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Final Activity Report Summary - PLASMA-SHEATH-LENS (Phenomenology and applications of three-dimensional plasma-sheath-lenses)

Plasma processing technologies are based on radical-assisted ion-induced surface modification where positive or negative ions accumulate energy within the sheath and then strike the surface with a certain energies and incidence angles. Due to the limited size of the treated electrode the sheath is a complex two- or three dimensional structure that may also include multi ion species. Recently it was shown that a sheath formed adjacent to a conductor-insulator interface exhibits the discrete and modal focusing effects.

The aim of this project was to further investigate by simulation and experiment the phenomenology related to the plasma-sheath-lens focusing effects and to propose new applications. The main objectives were to obtain a good correlation between experiments and simulations on etching yields for different gases and substrate materials; to clarify the range of parameters for magnetic field and sheath structure that could make it possible to use a magnetised plasma-sheath-lens for mass separation; to understand the effect of a dynamic sheath on discrete and modal focusing and to establish the geometric limits for the occurrence of discrete and modal focusing effect depending on plasma parameters.

The experiments have been performed in electronegative gases in inductively coupled plasma and a matrix electron cyclotron resonance discharge. Mass spectrometry and Langmuir probe have been used for plasma characterization. Disk and square samples made of silicon, brass, silver and gold and mounted in a plasma-sheath-lens configuration have been treated for different plasma parameters and applied biases. The resulted patterns by discrete and modal focusing effects were measured by phase-shifted laser interferometry or 3D contact profiler. In parallel with these experiments three-dimensional simulations of the potential distribution and ion kinetics within the sheath structure have been performed. The etching yield estimated from experiments was compared with that evaluated from simulations. Both the etching pattern by discrete and modal focusing and the resulted depth were found in good correlation with simulations.

Complex three-dimensional simulations of potential distribution of plasma-sheath-lenses with different geometries in the presence of magnetic field have been performed using customized software. From the resulted ion flux on the electrode surface we concluded that it would be possible to identify separated contours on the electrode surface resulted from different ions with rather large difference in mass.

Time dependent experiments performed in a pulse regime from 1 up to 100 microseconds have shown the correlation between the discrete and modal focusing effects formation with the transit time of the ions within the sheath. Plasma-sheath-lenses corresponding to disk and square structures of tens of micrometer order have also been successfully investigated.

The results of this project led to two patent applications concerning a new mechanism of ion extraction from plasma and a new type of mass spectrometer both of them based on discrete ion focusing. The publication of other results has been postponed in purpose as not to conflict with the patent applications.

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