Advances in piezoelectric thin film deposition technology have led to the creation of advanced Bulk-Acoustic-Wave (“BAW”) devices that enable GHz range acoustic resonators much higher than Quartz-Crystal-Microbalance (“QCM”) and Surface-Acoustic-Wave (“SAW”) resonator frequencies. Hexagonal crystals structures serving as piezoelectric thin films, such as zinc oxide (“ZnO”) and aluminum nitride (“AlN”), have been used to make film BAW resonators called Thin-Film-Bulk-Acoustic-Wave-Resonators (“FBARs”). FBARs comprise a piezoelectric thin film (a few hundred nanometers to a few micrometers) sandwiched between two electrodes. The whole structure is supported by another layer (a few micrometers thick) to provide mechanical ruggedness. This configuration is called as composite BAW Resonator. The frequency at which a BAW resonator resonates is inversely proportional to the thickness of the device, and the mass sensitivity of the device as a sensor is proportional to a power of the unperturbed resonant frequency. Thus, thinner piezoelectric films are required for higher mass sensitivity application. Unfortunately, this configuration becomes very fragile for gigahertz range operation. For robustness, BAW resonators may employ a Solidly Mounted Resonator (“SMR”) configuration. In SMRs, the piezoelectric film is deposited on top of an acoustic Bragg reflector. Conventionally, two different acoustic modes can be setup in the FBAR (i.e. the thickness shear mode (“TSM”) or the longitudinal, i.e. the thickness extensional mode (“TEM”)).
In conventional substantially c-axis oriented ZnO BAW resonators, the excitation of the TEM requires electrical citation parallel to the c-axis of the crystal structure, i.e. thickness excitation (“TE”). TE is accomplished by sandwiching the hexagonal crystal structure between two electrodes, thus providing an electric field parallel to the c-axis. On the other hand, excitation of the TSM requires electrical excitation orthogonal to the c-axis, i.e. lateral-field excitation (“LFE”). LFE is accomplished by placing the electrodes on the opposing sides of the hexagonal crystal structure to provide an electric field orthogonal to the c-axis. The acoustic wave resonating in the BAW will have a different velocity for TSM than for the TEM. Due to the different velocities of the TSM and TE modes, the corresponding wavelengths of the waves in any material are different. Thus, in conventional SMR BAWs, the reflector stack is either designed for the BAW to operate in the TSM or the TEM. In other words, a BAW operating in the TSM is typically not tuned for the TEM, and vice versa. Therefore different devices with different electrode and reflector designs are needed to excite the TSM and TEM bulk acoustic modes with each device optimized for a single mode.
Multiple modes excited in a single, solidly-mounted BAW resonator would provide a multi-band resonator operating at non-harmonically related frequencies, which could be in the gigahertz range. These types of devices could be useful in multi-band communication filter applications. Further, the advantages of a multi-mode device could be significant in bio sample analyses, especially because the TSM is desirable for liquid-phase sensing while the TEM is applicable in vapor-phase sensing. While devices operating in both the TSM and TEM have been created with devices employing an inclined/tilted c-axis growth of ZnO (as opposed to substantially c-axis oriented), these devices require sophisticated deposition and etching processes and have membrane structures that are inherently fragile when dealing in the GHz range due to the necessity of an extremely thin film.
Therefore, there is a desire for durable and more easily manufacturable solidly mounted BAW resonators capable of simultaneously operating in at least two non-harmonically-related modes. Various embodiments of the present invention address such a desire.