Design and experiment of capacitive micromachined ultrasonic transducer array for high-frequency underwater imaging

Background: Ultrasound is widely used in the applications of underwater imaging. Capacitive micromachined ultrasonic transducer (CMUT) is a promising candidate for the traditional piezoelectric ultrasonic transducer. In underwater ultrasound imaging, better resolutions can be achieved with a higher frequency ultrasound. Therefore, a CMUT array for high-frequency ultrasound imaging is proposed in this work. Methods: Analytical methods were used to calculate the center frequency in water and the pull-in voltage for determining the operating point of CMUT. A finite element method model was developed to finalize the design parameters. The CMUT array was fabricated with a five-mask sacrificial release process. Results: The CMUT array owned an immersed center frequency of 2.6 MHz with a 6 dB fractional bandwidth of 123 %. The pull-in voltage of the CMUT array was 85 V. An underwater imaging experiment was carried out with the target of three steel wires. Conclusion: In this study, we have developed CMUT for high-frequency underwater imaging. The experiment showed that the CMUT could detect the steel wires with a diameter of 100 µm and the axial resolution was 0.582 mm, which was close to one wavelength of ultrasound in 2.6 MHz.