The purpose of this project has been twofold: firstly, to develop further FTIR reflectance spectroscopy as a technique for the quantitative characterisation of SOI multilayer structures and secondly, to use it in combination with other techniques, such as RBS/Channelling, XTEM, SIMS, AES and SRP, to investigate the evolution and structure of novel SOI materials. In order to evaluate the FTIR results, many SIMOX samples were fabricated with a wide range of Si and buried SiO2 layer thicknesses with one or two buried oxide layers. The FTIR results are compared to those obtained with the other experimental techniques and with theoretical calculations. We show that, given an "a priori" knowledge of the structure which is used to define the initial structural model, FTIR can be used as a non-destructive, fast, and inexpensive control method for characterising SOI structures. FTIR offers +/-2 nm accuracy in layer thicknesses and +/-5 nm in interfacial region thicknesses, over a wide (0.5 to >10mum) depth range, while for shallower depths an error of 15% on average is likely. FTIR gives Si thickness values which are within 5%, and buried layer or interfacial region thickness values which are within 10% to 20%, from the values obtained by other techniques. The sensitivity to oxygen content variations is down to 5 x 1020 O cm-3 for the top of a Gaussian profile, and 1 X 1022 O cm-3 over a 20 nm thick layer. FTIR gives a value for the retained dose within 5% of the nominal dose for both unannealed and annealed samples. Novel SOI materials, such as deep buried oxide layers and double SIMOX structures were fabricated and characterised, and the processing parameters optimised. We show that, for 2 MeV oxygen implantation into Si at 700°C to a dose of 2 x 1018 O+ cm-2 and after annealing at 1300°C for 6 hours, a buried layer is formed which is continuous but contains Si islands. It is found that by increasing the annealing time to 12 hours, the homogeneity of the buried layer is improved. We propose that an increase of both the oxygen dose and the annealing temperature would result in a higher quality buried oxide layer. The adverse effect of high beam current densities to the sample structure after annealing is demonstrated for the high energy SIMOX samples. We show that double SIMOX structures can be fabricated by a three step process, with one only annealing step at the end. Such structures can be used for Si waveguiding applications and we present preliminary results of waveguiding loss measurements, where a value of 18 dB/cm was obtained.

The aim to provide preventive healthcare and high quality medical diagnostics and treatment to an increasingly ageing population caused a rapidly increasing demand for point-of-care diagnostic devices. Disposables have an advantage over re-usable units as cross-contamination is avoided, no cleaning and sterilising of equipment is required and devices can be used out of centralised laboratories. To remain cost-effective, costs for disposables should be kept low. This makes polymer materials an obvious choice. One method for the realisation of fluidic micro devices is the stacking of several layers of microstructured polymer films. Reel-to-reel manufacturing is a promising technique for high-volume manufacturing of disposable polymer bio-analytical devices. Polyethylene terephthalate (PET) and cycloolefin copolymer (COC) were selected as suitable polymer substrate materials and polydimethyl siloxane (PDMS) as membrane layer. Bonding of polymer films with the help of adhesives carries the risk of channel blocking. Despite this drawback, no other method of bonding PDMS to a structural layer could be identified. Bonding with solvents avoids channel blocking issues, but adversely affects biocompatibility. Thermal diffusion processes enable bonding of COC and PET without the use of any auxiliary material. The extensive process times requires for thermal diffusion bonding can be considerably shortened by pre-treating the material with plasma or UV exposure. Welding with the use of a laser energy absorbing dye was demonstrated to be particularly suitable for selective bonding around channels and reservoirs. None of the assessed bonding methods provide a generic solution to all bonding applications. Instead, the selection of an appropriate technique depends on the intended application and the required level of biocompatibility. Since this selection has implications on the feasibility and reliability of microfluidic structures on the device, design rules which ensure design for production have to be established and followed.

Cette thèse s'intéresse à l'évaluation de la qualité de l'adhésion dans des assemblages structuraux métal/adhésif/composite carbone-époxy, assemblages utilisés dans l'industrie automobile et aéronautique. La qualité du collage est évaluée par méthodes non destructives grâce à des ondes guidées ultrasonores de type Lamb et SH. Un nouveau modèle numérique par éléments finis a été développé au cours de ce travail : il s'agit d'un modèle rhéologique aux interphases qui permet de tracer les courbes de dispersion des modes guidés dans les échantillons étudiés. Il permet aussi de prédire l'évolution de l'amplitude de ces modes et la répartition de l'énergie dans la structure suivant le niveau d'adhésion ou la présence d'un défaut de type kissing bond. Des échantillons à niveau d'adhésion connus et maîtrisés sont réalisés avec l'aide de physico-chimistes, où l'époxy est partiellement ou totalement réticulé, et où l'interface substrat/adhésif a subi différents traitements chimiques. En particulier sont étudiés des échantillons dont la dégradation du collage dans la structure n'est pas symétrique. Les résultats expérimentaux sont confrontés à ceux issus du modèle numérique. Ces résultats montrent qu'il est possible de caractériser deux niveaux d'adhésion proches et aussi de déterminer sur quelle interface (métal/adhésif ou composite/adhésif) le collage a été dégradé. L'utilisation d'un banc de mesure à transducteurs électromagnétiques acoustiques (EMAT) a permis l'étude de l'amplitude des modes SH. Des variations importantes de l'amplitude de certains modes ont été constatées lors du passage de l'onde d'une zone de bonne adhésion à une zone où le collage est dégradé. / This thesis focuses on the evaluation of the quality of adhesion in metal/adhesive/carbon-epox: structural assemblies used in the automotive and aerospace industry. The quality of the bonding i assessed by non-destructive methods using ultrasonic guided waves of the Lamb and SH types. A nev finite element numerical model was developed during this work: it is an interphase rheological mode that allows the dispersion curves of the guided modes to be plotted in the samples studied. It als( makes it possible to predict the evolution of the amplitude of these modes and the distribution o energy in the structure according to the level of adhesion or the presence of a kissing bond type defect. Samples with known and controlled adhesion levels are made with the help of physico chemists, 'where the epoxy is partially or totally crosslinked, and where the substrate/adhesivl interface has undergone different chemical treatments. In particular, samples are studied whos degradation of the bonding in the structure is not symmetrical. The experimental results are compare( with those from the numerical model. These results show that it is possible to characterize two clos levels of adhesion and also to determine on which interface (metal/adhesive or composite/adhesive the bonding has been degraded. The use of a measurement bench with electromagnetic acousti( transducers (EMAT) allowed thè study of the amplitude of the SH modes. Significant variations in th( amplitude of some modes have been observed when the wave passes from an area of good adhesioi to an area where the bonding is degraded.

Made available in DSpace on 2014-12-17T14:56:00Z (GMT). No. of bitstreams: 1 RossanaMSC.pdf: 1964809 bytes, checksum: 4bf5121999d212cf5e61c5b931343038 (MD5) Previous issue date: 2005-08-15 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / In this work, the transmission line method is explored on the study of the propagation phenomenon in nonhomogeneous walls with finite thickness. It is evaluated the efficiency and applicability of the method, considering materials like gypsum, wood and brick, found in the composition of the structures of walls in question. The results obtained in this work are compared to those available in the literature, for several particular cases. A good agreement is observed, showing that the performed analysis is accurate and efficient in modeling, for instance, the wave propagation through building walls and integrated circuit layers in mobile communication and radar system applications. Later, simulations of resistive sheets devices such as Salisbury screens and Jaumann absorbers and of transmission lines made of metal-insulator-semiconductor (MIS) are made. Thereafter, it is described a study on frequency surface selective structures (FSS). It is proposed the development of devices and microwave integrated circuits (MIC) of such structures, for the accomplishment of experiments. Finally, future works are suggested, for instance, on the development of reflectarrays, frequency selective surfaces with dissimilar elements, and coupled frequency selective surfaces with elements located on different layers / Neste trabalho, ? utilizado o M?todo da Linha de Transmiss?o, para a investiga??o do fen?meno de propaga??o em paredes n?o-homog?neas e de espessura finita. A avalia??o da efici?ncia e aplicabilidade do m?todo da linha de transmiss?o ? realizada, considerando materiais como gesso, madeira e tijolo, encontrados na composi??o das estruturas de paredes em quest?o. Posteriormente, s?o apresentadas simula??es para superf?cies resistivas, como telas de Salisbury e absorvedores Jaumann, e para linhas de transmiss?o do tipo metal-isolante-semicondutor (MIS), al?m do estudo sobre superf?cies seletivas de freq??ncia (FSS). Em seguida, ? proposto o desenvolvimento de dispositivos e circuitos integrados de microondas (MIC) de tais estruturas, para a realiza??o de experimentos. Os resultados obtidos demonstram que a an?lise efetuada neste trabalho ? eficiente e precisa. Para diversas estruturas e aplica??es em circuitos, os valores num?ricos obtidos para os par?metros analisados foram comparados com os valores te?ricos e experimentais, inclusive de outros autores. Nestes casos, observa-se uma excelente concord?ncia. Estes resultados indicam o potencial da t?cnica adotada para a an?lise da propaga??o de ondas eletromagn?ticas atrav?s de estruturas de camadas m?ltiplas, com aplica??es em sistemas de comunica??es m?veis e radar. Finalmente, s?o apresentadas propostas para a realiza??o de trabalhos futuros relacionados, por exemplo, com o desenvolvimento de reflectarrays, superf?cies seletivas de freq??ncia com elementos dissimilares, localizados na mesma interface, e superf?cies seletivas de freq??ncias acopladas, com elementos localizados sobre camadas distintas

III-V nanostructures have attracted substantial research effort due to their interesting physical properties and their applications in new generation of ultrafast and high efficiency nanoscale electronic and photonic components. The advances in nanofabrication methods including growth/synthesis have opened up new possibilities of realizing one dimensional (1D) nanostructures as building blocks of future nanoscale devices. For processing of semiconductor nanostructure devices, simplicity, cost effectiveness, and device efficiency are key factors. A number of methods are being pursued to fabricate high quality III-V nanopillar/nanowires, quantum dots and nano disks. Further, high optical quality nanostructures in these materials together with precise control of shapes, sizes and array geometries make them attractive for a wide range of optoelectronic/photonic devices. This thesis work is focused on top-down approaches for fabrication of high optical quality nanostructures in III-V materials. Dense and uniform arrays of nanopillars are fabricated by dry etching using self-assembly of colloidal SiO2 particles for masking. The physico-chemistry of etching and the effect of etch-mask parameters are investigated to control the shape, aspect ratios and spatial coverage of the nanopillar arrays. The optimization of etch parameters and the utilization of erosion of etch masks is evaluated to obtain desired pillar shapes from cylindrical to conical. Using this fabrication method, high quality nanopillar arrays were realized in several InP-based and GaAs-based structures, including quantum wells and multilayer heterostructures. Optical properties of these pillars are investigated using different optical spectroscopic techniques. These nanopillars, single and in arrays, show excellent photoluminescence (PL) at room temperature and the measured PL line-widths are comparable to the as-grown wafer, indicating the high quality of the fabricated nanostructures. The substrate-free InP nanopillars have carrier life times similar to reference epitaxial layers, yet an another indicator of high material quality. InGaAs layer, beneath the pillars is shown to provide several useful functions. It effectively blocks the PL from the InP substrate, serves as a sacrificial layer for generation of free pillars, and as a “detector” in cathodoluminescence (CL) measurements. Diffusion lengths independently determined by time resolved photoluminescence (TRPL) and CL measurements are consistent, and carrier feeding to low bandgap InGaAs layer is evidenced by CL data. Total reflectivity measurements show that nanopillar arrays provide broadband antireflection making them good candidates for photovoltaic applications.  A novel post etch, sulfur-oleylamine (S-OA) based chemical process is developed to etch III-V materials with monolayer precision, in an inverse epitaxial manner along with simultaneous surface passivation. The process is applied to push the limits of top-down fabrication and InP-based high optical quality nanowires with aspect ratios more than 50, and nanostructures with new topologies (nanowire meshes and in-plane wires) are demonstrated.  The optimized process technique is used to fabricate nanopillars in InP-based multilayers (InP/InGaAsP/InP and InP/InGaAs/InP). Such multilayer nanopillars are not only attractive for broad-band absorption in solar cells, but are also ideal to generate high optical quality nanodisks of these materials. Finally, the utility of a soft stamping technique to transfer free nanopillars/wires and nanodisks onto Si substrate is demonstrated. These nanostructures transferred onto Si with controlled densities, from low to high, could provide a new route for material integration on Si. / <p>QC 20130205</p>

La réduction du cout de construction et d’exploitation des futurs accélérateurs d particules, a grande et petite échelles, dépend du développement de nouveaux matériaux pour les surfaces actives des structures supraconductrices en radiofréquence (SRF). Les propriétés SRF sont essentiellement un phénomène de surface vu que la profondeur de pénétration (profondeur de pénétration de London, λ) des micro-ondes (RF) est typiquement de l’ordre de 20 à 400 nm en fonction du matériau. Lorsque les procédés de préparation de surface sont optimises, la limite fondamentale du champ RF que les surfaces SRF peuvent supporter est le champ RF maximum, Hc₁, au-delà duquel le flux magnétique commence à pénétrer la surface du supraconducteur. Le matériau le plus utilise pour des applications SRF est le niobium (Nb) massif, avec un champ Hc₁ de l’ordre de 170 mT, qui permet d’atteindre un champ accélérateur de moins de 50 MV/m. Les meilleures perspectives d’amélioration des performances des cavités SRF sont liées à des matériaux et méthodes de production produisant la surface SRF critique de façon contrôlée. Dans cette optique, deux avenues sont explorées pour utiliser des couches minces pour augmenter les performances des structures SRF au-delà du Nb massif, en monocouche ou en structures multicouches Supraconducteur-Isolant-Supraconducteur (SIS) : La première approche est d’utiliser une couche de Nb déposée sur du cuivre (Nb/Cu) à la place du Nb massif. La technologie Nb/Cu a démontré, au cours des années, être une alternative viable pour les cavités SRF. Toutefois, les techniques de dépôt communément utilisées, principalement la pulvérisation magnétron, n’ont jusqu’à présent pas permis de produire des surfaces SRF adaptées aux performances requises. Le récent développement de techniques de dépôt par condensation énergétiques, produisant des flux d’ions énergétiques de façon contrôlée (telles que des sources d’ions ECR sous ultravide) ouvrent la voie au développement de films SRF de grand qualité. La corrélation entre les conditions de croissance, l’énergie des ions incidents, la structure et les performances RF des films produits est étudiée. Des films Nb avec des propriétés proches du Nb massif sont ainsi produits. La deuxième approche est basée sur un concept qui propose qu’une structure multicouche SIS déposée sur une surface de Nb peut atteindre des performances supérieures à celles du Nb massif. Bien que les matériaux supraconducteurs à haute Tc aient un champ Hc₁ inférieur à celui du Nb, des couches minces de tels matériaux d’une épaisseur (d) inférieure à la profondeur de pénétration voient une augmentation de leur champ parallèle Hc₁ résultant au retardement de la pénétration du flux magnétique. Cette surcouche peut ainsi permettre l’écrantage magnétique de la surface de Nb qui est donc maintenue dans l’état de Meissner à des champs RF bien plus importants que pour le Nb massif. La croissance et performance de structures multicouches SIS basées sur des films de NbTiN, pour le supraconducteur, et de l’AlN, pour le diélectrique, sont étudiées. Les résultats de cette étude montrent la faisabilité de cette approche et le potentiel qui en découle pour l’amélioration des performances SRF au-delà du Nb massif. / The minimization of cost and energy consumption of future particle accelerators, both large and small, depends upon the development of new materials for the active surfaces of superconducting RF (SRF) accelerating structures. SRF properties are inherently a surface phenomenon as the RF only penetrates the London penetration depth λ, typically between 20 and 400 nm depending on the material. When other technological processes are optimized, the fundamental limit to the maximum supportable RF field amplitude is understood to be the field at which the magnetic flux first penetrates into the surface, Hc₁. Niobium, the material most exploited for SRF accelerator applications, has Hc₁~170 mT, which yields a maximum accelerating gradient of less than 50 MV/m. The greatest potential for dramatic new performance capabilities lies with methods and materials which deliberately produce the sub-micron-thick critical surface layer in a controlled way. In this context, two avenues are pursued for the use of SRF thin films as single layer superconductor or multilayer Superconductor-Insulator-Superconductor structures: Niobium on copper (Nb/Cu) technology for superconducting cavities has proven over the years to be a viable alternative to bulk niobium. However the deposition techniques used for cavities, mainly magnetron sputtering, have not yielded, so far, SRF surfaces suitable for high field performance. High quality films can be grown using methods of energetic condensation, such as Electron Cyclotron Resonance (ECR) Nb ion source in UHV which produce higher flux of ions with controllable incident angle and kinetic energy. The relationship between growth conditions, film microstructure and RF performance is studied. Nb films with unprecedented “bulk-like” properties are produced. The second approach is based on the proposition that a Superconductor/Insulator/Superconductor (S-I-S) multilayer film structure deposited on an Nb surface can achieve performance in excess of that of bulk Nb. Although, many higher-Tc superconducting compounds have Hc₁ lower than niobium, thin films of such compounds with a thickness (d) less than the penetration depth can exhibit an increase of the parallel Hc₁ thus delaying vortex entry. This overlayer provides magnetic screening of the underlying Nb which can then remain in the Meissner state at fields much higher than in bulk Nb. A proof of concept is developed based on NbTiN and AlN thin films. The growth of NbTiN and AlN films is studied and NbTiN-based multilayer structures deposited on Nb surfaces are characterized. The results from this work provide insight for the pursuit of major reductions in both capital and operating costs associated with future particle accelerators across the spectrum from low footprint compact machines to energy frontier facilities.

The exploitation of the soft x-ray/extreme ultra-violet (EUV) region of the electromagnetic spectrum is possible mainly due to the development of multilayer (ML) mirrors. This region of the electromagnetic spectrum offers great opportunities in both science and technology. The shorter wavelength allows one to see smaller features in microscopy and write finer features in lithography. High reflectivity with moderate spectral bandwidth at normal/near-normal incidence can be achieved in soft x-ray/ EUV spectral range using these ML mirrors, where natural crystals with the required large periodicity are not available. These MLs are generally artificial Bragg’s reflectors, which consist of alternative high and low density materials with periodicity in the nanometer range. The main advantages of ML optics stem from the tunability of layer thickness, composition, lateral gradient, and the gradient along the normal to the substrate; these can be tailored according to the desired wavelength regime. They have the great advantage of being adaptable to figured surfaces, enabling their use as reflective optics in these spectral regions, for focusing and imaging applications. Broadband reflectivity and wavelength tunability are also possible by using MLs with normal and lateral gradient, respectively. However, fabrication of these ML mirrors requires the capability to deposit uniform, ultra-thin (a few angstroms-thick) films of different materials with thickness control on the atomic scale. Thus, one requires a proper understanding of substrate surfaces, individual layers, chemical reactivity at interfaces and, finally, of the ML structures required for particular applications. The performance of these MLs is limited by (the lack of) contrast in optical constants of the two materials, interfacial roughness, the chemical reactivity of two materials and, finally, errors in the thickness of individual layers. Soft x-ray/extreme ultra-violet ML mirrors have found a wide range of applications in synchrotron radiation beam lines, materials science, astronomy, x-ray microscopy, x-ray laser, x-ray lithography, polarizers, and plasma diagnostics. The Indus–1 synchrotron radiation (SR) source is an operational 450 MeV machine, which produces radiation up to soft x-rays. Indus-2 is a 2.5 GeV machine, which has been commissioned recently to produce hard x-rays (E > 25 keV). The combination of Indus-1 and Indus-2 will cover a broad energy spectrum from IR to hard x-rays. Therefore, there is a significant need and opportunity to study MLs of different pairs of materials, with different parameters such as periodicity and optimum thickness of individual layers. The goal of the present thesis is to fabricate MLs for soft x-ray optics and to study their physics for application as polarizers in the wavelength range from 67 Å to 160 Å on the Indus-1 synchrotron source. To accomplish this task, a UHV electron beam evaporation system has been developed indigenously for the fabrication of MLs. Three different ML systems viz., Mo/Si, Fe/B4C and Mo/Y have been fabricated, and their surfaces and interfaces were investigated thoroughly for the polarizer application. X-ray reflectivity (XRR) has been used extensively in the investigations of these MLs. This is because XRR is a highly sensitive non-destructive technique for the characterization of buried interfaces, and gives microscopic information (at atomic resolution) over a macroscopic length scale (a few microns). Numerical analysis of XRR data has been carried out using computer programs. Depth-graded x-ray photoelectron spectroscopy (XPS) has been used for compositional analysis at interfaces for some of the ML structures, as a technique complementary to XRR. The performance of some of these MLs has been tested in the soft x-ray region, using the Indus-1 synchrotron radiation (SR) source. Prior to studying the MLs, a detailed study of the surfaces and interfaces of thin films, bi-layers, and tri-layers was carried out using XRR and the glancing incidence fluorescence technique. The discontinuous-to-continuous transition and the mode of film growth, which are vital to the optimization of layer thickness (basically for the high-atomic number or high-Z layer) in the ML structures, were also investigated using in situ sheet resistance measurement method. Indus-1 is a soft x-ray SR source that covers atomic absorption edges of many low-Z materials. The present work demonstrates the possibilities of characterizing low-Z thin films and multilayers using soft x-ray resonant reflectivity. In one case, we have shown for first time that soft x-ray resonant reflectivity can be employed as a non-destructive technique for the determination of interlayer composition. In a second study using the Indus-1 SR source, we have shown, by observing the effect of the anomalous optical constant on reflectivity pattern when photon energy is tuned across the atomic absorption edge of the constituent low-Z element, that soft x-ray resonant reflectivity is an element-specific technique. This thesis is organized into 7 chapters. A brief summary of individual chapters is presented below. Chapter 1 gives a brief general introduction to x-ray ML optics. This is followed by a discussion of the importance of the soft x-ray region of electromagnetic radiation. The optical properties of x-rays are reviewed and optical constants are calculated for some of the important materials used for x-ray MLs. The refractive index in the x-ray region being less than unity (except absorption edges), the consequent limitation of conventional transmission lenses is discussed. The limitation of glancing angle incidence optics is presented, motivating the need for ML optics, which is discussed along with a theoretically calculated reflectivity profile. The procedure for materials for the MLs for application in different spectral regions is discussed, along with a survey of literature related to the present thesis. The importance of the quality of surfaces and interfaces on the performance of ML structures has been shown through simulations. The applications of soft x-ray MLs are discussed with emphasis on polarization. This is followed by a review of different modes of growth of thin films. Finally, the scope of the present work is highlighted. Chapter 2 provides brief descriptions of the experimental techniques used in the present investigations and of the numerical methods employed for quantitative data analysis. The XRR technique is discussed elaborately because it has been used extensively. Detailed calculations of x-ray reflectivity from single surfaces, thin films and bi-layers are presented, along with simulated values. The effect of critical angle and Brewster’s angle is also discussed. Data analysis methods for computing x-ray reflectivity from multilayer structures, based on dynamical and kinematical models, have been discussed. The effect of roughness on XRR has been discussed based on the recursion formalism of dynamical theory. Simulations of XRR and experimental XRR data fitting are carried out using computer programs. The XRR experimental set up is also outlined. A theoretical background is given for the electrical measurements on thin films. This is followed by a brief overview of x-ray photoelectron spectroscopy (XPS) and interpretation of spectra. Finally, the glancing incidence x-ray fluorescence (GIXRF) technique is outlined. Chapter 3 describes in detail the ultra-high vacuum electron beam evaporation system developed in house especially for the fabrication of thin films and x-ray multilayer optics. At the outset, a brief overview of different deposition techniques commonly used for the fabrication of x-ray optical elements is presented. Design, fabrication, and assembly of different accessories are discussed. The control of thickness and uniformity of the films deposited has been checked through the experiments, whose results are provided. The results obtained for ML test structures are presented to show the capability of system in carrying out fabrication of high quality x-ray ML structures. Finally, the versatility of evaporation system incorporating in situ characterization facilities such as -situ electrical measurements for different substrate temperatures is illustrated. Chapter 4 presents a study of the growth of ultra-thin Mo films at different substrate temperatures using in situ sheet resistance measurements. First, a theoretical background is given on the different stages of island growth and on factors affecting thin film growth, followed by a discussion of the possible electrical conduction phenomena in continuous and discontinuous metal films. The nature of thin film growth and a detailed microscopic picture at different growth stages are derived from a modeling of sheet resistance data obtained in situ. The various conduction mechanisms have been identified in different stages of growth. In the island growth stage, the isotropic and anisotropic growth of Mo islands is identified from the model. In the insulator-metal transition region, experimentally determined values of critical exponent of conductivity agrees well with theoretically predicted values for a two-dimensional (2D) percolating system, revealing that Mo films on float glass substrate is predominantly a 2D structure. The minimum thickness for which Mo films becomes continuous is obtained as 1.8 nm and 2.2 nm for Mo deposited at substrate temperatures 300 K and 100 K, respectively. An amorphous-to- crystalline transition is also observed, and discussed. Chapter 5 covers the detailed study of the surfaces and interfaces studies in three different ML structures viz., Mo/Si, Fe/B4C and Mo/Y, meant for the polarizer application in the wavelength range of 67 Å to 160 Å. Multilayers with varying periodicity, varying number of layer pairs, and different ratios of high-Z layer thickness to the period, were fabricated using the electron beam system. Initially, a brief overview of the design aspects of ML structures is given, along with the theoretically calculated reflectivity at Brewster’s angle from the best material combinations. In Mo/Si MLs, the interlayer formed at the interfaces due to interdiffusion of the two elements is asymmetric in thickness, i.e., Mo-on-Si interlayer is thicker than the Si-on-Mo interlayer. To take account of these interlayers in XRR data fitting, a four layer model is considered. The effect of interlayers on reflectivity pattern was studied using simulations, and differences with respect to roughness are also discussed. The mechanism of formation of asymmetric interlayers is also discussed. The interlayer composition has determined using depth-graded XPS. The results reveal the formation of the MoSi2 composition at both the interfaces. The experimental results agree well with theoretical calculations based on solid-state amorphization reaction, which is a result of large heat of mixing. The effective heat of formation model reveals the formation of MoSi2 as the first phase. The soft x-ray reflectivity performance of the Mo/Si ML structure at Brewster’s angle is tested using Indus-1 synchrotron radiation (SR). Using XRR and GIXFR, a study of the surfaces and interfaces of bilayers of B4C-on-Fe and Fe-on- B4C, and tri-layers of Fe-B4C-Fe was carried out, with a systematic variation of Fe and B4C layer thicknesses. A sharp interface was observed in Fe-on-B4C, whereas a low density (w.r.t. Fe) interlayer is observed at the B4C-on-Fe interface. The interlayer properties fluctuates w.r.t. the bottom Fe layer thickness and is independent of the top B4C layer thickness. The nature of fluctuations has been discussed in detail. A study of the surfaces and interfaces of Fe/B4C MLs is described. Finally, a study of the surfaces and interfaces of bilayers, tri-layers, and MLs of the Mo/Y system is discussed in detail. Chapter 6 describes the application of soft x-ray resonant reflectivity for the characterization of low-Z thin films and interfaces in multilayer structures. Initially, a discussion of the energy dependence of atomic scattering factors and hence of optical constants is provided with simulations, with emphasis on the atomic absorption edge. Then, a brief overview of synchrotron radiation, with particular emphasis on the parameters of the Indus-1 synchrotron source is given. The possibilities of determining the composition of the buried interlayer with sub-nanometer scale sensitivity using soft x-ray resonant reflectivity are discussed. The methodology has been applied to study the Mo/Si interface both by simulations and by experiments on the Indus-1 SR, by tuning the photon energy to the Si L-absorption edge. Finally, direct evidence of elemental specificity of soft x-ray resonant reflectivity through the observation of the effect of anomalous optical constants on the reflectivity pattern is discussed. We demonstrate the method through simulations and experiments on the B4C material in B4C thin films and Fe/ B4C bi-layers, using Indus-1 SR tuned to the boron Kedge. Chapter 7 summarizes the main findings of the present work, and provides an outlook for further investigations in the field.

Cette thèse s’intéresse au développement d’éléments finis solide–coques dédiés à la modélisation de structures multicouches sollicitées en vibrations. En effet, la plupart des modèles multicouches dans la littérature présentent des limitations dans certaines configurations géométriques et matérielles. Face à ce constat et dans un souci de proposer un outil moins coûteux en temps de calcul, nous avons proposé une approche basée sur le concept solide–coques. Il s’agit d’éléments finis tridimensionnels dont le comportement a été amélioré par l’Assumed Strain Method. Dans un premier temps, nous avons formulé le problème de vibrations de structures sandwichs à cœur viscoélastique. La dépendance en fréquence a ainsi été prise en compte en utilisant une loi constitutive complexe. Pour résoudre le problème discrétisé, la Méthode Asymptotique Numérique, couplée à l’homotopie, et utilisant l’approche DIAMANT, a été adoptée pour les excellents résultats qu’elle offre par rapport aux autres méthodes. Des tests ont permis de valider les modèles proposés et de montrer l’avantage par rapport aux éléments ayant la même cinématique. Poursuivant nos travaux, et dans un souci d’augmenter l’amortissement, nous nous sommes orientés vers un contrôle actif des vibrations. Pour ce faire, deux éléments finis piézoélectriques ont été formulés. Il s’agit des éléments SHB8PSE et SHB20E qui sont des extensions des éléments finis SHB8PS et SHB20, respectivement. Le couplage électromécanique a consisté en l’ajout d’un degré de liberté à chacun des nœuds des dits éléments. Quelques exemples en statique et en vibrations menés sur des structures multicouches allant de simples poutres aux structures présentant des non-linéarités géométriques ont permis de valider les éléments solide–coques proposés. Pour finir, une synthèse des acquis des chapitres 2 et 3 a permis de proposer une modélisation de structures multicouches comprenant des couches élastiques, viscoélastiques et piézoélectriques. À l’amortissement passif provenant du pouvoir amortissant des matériaux viscoélastiques, on ajoute un contrôle actif qui découle du courant électrique généré au cours de la déformation des couches piézoélectriques. Ainsi, un filtre a été installé entre les capteurs et actionneurs. Ce filtre permet d’amplifier ou d’atténuer le potentiel électrique généré dans le but de réduire les amplitudes de vibrations. Pour résoudre le problème résultant nous avons étendu le solveur utilisé au chapitre 2. Pour valider les modèles proposés, des tests de contrôle actif–passif ont été menés sur des structures plaques multicouches. Enfin, quelques lois de contrôle découlant de filtres ont permis de montrer comment cette procédure permet de réduire ou même d’éviter l’amplification des vibrations / This thesis deals with the development of solid–shell finite elements for vibration modeling of multilayer structures. Indeed, most of multilayer models in the literature show some limitations in certain geometric and material configurations. Considering these restrictions and in order to develop a more efficient calculation tool, we proposed an approach based on the solid–shell concept. This consists of three-dimensional finite elements enhanced through the Assumed Strain Method. First of all, we have formulated the problem of vibrations of sandwich structures with viscoelastic core. The frequency dependence has been taken into account by using a complex constitutive law. To solve the discretized problem, the Asymptotic Numerical Method, coupled with the homotopy technique and the DIAMANT toolbox approach, was adopted due to the excellent results it provides compared to other methods. Benchmark tests have validated the models and highlighted their advantages over existing elements having the same kinematics. In order to increase damping properties, we directed our attention towards an active vibration control. For this purpose, two piezoelectric finite elements have been developed. These finite elements SHB8PSE and SHB20E are extensions, of the elements SHB8PS and SHB20, respectively. The electromechanical coupling consisted in adding an electrical degree of freedom to each node of these elements. A variety of test problems both in static and vibration analysis conducted on multilayer structures ranging from simple beams to structures involving geometric nonlinearities allowed validating the proposed solid–shell elements. Finally, combining the achievements made in chapters 2 and 3, we proposed a modeling approach for multilayer structures composed of elastic, viscoelastic and piezoelectric layers. Active control is introduced using the piezoelectric properties in order to improve the reduction in vibration amplitudes. Thus, a filter has been mounted between the sensors and actuators. This filter allows amplifying or attenuating the generated electric potential in order to reduce the vibration amplitudes. To solve the resulting problem, we extended the resolution method used in chapter 2. To validate the proposed models, active–passive control tests have been conducted on multilayer plate structures. Finally, some control laws, associated with filters, have shown how this procedure can allow reducing or even avoiding amplification of vibrations