A dual lateral-field-excited bulk acoustic wave sensor array

The study of the influence of the geometry of a lateral electric field resonator on its resonant characteristics

An experimental study of the dependence of the electrical impedance of a lateral electric field resonator on its thickness and the size of the gap between the electrodes was carried out. The resonator was made of PZT-19 piezoceramics in the form of a rectangular parallelepiped with the shear dimensions of 18 × 20 mm2. Two rectangular electrodes with a gap that varied in the range from 4 to 14 mm were applied on one side of the resonator. For each gap width, the frequency dependences of the real and imaginary parts of the electrical impedance were measured using an impedance analyzer. It has been found that increasing the gap width leads to an increase in the resonant frequency and to an increase in the maximum value of the real part of the impedance. Three series of such experiments were carried out for three values of the resonator thickness: 3.02, 2.38 and 1.9 mm. The resonant characteristics of the resonator were also theoretically analyzed by finite element analysis using two models. One resonator model was based on a two-dimensional finite element method. In this case, the vibration modes that existed due to the finite size of the plate in the direction parallel to the gap between the electrodes were not taken into account. The second model of the resonator used a three-dimensional finite element method, which correctly took into account all vibration modes existing in the resonator. Comparison of theory with experiment has shown that the three-dimensional model provides a better agreement between theoretical and experimental results.

High-Frequency Vibration Analysis of Piezoelectric Array Sensor under Lateral-Field-Excitation Based on Crystals with 3 m Point Group

Based on Mindlin’s first-order plate theory, the high-frequency vibrations of piezoelectric bulk acoustic wave array sensors under lateral-field-excitation based on crystals with 3 m point group are analyzed, and the spectral-frequency relationships are solved, based on which, the optimal length–thickness ratio of the piezoelectric crystal plate is determined. Then, the dynamic capacitance diagram is obtained by a forced vibration analysis of the piezoelectric crystal plate. The resonant mode conforming to good energy trapping is further obtained. The frequency interferences between different resonator units are calculated, and the influences of the spacing between two resonant units on the frequency interference with different electrode widths and spacings are analyzed. Finally, the safe spacings between resonator units are obtained. As the electrode spacing value of the left unit increases, the safe spacing d₀ between the two resonator units decreases, and the frequency interference curve tends to zero faster. When the electrode spacings of two resonator units are equal, the safe distance is largest, and the frequency interference curve tends to zero slowest. The theoretical results are verified further by finite element method. The analysis model of high frequency vibrations of strongly coupled piezoelectric bulk acoustic array device based on LiTaO₃ crystals with 3 m point group proposed in this paper can provide reliable theoretical guidance for size optimization designs of strongly coupled piezoelectric array sensors under lateral-field-excitation.

Lateral field excited quartz crystal microbalances for biosensing applications

The most common bulk acoustic wave device used in biosensing applications is the quartz crystal microbalance (QCM), in which a resonant pure shear acoustic wave is excited via electrodes on both major faces of a thin AT-cut quartz plate. For biosensing, the QCM is used to detect the capture of a target by a target-capture film. The sensitivity of the QCM is typically based solely on the detection of mechanical property changes, as electrical property change detection is limited by the electrode on its sensing surface. A modification of the QCM called the lateral field excited (LFE) QCM (LFE-QCM) has been developed with a bare sensing surface as both electrodes are now on a single face of the quartz plate. Compared to the QCM, the LFE-QCM exhibits significantly higher sensitivity to both electrical and mechanical property changes. This paper presents theoretical and experimental aspects of LFE-QCMs. In particular, the presence and strength of the usual and newfound LFE-QCM modes depend on the electrical properties of the film and/or sensing environment. This work also presents examples of experimental setups for measuring the response of an LFE-QCM, followed by results of LFE-QCMs used to detect liquid electrical and mechanical properties, chemical targets, and biological targets. Finally, details are given about the attachment of various target-capture films to the LFE-QCM surface to capture biomarkers associated with diseases such as cancer.

Influence of the conductivity of a liquid contacting with a lateral electric field excited resonator based on PZT ceramics on its characteristics

The influence of the conductivity of a liquid contacting with a piezoelectric resonator with a lateral electric field based on a PZT piezoceramic plate was experimentally and theoretically investigated. In this resonator the shear component of the mechanical displacement, which does not lead to radiation losses upon contact with the liquid, was prevalent. The frequency dependences of the real and imaginary parts of the electrical impedance of the resonator in the frequency range 50-300 kHz showed the presence of three resonances at frequencies of 68.7, 97.8 and 264 kHz with the values of the electro-mechanical coupling coefficient of 12.2%, 14.7% and 6.5%, respectively. The quality factor of each resonance in contact with the liquid turned out to be significantly higher than the quality factor of the resonator with a longitudinal acoustic wave based on lithium niobate. The dependences of the maximum value of the real part of the electrical impedance of such a resonator on the conductivity of the liquid for each resonance were obtained. This experimental data turned out to be in a good agreement with theoretical results obtained by using the method of equivalent circuit.

The sensor for measuring the micro-displacements based on the piezoelectric resonator with lateral electric field

Theoretical and experimental studies of the influence of a thin metal film on the characteristics of a piezoelectric resonator with a lateral electric field showed the possibility of creating a micro-displacement meter in the range of 10-300 μm. For the experiments, two resonators based on the plates of PZT piezoceramics with a thickness of 3.56 and 4.46 mm with resonant frequencies of ~96 and ~260 kHz for both resonators were used. It has been experimentally established that in both cases, with an increase in the width of the gap between the free side of the piezoelectric resonator and thin aluminum film, the frequency of the parallel resonance and maximum value of the real part of the electrical impedance increase and reach saturation. Besides, it has been shown that the relative change of these values with a change in the width of the gap in the range 10-300 μm increases with decreasing the thickness of the resonator. In this case, the frequency of the series resonance practically does not change. It has been also established that the sensitivity of the resonator to the presence of a conducting film at the resonant frequency of ~96 kHz is significantly higher in comparison with the resonant frequency of ~260 kHz. The experimental results are in the qualitative agreement with the theoretical data.

Study on Dual Channel Lateral Field Excitation Quartz Crystal Microbalance for Measuring Liquid Electrical Properties

Lateral field excitation quartz crystal microbalance (LFE-QCM) can detect both the electrical properties (conductivity and permittivity) and mechanical properties (viscosity and density) of the liquid. In practical applications for detecting electrical properties, the viscosity and density of the liquid will also change. This research proposed a dual-channel LFE-QCM for reducing the influence of density and viscosity. The sensing layer of one resonant element is almost bare, and the other is covered by a metal film as a reference. Different organic solutions and NaCl solution were used to study the influence of mechanical properties and the temperature on electrical properties. The experimental results demonstrate that the dual-channel LFE-QCM is necessary for properly detecting electrical properties of the liquid.

Frequency interferences of two-unit quartz resonator arrays excited by lateral electric fields

The coupled thickness-twist and face-shear vibrations of the lateral-field-excitation (LFE) two-unit resonator array are analyzed, and the frequency interference and shift of LFE resonator arrays are both studied. Different from most of the quartz resonator arrays based on thickness-field-excitation, the resonator array in this study operates with LFE generated by a pair of electrodes on the top surface of the resonator. By using Mindlin's first-order theory of piezoelectric plates, the electrically forced vibrations are analyzed. The effects of various structural parameters on the frequency interference and shift are examined; furthermore, the corresponding mechanisms are discussed. Varying trends of the frequency interference and shift with various structural parameters are verified by the finite element method. The results are crucial for avoiding frequency interferences between two adjacent units in parameter design of the resonator array operating on LFE.

The influence of the conducting film on the characteristics of the lateral electric field excited piezoelectric resonator

The effect of a thin layer with the finite surface conductivity located near the lateral electric field excited resonator on its characteristics is studied theoretically and experimentally. It has been shown that for the fixed distance between the free side of the resonator and conducting layer with increasing the surface conductivity of the layer the resonant frequency of the parallel resonance remains initially practically constant, then sharply decreases in a certain range and then insignificantly changes. For the fixed value of the layer conductivity the parallel resonant frequency increases at the increase in the gap between the resonator and layer and then achieves the saturation. The maximum change in the frequency of the parallel resonance corresponds to a zero gap when the layer conductivity varies over the wide range is equal to ∼1%. The frequency of the series resonance decreases only by ∼0.08% due to the change in the layer conductivity. The obtained results may be useful for the development of the gas sensors based on the lateral electric field excited piezoelectric resonator conjugated to the gas sensitive film, the conductivity of which changes in the presence of the given gas.

Array of piezoelectric lateral electric field excited resonators

An array containing two resonators placed on X-cut lithium niobate plate has been experimentally investigated. The resonator's lateral electric field was directed along the Y-crystallographic axis. It has been shown that stable resonance exists for a longitudinal acoustic wave propagating along the X-axis in the area between the electrodes. A layer of special damping coating was deposited around the resonators and on the part of electrodes to suppress parasitic oscillations induced mainly by Lamb waves. Frequency dependences of the real and imaginary parts of electric impedance/admittance were measured for every resonator to find resonant frequency and Q-factor with series and parallel resonances. The optimal values of width of electrode coating for every resonator were revealed which provide good resonance quality. The measurements of parameter S12, which characterizes a degree of acoustical coupling between the resonators, have shown its value to be higher than 50dB in the absolute value in all the cases considered. This means that the resonators under study are entirely acoustically decoupled. Thus it has been demonstrated that the damping layer not only provides a sufficiently good quality of every resonator's resonance, but it also assures their entire acoustical decoupling.

The study of piezoelectric lateral-electric-field-excited resonator

The piezoelectric lateral-electric-field-excited resonator based on an X-cut lithium niobate plate has been investigated. Two rectangular electrodes were applied on one side of the plate so that the lateral electric field components were parallel to the crystallographic Y-axis and excited the longitudinal wave in the gap between the electrodes. The region around the electrodes was covered with a special absorbing varnish to suppress the spurious oscillations. The effect of the absorbing coating width on the resonant frequency and Q-factor of the lateral field-excited resonator was studied in detail with the series and parallel resonances for different width of the gap between the electrodes. As a result, we found experimentally the parameter regions of pure resonances and the boundaries of value variation for resonance frequency, Q-factor, and effective electromechanical coupling coefficient.