A device includes a semiconductor substrate, a black reference circuit in the semiconductor substrate, a metal pad on a front side of, and underlying, the semiconductor substrate, and a first and a second conductive layer. The first conductive layer includes a first portion penetrating through the semiconductor substrate to connect to the metal pad, and a second portion forming a metal shield on a backside of the semiconductor substrate. The metal shield is aligned to the black reference circuit, and the first portion and the second portion are interconnected to form a continuous region. The second conductive layer includes a portion over and contacting the first portion of the first conductive layer, wherein the first portion of the first conductive layer and the portion of the second conductive layer form a first metal pad. A dielectric layer is overlying and contacting the second portion of the first conductive layer.