A structure of semiconductor device includes a substrate, having a dielectric layer on top. The structure further includes at least two metal elements being adjacent, disposed in the dielectric layer, wherein an air gap is existing between the two metal elements. The air gap has a cross-section of substantially bottle shape with a flat top. A porous dielectric layer is disposed over the substrate, sealing the flat top of the air gap. An inter-layer dielectric layer disposed on the porous dielectric layer.