Sacrificial layer for channel surface retention and inner spacer formation in stacked-channel FETs
Field effect transistors include a stack of nanosheets of vertically arranged channel layers. A gate stack is formed over, around, and between the vertically arranged channel layers. Spacers are formed, with at least one top pair of spacers being positioned above an uppermost channel layer. The top pair of spacers each has a curved lower portion with a curved surface in contact with the gate stack and a straight upper portion that extends vertically from the curved portion along a straight sidewall of the gate stack.
1. A field effect transistor, comprising:
a stack of nanosheets of vertically arranged channel layers;
a gate stack formed over, around, and between the vertically arranged channel layers; and
a plurality of spacers, with at least one top pair of spacers being positioned above an uppermost channel layer, the at least one top pair of spacers each comprising a curved lower portion with a curved surface in contact with the gate stack and a straight upper portion that extends vertically from the curved lower portion along a straight sidewall of the gate stack.
2. The field effect transistor of claim 1 , wherein the plurality of spacers each have a crescent shape.
3. The field effect transistor of claim 2 , wherein the source and drain regions have bulges complementary to recesses in the crescent-shaped spacers.
4. The field effect transistor of claim 2 , wherein the gate stack is recessed to accommodate a crescent shape of the topmost pair of spacers.
5. The field effect transistor of claim 1 , wherein each of the stack of nanosheets has a cross-sectional width that is significantly greater than a cross-sectional height.
6. The field effect transistor of claim 1 , wherein a top channel layer of the stack of nanosheets is not damaged from an anisotropic etch.
7. The field effect transistor of claim 6 , wherein the top channel layer has a uniform thickness between portions that are between the top pair of spacers and a lower pair of spacers and portions that are in the gate stack.
8. A field effect transistor, comprising:
a stack of nanosheets of vertically arranged channel layers;
merged source and drain regions at respective ends of the vertically arranged channel layers, wherein each merged source and drain region contacts multiple channel layers;
a gate stack formed over, around, and between the vertically arranged channel layers; and
a pair of top spacers positioned above an uppermost channel layer, each comprising a crescent-shaped lower portion with a curved surface in contact with the gate stack and a straight upper portion that extends vertically from the curved portion along a straight sidewall of the gate stack.
9. The field effect transistor of claim 8 , wherein the merged source and drain regions share a crystalline structure with the channel layers.
10. The field effect transistor of claim 9 , wherein the each merged source and drain region is epitaxially grown from a respective set of ends of the channel layers.
11. The field effect transistor of claim 10 , wherein the source and drain regions have bulges complementary to recesses in the crescent-shaped internal spacers.
12. The field effect transistor of claim 10 , wherein the gate stack is recessed to accommodate a crescent shape of the topmost pair of spacers.
13. The field effect transistor of claim 8 , further comprising vestigial source and drain regions formed in lateral gate stacks on each side of the gate stack.
14. The field effect transistor of claim 8 , wherein each of the stack of nanosheets has a cross-sectional width that is significantly greater than a cross-sectional height.
15. The field effect transistor of claim 8 , wherein a top channel layer of the stack of nanosheets is not damaged from an anisotropic etch.
16. The field effect transistor of claim 15 , wherein the top channel layer has a uniform thickness between portions that are between the top pair of spacers and a lower pair of spacers and portions that are in the gate stack.
17. A field effect transistor, comprising:
a stack of nanosheets of vertically arranged channel layers, wherein a top channel layer of the stack of nanosheets is not damaged from an anisotropic etch;
a gate stack formed over, around, and between the vertically arranged channel layers; and
a plurality of spacers, with at least one top pair of spacers being positioned above an uppermost channel layer, the top pair of spacers each comprising a curved lower portion with a curved surface in contact with the gate stack and a straight upper portion that extends vertically from the curved portion along a straight sidewall of the gate stack.
18. The field effect transistor of claim 17 , wherein the spacers each have a crescent shape.
19. The field effect transistor of claim 18 , wherein the source and drain regions have bulges complementary to recesses in the crescent-shaped spacers.
20. The field effect transistor of claim 17 , wherein the top channel layer has a uniform thickness between portions that are between the top pair of spacers and a lower pair of spacers and portions that are in the gate stack.