Thickness Mode Resonance Research Articles

GHz Ultrasound and Electrode Chip-Scale Arrays Stimulate and Influence Morphology of Human Neural Cells.

This study describes the effects of chip-scale gigahertz (GHz) ultrasound (US) and electrical stimulus on the morphology, functionality, and viability of neural cells in vitro. The GHz frequency stimulation is achieved using aluminum nitride piezoelectric transducers fabricated on a silicon wafer, operating at 1.47 GHz, corresponding to the film's thickness mode resonance. These devices are used to stimulate SH-SY5Y neural cells in vitro and observe effects on the morphology and viability of the stimulated cells. It is possible to use these devices to deliver either ultrasonic stimulus alone or US stimulus in conjunction with electrical stimulus. Viability tests demonstrated that the neurons retained structural integrity and viability across a wide range of GHz US stimulus intensities (0-1.2 W/cm2), validating that measurements occur at nontoxic doses of US. Neural stimulation is validated with these devices following the outputs of a previous study, with the normalized fluorescence intensity of activated cells between 1.9 and 2.4. The 300-s ultrasonic stimulation at 1.47 GHz and 0.05 W/cm2 peak intensity led to a decrease in nuclear elongation by 17.5% and a cross-sectional area decrease by 17.8% across three independent trials of over 150 cells per category ( ). The F-actin governed cellular elongation increased in length by up to 16.3% in cells exposed to an ultrasonic stimulus or costimulus ( ). Neurite length increased following ultrasonic stimulation compared with control by 75.8% ( ). This article demonstrates new GHz US and electrical chip-scale arrays with apparent effects in both neural excitation and cell morphology.

Progress towards wafer-scale fabrication based on gel casting technique for 1–3 randomised piezocomposite μUS linear array

Microultrasound (μUS) linear arrays operating at frequencies over 25 MHz have applications in high resolution biomedical imaging. 1–3 connectively piezoceramic – polymer composite (“piezocomposite”) material is attractive for fabrication of these devices due to its high effective electromechanical coupling coefficient and low acoustic impedance for better acoustic matching between transducer and tissue. However, a major concern with this type of material comes from interference between the fundamental thickness-mode resonance and spurious modes, which is usually generated by wave propagation and reflection within the repetitive and symmetrical structure of classical piezocomposite. In general, a fine spatial scale is required of the material structure to suppress the spurious modes; however, the fabrication process is challenging using standard dice-and-fill methods at the fine scales required for high frequencies. A promising way to overcome this challenge is to manipulate the lateral geometry and spacing of the piezoceramic pillars with a random distribution. In this work, gel casting in association with a micromoulding technique has been developed for manufacturing 1–3 randomised piezocomposite active material for μUS linear arrays. 48 vol% solid loading of piezoceramic powder with 30 wt% Hydantoin resin content was employed to prepare a low viscosity aqueous suspension. Through varying powder size, it was found that the suspension with 1.22 µm powder had the highest viscosity, ~ 0.47 Pa.s, and a short gelation time, ~ 10 mins. However, all suspensions had viscosities less than 1 Pa.s at a shear rate of 100 s−1, indicating that they had good flowability. The green body samples showed mean flexural strength 49.7 ± 2.49 MPa. After piezocomposite fabrication with randomised pillars, surface planarisation was used to obtain reliable edge definition of photolithographically-defined electrodes. 20-element arrays with 50-μm element pitch were configured using a bilayer lift-off process. The 1–3 randomised piezocomposite demonstrated its capability to minimise the effects of spurious modes in the thickness mode frequency range, while the thickness resonances provided k33 = 0.67. Without a matching layer, the array produced a − 6 dB bandwidth of 38.4%- and − 20-dB pulse length of 0.26 μs. These results show that 1–3 randomised piezocomposite fabricated from gel-casting associated with a micromoulding technique is feasible for fabrication of μUS linear arrays and may offer a route to small wafer-scale production.

Ultrasonic Transducers Made From Freeze-Cast Porous Piezoceramics.

Porous and composite piezoelectric ceramics are of interest for underwater ultrasonic transducers due to their improved voltage sensitivity and acoustic matching with water, compared with their dense counterparts. Commonly, these materials are fabricated by dice-and-fill of sintered blocks of polycrystalline piezoceramic, which results in a high volume of waste. The freeze-casting technique offers a low waste and scalable alternative to the dice-and-fill method to produce porous piezoceramics with highly orientated, anisometric pores. In this article, we have fabricated underwater ultrasonic transducers from freeze-cast lead zirconate titanate (PZT) with a range of porosities. The porous PZT samples were characterized in terms of their piezoelectric and dielectric properties before being encapsulated for acoustic performance testing in water. Off resonance, the on- axis receive sensitivity of the manufactured devices was approximately [Formula: see text]; the transmit voltage response (TVR) was in the range of approximately [Formula: see text] at 60 kHz to [Formula: see text] at 180 kHz. The most porous transducer devices (0.51, 0.43, and 0.33 pore fraction) exhibited primarily a thickness mode resonance, whereas the least porous transducers (0.29 pore fraction and dense benchmark) exhibited an undesired radial mode, which was observed as an additional resonant peak in the electrical impedance measurements and lateral off-axis lobes in the acoustic beampatterns. Our results show that the acoustic sensitivities and TVRs of the porous freeze-cast transducers are comparable to those of a dense pressed transducer. However, the freeze-cast transducers with porosity exceeding 0.30 pore fraction were shown to achieve an effective structure with aligned porosity that suppressed undesired radial mode resonances.

A Film Bulk Acoustic Resonator Based on Ferroelectric Aluminum Scandium Nitride Films

This work reports on the first demonstration of the frequency tuning and intrinsic polarization switching of film bulk acoustic resonators (FBARs), based on sputtered AlScN piezoelectric thin films with Sc/(Al + Sc) ratio of approx. 30%. A box-like ferroelectric hysteresis behavior of 900 nm-thick Al0.7Sc0.3N sputtered films is obtained, showing a coercive electric field at ~3 MV/cm. The fundamental thickness-mode resonance of the bulk acoustic wave (BAW) resonator is measured at 3.17 GHz frequency with an excellent electromechanical coupling coefficient ( $k_{t}^{2}$ ) of 18.1%. The FBAR frequency response is studied, in both (i) the linear tuning regime, upon application of DC electric fields below the coercive field; as well as (ii) the polarization switching regime, upon application of electric fields above the coercive field. A large linear tuning range of 215 ppm $\times \,\,\mu \text{m}$ /V is obtained in case (i), resulting from the high scandium content. The series resonance frequency of the FBARs is switched ON and OFF in (ii) upon application of 350 V unipolar waveform across the Al0.7Sc0.3N thickness. This is the first demonstration of the intrinsically switchable AlN-based FBARs with a large tuning range; and record high $k_{t}^{2}$ reported for AlN-based FBARs to date. Furthermore, this work paves the way for realization of tunable and switchable wideband acoustic filters operating at super high frequency ranges (SHF). [2020-0203]

Over 10% of ${k}_{\text{eff}}^{{2}}$ Demonstrated by 2-GHz Spurious Mode-Free Sc0.12Al0.88N Laterally Coupled Alternating Thickness Mode Resonators

In this letter, a group of laterally coupled alternating thickness (LCAT) mode resonators based on 12% scandium-doped aluminum nitride (Sc <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.12</sub> Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.88</sub> N) are reported. The LCAT mode resonators are realized in-house by sandwiching a layer of 1-μm-thick Sc <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.12</sub> Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.88</sub> N between the top two sets and the bottom two sets of interdigitated electrode (IDE) fingers, all made of molybdenum (Mo). The experimental results show that an effective electromechanical coupling coefficient (k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) of more than 10% and loaded quality factors (QL) as high as 870 are obtained, while the motional resistance of the resonators is kept as low as 0.68 Ω. The figure-of-merit (FoM) which is defined as k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> QL and phase velocity (vp) is subsequently derived to be 87 and 7840 m/s, respectively. By changing the pitch of the IDE fingers, the resonators can operate from 1.96 GHz to 2.11 GHz without significant perturbation to their performance, corresponding to a lithographic frequency tuning range of 7.5%. In addition, clean spectra without any spurious modes are obtained within a 1-GHz frequency range for all fabricated devices of various pitches, making a significant step forward in the commercialization path of the reported resonators in the applications of the highband selection filters integrated on a single chip for mobile communication.

Propagation and scattering of Lamb waves with infinite wavelength

The dispersion characteristics of Lamb waves depend on the thickness and the elastic properties of a plate. At the cut-off frequencies (when the wavenumber goes to zero), Lamb waves are standing waves associated with plane longitudinal or shear waves reflecting up and down between the surfaces of the plate at normal incidence. In the general case, these thickness resonances have an infinite wavelength and do not carry energy along the plate. However, if the material properties are selected such that longitudinal and shear thickness mode resonances of the same symmetry coincide, then the elastic fields associated with the resonances couple to produce a wave with an infinite wavelength that propagates energy along the plate surface. Intriguing effects associated with this phenomenon can be observed in aluminum alloy plates where near-degeneracy between resonances occurs. We show that waves generated near the cutoff frequency in such cases spread from the excitation point and produce a spatially uniform oscillation over the plate surface. These infinite wavelength waves show angle independent mode conversion upon encountering the free edge of a plate or thickness step on the surface. Furthermore, we demonstrate that these peculiar waves flow around plate defects and are remarkably insensitive to scattering.

Performance Evaluation of CMUT-Based Ultrasonic Transformers for Galvanic Isolation.

This paper presents the development of a novel acoustic transformer with high galvanic isolation dedicated to power switch triggering. The transformer is based on two capacitive micromachined ultrasonic transducers layered on each side of a silicon substrate; one is the primary circuit, and the other is the secondary circuit. The thickness mode resonance of the substrate is leveraged to transmit the triggering signal. The fabrication and characterization of an initial prototype is presented in this paper. All experimental results are discussed, from the electrical impedance measurements to the power efficiency measurements, for different electrical load conditions. A comparison with a specifically developed finite-element method model is done. Simulations are finally used to identify the optimization rules of this initial prototype. It is shown that the power efficiency can be increased from 35% to 60%, and the transmitted power can be increased from 1.6 to 45 mW/Volt.

Resonance analysis of a high temperature piezoelectric disc for sensitivity characterization

Ultrasonic transducers for high temperature (200 °C+) applications are a key enabling technology for advanced nuclear power systems and in a range of chemical and petro-chemical industries. Design, fabrication and optimization of such transducers using piezoelectric materials remains a challenge. In this work, experimental data-based analysis is performed to investigate the fundamental causal factors for the resonance characteristics of a piezoelectric disc at elevated temperatures. The effect of all ten temperature-dependent piezoelectric constants (ε33, ε11, d33, d31, d15, s11, s12, s13, s33, s44) is studied numerically on both the radial and thickness mode resonances of a piezoelectric disc. A sensitivity index is defined to quantify the effect of each of the temperature-dependent coefficients on the resonance modes of the modified lead zirconium titanate disc. The temperature dependence of s33 showed highest sensitivity towards the thickness resonance mode followed by ε33, s11, s13, s12, d31, d33, s44, ε11, and d15 in the decreasing order of the sensitivity index. For radial resonance modes, the temperature dependence of ε33 showed highest sensitivity index followed by s11, s12 and d31 coefficient. This numerical study demonstrates that the magnitude of d33 is not the sole factor that affects the resonance characteristics of the piezoelectric disc at high temperatures. It appears that there exists a complex interplay between various temperature dependent piezoelectric coefficients that causes reduction in the thickness mode resonance frequencies which is found to be agreement in with the experimental data at an elevated temperature.

Conical dispersion of Lamb waves in elastic plates

Guided elastic waves in homogeneous isotropic plates exhibit conical dispersion at zero wave number for discrete values of Poisson's ratio where accidental degeneracy between longitudinal and transverse thickness resonances occur. Waves excited at the coincidence frequency have the infinite phase velocity associated with thickness mode resonances, but the group velocity remains finite. This leads to propagating waves that oscillate in time but are spatially uniform over the plate surface. Here, accidental degeneracy is induced in an aluminum plate by cooling the plate to tune the Poisson's ratio through the degenerate point. We measure linear dispersion in the transition from forward to backward propagating waves near zero wave number. In addition, we demonstrate that waves generated near the coincidence frequency exhibit spatially uniform phase over the plate surface, and show angle independent mode conversion upon encountering the free edge of the plate. We propose that elastic plates offer a simple system to explore the physics of conical dispersion at zero wave number, and that this phenomenon may find application in acoustic devices and nondestructive testing.

ScAlN-Based LCAT Mode Resonators Above 2 GHz With High FOM and Reduced Fabrication Complexity

This letter reports the first implementation of a new class of laterally coupled alternating thickness (LCAT) mode resonators operating above 2 GHz, realised by replacing the bottom interdigitated electrodes of the conventional LCAT mode resonator with an electrically floating plate. Besides saving one mask process and relaxing the tight resolution and alignment requirements for defining small features, the flat unpatterned bottom electrode metal is also more favorable for the growth of highly oriented piezoelectric thin film, which results in an improved resonator quality factor (Q). A group of 8% scandium doped aluminium nitride-based modified LCAT mode resonators with frequency difference over 100 MHz by varying the top interdigitated electrode pitches are fabricated achieving coupling coefficient (k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) of up to 6.45% and figure of merit of up to 54, making them promising candidates for integrated band selection filter banks.

Enhancing the bandwidth of piezoelectric composite transducers for air-coupled non-destructive evaluation

This paper details the development of a novel method for increasing the operational bandwidth of piezocomposites without the need for lossy backing material, the aim being to increase fractional bandwith by geometrical design. Removing the need for lossy backing materials, should in turn increase the transmit efficiency in the desired direction of propagation. Finite element analysis has been employed to determine the mode of operation of the new piezocomposite devices and shows good correlation with that derived experimentally. Through a series of practical and analytical methods it has been shown that additional thickness mode resonances can be introduced into the structure by a simple machining process. The shaped composites described in this paper offer increased operational bandwidth. A simple example of a two step thickness design is described to validate and illustrate the principle. A more complex conical design is presented that illustrates a possible tenfold increase in bandwidth from 30kHz to 300kHz, operating in air without backing. An illustration of the applicability of this type of transducer technology for frequency agile guided mode non-destructive evaluation is then presented.

Underwater characterizations of monolithic piezoceramic and 1–3 composite using a self-designed transducer

Underwater characterizations of (Pb 0.94 Sr 0.04 )(Zr 0.52 Ti 0.48 )O 3 (PZT) and PZT/araldite-F 1–3 composite were carried out through a self-designed transducer. Disc-shaped samples of bulk PZT and PZT/araldite-F composite were first characterized in air and then were assembled in the transducer individually. The transducer’s underwater voltage receiving sensitivity ( S h ) and transmitting voltage response ( S v ) were investigated in the frequency range of 10–200 kHz (well below thickness mode resonance) using a calibrated projector and receiver method with pulse technique. Results revealed that the transducer made with composite sample exhibited better ( S h ) values (−214 dB ref 1 V/ µ Pa) due to ~295% higher piezoelectric voltage coefficient g h (30 × 10 −3 Vm/N) of the composite compared to PZT. In addition, the transducer with the PZT sample showed better S v values (80 dB ref 1 µ Pa/1 V at 1 m) due to the presence of planar mode peaks in the frequency range of 10–200 kHz. These results indicate that the monolithic piezoceramic can exhibit underwater S v response in both planar and thickness resonance modes owing to the admittance peaks in these frequency regions.

Investigation of inorganic fillers on properties of 2–2 connectivity cement/polymer based piezoelectric composites

The 2–2 cement/polymer based piezoelectric composites containing inorganic fillers were prepared. The influences of filler content and composite thickness on the composite properties were investigated. The results show that graphite powder shows larger effect on dielectric property of the composites than silicon powder and strontium ferrite (Sr-ferrite) powder, especially when graphite powder is larger than 10wt.%. The Sr-ferrite can improve the thickness electromechanical coupling properties of the composites. When Sr-ferrite powder content is 15wt.%, the composite has a kt value of 68.6% at a 2mm thickness. The electromechanical quality factor (Qm) of the composites shows fluctuation variation between 10 and 60 with increasing the composite thickness, which is far less than that of the pure ceramic. The coupling resonance effects between planar mode and thickness mode of the composites was reduced by decreasing the composite thickness, and the pure thickness mode resonance can be obtained at a small composite thickness.

Stress- and temperature-compensated orientations for thickness-shear langasite resonators for high-temperature and high-pressure environment.

This paper describes an exhaustive study of the variations of the mean force sensitivity coefficients in the entire region of crystalline langasite (LGS). We also study the variation of temperature coefficients in the entire region of the crystalline LGS and its isomorphs. The computational results have been obtained from a procedure that has been successfully employed in the study of the planar and temperature stressinduced frequency shifts in thickness-mode resonators. Both the fast and slow thickness-shear modes have been studied. Among other things, the loci of orientations with zero stress and temperature coefficients of frequency have been identified for LGS.

Dielectric and electromechanical properties of modified cement/polymer based 1–3 connectivity piezoelectric composites containing inorganic fillers

Abstract Here the modified cement/polymer based 1–3 piezoelectric composites containing inorganic fillers were prepared. The influences of filler content and composite thickness on dielectric and electromechanical properties of the composites were investigated. The results indicate that the composites with fillers of graphite and strontium ferrite (Sr-ferrite) have larger static relative dielectric constant and smaller dielectric loss than those with silicon powder. The strontium ferrite can improve the thickness electromechanical coupling property of the composites. When the strontium ferrite content is 5 wt.% of the cement/polymer matrix, the piezoelectric composite has the largest thickness electromechanical coupling coefficient of 64.27% and the smallest mechanical quality factor 8.64. The graphite filler shows a certain suppression effect on the planar resonance of the composites. The coupling effects between the planar and thickness modes of the composites were reduced by decreasing the composite thickness, and pure thickness mode resonance can be obtained at a small composite thickness.

GaN Micromechanical Resonators with Meshed Metal Bottom Electrode.

This work describes a novel architecture to realize high-performance gallium nitride (GaN) bulk acoustic wave (BAW) resonators. The method is based on the growth of a thick GaN layer on a metal electrode grid. The fabrication process starts with the growth of a thin GaN buffer layer on a Si (111) substrate. The GaN buffer layer is patterned and trenches are made and refilled with sputtered tungsten (W)/silicon dioxide (SiO2) forming passivated metal electrode grids. GaN is then regrown, nucleating from the exposed GaN seed layer and coalescing to form a thick GaN device layer. A metal electrode can be deposited and patterned on top of the GaN layer. This method enables vertical piezoelectric actuation of the GaN layer using its largest piezoelectric coefficient (d33) for thickness-mode resonance. Having a bottom electrode also results in a higher coupling coefficient, useful for the implementation of acoustic filters. Growth of GaN on Si enables releasing the device from the frontside using isotropic xenon difluoride (XeF2) etch and therefore eliminating the need for backside lithography and etching.

A Temperature-Compensated Gallium Nitride Micromechanical Resonator

A GaN bulk acoustic wave resonator is presented in this letter, showing fundamental thickness-mode resonance at 2.18 GHz, with a quality factor ( Q ) of 655 and a coupling coefficient ( \({k_{t}}^{2}\) ) of 1%. The resonator is integrated with an AlGaN/GaN high electron mobility transistor (HEMT); the integrated resonator/HEMT structure is coated with a silicon dioxide (SiO2) passivation layer. It is shown that a 400-nm-thick SiO2 layer reduces the temperature coefficient of frequency (TCF) of the GaN-based resonator by >50%, while improving Q and \({k_{t}}^{2}\) of the fundamental thickness-mode resonance. The effect of SiO2 passivation layer is studied on \({k_{t}}^{2}\) , Q , and TCF of the device. Furthermore, the effects of temperature and input RF power on the resonator performance are characterized.

Significant shear mode softening in a c-axis tilt nanostructured hexagonal thin film induced by a self-shadowing effect

The mechanical properties of tilted AlN fiber structure caused by self-shadowing are investigated using gigahertz vibration induced through the piezoelectricity of AlN itself. We observed significant shear elastic softening in a highly tilted fiber nanostructure, but only a slight decrease in compressive elasticity. Abnormal 1/4 wavelength thickness mode resonance, indicating the decrease in the shear acoustic wave velocity, is reproducibly observed. We attribute this phenomenon to the weak fiber-to-fiber connection seen in a cross-sectional scanning electron microscopy image.

Experimental measurements of the force-frequency effect of thickness-mode langasite resonators.

Because of their excellent temperature behavior, high piezoelectric coupling, low acoustic loss, and high Q-factor, langasite resonators have been the subject of recent interest for use in a variety of applications. The force-frequency effect refers to the phenomenon of frequency changes resulting from the stress applied to the resonator. A clear understanding of this effect is essential for many design applications such as force sensors and stress-compensated resonators. In this article, we report on experimental measurements of the force- frequency effect of various doubly-rotated langasite resonator samples with plano-plano configurations. Comparisons with the available experimental data for the force-frequency effect of quartz resonators are made. The application of this effect for sensors and stress-compensated resonators is also discussed.

Experimental measurement of the electroelastic effect in thickness-mode langasite resonators

The electroelastic effect describes the shift in resonant frequency that a resonator experiences as a result of the application of a dc electrical field. We report on experimental measurements of the electroelastic effect observed in fourteen plano-plano configuration thickness-mode langasite (La3Ga5SiO14) resonators. The orientations of the fourteen samples provide a sufficient data set to extract all eight of the third-order piezoelectric constants of this material. The role of this type of measurement in determining third-order piezoelectric constants is discussed. We compare the experimentally observed behavior to that predicted when using the langasite material constants currently available in the literature.