Safa Labidi

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.

Modelling of argon-acetylene dusty plasma

The properties of an Ar/C2H2 dusty plasma (ion, electron and neutral particle densities, effective electron temperature and dust charge) in glow and afterglow regimes are studied using a volume-averaged model and the results for the glow plasma are compared with mass spectrometry measurements. It is shown that dust particles affect essentially the properties of glow and afterglow plasmas. Due to collection of electrons and ions by dust particles, the effective electron temperature, the densities of argon ions and metastable atoms are larger in the dusty glow plasma comparing with the dust-free case, while the densities of most hydrocarbon ions and acetylene molecules are smaller. Because of a larger density of metastable argon atoms and, as a result, of the enhancement of electron generation in their collisions with acetylene molecules, the electron density in the afterglow dusty plasma can have a peak in its time-dependence. The results of numerical calculations are in a good qualitative agreement with experimental results.

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.

Nanoparticle Growth in Ethanol Based Plasmas

Nanoparticles are grown in a capacitively-coupled radio-frequency discharge (ccrf) in argon from the sputtering of a carbonaceous film deposited on the electrodes. This brown film was previously formed from the ethanol decomposition obtained in argon/ethanol plasmas. During the nanoparticle growth, optical emission spectroscopy reveals the evolution of some typical carbonaceous molecules. The nanoparticle formation also disturbs the plasma equilibrium and induces several plasma instabilities consisting in some cases in regular plasma rotation at very low frequencies. Once nanoparticles are large enough to be observed, they constitute a dense cloud trapped in between the electrode with one central or two symmetrical voids. Ex-situ analysis by scanning electron microscopy evidences that grown nanoparticles can have original surface stuctures.

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.

Mass spectrometry to control dust particle growth in an acetylene plasma

Summary form only given. Dust particles are involved in a wide range of applications like thin film deposition or the manufacturing of microelectronic devices. They are an essential subject of research in the growing field of nanotechnology due to their interesting chemical and physical properties. In this study, nanoparticles are formed in a capacitively coupled asymmetric discharge running in an Ar/C2H2 mixture at a frequency of 13.56 MHz and RF-power of 9 W. The presence and growth of dust particles in a plasma change its properties like electron density or temperature. So to understand the formation and the growth of nanoparticles, it is necessary to study the complex physicochemical reactions in occuring hydrocarbon plasmas. Acetylene is used to provide the reactive species leading to dust particle growth. This cyclic process can be monitored in situ thanks to several methods [1]. Particularly, the different stages of dust particle growth like the nucleation and agglomeration can be evidenced by correlating the evolution of the electrical characteristics of the discharge with time resolved mass spectrometry. In this presentation, the evolution of the self-bias voltage [2] can be correlated with the acetylene concentration during nanoparticle growth.