Methods for forming isolation regions by depositing and oxidizing a silicon liner

1. A method comprising:

etching a semiconductor substrate to form a trench and a semiconductor strip, wherein a sidewall of the semiconductor strip is exposed to the trench;

depositing a silicon-containing layer extending into the trench, wherein a sidewall portion the silicon-containing layer is on the sidewall of the semiconductor strip;

filling the trench with a dielectric material, wherein the dielectric material is on a sidewall of the silicon-containing layer;

performing a planarization process to level a first top surface of the dielectric material;

after the planarization process, oxidizing the silicon-containing layer to form a first liner, wherein an entirety of an upper portion of the sidewall portion is oxidized, and a lower portion of the sidewall portion remains as a semiconductor layer that comprises silicon, wherein the first liner comprises oxidized silicon, and wherein the first liner and the dielectric material form parts of an isolation region; and

recessing the isolation region, wherein a portion of the semiconductor strip protrudes higher than a second top surface of the isolation region and forms a semiconductor fin.

2. The method of claim 1 further comprising, before the silicon-containing layer is deposited, depositing a silicon oxide layer in contact with the sidewall of the semiconductor strip.

3. The method of claim 2 , wherein the silicon oxide layer is in contact with the silicon-containing layer.

4. The method of claim 1 , wherein the filling the dielectric material comprises:

depositing the dielectric material as flowable; and

before the planarization process, partially solidifying the dielectric material that is flowable, wherein the oxidizing is performed after the planarization process.

5. The method of claim 1 , wherein the dielectric material is deposited as a flowable material, and the flowable material is solidified by the step of oxidizing the silicon-containing layer.

6. The method of claim 1 , wherein the silicon-containing layer comprises a top portion overlapping the semiconductor strip, and wherein an entirety of the top portion is fully oxidized into silicon oxide by the oxidizing.

7. The method of claim 1 , wherein the silicon-containing layer has a thickness greater than about 0.5 nm.

8. The method of claim 1 , wherein the silicon-containing layer is deposited using atomic layer deposition.

9. A method comprising:

etching a semiconductor substrate to form a semiconductor strip and a trench, wherein the semiconductor strip is on a side of, and has a first lengthwise direction parallel to, a second lengthwise direction of, the trench;

depositing a first liner extending into the trench and contacting a sidewall of the semiconductor strip, wherein the first liner comprises silicon oxide;

depositing a second liner on the first liner, wherein the second liner comprises silicon, and the second liner comprising:

a sidewall portion extending from a top surface of the semiconductor substrate to a bottom of the trench, wherein the sidewall portion comprises an upper portion and a lower portion lower than the upper portion; and

a bottom portion at the bottom of the trench and joined to the lower portion of the sidewall portion;

depositing a dielectric material to fill the trench, wherein a portion of the second liner is underlying the dielectric material;

curing the dielectric material to form an oxide layer; and

converting the second liner into a third liner, wherein an entirety of the upper portion of the sidewall portion of the second liner is oxidized, and wherein the lower portion remains to be a silicon layer.

10. The method of claim 9 , wherein a lower portion of the semiconductor strip is free from germanium.

11. The method of claim 9 , wherein the second liner has a thickness greater than about 0.5 nm.

12. The method of claim 9 further comprising performing an epitaxy process on a base substrate to form an epitaxy semiconductor layer, with the epitaxy semiconductor layer and the base substrate collectively forming the semiconductor substrate.

13. The method of claim 9 , wherein the curing the dielectric material and the converting the second liner are performed by a same annealing process.

14. The method of claim 9 further comprising:

recessing the first liner, the second liner, and the oxide layer; and

forming a gate stack extending over the recessed first liner, the second liner, and the oxide layer.

15. The method of claim 14 , wherein at a time after the gate stack is formed, a part of the lower portion remains to be the silicon layer.

16. A method comprising:

depositing a silicon-containing liner into a trench in a semiconductor substrate, wherein the silicon-containing liner comprises a vertical portion and a horizontal portion lower than and joined to the vertical portion, and the horizontal portion comprises a top surface;

oxidizing an entirety of a top portion of the silicon-containing liner into a first oxidized silicon liner, wherein a bottommost end of the top portion is higher than the top surface of the horizontal portion, and wherein a bottom portion of the silicon-containing liner remains after the oxidizing;

depositing a dielectric material into the trench, wherein the first oxidized silicon liner comprises a first portion underlying the dielectric material, and the dielectric material and the first oxidized silicon liner form isolation regions, wherein at a time after the dielectric material is deposited, the bottom portion of the silicon-containing liner remains;

recessing the isolation regions, wherein a portion of the semiconductor substrate between the recessed isolation regions forms a protruding semiconductor fin;

forming a gate dielectric extending over the isolation regions; and

forming a gate electrode over the gate dielectric to form a transistor, wherein at a time the transistor is formed, the bottom portion of the silicon-containing liner is oxidized.

17. The method of claim 16 , wherein the silicon-containing liner comprises crystalline silicon.

18. The method of claim 16 further comprising, before the silicon-containing liner is deposited, depositing a silicon oxide layer extending into the trench, wherein the silicon-containing liner comprises amorphous silicon.

19. The method of claim 16 , wherein the oxidizing the silicon-containing liner is performed using a process gas selected from the group consisting of oxygen (O 2 ), water steam, and combinations thereof.

20. The method of claim 16 , wherein, at a time the dielectric material is deposited, a horizontal portion of the silicon-containing liner contacting a top surface of the semiconductor substrate remains as being a silicon-containing semiconductor layer.