Semiconductor device with S/D bottom isolation and methods of forming the same

1. A semiconductor device comprising:

a first semiconductor stack over a substrate, wherein the first semiconductor stack includes first semiconductor layers separated from each other and stacked up along a direction substantially perpendicular to a top surface of the substrate;

a second semiconductor stack over the substrate, wherein the second semiconductor stack includes second semiconductor layers separated from each other and stacked up along the direction substantially perpendicular to the top surface of the substrate;

inner spacers between edge portions of the first semiconductor layers and between edge portions of the second semiconductor layers;

a bulk source/drain (S/D) feature between the first semiconductor stack and the second semiconductor stack, wherein the bulk S/D feature is separated from the substrate by a first air gap, and the bulk S/D feature is separated from the inner spacers by second air gaps;

a first gate spacer disposed over the first semiconductor stack;

a second gate spacer disposed over the second semiconductor stack;

a first dielectric layer disposed over the bulk S/D feature and sandwiched between the first gate spacer and the second gate spacer;

a metal gate structure wrapping each of the first semiconductor layers and each of the second semiconductor layers;

a second dielectric layer disposed over the first gate spacer, the second gate spacer and the first dielectric layer; and

an isolation layer covering a portion of the substrate below the bulk S/D feature,

wherein a top surface of the isolation layer is lower than a lowermost one of the first semiconductor layers,

wherein a bottom surface of the metal gate structure is above a top surface of the isolation layer by about 1 nm to about 2 nm.

2. The semiconductor device of claim 1 , wherein the isolation layer includes a dielectric material having an etch selectivity different from that of the first gate spacer or the second gate spacer.

3. The semiconductor device of claim 1 , wherein the first air gap is formed between the isolation layer and the bulk S/D feature.

4. The semiconductor device of claim 1 , wherein a thickness of the isolation layer is about 1 nm to about 10 nm.

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

a source/drain contact via extending through the second dielectric layer and the first dielectric layer to electrically couple to the bulk S/D feature.

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

a liner sandwiched between the first gate spacer and a topmost one of the first semiconductor layers.

7. The semiconductor device of claim 6 , wherein the liner comprises silicon oxide or silicon nitride.

8. 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;

depositing a dielectric liner over a topmost first semiconductor layer of the first semiconductor layers;

after the depositing of the dielectric liner, forming dummy gate structures over the first and second semiconductor layers;

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

forming an isolation layer on a bottom surface of the S/D trench; and

forming a bulk S/D feature in the S/D trench, wherein the bulk S/D feature is separated from the isolation layer,

wherein the forming the isolation layer on the bottom surface of the S/D trench includes:

depositing a dielectric layer in the S/D trench,

forming a buffer feature over a bottom portion of the S/D trench,

removing a portion of the dielectric layer over the buffer feature to form the isolation layer, and

removing the buffer feature.

9. The method of claim 8 , wherein the isolation layer includes a first dielectric material and the buffer feature includes a second dielectric material different from the first dielectric material.

10. The method of claim 9 , wherein the buffer feature includes an organic dielectric material.

11. The method of claim 8 , wherein a top surface of the buffer feature is below a bottom surface of a lowermost second semiconductor layer.

12. The method of claim 8 , wherein a thickness of the isolation layer is about 10% to about 50% of the bottom portion of the S/D trench.

13. The method of claim 8 , further comprising:

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

forming inner spacers to fill in the removed edge portions of the second semiconductor layers.

14. The method of claim 13 , wherein the forming the bulk S/D feature in the S/D trench includes:

epitaxially growing semiconductor material between the first semiconductor layers in the S/D trench, wherein the semiconductor material is merged to form the bulk S/D feature.

15. The method of claim 14 , wherein the bulk S/D feature and the inner spacers are separated by air gaps.

16. The method of claim 8 , wherein the removing of the buffer feature comprises use of an ashing process.

17. 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;

depositing a dielectric liner over a topmost first semiconductor layer of the first semiconductor layers;

after the depositing of the dielectric liner, forming dummy gate structures over channel regions of the first and second semiconductor layers;

removing a S/D region of the first and second semiconductor layers to form a source/drain (S/D) trench, such that the first semiconductor layers and the second semiconductor layers are truncated by the S/D trench and a recessed surface of the substrate is exposed in the S/D trench;

forming an isolation layer covering the recessed surface of the substrate exposed in the S/D trench; and

epitaxial growing semiconductor material between the truncated first semiconductor layers to form a bulk S/D feature,

wherein the forming the isolation layer covering the recessed surface of the substrate exposed in the S/D trench includes:

depositing a dielectric layer along sidewalls of the S/D trench and on the recessed surface of the substrate,

depositing an organic material over the dielectric layer,

etching back the organic material to form a buffer feature, wherein a top surface of the buffer feature is below a bottom surface of a lowermost second semiconductor layer,

removing a portion of the dielectric layer over the buffer feature to form the isolation layer, and

removing the buffer feature.

18. The method of claim 17 , further comprising:

forming inner spacers between edge regions of the first semiconductor layers, wherein the inner spacers are separated from the bulk S/D feature by air gaps.

19. The method of claim 17 , further comprising:

selectively removing the second semiconductor layers;

replacing the dummy gate structures with metal gate structures, wherein a bottom surface of the metal gate structures is above a top surface of the isolation layer; and

forming a contact feature contacting the bulk S/D feature.

20. The method of claim 17 ,

wherein the depositing of the dielectric layer comprises use of CVD, PVD or ALD,

wherein the depositing of the organic material comprises use of spin-on coating.