Structure and method for semiconductor devices

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

a semiconductor substrate having a top surface;

a first source/drain feature and a second source/drain feature disposed on the semiconductor substrate;

a plurality of semiconductor layers including a first semiconductor layer and a second semiconductor layer, the first semiconductor layer extending longitudinally in a first direction and connecting the first source/drain feature and the second source/drain feature, the second semiconductor layer extending longitudinally in the first direction and connecting the first source/drain feature and the second source/drain feature, wherein the first semiconductor layer is stacked over and spaced apart the second semiconductor layer in a second direction perpendicular to the first direction, the second direction being normal to the top surface of the semiconductor substrate; and

a gate structure engaging center portions of the first semiconductor layer and the second semiconductor layer, wherein a length of the first semiconductor layer along the first direction is less than a length of the second semiconductor layer along the first direction, and wherein a width of the first semiconductor layer is different from a width of the second semiconductor layer, wherein each of the width of the first semiconductor layer and the width of the second semiconductor layer is measured in a third direction perpendicular to the first direction and the second direction.

2. The IC device of claim 1 , wherein a difference between the length of the second semiconductor layer and the length of the first semiconductor layer is at least 0.5 nanometers.

3. The IC device of claim 2 , wherein the difference between the length of the second semiconductor layer and the length of the first semiconductor layer is at least 1 nanometer.

4. The IC device of claim 1 , wherein each of a thickness of the first semiconductor layer and a thickness of the second semiconductor layer, as measured in the second direction, is in a range from about 3 nanometers to about 10 nanometers.

5. The IC device of claim 1 , wherein a difference between the width of the first semiconductor layer and the width of the second semiconductor layer is in a range from about 0.5 nanometers to about 5 nanometers.

6. The IC device of claim 1 , wherein each of the width of the first semiconductor layer and the width of the second semiconductor layer is in a range from about 6 nanometers to about 50 nanometers.

7. The IC device of claim 1 , further comprising:

a first spacer disposed over side portions of the first semiconductor layer, the first semiconductor layer being a topmost layer of the plurality of semiconductor layers; and

a second spacer disposed between the side portions of the first semiconductor layer and side portions of the second semiconductor layer in the first direction, wherein the first spacer and the second spacer have different material compositions.

8. The IC device of claim 7 , wherein the second spacer includes an air gap.

9. The IC device of claim 1 , wherein a first portion of the gate structure engages the first semiconductor layer and a second portion of the gate structure engages the second semiconductor layer, wherein a length of the first portion of the gate structure along the first direction is less than a length of the second portion of the gate structure along the first direction.

10. The IC device of claim 9 , wherein a difference between the length of the first portion of the gate structure and the length of the second portion of the gate structure is at least 0.5 nanometers.

11. The IC device of claim 1 , wherein an acute angle subtended by a sidewall of the first source/drain feature and the top surface of the semiconductor substrate is in a range from about 80 degrees to about 88 degrees.

12. An integrated circuit (IC) device, comprising:

a semiconductor substrate having a top surface;

a first source/drain feature and a second source/drain feature disposed on the semiconductor substrate;

a plurality of semiconductor layers extending from the first source/drain feature to the second source/drain feature along a first direction, the plurality of semiconductor layers stacked over each other along a second direction normal to the top surface and perpendicular to the first direction, wherein each of the semiconductor layers has a center portion and two side portions laterally adjacent to the center portion;

a gate electrode engaging the center portion of each of the semiconductor layers;

a first spacer over the two side portions of a topmost semiconductor layer of the semiconductor layers; and

a second spacer between vertically adjacent side portions of the semiconductor layers along the second direction, wherein:

the first spacer includes a first dielectric material having a first dielectric constant;

the second spacer includes a second dielectric material having a second dielectric constant different from the first dielectric constant; and

a length of the topmost semiconductor layer, as measured along the first direction, is smaller than a length of each of the semiconductor layers vertically below the topmost semiconductor layer.

13. The IC device of claim 12 , further comprising a gate end dielectric layer contacting opposing ends of each of the plurality of semiconductor layers, wherein the gate end dielectric layer includes a third dielectric material having a third dielectric constant, the third dielectric constant being larger than the first dielectric constant and the second dielectric constant.

14. The IC device of claim 12 , wherein the first spacer includes a dielectric material selected from the group consisting of SiO2, SiON, SiOC, SiOCN, and combinations thereof, and wherein the second spacer includes a dielectric material selected from the group consisting of SiO2, Si3N4, carbon doped oxide, nitrogen doped oxide, porous oxide, and combinations thereof.

15. The IC device of claim 12 , wherein the second spacer includes an air gap therein.

16. An integrated circuit (IC) device, comprising:

a semiconductor substrate having a top surface;

a first source/drain feature and a second source/drain feature disposed on the semiconductor substrate;

a plurality of semiconductor layers extending longitudinally in a first direction and connecting the first source/drain feature and the second source/drain feature, wherein the semiconductor layers are stacked over and spaced apart from each other in a second direction perpendicular to the first direction, the second direction being normal to the top surface of the semiconductor substrate;

a gate structure engaging center portions of the semiconductor layers, wherein the semiconductor layers include respective lengths along the first direction, wherein the respective lengths are different from each other;

a first spacer disposed over side portions of the top semiconductor layer; and

a second spacer disposed between the side portions of the top semiconductor layer and side portions of the middle semiconductor layer in the first direction, wherein the first spacer and the second spacer have different material compositions.

17. The IC device of claim 16 , wherein

the semiconductor layers include a bottom semiconductor layer, a middle semiconductor layer stacked over the bottom semiconductor layer, and a top semiconductor layer stacked over the middle semiconductor layer;

the bottom semiconductor layer includes a first length along the first direction;

the middle semiconductor layer includes a second length along the first direction;

the top semiconductor layer includes a third length along the first direction; and

the second length is less than the first length and is greater than the third length.

18. The IC device of claim 17 , wherein

the bottom semiconductor layer includes a first width along a third direction perpendicular to the first direction and the second direction;

the middle semiconductor layer includes a second width along the third direction;

the top semiconductor layer includes a third width along the third direction; and

the second width is different from the first and third widths.

19. The IC device of claim 18 , wherein

the second spacer includes an air gap; and

an acute angle subtended by a sidewall of the first source/drain feature and the top surface of the semiconductor substrate is in a range from about 80 degrees to about 88 degrees.

20. The IC device of claim 16 , wherein the second spacer includes an air gap.