AVS 61st International Symposium & Exhibition
    Plasma Science and Technology Wednesday Sessions
       Session PS-WeA

Paper PS-WeA8
Quantitative Analysis of Neutral Species Generated in Styrene Low Pressure RF Plasma, as a Function of Plasma Power

Wednesday, November 12, 2014, 4:40 pm, Room 308

Session: Plasma Diagnostics, Sensors, and Control
Presenter: Laurent Houssiau, University of Namur, Belgium
Authors: X. Gillon, University of Namur, Belgium
J.-J. Jean-Jacques, University of Namur, Belgium
L. Houssiau, University of Namur, Belgium
Correspondent: Click to Email
Plasma polymerization processes enable unique polymer coatings unattainable with conventional wet chemistry. Among them, plasma polystyrene (pPS) deposition has been intensively studied, but very few studies report on plasma diagnostics of styrene discharges, which is however a necessary step to understand the fundamental mechanisms of plasma polymerization. In this work, pPS was produced from pure styrene vapor injected in a vacuum chamber at 50 mTorr (6.7 Pa) and 28 sccm. An inductively coupled plasma was ignited by a planar coil delivering a continuous wave RF power ranging from 30 W to 210 W. Plasma diagnostics was achieved by means of a mass spectrometer (MS) located in the post-discharge region, enabling only the detection of neutral species. A problem associated with electron impact MS is the cracking of organic molecules, which hampers species identification and quantification in the plasma phase. However, by reducing the electron energy as low as 12 eV, much below the standard 70 eV energy, we were able to suppress most of the molecular cracking, so that species measured by MS could be unambiguously assigned to neutral species existing in the plasma. The assignment was further confirmed by measuring the appearance potentials of all ions in the MS. This procedure revealed the existence of 55 neutral species in the styrene plasma, which is five times more than what has been reported so far. Their relative intensities in the MS spectrum help understanding the formation pathways of these species, either by direct fragmentation of the styrene molecule, or by recombination of small molecules. The most abundant species generated in the styrene plasma were H2, methane, acetylene, ethylene, benzene, toluene and naphthalene. In order to quantify the partial pressure of those species in the plasma, we determined their sensitivity factors by injecting them pure in the plasma reactor and measuring the molecular ion peak intensity at 50 mTorr pressure. The MS intensities measured in the styrene plasma were subsequently converted into partial pressures. The sum of partial pressures from the main species present in the plasma, including styrene, was found to match remarkably well with the measured pressure in the reactor, confirming the quantitativeness of the procedure. The main features observed in the plasma chemistry when the power was raised are: a decrease of styrene pressure (monomer consumption) along with a strong production of acetylene and hydrogen. A very sharp drop of the plasma pressure was measured around 185 W, corresponding to the disappearance of the monomer and a sudden increase of the deposition rate.