Semiconductor device active region profile and method of forming the same

1. A semiconductor structure, comprising:

a substrate;

a stack of semiconductor channels over the substrate;

a gate stack disposed on the stack of semiconductor channels and extended to wrap around each of the semiconductor channels of the stack of semiconductor channels, wherein the gate stack includes a gate dielectric layer and a gate electrode, wherein each one of the semiconductor channels spans a dimension greater than that of any one of the semiconductor channels below; and

a dielectric fin disposed adjacent the stack of semiconductor channels, wherein a first dimension in a first direction of a top portion of the dielectric fin is less than a second dimension in the first direction of a bottom portion of the dielectric fin.

2. The semiconductor structure of claim 1 , wherein a difference between the first dimension and the second dimension is between about 1 nm to about 5 nm.

3. The semiconductor structure of claim 1 , further comprising a shallow trench isolation structure under the dielectric fin.

4. The semiconductor structure of claim 3 , wherein the top portion of the dielectric fin includes a high-k dielectric, and the bottom portion of the dielectric fin includes silicon oxide or a low-k dielectric.

5. The semiconductor structure of claim 4 , wherein an interface between the top portion and the bottom portion of the dielectric fin is coplanar with a top surface of the stack of semiconductor channels.

6. The semiconductor structure of claim 5 , wherein a distance between a top surface of the shallow trench isolation structure to the top surface of the stack of semiconductor channels is between about 40 nm to about 60 nm.

7. The semiconductor structure of claim 1 , wherein a topmost channel of the stack of semiconductor channels spans about 24 nm.

8. The semiconductor structure of claim 7 , wherein a bottommost channel of the stack of semiconductor channels spans about 18 nm to 22 nm.

9. The semiconductor structure of claim 1 , wherein an angle between a top surface of the substrate and a sidewall of the dielectric fin is between about 85 degrees to about 87 degrees.

10. A method comprising:

providing a semiconductor stack over a substrate, the semiconductor stack having first and second semiconductor layers stacked in an interleaving fashion, the first and second semiconductor layers having different materials;

etching the semiconductor stack by a first etching process;

etching the semiconductor stack by a second etching process, wherein the second etching process has a lateral etch rate greater than that of the first etching process, and wherein the second etching process is longer than the first etching process; and

repeating the first etching process and the second etching process to form a trench that extends into the substrate; and

forming a dielectric fin in the trench.

11. The method of claim 10 , further comprising forming:

a shallow trench isolation structure in the trench; and

epitaxially growing a third semiconductor layer on sidewalls of the trench and on a top surface of the shallow trench isolation structure.

12. The method of claim 11 , wherein the first and third semiconductor layers have the same semiconductor materials.

13. The method of claim 12 , wherein the substrate and the second semiconductor layers have the same semiconductor materials.

14. The method of claim 10 , wherein the first etching process is performed with a chlorine-containing chemical and the second etching process is performed with a fluorine-containing chemical.

15. The method of claim 10 , further comprising:

flushing a byproduct from the semiconductor stack after etching the semiconductor layer stack as part of the second etching process; and

repeating the flushing after each repetition of the second etching process.

16. The method of claim 15 , wherein the flushing of the byproduct includes using SO2/O2 for the flushing.

17. The method of claim 10 , wherein the first etching process and the second etching process are repeated 4 to 6 times.

18. A method comprising:

receiving a semiconductor stack over a substrate, the semiconductor stack having first and second semiconductor layers stacked in an interleaving fashion, the first and second semiconductor layers having different materials;

etching the semiconductor stack by a first etching process, the first etching process etches at a higher etch rate in a vertical direction than in a horizontal direction;

etching the semiconductor stack by a second etching process, the second etching process etches at a higher etch rate in the horizontal direction than in the vertical direction, wherein the first etching process includes a chlorine-containing etchant, and the second etching process includes a fluorine-containing etchant, wherein the second etching process is longer in time duration than the first etching process; and

flushing a byproduct from the semiconductor layer stack after etching the semiconductor layer stack as part of the second etching process.

19. The method of claim 18 , further comprising:

etching the semiconductor stack as part of a third etching process, the third etching process etches at a higher etch rate in the vertical direction than in the horizontal direction;

etching the semiconductor stack as part of a fourth etching process, the fourth etching process etches at a higher etch rate in the horizontal direction than in the vertical direction, wherein the third etching process includes a chlorine-containing etchant, and the fourth etching process includes a fluorine-containing etchant, wherein the fourth etching process is longer in time duration than the third etching process; and

flushing a byproduct from the semiconductor layer stack after etching the semiconductor layer stack as part of the fourth etching process.

20. The method of claim 19 , further comprising:

etching the semiconductor stack to form a trench extending below the semiconductor stack; and

forming a dielectric fin in the trench.