Gate-all-around FETs having uniform threshold voltage

1. A semiconductor device comprising:

an N-type field effect transistor (NFET) and a P-type field effect transistor (PFET), each including an inner work function metal, an outer work function metal, a first nanosheet comprising an inner channel surface having a first threshold voltage, a second nanosheet comprising an outer channel surface having a second threshold voltage, and a third nanosheet; and

additional material in the outer work function metal so as to cause the outer channel surface for the second nanosheet to have the second threshold voltage that is within a predefined amount of the first threshold voltage for the inner channel surface of the first nanosheet, wherein a first thickness of the inner work function metal between the first nanosheet and the second nanosheet is greater than a second thickness of the inner work function metal between the second nanosheet and the outer work function metal, wherein an inner edge of the outer work function metal and an outer edge of the inner work function metal both extend in parallel continually from above an uppermost one of the first, second, and third nanosheets to below a lowermost one of the first, second, and third nanosheets to an underlayer so as to be in direct contact with the underlayer, the underlayer being directly below a middle portion of the first, second, and third nanosheets and extending in a width direction beyond sides edges of the first, second, and third nanosheets, wherein a stack of materials covers and laterally surrounds the first, second, and third nanosheets of the NFET.

2. The semiconductor device of claim 1 , wherein the additional material is different for the NFET and the PFET.

3. The semiconductor device of claim 1 , wherein the stack of materials comprises an additional layer formed over the inner work function metal in the NFET.

4. The semiconductor device of claim 1 , wherein the stack of materials comprises an additional layer formed over the outer work function metal in the PFET.

5. The semiconductor device of claim 1 , wherein the additional material in the outer work function metal is a uniform distribution of elements for the PFET.

6. The semiconductor device of claim 1 , the additional material comprises implanted material in the outer work function metal, the implanted material causing the second threshold voltage to change and become within the predefined amount of the first threshold voltage.

7. The semiconductor device of claim 1 , wherein the additional material comprises doped material that causes the second threshold voltage to change and become within the predefined amount of the first threshold voltage.

8. The semiconductor device of claim 1 , wherein the additional material comprises thermally annealed material having been thermally driven into the outer work function metal, the thermally annealed material causing the second threshold voltage to change and become within the predefined amount of the first threshold voltage.

9. The semiconductor device of claim 1 , wherein directionally deposited material is formed over the second nanosheet, the directionally deposited material causing the second threshold voltage to change and become within the predefined amount of the first threshold voltage.

10. The semiconductor device of claim 1 , wherein one or more of the first nanosheet and the second nanosheet comprise silicon.

11. The semiconductor device of claim 1 , wherein one or more of the first nanosheet and the second nanosheet comprise silicon carbide.

12. The semiconductor device of claim 1 , wherein one or more of the first nanosheet and the second nanosheet comprise silicon germanium.

13. The semiconductor device of claim 1 , wherein one or more of the first nanosheet and the second nanosheet comprise germanium.

14. The semiconductor device of claim 1 , wherein one or more of the first nanosheet and the second nanosheet comprise a silicon alloy.

15. A gate-all-around field effect transistor comprising: at least one stack comprising a first nanosheet, a second nanosheet, a third nanosheet, an inner work function metal, and an outer work function metal, wherein the outer work function metal has been modified so as to cause an outer threshold voltage of an outer channel to be within a predefined amount of an inner threshold voltage of an inner channel in the at least one stack, wherein a first thickness of the inner work function metal between the first nanosheet and the second nanosheet is greater than a second thickness of the inner work function metal between the second nanosheet and the outer work function metal, wherein an inner edge of the outer work function metal and an outer edge of the inner work function metal both extend in parallel continually from above an uppermost one of the first, second, and third nanosheets to below a lowermost one of the first, second, and third nanosheets to an underlayer so as to be in direct contact with the underlayer, the underlayer being directly below a middle portion of the first, second, and third nanosheets and extending in a width direction beyond sides edges of the first, second, and third nanosheets, wherein a stack of materials covers and laterally surrounds the first, second, and third nanosheets.

16. The gate-all-around field effect transistor of claim 15 , wherein the first nanosheet comprises the inner channel having the inner threshold voltage and the second nanosheet comprises the outer channel having the outer threshold voltage.

17. The gate-all-around field effect transistor of claim 15 , wherein the outer work function metal has been modified so as to cause the outer threshold voltage of the outer channel to be within the predefined amount of the inner threshold voltage of the inner channel by a process, the process being selected from the group consisting of implantation of material in the outer work function metal, plasma treatment to dope the material in the outer work function metal, and thermal annealing to diffuse the material in the outer work function metal.

18. The gate-all-around field effect transistor of claim 15 , wherein the at least one stack comprises a PFET and an NFET, material associated with the outer work function metal being different in the PFET and the NFET.

19. The gate-all-around field effect transistor of claim 15 , wherein the stack of materials comprises an additional layer formed on the at least one stack, the additional layer modifying the outer work function metal so as to cause the outer threshold voltage of the outer channel to be within the predefined amount of the inner threshold voltage of the inner channel in the at least one stack.

20. A semiconductor device comprising:

an N-type field effect transistor (NFET) and a P-type field effect transistor (PFET), each including an inner work function metal, an outer work function metal, a first nanosheet comprising an inner channel surface having a first threshold voltage, a second nanosheet comprising an outer channel surface having a second threshold voltage, and a third nanosheet; and

additional material in the outer work function metal so as to cause the outer channel surface for the second nanosheet to have the second threshold voltage that is within a predefined amount of the first threshold voltage for the inner channel surface of the first nanosheet, wherein an inner edge of the outer work function metal and an outer edge of the inner work function metal both extend continually from above an uppermost one of the first, second, and third nanosheets to below a lowermost one of the first, second, and third nanosheets to an underlayer so as to be in direct contact with the underlayer, wherein a stack of materials covers and laterally surrounds the first, second, and third nanosheets of the NFET.