Fabrication of TiO2 based broadband single layer anti-reflection (SLAR) 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 in the GLAD arrangement to enhance the degree of collimation of depositing vapour flux. Enhancement in ballistic growth of film on the substrate was observed with increasing distance from the vapour source. The substrate position near to vapour source (i.e. bottom region) showed high refractive index (~1.336 @ 550 nm wavelength) and less average film transmittance (~94.5% in 400-900 nm wavelength range) among the others. Whereas, the TiO2 coating deposited in distant position from the source (i.e. top region) showed remarkably low refractive index (~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) among the other substrate positions. The reduction in film refractive index was correlated qualitatively with the morphological alterations in the coating for different substrate positions. With further increase in distance from the vapour source, an ultimate reduction in refractive index of TiO2 to ~1.101 was observed, which is ~50% lower than the bulk TiO2 value (~2.221 @ 550 nm wavelength). The present study reports the lowest refractive index of TiO2 together with fabrication of TiO2 based broadband single layer anti-reflection (SLAR) coating.