The disclosure provides mechanisms of performing metal chemical-mechanical polishing (CMP) without significant loss of copper and a dielectric film of damascene structures. The mechanisms use a metal CMP stop layer made of a low-k dielectric film with a porogen, which significantly reduces the removal rate of the metal CMP stop layer by metal CMP. The metal CMP stop layer is converted into a porous low-k dielectric film after a cure (or curing) to remove or convert the porogen. The low-k value, such as equal to or less than about 2.6, of the metal CMP stop layer makes the impact of using of the metal CMP stop layer on RC delay from minimum to none. Further the CMP stop layer protects the porous low-k dielectric film underneath from exposure to water, organic compounds, and mobile ions in the CMP slurry.