Procedure for Designing Optical Coatings to Control Stress

Residual stress in optical thin film coatings can cause adhesion failures, cracking, buckling, and bending of the substrates. The stress in the coating depends on the materials used, deposition process, and the design. The bulk properties of the coating materials and the substrate such as thermal coefficient of expansion, Young’s Modulus, Poisson’s Ratio, etc., are basic parameters to be considered. The deposition process usually yields film properties which are different from the bulk properties. These properties depend at least upon process temperature, deposition rate, and plasma assist parameters. These plasma parameters include ion species, ion voltage, and ion to atom arrival rate. The resulting film structure includes porosity, crystal form, density, non-uniformity, and possible surface and chemical interactions. The design of an optical thin film coating is mostly the refinement of the thicknesses of the layers to produce the desired reflection versus wavelength. It has been found that considerable variation in layer thicknesses can still achieve the desired spectral result. A major factor in the stress contribution of each individual layer is its thickness. Thin film design software can balance both the optical performance and the residual stress in the overall coating on the specific substrate with specific deposition processes, but it is first necessary to characterize the properties of the materials produced by those processes.