IC structure with fin having subfin extents with different lateral dimensions

1. An integrated circuit (IC) structure, comprising:

a semiconductor substrate;

a semiconductor fin over the semiconductor substrate, the semiconductor fin having a first longitudinal extent and a second longitudinal extent, the semiconductor fin having:

an upper fin portion having a uniform lateral dimension in the first longitudinal extent and the second longitudinal extent,

a first subfin portion under the upper fin portion in the first longitudinal extent having a first lateral dimension, and

a second subfin portion under the upper fin portion in the second longitudinal extent having a second lateral dimension different than the first lateral dimension, wherein the second subfin portion and the semiconductor substrate include a same material composition.

2. The IC structure of claim 1 , wherein the second lateral dimension is less than the first lateral dimension.

3. The IC structure of claim 2 , wherein the second subfin portion has inwardly curved outer surfaces.

4. The IC structure of claim 1 , wherein each subfin portion is within a trench isolation, wherein the trench isolation has a bulbous cross-sectional shape adjacent the second subfin portion.

5. The IC structure of claim 1 , wherein the second subfin portion is within a drain extension region of a fin-type laterally-diffused metal oxide semiconductor (LDMOS) device.

6. The IC structure of claim 5 , wherein a portion of the first subfin portion is within the drain extension region.

7. The IC structure of claim 5 , wherein the LDMOS device includes a first gate structure over the semiconductor fin and a second, floating gate structure over the drain extension region.

8. The IC structure of claim 5 , wherein the LDMOS device includes a trench isolation in an n-well within the drain extension region and adjacent to the drain region.

9. The IC structure of claim 1 , wherein the semiconductor fin includes a plurality of fins.

10. A fin-type field effect transistor (FinFET) laterally-diffused metal-oxide semiconductor (LDMOS) device, comprising:

a semiconductor substrate;

a semiconductor fin, over the semiconductor substrate, the semiconductor fin having a first longitudinal extent and a second longitudinal extent, the semiconductor fin having:

an upper fin portion having a uniform lateral dimension in the first longitudinal extent and the second longitudinal extent,

a first subfin portion under the upper fin portion in the first longitudinal extent having a first lateral dimension, and

a second subfin portion under the upper fin portion in the second longitudinal extent having a second lateral dimension different than the first lateral dimension, wherein the semiconductor substrate narrows to the second lateral dimension within the second subfin portion;

a p-well in part of the first longitudinal extent;

an n-well in at least the second longitudinal extent;

a source region in the p-well;

a drain region in the n-well;

a drain extension region in the n-well, wherein the second subfin portion is within the drain extension region; and

a first gate structure extending over the p-well and n-well.

11. The FinFET LDMOS device of claim 10 , wherein the second lateral dimension is less than the first lateral dimension.

12. The FinFET LDMOS device of claim 10 , wherein the second subfin portion has inwardly curved outer surfaces.

13. The FinFET LDMOS device of claim 10 , wherein each subfin portion is within a trench isolation, wherein the trench isolation has a bulbous cross-sectional shape adjacent the second subfin portion.

14. The FinFET LDMOS device of claim 10 , wherein a portion of the first subfin portion is within the drain extension region.

15. The FinFET LDMOS device of claim 10 , further comprising a second, floating gate structure spaced from the first gate structure, wherein the second, floating gate structure is over the drain extension region.

16. The FinFET LDMOS device of claim 10 , further comprising a trench isolation in the n-well within the drain extension region and adjacent to the drain region.

17. A method, comprising:

forming a semiconductor fin within a semiconductor substrate and having a first longitudinal extent and a second longitudinal extent, the semiconductor fin having:

an upper fin portion having a uniform lateral dimension in the first longitudinal extent and the second longitudinal extent,

a first subfin portion under the upper fin portion in the first longitudinal extent having a first lateral dimension, and

a second subfin portion under the upper fin portion and in the second longitudinal extent, the second subfin portion having a second lateral dimension different than the first lateral dimension, and wherein the second subfin portion and the semiconductor substrate include a same material composition; and

forming a metal gate structure over the semiconductor fin.

18. The method of claim 17 , wherein forming the semiconductor fin includes:

forming a pair of trenches into a substrate to a first depth, the pair of trenches creating the semiconductor fin therebetween;

forming a first mask over the second longitudinal extent, leaving the first longitudinal extent exposed;

etching the pair of trenches in the first longitudinal extent to a second depth deeper than the first depth, creating the first subfin portion with the first lateral dimension;

removing the first mask over the second longitudinal extent;

forming a spacer layer over the first and second longitudinal extents;

forming a second mask over the first longitudinal extent, leaving the second longitudinal extent exposed;

etching the pair of trenches in the second longitudinal extent to a third depth deeper than the first depth;

etching the pair of trenches in the second longitudinal extent to widen the pair of trenches and create the second subfin portion with the second lateral dimension less than the first lateral dimension;

removing the second mask; and

filling the pair of trenches adjacent the first and second subfin portions with a dielectric.

19. The method of claim 18 , wherein the dielectric adjacent the second subfin portion has a bulbous cross-sectional shape.

20. The method of claim 17 , further comprising, prior to forming the metal gate structure:

forming a p-well in part of the first longitudinal extent;

forming an n-well in at least the second longitudinal extent;

forming a source region in the p-well;

forming a drain region in the n-well; and

forming a drain extension region in the n-well,

wherein the second subfin portion is within the drain extension region.