Encapsulation of low-refractive-index SiO₂ nanorods by Al ₂O₃ with atomic layer deposition

Thin films composed of SiO₂ nanorods or nanoporous SiO ₂ (npSiO₂) are attractive for use as a low refractive index material in various types of optical coatings. However, the material properties of these films are unstable because of the high porosity of the films. This is particularly apparent in dry versus humid atmospheres where both the refractive index and coefficient of thermal expansion (CTE) vary dramatically. In this article, we demonstrate that np-SiO₂ can be encapsulated by depositing Al₂O₃ with Atomic Layer Deposition (ALD), stabilizing these properties. In addition, this encapsulation ability is demonstrated successfully in a 4-pair distributed Bragg reflector (DBR) design. It is hoped that this technique will be useful in patterning specific regions of a film for optical and mechanical stability while other portions are ambient-interactive for sensing.