An image sensor comprises a semiconductor material having a front side and a back side opposite the front side; a dielectric layer disposed on the front side of the semiconductor material; a poly layer disposed on the dielectric layer; an interlayer dielectric material covering both the poly layer and the dielectric layer; an inter-metal layer disposed on the interlayer dielectric material, wherein a metal interconnect is disposed in the inter-metal layer; and a contact pad trench extending from the back side of the semiconductor material into the semiconductor material, wherein the contact pad trench comprises a contact pad disposed in the contact pad trench, wherein the contact pad and the metal interconnect are coupled with a plurality of contact plugs; and at least an air gap isolates the contact pad and side walls of the contact pad trench. The poly layer and the semiconductor material between adjacent two STI structures of a plurality of first and second STI structures are used as hard masks to form the plurality of contact plugs by selectively removing the dielectric materials between a first side of the plurality of first STI structures and the metal interconnect, wherein each of the plurality of contact plugs extends from the first side of each of the plurality of first STI structures through each of the plurality of first STI structures into the interlayer dielectric material and vertically abuts the metal interconnect.