Mechanical characterization of thin membranes with picosecond ultrasound

Abstract

Thin membranes are part of numerous microelectromechanical systems (MEMS) like sensors and bulk acoustic wave filters for example. In most applications the material properties of the membranes are key parameters for the correct working of the MEMS devices. Measuring bulk acoustic waves excited in MEMS-structures with ultra-short laser pulses is a powerful method for accurate and non-destructive evaluation as well as for the characterization of material properties. The laser based acoustic method generates acoustic bulk waves in a thermo-elastic way by absorbing the pump laser pulses at the surface of the MEMS-structure. The propagating acoustic pulses are partly reflected at any discontinuity of the acoustic impedance. Back at the surface the partly reflected acoustic pulses cause changes of the optical reflection coefficient, which are measured with the probe laser pulses. This technique is used for measuring the bulk wave propagation in very thin membranes. The bulk acoustic wave propagation in freestanding aluminium-silicon nitride multi-layer membranes with total thickness in the order 500 nanometers is measured and discussed. Furthermore comparisons of measurements on freestanding and supported membranes and of thermo-elastic models are presented. The measured results are used for the estimation of the Moduli of the aluminium-silicon nitride multi-layer. The technique presented in this work can also be applied for the characterization of material or geometrical properties of other components of MEMS like ultrasonic reflection layers and cantilevers. The advantage of the method lies in its non-destructive and non-contact approach, which is crucial for very thin and brittle structures.