BAW resonators based on aluminum nitride thin films

The fabrication and the characterization of solidly mounted (SMR) and membrane-based (TFBAR) bulk acoustic wave resonators made of piezoelectric aluminum nitride (AlN) thin films are presented. The resonance frequencies are 2.4 and 3.5 GHz, respectively. The active λ/2 AlN thin films were grown on Pt electrode layers by pulsed dc reactive magnetron sputtering. Simulation calculations were performed to fit experimental admittance curves. For both resonator types, a sound velocity of 11350 m/s was derived, which corresponds to a c₃₃ ^D of 420 GPa. The devices typically exhibited figures of merit Q·k_t² of 6 to 16. A zero thermal frequency drift was obtained in the TFBAR structure due to a SiO₂ layer as compensating element