Currently, the most advanced micromachined microphones on the market are based on a capacitive coupling principle. Capacitive micro-electromechanical-system-based (MEMS) microphones resemble their millimetric counterparts, both in function and in performance. The most advanced MEMS microphones reached a competitive level compared to commonly used measuring microphones in most of the key performance parameters except the acoustic overload point (AOP). In an effort to find a solution for the measurement of high-level acoustic fields, microphones with the piezoelectric coupling principle have been proposed. These novel microphones exploit the piezoelectric effect of a thin layer of aluminum nitride, which is incorporated in their diaphragm structure. In these microphones fabricated with micromachining technology, no fixed electrode is necessary, in contrast to capacitive microphones. This specificity significantly simplifies both the design and the fabrication and opens the door for the improvement of the acoustic overload point, as well as harsh environmental applications. Several variations of piezoelectric structures together with an idea leading to electrically controlled sensitivity of MEMS piezoelectric microphones are discussed in this paper.

Nowadays, the most advanced micromachined microphones on the market are represented with structures using the capacitive coupling principle.Structures and performances of these micro-devices resemble their millimetric counterparts, which are typically used as measuring microphones.In the past decade, thanks to technological progress of the microelectronics industry, microphones using a piezoelectric transduction have been proposed by several teams.Such novel microphones exploit the piezoelectric effect of a thin layer of aluminum nitride, incorporated in their diaphragm structure.In these microphones fabricated with micromachining technology, no fixed electrode is necessary, unlike capacitive microphones.This specificity significantly simplifies both design and fabrication and opens the door for a new improvement of acoustic overload point as well as harsh environmental applications.In this paper, we present and compare two most promising approaches to piezoelectric micromachined microphone design.The first approach is developed by using a flat, circular diaphragm fixed around its perimeter, having the piezoelectric layer with its upper electrode in the vicinity of the clamped region.The other approach involves a square diaphragm cut in a way to create several cantilevers.Such a structure enables a more compliant diaphragm compared to the first approach.

This work presents a lumped parameters equivalent model of condenser microphone based on analogies between acoustic, mechanical, fluidic, and electrical domains. Parameters of the model were determined mainly through analytical relations and/or finite element method (FEM) simulations. Special attention was paid to the air gap modeling and to the use of proper boundary condition. Corresponding lumped-parameters were obtained as results of FEM simulations. Because of its simplicity, the model allows a fast simulation and is readily usable for microphone design. This work shows the validation of the equivalent circuit on three real cases of capacitive microphones, including both traditional and Micro-Electro-Mechanical Systems structures. In all cases, it has been demonstrated that the sensitivity and other related data obtained from the equivalent circuit are in very good agreement with available measurement data.

This work demonstrates, with numerical simulations, the potential of an octagonal probe for the generation of radiation forces in a set of points following a path surrounding a breast lesion in the context of dynamic ultrasound elastography imaging. Because of the in-going wave adaptive focusing strategy, the proposed method is adapted to induce shear wave fronts to interact optimally with complex lesions. Transducer elements were based on 1-3 piezocomposite material. Three-dimensional simulations combining the finite element method and boundary element method with periodic boundary conditions in the elevation direction were used to predict acoustic wave radiation in a targeted region of interest. The coupling factor of the piezocomposite material and the radiated power of the transducer were optimized. The transducer's electrical impedance was targeted to 50 Ω. The probe was simulated by assembling the designed transducer elements to build an octagonal phased-array with 256 elements on each edge (for a total of 2048 elements). The central frequency is 4.54 MHz; simulated transducer elements are able to deliver enough power and can generate the radiation force with a relatively low level of voltage excitation. Using dynamic transmitter beamforming techniques, the radiation force along a path and resulting acoustic pattern in the breast were simulated assuming a linear isotropic medium. Magnitude and orientation of the acoustic intensity (radiation force) at any point of a generation path could be controlled for the case of an example representing a heterogeneous medium with an embedded soft mechanical inclusion.

In this work, we will present results showing a feasibility of a MEMS microphone, based on AMS 0.35 μm CMOS standard process, with only one step of a sacrificial SiO2 maskless etching on the substrate front-side. The microphone design, modeling and simulated performance will be studied. Fabrication of test structures obtained with a SiO2 etching will be shown as well as characterizations of these test structures.

Minimization of dissipative losses is a major goal in MEMS resonator design [1]-[2]. For an accurate simulation of a MEMS resonator vibrating in vacuum, thermoelastic damping phenomenon related to the irreversible heat dissipation induced by the coupling between heat transfer and strain rate during the resonator vibration and acoustic radiation into the substrate have to be taken into account. The finite element method (FEM) is suitable for structural simulation, especially for thermoelastic damped structures vibrating in vacuum. When the vibrating structure is deposited on an unbounded elastic medium, radiating conditions have to be taken into account. The boundary element method (BEM) is suitable for unbounded medium, because the radiating conditions are exact. It is often used to complete of the finite element method. The goal of our work is to develop and to validate a thermal-electromechanical FEM-BEM tool, which is helpful to predict and understand MEMS energy loss dissipation.

Pour les applications sous-marines et la fabrication de SONARs, de grandes surfaces de materiaux piezoelectriques avec de tres bonnes proprietes electromecaniques sont souvent necessaires. Les monocristaux piezoelectriques (typiquement les compositions PMN-PT ou PZN-PT) sont des materiaux tres performants mais sont difficiles a fabriquer en grandes surfaces tout en gardant des proprietes homogenes. Leur integration dans la fabrication de composites piezoelectriques de connectivite 1-3 avec la methode dite « dice and fill » delivre d'excellentes performances mais la taille finale des echantillons est limitee par celle du monocristal d'origine. Recemment, une methode prometteuse a ete proposee par empilement de couches successives [1] (dite « par lamination ») evitant ces restrictions dimensionnelles. Des piezo-composites de connectivite 1-3 ont ainsi ete fabriques avec les deux methodes. Au prealable, leur conception a ete realisee par analyse numerique (ATILA). Les proprietes electromecaniques mesurees sont comparables aux predictions numeriques. Les coefficients de couplage en mode epaisseur des composites fabriques sont compris entre 60 et 80% suivant les phases (piezoelectriques et inertes) utilisees. Finalement, plusieurs mono-transducteurs avec des frequences de resonance autour de 1 MHz ont ete fabriques, caracterises et compares. Les resultats confirment l'interet de cette methode par lamination [1] pour la fabrication de piezo-composites de connectivite 1-3. [1] Feifei Wang et al., Single-crystal 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3/epoxy 1-3 piezoelectric composites prepared by the lamination technique, Materials Chemistry and Physics, vol. 105, pp. 273-277, 2007.

This work demonstrates with numerical simulations, the feasibility of an ultrasound probe for the generation of radiation forces in set of points following a path surrounding a tumor. Such strategy is adapted to induce resonance elastography of breast tumors and/or to increase displacement magnitudes induced by low frequency shear waves. Transducer elements were based on 1-3 piezocomposite material. 3D simulations combining the finite element method and boundary element method with periodic boundary conditions in the elevation direction were used to predict acoustic wave radiation in the breast. The crosstalk between neighbor elements was not taken into account. The coupling factor of the piezocomposite material and the radiated power of the transducer were optimized. The transducer electrical impedance was targeted to 50 Ω. The final probe was simulated by assembling the designed transducer to build an octagonal phased-array, with 256 elements on each edge. Using dynamic transmitter beamforming techniques, the electrical excitation that generates the radiation force along a path and resulting acoustic pattern in the breast were evaluated. Transducers central frequency was 4.5 MHz; they were able to deliver enough power and could generate the radiation force with a relatively low level of voltage excitation. Magnitude and orientation of the acoustic intensity (radiation force) at any point of a path were controlled.

1-3 piezo-composites were fabricated by << dice and fill >> method (DFM) and on a lamination technique (LMT), well adapted to produce large area samples. Two piezoelectric materials (PMN-PT ceramic and single crystal) and epoxy resin have been considered. Design of these composites was performed with finite element analysis (ATILA). Particular attention was paid to sample characterization. An original technique of electrical impedance fitting with FEA was used to determine the properties of both components in their operating conditions. The measured thickness coupling factors of the composites obtained by both methods are between 60 and 80% according to the chosen constituents. Several single element transducers with a center frequency around 1 MHz were fabricated. Their characteristics were compared and results showed that sensitivity of the 1-3 LMT piezo-composite based transducer was slightly lower than that of the DFM one, but sufficient to find applications where large area apertures are required. Moreover, the LMT leaded to a non perfect alignment of the piezoelectric rods in the composites and this effect was quantified by FEA in terms of electromechanical performance.

This paper reports numerical and experimental analyses of negative refraction process using a phononic crystal, made of a periodic arrangement of holes in aluminium. Dispersion curves are discussed and conditions for which negative refraction can appear are identified. Negative refraction is clearly obtained for transverse waves. Experiments confirm numerical predictions.

A numerical analysis of negative refraction process is reported using a phononic crystal with an elastic solid matrix. The phononic crystal considered in this study is made of a periodic arrangement of holes in aluminum. Dispersion curves are discussed and conditions for which negative refraction can appear are identified. These conditions are obtained for the transverse waves, whereas the longitudinal waves are evanescent. A calculation is performed with a prism shaped phononic crystal, and it clearly exhibits a negative refraction angle. Several analyses are provided with a view to characterize the wave going out of the phononic crystal. Finally, improvements, with respect to the impedance matching and index tuning, are discussed.

Micro-machined ultrasonic transducers (MUT) are investigated for phased arrays in high frequency acoustic imaging to overcome resolution and frequency limits of bulk PZT transducers currently used. The advantage of structures on silicon substrate is that transducers could be integrated with the electronics of the system. From a design point of view, finite element (FEM) codes are commonly used to model MUT, but to take into account the fluid surrounding the device radiation, scattering and the inter-mutual acoustic influence of the MUT arrays cells - the boundary element method (BEM) coupled with the finite element method is advantageously used. This paper describes the design, the fabrication and the characterisation of bending mode circular piezoelectric membranes, with operating frequency around 10 MHz. The devices are based on a PZT (lead zirconate titanate) film 1 μm thick supported by a membrane of polysilicon, 2 μm thick and SiO2, 500 nm thick. The PZT thin film was deposited following a sol-gel route. Fully-supported membranes as well as suspended cells are considered with a view to increase the resonant frequency and decrease the cross coupling between cells.

A Solidly Mounted Resonator (SMR) is an ultra-thin piezoelectric layer deposited between two electrodes which operates in longitudinal thickness mode. Minimization of dissipative losses is a major goal in SMR designs. For an accurate simulation of a SMR vibrating in vacuum, thermoelastic damping phenomenon related to the irreversible heat dissipation induced by the coupling between heat transfer and strain rate during the resonator vibration and acoustic radiation into the substrate have to he taken into account The finite element method (FEM) is suitable for structural simulation, especially for thermoelastic damped structures vibrating in vacuum. When the vibrating structure is deposited on an unbounded elastic medium, the boundary element method (BEM) is suitable because it prescribes exact radiating conditions. Numerical FEM-BEM results are presented for an aluminum nitride resonator with molybdenum electrodes operating at 2.10 GHz and decoupled from the substrate by a tungsten-silicon dioxide or a silicon nitride-silicon oxycarbide Bragg reflector. Thermoelastic effect is shown to affect mainly SMR quality factor and temperature coefficient of frequency.

Micro-machined ultrasonic transducers (MUT) are investigated for phased arrays in high frequency acoustic imaging to overcome resolution and frequency limits of bulk PZT transducers currently used. The advantage of structures on silicon substrate is that transducers could be integrated with the electronics of the system. From a design point of view, finite element (FEM) codes are commonly used to model MUT, but to take into account the fluid surrounding the device radiation, scattering and the inter-mutual acoustic influence of the MUT arrays cells - the boundary element method (BEM) coupled with the finite element method is advantageously used. This paper describes the design, the fabrication and the characterisation of bending mode circular piezoelectric membranes, with operating frequency around 10 MHz. The devices are based on a PZT (lead zirconate titanate) film 1 µm thick supported by a membrane of polysilicon, 2 µm thick and SiO2, 500 nm thick. The PZT thin film was deposited following a sol-gel route. Fully-supported membranes as well as suspended cells are considered with a view to increase the resonant frequency and decrease the cross coupling between cells.

Solidly Mounted Resonators (SMR) are alternatives to membrane resonators to realize Bulk Acoustic Wave (BAW) filters for RF communication systems. SMR decoupling from substrate, which is realized with a multi-layered Bragg reflector, has a major influence on resonator performance (quality factor, spurious modes). A finite element-boundary element (FEM-BEM) method is presented for two-dimensional SMR simulation. The substrate is described by an elasto-dynamic half-space Green function which is discretized and implemented in the ATILA finite element software. Results are presented for an aluminum nitride resonator with molybdenum electrodes operating at 2.14 GHz and decoupled from the substrate by a tungsten (W)/silicon oxide(SiO2) or a silicon nitride (SiN)/silicon oxycarbide (SiOC) Bragg reflector. When compared to one-dimensional Mason's model results, FEM-BEM computed values show a decrease of the quality factor and of the effective coupling coefficient. Lowered quality factors at parallel resonance are attributed to elasto-dynamic radiation into the substrate at the vicinity of the electrode boundary.

Capacitive micromachined ultrasonic transducer (CMUT) arrays are seen as a promising way to improve ultrasound-imaging definition and they may supplant piezocomposite arrays for such applications. However, as a new technology it needs to be evaluated and modelling can help to design devices using cMUT technology. Implicit finite element codes are commonly used to model cMUT devices, but to take into account the fluid surrounding the device $radiation, scattering and the inter-mutual acoustic influence of the CMUT arrays cells - the boundary element method (BEM) coupled with the finite element method (FEM) can be advantageously used. Moreover the standard electromechanical coupling is either weak or lumped and cannot be used to model CMUT accurately. A FEM-BEM coupling together with an electro-mechanical direct coupling as add-on modules for ANSYSreg were developed. This paper will show how to identify exact nonlinear equivalent circuit lumped parameters from a FEM and FEM-BEM simulation. The electrical and motional branch of the circuit is identified by resonance and antiresonance modal analysis. The radiating impedance is computed using a FEM-BEM coupling in frequency domain. Finally the transducer insertion loss is evaluate

A numerical model is proposed to describe in the frequency domain the radiation of a piezoelectric transducer in a fluid-filled borehole surrounded by a formation of infinite extent. Finite elements are used to model the transducer, the borehole fluid, and the fluid–formation interface. The unbounded character of the domain is accounted for by using a wave number decomposition on the borehole surface and dampers on the top and bottom surfaces of the borehole mesh. The method is validated by studying three configurations with analytical solutions: (i) normal stress acting on an empty borehole; (ii) normal stress acting on a fluid-filled borehole; and (iii) point source acting on the fluid-filled borehole axis. The radiation of a piezoelectric ring transducer in an oil-filled tube surrounded by water is also studied experimentally and numerically.