Capacitive micromachined ultrasonic Lamb wave transducers

Many sensors and acoustic wave filters rely on the conversion of electrical energy into mechanical energy. This function is traditionally performed using a piezoelectric transducer in an interdigital configuration that launches acoustic waves along the substrate upon which the device is built. The acoustic wave of choice is often the asymmetric Lamb wave for two reasons: first, it can travel efficiently in a liquid environment, and second, it can exist in the 1--10 MHz range where the accompanying electronics are relatively easy to design and implement. This dissertation describes the fabrication, modeling and experimental results of a novel device built for the transduction of Lamb waves using the Capacitive Micromachined Ultrasonic Transducer (CMUT).; The CMUT is similar to other capacitance transducers in that it employs a vibrating membrane to send and receive ultrasound in air and in water. Its invention was reported in 1994, and it has since found applications in a variety of arenas. The presence of Lamb waves in devices fabricated for the purpose of transmitting an acoustic signal into the surrounding medium has a deleterious effect on the system behavior. This is because the wave that is excited creates a cross-coupling of energy between otherwise independent cells. If this excitation of Lamb waves is instead exploited and optimized, however, the foundation for a new device is created.; The Lamb wave device described in this dissertation uses high aspect ratio CMUTs manufactured using two different fabrication techniques. The first results in a capacitive transducer built using the standard sacrificial-layer CMUT manufacturing process, while the second employs the significantly more robust and less labor-intensive wafer bonding method. Both arrays and single element structures have been built on substrates that have a thickness ranging from 500 microns down to 8 microns. They have been characterized using S-parameter, pitch-catch, and laser doppler vibrometer techniques, and their behavior is consistent with results from simulations performed using both analytical and finite element models. Measurements further demonstrate an insertion loss of 16.4 dB at 2.6 MHz for an electrically matched device fabricated using a single 60 micron x 1 cm CMUT at both the transmit and receive ports.