Experimental characterization of collapse-mode CMUT operation.

Area of Science:

• Microelectromechanical Systems (MEMS)
• Ultrasonic Transducer Technology
• Acoustic Engineering

Background:

• Capacitive micromachined ultrasonic transducers (CMUTs) are typically operated below membrane collapse voltage, with maximum displacement at the membrane's center.
• Conventional CMUT operation involves a gap between the membrane and substrate, limiting performance characteristics.

Purpose of the Study:

• To experimentally characterize the performance of CMUTs in a novel 'collapse-mode' operation.
• To compare the efficiency, frequency response, and pressure generation of CMUTs in conventional versus collapse modes.

Main Methods:

• Experimental characterization using impedance measurements, pulse-echo experiments, and optical displacement measurements.
• Utilized a 205-µm x 205-µm 2D CMUT array with circular silicon nitride membranes.
• Employed a custom integrated circuit for high-frequency pulse-echo measurements.

Main Results:

• Collapse-mode CMUTs demonstrate significantly higher efficiency in generating and detecting ultrasound compared to conventional modes.
• The center frequency of collapsed membranes is tunable via applied DC bias, ranging from 20 MHz to 28 MHz (vs. 10 MHz in conventional mode).
• Collapse-mode operation achieved higher peak-to-peak pressure (590 kPa) with shorter pulse excitation than conventional mode (370 kPa).

Conclusions:

• Collapse-mode operation offers a viable pathway to enhance CMUT performance, enabling higher efficiency and tunable frequencies.
• This mode is particularly effective for applications requiring high-frequency ultrasound generation and detection with improved pressure output.