Défense de thèse de doctorat en sciences physiques "plasma polymerization"

Chemistry and physics of plasma polymerization probed by mass spectrometry

Jury

Achim von KEUDELL (Ruhr-Universität Bochum), Rony SNYDERS (UMons), François RENIERS (ULB), Laurent HOUSSIAU, promoteur (UNamur), Jean-Jacques PIREAUX, président (UNamur)

Résumé

Nowadays, deposition of thin films is one way to completely refashion our well-known glasses, metals and semiconductors: their surfaces are now able to produce light or to transform it into energy, to withstand wear, corrosion and oxidation, to conduct electricity in microprocessors or to be inserted inside bodies without rejection. Among deposition techniques of thin films, the ones using plasma – the fourth state of matter or the ionized gas – have many advantages. The deposited coating is tightly adherent to its substrate, covers it completely and can have a large diversity of tunable properties.

Plasma polymerization allows the thin film deposition of plastic, or polymeric, material. The present work investigated the low-pressure plasma polymerization in a radio-frequency reactor with planar inductive power coupling. The monomers studied were styrene (C₈H₈), benzene (C₆H₆), ethylene (C₂H₄) and acetylene (C₂H₂), which are pure hydrocarbons, but also methyl methacrylate (C₅H₈O₂ - MMA) which contains oxygen. The neutral by-products of their plasmas were investigated by mass spectrometry (MS) downstream of the discharge. In the case of styrene and MMA, the elemental composition of the plasma-deposited polymers was measured by X-Ray Photoeletron Spectroscopy (XPS) and the deposition rate by contact profilometry. The main results of the thesis are the following.

·       A systematic approach based on low electron impact energy and Threshold Ionization Mass Spectrometry (TIMS) allowed an extensive species identification of the by-products. To explain the many new species detected, light-weighted carbon containing radicals with low hydrogen content are proposed as intermediates of reaction. In the case of MMA, the dominating species suggested that oxygen containing radicals are not likely to participate to recombination reactions.

·       In addition, the partial pressures of 8 dominant species were quantified by MS in the discharges of styrene, benzene, ethylene and acetylene. Varying the plasma power revealed the systematic large production of hydrogen and acetylene. Effects due to hydrogen were proposed to explain the lower depletion of ethylene compared to the three other monomers.

·       Finally, the study of the MMA discharge pressure has led to the discovery of a method to quantify for the first time the partial flows of plasma pumped by a nitrogen trap.

In summary, the present work has developed a complete methodology to detect and quantify the neutral by-products of plasma polymerization. Its application allowed a deeper insight into the plasma chemistry of hydrocarbons and oxygen containing monomers. It opens the way to correlate this chemistry with the thin plasma polymer films.

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