Growth and structure of hyperthin SiO2 coatings on polymers

Transparent inorganic oxide coatings on polymers are playing an increasingly 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 methods. They possess barrier properties against permeation of gases or vapors 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 example, is roughly the same as that of the uncoated polymer. This fact is commonly 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 on various flexible polymeric substrates using plasma-enhanced chemical vapor deposition. The film thicknesses investigated here, well below d(c) (typically in the range 1-10 nm), were determined by Rutherford backscattering spectroscopy, which allows us to determine the surface concentration of silicon. 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 oxygen plasma with scanning electron and optical microscopy, we have been able to characterize the structure of the coatings: even for d less than or equal to2 nm, no island structure has been observed. Instead, we found continuous coatings which contain large concentrations, n, of tiny pinhole defects (with typical radii in the range of tens of nanometers), where n increases with decreasing d. These assertions are confirmed by grazing angle (80 degrees) angle-resolved x-ray photoelectron spectroscopy, which shows that even for d=2 nm, the structural features of the polymer substrate cannot be detected. (C) 2001 American Vacuum Society.