BSI image sensor and method of forming same

1. A method comprising:

forming a plurality of photosensitive pixels in a substrate, the substrate having a first surface and a second surface, the second surface being opposite to the first surface, the substrate having one or more active devices on the first surface; protecting a first portion of the second surface;

patterning a second portion of the second surface to form recesses in the substrate;

forming an anti-reflective layer on sidewalls of the recesses; and

forming a metal grid over the second portion of the second surface, the anti-reflective layer being interposed between the substrate and the metal grid.

2. The method of claim 1 , further comprising forming a metal shield over the first portion of the second surface, the anti-reflective layer being interposed between the substrate and the metal shield.

3. The method of claim 1 , wherein forming the metal grid comprises:

forming a metal layer over the second portion of the second surface; and

patterning the metal layer to form openings therein, the openings being aligned with respective photosensitive pixels.

4. The method of claim 3 , further comprising, after patterning the second portion of the second surface, filling the openings with a dielectric material.

5. The method of claim 3 , further comprising, prior to forming the metal layer, filling the recesses with a dielectric material, the anti-reflective layer being interposed between the sidewalls of the recesses and the dielectric material.

6. The method of claim 1 , wherein patterning the second portion of the second surface comprises:

forming a patterned mask on the second portion of the second surface, the patterned mask exposing portions of the substrate; and

etching exposed portions of the substrate to form the recesses in the substrate.

7. The method of claim 1 , wherein portions of the substrate interposed between neighboring recesses have a uniform width and a uniform pitch.

8. The method of claim 1 , wherein the sidewalls of the recesses are slanted.

9. A method comprising:

forming a plurality of photosensitive devices in a substrate, the substrate having a pixel array region and a black level correction (BLC) region adjacent the pixel array region; forming an interconnect structure on a first side of the substrate; patterning a second side of the substrate in the pixel array region to form recesses in the substrate, the second side being opposite to the first side;

blanket depositing an anti-reflective layer over the recesses in the pixel array region and over the second side of the substrate in the BLC region;

forming a first dielectric layer over the anti-reflective layer, the first dielectric layer overfilling the recesses;

forming a metal layer over the first dielectric layer; and

patterning the metal layer to form a metal grid over the pixel array region and a metal shield over the BLC region.

10. The method of claim 9 , wherein at least a portion of the metal shield extends into the first dielectric layer.

11. The method of claim 9 , wherein sidewalls of the recesses form a saw-tooth pattern in a cross-sectional view.

12. The method of claim 9 , further comprising forming a second dielectric layer over the metal grid and the metal shield, the second dielectric layer extending along sidewalls of the metal grid and a sidewall of the metal shield.

13. The method of claim 9 , wherein blanket depositing the anti-reflective layer over the recesses in the pixel array region and over the second side of the substrate in the BLC region comprises:

blanket depositing a first sublayer over the recesses in the pixel array region and over the second side of the substrate in the BLC region, the first sublayer comprising a first oxide material; and

blanket depositing a second sublayer over the first sublayer, the second sublayer comprising a second oxide material different from the first oxide material.

14. The method of claim 9 , further comprising performing a planarization process on the first dielectric layer.

15. The method of claim 9 , further comprising, before forming the metal layer over the first dielectric layer, forming a barrier layer over the first dielectric layer.

16. A method comprising:

forming an interconnect structure on a first side of a substrate, the substrate having a pixel array region and a black level correction (BLC) region adjacent the pixel array region;

forming a mask over a second side of the substrate, the second side being opposite to the first side, the mask protecting the second side of the substrate in the BLC region;

etching the second side of the substrate in the pixel array region to form recesses in the substrate;

blanket depositing an anti-reflective layer over the recesses in the pixel array region and over the second side of the substrate in the BLC region;

depositing a metal layer over the second side of the substrate in the pixel array region and the BLC region, the anti-reflective layer being interposed between the substrate and the metal layer; and

etching the metal layer to form a metal grid over the pixel array region and a metal shield over the BLC region.

17. The method of claim 16 , further comprising forming a dielectric layer over the anti-reflective layer, the dielectric layer overfilling the recesses, a portion of the metal shield extending through the dielectric layer.

18. The method of claim 16 , wherein a portion of the metal shield extends through the anti-reflective layer.

19. The method of claim 16 , wherein the anti-reflective layer comprises an oxide material.

20. The method of claim 16 , wherein portions of the substrate interposed between neighboring recesses are arranged in a rectangular array in a plan view.