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This thesis presents a detailed study of magnetic reconnection in a quasi-two-dimensional pulsed-power driven laboratory experiment. These experiments were performed at the 1.4 MA, 240 ns rise time Magpie facility at Imperial College London. Initial experiments are presented which demonstrate the viability of carbon as a wire material, and the use of exploding wire arrays as a platform for laboratory astrophysics. In the reconnection experiments, two exploding carbon wire arrays are placed side-by-side and driven in parallel by the Magpie current pulse. The carbon wires become plasma, creating super-sonic, sub-Alfvénic flows which advect anti-parallel magnetic fields towards the mid-plane between the two arrays, where the fields mutually annihilate inside a thin current sheet. A suite of temporally and spatially resolved diagnostics are used to study the reconnection process, including optical fast-framing, laser interferometry, Faraday Rotation imaging and Thomson Scattering. These detailed measurements allow the structure and dynamics of the reconnection layer to be determined, along with the nature of the inflows and outflows and the detailed energy partition during the reconnection process. The layer is unstable, exhibiting the repeated formation and ejection of plasmoids which have an associated magnetic structure measured by magnetic probes. The number and growth rate of these plasmoids agrees well with the predictions of semi-collisional tearing instability theory, and represent the first experimental observation of plasmoids in this regime. High electron and ion temperatures are observed, far in excess of what can be attributed to classical (Spitzer--Braginskii) resistivity or viscosity. Some possible anomalous heating mechanisms are discussed, including kinetic turbulence, and the plasmoid instability. Preliminary measurements of the out-of-plane velocity and magnetic field are presented along with the outlook for future experiments.

Plasma generated in contact with water has been extensively investigated in various electrode geometries and various discharge types for water treatment, of which the applications have been employed industrially on different scales. The reactive species such as OH radicals, O3, H2O2 and HO2 can be generated from the reactions that occur at the plasma-water interface. For discharges above water, the effect of positive gas ions, which lead to the formation of positive water ions, is considered the main pathway for OH radical formation; while for the discharge under water, the water dissociation by electron collisions is considered as the main pathway. However, the reaction zone for the production of reactive species (gas or liquid phase) is still controversial. This thesis presents a study of the plasma generated in the gas phase in contact with water by various discharge types for water treatment. The discharge characteristics, OH radical and H2O2 production, and solution conductivity and pH variation were investigated and compared under different experimental conditions. The degradation of methylene blue dye was investigated under DBD. The transition of impulsive current discharges into impulsive-diffuse discharges was recorded by increasing the solution conductivity; a further transition of the discharge type into a spark was recorded when the solution conductivity was increased to >2.4 mS/cm. The H2O2 energy efficiency of 1.1 g/kWh was recorded under positive impulsive current discharges in N2 and helium. The highest charge/H2O2 ratio of 1:1.26 was recorded under positive impulsive current discharges in O2 and N2. Under positive DC glow discharges, the H2O2 energy efficiency of 1.9 g/kWh was recorded in air discharges, and was slightly increased to 1.95 g/kWh when using a flow liquid electrode. Increased solution acidity and basicity from neutral solution have negative effects on H2O2 production. A significant amount of water vapour was observed under DC glow discharges, resulting in a negative effect on H2O2 production. Under negative discharges, no H2O2 production was detected in water after O2, N2, air and helium discharge treatments. In DBD, a threshold voltage is required to initiate electrical discharges between the glass plate and the water, through the micro-pores. The H2O2 production yield of 1.1 g/kWh was recorded in O2 discharge treatment. The degradation yield of methylene blue dye of 310 g/kWh was achieved within the first minute of O2 discharge treatment.

It has been shown that non-thermal plasma has great potential for chemical oxidation and bacterial inactivation. However, the mechanism of plasma-induced oxidation and bactericidal effects is not fully understood, and optimisation of the non-thermal plasma treatment is required to improve the efficiency of this technology. This research presents an investigation into the oxidation and bio-decontamination capabilities of steady-state corona discharges and impulsive transient plasma discharges in atmospheric air. Degree of decolorisation of blue dye by plasma discharges was obtained and used for evaluation of the oxidation efficiency of these discharges. The Gram-positive and Gram-negative bacteria, Staphylococcus aureus and Escherichia coli, respectively, were used for investigation of the bio-decontamination capability of the plasma discharges. It has been shown that conditions such as air humidity, electrode topology, and voltage levels may affect the efficiency of plasma treatment. The obtained results show that the oxidation and inactivation effects depend on the amount of charge delivered by the plasma. The charge-dependent decolorisation and inactivation rates of plasma discharge treatment, which indicate the oxidation efficiency and inactivation efficiency, were obtained and analysed. Different decolorisation and inactivation rates were achieved with various electrode topologies and energisation polarities. This study also investigated the production of reactive species by atmospheric plasma discharges. Ozone concentration was measured during the decolorisation and inactivation tests. The production of OH radicals by the plasma discharges have also been obtained in this study using terephthalic acid as the chemical probe. The obtained results confirm that the reactive oxygen species play a major role in the plasma discharge treatment. In addition, an attempt of using TiO2 as a catalyst to enhance oxidation and bio-decontamination effects of the plasma discharge treatment has been made. TiO2 was revealed to have the potential to improve the oxidation efficiency of atmospheric plasma discharges. The results obtained and presented in this thesis will help in optimisation of non-thermal plasma systems for chemical and biological decontamination.

Radio frequency atmospheric-pressure plasma jets have gained popularity in recent years, both in academia and industry, due to their ability to produce reactive chemical species at relatively cold gas temperatures. Operating at atmospheric-pressure allows for greater scalability than low-pressure plasma discharges, that are confined to operate in a vacuum chamber, offering advantages over current manufacturing techniques. Operation at atmospheric-pressure has also resulted in growing research into use of plasmas for therapeutic applications in biomedicine. Atmospheric-pressure plasma devices are beginning to be certified as medical devices in clinical settings, utilising their efficient production of reactive species in a cold, dry environment. The underlying mechanisms behind these processes are poorly understood, especially in the highly complex chemical conditions surrounding biomedical applications. Researchers require knowledge of the plasma chemistry to infer what subsequent interactions are taking place. Once a particular mechanism has been established, the plasma chemistry in atmospheric-pressure plasma devices can be tailored and optimised for a particular application. To achieve this, investigators require not only identification of which species are present, but also their concentrations, and how species can be maximised or minimised to yield the best therapeutic effect. Diagnostics are required which can measure reactive species in ambient air, but also identify the underlying plasma dynamics responsible. To this end, novel picosecond two-photon absorption laser induced fluorescence is implemented, allowing for the first time, spatially resolved measurement of plasma produced atomic species in ambient air. Production of atomic nitrogen and atomic oxygen is linked to various plasma parameters such as: molecular admixture, voltage, and operating frequency. A new methodology for measuring plasma power in small radio frequency atmospheric-pressure plasma devices is presented, and has allowed for better understanding of the plasma dynamics. This has identified how reactive species can be maximised through increased plasma electron density, but also how they can be produced most efficiently. Furthermore, this methodology has allowed for the confirmation of different operating modes inside the plasma, in agreement with phase resolved optical emission spectroscopy. With the knowledge gained from how plasma dynamics and plasma chemistry changes with input parameter variations, it has been possible to identify key reactive species in industrial scenarios, such as the case study of photoresist removal at atmospheric-pressure.

Ce travail porte sur l’étude de la cinétique des décharges et post-décharges dans l’azote et les mélanges azotés en écoulement à pressions modérées. Des travaux de modélisations cinétiques ont été effectués sur la base des résultats expérimentaux afin de comprendre les processus de production et destruction des différentes espèces sondées. La thèse est divisée en deux parties suivant les types de décharge et mélanges gazeux étudiés. Dans la partie A, un plasma généré dans des mélanges N2-Ar par une cavité hélicoïdale originale excitée à 27 MHz est étudiée par spectroscopie optique d’émission. Sur la base de ces résultats expérimentaux, un modèle cinétique global couplé à un modèle de la fonction de distribution vibrationnelle de l’état N2(C3Pu, v’=0-4) a été développé. Ces modèles permettent d’analyser nos résultats et démontrent l’importance, dans notre décharge, des mécanismes impliquant les espèces métastables d’azote et d’argon, N2(A3Su+) et Ar(3P0,2). Dans la partie B, des décharges dans des mélanges N2-O2 produites par une cavité coaxiale à 433 MHz et leur post-décharge ont été étudiées par spectroscopie optique d’émission et de spectrométrie de masse. Un modèle cinétique temporel 0D, basé sur les résultats expérimentaux et associé à une simulation de l’écoulement, nous permet de reproduire qualitativement les évolutions des concentrations des émetteurs et de prédire la nature du milieu produit par la décharge et ses caractéristiques en vue de son utilisation pour la chimie des dépôts de couches minces de matériaux organosiliciés pour la microtechnologie. / This work focuses on the study of the kinetics of discharges and post-discharges in flowing nitrogen and nitrogen mixtures at moderate pressures. The kinetic modeling was performed on the basis of experimental results in order to understand the production and destruction processes of the species presents in the plasma. The thesis is divided into two parts according to the type of the discharge and gas mixtures studied. In the frame of the part A, a N2-Ar discharge generated by an original helical cavity excited at 27 MHz is examined by optical emission spectroscopy. Based on experimental results, a global kinetic model coupled with a model of the vibrational distribution function of the N2(C3Pu, v’=0-4) state have been developed. These models enable the analysis of our results and demonstrate the importance of the mechanisms involving metastable nitrogen and argon species, i.e. N2(A3Su+) et Ar(3P0,2). In Part B, discharges in mixtures of N2-O2 generated by a 433 MHz coaxial cavity were studied along with the afterglows by optical emission spectroscopy and mass spectrometry methods. A 0D kinetic model, based on experimental results, is associated with a flow simulation in order to qualitatively reproduce the evolution of concentrations and predict the nature of the environment created by the discharge as well as its characteristics for the applications of thin layer deposits of organosilicon materials for microtechnology.

Le nitrure de gallium (GaN) est un semi-conducteur à grand gap dont les propriétés en font un bon candidat pour remplacer le Si dans le domaine de l’électronique de puissance. Dans cette optique, cette thèse s’est intéressée à l’une des étapes de la conception d’une diode de puissance Schottky à base de GaN épitaxié sur Si : la gravure. L’objectif était alors de développer des procédés permettant une reprise de contact sur une couche enterrée de GaN n+. Ils devaient combiner des performances de gravure compatibles avec des processus industriels et conduire à des profils et états de surface adaptés à la réalisation d’un contact ohmique. La réalisation d’une structure pseudo-verticale, au travers de la gravure par la face supérieure d’une couche de 5 à 6 μm de GaN a été étudiée avec quatre réacteurs plasma différents. Des analyses du plasma et du matériau après gravure ont mis en évidence que le meilleur compromis était obtenu pour des plasmas inductifs en chimie chlorée avec une tension d’auto-polarisation modérée. Il a également été déterminé que l’ajout d’un gaz fluoré dans un plasma de ce type permettait de générer une passivation à même de protéger la surface du GaN. Pour répondre à la chute de vitesse de gravure qu’elle entraîne, un procédé alternant des étapes de gravure et de passivation a été développé et étudié. D’autre part, des recherches ont été menées afin de créer une structure verticale. Afin d’atteindre le GaN n+, des gravures en face arrière de vias de Si d’une profondeur de 300 μm par les procédés cryogéniques standard et STiGer anisotrope ont été effectuées. Un procédé de révélation permettant l’étude des couches composant le tampon, situé entre le Si et le GaN, a été mis au point. Leur gravure par des plasmas chlorés s’est révélée efficace au travers ou non de vias de Si. / Gallium nitride (GaN) is a wide bandgap III-V semiconductor with interesting electrical properties in order to replace Si in the field of power devices. The subject of this thesis was then to study one of the essential steps to realize a GaN based Schottky diode : the etch. The aim was to develop etching processes allowing the realization of an ohmic contact on an embedded GaN n+ layer. They must combine GaN etching performances compatible with industrial requirements and etch profiles and surface states compatible with an ohmic contact. The etching of a 5 to 6 μm GaN layer by four different reactors was studied in order to realize a pseudovertical structure. Plasma and GaN surfaces analyses were performed during and after etching by five developed and optimized processes. These analyses showed that best compromise was obtained for inductive chlorine plasmas with a moderate bias. The creation of a passivation layer which is able to protect GaN surface, thanks to fluorine addition in plasma chemistries was also established. To overcome the etch rate decreases induced by fluorine addition, a time multiplexed etching process, alternating etch and passivation steps, was developed and studied. Researches were also performed to achieve a vertical diode. Etching of 300 μm depth Si vias by standard cryogenic and anisotropic STiGer processes were carried out. A revelation process was also developed in order to study buffer layers etching. Effective buffer etching by chlorine plasma was demonstrated with or without Si vias.

In recent years plasmonic devices have become an interesting area of research due to the sub-wavelength confinement and propagation of radiation, allowing the design of very compact structures. Compact structures are necessary to make smaller integrated optical circuits. Due to the use of metals, plasmonic guides usually show more losses compared to the conventional dielectric guides. Therefore, plasmonic waveguides are not normally used for long distance transmission. However, they are promising for inter-chip or intra-chip communication and also have seen a lot of sensor applications. There has been considerable interest in exploiting the frequency bands in the terahertz regime to open up new frontiers of research across a diverse range of applications. An array of opportunities for creating novel technologies using this frequency band had remained largely unexplored and undeveloped for a considerable period of time due to the lack of suitable sources, as well as lack of guiding and detecting devices. This thesis describes the design, analysis and optimisation of plasmonic devices in optical and terahertz frequencies. A fully vectorial H-field based finite element method has been used in the research reported in this thesis to reveal the modal characteristics of different plasmonic structures. A six layer planar contra-directional nano-coupler has been analysed at optical frequency. Three different modes of propagation were considered to study the characteristics of different properties of the structure, including the coupling length. A design approach has been proposed to make the coupler low loss as well as smaller in length. For the terahertz plasmonics, a rectangular metallic hollow core guide was considered at terahertz frequency. Several modes were considered for the modal analysis of the structure. Modal analysis was performed by changing metal, introducing different dielectric coating in the hollow core, changing the thickness of the metal and dielectric layers and changing the dimensions of the guide. A dispersion analysis was also performed. The criteria for designing very low loss, compact and low dispersion guide have been presented for the structure at the end of the study.

Des efforts de miniaturisation sont nécessaires dans le domaine des dispositifs actifs implantables afin de limiter l’invasivité et de réduire les risques de complications suite aux opérations chirurgicales. Les marges de progression pour la réduction des dimensions tendent à se réduire pour les systèmes actuels tels que les stimulateurs cardiaques, les neurostimulateurs ou les capteurs autonomes in vivo. Une rupture technologique est nécessaire pour permettre de repousser les limites des systèmes actuels. Le titane est un matériau possédant des propriétés de biocompatibilité. Il est stable et inerte en contact avec les tissus humains. De plus, ses caractéristiques mécaniques en font un matériau prometteur pour le développement de microsystèmes implantables. Dans le cadre du projet R&D MISTIC (Micro-Structuration du Titane pour Innovations Cardiologiques), l’objectif principal de cette thèse est de développer une brique technologique sur la gravure profonde du titane pour l’intégration de microsystèmes dans des dispositifs actifs implantables. Des études concernant la gravure profonde du titane en plasma de Cl₂ ont été menées afin de déterminer les mécanismes mis en jeu. L’ajout d’espèces fluorées permet, par la création d’un nouveau chemin réactionnel, d’accroître la vitesse de gravure du titane et d’augmenter la sélectivité par rapport au masque de Nickel. Un procédé de gravure du titane sur une profondeur de 300 μm a été mis au point sur des plaquettes de 100 mm de diamètre. L’application des résultats de ces études et le transfert du procédé vers la gravure pleine plaque ont permis de réaliser des démonstrateurs de traversées électriques en titane. / Miniaturization efforts are required in the field of implantable active devices in order to limit invasiveness and reduce the risks of complications following surgical operations. Progression margins for the reduction of dimensions tend to be reduced for current systems such as cardiac pacemakers, neurostimulators or in vivo autonomous sensors. A technological break is needed to push the limits of current systems. Titanium is a material with biocompatibility properties. It is stable and inert in contact with the human tissues.Moreover, its mechanical characteristics make it a promising material for the development of implantable microsystems. As a part of the MISTIC R&D project (Micro-Structuring of Titanium for Innovations in Cardiology), the main objective of this PhD thesis is to develop a technological brick on the deep etching of titanium for the integration of microsystems in active implantable devices. Studies on the deep etching of titanium into Cl₂ plasma have been carried out in order to determine the mechanisms involved. Further, by adding fluorinated species in the process through the creation of a new reaction pathway, an increase in the titanium etch rate and an improvement of the selectivity with the nickel hard mask were achieved. A process for titanium etching over a depth of 300 μm has been developed on 100 mm diameter wafers. The application of the results of these studies and the transfer of the process to the full wafer etching made it possible to produce the demonstrators with titanium-based electrical feedthrough.