Photonic device

1. A photonic device, comprising:

an optical coupler over a semiconductor substrate;

a photodetector over the semiconductor substrate;

a waveguide structure over the semiconductor substrate and laterally connected to the optical coupler, wherein a top of the waveguide structure is lower than a top of the optical coupler;

a metal-dielectric stack over the optical coupler, the photodetector, and the waveguide structure, wherein the metal-dielectric stack has a hole above the optical coupler;

a contact over the semiconductor substrate and connecting the photodetector to the metal-dielectric stack;

an interlayer dielectric layer below the metal-dielectric stack and surrounding the contact; and

a protection layer lining the hole of the metal-dielectric stack, wherein a bottom surface of the protection layer is higher than a top surface of the interlayer dielectric layer and lower than a top surface of the metal-dielectric stack, and the bottom surface of the protection layer is spaced apart from the top surface of the interlayer dielectric layer by a portion of the metal-dielectric stack.

2. The photonic device of claim 1 , wherein the metal-dielectric stack comprises a plurality of metallization layers, each of the metallization layers comprises a dielectric layer and a conductive feature in the dielectric layer, and the protection layer is in contact with sidewalls of the dielectric layers of the metallization layers.

3. The photonic device of claim 2 , wherein the bottom surface of the protection layer is below a top surface of a bottommost one of the metallization layers.

4. The photonic device of claim 2 , wherein the bottom surface of the protection layer is in contact with the dielectric layer of a bottommost one of the metallization layers.

5. The photonic device of claim 1 , further comprising:

an etch stop layer between the interlayer dielectric layer and the metal-dielectric stack.

6. The photonic device of claim 1 , further comprising:

a passivation layer over the metal-dielectric stack, wherein the passivation layer has an opening aligned with the hole.

7. The photonic device of claim 6 , wherein the protection layer further lines the opening of the passivation layer.

8. A photonic device, comprising:

an optical coupler over a semiconductor substrate;

a waveguide structure over the semiconductor substrate and laterally connected to the optical coupler;

a metal-dielectric stack over the optical coupler and the waveguide structure, wherein the metal-dielectric stack comprises a first etch stop layer, a first dielectric layer over the first etch stop layer, a second etch stop layer over the first dielectric layer, and a second dielectric layer over the second etch stop layer; and

a protection layer in the metal-dielectric stack and directly above the optical coupler, wherein the protection layer comprises a side portion extending along a sidewall of the second dielectric layer and a sidewall of the second etch stop layer and a bottom portion extending laterally from an end of the side portion of the protection layer, wherein a bottom surface of the bottom portion of the protection layer is higher than a top surface of the first etch stop layer and lower than a bottom surface of the second etch stop layer.

9. The photonic device of claim 8 , wherein the bottom portion of the protection layer extends along a top surface of the first dielectric layer.

10. The photonic device of claim 8 , wherein the protection layer has a recess directly above the optical coupler and surrounded by the side portion and the bottom portion of the protection layer.

11. The photonic device of claim 8 , wherein a top end of the side portion of the protection layer is higher than a top surface of the metal-dielectric stack.

12. The photonic device of claim 8 , wherein the first dielectric layer has a first portion directly below the protection layer and a second portion extending from the first portion of the first dielectric layer, and the second portion of the first dielectric layer has a greater thickness than a thickness of the first portion of the first dielectric layer.

13. A photonic device, comprising:

an optical coupler over a semiconductor substrate;

a waveguide structure over the semiconductor substrate and laterally connected to the optical coupler;

a metal-dielectric stack over the optical coupler and the waveguide structure, wherein the metal-dielectric stack has a hole above the optical coupler; and

a protection layer in the hole and having a concave bottom surface facing the optical coupler.

14. The photonic device of claim 13 , further comprising:

a contact over the semiconductor substrate and connected to the metal-dielectric stack, wherein the concave bottom surface of the protection layer is higher than a top surface of the contact.

15. The photonic device of claim 14 , further comprising:

a metal layer over and in contact with the contact, wherein the concave bottom surface of the protection layer is lower than a top surface of the metal layer.

16. The photonic device of claim 13 , wherein the metal-dielectric stack comprises a plurality of conductive features spaced apart from the protection layer.

17. The photonic device of claim 13 , wherein the waveguide structure comprises a plurality of protruding portions directly below the concave bottom surface of the protection layer.

18. The photonic device of claim 13 , wherein the protection layer has a convex top surface facing away from the optical coupler.

19. The photonic device of claim 5 , wherein the etch stop layer spaces the protection layer from the interlayer dielectric layer.

20. The photonic device of claim 8 , wherein the first dielectric layer spaces the protection layer from the first etch stop layer.