A model to predict baffle effects in linear array of cMUTs

International audiencecMUTs are a very promising alternative to the piezoelectricity in echographic imaging probes. Many authors have modeled cMUT arrays but only few works have been dedicated to the simulation of devices working in real configuration, i.e. where each element is made of a finite number of cMUTs. In previous studies, we pointed out that, in such conditions, interactions between cMUTs could cause some strong cut-off frequencies in the useful frequency bandwidth, leading to undesirable oscillations at the end of the impulse response. This phenomenon is comparable with baffle effects in multi-source radiators. Even if origins of these cut-off frequencies are well known in sonar arrays, this phenomenon have poorly been explored for arrays of cMUTs. This paper is a deep theoretical investigation of these interactions. The linear array of cMUTs is modeled like a linear system. Input electrical quantities (voltage applied to each element of the array) are linked to output acoustic quantities (pressure emitted by each element) with a global transfer matrix. The Eigenvalues decomposition of the transfer matrix is used to discuss and to explain origins of cut-off frequencies in the radiated pressure spectrum

Automatic Optimal Input Command for Linearization of cMUT Output by a Temporal Target

2014 IEEE. Reprinted, with permission, from Sébastien Ménigot, Dominque Certon, Dominque Gross and Jean-Marc Girault, Automatic Optimal Input Command for Linearization of cMUT Output by a Temporal Target, 2014 IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. This material is posted here with permission of the IEEE. Such permission of the IEEE does not in any way imply IEEE endorsement of any of the Université François Rabelais de Tours' products or services. Internal or personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution must be obtained from the IEEE by writing to [email protected] audienceCapacitive micromachined ultrasonic transducers (cMUTs) are a promising alternative to the piezoelectric transducer. However, their native nonlinear behavior is a limitation for their use in medical ultrasound applications. Several methods based on the pre-compensation of a preselected input voltage have been proposed to cancel out the harmonic components generated. Unfortunately, these existing pre-compensation methods have two major flaws. The first is that the pre-compensation procedure is not generally automatic, and the second is that they can only reduce the second harmonic component. This can, therefore, limit their use for some imaging methods, which require a broader bandwidth, e.g., to receive the third harmonic component. In this study, we generalized the presetting methods to reduce all nonlinearities in the cMUT output. Our automatic pre-compensation method can work whatever the excitation waveform. The precompensation method is based on the nonlinear modeling of harmonic components from a Volterra decomposition in which the parameters are evaluated by using a Nelder-Mead algorithm. To validate the feasibility of this approach, the method was applied to an element of a linear array with several types of excitation often encountered in encoded ultrasound imaging. The results showed that the nonlinear components were reduced by up to 21.2 dB

Approche temporelle de la simulation et de la caractérisation des transducteurs ultrasonores capacitifs micro-usinés

Les transducteurs ultrasonores capacitifs micro-usinés sont aujourd'hui une nouvelle alternative à la transduction d'ondes ultrasonores. En comparaison avec la technologie piézo-électrique, ils offrent des potentialités en termes de production, de miniaturisation et d'intégration d'une électronique associée mais aussi en termes de performances. Néanmoins,leur mise en œuvre n'en est encore qu'à ces balbutiements et la compréhension de leurs comportements nécessite d'être approfondie. C'est dans ce cadre que s'inscrit le présent travail de thèse. Nous proposons, dans un premier temps, à l'aide d'un modèle numérique basé sur une mécanique linéaire de plaques multicouches, d'étudier l'effet des contraintes initiales sur le comportement statique. Dans un second temps, l'impact de la non-linéarité de la dynamique d'une cellule, puis d'un réseau de cellules, est étudiée en s'appuyant à la fois sur des mesures d'interférométrie et sur un modèle temporel intégrant les effets du fluide. Enfin, nous proposons une optimisation de l'excitation et l'utilisation de ces dispositifs en régime forcée pour la génération d'onde basse fréquence dans l'air et dans l'eau.Capacitive tvIicromachined Ultrasound Transducers (cMUTs) are today a new alternative for the generation of ultrasonic waves. Compared lo the piezoelectric technology, theyoffer some potentialities in terms of reliability, production, miniaturization and electronicintegration but also in term of acoustic performance. Nevertheless, their implementationis relatively new and the understanding of their static and dynamic behaviors needs to bestudied further. This is in this context that this PhD is developed. We propose, in a firsttime, with the help of a numeric model based on the linear mechanic theory of multilayeredplates, to study the impact of initial stresses on the static behavior. In a second time, the impact of the nonlinearity on the dynamic of the cell first, and a cell array next, is studiedwith the help of a temporal model and measurements made by laser interferometry both.Finally, thanks to this dynamic study, a new operation mode of cMUTs is identified andverified. This one is based on the use of forced regime in air and water of these device togenerate low frequencies ultrasonic waves.TOURS-Bibl.électronique (372610011) / SudocSudocFranceF

Investigation of the Young's Modulus and the Residual Stress of 4H-SiC Circular Membranes on 4H-SiC Substrates

International audienceThe stress state is a crucial parameter for the design of innovative microelectromechanical systems based on silicon carbide (SiC) material. Hence, mechanical properties of such structures highly depend on the fabrication process. Despite significant progresses in thin-film growth and fabrication process, monitoring the strain of the suspended SiC thin-films is still challenging. However, 3C-SiC membranes on silicon (Si) substrates have been demonstrated, but due to the low quality of the SiC/Si heteroepitaxy, high levels of residual strains were always observed. In order to achieve promising self-standing films with low residual stress, an alternative micromachining technique based on electrochemical etching of high quality homoepitaxy 4H-SiC layers was evaluated. This work is dedicated to the determination of their mechanical properties and more specifically, to the characterization of a 4H-SiC freestanding film with a circular shape. An inverse problem method was implemented, where experimental results obtained from bulge test are fitted with theoretical static load-deflection curves of the stressed membrane. To assess data validity, the dynamic behavior of the membrane was also investigated: Experimentally, by means of laser Doppler vibrometry (LDV) and theoretically, by means of finite element computations. The two methods provided very similar results since one obtained a Young's modulus of 410 GPa and a residual stress value of 41 MPa from bulge test against 400 GPa and 30 MPa for the LDV analysis. The determined Young's modulus is in good agreement with literature values. Moreover, residual stress values demonstrate that the fabrication of low-stressed SiC films is achievable thanks to the micromachining process developed

Dual mode transducers based on cMUTs technology

International audienceMore and more medical ultrasonic applications are strongly interested by the development of dual acoustic sources enable to emit high frequency ultrasound (echographic imaging) and low frequency pressure field (therapeutic ultrasounds). The use of the piezoelectricity to fabricate such device requires overcoming strong technological bottlenecks. The objective of this paper is to demonstrate that the technology of capacitive micro-machined ultrasonic transducer is able to take up this challenge. To this end a demonstrator was designed and manufactured. The first part of the paper is devoted to the design of the low and high frequency cMUTs. For the low acoustic source, a dedicated time domain model was used, taking into account the nonlinearity of the cMUT. Several simulations were conducted to optimize the emitted pressure field intensity at 1 MHz for a set of diaphragm with different sizes and geometries. The high frequency source was designed on the help of a linear model, where output parameters were central frequency, bandwidth and collapse voltage. The second part of this paper reports a set of characterization results and performances of the fabricated device: electrical impedance, mechanical displacements performed in water and acoustic pressure fields

Belli Gigantei liber primus [-liber quartus]

Titre de départAuteur, éditeur et année d'après L'ère baroque en France de Roméo Arbou

Salomonis Certonis consiliarii et secretarii Regis, domus et coronae Franciae, Geneva

Sig. A-I⁴ [I₄ blanc

Salomonis Certonis consiliarii et secretarii Regis, domus et coronae Franciae : Geneva

En versSig. A-I⁴ [I₄ blanc

Design of Broadband Linear Micromachined Ultrasonic Transducer Arrays by Means of Boundary Element Method Coupled with Normal Mode Theory

International audienceIn view of the maturity of fabrication processes for capacitive micromachined ultrasonic transducers (cMUTs), engineers and researchers now need efficient and accurate modeling tools to design linear arrays according to a set of technological specifications, such as sensitivity, bandwidth, and directivity pattern. A simplified modeling tool was developed to meet this requirement. It consists of modeling one element as a set of cMUT columns, each being a 1-D periodic array of cMUTs. Model description and assessment of simulation results are given in the first part of the paper. The approach is based on the theory of linear systems so the output data are linked to input data through a large matrix, known as an admittance matrix. In the second part of the paper, we propose reorganization of matrix equations by applying the normal mode theory. From the modal decomposition, two categories of eigenmodes are highlighted, one for which all cMUTs vibrate in phase (the fundamental mode) and the others, which correspond to localized subwavelength resonances, known as baffle modes. The last part of the paper focuses mainly on the fundamental mode and gives several design strategies to optimize the frequency response of an element