Deriving a simulation model of a 40-khz CMUT cell from impedance and interferometer measurements

Abstract

A spring-mass-damper system and corresponding equivalent circuit help design and analyze capacitive micromachined ultrasonic transducers (CMUTs). The ability to extract similar models from measured devices helps design verification and development of simulation models that match fabricated devices. Impedance measurements yield the CMUT’s clamped capacitance and motional impedance, which fits a 4-element equivalent circuit. Additional measurements help extract a more complete equivalent circuit that includes the electromechanical transformer ratio and effective mechanical elements. In this work, we combine impedance measurements, the tightly specified mass of the CMUT plate, and interferometer displacement measurements to compute a more complete equivalent circuit. The circuit obtained for two 40-kHz CMUT cells exhibits a good fit to the model derived from the device geometry alone. The experimentally-obtained model exhibits larger damping which is likely due to energy loss in addition to radiated ultrasound.