2.2. Sample Characterisation

In order to gain information on the surface topography of the samples, both the average area roughness Sa and the root mean square area roughness Sq were detected (according to the standard ISO 4287/1-1997) after each manufacturing step. Here, an atomic force microscope (AFM) (Nanosurf GmbH, Langen, Germany, model easyScan 2) was applied where the measured area was 50 × 50 µm².

Moreover, the chemical composition of the surfaces was determined by high-resolution X-ray photoelectron spectroscopy (XPS) where a scanning XPS apparatus (Ulvac-phi, Inc., Chigasaki, Japan, model PHI VersaProbe II) with a monochromatic Al-K_α source and a photon energy of 1486.6 eV was used. The applied X-ray source has a power of 100 W, whereby the sample surface is scanned with a beam size of 100 µm over an area of 1400 × 200 µm². High-resolution spectra were recorded with a pass energy of 46.95 eV and a step size of 0.1 eV with a constant electron take-off angle of 45°. The spectrometer was calibrated to the copper and gold reference lines (932.62 eV and 83.96 eV); the minimum detector resolution measured at the silver peak (Ag3d_5/2) was 0.6 eV with a pass energy of 23.5 eV. The measurements were carried out at room temperature and a base pressure of 2 × 10⁻⁶ Pa. To avoid charging effects all measurements were performed with charge compensation with a cold cathode electron flood source and low energy argon ions. All spectra (N = 12) were shifted to the C1s carbon peak (248.6 eV). For data analyses the software MultiPak (version 9.9.0.8, Ulvac-phi, Inc., Chigasaki, Japan) was applied.

To ensure a normal distribution (α = 0.05), a Kolmogorov–Smirnov normality test was used. The data were analysed with a Tukey honestly significant difference test (α = 0.05) to control the results for statistical differences between the variants.