Semiconductor device with varying numbers of channel layers and method of fabrication thereof

1. A method, comprising:

providing a structure having a frontside and a backside, the structure including a substrate, two or more semiconductor channel layers disposed over the substrate and vertically stacked with respect to one another, a first source/drain (S/D) feature and a second S/D feature connected through the semiconductor channel layers therebetween, and a gate structure engaging the semiconductor channel layers, wherein the substrate is at the backside of the structure and the gate structure is at the frontside of the structure;

recessing the first S/D feature, thereby exposing a terminal end of one of the semiconductor channel layers; and

depositing a dielectric layer on the first S/D feature and covering the exposed terminal end of the one of the semiconductor channel layers.

2. The method of claim 1 , wherein after the recessing of the first S/D feature, the first S/D feature is not in direct contact with the one of the semiconductor channel layers.

3. The method of claim 1 , wherein the one of the semiconductor channel layers is a bottommost one of the semiconductor channel layers in the structure, and the recessing of the first S/D feature includes etching the first S/D feature from the backside of the structure.

4. The method of claim 3 , further comprising:

etching the substrate from the backside of the structure to form a trench exposing the first S/D feature, wherein the recessing of the first S/D feature includes etching the first S/D feature through the trench.

5. The method of claim 3 , wherein the gate structure is a high-k metal gate structure.

6. The method of claim 1 , wherein the one of the semiconductor channel layers is a topmost one of the semiconductor channel layers in the structure, and the recessing of the first S/D feature includes etching the first S/D feature from the frontside of the structure.

7. The method of claim 6 , wherein the gate structure is a sacrificial gate structure.

8. The method of claim 7 , further comprising:

removing the sacrificial gate structure, thereby exposing the semiconductor channel layers; and

forming a high-k metal gate structure engaging the semiconductor channel layers.

9. The method of claim 1 , wherein after the recessing of the first S/D feature, the second S/D feature remains in direct contact with each of the semiconductor channel layers.

10. The method of claim 1 , wherein the recessing of the first S/D feature includes recessing the second S/D feature, thereby exposing another terminal end of the one of the semiconductor channel layers.

11. The method of claim 1 , further comprising:

partially remove the dielectric layer, thereby exposing the first S/D feature; and

forming an S/D contact on the first S/D feature.

12. A method, comprising:

providing a structure having a frontside and a backside, the structure including a substrate, a semiconductor fin over the substrate, a first source/drain (S/D) feature and a second S/D feature over the semiconductor fin, two or more semiconductor channel layers over the semiconductor fin and connecting the first and second S/D features, and a gate structure engaging the semiconductor channel layers, wherein the substrate is at the backside of the structure and the gate structure is at the frontside of the structure;

thinning down the structure from the backside of the structure until the semiconductor fin is exposed;

etching the semiconductor fin from the backside of the structure to form a first trench exposing the first S/D feature;

recessing the first S/D feature through the first trench, such that a terminal end of a bottommost semiconductor channel layer is exposed in the first trench; and

depositing a dielectric layer in the first trench, wherein the dielectric layer covers the terminal end of the bottommost semiconductor channel layer.

13. The method of claim 12 , further comprising:

etching the semiconductor fin from the backside of the structure to form a second trench exposing the second S/D feature; and

forming an S/D contact landing on the second S/D feature.

14. The method of claim 13 , wherein the second S/D feature is in contact with the bottommost semiconductor channel layer.

15. The method of claim 13 , further comprising:

prior to the forming of the S/D contact, recessing the second S/D feature through the second trench, such that another terminal end of the bottommost semiconductor channel layer is exposed in the second trench.

16. The method of claim 12 , wherein the gate structure is a high-k metal gate structure.

17. A method, comprising:

providing a structure having a frontside and a backside, the structure including a substrate, a plurality of semiconductor channel layers vertically stacked with respect to one another and disposed over the substrate, a first epitaxial feature and a second epitaxial feature laterally sandwiching and in physical contact with each of the semiconductor channel layers, and a metal gate stack wrapping around each of the semiconductor channel layers, wherein the substrate is at the backside of the structure;

etching the substrate from the backside of the structure, thereby exposing the first epitaxial feature;

recessing the first epitaxial feature, thereby exposing at least an end of a bottommost one of the semiconductor channel layers;

depositing a dielectric layer in physical contact with the first epitaxial feature and covering the exposed end of the bottommost one of the semiconductor channel layers; and

forming a backside contact in physical contact with the second epitaxial feature.

18. The method of claim 17 , further comprising:

prior to the etching of the substrate from the backside of the structure, forming a first frontside contact in physical contact with the first epitaxial feature and a second frontside contact in physical contact with the second epitaxial feature.

19. The method of claim 17 , wherein the etching of the substrate from the backside of the structure also exposes the metal gate stack.

20. The method of claim 17 , further comprising:

prior to the forming of the backside contact, recessing the second epitaxial feature, thereby exposing an opposing end of the bottommost one of the semiconductor channel layers.