A ventilated platen/polishing pad assembly for chemical mechanical polishing copper conductors on a semiconductor wafer is disclosed. The ventilated platen is constructed by a platen having a multiplicity of apertures through a thickness of the platen, and a polishing pad that has a multiplicity of apertures for fluid communication with the multiplicity of apertures in the platen such that a gas can flow through the ventilated platen and the ventilated polishing pad to mix with a polishing slurry solution dispensed on top of the polishing pad. When an oxidizing gas is mixed with the slurry solution, the mass transfer process during the chemical mechanical polishing can be improved and thus improving the polishing uniformity of the copper surface. The invention further discloses a method for chemical mechanical polishing copper conductors on a semiconductor wafer by dispensing a polishing slurry/oxidizing gas mixture onto a top surface of a polishing pad for engaging a wafer surface and thus improving the polishing uniformity and preventing corrosion or erosion of the fresh copper surface by the acidic or basic components contained in the slurry solution.