Microfabrication and Experimental Evaluation of a Rotational Capacitive Micromachined Ultrasonic Transducer

The microfabrication, modeling, and experimental evaluation of an unconventional acoustic sensor are described. The sensor is comprised of two vacuum-sealed capacitively transduced pistons coupled with each other by a pivoting beam. The use of a pivoting beam can, in principle, enable high rotational compliance to in-plane small-signal acoustic pressure gradients, while resisting piston collapse against large background atmospheric pressure. A design path toward vacuum-sealed surface-micromachined broadband microphones is a motivation to explore the sensor concept. Fabrication of surface-micromachined prototypes is presented, followed by finite element modeling and experimental confirmation of successful vacuum sealing. Dynamic frequency response measurements are obtained using broadband electrostatic actuation and confirm a first fundamental rocking mode near 250 kHz. Successful reception of airborne ultrasound in air at 130 kHz is also demonstrated, and followed by a discussion of design paths toward improved signal-to-noise ratio beyond that of the initial prototypes presented.