Bulk acoustic wave transducers and resonators based on AlN thin films

Highly c-axis oriented piezoelectric AlN films were grown at 80°C by RF reactive magnetron sputtering technique on the metallized surface of Si, Si₃N₄ and Al₂O₃ substrates. The films were smooth and extremely adhesive to all the bare and Pt- or Al-coated substrates. The frequency response of AlN-based bulk acoustic wave (BAW) transducers implemented on semi-infinite (hundreds of ¿ms) substrates was tested for different piezoelectric film thicknesses. The BAW transducers operating frequencies, in the range 1 to 2.4 GHz, were found in good accordance with those theoretically calculated by using the one-dimensional physical model in the lossless materials approximation. The theoretical frequency response of the BAW transducers was also studied for different inner electrode/piezoelectric film thickness ratio values, in order to go insight the BAW device design tradeoffs performances. Thin film bulk acoustic resonators (TFBAR), consisting of an AlN film sandwiched between two metal electrodes, was implemented on a thin Si₃N₄ membrane and designed to operate in the fundamental thickness-extensional mode. The electrical performance of the TFBARs, showing different AlN film thicknesses, have been evaluated in the resonance frequency range from 1.16 to 2.7 GHz. The Butterworth-Van Dyke model was derived to simulate the electrical behavior of the TFBAR in the vicinity of the film resonance. Effective coupling coefficient and unloaded quality factor Q of about 3.9% and 150 were achieved. The obtained experimental results confirm that AlN films are suitable for high frequency application even in the region of low temperature deposition values.