We discuss AFM (Atomic Force Microscopy) characterization in terms of critical dimension and depth for large area
micro-optical elements. Results are shown and discussed in comparison with other techniques, such as SEM (Scanning
Electron Microscopy) for CD measurements and FIB (Focused Ion Beam)-SEM characterization for the structure profile.
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 an 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 and only possible in a qualified manner. 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 super smooth 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.
We have developed a novel approach to design ultra-hydrophobic surfaces with optical quality. The nanostructure necessary for the functional effect is realized through enhanced nanoroughness of optical thin films. At the same time, the optical appearance must not be disturbed by scattering from the roughness. We have found that through wide-scale roughness analysis, applying white light interferometry, AFM and STM, and subsequent data reduction the roughness characteristics can be directly related to the wetting properties. As, on the other hand, vector scattering theories connect the roughness properties with scatter losses, a formalism has been established, where both the wetting properties and scattering behavior can be expressed within the same "language". Using this tool, the optical thin film design and the surface nanoroughness can be tailored to fulfill demands on wetting properties as well as on sufficient low scatter levels. For the deposition of high index single layers on Borofloat 33 substrates, qualified substrate-film-combinations are predicted by "virtual" coating simulations. Experiments with single oxide layer as test coatings yielded surfaces with a high water contact angle and light scatter losses below defined scatter thresholds.
Optical coatings with enhanced roughness offer promising prospects for ultra-hydrophobic transparent surfaces with controlled scatter losses. Our coating design approach is based on roughness characterization by power spectral density (PSD) functions as a tool to describe both the wetting behavior and scattering. For the design of architectural glass coatings, the definition of scatter thresholds is necessary. These thresholds can be determined from investigations that link visual inspection and total scatter (TS) measurements. Experiments with rough oxide layers yielded surfaces with a high water contact angle.
This paper reports on an instrument designed to measure the total backward and forward scattering of optical components down to the DUV/VUV spectral region. The system is based on a Coblentz sphere imaging the light scattered into the backward or forward hemispheres within an angular range from 2 degree(s) to 85 degree(s) onto the detector according to ISO/DIS 13696. The equipment divides into two set-ups, one operating in air at several wavelengths from 10.6 micrometers to 193 nm, the other one working in a vacuum/nitrogen at 157 nm and 193 nm. The system is fully automated and capable of scanning large sample areas. Both a deuterium lamp and an excimer laser can be used as radiation sources at 193 nm and 157 nm. Results of measurements on fluoride multilayer coatings and CaF<SUB>2</SUB> substrates are presented.