High Electromechanical Coupling Coefficient Research Articles

Vibration Modes of Radially Polarized Thin Cylindrical Piezoceramic Rings

Radially polarized cylindrical piezoceramic rings are studied experimentally. It is shown that the inphase resonance is characterized by very high amplitudes and electromechanical coupling coefficient. The distribution of internal stresses along the height differs from cosinusoidal in the middle part of the ring. The edge resonance is characterized by high stresses near the ends and is very weak in the middle part. The amplitude–frequency and admittance–frequency responses are plotted and discussed

Micro‐electro‐mechanical systems capacitive ultrasonic transducer with a higher electromechanical coupling coefficient

Since the proposing of capacitive micromachined ultrasonic transducer by Khuri Yakub group in 1994 that this kind of transducer occupying the advantages of wide bandwidth, impedance matching well with the propagation medium especially in fluid and air and high sensitivity, has shown a great potential for wide ranges of applications. This Letter reports kind of micro-electro-mechanical systems (MEMS) capacitive ultrasonic transducer with the novel cavities embedded in the device layer of silicon on insulator wafer bonded with a glass substrate. The optimum geometric dimensions are confirmed by both mechanical vibrating of the membrane and the electrical characteristics analysis. Finite-element analysis is adopted to determine the operation mode. The safety and reliability of the proposed device is ensured by the obtained deflections and equivalent stress under operation/collapse voltage. The bottom electrodes of the proposed transducer are fabricated on the top surface of the glass substrate. The parallel parasitic capacitance is reduced, thus improving the electromechanical coupling coefficient. The test results show that the electromechanical coupling coefficient is 69.65%, which demonstrates that this proposed MEMS capacitive ultrasonic transducer structure can enhance the performance significantly.

Microstructure and Electrical Properties of Eu-Doped K0.5Na0.5NbO3 Ceramics

(K0.5Na0.5)1–3xEuxNbO3 (0≤x≤0.05) lead-free ceramics have been prepared by a conventional solid state reaction. XRD show that the ceramics possess a pure perovskite structure with orthorhombic symmetry. Dopant Eu restrains the grain growth, decreases the ferroelectric-paraelectric phase transition temperature and leads to a diffuse phase transition. With the increase of Eu, the observed remanent polarization (Pr) decreases while the coercive field (Ec) remains almost unchanged. Nevertheless, for the increase in relative permittivity, the sample with x = 0.005 exhibits the optimum piezoelectric properties, giving a large piezoelectric coefficient (d33 = 121 pC/N) and a high planar electromechanical coupling coefficient (kp = 0.41).

Optimal orientations of LiTaO3 for application in plate mode resonators

Optimal cuts of LiTaO3 for application in plate mode resonators were found via rigorous numerical investigations of zero- and higher-order plate modes propagating parallel or normal to the X-axis in rotated Y-cuts of LiTaO3; the plates were tested with a periodic metal grating on top of the plate and metal electrode present or absent on the plate bottom. In some cuts, high electromechanical coupling coefficients up to 20% could be combined with low or even zero temperature coefficients of frequency (TCF). Other cuts ensured moderate coupling of 12%–14% and low TCF in addition to high velocity of a higher-order plate mode up to 20 000 m/s. Metallization of a plate bottom helped to enhance coupling of certain modes. Interaction of a plate mode with electrodes of an interdigital transducer or with periodic metal gratings used for its excitation and reflection in resonators is illustrated by examples of dispersion plots. The nature of the analyzed modes was studied via visualization of the mechanical displacements accompanying wave propagation.

Methods for improving electromechanical coupling coefficient in two dimensional electric field excited AlN Lamb wave resonators

An AlN piezoelectric Lamb-wave resonator, which is excited by two dimensional electric field, is reported in this paper. Rhombus-shape electrodes are arranged on AlN thin film in a checkered formation. When out-of-phase alternating currents are applied to adjacent checkers, two dimensional acoustic Lamb waves are excited in the piezoelectric layer along orthogonal directions, achieving high electromechanical coupling coefficient, which is comparable to film bulk acoustic resonators. The electromechanical coupling coefficient of the 285.3 MHz resonator presented in this paper is 5.33%, which is the highest among AlN based Lamb-wave resonators reported in literature. Moreover, the spurious signal within a wide frequency range is significantly suppressed to be 90% lower than that of the resonance mode. By varying the electrode dimension and inter-electrode distance, resonators having different resonant frequencies can be fabricated on a single wafer, making single-chip broadband filters, duplexers, and multiplexers possible.

Ultrasonic Transducer Fabricated Using Lead-Free BFO-BTO+Mn Piezoelectric 1-3 Composite

Mn-doped 0.7BiFeO3-0.3BaTiO3 (BFO-0.3BTO+Mn 1% mol) lead-free piezoelectric ceramic were fabricated by traditional solid state reaction. The phase structure, microstructure, and ferroelectric properties were investigated. Additionally, lead-free 1–3 composites with 60% volume fraction of BFO-BTO+Mn ceramic were fabricated for ultrasonic transducer applications by a conventional dice-and-fill method. The BFO-BTO+Mn 1-3 composite has a higher electromechanical coupling coefficient (kt = 46.4%) and lower acoustic impedance (Za ~ 18 MRayls) compared with that of the ceramic. Based on this, lead-free piezoelectric ceramic composite, single element ultrasonic transducer with a center frequency of 2.54 MHz has been fabricated and characterized. The single element transducer exhibits good performance with a broad bandwidth of 53%. The insertion loss of the transducer was about 33.5 dB.

Preparation and characterization of lead zirconate titanate thick films prepared by chemical solution deposition for MEMS applications

Lead zirconate titanate (PbZrxTi1−xO3, PZT) films are widely used in silicon based microelectromechanical systems (MEMS) due to their high piezoelectric and electromechanical coupling coefficient. Si3N4 is often needed as a masking layer to protect Si during wet etching. In this paper, the preparation and characterization of Pb(Zr0.52Ti0.48)O3 thick films deposited on Pt/Ti/Si3N4/SiO2/Si substrate by chemical solution deposition were studied. PZT films with a thickness of 2μm were spin-coated on the substrates and pyrolyzed at 350°C, and then annealed at 650°C. PZT thick films exhibit highly (100) preferential orientation. The dielectric constant and loss tangent of the films are 2370 and 0.046 at 1kHz, respectively. The remnant polarization (Pr) and the coercive fields (Ec) are 32μC/cm2 and 56kV/cm, respectively. PZT films on patterned Si wafers have been studied for the applications on biosensors and medical ultrasound transducers.

Investigation of properties of surface acoustic waves generated by periodically patterned ZnO on silicon substrate

The paper presents the characteristics of vertically polarized surface waves generated in silicon substrate by acoustic coupling of bulk waves excited in a periodically patterned ZnO film on silicon. The finite element simulations are performed on the proposed patterned-ZnO/Si structure and vertically polarized modes in silicon are found to be dominant in the frequency dependent analysis. The generated modes in silicon are concentrated near the surface within a wavelength depth and exhibit surface wave properties. Dispersion curves of phase velocity and coupling coefficient for the surface modes are reported. The results indicate high electromechanical coupling coefficient of 6.4% as well as high phase velocity of 5332m/s for the surface mode generated in silicon owing to the acoustic coupling of the first order bulk mode in ZnO pattern observed at ZnO height to wavelength ratio of 0.19.

Differentially piezoresistive transduction of high-Q encapsulated SOI-MEMS resonators with sub-100 nm gaps.

A differentially piezoresistive (piezo-R) readout proposed for single-crystal-silicon (SCS) microelectromechanical systems (MEMS) resonators is implemented in a foundrybased resonator platform, demonstrating effective feedthrough cancellation using just simple piezoresistors from the resonator supports while maximizing their capacitively transduced driving areas. The SCS resonators are fabricated by a CMOS foundry using an SOI-MEMS technology together with a polysilicon refill process. A high electromechanical coupling coefficient is attained by the use of 50-nm transducer gap spacing. Moreover, a vacuum package of the fabricated resonators is carried out through wafer-level bonding process. In this work, the corner supporting beams of the resonator serve not only mechanical supports but also piezoresistors for detecting the motional signal, hence substantially simplifying the overall resonator design to realize the piezo-R sensing. In addition, the fabricated resonators are capable of either capacitive sensing or piezo-R detection under the same capacitive drive. To mitigate feedthrough signals from parasitics, a differential measurement configuration of the piezo-R transduction is implemented in this work, featuring more than 30-dB improvement on the feedthrough level as compared with the single-ended piezo-R counterpart and purely capacitive sensing readout. Furthermore, the high-Q design of the mechanical supports is also investigated, offering Q more than 10 000 with efficient piezo-R transduction for MEMS resonators.

Fabrication of a PMN-PT single crystal-based transcranial Doppler transducer and the power regulation of its detection system.

Doppler sonographic measurement of flow velocity in the basal cerebral arteries through the intact skull was developed using a pulsed Doppler technique and 2 MHz emitting frequency. Relaxor-based ferroelectric single crystals Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) were chosen to be the piezoelectric transducer material due to their ultrahigh piezoelectric coefficients, high electromechanical coupling coefficients and low dielectric loss. The pulse-echo response of the transducer was measured using the conventional pulse-echo method in a water bath at room temperature. The −6 dB bandwidth of the transducer is 68.4% and the sensitivity is −17.4 dB. In order to get a good match between transducer and detection system, different transmission powers have been regulated by changing the impedance of the transmitting electric circuit. In the middle cerebral artery (MCA) measurement photograph results, as the transmission power is increasing, the detection results become clearer and clearer. A comparison at the same transmission power for different transducers shows that the detection photograph obtained by the crystal transducer was clearer than that obtained with a commercial transducer, which should make it easier for doctors to find the cerebral arteries.

Dielectric, piezoelectric and damping properties of novel 2-2 piezoelectric composites

Here, a strip-shaped 2-2 cement/polymer-based piezoelectric composite was designed and fabricated. The dielectric, piezoelectric and electromechanical coupling properties of the composite were investigated as well as the coupling effects between the thickness and lateral modes of the piezoelectric composites. The dielectric and piezoelectric properties of the composites can be greatly influenced by variations of the piezoelectric ceramic volume fraction and the structural dimensions of the composites. Excellent properties have been achieved for ultrasonic transducer applications in civil engineering monitoring fields, such as large piezoelectric voltage constants, high thickness electromechanical coupling coefficients and low acoustic impedance. The damping property of the composites was especially studied. The maximum damping loss factor of the composites is between 0.28–0.32, and the glass transition temperature is between 55°–66 °C.

A Multiple Resonant Mode Film Bulk Acoustic Resonator Based on Silicon-on-Insulator Structures

We report a multiple resonant mode film bulk acoustic resonator (FBAR) with different AlN film thicknesses of 605 nm, 640 nm and 680 nm. With the tilted c-axis orientation of the AlN piezoelectric film providing polarization vertical to the c-axis, acoustic wave resonant peaks have been observed for both the thickness shear modes (TSM 0th, TSM 1st, TSM 2nd) and the thickness extension modes (TEM 0th, TEM 1st). The corresponding parallel resonant frequencies are around 1.60 GHz, 2.41 GHz, 3.45 GHz, 2.75 GHz and 4.10 GHz, respectively. The latter two TEM modes also have good quality factors, and high equivalent electromechanical coupling coefficients Keff2 of 647, 3.13% and 113, 6.23%, respectively. By etching the 1.8 μm silicon sacrificial layer, the air gap FBAR devices have been fabricated in an easier and cleaner way resulting in a low insertion loss of −2.2 dB. The overall device structure of the top electrode/AlN film/bottom electrode on SiO2/silicon-on-insulator (SOI) substrate potentially enables CMOS compatibility. These multiple resonant mode FBAR devices will promote the integration of multi-band filters on a single chip. Improvements of the fabrication process, the influence of different AlN film thicknesses and theoretical analyses of the coexistence of multiple resonant modes are presented.

Dielectric and piezoelectric properties of piezoceramic/polymer 1–3 composites fabricated by a modified align-and-fill technique

Piezoelectric ceramics with compositions (Pb0.94Sr0.04) (Zr0.52Ti0.48)O3 (PZT) and (Ba0.95Pb0.05) (Ti0.99Co0.01)O3 (BT) were synthesized by conventional solid state reaction technique. Three series of 1–3 composites; PZT/monothane-A70, BT/monothane-A70 and PZT/araldite-F were fabricated by a modified align-and-fill technique using the above mentioned piezoceramic compositions. Each series of the fabricated composite comprise five samples with variable ceramic contents (12–35 vol.%) and four samples with fixed ceramic volume fraction (28%) and varied ceramic aspect ratio (0.34–1.00). Dielectric, elastic and resonant characteristics of theses samples were measured. Effects of active piezoceramic composition and polymer influence on the electrical and mechanical properties of fabricated composites as a function of ceramic volume fraction were investigated. Results revealed that PZT based composites exhibit superior properties due to high piezoelectric charge coefficient (d33 ∼ 335 pC/N) and high electromechanical coupling coefficient (k33 ∼ 67%) compared to those of BT composites. It was also observed that electromechanical properties of composites are highly affected by the stiffness of the passive polymer matrix. Low stiffness (C33D ∼ 21–55 GPa) PZT/monothane-A70 composites have better acoustic impedance (Z ∼ 6–14 Mrayl), high piezoelectric charge coefficient (d33 ∼ 202–271 pC/N), high hydrostatic charge coefficient (dh ∼ 136–171 pC/N), high hydrostatic voltage coefficient (gh ∼ 33–114 × 10−3 Vm/N) and high figure of merit (FOM ∼ 4488–19,364 fm2/N) compared to PZT/araldite-F composites encompassing high stiffness (C33D ∼ 23–60 GPa). The influences of ceramic aspect ratio on the electrical properties of fabricated 1–3 composites are also discussed.

Piezoelectric thin films for double electrode CMOS MEMS surface acoustic wave (SAW) resonator

CMOS integration for RF-MEMS is desired to yield compact, low-power and portable devices. In this work, we illustrate the usage of double electrode CMOS SAW resonators using both ZnO and AlN as its piezoelectric material. Double electrode transducers were chosen, as they are better at suppressing undesired acoustic reflections compared to single electrodes. The structure and dimension of the device is based on 0.35 μm CMOS process where the IDTs are fabricated using standard CMOS fabrication process. 2D Finite element modeling of the CMOS SAW resonator using COMSOL Multiphysics® is presented. Two-step eigenfrequency and frequency domain analyses were performed. The acoustic velocities generated are 3,925 and 5,953 m/s for ZnO and AlN CMOS SAW resonator respectively. Higher acoustic displacement and surface potential were observed in ZnO compared to AlN. It can be concluded that ZnO thin films have higher electromechanical coupling coefficients and are more efficient than AlN thin films.

Electromechanical response of micromachined 1-3 piezoelectric composites: Effect of etched piezo-pillar slope

Micromachined single-crystal piezoelectric 1-3 composites are known for high electromechanical coupling coefficients, low acoustic impedance, high processing precision and uniformity, which are desired for high-frequency ultrasound transducers. In this article, based on Smith and Auld’s 1-3 composite thickness-mode oscillation model, the effect of etched side wall slope on the electromechanical characteristics of micromachined piezoelectric 1-3 composites was studied. In specific, strain constant, stiffness, dielectric constant, electromechanical coupling coefficient, acoustic impedance, longitudinal velocity, and frequency of micromachined 1-3 composites were deduced using the developed model. The analytical model was then verified by a COMSOL simulation and experimental measurements of a micromachined composite sample with pitch of 15.9 µm, thickness of 42.8 µm, and etched pillar slope angle of 83.8°. The measured center frequency was 49.05 MHz, electromechanical coupling coefficient was 0.66, dielectric constant was 1178, and strain constant was 26.90 C/m2, which all agreed well with the analytical calculations. These results will be helpful in design and fabrication of high-frequency micromachined ultrasound transducers.

Intrinsically tunable bulk acoustic wave resonators based on sol-gel grown PMN-PT films

Intrinsically tunable bulk acoustic wave resonators, based on sol-gel 0.70Pb(Mg1/3Nb2/3)O3-0.30PbTiO3 (PMN-PT) thin films, with high effective electromechanical coupling coefficient of 13% and tunability of the series resonance frequency up to 4.0% are fabricated and characterized. The enhanced electroacoustic properties of the PMN-PT resonators are attributed to the mechanism of polarization rotation occurring in the region of the morphotropic phase boundary. Electroacoustic performance of the PMN-PT resonators is analyzed using the theory of dc field-induced piezoelectric effect in ferroelectrics. Extrinsic acoustic loss in the PMN-PT resonators is analyzed using the model of the wave scattering at reflections from rough interfaces. Mechanical Q-factor of the resonators is up to 70 at 4.1 GHz and limited mainly by losses in the PMN-PT film.

Design of micromachined self-focusing piezoelectric composite ultrasound transducer.

Based on the Fresnel half-wave band interference, a micromachined self-focusing piezoelectric composite ultrasound transducer was proposed in this paper. The theoretical analysis was deduced based on the concept of constructive interference of acoustic waves and electromechanical response of piezoelectric composites. The calculated and simulation results showed that it combined the advantages of composite transducer and plate self-focusing transducer, and can achieve high electromechanical coupling coefficient, low acoustic impedance, high intensity, short focal length and micro size. Because it was based on the micro-electromechanical systems, the fabrication process was accurate and controllable, which made it have good potential for interventional ultrasound imaging, cellular microstructure imaging, skin cancer detection and industrial nondestructive testing applications.

High performance relaxor-based ferroelectric single crystals for ultrasonic transducer applications.

Relaxor-based ferroelectric single crystals Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) have drawn much attention in the ferroelectric field because of their excellent piezoelectric properties and high electromechanical coupling coefficients (d33∼2000 pC/N, kt∼60%) near the morphotropic phase boundary (MPB). Ternary Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PIN-PMN-PT) single crystals also possess outstanding performance comparable with PMN-PT single crystals, but have higher phase transition temperatures (rhombohedral to tetragonal Trt, and tetragonal to cubic Tc) and larger coercive field Ec. Therefore, these relaxor-based single crystals have been extensively employed for ultrasonic transducer applications. In this paper, an overview of our work and perspectives on using PMN-PT and PIN-PMN-PT single crystals for ultrasonic transducer applications is presented. Various types of single-element ultrasonic transducers, including endoscopic transducers, intravascular transducers, high-frequency and high-temperature transducers fabricated using the PMN-PT and PIN-PMN-PT crystals and their 2-2 and 1-3 composites are reported. Besides, the fabrication and characterization of the array transducers, such as phased array, cylindrical shaped linear array, high-temperature linear array, radial endoscopic array, and annular array, are also addressed.

Advances in the Growth and Characterization of Relaxor-PT-Based Ferroelectric Single Crystals

Compared to Pb(Zr1−xTix)O3 (PZT) polycrystalline ceramics, relaxor-PT single crystals offer significantly improved performance with extremely high electromechanical coupling and piezoelectric coefficients, making them promising materials for piezoelectric transducers, sensors and actuators. The recent advances in crystal growth and characterization of relaxor-PT-based ferroelectric single crystals are reviewed in this paper with emphases on the following topics: (1) the large crystal growth of binary and ternary relaxor-PT-based ferroelectric crystals for commercialization; (2) the composition segregation in the crystals grown from such a solid-solution system and possible solutions to reduce it; (3) the crystal growth from new binary and ternary compositions to expand the operating temperature and electric field; (4) the crystallographic orientation dependence and anisotropic behaviors of relaxor-PT-based ferroelectriccrystals; and (5) the characterization of the dielectric, elastic and piezoelectric properties of the relaxor-PT-based ferroelectriccrystals under small and large electric fields.

Theoretical research on ultrasonic sensors based on high-order Lamb waves

In an ultrasonic sensor, the electromechanical coupling coefficient, sensitivities related to the mass load and conductivity variation, insert loss and minimum detectable mass are important parameters determining the performance of the sensor, while it is challenging to optimize the abovementioned five parameters simultaneously. In this paper, we show that the multi-mode characteristic of Lamb wave provides possibilities to improve the performance of ultrasonic sensors by simultaneously considering the five parameters. According to the simulated results, piezoelectric films, relative thicknesses of films and substrates, structures of interdigital transducers, operating wavelengths and frequencies are optimized based on properly selected operating modes of Lamb wave. Then, high performance of Lamb wave sensors can be realized, in which high electromechanical coupling coefficients, mass sensitivities and conductivity sensitivities in addition to low insert losses and minimum detectable masses are simultaneously achieved.