Method for fabricating image sensor

1. A method, comprising:

preparing a first conductivity type substrate of an image sensor;

forming a second conductivity type impurity region in a predetermined portion of the first conductivity type substrate;

forming a mask pattern exposing the predetermined portion of the first conductivity type substrate;

etching the predetermined portion of the first conductivity type substrate using the mask pattern to form a trench;

forming a second conductivity type first epitaxial layer to fill the trench; and

forming a first conductivity type second epitaxial layer over the second conductivity type first epitaxial layer.

2. The method of claim 1 , further comprising forming a gate structure over the first conductivity type substrate next to the predetermined portion of the first conductivity type substrate.

3. The method of claim 1 , wherein the first conductivity type second epitaxial layer provides a pinning layer of a photodiode.

4. The method of claim 2 , wherein said forming a second conductivity type first epitaxial layer comprises growing a portion of the second conductivity type first epitaxial layer to overlap a bottom portion of the gate structure.

5. The method of claim 1 , wherein said forming a first conductivity type second epitaxial layer causes the first conductivity type second epitaxial layer to contact the first conductivity type substrate.

6. The method of claim 1 , wherein said forming a second conductivity type first epitaxial layer comprises forming the second conductivity type first epitaxial layer from a material that includes Si or SiGe.

7. The method of claim 1 , wherein said forming a first conductivity type second epitaxial layer comprises forming the first conductivity type second epitaxial layer from a material that includes Si or SiGe.

8. The method of claim 1 , wherein said etching forms the trench to a depth ranging from approximately 1,800 Å to approximately 2,200 Å.

9. The method of claim 1 , wherein said forming a second conductivity type first epitaxial layer comprises:

growing the second conductivity type first epitaxial layer; and

doping the second conductivity type first epitaxial layer with second conductivity type impurities in-situ during said growing.

10. The method of claim 1 , wherein said forming a first conductivity type second epitaxial layer comprises:

growing the first conductivity type second epitaxial layer; and

doping the first conductivity type second epitaxial layer with first conductivity type impurities in-situ during said growing.

11. The method of claim 1 , wherein said forming a mask pattern comprises using chemical vapor deposition (CVD) to form an oxide-based layer of the mask pattern.

12. The method of claim 1 , wherein said forming a mask pattern comprises performing a hydrogen chloride (HCL) etching process to expose the predetermined portion of the first conductivity type substrate.

13. The method of claim 12 , wherein the HCL etching process is performed at a temperature ranging from approximately 500° C. to approximately 5,000° C. with a pressure ranging from approximately 0.1 Torr to approximately 760 Torr.

14. The method of claim 1 , wherein said forming a second conductivity type first epitaxial layer comprises using a Ge source including germane (GeH 4 ) at a Ge concentration level of approximately 5% to approximately 100%.

15. The method of claim 1 , wherein said forming a second conductivity type first epitaxial layer comprises doping the second conductivity type first epitaxial layer using phosphine (PH 3 ) gas.

16. The method of claim 1 , wherein said forming a first conductivity type second epitaxial layer comprises doping the first conductivity type second epitaxial layer using diborane (B 2 H 6 ) gas at a concentration level ranging from approximately 1×10 17 cm −3 to approximately 1×10 22 cm −3 .

17. The method of claim 1 , wherein said forming a first conductivity type substrate forms a first conductivity type substrate comprising Si.

18. A method, comprising:

forming an impurity region of a photodiode in a substrate for an image sensor;

forming a first epitaxial layer of the photodiode on the impurity region; and

forming a pinning layer of the photodiode as a second epitaxial layer on the first epitaxial layer.

19. The method of claim 18 , further comprising:

etching the substrate to form a trench;

wherein said forming a first epitaxial layer comprises filling the trench with the first epitaxial layer.

20. The method of claim 19 , wherein said etching forms the trench to a depth ranging from approximately 1,800 Å to approximately 2,200 Å.

21. The method of claim 19 , further comprising:

forming a gate structure over the substrate and next to the trench;

wherein said forming a first epitaxial layer comprises growing a portion of the first epitaxial layer to overlap a bottom portion of the gate structure.

22. The method of claim 19 , wherein said forming a trench comprises:

forming a mask pattern using chemical vapor deposition (CVD) to form an oxide-based layer of the mask pattern;

performing a hydrogen chloride (HCL) etching process to expose a predetermined portion of the substrate in which to form the trench; and

etching the substrate using the mask pattern to form the trench.

23. The method of claim 18 , wherein said forming a second epitaxial layer forms the second epitaxial layer to contact the substrate.

24. The method of claim 18 , wherein said forming a first epitaxial layer comprises:

growing the first epitaxial layer; and

doping the first epitaxial layer with second conductivity type impurities in-situ during said growing.

25. The method of claim 18 , wherein said forming a second epitaxial layer comprises:

growing the second epitaxial layer; and

doping the second epitaxial layer with first conductivity type impurities in-situ during said growing.