Effect of water vapor on plasma processing at atmospheric pressure: Polymer etching and surface modification by an Ar/H₂O plasma jet

The authors evaluate the effect of water vapor on the plasma processing of materials using a model system consisting of a well-characterized radio-frequency plasma jet, controlled gaseous environment, and polystyrene as target material. The authors find that the effluent of Ar/H₂O plasma jet is capable of (1) etching polymers with relatively high etch rate and (2) weakly oxidizing the etched polymer surface by forming O containing moieties. When increasing the treatment distance between the polymer and the Ar/H₂O plasma, the authors find that the polymer etch rate drops exponentially, whereas the O elemental composition of the etched surface shows a maximum at intermediate treatment distance. The OH density in the Ar/H₂O jet was measured near the substrate surface by laser induced fluorescence, and the density change of the OH radicals with treatment distance is found to be consistent with the exponential decrease of polymer etch rate, which indicates that OH may play a dominant role in the polymer etching process. A control experiment of Ar/H₂ plasma shows that the observed fast polymer etching by Ar/H₂O plasma cannot be attributed to H atoms. By correlating the OH flux with the polymer etch rate, the authors estimated the etching reaction coefficient of OH radicals (number of C atoms removed per OH radical from the gas phase) as ∼10^-2. The polymer etch rate of Ar/H₂O plasma is enhanced as the substrate temperature is lowered, which can be explained by the enhanced surface adsorption of gas phase species. For the same molecular admixture concentration and plasma power, the authors find that Ar/H₂O/O₂ plasma has much reduced etching efficiency compared to either Ar/H₂O or Ar/O₂ plasma.