One of the major problems encountered in surface treatment by plasma technologies is the rescaling of the plasmas and of the processes to large areas. Specifically, the design of a plasma source requires the application of uniform electric field over its full span, fact which is not possible when the wavelength of the electromagnetic field and the size of the field applicator are of the same order of magnitude. The excitation of electrons by electric field and, hence, the plasma production, is no longer homogenous over the whole span of the applicator.
This issue can be circumvented and plasma homogeneity can be obtained by distributing the electric power on independent elementary plasma sources arranged evenly in space. The array can be either in in two dimensions (sources for surface) or in three dimensions (sources for volume). It based on this distribution concept that the CRPMN group has conceived and developed two generations of sources that excite the plasma by microwaves: the multi-dipolar sources which work at low pressure (0.5 – 5 mTorr) and are assisted by static magnetic field, and the matrix sources that work at intermediate pressure (0.1 – 1 Torr).
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Planar multi – dipolar plasma : front view of an oxygen plasma |
The first advantage of using microwaves for plasma heating comes from the fact that the waves can propagate and reach the plasma without the aid of polarized electrodes. Therefore, contamination by sputtering of the walls and electrodes is avoided. The second advantage lies in the fact that, in the low pressure range, microwaves combined with a static magnetic field can lead to very efficient excitation of electrons. The mechanism is called electron-cyclotron resonance (ECR) and it manifests when the frequency of electronic gyration equals the microwave frequency.
On one hand, these sources constitute the object of fundamental research work that connects experimental characterizations (electrical, optical) with numerical (trajectory, self-consistent model) simulations. On the other hand, they lead to prospective studies on new generations of plasma sources (ANR PLASMODIE project 2007 – 2010).
General literature
- Microwave Excited Plasmas, Michel MOISAN and Jacques PELLETIER, editors, Elsevier, Amsterdam (1992)
- High Density Plasma Sources : Concept, Design and Performance, Oleg A. POPOV, editors, Noyes, Park Ridge (1996)
- Physique des Plasmas Collisionnels / Application aux Décharges Haute Fréquence, Michel MOISAN and Jacques PELLETIER, editors, EDP Sciences, Grenoble Sciences, Les Ulis (2006)
