A method of fabricating a semiconductor structure includes forming a dielectric layer overlaying a substrate; forming a trench in the dielectric layer; forming a first barrier layer lining the trench; forming a conductive layer overlaying the first barrier layer; forming a second barrier layer overlaying the conductive layer; forming a metallic sacrificial layer to cover the second barrier layer and to fill the trench; and performing a polishing process to remove the materials above a bottom portion of the second barrier layer.