Image sensors with dummy pixel structures

1. A semiconductor device, comprising:

a substrate with a first surface and a second surface opposite to the first surface;

a first pixel region with a first pixel structure disposed on the first surface of the substrate,

wherein the first pixel structure comprises a first epitaxial structure disposed within the substrate and a first capping layer disposed on the first epitaxial structure, and

wherein the first epitaxial structure has a first top surface;

a second pixel region, surrounding the first pixel region, comprising a second pixel structure adjacent to the first pixel structure and electrically isolated from the first pixel structure,

wherein the second pixel structure comprises a second epitaxial structure disposed within the substrate and a second capping layer disposed on the second epitaxial structure,

wherein the second epitaxial structure has a second top surface that is substantially coplanar with the first top surface, and

wherein the first and second epitaxial structures comprise a same semiconductor material; and

a contact pad region with a pad structure disposed adjacent to the second pixel region.

2. The semiconductor device of claim 1 , wherein a ratio between a surface area of the first top surface and a surface area of the second top surface ranges from about 2:1 to about 1:2.

3. The semiconductor device of claim 1 , wherein a ratio between adjacent sides of the first and second top surfaces ranges from about 2:1 to about 1:2.

4. The semiconductor device of claim 1 , wherein the second epitaxial structure comprises an array of epitaxial structures surrounding the first epitaxial structure.

5. The semiconductor device of claim 1 , wherein a distance between the first and second top surfaces ranges from about 200 nm to about 1000 nm.

6. The semiconductor device of claim 1 , wherein vertical dimensions of the first and second epitaxial structures are substantially equal to each other.

7. The semiconductor device of claim 1 , wherein the first and second epitaxial structures comprise germanium or silicon germanium.

8. The semiconductor device of claim 1 , wherein top surfaces of the first and second capping layers are substantially coplanar with each other.

9. The semiconductor device of claim 1 , wherein the first and second capping layers comprise a same semiconductor material.

10. The semiconductor device of claim 1 , wherein the first and second capping layers comprise an other semiconductor material that is different from the semiconductor materials of the first and second epitaxial structures.

11. An image sensor, comprising:

a substrate with a front side surface and a back side surface opposite to the front side surface;

an array of active epitaxial structures disposed on the front side surface of the substrate;

an active capping layer disposed on each of the active epitaxial structures;

a plurality of dummy epitaxial structures, surrounding the array of active epitaxial structures, disposed on the front side surface of the substrate;

a dummy capping layer disposed on each of the dummy epitaxial structures,

wherein top surfaces of the active and dummy epitaxial structures are substantially coplanar with each other, and

wherein the active and dummy epitaxial structures comprise a same semiconductor material; and

an array of microlens disposed on the back side surface of the substrate.

12. The image sensor of claim 11 , wherein an outermost row of the array of active epitaxial structures is adjacent to an array of dummy epitaxial structures of the plurality of dummy epitaxial structures.

13. The image sensor of claim 11 , wherein an active epitaxial structure in the array of active epitaxial structures has a first top surface and a dummy epitaxial structure of the plurality of dummy epitaxial structures has a second top surface, and

wherein a ratio of a surface area of the first top surface to a surface area of the second top surface ranges from about 2:1 to 1:2.

14. The image sensor of claim 11 , wherein a distance between an outermost row of the array of active epitaxial structures and an array of dummy epitaxial structures of the plurality of dummy epitaxial structures adjacent to the outermost row ranges from about 200 nm to about 1000 nm.

15. A semiconductor device, comprising:

a dielectric layer disposed on a substrate;

a dummy epitaxial structure and an active epitaxial structure disposed in the dielectric layer and the substrate;

first and second capping layers disposed on the dummy and active epitaxial structures, respectively;

doped regions disposed in the active epitaxial structure and the second capping layer;

a silicide layer disposed on the doped regions;

an etch stop layer disposed on the silicide layer; and

conductive plugs disposed on the silicide layer through the etch stop layer.

16. The semiconductor device of claim 15 , wherein the dummy and active epitaxial structures comprise a same semiconductor material.

17. The semiconductor device of claim 15 , wherein the dummy epitaxial structure comprises a top surface area that is equal to or greater than about half of a top surface area of the active epitaxial structure.

18. The semiconductor device of claim 15 , wherein the dummy epitaxial structure comprises a top surface area that is equal to or less than about two times of a top surface area of the active epitaxial structure.

19. The semiconductor device of claim 15 , wherein the dummy epitaxial structure is separated from the active epitaxial structure by a distance of about 200 nm to about 1000 nm.

20. The semiconductor device of claim 15 , wherein the dummy epitaxial structure comprises an array of epitaxial structures surrounding the active epitaxial structure.