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In the last five years we have been actively developing capacitive micromachined ultrasonic transducers (cMUT) since they have potential advantages over piezoelectric transducers, such as ease of fabrication in single elements and arrays, broad bandwidth and high efficiency. We report on research efforts in the theoretical understanding of cMUT with an improved electrical equivalent circuit model as well as its actual implementation through microfabrication. First, we present a process sequence that has allowed us to make reproducible devices with sealed membranes. The impact of electrode metalization on the impedance, bandwidth and efficiency of the devices will be discussed, and experimental results will be compared to theoretical models. Our study in the paper indicates that the best cMUT performance can be achieved with the appropriate fabrication process under certain constraints. Our most recent devices have been designed to have an input impedance with areal part of 50 Ohms at a frequency around 5 Mhz. A through transmission measurement gave a dynamic range of better than 100 dB while operating in the frequency range of 1-10 MHz. Operated without tuning, the devices are capable of operation from dc to 10s of MHz which is achieved because the devices are not resonant. In summary, we will present a novel technology capable of delivering surface micromachined ultrasonic transducers that are efficient, and easy to make single element and multiple elements array transducers. These devices can also be integrated on chip with transmitter and receiver electronics.
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