Phase grating with three-dimensional configuration

1. A phase grating, comprising:

a substrate;

a first dielectric layer having a bulge with a tapered side on said substrate; and

a second dielectric layer disposed on said first dielectric layer and comprising a column and a plurality of rings, wherein the top sides of said column and of said rings align with one another to form a flat plane, said column and said rings are concentric and said rings are disposed on said tapered side of said bulge so that the height of each said ring is different.

2. The phase grating of claim 1 , wherein said substrate comprises an image sensor to correspond to said phase grating.

3. The phase grating of claim 1 , wherein said first dielectric layer and said second dielectric layer has different etching selectivity.

4. The phase grating of claim 1 , further comprising:

an etching-stop layer disposed on said first dielectric layer.

5. The phase grating of claim 4 , wherein said etching-stop layer is selected from the group consisting of nitride, oxide and oxynitride.

6. A phase grating, comprising:

a substrate;

a first dielectric layer disposed on said substrate and comprising a column and a plurality of rings, wherein the top sides of said column and of said rings align with one another to form a flat plane, said column and said rings are concentric; and

a plurality of annular trenches respectively sandwiched between said column and said rings so that the depths of any two annular trenches which are adjacent are different.

7. The phase grating of claim 6 , wherein said substrate comprises an image sensor to correspond to said phase grating.

8. The phase grating of claim 6 , wherein said annular trenches respectively have different width.

9. The phase grating of claim 8 , wherein the closer to the concentric center the smaller said width is.

10. The phase grating of claim 6 , wherein the closer to the concentric center the greater said depth is.