Stress analysis of optical coatings for improved durability and performance of high end optical components

Modern optical systems are often required to function in severe environmental conditions for prolonged periods without suffering from performance degradation. Essential parts of such electo-optical systems are windows, domes and other optical elements. These elements are almost always coated with an efficient, multispectral and highly durable antireflection coating. The complexity further increases when these coatings are applied on curved surfaces, such as hemispherical domes that are usually used in order to allow a wider field of regard. The durability of the coated optical component is dependent upon many different parameters such as deposition method and process parameters and the adhesion between layers and substrate. However, one very important parameter, which can have a significant impact on the durability and optical performance, is the stress state of the applied anti-reflection layers. This subject is mostly left untreated mainly due to the difficulty of characterization and modeling techniques and lack of thin film mechanical constants which sometimes significantly differ from the bulk constants of the same material. The stress state of the optical part is mainly determined by the mechanical properties of the coating materials and substrate, geometrical shape of the part and the thickness of the layers. In this work, both analytical and experimental approaches were used for characterization of stress distribution in thin optical films. Various single and multiple thin films were deposited using the electron beam evaporation technique onto Si 6 '' wafers. The film thickness was measured using an ellipsometer and the bi-axial module of the different films was studied by measuring the change in radius of curvature of the Si substrate and applying Stoney's equation. The results were compared to the analytical Timoshenko solution of stress state in a single and multiple film structure. Knowing the stress state of a multi-layer coating would allow the engineer to design a part with the desired stress state which alongside the optical design, is expected to bring about a significant improvement in the durability and optical performance.