Capacitive micromachined ultrasonic transducers (cmuts) with piston-shaped membranes

Abstract— Compared to PZT transducers in medical applications, CMUTs reported on so far have broader fractional bandwidth (FBW) but lower transduction efficiency (TX and RX) [1]. Most fabricated CMUTs reported in the literature carried membranes of uniform thickness. Since there is a performance trade-off between transduction efficiency and FBW when designing CMUTs with uniform membrane thickness, there is limited room for performance improvement in these devices. However, wafer-bonding-based CMUT fabrication provides design flexibility by allowing fabrication of membranes with different thickness profiles. Herein, CMUTs featuring piston-shaped membranes are developed to improve device performance. According to our theoretical predictions, piston-shaped membranes should improve the CMUT performance in terms of output pressure, sensitivity, and broader fractional bandwidth. The large ratio of second resonant harmonic frequency to first resonant frequency improves FBW. Increased electrical field intensity in the CMUT cavity (due to the larger equivalent spring constant of this membrane, compared to classical membranes) improves TX and RX. The device performance also benefits from a flatter membrane shape that allows greater average membrane displacement and intra-cavity electrostatic pressure. CMUTs featuring piston-shaped membranes with different geometric shapes were designed and fabricated. In order to make a fair comparison, all designs have a similar first resonant frequency, and all devices are equal in size. Fabricating CMUTs featuring piston-shaped membranes is a more complex process than fabricating CMUTs with uniformly thick membranes. However, no yield-loss was observed when CMUTs featuring piston-shaped membranes with different geometric shapes were fabricated. The device characterization was carried out with both pitch-catch (PC) and pulse-echo (PE) immersion tests in oil. These devices achieved ~100% improvement in transduction performance (TX and RX) over CMUTs with uniform membrane thickness. For CMUTs with square and rectangular membranes, FBW increased from ~110% to ~150% and from ~140% to ~175%, respectively, over CMUTs with uniformly thick membranes. The new devices produced a maximum output pressure exceeding 1 MPa. Finally, performance optimization using the geometric shape was the same in both CMUTs featuring piston-shaped membranes and CMUTs with uniform membrane thickness.