Proton-exchanged 36 degrees Y-X LiTaO3 waveguides for surface acoustic wave

A nontoxic proton source, octanoic acid, was adopted to fabricate proton-exchanged (PE) waveguides in 36 degrees Y-X lithium tantalate (LiTaO3) substrates. The PE ability of octanoic acid on LiTaO3, the penetration depth, was investigated by secondary-ion mass spectrometry (SIMS). The penetration depth of hydrogen ion exhibited an obviously step-like profile, which will be excellent for waveguide application. The relationship between waveguide depth (d) and exchanging time (t) was represented by d = 0.0653 X square root of t at T = 200 degrees C. To deserve to be mentioned, the octanoic acid has a slight dissociation coefficient and low activation energy, thus the accurate waveguide depth control can be obtained. For the application of acoustic wave guided acousto-optic devices, the leaky surface acoustic wave (LSAW) properties of PE 36 degrees Y-X LiTaO3 waveguides were investigated. The phase velocity slightly decreased with the increase of kd, where k was wavenumber. An indispensable parameter of acoustic wave device, the temperature coefficient of frequency (TCF), calculated from the frequency change of the output of LSAW delay line showed an increase with increased kd.     

Optimal cut for leaky SAW on LiTaO3 for high performance resonators and filters

The paper describes how the characteristics of leaky surface acoustic wave (LSAW) propagation depend on the thickness of Al grating electrodes on rotated Y-X LiTaO3. It is shown that the propagation loss arising from leaky nature changes parabolically with both the grating electrode thickness and rotation angle and becomes zero when electrode thickness and rotation angle are properly determined. This means that even when thick grating electrodes are needed in device design, zero propagation loss is always realized by properly determining the rotation angle. When the grating electrode thickness is 0.07 to 0.1 in wavelength for example, LSAWs on 40-42 degrees Y-X LiTaO3 give zero propagation loss without deteriorating other characteristics. Ladder-type filters for the 800-MHz range were fabricated, which essentially need thick Al grating electrodes of about 0.1 wavelength thickness. As predicted by theoretical calculation, experimental results showed that if the rotation angle is increased to circa 420 from a conventional value of 36 degrees, the insertion loss and shape factor are markedly improved compared with devices based on 36 degrees Y-X LiTaO3. This is essentially a result of the minimized propagation loss.     

Characterization of 36°YX-LiTaO3 wafers byline-focus-beam acoustic microscopy

Application of line-focus-beam (LFB) acoustic microscopy is extended to characterization of substrates for SH-type SAW devices. Theoretical and experimental studies on a wave mode for characterization are carried out on 36°Y-cut LiTaO₃ wafers. A Rayleigh-type mode of leaky surface acoustic waves (LSAWs) must be employed instead of an SH-type mode of leaky pseudo-surface waves (LPSAWs). Experimental results show that the LSAW propagation should be directed along the X-axis because the LSAW velocities are more sensitive to chemical composition and elastic inhomogeneities. The relations among the LSAW velocities, densities, and Curie temperatures are determined. The LSAW velocity increases linearly at the rate of 0.52 m/s/°C with the Curie temperature. A chemical composition change of 0.03 Li₂ O-mol%, corresponding to temperature resolution of better than 0.3°C, is easily detected by the velocity measurements. Elastic inhomogeneities due to residual multi-domains, produced during the poling process during wafer fabrication, are interpreted quantitatively by this ultrasonic technology     

Effect of An Initial Stress on SH-Type GuidedWaves Propagating in a Piezoelectric Layer Bonded on A Piezomagnetic Substrate

Propagation of SH-type guided waves in a layered structure with an invariant initial stress is studied, where a piezoelectric thin layer is perfectly bonded on a piezomagnetic substrate. Both the layer and the substrate possess transversely isotropic property. The dispersion relations of SH waves are obtained for four kinds of different electro-magnetic boundary conditions. The effects of initial stress, thickness ratio and electro-magnetic boundary conditions on the propagation behaviors are analyzed in detail. The numerical results show that: 1) The positive initial stresses make the phase velocity increasing, while the negative initial stresses decrease the phase velocity; 2) The smaller the thickness ratio of a piezoelectric layer to a piezomagnetic substrate, the larger the phase velocity of SH-type guided wave propagating in the corresponding layered structure; 3) The electrical boundary conditions play a dominant role in the propagating characteristics. Moreover the phase velocities for the electrically shorted surface are smaller than that for the open case. The obtained results are useful for understanding and design of the electromagnetic acoustic wave and microwave devices.     

Analysis of piezoelectric boundary acoustic wave in Cu electrode/Y-cut X-propagating LiNbO3 substrate structure partially covered with SiO2

This paper describes the existence of piezoelectric boundary acoustic wave (PBAW) propagating in a Cu electrode/Y-cut X-propagating (YX) LiNbO(3) substrate structure partially covered with a SiO2 layer. In the analysis, two types of structures are taken into consideration: one is the so-called slotted structure with SiO2 pillars placed in the grating slots; the other is the so-called topped structure with SiO(2) pillars placed on the top of grating electrodes. The top surface could be fully covered with an additional layer (like epoxy) to bridge the grating slots for encapsulation. Results show that SH-type PBAW begins to propagate in the slotted structure when the SiO(2) thickness exceeds 0.3 wavelength. Strong electromechanical coupling factor K(2) of 21%, and temperature coefficient of velocity (TCV) of -33 ppm/°C are obtained. In the topped structure, on the other hand, the boundary acoustic wave mode is not supported. Instead, the thickness resonance modes in the SiO2 pillar do exist. Comparison of the obtained results with those in the structure fully covered with the SiO(2) layer indicates that, as for the PBAW mode, the slotted structure offers improved K(2) but with worse TCV compared with the fully covered SiO(2) structure.     

Reflection of ultrasonic Lamb waves produced by thin conducting strips

The reflection of ultrasonic Lamb waves produced by a periodic array of thin conducting strips deposited on a piezoelectric plate was investigated both theoretically and experimentally. A repetitively mismatched transmission line model was used to analyze the performance of the reflector. The reflection coefficient of a single strip is given by R approximately Deltav/v, where Deltav/v is the fractional change in velocity produced by electrical shorting of the propagation surface. An attractive property of Lamb waves is that they can provide a much higher value of Deltav/v than is possible with surface acoustic waves. Therefore, efficient Lamb wave reflectors can be realized with relatively few strips in the reflector. For example, reflection coefficient very close to unity, R approximately 0.98, has been obtained in a Lamb wave reflector consisting of just 12 strips on a Y-X lithium niobate plate. The reflector has been used to realize a unidirectional transducer (UDT). A Lamb wave delay line consisting of two UDTs shows insertion loss of less than 3 dB with fractional bandwidth greater than 7%     

Optimization of single-phase,unidirectional transducers using three fingers per period

Electrode width controlled (EWC) single-phase, unidirectional transducers (SPUDT) is widely used for low loss surface acoustic wave (SAW) filters. The insertion loss and fractional bandwidth of the filters are strongly related to the reflectivity of EWC cells. In order to achieve wide band and low loss simultaneously, it is necessary to obtain higher reflectivity. The relationship between geometrical configuration of EWC cells and reflection coefficient, (and transduction coefficient as well) is studied. Simulation results indicate that the reflectivity of the EWC SPUDT cell could exceed 5% on a 128/spl deg/ Y-X lithium niobate (LiNbO/sub 3/) substrate. Using such structure, low loss SPUDT test filters without weighting are fabricated. The measured 3 dB bandwidth is 3.9% and the insertion loss is 2.9 dB. The theoretical calculation is verified by the experiment.     

Effects of bulk wave radiation on IDT admittance on 42 degrees YX-LiTaO3

This paper investigates the effects of bulk acoustic wave (BAW) radiation on the admittance of interdigital transducers (IDTs) with significant internal reflections of shear horizontal (SH) type leaky surface acoustic waves (SAWs) on 42°YX-LiTaO₃(42-LT). Theoretical analysis is made by using the discrete Green function theory, and synchronous one-port resonators are analyzed. It is shown that the BAW radiation significantly affects the IDT characteristics even for resonators; under certain circumstances, BAWs launched from an IDT are converted into SH-type SAWs by the strong internal reflections, and they interact with the BAWs radiated by the IDT. Then, the net amount of the radiated BAW power is highly dependent upon the number of IDT finger pairs. For the precise simulation of devices based on the SH-type SAWs with strong internal reflections, the BAW radiation should carefully be taken into consideration. If the BAW radiation is ignored, the radiated power of the SH-type SAWs may seem to be negative above the BAW cut-off frequency     

Analysis of the film thickness dependence of a single-phase unidirectional transducer using the coupling-of-modes theory and the finite-element method

A coupling-of-modes (COM) analysis is given for the film thickness dependence of a single-phase undirectional transducer (SPUDT), while the finite-element method (FEM) is employed for evaluating all the coefficients of COM equations. The relationship between the directivity and dispersion curves of the transducer is discussed. The theoretical analysis shows that when the electrode finger thickness increases through a threshold value, a mode conversion phenomenon occurs and the value of the reflection phase changes from the positive to the negative. This result predicts that the forward direction of a film thickness difference type SPUDT will move conversely when the electrode film thickness increases through the threshold thickness. A prototype step-type SPUDT, fabricated on 128 degrees Y-X LiNbO(3) substrate, showed a directivity of 10 dB/transducer at 481.5 MHz, and a low-loss surface acoustic wave (SAW) filter showed a minimum insertion loss of 3.8 dB.     

Half-thickness inversion layer high-frequency ultrasonic transducers using LiNbO3 single crystal.

Half-thickness inversion layer high-frequency ultrasonic transducers were fabricated using lithium niobate (LiNbO3) single crystal plate. The transducers developed for this study used a 36 degrees rotated Y-cut LiNbO3 thin plate with an active element thickness of 115 microm. The designed center frequency was in the range of 30 to 60 MHz. Half-thickness inversion layer was formed after the sample was annealed at a high temperature, and it is shown that the inversion layer thickness can be controlled by the temperature. Silver powder/epoxy composite and parylene were used as acoustic matching layers. A lossy silver epoxy was used as the backing material. Using an analytical method, the electrical impedance for different inversion layer ratios was determined. The measured resonant frequency was consistent with the modeled data. Even-order higher frequency broadband ultrasonic transducers with a center frequency at 60 MHz was obtained using half-thickness inversion layer of LiNbO3 single crystal.     

Investigation on SAW properties of LGS and optimal cuts for high-temperature applications

A promising perspective for surface acoustic wave (SAW) device applications at high temperature has been opened by langasite (LGS). The SAW properties of LGS in singly and doubly rotated cuts at 250 degrees C are investigated. Three noticeable regions for SAW-cut orientations and propagation directions at high temperature are put forward and are defined by Euler angles [0 degrees, 20 degrees --> 50 degrees, 35 degrees --> 45 degrees], [0 degrees, 85 degrees --> 110 degrees, 0 degrees --> 5 degrees], and [0 degrees, 138 degrees --> 145 degrees, 20 degrees --> 23 degrees], respectively. The first region includes zero or comparatively reduced temperature coefficient of delay (TCD) (<2 ppm) and smaller electromechanical coupling factor (K2) (0.2%-0.35%); the second one exhibits higher K2 (0.35%-0.45%) and moderate TCD (<5 ppm); and the highest K2 (>0.45%) and larger TCD (25-30 ppm) characterize the last region. For some typical orientations within the above-mentioned three regions, the temperature dependency of SAW characteristics (up to 1000 degrees C) is discussed. The second region [0 degrees, 85 degrees --> 110 degrees, 0 degrees --> 5 degrees], especially the orientation [0 degrees, 90 degrees, 0 degrees], has better comprehensive characteristics of SAW and is more suitable for high-temperature applications. Therefore, we should give a top priority to the orientation [0 degrees, 90 degrees, 0 degrees] in the design of SAW devices operating at high temperature. Comparison between published experimental results and numerical predictions based on LGS constants and their temperature coefficients available in the literature reveals that the theoretical results of TCD under 250 degrees C are in agreement with the experimental ones (the relative error of TCD is within 10%).     

Y旋转ST-石英基片上的准纵漏表面声波

文章主要研究了Y旋转ST-石英基片上准纵漏表面声波（快速声表面波）的传播特性。它的相速度是常规瑞利波的两倍,在某些传播方向可高达7000m／s;在理论与实验上寻找了束偏向和延时温度系数均较小的切割传播方向,例如,沿欧拉角（0°,155．25°,42°）方向的准纵漏表面声波的相速度为6153m／s,延时温度系数为11ppm／℃,其声波能流方向基本上与波的传播矢量方向相一致。实验证明放置于基片表面的液体对准纵漏表面声波的吸收不大,说明声波的质点振动与传播方向一致（沿表面方向）,具有纵波的性质。     

Analysis of SAW properties of epitaxial ZnO films grown on R-Al2O3 substrates

ZnO thin films with a high piezoelectric coupling coefficient are widely used for high frequency and low loss surface acoustic wave (SAW) devices when the film is deposited on top of a high acoustic velocity substrate, such as diamond or sapphire. The performance of these devices is critically dependent on the quality of the ZnO films as well as of the interface between ZnO and the substrate. In this paper, we report the studies on piezoelectric properties of epitaxial (112¯0) ZnO thin films grown on R-plane sapphire substrates using metal organic chemical vapor deposition (MOCVD) technique. The c-axis of the ZnO film is in-plane. The ZnO/R-Al₂O₃ interface is atomically sharp. SAW delay lines, aligned parallel to the c-axis, were used to characterize the surface wave velocity, coupling coefficient, and temperature coefficient of frequency as functions of film thickness to wavelength ratio (h/λ). The acoustic wave properties of the material system were calculated using Adler's matrix method, and the devices were simulated using the quasi-static approximation based on Green's function analysis     

Pure shear horizontal SAW biosensor on langasite

The undetected introduction of pathogens into food or water supplies can produce grave consequences in terms of economic loss and human suffering. Sensitive and selective sensors capable of quickly detecting microbial pathogens are urgently needed to limit the effects of bioterrorist incidents, accidents, or pollution. Shear horizontal surface acoustic wave (SH SAW) devices provide an attractive platform for the design of microbial biosensors that function in liquid media, where Rayleigh-type modes are rapidly attenuated. This paper reports on an exploratory SH SAW delay line designed and fabricated on langasite, La3Ga5SiO14 (LGS), along the novel Euler propagation direction (0 degrees, 22 degrees, 90 degrees). A liquid chamber was fabricated and attached to the top surface, and the device was submitted to liquid and biochemical tests. Moderate (6 dB) additional attenuation of the transmission coefficient, /S21/, was consistently observed when the SH SAW delay line was assembled in the test fixture and submitted to the liquid tests, indicating that LGS is an attractive candidate for liquid sensing. Sensor selectivity can be achieved by integrating the LGS SH SAW delay line with a biochemical recognition layer. A test setup was implemented for the characterization of LGS SH SAW-based biosensors. The delay line response to biomolecule binding was shown by detection of sequential binding of proteins to the SH SAW device delay path. The biotinylated sensor was exposed sequentially to biotin-binding deglycosylated avidin, biotin-modified rabbit IgG, and goat anti-rabbit IgG antibody. As each protein was bound to the sensing surface, marked changes in the delay-line phase were recorded. The reported results demonstrate the capability of these devices to act as biochemical detectors in aqueous solutions, and this work represents the first effort using the novel material LGS in SAW-based biosensor technology.     

High velocity SAW using aluminum nitride film on unpolished nucleation side of free-standing CVD diamond

High performances surface acoustic wave (SAW) filters based on aluminium nitride (AlN)/diamond layered structure have been fabricated. The C-axis oriented aluminum nitride films with various thicknesses were sputtered on unpolished nucleation side of free-standing polycrystalline chemical vapor deposition (CVD) diamond obtained by silicon substrate etching. Experimental results show that high order modes as well as Rayleigh waves are excited. Experimental results are in good agreement with the theoretical dispersion curves determined by software simulation with Green's function formalism. We demonstrate that high phase velocity first mode wave (so-called Sezawa wave) with high electromechanical coupling coefficient are obtained on AlN/diamond structure. This structure also has a low temperature coefficient of frequency (TCF), and preliminary results suggest that a zero TCF could be expected.     

A low-loss SAW-TV-IF filter with an extended impedance matching range

A novel low-loss SAW (surface acoustic wave) filter for an intermediate frequency (IF) circuit in a color TV receiver has been developed. It consists of an apodized bidirectional and an unapodized group-type unidirectional transducer. The unidirectional transducer is designed to use different numbers of finger pairs in sending and reflecting electrodes for extension of the impedance-matching range. A thin-film capacitor for use as a phase shifter is monolithically fabricated on a 128 degrees Y-X LiNbO(3) substrate. A low insertion loss (11.3 dB) and impedance matching without adjustment are achieved at the same time without increasing the device chip size or number of electrical parts.     

The effect of an SiO(2) buffer layer on the SAW properties of ZnO/SiO(2)/GaAs structure

The effect of an SiO(2) buffer layer on the surface acoustic wave (SAW) properties of ZnO/SiO(2)/GaAs structure is examined. Both theoretical and experimental results show that the coupling coefficient is increased appreciably by providing an SiO(2 ) film between the ZnO film and the GaAs substrate. Adding an SiO (2) film is also beneficial to the promotion of quality of ZnO thin film. The results could be useful for the further development of monolithic SAW devices.     

一种计算声表面波在周期栅阵中传播特性的方法

Hashimoto有限元程序是日本Chiba大学研究人员从1973年开始、历时几十年用Fortran语言编写、计算声表面波在周期栅阵中传波特性的一个软件包。该程序能计算Rayleigh波、漏表面波、SH型表面波、表面横波等多种声表面波的传输特性,同时它能适用于一个周期内单根指条、两根指条和多根指条及声表面波在栅阵中斜入射的情况。该程序已在声表面波（SAW）研究人员中得到了广泛的应用。     

Theoretical study on SAW characteristics of layered structuresincluding a diamond layer

Diamond has the highest surface acoustic wave (SAW) velocity among all materials and thus can provide much advantage for fabrication of high frequency SAW devices when it is combined with a piezoelectric thin film. Basic SAW properties of layered structures consisting of a piezoelectric material layer, a diamond layer and a substrate were examined by theoretical calculation. Rayleigh mode SAW's with large SAW velocities up to 12,000 m/s and large electro-mechanical coupling coefficients from 1 to 11% were found to propagate in ZnO/diamond/Si, LiNbO₃/diamond/Si and LiTaO₃/diamond/Si structures. It was also found that a SiO₂/ZnO/diamond/Si structure can realize a zero temperature coefficient of frequency with a high phase velocity of 8,000-9,000 m/s and a large electro-mechanical coupling coefficient of up to 4%     

New method of change in temperature coefficient delay of acoustic waves in thin piezoelectric plates

As is well-known, the development of high-effective and thermostable acoustic devices assumes using the acoustic waves with high coefficient of electromechanical coupling (K2) and low temperature coefficient of delay (TCD). At present, it also is well-known that fundamental shear horizontal (SH0) acoustic waves in thin piezoelectric plates possess by significantly more electromechanical coupling compared to surface acoustic waves (SAW) in the same material. However, although the value of TCD of SH0 waves is insignificantly less than for SAW, this is not enough for development of thermostable devices. This paper suggests a new way of decreasing TCD of SH0 waves in piezoelectric plates at a high level of electromechanical coupling. This way assumes to use the structure containing the piezoelectric plate and liquid with the special dependence of permittivity on temperature. Theoretical and experimental investigation showed that, for SH0 wave in YX LiNbO3 plate at hf = 700 m/s (h = plate thickness, f = wave frequency) the presence of butyl acetate can decrease the value of TCD by six times at K2 = 30%. In a whole the obtained results open the wide prospect of using SH0 wave in thin piezoelectric plate for development of high effective and thermo-stable acoustic devices.