Backside power rail structure and methods of forming same

1. A semiconductor device comprising:

a power rail;

a dielectric layer over the power rail;

a first channel region over the dielectric layer;

a second channel region over the first channel region;

a gate stack over the first channel region and the second channel region, wherein the gate stack is further disposed between the first channel region and the second channel region; and

a first source/drain region adjacent the gate stack and electrically connected to the power rail, wherein the first source/drain region is electrically connected to the power rail through a contact via, and wherein a top surface of the contact via directly contacts a bottom surface of the dielectric layer.

2. The semiconductor device of claim 1 , wherein sidewalls of the dielectric layer are convex or concave.

3. The semiconductor device of claim 1 , further comprising a silicide region between the contact via and the first source/drain region.

4. The semiconductor device of claim 1 , wherein the dielectric layer has a thickness in a range of 6 nm to 20 nm.

5. The semiconductor device of claim 1 , wherein the dielectric layer comprises silicon oxide, silicon nitride, silicon carbon nitride, silicon carbon oxynitride, a metal oxide, or a combination thereof.

6. The semiconductor device of claim 1 , wherein the gate stack extends over sidewalls of the dielectric layer.

7. The semiconductor device of claim 1 , wherein the dielectric layer comprises a trapezoid shape.

8. The semiconductor device of claim 1 , wherein the gate stack extends below a bottom surface of the dielectric layer.

9. A method of forming a semiconductor device, the method comprising:

depositing a sacrificial layer, a first semiconductor layer, a second semiconductor layer, and a third semiconductor layer sequentially over a semiconductor substrate; removing the sacrificial layer to form a first recess;

depositing a dielectric layer in the first recess;

forming an epitaxial source/drain region extending through the dielectric layer;

removing the second semiconductor layer to form a second recess;

forming a gate stack extending over a top surface of the third semiconductor layer and sidewalls of the third semiconductor layer and first semiconductor layer, wherein the gate stack fills the second recess;

thinning a surface of the semiconductor substrate to expose the epitaxial source/drain region and the dielectric layer;

forming a conductive contact connected to the epitaxial source/drain region; and

forming a backside power rail on an opposite side of the dielectric layer as the first semiconductor layer, wherein the backside power rail is electrically connected to epitaxial source/drain region through the conductive contact.

10. The method of claim 9 , wherein the dielectric layer insulates the backside power rail from the first semiconductor layer.

11. The method of claim 9 , wherein the sacrificial layer has a first germanium concentration that is lower than a second germanium concentration of the second semiconductor layer.

12. The method of claim ii, wherein the first germanium concentration is in a range from 10 10 low atoms/cm 3 to about 10 15 atoms/cm 3 , and the second germanium concentration is in a range from 10 20 atoms/cm 3 to about 10 35 atoms/cm 3 .

13. The method of claim 9 , wherein the conductive contact is wider than the epitaxial source/drain region at an interface where the conductive contact contacts the epitaxial source/drain region.

14. The method of claim 9 , wherein removing the sacrificial layer comprises:

etching sidewalls of the second semiconductor layer using an etchant that etches the second semiconductor layer at a faster rate than the sacrificial layer;

forming spacers on the sidewalls of the second semiconductor layer; and

etching the sacrificial layer, wherein the spacers mask the sidewalls of the second semiconductor layer while the sacrificial layer is etched.

15. A semiconductor device comprising:

a power rail;

a first channel region over the power rail;

a second channel region over the first channel region;

an isolation layer extending between the first channel region and the power rail;

a gate stack over the first channel region, second channel region, and isolation layer, wherein a portion of the gate stack is further disposed between the first channel region and the second channel region;

a first source/drain region adjacent the gate stack, the first source/drain region extending through the isolation layer; and

a contact plug directly contacting the power rail, wherein the contact plug electrically connects the power rail to the first source/drain region, and wherein a top surface of the contact plug is in direct contact with the isolation layer.

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

a first interconnect structure in direct contact with the power rail;

a second interconnect structure, wherein the first interconnect structure and the second interconnect structure are on opposite sides of the power rail; and

a first plurality of conductive features over the first interconnect structure, wherein the first plurality of conductive features comprise solder regions electrically coupled to the power rail.

17. The semiconductor device of claim 15 , wherein the isolation layer electrically isolates the power rail from the first channel region.

18. The semiconductor device of claim 15 , wherein a first width of the contact plug is smaller than a second width of the power rail.

19. The semiconductor device of claim 15 , further comprising a dielectric layer extending between the isolation layer and the power rail.

20. The semiconductor device of claim 15 , wherein the isolation layer comprises a metal oxide.