Investigations on the layer growth of plasma polymerized HMDSO coatings onmicro-structured polycarbonate (PC) samples

In this paper the growing process of plasma polymerized layers on micro-str uctured polycarbonate (PC) samples has been investigated. In order to creat e micro-structured surfaces, small samples (10 x 10 mm(2)) of extruded PC ( Makrolon, Bayer AG) were scratched with triangular notches (0.5-10 mu m in width) and samples with elevated rectangular structures (height: 1 mu m, wi dth: 1-10 mu m) were injection-molded. These structured surfaces were coate d by means of plasma polymerization of hexamethyldisiloxane (pp-HMDSO) or h examethyldisiloxane/oxygen mixtures (pp-HMDSO/O-2). Essentially both pp-HMD SO and pp-HMDSO/O-2 layers showed the same growth mechanisms on the substra te defects. After a certain coating time, a complete closure/coverage of th e investigated structures (notches or rectangular elevates) or of the layer s deposited on them was observed, which made the sample surface look smooth er. The layer thickness that is necessary for this effect depended on the l ateral extension of the defect and on the process gases. A characteristic q uantity to describe this effect is the ratio r between the layer thickness d and the width of the triangular notch or distance between the rectangular structures w (r = d/w), which is significantly dependent on the process ga s composition and the geometry of the structure. For triangular notches coa ted with pp-HMDSO for example, a change from open to closed structures was observed at r approximate to 1, respectively r approximate to 1,4 for pp-HD SO/O-2 coatings under similar process parameters.