CMOS image sensor and method for fabricating the same

1. A CMOS image sensor, comprising:

a substrate including silicon;

a silicon germanium (SiGe) epitaxial layer formed over the substrate, the SiGe epitaxial layer formed through epitaxial growth and doped with a predetermined concentration level of impurities, wherein the SiGe epitaxial layer includes layers which have different germanium (Ge) concentration gradients;

an undoped silicon epitaxial layer formed over the SiGe epitaxial layer by epitaxial growth; and

a photodiode region formed from a top surface of the undoped silicon epitaxial layer to a predetermined depth in the SiGe epitaxial layer.

2. The CMOS image sensor of claim 1 , wherein the SiGe epitaxial layer comprises:

a first SiGe epitaxial layer having a doping profile where a Ge concentration gradually increases from a bottom toward a top in the first SiGe epitaxial layer;

a second SiGe epitaxial layer highly doped with P-type impurities and formed over the first SiGe epitaxial layer; and

a third SiGe epitaxial layer lowly doped with P-type impurities and formed over the second SiGe epitaxial layer, wherein the second and the third SiGe epitaxial layers have a given Ge concentration level.

3. The CMOS image sensor of claim 2 , wherein the Ge concentration level gradually increases in the first SiGe epitaxial layer from approximately 0% to approximately 20%, and the given Ge concentration level in the second and third SiGe epitaxial layers is maintained uniformly at approximately 20%.

4. The CMOS image sensor of claim 2 , wherein the P-type impurities doped in the second and third SiGe epitaxial layers include boron.

5. The CMOS image sensor of claim 4 , wherein a concentration level of boron doped in the second SiGe epitaxial layer ranges from approximately 1×10 15 cm −3 to approximately 1×10 18 cm −3 .

6. The CMOS image sensor of claim 4 , wherein a concentration level of boron doped in the third SiGe epitaxial layer ranges from approximately 1×10 14 cm −3 to approximately 1×10 7 cm −3 .

7. The CMOS image sensor of claim 2 , wherein each of the first and second SiGe epitaxial layers has a thickness ranging from approximately 1 μm to approximately 5 μm, and the third SiGe epitaxial layer has a thickness ranging from approximately 3 μm to approximately 8 μm.

8. The CMOS image sensor of claim 1 , wherein the substrate is doped with P-type impurities.

9. The CMOS image sensor of claim 8 , wherein the substrate is doped with boron.

10. The CMOS image sensor of claim 9 , wherein a concentration level of boron doped in the substrate ranges from approximately 1×10 14 cm −3 to approximately 1×10 17 cm −3 .