A semiconductor structure includes a first low-k dielectric layer disposed over a semiconductor substrate, a first conductive feature and a second conductive feature disposed in the first low-k dielectric layer, a second low-k dielectric layer disposed in the first low-k dielectric layer and interposed between the first conductive feature and the second conductive feature, where the second low-k dielectric layer includes an air gap, and an etch-stop layer disposed at an interface between the first low-k dielectric layer and the second low-k dielectric layer. The first low-k dielectric layer includes carbon whose concentration is graded in a direction away from the etch-stop layer.