R. Lardat

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.

Finite-element analysis of periodic piezoelectric transducers

The need for optimized acoustic transducers for the development of high-quality imaging probes requires efficient simulation tools providing reliable descriptions of the behavior of real devices. The purpose of this work is the implementation of a finite-element model for the simulation of periodic transducer arrays. By using the assumption of harmonic excitation, the harmonic admittance of the studied structure can be derived. It is then shown how the mutual admittance is deduced from this feature, allowing one to estimate the amount of cross-talk effects for a given periodic transducer. Computation results are reported for standard linear acoustic probes, 2-2 (one-dimensional periodic) and 1-3 (two-dimensional periodic) piezocomposite materials. In the case of 2-2 connectivity composites, a comparison between nonperiodic and periodic computations of the mutual admittance is conducted, from which the minimum number of periods for which periodic computations can be trustfully considered can be estimated.

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.

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.

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

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.

3-D electrostatic hybrid element model for SAW interdigital transducers

In this work, the singular behavior of charges at corners and edges on the metallized areas in SAW transducers are investigated. In particular, it is demonstrated that a tensor product of the commonly used Tchebychev bases overestimates the singularities at corners, and, hence, it cannot be used in a proper boundary element method formulation. On the other hand, it is shown that a simple finite element method-like approach is impractical due to the enormous number of unknowns required to model the electrodes large length-to-width ratio. These considerations are then used for defining a hybrid element model, which combines Tchebychev and linear polynomials over differently meshed domains. Such an approach is shown to suitably account for charge singularities while greatly reducing the number of unknowns. Results are obtained for isotropic and anisotropic substrates for non-periodic configurations.

Sensitivity of interface acoustic waves to the nature of the interface

A finite element analysis/boundary integral method (FEA/BIM) model is used to investigate the influence of the geometry and the composition of the interface on the properties of interface acoustic waves (IAW). The method considers a finite region treated by FEA and submitted to periodic boundary conditions, taking into account radiation in both the bottom and the top half-spaces. The influence of the IDT thickness and of the material choice for the interfacial film are discussed for some cuts and structures. It is found that the most significant contribution of the interface geometry is to the ability of an IAW device of efficiently guiding waves, i.e. to the propagation losses.

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.

8E-5 Full 3D SAW IDT Boundary Element Model for Massless Electrodes

We present a boundary element model for calculating the mechanical displacements and surface charge distribution for SAW IDTs assuming perfect flat conductors with null mass. For this, we take into account the full 3D piezoelectric surface Greens dyad and solve the integral equation for the electric surface charge when a potential over the metallized area is known using a hybrid elements model. We then observe preliminary results for the charge distribution using the actual G44 component and give a simple formulation for the mechanical displacements.