A method of forming an ultrasonic transducer device includes forming a patterned metal electrode layer over a substrate, the patterned metal electrode layer comprising a lower layer and an upper layer formed on the lower layer； forming an insulation layer over the patterned metal electrode layer； and planarizing the insulation layer to the upper layer of the patterned metal electrode layer, wherein the upper layer comprises a electrically conductive material that serves as a chemical mechanical polishing (CMP) stop layer that has CMP selectivity with respect to the insulation layer and the lower layer, and wherein the upper layer has a CMP removal rate slower than that of the insulation layer.
