A semiconductor structure includes a semiconductive substrate including a first side and a second side opposite to the first side, a radiation sensing device disposed in the semiconductive substrate, and an ILD disposed over the first side of the semiconductive substrate, and a conductive pad disposed within the semiconductive substrate and the ILD, and electrically connected to an interconnect structure. A top surface of the conductive pad is between the first side of the semiconductive substrate and the second side of the semiconductor substrate.