Surface Acoustic Wave Technique Research Articles

Design optimization of surface acoustic wave devices and investigation of fabricated devices under temperature, humidity and steam environments

Surface Acoustic Wave (SAW) technique is one of the most promising Micro Electro Mechanical Systems (MEMS) based detection methods for a variety of applications. SAW sensor is based on the modulation of Rayleigh waves that travel near the surface of a piezoelectric substrate. Advent of MEMS technology has led to the development of SAW sensors which are widely used in chemical sensing, medical, industrial, telecommunication and biological applications. The main focus of this paper is to devise and characterize a generic SAW structure and study the acoustic properties at various environmental conditions before depositing the sensing layer/analyte for any particular application. In this work, a SAW device was designed using COMSOL Multiphysics for a resonant frequency of 100 MHz with 40 μm wavelength on Lithium Niobate (LiNbO3) piezoelectric substrate. The optimized device structure was fabricated using thin film deposition and UV photolithography. The resonant frequency of the fabricated device (97.63 MHz) matched closely with the simulation results (100 MHz) and theoretical results (100 MHz). The device was subjected to various environmental conditions to study the influence on the resonant frequency. The SAW structure showed a resonant frequency shift of 90 KHz in steam environment, ∼ 1 MHz when heated to 150 °C and ∼ 120 KHz when exposed to 80% RH. The device exhibited a linear shift in resonant frequency under all tested environmental conditions with a consistent baseline and good repeatability.

Effect of the strong electron correlations on the formation of Wigner crystal in n-GaAs/AlGaAs structure: Surface Acoustic Waves studies

The complex high-frequency conductance of high-mobility n-GaAs/AlGaAs heterostructures was determined in magnetic fields 12–18 T using the Surface Acoustic Waves technique. Analysis of frequency and temperature dependences of conductance showed that in the investigated magnetic field range and at low temperatures, T<200 mK, the electron system forms a Wigner crystal deformed due to pinning by disorder. We have also defined an effective melting temperature of the Wigner crystal.

Quantitative and nondestructive determination of residual stress for SiO2 thin film by laser-generated surface acoustic wave technique

The laser-generated surface acoustic wave (SAW) technique is a promising method to measure the mechanical properties of thin films quickly and nondestructively. Residual stress is inevitable during the processing and manufacturing of integrated circuits, which will have a major impact on the physical and mechanical properties of the thin film materials and cause deterioration to the structural strength. In this study, the SAW technique based method is proposed for quantitative and nondestructive measuring the residual stress in the nanostructured films. The method is verified by the experiment measuring the SiO2 films in the thickness range of 100–2000 nm. The experimental procedures, including signal excitation, reception and processing, are described in detail. By matching the SAW experimental dispersion curve with the calculated theoretical dispersion curve containing the residual stress, the residual stress of the SiO2 films along [110] and [100] crystallographic orientation of the Si wafer is successfully quantified. The determination results are ranged from −65.5 to 421.1 MPa and the stress value increases as the film thickness decreases, revealing the residual stress of the SiO2 film is compressive. Meanwhile, the conventional substrate curvature method as a comparison is used to verify the correctness and superiority of the proposed SAW method for the residual stress determination.

Effect of Liquid Crystalline Host on Structural Changes in Magnetosomes Based Ferronematics.

The effect of the liquid crystalline host on structural changes in magnetosomes based on ferronematics is studied using the surface acoustic wave (SAW) technique supported by some capacitance and light transmission measurements. The measurement of the attenuation response of SAW propagating along the interface between LC and the piezoelectric substrate is used to study processes of structural changes under magnetic field. The magnetosome nanoparticles of the same volume concentration were added to three different nematic LCs, 5CB, 6CB, and E7. Unlike to undoped LCs, the different responses of SAW attenuation under the influence of magnetic and electric fields in LCs doped with magnetosomes were observed due to characteristic structural changes. The decrease of the threshold field for doped LCs as compared with pure LCs and slight effects on structural changes were registered. The threshold magnetic fields of LCs and composites were determined from capacitance measurements, and the slight shift to lower values was registered for doped LCs. The shift of nematic-isotropic transition was registered from dependencies of SAW attenuation on temperature. The acoustic anisotropy measurement approved the previous supposition about the role of bulk viscosity in used SAW measurements. In addition, capacitance and light transmition investigations supported SAW results and pointed out conclusions about their magnetic field behavior. Obtained results are discussed and confronted with previous ones and coincide well with those observed using acoustic, optical, or dielectric techniques.

Surface acoustic wave (SAW) techniques in tissue engineering

Recently, the introduction of surface acoustic wave (SAW) technique for microfluidics has drawn a lot of attention. The pattern and mutual communication in cell layers, tissues, and organs play a critical role in tissue homeostasis and regeneration and may contribute to disease occurrence and progression. Tissue engineering aims to repair and regenerate damaged organs, depending on biomimetic scaffolds and advanced fabrication technology. However, traditional bioengineering synthesis approaches are time-consuming, heterogeneous, and unmanageable. It is hard to pattern cells in scaffolds effectively with no impact on cell viability and function. Here, we summarize a biocompatible, easily available, label-free, and non-invasive tool, surface acoustic wave (SAW) technique, which is getting a lot of attention in tissue engineering. SAW technique can realize accurate sorting, manipulation, and cells' pattern and rapid formation of spheroids. By integrating several SAW devices onto lab-on-a-chip platforms, tissue engineering lab-on-a-chip system was proposed. To the best of our knowledge, this is the first report to summarize the application of this novel technique in the field of tissue engineering.

H2S gas sensing performance and mechanisms using CuO-Al2O3 composite films based on both surface acoustic wave and chemiresistor techniques

Surface acoustic wave and chemiresistor based gas sensors integrated with a sensing layer of sol-gel CuO-Al2O3 composite film were fabricated and their performance and mechanisms for H2S sensing were characterized and compared. In the composite film, CuO nanoparticles provide active sites for adsorption and reaction of H2S molecules while Al2O3 nanoparticles help to form a uniform and mesoporous film structure, both of which enhance the sensitivity of the sensors by providing numerous active CuO surfaces. Through the comparative studies, the SAW based H2S sensor operated at room temperature showed a lower detection limit, higher sensitivity, better linearity and good selectivity to H2S gas with its concentration ranging from 5 ppb to 100 ppm, compared with those of the chemiresistor sensor, which are mainly attributed to the effective mass sensing properties of the SAW sensor, because a minor change in the mass of the film caused by adsorbed H2S molecules would lead to a significant and monotonous change of the resonant frequency of the SAW devices.

Time-frequency analysis of laser-excited surface acoustic waves based on synchrosqueezing transform

Laser-excited surface acoustic wave (LSAW) techniques can realize the measurement of surface properties through dispersion curve inversion. In addition to the conventional phase spectrum method, time-frequency analysis can also be used as a calculation tool for the dispersion curve. An accurate time-frequency analysis method is important in precisely inverting the surface properties. In this paper, synchrosqueezing transform (SST) is applied to the analysis of LSAW. By comparing the processing results of the constructed LSAW signal, SST shows higher time-frequency resolution and robustness than other common time-frequency analysis algorithms. Finite element model is used to simulate the propagation of LSAW in the film/substrate structure. And SST is used to calculate the dispersion curve of SAW information extracted during the simulation. A fast algorithm is introduced, where the film thickness is initially estimated by calculating the midrange. Through inversion, the resulting film thickness error is only 1.20% from the theoretical value. Therefore, SST is an effective tool for analyzing LSAW and calculating its dispersion curve.

On-chip surface acoustic wave and micropipette aspiration techniques to assess cell elastic properties.

The cytoskeletal mechanics and cell mechanical properties play an important role in cellular behaviors. In this study, in order to provide comprehensive insights into the relationship between different cytoskeletal components and cellular elastic moduli, we built a phase-modulated surface acoustic wave microfluidic device to measure cellular compressibility and a microfluidic micropipette-aspiration device to measure cellular Young's modulus. The microfluidic devices were validated based on experimental data and computational simulations. The contributions of structural cytoskeletal actin filament and microtubule to cellular compressibility and Young's modulus were examined in MCF-7 cells. The compressibility of MCF-7 cells was increased after microtubule disruption, whereas actin disruption had no effect. In contrast, Young's modulus of MCF-7 cells was reduced after actin disruption but unaffected by microtubule disruption. The actin filaments and microtubules were stained to confirm the structural alteration in cytoskeleton. Our findings suggest the dissimilarity in the structural roles of actin filaments and microtubules in terms of cellular compressibility and Young's modulus. Based on the differences in location and structure, actin filaments mainly contribute to tensile Young's modulus and microtubules mainly contribute to compressibility. In addition, different responses to cytoskeletal alterations between acoustophoresis and micropipette aspiration demonstrated that micropipette aspiration was better at detecting the change from actin cortex, while the response to acoustophoresis was governed by microtubule networks.

Influence factors of film adhesion criterion of the nondestructive CZM-SAW technique

Film adhesion can be determined by the surface acoustic wave (SAW) technique combined with the cohesive zone model (CZM) nondestructively and quantitatively. Moreover, the CZM-SAW technique can judge the adhesion quality quantitatively by the adhesion criterion. In this study, adhesion criterions of four kinds of different film materials with a wide thickness range from 50 nm to 6000 nm are simulated to study the influence factors of film adhesion criterion determined by the CZM-SAW technique. Studying influence factors of film adhesion criterion is essential for adhesion quality judgment. In addition, the adhesion criterion deviation caused by measurement error is analyzed. Adhesion properties of six film samples are determined by the CZM-SAW technique and are judged by the adhesion criterion with considering measurement errors. This paper exhibits the unique advantage of the CZM-SAW technique, which is a complete method for characterizing the film adhesion.

Influence of residual stress on the determination of Young’s modulus for SiO2 thin film by the surface acoustic waves

The surface acoustic wave (SAW) technique is attractive for nondestructive mechanical determination of thin films. In this paper, the influence of residual stress on the Young’s modulus determination for SiO2 thin film is discussed. The residual stress model is established by introducing the (effective elastic constants) into the ideal model. When SAWs propagate along the Si[110] and [100] direction, the dispersion curves obtained from ideal SAW model and residual stress model are compared, respectively. The ratio () is used to judge the impact. The modified formula for the Young’s modulus value determined by the ideal model is calculated. The scope of the application of the ideal SAW model is obtained, where the influence of residual stress could be safely ignored. This study indicates that the influence of residual stress on the determination of Young’s modulus for SiO2 thin film by SAWs are small, and it can be ignored or revised.

Quantitative Simultaneous Determination for Young’s Modulus and Adhesion of Low-k Thin Film by Non-destructive CZM-SAW Technique

Adhesion and Young’s modulus are of great interest in thin film science because these two properties are the most vital factors in determining the durability and stability of the device. Insufficient Young’s modulus and adhesion property are the most serious problems encountered in the application of low-k thin film which is widely used in microelectronic industry. In this study, the nondestructive surface acoustic wave (SAW) technique with cohesive zone model (CZM) is employed to determine the quantitative thin film Young’s modulus and adhesion simultaneously. By studying the influence of film adhesion on the determination of film Young’s modulus, a film adhesion and Young’s modulus simultaneous measuring method is proposed. Based on the original CZM-SAW technique, comprehensive matching process is executed in this simultaneous measuring method. By this method, interactive effects of these two parameters are taken into consideration in the determining process so that more accurate adhesion and Young’s modulus results can be acquired. In this paper, the adhesion and Young’s modulus of SiO2 films and porous Black Diamond film are measured by the CZM-SAW technique. This study presents a high-precision measuring method for film adhesion and Young’s modulus. This paper also shows the promising advantages of the CZM-SAW technique in characterization of thin film properties.

Response of Surface Acoustic Wave Propagation to Gasoline Gum Deposited on the Surface of Piezoelectric Substrate

Abstract In this article, theoretical analyses and experimental examinations have been performed on the response of surface acoustic wave (SAW) propagation to the film of gasoline gum deposited on the surface of a piezoelectric substrate, where the film of gasoline gum was thought of as viscoelastic. Using a commercial software (COMSOL Multiphysics), the influence of mass loading and viscoelasticity of the viscoelastic film on the SAW propagation has been numerically analyzed. It has been found that both the amplitude attenuation and the center frequency shift of SAW propagation change sensitively and monotonically with the change in mass loading and viscoelastic behavior of the viscoelastic film. To verify the theoretical analyses, the experimental examinations of the response of SAW propagation to the film of gasoline gum have been carried out. The experimental results show good consistency with the theoretical predictions. Both the amplitude attenuation and center frequency shift of SAW propagation indicate a significant sensitivity to change in the content of gasoline gum deposited on the surface of SAW propagation substrate. It is expected that the results obtained can provide a means for rapidly sensing the content of gum in automotive gasoline using the SAW technique.

Nondestructive determination of film thickness with laser-induced surface acoustic waves**Project supported by the National Natural Science Foundation of China (Grant No. 61571319).

The application of surface acoustic waves (SAWs) for thickness measurement is presented. By studying the impact of film thickness h on the dispersion phenomenon of surface acoustic waves, a method for thickness determination based on theoretical dispersion curve v(fh) and experimental dispersion curve v(f) is developed. The method provides a series of thickness values at different frequencies f, and the mean value is considered as the final result of the measurement. The thicknesses of six interconnect films are determined by SAWs, and the results are compared with the manufacturer’s data. The relative differences are in the range from 0.4% to 2.18%, which indicates that the surface acoustic wave technique is reliable and accurate in the nondestructive thickness determination for films. This method can be generally used for fast and direct determination of film thickness.

Young's modulus of plasma‐polymerized allylamine films using micromechanical cantilever sensor and laser‐based surface acoustic wave techniques

Mechanical properties of ultra‐thin organic films are fundamentally important in coating applications. Micromechanical cantilever sensor (MCS) and laser‐based surface acoustic wave (LA‐SAW) techniques were both used to measure Young's moduli (E) of plasma polymerized films at different humidities (H). For plasma polymerized allylamine (ppAA) films deposited at 5 W and 90 W, E of 1400 ± 350 MPa and 110 ± 20 MPa at H between 10 and 40%, and 1070 ± 250 MPa and 32 ± 10 MPa at H between 70 and 80% were measured. The LA‐SAW technique revealed E lower than 60% of that measured by MCS. The difference suggested either an enhanced swelling at the air interface or a gradient of cross‐linking density.

Finite Element Analysis of Laser-Generated Surface Acoustic Wave for Spinal Cord Tension Measurement

For patients with tethered cord syndrome (TCS), surgeons need accurate judgment of the tumor or bonding to be cut during the surgical procedures to release the spinal cord to normal tension. According to laser-generated surface acoustic wave (SAW) technology, using laser to irradiate the spinal cord with viscoelasticity to generate SAW, we can analyze the degree of traction in the spinal cord by the propagation conditions of SAW which is related to the physical and mechanical properties of the spinal cord surface. A finite element model was built in the environment of COMSOL Multiphasic. This simulation analyzed and verified the feasibility of laser-generated SAW technique for measuring spinal cord surface tension, and it will contribute to the development of the testing medical devices of spinal cord tension which enable doctors to achieve more accurate results during surgery.

Specific expression pattern of tissue cytokines analyzed through the Surface Acoustic Wave technique is associated with age-related spontaneous benign prostatic hyperplasia in rats

The aim of the study reported herein was to evaluate the suitability of the Surface Acoustic Wave (SAW) technique as a possible diagnostic tool in benign prostatic hyperplasia (BPH). Moreover, for the first time, the BPH model was a totally physiological using naturally aged rats with spontaneous, age-related BPH instead of the pharmacologically induced models usually used. Eighteen male Wistar rats were distributed according to their age: 6 weeks (young), 12 weeks (adult) and 12 months (old) old. Prostate gland was removed and analyzed by mini-arrays, Western blotting (WB) and SAW techniques. Mini-arrays indicated that there were significant differences in the expression of 29/34 inflammation-related cytokines. WB was carried out to confirm the results after selection of 4 cytokines from which one showed no changes, namely PDGF-AA, and the other three, which significantly increase in older animals, were CD86, β-NGF and VEGF. Notwithstanding, WB of old rats yielded confusing results due to an anomalous migration of proteins, dismissing this technique as an useful tool in these animals. Accurate results in old rats were uniquely obtained by using the SAW technique. Thus, SAW analysis showed that there were not differences among groups in the amount of PDGF-AA. On the contrary, SAW analysis showed that amounts of CD86, β-NGF and VEGF in old rats were 2.0, 1.9 and 5.7-fold higher than that from young ones, respectively. These results indicate that SAW is a highly accurate technique for determining changes in the cytokines expression in BPH.

Three-Dimensional Analysis of Surface Acoustic Resonator for Ultra-High Sensitive Ethanol Disclosure as Non-Invasive Biosensor

Due to the increasing demand on ethanol gas sensors for several applications such as automation transportation or environmental monitoring the need for more sensitive and reliable sensors with an on spot response time is required Indirect sensing invasive bulky time initializing and low reliability in strip tests are some drawbacks of ethanol sensing measurements at the present To overcome these leakages there is focused on modelling and simulation of a proposed ethanol sensor based on surface acoustic wave technique using finite element method

Real time size-dependent particle segregation and quantitative detection in a surface acoustic wave-photoacoustic integrated microfluidic system

We present an integrated microfluidic system capable of size-dependent separation, concentration, and quantitative detection of microparticles on a single piezoelectric (128° YX LiNbO3) substrate using surface acoustic wave (SAW) and photoacoustic (PA) technique. First, using a tilted-angle standing SAW highly efficient segregation of polystyrene beads of two different sizes was achieved in a continuous particle flow inside the channel. Flow-assisted migration of separated particles would then undergo particle aggregation using a SAW device, to preconcentrate the sample before detection The optical absorption of the pulsed laser by the polystyrene microparticles generates the PA wave. The detection scheme involves sensing the SAW generated by the optically induced mechanical resonance of the PA-wave inside the microfluidic channel. The SAW-PA signal was further correlated to the particle count. The developed integrated microfluidic system performs effectively, with separation efficiency above 95% for the 3- and 10-μm diameter polystyrene beads. The separated 10-μm particles were directed into the sensing chamber, where a SAW device concentrate the particles before real-time quantitative detection. The configuration and theoretical aspects of each integrated segments were studied and explained. The sensor presented a reliable (R2=0.98) detection of the 10-μm particles down to 7 particles in 10μL of the sample volume in 15min. This technique paves the way to the design and development of future lab-on-a-chip (LOC) devices for clinical analyte detection and quantification.

Elastic and structural properties of nanometer scale-thick refractory metal films

In this work, we investigate thickness-dependent elastic and structural properties of refractory W, Ta and Mo thin films fabricated by radio-frequency magnetron sputtering on silicon substrates. Elastic properties of studied hard thin coatings are measured by non-destructive nanosecond laser pulse induced surface acoustic wave technique, while their structural and dimensional properties are analysed using X-ray diffraction (XRD) and scanning electron microscopy, respectively. It was found that the Young’s modulus of these films increase with their nanoscale thickness. Young’s modulus of hard metals (W, Mo) was consistently higher than those of their bulk. However, the modulus of Ta films was higher than that of the corresponding bulk metal except for the film with the lowest thickness (22 nm). XRD analysis revealed that all films are under compressive stress, but this stress is diminished in thicker films.

Evaluation and criterion determination of the low-k thin film adhesion by the surface acoustic waves with cohesive zone model

The cohesive zone model (CZM) is introduced in the surface acoustic wave (SAW) technique to characterize the interfacial adhesion property of the low-k thin film deposited on the Silicon substrate. The ratio of the two parameters in the CZM, the maximum normal traction and normal interface characteristic length, is derived to evaluate the interfacial adhesion properties quantitatively. In this study, the adhesion criterion to judge the adhesion property is newly proposed by the CZM-SAW technique. The criterion determination processes of two kinds of film, dense and porous Black Diamond with different film thicknesses, are presented in this paper. The interfacial adhesion properties of the dense and porous Black Diamond films with different thicknesses are evaluated by the CZM-SAW technique quantitatively and nondestructively. The quantitative adhesion properties are obtained by fitting the experimental dispersion curves with maximum frequency up to 220MHz with the theoretical ones. Results of the nondestructive CZM-SAW technique and the destructive nanoscratch exhibit the same trend in adhesion properties, which means that the CZM-SAW technique is a promising method for determining the interfacial adhesion. Meanwhile, the adhesion properties of the detected samples are judged by the determined criterion. The test results show that different test film materials with different film thicknesses ranging from 300nm to 1000nm are in different adhered conditions. This paper exhibits the advantage of the CZM-SAW technique which can be a universal method to characterize the film adhesion.