An image sensor device includes a semiconductor substrate having a front side and a backside. A first dielectric layer is on the front side of the semiconductor substrate. A metal pad is in the first dielectric layer. A second dielectric layer is over the first dielectric layer and on the front side of the semiconductor substrate. An opening penetrates through the semiconductor substrate from the backside of the semiconductor substrate, wherein the opening includes a first portion extending to expose a portion of the metal pad and a second portion extending to expose a portion of the second dielectric layer. A metal layer is formed in the first portion and the second portion of the opening.