Ultrasonic transducers are used in many fields of daily life, e.g. as parking aids or medical devices. To enable their usage also for mass applications small and low- cost transducers with high performance are required. Capacitive, micro-machined ultrasonic transducers (CMUT) offer the potential, for instance, to integrate compact ultrasonic sensor systems into mobile phones or as disposable transducer for diverse medical applications. This work is aimed at providing fundamentals for the future commercialization of CMUTs. It introduces novel methods for the acoustic simulation and characterization of CMUTs, which are still critical steps in the product development process. They allow an easy CMUT cell design for given application requirements. Based on a novel electromechanical model for CMUT elements, the device properties can be determined by impedance measurement already. Finally, an end-of-line test based on the electrical impedance of CMUTs demonstrates their potential for efficient mass production.

Ultrasonic transducers are used in many fields of daily life, e.g. as parking aids or medical devices. To enable their usage also for mass applications small and low- cost transducers with high performance are required. Capacitive, micro-machined ultrasonic transducers (CMUT) offer the potential, for instance, to integrate compact ultrasonic sensor systems into mobile phones or as disposable transducer for diverse medical applications. This work is aimed at providing fundamentals for the future commercialization of CMUTs. It introduces novel methods for the acoustic simulation and characterization of CMUTs, which are still critical steps in the product development process. They allow an easy CMUT cell design for given application requirements. Based on a novel electromechanical model for CMUT elements, the device properties can be determined by impedance measurement already. Finally, an end-of-line test based on the electrical impedance of CMUTs demonstrates their potential for efficient mass production.

Ces travaux présentent l'étude de transformateurs par voie acoustique, basés sur la technologie CMUT (Capacitive Micromachined Ultrasonic Transducer ), visant à développer des composants monolithiques assurant l'isolation électrique au sein de la commande des interrupteurs à semi-conducteurs. S'agissant de microsystèmes électromécaniques, les CMUTs offrent des perspectives intéressantes en terme d'intégrabilité monolithique avec les interrupteurs à semi-conducteurs. L'architecture proposée est constituée de deux transducteurs CMUTs de part et d'autre d'un substrat en silicium. Un outil de modélisation a été développé dans le but de prédire le comportement du transformateur. Des protocoles de mesure du rendement des dispositifs fabriqués ont été mis en place permettant une évaluation quantitative des performances des prototypes (un rendement de 32 % est atteint avec une marge de progression à 60 %). L'exploitation du modèle développé, et validé par les résultats de caractérisation, a permis de mettre en évidence les limites et perspectives d'amélioration de ces dispositifs. / This work is a study of CMUT (Capacitive Micromachined Ultrasonic Transduer)based acoustical transformers as a step in the development of insulating components in semiconductor switches control chain. CMUT transducers being electromechanical systems (MEMS), their monolithic integration with semiconductor switches is full of interesting perspectives . The proposed architecture consists of two CMUTs layered on each side of a silicon substrate. A computational tool was designed to predict the behaviour of the transformer. Measurement protocols of the power efficiency of the constructed transformers were set up and allowed to quantify the prototypes' performances (A 32 % efficiency is currently reached, with improvements attainable up to 60 %). Exploring the results of the developed model, validated by bench measurements, allowed to determine the current limits of the transformers as well as perspectives of improvement.

O presente trabalho propõe o estudo de dispositivos CMUTs fabricados através de métodos e materiais alternativos com potencial aplicação em ensaios não destrutivos. O CMUT é constituído por uma membrana condutiva colocada sobre uma cavidade com o fundo metalizado, formando uma estrutura capacitiva. Ondas acústicas incidentes criam uma vibração na membrana que modifica de forma detectável a capacitância do dispositivo (modo de detecção). De modo inverso, é possível aplicar tensão alternada sobre a estrutura do CMUT fazendo com que a membrana vibre gerando ondas acústicas (modo de emissão). Hoje as principais tecnologias utilizadas para detecção e emissão de ondas acústicas utilizam materiais piezelétricos que são de difícil obtenção, possuem baixa integrabilidade e restrições em aplicações em altas temperaturas. O transdutor proposto baseia-se na tecnologia MEMS onde efeitos elétricos e mecânicos são combinados de forma a atender uma determinada aplicação. Os dispositivos MEMS são fabricados por técnicas de microfabricação, o que diminui o custo de produção e possibilita o uso de diversos tipos de materiais. Antes da fabricação dos CMUTs, foram feitas modelagens analíticas e computacionais visando determinar as dimensões ideais dos dispositivos a serem fabricados e também quais materiais seriam melhores se aplicados como membrana. A partir dos resultados da modelagem foi possível determinar que CMUTs com membrana de cobre fossem fabricados. Com os resultados da modelagem foram propostos métodos de fabricação baseados em técnicas de wafer-bonding. Dois materiais foram utilizados para produzir as cavidades, um deles foi o fotorresiste SU-8 e outro uma resina adesiva de secagem rápida, material este não convencional para produção de CMUTs. No primeiro caso, as cavidades foram abertas por processo de fotolitografia, já no segundo foi utilizado um método de corrosão a laser, também não convencional. Diferentes métodos foram utilizados na colagem das membranas, sendo que membranas coladas com resina adesiva e membranas deixadas suspensas sobre as cavidades apresentaram melhores resultados. As caracterizações elétricas e acústicas realizadas nos dispositivos fabricados mostraram que os CMUTs obtidos possuem características capacitivas e podem emitir e receber, comprovando assim seu funcionamento. Isto indica que os roteiros de fabricação propostos são eficientes e que os materiais e métodos utilizados se mostraram adequados. Além disso, a partir das caracterizações acústicas é possível dizer que os CMUTs fabricados podem ser potencialmente aplicados em ensaios não destrutivos, principalmente em análise de vibrações. / This thesis proposes the study of CMUT devices produced using alternative methods and materials with potential application in non-destructive analysis. CMUT is composed of a metallized substrate covered with an insulator cavity structure and a conductive membrane on its top, forming a capacitive structure. Incident acoustic waves cause vibration in the membrane modifying their capacitance (detection mode). AC voltage can be applied on the structure of the CMUT causing the membrane to vibrate generating acoustic waves (transmission mode). Today the main technologies used for the detection and emission of acoustic waves use piezoelectric materials that are difficult to obtain, have low integrability and restrictions in high temperature applications. The proposed transducer is based on MEMS technology where electrical and mechanical effects are combined to suit a particular application. MEMS devices are manufactured by microfabrication techniques, which reduce the cost of production and allow the use of various types of materials. Before the fabrication of CMUTs, analytical and computational modeling were made to determine the optimal dimensions of the devices to be manufactured and also what materials would be best if applied as membrane. From the modeling results CMUTs with copper membrane were manufactured. Based on the results of the modeling process, manufacturing methods based on wafer-bonding techniques were proposed. Two materials were used to produce the cavities: one of them was the SU-8 photoresist and the other an acrylic adhesive, an unconventional material for the production of CMUTs. In the first case the cavities were opened by the photolithographic process. In the second method the corrosion laser was used, which is also unconventional. Different methods were used to bond the membranes. The membranes bonded with acrylic adhesive and left suspended over the cavity wells showed better results. The electric and acoustic characterizations performed on fabricated devices showed that CMUTs present capacitive characteristics and can emit and receive acoustic waves, thus proving its operation. This indicates that the proposed manufacturing routes are efficient and that the materials and methods used are appropriate. Moreover, these results show that the manufactured CMUTs may be potentially applied to nondestructive analyses, especially in analyses of vibrations.

Les produits de contraste ultrasonore constituent un véritable apport pour l’imagerie échographique et sont aujourd’hui utilisés en clinique pour l’évaluation de la perfusion cardiaque ou encore la détection de tumeurs. Depuis quelques années, les transducteurs capacitifs micro-usinés (cMUTs) se présentent comme une alternative intéressante aux transducteurs piézoélectriques classiques et offrent certains avantages comme une large bande passante. Nous proposons dans cette thèse d’évaluer le potentiel de cette technologie pour l’imagerie de contraste. Dans un premier temps, notre étude s’est orientée vers l’adaptation des cMUTs à l’imagerie non linéaire. Ensuite, de nouvelles méthodes de détection de contraste, basées sur le comportement spécifique des microbulles, ont été développées pour exploiter les avantages de la technologie cMUT. Comparés aux méthodes conventionnelles, les résultats obtenus montrent une meilleure visualisation des agents de contraste par rapport aux tissus environnants. L’utilisation de cMUTs améliore l’efficacité de ces méthodes démontrant, ainsi, leur intérêt pour l’imagerie de contraste. / Using ultrasound contrast agents, many clinical diagnoses have now been improved thanks to new contrast dedicated imaging techniques. Contrast agents are now used routinely in cardiology and in radiology to improve the detection and visualization of blood perfusion in various organs (e.g. tumors). Since a few years, Capacitive Micromachined Ultrasonic Transducers (cMUTs) have emerged as a good alternative to traditional piezoelectric transducer. cMUTs provide several advantages such as wide frequency bandwidth which could be further developed for nonlinear imaging. In this dissertation, we propose to exploit cMUT for contrast agent imaging. Firstly, the excitation signal was adapted to suppress the inherent nonlinear behavior of cMUT. Then, new detection methods based on specific acoustic properties of microbubbles have been developed and evaluated with a cMUT probe. Results show a good suppression from tissue responses whereas echoes from microbubbles are enhanced. Furthermore, the efficiency of each method is improved by the use of cMUT revealing the potential of this new transducer technology for contrast agent detection.

La transduction électrostatique est utilisée depuis plusieurs décennies dans les fréquences du domaine audible, principalement sous la forme de microphones membranaires. La transposition du même principe de transduction, mais dans un domaine de fréquence au-dessus du MHz, et par l’utilisation de dispositifs micro-usinés, c'est-à-dire produits à l’aide de technologies de photolithographie, a été proposée à partir de la fin des années 1990. Ces transducteurs, désignés sous l’acronyme cMUT (capacitive micromachine ultrasonic transducers), se composent d’un assemblage de transducteurs élémentaires, chacun possédant une partie mobile conventionnellement appelée diaphragme ou membrane, actionnée par la pression électrostatique. Cette thèse s’inscrit dans le développement de transducteurs de ce type destinés au domaine de l’imagerie médicale ultrasonore. Ce secteur d’application utilise actuellement des transducteurs basés sur des céramiques (ou, dans certains cas précis, des polymères) piézoélectriques. Le cMUT est intéressant dans certains sous-domaines d’application des ultrasons médicaux en raison de sa bonne adaptation à une production en grande série, de son intégration plus facile avec des éléments électroniques, de son faible échauffement et de l’absence de matériaux toxiques dans son processus de fabrication. La partie théorique de cette thèse repose sur une approche de modélisation par différences finies. Un modèle basé sur la théorie des plaques minces est développé pour prendre en compte la mécanique du transducteur élémentaire cMUT (c'est-à-dire d’un seul diaphragme). Ce modèle est ensuite complété par l’intégration de l’effet d’un chargement acoustique par un fluide. De façon à modéliser un transducteur entier, il est nécessaire de prendre en compte le couplage acoustique existant entre les différentes membranes. Pour rendre cela possible, un circuit équivalent, permettant de réduire chaque membrane à un système à un seul degré de liberté, est développé. Il est validé en le comparant au modèle de différences finies dans des cas où celui-ci peut être utilisé. Les travaux expérimentaux ont fait appel principalement aux deux techniques de caractérisation suivantes : les mesures d’impédance électrique, et les mesures de déplacement effectuées par interférométrie laser. Ces mesures ont été utilisées dans une double optique. D’une part, dans un objectif de caractérisation, ils ont permis de vérifier la fonctionnalité des dispositifs fabriqués et d’évaluer leurs performances. D’autre part, en comparant différentes configurations entre elles, ils ont rendu possible une validation expérimentale du modèle qui a été mis au point. / Electro-acoustic transduction based on electrostatic force has a long history in the range of audible frequencies, mainly as membrane-based microphones. Starting in the late 1990’s, it has been proposed to use the same principle in the multi-MHz frequency domain, thanks to micro-machined devices – meaning they’re produced through lithography technology. Such transducers, known as cMUT for capacitive micromachine ultrasonic transducers, are made of an assembly of elementary vertically mobile cells, usually designated as membranes, driven by electrostatic force. This PhD work is part of the development of this kind of transducers designed for medical imaging applications. This area currently uses transducers based on piezoelectric ceramics (or piezoelectric polymers for some peculiar cases). CMUT is an interesting alternative for some subdomains of medical ultrasound applications, due to its volume production ability, its easier integration with electronic elements, its low heat dissipation and the absence of toxic materials.

Les transducteurs ultrasonores capacitifs micro-usinés (CMUT) représentent aujourd’hui une réelle alternative aux technologies piézoélectriques dans le domaine de l’imagerie échographique médicale. Au cours des années, les procédés de fabrication des transducteurs se sont enrichis en vue d’améliorer leurs performances. A contrario le choix du substrat, généralement en silicium, a été peu étudié. Il est cependant reconnu que le support contribue à la signature acoustique du dispositif ultrasonore. L’objectif de ces travaux de thèse a été d’intégrer une couche de silicium poreux afin d’absorber une partie des ondes élastiques qui se propagent dans le substrat et interfèrent avec le signal acoustique émis. Nous montrons alors qu’il été possible de réaliser une couche de silicium poreux en face arrière de composants, sur plaquettes 6 pouces, sans dégrader leurs performances. Finalement, par l’intermédiaire de caractérisations acoustiques et des signatures impulsionnelles des transducteurs, nous révélons le potentiel prometteur de ce matériaux pour la réalisation de milieu arrière atténuant dédié à la transduction ultrasonore. / Capacitive micromachined ultrasonic transducers (CMUT) have emerged as a potential alternative to traditional piezoelectric transducers for ultrasound imaging. Along the years, CMUT processes have been evolved to enhance the device performances. In the meantime, no particular attention was paid on the silicon substrate, even if it is well-known that it could contribute to the transducer efficiency. The aim of this PhD thesis was to use porous silicon as a backing material for ultrasonic transducers to absorb a piece of the acoustic wave propagating in the substrate and which induce crosstalks in the acoustic signal. We show that porous silicon layer can be obtained on the rear side of already processed wafers without any damage on the performances of capacitive micromachined ultrasonic transducers. Finally, by means of acoustic characterizations and the transducer electroacoustic responses, we reveal the potential interest of porous silicon as backing material for ultrasonic transducers.

O presente trabalho propõe o estudo de dispositivos CMUTs fabricados através de métodos e materiais alternativos com potencial aplicação em ensaios não destrutivos. O CMUT é constituído por uma membrana condutiva colocada sobre uma cavidade com o fundo metalizado, formando uma estrutura capacitiva. Ondas acústicas incidentes criam uma vibração na membrana que modifica de forma detectável a capacitância do dispositivo (modo de detecção). De modo inverso, é possível aplicar tensão alternada sobre a estrutura do CMUT fazendo com que a membrana vibre gerando ondas acústicas (modo de emissão). Hoje as principais tecnologias utilizadas para detecção e emissão de ondas acústicas utilizam materiais piezelétricos que são de difícil obtenção, possuem baixa integrabilidade e restrições em aplicações em altas temperaturas. O transdutor proposto baseia-se na tecnologia MEMS onde efeitos elétricos e mecânicos são combinados de forma a atender uma determinada aplicação. Os dispositivos MEMS são fabricados por técnicas de microfabricação, o que diminui o custo de produção e possibilita o uso de diversos tipos de materiais. Antes da fabricação dos CMUTs, foram feitas modelagens analíticas e computacionais visando determinar as dimensões ideais dos dispositivos a serem fabricados e também quais materiais seriam melhores se aplicados como membrana. A partir dos resultados da modelagem foi possível determinar que CMUTs com membrana de cobre fossem fabricados. Com os resultados da modelagem foram propostos métodos de fabricação baseados em técnicas de wafer-bonding. Dois materiais foram utilizados para produzir as cavidades, um deles foi o fotorresiste SU-8 e outro uma resina adesiva de secagem rápida, material este não convencional para produção de CMUTs. No primeiro caso, as cavidades foram abertas por processo de fotolitografia, já no segundo foi utilizado um método de corrosão a laser, também não convencional. Diferentes métodos foram utilizados na colagem das membranas, sendo que membranas coladas com resina adesiva e membranas deixadas suspensas sobre as cavidades apresentaram melhores resultados. As caracterizações elétricas e acústicas realizadas nos dispositivos fabricados mostraram que os CMUTs obtidos possuem características capacitivas e podem emitir e receber, comprovando assim seu funcionamento. Isto indica que os roteiros de fabricação propostos são eficientes e que os materiais e métodos utilizados se mostraram adequados. Além disso, a partir das caracterizações acústicas é possível dizer que os CMUTs fabricados podem ser potencialmente aplicados em ensaios não destrutivos, principalmente em análise de vibrações. / This thesis proposes the study of CMUT devices produced using alternative methods and materials with potential application in non-destructive analysis. CMUT is composed of a metallized substrate covered with an insulator cavity structure and a conductive membrane on its top, forming a capacitive structure. Incident acoustic waves cause vibration in the membrane modifying their capacitance (detection mode). AC voltage can be applied on the structure of the CMUT causing the membrane to vibrate generating acoustic waves (transmission mode). Today the main technologies used for the detection and emission of acoustic waves use piezoelectric materials that are difficult to obtain, have low integrability and restrictions in high temperature applications. The proposed transducer is based on MEMS technology where electrical and mechanical effects are combined to suit a particular application. MEMS devices are manufactured by microfabrication techniques, which reduce the cost of production and allow the use of various types of materials. Before the fabrication of CMUTs, analytical and computational modeling were made to determine the optimal dimensions of the devices to be manufactured and also what materials would be best if applied as membrane. From the modeling results CMUTs with copper membrane were manufactured. Based on the results of the modeling process, manufacturing methods based on wafer-bonding techniques were proposed. Two materials were used to produce the cavities: one of them was the SU-8 photoresist and the other an acrylic adhesive, an unconventional material for the production of CMUTs. In the first case the cavities were opened by the photolithographic process. In the second method the corrosion laser was used, which is also unconventional. Different methods were used to bond the membranes. The membranes bonded with acrylic adhesive and left suspended over the cavity wells showed better results. The electric and acoustic characterizations performed on fabricated devices showed that CMUTs present capacitive characteristics and can emit and receive acoustic waves, thus proving its operation. This indicates that the proposed manufacturing routes are efficient and that the materials and methods used are appropriate. Moreover, these results show that the manufactured CMUTs may be potentially applied to nondestructive analyses, especially in analyses of vibrations.

L’imagerie photoacoustique est une nouvelle modalité couplant imagerie optique et échographie. Non invasive, elle permet d’imager des absorbeurs optiques à quelques centimètres de profondeur et avec la résolution de l’échographie. La réception des signaux photoacoustiques se faisant à l’aide d’un échographe clinique, cette modalité hybride vient compléter idéalement l’imagerie ultrasonore en apportant des informations fonctionnelles aux informations structurelles de l’échographie. Ces atouts en font une technique d’imagerie très prometteuse pour la clinique, notamment comme outil de diagnostic précoce. Ce travail de thèse a pour objectif principal la mise en place des outils nécessaires au développement de cette thématique de recherche d’un point de vue expérimental, à des fins cliniques. En particulier, l’apport de l’imagerie photoacoustique pour le diagnostic de plaques d’athérome vulnérables est investigué sur fantômes, grâce à un protocole original. Pour cela un système d’imagerie photoacoustique a été développé et caractérisé à l’aide de fantômes bimodalités élaborés spécifiquement pour les différentes études présentées. Gardant à l’esprit le transfert de cette technique en clinique, un échographe clinique de recherche est utilisé et différentes spécificités du banc nécessaires à l’imagerie in vivo et au diagnostic médical ont été investiguées. Cela implique une amélioration des performances de détection du signal photoacoustique, notamment en termes de sensibilité et de contraste. Pour cela, une nouvelle technologie de sondes ultrasonores est évaluée en la comparant aux sondes actuellement utilisées. De plus, une excitation multispectrale permet l’identification de différents éléments présents dans les tissus. L’aspect temps-réel de l’échographie fait de cette modalité une des plus utilisées pour le diagnostic clinique. Par conséquent, une imagerie photoacoustique voire bimodale en temps réel présente un réel atout pour son transfert clinique. Cette possibilité est investiguée sur le système mis en place au cours de la thèse grâce à un échographe de recherche et une étude sur fantômes. Enfin, une autre contribution de ce travail concerne l’apport de l’imagerie photoacoustique à la caractérisation de la vulnérabilité de la plaque d’athérome. Cette indication de vulnérabilité est obtenue en déterminant la composition de la plaque, en particulier en termes de lipides. L’imagerie photoacoustique, couplée à l’échographie, peut permettre cette identification. Pour étudier cette possibilité, nous nous sommes intéressés à l’artère carotide pour son accessibilité et la place qu’elle occupe dans le diagnostic de la plaque d’athérome en échographie et échographie Doppler. Un protocole original a été élaboré afin d’apporter l’excitation optique au plus près de la carotide. La faisabilité de cette approche est investiguée sur un fantôme conçu spécifiquement pour cette étude et les résultats préliminaires sont présentés. / Photoacoustic (PA) imaging is a new imaging modality coupling ultrasound and optical imaging. This non-invasive technique achieves a penetration depth up to several centimeters with optical contrast and ultrasound resolution. Moreover, since PA signals are detected with a US scanner, PA imaging ideally complete US imaging, adding functional information to the structural ones brought by echography. Therefore PA imaging looks very promising, specifically as a clinical early diagnosis tool. The main objective of this thesis is to set up the required tools to develop the experimental investigation for this research topic and, in particular, to apply it to the diagnosis of vulnerable atheroma plaques. A PA imaging system has been set up and characterized using specifically designed bimodal phantoms. Additional studies have been made to evaluate the suitability of this imaging platform for clinical imaging. For example, in vivo imaging requires better signal detection in terms of contrast and sensitivity, achieved thanks to a new probe technology, and the identification of tissue composition using a multispectral optical excitation. Finally, PA and even PAUS real time imaging is a real asset for medical diagnosis that has been investigated. Another contribution of this work is the use of PA imaging to characterized atheroma plaques vulnerability with the detection of lipids inside these plaques. PA imaging, coupled to echography, can address this need. To study this possibility, the carotid artery has been considered and a new protocol has been elaborated to bring the optical excitation very close to this artery. A feasibility study has been realized on a specific phantom and the preliminary results are presented.

Les produits de contraste ultrasonore constituent un véritable apport pour l'imagerie échographique et sont aujourd'hui utilisés en clinique pour l'évaluation de la perfusion cardiaque ou encore la détection de tumeurs. Depuis quelques années, les transducteurs capacitifs micro-usinés (cMUTs) se présentent comme une alternative intéressante aux transducteurs piézoélectriques classiques et offrent certains avantages comme une large bande passante. Nous proposons dans cette thèse d'évaluer le potentiel de cette technologie pour l'imagerie de contraste. Dans un premier temps, notre étude s'est orientée vers l'adaptation des cMUTs à l'imagerie non linéaire. Ensuite, de nouvelles méthodes de détection de contraste, basées sur le comportement spécifique des microbulles, ont été développées pour exploiter les avantages de la technologie cMUT. Comparés aux méthodes conventionnelles, les résultats obtenus montrent une meilleure visualisation des agents de contraste par rapport aux tissus environnants. L'utilisation de cMUTs améliore l'efficacité de ces méthodes démontrant, ainsi, leur intérêt pour l'imagerie de contraste.