Asymmetric FET
After forming a first-side epitaxial semiconductor region and a second-side epitaxial semiconductor region on recessed surfaces of a semiconductor portion that are not covered by a gate structure, at least one dielectric layer is formed to cover the first-side and the second-side epitaxial semiconductor regions and the gate structure. A second-side contact opening is formed within the at least one dielectric layer to expose an entirety of the second-side epitaxial semiconductor region. The exposed second-side epitaxial semiconductor region can be replaced by a new second-side epitaxial semiconductor region having a composition different from the first-side epitaxial semiconductor region or can be doped by additional dopants, thus creating an asymmetric first-side epitaxial semiconductor region and a second-side epitaxial semiconductor region. Each of the first-side epitaxial semiconductor region and the second-side epitaxial semiconducting region can function as either a source or a drain for a transistor.
1. A semiconductor structure comprising:
a gate structure comprising a gate stack located over a channel portion of a semiconductor portion and a gate spacer located on each sidewall of the gate stack, wherein the semiconductor portion is provided on a substrate;
a first-side epitaxial semiconductor region located on a recessed surface of the semiconductor portion and adjoined to a first side of the gate structure, wherein a lower portion of the first-side epitaxial semiconductor region that is embedded in the semiconductor portion is surrounded by the semiconductor portion;
a second-side epitaxial semiconductor region located on another recessed surface of the semiconductor portion and adjoined to a second side of the gate structure opposite the first side; and
a dopant region located within the semiconductor portion and surrounding a lower portion of the second-side epitaxial semiconductor region that is embedded in the semiconductor portion.
2. The semiconductor structure of claim 1 , wherein the first-side epitaxial semiconductor region comprises a first semiconductor material and dopants of a first conductivity type, and the second-side epitaxial semiconductor region comprises the first semiconductor material and dopants of a second conductivity type opposite the first conductivity type.
3. The semiconductor structure of claim 1 , wherein a sidewall of the lower portion of each of the first-side epitaxial semiconductor region and the second-side epitaxial semiconductor region is vertically coincident with an inner sidewall of each gate spacer.
4. The semiconductor structure of claim 1 , wherein a top surface of each of the first-side epitaxial semiconductor region and the second-side epitaxial semiconductor region is located above a top surface of the semiconductor portion.
5. The semiconductor structure of claim 1 , further comprising a first-side contact structure laterally surrounded by at least one dielectric layer and in contact with at least a portion of a top surface of the first-side epitaxial semiconductor region, and a second-side contact structure laterally surrounded by the at least one dielectric layer and in contact with an entire top surface of the second-side epitaxial semiconductor region, wherein the second-side contact structure laterally contacts an upper portion of the gate spacer that is not contacted by the second-side epitaxial semiconductor region.
6. The semiconductor structure of claim 5 , wherein each of the first-side contact structure and the second-side contact structure comprises a contact liner and contact plug wherein a horizontal portion of the contact liner of the second-side contact structure is in contact with the entire top surface of the second-side epitaxial semiconductor region.
7. The semiconductor structure of claim 6 , wherein the contact liner of the first-side contact structure has a composition different from the contact liner of the second-side contact structure.
8. The semiconductor structure of claim 7 , wherein the contact liner of the first-side contact structure comprises NiPt, and the contact liner of the second-side contact structure comprises Ti.
9. The semiconductor structure of claim 5 , wherein the second-side contact structure contacts a portion of a top surface of the gate spacer.
10. The semiconductor structure of claim 1 , wherein each of the second-side epitaxial semiconductor region and the dopant region has a doping concentration greater than a doping concentration of the first-side epitaxial semiconductor region.
11. The semiconductor structure of claim 1 , wherein the dopant region contact a bottom surface and sidewalls of the lower portion of the second-side epitaxial semiconductor region.
12. The semiconductor structure of claim 1 , wherein a topmost surface of the dopant region is in contact with a bottommost surface of a portion of the gate stack.
13. The semiconductor structure of claim 1 , wherein the first-side epitaxial semiconductor region, the second-side epitaxial semiconductor region and the dopant comprise dopants having the same conductivity type.
14. The semiconductor structure of claim 10 , wherein the doping concentration of each of the second-side epitaxial semiconductor region and the dopant region is 1.5 to 3 times the doping concentration of the first-side epitaxial semiconductor region.
15. The semiconductor structure of claim 2 , wherein the semiconductor portion comprises a second semiconductor material that is different from the first semiconductor material.
16. The semiconductor structure of claim 1 , wherein the semiconductor portion is a semiconductor fin.
17. The semiconductor structure of claim 1 , wherein the substrate comprises a handle substrate and a buried insulator layer located on the handle substrate, wherein the semiconductor portion is located on the buried insulator layer.
18. The semiconductor structure of claim 1 , wherein the gate stack comprises a gate dielectric, a gate electrode and a gate cap.
19. The semiconductor structure of claim 1 , wherein the gate spacer laterally contacts an upper portion of each of the first-side epitaxial semiconductor region and the second-side epitaxial semiconductor region.