A chemical mechanical polishing process includes placing a substrate on a first polishing pad of a first platen, wherein the substrate has a bulk metal layer and a barrier layer; polishing the bulk metal layer by using the first polishing pad having a hardness of above 50 (Shore D) until the barrier layer is exposed; polishing the barrier layer on a second polishing pad of a second platen after removing the bulk metal layer, wherein the second polishing pad has a hardness ranging between 40 and 50 (Shore D) and includes an upper layer and a lower backing layer and the upper layer has a hardness less than 50 (Shore D).