Modélisation numérique d une décharge DC poussiéreuse (couplage entre équilibre de la décharge, croissance moléculaire et dynamique d aérosol)

L objectif était d améliorer la compréhension des phénomènes conduisant à la formation de particules solides de carbone produites par pulvérisation d une cathode en graphite dans une décharge DC en développant un ensemble d outils numériques. Nous avons tout d abord modélisé la décharge DC, puis la croissance moléculaire et le transport des précurseurs, couplés à la dynamique d aérosol des particules dans cette décharge électrique. La distribution des particules est appréhendée au travers de valeurs moyennes. Nous avons établi le schéma de croissance moléculaire en montrant le rôle essentiel des clusters négatifs. Nous avons ensuite étudié la croissance des particules qui se forment dans la zone d inversion de champ électrique et croissent principalement par collage. L effet de la coagulation sur la taille des particules reste faible bien que son impact sur leur densité ne puisse être négligé. Le modèle montre un bon accord avec les mesures expérimentales. Cependant, la coagulation des particules ne peut pas être fidèlement décrite. De plus, en fin de décharge, l hypothèse selon laquelle l équilibre de la décharge n est pas affecté par les clusters et les particules n est plus valide. C est pourquoi nous avons étendu notre modèle pour décrire la décharge poussiéreuse et l avons couplé aux modèles des clusters et des particules. Nous avons également développé un modèle sectionnel permettant de mieux décrire la distribution en taille des particules. Nous avons montré que la coagulation joue un rôle important sur la distribution en taille des particules. Les distributions simulées sont en très bon accord avec les observations expérimentales.The objective was to improve the understanding of the formation of solid particles of carbon produced by graphite cathode sputtering in a DC discharge with a set of numerical tools. We first modeled the DC discharge, then the growth and transport of molecular precursors, coupled to aerosol particle dynamics in the discharge. Particle distribution is represented by average values. We established the molecular growth scheme showing the importance of negative clusters. We then studied the growth of particles trapped in the electric field, which is controlled by molecular sticking. The influence of coagulation on particle size remains low although its impact on their density is important. The model shows good agreement with experimental measurements. However, coagulation of particles is not accurately described. Moreover, at the end of discharge, the assumption that the discharge equilibrium is not affected by clusters and particles is no longer valid. So we have extended our model to describe the dusty discharge and coupled it with models of clusters and particles. We also developed a sectional model for a better description of the particle size distribution. We have shown that coagulation plays an important role on the size distribution of particles. Simulated distributions are in good agreement with experimental observations.PARIS13-BU Sciences (930792102) / SudocSudocFranceF

On the Competition Between the Phenomena Involved in the Aerosol Dynamics in Sputtering Nonequilibrium Plasma

International audience— We made use of a combination of three models that describe plasma equilibrium, cluster kinetics and aerosol dynamics in order to investigate solid carbonaceous dust formation in a DC gas discharge where the carbon source is provided by a graphite cathode sputtering. This enabled us to determine the time-evolution of some of the particle characteristics, e., g. total density, mean-diameter, average-charge as function of the sputtering yield. We showed that for few minutes discharge duration, the particle density reaches 10 7-10 8 cm-3 , the particle mean diameter is approximately 40 nm and the particle size distributions are bimodal for all the investigated conditions. We also showed that the variation of the sputtering yield affects in a quite unexpected way the interplay between the phenomena involved in the aerosol dynamics. Basically, larger sputtering yields result in larger density and coagulation rate on one hand, and in a limited nucleation and surface sticking kinetics on the other hand

Stochastic models of systems for Nanotechnology: from micro to macro scale

Computer modeling technique based on the theory of stochastic processes have been used in order to provide a realistic simulation of the behavior of nanoscopic systems, related in particular to plasma reactors in microelectronic device production. Basing on decades of experience, we show here, with new results, that the universality of such methods allows the development of codes with the highest reusability and versatility, crossing the barrier of scale. At the smallest scale, the quantum calculation of the potential energy surface and spectroscopic properties of hydrogen species under nanoconfinement conditions display the effects due to the dimension and the symmetry of the confining potential well. Nanoparticles dispersed as aerosol in plasma feature strong fluctuations in temperature and charge which may affect the processing of silicon wafers. At the macroscopic scale, using a stochastic solution of transport equations, it is possible to describe laboratory or industrial systems for the production or treatment of nanomaterials, also exploiting the analogy between neutral particle transport and radiative transfer and information obtained by molecular simulations. These findings are relevant in the control of solid-particle contamination in the manufacture of electronic components and in other fields

DFT simulations for predicting tungsten potential energy surfaces for W-Wn and W+-Wn pair interaction

International audienc

Nanoparticle formation and dusty plasma effects in DC sputtering discharge with graphite cathode

International audienceWe developed a model for the nucleation, growth and transport of carbon dust particles in a DC discharge. The carbon source comes from the sputtering of a graphite cathode resulting in the production of primary clusters and then of nanoparticles. We consider the ionic cluster growth as well as the particle growth and charging and the influence of both on the discharge equilibrium. We found that the discharge becomes electronegative for long duration when particle density reaches 10 9 cm-3 and particle size 45 nm. The corresponding transition modifies the electric field profile in the vicinity of the field reversal region in the negative glow. We then analyze the space and time evolution of the different discharge characteristics and the mechanisms involved in the discharge. We showed that particle density is governed by nucleation, coagulation and transport, while particle size is mainly governed by the deposition of the small neural clusters emitted at the cathode on the particle surfac

Stochastic models of systems for Nanotechnology: from micro to macro scale

Computer modeling technique based on the theory of stochastic processes have been used in order to provide a realistic simulation of the behavior of nanoscopic systems, related in particular to plasma reactors in microelectronic device production. Basing on decades of experience, we show here, with new results, that the universality of such methods allows the development of codes with the highest reusability and versatility, crossing the barrier of scale. At the smallest scale, the quantum calculations of the potential energy surface of hydrogen species under nano-confinement conditions display the effects due to the dimension and the symmetry of the confining potential well. At a slightly larger scale, nanoparticles dispersed as aerosol in plasma feature strong fluctuations in temperature and charge which may affect the processing of silicon wafers. At the macroscopic scale, using a stochastic solution of transport equations, it is possible to describe laboratory or industrial systems for the production or treatment of nanomaterials, also exploiting the analogy between neutral particle transport and radiative transfer and information obtained by molecular simulations. These findings are relevant in the control of solid-particle contamination in the manufacture of electronic components and in other fields

Molecular dynamics approach for the calculation of the surface loss probabilities of neutral species in argon-methane plasma

International audienceMolecular dynamics simulations are carried out for calculating the surface loss probabilities of neutral species from an argon-methane plasma. These probabilities are the sum of the sticking and surface recombination probabilities. This study considers both the formation of reactive and nonreactive volatile species for evaluating recombination probabilities. Results show that stable species are reflected when hydrocarbon film starts growing on the surface. CH3 is mainly lost by surface recombination leading to formation of volatile products while very little contributes to film growth. C2H has surface loss probability in agreement with literature. While C2H loss is usually attributed to sticking on the surface, our results show that its main loss process is due to surface recombination

Molecular growth paths and dust-particles nucleation precursors in Ar/C2H2 low pressure discharges

International audienceA numerical model was used to investigate the effect of acetylene concentration in the feed gas on molecular growth and solid particle nucleation in argon-acetylene CCRF discharges. Results showed that for large acetylene conversion yields and small acetylene concentrations in the discharge, the ionization kinetic is driven by Penning process and the molecular growth is governed by neutral polyynes. The cluster size distribution predicted shows that nucleation is unlikely in this situation. On the opposite, for limited conversion yields and significant acetylene concentrations in the discharge, the ionization kinetic is driven by electron-impact process; the molecular growth is governed by positive ions and leads to significant concentrations of neutral polyynes and enhanced nucleation rate