Effect of Middle Membrane Dimensions on Dynamic Control Range in Dual Membrane Capacitive Micromachined Ultrasonic Transducers

Two sets of multiple moving membrane capacitive micromachined ultrasonic transducers (M-3-CMUTs) are designed and fabricated for a range of middle membrane radii, from 35 - 49 mu m and 34 - 44 mu m. All other design parameters were kept identical within each set of experiments to enable a fair comparison. To study the dynamic control range of the designed M-3-CMUTs, their performance was first evaluated through COMSOL simulation yielding results on the resonant frequency and corresponding cavity height. This was followed by fabrication of the same designs using multiuser microelectromechanical systems processes (MUMPs). The development of these prototypes enabled the comprehensive electrical characterization of the designs with responses that are demonstrated to be in full agreement with the simulation results. The middle membrane radii of the M-3-CMUTs impact the collapse voltage of these devices in addition to affecting the measured resonant frequencies when tested devices within the same measurement conditions. A 20 kHz increase in resonant frequency shift was recorded when the middle membrane radius of these devices was increased from 7 mu m to 10 mu m while operating under a 20V top membrane bias and -3V to -5V middle membrane bias. These observations indicate that the middle membrane radius has influence on the dynamic control of effective cavity height in M3-CMUTs, which enables the expansion of the devices' cavity heights beyond that of conventional capacitive micromachined transducers. This finding has potential in applications with a need for acoustic power where dynamic control of the cavity height could serve as a solution, such as in medical imaging.