providing a substrate having a front side and a back side opposite to each other;

forming a plurality of photodetectors in the substrate within a pixel region;

patterning the substrate from the back side of the substrate to form a first opening within the pixel region and a second opening within a periphery region;

forming a dielectric liner on sidewalls and bottom surfaces of the first and second openings;

removing a portion of the dielectric liner covering the bottom surfaces of the first and second openings;

forming a conductive material layer after the removing, wherein the conductive material layer is formed on the substrate and filling into the first and second openings, wherein the conductive material layer comprises a first conductive plug in the first opening, a second conductive plug in the second opening, and an upper portion over the back side of the substrate, and wherein the first conductive plug serves as a first portion of an isolation structure disposed between the plurality of photodetectors;

patterning the upper portion of the conductive material layer to form a conductive cap, wherein the conductive cap extends from the pixel region to the periphery region and is electrically connected to the first and second conductive plugs; and

forming a conductive contact on the second conductive plug over the front side of the substrate within the periphery region.