Semiconductor structures and methods thereof

1. A method, comprising:

providing a structure having a substrate and a stack of semiconductor layers over a surface of the substrate and spaced vertically one from another;

forming an interfacial layer wrapping around each of the semiconductor layers;

forming a high-k dielectric layer over the interfacial layer and wrapping around each of the semiconductor layers;

forming a capping layer over the high-k dielectric layer and wrapping around each of the semiconductor layers;

with the capping layer wrapping around each of the semiconductor layers, performing a thermal treatment to the structure, thereby increasing a thickness of the interfacial layer, wherein the capping layer includes an oxygen-scavenging oxide or an oxygen-scavenging nitride, and the thermal treatment is performed with ambient oxygen (O 2 ); and

after the performing of the thermal treatment, removing the capping layer.

2. The method of claim 1 , after the removing of the capping layer, further comprising:

forming a work function metal layer over the high-k dielectric layer and wrapping around each of the semiconductor layers.

3. The method of claim 2 , further comprising:

forming a bulk metal layer over the work function metal layer.

4. The method of claim 1 , wherein the capping layer includes one of TiN, TiSiN, TiO 2 , TiON, TaN, and TiSiN.

5. The method of claim 1 , wherein the thermal treatment is performed further with ambient nitrogen (N 2 ).

6. The method of claim 1 , wherein the thermal treatment increases the thickness of the interfacial layer by about 2 Å to about 10 Å.

7. The method of claim 1 , wherein the thermal treatment is spike annealing or soak annealing at temperature in a range from about 600° C. to about 1,000° C.

8. The method of claim 1 , wherein the thermal treatment is furnace annealing at temperature in a range from about 300° C. to about 600° C.

9. The method of claim 1 , wherein the capping layer is the outmost layer over each of the semiconductor layers during the thermal treatment.

10. The method of claim 1 , wherein the capping layer is formed such that there is vertical space between the capping layer on adjacent semiconductor layers.

11. A method, comprising:

forming first nanostructures in a first region over a substrate;

forming second nanostructures in a second region over the substrate;

forming an interfacial layer wrapping around the first nanostructures and the second nanostructures;

forming a high-k dielectric layer over the interfacial layer and wrapping around the first nanostructures and the second nanostructures;

forming a capping layer over the high-k dielectric layer and wrapping around the first nanostructures and the second nanostructures, wherein the capping layer is formed such that there is vertical space between the capping layer on adjacent first nanostructures and there is vertical space between the capping layer on adjacent second nanostructures;

removing the capping layer in the first region to expose the high-k dielectric layer in the first region while keeping the capping layer over the high-k dielectric layer in the second region;

performing a thermal treatment to the first nanostructures and the second nanostructures, wherein the interfacial layer in the second region becomes thicker than the interfacial layer in the first region after the thermal treatment is performed; and

after the thermal treatment, removing the capping layer in the second region.

12. The method of claim 11 , wherein the removing of the capping layer in the first region includes:

forming a hard mask layer filling space between the first nanostructures and filling space between the second nanostructures;

forming a coating layer over the first and the second regions;

patterning the coating layer into a patterned coating layer that exposes the first region and covers the second region;

using the patterned coating layer as an etch mask, removing the hard mask layer and the capping layer from the first region;

removing the patterned coating layer; and

after the removing the patterned coating layer, removing the hard mask layer from the second region.

13. The method of claim 12 , wherein the hard mask layer includes one of aluminum oxide, silicon nitride, lanthanum oxide, silicon, silicon carbonitride, silicon oxy-carbonitride, aluminum nitride, aluminum oxynitride.

14. The method of claim 11 , wherein the thermal treatment is performed with ambient oxygen (O 2 ) and the capping layer includes an oxygen-scavenging oxide or an oxygen-scavenging nitride.

15. The method of claim 14 , wherein the capping layer includes one of TiN, TiSiN, TiO 2 , TiON, TaN, and TiSiN.

16. The method of claim 11 , wherein the capping layer is the outmost layer in the second region during the thermal treatment.

17. A semiconductor structure, comprising:

first nanostructures vertically spaced one from another over a substrate in a first region of the semiconductor structure;

a first interfacial layer wrapping around each of the first nanostructures;

a first high-k dielectric layer over the first interfacial layer and wrapping around each of the first nanostructures;

a first work function metal layer over the first high-k dielectric layer and wrapping around each of the first nanostructures;

second nanostructures vertically spaced one from another over the substrate in a second region of the semiconductor structure;

a second interfacial layer wrapping around each of the second nanostructures;

a second high-k dielectric layer over the second interfacial layer and wrapping around each of the second nanostructures;

a second work function metal layer over the second high-k dielectric layer and wrapping around each of the second nanostructures, wherein a first thickness of the first interfacial layer is smaller than a second thickness of the second interfacial layer by about 2 Å to about 10 Å;

two first dielectric fins disposed on opposite sides of the first nanostructures, wherein a first outer surface of the two first dielectric fins includes a material that is different from a material in the first interfacial layer, and the first high-k dielectric layer directly contacts the first outer surface; and

two second dielectric fins disposed on opposite sides of the second nanostructures, wherein a second outer surface of the two second dielectric fins includes a material that is different from a material in the second interfacial layer, and the second high-k dielectric layer directly contacts the second outer surface.

18. The semiconductor structure of claim 17 , wherein a vertical dimension of one of the first nanostructures is greater than a vertical dimension of one of the second nanostructures by about 4 Å to about 20 Å.

19. The semiconductor structure of claim 17 , wherein the first nanostructures are spaced one from another by a vertical distance in a range from about 6 nm to about 12 nm, and the second nanostructures are spaced one from another by a vertical distance in a range from about 6 nm to about 12 nm.

20. The semiconductor structure of claim 17 , wherein a first vertical pitch of the first nanostructures is about equal to a second vertical pitch of the second nanostructures.