Thermomechanical model to assess stresses developed during elevated-temperature cleaning of coated optics.

A thermomechanical model is developed to estimate the stress response of an oxide coating to elevated-temperature chemical cleaning. Using a hafnia-silica multilayer dielectric pulse compressor grating as a case study, we demonstrate that substrate thickness can strongly affect the thermal stress response of the thin-film coating. As a result, coatings on large, thick substrates may be susceptible to modes of stress-induced failure (crazing or delamination) not seen in small parts. We compare the stress response of meter-scale optics to the behavior of small-scale test or "witness" samples, which are expected to be representative of their full-size counterparts. The effects of materials selection, solution temperature, and heating/cooling rates are explored. Extending the model to other situations, thermal stress results are surveyed for various combinations of commonly used materials. Seven oxide coatings (hafnia, silica, tantala, niobia, alumina, and multilayers of hafnia-silica and alumina-silica) and three glass substrates (BK7, borosilicate float glass, and fused silica) are examined to highlight some interesting results.

Fracture mechanics of delamination defects in multilayer dielectric coatings.

During the fabrication of multilayer-dielectric (MLD) thin-film-coated optics, such as the diffraction gratings used in OMEGA EP's pulse compressors, acid piranha cleaning can lead to the formation of chemically induced delamination defects. We investigate the causes of these defects and describe a mechanism for the deformation and failure of the MLD coating in response to hydrogen peroxide in the cleaning solution. A fracture mechanics model is developed and used to calculate the crack path that maximizes the energy-release rate, which is found to be consistent with the characteristic fracture pattern observed in MLD coating delamination defects.