Th. Pastureaud

Stable scattering-matrix method for surface acoustic waves in piezoelectric multilayers

A scattering matrix approach is proposed to avoid numerical instabilities arising with the classical transfer matrix method when analyzing the propagation of plane surface acoustic waves in piezoelectric multilayers. The method is stable whatever the thickness of the layers, and the frequency or the slowness of the waves. The computation of the Green’s function and of the effective permittivity of the multilayer is outlined. In addition, the method can be easily extended to the case of interface acoustic waves.

A mixed finite element/boundary element approach to simulate complex guided elastic wave periodic transducers

The development of new surface acoustic wave devices exhibiting complicated electrode patterns or layered excitation transducers has been favored by an intense innovative activity in this area. For instance, devices exhibiting interdigital transducers covered by piezoelectric or dielectric layers have been fabricated and tested, but the design of such structures requires simulation tools capable to accurately take into account the actual shape of the wave guide elements. A modeling approach able to address complicated surface acoustic wave periodic structures (defined in the saggital plane) exhibiting any geometry then has been developed and implemented. It is based on the combination of a finite element analysis and a boundary element method. A first validation of the computation is reported by comparison with standard surface wave devices. Surface transverse wave resonators covered by amorphous silica have been built and consequently used for theory/experiment assessment. Also the case of recessed electrodes has been considered. The proposed model offers large opportunities for modeling any two-dimensional periodic elastic wave guide.

Generally polarized acoustic waves trapped by high aspect ratio electrode gratings at the surface of a piezoelectric material

It has been shown theoretically and demonstrated experimentally that shear horizontal (SH) surface waves can exist when the surface of an isotropic substrate is perturbed by a strong corrugation, for instance consisting of deep grooves etched in the substrate, whereas these waves cannot exist without this perturbation. It is shown in this article that a periodic array of metallic electrodes (wires) exhibiting large aspect ratios deposited over a piezoelectric substrate give rise to surface acoustic waves with general polarization. The admittance of an interdigitated transducer, which is a basic tool for predicting the waves parameters, is calculated by a combination of finite element analysis and a boundary integral method. This approach has been extended to obtain the polarization of the acoustic waves. For different piezoelectric substrates, we predict various surface acoustic modes and their polarization. Along with mostly SH modes, we also find modes mostly polarized in the sagittal plane. We discuss ...

A FEA/BEM approach to simulate complex electrode structures devoted to guided elastic wave periodic transducers

A modelling approach able to address complicated SAW periodic structures with non homogeneous geometry has been developed and implemented. It is based on the combination of finite element analysis and a boundary element method. Validation of the computation is reported. An example of simulation of a passivated STW resonator is used for theory/experiment assessment.

Evaluation of the P-matrix parameters frequency variation using periodic FEM/BEM analysis [SAW device simulation]

An original approach is proposed to predict and to take into account the frequency variation of the P-matrix parameters in the simulations. The proposed method is applied to well known Rayleigh and dispersive leaky waves for validation. The computed dispersion laws are compared to experimental results for different kinds of substrates and waves.

Characterization and prediction of transverse plate resonators built using mixed strip and groove gratings

Two approaches are developed to model accurately the physical characteristics of plate mode devices, and more specifically of resonators. They are respectively based on pure finite element computation and on mixed boundary integral methods and finite element analysis (BIM/FEA). The main parameters of wave propagation (velocity, coupling factor, reflection, and so on) can be estimated, considering an infinite periodic structure and using the harmonic admittance, and can be inserted in a mixed matrix model to simulate the electrical response of devices. A comparison between theory and experiments is reported for transverse plate resonators built using mixed strip and groove gratings.

Stabilization of the simulation of saw devices on stratified structures: application to transverse plate mode resonators

Two approaches are investigated to model accurately the physical characteristics of plate mode devices, and more particularly of resonators. They are respectively based on finite element analysis (FEA) and on mixing FEA with a boundary integral method (FEA/BIM). In the later case, using a transfer matrix approach for the computation of the spectral Greens function results in numerical instabilities for large layer thickness or large slowness. A new stable algorithm is described for the computation of the spectral Greens function of a multilayer structure, that is inherently numerically stable. The main parameters of wave propagation (velocity, coupling factor, reflection coefficient) can then be estimated considering an infinite periodic structure and computing the harmonic admittance. For comparison with measurements of quartz transverse plate mode resonators, the estimated parameters can be inserted in a P-matrix model. Theory and experiments are found to comply well for both the pure FEA and the FEA/BIM approaches.

Analysis of saw propagation under a periodic multi-electrode-type grating

A general approach is proposed to model complicated cells, made of an arbitrary number of electrodes and electrical ports. Simulations are based on a periodic FEM/BEM analysis, and emphasis is put on the study of one wavelength long cell. The proposed method is applied to well known basic cells and Rayleigh waves for validation, and comparisons to experimental results are given. Using this approach, propagation losses due to bulk radiation in DART cells are evaluated. Finally, discussion is opened about the phase references required for accurate simulation of complicated cells showing directivity in a non- periodic environment.