US Patent 11766696 Bi-layer metal electrode for micromachined ultrasonic transducer devices

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.