Next-generation high performance MEMS ultrasonics

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It has long been hypothesized that capacitive micromachined ultrasound transducers (CMUTs) could potentially outperform piezoelectric technologies. However, challenges with dielectric charging, operational hysteresis, and transmit sensitivity have stood as obstacles to these performance outcomes. Typically, a CMUT element is designed with an ensemble of smaller membranes oscillating together to transmit or detect ultrasound waves. However, this approach can lead to unreliable behavior and suboptimal transmit performance if these smaller membranes oscillate out of phase, or collapse at different voltages. After nearly 15 years of work, we recently designed reliable CMUT elements composed of a single long rectangular membrane, that can outperform piezoelectrics. We explored various architectural modifications to the CMUT cavity in order to improve robustness to charging and minimize hysteresis without compromising performance. In order to fabricate CMUTs with these architectural modifications, we developed a double-SOI wafer bonded process with near-100% bonding yield, without the need for aligned bonding. The fabricated single-membrane CMUTs achieved electromechanical efficiency values as high as 0.95, higher than values reported with either piezoelectric transducers or CMUT elements. Moreover, these single-membrane CMUTs exhibited transmit efficiency 2-5 times greater than published CMUT or piezoelectric transducer elements in the 1.5-2.0 MHz range. Our devices demonstrated considerable charging robustness, demonstrating minimal charging over millions of collapse-snapback actuation cycles, while also mitigating hysteresis. In this proposal, we aim to further improve these devices for next-generation high-performance MEMS ultrasonics. Improvements will include high-K dielectrics enabling higher combined transmit and biasing voltages. We also propose an inverted CMUT architecture for piston-like motion, development of high-density through-silicon via technology, and development of prototype linear and 2D arrays. We aim to demonstrate the advantages of our technology using head-on comparisons against state-of-the art commercial probes.