How the chemical structure of the plasma-deposited SiOx film modifies its stability and barrier properties: FTIR study

Abstract

Organosilicon thin films issued from the gas mixture of oxygen and hexamethyledisiloxane (HMDSO), 2,4,6,8-tetramethylcyclotetrasiloxane (TMCTS), or fluorotriethoxysilane (FTEOS) deposited on polycarbonate (PC) substrate were obtained thanks to a RF plasma source. The chemical structure of the two types of inorganic SiOx (HMDSO/O2, TMCTS/O2) and fluorinated SiOxCyFz (FTEOS/O2, FTEOS/O2/TMCTS) coatings have been investigated by FTIR and XPS spectroscopies, and wettability measurement. Network structure and density of SiOx and SiOxCyFz thin films were discussed as a function of the νSiOSi band shifting observed in their respective FTIR spectra. The influence of various plasma parameters (such as discharge power, precursor/O2 ratio and, duration) on the film properties was studied in order to optimize the growth of both inorganic and fluorinated thin films. The ageing of such layers either at ambient atmosphere or even at reduced pressure showed a chemical alteration of the thin films surface and bulk, which stabilized after 15 days of storage. Such phenomenon is explained by the hydrophobicity recovery that may induce the observed network alteration. Water and oxygen permeation and transport properties of treated PC films were correlated with the coating structure and composition; then TMCTS was identified as the most efficient precursor for enhancing water barrier property. Furthermore, the permeation properties of these deposits seem to be more controlled by the material density than by their chemistry.