A method includes forming a first conductive line and a second conductive line in a dielectric layer, etching a portion of the dielectric layer to form a trench between the first conductive line and the second conductive line, and forming a first etch stop layer. The first etch stop layer extends into the trench. A second etch stop layer is formed over the first etch stop layer. The second etch stop layer extends into the trench, and the second etch stop layer is more conformal than the first etch stop layer. A dielectric material is filled into the trench and over the second etch stop layer. An air gap is formed in the dielectric material.