Multi-gate device with air gap spacer and fabrication methods thereof

1. A semiconductor device, comprising:

a substrate;

first and second dielectric fins disposed above the substrate;

a semiconductor channel layer sandwiched between the first and second dielectric fins;

a gate structure engaging the semiconductor channel layer;

a source/drain (S/D) feature abutting the semiconductor channel layer and sandwiched between the first and second dielectric fins; and

an air gap sandwiched between the first and second dielectric fins, wherein the air gap exposes a first sidewall of the S/D feature facing the first dielectric fin and a second sidewall of the S/D feature facing the second dielectric fin.

2. The semiconductor device of claim 1 , further comprising:

a silicide feature atop the S/D feature, wherein the silicide feature has a larger width than the S/D feature.

3. The semiconductor device of claim 1 , further comprising:

an isolation feature disposed on sidewalls of each of the first and second dielectric fins, wherein the isolation feature separates each of the first and second dielectric fins from contacting the substrate.

4. The semiconductor device of claim 1 , further comprising:

an isolation feature disposed on sidewalls of each of the first and second dielectric fins, wherein each of the first and second dielectric fins is in contact with the substrate.

5. The semiconductor device of claim 1 , further comprising:

a gate sidewall spacer disposed on sidewalls of the gate structure, wherein a portion of the semiconductor channel layer directly under the gate sidewall spacer is fully surrounded by the air gap in a cross-sectional plane perpendicular to a top surface of the substrate.

6. The semiconductor device of claim 1 , further comprising:

an interlayer dielectric layer disposed above the S/D feature, wherein the air gap exposes a bottom surface of the interlayer dielectric layer.

7. The semiconductor device of claim 1 , wherein the air gap extends horizontally from the gate structure to the S/D feature.

8. The semiconductor device of claim 1 , wherein the air gap fully surrounds a portion of the S/D feature in a horizontal plane parallel to a top surface of the substrate.

9. The semiconductor device of claim 1 , wherein the semiconductor channel layer is suspended above the substrate.

10. A semiconductor device, comprising:

a substrate;

first and second dielectric features disposed over the substrate, each of the first and second dielectric features having a first sidewall and a second sidewall;

a dielectric layer disposed on the first sidewall of the first dielectric feature and the second sidewall of the second dielectric feature, wherein the second sidewall of the first dielectric feature directly faces the first sidewall of the second dielectric feature; and

an epitaxial feature sandwiched between the second sidewall of the first dielectric feature and the first sidewall of the second dielectric feature.

11. The semiconductor device of claim 10 , further comprising:

a semiconductor layer abutting the epitaxial feature; and

a gate structure engaging the semiconductor layer.

12. The semiconductor device of claim 11 , wherein the semiconductor layer is disposed between the first and second dielectric features.

13. The semiconductor device of claim 11 , further comprising:

a gate sidewall spacer disposed on sidewalls of the gate structure, wherein the gate sidewall spacer is in contact with top surfaces of the first and second dielectric features and the dielectric layer.

14. The semiconductor device of claim 10 , further comprising:

interlayer dielectric layer disposed above the first and second dielectric features and the dielectric layer; and

a silicide feature disposed above the epitaxial feature, wherein a portion of the silicide feature is directly under the interlayer dielectric layer.

15. The semiconductor device of claim 10 , wherein the epitaxial feature is spaced from the second sidewall of the first dielectric feature and the first sidewall of the second dielectric feature by an air gap.

16. A method of manufacturing a semiconductor device, comprising:

forming a semiconductor fin protruding from a substrate, the semiconductor fin having a channel region and a source/drain (S/D) region;

forming first and second dielectric fins sandwiching the semiconductor fin;

depositing a first dielectric layer filling space between the semiconductor fin and the first and second dielectric fins;

recessing the semiconductor fin in the S/D region, resulting in a trench exposing sidewalls of the first dielectric layer;

forming an epitaxial feature in the trench;

depositing a second dielectric layer over the epitaxial feature and the first dielectric layer;

patterning the second dielectric layer, thereby forming an opening in the second dielectric layer, the opening exposing the epitaxial feature and the first dielectric layer; and

partially removing the first dielectric layer from the opening.

17. The method of claim 16 , wherein the partially removing of the first dielectric layer results in an air gap between the epitaxial feature and the first dielectric fin and between the epitaxial feature and the second dielectric fin.

18. The method of claim 16 , wherein a portion of a top surface of the first dielectric layer exposed by the opening has a width about one eighth to about one third of a width of the epitaxial feature.

19. The method of claim 16 , further comprising:

forming a gate structure over the channel region;

forming a spacer feature between the gate structure and the epitaxial feature; and

etching the spacer feature through the opening.

20. The method of claim 16 , further comprising:

forming a conductive feature in the opening, wherein a portion of the conductive feature extends directly under a bottom surface of the second dielectric layer.