Global and local planarization of surface roughness by chemical vapor deposition of organosilicon polymer for barrier applications

Particulates and asperities on the surface of plastic substrates limit the performance of the current protective barrier coatings for flexible electronics. By applying a smoothing layer to the substrate, prior to barrier deposition, permeation is reduced. While application of smoothing layers from the liquid-phase application and curing of acrylate monomers is well known, reports of planarization achieved by vapor deposition are quite limited. In the current work, the chemical vapor deposition (CVD) of a flexible smoothing layer, requiring no curing, is implemented in the same reactor chamber and from the same organosilicon monomer used for depositing the multilayer barrier stack. The process similarity between the smoothing and barrier layer deposition steps has the potential to lower the overall cost of the process and to improve interfacial properties, such as adhesion between the smoothing layer and the barrier stack. The current methods adapts and combines features of two well established methods for CVD of organic layers, plasma enhancement (PECVD) and the specific use of an initiator species (iCVD). The novel, initiated plasma enhanced chemical vapor deposition (iPECVD) method achieves a far greater degree of planarization of flexible organic layer than either of its predecessors. Polystyrene microspheres serve as model defects and allow the degree of planarization to be quantitatively measured. Both cross-sectional scanning electron micrographs and atomic force micrographs demonstrate that when the iPECVD organic layer is 1.8 μm thick, the degree of global planarization is 99. A model demonstrates that the planarization is achieved as a result of the coating viscosity and the surface tension. Finally, the water vapor barrier performance of a 20-nm-thick SiO _x layer is two orders of magnitude improved when it is deposited on a planarized substrate.