Conventional Surface Acoustic Wave Research Articles

Simple, and highly efficient edge-effect surface acoustic wave atomizer

Conventional surface acoustic wave (SAW) atomizers require a direct water supply on the surface, which can be complex and cumbersome. This paper presents a novel SAW atomizer that uses lateral acoustic wetting to achieve atomization without a direct water supply. The device works by simply pressing a piece of wetted paper strip against the bottom of an excited piezoelectric transducer. The liquid then flows along the side to the unmodified surface edge, where it is atomized into a well-converging mist in a stable and sustainable manner. We identified this phenomenon as the edge effect, using numerical simulation results of surface displacement mode. The feasibility of the prototype design was demonstrated by observing and investigating the integrated process of liquid extraction, transport, and atomization. We further explored the hydrodynamic principles of the change and breakup in liquid film geometry under different input powers. Experiments demonstrate that our atomizer is capable of generating high-quality fine liquid particles stably and rapidly even at very high input power. Compared to conventional SAW atomizer, the dispersion of mist width can be scaled down by 70%, while the atomization rate can be increased by 37.5%. Combined with the advantages of easy installation and robustness, the edge effect-based atomizer offers an attractive alternative to current counterparts for applications requiring high efficiency and miniaturization, such as simultaneous synthesis and encapsulation of nanoparticles, pulmonary drug delivery and portable inhalation therapy.

Removable and replaceable micro-mixing system with surface acoustic wave actuators

Abstract A micromixer based on a surface acoustic wave (SAW) has attracted a great deal of attention because of their ability to mix liquids and multiphase flows with very high mixing efficiency. However, since microfluidic channels cloud only be formed directly on the piezoelectric substrate in conventional SAW devices, contamination problems have arisen. In this study, by placing a solution that propagates sound waves between the microfluidic channel and the SAW excitation substrate, vibrations caused by SAWs are propagated into the micro-well to form a micro-mixing system. This system can provide a micromixer that is removable, replaceable, and contamination-free.

Ultrahigh frequency shear-horizontal acoustic wave humidity sensor with ternary nanocomposite sensing layer

Surface acoustic wave (SAW) technology is promising for humidity monitoring due to its digital output, small size, large-scale production and wireless passive capability, but there are major challenges to achieve ultra-high sensitivity and fast responses using the conventional SAW devices. Herein, ultrahigh frequency (4.7 GHz and 5.9 GHz) shear-horizontal (SH) SAW devices were developed and a ternary nanocomposite strategy of graphene quantum dots/polyethyleneimine/silicon dioxide nanoparticles (GQDs-PEI-SiO2 NPs) was proposed as a sensitive layer to achieve ultrahigh sensitivity and fast response. This ternary material system was constructed by modifying the surface of SiO2 NPs with the PEI through an electrostatic force, and then adsorbing the GQDs onto the PEI through hydrogen bonds. Compared with the conventional low frequency SAW devices, the ultrahigh frequency SH-SAW devices showed exceptionally ultra-high sensitivity (2.4 MHz/%RH, 1000 times as high as a 202 MHz SAW device), fast response (20 s/5 s), excellent linearity, and good repeatability in the range of 20–80% RH. These superior performances are attributed to ultrahigh frequency of SAW devices, large specific surface areas of the nanocomposite (which exposed multiple hydrophilic groups in PEI and GQDs), and high vapor pressure of convex spherical curved liquid surface (which accelerated the adsorption and desorption of water molecules).

Femtosecond Laser Micromachining of the Mask for Acoustofluidic Device Preparation.

Surface acoustic wave (SAW)-based acoustofluidic devices have shown broad applications in microfluidic actuation and particle/cell manipulation. Conventional SAW acoustofluidic device fabrication generally includes photolithography and lift-off processes and thus requires accessing cleanroom facilities and expensive lithography equipment. In this paper, we report a femtosecond laser direct writing mask method for acoustofluidic device preparation. By micromachining of steel foil to form the mask and direct evaporation of metal on the piezoelectric substrate using the mask, the interdigital transducer (IDT) electrodes of the SAW device are generated. The minimum spatial periodicity of the IDT finger is about 200 μm, and the preparation for LiNbO3 and ZnO thin films and flexible PVDF SAW devices is verified. Meanwhile, we have demonstrated various microfluidic functions, including streaming, concentration, pumping, jumping, jetting, nebulization, and particle alignment using the fabricated acoustofluidic (ZnO/Al plate, LiNbO3) devices. Compared to the traditional manufacturing process, the proposed method omits spin coating, drying, lithography, developing, and lift-off processes and thus has advantages of simple, convenient, low cost, and environment friendliness.

Record-Breaking Frequency of 44 GHz Based on the Higher Order Mode of Surface Acoustic Waves with LiNbO3/SiO2/SiC Heterostructures

Surface acoustic wave (SAW) technology has been extensively explored for wireless communication, sensors, microfluidics, photonics, and quantum information processing. However, due to fabrication issues, the frequencies of SAW devices are typically limited to within a few gigahertz, which severely restricts their applications in 5G communication, precision sensing, photonics, and quantum control. To solve this critical problem, we propose a hybrid strategy that integrates a nanomanufacturing process (i.e., nanolithography) with a LiNbO3/SiO2/SiC heterostructure and successfully achieve a record-breaking frequency of about 44 GHz for SAW devices, in addition to large electromechanical coupling coefficients of up to 15.7%. We perform a theoretical analysis and identify the guided higher order wave modes generated on these slow-on-fast SAW platforms. To demonstrate the superior sensing performance of the proposed ultra-high-frequency SAW platforms, we perform micro-mass sensing and obtain an extremely high sensitivity of approximately 33151.9 MHz·mm2·μg−1, which is about 1011 times higher than that of a conventional quartz crystal microbalance (QCM) and about 4000 times higher than that of a conventional SAW device with a frequency of 978 MHz.

Mm-band surface acoustic wave devices utilizing two-dimensional boron nitride

The simple structure, low power consumption, and small form factor have made surface acoustic wave (SAW) devices essential to mobile communication as RF filters. For instance, the latest 5G smartphones are equipped with almost 100 acoustic wave filters to select a specific frequency band and increase communication capacity. On the arrival of the newest communication standard, 5G, mm-band up to 39 GHz is supposed to be utilized, whereas the conventional SAW filters are limited to below 3 GHz, leaving a critical component missing. Here, we show an emerging 2D material—hexagonal boron nitride—can become a key enabler of mm-band SAW filter. Our study, based on first principles analysis and acousto-electric simulation, shows the operating frequency of SAW devices can reach over 20 GHz in its fundamental mode and 40 GHz in its interface mode with high electromechanical coupling coefficient (K2) and low insertion loss. In addition to the orders of magnitude improvement compared to the conventional SAW devices, our study provides a systematic approach to utilizing van der Waals crystals with highly anisotropic acoustic properties for practical applications.

Strategies for Giant Mass Sensitivity Using Super-High-Frequency Acoustic Waves

Surface acoustic wave (SAW) devices are powerful platforms for mass sensing, chemical vapor or gas detection, and biomolecular identification. Great efforts have been made to achieve high sensitivities by using super-high-frequency SAW devices. Conventional SAW sensing is based on mass-loading effects at the acoustic wave propagation (or delay line) region between two interdigitated transducers (IDTs). However, for many super-high-frequency SAW devices with their small sizes, there is a huge challenge that the sensitivity is difficult to be further increased, simply because there are very limited areas between the IDTs to deposit a sensing layer. Herein, we proposed a novel strategy based on giant mass-sensitivity effects generated on the global area of acoustic wave device (defined as areas of both delay line region and IDTs), which significantly enhances sensitivity and reduces the detection limit of the SAW device. Both theoretical analysis and experimental results proved this new strategy and mechanism, which are mainly attributed to the efficient energy confinement at the IDTs’ region for the super-high-frequency SAW devices. The achieved mass sensitivity using this new strategy is as high as 2590 MHz <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\cdot $ </tex-math></inline-formula> mm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{{2}} \cdot \mu \text{g}^{-{1}}$ </tex-math></inline-formula> , which is about 500 times higher than that obtained from only using the acoustic wave propagation region with a SAW frequency of 4.43 GHz. Hypersensitive humidity detection has been demonstrated using this newly proposed sensing platform, achieving an extremely high sensitivity of 278 kHz/%RH and the fast response and recovery times of ~37 and ~35 s, respectively.

Ultrafast dynamical charge-lattice coupling in rare-earth nickelate thin films studied by time-resolved terahertz spectroscopy

Rare-earth nickelates exhibit a rich phase diagram formed by the complex interplay of intertwined and competing energetics of fundamental entities. To unwind the coupling and interaction mechanisms of fundamental entities underneath, time-resolved terahertz (THz) spectroscopy was implemented to understand non-equilibrium carrier and lattice dynamics of epitaxial thin films of La x Eu1−x NiO3 (x = 0, 0.25, 0.50, 1) systems, where x = 0 is insulating while remaining are metallic at room temperature. The THz transmittance of the insulating and metallic phases exhibit contrasting photo-induced phases associated with bi-exponential and mono-exponential relaxation mechanisms, respectively. A pronounced oscillatory feature superimposed on the mono-exponential relaxation manifests only in the metallic phase. As ascribed to the acoustic phonons, the ‘x’ dependent behavior of this feature reveals an inverse relation between the strength of electron–phonon coupling and the magnitude of conductivity. In the insulating state, in contrast, the relaxation time constants are associated with the recovery of charge-ordering and electron–phonon thermalization. This dynamical lattice–charge interaction study demonstrates use of this ultrafast phenomena in nickelate thin films in new generation ultrafast photo-acoustic devices as an alternate to conventional surface acoustic wave device.

Ultrahigh-Frequency Surface Acoustic Wave Sensors with Giant Mass-Loading Effects on Electrodes.

Surface acoustic wave (SAW) devices are widely used for physical, chemical, and biological sensing applications, and their sensing mechanisms are generally based on frequency changes due to mass-loading effects at the acoustic wave propagation area between two interdigitated transducers (IDTs). In this paper, a new sensing mechanism has been proposed based on a significantly enhanced mass-loading effect generated directly on Au IDT electrodes, which enables significantly enhanced sensitivity, compared with that of conventional SAW devices. The fabricated ultrahigh-frequency SAW devices show a significant mass-loading effect on the electrodes. When the Au-electrode thickness increased from 12 to 25 nm, the Rayleigh mode resonant frequency decreased from 7.77 to 5.93 GHz, while that of the higher longitudinal leaky SAW decreased from 11.87 to 9.83 GHz. The corresponding mass sensitivity of 7309 MHz·mm2·μg-1 (Rayleigh mode) is ∼8.9 × 1011 times larger than that of a conventional quartz crystal balance (with a frequency of 5 MHz) and ∼1000 times higher than that of conventional SAW devices (with a frequency of 978 MHz). Trinitrotoluene concentration as low as 4.4 × 10-9 M (mol·L-1) can be detected using the fabricated SAW sensor, proving its giant mass-loading effect and ultrahigh sensitivity.

30 GHz surface acoustic wave transducers with extremely high mass sensitivity

A nano-patterning process is reported in this work, which can achieve surface acoustic wave (SAW) devices with an extremely high frequency and a super-high mass sensitivity. An integrated lift-off process with ion beam milling is used to minimize the short-circuiting problem and improve the quality of nanoscale interdigital transducers (IDTs). A specifically designed proximity-effect-correction algorithm is applied to mitigate the proximity effect occurring in the electron-beam lithography process. The IDTs with a period of 160 nm and a finger width of 35 nm are achieved, enabling a frequency of ∼30 GHz on lithium niobate based SAW devices. Both centrosymmetric type and axisymmetric type IDT structures are fabricated, and the results show that the centrosymmetric type tends to excite lower-order Rayleigh waves and the axisymmetric type tends to excite higher-order wave modes. A mass sensitivity of ∼388.2 MHz × mm2/μg is demonstrated, which is ∼109 times larger than that of a conventional quartz crystal balance and ∼50 times higher than a conventional SAW device with a wavelength of 4 μm.

Langasite-based surface acoustic wave resonator for acetone vapor sensing

High concentration of acetone sensing is of an increasing interest due to its particular application in health monitoring for medical use as well as chemical industries. This paper proposes an advanced sensor that is based off of a Langasite (LGS) surface acoustic wave (SAW) resonator for acetone vapor sensing/detection. The detection of acetone vapor is based off of the chemo-resistive adsorption of pre-installed air on the LGS resonator surface, which will be significantly reduced via chemical reaction and lead to resonant frequency drift of the resonator. Lab tests of acetone vapor sensing are carried out with various concentration at various operating temperature (50 °C–200 °C). The sensitivity is investigated numerically and experimentally, where a rigorous proportionality between frequency drift and acetone concentration is observed and the temperature effect is further discussed. Compared to the conventional SAW gas sensor, the proposed acetone sensor employs the SAW piezoelectric substrate, LGS, as the sensing material, which not only eliminates the use of extra sensing thin film and consequently reduce the cost of fabrication, but also provides a promising high temperature stability due to the nature of LGS. Therefore, the proposed acetone sensor is of low cost, high sensitivity, and low maintenance, which can be further miniaturized and simplified for MEMS application and real environment acetone sensing in health monitoring.

Graphene-Based Fully Transparent Thin Film Surface Acoustic Wave Devices for Sensing and Lab-on-Chip Applications

This paper explores to use graphene as transparent interdigital transducer (IDT) electrode for a fully transparent surface acoustic wave (SAW) device due to its extraordinary electrical, physical and mechanical properties. The number of graphene atomic layers was firstly optimized for its best performance as the SAW electrode, and a 4-layered graphene IDT electrode, with aluminum doped zinc oxide, AZO, as the bus bar and wire bonding pad, was selected to fabricate fully transparent ZnO/glass SAW devices. The SAW resonators exhibited obvious resonant response at different wavelengths, and resonance signals with amplitude up to 20 dB were obtained with the transparency above 80%. The graphene-based transparent SAW sensor has been used for different sensing applications. Temperature sensing tests showed that the frequencies increase linearly with the increase of temperature, which has an opposite trend compared to that obtained from a conventional LiNbO3 SAW device. The humidity sensing and human breathing detection have been demonstrated, and discontinuous respiration measurement can be used to distinguish the human respiration at the normal state or the state after exercise. Strong acoustic streaming and particle concentration using the transparent SAW devices have been achieved, which are suitable for microfluidic and lab-on-chip applications.

Dual Transduction Surface Acoustic Wave Gas Sensor for VOC Discrimination

A novel dual transduction surface acoustic wave (SAW) resonator gas sensor is proposed to enable single sensor gas identification. In contrast to conventional gas sensors, which only detect a single physical property of the sensing material, the proposed dual transduction sensor can simultaneously detect variations in both the mass and resistance of the sensing material. As a result, this single sensor can achieve gas identification by exploiting the unique relationship between the resistance and mass responses to the sensed gas. Reported experimental results show that the proposed sensor can identify volatile organic compounds, including ethanol, acetone, and tetrahydrofuran. The proposed dual transduction scheme can be readily applied to conventional SAW resonators, thereby enabling low-cost implementations through the use of existing SAW fabrication lines. In addition, it alleviates the need to fabricate large sensor arrays to discriminate between gases.

Sub-limit detection in SAW sensors by FFT spectral analysis of frequency time instability

PurposeThe detection of H2 concentrations in concentrations undetectable by the conventional detection method of surface acoustic wave (SAW) sensors based on frequency shift, by correlating analyte presence with Fourier spectra components.Design/methodology/approachFast Fourier Transform (FFT) and autocorrelation analysis of phase noise in a SnO2-coated SAW sensor was performed. Fourier spectra were obtained by FFT from the SAW sensor resonance frequency instability, in the absence of analyte, and for H2 concentrations between 0.08 and 0.4 per cent.FindingsAll analyte concentrations are below the sensor limit of detection, which is 0.8 per cent for H2. Although these analyte concentrations caused no significant change in the resonance frequency of the SAW resonator, the FFT spectra presented several modifications, namely, the appearance of a new peak and the decrease of randomness. The authors consider that the effect is because of the chaotic behavior of the temporal dependence of the SAW resonance frequency. This explanation is substantiated by the decrease observed in the SAW oscillator autocorrelation function, which is an indication for a chaotic behavior.Practical implicationsAs chaotic systems are extremely sensitive to perturbation, measurement methods based on chaos diagnosis could potentially greatly improve the SAW detection.Originality/valueFourier spectra components were correlated with analyte presence in concentrations undetectable by the conventional SAW detection method based on frequency shift.

Hybrid Surface Acoustic Wave-Electrohydrodynamic Atomization (SAW-EHDA) For the Development of Functional Thin Films.

Conventional surface acoustic wave - electrostatic deposition (SAW-ED) technology is struggling to compete with other thin film fabrication technologies because of its limitation in atomizing high density solutions or solutions with strong inter-particle bonding that requires very high frequency (100 MHz) and power. In this study, a hybrid surface acoustic wave - electrohydrodynamic atomization (SAW-EHDA) system has been introduced to overcome this problem by integrating EHDA with SAW to achieve the deposition of different types of conductive inks at lower frequency (19.8 MHZ) and power. Three materials, Poly [2-methoxy-5-(2-ethylhexyloxy)-1, 4-phenylenevinylene] (MEH-PPV), Zinc Oxide (ZnO), and Poly(3, 4-ethylenedioxythiophene):Polystyrene Sulfonate (PEDOT:PSS) have been successfully deposited as thin films through the hybrid SAW-EHDA. The films showed good morphological, chemical, electrical, and optical characteristics. To further evaluate the characteristics of deposited films, a humidity sensor was fabricated with active layer of PEDOT:PSS deposited using the SAW-EHDA system. The response of sensor was outstanding and much better when compared to similar sensors fabricated using other manufacturing techniques. The results of the device and the films’ characteristics suggest that the hybrid SAW-EHDA technology has high potential to efficiently produce wide variety of thin films and thus predict its promising future in certain areas of printed electronics.

Acoustothermal heating of polydimethylsiloxane microfluidic system

We report an observation of rapid (exceeding 2,000 K/s) heating of polydimethylsiloxane (PDMS), one of the most popular microchannel materials, under cyclic loadings at high (~MHz) frequencies. A microheater was developed based on the finding. The heating mechanism utilized vibration damping in PDMS induced by sound waves that were generated and precisely controlled using a conventional surface acoustic wave (SAW) microfluidic system. The refraction of SAW into the PDMS microchip, called the leaky SAW, takes a form of bulk wave and rapidly heats the microchannels in a volumetric manner. The penetration depths were measured to range from 210 μm to 1290 μm, enough to cover most sizes of microchannels. The energy conversion efficiency was SAW frequency-dependent and measured to be the highest at around 30 MHz. Independent actuation of each interdigital transducer (IDT) enabled independent manipulation of SAWs, permitting spatiotemporal control of temperature on the microchip. All the advantages of this microheater facilitated a two-step continuous flow polymerase chain reaction (CFPCR) to achieve the billion-fold amplification of a 134 bp DNA amplicon in less than 3 min.

Wideband Bandpass Filters With SAW-Filter-Like Selectivity Using Chip SAW Resonators

Bandpass filters with surface acoustic wave (SAW)-filter-like selectivity and much wider bandwidth compared to conventional SAW filters are presented. The proposed topology consists of SAW resonator networks and parallel coupled lines. Each ladder type resonator network has a series resonator and two shunt resonators to define the upper and lower cutoff frequencies and realize the sharp passband selectivity. Analysis of the proposed SAW resonator network and a comparison with a traditional ladder type SAW filter is provided. Two filters are designed, fabricated, and measured using chip SAW resonators and microstrip coupled lines on Rogers RO4003C. As the available SAW resonators are discrete, they are connected together using 1-mil bond wires to form the SAW resonator network. The resonators are also wire bonded to the coupled lines and ground patch. The bandwidth of the two filters is 10.5% and 17%, respectively, which is much wider than conventional SAW filters with a bandwidth of a few percent. An analysis is provided for the bulk wave effects, ripples due to finite transducer, gratings of resonators, and second-order effects from series and parallel resonators, which cause deterioration of the passband performance. The impact of bond wire inductance and parasitic coupling, which cause spurious responses at the stopbands, are also discussed.

Lamb Wave Characteristics of Composite Plates Including a Diamond Layer with Distinct Electrode Arrangements

Diamond films have been utilized to develop surface acoustic wave filters and micromechanical resonators because of the highest acoustic wave velocity and largest product of frequency and quality factor (f·Q) of diamond among all materials. A theoretical analysis of Lamb wave characteristics in multilayer piezoelectric plates including a diamond layer is presented in this paper. Formulae for effective permittivity are derived using the transfer matrix method and are further employed to calculate Lamb wave phase velocity dispersions. The electromechanical coupling coefficients (ECCs) are also calculated exactly by Green's function method. Detailed calculations are carried out for ZnO/diamond and AlN/diamond composite plates with four distinct electrode arrangements. Results show that the ZnO/diamond structure yields a phase velocity of 6420 m/s and a large ECC of 7.41%, which makes it suitable for high-frequency wideband filter applications. Moreover, in the AlN/diamond structure, the S0 mode exhibits a large phase velocity of up to 10.3 km/s and a moderate ECC of 1.97%. Such favorable characteristics are expected to contribute to the development of AlN/diamond Lamb wave oscillators operating at approximately 5–10 GHz without the need for a sub-micrometer-resolution lithographic process. Therefore, both ZnO/diamond and AlN/diamond Lamb wave devices are highly promising candidates for RF devices in modern communication systems with advantages over conventional surface acoustic wave devices.

Surface acoustic wave devices on AlN/3C–SiC/Si multilayer structures

Surface acoustic wave (SAW) propagation characteristics in a multilayer structure including a piezoelectric aluminum nitride (AlN) thin film and an epitaxial cubic silicon carbide (3C–SiC) layer on a silicon (Si) substrate are investigated by theoretical calculation in this work. Alternating current (ac) reactive magnetron sputtering was used to deposit highly c-axis-oriented AlN thin films, showing the full width at half maximum (FWHM) of the rocking curve of 1.36° on epitaxial 3C–SiC layers on Si substrates. In addition, conventional two-port SAW devices were fabricated on the AlN/3C–SiC/Si multilayer structure and SAW propagation properties in the multilayer structure were experimentally investigated. The surface wave in the AlN/3C–SiC/Si multilayer structure exhibits a phase velocity of 5528 m s−1 and an electromechanical coupling coefficient of 0.42%. The results demonstrate the potential of AlN thin films grown on epitaxial 3C–SiC layers to create layered SAW devices with higher phase velocities and larger electromechanical coupling coefficients than SAW devices on an AlN/Si multilayer structure. Moreover, the FWHM values of rocking curves of the AlN thin film and 3C–SiC layer remained constant after annealing for 500 h at 540 °C in air atmosphere. Accordingly, the layered SAW devices based on AlN thin films and 3C–SiC layers are applicable to timing and sensing applications in harsh environments.

Quantitative Determination of Escherichia Coli in Water Sources in the Environment Using a Surface Acoustic Wave Impedance System Modified with a Syringe Filter

A novel surface acoustic wave (SAW) sensor was developed in the study to measure the density of Escherichia coli in water sources in the environment. With a modified syringe filter sealed by a Teflon cover, the sensor system ensured that the CO2 gas produced by bacteria flowed upward and accumulated at the headspace of the syringe filter. The measurements were obtained by immersing a pair of electrodes into a sealed syringe filter which contained a culture solution containing lauryl sulfate trytose (LST). The changes in frequency of the two electrodes were caused by generation of small CO2 bubbles on the surface of the electrodes, which were connected in series to a SAW resonator. The signals obtained by the system were 10 times greater than those obtained by conventional SAW systems. The calibration curve of detection time versus density of E. coli showed a linear relationship with a correlation coefficient (R 2 ) of 0.9434 over the range of 102 to 107 cells/mL. Further integration of this sensing system with a suitable automated sampling device will enable this device to be used on-site for analysis and field studies.