Modeling a CMUT covered with passivation layer by means of boundary element method

This paper presents the development of a model to predict the response of CMUTs arrays covered by a viscoelastic material layer, i.e. the passivation layer. Theoretical approach developed here combines two models: on one side a CMUT model based on boundary element method and, on the other side a model of propagation in elastic media for computing the Green function associated to the semi-infinite loading medium. A first confrontation between theory and experiments is presented through a set of electrical impedance measurements obtained with one prototype of CMUT array element. Several loading media with variable viscoelastic properties were tested. It results that for media which rheological behavior obeys to Newtonian model, even in the MHz frequency range, the model validation was achieved. For the others which rheological behavior is poorly understood, it was necessary to implement more complex models like Kelvin-Voigt or Maxwell, in order to improve concordance between theory and experiments.