a dual detection method for end point in a chemical mechanical polishing process is described . the dual detection method utilizes both an optical detection device and an acoustical detection device . the acoustical detection device may also be used independently in certain applications without the optical detection device . the acoustical detection device determines an end point and stops the cmp process when a volume of the acoustical emission changes by at least 30 % from its initial volume , or preferably changes by at least 50 % from its initial volume .