Super Hydrophilic Low Refractive Index SiO₂ Optical Thin Films Deposited by Using a Combination Coating Method

Abstract

In this study, we evaluated the hydrophilicity of low-refractive-index SiO₂ optical thin films deposited using a combination coating method of electron beam (EB) deposition and direct current pulse sputtering. In this deposition method, the introduction of process gas during deposition deteriorates the vacuum level, shortens the mean free path of deposited particles, and produces low-refractive-index films due to the low density associated with the columnar structure. Furthermore, the refractive index of the thin film deposited by using the combination coating method was 1.28. The low density of the thin film increases the surface roughness of the thin film, resulting in hydrophilic properties (contact angle, θ ≤ 10°). Although conventional EB deposition also exhibited hydrophilicity (contact angle, θ ≤ 10°) immediately after film deposition, the contact angle increased about 1 week after film deposition and lost its hydrophilicity after 1 year (contact angle, θ = 47°). The cause of this factor is the deposition of hydrocarbons in the air. However, the combination coating method obtained high hydrophilicity (contact angle, θ ≤ 10°) even after 1 year of deposition. The possible reason is the chemical termination by sputtering plasma besides the structural factor of the surface roughness, which is sufficient to sustain the hydrophilicity for 1 year.