Model PolymerEtching Mechanisms and Surface Modification by a Time- Modulated RF Plasma Jet

Cold atmospheric pressure plasma jets (APPJs) are able to generate chemically reactive species desired for material processing and biomedical applications. A great amount of attention has been devoted to characterizing these APPJ sources and expanding their applications. Less is known on the interaction mechanisms between cold atmospheric pressure plasma and material surfaces. Since both plasma sources and material surfaces are constantly exposed to the surrounding ambient, the gaseous environment is coupled with the plasma and material surface and influences plasma-surface interactions. The lack of quantified reactive species measurements for these plasma sources also impedes the interpretation of material surface response with plasma treatment. In this work, surface interaction of a well-characterized time modulated radio frequency (RF) plasma jet [1] with model polymers, namely polystyrene , poly(methyl methacrylate) (PMMA) and poly(vinyl alcohol) (PVA), is investigated using a controlled gaseous environment. The RF plasma jet has been previously studied in detail including its biochemically reactive species densities [2] such as NO, O and O 3 which provides a unique opportunity to improve the understanding of plasma surface interaction. We find that the RF jet can induce fast organic material etching, whereas X-ray photo-electron spectroscopy after vacuum transfer only detects minor surface chemical modifications of the polymers, including slight oxidation. This is in contrast to surface interaction mechanisms with other atmospheric pressure plasma sources (e.g. surface micro-discharges). The measured etch rates and surface chemistry reflect plasma treatment conditions (e.g. feed gas, environment, treatment distance). In particular, we find that etching depth mirrors the measured decay of O atoms in the gas phase as the nozzle-surface distance increases. The etching reaction probability of O atoms with C atoms ranges from 10-4 to 10-3 and is consistent with low pressure plasma research. The elucidation of the role of oxygen atoms in polymer etching was possible, since the source operating conditions were chosen so that other surface interaction mechanisms (O 3 , VUV, direct filament interaction) were negligible. Figure 1 – Polymer etching induced by Ar/1% air plasma. Relationship of C flux leaving the surface (corresponding to measured polymer etching rates) with estimated O flux arriving at the polymer surface (based on O atom density measurements).