Intrinsic and thermal stress modeling for thin-film multilayers

The measurement of intrinsic and thermally induced stresses in multilayer dielectric coatings was investigated to improve reliability and durability predictions of thin-film coatings, particularly, those coatings that will be utillized under extreme thermal conditions (e.g., in high-energy laser systems). Principally, the investigation was conducted using thorium tetrafluoride, zinc selenide, and thallium iodide films as antireflection coatings on potassium chloride at 10.6 micrometers. A four-channel stress interferometer system that measures stress levels in single or multilayer films as they are being deposited was fabricated and tested. A new experimental technique was established whereby the stress interferometer system can also be used to compute Young's modulus and the coefficient of thermal expansion for the thin film materials. A computer code was developed that utilizes the experimentally measured values of intrinsic stress, expansion coefficient, and Young's modulus to model stress levels in multilayer coatings when subjected to a changing uniform environment.