Excitation of ultrasonic Lamb waves in composite membranes

The excitation of ultrasonic Lamb waves by an interdigital transducer in a composite plate consisting of a piezoelectric thin film deposited on a nonpiezoelectric membrane is analyzed using the Green's function method. The amplitudes of the generated waves are obtained in terms of the charge density on transducer electrodes. An electrostatic analysis that neglects piezoelectric coupling is used to relate this charge density to the voltage applied across the IDT. This is then used to calculate the input characteristics of the IDT. The characteristics of Lamb wave devices fabricated on a zinc oxide-on-silicon nitride membrane are measured and compared with theoretical calculations. It is shown that strong internal reflections are inherently present in an IDT fabricated on thin plates. A knowledge of these is essential in order to correctly calculate the characteristics of the IDT. It appears that the internal reflections can be utilized to realize unidirectional transducers for Lamb waves.