Novel mode-coupling vibrations of AlN thin film bulk acoustic resonator operating with thickness-extensional mode

The dispersion curves of bulk waves propagating in both AlN and ZnO film bulk acoustic resonators (FBARs) are presented to illustrate the mode flip of the thickness-extensional (TE) and 2nd thickness-shear (TSh2) modes. The frequency spectrum quantitative prediction (FSQP) method is used to solve the frequency spectra for predicting the coupling strength among the eigen-modes in AlN and ZnO FBARs. The results elaborate that the flip of the TE and TSh2 branches results in novel self-coupling vibration between the small-wavenumber TE and large-wavenumber TE modes, which has never been observed in the ZnO FBAR. Besides, the mode flip leads to the change in the relative positions of the frequency spectral curves about the TE cut-off frequency. The obtained frequency spectra can be used to predict the mode-coupling behaviors of the vibration modes in the AlN FBAR. The conclusions drawn from the results can help to distinguish the desirable operation modes of the AlN FBAR with very weak coupling strength from all vibration modes.