner spacer comprises SiCN, and wherein the second inner spacer comprises SiN.

20. The method of claim 16 , wherein the first inner spacer and the second inner spacer each extend from a top surface of the first channel region to a bottom surface of the second channel region.