Deep trench isolation structure for image sensor

1. A method of forming an image sensor, comprising:

receiving a workpiece having a first substrate;

etching a frontside of the first substrate, thereby forming a first trench;

depositing a fill layer in the first trench;

removing a portion of the fill layer from a backside of the first substrate, thereby forming a second trench surrounded by the fill layer; and

depositing a metal layer in the second trench, thereby forming an isolation structure comprising the metal layer and the fill layer, wherein the isolation structure separates adjacent pixel regions of the image sensor.

2. The method of claim 1 , wherein the depositing of the fill layer comprises:

depositing polysilicon in the first trench; and

doping the polysilicon.

3. The method of claim 1 , wherein the depositing of the metal layer comprises depositing metal material using a chemical vapor deposition (CVD) process.

4. The method of claim 1 , further comprising:

prior to the removing of the portion of the fill layer, thinning the backside of the first substrate, thereby exposing a seam sealed in the fill layer from the backside of the first substrate, wherein the seam is formed during the depositing of the fill layer in the first trench.

5. The method of claim 4 , wherein the removing of the portion of the fill layer comprises etching a center portion of the fill layer from the exposed seam using a wet etchant.

6. The method of claim 1 , further comprising:

prior to the depositing of the fill layer, forming a dielectric liner in the first trench, wherein the dielectric liner separates the fill layer from the first substrate.

7. The method of claim 1 , wherein the first substrate comprises an isolation feature formed in the frontside of the first substrate, and wherein the first trench extends through the isolation feature.

8. The method of claim 1 , further comprising:

prior to the removing of the portion of the fill layer, bonding a second substrate to the frontside of the first substrate.

9. The method of claim 1 , wherein the depositing of the metal layer in the second trench comprises:

depositing the metal layer in the second trench and on the backside of the first substrate; and

planarizing the workpiece, thereby exposing the backside of the first substrate.

10. The method of claim 1 , wherein the depositing of the metal layer in the second trench comprises:

depositing the metal layer in the second trench and on the backside of the first substrate; and

patterning the metal layer, thereby forming a metal grid over the isolation structure.

11. A method of forming an image sensor, comprising:

receiving a substrate comprising an isolation structure formed in a frontside of the substrate;

forming a first trench extending through the isolation structure and into a portion of the substrate;

filling the first trench with a first conductive material;

removing a bottom portion of the substrate, thereby exposing the first conductive material from a backside of the substrate;

removing a center portion of the first conductive material from the backside of the substrate, thereby forming a second trench;

filling the second trench with a second conductive material; and

removing a portion of the second conductive material from the backside of the substrate.

12. The method of claim 11 , wherein the first conductive material comprises doped polysilicon and the second conductive material comprises aluminum.

13. The method of claim 11 , further comprising:

prior to the removing of the bottom portion of the substrate, forming a photodiode in a region of the substrate surrounded by multiple first trenches.

14. The method of claim 11 , wherein the filling of the first trench with the first conductive material comprises forming an air gap sealed in the first conductive material.

15. The method of claim 14 , wherein the forming of the second trench comprises expanding a width of the air gap in a wet etching process.

16. The method of claim 11 , wherein the removing of the portion of the second conductive material comprises patterning the second conductive material to form an electrode, wherein the electrode is configured to couple a bias voltage to the second conductive material in the second trench.

17. A method of forming an image sensor, comprising:

receiving a substrate;

forming a first trench extending into a top portion of the substrate;

filling the first trench with a first conductive material;

forming a photodiode in a region of the substrate adjacent the first trench;

after the forming of the photodiode, thinning a bottom portion of the substrate, thereby exposing a bottom portion of the first conductive material;

partially recessing the bottom portion of the first conductive material to form a second trench;

filling the second trench with a second conductive material; and

forming a color filter overlying the photodiode.

18. The method of claim 17 , wherein the first conductive material comprises doped polysilicon and the second conductive material comprises a metal.

19. The method of claim 17 , wherein the filling of the second trench with the second conductive material includes capping a first airgap in the first conductive material and trapping a second airgap in the second trench.

20. The method of claim 17 , wherein the color filter is in physical contact with the first conductive material and the second conductive material.