Semiconductor structure and fabrication method thereof

1. A semiconductor structure, comprising:

a base substrate;

an optical waveguide layer over the base substrate;

a first dielectric layer over the base substrate, wherein a cavity is formed between the first dielectric layer and the optical waveguide layer, and the cavity is located on sidewall surfaces of the optical waveguide layer and has a bottom coplanar with a bottom of the optical waveguide layer, and the cavity is filled with air; and

a second dielectric layer on the first dielectric layer and the optical waveguide layer, wherein the second dielectric layer is located on a top of the cavity and seals the cavity.

2. The structure according to claim 1 , wherein:

the optical waveguide layer is made of a material with a first refractive index;

the cavity has a second refractive index; and

the second refractive index is less than the first refractive index, and is also less than a refractive index of silicon dioxide.

3. The structure according to claim 2 , wherein:

the optical waveguide layer is made of a semiconductor material including silicon or SiN x .

4. The structure according to claim 1 , wherein:

a size of the cavity along a direction perpendicular to sidewalls of the cavity is about 10 nm to about 200 nm.

5. The structure according to claim 1 , wherein:

a size of a top of the cavity is smaller than a size of a bottom of the cavity along a direction perpendicular to sidewalls of the cavity.

6. The structure according to claim 1 , wherein the base substrate includes a substrate and an insulation layer on the substrate.

7. The structure according to claim 6 , wherein the substrate is made of a semiconductor material including silicon, silicon germanium, or a combination thereof.

8. The structure according to claim 6 , wherein the insulation layer is made of SiO x , SiN x , SiNC, SiNB, SiNCO, SiNO, or a combination thereof.

9. The structure according to claim 1 , wherein the first dielectric layer is made of SiO x , SiN x , SiNC, SiNB, SiNCO, SiNO, or a combination thereof.

10. The structure according to claim 1 , wherein the second dielectric layer is made of SiO x , SiN x , SiNC, SiNB, SiNCO, SiNO, or a combination thereof.

11. A fabrication method of a semiconductor structure, comprising:

providing a base substrate with an optical waveguide layer over the base substrate, wherein the optical waveguide layer is made of a material with a first refractive index;

forming a sacrificial layer on sidewall surfaces of the optical waveguide layer;

forming a first dielectric layer over the base substrate, wherein the first dielectric layer exposes a top surface of the sacrificial layer;

removing the sacrificial layer to form a cavity between the first dielectric layer and the optical waveguide layer; and

forming a second dielectric layer on the first dielectric layer and the optical waveguide layer,

wherein:

the cavity has a second refractive index, and the cavity is filled with air;

the second refractive index is less than the first refractive index, and is also less than the refractive index of silicon dioxide; and

the second dielectric layer is located on a top of the cavity and seals the cavity.

12. The method according to claim 11 , before forming the sacrificial layer, further including:

forming a protection layer on a top surface of the optical waveguide layer.

13. The method according to claim 12 , wherein:

the protection layer and the sacrificial layer are made of a same material; and

the protection layer is made of SiO x , SiN x , SiNC, SiNB, SiNCO, SiNO, or a combination thereof.

14. The method according to claim 11 , wherein the sacrificial layer is formed by:

forming a sacrificial material layer on the surface of the base substrate, on a top of the optical waveguide layer, and on the sidewall surfaces of the optical waveguide layer; and

etching back the sacrificial material layer until exposing the surface of the base substrate and the top surface of the optical waveguide layer, to form the sacrificial layer on the sidewall surfaces of the optical waveguide layer.

15. The method according to claim 13 , wherein the first dielectric layer is formed by:

forming a dielectric material layer over the base substrate and the protection layer, wherein the dielectric material layer covers a surface of the sacrificial layer; and

planarizing the dielectric material layer until exposing the top surface of the sacrificial layer and the top surface of the protection layer, to form the first dielectric layer.

16. The method according to claim 11 , wherein:

the second dielectric layer is formed by a plasma-enhanced chemical vapor deposition method.

17. The method according to claim 12 , after forming the first dielectric layer and before forming the second dielectric layer, further including:

removing the protection layer.

18. The method according to claim 11 , wherein:

the base substrate includes a substrate and an insulation layer on a surface of the substrate.

19. The method according to claim 18 , wherein the base substrate and the optical waveguide layer are formed by:

providing a silicon-on-insulator substrate, wherein the silicon-on-insulator substrate includes a first substrate, an initial insulation layer on a surface of the first substrate, and a second substrate on a surface of the initial insulation layer; and

patterning the second substrate of the silicon-on-insulator substrate until exposing the surface of the initial insulation layer, to form the base substrate and the optical waveguide layer over the base substrate.

20. The structure according to claim 1 , wherein:

a bottom surface of the first dielectric layer is coplanar with a bottom surface of the optical waveguide layer;

a top surface of the first dielectric layer is higher than a top surface of the optical waveguide layer; and

a size of the top surface of the first dielectric layer is larger than a size of the bottom surface of the first dielectric layer.