Controlling the shape of source/drain regions in FinFETs

1. A method comprising:

forming a fin field-effect transistor (FinFET) comprising:

forming a semiconductor fin over and adjacent insulation regions, wherein the;

forming a gate dielectric on a top surface, and extending on sidewalls, of the semiconductor fin;

forming a gate electrode on the gate dielectric; and

forming a source/drain region over the insulation regions and adjoining the semiconductor fin, wherein the forming the source/drain region comprises:

forming a first semiconductor region comprising silicon and an element selected from the group consisting of germanium and carbon, wherein the element has a first atomic percentage in the first semiconductor region, and wherein the first semiconductor region comprises an up-slant facet and a down-slant facet; and

forming a second semiconductor region comprising silicon and the element, wherein the element has a second atomic percentage in the second semiconductor region with the second atomic percentage being lower than the first atomic percentage, wherein the second semiconductor region comprises a first portion on the up-slant facet and having a first thickness, and wherein a second portion of the second semiconductor region on the down-slant facet has a second thickness smaller than the first thickness.

2. The method of claim 1 , wherein, before the step of forming the source/drain region, the semiconductor fin comprises a portion not covered by the gate dielectric and the gate electrode, and wherein the method further comprises:

removing the portion of the semiconductor fin not covered by the gate dielectric and the gate electrode to form a recess;

performing the step of forming the first semiconductor region in the recess using an epitaxial growth; and

performing the step of forming the second semiconductor region using an additional epitaxial growth.

3. The method of claim 1 further comprising siliciding at least a top portion of the second semiconductor region.

4. The method of claim 3 , wherein the second semiconductor region is fully silicided.

5. The method of claim 3 , wherein a bottom portion of the second semiconductor region is un-silicided.

6. The method of claim 1 , wherein the second atomic percentage is lower than the first atomic percentage by greater than about five percent.

7. The method of claim 1 , wherein the second thickness is less than 20 percent of the first thickness.

8. The method of claim 1 , wherein there is substantially no second portion of the second semiconductor region on the down-slant facet.

9. The method of claim 1 , wherein the up-slant facet and the down-slant facet are on (111) planes.

10. The method of claim 1 , wherein the down-slant facet and top surfaces of the insulation regions form an angle equal to about 54.7 degrees.

11. A method comprising:

forming two shallow trench isolation (STI) regions in a silicon substrate and facing each other, with a portion of the silicon substrate therebetween; and

forming a silicon fin over, and horizontally between, the two STI regions;

forming a gate dielectric on a top surface, and extends on sidewalls, of the silicon fin;

forming a gate electrode on the gate dielectric;

removing a portion of the silicon fin not covered by the gate dielectric to form a recess;

epitaxially growing a first SiGe region having a first germanium atomic percentage in the recess, wherein the first SiGe region has an up-slant facet on a (111) plane, and a down-slant facet on an additional (111) plane;

epitaxially growing a second SiGe region having a second germanium atomic percentage lower than the first germanium atomic percentage, wherein the second SiGe region contacts the up-slant facet, and wherein substantially none of the second SiGe region is formed on the down-slant facet; and

siliciding at least a portion of the second SiGe region.

12. The method of claim 11 , wherein the second SiGe region is fully silicided.

13. The method of claim 11 , wherein a bottom portion of the second SiGe region is un-silicided.

14. The method of claim 11 , wherein the second germanium atomic percentage is lower than the first germanium atomic percentage by greater than about five percent.

15. The method of claim 11 , wherein the down-slant facet and top surfaces of the two STI regions form an angle equal to about 54.7 degrees.

16. A method comprising:

epitaxially growing a first SiGe region having a first germanium atomic percentage from a space between two shallow trench isolation (STI) regions, wherein the first SiGe region has an up-slant facet on a (111) plane, and a down-slant facet on an additional (111) plane; and

epitaxially growing a second SiGe region having a second germanium atomic percentage lower than the first germanium atomic percentage, wherein the second SiGe region contacts the up-slant facet, and wherein substantially none of the second SiGe region is formed on the down-slant facet.

17. The method of claim 16 , wherein the first SiGe region is grown from a level lower than top surfaces of the two STI regions, and wherein the up-slant facet are higher than the top surfaces of the two STI regions.

18. The method of claim 16 further comprising siliciding at least a portion of the second SiGe region.

19. The method of claim 18 , wherein a top portion of the second SiGe region on is fully silicided, and wherein a silicide formed the down-slant facet is substantially free from silicide.

20. The method of claim 16 comprising forming a fin field-effect transistor (FinFET), wherein the first SiGe region and the second SiGe region form a source or drain region of the FinFET.