Gas barrier property of silica-based films on PET synthesized by atmospheric pressure plasma enhanced CVD

In this study, silica-based films were synthesized on polyethylene terephthalate (PET) using the mixture gas of trimethylsilane (TrMS), N₂, and O₂ by the atmospheric pressure plasma enhanced CVD (AP-PECVD) method. We investigated the effect of the oxygen flow rate on the gas barrier property of silica-based films with various O₂ flow rates ranging from 0 mL/min to 1000 mL/min. As the O₂ flow rate increased to 500 mL/min, the oxygen transmission rate (OTR) of the films decreased to 8.2 cm³/m²/24 h/atm, which is approximately one third of that of the uncoated PET. On the other hand, the OTR increased when the O₂ flow rate exceeded 500 mL/min. From the fourier-transform infrared (FT-IR) spectra, the intensity of Si-O-Si peaks increased and the shape of Si-O-Si peaks changed to be sharp with increasing the O₂ flow rate, indicating that the structure of the films became dense. Oxygen molecules could not permeate through the highly dense films, so that the gas barrier property was improved. Using the atomic force microscope (AFM), many pinholes with a diameter of approximately 30 nm were observed on the film surface at the O₂ flow rate over 500 mL/min. From the observation of the uncoated PET exposed to N₂/O₂ plasma, the surface texture hardly changed and kept its smooth surface at less O₂ flow rate up to 500 mL/min, while the rugged texture were obviously observed at the O₂ flow rate over 500 mL/min because of the etching effect in plasma. The holes were formed because the films were synthesized at the PET surface in the rugged shape. Therefore, the formation of the pinholes on the film surface could allow oxygen molecules to diffuse into the films.