Dépôt de composés inorganiques du Si par plasma CVD sur substrats polymériques: Caractérisation structurale et fonctionnelle

Structural and functional characterization studies of plasma-enhanced chemically vapor deposited (PECVD) silicon compounds (SiO₂ and Si₃N₄, hereafter “SiN”) on 13 μm polyethylene terephthalate (PET) substrates have been carried out. Barrier coatings were deposited in a dual-frequency (microwave/radio frequency) pilot-scale PECVD reactor for continuously-moving flexible webs up to 30 cm in width. The volatile silicon compounds used for SiO₂ deposition were HMDSO (C₆ H₁₈Si₂O) and SiH₄, while the latter served to deposit SiN. Coating thicknesses, d, in the range 8 nm ≤ d ≤ 200 nm, were measured using a variety of techniques.

The “interphase” region between the deposited layers (eg. PECVD SiO₂ or SiN) and the PET substrate has been investigated and compared to PVD (electron beam evaporated) SiO₂. Composition profiles determined by electron microprobe analysis (EMA), TOF-ERD, and XPS all show an extended interphase region more than 50 nm in width, while that of the PVD SiO₂ is narrower. We have also examined ultra-thin (about 10 and 27 nm) SiO₂ and SiN PECVD layers on 50 nm spin-coated PET substrates by non-destructive infrared (IR) techniques. The IR spectra also confirm that the thin PECVD deposits comprise an organosilicon phase with Si-CH_x bonds.

Oxygen transmission (OTR) and water vapor transmission (WVTR) measurements were carried out with MOCON “Oxtran” and “Permatran-W” instruments, respectively. We typically found OTR values of about 0.5 scc/m ²-day and WVTR about 0.3 g/m²-day, for barrier thicknesses exceeding a “critical” value (d_c, about 8nm for SiN, and 15nm for SiO₂), but the minimum permeation values depend upon the concentration of defect sites in the coating (partly related to substrate microroughness). In order to confirm this correlation, we have developed a technique to characterize the types, origins and number densities of coating defects. We found an excellent correlation between measured OTR values and the number densities and size distributions of defects in SiO ₂ and SiN barrier coatings on PET.

We have developed a model to calculate gas permeation through defects in barrier coatings. We also present the temperature-dependence of OTR and WVTR for SiO₂ and SiN coatings deposited on one or on both sides of PET films. The apparent activation energies evaluated from these measurements support the model of defect-dominated mass transport, but suggest different mechanisms for oxygen and water vapor transmission. (Abstract shortened by UMI.)