Low-refractivity grid structure and method forming same

1. A method comprising:

forming image sensors in a semiconductor substrate;

thinning the semiconductor substrate from a backside of the semiconductor substrate;

forming a dielectric layer on the backside of the semiconductor substrate;

forming a polymer grid on the backside of the semiconductor substrate, wherein the polymer grid has a first refractivity value, and wherein the forming the polymer grid comprises:

applying a photo-sensitive material;

light-exposing the photo-sensitive material using a lithography mask; and

performing a development process on the photo-sensitive material to remove some portions of the photo-sensitive material, wherein remaining portions of the photo-sensitive material form the polymer grid;

forming color filters in the polymer grid, wherein the color filters has a second refractivity value higher than the first refractivity value; and

forming micro-lenses on the color filters.

2. The method of claim 1 , wherein the lithography mask comprises opaque patterns and transparent patterns.

3. The method of claim 1 further comprising, before the polymer grid is formed, forming a metal grid on the backside of the semiconductor substrate, wherein the polymer grid is vertically aligned to the metal grid, and wherein the polymer grid is vertically spaced apart from the metal grid by a transparent layer.

4. The method of claim 1 further comprising forming backside high-absorption regions extending into the semiconductor substrate from a back surface of the semiconductor substrate, wherein grid openings of the polymer grid are aligned to the backside high-absorption regions.

5. The method of claim 1 further comprising forming a deep-trench isolation grid extending into the semiconductor substrate from the backside of the semiconductor substrate, wherein the deep-trench isolation grid is aligned to the polymer grid.

6. The method of claim 1 , wherein the second refractivity value is greater than the first refractivity value by a difference greater than about 0.5.

7. The method of claim 1 , wherein the polymer grid is formed before the color filters are formed, and the color filters are filled into grid openings of the polymer grid.

8. A method comprising:

forming a Backside Illumination (BSI) image sensor chip comprising:

forming image sensors in a semiconductor substrate;

forming a polymer grid over the semiconductor substrate, wherein the polymer grid is on a backside of the semiconductor substrate, and wherein the forming the polymer grid comprises:

forming a blanket sacrificial layer;

patterning the blanket sacrificial layer to form an opening, wherein the opening has a pattern of an additional grid; and

filling the opening to form the polymer grid; and

forming color filters, wherein the color filters are inside grid openings of the polymer grid, and wherein the color filters are in contact with the polymer grid.

9. The method of claim 8 , wherein the polymer grid has a first refractivity value, and the color filters have a second refractivity value higher than the first refractivity value.

10. The method of claim 8 further comprising performing a planarization process to level top surfaces of the color filters with top surfaces of a polymer material of the polymer grid.

11. The method of claim 8 , wherein the filling the opening to form the polymer grid comprises filling a photo-sensitive material.

12. The method of claim 8 further comprising planarizing a top surface of the polymer grid with top surfaces of the color filters, wherein after the planarizing, a polymer material of the polymer grid is exposed.

13. The method of claim 8 , wherein the forming the polymer grid comprises forming a resin grid.

14. The method of claim 8 further comprising, before the polymer grid is formed, forming a metal grid over the semiconductor substrate, wherein the polymer grid is vertically aligned to the metal grid.

15. The method of claim 8 further comprising, before the polymer grid is formed, forming a metal grid over the semiconductor substrate, wherein the polymer grid is horizontally offset from the metal grid.

16. The method of claim 8 further comprising forming a deep-trench isolation grid in the semiconductor substrate, wherein the polymer grid is vertically aligned to the deep-trench isolation grid.

17. A method comprising:

forming image sensors in a semiconductor substrate;

forming a metal grid over the semiconductor substrate;

forming a polymer grid over the metal grid, wherein first grid lines of the polymer grid and second grid lines of the metal grid are vertically aligned, and wherein grid openings of the polymer grid and the metal grid are vertically aligned to the image sensors, and wherein the forming the polymer grid comprises:

applying a photoresist;

light-exposing the photoresist using a lithography mask, wherein the lithography mask comprises opaque patterns and transparent patters; and

performing a development process on the photoresist to remove some portions of the photoresist, wherein remaining portions of the photoresist form the polymer grid;

forming color filters in the grid openings of the polymer grid, wherein a first refractivity value of the polymer grid is smaller than a second refractivity value of the color filters; and

forming micro-lenses over the color filters.

18. The method of claim 17 , wherein a difference between the second refractivity value and the first refractivity value is greater than about 0.5.

19. The method of claim 17 further comprising filling a dielectric layer into the metal grid, wherein a portion of the dielectric layer extends higher than the metal grid.

20. The method of claim 17 , wherein the polymer grid comprises a resin.