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. While almost all acoustic wave devices operate efficiently in a gaseous or vacuum ambient, only those that generate primarily shear motion in their substrate remain efficient in a liquid environment due to excessive damping. The exception to this rule occurs for devices that propagate waves with a velocity lower than that of sound in the liquid. Asymmetric Lamb waves, also known as flexural plate waves, fit this criterion. Furthermore, these waves generally operate in the 1-10 MHz range where the design and implementation of the accompanying electronics is relatively simple. As a result, asymmetric Lamb waves are often used in both sensor and filter applications. 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 built using two different fabrication techniques. The firrst results in a capacitive transducer built using the standard sacricial-layer CMUT manufacturing process, while the second employs the signicantly 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 m x 1 cm CMUT at both the transmit and receive ports.
