Potential applications such as high intensity focused ultrasound in medical therapeutics require larger output pressures. To offer unprecedented acoustic output pressure in transmit without the limitations, Capacitive Micromachined Ultrasonic Transducer operation modes of collapse and collapse-snapback are introduced in literature. Both operation modes require the membrane to contact the substrate surface, which poses a problem on the durability of the membrane in terms of structural integrity and tribological property. Based on the additional requirements of these modes, diamond is proposed as the ultimate solution to be used as the membrane material. Mechanical, thermal, and electrical properties of diamond are all in favor of its use in the microfabrication of CMUTs. This thesis introduces the design and test results of the first diamond-based CMUTs as an alternative to silicon and silicon nitride based CMUTs. Simulations are performed using Finite Element Methods (FEM) using a commercially available software package, ANSYS. The diamond-based CMUT is operated successfully both in air and immersion for the first time. Fully customizable in-house software is developed to command and control the test setup equipments for current dissertation and future work. Fresnel and Fraunhofer regions of the CMUT are characterized in sunflower oil using a combination of advanced hardware and software. The experimental results of radiation and diffraction for the diamond-based circular CMUT are verified by the theoretical calculations for a circular piston transducer. The results obtained from the first generation diamond-based CMUTs presented the diamond as a promising material for membranes in CMUTs.

Capacitive Micromachined Ultrasonic Transducers (CMUTs) have been introduced as a viable alternative to piezoelectric transducers in medical ultrasound imaging in the last decade. CMUTs are especially suitable for applications requiring small size such as catheter based cardiovascular applications. Despite these advantages and their broad bandwidth, earlier studies indicated that the overall sensitivity of CMUTs need to be improved to match piezoelectric transducers. This dissertation addresses this issue by introducing the dual-electrode CMUT concept. Dual electrode configuration takes advantage of leveraged bending in electrostatic actuators to increase both the pressure output and receive sensitivity of the CMUTs. Static and dynamic finite element based models are developed to model the behavior of dual-electrode CMUTs. The devices are then successfully fabricated and characterized. Experiments illustrate that the pulse echo performance is increased by more than 15dB with dual-electrode CMUTs as compared to single electrode conventional CMUT. Further device optimization is explored via membrane shape adjustment by adding a center mass to the design. Electromechanical coupling coefficient (kc2) is investigated as a figure of merit to evaluate performance improvement with non-uniform/uniform membrane dual-electrode CMUTs. When the center mass is added to the design, the optimized non-uniform membrane increases the electromechanical coupling coefficient from 0.24 to 0.85 while increasing one-way 3dB fractional bandwidth from 80% to 140% and reducing the DC bias requirement from 160V to 132V. The results of this modeling study are successfully verified by experiments. With this membrane shape adjustment, significant performance improvement (nearly 20dB) is achieved with the dual-electrode CMUT structure that enables the CMUT performance to exceed that of piezoelectric transducers for many applications.

Capacitive Micromachined Ultrasonic Transducers (CMUTs) have demonstrated significant potential to advance the state of medical ultrasound imaging beyond the capabilities of the currently employed piezoelectric technology. Because they rely on well-established micro-fabrication techniques, they can achieve complex geometries, densely populated arrays, and tight integration with electronics, all of which are required for advanced intravascular ultrasound (IVUS) applications such as high-frequency or forward-looking catheters. Moreover, they also offer higher bandwidth than their piezoelectric counterparts. Before CMUTs can be effectively used, they must be fully characterized and optimized through experimentation and modeling. Unfortunately, immersed transducer arrays are inherently difficult to simulate due to a phenomenon known as acoustic crosstalk, which refers to the fact that every membrane in an array affects the dynamic behavior of every other membrane in an array as their respective pressure fields interact with one another. In essence, it implies that modeling a single CMUT membrane is not sufficient; the entire array must be modeled for complete accuracy. Finite element models (FEMs) are the most accurate technique for simulating CMUT behavior, but they can become extremely large considering that most CMUT arrays contain hundreds of membranes. This thesis focuses on the development and application of a more efficient model for transducer arrays first introduced by Meynier et al. [1], which provides accuracy comparable to FEM, but with greatly decreased computation time. It models the stiffness of each membrane using a finite difference approximation of thin plate equations. This stiffness is incorporated into a force balance which accounts for effects from the electrostatic actuation, pressure forces from the fluid environment, mass and damping from the membrane, etc. For fluid coupling effects, a Boundary Element Matrix (BEM) is employed that is based on the Green's function for a baffled point source in a semi-infinite fluid. The BEM utilizes the nodal mesh created for the finite difference method, and relates the dynamic displacement of each node to the pressure at every node in the array. Use of the thin plate equations and the BEM implies that the entire CMUT array can be reduced to a 2D nodal mesh, allowing for a drastic improvement in computation time compared with FEM. After the model was developed, it was then validated through comparison with FEM. From these tests, it demonstrated a capability to accurately predict collapse voltage, center frequency, bandwidth, and pressure magnitudes to within 5% difference of FEM simulations. Further validation with experimental results revealed a close correlation with predicted impedance/admittance plots, radiation patterns, frequency responses, and noise current spectrums. More specifically, it accurately predicted how acoustic crosstalk would create sharp peaks and notches in the frequency responses, and enhance side lobes and nulls in the angular radiation pattern. Preliminary design studies with the model were also performed. They revealed that membranes with larger lateral dimensions effectively increased the bandwidth of isolated membranes. They also demonstrated potential for various crosstalk reduction techniques in array design such as disrupting array periodicity, optimizing inter-membrane pitch, and adjusting the number of membranes per element. It is expected that the model developed in this thesis will serve as a useful tool for future iterations of CMUT array optimizations.

Thin films have become very important in the past years as there is a tremendous increase in the need for small-scale devices. Thin films are preferred because of their electrical, mechanical, chemical, and other unique properties. They are often used for coatings, and in the fabrication of Microelectronic devices and Micro-electro Mechanical Systems (MEMS). Internal (residual) stress always exists when a thin film is employed in the device design. Residual and thermal stresses cause membrane bow, altering the anticipated dynamic response of a membrane-based MEMS design. The device may even become inoperable under the high stresses conditions. As a result, the stresses that act upon the membrane should be minimized for optimum operation of a MEMS device. In this research, the fabrication process parameters leading to low stress silicon nitride films were investigated. Silicon nitride was deposited using Plasma Enhanced Chemical Vapor Deposition (PECVD) and the residual stresses on these films were determined using a wafer curvature technique. By adjusting the silane (SiH4) and nitrogen (N2) gas flow rates, and the radiofrequency (RF) power; high quality silicon nitride films with residual stress as low as 11 MPa were obtained. Furthermore, an analytical study was also conducted to explore the effect of thermal stresses between layers of thin films on the MEMS device operation. In this thesis, we concentrated our efforts on three layers of thin films, as that is the most commonly encountered in a membrane based MEMS device. The results obtained from a parametric study of the membrane center deflection indicate that the deflection can be minimized by the appropriate choice of materials used. In addition, our results indicate that thin films with similar coefficient of thermal expansion should be employed in the design to minimize the deflection of the membrane, leading to anticipated device operation and increased yield. A complete understanding of the thermal and residual stress in MEMS structures can improve survival rate during fabrication, thereby increasing yield and ultimately reducing the device cost. In addition, reliability, durability, and overall performance of membrane-based structures are improved when substrate curvature and membrane deflection caused by stresses are kept at a minimum.

Capacitive micromachined ultrasonic transducers (CMUT:s) are often used in medical imaging and they show some promise as underwater transducers. This thesis collates the available information about how CMUT:s operate, their strengths and weaknesses and investigates their efficiency as an underwater transducer. The accumulated knowledge was channelled into a simulation of a CMUT as a dampened spring system done in MATLAB and Simulink. The simulation investigated the resonance frequency and bandwidth through simulation and compared the results to experimental results from literature.  CMUT:s have good acoustic matching with water making them sensitive, broadband transducers when used under water. Special care must be taken when choosing the CMUT so that materials and designs can fulfil the task for which it is intended, such as the radius of the membrane, the material of the membrane, the insulating layers in or around the CMUT and the height of the air gap inside. CMUT:s are, for the transmission of sound, less capable than existing lead zirconate-titanate-transducers (PZT-transducers). This problem can be somewhat alleviated through operating the CMUT in collapse-mode but care must be taken so that the CMUT is not damaged during this operation. Simulation results and results from literature show that it is possible to simulate CMUT:s with accuracy. By simulating 10 different CMUT:s, using the geometries and material properties of experimentally tested devices and testing for resonance frequency and bandwidth the results were as follows:The average relative error of resonance frequency was found to be -14 %, if outlier results are excluded and the average relative error of bandwidth proved inaccurate at -54 %

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.

The objective of this research is to develop novel capacitive micromachined ultrasonic transducer (CMUT) arrays for intravascular ultrasonic (IVUS) imaging along with the fabrication processes to allow for monolithic integration of CMUTs with custom CMOS electronics for improved performance. The IVUS imaging arrays include dual-ring arrays for forward-looking volumetric imaging in coronary arteries and annular-ring arrays with dynamic focusing capabilities for side-looking cross sectional imaging applications. Both are capable of integration into an IVUS catheter 1-2 mm in diameter. The research aim of monolithic integration of CMUTs with custom CMOS electronics has been realized mainly through the use of sloped sidewall vias less than 5 µm in diameter, with only one additional masking layer as compared to regular CMUT fabrication. Fabrication of CMUTs has been accomplished with a copper sacrificial layer reducing isolation layers by 50%. Modeling techniques for computational efficient analysis of CMUT arrays were developed for arbitrary geometries and further expanded for use with larger signal analysis. Dual-ring CMUT arrays for forward-looking volumetric imaging have been fabricated with diameters of less than 2 mm with center frequencies at 10 MHz and 20 MHz, respectively, for an imaging range from 1 mm to 1 cm. These arrays, successfully integrated with custom CMOS electronics, have generated 3D volumetric images with only 13 cables necessary. Performance from optimized fabrication has reduced the bias required for a dual-ring array element from 80 V to 42 V and in conjunction with a full electrode transmit array, it was shown that the SNR can be improved by 14 dB. Simulations were shown to be in agreement with experimental characterization indicated transmit surface pressure in excess of 8 MPa. For side-looking IVUS, three versions of annular CMUT arrays with dynamic focusing capabilities have been fabricated for imaging 1 mm to 6 mm in tissue. These arrays are 840 µm in diameter membranes linked to form 8 ring elements with areas that deviate by less than 25 %. Through modeling and simulation undesirable acoustic cross between ring elements was reduced from -13 dB to -22 dB.

Les cMUT sont des microsystèmes principalement utilisés pour de l’imagerie médicale. Afin de développer de nouvelles architectures de sondes, intégrer l’électronique de commande devient impératif. Pour y parvenir, la température du procédé de réalisation ne doit pas excéder 400°C. Cela nécessite donc de revoir les procédés et matériaux utilisés. Pour répondre à cette problématique, nous avons utilisé une électrode originale en siliciure de nickel obtenu à 400°C, une couche sacrificielle en nickel et une membrane en nitrure de silicium déposée à 200°C. Des cMUT ont été fabriqués sur un substrat silicium. Ils présentent les caractéristiques souhaitées à savoir une forte fréquence de résonance (16,4MHz), une tension de collapse maitrisée (65V) et un coefficient de couplage électromécanique satisfaisant (0,6). De plus, le procédé développé peut être étendu à d’autres types de substrats. / CMUTs are innovating microsystems for ultrasonic medical imaging. To develop new array architectures, monolithic integration of integrated circuits is required. In this context, microsystems must be achieved using process temperature limited to 400°C. The main objective of this PhD thesis is the development of alternative processes and materials to replace usual ones done at high temperature. We have developed a nickel silicide bottom electrode at 400°C, a metallic sacrificial layer and a silicon nitride membrane deposited at 200°C. The devices, fabricated on silicon substrates, are functional with a high resonance frequency (16.4MHz), a mastered collapse voltage (65V) and an efficient electromechanical coupling coefficient (0.6). Moreover, this low temperature process was successfully applied on other substrates such as glass.

This thesis presents the design and analysis for development of a Capacitive Micromachined Ultrasonic Transducer (CMUT), a novel sensor and actuator, aimed at replacing the conventional piezoelectric transducers for air-coupled ultrasonic imaging applications. These CMUTs are fabricated using the silicon micromachining technology wherein all fabrication is done on the surface of a silicon wafer by means of thin-film depositions, patterning with photolithography and etching. The main emphasis of this study is on developing analytical models that serve as effective design tools for the development of these devices. A desirable goal of such study is to create reasonable mathematical models, obtain analytical solutions, wherever possible, for various measures of transducer performance and provide design aids. A logical start is the lumped parameter modeling wherein the explicit dependence of the physical parameters on the spatial extent of the device is ignored. The system lumped parameters, such as the equivalent stiffness, the equivalent mass, and the equivalent damping are extracted from reasonable analytical or numerical models and subsequently used in the static and dynamic analysis of the device. Useful predictions are made with regard to the key transducer parameters, such as, the pull-in voltage, the static deflection, the dynamic response and the acoustic field produced. The modeling work presented embodies two main objectives: (i) it serves to provide direction in the design phase, and, (ii) it serves to aid in the extraction of critical parameters which affect the device behavior. Comparison of the results with the more rigorous FEM simulations as well as with those present in the existing literature assure that the developed models are accurate enough to serve as useful design tools. The distributed parameter modeling is presented next. Analysis of MEMS devices which rely on electrostatic actuation is complicated due to the fact that the structural deformations alter the electrostatic forces, which redistribute and modify the applied loads. Hence, it becomes imperative to consider the electro-elastic coupling aspect in the design of these devices. An approximate analytical solution for the static deflection of a thin, clamped circular plate caused by electrostatic forces which are inherently nonlinear, is presented. The model is based on the Kirchhoff-Love assumptions that the plate is thin and the deflections and slopes are small. The classical thin-plate theory is adequate when the ratio of the diameter to thickness of the plate is very large, a situation commonly prevalent in many MEMS devices, especially the CMUTs. This theory is used to determine the static deflection of the CMUT membrane due to a DC bias voltage. The thin-plate electro-elastic equation is solved using the Galerkin weighted residual technique under the assumption that the deflections are small in comparison to the thickness of the plate. The results obtained are compared to those obtained from ANSYS simulations and an excellent agreement is observed between the two. The pull-in voltage predicted by our model is close to the value predicted by ANSYS simulations. A simple analytical formula, which gives fairly accurate results (to within 3% of the value predicted by ANSYS simulations) for determination of the pull-in voltage, is also presented. As stated, this formula accounts for the elastic deflection of the membrane due to the coupled interaction with the electrostatic field. The effect of vacuum sealing the backside cavity of a CMUT is investigated in some detail. The presence or absence of air inside the cavity has a marked effect upon the system parameters, such as the natural frequency and the pull-in voltage. The possibility of using sealed CMUT cavities with air inside at ambient pressure is explored. In order to estimate the transducer loss due to the presence of air in the sealed cavity, the squeeze film forces resulting from the compression of the trapped air film are evaluated. Towards this end, the linearized Reynolds equation is solved in conjunction with the appropriate boundary conditions, taking the flexure of the membrane into account. From this analysis, it is concluded that, for a sealed CMUT cavity, the presence of air does not cause any squeeze film damping even when the flexure of the membrane is taken into account (the case of a rigid plate is already known). Although the emphasis of the study undertaken here is not on the physical realization of a working CMUT, a single cell as well as a linear array based on the design presented here, were fabricated (in a foundry elsewhere) in order to verify some of the most fundamental device parameters from experimental measurements. The fabricated devices have been characterized for their resonant frequency, quality factor, and structural integrity. These tests were conducted using the laser Doppler vibrometer and the Focused Ion Beam milling. Having investigated thoroughly the behavior of a single cell, we proceed to demonstrate how these cells can be arranged optimally in the form of an array to provide a comprehensive ultrasonic imaging system. A thorough analysis of the requirements for the array architecture is undertaken to determine the optimal configuration. The design constraints that need to be taken into account for CMUT arrays, especially for NDE applications, are presented. The main issue of designing an array consisting of a large number of CMUT cells required for producing a pressure wave of sufficient strength which is detectable upon reflection from the desired location even after suffering severe attenuation resulting from propagation in various media is addressed. A scalable annular array architecture of CMUT cells is recommended based on the analysis carried out.

Les dispositifs d’isolation galvanique intégrés au sein des systèmes de commande d’interrupteurs de puissance doivent répondre à une demande accrue en performance, facilité d’intégration et efficacité énergétique. Les transducteurs ultrasonores capacitifs micro-usinés (cMUT : capacitive Micromachined Ultrasonic Transducer), capables d’émettre et de recevoir des ondes ultrasonores, semblent une alternative tout à fait nouvelle à la fonction d’isolation galvanique. Ces travaux de thèse ont pour objectif de démontrer la faisabilité d’un dispositif basé sur la technologie cMUT. Le principe de fonctionnement consiste à transmettre une information grâce à une communication par onde acoustique de volume entre deux réseaux de cMUT placés de part et d’autre d’un substrat. Nous focalisons, en premier lieu, ces travaux sur le processus de fabrication par micro-usinage de surface des cMUT ainsi que les techniques de réalisation des dispositifs en structure double face sur substrat de silicium. L’étude permet d’identifier le collage de substrat comme une solution de fabrication industrialisable. Suite à la réalisation des dispositifs, la caractérisation électro-mécanique des cMUT est une étapeessentielle à la validation de leur fonctionnalité en tant que dispositifs émetteurs. L’étude débute par uneévaluation des propriétés mécaniques du matériau constituant la membrane et qui impactent directementle comportement global des cMUT. Puis, la caractérisation du comportement statique et dynamique descMUT permet d’extraire les paramètres tels que la fréquence de résonance, la tension de collapse etl’efficacité électro-mécanique qui définissent le mode de pilotage d’un tel système.Finalement, la validation du concept de transmission et de détection d’ondes ultrasonores est réaliséegrâce à des mesures de vibrométrie laser Doppler. Les résultats apportent des éléments de réponse quantau mode de propagation des ondes et permettent d’identifier les topologies de meilleure efficacité entransmission acoustique. Enfin, l’intégration du prototype dans l’application de commanded’interrupteur de puissance démontre la faisabilité du concept de transformateur acoustique basé sur latechnologie cMUT. / Galvanic isolation devices integrated into switch command systems must be able to answer all of the increasing demand for performance, energetic efficiency and integration easiness. The capacitive micro machined ultrasonic transducers (cMUT), able to emit and receive ultrasounds, could be an entirely new alternative to the function of galvanic isolation. This work aims to demonstrate the feasibility of a cMUT-based device. The operating principle consists in transmitting information thanks to a bulk acoustic wave between two cMUT arrays located on both sides of a substrate. We first focus on cMUT surface micromachining fabrication process and techniques of double-side device manufacturing. Our study allows us to identify wafer bonding as a realistic industrial solution. After device fabrication, electro-mechanical of cMUT is an essential step to validate their functionality as ultrasonic emitters. The study starts with the mechanical properties evaluation of the membrane material. These properties directly impact the global behavior of cMUT. Then, the characterization of cMUT static and dynamic behavior allows extracting parameters as resonance frequency, collapsing voltage and electro-mechanical efficiency which define the actuation mode of such a system. Finally, the validation of transmission and reception of ultrasonic waves is evaluated by vibrometer laser Doppler measurements. Results bring elements concerning the waves propagation modes and allow identifying the best acoustical efficiency in regard to the topology. In conclusion, the prototype integration in the application of power switch command demonstrates the feasibility of acoustic transformer concept based on cMUT technology.