A Method for Expanding the Bandwidth and Decreasing the Actuation Voltage of CMUT Devices

[Figures/graphical abstract.pdf] This paper presents a novel capacitive micromachined ultrasonic transducer (CMUT) fabricated with the MEMSCAP PolyMUMPs process and incorporating two new design elements: spring arms and rocker stems. The structure enables low-Q-factor, broadband operation in a commercial surface-micromachining technology, making it suitable for air-coupled applications. The device operates at a bias voltage as low as 26 V, which is advantageous for portable systems. Electrical characterization demonstrates a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$6.5\times$</tex-math></inline-formula> reduction in bias voltage compared with traditional designs. Four CMUTs were fabricated to isolate and assess the influence of each design element. The frequency spectra showed that the device with the novel design features had the broadest bandwidth (a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$Q$</tex-math></inline-formula>-factor of approximately 1.2 compared to approximately 34 for the traditional design). These results highlight the potential of the proposed CMUT architecture for high axial resolution pulse-echo systems.