On the properties of nanoporous SiO₂ films for single layer antireflection coating

Single layer antireflection coatings (SLAR) consisting of nanoporous silica (NP SiO₂) films are developed by selective chemical etching of atomic layer deposited (ALD) Al₂O₃:SiO₂ composite films. The reflective index of the final NP SiO₂ film is finely adjusted from 1.132 to 1.400 at 600 nm wavelength by applying an appropriate ratio in the composite. To meet the requirements of the SLAR coatings from the deep UV (DUV) to the near IR (NIR) spectral range, the film thickness is controlled with nanometer precision by the ALD process. The SLAR are simultaneously applied on both sides of flat or highly curved substrates. Transmittance values above 99.4% are achieved even at a wavelength of 193 nm on fused silica substrates. Various characterization methods demonstrate the advantages of these SLAR with regard to impurities, optical losses, laser induced damage threshold (LIDT) properties, and surface super-hydrophilicity. The absorption losses at 193 nm wavelength as determined by laser induced deflection measurements amount to approximately 200 ppm, and to approximately 2 ppm at a wavelength of 1064 nm, while the scattering losses are around 30 ppm at 532 nm wavelength for quarter-wave layers. The LIDT values at 1064 nm are in the range of 93 J cm⁻² being close to the values measured on the uncoated substrate.