Characterization of capacitive micromachined ultrasonic transducers in air using optical measurements

Capacitive micromachined ultrasonic transducers (CMUTs) are efficient transmitters and receivers for air-coupled nondestructive evaluation applications. In this paper, we present optical measurements on CMUTs with circular and rectangular membranes. Use of laser interferometer permits accurate measurement of individual membrane displacements as well as characterization of mode shapes on the membrane. When optical displacement measurements are combined with electrical impedance measurements, all elements of the equivalent circuit model can be evaluated. In particular, an estimate of the loss in the transducer is possible. Loss mechanisms include structural losses and squeeze-film effects of air behind the membrane, the latter of which has both frequency-shifting and dissipative effects. Recently fabricated circular and rectangular membrane designs typically exhibit a real loss on the order of 500 rayls/spl plusmn/200 rayls. The degree of resonant frequency shift for unsealed membranes in air depends strongly on the particular geometry, but can be as much as 30% of the transducer's resonant frequency in vacuum. Measurements of CMUT loss due to air are compared to squeeze-film theory and are included in an equivalent circuit model. This model is compared against the measurement results.