Thin-film Transducers Research Articles

Enhancing Ultrasonic Echo Response of AlN Thin Film Transducer Deposited by RF Magnetron Sputtering.

Accurate measurement of the pretightening stress for bolts has great significance for improving the assembly quality and safety, especially in severe environments. In this study, AlN thin film transducers were deposited on GH4169 nickel base alloy bolts using the RF magnetron sputtering, enabling a systematic investigation into the correlation between structures and the intensity of ultrasonic echo signals. Employing the finite element method resulted in consistency with the experimental data, enabling further exploration of the enhancement mechanism. With the increasing thickness of both the piezoelectric layer and the electrode layer, the intensity of the ultrasonic echo signals saw a great enhancement. The maximum-intensity observed increase is 14.7 times greater than that of the thinnest layers. Specifically, the thicker piezoelectric layer improves its mechanical displacement, while the increased thickness of the electrode layer contributes to better densification. An electrode diameter of nearly 4 mm is optimal for an AlN thin film transducer of M8 bolts. For pretightening the stress measurement, the sample with a strong and stable echo signal shows a low measurement error of pretightening below ±2.50%.

Critical Issues on the Surface Functionalization of Plasmonic Au-Ag/TiO2 Thin Films with Thiolated Oligonucleotide-Based Biorecognition Elements.

This work reports on the surface functionalization of a nanomaterial supporting localized surface plasmon resonances (LSPRs) with (synthetic) thiolated oligonucleotide-based biorecognition elements, envisaging the development of selective LSPR-based DNA biosensors. The LSPR thin-film transducers are composed of noble metal nanoparticles (NPs) embedded in a TiO2 dielectric matrix, produced cost-effectively and sustainably by magnetron sputtering. The study focused on the immobilization kinetics of thiolated oligonucleotide probes as biorecognition elements, followed by the evaluation of hybridization events with the target probe. The interaction between the thiolated oligonucleotide probe and the transducer's surface was assessed by monitoring the LSPR signal with successive additions of probe solution through a microfluidic device. The device was specifically designed and fabricated for this work and adapted to a high-resolution LSPR spectroscopy system with portable characteristics. Benefiting from the synergetic characteristics of Ag and Au in the form of bimetallic nanoparticles, the Au-Ag/TiO2 thin film proved to be more sensitive to thiolated oligonucleotide binding events. Despite the successful surface functionalization with the biorecognition element, the detection of complementary oligonucleotides revealed electrostatic repulsion and steric hindrance, which hindered hybridization with the target oligonucleotide. This study points to an effect that is still poorly described in the literature and affects the design of LSPR biosensors based on nanoplasmonic thin films.

Skin-Attached Arrayed Piezoelectric Sensors for Continuous and Safe Monitoring of Oculomotor Movements.

Abnormal oculomotor movements are known to be linked to various types of brain disorders, physical/mental shocks to the brain, and other neurological disorders, hence its monitoring can be developed into a simple but effective diagnostic tool. To overcome the limitations in the current eye-tracking system and electrooculography, a piezoelectric arrayed sensor system is developed using single-crystalline III-N thin-film transducers, which offers advantages of mechanical flexibility, biocompatibility, and high electromechanical conversion, for continuous monitoring of oculomotor movements by skin-attachable, safe, and highly sensitive sensors. The flexible piezoelectric eye movement sensor array (F-PEMSA), consisting of three transducers, is attached to the face temple area where it can be comfortably wearable and can detect the muscles' activity associated with the eye motions. Output voltages from upper, mid, and lower sensors (transducers) on different temple areas generate discernable patterns of output voltage signals with different combinations of positive/negative signs and their relative magnitudes for the various movements of eyeballs including 8 directional (lateral, vertical, and diagonal) and two rotational movements, which enable various types of saccade and pursuit tests. The F-PEMSA can be used in clinical studies on the brain-eye relationship to evaluate the functional integrity of multiple brain systems and cognitive processes.

Pump-probe reflectivity studies of ultrashort laser-induced acousto-mechanical strains in ZnO films

In the current work we report on the generation of acoustic strains in thin ZnO layers using optoacoustic transduction of ultrashort laser pulses into acoustic waves on an Au thin film transducer. After absorption of energy by the electron system of the metal, energy conversion, thermal expansion and mechanical deformation takes place. The generation and propagation of the induced acoustic strains are monitored in time via a degenerate pump-probe transient reflectivity optical setup at 800 nm, as opposed to most commonly used schemes that employ different wavelengths for the pump and probe beams, mostly in the vicinity of ZnO maximum absorption. The experimental results include energy relaxation times and phonon scattering frequencies and are supported by a thermal vibro-acoustic finite element model. The model is based on the combination of a revised two-temperature approach and elasticity theory, and considers anisotropic properties for the ZnO film and the computation of the elastic wave velocity for the first time. The outcomes are discussed in the context of electron–phonon coupling factors and other material properties. A good agreement between the experimental findings and the results from the numerical simulations has been established, regarding outcomes like the mean velocity of the strain waves, establishing a novel characterization method applicable to a variety of materials and structures.

Study on TaN Thin Film Transducer Fabricated on the AlN Surface

In order to develop a new type of transducer for insensitive explosive devices, the problems in the film forming process of AlN ceramic surface were studied and the solutions were given. By comparison of ignition experiments, the surface roughness and average ignition current of the TaN film energy commutator prepared on the surface of polished AlN ceramics were smaller and higher. It met the technical requirements of insensitive explosive devices, which provided a new way for insensitive explosives.

Picosecond magneto-optic thermometry measurements of nanoscale thermal transport in AlN thin films

The thermal conductivity Λ of wide bandgap semiconductor thin films, such as AlN, affects the performance of high-frequency devices, power devices, and optoelectronics. However, accurate measurements of Λ in thin films with sub-micrometer thicknesses and Λ > 100 W m−1 K−1 is challenging. Widely used pump/probe metrologies, such as time–domain thermoreflectance (TDTR) and frequency–domain thermoreflectance, lack the spatiotemporal resolution necessary to accurately quantify thermal properties of sub-micrometer thin films with high Λ. In this work, we use a combination of magneto-optic thermometry and TiN interfacial layers to significantly enhance the spatiotemporal resolution of pump/probe thermal transport measurements. We use our approach to measure Λ of 100, 400, and 1000 nm AlN thin films. We coat AlN thin films with a ferromagnetic thin-film transducer with the geometry of (1 nm-Pt/0.4 nm-Co)x3/(2 nm-TiN). This PtCo/TiN transducer has a fast thermal response time of <50 ps, which allows us to differentiate between the thermal response of the transducer, AlN thin film, and substrate. For the 100, 400, and 1000 nm thick AlN films, we determine Λ to be 200 ± 80, 165 ± 35, and 300 ± 70 W m−1 K−1, respectively. We conclude with an uncertainty analysis that quantifies the errors associated with pump/probe measurements of thermal conductivity, as a function of transducer type, thin-film thermal conductivity, and thin-film thickness. Time resolved magneto-optic Kerr effect experiments can measure films that are three to five times thinner than is possible with standard pump/probe metrologies, such as TDTR. This advance in metrology will enable better characterization of nanoscale heat transfer in high thermal conductivity material systems like wide bandgap semiconductor heterostructures and devices.

Development of Double-Insulated Ring Sensors for Hollow Pin Preload Measurement

The hinge mechanism of aviation flap is composed of joint bearing, single lug, double lugs, hollow pins and self-locking nuts, etc. This hinge mechanism has accurate requirements for the preload of the hollow pin. When no seizure of the joint bearing occurs, the bolts are required to have sufficient preload to clamp the inner ring of the bearing. Meanwhile, when the seizure of the joint bearing occurs, there shall be no movement between the bearing and pin shaft or the end face of the bushing shall arise. This requires that the tension of the hollow pin shaft shall not be too high, otherwise the friction torque isn’t easy to overcome. However, the method to directly measure the preload in the hollow pin is not available, and the influence law of preload decline on the seizure resistance of the bearings also can’t be evaluated. This paper presents a method suitable for measuring the preload of hollow thin-walled pin parts. By applying permanent piezoelectric thin film transducer onto the end face of the hollow pin, the standardization technology is used to measure the propagation time change of the ultrasonic signal generated by the transducer in the pin as a function of the temperature and preload, thus realizing the accurate measurement for the preload of the hollow pin during installation. Finally, the application verification is conducted by using the measurement method proposed in this paper. The results show that this method can effectively optimize the installation and service performance for the hollow pin.

3D-Printed Encapsulation of Thin-Film Transducers for Reliable Force Measurement in Biomedical Applications

In biomedical studies as well as in clinical trials, it is often useful to have a reliable measure of the force exerted by the body (e.g., clenching force at the teeth or pinch force at fingertips) or on the body by external stimuli (e.g., taps to elicit reflexes or local pressure for nociceptive stimulation). Thin-film sensors such as FlexiForce® provide a very handy and versatile solution for these applications, but can be easily damaged and offer poor accuracy and repeatability, being heavily affected by the surface material they come into contact with. The aim of the study is the realization of a 3D-printed housing that completely embeds the sensor, thus providing mechanical protection and increasing the reliability of the measurement. The increasing availability of 3D printers and of printing materials for medical use allows the user to shape the housing according to specific needs, with short developing time and low cost.

Simulation and analysis of the temperature change affected by the material and shape of MEMS transducer

MEMS electric energy utilization rate is the key parameter affecting firing performance. It has important relationship with its preparation technology, microscale and structure morphology. In order to improve the theory of energy exchange element design for MEMS pyrotechnics, reduce the design error of transducer parameters at microscale. To ensure the performance of MEMS pyrotechnics, in this paper, the influence of the central line of the thin film transducer element on the electrothermal conversion is simulated and analyzed. The electrothermal conversion and temperature characterization of different central lines in the transducer bridge area are calculated by the finite element method. The influence of the central line on the thermal efficiency under microscale is revealed. The corresponding data are obtained.

Morphology of lithium niobium oxide thin film ultrasonic transducers deposited by RF magnetron sputtering

Lithium niobium oxide (LNO) thin film high temperature ultrasonic transducers have been deposited using radio frequency magnetron sputtering. Effects of the sample position in the sample plane on the thin film morphology are studied. In the center of the sputtering area, where the radial distance is 0 mm, the LNO thin film transducers are a mixture of LiNbO3/LiNb3O8 phases with well developed columnar structure and good ultrasonic response. At the radial distance of 10–40 mm, complex “sunflower-like” morphological features are formed, which may develop due to preferable crystallization of the sputtered material on particle seeds, significant difference in atomic mass of Li and Nb, and space confinement. The formation of wedge-like “sunflowers” reduces the performance of the ultrasonic transducers. In addition, the impacts of annealing temperature and time on morphology, crystal structure, element distribution, and ultrasonic response are also observed. The long-term annealing at 700 °C (for 100 h) upgrades the ultrasonic response, and for prolonged annealing up to 312 h, the ultrasonic response gradually degrades.

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.

Chlorine Gas Sensor with Surface Temperature Control.

The work describes the design, manufacturing, and user interface of a thin-film gas transducer platform that is able to provide real-time detection of toxic vapor. This proof-of-concept system has applications in the field of real-time detection of hazardous gaseous agents that are harmful to the person exposed to the environment. The small-size gas sensor allows for integration with an unmanned aerial vehicle, thus combining high-level mobility with the ability for the real-time detection of hazardous/toxic chemicals or use as a standalone system in industries that deal with harmful gaseous substances. The sensor was designed based on the ability of thin-film metal oxide sensors to detect chlorine gas in real time. Specifically, a concentration of 10 ppm of Cl2 was tested.

Multi-Tube Helmholtz Resonator Based Triboelectric Nanogenerator for Broadband Acoustic Energy Harvesting

Acoustic energy, especially broadband low-frequency sound energy is part of the environmental mechanical energy acquisition cannot be ignored. Herein, a multi-tube parallel Helmholtz resonator-based triboelectric nanogenerator (MH-TENG) is investigated to reap sound energy in low-frequency noise environments. The designed MH-TENG consists of a modified Helmholtz resonator and a thin-film TENG transducer. The core materials of the TENG transducer are aluminum, FEP film, and carbon. To further clarify the influence of the modified Helmholtz resonator on the conversion performance of MH-TENG, the acoustic characteristics of the improved resonators are systematically studied. A series of experiments show that the multi-tube parallel Helmholtz resonator structure has a better sound wave collection effect. Meanwhile, the flexible film TENG can reduce the optimal output frequency of the device. The power generation performance and the bandwidth of the MH-TENG are significantly improved by adopting a multi-tube Helmholtz resonator. Within the frequency bandwidth range of 230 Hz, MH-TENG can effectively improve the efficiency of acoustic energy harvesting. 110 LEDs and an electronic thermometer can be powered by the sound-driven MH-TENG. In addition, the MH-TENG has a good capacitor charging performance, which is conducive to its application in ambient sound energy harvesting.

Fabrication, Characterization, and Simulation of Glass Devices with AlN Thin-Film Transducers for Excitation of Ultrasound Resonances

We present fabrication of 570-um-thick, millimeter-sized soda-lime-silicate float glass blocks with a 1-um-thick AlN-thin-film piezoelectric transducer sandwiched between thin metallic electrodes and deposited on the top surface. The electro-mechanical properties are characterized by electrical impedance measurements in the frequency range from 0.1 to 10 MHz with a peak-to-peak voltage of 0.5 V applied to the electrodes. We measured the electrical impedance spectra of 35 devices, all of width 2 mm, but with 9 different lengths ranging from 2 to 6 mm and with 2-7 copies of each individual geometry. Each impedance spectrum exhibits many resonance peaks, of which we carefully measured the 5 most prominent ones in each spectrum. We compare the resulting 173 experimental resonance frequencies with the simulation result of a finite-element-method model that we have developed. When using material parameters from the manufacturer, we obtain an average relative deviation of the 173 simulated resonance frequencies from the experimental ones of (-4.2 +/-0.04)%. When optimizing the values of the Young's modulus and the Poisson ratio of the float glass in the simulation, this relative deviation decreased to (-0.03 +/- 0.04)%. Our results suggest a method for an accurate in-situ determination of the acoustic parameters at ultrasound frequencies of any elastic solid onto which a thin-film transducer can be attached

Numerical study of bulk acoustofluidic devices driven by thin-film transducers and whole-system resonance modes.

In bulk acoustofluidic devices, acoustic resonance modes for fluid and microparticle handling are traditionally excited by bulk piezoelectric (PZE) transducers. In this work, it is demonstrated by numerical simulations in three dimensions that integrated PZE thin-film transducers, constituting less than 0.1% of the bulk device, work equally well. The simulations are performed using a well-tested and experimentally validated numerical model. A water-filled straight channel embedded in a mm-sized bulk glass chip with a 1- μm-thick thin-film transducer made of Al0.6Sc0.4N is presented as a proof-of-concept example. The acoustic energy, radiation force, and microparticle focusing times are computed and shown to be comparable to those of a conventional bulk silicon-glass device actuated by a bulk lead-zirconate-titanate transducer. The ability of thin-film transducers to create the desired acoustofluidic effects in bulk acoustofluidic devices relies on three physical aspects: the in-plane-expansion of the thin-film transducer under the applied orthogonal electric field, the acoustic whole-system resonance of the device, and the high Q-factor of the elastic solid, constituting the bulk part of the device. Consequently, the thin-film device is remarkably insensitive to the Q-factor and resonance properties of the thin-film transducer.

A Robust Infrared Transducer of an Ultra-Large-Scale Array.

A robust micro-electro-mechanical systems (MEMS) infrared thin film transducer of an ultra-large-scale array was proposed and fabricated on a 4-inch silicon wafer. The silicon substrate and micro cavities were introduced. This novel transducer had excellent mechanical stability, time response, and state-of-the-art pixel scale. It could bear a load of 1700 g and its load pressure was improved by more than 5.24 times and time constant decreased by 50.7% compared to the traditional soft infrared thin film transducer. The array scale of its pixels exceeded 2k × 2k. The simulation and measured results of the transient temperature and radiation intensity were well consistent. Illuminated by a 532 nm laser with a frequency of 50 Hz and 50% duty cycle, the thermal decay time of the proposed transducer was 6.0 ms. A knife-edge image was utilized for spatial resolution test and the full width at half maximum (FWHM) of the proposed transducer was 24% smaller than the traditional soft one. High-resolution infrared images were generated using the proposed robust transducer. These results proved that the robust transducer was promising in infrared image generation.

Fully Printed Flexible Chemiresistors with Tunable Selectivity Based on Gold Nanoparticles

This study presents a method for printing flexible chemiresistors comprising thin film transducers based on cross-linked gold nanoparticles (GNPs). First, interdigitated silver paste electrodes are printed onto polyimide (PI) foil via dispenser printing. Second, coatings of GNPs and dithiol/monothiol blends are inkjet-printed onto these electrode structures. 1,9-Nonanedithiol (9DT) is used as cross-linking agent and a variety of monothiols are added to tune the sensors’ chemical selectivity. When dosing these sensors with different analyte vapors (n-octane, toluene, 4-methyl-2-pentanone, 1-butanol, 1-propanol, ethanol, water; concentration range: 25–2000 ppm) they show fully reversible responses with short response and recovery times. The response isotherms follow a first-order Langmuir model, and their initial slopes reveal sensitivities of up to 4.5 × 10−5 ppm−1. Finally, it is demonstrated that arrays of printed sensors can be used to clearly discern analytes of different polarity.

Investigation of AlGaN/GaN HFET and VO2 Thin Film Based Deflection Transducers Embedded in GaN Microcantilevers.

The static and dynamic deflection transducing performances of piezotransistive AlGaN/GaN heterojunction field effect transistors (HFET) and piezoresistive VO2 thin films, fabricated on GaN microcantilevers of similar dimensions, were investigated. Deflection sensitivities were tuned with the gate bias and operating temperature for embedded AlGaN/GaN HFET and VO2 thin film transducers, respectively. The GaN microcantilevers were excited with a piezoactuator in their linear and nonlinear oscillation regions of the fundamental oscillatory mode. In the linear regime, the maximum deflection sensitivity of piezotransistive AlGaN/GaN HFET reached up to a 0.5% change in applied drain voltage, while the responsivity of the piezoresistive VO2 thin film based deflection transducer reached a maximum value of 0.36% change in applied drain current. The effects of the gate bias and the operation temperature on nonlinear behaviors of the microcantilevers were also experimentally examined. Static deflection sensitivity measurements demonstrated a large change of 16% in drain-source resistance of the AlGaN/GaN HFET, and a similarly high 11% change in drain-source resistance in the VO2 thin film, corresponding to a 10 μm downward step bending of the cantilever free end.

Switching performance of bistable membranes activated with integrated piezoelectric thin film transducers

In this paper we report on the fabrication of bistable micro electromechanical systems (MEMS) membranes, which have diameters in the range of 600–800 µm, a total thickness of 3.13 µm and feature integrated low power piezoelectric transducers based on aluminium nitride. To estimate the impact of the membrane asymmetry due to the integrated piezoelectric transducers, an asymmetric constant in the potential energy calculation of the bistable system is introduced, thus enabling a proper theoretical prediction of the membrane behaviour. To switch between the two bistable ground states, rectangular pulses with frequencies in the range of 50–100 kHz and a peak-to-peak voltage of 30 Vpp are applied. Two different actuation schemes were investigated, whereas one shows positive and the other negative pulse amplitudes. With a Laser-Doppler Vibrometer the velocity of the membranes during the bistable switching process is measured and integrated over time to calculate the membrane displacement in the centre. FFT (fast Fourier transform) spectra of an applied broadband white noise signal were determined in both ground states and showed a strongly decreased dominant resonance frequency in the lower ground state. The results also showed, that the asymmetry of the system causes different switching behaviours for each bistable ground state, whereas it requires less energy to switch from the lower to the upper ground state. Furthermore, it was demonstrated that a minimum of two pulses are needed for switching when using positive rectangular pulses of 30 Vpp in contrast to four when applying negative pulses. The pulse frequency causing switching was in the range of 60–110 kHz, strongly depending on the geometry and applied signal scheme. Additionally, a positive voltage offset applied to the pulse signal characteristics resulted in both a wider range of frequencies suitable for switching and in a decrease of the dominant resonance frequency, which is also beneficial for the switching process and indicates the potential for efficient switching of bistable MEMS membranes.

Integrated nanosecond laser full-field imaging for femtosecond laser-generated surface acoustic waves in metal film-glass substrate multilayer materials

Femtosecond laser pulses are used for the excitation of surface acoustic waves (SAWs) in a gold thin film transducer deposited on a glass substrate, while a single longitudinal nanosecond laser source is employed for their full-field, high-resolution dynamic interferometric imaging in a pump–probe experimental set-up. The successful combination of the two laser sources in one integrated pump–probe set-up, poses no restrictions in the time-delays. The experimental results are supported by numerical simulations implemented by the connection of a finite difference two-temperature model (TTM) with a finite element method (FEM) model. The new integrated method allows for the investigation of the dynamic response of matter for long timescale ranges. Representative applications for SAWs characterization and surface structural defects detection on Au thin metal film deposited on BK7 glass substrate samples, demonstrate the efficiency of the proposed method and the influence of the pulse duration of the excitation pulses.