Optical and nanostructural insights of oblique angle deposited layers applied for photonic coatings

Abstract

Oblique angle deposition (OAD) is a nanostructuration method widely used to tune the optical properties of thin films. The introduction of porosity controlled by the deposition angle is used to develop the architecture of each layer. However, optical properties of these porous layers may differ greatly from those of dense layers due to the presence of anisotropy, refractive index gradient and scattering. This work focuses on OAD thin film of SiO2 and it aims at considering all these effects to describe the optical response. For that, the nanostructure has been analyzed with a complete SEM study and key parameters like the porosity gradient profile and aspect ratio of the nanocolumns were extracted. The samples were then characterized by generalized ellipsometry to evaluate the influence of the microstructural properties on the optical response of the films. An original optical model is then presented to fit these new optical properties. A reliable correspondence is observed between the optical model parameters and the microstructure characteristics like the column angle and porosity gradient. This demonstrates that such complex microstructural parameters can be accessed solely from optical measurements. All the work has enabled us to develop a two-layer anti-reflective coating that demonstrate high transmission level.