Dual-electrode capacitive micromachined ultrasonic transducers for medical ultrasound applications
Guldiken, Rasim Oytun
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Capacitive Micromachined Ultrasonic Transducers (CMUTs) have been introduced as a viable alternative to piezoelectric transducers in medical ultrasound imaging in the last decade. CMUTs are especially suitable for applications requiring small size such as catheter based cardiovascular applications. Despite these advantages and their broad bandwidth, earlier studies indicated that the overall sensitivity of CMUTs need to be improved to match piezoelectric transducers. This dissertation addresses this issue by introducing the dual-electrode CMUT concept. Dual electrode configuration takes advantage of leveraged bending in electrostatic actuators to increase both the pressure output and receive sensitivity of the CMUTs. Static and dynamic finite element based models are developed to model the behavior of dual-electrode CMUTs. The devices are then successfully fabricated and characterized. Experiments illustrate that the pulse echo performance is increased by more than 15dB with dual-electrode CMUTs as compared to single electrode conventional CMUT. Further device optimization is explored via membrane shape adjustment by adding a center mass to the design. Electromechanical coupling coefficient (kc2) is investigated as a figure of merit to evaluate performance improvement with non-uniform/uniform membrane dual-electrode CMUTs. When the center mass is added to the design, the optimized non-uniform membrane increases the electromechanical coupling coefficient from 0.24 to 0.85 while increasing one-way 3dB fractional bandwidth from 80% to 140% and reducing the DC bias requirement from 160V to 132V. The results of this modeling study are successfully verified by experiments. With this membrane shape adjustment, significant performance improvement (nearly 20dB) is achieved with the dual-electrode CMUT structure that enables the CMUT performance to exceed that of piezoelectric transducers for many applications.