Surface Acoustic Wave Properties Research Articles

The effect of geometry and post-annealing on surface acoustic wave characteristics of AlN thin films prepared by magnetron sputtering

This paper describes experimental relationship between surface acoustic wave (SAW) properties of two-port SAW resonators based on polycrystalline aluminum nitride (AlN) thin films grown on Si substrates by using a pulsed reactive magnetron sputtering system and their geometry's parameters. Moreover, the influence of post-deposition heat treatment on SAW properties of AlN thin films was investigated at different annealing temperature (600 °C and 900 °C). The measurement results show the number of the inter-digital transducers (IDT) finger pairs ( N), the number of reflectors grating pairs ( R) and the IDT center-to-center distance ( L) related to insertion loss of SAW resonators. The best result of insertion loss was 15.6 dB for SAW resonators with R = 160 pair, N = 5 pair and L = 750 μm. At the same geometry parameters, the SAW velocity and insertion loss were improved slightly after annealing at 600 °C and were worse for the films annealed at 900 °C by changes in the surface morphology and stress on the film.

Theoretical analysis of ultrahigh electromechanical coupling surface acoustic wave propagation in Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 crystals

The propagation characteristics of surface acoustic waves (SAWs) on the Y-cut X-propagating Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 (PIN–PMN–PT) substrate are theoretically analyzed. The results indicate the existence of a shear horizontal (SH)-type SAW with an ultrahigh electromechanical coupling factor K2. Different metal layers (Au, Al, and Cu) were examined as the electrode material. With gold, the maximum K2 for the SH SAW reaches 72.6%, while it is smaller (69.7%) for Al or Cu. There also exists a Rayleigh SAW causing the spurious response. It is shown that the response can be suppressed sufficiently by properly adjusting the thickness of the metal layer. This feature is very attractive for the realization of ultrawideband SAW filters. Variation of SAW properties with the substrate rotating angle θ was also investigated. The result indicates that θ = 0° is the best due to the relatively larger K2 for SH SAW and lower value for Rayleigh spurious response.

Study on the structure and surface acoustic wave properties of SNGS and STGS piezoelectric crystals

The geometry structure, band structure and density of states of Sr 3NbGa 3Si 2O 14 (SNGS) and Sr 3TaGa 3Si 2O 14 (STGS) single crystals were investigated by using density functional theory (DFT) method. Parameters including the phase velocity, electromechanical coupling factor and power flow angle (PFA) were calculated for SNGS and STGS at X, Y, Z cuts, respectively. Our calculated data are in good agreement with the experimental results. It is found that the peaks of partial density of states of Sr, Ga, Si and O atoms of STGS have a tendency of shifting to the higher energy levels relative to those of SNGS. Compared with quartz, SNGS and STGS crystals are of lower phase velocities and higher electromechanical coupling factor. The propagation direction of 167.5° of X-cut SNGS, 0° of Y-cut SNGS and 170° of X-cut STGS are found to be optimum direction for SAW device applications.

Bleustein-Gulyaev SAWS with Low Losses: Approximate Direct Solution

The main properties (attenuation along the surface, attenuation in depth, additional radiation in depth, dispersion in propagation space) of Bleustein-Gulyaev surface acoustic waves (SAWs) in electroelasticity are determined in terms of a perturbation due to viscosity. This paves the way for a study of the perturbed motion of associated quasi-particles in the presence of low losses.

Effect of Alumina Film on Surface Acoustic Wave Properties of ZnO Thin Film Surface Acoustic Wave Devices

ZnO films with c-axis (0002) orientation have been successfully grown by RF magnetron sputtering on Al2O3/SiO2/Si substrates. The alumina films were deposited on SiO2/Si substrates by electron beam evaporation. Crystalline structures of the films were investigated by X-ray diffraction, atomic force microscopy, and scanning electron microscopy. The phase velocity of the surface acoustic wave (SAW) device with a 8-µm-thick alumina film was measured to be about 4733 m/s, which is much higher than that (4069 m/s) of the ZnO/SiO2/Si structure and approaches (89.6%) that (5283 m/s) of ZnO/sapphire. The coupling coefficient and temperature coefficient of the frequency of the ZnO/Al2O3/SiO2/Si structure with 8-µm-thick alumina film were close to those of the ZnO/sapphire structure. The experimental result is beneficial to the replacement of the expensive single-crystalline sapphire substrate with alumina film at lower cost for high-frequency SAW devices, and is also useful for integrating the semiconductor and high-frequency SAW devices on the same Si substrate.

Influence of substrate temperature to prepare (1 0 3) oriented AlN films

(1 0 3) Oriented AlN films is an attractive piezoelectric material for the applications on surface acoustic wave (SAW) and film bulk acoustic wave (FBAR) devices. As regards the SAW properties of the (1 0 3) oriented AlN films, the electromechanical coupling constant ( K 2) is larger than the (0 0 2) oriented AlN films. As regards the bulk acoustic wave (BAW) properties of (1 0 3) oriented AlN films, it can excite a quasi-shear mode (velocity = 5957 m/s, K 2 = 3.8%) that can be used for FBAR liquid sensor. In this research, the (1 0 3) oriented AlN films were successfully prepared on the silicon substrate by rf magnetron sputtering. Different temperatures (100 °C, 200 °C, 300 °C, and 400 °C) were used in this experiment process. The crystalline structure of films was determined by X-ray diffraction (XRD) and the surface microstructure was investigated by the atomic force microscope (AFM). The result exhibited the optimal substrate temperature is 300 °C. The optimal (1 0 3) oriented AlN films have the strongest XRD intensity, the smallest full width at half maximum (FWHM) value (0.6°), the largest grain size (15.8 nm) and the smoothest surface (Ra = 3.259 nm).

Fabrication and Evaluation of Highly Oriented Ta2O5 Piezoelectric Thin Films Prepared by Radio Frequency Magnetron Sputtering

Highly X-axis-oriented tantalum pentoxide (Ta2O5) piezoelectric thin films were deposited on a SiO2 substrate using an RF-magnetron sputtering system with a metal tantalum target and an O2-radical source. The degree of orientation, Rayleigh-type surface acoustic wave properties, and surface morphology were evaluated. The deposition condition with the substrate temperature TS of 700 °C and O2 flow rate of 10 ccm was found to be optimum for obtaining a strongly piezoelectric property. Under the optimum condition, the coupling factor of the oriented Ta2O5 thin film with a normalized thickness h/λ of 0.21 was determined to be 0.88% and was 75% of the reported value. The diffraction angle of the preferential peak under the optimum condition was equal to that of the (200)-plane spacing d(200) in the unit cell of monoclinic Ta2O5. A larger plane spacing with Δd/d(200)=2.9% exists preferentially at TS=600 °C, and the piezoelectricity is considered to be zero or very weak. When TS was higher than 700 °C and the O2 flow rate was more than 8 ccm, a smooth surface with the rms roughness of approximately 8–10 nm was obtained. A correlation was found in which a strongly piezoelectric property was obtained when the thin film had a smooth surface.

Transport properties of surface acoustic wave based single electron transport device in the presence of an impurity potential

We report the transport properties of a surface acoustic wave based single electron transport device, which contains an unintentional quantum dot induced by background impurity potential fluctuations. It is found that the presence of the impurity potential can cause a deviation of the acoustoelectric current from its quantized value. Through the charging effect of the quantum dot induced by the impurity, we get an approximate relationship between the applied gate voltage and its corresponding electrostatic potential barrier height, together with the Coulomb charging energy needed to add a second electron into the dynamic quantum dot. Moreover, the amplitude of the surface acoustic wave is also estimated within a simple model.

Surface acoustic wave properties of (100) AIN films on diamond with different IDT positions

(100) AlN films have better surface acoustic wave (SAW) properties than (002) AlN films. In this research, (100) AlN films were combined with diamonds as a new composite SAW substrate. The SAW properties of (100) AlN films on diamonds were analyzed with 4 composite structures: interdigital transducer (IDT)/(100) AlN/diamond, (100) AlN/IDT/diamond, IDT/(100) AlN/metal/diamond, and metal/IDT/(100) AlN/diamond, and they exhibited some excellent SAW properties. Our research results provide a predictable and theoretical basis for further application on high-velocity SAW devices.

Numerical Calculation of SAW Propagation Properties at the x-Cut of Ferroelectric PMN-33%PT Single Crystals

Surface acoustic wave (SAW) properties at the x-cut of relaxor-based 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 (PMN-33%PT) ferroelectric single crystals are analyzed theoretically when poled along the [001]c cubic direction. It can be found that PMN-33%PT single crystal is a kind of material with a low phase velocity and high electromechanical coupling coefficient, and the single crystal possesses some cuts with zero power flow angle. The results are based on the material parameters at room temperature. The conclusions provide device designers with a few ideal cuts of PMN-33%PT single crystals. Moreover, choosing an optimal cut will dramatically improve the performance of SAW devices, and corresponding results for crystal systems working at other temperatures could also be figured out by employing the method.

High velocity shear horizontal surface acoustic wave modes of interdigital transducer/(100) AlN/(111) diamond

In this research, shear horizontal (SH) surface acoustic wave (SAW) modes of interdigital transducer (IDT)/(100) AlN/(111) diamond propagated along the y-axis were theoretically analyzed and they exhibited excellent SH SAW properties. This is special for mode 0 where the K2 curve shows a maximum value (1.27%) at h/λ=0.28 and the velocity is 7496 m/s. The research results provide a predictable and theoretical basis for further application on high velocity SH SAW devices.

Existence and spectral properties of shear horizontal surface acoustic waves in vertically periodic half-spaces

The paper is concerned with the propagation of shear horizontal surface waves (SHSW) in semi-infinite elastic media with vertically periodic continuous and/or discrete variation of material properties. The existence and spectral properties of the SHSW are shown to be intimately related to the shape of the properties variation profile. Generally, the SHSW dispersion branches represent randomly broken spectral intervals on the ( ω , k ) plane. They may, however, display a particular regularity in being confined to certain distinct ranges of slowness s = ω / k , which can be predicted and estimated directly from the profile shape. The SHSW spectral regularity is especially prominent when the material properties at the opposite edge points of a period are different. In particular, a unit cell can be arranged so that the SHSW exists within a single slowness window, narrow in the measure of material contrast between the edges, and does not exist elsewhere or vice versa. Explicit analysis in the ( ω , k ) domain is complemented and verified through the numerical simulation of the SH wave field in the time–space domain. The results also apply to a longitudinally periodic semi-infinite strip with a homogeneous boundary condition at the faces.

Love waves in a smart functionally graded piezoelectric composite structure

The propagation of Love waves in a smart functionally graded piezoelectric structure is analyzed by applying elastic wave theory. There is an additional functionally graded layer between the piezoelectric layer and the substrate in this smart structure. When the piezoelectric and dielectric constants vary individually in a functionally graded layer, the asymptotic solutions of Love waves are obtained by applying the WKB method and solving the fourth order differential equation with variable coefficients. The effects of gradient variation on the phase velocity and the coupled electromechanical factor are discussed in detail. The analysis shows that the number of vibration modes is greater than that in the non-graded layer structure, and the coupled electromechanical factor increases with the increase of piezoelectric constant graded variation. Presented results are useful for the improvement of properties of surface acoustic wave (SAW) devices.

Surface Acoustic Waves in Reverse-Biased AlGaN/GaN Heterostructures

Properties of surface acoustic waves (SAWs) in reverse-biased AlGaN/GaN heterostructures on (0001) sapphire substrates were studied by examining the characteristics of SAW filters composed of interdigital Schottky and ohmic contacts. The fundamental and higher frequency SAW signals in measured -parameters were attributed to Rayleigh and Sezawa modes, respectively. The onsets of the SAW signals, which were close to the threshold voltage of HEMTs in the vicinities of the respective filters, changed in response to the spatial variation of the threshold voltage. The onset of Sezawa mode was deeper than that of Rayleigh mode, and the difference in onset was larger for longer SAW wavelengths. These results are possibly explained by the change of the input capacitance of interdigital transducers due to the reverse-bias voltages or by the difference in the distribution of SAW energy between the two modes.

The theoretical analyse of the surface acoustic waves propagated on the surface of the bulk materials with subsurface defect

A theoretical model based on finite element method is used to analyse the properties of surface acoustic wave (SAW) propagated on the surface of the bulk materials with subsurface defect. The simulation results reveal the modes conversion between SAW and Lamb wave at the border of the defect and explain the diffusion properties of SAW propagated on the bulk material with subsurface defect. The modes conversion and wave reflection at the border of the defect are theoretically explained why there are a lot of riffles after the SAW passed through Al sample with circle subsurface defect, which detected by laser ultrasonic method. The relationships between the wavelength of wave and the physical state of the defect in the bulk material are also fully described in theoretical simulation.

Propagation and Resonance Properties of Love-Type Surface Acoustic Wave on Y–X LiTaO3 with TeO2 Thin Film

The propagation characteristics of a Love-type surface acoustic wave (SAW) on Y-cut X-propagating LiTaO3 with an amorphous tellurium dioxide (TeO2) thin film with a low phase velocity were investigated theoretically and experimentally, and an interdigital transducer (IDT) resonator using this structure was fabricated and evaluated. The K2 of the Love-type SAW on TeO2/Y–X LiTaO3 was calculated to be 6.6% at a transition thickness of 0.07λ. The insertion loss of a simple delay line was reduced by loading with TeO2 film because the bulk wave radiation in excitation and/or propagation of the LSAW was markedly suppressed. Beyond the experimental transition thickness of 0.12λ, the suppression of the bulk wave radiation was saturated. According to the propagation characteristics for IDT resonators fabricated using this structure, the admittance ratio increased and the minimum phase decreased until the TeO2 film thickness reached the experimental transition thickness. When the TeO2 film thickness was 0.12λ, such resonance properties as admittance ratio, minimum phase, and resonance quality factor of the Love-type SAW on TeO2/Al/Y–X LiTaO3 were almost equal to those for Au/Y–X LiTaO3 and were better than those for LSAW on Al/39° Y–X LiTaO3, except for the band-width ratio. Moreover, the TCD was initially decreased by loading with TeO2 film but then increased as film thickness increased.

Review of phononic crystals, nonlinear processes, devices and prospects

The review of current state of the art of bulk and surface linear and nonlinear acoustic wave propagation in phononic crystals (PhC) is given. First, theoretical analysis of bulk acoustic waves propagation in 2D phononic crystals composed of elastic medium with periodic systems of air holes with different symmetry is considered. The properties of hypersonic bulk and surface acoustic waves (Love, Lamb) in PhC are considered theoretically and expermentally. We studied bulk waves propagation in microstructured optical fiber preforms made of quartz. The study of simultaneous propagation of acoustic waves and light in structures being both photonic and phononic crystals was done. We study also properties of guided waves (Lamb modes) propagating in layered structures containing the magnetic films with two‐dimensional periodic structures. In these structures we discovered a strong coupling between elastic and magnetic properties leading to effective waves transformation between magnetic and elastic systems. Such structures are so called magnonic‐phononic crystals. The recommendations of use such unusual BAW and SAW properties in PhC for development of solid state devices are given. This work is supported by RFBR grant 08‐02‐00785.

SAW and pseudo SAW properties of NdCa4O(BO3)3 crystal

The surface acoustic wave (SAW) and pseudo SAW properties of NdCa4O(BO3)3 crystal were measured for the first time in the temperature range from −10°C to 100°C. For SAW, the largest electromechanical coupling coefficient of about 0.7% was obtained for the y–x orientation, while for the pseudo SAW the above coefficient was about 1.4% for the x–y and z–y orientations. For the y–x orientation, a second-order temperature coefficient of frequency and a turnover temperature of about −50 ppb/°C2 and 43°C, respectively, were obtained. Other investigated orientations showed approximate linear frequency changes against temperature for both SAW and pseudo SAW. This crystal is promising for application in low-loss temperature stable SAW devices and for high-temperature sensors.

Low pass effect of surface defect metal based on laser ultrasonic

The travelling properties of laser-generated surface acoustic wave with broad bandwidth, which propagated on a sample with surface defects, have been investigated in theory. The numerical results of theoretical models with different defects indicate that the surface defects worked as low-pass filter for the travelling surface acoustic waves. The cut-off frequency of low-pass filter is linearly related to the depth of the defect, and decreased against the increasing depth of the defect. The amplitude of the travelling surface acoustic wave is seriously attenuated with increasing width of surface defect. The numerical results provide a theoretical basis for quantitative detection of the surface defects.

Acousto-optical and SAW propagation characteristics of temperature stable multilayered structures based on LiNbO3 and diamond

Theoretical studies on the surface acoustic wave (SAW) properties of c-axis oriented LiNbO3/IDT/diamond and diamond/IDT/128° rotated Y–X cut LiNbO3 multilayered structures have been considered. Both layered structures exhibit a positive temperature coefficient of delay (TCD) characteristic, and a zero TCD device is obtained after integrating with an over-layer of either tellurium dioxide (TeO2) or silicon dioxide (SiO2). The presence of a TeO2 over-layer enhanced the electromechanical coupling coefficients of both multilayered structures, which acts as a better temperature compensation layer than SiO2. The temperature stable TeO2/LiNbO3/IDT/diamond layered structure exhibits good electromechanical coefficient and higher phase velocity for SAW device applications. On the other hand, a high acousto-optical (AO) figure of merit (30–37) × 10−15 s3 kg−1 has been obtained for the temperature stable SiO2/diamond/IDT/LiNbO3 layered structure indicating a promising device structure for AO applications.