Piezoelectric Thin Films Research Articles

Electro-Acoustic Properties of Scandium-Doped Aluminum Nitride (ScxAl1-xN) Material and its Application to Phononic Crystal-Coupled SAW Devices

Within the framework of the Density Functional Theory, the elastic, dielectric, and piezoelectric coefficients of w-ScxAl1−xN material were investigated for scandium (Sc) concentrations x = 0 to 0.375. The electro-acoustic properties are used to investigate the frequency response of the SAW delay line, based on the tilt θ° of the normal c-axis of the w-ScxAl1−xN piezoelectric thin film. We found that the piezoelectric response is improved as the Sc concentration increases, which is consistent with existing works in the literature. A 2D-phononic crystal pillars was then grafted on top of the surface, and the dependence of the acoustic band gaps is investigated with the help of the finite element method as a function of the Sc concentration and the tilted angle of w-Sc0.375Al0.625N. It was found that the two first band gaps exhibit a shift toward low frequencies with increasing Sc concentration. Moreover, the second acoustic bandgap is more sensitive to the inclination angle than the first. Furthermore, the insertion loss (S21) of w-Sc0.375Al0.625N is improved by 22 dB at θ° = 60°. The c-axis tilted Sc0.375Al0.625N-SAW delay line coupled with 2D-phononic crystals is a promising structure for low-loss and high-frequency SAW devices.

Laser Ultrasound Investigations of AlScN(0001) and AlScN(11-20) Thin Films Prepared by Magnetron Sputter Epitaxy on Sapphire Substrates.

The laser ultrasound (LU) technique has been used to determine dispersion curves for surface acoustic waves (SAW) propagating in AlScN/Al2O3 systems. Polar and non-polar Al0.77Sc0.23N thin films were prepared by magnetron sputter epitaxy on Al2O3 substrates and coated with a metal layer. SAW dispersion curves have been measured for various propagation directions on the surface. This is easily achieved in LU measurements since no additional surface structures need to be fabricated, which would be required if elastic properties are determined with the help of SAW resonators. Variation of the propagation direction allows for efficient use of the system’s anisotropy when extracting information on elastic properties. This helps to overcome the complexity caused by a large number of elastic constants in the film material. An analysis of the sensitivity of the SAW phase velocities (with respect to the elastic moduli and their dependence on SAW propagation direction) reveals that the non-polar AlScN films are particularly well suited for the extraction of elastic film properties. Good agreement is found between experiment and theoretical predictions, validating LU as a non-destructive and fast technique for the determination of elastic constants of piezoelectric thin films.

Large out-of-plane piezoelectricity of VIA group functionalized MXenes thin films for MEMS

The integration of piezoelectric and micro-electro-mechanical system (MEMS) technologies is promoting the rapid development of the MEMS industry. However, due to the lack of excellent in-plane and out-of-plane piezoelectric coefficients, good ambient temperature stability, and high mechanical durability, the reported two-dimensional piezoelectric thin films are facing severe challenges in the development of MEMS. Therefore, based on density functional theory calculations, we simulated the VIA group (the sixth main family in the periodic table of elements) functional MXenes, namely, Sc2CXY (Sc2COS, Sc2COSe, Sc2CSO, and Sc2CSeO). The results show that this kind of material has good dynamic, thermal, and mechanical stability. Importantly, the structure exhibits good in-plane and out-of-plane piezoelectric effects due to the inversion asymmetry, bandgap adjustability, flexibility, and different electronegativity. Among them, the piezoelectric strain value of d33 is one to three orders of magnitude higher than that of d22 or d31, which makes these materials can become a strong candidate for advanced equipment such as nano-wearable electronics, robot bionic skin, and piezoelectric products for MEMS.

Ultrathin Piezoelectric Resonators Based on Graphene and Free-Standing Single-Crystal BaTiO3.

Suspended piezoelectric thin films are key elements enabling high-frequency filtering in telecommunication devices. To meet the requirements of next-generation electronics, it is essential to reduce device thickness for reaching higher resonance frequencies. Here, the high-quality mechanical and electrical properties of graphene electrodes are combined with the strong piezoelectric performance of the free-standing complex oxide, BaTiO3 (BTO), to create ultrathin piezoelectric resonators. It is demonstrated that the device can be brought into mechanical resonance by piezoelectric actuation. By sweeping the DC bias voltage on the top graphene electrode, the BTO membrane is switched between the two poled ferroelectric states. Remarkably, ferroelectric hysteresis is also observed in the resonance frequency, magnitude and Q-factor of the first membrane mode. In the bulk acoustic mode, the device vibrates at 233 GHz. This work demonstrates the potential of combining van der Waals materials with complex oxides for next-generation electronics, which not only opens up opportunities for increasing filter frequencies, but also enables reconfiguration by poling, via ferroelectric memory effect.

Evaluation of acousto-optic effect in liquid using high-frequency and high-intensity ultrasound

Piezoelectric thin film transducers that can receive and transmit high-frequency and high-intensity ultrasound are powerful tools for future medical and industrial applications. Our group has developed acousto-optic devices where the refractive index can be controlled at high speed and high precision using ultrasound, but there are few reports on the interaction between high-frequency (tens to hundreds MHz) and high-intensity (MPa) ultrasound and visible light in liquids. In this study, we evaluated the acousto-optic interaction using polarized CW laser (532 nm) and a piezoelectric KNbO3 thin film transducer formed by hydrothermal method. The transducer was driven continuously at the fundamental or higher resonance frequencies (32, 96, 160, or 210 MHz) in a water cell, and the projection light of a laser beam penetrating perpendicularly to the sound axis was observed. The length of the projection light was elongated in the sound axis on the screen as the driving frequency increased. These results implies that the relationship between the wavelength of ultrasound and the laser beam width is the critical parameter for the acousto-optic phenomenon; the profile of the projection light is dependent on a ratio of the wavelength of ultrasound and the beam width of the laser beam.

Tuning the Porosity of Piezoelectric Zinc Oxide Thin Films Obtained from Molecular Layer-Deposited "Zincones".

Porous zinc oxide (ZnO) thin films were synthesized via the calcination of molecular layer-deposited (MLD) “zincone” layers. The effect of the MLD process temperature (110 °C, 125 °C) and of the calcination temperature (340 °C, 400 °C, 500 °C) on the chemical, morphological, and crystallographic properties of the resulting ZnO was thoroughly investigated. Spectroscopic ellipsometry reveals that the thickness of the calcinated layers depends on the MLD temperature, resulting in 38–43% and 52–56% of remaining thickness for the 110 °C and 125 °C samples, respectively. Ellipsometric porosimetry shows that the open porosity of the ZnO thin films depends on the calcination temperature as well as on the MLD process temperature. The maximum open porosity of ZnO derived from zincone deposited at 110 °C ranges from 14.5% to 24%, rising with increasing calcination temperature. Compared with the 110 °C samples, the ZnO obtained from 125 °C zincone yields a higher porosity for low calcination temperatures, namely 18% for calcination at 340 °C; and up to 24% for calcination at 500 °C. Additionally, the porous ZnO thin films were subjected to piezoelectric measurements. The piezoelectric coefficient, d33, was determined to be 2.8 pC/N, demonstrating the potential of the porous ZnO as an, e.g., piezoelectric sensor or energy harvester.

Reliable Pb(Zr,Ti)O3-based thin film piezoelectric micromirrors for space-applications

Although Pb(Zr,Ti)O3 (PZT) piezoelectric thin films are finding widespread applications on Earth, it is yet unclear if they are suitable for space-related applications. In space, their long-term reliability is a significant concern due to the difficulties of repairing and replacing malfunctioning devices. In this work, PZT thin film micromirrors for compact interferometric 3D imaging systems have been exposed to operating conditions encountered on a space mission and tested according to criteria set by the European Space Agency. Thermal cycling in vacuum, sinusoidal and random mechanical vibrations, and γ-radiation with and without bias did not degrade key functional device properties of the micromirror such as angular deflection, resonance frequency, polarization, and permittivity. Apart from γ-radiation, stressing the devices enhanced their large-signal angular deflection and improved their electrical lifetime compared to pristine devices. Their dielectric and ferroelectric characteristics remained comparable to that of a lab-scale environment. Simultaneously applying a 10 V field-down bias while γ-radiating the micromirrors changed the capacitance-field and polarization-field characteristics and enhanced the electrical imprint. After stress-testing, the median time-to-failure in moderate acceleration conditions of 150 kV/cm and 175 °C ranged from 1.95 to 2.64 h, close to 2.11 h as measured for a reference group. All actuator membranes had shorter electrical lifetimes, smaller voltage acceleration factors, and smaller activation energies, ranging from 2.56 to 2.88 V−1 and 1.03 to 1.09 eV, than simple bonding pads. This work is a device-level report covering a full set of space-relevant tests demonstrating that PZT-based thin film piezomicroelectromechanical systems technology is space-ready.

Fabrication of Hollow Needle Tips with Nanoholes by Plasma Maskless Processing and Calculation and Detection of Complex Surface Edges

The plasma maskless processing method combines the advantages of plasma etching and scanning probe processing, such as high etching efficiency, wide range of applicable materials, and high resolution. This method is based on the probe driving and detection of piezoelectric thin films, which enables all probes in the array to be processed completely independently, greatly improving the overall processing efficiency, and can effectively etch specific nano-scale regions of various materials. And it provides an effective solution for efficiently processing small batches of multi-variety micro-nano devices. In this study, a hollow needle tip with nanoholes was fabricated by plasma maskless processing, and use edge computing to calculate the complex surface shape of the needle tip. By establishing a plasma reactor, using the calculation method of the complex surface edge of the needle tip, and testing the stability of the processed nanotip by means of filling air pressure and power excitation in the experiment. The results show that when the breakdown voltage SF6 discharges, with the increase of air pressure, the breakdown voltage of the arrester first decreases and then increases gradually, and it is the smallest at 5 kPa and reaches the optimal 490 V, (PD) m = 0.5 Pa·m. And the Ar reaction ionization potential (15.76 eV) and excitation potential (11.53 eV, 11.72 eV) are lower than the splitting potential (≥16 eV) of SF6. As time goes on, high-energy particles are neutralized with the vessel wall, so the ion concentration is gradually smaller, sparse between 200 and 250, and increases from 100 to 300, which is close to the peak value, according to the gradual increase of the discharge voltage. The data obtained from the experimental test proves the reliability of the nanotips prepared in this study, and the properties remain stable under different breakdown voltages. Therefore, the nanohole hollow tip processed by maskless scanning plasma in this study has reached the standard of process use and has important application significance.

PVDF/RGO based piezoelectric nanocomposite films for enhanced mechanical and dielectric properties

Polymer nanocomposites are dominating today in the global scenario due to their diverse applications. The study involves the investigation of the role of Reduced graphene oxide (RGO) as nanofiller on mechanical and dielectric properties of Polyvinylidene difluoride (PVDF). Flexible piezoelectric thin films of PVDF loaded with RGO nanofillers were synthesized using a solvent casting technique. The crystal structure of neat PVDF and RGO loaded PVDF films were analysed using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and Fourier transform infrared spectroscopy (FTIR). Tensile strength and Young’s modulus of polymer nanocomposite films were analysed by performing tensile testing on a universal testing machine (UTM). The results showed an improvement in elastic modulus up to a certain nanofiller percentage (2 wt% RGO). Measurement of dielectric properties of thin films was conducted using an impedance and material analyser (IMA). The dielectric constant of the film increases with increment in RGO loading. Thus, overall increment in piezoelectric properties take place. This nanocomposite can be used for developing flexible and highly sensitive sensors and actuators.

Domain structure and optical band gap of MnO2 doped (K,Na)NbO3-LiTaO3 thin films

In this paper, potassium sodium niobate lead-free piezoelectric thin films were prepared by sol-gel method. The electric domain structure and optical band gap of the thin films were regulated by doping Mn. It is found that Mn doping increases the grain size and density of KNNLT piezoelectric film surface. In addition, Mn doping can regulate the electric domain of thin films. Increasing Mn doping concentration can regulate the size and distribution of electric domain, and the ratio of 71° electric domain increases with the increase of doping concentration. The results show that the optical band gap decreases gradually with the increase of doping amount when the annealing temperature is 650 °C. The lowest optical band gap reaches 3.426 eV, and is lower than the undoped KNNLT films.

A Piezoelectric Smart Textile for Energy Harvesting and Wearable Self-Powered Sensors

Today’s wearable electronics have dramatically altered our daily lives and created an urgent demand for new and intelligent sensor technologies. As a new energy source, self-powering sensors are currently seen as critically important units for wearable and non-wearable textile–electronic systems. To this aim, this paper presents a smart textile-based piezoelectric energy-autonomous harvester and a self-powered sensor for wearable application, where the sandwich structure of the wearable sensor consists of top and bottom textile conductors, and in between the two textile electrodes there is a piezoelectric PVDF thin film. The generating voltage, current, charge, power, and capacitor charging–discharging behaviour of the device were confirmed using multimeter, oscilloscope, Keithley, etc., analyses. Finally, a piezoelectric-textile sensor was integrated into wearable clothes for breathing detection; a shoe insole for footstep recognition; and it can store energy by tapping, to power electronics, such as a calculator, timer, LED, etc., at a later time. The sensitivity of the sensor was enough for generating voltage from a tiny water droplet. Thus, we can assume raindrops to be utilized as a power-generating source on days when no sun is available to solar cells.

Design and Characterization of a Planar Micro-Conveyor Device Based on Cooperative Legged Piezoelectric MEMS Resonators.

This paper reports the design, fabrication, and performance of a hybrid piezoelectric planar micro-conveyor based on Micro-Electromechanical Systems (MEMS) bridge resonators and featuring 3D-printed vertical legs. The device includes two cooperating silicon plate resonators with an area of 5 × 1 mm2, actuated by an integrated aluminum-nitride (AlN) piezoelectric thin film. An optimally designed array of 3D-printed projection legs was attached to the plates, to convert the standing-wave (SW) vertical vibrations into horizontal rotations or translations of the supported slider. An open-loop control strategy based on burst-type driving signals, with different numbers of sinusoidal cycles applied on each of the resonators, allowed the cooperation of the two bridges to set up prescribed trajectories of small flat objects, up to 100 mg, with positional accuracy below 100 nm and speeds up to 20 mm/s, by differential drive actuation. The effect of the leg tip and sliders’ surface finish on the conveyor performance was investigated, suggesting that further optimizations may be possible by modifying the tribological properties. Finally, the application of the micro-conveyor as a reconfigurable electronic system, driven by a preprogrammed sequence of signals, was demonstrated by delivering some surface-mount technology (SMD) parts lying on a 65 mg glass slider.

Polarization Rotation Control Domain Dynamic Response Modulates Piezoelectric Properties of Lead‐Free Thin Films

AbstractDomain orientation engineering regulates the polarization direction of thin film and facilitates the enhancement of piezoelectric properties. A change in polarization direction implies a structural change in the piezoelectric thin film. In this work, some unexpected results are obtained by preparing three orientations of Bi0.5Na0.5TiO3‐based lead‐free piezoelectric thin films to amplify this structural change. In distinction to the conventional perception, the piezoelectric properties decrease with increasing polarization direction. This anomaly is influenced by the concentration of structural defects, and rotation of the polarization direction may induce a change in the electronic structure of the material and hence in the concentration of structural defects. Ultimately, the suppression of the domain dynamics response by increasing structural defects leads to the worst piezoelectric properties for thin films with the traditionally perceived optimal (100) orientation. The revelation of this anomaly has implications for the design of the next generation of high‐performance piezoelectric thin films.

A Microfluidic System of Gene Transfer by Ultrasound.

Ultrasonic gene transfer has advantages beyond other cell transfer techniques because ultrasound does not directly act on cells, but rather pushes the gene fragments around the cells into cells through an acoustic hole effect. Most examples reported were carried out in macro volumes with conventional ultrasonic equipment. In the present study, a MEMS focused ultrasonic transducer based on piezoelectric thin film with flexible substrate was integrated with microchannels to form a microfluidic system of gene transfer. The core part of the system is a bowl-shaped curved piezoelectric film structure that functions to focus ultrasonic waves automatically. Therefore, the low input voltage and power can obtain the sound pressure exceeding the cavitation threshold in the local area of the microchannel in order to reduce the damage to cells. The feasibility of the system is demonstrated by finite element simulation and an integrated system of MEMS ultrasonic devices and microchannels are developed to successfully carry out the ultrasonic gene transfection experiments for HeLa cells. The results show that having more ultrasonic transducers leads a higher transfection rate. The system is of great significance to the development of single-cell biochip platforms for early cancer diagnosis and assessment of cancer treatment.

Investigation of support transducer enabled higher-order radial bulk mode MEMS resonator and low phase noise oscillator

This work reports the successful excitation of a novel bulk acoustic mode whose actuation and sensing are facilitated by the support transducer topology (STT). The design methodology concurrently supports low motional impedance and high energy confinement features which are crucial for frequency reference components in Radio Frequency communication. A conventional bulk mode operating in the width extension (WE) mode is deployed to efficiently excite the higher-order radial mode using the STT design feature of resonant frequency matching. The thin-film piezoelectric on substrate support transducers excited in a WE mode is mechanically coupled to the bulk silicon allowing the acoustic energy of the MEMS resonator to be stored maximally in the high-quality factor (Q) resonant tank, thereby alleviating the overall losses due to low-Q of the piezoelectric thin film and electrode material. The resonator exhibits a Q of 19 728 at 63.56 MHz resonant frequency with a motional resistance (R m) of 368 Ω when measured in vacuum at a power level of 0 dBm. Under cryogenic measurement conditions, the device recorded a Q of 24 153 at 15 K. A standalone WE resonator is studied to put a spotlight on the quality factor enhancement technique using the STT. The STT enabled novel bulk mode enhances the overall Q by 320% and halves the R m. When implemented as an oscillator, its performance exceeds the Global System for Mobile communication standards phase noise (PN) requirements. PN of −136.95 dBc Hz−1 and −161.52 dBc Hz−1 at 1 kHz and 1 MHz offset, respectively, were recorded when normalized to a carrier frequency of 13 MHz.

Thin Films (FTO/BaTiO3/AgNPs) for Enhanced Piezo-Photocatalytic Degradation of Methylene Blue and Ciprofloxacin in Wastewater.

In this study, we investigate the ability of barium titanate/silver nanoparticles (BaTiO3/AgNPs) composites deposited on a fluorine-doped tin oxide (FTO) glass using tape-casting method to produce piezoelectric thin film (FTO/BaTiO3/AgNPs) for piezocatalytic, photocatalytic, and piezo-photocatalytic degradation of methylene blue (MB) and ciprofloxacin (CIP) in wastewater. The prepared piezoelectric materials (BaTiO3 and BaTiO3/AgNPs) were characterized using XRD, SEM, TEM, EDS, UV-DRS, TGA, PL, BET, EIS, and chronoamperometry. The UV-DRS showed the surface plasmon resonance (SPR) of Ag nanoparticles on the surface of BaTiO3 at a wavelength of 505 nm. The TEM images revealed the average Ag nanoparticle size deposited on the surface of BaTiO3 to be in the range of 10–15 nm. The chronoamperometry showed that the photoreduction of silver nanoparticles (AgNPs) onto BaTiO3 (BTO) resulted in a piezo-electrochemical current enhancement from 0.24 to 0.38 mA. The composites (FTO/BaTiO3/AgNPs) achieved a higher degradation of MB and CIP when the photocatalysis and piezocatalysis processes were merged. Under both ultrasonic vibration and UV light exposure, FTO/BTO/AgNPs degraded about 72 and 98% of CIP and MB from wastewater, respectively. These piezoelectric thin films were shown to be efficient and reusable even after five cycles, suggesting that they are highly stable. Furthermore, the reactive oxygen species studies demonstrated that hydroxyl radicals (·OH) were the most effective species during degradation of MB, with minor superoxide radicals (·O2–) and holes (h+). From this study, we were able to show that these materials can be used as multifunctional materials as they were able to degrade both the dye and pharmaceutical pollutants. Moreover, they were more efficient through the piezo-photocatalytic process.

Anisotropic Acoustodynamics in Gigahertz Piezoelectric Ultrasonic Transducers

In this work, we employed our newly developed optical imaging method to probe detailed acoustodynamic physics in gigahertz unreleased ultrasonic transducers based on an AlN-on-silicon system, revealing mode superposition, anisotropic transduction, and dynamic mode evolution. Superpositioned upon the dominant breathing mode along the vertical direction of the AlN layer, multiple resonant lateral modes are identified, and they are shown to evolve into a surface mode beyond the piezoelectric transduction envelope, with strong anisotropic transduction brought by the shear motion of silicon. This acoustodynamic property is important for verifying and further improving design theories of broadband piezoelectric transducers and thin film piezoelectric-on-substrate systems in general.

A Battery-free Pressure Sensing System Based on Soft Piezoelectric Device for Tennis Training

In this work, a battery-free position and pressure monitoring system based on soft polyvinylidene fluoride (PVDF) piezoelectric thin film with helical coils circuit design is designed and used for tennis training. The racket surface is firstly divided into block areas and the sensing transducers are placed in each of them to detect the hitting position and force in these areas. To modify the extensibility of the system for tennis training, all the devices are connected by the S-shape helical line, which has two states: curled at usual and straight at work. Four piezoelectric patches form a series system to power the energy harvest system and wireless communication module, which help to realize the battery-free and wearable functions. A linear relation between the hitting force and induced electric voltage is experiment verified with the slope of 0.05 V/N. The system is also verified in the real tennis ball game monitoring for the hitting time and hitting position, benefiting the future application of wearable devices in artificial intelligence and the Internet of Things (IoT).

Stress-balancing in piezoelectric adjustable x-ray optics

Next-generation x-ray observatories require lightweight, high throughput optics that maintain a <0.5 arcsec resolution to probe the physics of black holes and gain understanding of the early universe. One potential type of x-ray mirror consists of a 400-μm thick curved Corning EAGLE XG® glass substrate with a Cr/Ir x-ray mirror coating deposited on the front (concave) side and an array of radio frequency sputtered Pb0.995 ( Zr0.52Ti0.48)0.99Nb0.01O3 piezoelectric thin film actuators on the back (convex) side to enable correction of figure errors. A stress-balancing process was developed to correct the figure distortion arising from thin film stresses in the actuator layers. Compressively stressed SiO2 films were deposited on the convex side of the mirror to balance the tensile integrated stress of the actuator array while also matching the film thickness distribution. Finite-element methods were used to assess the impact of film thickness distributions on the convex and concave substrate surfaces. The resulting models show peak-to-valley figure errors of 105 nm, well within the 1-μm peak-to-valley dynamic range of the piezoelectric adjusters. In contrast, when stress compensation was done with an iridium mirror film deposited on the front side, the mismatched thickness distribution results in peak-to-valley figure errors over 3 μm.

Biocompatible a-SiC:H-Based Bistable MEMS Membranes With Piezoelectric Switching Capability in Fluids

In this paper, we demonstrate biocompatible micromachined buckled membranes for the operation in liquids. The membranes feature diameters between 600 and 800 μm as well as integrated piezoelectric thin film actuators, thus enabling switching between the bistable states. The membrane material is known to be not biocompatible, hence a hydrogenated amorphous silicon carbide (a-SiC:H) layer is deposited on the surface. For demonstration purposes, a 70 nm ±3 nm thin a-SiC:H coating with a specific silicon to carbon ratio was chosen, with a negligible impact on the overall switching performance of the bistable membranes. Furthermore, a relation between the membrane center velocity at the first characteristic resonance frequency and the switching ability of a membrane in different viscous fluids is shown. Based on a small signal analysis the switching behavior can be predicted. The membranes were successfully switched in liquids with a dynamic viscosity up to 286 mPa·s. The biocompatibility of the membranes was examined by growing Caco-2 cells, a human carcinoma cell line, on a-SiC:H thin films, featuring different carbon contents and organic surface treatments. The proliferation and adhesion of the cells on the substrates are examined in an empirical cell growth and removal study. Only a-SiC:H surfaces pre-treated with an O2-plasma and coated with Collagen Type I indicated to provide an environment of improved cell adhesiveness compared to other surface treatments. The biological investigations resulted in good cell proliferation, that also depends on the altered hydrophilicity of the surface, as well as on the carbon content of the a-SiC:H thin films. This study reveals that a broad range of biocompatible a-SiC:H surfaces can be prepared, whereby the cell growth can be tailored in terms of proliferation and adhesion for different biomedical application scenarios. Finally, this paper reports on the mechanical features of bistable, buckled membranes and their suitability as a growth substrate for human cell cultures, due to the good biocompatibility of a-SiC:H thin films. We therefore suggest that it will be feasible to grow cells on bistable MEMS membranes, enabling cell experiments in liquid medical environments, with both mechanically excitable and biocompatible surfaces. [2022-0006]