SAW devices for consumer communication applications

An overview of surface acoustic wave (SAW) filter techniques available for different applications is given. Techniques for TV IF applications are outlined, and typical structures are presented. This is followed by a discussion of applications for SAW resonators. Low-loss devices for mobile communication systems and pager applications are examined. Tapped delay lines (matched filters) and convolvers for code-division multiaccess (CDMA) systems are also covered. Although simulation procedures are not considered, for many devices the theoretical frequency response is presented along with the measurement curve.     

Transduction magnitude and phase for COM modeling of SAW devices

An analytic formulation for transduction parameter required by the coupling-of-modes analysis of surface acoustic wave (SAW) transducers is derived by comparing solutions obtained using the coupling-of-modes analysis and an equivalent quasi-static analysis using the Green function method. This analysis results in the derivation of a convenient representation for the transduction strength of periodic SAW transducers. Using the formulation, analytic solutions are obtained for the transduction magnitude and phase of regular SAW transducers. Nonregular transducers are also modeled with the help of an electrostatic field analysis of the transducer's structure.     

New capabilities for optimizing SAW gas sensors

It is shown how the performances of SAW gas sensors can be optimized based on pure acoustic peculiarities of SAW propagation in anisotropic single crystals. For a given gas and sensitive membrane, the calibration curve (dependence of the response R versus gas concentration n), the sensitivity S (slope of the calibration curve: S=dR/dn), the detection limit n_thr, (cut-off of the curve at the threshold R_thr), and the resolution Δn of the sensor (recognition of two close concentrations) can be controlled by a proper choice of the substrate material and its crystallographic orientation (cut and direction of the SAW propagation). An experimental test of this property is performed on SAW devices implemented on different substrate materials and crystallographic orientations, both uncoated or coated, with a sorbent membrane of polycrystalline Pd or Pd:Ni film, upon exposure to humid air as a test analyte     

Effects of layer thickness for SAW, PSAW, and HVPSAW devices

Considerable efforts in recent SAW device design and development have been aimed at obtaining high frequency, low loss, and high performance. A large number of applications relate to cellular and mobile telephony, pagers, local area networks, cordless phones, global positioning systems (GPS), and security systems. Pseudo-SAW (PSAW) and high velocity PSAW (HVPSAW) have received great attention because of their high phase velocities and, therefore, the high frequencies of operation that these modes provide. In addition to high phase velocities, the pseudomodes must also present low propagation losses and considerably high electromechanical coupling coefficients to be considered for surface acoustic wave (SAW) devices. This paper verifies that the metallic layer thickness is a relevant SAW device parameter, which must be considered to achieve lower losses for high frequency, low loss SAW devices. Popular PSAW and HVPSAW material orientations, such as 64 degrees YX LiNbO3 (0 degree -26 degrees 0 degree), 36 degrees YX LiTaO3 (0 degree -54 degrees 0 degree), LiNbO3 (90 degrees 90 degrees 36 degrees), LiTaO3 (90 degrees 90 degrees 31 degrees), and Li2B4O7 (0 degree 47.3 degrees 90 degrees), are considered as examples. In addition to the reduced loss analysis and the dispersion analysis for the pseudo modes, the present work discusses the transitions with respect to the layer thickness from the PSAWs and HVPSAWs to the generalized SAWs (GSAWs) and Rayleigh (sagittal particle motion) modes. In addition to contributing to the knowledge of the pseudomodes behavior with layer thickness, this mode transition analysis enlightens the situation in which the losses in the pseudo modes go to zero because of the merging of the pseudo modes into the SAWs (GSAWs and Rayleigh). The fact that the SAWs are a continuation as a function of thickness for the pseudo modes may be conveniently used in the fabrication of low loss devices. In addition, the effects of heavy layer metals, such as gold, in reducing the layer thickness at which the pseudo modes merge to the SAWs are discussed. Numerical results are compared with experimental data available in the literature, and the present analysis elucidates experimentally observed higher order pseudo modes and values of layer thickness for which lower losses are achieved.     

Possible design procedure for low-loss 180 degrees reflecting arrays in SAW devices

A procedure for designing highly reflecting arrays to specified responses in surface acoustic-wave (SAW) devices is proposed. Multiple reflections in the low-loss arrays are fully taken into account. The problem is made tractable by using a 180 degrees reflecting geometry rather than reflecting the SAW twice through 90 degrees.     

Minimizing the bulk-wave scattering loss in dual-mode SAW devices

The losses arising from the scattering of SAW into bulk waves in the nonsynchronous areas of SAW devices are studied numerically using the boundary element method combined with the finite element method. As a reference structure, we use a typical one-port hiccup resonator on 42 degrees Y-LiTaO3. Strong scattering into bulk wave occurs in the central gap due to an abrupt change in periodicity. To reduce the scattering, we replace the gap with electrodes having reduced pitches. We show that it is possible to significantly increase the Q-factor of the resonator while keeping the resonant frequency unchanged. Two types of structures are studied: the "distributed" gap and the "accordion" gap. To minimize the bulk-wave scattering in dual-mode SAW filters, we replace the metallized gaps in the traditional filter with distributed gaps. We find an optimal combination of pitch and metallization ratio in the gaps, reducing the insertion loss by 0.3 dB.     

Thin-film induced effects on the stability of SAW devices

Measurements show an upward shift on the order of 50 ppm in the resonant frequency of a surface acoustic wave (SAW) resonator, as taken before and after the device is hermetically sealed in vacuum following a certain glass-frit sealing process. The authors analyze some of the thin-film phenomena that are potential sources of the observed frequency shift and that may affect the long-term stability of such devices. Various factors contributing to the shifts include: 1) intrinsic or structural stresses in the bonding layers as well as in the interdigital transducer (IDT) fingers; 2) thermal stresses due to the differences in thermal expansion coefficients of the metallic IDT fingers and the bonding agent (glass frits) from those of quartz; 3) partial oxidation of the IDT fingers and transmission lines during the frit glazing process; and 4) possible metal diffusion into quartz. Quantitative estimates of the contribution of two factors to the total observed frequency shift after a certain glass-frit sealing process are provided. Rough estimates of the frequency shifts due to the oxidized film are made from the dispersion curves for a uniform thin aluminum film and for its oxide film as fully plated on a quartz substrate. It is concluded that the results may provide a way of estimating the magnitude of the intrinsic stress for a given long-term stability of the SAW device.     

Diffraction minimization in SAW devices using wide aperture compensation

A relationship is outlined between the previously derived uniform asymptotic expansion (UAE) for surface acoustic waves on anisotropic substrates and the intuitive geometrical theory of diffraction (GTD). It is demonstrated that even for an anisotropic medium, the leading terms in the UAE have a direct interpretation in terms of the simple GTD. This model allows a physical explanation of the mechanism of ;wide aperture diffraction minimization', a simple technique that allows an almost complete elimination of diffractive effects in SAW devices by merely increasing the aperture of the unapodized transducer by a calculable amount. These techniques are illustrated with both theoretical and experimental results.     

Correction factor for low-loss 180 degrees reflecting linear chirp arrays in SAW devices

Low-loss, 180 degrees -reflecting linear chirps with constant or slowly varying weighting and large time-bandwidth product are analyzed. Multiple reflections are fully taken into account, giving a universal correction factor as a function of array parameters. This should enable such chirps to be designed without making iterations. Surface acoustic-wave (SAW) device layouts making use of the 180 degrees arrays are suggested.     

Accurate measurements of the acoustical physical constants of synthetic alpha-quartz for SAW devices

Accurate measurements of the acoustical physical constants (elastic constants, piezoelectric constants, dielectric constants, and density) of commercially available and widely used surface acoustic wave (SAW)-grade synthetic α-quartz are reported. The propagation directions and modes of bulk waves optimal for accurately determining the constants were selected through numerical calculations, and three principal X-, Y-, and Z-cut specimens and several rotated Y-cut specimens were prepared from a single crystal ingot to determine the constants and to confirm their accuracy. All of the constants were determined through highly accurate measurements of the longitudinal velocities, shear velocities, dielectric constants, and density. The velocity values measured for the specimens that were not used to determine the constants agreed well with those calculated from the determined constants, within a difference of ±0.20 m/s (±0.004%)     

Optimal cuts of langasite, La3Ga5SiO14 for SAW devices

The results of a theoretical and experimental investigation of the SAW propagation characteristics in an optimal region of langasite defined by the Euler angles φ from -15° to +10°, &thetas; from 120° to 165°, and ψ from 20° to 45° are presented. Based on temperature coefficients of the elastic constants derived from experimental data, some optimal orientations of langasite characterized by high electromechanical coupling factor (k²), zero power flow angle (PFA) and low or zero temperature coefficient of frequency (TCF) were found. The SAW velocity in the region of interest is highly anisotropic; this results in a significant amount of diffraction, which must be taken into account in the search for orientations useful for SAW devices. An orientation having simultaneously zero PFA, zero TCF, negligible diffraction, and relatively high piezoelectric coupling has been found and verified experimentally. The experimental results are in excellent agreement with the calculated SAW characteristics. The frequency response of a SAW device fabricated on the optimal cut of langasite is presented and demonstrates that high performance SAW filters can be realized on this optimal cut of langasite     

High-pin density feedthrough for superfluid helium applications

A vacuum tight high-pin density feedthrough for use in superfluid helium has been developed. The design and construction based on commercially available Framatome connectors is described.     

Behavior of platinum/tantalum as interdigital transducers for SAW devices in high-temperature environments

In this paper, we report on the use of tantalum as adhesion layer for platinum electrodes used in high-temperature SAW devices based on langasite substrates (LGS). Tantalum exhibits a great adhesive strength and a very low mobility through the Pt film, ensuring a device lifetime at 900°C of about one hour in an air atmosphere and at least 20 h under vacuum. The latter is limited by morphological modifications of platinum, starting with the apparition of crystallites on the surface, followed by important terracing and breaking of the film continuity. Secondary neutral mass spectroscopy (SNMS), Auger electron spectroscopy (AES), X-ray diffraction (XRD) measurements, and comparison with iridium-based electrodes allowed us to show that this deterioration is likely intrinsic to platinum film, consisting of agglomeration phenomena. Finally, based on these results, we present a solution that could significantly enhance the lifetime of Pt-based IDTs placed in high-temperature conditions.     

New orientations for bulk-acoustic-wave devices

Several new materials/orientations for which strong BAW (bulk acoustic wave) response is predicted are presented. These new materials/orientations have much lower insertion losses and therefore present the possibility of superior BAW devices. The results also point out some correspondence between pseudo-SAW and bulk acoustic waves. The pseudo-SAW waves cannot theoretically exist in a semiinfinite piezoelectric half-space. The presence of a pseudo-SAW pole with high coupling coefficient (related to the residue of the pole) indicates a strong bulk wave being launched into the medium.     

New properties of SAW gas sensing

Novel attractive properties of SAW gas sensing are theoretically predicted and experimentally verified. The response upon gas exposure of SAW-based gas sensors can be increased, decreased, reversed, cancelled, speeded up, aged down, and selected for a given sensitive layer, simply by changing the substrate material and orientation. When utilized as a tool for analytical chemistry, the steady-state and kinetic properties of adsorption, desorption, and diffusion, together with other related processes, can be simply deduced from pure acoustic measurements. These new properties are shown to be produced by the change of the components of the elliptic polarization of the wave, varying with the propagation direction and the substrate material. Experimental results, obtained for quartz substrates coated with polycrystalline palladium and palladium-nickel films exposed to H(2), CO, N(2)O, and different concentrations of relative humidity in air are presented as an example.     

Approximate green?s function representations for the analysis of SAW and leaky wave devices

The Green?s function or boundary element method (BEM) is the preferred technique for rigorous SAW device analysis. However, because of its computational cost, its principal application is the analysis of mode propagation in periodic structures to determine parameters that can then be used in simplified coupling of modes (COM) or P-matrix models. In this paper, rigorous representations are derived that express the Green?s function in terms of a continuous superposition of modes. The derivations include detailed analysis of the Green?s function properties as a function of both frequency and wavenumber, and representations are obtained for both the slowness and spatial domains. Approximate forms are then generated by replacing the continuous mode superposition by a discrete one. The Green?s function can be approximated to any required degree of accuracy, and the resulting approximations are applicable to any type of wave on any type of substrate. The long-range spatial components in the approximate forms are represented by exponential terms. The separable properties of these terms allow this class of approximation to be applied to general SAW and leaky wave device analysis in such a way that the computational effort increases only linearly with device size.