Development of a Novel CMUT-Based Concentric Dual-Element Ultrasonic Transducer: Design, Fabrication, and Characterization

Capacitive micromachined ultrasonic transducers (CMUTs) have shown great potential to make a strong impact on ultrasound imaging technologies over the years. However, a critical weakness of CMUTs is their relatively low output pressure at transmission. To address this, we present a novel dual-element ultrasonic transducer that incorporates two transducer technologies by using a circular commercial piezoelectric element for ultrasound transmission and an annular CMUT element for reception. The hybrid transducer combines the broad bandwidth and high receive sensitivity of the CMUT and the high output pressure of the piezoelectric transducer to improve the overall sensitivity and axial resolution. The annular CMUT is designed, fabricated, and concentrically aligned with the piezoelectric probe via a custom housing. A through-wafer etching scheme based on deep reactive-ion etching is developed to create the CMUT annulus. Immersion characterization reveals that the CMUT has a center frequency of 2.63 MHz, a −6-dB fractional bandwidth of 131%, and a receive sensitivity of 22.57 mV/kPa, compared with 2.34 MHz, 42%, and 4.28 mV/kPa of the piezoelectric probe. The round-trip measurements show that the hybrid dual-element transducer improves the axial resolution by 25.58% and the signal-to-noise ratio by 8.55 dB over the commercial piezoelectric probe. [2017-0305]