Shallow trench isolation structures having uniform step heights

1. A method, comprising:

forming a fin protruding from a substrate, the fin comprising a first sidewall and a second sidewall formed opposite to the first sidewall;

depositing a shallow-trench isolation (STI) material on the substrate, comprising:

depositing a first portion of the STI material in contact with the first sidewall; and

depositing a second portion of the STI material in contact with the second sidewall;

performing a first etching process on the STI material to remove a first height of the STI material in the first portion at a first etching rate and a second height of the STI material in the second portion at a second etching rate greater than the first etching rate;

performing a second etching process on the STI material to remove a third height of the STI material in the first portion at a third etching rate and a fourth height of the STI material in the second portion at a fourth etching rate less than the third etching rate; and

performing a radical treatment process on the STI material after the first and second etching processes.

2. The method of claim 1 , wherein the first etching process comprises dispensing ammonia and hydrogen fluoride precursors into an etching chamber.

3. The method of claim 2 , wherein the first etching process further comprises maintaining a temperature of the etching chamber between about 25° C. and about 50° C.

4. The method of claim 1 , wherein the second etching process comprises dispensing ammonia and nitrogen trifluoride precursors into an etching chamber.

5. The method of claim 4 , wherein the second etching process further comprises generating plasma on the ammonia and nitrogen trifluoride precursors.

6. The method of claim 1 , further comprising forming an other fin protruding from the substrate and adjacent to the fin, wherein the other fin comprises:

a third sidewall opposing the second sidewall, wherein the first portion of the STI material is formed between the second and third sidewalls; and

a fourth sidewall formed opposite to the third sidewall.

7. The method of claim 1 , wherein a ratio of the first etching rate over the second etching rate is greater than or equal to about 1 and less than about 2.

8. The method of claim 1 , wherein a ratio of the third etching rate over the fourth etching rate is less than or equal to 1 and greater than about 0.5.

9. The method of claim 1 , wherein the radical treatment process comprises dispensing hydrogen and an inert gas into an etching chamber.

10. A method, comprising:

forming a plurality of fins protruding from a substrate, the plurality of fins comprising first and second outermost fins, wherein:

the first outermost fin comprises a first inner sidewall and a first outer sidewall formed opposite to the first inner sidewall; and

the second outermost fin comprises:

a second inner sidewall opposing the first inner sidewall; and

a second outer sidewall formed opposite to the second inner sidewall;

depositing a shallow-trench isolation (STI) material on the substrate, comprising:

depositing a first portion of the STI material between the first and second inner sidewalls; and

depositing a second portion of the STI material in contact with the first and second outer sidewalls;

performing a first etching process on the STI material to etch the first portion of the STI material at a first etching rate and the second portion of the STI material at a second etching rate greater than the first etching rate;

performing a second etching process on the STI material to etch the first portion of the STI material at a third etching rate and the second portion of the STI material at a fourth etching rate less than the third etching rate; and

performing a radical treatment process on the STI material after the first and second etching processes.

11. The method of claim 10 , wherein the first etching process comprises dispensing ammonia and hydrogen fluoride precursors into an etching chamber.

12. The method of claim 11 , wherein the first etching process further comprises maintaining a temperature of the etching chamber between about 25° C. and about 50° C.

13. The method of claim 10 , wherein the second etching process comprises dispensing ammonia and nitrogen trifluoride precursors into an etching chamber.

14. The method of claim 13 , wherein the second etching process further comprises generating plasma on the ammonia and nitrogen trifluoride precursors.

15. A method, comprising:

forming first and second fins protruding from a substrate:

depositing a shallow-trench isolation (STI) material on the substrate, wherein the STI material comprises a first portion in contact with outer sidewalls of the first and second fins and a second portion between inner sidewalls of the first and second fins;

performing a first etching process on the STI material to etch the first portion of the STI material at a first etching rate and the second portion of the STI material at a second etching rate different from the first etching rate;

performing a radical treatment process with hydrogen and an inert gas on the STI material after the first etching process; and

performing a second etching process on the STI material to etch the first portion of the STI material at a third etching rate and the second portion of the STI material at a fourth etching rate different from the third etching rate.

16. The method of claim 15 , further comprising performing an additional radical treatment process on the STI material after the second etching process.

17. The method of claim 15 , wherein performing the radical treatment process comprises dispensing the hydrogen and the inert gas into an etching chamber.

18. The method of claim 15 , wherein a ratio of the first etching rate over the second etching rate is greater than or equal to about 1 and less than about 2.

19. The method of claim 15 , wherein a ratio of the third etching rate over the fourth etching rate is less than or equal to 1 and greater than about 0.5.