A method of making a backside illuminated image sensor includes forming a first isolation structure in a pixel region of a substrate, where a bottom of the first isolation structure is exposed at a back surface of the substrate. The method further includes forming a second isolation structure in a peripheral region of the substrate, where the second isolation structure has a depth less than a depth of the first isolation structure. Additionally, the method includes forming an implant region adjacent to at least a portion of sidewalls of the first isolation structure, where the portion of the sidewalls is located closer to the back surface than a front surface of the substrate, and where the second isolation structure is free of the implant region.