Isabelle Géraud-Grenier

Influence of the power on the particles generated in a low pressure radio frequency nitrogen-rich methane discharge

Particles are generated in low pressure radio frequency (13.56 MHz) CH4/N2 discharges containing 90% of nitrogen. The influence of the radio frequency power supply on the particle presence within the plasma is studied. Particles are evidenced by laser light scattering. The particle formation leads to modifications in the discharge electrical parameters such as the dc self-bias voltage and the phase angle third harmonic. The plasma is analyzed by optical emission spectroscopy by following the temporal evolution of excited species such as CN, N2, N2+, Ar, and He. Finally, the particle morphology and size are analyzed by scanning electron microscopy. The correlation between these results allows a better understanding of the power influence on the particle growth within the plasma.

Nitrogen effect on the dust presence and behavior in a radio frequency CH4∕N2 discharge

In this paper, we have studied the effects of the nitrogen percentage on particles generated in low pressure radio frequency CH4∕N2 discharges. The particle behavior has been analyzed by laser beam extinction and scattering. The nitrogen percentage in the mixture influences the particle presence, behavior, and size in the discharge. For nitrogen percentages greater than 50%, we have evidenced a particle multigeneration and oscillations in particle clouds. These oscillations have been correlated with the discharge electrical parameters.

Determination of the electron temperature by optical emission spectroscopy in a 13.56 MHz dusty methane plasma: Influence of the power

Optical emission spectroscopy is applied to the study of a radiofrequency (13.56 MHz) discharge in methane used to obtain hydrogenated carbon films and particles. The methane dissociation allows the creation of species in the plasma bulk as H2, H, and CH. The emission lines of these species are studied as a function of time and of incident rf power. The electron temperature is determined from the two line radiance ratio method and the corona balance model using the Balmer lines (Hα, Hβ, and Hγ). The incident rf power enhancement in the range 40–120 W leads to the increase in the emission line intensities as the electron temperature decreases. The temporal variations of CH and hydrogen emission lines, of the dc self-bias voltage, and of the electron temperature are correlated both with the particle behavior and growth in the plasma, and with the coating that grows onto the powered electrode.

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.

Photoluminescence of hydrogen amorphous carbon nitrile particles obtained in a 13.56 MHz dusty plasma

In this article, we report the photoluminescence (PL) of small particles generated in CH4/N2 radiofrequency (13.56 MHz) discharges. The particles have been produced with various mixtures of N2 and CH4 gases. The particle PL has been analysed using fluorescence microscopy in air with an Ar+ laser at 488 nm. It appears that the photoluminescence intensity is in relation to the particle size. However, the incorporation of nitrogen modifies the peak position of the PL spectrum.

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.

Influence of the RF electrode cleanliness on plasma characteristics and dust-particle generation in methane dusty plasmas

The methane decomposition in a planar RF discharge (13.56 MHz) leads both to a dust-particle generation in the plasma bulk and to a coating growth on the electrodes. Growing dust-particles fall onto the grounded electrode when they are too heavy. Thus, at the end of the experiment, the grounded electrode is covered by a coating and by fallen dust-particles. During the dust-particle growth, the negative DC self-bias voltage (VDC) increases because fewer electrons reach the RF electrode, leading to a more resistive plasma and to changes in the plasma chemical composition. In this paper, the cleanliness influence of the RF electrode on the dust-particle growth, on the plasma characteristics and composition is investigated. A cleanliness electrode is an electrode without coating and dust-particles on its surface at the beginning of the experiment.

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.

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.