Short reflectors operating at the fundamental and second harmonics on 128 degree LiNbO3

In this work, we study numerically the operation of surface acoustic wave (SAW) reflectors comprising a small number of electrodes on the 128 degree YX-cut lithium niobate (LiNbO3) substrate. The electrodes have a finite thickness, and they are either open circuited or grounded. The center-to-center distance between adjacent electrodes d corresponds roughly either to half of the characteristic wavelength d proportional to lambda0/2 or to d proportional to lambda0, for the reflectors operating at the fundamental and second harmonic modes, respectively. We use software based on the finite-element and boundary-element methods (FEM/BEM) for numerical experiments with a tailored test structure having 3 interdigital transducers (IDTs), simulating experimental conditions with an incident wave and reflected and transmitted SAWs. Using the fast Fourier transform (FFT) and time-gating techniques, calculation of the Y-parameters in a wide frequency range with rather a small step allows us to determine the reflection coefficients, and to estimate the energy loss due to bulk-wave scattering. The detailed dependences of the attenuation and reflectivity on the metallization ratio and the electrode thickness are given for the classic 128 degree-cut of LiNbO3.     

Propagation characteristics of longitudinal leaky SAW in Al-grating structure

New orientations for longitudinal leaky SAW with small propagation losses and high velocities in lithium niobate (LN) and lithium tantalate (LT) have been found. Their velocities are 6100 to 6500 m/s in LN and 5200 to 5800 m/s in LT. Their propagation losses are almost negligible only in the case of an Al-grating structure. For example, the Al thickness relative to the SAW wavelength must be 8.4% for 171 degrees YZ'-LN; for 133 degrees YZ'-LT, the value must be 9.2%. These propagation properties were obtained by using a simulator that combined both finite-element and analytical methods. These results were also well confirmed by measuring the impedance responses of one-port SAW resonators.     

Phases of the SAW reflection and transmission coefficients for short reflectors on 128 degree LiNbO3

We study numerically the phase of surface acoustic waves reflected by or transmitted through short reflectors comprising only 1-3 aluminium electrodes on 128 degree YX-cut lithium niobate (LiNbO3). The electrodes have a finite thickness, and they are either open-circuited or grounded. The center-to-center distance between adjacent electrodes d corresponds roughly either to half of the characteristic wavelength d proportional to lambda0/2 or to d proportional to lambda0, for the reflectors operating at the fundamental and second harmonic modes, respectively. We use software based on the finite-element and boundary-element methods (FEM/BEM) for numerical experiments with a tailored test structure having 3 interdigital transducers (IDTs), simulating experimental conditions with an incident wave and reflected and transmitted surface acoustic wave (SAW). Using artificial enhancement of time resolution in conjunction with the fast Fourier transform (FFT) and time-gating, calculation of the Y-parameters in a relatively wide frequency range allows us to determine the phase of the reflection and transmission coefficients.     

Minimum-loss short reflectors on 128 degrees LiNbO3

We consider the interaction of surface acoustic waves (SAWs) with short electrode gratings encompassing only few electrodes on 128/spl deg/ lithium niobate (LiNbO/sub 3/). The qualifications of the reflectors are evaluated by comparing the part of incident SAW energy scattered by the structure into the bulk to the energy reflected back as a SAW.     

Unidirectional SAW transducer for gigahertz frequencies

A single-phase unidirectional transducer (SPUDT) structure using /spl lambda//4 and wider electrodes is introduced. The considerable difference between the reflectivity of short-circuited /spl lambda//4 electrodes and that of floating /spl lambda//2-wide electrodes on 128/spl deg/ lithium niobate (LiNbO/sub 3/) is exploited. The surface acoustic wave (SAW) device operating at 2.45 GHz has critical dimensions of about 0.4 /spl mu/m, accessible for standard optical lithography.     

Side radiation of Rayleigh waves from synchronous SAW resonators

Surface acoustic wave (SAW) resonators on lithium tantalate (LiTaO3) and lithium niobate (LiNbO3) are investigated. The amplitude of the acoustic fields in the resonators are measured using a scanning laser interferometer. The amplitude profiles of the surface vibrations reveal the presence of distinct acoustic beams radiated from the transducer region of the SAW resonators and propagating with low attenuation. We suggest that this radiation is generated by the charges accumulating at the tips of the finger electrodes. The periodic system of sources, namely oscillating charges at the fingertips, generates Rayleigh-wave beams in the perpendicular and oblique directions. Green's function theory is used to calculate the coupling strength and slowness of the Rayleigh waves on 42 degrees Y-cut LiTaO3 and Y-cut LiNbO3 substrates as a function of the propagation direction. Furthermore, the propagation angles of the Rayleigh-wave beams as a function of frequency are calculated. The computed angles are compared with the measured ones for both the LiTaO3 and LiNbO3 substrates.     

SPUDT filters for the 2.45 GHz ISM band

Filters based on using single-phase, unidirectional transducers (SPUDT) consisting of lambda/4 and wider electrodes are presented. The design variants exploit the significant difference between the reflectivity of short-circuited lambda/4 electrodes and that of floating wide electrodes on 128 degree lithium niobate (LiNbO3). The surface acoustic wave (SAW) devices operating at 2.45 GHz have critical dimensions of about 0.3-0.4 microm, accessible to standard optical lithography. When matched, the fabricated SPUDT filters exhibit minimum insertion losses of 5.5-7.9 dB together with 3 dB passbands of 89-102 MHz. The majority of the insertion loss can be attributed to the attenuation on free surface and inside the grating, and to the resistivity of the electrodes.     

Multiparameter tuning of surface-acoustic-wave filters

An experimental methodology for tuning the frequency response of surface-acoustic-wave (SAW) filters after initial fabrication is demonstrated. The sensitivity of such a filter to perturbations in the length of individual electrodes of the interdigital transducer is determined from highly accurate swept-frequency measurements. Sensitivity measurements from several electrodes are then used to synthesize improved filter responses. Sensitivity measurements, measurement accuracy, and control of frequency-response sidelobes are demonstrated for devices fabricated on (YZ) lithium niobate. The tuning method provides predictions of improvement that closely match actual performance.     

Transformation of acoustic waves in periodic metal grating sandwiched between piezoelectric and dielectric

The mechanism of SAW transformation with variation of film thickness is investigated in a piezoelectric substrate with a metal grating overlaid by a dielectric film, via simulation and visualization of the acoustic fields. By way of example, two orientations of lithium niobate substrates are analyzed, YX-LN and 128°YX-LN, with a Cu grating and an isotropic silica glass overlay. The motions, which follow the wave propagation in the sagittal plane, are visualized within two periods of the grating, with added contour plots showing the shear horizontal displacements. The continuous transformation of the wave's nature is investigated for each wave propagating in the analyzed material structures when the film thickness is increased from zero to a few wavelengths. The examples of the SAW transformation into boundary waves and into plate modes of different polarization have been found and investigated. The behavior of the SAW characteristics in the grating is correlated with transformation of the wave structure with increasing overlay thickness.     

Longitudinal leaky SAW resonators and filters on YZ-LiNbO3.

The high-phase velocity (above 6100 m/s in an aluminum (Al) grating on lithium niobate (LiNbOs)) of the longitudinal leaky surface acoustic wave (SAW) (LLSAW) mode makes it attractive for application in high-frequency SAW ladder filters in the 2-5 GHz range. We investigate the dependence of one-port synchronous LLSAW resonator performance on YZ-LiNbO3 on the metallization thickness and metallization ratio, both experimentally and theoretically. Our results indicate a strong dependence of the Q factor and resonance frequency on the aluminum thickness, with the optimal thickness that produces the highest Q values being about 8%. The optimal thickness increases with the metallization ratio. The observed behavior is interpreted with the help of simulations using a combined finite element method (FEM)/boundary element method (BEM) technique. As an application, bandpass filters have been fabricated in the 2.8 GHz frequency regime, based on LLSAWs. The synchronous resonators constituting the ladder filters operate in the fundamental mode. The filters feature low insertion losses below 3 dB and wide relative passbands of 4.5-5%.     

Longitudinal leaky SAW resonators and filters on YZ-LiNbO/sub 3/

The high-phase velocity (above 6100 m/s in and aluminum (Al) grating on lithium niobate (LiNbO/sub 3/)) of the longitudinal leaky surface acoustic wave (SAW) (LLSAW) mode makes it attractive for application in high-frequency SAW ladder filters in the 2-5 GHz range. We investigate the dependence of one-port synchronous LLSAW resonator performance or YZ-LiNbO/sub 3/ on the metallization thickness and metallization ratio, both experimentally and theoretically. Our results indicate a strong dependence of the Q factor and resonance frequency on the aluminum thickness, with the optimal thickness that produces the highest Q values being about 8%. The optimal thickness increases with the metallization ratio. The observed behavior is interpreted with the help of simulations using a combined finite element method (FEM)/boundary element method (BEM) technique. As an application, bandpass filters have been fabricated in the 2.8 GHz frequency regime, based on LL-SAWs. The synchronous resonators constituting the ladder filters operate in the fundamental mode. The filters feature low insertion losses below 3 dB and wide relative passbands of 4.5-5%.     

Surface acoustic wave properties of proton-exchanged LiNbO3 waveguides with SiO2 film

Surface acoustic wave (SAW) properties of proton-exchanged (PE) z-cut lithium niobate (LiNbO3) waveguides with silicon dioxide (SiO2) film layers were investigated using octanoic acid. The distribution of hydrogen measured by secondary ion mass spectrometry (SIMS) showed a step-like profile, which was assumed to be equal to the waveguide depth (d). The SiO2 film was deposited on z-cut LiNbO3 waveguide by radio frequency (rf) magnetron sputtering. We investigated the important parameters for the design of SAW devices such as phase velocity (Vp), insertion loss (IL) and temperature coefficient of frequency (TCF) by a network analyzer using thin-film aluminum interdigital transducer electrodes on the upper SiO2 film surface. The experimental results showed that the Vp of SAW decreased slightly with the increase of h/lambda, where h was the thickness of SiO2 films and lambda was the wavelength. The IL of SAW increased with increased h/lambda. The TCF of SAW calculated from the frequency change of the output of SAW delay line showed an evident decrease with the increase of h/lambda. The TCF for PE z-cut LiNbO3 was measured to be about -54.72 ppm/degreees C at h/lambda = 0.08. It revealed that the SiO2 films could compensate and improve the temperature stability as compared with the TCF of SAW on PE samples without SiO2 film.     

Analysis of leaky-surface-wave propagating under periodic metal grating

A detailed field analysis is presented for a leaky surface wave propagating under a periodic metal grating, using a theory that neglects the effect of mass loading due to the grating. The approach is based on Floquet's theorem and the coupled equations of wave motion with unperturbed mechanical and perturbed (or periodic) electrical boundary conditions, yielding a general field solution applicable to any material and to arbitrary connections to the grating. As a key step, the periodic boundary equations are solved by combining them into a set of infinite homogeneous equations through algebraic treatment and performing orthogonal integration with respect to space harmonics. The advantage in using this method results from there being no need to use assumptions or complicated expressions anticipating an accurate solution if sufficient space harmonics are considered. It is shown that the theory proposed here can be directly extended to solve simpler SAW problems. An analysis is carried out for LiNbO(3) for both the leaky wave and Rayleigh wave, taking into account dispersion relations, propagation attenuation of the leaky wave, and other field distributions. Theoretical and experimental results for the width of the first stopband are discussed.     

Periodic Green's functions analysis of SAW and leaky SAWpropagation in a periodic system of electrodes on a piezoelectriccrystal

A method of periodic Green's functions with a propagation factor exp(iβx), unknown in advance, is used to calculate dispersion curves and attenuation coefficients for Rayleigh- and leaky- waves propagating in a periodic system of thin electrodes on a piezoelectric surface. To describe the charge distribution on the electrodes both a step approximation and Chebyshev polynomials are used, the last being more adequate in most cases. Numerically determined values of the Green's function are used and interpolated either linearly or using a modified variant of Ingebrigtsen's formula. Such basic parameters as stopband width, stopband center frequency, wave velocity and attenuation in the stopband are found. These parameters can be used in the coupling-of-modes (COM) analysis and design of SAW devices. The analysis includes bulk wave radiation and scattering. The dependence of the corresponding attenuation coefficient on frequency is determined. Results obtained allow the determination directly and properly of the COM parameters and the design of SAW devices having large number of electrodes most precisely and rapidly. Numerical results for Rayleigh waves on YZ-LiNbO₃ and leaky waves on 36°YX-LiTaO₃ substrates are presented     

Propagation properties of longitudinal leaky surface waves on lithium tetraborate

Theoretical and experimental results of longitudinal leaky surface waves with a higher phase velocity than that of ordinary leaky surface waves and a low propagation loss on lithium tetraborate (LBO) are investigated in detail. They propagate along the surface with a phase velocity close to that of longitudinal bulk wave, slightly radiating two kinds of shear bulk waves (or one shear bulk wave in the case that one of two shear wave terms is uncoupled) into the solid. Most surface components of the mode consist of a longitudinal wave term and an electromagnetic wave term. The detailed propagation properties of the longitudinal leaky surface waves on LBO with the Euler angles (phi, theta, 90 degrees ) are investigated theoretically and experimentally. The (011) cut of LBO was found to be desirable for higher frequency SAW devices. One of the reasons why that mode on LBO has a low propagation loss is also discussed.     

SAW diffraction using the thin-element decomposition method

We adapt the angular spectrum of plane waves (ASPW) decomposition to numerical simulations of the diffraction of surface-acoustic waves (SAWs) on anisotropic model substrate, such as YZ lithium niobate. We utilize the thin-element decomposition (TED) method, appropriately modified for an anisotropic substrate; we also introduce a novel "average-wavenumber" variation of this scheme; these are numerically found to be mutually consistent. We apply the TED method to simulate wave propagation both in infinite periodic structures of metallized gratings and also in finite gratings. We demonstrate that the ASPW provides a convenient and numerically fast tool for precise diffraction calculations in practical SAW devices which also display structural variations in the lateral direction, perpendicular to the principal direction of wave propagation. Additionally, we compare the present TED method to waveguide theory in an analysis of the role of SAW reflections from the electrodes.     

FEM/BEM for simulation of LSAW devices

This paper presents a modeling of the propagation of surface acoustic, leaky acoustic, and surface skimming bulk waves in piezoelectrics with a finite array of metallic electrodes over their surface. A combined method of matrix Green's function and the finite element method for computation of all acoustic wave fields is an effective tool for simulation of the propagation of acoustic waves in such structures. The proposed method is optimized in the speed of computation of all matrix Green's function components originally obtained. The Fourier transformations of Green's function from kappa-space domain to real space domain are performed by combined trapezoidal and Filon's integration methods for rapidly oscillating functions. The trapezoidal integration method is used on a distance from a point source from zero to a few wavelengths long, but the other has the advantage for a distance from some wavelength to infinity. That allows one, by selectively condensing computation grids around branch and singular points of the sharp behavior of Green's function, to maximize speed and accuracy of computation of integrals. FEM is used, modified originally to achieve acceleration without loss accuracy. Because of the simple geometry of the electrodes, unknown elastic fields are presented as a series of known eigenfunctions with unknown coefficients over the whole region of electrodes. All unknown coefficients are determined by applying the Galerkin method. There is good agreement between numerical and experimental conductances of acoustic wave transducers on materials such as lithium niobate and lithium tantalate.     

Propagation of transverse bulk and surface acoustic waves in LiNbO (3) variable time-delay devices

Experimental measurements are reported on voltage-controlled acoustic time-delay lines operating at 1 GHz in the nearly pure shear-horizontal (S-H) mode in 38 degrees rotated Y-cut LiNbO(3). The high-acoustic velocity (4800 m/s) in conjunction with the large electroacoustic effect exhibited by this orientation allows high-frequency operation and optimum time-delay tuning sensitivity with a planar, single surface, device geometry. The authors demonstrate fractional time delay of 0.3x10(-6) V(-1 ) for surface electrodes that produce an in-plane E-field. However, the simultaneous excitation and propagation of both a leaky surface-acoustic wave (LSAW) and surface skimming bulk wave (SSBW), both as (nearly pure) S-H waves in these devices, seriously restricts the extent to which it is possible to maximize the time delay modulation sensitivity by reducing electrode gap spacing as done in similar SAW devices. The LSAW and surface-skimming body wave (SSBW) propagate at nearly the same velocity on a free surface, and perturbation of their velocity and relative attenuation rates by surface electrodes causes pronounced interference effects between the two modes for some device geometries.     

Propagation of a quasi-shear horizontal acoustic wave in Z-Xlithium niobate plates [and conductivity sensor application]

It is found that an acoustic wave which is nearly polarized in the shear horizontal (SH) direction can propagate along the X axis of a Z-cut lithium niobate plate if the ratio h/λ, where h=plate thickness and λ=acoustic wavelength, is less than about 0.5. Attractive properties of this quasi-SH wave include: (1) phase velocity nearly constant for all values of h/λ; (2) ability to propagate in contact with a liquid medium; and (3) electromechanical coupling coefficient as high as 0.15. These properties make the wave attractive for use in a variety of sensor and signal processing applications. An example of sensor applications is illustrated by using the wave to measure conductivity of liquids (aqueous KCl solution). The frequency of a 12-MHz quasi-SH mode oscillator fabricated on a 0.48 wavelength thick Z-X lithium niobate plate is found to vary by more than 80 kHz for variation in KCI concentration from 0 to 0.15%