Pascal Ventura

Numerical methods for SAW propagation characterization

Due to more and more stringent requirements on SAW filter performance, it is mandatory to precisely characterize the SAW propagation characteristics as a function of manufacturing variations (metal thickness, mark-to-space ratio, etc.). Several authors have already proposed experimental characterizations using sets of test devices. One of the main difficulties of this experimental approach is the accuracy of both the geometrical and electrical measurements. On the other hand, precise numerical methods have been developed to advantageously replace experiments. Generally speaking, these methods are CPU time consuming. However, due to the continuing improvement of computer power, they are becoming more and more time efficient when applied to the analysis of SAW filters. The overall efficiency depends on the numerical integration methods used. In this paper we present a review of the numerical programs that have been developed during the past few years. For both infinite and finite grating modeling, we developed numerical mixed FEM/BEM (Finite Element Method-Boundary Element Method) models using an efficient interpolation function basis that takes into account the singularity at both edges of each electrode. First we propose a model for the simulation of finite transducers with arbitrary geometries. This numerical code has been successfully used to analyze a SPUDT (Single Phase UniDirectional Transducer) on the Y+36/spl deg/ cut of LiTaO/sub 3/. For an infinite periodic grating, it is convenient to solve the propagation problem in the Fourier domain (wave number space and harmonic excitation) and important efforts have been spent to properly integrate the so-called periodic harmonic Greens function. Using this numerical model together with the general P-matrix formalism, it is possible to compute all the basic parameters with a very good accuracy: these consist of the single strip reflectivity, acoustic wave-phase velocity, and phase offset between reflection and transduction centers. Simulations and comparisons with experiments are shown for each model.

Rigorous analysis of finite SAW devices with arbitrary electrode geometries

This paper proposes a rigorous numerical model allowing the analysis of finite SAW filters with only the 2D approximation: all the acoustical and electrical interactions are taken into account as well as mass loading effects. Areas outside the interdigital transducers can be either free surface or fully metallized: in the first case, a charge distribution formulation is used while, in the second one, a transverse E-field formulation is the most convenient representation. This method combines a BEM formulation using a semi-infinite dyadic Greens function and a FEM computation of the mechanical behavior of each metallic electrode. In order to limit the CPU time needed to perform such an analysis it has been necessary to develop a new powerful method to integrate the Greens functions. It has been possible to reduce this integration from a double integral of rapidly varying functions to a single integral of a slowly varying function, leading to easy numerical integration and allowing the use of this new numerical tool to rigorously analyze SAW filters with a total number of electrodes up to 200. It has been successfully used to analyze SAW filters based on short transducers for the 36/spl deg/ Y rotated cut of LiTaO/sub 3/. Comparisons between simulations and experiments will be presented, which show very good agreement.

Combined FEM and Green's function analysis of periodic SAW structure, application to the calculation of reflection and scattering parameters

Because of more and more stringent requirements on SAW filter performances, it is important to compute, with very good accuracy, the SAW propagation characteristics, which include the calculation of reflection and scattering parameters. For that reason, the analysis of periodic structures on a semi-infinite piezoelectric substrate is one of the most important problems being investigated by SAW researchers. For infinite periodic grating modeling, we developed numerical mixed FEM/BEM (finite element method-boundary element method) models using an efficient interpolation basis function that takes into account the singularity at both edges of each electrode. In this paper, a review of the numerical program that has been developed during the past few years will be presented. For an infinite periodic grating, it is convenient to solve the propagation problem in the Fourier domain (wave number space and harmonic excitation), and important efforts have been spent to properly integrate the so-called periodic harmonic Green function. Using this numerical model together with the general P-matrix formalism, it is possible to compute all of the basic parameters with a very good accuracy. These consist of the single strip reflectivity, acoustic wave-phase velocity, and position offset between reflection and transduction centers. Simulations and comparisons with experiments are shown for each model.

SPUDT-based filters: design principles and optimization

Tremendous work has been done all over the world to overcome the difficult problems of triple transit and insertion loss in SAW filters. Most of research efforts towards low losses have consisted of introducing reflections in the filter designs. Two major classes of low loss filters have emerged during the past 20 years, resonator filters and Single Phase UniDirectional Transducer (SPUDT) filters. This paper reviews the design principles of this second class, including some representative examples of actual devices.

A new concept in SPUDT design: the RSPUDT (resonant SPUDT)

Presents new concepts in SPUDT design, leading to a new low loss transducer structure called Resonant SPUDT (RSPUDT). Resonant acoustic cavities are created inside the transducer while keeping a global unidirectionality required for low loss. As a result, some areas of the transducer are locally backward directional. SAW filters utilizing the RSPUDT exhibit improved filter characteristics (e.g. passband ripple, shape factor) and result in more compact filters. Two kinds of RSPUDT elementary cells were developed. The first one is the generalization of classical EWC cells to RSPUDT concept while the second improves the reflectivity per wavelength by 50% with the same critical dimensions. Measurements on both 200 MHz and 110 MHz lowloss filters on 38° Y rotated quartz are presented to illustrate the interest in these new SPUDT structures. The 200 MHz filter exhibits a 2 dB bandwidth of 0.30 MHz and a 40 dB bandwidth of 1.01 MHz while the 110 h MHz filter combines a 40 dB bandwidth 912.25 MHz and a 0.5 dB bandwidth of 0.9 MHz

Synthesis of SPUDT filters with simultaneous reflection and transduction optimization

The general design problem for the single-phase unidirectional transducer (SPUDT) is solved, resulting in a new nonlinear algorithm which computes quasi-optimum transduction and reflection functions for a given template. Load impedance effects are taken into account allowing the placing of constraints on the amplitude and phase of the transducer function as well as on the triple transit level. Several filters are presented showing how SPUDT can be used to design very linear and low triple transit levels or very compact filters for mobile radio applications.<<ETX>>

Optimized STW devices with buried electrodes based on a mixed FEM/BEM numerical model

STW resonators on quartz are preferred over SAW due to their superior acceleration sensitivity, power handling, improved aging behavior, and 60% higher velocity. The achieved resonator Q at high frequencies using a metallic grating is lower than expected, mainly due to bulk mode scattering losses. The purpose of this study is to find the grating geometry which can reduce the propagation attenuation of the STW. STW propagation on Y+α°, X+90° propagation quartz cut (Euler angles 0,90-α,90) has been examined using a mixed FEM/BEM numerical model developed for buried electrodes for SAW propagation. It was found that by adjusting the a/p of the metal strips and by partially burying the electrodes one obtains SAW-like propagation properties with low propagation attenuation. For practical device design it is also necessary to be able to predict the turn-over temperature. To this end, simplified assumptions have been made to model the electrode and the piezoelectric substrate dilatations with temperature, and incorporated in the mixed FEM/BEM numerical model PerIDT presented at last years Ultrasonics Symposium. The optimized partially buried strip structure is used to obtain STW resonators with increased Q with predicted turnover temperature. Comparison with experiments will be shown. Partially buried electrode STW resonators with improved Q have been developed with the aid of the mixed FEM/BEM numerical model PerIDT.

SSBW to PSAW conversion in SAW devices using heavy mechanical loading

The development of efficient computation tools based on mixed analytical and numerical calculation approaches allows precise descriptions and characterizations of Surface Acoustic Wave (SAW) propagation. In this work, such a theoretical model is used to analyze the evolution of Pseudo Surface Acoustic Waves (PSAW) on standard YX Lithium Tantalate cuts versus Aluminum strip height. It is shown that the Surface Skimming Bulk Wave (SSBW) which accompanies the PSAW on such crystal orientations may be trapped by the grating, exhibiting a PSAW-like behavior when close to the Bragg condition. The consequences of this phenomenon are discussed.

Innovative SPUDT based structures for mobile radio applications

The IF filter is a key component in modern mobile radio receivers. Depending on the system design, the filter requirements goes from a very smooth prefiltering to a complete channel filtering. SPUDTs based structures are well suited for fractional bandwidths typically between 0.1% and 0.5%. Due to the reflectors presence inside the transducers, SPUDTs allows many different filters structures mostly using its reflective characteristic, its transduction characteristics or both. For base station applications, rejections in the 80 dB range are required. The first way to obtain as high a rejection is to use a cascade of 2 filters. We present a cascade of 2 filters at 201 MHz. The first is a classical SPUDT while the second is a new double track structure. A simple means to obtain as high a rejection in a single filter is to use a reflective array structure. We present a 71 MHz filter using this technique. The rejection is more than 75 dB for a 170 kHz 3 dB bandwidth and a 470 kHz 50 dB bandwidth. The simplest SPUDT structure consists of two SPUDTs on a single track. Due to its lack of symmetry, this structure is not well suited for balanced drive. A symmetric SPUDT structure was developed to fix this problem. Two balanced drive SPUDT filters are shown. The first is a 71 MHz GSM prefilter while the second is 85.4 MHz filter for CDMA. This filter exhibits a 0.85 MHz ±0.2 dB bandwidth, a 1.7 MHz 40 dB bandwidth for a 14 dB insertion loss

Analysis of SAW transducer having aperiodic multi-electrode cells using a coupled FEM/BIE numerical model

Low loss SAW filters sometimes require a structure with a complex geometry in order to improve the electro acoustical response (coupling coefficient, reflection coefficient, and static capacitance). Most of the coupling of mode models and P matrix models use parameters obtained from a single electrode periodic transducer analysis. In the case of complex electro acoustical cells (like HanmaHunsinger cells), it is sometimes necessary to obtain the COM parameters from the analysis of the entire electro acoustical cell, which means assuming that the filter contains an infinite array of identical electro acoustical cells the elementary cell of which is made of several electrodes. At the 2011 IEEE Ultrasonics Symposium, an original coupled Finite Element Model/Boundary Integral Equation (FEM/BIE) was presented in order to simulate an infinite array of single metallic electrodes [1]. In this paper, a generalization of this model to elementary cells more complex than one electrode will be presented. Like in the 2011 publication, the Finite Element Method is used for the finite part of geometry (the electrodes, part of the dielectric, and part of the piezoelectric substrate), while the semi infinite part of the geometry (piezoelectric part, and dielectric part) are taken into account using a Boundary Integral Equation. The theoretical and the numerical parts of the model will be presented, as well as Coupling of Modes or P matrix parameters determination for Hanma-Hunsinger low loss cells with various substrate materials and orientations. Using the results of the FEM/BIE analysis, a simulation of a filter containing modified Hanma-Hunsinger electro acoustical cells is made and is compared with measurements.