Field effect transistors with gate fins and method of making the same

1. A field effect transistor, comprising:

at least one line trench extending downward from a top surface of a first transistor active region located in a substrate, and laterally extending along a first horizontal direction;

a gate dielectric contacting a bottom surface and sidewalls of each of the at least one line trench and comprising a planar gate dielectric portion overlying the substrate, at least one bottom gate dielectric portion contacting a bottom surface of a respective one of the at least one line trench, and vertically-extending gate dielectric portions contacting sidewalls of the at least one line trench and connecting the planar gate dielectric portion and a respective one of the at least one bottom gate dielectric portion;

a gate electrode comprising a planar gate electrode portion that overlies the planar gate dielectric portion and at least one gate electrode fin portion located within the at least one line trench;

a dielectric gate spacer laterally surrounding the gate electrode and comprising at least one pair of downward-protruding portions vertically extending into the at least one line trench, wherein each downward-protruding portion of the dielectric gate spacer comprises a respective bottom surface that contacts a top surface segment of a respective bottom gate dielectric portion of the gate dielectric; and

a source extension region in contact with end segments of a bottom surface and sidewalls of the at least one line trench and contacting a bottom surface of the planar gate dielectric portion.

2. The field effect transistor of claim 1 , wherein the dielectric gate spacer contacts a segment of a top surface of the planar gate dielectric portion and sidewalls of vertically-extending portions of the gate dielectric that are located in the at least one line trench.

3. The field effect transistor of claim 1 , wherein bottom surfaces of the at least one pair of downward-protruding portions of the dielectric gate spacer are located within a same horizontal plane as a bottom surface of each of the at least one gate electrode fin portion.

4. The field effect transistor of claim 1 , wherein the at least one pair of downward-protruding portions of the dielectric gate spacer contacts sidewalls the at least one gate electrode fin portion.

5. The field effect transistor of claim 4 , wherein interfaces between the at least one pair of downward-protruding portions of the dielectric gate spacer and the at least one gate electrode fin portion are perpendicular to the first horizontal direction.

6. The field effect transistor of claim 1 , wherein:

the planar gate electrode portion has widthwise gate sidewalls that are perpendicular to the first horizontal direction and laterally spaced apart along the first horizontal direction by a gate length; and

the at least one gate electrode fin portion has respective widthwise fin sidewalls that are perpendicular to the first horizontal direction and laterally spaced apart along the first horizontal direction by the gate length.

7. The field effect transistor of claim 6 , wherein each of the widthwise fin sidewalls is vertically coincident with a respective one of the widthwise gate sidewalls.

8. The field effect transistor of claim 1 , further comprising a deep source region and a deep drain region located in the substrate and laterally spaced from each other by the at least one line trench, wherein the source extension region is adjoined to the deep source region to provide a source region.

9. The field effect transistor of claim 8 , wherein a bottommost surface of the source extension region is located at a greater depth than a bottom surface of the deep source region from a horizontal plane including an interface between the substrate and the planar gate dielectric portion.

10. The field effect transistor of claim 9 , wherein a bottom surface of a horizontally-extending portion of the source extension region that is laterally offset from the at least one line trench is located at a lesser depth than the bottom surface of the deep source region from the horizontal plane.

11. The field effect transistor of claim 1 , wherein the gate dielectric contacts all surfaces of the at least one line trench.

12. The field effect transistor of claim 1 , wherein the at least one line trench comprises a plurality of line trenches each having a respective pair of lengthwise sidewalls that laterally extend along the first horizontal direction and laterally spaced from each other along a second horizontal direction that is perpendicular to the first horizontal direction.

13. The field effect transistor of claim 12 , wherein the planar gate electrode portion comprises:

a pair of lengthwise sidewalls that laterally extend along the first horizontal direction; and

a pair of widthwise sidewalls that laterally extend along the second horizontal direction and having a respective bottom edge that coincides with a top edge of a respective sidewall of the gate dielectric.

14. The field effect transistor of claim 1 , wherein each of the downward-protruding portions of the dielectric gate spacer further comprises a respective first sidewall that vertically extends from a first edge of the top surface segment of the respective bottom gate dielectric portion of the gate dielectric to an edge of a top surface of the gate electrode, and a respective second sidewall that vertically extends from a second edge of the top surface segment of the respective bottom gate dielectric portion of the gate dielectric to an inner periphery of a top surface of the planar gate dielectric portion.

15. A method of forming field effect transistor, comprising:

forming at least one line trench in a transistor active region in a semiconductor substrate;

forming a gate dielectric in the at least one line trench and over a top surface of the semiconductor substrate, wherein the gate dielectric comprises a planar gate dielectric portion overlying the substrate, at least one bottom gate dielectric portion contacting a bottom surface of a respective one of the at least one line trench, and vertically-extending gate dielectric portions contacting sidewalls of the at least one line trench and connecting the planar gate dielectric portion and a respective one of the at least one bottom gate dielectric portion;

forming a gate electrode over the gate dielectric, wherein the gate electrode comprises a planar gate electrode portion that overlies the transistor active region and at least one gate electrode fin portion located within a center portion of the at least one line trench;

forming a dielectric gate spacer around the gate electrode, wherein the dielectric gate spacer comprises downward-protruding portions vertically extending into a respective end portion of the at least one line trench, wherein each downward-protruding portion of the dielectric gate spacer comprises a respective bottom surface that contacts a top surface segment of a respective bottom gate dielectric portion of the gate dielectric; and

forming a source region and a drain region in the semiconductor substrate by implanting electrical dopants, wherein the source region comprises a source extension region having a bottommost surface that underlies a horizontal plane including each bottom surface of the at least one line trench.

16. The method of claim 15 , wherein the at least one gate electrode fin portion and a respective pair of downward-protruding portions are formed in the at least one line trench.

17. The method of claim 15 , further comprising:

forming a gate dielectric layer directly on all surfaces of the at least one line trench and on a top surface of the semiconductor substrate;

depositing at least one gate electrode material layer over the gate dielectric layer; and

patterning the at least one gate electrode material layer by performing an anisotropic etch process that etches unmasked portions of the at least one gate electrode material layer selective to the gate dielectric layer, wherein:

a patterned portion of the at least one gate electrode material layer comprises the gate electrode; and

a pair of voids is formed within each line trench of the at least one line trench.

18. The method of claim 17 , wherein the dielectric gate spacer is formed by depositing at least one dielectric spacer material layer around the gate electrode and in the voids in the at least one line trench, and by anisotropically etching the at least one dielectric spacer material layer.

19. The method of claim 15 , wherein:

the at least one line trench comprises a plurality of line trenches including a respective pair of lengthwise sidewalls that laterally extend along a first horizontal direction and laterally spaced apart along a second horizontal direction; and

each of plurality of line trenches comprises a respective upper periphery that coincides with a periphery of a respective opening in the top surface of the semiconductor substrate.

20. The method of claim 15 , wherein each of the downward-protruding portions of the dielectric gate spacer further comprises a respective first sidewall that vertically extends from a first edge of the top surface segment of the respective bottom gate dielectric portion of the gate dielectric to an edge of a top surface of the gate electrode, and a respective second sidewall that vertically extends from a second edge of the top surface segment of the respective bottom gate dielectric portion of the gate dielectric to an inner periphery of a top surface of the planar gate dielectric portion.