High Electromechanical Coupling Research Articles

Compressive stress reduction in sputter-deposited yttrium aluminum nitride (Y0.2Al0.8N) thin films for BAW resonators with high electromechanical coupling

In the past decade, rare earth elements have found increasing interest in enhancing the piezoelectric coefficient of aluminum nitride (AlN) thin films. Until now, scandium has been the most successful rare earth element to enhance the piezoelectric coefficient (d33) up to 600 %. In this research paper, we present a 250 % increase in d33 to 12.5 pC/N for sputter-deposited yttrium alloyed aluminum nitride (Y0.2Al0.8N) thin films compared to pure AlN. However, this increase in d33 comes with a substantial increase in compressive layer stress well above 1 GPa. Therefore, a chamber pressure sweep technique is introduced, resulting in a 40 % reduction in the stress level without significantly affecting the crystallographic film parameters to facilitate device integration. Even more, high compressive stress in thin films is a critical concern in piezoelectric MEMS devices such as bulk acoustic wave (BAW) resonators, as it can adversely affect the piezoelectric material properties and hence, the electromechanical performance. For this purpose, we demonstrate that the pressure sweep technique improves the electromechanical coupling factor of Y0.2Al0.8N to 7.02 %, being substantially above the reference value of 6.02 % set by pure AlN. In summary, the findings from this study may stimulate further effort to integrate YxAl1-xN thin films in BAW filter devices for future telecommunication applications.

3.2 GHz first shear horizontal-mode plate wave resonator on a 175°YX LiNbO3 thin plate showing 24.6% bandwidth

In this study, a first shear horizontal (SH1) mode plate wave resonator for HF and wide-band filters was investigated. From FEM simulation, the SH1 mode plate wave exhibited a high phase velocity of 23 km s−1 and a high electro-mechanical coupling factor (k 2) of 37% on (0°, 85°, 0°) LiNbO3 (LN) with a thickness of 0.1λ (λ: wavelength). The SH1 mode resonator was fabricated on a 0.5–0.57 μm thick LN film. An 80 nm Al IDT electrode was formed with λ varied from 4–16 μm. Finally, 50 nm thick Al was deposited to form an electrically short bottom plane, which was required to generate a SH1 mode wave. The fabricated device exhibited a resonance frequency (f r) of 2.24 GHz, an anti-resonance frequency (f a) of 2.8 GHz, a wide fractional bandwidth (FBW) of 20%, an impedance (Z) ratio of 35.4 dB, and TCF of −90.8 ppm/°C at f r and −74.8 ppm/°C at f a.

0.36BiScO3-0.64PbTiO3/Epoxy 1-3-2 High-Temperature Composite Ultrasonic Transducer for Non-destructive Testing Applications.

The development of high-temperature nondestructive testing (NDT) requires ultrasonic transducers with good temperature resistance and high sensitivity for improved detection efficiency. Piezoelectric composite can improve the performance of transducers because of its high electromechanical coupling coefficient and adjustable acoustic impedance. In this study, 1-3-2 composites and 1-3-2 high-temperature composite ultrasonic transducers (HTCUTs) based on 0.36BiScO3-0.64PbTiO3 (BSPT), which are preferred piezoelectric materials at 200°C-300°C, and high-temperature epoxy with a center frequency of 6 MHz were designed and fabricated. From 25°C to 250°C, 1-3-2 composites show a higher electromechanical coupling coefficient kt especially at high temperatures (~0.53 at 25°C, and ~0.64 at 250°C) than monolithic BSPT (~0.5). The signal of the pulse-echo response of 1-3-2 HTCUTs is distinguishable up to 250 °C and remains stable (Vpp~500 mV) below 150°C, exhibiting higher sensitivity (improved by 7 dB) than that of monolithic BSPT high-temperature ultrasonic transducers (HTUTs). Bandwidth has been greatly enhanced especially at high temperatures (~103250°C) compared with that of monolithic BSPT HTUTs(~30250°C). To verify the excellent performance, B-mode scanning imaging measurement of a stepped steel block and defect location detection of a steel block were performed, showing the potential for high-temperature NDT applications.

Recent progress in piezoelectric thin films as self-powered devices: material and application

Piezoelectric materials have become a key component in sensors and actuators in many industrial fields, such as energy harvesting devices, self-powered structures, biomedical devices, nondestructive testing, owing to the novel properties including high piezoelectric coefficient and electromechanical coupling factors. Piezoelectric thin films integrated on silicon substrates are widely investigated for their high performance and low manufacturing costs to meet the requirement of sensor networks in internet of things (IoT). The aim of this work is to clarify the application and design structure of various piezoelectric thin films types, synthesis methods, and device processes. Based on latest literature, the process of fabricating thin film sensors is outlined, followed by a concise overview of techniques used in microelectromechanical systems (MEMS) processing that can integrate more complex functions to obtain relevant information in surrounding environment. Additionally, by addressing piezoelectric thin films sensors as a cutting-edge technology with the ability to produce self-powered electronic devices, this work delivers incisive conclusions on all aspects of piezoelectric sensor related features. A greater understanding of piezoelectricity is necessary regarding the future development and industry challenges.

Analytically assisted FEM approach for the design and optimization of laterally excited bulk acoustic wave resonators (XBARs) with a high electromechanical coupling

Analytically assisted FEM approach for the design and optimization of laterally excited bulk acoustic wave resonators (XBARs) with a high electromechanical coupling

Coupled Shear SAW Resonator With High Electromechanical Coupling Coefficient of 34% Using X-Cut LiNbO₃-on-SiC Substrate

Coupled Shear SAW Resonator With High Electromechanical Coupling Coefficient of 34% Using X-Cut LiNbO₃-on-SiC Substrate

Effects of heterovalent ions doping-induced oxygen octahedral distortion and defect chemical change on piezoelectric characteristics and thermal stability of PHT-PIN ceramics

Electromechanical devices require functional materials possessing excellent piezoelectric properties, good thermal stability, and high electromechanical coupling coefficients. To modulate the piezoelectric, dielectric, and electromechanical characteristics of 0.89Pb(Hf0.47Ti0.53)O3-0.11Pb(In1/2Nb1/2)O3 (PHT-PIN) piezoelectric ceramics, heterovalent ions of lithium (Li) and tungsten (W) were doped through a traditional solid-phase reaction approach. On the one hand, large lattice distortions caused by the doped W6+ resulted in the shortening of BO bonds while an increase in the local displacement of B-site ions. Moreover, a mismatch occurred for the oxygen octahedral dimensions, and a subsequent disruption of the long-ranged ordered ferroelectric domains was observed, which resulted in the formation of microdomains. On the other hand, the changes in the concentration of oxygen vacancy defects induced by the Li and W doping inside the ceramic were comprehensively analyzed. The decrease in the concentration of oxygen vacancies was found to be accompanied by the enhancement of piezoelectric properties of the material. Excellent comprehensive performance (piezoelectric coefficient d33 = 655 pC N−1, Curie temperature Tc = 321.1 °C, dielectric constantεr = 3352.31, electromechanical coupling coefficient kp = 0.67, and residual polarization strength Pr = 55 μC cm−2) was obtained for 0.2 mol % doping amount of Li+ and W6+ ions. This work, therefore, proposes a novel strategy for the origin of piezoelectric properties and inspires doping of other heterovalent ions to improve the piezoelectric properties of ceramics. It further promotes the development of Pb-based piezoelectric ceramics for electromechanical devices that can efficiently operate under high-temperature environments.

Miniature Ultrasound Transducer Incorporating Sm-PMN-PT 1-3 Composite

Piezoelectric 1-3 composite materials have become extensively utilized in diagnostic ultrasound transducers owing to their high electromechanical coupling coefficient, low acoustic impedance, and low dielectric loss. In this study, Sm-doped PMN-PT ceramic/epoxy 1-3 composite with a ceramic volume fraction of 60% is fabricated by the dice-and-fill method, resulting in a high piezoelectric constant (650 pC/N) and clamped dielectric constant (2350). Utilizing the exceptionally high clamped dielectric constant, a low-frequency (12.4 MHz) ultrasound transducer is developed with a miniature aperture size (0.84 mm × 0.84 mm), exhibiting a −6 dB bandwidth of 70% and an insertion loss of −20.5 dB. The imaging capability of the miniature composite transducer is validated through both phantom and ex vivo imaging. The satisfactory results indicate that Sm-doped ceramic/epoxy composites possess significant potential for miniature devices in biomedical imaging applications.

Integration of epitaxial LiNbO3 thin films with silicon technology

Development of bulk acoustic wave filters with ultra-wide pass bands and operating at high frequencies for 5th and 6th generation telecommunication applications and micro-scale actuators, energy harvesters and sensors requires lead-free piezoelectric thin films with high electromechanical coupling and compatible with Si technology. In this paper, the epitaxial growth of 36°Y-X and 30°X-Y LiNbO3 films by direct liquid injection chemical vapour deposition on Si substrates by using epitaxial SrTiO3 layers, grown by molecular beam epitaxy, has been demonstrated. The stability of the interfaces and chemical interactions between SrTiO3, LiNbO3 and Si were studied experimentally and by thermodynamical calculations. The experimental conditions for pure 36°Y-X orientation growth have been optimized. The piezoelectricity of epitaxial 36°Y-X LiNbO3/SrTiO3/Si films was confirmed by means of piezoelectric force microscopy measurements and the ferroelectric domain inversion was attained at 85 kV.cm−1 as expected for the nearly stoichiometric LiNbO3. According to the theoretical calculations, 36°Y-X LiNbO3 films on Si could offer an electromechanical coupling of 24.4% for thickness extension excitation of bulk acoustic waves and a comparable figure of merit of actuators and vibrational energy harvesters to that of standard PbZr1−x Ti x O3 films.

Electromechanical coupling of a 3.88 W harvester with circumferential step-size field: modeling, validation and self-powered wearable applications

The fast advances in wearable electronic devices require clean and wearable power sources. This study presents a wearable electromagnetic energy harvester (EMEH) with high output performance mounted on the knee to obtain human vibration energy. The design forms a circumferential step-change magnetic field with high electromechanical coupling for high-efficiency energy conversion. We first formulate a theoretical model and simulate the analytical voltage via MATLAB. To predict the output performance of the EMEH, we conduct simulations via ANSYS. Subsequently, experiments are conducted to explore the output performance of the harvester in terms of the voltage, the output power, and the charging rate. The prototype generates a peak power of 3.88 W with a 449 Ω resistor under the excitation of 2.0 Hz. Additionally, the prototype charges a battery to 33.9% within 300 s at a running speed of 8 km h−1. This study provides a new perspective for advancing the development of watt-level self-powered wearables.

Lithium Niobate MEMS Antisymmetric Lamb Wave Resonators with Support Structures.

The piezoelectric thin film composed of single-crystal lithium niobate (LiNbO3) exhibits a remarkably high electromechanical coupling coefficient and minimal intrinsic losses, making it an optimal material for fabricating bulk acoustic wave resonators. However, contemporary first-order antisymmetric (A1) Lamb mode resonators based on LiNbO3 thin films face specific challenges, such as inadequate mechanical stability, limited power capacity, and the presence of multiple spurious modes, which restrict their applicability in a broader context. In this paper, we present an innovative design for A1 Lamb mode resonators that incorporates a support-pillar structure. Integration of support pillars enables the dissipation of spurious wave energy to the substrate, effectively mitigating unwanted spurious modes. Additionally, this novel approach involves anchoring the piezoelectric thin film to a supportive framework, consequently enhancing mechanical stability while simultaneously improving the heat dissipation capabilities of the core.

Simulation and structural design of 2–2 PIMNT/epoxy piezoelectric composites

Piezoelectric materials are commonly used in transducers to convert electromechanical signals due to their energy conversion characteristics. We designed a PIMNT/epoxy 2–2 composite to take full advantage of the excellent beam-mode piezoelectric and acoustic features of PIMNT single crystals. Following the approach used for piezoelectric ceramic composites, we selected PIMNT piezoelectric single crystal and EPO-TEK301-2 epoxy resin for the composite, and combined finite element analysis with experimental preparation. We prepared, tested and analyzed 2–2 piezoelectric single crystal composites with varying volume fractions, which showed high electromechanical coupling properties ([Formula: see text]%) and low acoustic impedance ([Formula: see text][Formula: see text]MRayl). These encouraging findings suggest the possibility of devising high-performance ultrasonic transducers utilizing the PIMNT/epoxy 2–2 composite.

Design and Fabrication of Annular-Array Ultrasound Transducer Based (K, Na) NbO3 Lead-Free 1-3 Piezoelectric Composite.

This article reports the design, fabrication, and performance of an annular-array ultrasound transducer using (K, Na)NbO3 (KNN)-based lead-free 1-3 piezoelectric composite. The 1-3 piezoelectric composite based on lead-free 0.965(K0.45Na0.55)(Nb0.96Sb0.04)O3-0.035 Bi0.5Na0.5Zr0.85Hf0.15O3 (KNNS-BNZH) was first prepared with the dice-and-fill method. Different from previous reported studies, by patterning annular electrodes on this kind of 1-3 composite, a five-element annular-array transducer in diameter of 6 mm was designed and successfully fabricated and its working center frequency was increased from 5 MHz to around 15 MHz. Meanwhile, a -6 dB bandwidth of 50% was achieved for each annular-array element via a pulse-echo response measurement. Moreover, the obtained annular-array transducer exhibits low crosstalk (<-40 dB) and a high electromechanical coupling coefficient of around 0.7. The application of ultrasonic imaging was further demonstrated to show its excellent performance. Both theoretical analysis and experimental results show that the obtained annular-array transducer presented in this work has a competitive capability for ultrasonic application.

Characteristics of NaNbO3 templates and improvement of electrical properties of textured KNLNS-BNKT ceramics

In this study, plate-like perovskite NaNbO3 (NN) templates were formed from Bi2.5Na3.5Nb5O18 (BiNN5) precursors via a molten salt method. The structure and microstructure of BiNN5 precursors and NN templates were studied. As a result, the platelike BiNN5 precursor particles have a bismuth layer perovskite structure, which is the basis for forming the synthesized plate-like NN particles that have a high aspect ratio with a width of 2-7 μm, a length of 5-12 μm and a thickness of 0.5-1 μm. From NN templates, we fabricated textured lead-free 0.98(K0.48Na0.48Li0.04)(Nb0.95Sb0.05)O3 − 0.02Bi0.5(Na0.4K0.1)TiO3 (KNLNS- BNKT) ceramic by the grain orientation technique. Experimental results show that the KNLNS-BNKT ceramics have a single-phase orthorhombic structure in which textured KNLNS-BNKT ceramics with 67% grain orientation. The electrical properties of the oriented ceramic are significantly improved compared to the non-oriented ceramic fabricated by the traditional method. Specifically, the electrical properties of the resulting KNLNS-BNKT oriented ceramics have a high dielectric constant (ε = 1323, an increase of 17%); large remnant polarization (Pr = 13.5 mC/cm2, an increase of 11.9%); high piezoelectric coefficient (d33 = 205 pC/N, an increase of 32%) and high electromechanical coupling coefficient (kp = 0.4, an increase of 11%).

Frequency Response for Sweep-Up and Sweep-Down of Piezoelectric Energy Harvester

PurposeThe aim of this work is to study the frequency response of linear and nonlinear piezoelectric vibration energy harvester (pVEH) based on Duffing-type nonlinearity and Kirchhoff’s law, and the effect of the effective electromechanical coupling on the frequency response of nonlinear pVEH.MethodsThe analytical technique based on the harmonic balance method (HBM) is used to obtain the mechanical and electrical frequency responses of linear and nonlinear pVEH, and the Jacobian matrix is used to analyze the solutions’ stability. The Runge–Kutta fourth-order method as a numerical technique is used to solve the ordinary differential equations of pVEH.ResultsThe analytical calculations are completed, and the frequency response solution of displacement of pVEH is contained in real and complex values and those of electrical responses are presented. The harmonic balance results are compared with numerical simulations for various hardening nonlinear springs. Simple analytical expressions for the jump points are suggested for the sweep-up and sweep-down frequencies of pVEH.ConclusionsThe analytical analysis shows that the jump-down frequency ratios have definite behaviors for each low and high effective electromechanical coupling of hardening-nonlinear pVEHs. Moreover, the jump-up frequency ratio is directly proportional to the effective electromechanical coupling of hardening nonlinear pVEHs.

Influence of growth temperature on the properties of aluminum nitride thin films prepared by magnetron sputter epitaxy

High quality, uni-polar, epitaxial AlN with minimum oxygen content promises excellent surface acoustic wave and bulk acoustic wave resonator characteristics such as high electromechanical coupling coefficient and power handling capabilities, which is particularly useful for RF filter applications. By systematically varying the growth temperature, the study investigates its impact on the oxygen levels, defect states, and crystallographic texture of the AlN thin films using a combination of atomic force microscopy, X-ray diffraction, time-of-flight secondary ion mass spectrometry, spectroscopic ellipsometry, scanning transmission electron microscopy, as well as room temperature and temperature dependent I–V measurements. The research demonstrates that the films grown at a temperature of 700°C exhibit the most favorable results. These films exhibit the lowest oxygen levels, possess epitaxial growth, and display the highest crystalline quality (XRD AlN 0002 ω−FWHM=1.3°). Additionally, these films demonstrate a significant reduction in sub-bandgap absorption. By comparing the cathode current measured during deposition, we suggest that the presence of an impurity layer formed during idle time between depositions as a possible source of oxygen in the sputter chamber. In addition, the study presents a possible model to explain the mixed polarity observed in AlN and proposes various ways to achieve uni-polar AlN on silicon substrates.

High Electromechanical Coupling Coefficient of Longitudinally Excited Shear Wave Resonator Based on Optimized Bragg Structure.

In this work, a longitudinally excited shear-wave resonator (YBAR) based on single-crystalline lithium tantalate (LiTaO3, LT) thin film is proposed. The YBAR has a 200 nm X-cut thin film and molybdenum electrode. A high effective electromechanical coupling coefficient (k2eff) of up to 19% for the suspension-type structure was obtained. Furthermore, a Bragg reflector (SiO2/Pt) with optimized layer thickness ratio was employed to improve the performance of the YBAR. Compared to the acoustic wave resonators with the conventional quarter-wave (λ/4) Bragg reflector, the proposed YBAR with an optimized Bragg reflector can reflect both the longitudinal and shear waves efficiently, and resonators with spurious-free response and high quality (Q) value were achieved. This work provides a potential solution to enabling high coupling micro-acoustic resonators with high Q factor in the 5G/6G communication system.

Near Spurious-Free Thickness Shear Mode Lithium Niobate Resonator for Piezoelectric Power Conversion.

Piezoelectric power converters, where acoustic resonators replace the inductors as energy storage elements, promise much higher power density and higher efficiency compared to conventional circuits. Recently, lithium niobate (LiNbO3) piezoelectric resonators have been integrated within power converter circuits, showing good conversion efficiency, thanks to their high quality factor (Q) and electromechanical coupling (kt2). However, the converter output power range is limited by large spurious modes near resonance. This work reports a near-spurious-free LiNbO3 thickness shear (TS) resonator, demonstrating high Q of 3500 and kt2 of 45% at 5.94 MHz, with a fractional suppressed region of 35%. First, we identify the best LiNbO3 crystal orientation for efficient TS resonators. Then, we propose a novel acoustic design without busbars for spurious suppression, which is extensively simulated, fabricated, and characterized. Further analysis is done to identify existing spurious modes in our proposed design, specifically the effect of dicing on our TS resonator design. Upon optimization, LiNbO3 TS resonators could potentially empower a new design space for low-loss and compact power converters.

Research on Stacked Piezoelectric Cymbal Vibrator.

As demand for haptic feedback increases, piezoelectric materials have become one of the best candidate materials due to their small size, high electromechanical coupling coefficient, and fast response. A stacked piezoelectric cymbal vibrator is proposed based on the common cymbal-type transducer, which is composed of a piezoelectric stack to drive and a cymbal disk to amplify displacement. A coupling theoretical model between the piezoelectric stack and the cymbal-type structure is established. The longitudinal and radial displacements of the stacked piezoelectric cymbal vibrator are calculated in the low frequency range (<1000 Hz) by the theoretical model and the finite element method. The theoretical and numerical results are in good agreement. The results show that the radial displacement can be converted into longitudinal displacement and then effectively amplified by the cymbal disk with an amplification ratio of 30. The feature is conducive to its widespread application in the field of consumer electronics.

6-20 GHz 30% ScAlN Lateral Field-Excited Cross-Sectional Lamé Mode Resonators for Future Mobile RF Front Ends.

This article reports on 30% scandium-doped AlN (ScAlN) lateral field-excited (LFE) cross-sectional Lame' mode resonators (CLMRs) with unprecedented performance in the 6-20 GHz range. By combining high-crystallinity 30% ScAlN piezoelectric thin films, a lithographic tunability of the resonance frequency, and a simple three-mask post-CMOS compatible fabrication process, we propose a technology platform that can enable the mass production of low-loss, wideband, and compact microacoustic filtering devices spanning a wide spectrum portion on the same chip for the next-generation radio frequency front ends (RFFEs) of handsets. This article demonstrates a successful scaling of the microacoustic technology well beyond the sub-6-GHz fifth-generation (5G) band, as well as the outstanding capabilities of high-crystallinity 30% ScAlN piezoelectric layers in delivering high-quality factor ( Q ) and high-electromechanical coupling ( kt2 ) resonators, notably exceeding the state of the art in terms of relevant figures of merit (FOMs). Furthermore, we experimentally investigate the impact of geometrical parameters, such as tethering configuration and width-over-length ratio on the devices' 3-dB quality factor ( [Formula: see text]), power linearity (PL), and temperature coefficient of frequency (TCF). By adopting a statistical approach for data analysis, we determine the optimal geometry to maximize the Q value. Moreover, we experimentally demonstrate that a fully tethered device's configuration ensures superior PL, lower TCF, and higher device yield and select that as the best design tradeoff between all the variables under consideration. Finally, we discuss a further scaling of LFE CLMRs, both in terms of higher doping levels in the piezoelectric layer, in order to enhance the performance of microacoustic filters, and in terms of higher operation frequencies, in order to reach and cover the mm-wave spectrum.