Radical etching in gate formation

1. A method, comprising:

providing a structure having a substrate and a fin protruding from the substrate;

forming a dummy gate stack over the fin;

forming a gate spacer on sidewalls of the dummy gate stack;

generating a plasma from a mixture of a nitrogen-containing gas and a nitrogen-free gas;

separating radicals from the plasma;

removing the dummy gate stack using the radicals in a radical etch process, resulting in a gate trench; and

forming a metal gate stack in the gate trench.

2. The method of claim 1 , wherein the radical etch process is isotropic.

3. The method of claim 1 , wherein

the separating of the radicals from the plasma is prior to having the radicals contact the dummy gate stack.

4. The method of claim 1 , further comprising:

after the separating of the radicals from the plasma, combining the radicals with an etching precursor.

5. The method of claim 1 , wherein the mixture is free of oxygen and chlorine.

6. The method of claim 1 , wherein the radical etch process includes applying radicals containing fluorine and hydrogen.

7. The method of claim 1 , wherein the radical etch process results in a capping film covering the gate trench.

8. The method of claim 7 , wherein the capping film includes silane.

9. The method of claim 7 , wherein the radical etch process includes applying etchants with a ratio of number of fluorine atoms to number of hydrogen atoms less than about 93:1000.

10. The method of claim 6 , wherein the removing of the dummy gate stack is free of applying a biased electric field towards the structure.

11. A method, comprising:

receiving a structure in an etch processing region of a process chamber, the structure including a dummy gate stack over a substrate and a gate spacer on sidewalls of the dummy gate stack;

generating plasma from a precursor gas in a plasma region of the process chamber, the precursor gas being a mixture of a first gas and a second gas that is different from the first gas, the plasma including radicals and charged ions;

flowing the radicals into the etch processing region while substantially excluding the charged ions from entry into the etch processing region;

receiving an unexcited gas in the etch processing region to mix with the radicals, the unexcited gas including the second gas;

etching the dummy gate stack with the radicals, resulting in a gate trench; and

forming a metal gate stack in the gate trench.

12. The method of claim 11 , wherein the radicals and the unexcited gas both include hydrogen.

13. The method of claim 11 , wherein during the etching of the dummy gate stack, the radicals and the unexcited gas include fluorine and hydrogen.

14. The method of claim 13 , wherein a ratio of number of fluorine atoms to number of hydrogen atoms is larger than about 93:1000.

15. The method of claim 13 , wherein a ratio of number of fluorine atoms to number of hydrogen atoms is smaller than 93:1000, such that the etching of the dummy gate stack results in a capping film over sidewalls of the gate trench.

16. The method of claim 15 , further comprising:

performing a wet cleaning process to remove the capping film prior to the forming of the metal gate stack.

17. The method of claim 11 , wherein the first gas contains nitrogen and the second gas is free of nitrogen.

18. A method, comprising:

receiving a structure having a fin extending lengthwise in a first direction, a dummy gate stack over the fin and extending lengthwise in a second direction perpendicular to the first direction, and a gate spacer including an inner sidewall facing the dummy gate stack and an outer sidewall opposing the inner sidewall; and

removing the dummy gate stack in an isotropic etch process without applying a biased electric field surrounding the structure, thereby forming a gate trench, wherein the isotropic etch process uses radicals as an etchant,

wherein after the removing of the dummy gate stack, in a cross-sectional view along the first direction, the inner sidewall of the gate spacer has a first height and a bowed structure extending towards the gate trench for a first lateral distance measured from a middle point of the inner sidewall along the first direction, wherein the first lateral distance is less than about 8% of the first height.

19. The method of claim 18 , wherein after the removing of the dummy gate stack, in the cross-sectional view along the first direction, the outer sidewall of the gate spacer has a second height lower than the first height.

20. The method of claim 18 , wherein after the removing of the dummy gate stack, in the cross-sectional view along the first direction the inner sidewall has a footing structure laterally extending towards the gate trench for a second lateral distance measured from the middle point of the inner sidewall along the first direction, wherein the second lateral distance is less than about 8% of the first height.