Growth and structure of hyperthin SiO2 coatings on polymers

Transparent inorganic oxide coatings on polymers are playing an increasingl y important role in pharmaceutical, food, and beverage packaging, and more recently in encapsulation of organic, light-emitting display devices. Such coatings are being prepared by physical or by chemical vacuum-deposition me thods. They possess barrier properties against permeation of gases or vapor s when they are thicker than a certain critical thickness, d(c); for d <d(c ), the "oxygen transmission rate" (in standard cm(3)/m(2)/day/bar), for exa mple, is roughly the same as that of the uncoated polymer. This fact is com monly attributed in the literature to a "nucleation" phase of the coating's growth, during which it is thought to present an island-like structure. In order to test this hypothesis, we have deposited hyperthin SiO2 coatings o n various flexible polymeric substrates using plasma-enhanced chemical vapo r deposition. The film thicknesses investigated here, well below d(c) (typi cally in the range 1-10 nm), were determined by Rutherford backscattering s pectroscopy, which allows us to determine the surface concentration of sili con. This was found to be a linear function of the deposition time, t, for t greater than or equal to0.5 s. Then, combining reactive ion etching in ox ygen plasma with scanning electron and optical microscopy, we have been abl e to characterize the structure of the coatings: even for d less than or eq ual to2 nm, no island structure has been observed. Instead, we found contin uous coatings which contain large concentrations, n, of tiny pinhole defect s (with typical radii in the range of tens of nanometers), where n increase s with decreasing d. These assertions are confirmed by grazing angle (80 de grees) angle-resolved x-ray photoelectron spectroscopy, which shows that ev en for d=2 nm, the structural features of the polymer substrate cannot be d etected. (C) 2001 American Vacuum Society.