Fabrication of TiO2-based broadband single-layer anti-reflection coating by collimated glancing angle deposition technique.

Single-sided TiO2 thin films were prepared using a modified glancing angle deposition (GLAD) technique. An additional flux collimation plate was introduced into the GLAD arrangement to enhance the degree of collimation of depositing vapour flux. Enhancement in the ballistic growth of film on the substrate was observed with increasing distance from the vapour source. The substrate position near to the vapour source (i.e. bottom region) showed a high refractive index (RI, ∼1.336 @ 550 nm wavelength) and lower average film transmittance (∼94.5% in 400-900 nm wavelength range) compared to the others. In contrast, the TiO2 coating deposited at a distant position from the source (i.e. top region) showed a remarkably low RI (∼1.190 @ 550 nm wavelength) and excellent anti-reflection over a broad spectral region with a maximum average transmittance (∼95.3% in 400-900 nm wavelength) compared to the other substrate positions. The reduction in film RI was correlated qualitatively with the morphological alterations in the coating for different substrate positions. With a further increase in distance from the vapour source, an ultimate reduction in the RI of TiO2 to ∼1.101 was observed, which was ∼50% lower than the bulk TiO2 value (∼2.221 @ 550 nm wavelength). The present study reports the lowest RI of TiO2 together with fabrication of a TiO2-based broadband single-layer anti-reflection coating.