A method comprises forming a first conductive line and a second conductive line in a first dielectric layer over a substrate, each having a planar top surface, applying an etch-back process to the first dielectric layer until a dielectric portion between the first conductive line and the second conductive line has been removed, and the first conductive line and the second conductive line have respective cross sectional shapes including a rounded surface and two rounded corners and depositing a second dielectric layer over the substrate, while leaving a first air gap between the first conductive line and the second conductive line.