1. A backside illumination image sensor, comprising: a semiconductor substrate, the semiconductor substrate having an upper surface and a lower surface; photodiodes in the semiconductor substrate; isolation regions adjacent to the lower surface of the semiconductor substrate and between the photodiodes, and metal interconnections below the semiconductor substrate, wherein: each of the photodiodes includes: an N-type region, a lower P-type region below the N-type region, and an upper P-type region on the N-type region, the upper P-type region includes at least a first P-type high-concentration region in direct physical contact with a second P-type high-concentration region, the upper P-type region has a stepwise concentration distribution of impurities between the first and second P-type high-concentration regions, the second P-type high-concentration region is directly on the N-type region, the first P-type high-concentration region and the second P-type high-concentration region are continuously disposed on the N-type region and between the photodiodes, and both the first P-type high-concentration region and the second P-type high-concentration region extend horizontally continuously to be parallel with the upper surface of the semiconductor substrate, respectively, and wherein the second P-type high-concentration region is not in contact with the isolation regions.