A method for is used for forming a semiconductor device. The method includes forming an ILD layer on a substrate and a buffer layer on the ILD layer, wherein at least one contact is formed in the ILD layer; forming an opening through the buffer layer, the ILD layer, and the substrate; forming a liner structure layer over the substrate, wherein an exposed surface of the opening is covered by the liner structure layer; depositing a conductive material over the substrate to fill the opening; performing a polishing process, to polish over the substrate and stop at the buffer layer, wherein the liner structure layer and the conductive material remaining in the opening form a conductive via; performing an etching back process, to remove the buffer layer and expose the ILD layer, wherein a top portion of the conductive via is also exposed and higher than the ILD layer.