Lateral Field Excited Sensor Research Articles

Energy-Trapping Characteristics of Lateral Field Excited GdCOB Crystal Bulk Acoustic Wave Devices Based on Stepped Electrodes.

In this work, high-frequency forced vibrations of lateral field excitation (LFE) devices with stepped electrodes based on monoclinic crystals GdCOB are modeled, and the influence laws of the device parameters (the step number, size, and thickness of the stepped electrodes) on the energy-trapping effects of the device are revealed. The results show that the step number has a significant effect on the energy-trapping effect of the device: with the increase in the step number, the stronger energy-trapping effect of the device can be obtained; with the increase in the thickness difference of two layers of electrodes, the energy-trapping effect of the device becomes stronger; with the increase in the difference of the electrode radius, the energy-trapping effect of the device is enhanced gradually. The results of this work can provide an important theoretical basis for the design of stepped-electrode LFE resonators and sensors with high-quality factors based on monoclinic crystals.

Effects of Polarity Inversion Layer on Performances of Lateral-Field-Excitation Piezoelectric Sensors Based on Lithium Niobate Single Crystal

In this work, lateral-field-excitation (LFE) piezoelectric sensors based on polarity inversion layer are designed and fabricated, then frequency stabilities and sensitivities on electric property changes of liquids are tested. Because the polarity inversion layer can suppress the spurious modes, the stabilities of the LFE devices with a polarity inversion layer are obviously better than that of LFE devices with no polarity inversion layer. On the changes of liquid conductivity and permittivity, the sensitivities of the LFE devices with a polarity inversion layer are 2.4 times and 2.1 times higher than that of LFE devices with no polarity inversion layer, respectively. The polarity inversion layer of the lithium niobate crystal plate can be realized conveniently by heat treatment, therefore, the technology of the polarity inversion layer can play an important role in improving the sensing performances of LFE sensors.

Double-mode lateral-field-excitation bulk acoustic wave characteristics of Ca3TaGa3Si2O14 crystals.

The double-mode lateral-field-excitation (LFE) bulk acoustic wave characteristics of Ca3TaGa3Si2O14 (CTGS) crystals are investigated. It is found that LFE devices based on (yxl)-57° CTGS crystals can work on both pure-LFE and pseudo-LFE modes when the driving electric field direction is normal to the crystallographic x axis of the piezoelectric substrate. Several double-mode LFE bulk acoustic wave devices based on CTGS crystals are designed and tested. The experimental results conform to the theoretical prediction well. Being able to operate in pure-LFE and pseudo-LFE modes, the double-mode LFE sensors show high sensitivity to both mechanical and electrical property changes of analytes. The results provide the crystal cut for double-mode LFE sensors, which is a critical basis of designing high-performance chemical and biological sensors by using double-mode LFE devices.

Structure ordered Ca3TaGa3Si2O14 crystal-based liquid phase sensors with enhanced stability and sensitivity

In this study, lateral-field-excitation (LFE) bulk acoustic wave sensors based on the structure ordered Ca3TaGa3Si2O14 (CTGS) crystal are investigated. It is found that LFE devices using (yxl)-6° CTGS crystal can operate with the pseudo-LFE mode. The impedance characteristics and sensitivities of the devices are measured and compared with the LFE devices based on structure disordered langasite (LGS) crystal. The experimental results show that the CTGS LFE sensor can achieve higher Q-factors than the LGS LFE sensor, which indicates that the former has higher frequency-stability. In addition, the CTGS LFE sensor is nearly 2 times more sensitive to both liquid conductivity and permittivity changes compared to the LGS LFE sensor. For liquid acoustic viscosity changes, the CTGS LFE sensor is approximately 1.3 times more sensitive than the latter. In some chemical and biological analysis process, mechanical and electrical property changes of analytes need to be measured simultaneously, therefore the LFE sensors based on the structure ordered CTGS crystal have a good potential for use in liquid-phase sensing of chemical and biological systems.

Investigation of pseudo lateral field excited acoustic wave gas sensors with finite element method

To improve the weak piezoelectric excitation of lateral field excited (LFE) acoustic wave sensors in air, a novel bulk acoustic wave device, named pseudo LFE acoustic wave sensor, is proposed in this paper. The commercially available finite element software, COMSOL Multiphysics, is adopted to analyze pseudo LFE acoustic wave gas sensors and further calculate their sensitivities to electric conductivity variation of the polyisobutylene (PIB) sensing film. An AT-cut quartz plate with a thickness of 185 µm is chosen as the piezoelectric substrate due to its tiny temperature coefficient to frequency for thickness shear mode (TSM). After the mesh convergence analysis, a mesh level is decided for following calculations. In frequency response analysis, a bare pseudo LFE sensor shows a pure TSM and a stronger excitation than a bare LFE sensor. The experiments also verify this result. In sensitivity analysis, a gradual conductivity increase of the sensing film from 0 to 3 S/m causes a significant resonance frequency decrease of the pseudo LFE sensor. Besides, the pseudo LFE sensor exhibits an excellent sensitivity to the film conductivity variation, about 30 times as high as the QCM sensor. Finally, the electrode dimensions of pseudo LFE sensors are discussed for optimizing device performances. One can find ringed floating electrodes exhibit greater sensitivity than circular ones. Moreover, a larger gap width between the LFE electrodes also contributes to larger sensitivity but weakens piezoelectric excitation. The results conclude that the pseudo LFE sensor exhibits a satisfactorily large admittance and an excellent sensitivity to external electrical fluctuation.

Coupled extension and thickness-twist vibrations of lateral field excited AT-cut quartz plates

In this paper, the coupled extension and thickness-twist vibrations are studied for AT-cut quartz plates under Lateral Field Excitation (LFE) with variations along the x1-direction. Mindlin’s two-dimensional equations are used for anisotropic crystal plates. Both free and electrically forced vibrations are considered. Important vibration characteristics are obtained, including dispersion relations, frequency spectra, and motional capacitances. It is shown that, to avoid the effects of the couplings between extension and thickness-twist vibrations, a series of discrete values of the length/thickness ratio of the crystal plate need to be excluded. The results are of fundamental significance for the design of LFE resonators and sensors. Open image in new window

Investigation of quasi lateral-field-excitation on (yxl)-17° LiNbO3 single crystal

Quasi lateral-field-excitation (LFE) on LiNbO3 crystal is investigated both theoretically and experimentally. It is found that when the driving electric field direction is parallel to the crystallographic X-axis of the piezoelectric substrate, (yxl)-17° LiNbO3 LFE bulk acoustic wave devices work on quasi-LFE mode. The experimental results agreed with the theoretical prediction well. The results provide the cut of LiNbO3 crystal for quasi-LFE bulk acoustic wave devices, which is important for designing high performance LFE sensors on LiNbO3 substrates.

Lateral-field-excited bulk acoustic wave sensors on langasite working on different operational modes

Lateral-field-excited (LFE) bulk acoustic wave sensors on langasite working on different operational modes are investigated and compared. Langasite LFE sensors on pure-LFE mode and pseudo-LFE mode are designed and fabricated. The sensitivities of the devices to changes in liquid electrical properties (e.g., conductivity or permittivity) and micro-mass changes on their surfaces were tested and compared. The experimental results show that compared with langasite pure-LFE devices, langasite pseudo-LFE devices have superior sensitivities to both liquid conductivity and liquid permittivity changes. For the micro-mass changes, the pure- LFE devices showed high sensitivity, whereas the pseudo-LFE devices do not, because they cannot resonate in air. On this basis, the physical interpretation of these results is discussed.

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

Recently, there has been interest in the fabrication of multiple quartz crystal microbalances (QCMs) on a single substrate to create a sensor array. However, such devices are ultimately subject to the limitations of the QCM configuration, requiring electrodes and wires on the sensing surface of the crystal substrate, resulting in a cumbersome arrangement that is unable to detect electrical property changes. The lateral-field-excited (LFE) sensor is a novel sensing device that only requires electrodes on the back side of the substrate. With a bare sensing surface, the LFE sensor is a better choice for implementing a sensor array. The purpose of this paper is to report on the fabrication and testing of two independent LFE devices on a single substrate. The individual LFE elements are shown to respond to both electrical and mechanical property changes, with minimal crosstalk between the LFE elements.

A lateral field excited (yxl)88° LiTaO3 bulk acoustic wave sensor with interdigital electrodes

In this work, to improve the sensitivity of lateral field excited (LFE) sensors to changes of liquid conductivity, LFE bulk acoustic wave sensors with interdigital electrodes are investigated. LFE bulk acoustic wave on thickness-shear mode is excited successfully by interdigital electrodes on (yxl)88° LiTaO3 and applied in liquid-phase sensor. The electric field direction of LFE (yxl)88° LiTaO3 plates is determined. Based on this, several LFE bulk acoustic wave sensors with interdigital electrodes are designed and fabricated to increase the sensitivity of LiTaO3 LFE sensors. The results show that the (yxl)88° LiTaO3 LFE sensor with interdigital electrodes is 10.9 times and 2.1 times more sensitive to changes in liquid conductivity compared to traditional LFE devices with single gap circular electrodes and Archimedes spiral electrodes, respectively. The results are important for investigating high-sensitivity LFE bulk acoustic wave sensors by using LiTaO3 crystal.

Sensitivity Analysis of Lateral Field Excited Acoustic Wave Gas Sensors with Finite Element Method

In the last decade, there are increasing investigations on lateral field excited (LFE) acoustic wave sensors in biochemical liquid sensing applications due to their high sensitivity and simple fabrication. However, the research on this kind of sensor for gas detection is still awaited. This paper presents a theoretical modeling of the LFE acoustic wave gas sensor with a nanostrustured selective film for the first time. We developed this model by adopting a finite element software, COMSOL. Besides the eigenfrequency and frequency-response analyses, the sensitivities to the variations of mass density and electrical conductivity of the selective film caused from gas concentration were calculated. In the meantime, quartz crystal microbalance (QCM) sensors were also analyzed for comparison. Finally, the effect of geometry of the LFE gas sensor on the sensitivity was discussed. Results show that the LFE sensor exhibits larger sensing range and higher sensitivity to external electrical variation than the QCM sensor. This is because no shielding electrode exists on sensing surface of the LFE sensor, and hence the electric field can penetrate into the selective film. The simulation results provide useful guidelines for designing LFE acoustic wave gas sensors.

Lateral field excited Y-cut langasite bulk acoustic wave sensor

Previous studies have shown that lateral field excited (LFE) devices on AT-cut quartz are sensitive to liquid electrical property changes. AT-cut quartz LFE devices have low piezoelectric coupling factors. To further improve electrical sensitivities of LFE sensors, it is necessary to study LFE sensors based on other piezoelectric materials with higher coupling factors. In this paper, LFE sensors on Y-cut langasite, which has twice the higher piezoelectric coupling factor of AT-cut quartz, are investigated. Several Y-cut langasite LFE sensors are designed, fabricated, and tested. The experimental results show that the devices are over 6 times more sensitive to liquid conductivity changes and about 2.5 times more sensitive to liquid permittivity changes compared to AT-cut quartz LFE devices. It was also found that the Y-cut langasite LFE devices are about 1.3 times more sensitive to changes in liquid acoustic viscosity compared to the AT-cut quartz LFE devices.

Optimization of the lateral field excited platform for liquid sensing applications

The Lateral Field Excited (LFE) platform is sensitive to mechanical and electrical property changes occurring in adjacent media. Using the LFE sensor responses have been analyzed for the piezoelectric resonator materials alpha-quartz, lithium tantalate (LiTaO3), and lithium niobate (LiNbO3). Since the impact of the mechanical load parameters on piezoelectric resonators is governed by the piezoelectric coupling factor, LiTaO3 and LiNbO3 are well suitable for operation in highly viscous environments. This benefit is achieved at the cost of a reduced sensitivity to the mechanical and electrical properties of the medium. A second approach proposes a new 3-electrode configuration. The response of this LFE sensor design to low electric permittivity has been modeled using a novel approach to simulate the low permittivity boundary conditions. An enhanced sensitivity of the sensor has been achieved.

Recent advances in lateral field excited and monolithic spiral coil acoustic transduction bulk acoustic wave sensor platforms

The quartz crystal microbalance (QCM) has been used extensively as a bulk acoustic wave (BAW) platform for applications such as chemical and biological sensors and rate monitors in thin film deposition systems. Although the QCM is capable of measuring mechanical property changes critical in many thin film deposition systems, it cannot measure electrical property changes that can occur in many sensor applications. In this paper we review the recent developments of two novel transducer configurations for BAW sensors. In the first sensor, called the lateral field excited (LFE) sensor, the transverse shear mode (TSM) in AT-cut quartz is excited by two electrodes on the reference surface, resulting in a bare sensing surface which allows both electrical and mechanical properties of target analytes to be measured. In the second sensor, called the monolithic spiral coil acoustic transduction (MSCAT) sensor, the TSM is excited by a photolithographically deposited spiral antenna on the reference surface which can excite high-order harmonics in the substrate, and potentially lead to increased sensitivity. The responses of both the LFE and MSCAT sensors to electrical and mechanical property changes of liquids have been examined and compared to the response of the standard QCM. In addition, results relating to the detection of chemical and biological target analytes using the LFE and MSCAT sensor platforms are presented.

A lateral-field-excited LiTaO3high-frequency bulk acoustic wave sensor

The most popular bulk acoustic wave (BAW) sensor is the quartz crystal microbalance (QCM), which has electrodes on both the top and bottom surfaces of an AT-cut quartz wafer. In the QCM, the exciting electric field is primarily perpendicular to the crystal surface, resulting in a thickness field excitation (TFE) of a resonant temperature compensated transverse shear mode (TSM). The TSM, however, can also be excited by lateral field excitation (LFE) in which electrodes are placed on one side of the wafer leaving a bare sensing surface exposed directly to a liquid or a chemi/bio selective layer allowing the detection of both mechanical and electrical property changes caused by a target analyte. The use of LFE sensors has motivated an investigation to identify other piezoelectric crystal orientations that can support temperature-compensated TSMs and operate efficiently at high frequencies resulting in increased sensitivity. In this work, theoretical search and experimental measurements are performed to identify the existence of high-frequency temperature-compensated TSMs in LiTaO(3). Prototype LFE LiTaO(3) sensors were fabricated and found to operate at frequencies in excess of 1 GHz and sensitively detect viscosity, conductivity, and dielectric constant changes in liquids.

Advanced application of the impedance spectrum of a lateral field excited sensor

An AT-cut quartz crystal in lateral field excitation has been theoretically and experimentally investigated. The effect of a z-directed electrical field in the plane of the quartz crystal has been studied and conclusions regarding the generated mechanical stresses are made. Further, the influence of external conditions on the excitation mechanism has been discussed. Measurements are performed on the lateral field excited (LFE) sensor exposed to deionised (DI) water and 2-propanol–water-mixtures of different concentrations. It is shown that the impedance spectrum is mainly dependent on electrical material parameters instead of mechanical material parameters. The impedance spectrum in the presence of a shielding electrode connected to ground potential allows the discussion of changes in the field distribution without adding an acoustic load onto the sensor.

Pesticide detection using a lateral field excited acoustic wave sensor

Although organophosphate pesticides have been widely used in agriculture to control pests, they can have adverse effects on the environment and also enter the food chain causing adverse health effects in humans. Therefore, a need exists to develop a low-cost, robust, sensitive, and portable sensor for organophosphate pesticides. A lateral field excited (LFE) acoustic wave sensor has been developed in which the sensing surface is bare and the external excitation is applied to the reference surface. The LFE sensor is sensitive to both mechanical and electrical property changes in the monitored solution. In the present work, the LFE sensor is used as the sensing platform to detect the organophosphate pesticide, phosmet (C 11H 12NO 4PS 2). A polyepichlorohydrin (PECH) polymer film is applied to the sensing surface to detect phosmet in solution. The results show that the PECH-coated LFE sensor displays a nearly linear response to the addition of phosmet with a slope of 20 Hz/ppm and detection limit in the ppb range.

A lateral field excited liquid acoustic wave sensor

Lateral field excited (LFE) AT-cut quartz acoustic wave sensors in which the electrodes are located on the reference surface have been fabricated and tested in liquid environments. The sensing surface, which is opposite to the reference surface, is free allowing the electric field of the thickness shear mode (TSM) to penetrate into the liquid. This results in increased sensitivity to both mechanical and electrical property changes of the liquid. In the present paper, several 5-MHz LFE sensors with a range of electrode spacings were exposed to liquid environments in which the viscosity, relative permittivity, and conductivity were varied. The LFE sensors demonstrate sensitivity to viscosity that is more than twice that obtained for the standard quartz crystal microbalance (QCM), and sensitivity to relative permittivity and conductivity about 1.5 times that of the QCM sensors with modified electrodes. The present results clearly indicate that the LFE sensors may have a wide range of liquid phase applications in which sensitivity is crucial.