Quest for Durable Low-Index Optical Coatings: From Plasma Polymerized Fluorocarbons to Hybrid Organic-Inorganic and Nanostructured Films
(Room Room 104-C)
29 Apr 19
4:40 PM
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5:00 PM
Low (n less than about 1.55@550nm) and ultra-low refractive index materials (n less than 1.38@550nm) are of particular interest in the context of enhanced optical performance of interference filters (IF, including anti-reflective coatings). In addition, for many years now, there has been a lot of effort to develop such low and ultra-low index films with multifunctional characteristics making them suitable for the application of IF on plastic and flexible substrates. This includes a necessity to better understand and optimize the thermo-mechanical properties (hardness, Young's modulus, adhesion, thermal expansion coefficient), surface energy (hydrophobicity), and possibly provide complementary functionalities (color, electrical conductivity, etc.). This presentation provides an overview of the progress in the field outlined above. In the first part, we describe the main optical, mechanical and dielectric characteristics of plasma polymerized fluorocarbons and related metal-doped plasma polymers. In the second part, we introduce hybrid organic-inorganic (SiOCH) materials that are shown to combine high mechanical resistance of ceramics with high elasticity of polymers. We demonstrate, that the refractive index of such films can vary from 1.50 to 1.53, while the extinction coefficient is kept below 10-4. At the same time, the hardness-to-Young’s modulus ratio, H/E, is substantially increased (to 0.16) compared to 0.08 for pure SiO2, a result that is well correlated with the increased resistance to crack formation and propagation. Such layers, incorporated in multilayer structures on plastic substrates, provide significantly enhanced optical and mechanical stability in environments involving large temperature and humidity excursions. In the third part, we explore the characteristics of a new category of hybrid SiOCH thin films prepared by glancing angle deposition (GLAD) using electron beam evaporation of SiO2, in the presence of an organosilicon precursor. The resulting layers are shown to exhibit n as low as 1.2, high elastic rebound and high hydrophobicity, and better mechanical properties than their inorganic counterparts. Their performance is discussed in terms of their anisotropy and additional functionalities assessed by the nanostructure-based spectroscopic ellipsometry model. Finally, we demonstrate successful implementation of the ultra-low index material in a complete IF stack.