A semiconductor structure includes a substrate, a dam element, a first isolation layer, a second isolation layer, and a conductive layer. The substrate has a conductive pad, a trench, a sidewall, a first surface, and a second surface opposite to the first surface. The conductive pad is located on the second surface. The trench has a first opening at the first surface, and has a second opening at the second surface. The dam element is located on the second surface and covers the second opening. The dam element has a concave portion that is at the second opening. The first isolation layer is located on a portion of the sidewall. The second isolation layer is located on the first surface and the sidewall that is not covered by the first isolation layer, such that an interface is formed between the first and second isolation layers.