Structural Studies of Hyper-Thin SiO₂ and Si₃N₄ Coatings on Polymers

Transparent barriers, such as SiO₂, against oxygen and/or water vapor permeation through polymers are the object of increasing interest in the food and pharmaceutical packaging industries, and more recently in encapsulation of organic-based displays. It is well known now that these layers possess barrier properties only if they are thicker than a certain critical thickness, d_c. Below this value, the “oxygen transmission rate” (OTR, in standard cm³/m²/day/bar) is roughly the same as that of the uncoated polymer. This is explained in the literature in terms of an island-like structure of the coating below d_c. In order to verify this hypothesis, we have deposited hyper-thin PECVD SiO₂ and Si₃N₄ layers on various polymeric substrates (polyester, PET; polyimide, PI; polypropylene, PP). Values of d, typically below 15 nm, were evaluated by variable angle spectroscopic ellipsometry (VASE) and by Rutherford backscattering spectroscopy (RBS). This latter method has revealed the surface concentration of silicon atoms to be a perfectly linear function of the deposition time, t, for 0.5 = t = 100 seconds (0.5 nm = d = 100 nm). Then, using Scanning Electron Microscopy (SEM) after reactive ion etching (RIE) in oxygen, we have observed the coating structures to be continuous, not island-like. However, the layers contain large numbers ( 10⁹ cm^-2) of tiny pinhole defects (with radius, r  20 nm) which can account for the absence of barrier characteristics. Finally, X-ray photoelectron spectroscopy (XPS) measurements, at high take-off angles ( 80 degrees) show that even for a 2 nm thin SiO₂ coating, the polymer substrate cannot be detected, again confirming the continuous film structure.