Micromachined transducers for ultrasound applications

Within the last decade many interesting solutions for micromachined electroacoustic transducers have been presented, most of them microphones for the audio range. So far they could not compete significantly on the market with miniaturized conventional transducer techniques such as electret microphones. One of the main reasons for this was a limited sensitivity due to a high noise level without offering significant cost advantages. Now two facts raised new attention to micromachined transducers: For high-frequency ultrasound imaging the reduction in size of a single transducer element for 1D and even more for 2D arrays is more and more limited by fabrication and cabling technology. On the other hand, new microfabrication technologies have emerged, allowing a highly reproducible fabrication of electrostatically driven membranes with gap heights below 400 nm. One of the most interesting facts is that with a recently developed process step micromechanical membranes can be fabricated within a modified BiCMOS process. This allows the combination of transducer elements with the driving, preamplifying and multiplexing electronics on a single chip, thus reducing parasitic capacities and noise level significantly. This paper first outlines the history of micromachined transducers. Then it describes the internal structure of a micromechanical transducer element and its acoustical properties. The main differences in comparison to piezoelectric bulk transducers are the significantly lower acoustic impedance of the membranes and the nonlinear electromechanical working principle, leading to consequences in array design, which will be discussed. Mathematical models and experimental results for transducer bandwidth, membrane deflection and radiation patterns of transducer arrays are presented and compared with the properties of piezoelectric transducer arrays. The influence of the membrane's relative deflection, the poling voltage, the cabling capacity and the preamplifier characteristics upon transmission level and signal-to-noise ratio are discussed. Finally an outlook for potential applications of micromachined transducers, especially in array configurations, is given.