Jean-François Lagrange

Optical diagnostics of dusty plasmas during nanoparticle growth

Carbon-based thin films deposited on surfaces exposed to a typical capacitively-coupled RF plasma are sources of molecular precursors at the origin of nanoparticle growth. This growth leads to drastic changes of the plasma characteristics. Thus, a precise understanding of the dusty plasma structure and dynamics is required to control the plasma evolution and the nanoparticle growth. Optical diagnostics can reveal some particular features occurring in these kinds of plasmas. High-speed imaging of the plasma glow shows that instabilities induced by nanoparticle growth can be constituted of small brighter plasma regions (plasmoids) that rotate around the electrodes. A single bigger region of enhanced emission is also of particular interest: the void, a main central dust-free region, has very distinct plasma properties than the surrounding dusty region. This particularity is emphasized using optical emission spectroscopy with spatiotemporal resolution. Emission profiles are obtained for the buffer gas and the carbonaceous molecules giving insights on the changes of the electron energy distribution function during dust particle growth. Dense clouds of nanoparticles are shown to be easily formed from two different thin films, one constituted of polymer and the other one created by the plasma decomposition of ethanol.

Electron Temperature Evolution in a Low-Pressure Dusty RF Nitrogen-Rich Methane Plasma

Particles are generated in a classical planar RF (13.56-MHz) reactor in nitrogen-rich methane at low pressure (120 Pa). The gas decomposition leads to particle generation and growth. During their formation, the particles become negatively charged. The electrical and the optical parameters of the discharge are disturbed. The presence of particles in the plasma is put in evidence by laser light scattering and is correlated to the DC self-bias voltage. A small quantity of argon is introduced in the gas mixture in order to estimate the electron temperature. The temporal evolutions of both the electron temperature and the optical emission intensities of excited argon are correlated with the particle growth and behavior in the plasma.

Optical diagnostic and electrical analysis in dusty RF discharges containing plasmoids

The presence of hydrogenated carbon nitride a-CNx:H particles confined in an argon dusty discharge induces the appearance of instabilities. Those instabilities, also called plasmoids, are luminous regions which move through the plasma and rotate around the biased electrode circumference. Electrical characteristics of the plasma have been used to evidence the presence of dust particles and to demonstrate that plasmoid appearance is triggered by particles. The light emitted by the plasma is analysed by optical emission spectroscopy. This paper presents the spatial distribution of excited species, such as CN, Ar I… between electrodes both inside plasmoids and in the surrounding dusty plasma. Obtained results allow to get information for the electron energy distribution function. Moreover, the interplay between plasmoid behaviour and particle presence in the plasma is shown.

Plasma response to nanoparticle growth

The influence of nanoparticle growth on the plasma characteristics is studied by analyzing the spatiotemporal evolution of several argon lines. It appears that some lines are promoted in dusty areas while other ones have an enhanced emission in dust-free regions like the void. This effect is related to the spatial dependence of the electron energy distribution: the particularly small electron energy in dust-free regions of a dusty plasma results in enhanced excitation of certain argon energy levels from metastable atoms.

How the emission spectroscopy can determine the effects of dust particles on the plasma

Summary form only given. Dusty plasmas [1] are found in many astrophysical environments such as comet tails, planetary nebulae and rings or in fusion devices like the future ITER. In industrial and laboratory reactors, these dust particles [2] become a huge problem, particularly in microelectronics. However, these particles could be used in many industrial applications related to nanotechnology. So it is important to study the production of these solid particles. At GREMI laboratory, several methods are used to create dust particles in a plasma. They are mainly based on reactive gases or material sputtering. In this work, experiments are performed in a capacitively-coupled RF discharge in the PKE-Nefedov reactor [3], where dust particles are grown by sputtering a polymer layer in Ar or Kr plasmas. The presence of dust particles in plasmas can strongly change their properties like the light emission. This modification is due to a change in the plasma parameters such as electron temperature and density. Emission spectroscopy is used to analyze the light emission, more precisely to study the spatiotemporal evolution of the Ar emission and the molecules involved in the dust particle growth like: CN, CH and C2. When dust particles are growing in the plasma, a laser at 685 nm is also used to highlight their presence. Their localization is determined by recording the scattered light with the spectrometer. Other diagnostics are also used to follow dust particle growth like a CCD camera and the measurement of the discharge current.

Particle Movement in a Dusty RF Plasma at Power Switch-OFF

Particles are generated in a capacitive radiofrequency methane-nitrogen discharge. Forces act on them; they levitate and form two clouds at the sheath boundaries close to the electrodes. In each cloud, particle size segregation is observed. At the plasma extinction, dust particle clouds show different movements. In addition, particle movement within each cloud depends on the particle size. The movements are due to the plasma extinction, the electrostatic interaction between clouds and the particle residual charge.