Studies on the performance of silicon-based miniaturized PVDF ultrasonic transducers.

Abstract

It is widely believed that increased use of microelectronics is one of the keys to the development of future ultrasound systems. One approach to the implementation of such systems is the monolithic fabrication of transducers on silicon integrated circuit (IC) substrates, resulting in integrated ultrasonic transducers and arrays. This thesis describes in detail experimental and theoretical studies of the performance of such devices, using the piezoelectric polymer PVDF, because of its compatibility with standard IC fabrication techniques. A fabrication process for such transducers, compatible with standard silicon IC and integrated sensor technologies, was developed. The resulting devices were used for experimental studies of sensitivity, crosstalk, and pulse-echo efficiency. A key component in the process is micromachining of the silicon substrate, which improves transducer sensitivity and decreases crosstalk in arrays. Theoretical studies were performed on noise in integrated PVDF transducer/MOSFET pre-amplifier systems, using a modified Mason model which accounts for both dielectric and mechanical losses in PVDF. For a given transducer geometry (area and thickness), there is an optimum MOSFET width which allows minimization of the power dissipation required to reduce the pre-amplifier contribution to a negligible level. Theoretical studies were performed on insertion loss and signal loss for PVDF transducers in pulse-echo operation. Different electrical driving and loading conditions were considered, and optimal configurations were determined for either best possible efficiency or widest bandwidth for the frequency range (1-50 MHz) and transducer size (0.01-100 mm$\sb2$) of interest. Significant advances have been made in understanding interactions between the silicon substrate and PVDF transducers in integrated transducers and integrated arrays. Systematic approaches have been developed for characterization of crosstalk, for optimal noise design of a transducer/MOSFET pre-amplifier system, and for determining the optimal electrical driving and loading conditions for a particular transducer. These approaches are not only important for IC-based PVDF transducers, but should be applicable to other piezoelectric ultrasonic transducers as well. Overall, this thesis represents a significant step in the advancement of the application of microelectronics and microfabrication techniques to ultrasonic transducers and arrays.