Stacked Gate-All-Around FinFET and method forming the same

1. A device comprising:

a first semiconductor strip;

a first gate dielectric encircling the first semiconductor strip;

a second semiconductor strip overlapping the first semiconductor strip;

a second gate dielectric encircling the second semiconductor strip, wherein the first gate dielectric contacts the second gate dielectric, wherein each of the first gate dielectric and the second gate dielectric comprises:

a silicon oxide layer encircling the respective one of the first and the second semiconductor strips, and the silicon oxide layer encircling the first semiconductor strip is in contact with the silicon oxide layer encircling the second semiconductor strip; and

a high-k dielectric layer comprising portions on opposite sides of, and contacting, the silicon oxide layer; and

a gate electrode comprising a first portion over the second semiconductor strip, and portions on opposite sides of the first and the second semiconductor strips and the first gate dielectric and the second gate dielectric.

2. The device of claim 1 further comprising:

a first shallow trench isolation region and a second shallow trench isolation region; and

a silicon germanium oxide layer between the first and the second shallow trench isolation regions, wherein the first and the second semiconductor strips overlap a middle portion of the silicon germanium oxide layer.

3. The device of claim 1 , wherein a portion of the gate electrode is overlapped by a portion of the second semiconductor strip, and overlaps a portion of the first semiconductor strip.

4. The device of claim 2 , wherein the silicon germanium oxide layer has a first top surface, and a second and a third top surface on opposite sides of the first top surface, with the second and the third top surfaces being lower than the first top surface.

5. The device of claim 2 , wherein the silicon germanium oxide layer is separated from the first gate dielectric by a portion of the gate electrode.

6. The device of claim 1 further comprising a source/drain region connected to both the first semiconductor strip and the second semiconductor strip.

7. A device comprising:

a substrate;

a first Shallow Trench Isolation (STI) region and a second STI region extending into the substrate;

a silicon germanium oxide layer between the first and the second STI regions, wherein sidewalls of the silicon germanium oxide layer are in contact with sidewalls of the first and the second STI regions;

a plurality of semiconductor strips stacked to overlap the silicon germanium oxide layer;

a gate dielectric encircling each of the plurality of semiconductor strips, wherein portions of the gate dielectric encircling the plurality of semiconductor strips are joined together to form a continuous region, wherein the gate dielectric comprises an extension portion having a bottom surface in contact with a top surface of the silicon germanium oxide layer;

a gate electrode on the gate dielectric, wherein a portion of the gate electrode overlaps a portion of a first one of the plurality of semiconductor strips, and is overlapped by a portion of a second one of the plurality of semiconductor strips; and

source and drain regions connected to opposite ends of the plurality of semiconductor strips.

8. The device of claim 7 , wherein a first gate dielectric encircling a first one of the plurality of semiconductor strips and a second gate dielectric encircling a second one of the plurality of semiconductor strips are continuously connected to each other with no distinguishable interface therebetween.

9. The device of claim 7 , wherein the silicon germanium oxide layer comprises:

a lower portion lower than top surfaces of the first and the second STI regions; and

an upper portion overlying the lower portion, wherein the gate dielectric extends down to contact opposite sidewalls of the upper portion of the silicon germanium oxide layer.

10. The device of claim 7 , wherein the plurality of semiconductor strips is intrinsic.

11. The device of claim 7 , wherein the silicon germanium oxide layer is spaced apart from the gate dielectric by a portion of the gate electrode.

12. A device comprising:

a semiconductor substrate;

a first semiconductor strip over the semiconductor substrate;

a first gate dielectric encircling the first semiconductor strip;

a second semiconductor strip overlapping the first semiconductor strip;

a second gate dielectric encircling the second semiconductor strip, wherein the first gate dielectric contacts the second gate dielectric, wherein each of the first gate dielectric and the second gate dielectric comprises:

a silicon oxide layer encircling the respective one of the first and the second semiconductor strips, wherein the silicon oxide layer of the first gate dielectric contacts the silicon oxide layer of the second gate dielectric; and

a high-k dielectric layer comprising portions on opposite sides of, and contacting, the silicon oxide layer, wherein the portions of the high-k dielectric layer are overlapped by the second semiconductor strip;

a gate electrode comprising a first portion over the second semiconductor strip, and portions on opposite sides of the first and the second semiconductor strips and the first gate dielectric and the second gate dielectric; and

a silicon germanium oxide layer underlying and in contact with the first gate dielectric, wherein the first gate dielectric comprises an extension portion contacting a top surface of the silicon germanium oxide layer.

13. The device of claim 12 , wherein the first gate dielectric and the second gate dielectric are continuously connected to each other with no distinguishable interface therebetween.

14. The device of claim 12 further comprising:

a first isolation region and a second isolation region extending into the semiconductor substrate, wherein the silicon germanium oxide layer is between, and contacting sidewalls of, the first isolation region and the second isolation region.

15. The device of claim 12 , wherein the first and the second semiconductor strips are directly over a middle portion of the silicon germanium oxide layer.

16. The device of claim 12 , wherein the silicon oxide layer of the first gate dielectric and the silicon oxide layer of the second gate dielectric share a middle portion aligned to a straight line interconnecting centers of the first and the second semiconductor strips, and the high-k dielectric layer of the first gate dielectric and the high-k dielectric layer of the second gate dielectric share a first portion and a second portion overlapping the first semiconductor strip, and the first portion and the second portion are separated from each other by the middle portion.

17. The device of claim 12 , wherein the gate electrode comprises:

a first portion overlapped by a first portion of the second semiconductor strip, and overlaps a first portion of the first semiconductor strip; and

a second portion overlapped by a second portion of the second semiconductor strip, and overlaps a second portion of the first semiconductor strip, wherein the first portion and the second portion of the gate electrode are separated from each other by a common gate dielectric region comprising a portion of the first gate dielectric and a portion of the second gate dielectric.

18. The device of claim 2 , wherein the first gate dielectric comprises a portion having a bottom surface contacting a top surface of the silicon germanium oxide layer.

19. The device of claim 9 , wherein the gate dielectric further contacts a top surface of the lower portion of the silicon germanium oxide layer.

20. The device of claim 12 , wherein the silicon oxide layer of the first gate dielectric is in contact with the silicon oxide layer of the second gate dielectric.