Electrode optimization for bulk acoustic wave resonators based on ZnO

In this study we have employed a one-dimensional transfer matrix method to obtain the input electrical impedance (Zin) 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 keff and the quality factor of the device (QD). We use a figure of merit (FOM) defined as the product of keff2×QD 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 0.15 μm. In addition, the calculated values show that for copper and silver electrodes the FOM is smaller but close to that obtained for gold. Then, copper or electrodes can substitute aluminum electrodes for achieving low cost filters with good electrical performance.