A semiconductor device is provided. The semiconductor device includes a device substrate, having a device structure layer and a buried dielectric layer, wherein the buried dielectric layer is disposed on a semiconductor layer of the device structure layer and the device substrate comprises a device structure. A metal layer is disposed on the buried dielectric layer and surrounded by a first inter-layer dielectric (ILD) layer. A region of the metal layer has a plurality of openings. The buried dielectric layer has an air gap under and exposing the region of the metal layer with the openings, wherein the air gap is located above the device structure in the device substrate. A second ILD layer is disposed on the metal layer and sealing the air gap at the openings of the metal layer.