Replacement body FinFET for improved junction profile with gate self-aligned junctions

1. A method of forming a semiconductor structure comprising:

forming a sacrificial gate structure over a portion of a semiconductor fin, the sacrificial gate structure comprising a sacrificial gate stack and a gate spacer present on sidewalls of the sacrificial gate stack;

forming an epitaxial semiconductor layer comprising dopants of a first conductivity type over portions of the semiconductor fin that is not covered by the sacrificial gate structure;

forming a dopant-containing semiconductor fin by diffusing the dopants from the epitaxial semiconductor layer into the semiconductor fin, wherein a dopant concentration profile across the dopant-containing semiconductor fin is graded such that a portion of the dopant-containing semiconductor fin underneath the sacrificial gate stack has a lower dopant concentration than another portion of the dopant-containing semiconductor fin;

removing the sacrificial gate stack to form a gate cavity exposing a portion of the dopant-containing semiconductor fin;

removing the exposed portion of the dopant-containing semiconductor fin to provide an opening underneath the gate cavity, wherein the removing the exposed portion of the dopant-containing semiconductor fin removes an entirety of the exposed portion of the dopant-containing semiconductor fin to expose a top surface of a semiconductor substrate;

forming a channel region in the opening, wherein the forming the channel region is performed by epitaxially growing a semiconductor material from sidewalls of portions of the dopant-containing semiconductor fin that are covered by the gate spacer and the top surface of the semiconductor substrate; and

forming a functional gate stack in the gate cavity over the channel region.

2. The method of claim 1 , wherein the forming the dopant-containing semiconductor fin is performed by an anneal process.

3. The method of claim 1 , wherein the another portion of the dopant-containing semiconductor fin extends beyond inner sidewalls of the gate spacer and underneath peripheral portions of the sacrificial gate stack.

4. The method of claim 1 , wherein after the removing the exposed portion of the dopant-containing semiconductor fin, remaining portions of the dopant-containing semiconductor fin that are covered by the sacrificial gate structure have sidewalls vertically coincident with inner sidewalls of the gate spacer.

5. The method of claim 1 , further comprising forming a sacrificial spacer on sidewalls of the exposed portion of the dopant-containing semiconductor fin prior to the removing the exposed portion of the dopant-containing semiconductor fin.

6. The method of claim 5 , further comprising removing the sacrificial spacer after the forming the channel region.

7. The method of claim 1 , wherein the channel region is undoped or doped with dopants of a second conductivity type opposite to the first conductivity type.

8. A method of forming a semiconductor structure comprising:

forming a sacrificial gate structure over a portion of a semiconductor fin, the sacrificial gate structure comprising a sacrificial gate stack and a gate spacer present on sidewalls of the sacrificial gate stack;

forming an epitaxial semiconductor layer comprising dopants of a first conductivity type over portions of the semiconductor fin that is not covered by the sacrificial gate structure;

forming a dopant-containing semiconductor fin by diffusing the dopants from the epitaxial semiconductor layer into the semiconductor fin, wherein a dopant concentration profile across the dopant-containing semiconductor fin is graded such that a portion of the dopant-containing semiconductor fin underneath the sacrificial gate stack has a lower dopant concentration than another portion of the dopant-containing semiconductor fin;

removing the sacrificial gate stack to form a gate cavity exposing a portion of the dopant-containing semiconductor fin;

removing the exposed portion of the dopant-containing semiconductor fin to provide an opening underneath the gate cavity, wherein the removing the exposed portion of the dopant-containing semiconductor fin removes an entirety of the exposed portion of the dopant-containing semiconductor fin to expose a top surface of a buried insulator layer;

forming a channel region in the opening, wherein the forming the channel region is performed by epitaxially growing a semiconductor material from sidewalls of portions of the dopant-containing semiconductor fin that are covered by the gate spacer; and

forming a functional gate stack in the gate cavity over the channel region.

9. A method of forming a semiconductor structure comprising:

forming a sacrificial gate structure over a portion of a semiconductor fin, the sacrificial gate structure comprising a sacrificial gate stack and a gate spacer present on sidewalls of the sacrificial gate stack;

forming an epitaxial semiconductor layer comprising dopants of a first conductivity type over portions of the semiconductor fin that is not covered by the sacrificial gate structure;

forming a dopant-containing semiconductor fin by diffusing the dopants from the epitaxial semiconductor layer into the semiconductor fin, wherein a dopant concentration profile across the dopant-containing semiconductor fin is graded such that a portion of the dopant-containing semiconductor fin underneath the sacrificial gate stack has a lower dopant concentration than another portion of the dopant-containing semiconductor fin;

removing the sacrificial gate stack to form a gate cavity exposing a portion of the dopant-containing semiconductor fin;

removing the exposed portion of the dopant-containing semiconductor fin to provide an opening underneath the gate cavity, wherein the removing the exposed portion of the dopant-containing semiconductor fin removes an outer portion of the exposed portion of the dopant-containing semiconductor fin, leaving an interior portion of the exposed portion of the dopant-containing semiconductor fin intact, wherein the interior portion is a least doped portion in the dopant-containing semiconductor fin;

forming a channel region in the opening, wherein the forming the channel region is performed by epitaxially growing a semiconductor material from sidewalls of portions of the dopant-containing semiconductor fin that are covered by the sacrificial gate stack and the interior portion of the exposed portion of the dopant-containing semiconductor fin; and

forming a functional gate stack in the gate cavity over the channel region.

10. The method of claim 9 , wherein the outer portion of the exposed portion of the dopant-containing semiconductor fin is removed by an isotropic etch.