Ultra-low Refractive Index SiO₂ Optical Thin Film with High Mechanical Strength by Sputtering and Electron Beam Evaporation

Abstract

In this study, we evaluated the refractive indices of silicon dioxide (SiO₂) optical thin films deposited using a combinatorial coating method comprising electron beam (EB) evaporation and direct-current pulse sputtering. The combinatorial deposition system can fabricate optical thin films by simultaneously operating EB evaporation and sputtering within the same vacuum chamber. The refractive indices of the deposited films can be reduced by increasing the degree of vacuum during deposition and decreasing the deposition rate ratio of sputtering, substrate temperature, and sputtering output owing to a decrease in the film packing density due to the reduced mean free path of deposited particles and surface diffusion. When the sputter output and deposition rate ratio of sputtering were low, cracks were observed during simultaneous deposition, indicating that sputtered particles can suppress cracks in the film. Simultaneous deposition can be used to fabricate SiO₂ optical thin films with a refractive index of 1.17. The films were crack-free and exhibited a low light scattering intensity ratio of less than 0.1%. Further, they were not peeled off upon crosshatch testing or ultrasonic cleaning and had sufficient adhesion for practical use.