Piezoelectric Thin Films Research Articles

Contactless, Battery-free, and Stretchable Wearable for Continuous Recording of Seismocardiograms

Seismocardiography (SCG) is the technique to measure microscale vibrations of the chest, which is closely related to the mechanical activity of the heart. To date, only a few studies have reported technical solutions to remotely measure SCG signals with soft electronics using stretchable, piezoelectric thin films. In this study, however, we demonstrate a contactless wearable patch using a 28-μm-thick foil of the piezoelectric polymer polyvinylidene fluoride. As a technical advancement, we utilize near-field communication (NFC) technology to power the patch and to record the SCG data wirelessly. A separated electrocardiogram (ECG) is used as a reference. Our results show expected waveforms of the recorded SCG signals with good signal-to-noise ratios and a close correlation between SCG and ECG recordings. The SCG technique adds an additional tool for cardiac monitoring, which is tightly associated with an ECG. The SCG method could be combined with other techniques in wearable on-skin electronics with multiple functionality for advanced cardiovascular monitoring.

Microstructure and Electrical Properties of Novel piezo-optrodes Based on Thin-Film Piezoelectric Aluminium Nitride for Sensing

Thin-film piezoelectric materials are currently employed in micro- and nanodevices for energy harvesting and mechanical sensing. The deposition of these functional layers, however, is quite challenging onto non-rigid/non-flat substrates, such as optical fibers (OFs). Besides the recent novel applications of OFs as probes for biosensing and bioactuation, the possibility to combine them with piezoelectric thin films and metallic electrodes can pave the way for the employment of novel opto-electro-mechanical sensors (e.g., waveguides, optical phase modulators, tunable filters, energy harvesters or biosensors). In this work the deposition of a thin-film piezoelectric wurtzite-phase Aluminium Nitride (AlN), sandwiched between molybdenum (Mo) electrodes, on the curved lateral surface of an optical fiber with polymeric cladding, is reported for the first time, without the need of an orientation-promoting interlayer. The material surface properties and morphology are characterized by microscopy techniques. High orientation is demonstrated by SEM, PFM and X-ray diffraction analysis on a flat polymeric control, with a resulting piezoelectric coefficient (d33) of ∼5.4 pm/V, while the surface roughness Rms measured by AFM is 9 ÷ 16 nm. The output mechanical sensing capability of the resulting AlN-based piezo-optrode is investigated through mechanical buckling tests: the peak-to-peak voltage for weakly impulsive loads increases with increasing relative displacements (up to 30%), in the range of 20 ÷ 35 mV. Impedance spectroscopy frequency sweeps (10 kHz-1 MHz, 1 V) demonstrate a sensor capacitance of ∼8 pF, with an electrical Q factor as high as 150. The electrical response in the long-term period (two months) revealed good reliability and durability.

Giant Piezoelectric Properties of ZnO Film Doped with Acceptor-Donor Ionic Pair

Piezoelectric thin film materials are high energy density materials that scale very favorably upon miniaturization and that has led to an ever-growing interest for MEMS applications. Thus extensive investigation of lead-free piezoelectric has been triggered out of environmental awareness. Here, a concrete lead-free paradigm is presented, Zn1-2x(FexLix)O thin films, which exhibits a splendid d33* value (~415 pm/V) and electrostrain (~0.68%) after thermal-electric treatment in the co-doped film with the x value of 0.06. It is considered that the local lattice distortion generated by preferential distributed Fe3+-Li+ ionic pairs is responsible for the outstanding piezoelectric properties and obvious ferroelectricity response. The defect engineering strategy presented in this work open a new development window for obtaining excellent piezoelectricity in a wide range of binary metal oxide systems and have profound implications for the potential utilization of lead-free piezoelectrics in microelectromechanical systems and surface acoustic wave devices.

Highly‐Sensitive Skin‐Attachable Eye‐Movement Sensor Using Flexible Nonhazardous Piezoelectric Thin Film

AbstractAccurate and continuous monitoring of eye movements using compact, low‐power‐consuming, and easily‐wearable sensors is necessary in personal and public health and safety, selected medical diagnosis techniques (point‐of‐care diagnostics), and personal entertainment systems. In this study, a highly sensitive, noninvasive, and skin‐attachable sensor made of a stable flexible piezoelectric thin film that is also free of hazardous elements to overcome the limitations of current computer‐vision‐based eye‐tracking systems and piezoelectric strain sensors is developed. The sensor fabricated from single‐crystalline III‐N thin film by a layer‐transfer technique is highly sensitive and can detect subtle movements of the eye. The flexible eye movement sensor converts the mechanical deformation (skin deflection by eye blinking and eyeball motion) with various frequencies and levels into electrical outputs. The sensor can detect abnormal eye flickering and conditions caused by fatigue and drowsiness, including overlong closure, hasty eye blinking, and half‐closed eyes. The abnormal eyeball motions, which may be the sign of several brain‐related diseases, can also be measured, as the sensor generates discernable output voltages from the direction of eyeball movements. This study provides a practical solution for continuous sensing of human eye blinking and eyeball motion as a critical part of personal healthcare, safety, and entertainment systems.

Plasma Treatment of Composite Piezoelectric Thin Films for Good Adhesion of Printed Conductive Ink

Lead zirconate titanate and polydimethylsiloxane (PZT–PDMS) thin films are an attractive choice for a flexible piezoelectric substrate. In this work, the surface modification of the PZT–PDMS compos...

Nonlinear Response of PZT-Actuated Resonant Micromirrors

In this work, we address the modelling of piezo-micromirrors with large opening angles. We focus on various sources of nonlinearites induced by the interaction with the surrounding fluid and by occurrence of geometric large transformations. Specific attention is also devoted to the piezoelectric coupling coefficient. Piezoelectric thin films have gained attention as key materials for the actuation of micro devices since they provide high drive forces, enabling devices with higher performances compared to electrostatic actuation. However they also induce material nonlinearities that cannot be neglected. The model is validated using experimental data obtained for various excitation levels.

High-frequency surface acoustic wave resonator with ScAlN/hetero-epitaxial diamond

High-frequency and wideband surface acoustic wave (SAW) devices have become an important topic in 5G communication systems and beyond. For this purpose, piezoelectric ScAlN thin films deposited on high SAW velocity diamond are studied in SAW resonators on the polycrystalline diamond (PCD) and hetero-epitaxial diamond (HED) substrates. A very strong c-axis orientation of ScAlN on HED was confirmed. Interdigital transducers of 0.8 μm and 0.5 μm were fabricated to realize 2.2 to 3.5 GHz high-frequency resonators. High Q values were obtained for the 2.3–2.5 GHz device on both PCD and HED. Additionally, a high electro-mechanical coupling coefficient (K2) of 5.40–5.52% and high SAW velocities of 7400–7766 m/s were obtained in the 2.3–2.5 GHz devices. In comparison to the simulated coupling coefficient results of a previous report, ScAlN/diamond might have higher K2 for the Sezawa wave.

Peculiarities of energy trapping of the UHF elastic waves in diamond-based piezoelectric layered structure. II. Lateral energy flow

Features of an acoustic energy trapping and outflow in a High-overtone Bulk Acoustic Resonator (HBAR) were considered. On the example of a specified resonator based on a piezoelectric layered structure (PLS) as “Al/AlN/Mo/(100) diamond”, the simulation of the propagation of microwave acoustic waves in the range of 10–6000 MHz by Finite Element Method (FEM) in COMSOL Multiphysics has been carried out. It was found that the magnitude of acoustic energy retained in the substrate under the bottom electrode depends substantially on the relation between the HBAR’s antiresonance frequency fa,n and the cut-off frequencies fs,n-k and fs,n-k+1 of the substrate. The term “cut-off frequency” has the meaning of the frequency point when integer value of half-waves fits along the thickness of substrate. Parameter gk,n estimating the degree of the HBAR’s energy trapping has been proposed. If 0.5 < gk,n < 1, then a retention of acoustic energy in the HBAR’s center as a maximal ET-effect at the overtone frequency fa,n should be fulfilled. Increasing the width of the top electrode as well as the HBAR’s aperture should expand the frequency interval in which the retained elastic energy in the vicinity of the HBAR’s center will be greater than the similar outside. It was shown that the area-averaged Umov-Poynting’s power energy flow vector from the HBAR’s center in the lateral directions increases when approaching the Thin Film Piezoelectric Transducer half-wave resonance frequency from below, and then decreases sharply in the point of complete energy trapping. This can be explained by the outflow of acoustic wave energy from the center to the periphery as a consequence of the transformation of an operational longitudinal acoustic mode into waves of Lamb type mainly. Results on the peculiarities of an energy trapping effect will be useful developing the HBARs and HBAR-based acoustic sensors operating at microwave frequencies.

Development of Sputter Epitaxy Technique of Pure-Perovskite (001)/(100)-Oriented Sm-Doped Pb(Mg1/3, Nb2/3)O3-PbTiO3 on Si.

We have developed a unique sputter deposition technique for a pure-perovskite (001)/(100)-oriented samarium-doped Pb(Mg1/3, Nb2/3)O3-PbTiO3 (Sm-PMN-PT) epitaxial thin film on Si as a future piezoelectric transducer thin film in microelectromechanical systems (MEMSs). This technique bases on the use of a "Pb(Zr,Ti)O3 (PZT)-based seed layer" and "separate sputter deposition." Undesired orientations and phases of such a relaxor-based ferroelectric are usually generated during the sputter deposition. This technique was demonstrated to provide preferential (001)/(100) orientation and pure-perovskite phase to the monocrystalline thin film. The fabricated film had excellent homogeneousness of the content distribution. Considering a practical thickness, a 2- [Formula: see text]-thick monocrystalline thin film was grown on an Si substrate with this technique. Then, the piezoelectricity |e31,f| of the Sm-PMN-PT/PZT stacked film was evaluated through an actuation test of the unimorph cantilever. As a result, it measured 16-17 C/m2, which is almost comparable with intrinsic PZT polycrystalline thin films with high |e31,f| values. Considering that the actuation voltage was divided into Sm-PMN-PT and PZT layers, the inherent piezoelectricity of the Sm-PMN-PT thin film is expected to be higher. Optimization of the phase in the film by tuning the composition ratio also will further improve the piezoelectricity. We believe that this achievement is a great step to discover a giant piezoelectricity relaxor-based thin film beyond PZT for MEMS.

Development and characterization of confocal sputtered piezoelectric zinc oxide thin film

Thin film zinc oxide (ZnO) is often used as a piezoelectric material, but its material properties are dependent on the deposition method and parameters. Direct sputtering using reactive DC or RF sputtering are the most common deposition methods for thin film piezoelectric ZnO. This paper investigates the influence of varying sputtering parameters on piezoelectric and mechanical properties of ZnO deposited by confocal sputtering using pulsed DC on a silicon substrate. Parameters such as varying power, duty cycle and temperature were examined. Crystallinity of the films were investigated systematically by X-ray Diffraction. Mechanical properties such as hardness and Youngs Modulus of thin films were measured via nanoindentation. Piezoelectric coefficient d33 was also measured for the films. Results demonstrated that the ZnO thin film exhibited high c-axis (002) textured films with rocking curve full width half maximum values of 1.69°. It was observed that power, duty cycle and temperature significantly affected the crystallinity, mechanical properties and d33 values of the ZnO thin film. This paper demonstrates that pulsed DC confocal sputtering can be utilized to deposit piezoelectric ZnO films and demonstrates key sputtering parameters that influence the piezoelectric, mechanical and morphology properties of the film.

Piezoelectric MEMS Resonators: A Review

Since two decades piezoelectric MEMS resonator research has been making a headway by leaps and bounds in multiple fronts such as micro/nanofabrication techniques, device and system design, monolithic integration, packaging, material engineering, etc. Piezoelectricity based frequency selective resonant tanks are an integral part of radio frequency communication, inertial sensors, infrared sensors, environmental monitoring modules, energy harvesters, etc. To improve the functionality of the aforementioned modules it is cardinal to understand the underlying concepts of resonator design, material selection, and performance enhancement techniques. Here in this review paper, a comprehensive overview of various piezoelectric thin-film material properties along with different commercial microfabrication platforms with CMOS integration facility is presented. The device characteristics greatly depend on the resonator mode shape and thus suitable modes and thin-film material configurations can be chosen depending on the target frequency, application, and power budget requirements. In addition to device modeling, a synopsis of the several acoustic and material engineering approaches to enhance the figure of merit of the resonator has been presented. The future avenues of piezoelectric MEMS cover a wide spectrum of possibilities such as 5G communication, quantum processing, frequency combs, photonics integration, gesture sensors, etc.

2D Scanning Micromirror with Large Scan Angle and Monolithically Integrated Angle Sensors Based on Piezoelectric Thin Film Aluminum Nitride.

A 2D scanning micromirror with piezoelectric thin film aluminum nitride (AlN), separately used as actuator and sensor material, is presented. For endoscopic applications, such as fluorescence microscopy, the devices have a mirror plate diameter of 0.7 mm with a 4 mm2 chip footprint. After an initial design optimization procedure, two micromirror designs were realized. Different spring parameters for x- and y-tilt were chosen to generate spiral (Design 1) or Lissajous (Design 2) scan patterns. An additional layout, with integrated tilt angle sensors, was introduced (Design 1-S) to enable a closed-loop control. The micromirror devices were monolithically fabricated in 150 mm silicon-on-insulator (SOI) technology. Si (111) was used as the device silicon layer to support a high C-axis oriented growth of AlN. The fabricated micromirror devices were characterized in terms of their scanning and sensor characteristics in air. A scan angle of 91.2° was reached for Design 1 at 13 834 Hz and 50 V. For Design 2 a scan angle of 92.4° at 12 060 Hz, and 123.9° at 13 145 Hz, was reached at 50 V for the x- and y-axis, respectively. The desired 2D scan patterns were successfully generated. A sensor angle sensitivity of 1.9 pC/° was achieved.

Sensitivity and Directivity Analysis of Piezoelectric Ultrasonic Cantilever-Based MEMS Hydrophone for Underwater Applications

In this paper, we report on the characterization of the sensitivity and the directionality of a novel ultrasonic hydrophone fabricated by micro-electro-mechanical systems (MEMS) process, using aluminum nitride (AlN) thin film as piezoelectric functional layer and exploiting a stress-driven design. Hydrophone structure and fabrication consist of four piezoelectric cantilevers in cross configuration, whose first resonant frequency mode in water is designed between 20 kHz and 200 kHz. The MEMS fabricated structures exploit 1 µm and 2 µm thick piezoelectric AlN thin film embedded between two molybdenum electrodes grown by DC magnetron sputtering on silicon (Si) wafer. The 200 nm thick molybdenum electrodes thin layers add a stress-gradient through cantilever thickness, leading to an out-of-plane cantilever bending. A water resistant parylene conformal coating of 1 µm was deposited on each cantilever for waterproof operation. AlN upward bent cantilevers show maximum sensitivity up to −163 dB. The cross configuration of four stress-driven piezoelectric cantilevers, combined with an opportune algorithm for processing all data sensors, permits a finer directionality response of this hydrophone.

Decoding of facial strains via conformable piezoelectric interfaces.

Devices that facilitate nonverbal communication typically require high computational loads or have rigid and bulky form factors that are unsuitable for use on the face or on other curvilinear body surfaces. Here, we report the design and pilot testing of an integrated system for decoding facial strains and for predicting facial kinematics. The system consists of mass-manufacturable, conformable piezoelectric thin films for strain mapping; multiphysics modelling for analysing the nonlinear mechanical interactions between the conformable device and the epidermis; and three-dimensional digital image correlation for reconstructing soft-tissue surfaces under dynamic deformations as well as for informing device design and placement. In healthy individuals and in patients with amyotrophic lateral sclerosis, we show that the piezoelectric thin films, coupled with algorithms for the real-time detection and classification of distinct skin-deformation signatures, enable the reliable decoding of facial movements. The integrated system could be adapted for use in clinical settings as a nonverbal communication technology or for use in the monitoring of neuromuscular conditions.

Enabling Higher Order Lamb Wave Acoustic Devices With Complementarily Oriented Piezoelectric Thin Films

In this work, we present a new paradigm for enabling gigahertz higher-order Lamb wave acoustic devices using complementarily oriented piezoelectric (COP) thin films. Acoustic characteristics are first theoretically explored with COP lithium niobate (LiNbO3) thin films, showing their excellent frequency scalability, low loss, and high electromechanical coupling (k2). Acoustic resonators and delay lines are then designed and implemented, targeting efficient excitation of higher-order Lamb waves with record-breaking low loss. The fabricated resonator shows a 2nd-order symmetric (S2) resonance at 3.05 GHz with a high quality factor (Q) of 657, and a large k2 of 21.5% and a 6th-order symmetric (S6) resonance at 9.05 GHz with a high Q of 636 and a k2 of 3.71%, both among the highest demonstrated for higher-order Lamb wave devices. The delay lines show an average insertion loss (IL) of 7.5 dB and the lowest reported propagation loss of 0.014 dB/μm at 4.4 GHz for S2. Notable acoustic passbands up to 15.1 GHz are identified. Upon further optimizations, the proposed COP platform can lead to gigahertz low-loss wideband acoustic components.

Deciphering facial movements.

Wearable piezoelectric thin films integrated with multiphysics modelling and three-dimensional digital image correlation can decode facial expressions in real time.

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]

The impact of residual stress on resonating piezoelectric devices

Resonating piezoelectric devices, such as aluminum nitride (AlN) piezoelectric micromachined ultrasonic transducers (PMUTs), display superior performance to previous generations of resonating microelectromechanical systems (MEMS). However, the quality of the piezoelectric thin film can greatly impact operating characteristics, such as the resonant frequency. Several AlN PMUT devices fabricated on the same silicon wafer exhibited a range of resonance frequencies (400-600 kHz), indicating that there is nonuniformity across the processed wafer. AlN film nonuniformity is likely introduced during the reactive sputtering process. Two key parameters identified as influencing the resonance frequency include: (i) the membrane diameter and (ii) residual stress. This work focuses on the residual stress, and uses X-ray diffraction technique (XRD) to determine the in-plane biaxial residual stress values, as a function of die position on the wafer. Results show that there is a compressive stress gradient along the wafer, ranging from -357 MPa to -56 MPa. A plot of in-plane biaxial residual stress as a function of resonance frequency shows a relation between the measured stress and frequency. As resonating piezoelectric devices require well defined operating frequencies and bandwidths, this work demonstrates the importance of studying not only global stresses, but also local residual stresses.

An evaluation method for direct piezoelectric coefficients of thin films using both-ends-hinge-supported unimoprh beams

We have developed a new method for measuring the direct piezoelectric effect of piezoelectric thin films, consisting of both-ends-hinge-supported unimorph beams (BUBs). This method can determine the precise direct transverse piezoelectric coefficient e31,f of arbitrary piezoelectric films by measuring the displacement and the output voltage between the electrodes of the BUBs. We formulated the e31,f values based on the flexure basic formula, and a constitutive equation of electromagnetic field for the BUBs, including measuring systems such as oscilloscopes, which have an arbitrary input impedance. In this study, we were able to obtain a piezoelectric coefficient of a lead zirconate titanate (PZT) thin film on a Si substrate of e31,f = 4.7 Cm−2, regardless of the input impedance of the measurement instrument. Using this method, we then measured the temperature characteristics of e31,f for the PZT thin film. We clearly observed the increase of e31,f , and the relative dielectric constant, with an increase in temperature. We obtained a temperature gradient for of a = 0.01 Cm−2K−1, a value which is in good agreement with previous research. We also measured the time dependences of the e31,f after polling piezoelecric films. The e31,f value decreased over time, and we were able to obtain a logarithmic time gradient for normalized e31,f of p = −0.01, a value consistent with the results of previous studies.

Field-induced polarization response and energy storage behavior of lead-free BNT-BKT-SZ films

Lead-free piezoelectric thin films of composition (1-x)Bi0.5Na0.5TiO3-xBi0.5K0.5TiO3-0.03SrZrO3 (x = 15, 17.5, 20 and 22.5%) have been created using the sol-gel spin coating approach over a Pt/Ti/SiO2/Si substrate. The X-ray diffraction profiles suggest that a remarkable transition from tetragonal to rhombohedral-phase passing by a morphotropic phase boundary (MPB) occurs at x values from 17.5 to 22.5%. Additionally, the surface morphology, piezoelectric response, ferroelectric behavior, and energy storage properties were also investigated. Piezoresponse force microscopy (PFM) depicted a strong piezoelectric and ferroelectric behavior at a very localized size; at x = 17.5%. The piezoelectric behavior was enhanced and reached values of the recoverable energy storage density (Jrec) and the energy storage efficiency (η) of 3.27 J/cm3, and 39.9%, respectively. Such materials are anticipated to play an important role in lead-free devices.