Semiconductor devices and methods of fabrication thereof

1. A semiconductor device, comprising:

a nanosheet stack comprising:

two or more channel layers; and

a dielectric layer disposed over the two or more channel layers, wherein the dielectric layer has a first surface facing a topmost layer of the two or more channel layers, and a second surface opposing the first surface;

a gate dielectric layer formed around the two or more channel layers and on the first surface of the dielectric layer;

a metal gate liner formed directly on the second surface of the dielectric layer; and

a sidewall spacer in contact with the second surface of the dielectric layer, wherein a first air gap is disposed between the first surface of the dielectric layer and the topmost layer of the two or more channel layers and a second air gap disposed between the two or more channel layers.

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

a gate electrode layer disposed in spacings in the nanosheet stack on the gate dielectric layer, wherein a first surface of the gate electrode layer coplanar with the second surface of the dielectric layer.

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

a self-alignment contact (SAC) layer formed on the metal gate liner, wherein the metal gate liner is formed on the first surface of the gate electrode layer.

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

a conductive feature formed in the self-alignment contact (SAC) layer, wherein the conductive feature is in contact with the gate electrode layer.

5. The semiconductor device of claim 1 , further comprising an inner spacer disposed between the first surface of the dielectric layer and the two or more channel layers.

6. The semiconductor device of claim 5 , wherein the inner spacer includes the first air gap and the second air gap.

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

a boundary feature in contact with the inner spacer, the two or more channel layers, and the dielectric layer, wherein the dielectric layer has a third surface connecting the first and second surfaces, and the boundary feature is in contact with the third surface of the dielectric layer.

8. A semiconductor device, comprising:

a first source/drain feature;

two or more first channel layers in contact with the first source/drain feature, wherein each of the two or more first channel layers includes:

a first surface;

a second surface opposing the first surface; and

an end surface connecting the first surface and second surface, wherein the first surface and second surface are exposed to air gaps at portions adjacent the end surface; and

a dielectric layer formed over the two or more first channel layers;

a gate dielectric layer formed on the two or more first channel layers and a bottom surface of the dielectric layer;

a metal gate liner formed directly on a top surface of the dielectric layer; and

a gate electrode layer formed on the gate dielectric layer.

9. The semiconductor device of claim 8 , wherein one of the air gaps is formed between the dielectric layer and a topmost of the two or more first channels near the end surface.

10. The semiconductor device of claim 9 , further comprising:

a fin sidewall spacer in contact with the dielectric layer and the two or more first channel layers, wherein the fin sidewall spacer is exposed to the air gaps.

11. The semiconductor device of claim 9 , further comprising:

a boundary structure in contact in contact with the dielectric layer and the two or more first channel layers, wherein the air gaps are formed between the boundary structure and the fin sidewall spacer, and between the gate dielectric layer and the first source/drain feature.

12. The semiconductor device of claim 9 , further comprising:

a self-alignment contact (SAC) layer formed on the metal gate liner.

13. The semiconductor device of claim 12 , further comprising:

a second source/drain feature; and

two or more second channel layers in contact with the second source/drain feature, wherein the gate electrode is formed around the two or more second channels.

14. The semiconductor device of claim 12 , wherein the metal gate liner is in contact with a top surface of the gate electrode layer, and the top surface of the gate electrode layer is coplanar with a top surface of the nanosheet of the hard mask.

15. A method for forming a semiconductor device, comprising:

forming fin structures including two or more channel layers and a dielectric layer over the two or more channel layers with spacing layers formed between adjacent channel layers and the dielectric layer;

forming a sacrificial gate structure over the fin structures and sidewall spacers on the sacrificial gate structure;

recess etching the fin structures;

removing a portion of the spacing layers to form inner spacers between the of channel layers and the dielectric layer;

forming source/drain features;

removing the sacrificial gate structure;

forming a replacement gate structure comprising:

depositing a gate dielectric layer on the two or more channel layers and the dielectric layer; and

depositing a gate electrode layer over the gate dielectric layer;

etching back the gate electrode layer using the dielectric layer as an etch stop; and

etching the inner spacers to form air gaps therein.

16. The method of claim 15 , further comprising:

prior to etching the inner spacers, forming a trench along a direction perpendicular to the replacement gate structure to remove a portion of the fin structures; and

etching the inner spacers to form air gaps from the trench.

17. The method of claim 15 , further comprising:

depositing a self-aligned contact (SAC) layer between the sidewall spacers.

18. The method of claim 15 , wherein each of the nanosheet of channel layers comprises an epitaxial semiconductor layer.

19. The method of claim 18 , wherein the nanosheet spacing layer comprises SiGe.