Wide-band acoustically coupled thin-film BAW filter
In a bulk acoustic wave (BAW) filter based on laterally acoustically coupled resonators on piezoelectric thin films, one can utilize two distinct acoustic plate wave modes of different nature, for example the thickness extensional (longitudinal) TE 1 and the second harmonic thickness shear (TS 2 ) mode to form a bandpass response. The invention is based on the excitation of at least two lateral standing wave resonances belonging to different plate wave modes that facilitate the transmission of signal. The passband is designed by tailoring the wave propagation characteristics in the device such that the resonances are excited at suitable frequencies to form a passband of a desired shape. The bandwidth of the filter described herein may therefore be more than twice that of the existing state-of-the-art microacoustic filters. Consequently, it has significant commercial and technological value.
1. An acoustic filter comprising;
a stack of materials suitable for an acoustic filter, the stack comprising at least one piezoelectric layer,
at least two resonators having a width and a gap between them and comprising electrodes on different sides of the piezoelectric layer, where the electrodes on at least one side of the piezoelectric layer are separated to form at least two resonator regions, and
wherein the stack of materials, width and gap between the separated electrodes being adapted to produce at least two distinct acoustic plate wave modes, each acoustic plate wave mode contributing at least one lateral standing wave resonance in the acoustic filter, wherein the acoustic filter is adapted to trap at least two distinct plate wave modes having a total of at least three lateral standing wave resonances in the acoustic filter.
2. The acoustic filter according to claim 1 , wherein the electrodes on only one side of the piezoelectric layer are separated to form at least two resonator regions and there is provided a continuous electrode on the other side of the piezoelectric layer.
3. The acoustic filter according to claim 1 , wherein the electrodes on both sides of the piezoelectric layer are separated to form at least two resonator regions.
4. The acoustic filter according to claim 3 , wherein at least two of the electrodes are electrically connected to provide a balanced electrical port.
5. The acoustic filter according to claim 1 , wherein at least two of said at least three lateral standing wave resonances arise for one plate wave mode and at least one of said at least three lateral standing wave resonances arise for another plate wave mode.
6. The acoustic filter according to claim 1 , wherein the acoustic filter is adapted to trap at least two distinct plate wave modes having a total of at least four lateral standing wave resonances in the acoustic filter.
7. The acoustic filter according to claim 1 , wherein the at least two distinct acoustic plate wave modes comprising at least one longitudinal mode and at least one shear mode.
8. The acoustic filter according to claim 1 , further comprising at least two piezoelectric layers in stacked configuration.
9. The acoustic filter according to claim 1 , wherein the piezoelectric layer(s) is (are) selected from symmetry group 6 mm, with their c-axis in vertical direction.
10. The acoustic filter according to claim 1 , wherein the piezoelectric layer is formed from AlN or ZnO.