A chemical-mechanical polishing process includes the steps of providing a semiconductor substrate having a first conductive line thereon, and then forming at least one dielectric layer over the substrate and the first conductive line. Next, a chemical-mechanical polishing method is used to polish the surface of the dielectric layer. Thereafter, a cap layer is formed over the polished dielectric layer. The method of forming the cap layer includes depositing silicon oxide using a chemical vapor deposition method with silicane (SiH4) or tetra-ethyl-ortho-silicate (TEOS) as the main reactive agent. Alternatively, the cap layer can be formed by depositing silicon nitride using a chemical vapor deposition method with silicane or silicon dichlorohydride (SiH2Cl2) as the main reactive agent. Finally, a via opening is formed through the dielectric layer and the cap layer, and a second conductive line that couples electrically with the first conductive line through the via opening.