Evaluation of parylene as protection layer for capacitive micromachined ultrasonic transducers

Due to their enhanced performance, capacitive Micromachined Ultrasonic Transducers (cMUTs) are on the way to replacing standard piezoelectric ceramics technology in ultrasound medical imaging. The cMUT device requires the implementation of a final protection layer in order to avoid electrode degradation and achieve biocompatibility. Parylene C is a material of choice for such applications as it can be deposited in exceptionally conformal and chemically inert thin films. In this work, the influence of 1μm thick protective coating of Parylene film on cMUT performance and reliability is studied through simulations and dynamic characterization. Data obtained by laser Doppler vibrometry show a good agreement between simulations and experiments with a slight increase of the collapse voltage. Device aging provides evidence of the necessity and the effectiveness of Parylene as a protective layer. Finally, this work reveals that cMUT resonant frequency can be easily tuned by controlling the deposited Parylene film thickness.