Robert Hugon

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.

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.

Spectroscopic investigation of a temporal post-discharge plasma for iron nitriding

A pure nitrogen plasma operating in a low-frequency (<1 kHz) DC pulsed mode is used for iron nitriding and is characterized by means of optical emission spectroscopy. Line intensities related to N2 or N2+ states involved in the surface treatment, as well as an iron line linked to the cathode sputtering, are maintained during the discharge and the temporal post-discharge, and are compared with the electron density evolution. The processes governing the population of excited states are quite different when the discharge is on (electron collisions) and when the discharge voltage is cut off (importance of long lifetime metastable N2 states)-justifying the interest of the post-discharge phase during which active species are created in a surface treatment process.

a-SiC x N y thin films deposited by a microwave plasma assisted C VD process using a CH 4 /N 2 /Ar/HMDSN mixture: methane rat e effect

Amorphous silicon carbonitride thin films were deposited using a microwave plasma assisted chemical vapour deposition process fed with a mixture of methane, nitrogen, argon and hexamethyldisilazane (Si2C6H19N). Effects of the methane rate on thin films composition, n anostructuration and characteristics are investigated by means of various techniques such as X-ray Photoelectron Spectroscopy, Fourier Transform Infrared Spectroscopy, Transmission Electron Microscopy and UV-Visible absorption. The raise of the methane rate results in less organic, denser films and in an increase of refractive index.

Diagnostics of a d.c. pulsed-plasma-assisted nitriding process

Abstract Plasma diagnostics is an in-situ tool well adapted to study the processes involved in surface modification. In this paper, we report some experiments carried out using optical emission spectroscopy and electrostatic probes, and performed in an iron nitriding d.c. pulsed plasma. We show that some excitation processes during the temporal postdischarge are the same as those which occur in the discharge. Moreover, results about the effect of the discharge power on the excitation of particular nitrogen levels are discussed. Some first results about the experimental determination of the electron energy distribution function are also presented.

Influence of gas temperature on the loss mechanisms of H-atoms in a pulsed microwave discharge identified by time-resolved LIF measurements

The present work deals with investigating the loss kinetics of atomic hydrogen in an H2/CH4 pulsed microwave discharge used for diamond chemical vapour deposition. The temporal evolution of the hydrogen atom density, particularly in the afterglow phase, was investigated in order to determine the fundamental loss processes occurring in such discharge. Time resolved two-photon laser induced fluorescence (LIF) measurements of the relative concentration of H-atoms in the ground state and observed by Hα transition have been established. The gas temperature was estimated from the Doppler broadening of the LIF Gaussian profile obtained by scanning the laser frequency. The wavelength calibration and the estimation of the effective bandwidth of the LIF profile were accomplished by measuring the spectral response of atomic hydrogen and deuterium. The decay of H-atom density in the afterglow is characterized by a fast decrease in the early afterglow up to 2 ms, followed by a slow evolution. The effect of the gas cooling on the diffusion coefficient and then on the atom density decay in the afterglow was studied. It is found that the fundamental loss processes of atomic hydrogen are governed by the diffusion phenomenon and surface recombinations, especially on the diamond substrate.

Time resolved determination of the electron energy distribution function in a DC pulsed plasma

We present an experimental device and a numerical procedure which allows the time-resolved sampling and analysis of electrical I - V probe characteristics in a low frequency DC pulsed plasma used for iron nitriding. Plasma parameters such as electron density and energy and plasma potential are deduced from the experimental determination of the electron energy distribution function f(E). The use of the second derivative technique allows us to follow the time variation of the electron population in the early afterglow. The non-Maxwellian form of f(E) in the afterglow is explained in terms of kinetic processes leading to vibrationally excited states of molecular nitrogen which are involved in the plasma reactivity. Particular study of the real duration of the afterglow shows that the charged species remain in the gas phase up to 14 ms after the discharge is cut off. These results, combined with the study of f(E) in the afterglow, lead us to establish time ranges for the discharge and post-discharge durations which appear favourable for the nitriding process.

Study of the surface mechanisms in an Ar–N2 post-discharge cleaning process

The purpose of the present investigation is to gain understanding of the mechanisms responsible for the cleaning properties of Ar–N2 post-discharges at room temperature. The evolution of the gas-phase composition is monitored by optical emission spectroscopy, and X-ray photoelectron spectroscopy is used to characterize the surface of iron samples initially covered by a contamination layer at various treatment times. Both techniques suggest that cleaning is at least a two-step process, beginning with the removal of aliphatic carbon from the surface.

Surface cleaning and passivation of an iron foil by a nitrogen post-discharge surface treatment

Abstract Interactions occurring between an argon–nitrogen post-discharge and oxidised iron surfaces are studied by measurements based on optical emission spectroscopy. Transient effects observed for the first 10 min shed light on the partial cleaning which permanently modifies the topmost surface of oxidised iron substrates, as shown by XPS analysis. A special device was built to analyse the treated substrates without breaking the vacuum. The influence of the temperature on surface cleaning is determined. At room temperature, nitrogen is grafted on the iron topmost surface, very likely by means of NC  O groups. This is no longer the case at 323 K and 373 K. In any case, the iron foil is not completely cleaned but covered by a thin film which partially protects the iron foil against corrosion.