Ion-ion plasma in propulsion and processing applications

Plasma thrusters were developed in the 1970's with the aim, among other, to correct the orbit of satellites. Their operation is based on the extraction and acceleration of an ion beam from a plasma in order to provide thrust. Usually, the plasma used in these ion sources is electro-positive. The extracted ion beam forms a positive space charge and current that are usually neutralized by electrons. The disadvantage of these thrusters is that the electron-ion recombination is inefficient and part of the extracted ions can therefore interacts with the satellite causing damage. The PEGASES¹ thruster is developed in order to get rid of this constraint. In the PEGASES ion source, the ion beam is extracted from an electro-negative plasma which contains almost no electrons (ion-ion plasma). The positive and the negative ions are extracted successively, by alternatively polarized grids, leading to a neutral ion beam. The ion-ion recombination, which is significantly more efficient than electron-ion recombination, induces a rapid decrease of the charged particle density in the beam and might reduce significantly the damage induced by the extracted beam to the satellite. The use of electro-positive plasma in the etching applications can lead, more or less, to an isotropic etch. This is due to the fact that, in the plasma sheath, the velocity distribution is strongly anisotropic for the ions and almost isotropic for the electrons. The positive and negative charges are deposited, respectively, essentially in the bottom and sidewalls of the cavities. The resulting electric field deflects the ion trajectories which leads the ions to strike the sidewalls. When the ion-ion plasmas are used for etching both of negative and positive charges are deposited in the bottom of the cavities and resulting etch is much more anisotropic. Ion-ion plasmas are also suited for polymers processing, because of the absence of energetic species (no electrons) in these plasmas. In this work a one dimensional model of pure ion-ion plasma is developed and used in order to investigate the ion-ion plasma behavior exposed to electrodes or grids biased with a various voltage wave forms in the kHz to MHz range. The effect of positive and negative plasma sheath formation, ion flux toward walls and ion beam extraction are investigated. A systematic comparison between numerical and experimental results will be presented in this work.