Piezoelectric Thin Films Research Articles

Stretchable Piezoelectric Sensing Systems for Self‐Powered and Wireless Health Monitoring

AbstractContinuous monitoring of human physiological signals is critical to managing personal healthcare by early detection of health disorders. Wearable and implantable devices are attracting growing attention as they show great potential for real‐time recording of physiological conditions and body motions. Conventional piezoelectric sensors have the advantage of potentially being self‐powered, but have limitations due to their intrinsic lack of stretchability. Herein, a kirigami approach to realize a novel stretchable strain sensor is introduced through a network of cut patterns in a piezoelectric thin film, exploiting the anisotropic and local bending that the patterns induce. The resulting pattern simultaneously enhances the electrical performance of the film and its stretchability while retaining the mechanical integrity of the underlying materials. The power output is enhanced from the mechano‐electric piezoelectric sensing effect by introducing an intersegment, through‐plane, electrode pattern. By additionally integrating wireless electronics, this sensing network could work in an entirely battery‐free mode. The kirigami stretchable piezoelectric sensor is demonstrated in cardiac monitoring and wearable body tracking applications. The integrated soft, stretchable, and biocompatible sensor demonstrates excellent in vitro and ex vivo performances and provides insights for the potential use in myriad biomedical and wearable health monitoring applications.

A Geometrical Study on the Roof Tile-Shaped Modes in AlN-Based Piezoelectric Microcantilevers as Viscosity⁻Density Sensors.

Cantilever resonators based on the roof tile-shaped modes have recently demonstrated their suitability for liquid media monitoring applications. The early studies have shown that certain combinations of dimensions and order of the mode can maximize the Q-factor, what might suggest a competition between two mechanisms of losses with different geometrical dependence. To provide more insight, a comprehensive study of the Q-factor and the resonant frequency of these modes in microcantilever resonators with lengths and widths between 250 and 3000 µm and thicknesses between 10 and 60 µm is presented. These modes can be efficiently excited by a thin piezoelectric AlN film and a properly designed top electrode layout. The electrical and optical characterization of the resonators are performed in liquid media and then their performance is evaluated in terms of quality factor and resonant frequency. A quality factor as high as 140 was measured in isopropanol for a 1000 × 900 × 10 µm3 cantilever oscillating in the 11th order roof tile-shaped mode at 4 MHz; density and viscosity resolutions of 10−6 g/mL and 10−4 mPa·s, respectively are estimated for a geometrically optimized cantilever resonating below 1 MHz.

Coupled BAW/SAW Resonators Using AlN/Mo/Si and AlN/Mo/GaN Layered Structures

We demonstrate coupled bulk and surface acoustic wave (BAW/SAW) resonators with significantly improved coupling efficiency as compared to SAW only devices. Two sets of stacks are investigated: one uses a thin film piezoelectric material with a bottom electrode on GaN substrate (i.e., AlN/Mo/GaN) and another includes a piezoelectric thin film with bottom metal on a Si substrate (i.e., AlN/Mo/Si). A vibrating BAW transducer induces a SAW in the substrate and between two sets of interdigitated transducers improving the coupling efficiency by a factor of ${\geq }\, \mathsf {8}{x}$ as compared to the conventional SAW. The performance of the coupled BAW/SAW resonators using each of the Si and GaN substrates is compared using FEM analysis. By using FEM, coupling efficiency of the hybrid BAW/SAW with AlN/Mo/Si stack is 2.4%, whereas it is 3.2% for the SAW using AlN/Mo/GaN stack.

Individually free-standing piezoelectric nanotubes-based BaTiO3/CoFe2O4 magnetoelectric thin films

In this study, individually free-standing piezoelectric nanotubes-based magnetoelectric thin films were synthesized by complexation of one-dimensional (1D) BaTiO3 as the piezoelectric phase, and CoFe2O4 as the magnetostrictive phase. Particularly, the individually TiO2 free-standing nanotubes were developed onto a Ti substrate via metal anodizing to obtain a high standing stability of 1D piezoelectric structure and a filling volume of magnetostrictive phase, simultaneously for fabrication of magnetoelectric thin films. After BaTiO3 perovskite conversion from the TiO2 nanotubes, an enough volume still remains for sufficient self-filling of CoFe2O4 between the BaTiO3 nanotubes. Finally, BaTiO3/CoFe2O4 magnetoelectric films were found to exhibit a dense structure being suitable for strong magnetostriction-mediated piezoelectric voltages. The structure design, based on individually free-standing nanotubes, would be feasible for flexible magnetoelectric devices such as health monitoring sensors, energy harvesters, and memory devices.

Large piezoelectricity on Si from highly (001)-oriented PZT thick films via a CMOS-compatible sputtering/RTP process

Integration of lead zirconate titanate (PZT) ferroelectrics into CMOS-Si technology has become a perennial challenge due to the continuously shrinking thermal budget in semiconductor processing. In this work, Pb(Zr0.53Ti0.47)O3 (PZT) thick films (∼1 µm) were prepared on LaNiO3 buffered (111)Pt/Ti/SiO2/(100) Si substrates via a low temperature (350 °C) sputtering deposition followed by a rapid thermal process (RTP). This two-step fabrication process resulted in a highly (001)-oriented perovskite PZT film with a dense, fine-grain morphology and reduced strains, hence an optimal electrical performance intrinsic to the chemical composition was achieved. EDS and XPS analyses verified a desirable evolution of the chemical stoichiometry in the RTP, as well as the expected chemical bonding states of the elements in the annealed films. Moreover, prototypical piezoelectric thin film cantilevers fabricated from the PZT/LaNiO3/Pt/Ti/SiO2/(100)Si heterostructure yielded a large transverse piezoelectric coefficient (e31,f) up to 15.7 C/m2, together with a high coupling coefficient k2∼0.23 for energy conversion efficiency. The PZT films showed high e31,f coefficients (∼10 C/m2) even when the RTP annealing time was reduced down to 2 min. Such a piezoelectric performance is close to the highest ones reported in the literature from epitaxial or highly-oriented PZT films, which require a much higher thermal budget for processing. These high quality PZT films open up many possibilities for the integration of piezoelectricity into Si-based micro-electro-mechanical systems (MEMS).

Recent progress in piezoelectric thin film fabrication via the solvothermal process

The reaction conditions are regulated to realize the preparation of a high piezoelectric thin film via a solvothermal process.

Piezoelectric ZnO thin films for 2DOF MEMS vibrational energy harvesting

Zinc oxide (ZnO) is an environmental-friendly semiconducting, piezoelectric and non-ferroelectric material, and plays an essential role for applications in microelectromechanical systems (MEMS). In this work, a fully integrated two-degree-of-freedom (2DOF) MEMS piezoelectric vibration energy harvester (p-VEH) was designed and fabricated using ZnO thin films for converting kinetic energy into electrical energy. The 2DOF energy harvesting system comprises two subsystems: the primary one for energy conversion and the auxiliary one for frequency adjustment. Piezoelectric ZnO thin film was deposited using a radio-frequency magnetron sputtering method onto the primary subsystem for energy conversion from mechanical vibration to electricity. Dynamic performance of the 2DOF resonant system was analyzed and optimized using a lumped parameter model. Two closely located but separated peaks were achieved by precisely adjusting mass ratio and frequency ratio of the resonant systems. The 2DOF MEMS p-VEH chip was fabricated through a combination of laminated surface micromachining process, double-side alignment and bulk micromachining process. When the fabricated prototype was subjected to an excitation acceleration of 0.5 g, two close resonant peaks at 403.8 and 489.9 Hz with comparable voltages of 10 and 15 mV were obtained, respectively.

Preparation of high-quality stress-free (001) aluminum nitride thin film using a dual Kaufman ion-beam source setup

We proposed and demonstrated a preparation method of (001) preferentially oriented stress-free AlN piezoelectric thin films. The AlN thin films were deposited by a reactive sputtering technique at substrate temperatures up to 330 °C using a dual Kaufman ion-beam source setup. We deposited the AlN on Si (100), Si (111), amorphous SiO2, and a (001) preferentially oriented Ti thin film and compared their crystallographic, optical, and piezoelectric properties. The AlN thin films deposited on the (001) preferentially oriented Ti thin films have the highest crystallographic quality. The stress-free AlN reached a high value of the piezoelectric coefficient d33 = (7.33 ± 0.08) pC·N−1. The properties of the AlN thin film prepared at such low temperatures are suitable for numerous microelectromechanical systems, piezoelectric sensors, and actuators monolithically integrated with complementary metal-oxide-semiconductor signal-processing circuits.

Strain-Mediated Substrate Effect on the Dielectric and Ferroelectric Response of Potassium Sodium Niobate Thin Films

If piezoelectric thin films sensors based on K0.5Na0.5NbO3 (KNN) are to achieve commercialization, it is critical to optimize the film performance using low-cost scalable processing and substrates. Here, sol–gel derived KNN thin films are deposited using a solution with 5% of potassium excess on Pt/TiO2/SiO2/Si and Pt/SrTiO3 substrates, and rapid thermal annealed at 750 °C for 5 min. Despite an identical film morphology and thickness of ~335 nm, an in-plane stress/strain state is found to be tensile for KNN films on Pt/TiO2/SiO2/Si, and compressive for those on Pt/SrTiO3 substrates, being related to thermal expansion mismatch between the substrate and the film. Correspondingly, KNN films under in-plane compressive stress possess superior dielectric permittivity and polarization in the parallel-plate-capacitor geometry.

Lamb Wave Focusing Transducer for Efficient Coupling to Wavelength-Scale Structures in Thin Piezoelectric Films

This paper describes the theoretical and experimental investigation of interdigitated transducers capable of producing focused acoustical beams in thin film piezoelectric materials. A mathematical formalism describing focused acoustical beams, lamb beams, is presented and related to their optical counterparts in two- and three-dimensions. A novel Fourier domain transducer design methodology is developed and utilized to produce near diffraction limited focused beams within a thin film AlN membrane. The properties of the acoustic beam formed by the transducer were studied by means of Doppler vibrometry implemented with a scanning confocal balanced homodyne interferometer. The Fourier domain modal analysis confirmed that 83% of the acoustical power was delivered to the targeted focused beam which was constituted from the lowest order symmetric mode, while 1% was delivered unintentionally to the beam formed from the anti-symmetric mode, and the remaining power was isotropically scattered. The transmission properties of the acoustic beams as they interact with devices with wavelength scale features were also studied, demonstrating minimal insertion loss for devices in which a subwavelength and pinhole apertures were included. [2018-0059]

Probing the dielectric, piezoelectric and magnetic behavior of CoFe2O4/BNT-BT0.08 composite thin film fabricated by sol-gel and spin-coating methods

We investigated in this paper a novel bilayer composite obtained by sol-gel and spin coating of the ferroelectric 0.92Na0.5Bi0.5TiO3–0.08BaTiO3 (abbreviated as BNT-BT0.08) and ferromagnetic CoFe2O4 phases, for miniature low-frequency magnetic sensors and piezoelectric sensors. This heterostructure, deposited on Si-Pt substrate (Si-Pt/CoFe2O4/BNT-BT0.08), was characterized using selected method such as: X-ray diffraction, dielectric spectroscopy, piezoelectric force microscopy, SQUID magnetometry, atomic force microscopy/magnetic force microscopy, and advanced methods of transmission electron microscopy. CoFe2O4/BNT-BT0.08 ferromagnetic–piezoelectric thin films show good magnetization, dielectric constant and piezoelectric response. The results of analyses and measurements reveal that this heterostructure can have applications in high-performance magnetoelectric devices at room temperature.

Tuneable Q-Factor of MEMS Cantilevers with Integrated Piezoelectric Thin Films.

In atomic force microscopes (AFM) a resonantly excited, micro-machined cantilever with a tip is used for sensing surface-related properties. When targeting the integration of AFMs into vacuum environments (e.g., for enhancing the performance of scanning electron microscopes), a tuneable Q-factor of the resonating AFM cantilever is a key feature to enable high speed measurements with high local resolution. To achieve this goal, in this study an additional mechanical stimulus is applied to the cantilever with respect to the stimulus provided by the macroscopic piezoelectric actuator. This additional stimulus is generated by an aluminum nitride piezoelectric thin film actuator integrated on the cantilever, which is driven by a phase shifted excitation. The Q-factor is determined electrically by the piezoelectric layer in a Wheatstone bridge configuration and optically verified in parallel with a laser Doppler vibrometer. Depending on the measurement technique, the Q-factor is reduced by a factor of about 1.9 (electrically) and 1.6 (optically), thus enabling the damping of MEMS structures with a straight-forward and cheap electronic approach.

Electrical extraction of piezoelectric constants

The piezoelectric materials are incorporated in smart structure to exhibit specific functionality. The activity of piezoelectric material dimension and electrical properties can be changed with an applied stress. These variations are translated to a change in the capacitance of the structure. This work takes a close outlook on the use of the capacitance-voltage measurements for the extraction of double piezoelectric thin film material deposited at the two faces of a flexible steel sheet. The piezoelectric thin film materials have been deposited using RF sputtering techniques. Gamry analyzer references 3000 was used to collect the capacitance-voltage measurements from both layers. The developed algorithm extracts directly the piezoelectric coefficients knowing the film thickness, the applied voltage, and the capacitance ratio. The capacitance ratio is the ratio between the capacitances of the film when the applied field in antiparallel and parallel to the polling field direction, respectively. The method has been calibrated using a piezoelectric bulk ceramic and validated by comparing the result with the reported values in the literature. The extracted values using the current approach match well the values extracted by other existing methods.

Flexible vibrational energy harvesting devices using strain-engineered perovskite piezoelectric thin films

The material properties of Pb(Zr,Ti)O3 (PZT) thin film with a LaNiO3 (LNO) buffer layer on an ultra-thin Ni-Cr-based austenitic steel metal foil substrate are systematically investigated for flexible piezoelectric vibrational energy harvesting device applications. The use of the flexible austenitic metal foil substrate with a high thermal expansion coefficient results in the large compressive stress in the deposited PZT thin film, and subsequently produces much enhanced ferroelectric polarization of the film. The measured polarization value of the PZT film on the flexible metal foil substrate is over 50 µC/cm2, showing a great improvement in comparison with the film on the conventional Pt/Ti/SiO2/Si substrate. Simultaneously, the LNO buffer layer prevents any undesirable reactions, provides uniform nucleation sites and promotes easy crystallization of the PZT thin film, achieving the highly dense and uniform microstructure of the film with a smooth surface. Using the strain-engineered PZT thin film, a small-scale and flexible vibrational energy harvester is fabricated using a simple punching process without any complex microelectromechanical system (MEMS) or etching processes. The maximum power and the corresponding peak voltage of the device are 5.6 µW and 690 mV, respectively, which are much higher values than those of MEMS-based vibrational energy harvester. The improved device performance of our flexible small-scale vibrational energy harvester is due to the enhanced PZT film properties by the stress engineering, the increased flexure and deflection of the flexible structure, and the easy vibrational sensitivity at low acceleration of 0.5g, compared to the MEMS-based device integrated with conventional Si-based substrates. The results prove that our strain-tuned PZT thin film-based flexible vibrational piezoelectric energy harvester is a promising candidate for autonomous power systems and future small-scale mobile platforms.

Fabrication of ZnO/Si lamb wave acoustic devices

Surface acoustic wave devices have widespread applications in various fields including in communication and wireless sensing devices. However some shortcomings like lower sensitivity and the corrosion of IDT’s limit their application as sensing devices. Lamb wave SAW devices have proved to be promising for sensing applications. In the present work effort has been made to find out the optimum etching temperature in order to get the sufficiently fast etch rate and smooth surface for the fabrication of SiO2/Si membrane for Lamb wave acoustic (LWA) devices. Piezoelectric ZnO thin film deposited using rf sputtering technique has been integrated with the SiO2/Si membrane. The bulk etching of Silicon has been carried out from back side of each LWA device using wet etching. The devices are packaged and frequency response is studied using a network analyzer. The antisymmetric and symmetric modes of the Lamb wave devices have been identified.

Process optimization and device variation of Mg-doped ZnO FBARs

Thin film bulk acoustic resonator (FBAR) plays a very important role in radio frequency (RF) filters used in cell phone and other wireless systems. Although FBAR is commercialized, the design/process interactions on the frequency response variation in FBAR device are still lacking. Design and fabrication are two crucial aspects affecting FBAR device performance. In this report, various solidly mounted resonators (SMR) were designed, fabricated and analyzed to study wafer-level site-to-site RF variation on design and fabrication process. As a key process step for SMR FBAR, the optimization process of Mg-doped ZnO piezoelectric thin film deposition was studied by varying thin film sputtering conditions using various sputtering targets and by post annealing treatment after the deposition. The quality of this crucial layer was verified by XRD on its (0 0 2) crystallization and wafer-level FBAR RF characterization. FBAR devices with high quality were fabricated with an excellent resonant behavior near 2 GHz and a maximum return loss of −15 to 25 dB. Quality factor Q ranges from 400 to 800, with a coupling coefficient keff2 of 1.5–3%. Wafer-level and wafer-to-wafer variation of central frequency are within 1.8–2.1 GHz. Computer simulation verified that this frequency variation correlates to the piezoelectric film variation of 1.6–1.9 μm. Process control on this piezoelectric thin film is essential to maintaining the resonator frequency-controlled value when building duplexer RF circuits. The dependency of RF performance on FBAR size, density and orientation is not obvious, compared to that of the wafer-level FBAR device variation on fabrication process. Regarding to the Mg-doping effect in MgxZn1-xO piezoelectric film, the amount of Mg in MgxZn1-xO film during the sputtering process must be properly controlled within 30% to keep the piezoelectric quality. The average acoustic speed of the Mg-doped ZnO film is 6870 m/s with the estimated range of 5760–7980 m/s, which is better than that of pure ZnO film (6330 m/s).

Sol‐gel processed highly (100)‐textured (K, Na)NbO3‐based lead‐free thin films: Effect of pyrolysis temperature

AbstractRecently, lead‐free piezoelectric thin films have received increasing attention due to the growing demands for mircoelectromechanical systems and the significant progress in lead‐free piezoelectric research. Here, potassium sodium niobate [(K, Na)NbO3 (KNN)]‐based thin films were fabricated via a sol‐gel method. The effects of pyrolysis temperature on the resulting microstructure and electrical properties of KNN‐based films were investigated. The KNN‐based film pyrolyzed at 550°C and annealed at 700°C shows a dominant (100) orientation with a high texturing degree of 91.7%. The microstructures, morphologies, piezo‐ and ferroelectric properties of the KNN‐based films were discussed in association with different pyrolysis temperatures. The crystallization mechanism of the (100) textured KNN‐based thin films was elaborated in detail.

Ultrahigh Frequency Ultrasonic Transducers Design with Low Noise Amplifier Integrated Circuit.

This paper describes the design of an ultrahigh frequency ultrasound system combined with tightly focused 500 MHz ultrasonic transducers and high frequency wideband low noise amplifier (LNA) integrated circuit (IC) model design. The ultrasonic transducers are designed using Aluminum nitride (AlN) piezoelectric thin film as the piezoelectric element and using silicon lens for focusing. The fabrication and characterization of silicon lens was presented in detail. Finite element simulation was used for transducer design and evaluation. A custom designed LNA circuit is presented for amplifying the ultrasound echo signal with low noise. A Common-source and Common-gate (CS-CG) combination structure with active feedback is adopted for the LNA design so that high gain and wideband performances can be achieved simultaneously. Noise and distortion cancelation mechanisms are also employed in this work to improve the noise figure (NF) and linearity. Designed by using a 0.35 μm complementary metal oxide semiconductor (CMOS) technology, the simulated power gain of the echo signal wideband amplifier is 22.5 dB at 500 MHz with a capacitance load of 1.0 pF. The simulated NF at 500 MHz is 3.62 dB.

Design and fabrication of prototype piezoelectric adjustable X-ray mirrors.

Lynx, a next generation X-ray observatory concept currently under study, requires lightweight, high spatial resolution X-ray mirrors. Here we detail the development and fabrication of one of the candidate technologies for Lynx, piezoelectric adjustable X-ray optics. These X-ray mirrors are thin glass shell mirrors with Cr/Ir X-ray reflective coatings on the mirror side and piezoelectric thin film actuators on the actuator side. Magnetron sputtering was used to deposit metal electrodes and metal-oxide piezoelectric layers. The piezoelectric (Pb0.995(Zr0.52Ti0.48)0.99Nb0.01O3) was divided into 112 independent piezoelectric actuators, with 100% yield achieved. We discuss the fabrication procedure, residual thermal stresses and tuning of the Cr/Ir coating stress for the purposes of stress balancing.

Flexible Piezoelectric Energy-Harvesting Exploiting Biocompatible AlN Thin Films Grown onto Spin-Coated Polyimide Layers

The increasing demand of piezoelectric energy harvesters for wearable and implantable applications requires biocompatible materials and careful structural device design, paying special attention to the conformability characteristics, properly tailored to scavenge continuously electrical energy even from the tiniest body movements. This paper provides a comprehensive study on a flexible and biocompatible aluminum nitride (AlN) energy harvester based on a new alternative fabrication approach, exploiting a thin polyimide (PI) substrate, prepared by spin coating of precursors solution. This strategy allows manufacturing substrates with adjustable thickness to meet conformability requirements. The device is based on a piezoelectric AlN thin film, sputtered directly onto the soft PI substrate, without poling/annealing processes and patterned by simple and low cost microfabrication technologies. AlN active layer, grown on soft substrate, exhibits good morphological and structural properties with roughness root m...