Transparent barrier coatings on PET by single- and dual-frequency PECVD

Resumo

Plasma-enhanced chemical vapor deposition (PECVD) is among several competing techniques which can produce thin layers of inorganic glassy barrier materials for packaging and related applications. In this paper we describe the performance of SiO2 on 13 μm PET substrates, the barrier coatings being deposited from organosilicon (HMDSO) precursor. Microwave (MW), radiofrequency (RF), or dual-frequency plasmas are generated in a pilot-scale PECVD reactor for continuously-moving webs up to 30 cm in width. Coating thicknesses, d, in the range 8 nm≤d≤200 nm, have been examined, where d is measured by a variety of techniques, including spectroscopic ellipsometry, x-ray fluorescence, and transmission electron microscopy (TEM), while film compositions are determined by x-ray photoelectron spectroscopy (XPS). As also reported by other workers, typical values of oxygen permeation (OTR) are about 0.4 scc/m2-day, provided d exceeds a certain `critical' value, dc. The residual permeation has been correlated with the existence of coating defects, mostly resulting from substrate microroughness. We have developed a technique based on reactive ion etching (RIE) and undercutting under defect sites, which render these readily detectable by optical microscopy. This, in turn, has allowed us to characterize the types and number densities of coating defects, and to correlate these with OTR measurements.