Atmospheric pressure PE-CVD of silicon based coatings using a glow dielectric barrier discharge

The aim of this work is to determine the properties and to understand the growth mechanisms of silicon-based coatings realised with an atmospheric pressure glow dielectric barrier discharge. Mixtures of SiH4 and N2O, diluted in N2 to reach the atmospheric pressure are used to obtain silicon oxide thin films. A longitudinal gas injection allows to study the coating properties as a function of the reactive gas decomposition and transformations level. The other parameters of this study are the silane rate and the plasma power. The thickness profile as a function of the gas residence time in the discharge clearly shows two maxima. SEM observations, ellipsometry and infrared results show that the first maximum which typically corresponds to a 400 μs residence time is due to radicals directly interacting with the surface while the second one observed for 10 ms residence time is due to the accumulation of aggregates formed in the gas phase following the interaction of radicals in the gas bulk. The first mechanism leads to dense silicon oxide coatings. Higher is the plasma power, denser is the coating: values of 98% of SiO2 are reached. The growth rate increases with the silane concentration and the plasma power but saturation is observed around some tens of nanometers per minute. The second mechanism leads to porous deposit typically made of 30% of SiO2 and 70% of void. The growth rate is proportional to the power and increases as the square of the silane concentration. The maximum coating roughness is observed when the two mechanisms significantly contribute to the film growth leading to the formation of ‘cauliflower’ type structure.