Plasma-surface interaction at atmospheric pressure: A case study of polystyrene etching and surface modification by Ar/O₂ plasma jet

In this paper, the authors studied atmospheric pressure plasma-surface interactions using a well-characterized radio-frequency Ar/O₂ plasma jet with polystyrene (PS) polymer films in controlled gas environments as a model system. A number of plasma processing parameters, such as the treatment distance, environmental gas composition, and substrate temperature, were investigated by evaluating both the changes in the thickness and the surface chemical composition of PS after treatment. The authors found that the polymer average etch rate decayed exponentially with the nozzle-surface distance, whereas the surface oxygen composition increased to a maximum and then decreased. Both the exponential decay constant and the oxidation maximum depended on the composition of the gaseous environment which introduced changes in the density of reactive species. The authors previously reported a linear relationship between measured average etch rates and estimated atomic O flux based on measured gas phase atomic O density. In this work, the authors provided additional insights into the kinetics of surface reaction processes. The authors measured the substrate temperature dependence of the PS etch rate and found that the apparent activation energy (E_a) of the PS etching reaction was in the range of 0.10-0.13 eV. Higher values were obtained with a greater nozzle-to-surface distance. This relatively low E_a value suggests that additional energetic plasma species might be involved in the etching reactions, which is also consistent with the different behavior of etching and surface oxidation modification reactions at the polymer surface as the treatment distance is varied.