A semiconductor structure includes a conductive line, a pad layer, and a barrier layer. The conductive line is embedded in a multi-level interconnect structure. The pad layer is over the conductive line. The barrier layer is between the conductive line and the pad layer. The pad layer is electrically connected to the conductive line through the barrier layer, and the barrier layer includes a first poly-crystalline layer and a second poly-crystalline layer. A boundary is between the first poly-crystalline layer and the second poly-crystalline layer.