A method of forming a semiconductor device is disclosed. Provided is a substrate having at least one MOS device, at least one metal interconnection and at least one MOS device formed on a first surface thereof. A first anisotropic etching process is performed to remove a portion of the substrate from a second surface of the substrate and thereby form a plurality of vias in the substrate, wherein the second surface is opposite to the first surface. A second anisotropic etching process is performed to remove another portion of the substrate from the second surface of the substrate and thereby form a cavity in the substrate, wherein the remaining vias are located below the cavity. An isotropic etching process is performed to the cavity and the remaining vias.