Backside contact

1. A semiconductor structure, comprising:

an epitaxial source feature and an epitaxial drain feature;

a vertical stack of channel members disposed over a backside dielectric layer, the vertical stack of channel members extending between the epitaxial source feature and the epitaxial drain feature along a direction;

a gate structure wrapping around each of the vertical stack of channel members;

a backside source contact disposed in the backside dielectric layer; and

a silicide layer sandwiched between the epitaxial source feature and the backside source contact such that the epitaxial source feature is electrically coupled to the backside source contact,

wherein the backside source contact includes a top portion adjacent the epitaxial source feature and a bottom portion away from the epitaxial source feature,

wherein the top portion and the bottom portion comprise a step width change along the direction.

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

a frontside drain contact over the epitaxial drain feature.

3. The semiconductor structure of claim 1 , wherein a width of the top portion is greater than a width of the bottom portion along the direction.

4. The semiconductor structure of claim 1 , wherein a width of the top portion is smaller than a width of the bottom portion along the direction.

5. The semiconductor structure of claim 1 , wherein a center line of the bottom portion is offset from a center line of the top portion.

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

a dummy epitaxial feature disposed directly below the epitaxial drain feature,

wherein the dummy epitaxial feature extends into the backside dielectric layer.

7. The semiconductor structure of claim 6 ,

wherein the epitaxial drain feature comprises a first epitaxial layer in contact with the dummy epitaxial feature and a second epitaxial layer over the first epitaxial layer,

wherein the dummy epitaxial feature comprises silicon germanium.

8. The semiconductor structure of claim 7 , wherein the second epitaxial layer is in contact with the vertical stack of channel members and the first epitaxial layer is spaced apart from the vertical stack of channel members.

9. A semiconductor structure, comprising:

a source feature and a drain feature;

a plurality of channel members disposed over a backside dielectric layer, the plurality of channel members extending between the source feature and the drain feature along a first direction, the plurality of channel members being stacked along a second direction perpendicular to the first direction;

a gate structure wrapping around each of the plurality of channel members; and

a backside source contact disposed in the backside dielectric layer, the backside source contact comprising a first portion adjacent the source feature and a second portion away from the source feature,

wherein the first portion comprises a first width along the first direction and the second portion comprises a second width along the first direction,

wherein the first portion comprises a first height along the second direction and the second portion comprises a second height along the second direction,

wherein the first width is different from the second width,

wherein the first height is smaller than the second height,

wherein the gate structure is spaced apart from the backside dielectric layer by a liner,

wherein a composition of the backside dielectric layer is different from a composition of the liner.

10. The semiconductor structure of claim 9 , wherein a ratio of the second height to the first height is between about 1.1 and about 2.

11. The semiconductor structure of claim 9 , wherein a center line of the second portion is offset from a center line of the first portion.

12. The semiconductor structure of claim 9 , further comprising a dummy epitaxial feature extending into the backside dielectric layer and disposed directly below the drain feature, wherein the dummy epitaxial feature is spaced apart from the backside dielectric layer by the liner.

13. The semiconductor structure of claim 12 ,

wherein the drain feature comprises a first epitaxial layer in contact with the dummy epitaxial feature and a second epitaxial layer over the first epitaxial layer.

14. The semiconductor structure of claim 13 ,

wherein the first epitaxial layer and the second epitaxial layer comprise a dopant,

wherein a concentration of the dopant in the second epitaxial layer is greater than a concentration of the dopant in the first epitaxial layer.

15. The semiconductor structure of claim 13 , wherein the second epitaxial layer is in contact with the plurality of channel members and the first epitaxial layer is spaced apart from the plurality of channel members.

16. A semiconductor structure, comprising:

a first epitaxial feature and a second epitaxial feature;

a plurality of nanostructure disposed over a backside dielectric layer, the plurality of nanostructure extending between the first epitaxial feature and the second epitaxial feature along a direction;

a backside contact disposed in the backside dielectric layer and directly below the first epitaxial feature; and

a silicon germanium feature extending into the backside dielectric layer and directly below the second epitaxial feature, the silicon germanium feature being in direct contact with the second epitaxial feature,

wherein the backside contact includes a top portion adjacent the first epitaxial feature and a bottom portion away from the first epitaxial feature,

wherein a width of the top portion along the direction is different from a width of the bottom portion along the direction.

17. The semiconductor structure of claim 16 , further comprising:

a frontside contact disposed directly over the second epitaxial feature.

18. The semiconductor structure of claim 16 , wherein the first epitaxial feature and the second epitaxial feature comprise silicon and an n-type dopant.

19. The semiconductor structure of claim 16 , further comprising:

a backside silicide feature sandwiched between the first epitaxial feature and the backside contact.

20. The semiconductor structure of claim 16 , wherein the width of the bottom portion is greater than the width of the top portion.