Design of bulk acoustic wave resonators based on ZnO for filter applications

In this study we have employed the one-dimensional transfer matrix method, derived from Mason´s model, to obtain the input electrical impedance (Z_in) for a four layer (metal/piezoelectric/metal/substrate) thin film bulk acoustic wave resonator (FBAR). The input electrical impedance was calculated taking into account the electromechanical properties of the ZnO thin films, the metal used for the contacts and the silicon oxide (SiO₂) supporting layer in order to calculate the electromechanical effective coupling coefficient K² _eff and the quality factor of the device (Q_D). We use a figure of merit (FOM) defined as the product of K_eff ² × Q_D to optimize both parameters simultaneously for their use in microwave band-pass filters. In this analysis, several metals were employed as electrodes in the FBAR device, and we have found that for gold we obtain a higher value for the FOM than for aluminum, copper or silver. In this case, the optimal metal thickness is around 1.5 micrometers. In addition, the calculated values show that for copper and silver electrodes the FOM is almost the same than for gold electrodes. Then, these electrodes could substitute gold electrodes for achieving low cost filters with good electrical performance.