Dielectric constant reduction of gate spacer

1. A method, comprising:

forming a dummy gate structure over a substrate;

forming a plurality of gate spacers respectively on opposite sidewalls of the dummy gate structure, the gate spacers having a first dielectric constant;

removing the dummy gate structure to form a gate trench between the gate spacers;

forming a dopant source layer to line the gate trench;

annealing the dopant source layer to diffuse k-value reduction impurities from the dopant source layer into the gate spacers to lower the first dielectric constant of the gate spacers to a second dielectric constant; and

forming a replacement gate stack in the gate trench.

2. The method of claim 1 , further comprising:

removing the dopant source layer from the gate trench prior to forming the replacement gate stack.

3. The method of claim 2 , wherein removing the dopant source layer is performed using an etchant same as an etchant used in removing the dummy gate structure.

4. The method of claim 2 , wherein an etch duration of removing the dopant source layer is shorter than an etch duration of removing the dummy gate structure.

5. The method of claim 2 , further comprising:

forming a gate dielectric layer over the substrate prior to forming the dummy gate structure, wherein the gate dielectric layer remains over the substrate after removing the dopant source layer.

6. The method of claim 5 , further comprising:

removing the gate dielectric layer prior to forming the replacement gate stack.

7. The method of claim 1 , wherein a material of the dopant source layer is same as a material of the dummy gate structure.

8. The method of claim 1 , where the k-value reduction impurities are fluorine.

9. The method of claim 1 , wherein the dopant source layer comprises polysilicon.

10. A method, comprising:

forming a first gate dielectric layer over a substrate and a first dummy gate structure over the first gate dielectric layer;

forming a plurality of first gate spacers alongside the first dummy gate structure;

etching the first dummy gate structure to form a first gate trench between the first gate spacers;

forming a dopant source layer over the first gate dielectric layer after etching the first dummy gate structure;

annealing the dopant source layer such that first k-value reduction impurities in the dopant source layer are diffused into the first gate dielectric layer; and

forming a first replacement gate stack in the first gate trench.

11. The method of claim 10 , further comprising:

removing the first gate dielectric layer from the first gate trench prior to forming the first replacement gate stack.

12. The method of claim 10 , further comprising:

forming a second gate dielectric layer over the substrate and a second dummy gate structure over the second gate dielectric layer;

forming a plurality of second gate spacers alongside the second dummy gate structure; and

etching the second dummy gate structure to form a second gate trench between the second gate spacers, wherein the second gate dielectric layer remains over the substrate after etching the second dummy gate structure,

wherein annealing the dopant source layer is performed such that second k-value reduction impurities in the dopant source layer are diffused into the second gate dielectric layer.

13. The method of claim 12 , wherein forming the dopant source layer is performed such that the dopant source layer is further formed over the second gate dielectric layer.

14. The method of claim 10 , wherein forming the dopant source layer comprises:

depositing a polysilicon layer over the first gate dielectric layer; and

doping the polysilicon layer with fluorine.

15. The method of claim 14 , wherein doping the polysilicon layer is in-situ performed with depositing the polysilicon layer.

16. A method, comprising:

forming a first dummy gate electrode over an n-type field-effect transistor (NFET) region of a substrate, and a second dummy gate electrode over a p-type field-effect transistor (PFET) region of the substrate;

forming first gate spacers on either side of the first dummy gate electrode, and second gate spacers on either side of the second dummy gate electrode;

etching the first and second dummy gate electrodes, resulting in a first gate trench between the first gate spacers and a second gate trench between the second gate spacers;

after etching the first and second dummy gate electrodes, depositing a dopant source layer spanning across both the NFET region and the PFET region;

performing an annealing process such that k-value reduction impurities in the dopant source layer are diffused into the first gate spacers and the second gate spacers; and

after performing the annealing process, forming an NFET gate structure in the first gate trench and a PFET gate structure in the second gate trench.

17. The method of claim 16 , further comprising:

forming an interlayer dielectric (ILD) layer laterally around the first gate spacers and the second gate spacers, wherein the annealing process also results in diffusing the k-value reduction impurities into the ILD layer.

18. The method of claim 16 , further comprising:

forming n-type epitaxy structures on either side of the first dummy gate electrode, and p-type epitaxy structures on either side of the second dummy gate electrode, wherein after the annealing process, the n-type epitaxy structures and the p-type epitaxy structures are free of the k-value reduction impurities.

19. The method of claim 16 , further comprising:

forming a first interfacial layer below the first dummy gate electrode, and a second interfacial layer below the second dummy gate electrode, wherein the dopant source layer is deposited on both the first interfacial layer and the second interfacial layer.

20. The method of claim 19 , wherein the annealing process also results in diffusing the k-value reduction impurities into both the first interfacial layer and the second interfacial layer.