Measurement of the bulk acoustic wave propagation in ultra-thin membranes

The measurement of bulk acoustic waves (BAW) excited in thin films or micro-structures with ultra-short laser pulses is a powerful method for accurate and non-destructive evaluation of material or geometrical properties. Optical techniques like the pump-probe laser-based acoustic method generate BAW in a thermo-elastic way by absorbing the pump laser pulses at the surface of the specimen. The acoustic waves are partly reflected at any discontinuity of the acoustic impedance. Back at the surface the reflected acoustic pulses cause changes of the optical reflection coefficient, which are measured with the probe laser pulses. The measurement technique is explained for the case of an aluminium thin film on sapphire. The influence of the film thickness and the deposition method of the thin films on the bulk wave speed is shown. In the second part of the paper this technique is used for measuring the bulk wave propagation in very thin membranes. The BAW propagation in freestanding silicon-nitride aluminium multi-layer membranes with total thickness in the order of several hundred nanometers is measured. The measurements of the freestanding membranes are compared with measurements of the supported case. The technique presented in this paper can also be applied for the characterization of material or geometrical properties of thin film BAW resonators. The advantage of the method lies in its non-destructive and non-contact approach, which is necessary for ultra-thin and brittle structures.