Hongbin Cheng

Analytical study of dual-mode thin film bulk acoustic resonators (FBARs) based on ZnO and AlN films with tilted c-axis orientation

In this paper, we present the analytical study of thin film bulk acoustic wave resonators (FBARs) using ZnO and AlN films with a c-axis tilt angle (off-normal) from 0° to 180°. The tilted c-axis orientation induces normal plane and inplane polarizations, which leads to the coexistence of the longitudinal mode and shear mode in the resonator. The equation for predicting electric impedance of FBARs was derived from the basic piezoelectric constitutive equations. Material properties including elastic, dielectric, and piezoelectric coefficients, bulk wave properties including acoustic velocity and electromechanical coupling coefficient, and impedance of FBARs were calculated and showed strong dependence on the tilt angle. Interestingly, it was found that for ZnO FBARs, pure thickness longitudinal modes occur at 0° and 65.4°, and pure thickness shear modes occur at 43° and 90°. For AlN FBARs, pure longitudinal modes occur at 0° and 67.1°, and pure shear modes occur at 46.1° and 90° for AlN. In other words, pure thickness longitudinal and shear modes exist in ZnO and AlN FBARs at specific tilted polarization angles. In addition, two peaks of shear mode electromechanical coefficient are found at 33.3° and 90° for ZnO, and 34.5° and 90° for AlN. Therefore, ZnO and AlN films with specific tilt angles may provide options in the design and fabrication of FBARs, considering their strong shear resonance with high electromechanical coefficients. The use of dual-mode FBARs for mass sensors is also analyzed; the calculated large resonant frequency shift caused by mass loading shows that they have good prospects for use in sensor applications with high sensitivity. The simulation results agreed well with the reported experiment results, and can be used for design and application of FBARs.

Viscosity sensor using ZnO and AlN thin film bulk acoustic resonators with tilted polar c-axis orientations

We present a theoretical analysis of liquid viscosity sensors using ZnO and AlN thin film bulk acoustic wave resonators (FBARs) with tilted polar c-axis orientations. Besides the thickness longitudinal mode, the tilted c-axis orientation induces thickness shear mode in the resonator, which allows resonators operated in liquid medium without significant damping for sensory application. The equation for predicting electric impedance of shear mode film bulk acoustic wave resonators (FBARs) with a viscous liquid loading was derived from the basic piezoelectric constitutive equations. The viscosity sensitivity of shear mode ZnO and AlN resonators was achieved by calculation of resonant frequency shift due to viscous liquid loading. In the simulation, it is assumed that all the resonators have 2 μm thickness and 300 μm×300 μm electrode area; three different liquids (water, acetone, and olive oil) were chosen as the liquid loadings; and different tilt c-axis angles for both ZnO FBARs and AlN FBARs have been exam...

P1D-2 High Temperature Langasite BAW Gas Sensor Based on ZnO Nanowire Arrays

In this paper, we present our recent study on the fabrication and characterization of the ZnO nanowires gas sensor using thickness shear mode (TSM) langasite resonators. C-axis vertically aligned ZnO nanowire arrays were synthesized on the langasite resonator by using an utrathin ZnO seed layer through a simple hydrothermal route. The nanowires were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). N02 as the target gas was particularly used to investigate the sensing performance of langasite resonator sensors with nanostructured ZnO interfacial layer at high temperature conditions. The results indicated the use of the ZnO nanowire arrays on acoustic wave resonator can greatly enhance the sensitivity and sensor response speed due to the fast surface/interface reaction and surface roughness.

Acoustic wave flow sensor using quartz thickness shear mode resonator

A quartz thickness shear mode (TSM) bulk acoustic wave resonator was used for in situ and real-time detection of liquid flow rate in this study. A special flow chamber made of 2 parallel acrylic plates was designed for flow measurement. The flow chamber has a rectangular flow channel, 2 flow reservoirs for stabilizing the fluid flow, a sensor mounting port for resonator holding, one inlet port, and one outlet port for pipe connection. A 5-MHz TSM quartz resonator was edge-bonded to the sensor mounting port with one side exposed to the flowing liquid and other side exposed to air. The electrical impedance spectra of the quartz resonator at different volumetric flow rate conditions were measured by an impedance analyzer for the extraction of the resonant frequency through a data-fitting method. The fundamental, 3rd, 5th, 7th, and 9th resonant frequency shifts were found to be around 920, 3572, 5947, 8228, and 10 300 Hz for flow rate variation from 0 to 3000 mL/min, which had a corresponding Reynolds number change from 0 to 822. The resonant frequency shifts of different modes are found to be quadratic with flow rate, which is attributed to the nonlinear effect of quartz resonator due to the effective normal pressure imposing on the resonator sensor by the flowing fluid. The results indicate that quartz TSM resonators can be used for flow sensors with characteristics of simplicity, fast response, and good repeatability.

Mass sensitivity of thin film bulk acoustic wave resonator based on c-axis tilted ZnO film

In this paper, the mass sensitivity of dual mode thin film bulk acoustic wave resonator (FBAR) based on ZnO film with tilted polar c-axis has been theoretically investigated. The tilted c-axis orientation induces normal plane and in-plane polarizations, which leads to the coexistence of the longitudinal mode and shear mode in the resonator. Equation for predicting electric impedance of dual mode ZnO FBAR with a mass loading layer was derived from the basic piezoelectric constitutive equations. The mass sensitivity of dual mode ZnO resonator was achieved by calculation of resonant frequency shifts of thickness shear mode and longitudinal mode caused by a thin mass loading layer. In the simulation, ZnO thin film has a c-axis tilt angle from 0°–90°, 2µm thickness and 300µm by 300µm electrode area; Al, SiO2, Au and Pt thin films were considered as the mass loading layers. It was found that both thickness longitudinal resonance frequency and shear resonance frequency for different c-axis tilt angle have a large shift due to the mass loading; the mass sensitivities of longitudinal and shear mode for this four mass loading materials are very close, and do not change much with c-axis tilt angle with a value rang around −900 cm2/g. The simulation results can be used for design and application of ZnO FBARs.

Ammonia sensing characteristics of quartz resonator coated with ZnO nanowires sensitive layer

In this paper, we present our recent study on the fabrication and characterization of ammonia gas sensors based on quartz thickness shear mode (TSM) resonators employing ZnO nanowires as the sensitive coating layer. c-axis vertically aligned ZnO nanowire arrays were synthesized on the quartz resonator through a simple hydrothermal synthesis route. The ZnO nanowires were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The sensing characteristics, including sensitivity, stability, reproducibility, and response time of the acoustic wave gas sensors have been studied under different concentration levels of ammonia at room temperature. It is demonstrated that the use of the ZnO nanowire arrays on quartz TSM acoustic wave resonator can greatly enhance the sensitivity and sensor response speed due to the fast surface/interface reaction and large surface/volume ratio of the nanowire arrays.