Fabrication and Testing of a 64-Element CMUT Ring Array for Underwater Ultrasound Imaging

Capacitive micromachined ultrasonic transducers (CMUTs) have demonstrated great potential in ultrasonic imaging due to their wide bandwidth, high electromechanical coupling coefficient, flexible design, and ease of integration. However, traditional CMUT devices require high drive voltages, which may pose potential safety risks to humans and limit their widespread application in imaging fields. To address this issue, this study innovatively designs and fabricates a large-diameter (20 cm) CMUT annular array with low drive voltage (25-V dc and 15-V ac). Finite element simulation results show that the collapse voltage of the CMUT is 52 V. Devices fabricated using wafer bonding technology exhibit excellent linear I–V characteristics and a “U” shaped C–V curve. In air, the resonant frequency is 4.71 MHz; after polydimethylsiloxane (PDMS) electrical insulation encapsulation, the resonant frequency drops to 2.74 MHz when submerged in water. The fabricated CMUT elements exhibit a −6-dB bandwidth of 118%, a −6-dB beamwidth of 13°, and a receive sensitivity of −205 dB @ 2.5 MHz. The maximum normalized consistency error among the 64 array elements is 0.3. In the underwater imaging experiment, five targets with varying sizes, positions, and sound speeds embedded in a tissue-mimicking phantom were successfully reconstructed. The maximum radial error in the reconstructed target center positions was 5%. These results demonstrate that the designed low-voltage CMUT ring array possesses excellent imaging performance and significant potential for underwater ultrasound imaging applications.