Influence of Etching Trench on Keff2 of Film Bulk Acoustic Resonator

As radio-frequency (RF) communication becomes more ubiquitous globally, film bulk acoustic resonators (FBAR) have attracted great attention for their superior performance. One of the key parameters of an FBAR, the effective electromechanical coupling coefficient (Keff2), has a great influence on the bandwidth of RF filters. In this work, we propose a feasible method to tune the Keff2 of the FBAR by etching the piezoelectric material to form a trench around the active area of the FBAR. The influence of the position of the etching trench on the Keff2 of the FBAR was investigated by 3D finite element modeling and experimental fabricating. Meanwhile, a theoretical electrical model was presented to test and verify the simulated and measured results. The Keff2 of the FBAR tended to be reduced when the distance between the edge of the top electrode and the edge of the trench was increased, but the Q value of the FBAR was not degraded. This work provides a new possibility for tuning the Keff2 of resonators to meet the requirements of different filter bandwidths.

Effect of c-axis tilted orientation ZnO thin film on shear-mode bulk acoustic resonator in liquid environment

ZnO has been widely used as piezoelectric materials in bulk acoustic resonators, especially in shear-mode film bulk acoustic resonators (FBAR). In this paper, we studied the effect of c-axis tilted ZnO thin film on shear mode FBAR in liquid environment. Mechanical impedances of FBARs are compared between shear-mode and longitudinal-mode wave due to their different orientations of ZnO thin film. It is found ZnO thin film with 43° c-axis tilted angels induces quasi-shear-mode acoustic wave only. The Q values of FBAR with 43° c-axis tilted ZnO are 256.94 in air at 0.8148 GHz, and 34.34 in water at 0.814 GHz, respectively. The shifts of the resonant frequencies are proportional to the square root of the product of viscosity and density of liquid medium, which is consistent with Kanazawa-Gordon theory and verified by actual experiment.

Electro-mechanical Casimir effect

The dynamical Casimir effect is an intriguing phenomenon in which photons are generated from vacuum due to a non-adiabatic change in some boundary conditions. In particular, it connects the motion of an accelerated mechanical mirror to the generation of photons. While pioneering experiments demonstrating this effect exist, a conclusive measurement involving a mechanical generation is still missing. We show that a hybrid system consisting of a piezoelectric mechanical resonator coupled to a superconducting cavity may allow to electro-mechanically generate measurable photons from vacuum, intrinsically associated to the dynamical Casimir effect. Such an experiment may be achieved with current technology, based on film bulk acoustic resonators directly coupled to a superconducting cavity. Our results predict a measurable photon generation rate, which can be further increased through additional improvements such as using superconducting metamaterials.