In this thesis, the c-axis-oriented AlN films were deposited on piezoelectric substrates, lithium niobate (LiNbO3), ST-Quartz, and non-piezoelectric substrate, silicon (Si), by reactive rf magnetron sputtering. AlN films were deposited with the nitrogen concentration (N2/Ar+N2) of 20¡ã80%, the chamber pressure of 1¡ã15mTorr, the rf power of 200¡ã450W, the deposition time of 1~3 hours and the substrate temperature of 100¡ã400¢J. The correlation between growth parameters and piezoelectric coefficients will be investigated by XRD¡Bd33 and K2 analysis in this study. The experimental results showed that the values of d33 become larger as the intensity of X-ray is stronger. It can also be concluded that the smaller the FWHM of (002) XRD peak is, the larger the value of d33 is. With various sputtering parameters, the K2 values exhibit diversely. The multilayer structures of AlN/LiNbO3 and AlN/ST-Quartz both make lower values of K2. In general, by combining the higher K2 and d33 values of LiNbO3 and ST-Quartz with high wave velocity of AlN, the high-frequency with high performance SAW devices can be obtained.

Cette thèse a été effectuée dans un contexte industriel de forte concurrence en lien avec les capteurs miniatures en silicium, destinés au gigantesque marché dit "consumer", dont l'application phare est le "Smartphone", pour laquelle les fonctionnalités accrues engendrent un besoin en matière de multi-capteurs inertiels dits 10-axes (accéléromètre 3-axes, magnétomètre 3-axes, gyromètre 3-axes et capteur de pression). Tout comme les circuits intégrés, les contraintes de coût de tels capteurs se traduisent par une exigence en termes de densité d'intégration. La technologie M&NEMS (Micro- & Nano- Electro Mechanical Systems) a été développée pour répondre à cette attente. Elle repose sur l'intégration de jauges de contraintes de dimensions nanométriques (~250 nm) avec des structures électromécaniques micrométriques, ce qui prodigue une compacité hors-pair des capteurs, ouvrant la voie à la co-intégration de multi-capteurs sur la même puce de silicium. Toutefois, la nature différente des grandeurs physiques à mesurer impose des contraintes supplémentaires, parfois opposées, ce qui rend leur co-intégration difficile. Partant de ce constat, nous avons exploré et développé, des solutions devant permettre le fonctionnement sous une même pression environnante, d'accéléromètres et de gyromètres à force de Coriolis. Cette problématique de co-intégration, s'étend au-delà du couple accéléromètre-gyromètre. Des questions inhérentes au capteur de pression ainsi qu'aux 3 axes de mesure d'un accéléromètre, sont également traitées dans cette thèse / This thesis was carried out in an industrial context of strong competition in connection with miniature silicon sensors for the huge so-called “consumer” market, where the “Smartphone” is the killer application; its increasing functionality creates a need for the so-called ‘10-axis' inertial multi-sensors (3-axis accelerometer, 3-axis magnetometer, 3-axis gyro sensor and pressure). Similarly to integrated circuits, cost constraints on such sensors translate into a requirement in terms of integration density. The M & NEMS (Micro- & Nano- Electro-Mechanical-Systems) technology has been developed to meet this expectation. It is based on the integration of nanoscale (~ 250 nm) strain gauges together with micrometric electromechanical structures, which ensure unrivaled compactness, paving the way for the co-integration of multiple inertial sensors on the same silicon chip. However, the different nature of the physical quantities to be measured imposes additional constraints, sometimes conflicting, which leads to a difficult co-integration. Based on this observation, we have explored and developed solutions to allow operation under the same ambient pressure, of accelerometers together with Coriolis force based gyroscopes. This issue of co-integration extends beyond the accelerometer-gyroscope couple. Issues inherent to the pressure sensor and to the 3-axis accelerometer measurements, are also addressed in this thesis

Le coefficient de couplage électromécanique (CCEM) est un paramètre essentiel pour la description des matériaux piézoélectriques, il traduit la conversion d’énergie électrique en énergie mécanique et vice versa. Ce coefficient de couplage est étudié et déterminé dans le cadre de cette thèse pour des céramiques piézoélectriques. Ces dernières sont utilisées pour trois structures différentes ; la première structure étudiée est constituée d’une poutre longue et mince avec des petits patchs collés symétriquement sur les deux faces de la poutre en Aluminium, la seconde structure se compose d’une poutre courte et épaisse avec deux grands patchs. La dernière structure étudiée est une plaque composite multicouche du type aéronautique avec un seul grand patch. Ces trois structures ont été étudiées afin de déterminer le coefficient de couplage électromécanique effectif qui est considéré comme un indicateur de performance de l’amortissement passif shunté. Ce coefficient de couplage a été évalué de différentes manières en utilisant différents paramètres dont les conditions limites électriques, les propriétés élastiques des patchs, les propriétés modales de la poutre seule ainsi que les facteurs de couplages piézoélectriques. Une première étude expérimentale a été menée sur la poutre longue pour deux types de configurations en court circuit et circuit ouvert pour identifier ses propriétés modales. La poutre longue a été simulée pour deux types de polarisations, identiques et opposées, et simulée dans les deux codes Ansys® et Abaqus®. L’influence de la condition d’équipotentielle sur le coefficient de couplage a été étudiée. Une seconde campagne expérimentale et numérique sur une autre structure a été nécessaire pour valider les résultats obtenus. Pour pouvoir atteindre cet objectif, il était nécessaire de travailler sur une structure plus courte et plus rigide. Ainsi, la poutre courte a été simulée dans Ansys® et les résultats obtenus ont confirmé la nécessité de prendre en compte l’équipotentialité sur les faces des patchs. Cette condition a pour effet de réduire le couplage électromécanique et parfois de découpler certains modes. L’écart résultant de la corrélation expérimentale / numérique des deux poutres instrumentées a incité à recaler les modèles numériques. Ce recalage peut se présenter sous trois formes : mécanique en remplaçant l’encastrement par des ressorts linéaires, électrique en remplaçant les capacités fournies par le fabricant par les valeurs mesurées expérimentalement et électromécanique en utilisant les deux recalages précédents simultanément. Les deux poutres ont ensuite été simulées en déformations planes et contraintes planes et recalées afin d’approcher les résultats expérimentaux. L’étude de ces deux structures a permis de confronter les différentes méthodes d’évaluation du CCEM effectif, d’évaluer l’influence de l’équipotentialité sur les faces des électrodes et de comparer les simulations bidimensionnelles aux tridimensionnelles. Une plaque composite multicouche du type aéronautique a été ensuite étudiée pour généraliser la méthode d’évaluation du CCEM effectif pour les structures minces composites. La plaque seule a d’abord été simulée dans Ansys® pour valider le modèle numérique. Des tests sur la structure adaptative ont ensuite été menés pour l’évaluation du CCEM expérimental. La position choisie du patch a été déterminée par une analyse de l’énergie de déformation de la plaque seule pour les modes d’intérêt. Cette méthode de placement du patch s’est avérée efficace dans le sens où elle a conduit à des CCEM effectifs élevés pour certains modes de la bande de fréquence retenue. / The electromechanical coupling coefficient (EMCC) is an important parameter for the description of piezoelectric materials; it measures the conversion of electrical energy into mechanical one and vice versa. The coupling coefficient is studied and determined in this dissertation for piezoelectric ceramics. The latter are used for different structures: the first studied one is a long and thin Aluminium beam with small patches bonded symmetrically on its faces, the second one is a short and thick Aluminium beam with symmetrically bonded two large patches, and the third structure is considered more complex because it is an aeronautic-type multilayer composite plate with a single large patch. These three structures were studied to determine the electromechanical coupling coefficient which is considered as a performance indicator for passive shunted damping. The coupling coefficient was evaluated in different ways using different parameters, including the electrical boundary conditions, the elastic properties of the patches, the modal properties of the base beam and the piezoelectric coupling factor. A first experimental study was conducted on the long beam for two configurations, short circuit and open circuit, to identify its modal properties. The long beam was simulated for two configurations of polarization, same and opposite, in Ansys® and Abaqus® commercial codes. The equipotential condition influence on the coupling coefficient has been studied. A second experimental and numerical campaign for a different structure was necessary to validate the obtained results. For this purpose, it was necessary to work on a shorter and more stiff structure. Thus, the short beam was simulated in Ansys® which results have confirmed the necessity to consider the equipotentiality of the patches faces. This condition was found to reduce the electromechanical coupling and to uncouple some modes. The difference between experimental and numerical results of both adaptive structures was reduced by updating the numerical models. This updating is made in three ways: mechanically, by replacing the theoretical clamp conditions by linear springs, electrically, by replacing the capacities provided by the supplier by the experimental measured values, and electromechanically by considering previous updatings simultaneously. Both beams were simulated in 2D plane-strain and plane-stress and updated in order to approximate the experimental results. The study of these two structures allowed to assess different methods for the evaluation of the EMCC, to evaluate the influence of the equipotentiality constraints on the electroded faces, and to compare two-dimensional simulations to three-dimensional ones. Finally, an aeronautic-type multilayer plate composite has been studied in order to generalize the evaluation method of the EMCC for thin composite structures. The base plate was first simulated in Ansys® in order to validate the numerical model, then tests of the adaptive plate were conducted in order to evaluate the experimental EMCC. The selected position of the patch results from a strain energy analysis of the base plate for the mode of interest. The patch placement method was efficient in the sense that it provided high EMCC for some modes in the retained frequency range.

Abstract Compared with the other micro acoustic wave devices, the flexural plate-wave (FPW) device is more suitable for being used in liquid-sensing applications due to its higher mass sensitivity, lower phase velocity and lower operation frequency. However, conventional FPW devices usually present a high insertion loss and low fabrication yield. To reduce the insertion loss and enhance the fabrication yield of FPW device, a 1.5 £gm-thick silicon-trench reflective grating structure (RGS), a high electromechanical coupling coefficient ZnO thin-film and a 5 £gm-thick silicon oxide membrane substrate are adopted in this research. The influences of the amount of silicon trench and the distance between inter-digital transducer (IDT) and RGS on the insertion loss and quality factor of FPW device are investigated. The main fabrication technology adopted in the study is bulk micromachining technology and the main fabrication steps include six thin-film deposition and five photolithography processes. Under the optimized conditions of the sputtering deposition processes (200¢J substrate temperature, 200 W radio-frequency power and 75% gas flow ratio), a high C-axis (002) orientation ZnO piezoelectric thin-film with 31.33% electromechanical coupling coefficient can be demonstrated. The peak of XRD intensity of the standard ZnO film occurs at diffraction angle 2£c = 34.422¢X, which matches well with our results (2£c = 34.282¢X). By controlling the thickness of ZnO/Au/Cr/SiO2/Si3N4 sensing membrane less than 6.5 £gm-thick, the fabrication yield of FPW device can be improved and a low operation frequency (6.286 MHz) and high mass sensitivity (-113.63 cm2 / g) can be achieved. In addition, as the implemented FPW device with four silicon trenches RGS and 37.5 £gm distance between IDT and RGS, a low insertion loss (-40.854 dB) and very high quality factor (Q=206) can be obtained. Keywords¡Gflexural plate-wave; silicon-trench reflective grating structure; electromechanical coupling coefficient; ZnO; bulk micromachining technology

Σκοπός της διατριβής είναι η πρόβλεψη των συζευγμένων ηλεκτρομηχανικών ιδιοτήτων ενός πολυμερούς ενισχυμένου με νανοσωλήνες άνθρακα σε επίπεδο στρώσης υλικού (μικροκλίμακα), χρησιμοποιώντας δεδομένα και αναλύσεις ατομικής και υποατομικής κλίμακας. Αρχικά γίνεται η πρόβλεψη των μηχανικών ιδιοτήτων των νανοσωλήνων χρησιμοποιώντας μια προσέγγιση που συνδυάζει εξισώσεις μοριακής μηχανικής και ανάλυση με πεπερασμένα στοιχεία. Στη συνέχεια γίνεται πρόβλεψη των ηλεκτρικών ιδιοτήτων των νανοσωλήνων χρησιμοποιώντας τη μέθοδο Ισχυρού Δεσμού (ή Δέσμιας Κατάστασης). Ο συνδυασμός των δυο αυτών παρέχει πληροφορίες για τη συζευγμένη ηλεκτρομηχανικής απόκριση των νανοσωλήνων άνθρακα. Τέλος, γίνεται ανάλυση της μικροδομής ενός νανοσυνθέτου υλικού και πώς αυτή μεταβάλεται υπό την επίδραση μηχανικού φορτίου. Ο συνδυασμός όλων των αναπτυχθέντων προτύπων οδηγεί στην πρόβλεψη της συζευγμένης ηλεκτρομηχανικής συμπεριφοράς του νανοσυνθέτου υλικού. Τα μοντέλα που αναπτύχθηκαν καθώς και οι προβλέψεις τους επιβεβαιώθηκαν σε αντιπαράθεση με θεωρητικές προβλέψεις και πειραματικές μετρήσεις άλλων διακεκριμένων ερευνητών. / The goal of this thesis is the prediction of the coupled electromechanical response of carbon nanotube doped polumer in ply-level (microscale), using minimal input from atomic and subatomic analyses. First, the mechanical properties of carbon nanotubes are predicted using an approach that combines molecular mechanics and finite element analysis. Next, the electrical properties of carbon nanotubes are predicted using the tight-binding method. The coupling of these two models leads to the prediction of the electromechanical response of individual carbon nanotubes. Finally, the microstructure of a nanocomposite is analyzed, along with the effects of strain. All these models, when combined, predict the coupled electromechanical response of the nanocomposite. All models and predictions have been successfully correlated with theroetical predictions and experimental measurements of other researchers, found in the open literature.

The focus of this thesis is on using mechanical strain to tailor the electronic properties of graphene. The first half covers the electro-mechanical coupling for graphene in different configurations, namely a hexagonal Y-junction, various shaped bubbles on different substrates, and with kirigami cuts. For all of these cases, a novel combination of tight-binding electronic structure calculations and molecular dynamics is utilized to demonstrate how mechanical loading and deformation impacts the resulting electronic structure and transport. For the Y-junction, a quasi-uniform pseudo magnetic field induced by strain restricts transport to Landau-level and edge-state-assisted resonant tunneling. For the bubbles, the shape and the nature of the substrate emerge as decisive factors determining the effectiveness of the nanoscale pseudo magnetic field tailoring in graphene. Finally, for the kirigami, it is shown that the yield and fracture strains of graphene, a well-known brittle material, can be enhanced by a factor of more than three using the kirigami structure, while also leading to significant enhancements in the localized pseudo magnetic fields. The second part of the thesis focuses on dissipation mechanisms in graphene nanomechanical resonators. Thermalization in nonlinear systems is a central concept in statistical mechanics and has been extensively studied theoretically since the seminal work of Fermi, Pasta, and Ulam (FPU). Using molecular dynamics and continuum modeling of a ring-down setup, it is shown that thermalization due to nonlinear mode coupling intrinsically limits the quality factor of nanomechanical graphene drums and turns them into potential test beds for FPU physics. The relationship between thermalization rate, radius, temperature and prestrain is explored and investigated.

abstract: This work is focused on modeling the reliability concerns in GaN HEMT technology. The two main reliability concerns in GaN HEMTs are electromechanical coupling and current collapse. A theoretical model was developed to model the piezoelectric polarization charge dependence on the applied gate voltage. As the sheet electron density in the channel increases, the influence of electromechanical coupling reduces as the electric field in the comprising layers reduces. A Monte Carlo device simulator that implements the theoretical model was developed to model the transport in GaN HEMTs. It is observed that with the coupled formulation, the drain current degradation in the device varies from 2%-18% depending on the gate voltage. Degradation reduces with the increase in the gate voltage due to the increase in the electron gas density in the channel. The output and transfer characteristics match very well with the experimental data. An electro-thermal device simulator was developed coupling the Monte Caro-Poisson solver with the energy balance solver for acoustic and optical phonons. An output current degradation of around 2-3 % at a drain voltage of 5V due to self-heating was observed. It was also observed that the electrostatics near the gate to drain region of the device changes due to the hot spot created in the device from self heating. This produces an electric field in the direction of accelerating the electrons from the channel to surface states. This will aid to the current collapse phenomenon in the device. Thus, the electric field in the gate to drain region is very critical for reliable performance of the device. Simulations emulating the charging of the surface states were also performed and matched well with experimental data. Methods to improve the reliability performance of the device were also investigated in this work. A shield electrode biased at source potential was used to reduce the electric field in the gate to drain extension region. The hot spot position was moved away from the critical gate to drain region towards the drain as the shield electrode length and dielectric thickness were being altered. / Dissertation/Thesis / Ph.D. Electrical Engineering 2013

The accurate evaluation of interlaminar stresses is of great significance in the analysis and design of laminated and piezoelectric laminated structures because complex behaviours of these stresses near free edges initiate edge delamination that raises concerns about the structural integrity and reliability. This thesis presented 3D hybrid analyses on the interlaminar stresses to investigate the electromechanical coupling and free edge effects of piezoelectric laminated plates with an emphasis on the realistic distributions of the 3D stress and electric fields near free edges. In this research, the state space equations for simply-supported and free-edge piezoelectric laminates under transverse loads and infinite long free-edge piezoelectric laminates under uniaxial extension were obtained in the framework of 3D piezoelasticity by considering all the independent elastic and piezoelectric constants. The equations satisfy the traction-free and open-circuit boundary conditions at free edges and the continuity conditions across all interfaces. On the basis of the transfer matrix and recursive solution approaches, 3D exact solutions were sought by a novel non-uniform layer refinement technique to evaluate the accuracy of the finite element method (FEM), and realistic gradients of interlaminar stresses and electric fields were captured. The FEM results were in good agreement with those from the present solutions except for the regions near free edges. For simply-supported and free-edge laminates, stress variations with material properties, geometries and stacking sequences were obtained. The interlaminar stress τxz was dominant at corners and τyz also tended to contribute to delamination. In the infinite long free-edge laminates, σz, τyz, Ey and Ez exhibited significant gradients near free edges. Furthermore, the considerable influence of the electromechanical coupling effect on interlaminar stresses revealed that piezoelectric laminates were more susceptible to edge delamination and the application of closed-circuited surface conditions might prevent such edge delamination. The present analytical solution demonstrated an improvement in precision over other 2D analytical and numerical solutions and could serve as a benchmark for the determination of interlaminar stresses and electric fields near the free edges of the piezoelectric laminates.

In this thesis we use a new numerical code, based upon a mixed FEM-Runge-Kutta method, for the analysis and the design of plane 2-dimensional smart structures. We applied the developed code to the study of arbitrarily shaped piezo-electromechanical (PEM) plates. This code is based on a weak formulation of their governing equations as found in [18]. The optimal parameters needed to synthesize the appropriate electric networks are computed, and the overall performances of such plates are investigated. In particular, two examples are studied: firstly, a simple case is used to test the main features of the code; secondly, a more complex PEM plate is designed and analyzed by means of the proposed numerical approach. / Master of Science

In this thesis a piezo-electro-mechanical system, constituted of an aluminum beam with five piezoelectric patches glued on it, each of them shunted with an RL electrical circuit, has been numerically and experimentally investigated, in order to determine the optimal electric tuning parameters for vibration damping. A numerical code based upon Galerkin weighted-residual method is developed and the complete piezo-electro-mechanical system is designed, realized and finally tested by a standard modal testing technique. Comparisons between different shunting configurations of the system are given and finally the experimental data are compared with ones obtained by the developed numerical code in order to verify the accuracy of the latter. / Master of Science