Eylül Demir, Hasan Hüseyin Güllü, Haci Batman, ASELSAN, Ankara, Turkey
Optical coatings are an essential tool to match with optical requirements in the electro-optical imaging systems. The technology arises from multilayer thin film coatings on different optical components to improve optical performance in particular spectral regions. Related to the requirements on transmission, reflection and absorption of incoming light, the choice of layer materials and the coating design with optimum layer thicknesses are the key points to reach these interference coatings. From the point of mechanical strength, in addition to material characteristics, the kind of deposition process affects adhesion to the substrate surface and layer partner, intrinsic film stress and as a result resistance to environmental conditions. In this work, deposition technique and post-annealing are in search for the concept of minimizing stress in coating-substrate system. The case study is about an ultra-violet and infrared cut filter coating. Multilayer coating is constructed by sequential HfO₂ (hafnia) and SiO₂ (silica) film layers on BK7 glass substrate. Thin films are generally in a state of elastic mechanical stress after deposition and hafnia/silica multilayer is a source of high tensile stress where hafnia is in high and silica is moderate stress characteristics. Although the effects of coating stress can be practically diminished by using thick substrate or redesigning optical coating with decreasing total film thickness, the current approach to minimizing stress effect is divided to three methods as thick backside AR coating, prefiguring substrate surface and post-annealing. The experiments are concentrated on stress characteristics of plasma enhanced electron-beam evaporated and magnetron sputtered coatings. Following to measurement of interference fringe patterns, the techniques of designing a silica film-thickness profile to the back surface or fabricating the substrate surface with a curvature to compensate effect of the film stress are discussed. Optimum annealing temperature and process time are also investigated to improve the optical surface flatness.