Integrated circuit structure and fabrication thereof

1. A method comprising:

forming a fin structure extending above a substrate;

forming dummy gate structures extending across the fin structure, each of the dummy gate structures including a dummy gate electrode layer and a hard mask layer over the dummy gate electrode layer;

performing an ion implantation process to dope the hard mask layers of the dummy gate structures;

after performing the ion implantation process to dope the hard mask layers of the dummy gate structures, performing a first etching process to etch a source/drain region of the fin structure between the dummy gate structures to form a recess in the source/drain region of the fin structure;

forming an epitaxial structure in the recess; and

replacing the dummy gate structures with metal gate structures.

2. The method of claim 1 , wherein the ion implantation process is performed at a tilt angle.

3. The method of claim 1 , wherein the fin structure is not doped by the ion implantation process.

4. The method of claim 1 , wherein the ion implantation process forms doped regions in the hard mask layers of the dummy gate structures, while leaving other regions of the hard mask layers of the dummy gate structures un-doped.

5. The method of claim 4 , wherein the doped regions of the hard mask layers have a higher etch resistance to the first etching process than that of the un-doped regions of the hard mask layers.

6. The method of claim 1 , further comprising:

after performing the ion implantation process and before performing the first etching process, forming a patterned organic material over the substrate, the patterned organic material having an opening over the source/drain region of the fin structure,

wherein the first etching process etches the patterned organic material at a faster etch rate than etching the doped hard mask layers.

7. The method of claim 1 , further comprising:

forming a source/drain epitaxial structure on the epitaxial structure; and

after forming the source/drain epitaxial structure on the epitaxial structure, replacing the epitaxial structure with a backside metal via.

8. The method of claim 1 , further comprising:

after forming the dummy gate structures and before performing the ion implantation process, performing a second etching process to etch the source/drain region of the fin structure, wherein the fin structure comprises a stack of alternating first semiconductor layers and second semiconductor layers, and the second etching process is performed until a top surface of the source/drain region of the fin structure is below a bottommost one of the second semiconductor layers.

9. The method of claim 8 , wherein the first semiconductor layers and the second semiconductor layers are not doped by the ion implantation process.

10. The method of claim 8 , further comprising:

after performing the second etching process, laterally recessing the first semiconductor layers; and

forming inner spacers on end surfaces of the laterally recessed first semiconductor layers, wherein the ion implantation process is performed after the inner spacers are formed.

11. The method of claim 10 , wherein the inner spacers are not doped by the ion implantation process.

12. The method of claim 8 , wherein replacing the dummy gate structures with the metal gate structures comprises:

removing the dummy gate structures;

after removing the dummy gate structures, removing the first semiconductor layers such that the second semiconductor layers are suspended above the substrate; and

forming the metal gate structures surrounding each of the suspended second semiconductor layers.

13. An integrated circuit (IC) structure comprising:

a gate structure;

gate spacers on either side of the gate structure;

source/drain epitaxial structures adjacent to the gate spacers, respectively;

a front-side interconnection structure on a front-side of the source/drain epitaxial structures;

a backside dielectric layer on a backside of the source/drain epitaxial structures; and

a backside via extending through the backside dielectric layer to the backside of a first one of the source/drain epitaxial structures, wherein the gate spacers have a concentration of a dopant decreasing toward the backside via.

14. The IC structure of claim 13 , further comprising:

a plurality of semiconductor channel layers arranged one above another in a spaced apart manner, the plurality of semiconductor channel layers laterally extending from the first one of the source/drain epitaxial structures to a second one of the source/drain epitaxial structures, the gate structure surrounding each of the plurality of semiconductor channel layers.

15. The IC structure of claim 14 , further comprising:

a plurality of inner spacers alternately arranged with the plurality of semiconductor channel layers, wherein the plurality of inner spacers has a lower concentration of the dopant of the gate spacers.

16. The IC structure of claim 14 , wherein the plurality of semiconductor channel layers has a lower concentration of the dopant of the gate spacers.

17. The IC structure of claim 13 , wherein the dopant is a Group III element or a Group V element.

18. A method comprising:

forming a dummy gate structure over a substrate, and gate spacers on opposite sides of the dummy gate structure;

epitaxially growing source/drain structures on regions of the substrate beyond the dummy gate structure and the gate spacers;

forming an interlayer dielectric (ILD) layer over the source/drain structures;

replacing the dummy gate structure with a metal gate structure;

recessing the metal gate structure to fall below top ends of the gate spacers;

forming a gate hard mask over the recessed metal gate structure;

performing an ion implantation process to dope the gate hard mask;

after performing the ion implantation process to dope the gate hard mask, etching the ILD layer to form source/drain contact openings over the source/drain structures, respectively; and

forming source/drain contacts in the source/drain contact openings, respectively.

19. The method of claim 18 , wherein the ion implantation process is performed at a tilt angle.

20. The method of claim 18 , wherein the ion implantation process also dopes the gate spacers.