Sylvain Ballandras

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.

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.

Design of coupled resonator filters using admittance and scattering matrices

The longitudinal coupling of thin film bulk acoustic resonator (FBAR) is investigated in order to design coupled resonator filter (CRF). Such devices are dedicated to high-frequency filter applications. A scattering matrix algorithm that was already used to simulate simple FBARs devices is used to study the behavior of these structures. The principle of the algorithm is recalled and is explained how it can be adapted to the simulation of multiport devices. Then an example of filter design is given.

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.

High-frequency surface acoustic wave device based on thin-film piezoelectric interdigital transducers

Using high-quality epitaxial c-axis Pb(Zr0.2Ti0.8)O3 films grown by off-axis magnetron sputtering onto metallic (001) Nb-doped SrTiO3 substrates, a nonconventional thin-film surface acoustic wave device based on periodic piezoelectric transducers was realized. The piezoelectric transducers consist of a series of ferroelectric domains with alternating polarization states. The artificial modification of the ferroelectric domain structure is performed by using an atomic force microscope tip as a source of electric field, allowing local switching of the polarization. Devices with 1.2 and 0.8μm wavelength, defined by the modulation period of the polarization, and corresponding to central frequencies in the range 1.50–3.50GHz have been realized and tested.Using high-quality epitaxial c-axis Pb(Zr0.2Ti0.8)O3 films grown by off-axis magnetron sputtering onto metallic (001) Nb-doped SrTiO3 substrates, a nonconventional thin-film surface acoustic wave device based on periodic piezoelectric transducers was realized. The piezoelectric transducers consist of a series of ferroelectric domains with alternating polarization states. The artificial modification of the ferroelectric domain structure is performed by using an atomic force microscope tip as a source of electric field, allowing local switching of the polarization. Devices with 1.2 and 0.8μm wavelength, defined by the modulation period of the polarization, and corresponding to central frequencies in the range 1.50–3.50GHz have been realized and tested.

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.

Microgrippers fabricated by the LIGA technique

A study devoted to the design and fabrication of microgrippers using the LIGA technique is described in this paper. The design method is presented and validated by the use of finite-element analysis. Technological topics are detailed to illustrate the fabrication process. Also, experimental data concerning the mechanical behaviour of one of the microgrippers are reported. These data are used to improve the previous finite-element analysis. Finally, a comparison between experiments and theoretical predictions is discussed.

Lateral optical accelerometer micromachined in (100) silicon with remote readout based on coherence modulation

Abstract A novel type of optical silicon accelerometer is demonstrated. A non-conventional wet etching technique for (100) silicon allows us to make a highly symmetrical seismic mass with a pure lateral translation movement. This ensures a low sensitivity to accelerations that are not along the sensing axis. To detect the displacement of the seismic mass due to accelerations, an optical fiber can be easily and precisely implemented to form a Fabry-Perot interferometer. A detection method based on coherence modulation allows remote acceleration sensing through a fiber-optic link without any electrical link between the measurement region and the signal output. The sensitivity of the demonstrated system is 1.8 V g −1 for a measurement range of ±10 g and a resolution of less than 1mg. Moreover, the fabrication technique and the multiplexing capability of the detection method open the way to a 3D single-chip accelerometer, the measurements of which could be sent directly through a single optical link.

Surface acoustic wave trapping in a periodic array of mechanical resonators

The existence of two families of surface acoustic modes trapped by steep ridges on a piezoelectric substrate, shear horizontal and vertically polarized surface modes, is demonstrated experimentally using high aspect ratio interdigital transducers fabricated on lithium niobate. The experimental variation of the resonance frequencies of the various surface modes is obtained experimentally, and up to an order of magnitude slowing of surface waves is observed, with the phase velocity dropping from 4000 down to 450m∕s. It is argued that the observed resonances are surface modes trapped by the ridge electrodes.