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 Physicochemical processes in nonequilibrium plasma

par Agnès Gaunie Picart - publié le

This topic can be separated into two parts :

Investigations of low-temperature plasma of gas discharges at atmospheric pressure

The development of a plasma enhanced chemical vapor deposition (PECVD) process working at atmospheric pressure and which allows an easy on-line treatment of films, fibbers and plates is a challenge of great interest. The successful solution will avoid batch treatments and thus significantly reduces the cost of the coating, contributing to an expansion of plasma technologies outside microelectronics.

Dielectric Barrier Discharges (DBDs) appear as ones of the most suitable discharges because they are non-thermal plasmas, robust and not disturbed by the motion of the substrate. DBDs normally operate in the usual filamentary mode, but it is now well-known that depending on the gas, electrical parameters, and electrode configuration, DBDs can also operate in homogeneous modes.

Depending on the main gas in which they are ignited, the homogeneous DBDs generally present different features, which lead to their classification in at least two categories : the atmospheric
pressure glow discharges-APGD (high current densities and an electric field profile between the electrodes showing a cathode fall, a negative glow, a Faraday dark space, and a positive column) and the atmospheric pressure Townsend discharges-APTD (lower current densities and a constant high field in between the electrodes).

However, the working domain of these homogeneous DBDs is still restricted : gas composition has to be carefully controlled and the power which can be dissipated in the discharge keeping the homogeneous mode is still rather low.

Thus, the study of the transition between the filamentary mode normally observed at atmospheric pressure and the homogeneous discharges is one of our main axes of research activities.

Homogeneous (top) and filamentary (bottom) DBDs in air at atmospheric pressure. The photographs are realized with an exposure time of 100 ns. Stabilization of discharge is obtained by mounting of resistive layer on the dielectric.

Under this frame, we have demonstrated these last years that :

  • Electronegativity of gas plays a preponderant role in the homogenization of discharges (comparison between N2O et O2).
  • Parasitics elements of power supply (capacitive or inductive) could be responsible for instabilities in plasma (collaboration OSDP).
  • Adding a resistive layer on dielectric can control the transition between filamentary and uniform regimes. Actually, we have demonstrated that homogeneous discharge can even be obtained in air using such configuration.
  • The mechanisms of surface absorption and desorption can play a key role on the production of active species (radicals) in discharge, for example the nitrogen atoms.

Also, investigation of the mechanisms of secondary emission from electrodes and determination of their role on Townsend breakdown mechanism at atmospheric pressure are under consideration.

Moreover, new tools have been developed these last years :

  • Laser-induced fluorescence (LIF) and two-photon absorption laser-induced fluorescence (TALIF) diagnostics applied to DBD at atmospheric pressure.

The nitrogen atomic density measured as a function of the energy dissipated in discharge. The measurements are realized by using TALIF in homogenous discharge in N2 at atmospheric pressure.
  • An electrical circuit model for the homogeneous discharges. This model allows not only to study the coupling between the power supply and the discharge but also to get simultaneously a macroscopic understanding of the discharge physics (memory effect from one discharge to the following one, ...).

Equivalent electrical circuit of Townsend discharge at atmospheric pressure in N2.
  • A 0D freeware software, « ZDPlaskin », designed to follow the time evolution of the species densities and gas temperature in a non-thermal plasma with an arbitrarily complex chemistry – (collaboration GREPHE).
  • A specific-state complete plasma chemical scheme on nitrogen-oxygen mixtures.
  • An adaptive mesh refinement method in 1, 2 and 3 dimensions based on freeware library “PARAMESH”.

Electric field and electron density distributions in a positive streamer development. The results of 2D simulation obtained with the use of adaptive mesh of 10-4 cm. The blocks represent 8 x 8 cells.
  • New fast and accurate methods for taking into account the photoionization and photoemission processes which are important in discharge at atmospheric pressure.


References :

Thesis :

  • Nicolas Naudé (2005) « Étude électrique de la physique d’une décharge de Townsend à la pression atmosphérique et de son interaction avec un générateur : Modèle et Expérience »
  • Et-touhami Es-Sebbar (2007) « Etude électrique et analyse par Fluorescence induite par Laser des décharges de Townsend à la pression atmosphérique dans N2, N2/N2O et N2/O2 »

Main publications :

Contractual activities :

Internal collaborations :

Scientific collaborations :
PROMES (Perpignan), EM2C (Paris), LGE (Pau), CWI (Netherlands), USTO (Algeria)


Physico-chemistry in gas phase during deposition process

The chemical composition and so the derived properties of the coatings realized in PECVD processes depend on the nature of the species which reach the surfaces. Our team made a significant effort on these questions by considering that the variety of the types of plasmas (from low pressure to the atmospheric pressure) and that the detailed analysis of the deposited layers (cf Plasma and materials) gave it considerable assets. The most part of our activity is related to organosilicon, silicon and hydrocarbon precursors, because these precursors allow the obtaining of a broad range of materials. The most important species which contribute to the formation of films were identified by comparing the infrared analysis of both the gas phase and the deposit. The competition between etching and deposition processes has been carefully studied, in particular in a RCER microwave process of CH4, connected with the study of the etching of hydrocarbon material by atomic a hydrogen flow created in post-discharge (Surfatron) - (collaboration PRHE). Thus we have shown the important role of the species flows (ions and atoms), of the energy of the ions and of the temperature of the substrate.

Thus, such processes can be useful also in the study of the cleaning of organic surfaces or even for the sterilization.

Simplified kinetic scheme of the HMDSO decomposition and its interaction with the surfaces, deduced from FTIR analysis in a RCER microwave process (top : low power, bottom : high power) : for example, formation of PMDSO, CH4 and TMS.

Description, by in situ ellipsometry, of the effect of the substrate temperature in the competition etching/deposition in a RCER microwave process of CH4.

A study on the effect of diluting organosilicon precursors in N2 was initiated, in order to help in the understanding of the physico-chemical mechanisms occuring in the processes at atmospheric pressure, using our knowledge in low pressure processes. Moreover, we have shown that at atmospheric pressure the reactive transport (convection, diffusion, gas recirculations) needs to be carefully taken into account. Hence, the PECVD process has been simulated using FLUENT©. For mixtures with HMDSO, our results show that the dissociation of the precursor is the limiting step of the process. For mixtures using SiH4 (still at atmospheric pressure), the model showed that the kinetics of formation of the powders is faster than what which can be computed using the highest possible collision rate (resulting from the kinetic theory of gases). In addition, we established the effect of gas recirculations on the stability of the discharges and so on the homogeneity of the coatings.

Gas recirculation in a PECVD process at atmospheric pressure (simulation using FLUENT©)


References :

Thesis :

  • David Escaich (2006) « Caractérisation et élaboration de couches de carbones amorphes hydrogénés à propriétés optiques par procédés plasmas »
  • Ionut Enache (2007) « Etude expérimentale et modélisation du transfert de matière et des instabilités dans les décharges de Townsend à pression Atmosphérique en mélange HMDSO-N2O–N2 et SiH4–N2O–N2 »
  • Richard Murillo (2006) « Nettoyage de surface par plasma froid : Etude de l’interaction plasma - Molécule organique »

Main publications :

Contractual activities :
Projet européen Napolyde, SIDUR, ACI Technosurf, ARCELOR Mittal, CEA-LITEN, Applied Materials

Internal collaborations :

Scientific collaborations :
GDR TEMPS, GDR Arches, LSGS (Nancy), Université du Maine, University of Barcelona (Spain), Université de Constantine (Algérie), Université de Montréal (Canada)