Surface Acoustic Wave Properties Research Articles

Enhanced surface acoustic wave microfluidic performance through FDTS self-assembled monolayers on [formula omitted]

Microfluidics technology has been extensively applied in biochemical analysis and medical diagnostics, where precise and efficient control of microfluids is crucial. However, existing passive microfluidic control methods (e.g., microchannels) and active strategies (e.g., optical, thermal, magnetic field, and electrokinetic manipulation) are often hindered by complex device structures and operational uncertainties. Surface acoustic wave (SAW) microfluidic manipulation offers a promising alternative, providing flexible and efficient digital microfluidic control through the modulation of input signals. Nevertheless, the intrinsic hydrophilicity of substrates such as LiNbO3 results in inefficient microfluidic driving and jetting, primarily due to high surface tension. In this study, we employed chemical vapor deposition (CVD) to coat LiNbO3 substrates with 1H,1H,2H,2H-Perfluorododecyltrichlorosilane (FDTS) self-assembled monolayers (SAMs), creating hydrophobic surfaces with a contact angle of approximately 120°. In comparison to commercially available Glaco hydrophobic coatings, which reduce SAW amplitude by approximately threefold and increase substrate temperature by up to 1.5 times, FDTS SAMs exhibit negligible impact on SAW properties. Furthermore, when compared to untreated SAW devices, FDTS SAMs-coated devices demonstrated a sixfold increase in droplet driving speed under low power input (e.g., 0.5 W), enabling faster and more precise liquid jetting at velocities reaching 5 m/s. This work highlights the potential of FDTS SAMs in enhancing the performance of SAW-driven microfluidic devices, particularly in applications requiring high-efficiency droplet manipulation.

Investigation of the Properties of Surface Acoustic Waves in a Lithium Niobate Single Crystal with a Silicon Dioxide Film by the Finite-Element Method

Investigation of the Properties of Surface Acoustic Waves in a Lithium Niobate Single Crystal with a Silicon Dioxide Film by the Finite-Element Method

Analysis of propagation and resonance properties of longitudinal leaky surface acoustic wave on LiNbO3/SiC bonded structure

Abstract The propagation and resonance properties of longitudinal leaky surface acoustic waves (LLSAW) on a bonded structure comprising an X-cut LiNbO3 (LN) thin plate and a 4H-SiC support substrate are theoretically investigated. The strong LLSAW responses with high Q factors were obtained at the LN thin-plate thickness h where the LLSAW phase velocity was slower than the bulk shear wave of 4H-SiC of 7126 m/s, and a fractional bandwidth (FBW) of 9–10% was obtained for the normalized Al film thickness by wavelength h Al/λ = 0.06–0.07 and h/λ = 0.30–0.40. Moreover, even at h/λ with a faster phase velocity than the bulk shear wave of 4H-SiC, the strong LLSAW responses without spurious response owing to the LLSAW higher-order mode were obtained. Finally, h Al/λ = 0.031 and h/λ = 0.19 were extracted to obtain a phase velocity of 7800 m/s, high Q factors, and FBW of 7.6%.

Surface Acoustic Wave Interaction with a Mixture of Oxygen and Nitrogen

Parameters of surface acoustic waves (SAW) are very sensible to change of physical conditions of a propagation medium. In the classical theory formulation, the waves are guided along the boundary of semi-infinity solid state and free space. A real situation is more complex and a medium commonly consists of two physical components: a solid substrate and a gaseous or liquid environment. In the case of stress-free substrate, the strongest impact on SAW properties have surface electrical and mechanical conditions determined by solids or liquids adhering to the boundary. This impact is utilised for constructing sensors for different gases and vapours e.g. (Jakubik et al., 2007; Hejczyk et al., 2011; Jasek et al., 2012). The influence of gaseous environment on the SAW properties is usually very weak and ignored. However, in certain condition it can be significant enough to be applied to sensor construction. In general, it concerns Rayleigh wave devices where energy leakage phenomenon is perceptible, especially when the gas being detected considerably changes the density of environment. The paper presents the results of experiments with oxygen-nitrogen mixture. Their primary aim was focused on finding the dependence of resonant frequency and attenuation in SAW resonator on parameters and concentrations of the gas in the environment.

Analysis of longitudinal leaky surface acoustic waves on piezoelectric thin plates bonded to diamond substrate

Using the finite element method, we analyzed the resonance properties of a longitudinal leaky surface acoustic wave (LLSAW) on the structure of a piezoelectric LiTaO3 (LT) or LiNbO3 (LN) thin plate bonded to a diamond support substrate. When the plate thickness was 0.3 wavelength or larger, the particle displacements of the LLSAW were substantially concentrated near the surface, and the resonance properties were significantly improved compared to those of the single LT or LN. However, spurious responses due to other SAW modes also emerged. Regarding the LN thin plate, we obtained a phase velocity and fractional bandwidth of approximately 7300 m s−1 and 9.4%, respectively. Furthermore, a structure was proposed to suppress spurious responses in which the piezoelectric thin plate is divided into two layers with different Euler angles. The spurious responses were significantly suppressed after dividing the thin plate, while the main response of the LLSAW was maintained.

Analysis of longitudinal leaky surface acoustic waves on bonded structures consisting of similar and dissimilar materials

The propagation and resonance properties of longitudinal leaky surface acoustic waves on structures consisting of a LiTaO3 (LT) thin plate bonded to a quartz (Qz) similar-material bonded structure were investigated theoretically. It was found by surface acoustic wave (SAW) propagation analysis that a small attenuation can be obtained by combining an LT thin plate and a Qz similar-material bonded structure with appropriate Euler angles. Furthermore, regarding SAW resonance analysis, resonances with admittance ratios exceeding 50 dB and Q factors exceeding 1000 were obtained in the LT/Qz/Qz bonded structure. Such a small propagation loss and high Q can be obtained with a larger LT plate thickness than in the case of the LT/Qz bonded structure.

Resonance properties of shear-horizontal surface acoustic waves on Ca3TaGa3Si2O14 at room and high temperatures

The resonance properties of shear-horizontal surface acoustic waves on Ca3TaGa3Si2O14 (CTGS) with Au or Al interdigital transducers (IDTs) were investigated. IDT-type resonators were fabricated on CTGS (0°, 134° or 155°, 90°) using Au- or Al-IDT, and the strong resonance properties and near-zero temperature coefficient of frequency (TCF) were measured at a certain electrode film thickness. From the measured and simulated results, an effective coupling factor of approximately 1.2% and zero TCF can be simultaneously obtained by using not only a high-density Au-IDT but also an Al-IDT and by adjusting the cut angle and electrode film thickness. Moreover, the resonance properties on Au-IDT/CTGS were evaluated at temperatures up to 570 °C. From the cut-angle dependences of the frequency shift calculated using the determined second-order temperature coefficients of the elastic constants, the potential for sensor applications that measure not only temperature but also pressure in such high-temperature environments can be expected.

Analysis of surface acoustic wave resonance properties on piezoelectric substrates with periodic voids

The resonance properties of leaky surface acoustic waves (LSAWs) and longitudinal LSAWs (LLSAWs) on bonded structures consisting of a LiNbO3 (LN) thin plate and a support substrate, with periodic voids exhibiting a rectangular cross-section, were simulated using the finite element method. The voids were formed below the electrode at the boundary between the LN thin plate and support substrate and presented the same pitch and period as the electrode. LN, quartz, glass, or Si were used as support substrates. Simulations revealed that by introducing periodic voids, resonance properties similar to those of the SH0-mode plate and S0-mode Lamb waves can be obtained. The values of the simulated fractional bandwidths of approximately 13 and 18% were found for LSAW and LLSAW, respectively, when the ratio of the width of the void to the pitch of the electrode was 0.7.

Analysis of SAW Temperature Properties in KTiOPO4 Single Crystal

The surface acoustic wave (SAW) properties of potassium titanyl phosphate (KTiOPO4, KTP) single crystal were evaluated by numerical methods. The phase velocity, electromechanical coupling coefficient, power flow deflection angle, and temperature coefficient of delay (TCD) were determined for different crystal cuts of KTP. It was shown that SAW has the electromechanical coupling coefficient of 0.59% and the TCD of 62 ppm/°C on the Z-cut and wave propagation direction along the crystal X + 70°-axis. For the Z-cut and wave propagation direction along the X-axis, the pseudo-surface wave (PSAW) is characterized by the coupling coefficient of 0.46% and the TCD value of 57 ppm/°C. The Bleustein-Gulyaev (BG) wave has the TCD value of 35 ppm/°C and 41 ppm/°C on the Y- and X-cuts of KTP, respectively.

Finite element analysis on tunable solid/fluid phononic crystal for surface acoustic wave bandgaps with various fluid heights

In this study, the propagations of surface acoustic waves (SAWs) in two-dimensional solid/fluid phononic crystal (PC) structures were investigated. The PC structures are composed of a periodic sequence of hollow pillars deposited on a semi-infinite substrate, which can be filled with various kinds/heights of fluid. Finite element analysis was used to study the characteristics of SAW bandgaps for the unit cell of PCs. The results showed that the distribution of bandgaps varies with the fluid height. Additionally, the change of bandgaps is more sensitive to mercury than water. Furthermore, transmission properties of SAWs for the PCs filled with different mercury heights are analyzed. It can be found that the PCs can inhibit the propagation of SAWs with a frequency corresponding to the bandgaps validly. Meanwhile, comparing with the transmission spectra, it could be concluded that the transmission troughs shift to lower frequency overall as the mercury heights increase. The results obtained in this study are instructive and meaningful for the practical design of tunable SAW PCs.

Effect of Guiding Layers and Interdigitated Electrode Structures on the Frequency Behaviour of SAW Sensors

The properties of surface acoustic wave (SAW) sensor can be optimized by using a rational structural design. In this work, simulation and experimental studies of ZnO/quartz structure are presented. The effect of the guiding layer on the propagation characteristic and surface displacement of the designed SAW sensor is mainly discussed. SAW sensors with different structures (different input-output interdigital transducer (IDT) ratio, different aperture and different guiding layer) are fabricated via conventional photolithography techniques and measured by network analyzer. The ZnO-based SAW sensor with symmetrical structure (50:50) and larger aperture (1800 μm) shows good frequency behaviour and obtains lower insertion loss. The experimental results are in agreement with the simulation results.

Theoretical investigation of the piezoelectric and surface acoustic wave properties of GeSn alloys

The piezoelectric and dielectric constants of Ge1-xSnx with varying concentrations (x∈(0–1.0) were calculated using the first-principles method based on the density functional theory, and the surface acoustic wave (SAW) properties such as phase velocity, electromechanical coupling coefficient k2, and power flow angle (PFA) were obtained for Ge0.5Sn0.5 from using the Christopher equations in different orientations (0–90°, 0–90°, and 0–180°). The results demonstrate that the piezoelectric and dielectric constants of Ge1-xSnx deviate from Vegard's law when the x is varied, and the piezoelectric constant value reaches a maximum when x = 0.5. Additionally, the SAW velocity of Ge1-xSnx can be tuned by changing the Sn concentration. It can be seen from the contour maps of Ge0.5Sn0.5 at orientations (45°, 0–90°, and 0–180°), many orientations exhibit outstanding SAW performance. Orientations with large k2 and zero PFA are selected as promising candidates for fabricating SAW devices, which has important significance for experimental research.

Analysis of longitudinal leaky surface acoustic waves on quartz thin plate bonded to similar-material substrate

The propagation and resonance properties of longitudinal leaky surface acoustic waves (LLSAWs) on bonded structures consisting of a quartz (Qz) thin plate and a Qz support substrate with different Euler angles were investigated theoretically. By using both an X-cut Qz thin plate and a Qz support substrate with optimal Euler angles, we obtained LLSAWs with a larger coupling factor, a smaller attenuation, and a lower temperature coefficient of frequency than those on a single Qz substrate. Furthermore, from the resonance properties simulated by the finite element method, the bonded structures were found to exhibit a large admittance ratio and a high quality factor, which could not be obtained when using a single Qz substrate; the bandwidth, however, was as small as 0.016%–0.086%.

Analysis of surface acoustic wave properties in CaYAl3O7 single crystal

The success on the growth of new piezoelectric materials allows sufficiently increase the operating temperature of the surface acoustic wave (SAW) devices from 300°C to 1000°C. A new calcium yttrium aluminate (CaYAl3O7) single crystal of the tetragonal symmetry has piezoelectric properties up to the temperature of 1000°C. The paper presents a numerical study of the surface acoustic wave properties in the crystal. The SAW velocity, electromechanical coupling coefficient and power flow angle are studied for different crystal cuts of CaYAl3O7. It is shown that the maximum value of SAW coupling coefficient (0.24%) is on the Z+60°-cut and wave propagation direction along the X-axis of the crystal. For the Z-cut and wave propagation direction along the X+45°-axis of crystal, the SAW coupling coefficient is equal to 0.2%. These two cuts of the crystal are potentially useful for SAW device applications.

Analysis of leaky surface acoustic waves on quartz thin plates bonded to similar-material substrate

The propagation and resonance properties of a leaky surface acoustic wave (LSAW) on quartz thin plates bonded to a similar-material substrate are investigated theoretically. The electromechanical coupling factor K 2 on Z-cut quartz (Z–Q) thin plates bonded to an AT-cut 0°X-propagating quartz (AT0°X-Q) support substrate is calculated to be 0.43%, which is approximately three times larger than the maximum value of a single quartz substrate. A positive temperature coefficient of frequency for LSAW can be produced on a quartz thin plate bonded to a quartz substrate with a different cut angle. By the finite element method, the aluminum thin-film thickness dependence of the resonance properties of LSAW on LST-cut quartz (LST-Q) and LST-Q/AT0°X-Q is analyzed. In the simulation with the optimal Al thin-film thickness, the admittance ratio on the LST-Q/AT0°X-Q is found to be larger than that for the single LST-Q.

Molecular dynamics modeling of nonlinear propagation of surface acoustic waves

A new computational setup suitable for the exploration of nonlinear effects in free propagation and dissipation of surface acoustic waves (SAWs) is developed based on the molecular dynamics (MD) simulation method. First applications of the computational model demonstrate the ability of atomistic simulations to reproduce the key features of the nonlinear SAW evolution, which are distinct from their well-known counterparts in bulk wave propagation. In particular, the MD simulations predict the increasing localization of the acoustic energy near the surface of the substrate during the nonlinear sharpening of the wave profile, which leads to the formation of a shock front with characteristic cusps in the horizontal strain and velocity profiles. The peak values of surface strain and velocity associated with the cusps can significantly exceed those of the initial wave. Some of the effects revealed in the MD simulations are outside the capabilities of continuum-level models and have not been explored so far. These include the observation of an unusual quadratic correction to the dispersion relation at short wavelengths defined by the frequency-dependent localization of SAWs near the surface of the substrate, the establishment of a new mechanism of the energy dissipation at the SAW shock front, where SAW harmonics generated at the limit of frequency up-conversion transform very effectively into clouds of phonon wave packets descending into the substrate bulk, and the generation of localized zones of plastic deformation at a substantial distance from the wave source. Overall, the MD methodology developed for atomistic modeling of free SAW propagation not only enables detailed analysis of the intrinsic properties of nonlinear SAWs and verification of theoretical models but also opens up a broad range of opportunities for investigation of acoustically induced surface processes, material modification by SAWs, and the interaction of SAWs with preexisting crystal defects and other material heterogeneities.

Propagation properties of leaky surface acoustic wave on water-loaded LiTaO3/quartz bonded structure

Recently, bonded structures such as LiTaO3/quartz structures have been attracting attention as a means of obtaining high-performance surface acoustic wave (SAW) devices. To investigate the possibility of evaluating the acoustical loss in such bonded structures by subtracting the calculated leakage loss into the water from the measured normalized attenuation factor, propagation properties such as the velocity and normalized attenuation factor of leaky SAWs on a water-loaded 36°YX-LiTaO3/AT0°X-quartz bonded structure were investigated using a line-focus-beam ultrasonic material characterization system and were found to be in good agreement with the calculated values for both the 0°X and 90°X propagation directions.

Tunable surface acoustic waves on strain-engineered relaxor K0.7Na0.3NbO3 thin films

In this work, we demonstrate the electronic tunability of surface acoustic waves (SAWs) in epitaxially strained relaxor-type ferroelectric thin films. Epitaxial K0.7Na0.3NbO3 thin films of typically 30 nm in thickness are grown via pulsed laser deposition on (110)-oriented TbScO3. A partial plastic lattice relaxation of the epitaxial strain in these samples leads to a relaxor-type ferroelectricity of these films, which strongly affects the SAW properties. Without electronic bias, only tiny SAW signals of ∼0.2 dB can be detected at room temperature, which can be boosted up to ∼4 dB by a static voltage bias added to the high frequency driving current of the SAW transducers. Upon field cooling below the freezing temperature of polar nanoregions (PNRs), this strong SAW signal can be preserved and is even enhanced due to a release of the electronically fixed PNRs if the bias is removed. In contrast, at elevated temperatures, a reversible switching of the SAW signal is possible. The switching shows relaxation dynamics that are typical for relaxor ferroelectrics. The relaxation time τ decreases exponentially from several hours at freezing temperature to a few seconds (<5 s) at room temperature.

Theoretical analysis of high electromechanical coupling surface acoustic wave propagating on lead-free Na0.5Bi0.5TiO3–BaTiO3 single crystal

The propagation properties of surface acoustic wave (SAW) on the Na0.5Bi0.5TiO3–BaTiO3 (NBBT) substrate with different crystal cuts were theoretically analyzed by Finite Element Method (FEM). The calculated results, for the NBBT with the optimized electrode type and thickness, indicate the existence of Rayleigh and Sezawa modes with high electromechanical coupling factor K2 ~18% and ~6.8% respectively, which are considerably higher than that of the conventional lithium niobate (LiNbO3) single crystal for SAW applications. The research provides a comprehensive understanding of the SAW propagation characteristics of lead-free NBBT, and explores the promising applications of NBBT in wideband SAW devices.

Numerical analysis of SAW propagation characteristics in Ca2Al2SiO7 crystal

The importance of surface acoustic wave (SAW) technology is to integrate signal processing with sensor functions. Recently, SAW sensors can operate at room temperature or around 100°C-200°C. The task associated with materials becomes relevant if it is necessary to increase the operating temperature of the device to 1000°C. A new piezoelectric calcium aluminate silicate (Ca2Al2SiO7) crystal of the tetragonal symmetry has stable piezoelectric properties up to the melting point temperature of 1500°C. The properties of surface acoustic waves a new Ca2Al2SiO7 single crystal are numerically modeled in the paper. The SAW velocity, the electromechanical coupling coefficient, and the power flow angle are modeled for various cuts of the Ca2Al2SiO7 crystal. The SAW has the maximal electromechanical coupling coefficient value (∼0.1%) on Z,X+45°-cut of the crystal. In the Z+63°, X-cut, the SAW electromechanical coupling coefficient is equal to 0.08%. Both of these crystal cuts show the absence of bulk acoustic wave generation by an interdigital transducer, and the Ca2Al2SiO7 crystal is a promising piezoelectric material for use in the SAW device applications.