A semiconductor device includes a substrate, having a silicon layer on top. A device structure is disposed on the substrate. A dielectric layer is disposed on the substrate and covering over the device structure. The dielectric layer has a first air gap above the device structure. The first air gap is enclosed by a dielectric wall constituting as a part of the dielectric layer and the dielectric wall is disposed on the device structure. The dielectric layer has a second air gap, exposing a top of the device structure and adjacent to the dielectric wall.