A semiconductor structure includes a semiconductor substrate and an interconnect structure on the semiconductor structure. The interconnect structure includes a first layer, a second layer over the first layer, a third layer over the second layer, and a fourth layer over the third layer. A first through via extends through the semiconductor substrate, the first layer, and the second layer. A second through via extends through the third layer and the fourth layer. A bottom surface of the second through via contacts a top surface of the first through via.