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A microwave driven low pressure plasma reactor is developed based on a modified Plasmaline antenna for plasma processing of polyethylene terephthalate (PET) foils and bottles. It allows for the treatment of thermolabile packaging materials, e.g. plasma sterilization and permeation barrier coating. Silicon oxide films are deposited on PET foils as a permeation barrier coating. A pulsed hexamethyldisiloxane:oxygen plasma is ignited under various conditions and the oxygen permeation is investigated. A criterion for the homogeneous deposition of SiO x coatings is described depending on the residence time of process gases. Additionally, the composition of the coatings is analyzed by means of Fourier transform infrared spectroscopy regarding carbon and hydrogen content. A strong relation between barrier properties and film composition is found: good oxygen barriers are observed as carbon content is reduced and films become inorganic, quartz-like. A residual permeation as low as J = 1.0 ± 0.3 cm 3 m − 2 day − 1 bar − 1 for SiO x coated PET foils is achieved. The dependencies of important plasma parameters, such as gas mixture, process pressure, power and pulse conditions on oxygen permeation through packaging foil are shown to optimize the coating process.