Methods of forming epitaxial structures in fin-like field effect transistors

1. A semiconductor structure, comprising:

a semiconductor substrate having a first region for forming p-type metal-oxide-semiconductor (PMOS) devices and a second region for forming n-type metal-oxide-semiconductor (NMOS) devices;

a first source/drain feature over a first fin structure in the first region, the first source/drain feature being a p-type source/drain feature;

a second source/drain feature over a second fin structure in the second region, the second source/drain feature being an n-type source/drain feature; and

isolation features between the first source/drain feature and the second source/drain feature, the isolation features including a first portion adjacent to the first source/drain feature, a second portion adjacent to the second source/drain feature, and a third portion between the first portion and the second portion, wherein a width of the first portion is greater than a width of the second portion.

2. The semiconductor structure of claim 1 , wherein the first fin structure includes one fin and the second fin structure includes two fins.

3. The semiconductor structure of claim 2 , wherein the second source/drain feature merges the two fins of the second fin structure together.

4. The semiconductor structure of claim 1 , wherein a vertical distance between a lowest point and a highest point of the first portion of the isolation features is greater than a vertical distance between a lowest point and a highest point of the second portion of the isolation features.

5. The semiconductor structure of claim 1 , wherein a ratio of the width of the first portion of the isolation features to the width of the second portion of the isolation features is between about 1:5 and about 3:5.

6. The semiconductor structure of claim 1 , wherein a top surface of the first fin structure is below a top surface of the second fin structure.

7. The semiconductor structure of claim 1 , wherein a width of the third portion is less than the width of the first portion.

8. A semiconductor structure, comprising:

a first fin structure disposed in a first region of a semiconductor substrate for forming a p-type metal-oxide-semiconductor (PMOS) device;

a p-type source/drain (S/D) feature disposed on a top surface of the first fin structure;

a second fin structure disposed in a second region of the semiconductor substrate for forming an n-type metal-oxide-semiconductor (NMOS) device, wherein the second region is adjacent to the first region, and wherein a top surface of the second fin structure is above the top surface of the first fin structure;

an n-type S/D feature disposed on the top surface of the second fin structure; and

a dielectric layer disposed between bottom portions of the first fin structure from the second fin structure, wherein a height of the dielectric layer decreases from a middle portion of the dielectric layer toward the first fin structure and the second fin structure, respectively.

9. The semiconductor structure of claim 8 , wherein the first fin structure includes a single fin, and wherein the second fin structure includes two fins connected at their bottom portions.

10. The semiconductor structure of claim 9 , wherein the n-type S/D feature merges the two fins of the second fin structure together.

11. The semiconductor structure of claim 8 , wherein a size of the p-type S/D feature is smaller than a size of the n-type S/D feature.

12. The semiconductor structure of claim 8 , wherein a lowest point of the dielectric layer contacts a bottom portion of the p-type S/D feature.

13. The semiconductor structure of claim 8 , wherein a highest point of the dielectric layer is laterally separated from the top surface of the first fin structure and the top surface of the second fin structure by a first distance and a second distance, respectively, and wherein the first distance is greater than the second distance.

14. A semiconductor structure, comprising:

a semiconductor substrate;

a first fin structure disposed over the semiconductor substrate;

a first source/drain (S/D) feature disposed on the first fin structure, wherein the first S/D feature is a p-type S/D feature;

a second fin structure disposed over the semiconductor substrate and adjacent to the first fin structure;

a second S/D feature disposed on the second fin structure, wherein the second S/D feature is an n-type S/D feature; and

an isolation structure separating the first and the second fin structures, wherein the isolation structure includes a first region surrounding the first fin structure, a second region surrounding the second fin structure, and an island region disposed between the first and the second regions, and wherein a lowest point of the first region is below a lowest point of the second region.

15. The semiconductor structure of claim 14 , further comprising an etch-stop layer disposed conformally over the first and the second S/D features and the isolation structure.

16. The semiconductor structure of claim 14 , wherein the island region is disposed above the lowest point of the second region.

17. The semiconductor structure of claim 14 , wherein the island region is laterally separated from a bottom surface of the first S/D feature by a first distance, wherein the island region is laterally separated from a bottom surface of the second S/D feature by a second distance, and wherein the second distance is less than the first distance.

18. The semiconductor structure of claim 17 , wherein a top surface of the island region laterally expands a third distance, and wherein the third distance is less than the first distance.

19. The semiconductor structure of claim 18 , wherein a ratio of the third distance to the first distance is greater than a ratio of the second distance to the first distance.

20. The semiconductor structure of claim 18 , wherein a ratio of the second distance to the first distance is greater than a ratio of the third distance to the first distance.