Piezoelectric Thin Films Research Articles

Giant piezoelectricity in oxide thin films with nanopillar structure.

High-performance piezoelectric materials are critical components for electromechanical sensors and actuators. For more than 60 years, the main strategy for obtaining large piezoelectric response has been to construct multiphase boundaries, where nanoscale domains with local structural and polar heterogeneity are formed, by tuning complex chemical compositions. We used a different strategy to emulate such local heterogeneity by forming nanopillar regions in perovskite oxide thin films. We obtained a giant effective piezoelectric coefficient [Formula: see text] of ~1098 picometers per volt with a high Curie temperature of ~450°C. Our lead-free composition of sodium-deficient sodium niobate contains only three elements (Na, Nb, and O). The formation of local heterogeneity with nanopillars in the perovskite structure could be the basis for a general approach to designing and optimizing various functional materials.

Fully Transparent Friction‐Modulation Haptic Device Based on Piezoelectric Thin Film

AbstractStanding Lamb waves in vibrating plates enable haptic interfaces. If the out‐of‐plane displacement of these waves exceeds 1 µm at frequencies above 25 kHz, a silent friction modulation can be created between a human finger and a vibrating plate. A fully transparent friction‐modulation haptic device based on a piezoelectric thin film is demonstrated. The antisymmetric Lamb mode induced at 73 kHz allows for a functional performance that fulfills all conditions for practical use. Out‐of‐plane displacement reaches 2.9 µm when 150 V unipolar voltage is applied. The average transmittance of the whole transducer reaches 75%. The key points of this technology are: 1) a thin HfO2 layer between lead zirconate titanate film and substrate that prevents chemical reaction between them; 2) the efficient integration of transparent indium tin oxide electrodes and solution‐derived piezoelectric lead zirconate titanate thin film onto optical‐grade fused silica; and 3) the use of a transparent insulating layer made of SU‐8 photoresist.

Superior Transverse Piezoelectricity in a Halide Perovskite Molecular Ferroelectric Thin Film.

Piezoelectric materials with inherent mechanical-electric coupling effect are a crucial family of functional materials in high-end information technology. For practical applications, the transverse piezoelectric performance (d31 or d32) is mainly considered, because this parameter is a vitally important index to characterize the performance of piezoelectric thin films. However, the transverse piezoelectricity of the thin films as a key figure of merit is seldom mentioned in molecular ferroelectrics. Herein, we report that a new 1D halide perovskite ferroelectric N,N-dimethylallylammoniumCdCl3 (DMAACdCl3) exhibits an above room-temperature ferroelectric phase transition with a saturated polarization of 1.9 μC cm-2 and a coercive field of 5.0 kV cm-1. The thin film of DMAACdCl3 is successfully fabricated using an easy processing spinning method and maintains well ferroelectric properties verified by piezoresponse force microscopy (PFM). More significantly, the ferroelectric thin film offers superior transverse piezoelectricity with an in-plane piezoelectric response of about 41 pC N-1, which is about twice that of well-known piezoelectric polymer PVDF (21 pC N-1). Transverse piezoelectricity has been scarcely studied in molecular ferroelectrics, and its exploitation would play an important role in the design of next-generation smart piezoelectric devices.

Design and Characterization of High-Q SAW Resonators Based on the AlN/Sapphire Structure Intended for High-Temperature Wireless Sensor Applications

Aluminium nitride piezoelectric thin films grown on sapphire are strong candidates for high-temperature surface acoustic wave (SAW) sensors, due to their thermal stability, large bandgap, high acoustic velocity and suitable electromechanical coupling. However, thin-film resonators need more design efforts than those based on bulk crystals, due to the usually limited thickness of the piezoelectric films, and to acoustic properties disparities between the latters and their host substrate. This work presents an optimization of AlN/Sapphire-based SAW resonators with high quality factors for high-temperature applications. It combines specifically grown, 3 μm-thick aluminium nitride films, with the use of aluminium electrodes for their low density and resistivity, as an alternative to heavier electrodes like Pt. These electrodes allow for much lower mechanical losses and higher quality factors, in spite of needing passivation for increased lifetime. A standard resonator design is first presented and used for preliminary tests, in order to monitor the AlN/Sapphire structure with unprotected aluminium electrodes, for temperatures up to 600°C. A quasi-synchronous, optimized design is then proposed for higher quality factors and wireless sensing compliance. The high temperature characterizations confirmed that much larger quality factors can be retrieved from this optimized design. The quasi-synchronous resonators proposed in this study remain well-tuned for temperatures up to 400°C, and show high quality factors, as high as 3400 at 400°C.

Isomorphic contact resonance force microscopy and piezoresponse force microscopy of an AlN thin film: demonstration of a new contact resonance technique

We present a new contact resonance force microscopy (CRFM) imaging technique, isomorphic contact resonance (iso-CR), that acquires data at a constant contact resonance (CR) frequency, and hence constant tip-sample contact stiffness across the scan area. Constant CR frequency is obtained by performing force versus distance measurements to vary the applied force at each pixel (i.e. force-volume mapping mode). The CR frequency increases with increasing applied force; thus, a carefully selected target frequency will be reached for most pixels at some point in the force versus distance curve. In the iso-CR mode, the cantilever maintains an invariant vibrational shape and a constant environmental damping, thus simplifying interpretation of amplitude and quality factor contrast compared to conventional CRFM. Iso-CR imaging of a piezoelectric AlN thin film sample is demonstrated. Iso-CRFM images were obtained by mechanically driving the base of the cantilever, and iso-CR piezoresponse force microscopy (iso-CR-PFM) images were obtained by electrically biasing the tip. The PFM phase images reveal that the sample contains nanoscale Al-polar (or ‘up’) and N-polar (or ‘down’) domains, with ≈180° phase contrast between oppositely polarized domains. The PFM amplitude and Q-factor images also show ‘up’ vs. ‘down’ domain contrast, which decreases with increasing CR frequency. The frequency-dependent amplitude and Q contrast is ascribed to a frequency-dependent electrostatic contribution to the signal. Domain contrast is not observed in the CRFM (mechanically driven) images. To summarize, the iso-CR capability to control the resonance frequency across multiple excitation schemes helps elucidate the origin of the electromechanical and nanomechanical image contrast.

RETRACTED: Preparation of high-performance polyacrylonitrile piezoelectric thin film by temperature control

This article has been retracted: please see Elsevier Policy on Article Withdrawal (https://www.elsevier.com/about/our-business/policies/article-withdrawal). This article has been retracted at the request of the authors. The authors have found that the test methods used in the article are not rigorous. They simply considered the force produced by different weights. In fact, because the bottom area of the weight is different, the pressure on the film is not equal to the force. Agreed by all co-authors, we retract this paper and apologize to readers of the journal that this was not detected during the submission process.

Piezoelectric Scanning Micromirror With Built-In Sensors Based on Thin Film Aluminum Nitride

A micromirror with piezoelectric thin film aluminum nitride (AIN) as transducer material for actuation and detection is presented. Four sensor elements are integrated for closed-loop monitoring of the resonant driven microscanner with an entire footprint of 6 mm2. The sensor and actuator elements are monolithically fabricated in 150 mm SOI technology. The micromirror is characterized in terms of its scanning and sensor characteristics. A large optical scan angle of 137.9° (34.5° mechanical tilt angle corresponding) at 3385 Hz and 20 V actuation voltage is achieved. The median resonant voltage sensitivity is 4.7 °/V. For the sensor characterization, a cross-talk compensation is introduced, which significantly improves the signal quality and thus enables an accurate charge determination. The sensor signal increases linearly to the deflection. A dynamic sensor angle sensitivity of 48.4 fC/° is reached. Possible applications of the micromirror are consumer electronics, such as LIDAR or medical applications, such as fluorescence microscopy.

Thin film formation of PEDOT conductive polymer and PVDF piezoelectric polymer by dip‐coating method assuming application to flexible power generation element

AbstractPoly(3,4‐ethylenedioxythiophene) (PEDOT) conductive polymer and polyvinylidene difluoride (PVDF) piezoelectric polymer thin films were formed by dip‐coating method. The relationship between drawing speed and film thickness was experimentally investigated. Dip‐coated PEDOT and PVDF thin films were characterized and confirmed that both had surely worked as electrode and piezoelectric film of a flexible power generation element, respectively. Using dip‐coating method, these thin films were successfully formed on 3D surface of a bellows shaped spring made of silicone rubber.

On the design of piezoelectric MEMS microspeaker for the sound pressure level enhancement

This study aims to design and realize MEMS microspeakers with high sound pressure level (SPL) for in-ear applications. On the basis of high piezoelectric coefficient PZT thin film, the proposed suspension-spring and dual-electrode designs enhance the out-of-plane displacement of central diaphragm to improve low-frequency SPL. The preliminary tests show that both proposed single-curve and dual-curve spring designs have higher low-frequency SPL than that of the reference clamped diaphragm microspeaker. Measured in the standard ear simulator systems with 2 Vpp driving voltage, the proposed dual-curve spring design demonstrates good performance at low frequencies. At the resonant frequency of 1.85 kHz, the dual-curve spring design is 28 dB SPL higher than the clamped diaphragm. In addition, from 100 Hz to 3 kHz, more than 10 dB SPL enhancement is achieved. Furthermore, the total harmonic distortion (THD) of dual-curve spring design is less than 2% in most of the audio range. Compared with the published literature of piezoelectric MEMS microspeakers, this study shows a reasonable SPL with the smallest diaphragm area of 1 mm2.

Quality factor boosting of bulk acoustic wave resonators based on a two dimensional array of high-Q resonant tanks

An innovative resonance excitation methodology of a two-dimensional array of uniformly patterned structure into extremely high-quality factor (Q) mode using engineered frequency-matched thin piezoelectric film on substrate (TPoS) transducer is demonstrated in this Letter. The passive Q-enhancement can be attained lithographically without adding to the power requirement of the system. A 500 nm thick piezoelectric material facilitates the inter-electrical-mechanical domain coupling, and the displacement generated at the transducer periphery drives the patterned 10 μm thick single crystal silicon plate into various resonant modes. Using this topology, unexplored modes can be generated as well by the forced vibration feature of the interacting resonant tanks. Utilizing a multiport port excitation scheme, multiple modes are explored, while the primary focus is maintained on the Lamé mode of the array. In vacuum for an operational frequency of 58.56 MHz, an exceptionally high-quality factor of 58 276 is attained with a motional resistance of 1250 Ω and this is the record-high figure of merit (f × Q = 3.4 × 1012) attained for aluminum nitride-based TPoS based resonators. This successful demonstration of the energy coupling scheme paves a path for the study of existing and investigation of unexplored high-Q modes spanning over a wide range of frequency, making these devices a potential candidate for radio frequency front-end applications. The CMOS compatibility of aluminum nitride makes it an attractive solution for achieving low phase noise monolithically integrated CMOS–microelectromechanical systems oscillators.

RETRACTED: Preparation of high-performance polymer piezoelectric thin film by electric field induction technology

This article has been retracted: please see Elsevier Policy on Article Withdrawal ( https://www.elsevier.com/about/our-business/policies/article-withdrawal ). This article has been retracted at the request of the authors. The authors have found that the test methods used in the article are not rigorous. They simply considered the force produced by different weights. In fact, because the bottom area of the weight is different, the pressure on the film is not equal to the force. All of the co-authors have agreed to retract this paper and apologise to readers of the journal.

Thickness Dependence of crack initiation and propagation in stacks for piezoelectric microelectromechanical systems

Piezoelectric thin films are vulnerable to fracture, which results in degradation of the structural integrity and device performance in piezoelectric microelectromechanical systems (PiezoMEMS). This work explains the fracture process as a combination of a crack initiation event in the piezoelectric film followed by crack propagation through the remaining layers. Biaxial bending tests using the Ball-on-three-Balls (B3B) technique were performed on stacks containing Pb(Zr0.52Ti0.48)O3 (PZT) thin films of varying thicknesses grown on Si wafers (coated with thin LaNiO3/SiO2 layers). The fracture initiates in the PZT film, arrests in the compressive SiO2 layer, prior to failure of the Si substrate. Weibull analyses show a significant effect of the thin film thickness on the stack's strength; the characteristic strength and Weibull modulus being σ0 ∼1110 MPa and m ∼28, σ0 ∼1060 MPa and m ∼26, and σ0 ∼880 MPa and m ∼10 for the 0.7 µm, 1.3 µm, and 1.8 µm film stack, respectively and σ0 ∼ 1820 MPa and m ∼3 for the Si wafer. A stress-energy criterion using finite fracture mechanics explains the dependence of crack initiation load on the PZT layer thickness, whereas linear elastic fracture mechanics is employed to rationalize crack propagation through the stack.

A Low Temperature Drifting Acoustic Wave Pressure Sensor with an Integrated Vacuum Cavity for Absolute Pressure Sensing.

In this paper we demonstrate a novel acoustic wave pressure sensor, based on an aluminum nitride (AlN) piezoelectric thin film. It contains an integrated vacuum cavity, which is micro-fabricated using a cavity silicon-on-insulator (SOI) wafer. This sensor can directly measure the absolute pressure without the help of an external package, and the vacuum cavity gives the sensor a very accurate reference pressure. Meanwhile, the presented pressure sensor is superior to previously reported acoustic wave pressure sensors in terms of the temperature drift. With the carefully designed dual temperature compensation structure, a very low temperature coefficient of frequency (TCF) is achieved. Experimental results show the sensor can measure the absolute pressure in the range of 0 to 0.4 MPa, while the temperature range is from 20 °C to 220 °C with a TCF of −14.4 ppm/°C. Such a TCF is only about half of that of previously reported works.

Features of the receiving of piezoelectric thin films by plasma spraying of powdery AlN

Оne of the promising materials in solid state electronics is the AlN compound. A wide range of semiconductor devices are produced from it, such as photodetectors, LEDs, piezoelectric converters, etc. But the widespread use of products based on AlN prevents low manufacturability designs based on it. In this regard, the development of new technologies for the production of devices based on AlN is relevant. The work is devoted to the study of thin AlN films obtained by plasma spraying from AlN powder. The review of existing technologies of production of thin films AlN is carried out. Their advantages and disadvantages are discussed. Information on the modernization of the VUP-5 installation, which allowed to spray the AlN film from the powdered state, is given. One of the significant advantages of the process developed in this work is that the substrate is heated to temperatures no higher than 300 oC, which in turn allows to combine this technology with the technology of silicon semiconductor devices. As a result, films with a thickness of 200 nm on various substrates were obtained and their surface structure was studied. It is shown that AlN films deposited on single crystal substrates such as diamond and silicon have the least roughness, while films on sitall have the worst roughness. The transmission spectra of the obtained AlN films were investigated by IR spectroscopy. With their help, it was shown that a polycrystalline AlN layer oriented in the crystallographic direction 002 is formed on the substrate. The piezoelectric properties of the obtained films were investigated by scanning probe microscopy. It is shown that their piezoelectric coefficient d 33 is 60% of the value for a single-domain single-crystal sample for a diamond substrate, which indicates a sufficiently high quality of the resulting film. It is concluded that, although the quality of the layers strongly depends on the substrate, nevertheless, they exhibit a significant piezoelectric effect, which allows the use of this method for the manufacture of piezoelectric sensors, ultrasonic emitters, etc.

Deposition and characterization of ZnO thin films on GaAs and Pt/GaAs substrates

This work reports the deposition and characterization of piezoelectric ZnO thin films on semi-insulating GaAs substrates for the fabrication of bulk acoustic waves sensors. ZnO films are deposited at 350 °C and low deposition rate using reactive radio frequency magnetron sputtering. The use of a Pt bottom electrode, between ZnO and GaAs, with and without Ti buffer layer, as well as the effect of the substrate crystallographic orientation are investigated. The characterization of the deposited films is performed to determine the optimal parameters for obtaining high-quality films and ZnO residual conductivity. ZnO films are textured along the c-axis for all GaAs cuts. The highest structural quality is obtained on (100) GaAs substrates. Moreover, the presence of the Ti buffer layer improves the texture quality, surface roughness and residual stresses. The lowest residual conductivity is determined for the ZnO/Pt/Ti/GaAs structure. However, a slight diffusion of Ga and Ti into Pt is observed, which may be of concern if Pt is used as a bottom electrode. Therefore, for thickness excitation functioning, further optimization of the Pt/GaAs interface has to be considered.

Structure and piezoelectricity properties of V-doped ZnO thin films fabricated by sol-gel method

Vanadium doped (2.5 at%) ZnO piezoelectric thin films were synthesized on Pt/Ti/SiO2/Si by sol-gel technology. The effects of annealing process on the structure and piezoelectric properties of films were studied. The crystal structure and micro morphology were analyzed by X-ray diffractometer (XRD) and scanning electron microscope (SEM) respectively. The results showed that V-doped ZnO thin films were oriented in (002) direction to some extent. With the increasing of annealing temperature or time, grain size of the film increased, and the density decreased obviously. The piezoelectric properties were measured by piezoresponse force microscopy (PFM) method. As the annealing temperature or time increased, the piezoelectric performance of V-doped ZnO thin film firstly increased and then decreased. The piezoelectric coefficient d33 of V-doped ZnO thin film annealed at 600 °C for 45min followed by 700 °C for 30min was 240 pm/V. Compared with other preparation methods, the sol-gel improved the piezoelectric performance of V-doped ZnO thin film due to the more sufficient and homogeneous doping.

Flexible piezoelectric nanogenerators using metal-doped ZnO-PVDF films

Piezoelectric nanomaterial-polymer composites represent a unique paradigm for making flexible energy harvesting and sensing devices with enhanced devices’ performance. In this work, we studied various metal doped ZnO nanostructures, fabricated and characterized ZnO nanoparticle-PVDF composite thin film, and demonstrated both enhanced energy generation and motion sensing capabilities. Specifically, a series of flexible piezoelectric nanogenerators (PENGs) were designed based on these piezoelectric composite thin films. The voltage output from cobalt (Co), sodium (Na), silver (Ag), and lithium (Li) doped ZnO-PVDF composite as well as pure ZnO-PVDF samples were individually studied and compared. Under the same experimental conditions, the Li-ZnO based device produces the largest peak-to-peak voltage (3.43 Vpp) which is about 9 times of that of the pure ZnO based device, where Co-ZnO, Na-ZnO and Ag-ZnO are 1.2, 4.9 and 5.4 times, respectively. In addition, the effect of doping ratio of Li-ZnO is studied, and we found that 5 % is the best doping ratio in terms of output voltage. Finally, we demonstrated that the energy harvested by the device from finger tapping at ∼2 Hz can charge a capacitor with a large output power density of 0.45 W/cm3 and light up an ultraviolet (UV) light-emitting diode (LED). We also showed the device as a flexible wearable motion sensor, where different hand gestures were detected by the device with distinctive output voltage amplitudes and patterns.

Fabrication and application of flexible AlN piezoelectric film

A fabrication method for high quality flexible aluminum nitride piezoelectric thin films based on micro-nano fabrication technology is reported in this paper. Molybdenum (Mo)/aluminum nitride (AlN)/aluminum (Al) structure was prepared on silicon(100) by sputtering. Then, deep-reactive ion etching technology was used to remove the silicon material used for support, then the Mo/AlN/Al flexible sandwich film was obtained. The full-width at half-maximum of the AlN film (002) peak before and after removal of silicon was only 1.47° and 1.49°, respectively. No obvious cracks in the film under bending conditions were observed with a scanning electron microscope. Capacitance–voltage (C–V) test results showed that the capacitance curve of the sandwich structure was consistent with that of the double-sided Schottky junction. The flexible piezoelectric film with an active area of 2 × 2 mm2 exhibited an average peak-to-peak generated voltage of 0.6 V in energy harvesting tests, which indicates that the method could be used to fabricate high quality flexible energy collectors based on AlN film. The preparation scheme in this paper is compatible with integrated circuit processes and simple to implement, which lays a foundation for the wide application of flexible piezoelectric devices.

Flexible Piezoelectric Nanofibers/Polydimethylsiloxane‐Based Pressure Sensor for Self‐Powered Human Motion Monitoring

Self‐powered wearable devices have attracted significantly increased attention in human motion monitoring. These flexible physical sensors conformally attach to the surface of the skin to provide new applications in human‐activity monitoring. Herein, a highly flexible and self‐powered piezoelectric pressure sensor for real‐time human motion detecting using a piezoelectric thin film with high filling ratio of lead zirconate titanate (PZT) nanofibers (NFs) in polydimethylsiloxane (PDMS) is reported. Due to the large dielectric constant of PZT and elastic properties of PDMS, the sensor performs in a wide linear region (1.25–250 kPa) with great linearity (pressure–voltage 0.9909) and good reproducibility over 2000 cycles. Using the sensor, various human body motions are detected, including joint bending, subtle/large wrist deformations, wrist and some common sporting movements. In addition, a sensor array is successfully developed to realize motion tracking, showing strong potential for application in personal recognition.

A new blood pressure prediction method using wrist pulse examination

Wrist pulse examination plays an imperative role in clinical practice including contemporary medicine systems like Ayurveda. Wrist pulse measurement system can also be used for blood pressure (BP) monitoring. This article proposes an extension of wrist pulse analysis system for BP monitoring. Wrist pulse signals are captured using three piezoelectric thin film sensors at the radial artery of the wrist as per Ayurvedic pulse diagnosis method. The wave features such as Spatial Pulse Transit Time (SPTT) and Spatial Pulse Wave Velocity (SPWV) are extracted from the captured wrist pulses. Pearson’s correlation coefficient is computed to find the correlation between SPTT and BP measured using standard measuring instrument (Omron HEM-6131). A moderate correlation is found between SPTT and Systolic Blood Pressure (SBP) with correlation coefficient of 0.72. SPWV values computed from SPTT are used to develop an empirical model for BP in terms of SPWV. Limitations of the empirical model lead to a theoretical model using Poiseuilli law. The Mean Artery Pressure (MAP) using theoretical model is recorded for 41 participants. The deviation of MAP thus computed as compared to the readings taken from clinically validated BP monitoring device is found to be varying in the range of 4.5 ± 2.3 mmHg.