Semiconductor device with facet S/D feature and methods of forming the same

1. A method of forming a semiconductor device, comprising:

alternately forming first semiconductor layers and second semiconductor layers over a substrate, wherein the first semiconductor layers and the second semiconductor layers include different materials and are stacked up along a direction substantially perpendicular to a top surface of the substrate;

forming a dummy gate structure over the first and second semiconductor layers;

forming a source/drain (S/D) trench along a sidewall of the dummy gate structure;

forming inner spacers between edge portions of the first semiconductor layers, wherein the inner spacers are bended towards the second semiconductor layers;

forming a hard mask layer directly on the inner spacers and the dummy grate structure thereby covering the inner spacers and the dummy gate structure; and

epitaxially growing a S/D feature in the S/D trench, wherein the S/D feature contacts the first semiconductor layers and includes facets forming a recession away from the inner spacers.

2. The method of claim 1 , wherein the forming the inner spacers between edge portions of the first semiconductor layers includes:

selectively removing edge portions of the second semiconductor layers from the S/D trench;

conformally depositing an inner spacer layer in the S/D trench to partially fill in the removed edge portions of the second semiconductor layers; and

removing portions of the inner spacer layer along the direction substantially perpendicular to the top surface of the substrate to form the inner spacers.

3. The method of claim 2 , wherein the forming the inner spacers between edge portions of the first semiconductor layers further includes:

forming a sacrificial layer over the inner spacer layer in the S/D trench, wherein the sacrificial layer and the inner spacer layer fill up the removed edge portions of the second semiconductor layers; and

removing portions of the sacrificial layer along the direction substantially perpendicular to the top surface of the substrate to expose sidewalls of the first semiconductor layers in the S/D trench.

4. The method of claim 2 , wherein the inner spacer layer is conformally deposited for a thickness of about 1 nm to about 5 nm.

5. The method of claim 1 , wherein the epitaxially growing a S/D feature in the S/D trench includes epitaxially growing a first epitaxial S/D layer directly on the first semiconductor layers.

6. The method of claim 5 , wherein the epitaxially growing a S/D feature in the S/D trench includes epitaxially growing the first epitaxial S/D layer with hydrogen chloride (HCl) at a flow rate of about 100 sccm to about 300 sccm, and under a temperature of about 600° C. to about 800° C.

7. The method of claim 1 , further comprising replacing the dummy gate structure with a metal gate structure.

8. The method of claim 1 , further comprising removing the hard mask layer to expose the inner spacers prior to epitaxially growing the S/D feature in the S/D trench.

9. A method of forming a semiconductor device, comprising:

forming a first semiconductor stack over a first region of a substrate and forming a second semiconductor stack over a second region of the substrate, wherein each of the first semiconductor stack and the second semiconductor stack includes first semiconductor layers and second semiconductor layers having different materials, and wherein the first semiconductor layers and second semiconductor layers are stacked up along a direction substantially perpendicular to a top surface of the substrate;

forming a dummy gate structure over channel regions of the first semiconductor stack and the second semiconductor stack;

selectively removing edge portions of the second semiconductor layers to form first gaps between edge portions of the first semiconductor layers;

conformally forming inner spacers partially filling in the first gaps;

forming sacrificial features to fill up the first gaps within the inner spacers;

forming a hard mask layer directly on the sacrificial features filling the first gaps in the second region of the substrate such that the sacrificial features are covered by the hard mask layer in the second region of the substrate;

removing the sacrificial features in the first region of the substrate to form second gaps surrounded by the inner spacers; and

epitaxially growing a source/drain (S/D) feature over a S/D region in the first region of the substrate, wherein the S/D feature is epitaxially grown from the sidewalls of the first semiconductor layers and merged along the direction substantially perpendicular to the top surface of the substrate to form third gaps away from the inner spacers.

10. The method of claim 9 , wherein the S/D feature is epitaxially grown to form facets along the direction <111> to form the third gaps.

11. The method of claim 9 , wherein a depth of each of the first gap is about 5 nm to about 20 nm.

12. The method of claim 9 , wherein an opening of each of the second gap is about 2 nm to about 18 nm.

13. The method of claim 9 , wherein an opening of each of the third gap is about 4 nm to about 20 nm.

14. A method comprising:

forming a first semiconductor layer and a second semiconductor layer over a substrate;

forming a gate structure over the first and second semiconductor layers;

removing a first portion of the first and second semiconductor layer thereby forming a trench;

removing a second portion of the first semiconductor layer to thereby form a recess within the first semiconductor layer;

forming a spacer layer within the recess and along a sidewall of the second semiconductor layer;

forming a sacrificial layer within the recess and on the spacer layer disposed along the sidewall of the second semiconductor layer;

removing, during the same etching process, the sacrificial layer disposed along the sidewall of the second semiconductor layer and the spacer layer disposed on the sidewall of the second semiconductor layer such that the sidewall of the second semiconductor layer is exposed;

removing the sacrificial layer from within the recess, wherein the spacer layer is disposed within the recess after the removing of the sacrificial layer from within the recess; and

forming a source/drain feature in the trench, wherein an air gap extends from within the recess to the trench after forming the source/drain feature in the trench, wherein the spacer layer defines a first edge of the air gap and the source/drain feature defines a second edge of the air gap.

15. The method of claim 14 , wherein the forming of the sacrificial layer within the recess and on the spacer layer disposed along the sidewall of the second semiconductor layer further includes forming the sacrificial layer over the gate structure.

16. The method of claim 14 , wherein the second edge of the air gap defines by the source/drain feature includes a facet of the S/D features formed along a direction <111>.

17. The method of claim 14 , wherein a first portion of the air gap has a cross-section view of an isosceles triangle-shape, and

wherein a second portion of the air gap has a cross-section view of a half-ellipse-shape, a triangle-shape with curved sides, or a rectangular-shape with round corners.

18. The method of claim 14 , wherein the first semiconductor layer is formed of a different material than the second semiconductor layer.

19. The method of claim 14 , wherein the first semiconductor layer includes silicon germanium.

20. The method of claim 14 , wherein an outer edge of the sacrificial layer and the sidewall of the sidewall spacer are coplanar after the removing, during the same etching process, the sacrificial layer disposed along the sidewall of the second semiconductor layer and the spacer layer disposed on the sidewall of the second semiconductor layer.