We can predict and compute the behavior of elementary transducers in some particular conditions (e.g.simple geometric shapes in rigid baffles).In a practical situation, the transducers will rarely have a simple shape and all

La notion de filtrage est nee de la volonte de transmettre plusieurs communications sur une meme ligne. A la reception, le filtrage du signal permet de ne selectionner que la bande utile. Les techniques ont largement evolue avec l'utilisation de composants acoustoelectriques, dont la destination est d'etre inseres dans un circuit electronique afin de realiser une fonction de filtrage en frequence. La modelisation des ondes elastiques guidees et des methodes numeriques permet la conception et l'analyse de filtres de maniere extremement precise. La combinaison de methodes de simulation elaborees et de la connaissance precise des materiaux conduit a des accords simulation-mesure extraordinaires. Cet article presente donc les equations de base en acoustique, les phenomenes de propagation d'ondes ainsi que la modelisation des dispositifs a ondes de surface.

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.

In this work, we report on an original approach for increasing the coupling factor of Harmonic Bulk Acoustic Resonators (HBARs). It consists in adding a layer onto the top electrode to optimize the operation of the device. We avoid using metal layers and used an extra Aluminum Nitride (AlN) film to optimize the excitation energy localization in the stack and try and preserve the Q factor of the structure. We present simulation results demonstrating the efficiency of the optimization approach. Based on these simulations, test devices have been manufactured and tested, demonstrating the possibility to gain more than a factor of 2 on the coupling along the proposed approach. Finally, the possibility to wirelessly interrogate these resonators is demonstrated.

Spectacular advances have been made during the past years in the development of surface acoustic wave thanks to technology improvements but also to a strong effort in the modelling and design of such devices, taking into account their actual structure to benefit from second order effects (for instance the influence of the metal strip shape used in inter-digital transducers on the wave propagating under periodic gratings). In this paper, we present a 2D-periodic 3D-simulation approach to compute the spectral response of one period of a surface wave transducer, accounting for the actual shape of the electrodes as well as realistic guiding conditions and acoustic properties of the propagation surface. The idea consists in meshing the inhomogeneous part of the transducer, i.e. the electrodes and the bus bar, as well as a thin layer of the substrate to match the finite element section with surface boundary elements simulating the acoustic contribution of the substrate (propagation and radiation). Periodic boundary conditions are applied in both surface direction, allowing for the exploitation of spectral Green's function-based boundary elements, but additional absorbing conditions are added in the transverse direction (from both part of the bus bars) to suppress any unwanted modes generated by the mesh.

La notion de filtrage electrique est apparue avec les premiers systemes de communication utilisant des bandes spectrales tres etroites. Dans les annees 1910, les premiers ingenieurs radioelectriciens envisagent la possibilite de transmettre plusieurs communications telephoniques sur une meme ligne : ils utilisent pour ce faire une modulation du signal autour d'une frequence porteuse, differente pour chaque canal de communication. C'est la meme idee qui preside encore aujourd'hui dans pratiquement toutes les communications multiplexes (comme dans le cas du degroupage ou la telephonie, la television et les transmissions numeriques coexistent sur une meme ligne de transmission). De cette maniere, le spectre du signal est reparti dans une bande de frequence centree autour de la frequence porteuse. A la reception, la selection d'un canal specifique necessite un filtrage du signal pour ne selectionner que la bande utile. On en arrive donc a la notion de filtre selectif , capable d'eliminer efficacement tout signal hors bande utile sans pour autant alterer de facon trop importante l'information transmise. Au depart, des filtres lineaires a elements discrets de type resistance, inductance et capacite ( R , L , C ) sont utilises pour assurer des operations de filtrage a des frequences allant de 10 a 40 kHz. L'utilisation de methodes de communication exploitant les ondes hertziennes et le besoin d'elargir les bandes utiles pour augmenter la quantite d'information vehiculee imposent rapidement la mise au point d'autres principes de filtrage, mieux adaptes a de tels besoins. La realisation d'elements passifs stables et compacts pour le filtrage a egalement pousse a rechercher des solutions alternatives aux guides d'ondes et cavites resonantes electromagnetiques generalement efficaces mais cheres et encombrantes. Les techniques de filtrage ont ete rapidement etendues par l'introduction d'elements actifs permettant de resoudre entre autre le probleme des pertes dans la bande utile, generalement importantes, associees a l'utilisation d'elements passifs ( RLC ). Neanmoins, et malgre les progres incessants de la microelectronique, les bandes passantes des circuits actifs d'amplification restent souvent peu compatibles avec les applications « telecoms » pour lesquelles les frequences de travail ne cessent d'augmenter. De nombreuses tentatives ont ete engagees pour remplacer le filtrage lineaire analogique par des techniques numeriques. Celles-ci supposent l'existence de composants electroniques capables d'echantillonner et de traiter le signal a des frequences egalement tres elevees, et s'averent moins immunes aux parasites et aux effets de recouvrement spectral que les composants passifs. On s'interesse ici aux filtres « electriques », fondes sur l'utilisation de composants acoustoelectriques, dont la destination est d'etre inseres dans un circuit electronique afin de realiser une fonction de filtrage en frequence. L'objet de cet article est de decrire et de comparer entre elles des technologies de filtres passifs appartenant a une categorie particuliere que sont les filtres « acoustiques » bases sur une conversion d'energie electrique en energie acoustique et vice versa . Un second article [E 2 001] sera consacre a la modelisation de ces filtres. On limitera l'analyse aux filtres de bande, non dispersifs, parce que c'est le seul terrain commun a l'ensemble des filtres electroacoustiques. La technologie des ondes de surface permet par ailleurs d'adresser le domaine des filtres analogiques dispersifs, mais il s'agit la d'un sujet tres specifique.

Surface acoustic wave (SAW) radio-frequency identification (RFID) tags are soon expected to be produced in very high volumes. The size and cost of a SAW RFID tag will be key parameters for many applications. Therefore, it is of primary importance to reduce the chip size. In this work, we describe the design principles of a 2.4-GHz SAW RFID tag that is significantly smaller than earlier reported tags. We also present simulated and exper-

Surface acoustic wave (SAW) radio-frequency identification (RFID) tags are soon expected to be produced in very high volumes. The size and cost of a SAW RFID tag will be key parameters for many applications. Therefore, it is of primary importance to reduce the chip size. In this work, we describe the design principles of a 2.4-GHz SAW RFID tag that is significantly smaller than earlier reported tags. We also present simulated and experimental results. The coded signal should arrive at the reader with a certain delay (typically about 1 mus), i.e., after the reception of environmental echoes. If the tag uses a bidirectional interdigital transducer (IDT), space for the initial delay is needed on both sides of the IDT. In this work, we replace the bidirectional IDT by a unidirectional one. This halves the space required by the initial delay because all the code reflectors must now be placed on the same side of the IDT. We reduce tag size even further by using a Z-path geometry in which the same space in x-direction is used for both the initial delay and the code reflectors. Chip length is thus determined only by the space required by the code reflectors.

Surface acoustic wave (SAW) resonators on lithium tantalate (LiTaO3) and lithium niobate (LiNbO3) are investigated. The amplitude of the acoustic fields in the resonators are measured using a scanning laser interferometer. The amplitude profiles of the surface vibrations reveal the presence of distinct acoustic beams radiated from the transducer region of the SAW resonators and propagating with low attenuation. We suggest that this radiation is generated by the charges accumulating at the tips of the finger electrodes. The periodic system of sources, namely oscillating charges at the fingertips, generates Rayleigh-wave beams in the perpendicular and oblique directions. Green's function theory is used to calculate the coupling strength and slowness of the Rayleigh waves on 42 degrees Y-cut LiTaO3 and Y-cut LiNbO3 substrates as a function of the propagation direction. Furthermore, the propagation angles of the Rayleigh-wave beams as a function of frequency are calculated. The computed angles are compared with the measured ones for both the LiTaO3 and LiNbO3 substrates.

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.

Rayleigh wave side radiation from a synchronous 1.6 GHz longitudinal leaky SAW (LLSAW) resonator on YZ-LiNbO3 substrate is studied in this paper. Acoustic wave fields measured from the LLSAW resonator using a scanning laser interferometer are presented. The radiation angles extracted from the interferometric measurements are compared to theoretical calculations in a wide frequency range. Furthermore, the relative strength of the escaping Rayleigh wave beams evaluated from the interferometric measurements as a function of frequency is compared to the calculated coupling strength. These measurements reveal interesting effects not observed earlier. While the number of beams, the angles of radiation and their dependence on frequency can be reasonably explained, there are other effects whose explanation remains uncertain.

Recent developments in wide bandwidth SAW filter design led to the use of complex electroacoustic cells, such as the Hanma-Hunsinger cell. For this kind of structure, it is not sufficient to use only a single electrode periodic FEM/BEM model to derive the P-matrix parameters. The present paper proposes a mixed FEM/BEM numerical model for the simulation of a periodic array of metallic electrodes, the elementary cell of which can be as complex as necessary: it can contain several electrodes, connected to active ports, or short-circuited, or floating.

This paper proposes an original approach to design a new kind of transducer dedicated to high frequency applications. Using either multilayer structure, membrane, or plate, the device is composed of an isopotential interdigital transducer (IDT) together with a back counter electrode. Although electrical excitation is the same as a BAW device, modal propagation is settled onto the surface of the guiding structure, as for a SAW component. These Hybrid IDTs work naturally on the second Bragg harmonic. Consequently, with a given lithography process, central frequencies are twice higher than standard SAW IDT. Numerical models required to validate the principles of this structure and characterize the devices are presented. They are based on FEM/BEM analysis. Physical mechanisms involved are discussed in the paper, in order to find general rules to design Hybrid IDTs. Finally, an experimental validation is reported in the case of a piezoelectric plate

Surface acoustic wave (SAW) -based radio-frequency identification (RFID) tags are soon expected to be produced in very high volumes. The size and cost of a SAW RFID tag will be key parameters for many applications. Therefore, it is of primary importance to reduce the chip size. However, the number of distinct codes to be realized and the used frequency band impose limitations on the delays of coded responses and, consequently, on the tag size. The coded signal should arrive at the reader with a certain delay, that is, after the reception of the environmental echoes. An adequate initial delay is typically about 1 mus. If the tag uses a bidirectional interdigital transducer (IDT), the initial delay is needed on both sides of the IDT. In this work, we have replaced the bidirectional IDT by a unidirectional IDT. This allows to halve the space required by the initial delay, since all the reflectors must now be placed on the same side of the IDT. We have reduced the tag size even further by using a Z-path geometry with two strong inclined reflectors. In this configuration, the same space in the x-direction (the initial propagation direction) is used for both the initial delay and the code reflectors, which means that the chip length is finally determined only by the space required by the code reflectors. In this way, the tag length is reduced by about 2 mm compared to an equivalent single-track configuration using a unidirectional IDT with the code reflectors placed in line. The proposed configuration is especially advantageous for tags having a relatively long initial delay compared to the space required by the code reflectors. For such devices, a chip size of less than 2 mm by 1 mm (at 2.45 GHz) is realizable

An example of SAW P&T sensor is shown. This sensor is to operate for TPMS using the 434 MHz ISM band. Design of this sensor is based on simplicity, leading to micro-machined all quartz SAW device. Details of design process, realization and sensor responses are shown

The high-phase velocity (above 6100 m/s in and aluminum (Al) grating on lithium niobate (LiNbO/sub 3/)) of the longitudinal leaky surface acoustic wave (SAW) (LLSAW) mode makes it attractive for application in high-frequency SAW ladder filters in the 2-5 GHz range. We investigate the dependence of one-port synchronous LLSAW resonator performance or YZ-LiNbO/sub 3/ on the metallization thickness and metallization ratio, both experimentally and theoretically. Our results indicate a strong dependence of the Q factor and resonance frequency on the aluminum thickness, with the optimal thickness that produces the highest Q values being about 8%. The optimal thickness increases with the metallization ratio. The observed behavior is interpreted with the help of simulations using a combined finite element method (FEM)/boundary element method (BEM) technique. As an application, bandpass filters have been fabricated in the 2.8 GHz frequency regime, based on LL-SAWs. The synchronous resonators constituting the ladder filters operate in the fundamental mode. The filters feature low insertion losses below 3 dB and wide relative passbands of 4.5-5%.

Sylvain BALLANDRAS, Raphaël LARDAT, Mikaël WILM, Alexandre REINHARDT, Thomas PASTUREAUD, Vincent LAUDE, William DANIAU, Raphaël ARMATI, William STEICHEN, Olivier BURAT 1 Institut FEMTO-ST, UMR CNRS 6174, Département LPMO, Besançon, France 2 TEMEX SA, Sophia Antipolis, France 3 LUSSI, FRE CNRS 2448, Tours, France 4 Department of Physics, University of Bath, Bath, United Kingdom 5 FRAMATOME-ANP, Saint-Marcel, France Abstract. This paper is devoted to the description of a mixed finite element/boundary element analysis for the simulation of any periodic transducer radiating in any combination of solid and fluids assuming flat interfaces and linear operation regime. The theoretical developments required in that purpose are described and different examples of transducers are considered to demonstrate the interest of the proposed approach.