## 3 Results  
### Plasma Etching of Thin Surface Polymer Layer  
Pure atmospheric-pressure hydrogen plasma was employed to etch out the polymer film from the surface of mesh foil and to expose free metal wire surface (Figure 2). The thickness of the polymer  
Figure 1: Reactor based on diffuse coplanar surface barrier discharge (DCSBD) with concavely curved alumina ceramics for roll-to-roll treatment of flexible substrates. 1—Concavely curved DCSBD unit; 2—DCSBD plasma; 3—roller; 4—sample; 5—reactor chamber; 6—gas inlet; 7—gas outlet. layer at the thinnest point over the top of a metal wire was ranging in the order of 1 nm up to 100 nm. The removal of a thin surface polymer film is important to make conductive connection sites on the surface before next processing steps. As seen in Figure 3, already a short plasma exposure time of 1 s can expose \"isles\" of free metal surface. Figure 3a shows the surface of the untreated sample. More bright, uncovered metal areas are visible in Figure 3b showing etched surface with details on single uncovered wires in Figure 3c,d. Both SEM images Figure 3a,b were taken at the same magnification 26\(\times\), working distance 15.0 mm and view field 10.7 mm at accelerating voltage 15.0 kV. The shorter exposure times (0.25 s and 0.5 s) led to the etching of surface polymer film, however, not all metal wires tops were exposed (uncovered) on the surface after such short plasma etchings. Figure 4 shows SEM micrograph of a single uncovered wire after 1 s etching in pure H\({}_{2}\) plasma. The SEM image is shown using secondary electrons (SE image) and also back-scattered electrons (BSE image). Figure 3: The surface of (**a**) untreated mesh foil and (**b**) the mesh foil after H\({}_{2}\) plasma etching (1 s) with details on cross-section and upper surface of exposed wires (**c**) and upper surface detail (**d**). The distance of the sample surface from the surface of concave DCSBD electrode system was H = 0.35 mm. Figure 2: Schematic sketch of plasma etching. The average thickness of the etched polymer over the wire tops was about 10 nm. 1—polymer foil; 2—mesh of polymer and metal wires; 3—metal wire; 4—uncovered metal wires tops. Secondary electrons originate within a few nanometers from the sample surface [20]. Therefore, SE image shows the best the topography of the analyzed surface. Since heavy elements in the specimen reflect or backscatter electrons more effectively than the light elements, the atoms with a high atomic number appear brighter in the BSE image [20]. Backscattered electrons can be then used to detect contrast between areas with different chemical compositions. As seen in Figure 4 (BSE image), we can distinguish a bright uncover metal wire from the dark area composed of light polymer material. Figure 5a,b shows the EDX line scans in lateral and longitudinal direction, respectively. As observed, surface polymer film was successfully removed uncovering the metal wire. Figure 6 shows composite element map of single wire indicating removal of polymer on the entire area of the uncovered wire. In the area of uncovered wire a strong signal of silver was evident, while only a small signal of copper indicating concentration of copper less than 0.5 wt.% was discernible (Figure 6 and Table 1). This indicates that the thin polymer film was removed from the area above the wire without damage of metal wire and its silver coating. The presence of carbon and a small concentration of oxygen in the area of uncovered wire as shown in Figure 5 can be explained by the surface contamination during the manipulation with the sample, when the sample was exposed to air after the plasma etching for several hours before the SEM EDX analysis. We suppose that this is also the reason for the higher concentration of carbon in the plasma exposed area of pure polymer compared to the value expected on the clean PMMA surface. Figure 4: SEM micrograph of the uncovered wire top after 1 s H\({}_{2}\) plasma exposure (distance H = 0.35 mm).