Barrier CMP can reduce the topography generated during Cu CMP. In case of selective barrier CMP slurry like 1:10:1, it is expected that the topography reduction could not exceed the Ta thickness. Some recent observations made at Fujimi show that the topography reduction can be twice larger than the Ta thickness without dielectric loss. This paper presents the chemical mechanical phenomenon that is responsible for this 40 to 50 nm topography reduction during barrier CMP. At the end of the Cu CMP, Ta is exposed to oxidizer. This process can oxidize an important part of the Ta barrier which becomes Ta2O5 according to Pourbaix diagram [1]. Ta2O5 can be as much as 2.3X thicker than original Ta layer. This phenomenon explains why topography reduction can be twice higher than initial Ta thickness. This mechanism explains why after Cu CMP it is likely to have more than 30 nm dishing on copper lines. The last and more important consequence is that this reduces final topography and total copper metal loss at the same time by about 30 nm. Obviously very high selectivity Cu CMP slurry (Cu:Ta ∼ 1000:1) is necessary to have neither Ta erosion nor Ta2O5 erosion during copper CMP. High selectivity slurry is required during barrier CMP in order to reduce the loss of copper and Dielectric during Barrier CMP.