Image sensor with absorption enhancement structure

1. A method for forming an image sensor, the method comprising:

forming a hard mask over a substrate;

patterning the hard mask to define a patterned hard mask including a first patterned hard mask region and a second patterned hard mask region laterally surrounding the first patterned hard mask region;

performing one or more etches into the substrate with the patterned hard mask in place to form a substrate depression along a first side of the substrate, wherein the substrate depression establishes an absorption enhancement structure, wherein the substrate depression is defined by a first plurality of sidewalls that slope toward a first common point and by a second plurality of sidewalls that slope toward at a second common point, and wherein the first plurality of sidewalls extend over the second plurality of sidewalls; and

removing the first patterned hard mask region from over the substrate during the performing of the one or more etches and before removing the second patterned hard mask region from over the substrate.

2. The method of claim 1 , wherein the second patterned hard mask region is separated from the first patterned hard mask region by a cavity that surrounds the first patterned hard mask region.

3. The method of claim 1 , wherein the one or more etches undercut the first patterned hard mask region, and wherein the first patterned hard mask region is removed once the one or more etches undercut the first patterned hard mask region.

4. The method of claim 1 , wherein both the first common point and the second common point are below a top surface of the substrate, and wherein the first common point and the second common point are disposed at different depths below the top surface of the substrate.

5. The method of claim 1 , further comprising:

removing the second patterned hard mask region from over the substrate after the first patterned hard mask region is removed from over the substrate.

6. The method of claim 1 , wherein the performing of the one or more etches continues for a period of time after the first patterned hard mask region is removed from over the substrate.

7. The method of claim 3 , wherein the first patterned hard mask region is washed away during the performing of the one or more etches in response to the one or more etches undercut the first patterned hard mask region.

8. The method of claim 2 , wherein the cavity is delimited by sidewalls of the first patterned hard mask region that face the second patterned hard mask region and sidewalls of the second patterned hard mask region that face the first patterned hard mask region.

9. A method for forming an image sensor, the method comprising:

forming a photodetector in a substrate;

depositing a hard mask over the substrate;

patterning the hard mask to define a patterned hard mask; and

etching the substrate with the patterned hard mask in place to form a depression in the substrate, wherein the depression is defined by a first pair of sidewalls of the substrate and by a second pair of sidewalls of the substrate, wherein the second pair of sidewalls extend between the first pair of sidewalls, wherein the second pair of sidewalls are separated from each other by a first non-zero angle, and wherein the first pair of sidewalls extend over the second pair of sidewalls, wherein etching the substrate comprises:

performing a first etching process into the substrate to form a first recess in the substrate;

performing a second etching process into the substrate to form a second recess in the substrate below the first recess; and

performing a third etching process into the substrate to enlarge the second recess.

10. The method of claim 9 , wherein the first etching process undercuts the patterned hard mask.

11. The method of claim 9 ,

wherein the first recess is defined by the first pair of sidewalls of the substrate, and wherein the second recess is defined by the second pair of sidewalls after the third etching process.

12. The method of claim 11 , wherein etching the substrate further comprises:

depositing a dielectric layer on the first pair of sidewalls before performing the second etching process, wherein the second etching process extends through the dielectric layer and into the substrate to form the second recess.

13. The method of claim 11 , wherein the second recess is further defined by a third pair of sidewalls of the substrate after the third etching process, wherein the first pair of sidewalls and the third pair of sidewalls are separated by a pair of second non-zero angles, and wherein the second pair of sidewalls and the third pair of sidewalls are separated by a pair of third non-zero angles.

14. The method of claim 9 , wherein the first etching process comprises a first wet etching process, the second etching process comprises a dry etching process, and the third etching process comprises a second wet etching process.

15. The method of claim 12 , wherein the dielectric layer is deposited directly under the patterned hard mask.

16. The method of claim 12 , further comprising:

removing the dielectric layer and the patterned hard mask after the third etching process is performed.

17. A method for forming an image sensor, the method comprising:

forming a photodetector in a substrate;

forming a hard mask over the substrate;

patterning the hard mask to define a patterned hard mask having one or more openings over the substrate, the patterned hard mask including a first patterned hard mask region, a second patterned hard mask region laterally surrounding the first patterned hard mask region, and a plurality of patterned hard mask segments that extend between the first patterned hard mask region and the second patterned hard mask region at corners of the first patterned hard mask region; and

etching the substrate with the patterned hard mask in place to form a depression in the substrate, wherein the depression is delimited by a first pair of sidewalls and a second pair of sidewalls of the substrate, wherein the first pair of sidewalls slope toward a first common point and the second pair of sidewalls slope toward a second common point, wherein the first common point is disposed at a first depth below a top of the substrate, and wherein the second common point is disposed at a second depth below the top of the substrate, and wherein the second depth is different than the first depth.

18. The method of claim 17 , wherein the first pair of sidewalls and the second pair of sidewalls are directly over the photodetector, and wherein tops of the first pair of sidewalls are disposed above tops the second pair of sidewalls.

19. The method of claim 17 , wherein the plurality of patterned hard mask segments join the first patterned hard mask region and the second patterned hard mask region such that the patterned hard mask extends continuously from the first patterned hard mask region to the second patterned hard mask region via the plurality of patterned hard mask segments.

20. The method of claim 17 , wherein the first patterned hard mask region is separated from the second patterned hard mask region at sidewalls of the first patterned hard mask region.