Sonde de Langmuir | EPFL Graph Search

Une sonde de Langmuir est un instrument qui permet de mesurer la température et la densité électronique, ainsi que le potentiel électrique d'un plasma. Elle consiste à insérer une ou plusieurs électrodes dans un plasma, avec un potentiel électrique constant ou variable dans le temps entre les électrodes ou entre celles-ci et le contenant. Les potentiels et courants électriques mesurés permettent de déterminer les grandeurs physiques du plasma. Étymologie La sonde est nommée d'après le nom du physicien Irving Langmuir. Considération pratiques En laboratoire, les électrodes sont normalement faites de fils de tungstène d'une fraction de millimètre (mm) d'épaisseur. Ce choix s'explique par la haute température de fusion du matériau, ainsi que la possibilité de lui donner une petite taille. Le molybdène est parfois utilisé. Dans les plasmas basse température, où la sonde ne devient pas trop chaude, la contamination de surface peut devenir non négligeable. Il peut en

Experiments and modelling for glow discharge plasmas applied to niobium sputter deposition in superconducting radiofrequency cavities

Thibaut François Hugo Richard

Thin film coatings are ubiquitous in modern daily lives, from the semi-conductor industry to aesthetic applications, and rely on the modification of a given material to enhance its surface properties. At CERN (European Organization for Nuclear Research), thin film coatings are used for several applications related to the accelerator technology field, such as electron cloud mitigation, continuous pumping or radio-frequency (RF) superconductivity. This PhD thesis aims at studying the use of a Particle-in-Cell Monte Carlo/ Direct Simulation Monte Carlo (PICMC/DSMC) numerical code applied to the modelling of deposition processes. The present study focuses on niobium thin film deposition on copper RF cavities of peculiar geometries and large sizes. Numerical simulations can help to understand and improve process parameters in existing systems, and guide the design of future coating systems such that thin ï¬lm characteristics can be predicted and expensive times of Research and Development can be reduced. In the first part of this thesis, critical parameters are identified to provide an accurate modelling of glow discharge plasmas commonly used in coating applications with the PICMC module of the numerical code. A dedicated experimental system is designed to enable both DC diode and DC magnetron operation with a coaxial cylindrical plasma source. Suitable ion-induced secondary electron emission yield and energy distribution are numerically assessed by matching simulated discharge voltages and currents with experimental ones. Simulated local plasma parameters, such as electron density and energy, are compared with experimental measurements obtained with a Langmuir probe. In the second part, transport simulations of sputtered neutral niobium atoms performed with the DSMC method are validated by comparing simulated deposition rates on a substrate with experimental ones with a compact hollow cathode magnetron sputtering source. At last, the validated ab initio methodology for coating process modelling, from plasma simulation and extraction of sputtering profiles to sputtered niobium atoms transport, is applied to typical coating systems used at CERN. The scalability of low discharge power simulation results to realistic powers is first demonstrated for a planar magnetron source in terms of cathode erosion and deposited thin film thickness profiles. Then, an elliptical RF cavity is used as a case study to apply the full methodology in a real substrate of complex shape. PICMC and DSMC simulation results are further extended towards thin film growth modelling, and simulated thin film morphology is compared with experimental Focused Ion Beam/Scanning Electron Microscopy imaging of the niobium layer.

EPFL2019

Design, characterisation and modelling of a high current DC arc plasma source for silicon and silicon carbide processing at low pressure

Lukas Derendinger

In the frame of this thesis, two similar high current DC arc (HCDCA) plasma sources were investigated in a low gas pressure regime (10-3-10-2 mbar). One of them was initially designed for the epitaxial growth of silicon and silicon germanium (LEP), the other for the industrial deposition of diamond (BAI). The LEP source was analysed using pure argon plasmas. Measurements of the ion saturation current were performed with a custom-built multi-Langmuir probe to analyse plasma density homogeneity. Plasma instabilities at 50 Hz were observed and studied by different means. One source of the instability is the AC current used for the filament heating, whereas lower frequency instabilities are due to the use of a ring shaped anode. A sensitive Hall sensor was used to measure the magnetic field induced by the discharge current. It was found that depending on the plasma parameters gas pressure and external magnetic field the current tends to attach at different points on the anode, leading to a complete loss of reactor symmetry. The installation of an additional cusp field around the reactor chamber lead to an increase of the overall homogeneity of the plasma density, but it could not resolve the problems of current attachment. In the following the ring anode was replaced by a point anode and the maximum external magnetic field strength was increased by a factor of ten. With a novel multi-Hall probe the current density of the now columnar shaped plasma was measured, showing a strongly peaked current density profile, when an external magnetic field is applied. Together with ion saturation current measurements made with a Langmuir probe, the electron temperature inside the plasma column was estimated to be about 4 eV. In earlier works made on the BAI reactor the high dissociation efficiency of the HCDCA plasma source has already been shown. Optical emission spectra were compared between RF plasmas, low pressure (1.5mbar) HCDCA plasmas and our very low pressure (10-3-10-2 mbar) HCDCA plasmas. The dominating species found in RF plasma spectra are molecular, the spectra of low pressure HCDCA plasmas are dominated by atomic species and the spectra of very low pressure HCDCA plasmas are dominated by ions, emphasising the high dissociation efficiency of this system. Silane and Methane was used as precursor gases in the BAI reactor for the deposition of silicon carbide films. Promising high deposition rates up to 9 nm/s were found, but FTIR spectroscopy showed high hydrogen and oxygen concentrations in the porous films, making the not optimised deposited material useless for the application of wear-resistant coatings. Microcrystalline hydrogenated silicon (µc-Si:H) is viewed as a cost- and energy-effective alternative to crystalline silicon for the production of solar cells. A detailed analysis of the deposition rate and the Raman crystallinity of µc-Si:H films deposited in the BAI reactor showed deposition rates up to 6.5 nm/s and a wide range of crystallinity from 0-80%. Film thickness inhomogeneity in silicon solar cells has to be less than 5%. To meet this standard a first substantial improvement was achieved with the installation of a linear gas injection along the plasma column. Tests on large surface glasses (47 × 37 cm2) revealed strong diffusive effects, which could be reproduced with a simple gas diffusion model. The model showed the necessity to reduce the dead volume around the plasma and to set the substrates as close as possible to the plasma column in order to minimise film thickness inhomogeneity due to diffusion. Deposition rate measurements made in these conditions assured the results of the model. The development of a method to estimate the dissociation efficiency of the plasma by simple pressure measurements showed also an important increase of the silane dissociation from 75-92% when the plasma is confined in a smaller volume. Therefore, compared to the reactor with a large dead volume, only a third of the initial silane is lost to the pumps.

EPFL2008

Influence of ion-induced secondary electron emission parameters in PICMC plasma simulations with experimental validation in DC cylindrical diode and magnetron discharges

Ivo Furno, Thibaut François Hugo Richard, Alban Sublet

Niobium thin lms are used at CERN (European Organization for Nuclear Research) for coatings of superconducting radio-frequency (SRF) accelerating cavities. Numerical simulations can help to better understand the physical processes involved in such coatings and provide predictions of thin lm properties. In this article, Particle-in-Cell Monte Carlo (PICMC) 3D plasma simulations are validated against experimental data in a coaxial cylindrical system allowing both DC diode and DC magnetron operation. A proper choice of ion induced secondary electron emission (IISEE) parameters enables to match experimental and simulated discharge currents and voltages, with argon as the process gas and niobium as the target element. Langmuir probe measurements are presented to further support simulation results. The choice of argon gas with a niobium target is driven by CERN applications, but the methodology described in this paper is applicable to other discharge gases and target elements. Validation of plasma simulations is the rst step towards developing an accurate methodology for predicting thin lm coatings characteristics in complex objects such as SRF cavities.

2020