Stacked multi-gate structure and methods of fabricating the same

1. A semiconductor device, comprising:

a stack of first channel layers;

first and second source/drain (S/D) epitaxial features adjacent to opposite sides of at least a portion of the first channel layers, respectively, wherein the first and second S/D epitaxial features have a first conductivity type;

a stack of second channel layers stacked over the first channel layers;

third and fourth source/drain (S/D) epitaxial features adjacent to opposite sides of at least a portion of the second channel layers, respectively, wherein the third and fourth S/D epitaxial features have a second conductivity type; and

a dielectric isolation layer disposed vertically between the first and second S/D epitaxial features and the third and fourth S/D epitaxial layers,

wherein a total active channel layer number of the first channel layers is different from a total active channel layer number of the second channel layers, and wherein the dielectric isolation layer is in physical contact with at least one of the first and second channel layers and separates the at least one of the first and second channel layers from physically contacting any of the first, second, third, and fourth S/D epitaxial features.

2. The semiconductor device of claim 1 , wherein a difference between the total active channel layer numbers of the first channel layers and the second channel layers is equal to or larger than two.

3. The semiconductor device of claim 1 , wherein the dielectric isolation layer isolates both the third and fourth S/D epitaxial features from adjoining a bottommost channel layer of the second channel layers.

4. The semiconductor device of claim 1 , wherein the dielectric isolation layer is a first dielectric isolation layer, the semiconductor device further comprising:

a second dielectric isolation layer disposed below a topmost channel layer of the first channel layers, wherein the second dielectric isolation layer isolates both the first and second S/D epitaxial features from adjoining a bottommost channel layer of the first channel layers.

5. The semiconductor device of claim 1 , wherein at least one of the first and second S/D epitaxial features has a top surface below a topmost channel layer of the first channel layers.

6. The semiconductor device of claim 1 , wherein at least one of the third and fourth S/D epitaxial features has a top surface below a topmost channel layer of the second channel layers.

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

a first power rail under the first channel layers; and

a second power rail above the second channel layers,

wherein the first and third S/D epitaxial features are electrically coupled to the first power rail, and the second and fourth S/D epitaxial features are electrically coupled to the second power rail.

8. The semiconductor device of claim 7 , wherein the third S/D epitaxial feature is directly above the first S/D epitaxial feature, and the fourth S/D epitaxial feature is directly above the second S/D epitaxial feature.

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

a gate structure that wraps around each of the first and second channel layers.

10. The semiconductor device of claim 1 , wherein the first and second conductivity types are opposite.

11. The semiconductor device of claim 1 , wherein the dielectric isolation layer isolates the first and second S/D epitaxial features from adjoining a topmost channel layer of the first channel layers.

12. A semiconductor device, comprising:

a substrate;

a first transistor over the substrate, the first transistor including first channel layers and a first source/drain (S/D) feature adjoining active members of the first channel layers;

a second transistor over the first transistor, the second transistor including second channel layers and a second S/D feature adjoining active members of the second channel layers; and

an isolation layer disposed vertically between the first S/D feature and the second S/D feature,

wherein a number of the active members of the first channel layers is different from a number of the active members of the second channel layers, and wherein the isolation layer either separates a topmost one of the first channel layers from the first S/D feature or separates a bottommost one of the second channel layers from the second S/D feature.

13. The semiconductor device of claim 12 , wherein a number of the first channel layers is different from a number of the second channel layers.

14. The semiconductor device of claim 12 , wherein a number of the first channel layer equals a number of the second channel layers.

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

a gate structure that wraps around each of the first and second channel layers.

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

a first gate structure that wraps around each of the first channel layers; and

a second gate structure that wraps around each of the second channel layers.

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

a power rail under the first channel layers, wherein both of the first and second S/D features are electrically coupled to the power rail.

18. A method, comprising:

receiving a workpiece including a substrate portion and a stack portion over the substrate portion, the stack portion including a first stack of first channel layers interleaved by first sacrificial layers and a second stack of second channel layers interleaved by second sacrificial layers, the second stack being above the first stack;

forming a fin-shaped structure from the stack portion and the substrate portion, the fin-shaped structure including a source region and a drain region;

forming a first source feature in the source region and a first drain feature in the drain region;

depositing an isolation layer over the first source feature and the first drain feature, the isolation layer adjoining at least a bottommost one of the second channel layers; and

forming a second source feature in the source region and over the isolation layer and a second drain feature in the drain region and over the isolation layer, wherein the isolation layer separates each of the second source feature and the second drain feature from the bottommost one of the second channel layers.

19. The method of claim 18 , further comprising:

recessing the second source feature and the second drain feature below a topmost one of the second channel layers; and

depositing a dielectric layer over the second source feature and the second drain feature, the dielectric layer adjoining the topmost one of the second channel layers.

20. The method of claim 18 , further comprising:

forming a first power rail under the first stack;

forming a second power rail above the second stack;

forming first interconnection features that electrically couple the first source feature and the second source feature to the first power rail; and

forming second interconnection features that electrically couple the first drain feature and the second drain feature to the second power rail.