Nadja Felde, Tobias Herffurth, Anne-Sophie Munser, Michael Scheler, Thomas Gischkat, Sven Schröder
Fraunhofer Institute for Applied Optics and Precision Engineering IOF, Jena, Thuringia, Germany
Light scattering due to interface or coating imperfections is a significant concern in optical component applications, even when using advanced polishing and coating techniques. However, it is worth noting that scattered light contains on the other hand valuable information about its source, making light scattering techniques an excellent tool for characterizing surfaces and thin film coatings. These techniques not only enable the detection of surface or subsurface defects but also provide means to measure the surface or coating roughness. Characterizing surface properties is vital for the development of functional coatings, and while standard inspection systems like AFM, WLI, or LSM are used during inspection in labs for surface roughness analysis, light scattering measurements have distinct advantages. They are non-contact, non-destructive, robust, and fast - making them suitable for integration into fabrication processes or test environments.
In a collaborative project, a light scattering sensor was integrated into a roll-to-roll process for fabrication of colorshift foil fabrication by evaporation. Such high-speed processes can result in the disposal of kilometers of finished products due to quality defects, which makes early intervention and regulation during the manufacturing process particularly crucial. By combining in-situ spectrophotometric measurements with light scattering information, real-time data can be recorded, provided, and evaluated during production while the process is ongoing. Thus, even coating process adjustment might become possible.
Recently, a light scattering sensor was successfully integrated even into a magnetron sputtering coating system. This breakthrough integration allowed the monitoring of roughness evolution and defect growth during deposition while maintaining a vacuum environment. Examples focused on different metal mirrors. The integration enables precise determination of the optimal time for closed metal films without pinholes. Additionally, the sensor can be used in this way to effectively control the impact of particles and contamination, leading to the development of various cleaning strategies.