Academic literature on the topic 'CMUT'

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.

Close integration of front-end electronics and the transducer array within the catheter is critical for successful implementation of CMUT-based intravascular ultrasound (IVUS) imaging catheters to enable next generation imaging tools. Therefore, this research developed and implemented custom-designed electronic circuits and systems integrated with an IC compatible transducer technology for realization of miniature IVUS imaging catheters operating at 10-50 MHz frequency range. In one path of this research, an IC is custom designed in a 0.35-um CMOS process to monolithically integrate with a CMUT array (CMUT-on-CMOS) to realize a single-chip, highly-flexible, forward-looking (FL) IVUS imaging system. The amplifiers that are custom-designed achieved transducer thermal-mechanical noise dominated receive performance in a CMUT-on-CMOS implementation. In parallel to the FL-IVUS effort, for realization of a side-looking IVUS catheter based on an annular phased array, a dynamic receive beamformer IC is custom designed also in a 0.35-um CMOS process. Overall, the circuits and systems developed as part of this dissertation form a critical step in the translation of the research on CMUT-based IVUS catheters into real clinical applications for better management of coronary arterial diseases.

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.

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<br>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

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<br>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

Ultrasound imaging is a well-established medical diagnostic technique. Compared with other imaging modalities, such as for example X-ray, ultrasound is harmless to the patient and less expensive while providing real-time imaging capability with adequate resolution for most applications. Piezoelectric materials have dominated the ultrasound transducers technology for a long time but, thanks to the intense research activity in recent years, capacitive micromachined ultrasonic transducers (CMUT) are emerging as a competitive alternative for next generation imaging systems. The objective of the thesis is to analyze the ultrasound system, when a CMUT is used instead of a piezoelectric transducer, to identify and design the best integrated solution to optimize the front-end performance. After giving an overview of the ultrasound system and the Capacitive Micromachined Ultrasonic Transducer (CMUT) in Chapter 1, Chapter 2 presents a thorough comparison between RX amplifier alternatives. The impact on the pulse-echo frequency response and SNR is assessed. The study demonstrates that a capacitive-feedback stage provides a remarkable improvement in the noise-power performance compared to the very popular resistive-feedback amplifier, at the expense of a low-frequency shift of the pulse-echo response, making it suitable for integration of dense CMUT arrays for low and mid-frequency ultrasound imaging applications. Then, Chapter 3 proposes the design of a CMUT front-end circuits comprising a TX driver, T/R switch and RX amplifier. Realized in BCD8-SOI technology from STMicroelectronics, the TX delivers up to 100V pulses, while the RX shows 70dB dynamic range with very low noise at 1mW only power dissipation. Measurement results and imaging experiments are presented and discussed. In Chapter 4, the non-linear behavior of the CMUT is discussed and possible solution proposed. Experimental results demonstrate a significant reduction of the second-harmonic distortion, estimated to be lower than -30 dB, resulting in good linearization for typical nonlinear imaging operation. In addition, Chapter 5 shows a novel amplifier architecture exploiting the regeneration feature of the cross-coupled pair. It will be used as Programmable Gain Amplifier (PGA) in the ultrasound chain. A test-chip in 0.18 μm CMOS provides 15dB to 66dB gain over 50MHz bandwidth. With state-of-the-art noise and linearity performance, a record GBW up to 100GHz is demonstrated with only 420 μW power dissipation.

Recent advances in medical imaging have greatly improved the success of cardiovascular and intracardiac interventions. This research aims to improve capacitive micromachined ultrasonic transducers (CMUT) based imaging catheters for intravascular ultrasound (IVUS) and intra-cardiac echocardiography (ICE) for 3-D volumetric imaging through integration of high-k thin film material into the CMUT fabrication and array design. CMUT-on-CMOS integration has been recently achieved and initial imaging of ex-vivo samples with adequate dynamic range for IVUS at 20MHz has been demonstrated; however, for imaging in the heart, higher sensitivities are needed for imaging up to 4-5 cm depth at 20MHz and deeper at 10MHz. Consequently, one research goal is to design 10-20MHz CMUT arrays using integrated circuit (IC) compatible micro fabrication techniques and optimizing transducer performance through high-k dielectrics such as hafnium oxide (HfO2). This thin film material is electrically characterized for its dielectric properties and thermal mechanical stress is measured. Experiments on test CMUTs show a +6dB improvement in receive (Rx) sensitivity, and +6dB improvement in transmit sensitivity in (Pa/V) as compared to a CMUT using silicon nitride isolation (SixNy) layer. CMUT-on-CMOS with HfO2 insulation is successfully integrated and images of a pig-artery was successfully obtained with a 40dB dynamic range for 1x1cm2 planes. Experimental demonstration of side looking capability of single chip CMUT on CMOS system based FL dual ring arrays supported by large signal and FEA simulations was presented. The experimental results which are in agreement with simulations show promising results for the viability of using FL-IVUS CMUT-on-CMOS device with dual mode side-forward looking imaging. Three dimensional images were obtained by the CMUT-on-CMOS array for both a front facing wire and 4 wires that are placed perpendicular to the array surface and ~4 mm away laterally. For a novel array design, a dual gap, dual frequency 2D array was designed, fabricated and verified against the large signal model for CMUTs. Three different CMUT element geometries (2 receive, 1 transmit) were designed to achieve ~20MHz and ~40MHz bands respectively in pulse-echo mode. A system level framework for designing CMUT arrays was described that include effects from imaging design requirements, acoustical cross-talk, bandwidths, signal-to-noise (SNR) optimization and considerations from IC limitations for pulse voltage. Electrical impedance measurements and hydrophone measurements comparisons between design and experiment show differences due to inaccuracies in using SixNy homogenous material in simulation compared to fabricated thin-film stacks (HfO2-AlSi-SixNy). It is concluded that for “thin” membranes the effect of stiffness and mass of HfO2 and AlSi (top electrode) cannot be ignored in the simulation. Also, it is understood that aspect ratio (width to height) <10 will have up to 15% error for center frequency predicted in air when the thin-plate approximation is used for modelling the bending stiffness of the CMUT membrane.

A metasurface or 2D metamaterial composed of a membrane array can support an interesting acoustic wave field. These waves are evanescent in the direction normal to the array and can propagate in the immersion fluid immediately above the metasurface. These waves are a result of the resonant membranes coupling to the fluid medium and propagate with a group and phase speed lower than that of the bulk waves in the surrounding fluid. This work examines and utilizes these evanescent surface waves using Capacitively Micromachined Ultrasonic Transducers (CMUT) as a specific example. CMUT arrays can generate and detect membrane displacement capacitively, and are shown to support the surface waves capable of subwavelength focusing and imaging. A model is developed that can solve for the modes of the membrane array in addition to transiently modeling the behavior of the array. It is found that the dispersive nature of the waves is dependent on the behavior of the modes of the membrane array. Two-dimensional dispersion analysis of the metasurface shows evidence of four distinct frequency bands of surface wave propagation: isotropic, anisotropic, directional band gap, and complete band gap around the first resonant frequency of the membrane. Some of the frequencies in the partial band gap show concave equifrequency contours capable of negative refraction. The dispersion and modal properties are also examined as to how they are affected by basic array parameters. Potential applications of this wave field are examined in the context of subwavelength focusing and imaging. Several methods of acoustic focusing are used on an array consisting of dense grid of membranes and several membranes spatially removed from the structure. Subwavelength acoustic focusing to a resolution of λ/5 is shown in simulations and verified with experiments. An imaging test is also performed in which a subwavelength defect is localized. This fundamental work in characterizing the waves above the membrane metasurfaces is expected to have impact and implications for transducer design, resonant sensors, 2D acoustic lenses, and subwavelength focusing and imaging.

Présentés pour la première fois en 1994, les transducteurs ultrasonores capacitifs micro- usinés, ou CMUTs (Capacitive Micromachined Ultrasonic Transducers), représentent une technologie alternative aux matériaux piézoélectriques pour la transduction électroacoustique. En particulier, leur souplesse de conception et leur haut degré de miniaturisation en font des candidats privilégiés pour le développement de sondes mixtes complexes alliant thérapie et imagerie par ultrasons. C’est dans ce contexte que s’est inscrit ce travail de thèse, dédié au développement d’une première sonde CMUT mixte. Le document débute par une présentation générale de la technologie et du contexte du projet. Puis, le développement est présenté, en commençant par les étapes préliminaires de modélisation numérique jusqu’aux caractérisations les plus avancées du prototype fabriqué. Les résultats démontrent l’intérêt de la technologie pour les applications visées<br>Presented for the first time in 1994, capacitive micromachined ultrasonic transducers (CMUT) are a promising alternative to the piezoelectric technology for electroacoustic transduction. Particularly, their intrinsic design flexibility and miniaturization capability are strong advantages for the manufacturing of high-end Ultrasound-guided Focused Ultrasound (USgFUS) probes. The work presented in this Ph.D. dissertation is devoted to the f irst development of a USgFUS CMUT probe. After a general introduction of the CMUT technology and the context of this research project, the development is reported starting from the preliminary numerical studies to the most advanced characterizations of the fabricated device. The first results demonstrate the benefits of this technology for the targeted applications