Energy loss mechanisms in SMR-type BAW devices

Bulk acoustic wave (BAW) filters owe their performance advantages particularly to the excellent Q-values of the acoustic resonators. Solidly mounted resonator (SMR) type BAW devices achieve Q-values greater than 1500 when acoustic mirror layers with a large impedance ratio and, hence, a high reflectivity are used. Tungsten and silicon oxide are the materials of choice for low loss BAW resonators. This paper describes the loss mechanism still present in low loss BAW resonators and presents comprehensive methods to acquire quantitative data about energy losses. These methods are based on electrical measurements, 1D transmission line models and higher dimensional FEM simulations, and laser interferometer measurements. The lateral currents across the electrodes and the resulting ohmic power dissipation are derived from surface vibration measurements and an inverse calculation of local charge distribution. It is demonstrated that combining these methods allows to detect acoustic leaks and to analyze resistive losses and, thus, the major energy losses in the respective operating conditions can be identified.