Effects of electrodes on performance figures of thin film bulk acoustic resonators

The effects of mechanical losses and elastic properties of the electrodes on the performance figures of a thin film bulk acoustic resonator (FBAR) are analyzed by numerical simulation. Results indicate that the material loss of the electrode has no visible effect on the characterization of the effective electromechanical coupling factor, k(eff)2. The acoustic impedance ratio of the electrode to the piezo-film dominantly determines the behaviors of the k(eff)2 variation with the electrode thickness. The resonance Q value, Q(s), of the FBAR closely relies on the material Q values of film and of electrodes as expected. Besides, the variation of Q(s) versus the thickness of the electrodes crucially depends on the acoustic impedance ratio as well. Especially, three characteristic parameters, i.e., the maximum value of k(eff)2, the sectional mass ratio of the electrode to the piezo film corresponding to the maximum k(eff)2, and the tolerance range of the ratio to keep k(eff)2 near the maximum, are calculated for some typical samples. These results would be useful for optimizing FBAR designs and performances.