An image sensor includes a metal interconnection and readout circuitry over a first substrate, an image sensing device, and an ion implantation isolation layer. The image sensing device is over the metal interconnection, and an ion implantation isolation layer is in the image sensing device. The image sensing device includes first, second and third color image sensing units, and ion implantation contact layers. The first, second and third color image sensing units are stacked in or on a second substrate. The ion implantation contact layers are electrically connected to the first, second and third color image sensing units, respectively.