Low drift in polysilicon-oxide micromachined ultrasonic transducers

The trapping of charges in dielectric materials in micromachined membranes has impeded the development of commercially-viable Capacitive Micromachined Ultrasonic Transducers (CMUTs) fabricated through sacrificial release techniques. Unlike previous work focused on metal/silicon nitride material sets, this paper examines the possibility of engineering polysilicon and silicon dioxide to create stable and reliable emitters and receivers of ultrasonic waves for medical diagnostic and therapeutic tools, even after several years of operation. Less than 0.25% drift of zero-bias capacitance is observed after a one-year accelerated life test. This stability is more than 10 times better than past reports in the literature for conventional CMUTs.