Semiconductor device fabrication methods and structures thereof

1. A method comprising:

providing semiconductor channel layers over a substrate;

forming a first dipole layer wrapping around the semiconductor channel layers;

forming an interfacial dielectric layer wrapping around the first dipole layer;

forming a high-k dielectric layer wrapping around the interfacial dielectric layer;

forming a second dipole layer wrapping around the high-k dielectric layer;

performing a thermal process to drive at least some dipole elements from the second dipole layer into the high-k dielectric layer;

removing the second dipole layer; and

forming a work function metal layer wrapping around the high-k dielectric layer.

2. The method of claim 1 , wherein the first dipole layer includes a p-dipole material and the second dipole layer includes an n-dipole material.

3. The method of claim 2 , wherein the first dipole layer includes germanium oxide, aluminum oxide, gallium oxide, or zinc oxide.

4. The method of claim 2 , wherein the second dipole layer includes lanthanum oxide, yttrium oxide, or titanium oxide.

5. The method of claim 1 , wherein the work function metal layer is free of aluminum.

6. The method of claim 5 , wherein the work function metal layer includes titanium nitride, tantalum nitride, tungsten carbonitride, or titanium silicon nitride.

7. A method comprising:

providing first channel layers and second channel layers over a substrate;

forming a first dipole layer wrapping around the second channel layers but not the first channel layers;

forming an interfacial dielectric layer wrapping around the first dipole layer and the first channel layers;

forming a high-k dielectric layer wrapping around the interfacial dielectric layer;

forming a second dipole layer wrapping around the high-k dielectric layer over the second channel layers but not the high-k dielectric layer over the first channel layers;

performing a thermal process to drive at least some dipole elements from the second dipole layer into the high-k dielectric layer over the second channel layers;

removing the second dipole layer; and

forming a work function metal layer wrapping around the high-k dielectric layer over both the first and the second channel layers.

8. The method of claim 7 , further comprising:

forming a gate electrode layer over the work function metal layer over both the first and the second channel layers.

9. The method of claim 8 , wherein the work function metal layer fills gaps between adjacent first channel layers, fills gaps between adjacent second channel layers, and is free of aluminum.

10. The method of claim 7 , wherein the first dipole layer includes germanium oxide, aluminum oxide, gallium oxide, or zinc oxide.

11. The method of claim 7 , wherein the second dipole layer includes lanthanum oxide, yttrium oxide, or titanium oxide.

12. The method of claim 7 , wherein the forming of the first dipole layer and the forming of the interfacial dielectric layer include:

forming a patterned hard mask covering the first channel layers and exposing the second channel layers;

selectively depositing a cladding layer over the second channel layers but not over the patterned hard mask;

selectively removing the patterned hard mask but not the cladding layer; and

performing a cleaning process with an oxygen-containing cleaning solution to the first channel layers, the cladding layer, and the second channel layers, resulting in the first dipole layer wrapping around the second channel layers and the interfacial dielectric layer wrapping around the first channel layers and the first dipole layer.

13. The method of claim 12 , wherein the first and the second channel layers include crystalline silicon, the cladding layer include germanium, the first dipole layer includes germanium dioxide, and the interfacial dielectric layer includes silicon dioxide.

14. The method of claim 12 , wherein the oxygen-containing cleaning solution includes standard clean 1 (SC1) solution or standard clean 2 (SC2) solution.

15. The method of claim 7 , wherein the forming of the second dipole layer includes:

depositing the second dipole layer wrapping around the high-k dielectric layer over both the first and the second channel layers;

forming a patterned hard mask covering the second dipole layer over the second channel layers and exposing the second dipole layer over the first channel layers;

selectively removing the second dipole layer over the first channel layers to expose the high-k dielectric layer over the first channel layers; and

selectively removing the patterned hard mask to expose the second dipole layer over the second channel layers.

16. A semiconductor structure, comprising:

a substrate;

semiconductor channel layers over the substrate;

a p-dipole material around the semiconductor channel layers;

an interfacial dielectric layer over the p-dipole material and the semiconductor channel layers;

an n-dipole material over the interfacial dielectric layer;

a high-k dielectric layer over the n-dipole material and the interfacial dielectric layer; and

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

17. The semiconductor structure of claim 16 , wherein:

the semiconductor channel layers include crystalline silicon; and

the p-dipole material includes germanium, aluminum, gallium, or zinc.

18. The semiconductor structure of claim 17 , wherein:

the interfacial dielectric layer includes silicon dioxide; and

the n-dipole material includes lanthanum, yttrium, or titanium.

19. The semiconductor structure of claim 16 , wherein the work function metal layer is substantially free of aluminum.

20. The semiconductor structure of claim 16 , further comprising a gate electrode layer over the work function metal layer.