Transducers generally convert electrical signals to mechanical signals or vibrations, and/or mechanical signals or vibrations to electrical signals. Acoustic transducers, in particular, convert electrical signals to acoustic signals (sound waves) in a transmit mode and/or convert received acoustic waves to electrical signals in a receive mode. Acoustic transducers generally include acoustic resonators, such as thin film bulk acoustic resonators (FBARs), surface acoustic wave (SAW) resonators or bulk acoustic wave (BAW) resonators, and may be used in a wide variety of electronic applications, such as cellular telephones, personal digital assistants (PDAs), electronic gaming devices, laptop computers and other portable communications devices. For example, FBARs may be used for electrical filters and voltage transformers. Generally, an acoustic resonator has a layer of piezoelectric material between two conductive plates (electrodes), which may be formed on a thin membrane. FBAR devices, in particular, generate longitudinal acoustic waves and lateral (or transverse) acoustic waves when stimulated by an applied time-varying electric field, as well as higher order harmonic mixing products. The lateral modes and the higher order harmonic mixing products may have a deleterious impact on functionality.
A stacked bulk acoustic resonator, also referred to as a single cavity acoustic resonator, includes two layers of piezoelectric materials between three electrodes in a single stack, forming a single cavity. That is, a first layer of piezoelectric material is formed between a first (bottom) electrode and a second (middle) electrode, and a second layer of piezoelectric material is formed between the second (middle) electrode and a third (top) electrode. Generally, the stacked bulk acoustic resonator device allows reduction of the area of a single bulk acoustic resonator device by about half. Examples of stacked bulk acoustic resonators, as well as their materials and methods of fabrication, may be found in U.S. Patent App. Pub. No. 2010/0052815 to Bradley et al., published Mar. 4, 2010, which is hereby incorporated by reference.
Conventional solutions for reducing effects of spurious modes in the filter/duplexer response include increasing the path of the lateral acoustic wave until it reaches its lateral resonance condition. It could be implemented in the stacked acoustic resonator device either by increasing the area of resonators or by creating an apodized shape of the third (top) electrode. These solutions are capable of attenuating the effect of the spurious resonance in the filter response, but they cannot recover the energy in the lateral modes.