This step follows the primary Cu CMP step, which clears the bulk Cu overburden and selectively stops on the Ta-based liner P1.The purpose of the Ta-based liner CMP is to completely remove the exposed Ta/TaN barrier film from the inter-level dielectric (ILD) field regions, thereby electrically isolating the adjacent Cu interconnect lines in the MET5 layer A1.Because Cu readily diffuses into surrounding dielectrics and causes device failure, the Ta-based liner must remain intact within the trenche