Characterization Of Cross-Coupling In Capacitive Micromachined Ultrasonic Transducers

This paper analyzes element-to-element and cell-to-cell cross-coupling in capacitive micromachined ultrasonic transducers (cMUTs) using an interferometer. In a 1-D linear cMUT array immersed in oil, a single element was excited, and membrane displacements were measured at different positions along the array with an interferometer. Electrical measurements of the received voltage on each array element were also performed simultaneously to verify the optical measurements. The array was then covered with a polydimethylsiloxane (PDMS) layer, and the cross-coupling measurements were repeated. The cross-coupling levels for conventional and collapsed operation of the cMUT were compared. Since the cMUTs were immersed in oil, the optical measurements were corrected for acousto-optic interaction, and the results were reviewed in time-spatial and frequency-spatial domains. The main cross-coupling mechanism was due to the dispersive guided modes supported by the membrane periodicity. In both modes of operation, cross-coupling dispersion curves predicted a gradual reduction in phase velocity at higher frequencies. At lower frequencies, this phase velocity tended to approach 1480 m/s asymptotically. Better cross-coupling suppression was observed in the collapsed (-34 dB) than the conventional operation (-23 dB). The element-to-element cross-coupling experiments showed that a 5-mu m PDMS layer reduced the measured cross-coupling levels down to -39 dB in the collapsed operation.