Three methods for measuring the ultrasonic velocity in thin films

Abstract We present three methods of measuring the velocity of ultrasound in self-supported thin films (1 μm or thicker). The first method involves time-of-flight measurements of sound pulses excited by a pulsed laser. These pulses are detected with a wide band laser interferometer and we measure the propagation delay time as a function of the path length. In the second method, we continuously excite the film using an interdigital transducer (IDT) while performing narrow band detection using the laser interferometer. The interferometer allows us to determine the position dependence of both the in-phase and the out-of-phase components of the film vibration (relative to the emitted signal). The third method is similar to the second, but both the generation and the detection of the ultrasonic wave are accomplished with IDTs. The receiving IDT is translated across the surface of the film and the ultrasonic continuity is maintained through a liquid bond. All three methods have been tested on the same film and yield similar values (within experimental limits) of the velocity of sound of the various modes excited. From these results, we deduce the shear and flexural moduli of the film and compare them with values quoted in the literature.