Scanning force microscopy for optical surface metrology
Surface structures with lateral dimensions in the nanometer range (? 100 nm) have a significant impact on the optical and functional surface properties. Scanning Force Microscopy (SFM) has been increasingly used to investigate the nanotopography of substrates and thin films. SFM data evaluation is nevertheless so far mainly restricted to qualitative image information or single roughness parameters. Appropriate description of statistical surface roughness needs advanced quantitative data analysis, which can be accomplished by Power Spectral Density (PSD) functions. For nanostructures, conclusions about the information content of measurement results are difficult. The results can be strongly influenced by the geometry of the probe tip, whose lateral dimension is in the nanometer range, too. Based on experimental / empirical work, we estimated tip size effects on the PSDs of thin films. Especially the SFM measurement of supersmooth samples (e.g. substrates for EUV coatings) can also be affected by inherent noise of the system. We therefore also present and discuss methods of noise analysis.