Film bulk acoustic resonator using high-acoustic-impedance electrodes

A bulk acoustic wave filter composed of piezoelectric thin-film resonators has many features superior to those of other small filters such as a surface acoustic wave (SAW) filter and a ceramic filter. A higb-Q factor and low loss are big advantages for a film bulk acoustic resonator (FBAR). The electrode material that is selected is very important for maintaining a higb-Q and appropriate coupling factor. An analysis of the effects of acoustic impedance on A1N-based FBAR performance is described. The electromechanical coupling factor (k(2)) and filter loss were found to be influenced by acoustic impedance and to be mainly dominated by Young's modulus. Ru was found by analysis and experiments to be a suitable material for the electrode. Finally, a 2 GHz wide band code division multiple access (WCDMA) filter was demonstrated to have a good performance.