Interphase characterization of PECVD silicon-compound layers on PET

Abstract

Thin films of silicon nitride or -oxide (SiN and SiO2) on flexible or rigid polymeric substrates are receiving much attention in numerous application areas, since they are excellent barriers against the permeation of gases (e.g. O2) or vapors (e.g. H2O), are hard, optically transparent, and chemically resistant. In this laboratory SiN and SiO2 films are deposited onto polyethylene terephthalate (PET) using `dual'-frequency (microwave-radiofrequency) plasma-enhanced CVD (PECVD). Such films adhere very well to the polymer substrates, a likely reason for this being found in an extended `interphase' region. In this paper we report results obtained by probing the interphase by various complementary destructive and non-destructive techniques (ERD, XPS, ATR-FTIR, IRRAS, and spectroscopic ellipsometry). The measured thickness of the interphase is found to range from a few tens to about 80 nm, and all the above-mentioned techniques identify it to be an `organosilicon' layer of varying composition. We propose a fragmentation/redeposition mechanism at the earliest stage of the coating process, during which the plasma-surface interaction leads to a mobilization of volatile organic fragments from the polymer surface into the plasma; these fragments intermix with the feed-gas, and then re-deposit in the form of the observed organosilicon interphase region.