Thomas Pastureaud

A full 3D plane-wave-expansion model for 1-3 piezoelectric composite structures

The plane-wave-expansion (PWE) approach dedicated to the simulation of periodic devices has been extended to 1-3 connectivity piezoelectric composite structures. The case of simple but actual piezoelectric composite structures is addressed, taking piezoelectricity, acoustic losses, and electrical excitation conditions rigorously into account. The material distribution is represented by using a bidimensional Fourier series and the electromechanical response is simulated using a Bloch-Floquet expansion together with the Fahmy-Adler formulation of the Christoffel problem. Application of the model to 1-3 connectivity piezoelectric composites is reported and compared to previously published analyses of this problem.

Scattering matrix method for modeling acoustic waves in piezoelectric, fluid, and metallic multilayers

Many ultrasonic devices, among which are surface and bulk acoustic wave devices and ultrasonic transducers, are based on multilayers of heterogeneous materials, i.e., piezoelectrics, dielectrics, metals, and conducting or insulating fluids. We introduce metal and fluid layers and half spaces into a numerically stable scattering matrix model originally proposed for solving the problem of plane wave propagation in piezoelectric and dielectric multilayers. The method is stable for arbitrary thicknesses of the layers. We discuss how the surface Green’s functions can be computed for an arbitrary stack of homogeneous materials with plane interfaces. Aditionnally, we set up a backscattering algorithm to compute the distribution of electromechanical fields at any point in the stack. The model is assessed by considering some well-known examples.

Simulations of surface acoustic wave devices built on stratified media using a mixed finite element/boundary integral formulation

The demand for high frequency surface acoustic wave devices for modern telecommunication applications imposes the development of devices able to answer the manufacturer requirements. The use of high velocity substrates for which a piezoelectric layer is required to excite and detect surface waves has been widely investigated and requires the implementation of accurate theoretical tools to identify the best combinations of material. The present paper proposes a mixed formulation combining finite element analysis with a boundary integral method to accurately simulate the capability of massive periodic interdigital transducers to excite and detect guided acoustic waves in layered media. The proposed model is exploited for different typical configurations.

High-frequency surface acoustic waves excited on thin-oriented LiNbO/sub 3/ single-crystal layers transferred onto silicon

The need for high-frequency, wide-band filters has instigated many developments based on combining thin piezoelectric films and high acoustic velocity materials (sapphire, diamond-like carbon, silicon, etc.) to ease the manufacture of devices operating above 2 GHz. In the present work, a technological process has been developed to achieve thin-oriented, single-crystal lithium niobate (LiNbO3) layers deposited on (100) silicon wafers for the fabrication of radio-frequency (RF) surface acoustic wave (SAW) devices. The use of such oriented thin films is expected to favor large coupling coefficients together with a good control of the layer properties, enabling one to chose the best combination of layer orientation to optimize the device. A theoretical analysis of the elastic wave assumed to propagate on such a combination of material is first exposed. Technological aspects then are described briefly. Experimental results are presented and compared to the state of art

Oriented lithium niobate layers transferred on 4" [100] silicon wafer for RF SAW devices

A technological process has been developed to achieve a thin oriented Lithium Niobate layer deposited on [100] Si wafers for the fabrication of SAW devices integrable on silicon. The theoretical analysis of the elastic wave assumed to propagate on such combination of material is first reported. Technological aspects are then briefly described. Finally, experimental results are presented and compared to the state of art.

Interface acoustic waves properties in some common crystal cuts

Interface acoustic waves (IAWs), also termed boundary waves, propagate at the interface between two solids. We present two IAW numerical analysis tools, inspired from well established surface acoustic wave (SAW) methods. First, the interface effective permittivity is derived for arbitrary piezoelectric solids and is used to estimate some basic parameters of IAWs. The harmonic admittance for an interface excitation is then derived from the interface effective permittivity, in much the same way the harmonic admittance for surface excitation is obtained from the (surface) effective permittivity. The finite electrode thickness is neglected in this problem analysis. The harmonic admittance is used to model propagation in the case when an infinite periodic interdigital transducer is located at the interface. Simulation results are commented upon for some usual piezoelectric material cuts and the paper outlines a modal selection specific to IAWs as compared with SAWs. The temperature dependence of the resonance frequency is also estimated.

Numerical simulation and comparison of membrane and solidly mounted FBAR's

With the increase in operating frequencies of new wireless communication standards, Surface Acoustic Wave (SAW) devices are now facing competitors, such as thin film bulk acoustic resonators (FBARs). FBAR components are expected to overcome some of the limits of SAW components, and to provide miniature, high-performance and high-frequency filters. In this paper, we compare single resonator responses to more realistic configurations:a piezoelectric layer with relatively thick electrodes and a Solidly Mounted Resonator (SMR) with an acoustic mirror, and a resonator supported by a membrane. We then discuss how to adjust the relative thickness of the piezoelectric layer and of the electrodes to comply with frequency requirements for filtering applications.

Simulation of cMUT radiating in water using a mixed finite element/boundary element approach

A 2D finite element analysis of cMUT is proposed, taking into account periodicity and radiation in fluids. The convergence of the calculation is verified using non periodic computations. The capability of cMUT radiating in water to generate low velocity wave guided at the fluid/silicon interface is theoretically pointed out.

Optimisation and improved convergence of coupled finite element/boundary element analyses

In this paper, different approaches are investigated to improve the convergence and computation delay of mixed finite element/ boundary element analysis dedicated to periodic devices. Hence, different kinds of elements have been implemented to reduce the number of degree of freedom to the strictly needed ones according to the treated problem. The mesh also has been reduced to the actual inhomogeneous part of the problem by combining segment and more classical 2D elements (triangles and/or quadrangles). Finally, the asymptotic behavior of the Greens function used in the boundary elements is studied to try and reduce the number of space harmonics as far as possible.

Surface acoustic waves propagating on piezoelectric substrates under periodic arrays with large electrode thickness

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 paper that a periodic array of metallic electrodes (wires) exhibiting large aspect ratios deposited over a piezoelectric substrate gives rise to surface acoustic waves with general polarization. The admittance of an interdigital transducer (IDT), which is a basic tool for predicting the wave 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 the frequency behavior of the surface modes as a function of the electrode height compared to the period.