A semiconductor structure includes a first dielectric layer, a first via and a second via disposed in the first dielectric layer, a second dielectric layer disposed over the first dielectric layer, the first via, and the second via, a first conductive line disposed on the first via and in a bottom portion of the second dielectric layer, a second conductive line disposed on the second via and in the bottom portion of the second dielectric layer, a first barrier layer extending along sidewalls and a top surface of the first conductive line, and a second barrier layer extending along sidewalls and a top surface of the second conductive line. The bottom portion of the second dielectric layer includes an air gap between the first conductive line and the second conductive line.