Gérard Henrion

Effects of hydrogen on iron nitriding in a pulsed plasma

A low frequency DC pulsed plasma used for iron nitriding is studied by means of an electrostatic probe and optical emission spectroscopy. The evolution of the electron density leads to the determination of the electron-ion recombination coefficient. The variations of this coefficient and the measurement of some emission lines of neutral iron and of excited states of neutral and ionic molecular nitrogen as a function of the percentage of H2 give additional insights into the role of hydrogen during the nitriding process. Thus the presence of vibrational states of nitrogen-N2(C3 Pi u, B3 Pi g, A3 Sigma u+), N2+(B2 Sigma u+)-late in the afterglow confirms their importance in such plasma assisted surface treatments.

The influence of the respective durations of the discharge and the afterglow on the reactivity of a DC pulsed plasma used for iron nitriding

The optical diagnostics of a DC pulsed plasma used for iron nitriding is carried out for various plasma conditions. The fluorescence of the first negative and the second positive systems of is observed in the time afterglow, in order to obtain some insights into the creation and the loss processes of and states. An iron emission line is also observed which is indicative of the sample sputtering. Particular attention is paid to the influence of the gas pressure on the respective durations of the discharge and the afterglow. Analysis of the plasma both during the discharge and during the post-discharge leads to the determination of favourable conditions for the surface treatment. It is also shown that the electron density in the discharge can be deduced from the line intensity decay in the afterglow.

Time-resolved plasma diagnostics for a better understanding of the improvement of pulsed MWPACVD of diamond

The deposition of diamond layers from CH4-H2 microwave discharge operating in pulsed mode has been achieved. It has been shown that the variation of the time parameters of the process (frequency and duty cycle) leads to noticeable modifications of the deposited layers. From plasma diagnostic measurements, the change in plasma composition has been determined and correlated with the quality and growth rate of the diamond thin films. Particular attention has been paid to the concentration of H-atoms, CH and C2 radicals and their evolution during the discharge regime and the afterglow. Indeed, these species are well known either as agents for graphite etching (H), or diamond precursors (CHx imaged by CH) or graphite precursors (C2Hx imaged by C2). Optimum values of the power pulse repetition rate (500 Hz) and duty cycle (50%) have been found which are correlated with the variation of the relative concentrations of H, CH and C2 with time, especially during the afterglow. It has been shown that these optimum conditions correspond to a minimization of C2 in the afterglow while H and CH concentrations remain high enough to continue the diamond deposition process after the power is switched off.

Diamond thin film growth by pulsed microwave plasma at high power density in a CH4–H2 gas mixture

Abstract Microwave plasma assisted chemical vapour deposition in hydrogen–methane gas mixture is one of the most widespread methods used for the growth of diamond thin films. It has been proved that at low microwave power (300 W) the modulation of the power can lead to a significant improvement of the growth rate or the quality of diamond films. In this work, we show that this advantage can be validated at an industrial scale in reactors working at higher power (peak power up to 6 kW). The pulsed mode operation shows that whatever the frequency or the duty cycle, the quality of diamond (according to the micro-Raman spectroscopy) can be appreciably improved at the same growth rate. An optimal range of temporal parameters as the pulse and afterglow duration, corresponding to an optimal range of frequency and duty cycle has been determined. The main reactive species observed by optical emission spectroscopy in such plasmas are the CH and C 2 radicals as well as atomic hydrogen. The chemistry of CH 4 /H 2 discharge is very rich in hydrocarbon radicals. Nevertheless, some works have shown that the CH x radicals are the key species for the diamond deposit whereas C 2 H y is rather responsible for the graphite growth and the atomic hydrogen preferentially for the graphite etching. Time resolved optical emission spectroscopy and double pulse technique have been carried out to investigate the evolution of the active species during the discharge on-time as well as during the afterglow. These plasma studies lead to a better understanding of the advantages in modulating the power and allow establishing some correlations between the temporal evolution of the observed species and the characteristics of the deposited films.

Impacts created on various materials by micro-discharges in heptane: Influence of the dissipated charge

Modes of energy dissipation in impacts made on various materials (Al, Cu, Fe, and Si) by discharges in heptane are investigated for micro-gap conditions. Bulk metals and thin films of 300u2009nm in thickness deposited on silicon wafers are used as samples. Positive high voltage pulses with nanosecond rise times make it possible to isolate a single discharge and to study the way the charge delivered by the power supply is transferred to the larger electrode (the sample) in a pin-to-plate configuration. The diameter of the impacts created by the plasma varies linearly versus the charge raised at a power close to 0.5. However, the exact value of the power depends on the material. We also show how the impact morphologies change with the applied charge. At high charges, the diameters of impacts on thin films behave as those made on silicon. At low charges, they behave as the bulk material. Finally, we show that the energy dissipated in impacts is below a few percent.

Modelling the pulsed glow discharge of a nitriding reactor

Abstract Through a detailed study, involving a close coupling between numerical modelling and experimental investigations, a better understanding of the underlying physical mechanisms in the low-frequency DC pulsed regime of a nitriding plasma reactor has been obtained. The modelling, based on a one-dimensional electric model with two populations of electrons (beam and bulk), leads to an accurate description of the different regions of the discharge, including the negative glow. Current–voltage characteristics of the discharge as well as the evolution of line emission intensities measured by space- and time-resolved optical emission spectroscopy, have been obtained for various operation regimes. In analysing the experimental results obtained, together with relevant simulations, clear evidence has been obtained that a heating process of the neutral gas occurs and plays an important role in usual operation regimes. An actual self-consistent modelling including this effect is under way.

The evidence of cathodic micro-discharges during plasma electrolytic oxidation process

Plasma electrolytic oxidation (PEO) processing of EV31 magnesium alloy has been carried out in fluoride containing electrolyte under bipolar pulse current regime. Unusual PEO cathodic micro-discharges have been observed and investigated. It is shown that the cathodic micro-discharges exhibit a collective intermittent behavior, which is discussed in terms of charge accumulations at the layer/electrolyte and layer/metal interfaces. Optical emission spectroscopy is used to determine the electron density (typ. 1015u2009cm−3) and the electron temperature (typ. 7500u2009K) while the role of F− anions on the appearance of cathodic micro-discharges is pointed out.

Plasma diagnostics for the control of reactive magnetron deposition process

Abstract Many plasma diagnostic methods are usually studied in laboratories in order to understand the fundamental processes that govern the plasma. This paper aims to study the potentialities of such diagnostics performed in a specific magnetron industrialtype deposition reactor in which the form of the time variation of the electrical plasma parameters ( U ( t ); I ( t )) depends on the mean power applied to the electrodes. In particular, Langmuir probe and optical emission spectroscopy (OES) measurements are carried out in the case of aluminium nitride and oxide deposition. Ex-situ coating analysis is performed by means of optical microscopy, scanning electron microscopy, microprobe analysis and Auger spectroscopy. In this paper we show that the Langmuir probe is not a well-adapted diagnostic method to study such reactive plasmas. Indeed, the high deposition rate implies a fast modification of the probe geometry and electrical properties due to the deposition of a thin film on the probe tip. On the other hand, OES measurements are related to the stoichiometry of the deposited coatings. Some phenomenological interrelations between the mean lines intensity and the coating stoichiometry are found, which may be used to control the deposition process.

Theoretical background of optical emission spectroscopy for analysis of atmospheric pressure plasmas

This review contains a theoretical background of optical emission spectroscopy and some selected examples of issues in the field of atmospheric plasmas. It includes elements like line broadening, emission of continua and molecules, radiation models, etc. Modernized expressions figuring the terms hidden in global constants where cgs units prevail are given together with restrictions of use. Easy-to-use formulas are provided to give access to essential plasma parameters.

Effects of Ar―H2―N2 microwave plasma on chitosan and its nanoliposomes blend thin films designed for tissue engineering applications

This work addresses the functionalization of chitosan thin films and its nanoliposomes blend films by a microwave-excited Ar/N2/H2 surface-wave plasma treatment which was found an effective tool to modify surface properties. Changes in the film properties (wettability, chemical composition, morphology) induced by the plasma treatment are studied using water contact angle measurements, X-ray photoelectron spectroscopy and scanning probe microscopy. The results suggest that hydrophilicity of the films is improved by plasma treatment in a plasma condition dependency manner. Water contact angle of chitosan films before and after plasma treatment are, respectively, 101° and 27°. Besides chemical changes on the surface, the nanoliposomes incorporation and plasma treatment also induce morphological modifications. Moreover, a correlation is found between the nanoliposomes composition and size, and the effects of plasma treatment. It is shown that the plasma treatment significantly improves the chitosan film functionalization. The effect of N2 content (88% and 100%) in the plasma gas mixture on the film etching is also pointed out.