Pad structure for front side illuminated image sensor

The present disclosure relates to a semiconductor structure. The semiconductor structure includes a dielectric layer having a first dielectric surface and a second dielectric surface opposite to the first dielectric surface. The dielectric layer defines a recess in the first dielectric surface, and the recess includes a sidewall of the dielectric layer. A first conductive layer contacts a bottom surface of the dielectric layer. The sidewall of the dielectric layer is directly over the first conductive layer. A second conductive layer contacts the first conductive layer and the dielectric layer. The second conductive layer vertically extends from the first conductive layer to above the dielectric layer. A third conductive layer contacts the second conductive layer. The third conductive layer is laterally separated from a sidewall of the second conductive layer that faces the third conductive layer by a non-zero distance.