Viscoelasticity Assessment Using Quartz Crystal Microbalance for Accurate Loaded CMUT Modeling

The main challenge with Capacitive Micromachined Ultrasonic Transducer (CMUT) probes packaging is to preserve their intrinsic performances. Indeed, careful attention should be paid to the encapsulation of CMUTs and the impact of a viscoelastic acoustic loading (e.g., polymer) on the array performance. Our group developed a Boundary Element Matrix (BEM) dedicated to the acoustic coupling computation between a viscoelastic medium and a CMUT. As a complementary work, the present study aims to set up an experimental protocol to extract viscoelastic properties of different media and use them as model data inputs. This task is performed using Quartz Crystal Microbalance (QCM) with electrical admittance measurements performed in air and in each viscoelastic medium. A complex viscosity value was obtained for vegetable oils and the tested polymer. Then, using a CMUT array, electrical impedance measurements were performed in each medium. Using the QCM extracted media properties, a very good agreement was obtained between CMUT modeling and measurements where viscoelasticity is proven to reduce inter-cell coupling effects. Full electrode loading of a highly viscous polymer overdamp the QCM resonance and highlight its first order equation limits.