An interposer includes an interconnection structure and a redistribution layer. The interconnection structure includes a metal layer, at least one metal via and an isolation material. The metal layer defines at least one through hole having a side wall. The at least one metal via is disposed in the through hole. A space is defined between the at least one metal via and the side wall of the through hole, and the isolation material fills the space. The redistribution layer is disposed on a surface of the interconnection structure and is electrically connected to the metal via.