Acoustic characteristics of CMUT with rectangular membranes caused by higher order modes

The precise relationship between fundamental and higher order modes of capacitive micro-machined ultrasonic transducers (CMUTs) with rectangular membranes were determined. We investigated the effects of higher order modes on acoustical characteristics of CMUTs with rectangular membranes, especially effects on the transmission gain spectrum, through experiments using fabricated rectangular CMUT cells and a modal analysis using 3D simulation. The dips in the transmission gain spectrum corresponded to higher order mode excitation and first-order mode suppression. The frequency and depth of these dips depended on the aspect ratio. The interactions between individual modes in immersion become more complex as the aspect ratio of the rectangular membrane increases.