Method of fabricating trimmed fin and fin structure

1. A method of fabricating a trimmed fin, comprising:

forming a preliminary fin comprising silicon and germanium protruding from a substrate, wherein the preliminary fin has a first germanium concentration at a top surface of the preliminary fin and a second germanium concentration at a position beneath the top surface of the preliminary fin, and the first germanium concentration is less than the second germanium concentration;

oxidizing an exposed surface of the preliminary fin to form a trimmed fin covered by an oxide layer; and

removing the oxide layer to obtain the trimmed fin.

2. The method of claim 1 , wherein the oxide layer has a first thickness on a top surface of the trimmed fin and a second thickness on a side surface of the trimmed fin, and the first thickness is less than or equal to the second thickness.

3. The method of claim 1 , further comprising:

forming an insulating layer laterally adjacent to a bottom of the trimmed fin after removing the oxide layer to obtain the trimmed fin.

4. The method of claim 1 , further comprising:

forming an insulating layer laterally adjacent to a bottom of the preliminary fin before oxidizing the exposed surface of the preliminary fin.

5. The method of claim 1 , wherein the preliminary fin has a maximum germanium concentration at a bottom of the preliminary fin.

6. The method of claim 1 , wherein the germanium concentration of the preliminary fin is increased from the top surface of the preliminary fin to a bottom of the preliminary fin.

7. The method of claim 1 , wherein the germanium concentration of the preliminary fin is constant from the top surface of the preliminary fin to another position beneath the top surface of the preliminary fin.

8. The method of claim 1 , wherein the preliminary fin has a germanium concentration peak adjacent to a bottom of the preliminary fin or at an intermediate portion of the preliminary fin.

9. The method of claim 1 , wherein the preliminary fin has Si 1-x Ge x and Si 1-y Ge y respectively at the top surface and the position, and x is greater than or equal to 0, and y is less than or equal to 1, and y minus x is greater than or equal to 0.1.

10. The method of claim 1 , further comprising:

oxidizing an exposed surface of the trimmed fin to form a further trimmed fin covered by another oxide layer; and

removing the other oxide layer to obtain the further trimmed fin.

11. A method of fabricating a trimmed fin, comprising:

forming a preliminary fin comprising silicon and germanium protruding from a substrate, wherein the preliminary fin has a first germanium concentration at a top surface of the preliminary fin and a second germanium concentration at a position beneath the top surface of the preliminary fin, and the first germanium concentration is less than the second germanium concentration;

oxidizing an exposed surface of the preliminary fin to form a trimmed fin covered by an oxide layer, such that an angle formed between a horizontal cross-sectional plane through the position of the trimmed fin and a side surface of the trimmed fin is in a range of from 80 degrees to 100 degrees; and

removing the oxide layer to obtain the trimmed fin.

12. The method of claim 11 , further comprising:

forming an insulating layer laterally adjacent to a bottom of the trimmed fin after removing the oxide layer to obtain the trimmed fin.

13. The method of claim 12 , wherein the insulating layer is formed such that the position of the trimmed fin has a height the same as a height of a top surface of the insulating layer.

14. The method of claim 11 , further comprising:

forming an insulating layer laterally adjacent to a bottom of the preliminary fin before oxidizing the exposed surface of the preliminary fin.

15. The method of claim 11 , wherein oxidizing the exposed surface of the preliminary fin is performed such that the trimmed fin has a first side surface, a second side surface beneath the first side surface and misaligned with the first side surface.

16. The method of claim 11 , wherein oxidizing the exposed surface of the preliminary fin is performed such that a side surface of the trimmed fin has at least one corner, and the corner of the side surface of the trimmed fin has a height higher than a height of a top surface of an insulating layer.

17. The method of claim 16 , wherein the corner has an inner angle greater than or equal to 170 degrees and less than 180 degrees.

18. A method of fabricating a semiconductor device, comprising:

forming a preliminary fin comprising silicon and germanium protruding from a substrate;

forming an insulating layer laterally adjacent to a bottom portion of the preliminary fin, wherein an upper portion of the preliminary fin protrudes over a top surface of the insulating layer, and a top surface and sidewalls of the upper portion of the preliminary fin are exposed;

oxidizing the exposed top surface and sidewalls of the upper portion of the preliminary fin to form a trimmed fin covered by an oxide layer, wherein the oxide layer has a first portion extending along a top surface of the trimmed fin and a second portion extending along sidewalls of the trimmed fin, the first portion of the oxide layer has a first thickness, a bottom of the second portion of the oxide layer has a second thickness, and the second thickness is greater than the first thickness;

removing the oxide layer to obtain the trimmed fin; and

forming a gate dielectric layer on the insulating layer and the trimmed fin after removing the oxide layer.

19. The method of claim 18 , further comprising:

forming a gate structure above the gate dielectric layer.

20. The method of claim 19 , further comprising:

removing a portion of the gate dielectric layer exposed by the gate structure after forming the gate structure.