Piezoelectric materials for BAW resonators and filters

Thin film bulk acoustic wave (BAW) resonators and filters are appropriate for mobile communication systems operating at high frequencies between 0.5 and 10 GHz. c-axis orientated AlN films are used for medium bandwidth applications. Electromechanical coupling factors k/sub t/ of 0.25/spl plusmn/0.03, which are close to the bulk data, have been found in AlN thin films using optimum sputter conditions. The correlation between k/sub t/ and AlN film orientation is discussed. Resonator Q values of approx. 450 are reported at 2.6 GHz and a Q of 250 was realized at 9.2 GHz. A physical model is shown to accurately predict the frequency responses of BAW resonators and band-pass filters. For wide bandwidth applications PbZr/sub x/Ti/sub 1-x/O/sub 3/ (PZT) films are investigated. They exhibit large coupling coefficients.