Optimization of optical and structural properties of Al₂O₃/TiO₂ nano-laminates deposited by atomic layer deposition for optical coating

Optimizing the atomic layer deposition (ALD) process of films is particularly important in preparing multilayer interference films. In this work, a series of Al 2 O 3 /TiO 2 nano-laminates with a fixed growth cycle ratio of 1:10 were deposited on Si and fused quartz substrates at 300 °C by ALD. The optical properties, crystallization behavior, surface appearance and microstructures of those laminated layers were systematically investigated by spectroscopic ellipsometry, spectrophotometry, X-ray diffraction, atomic force microscope and transmission electron microscopy. By inserting Al 2 O 3 interlayers into TiO 2 layers, the crystallization of the TiO 2 is reduced and the surface roughness becomes smaller. The TEM images show that excessively dense distribution of Al 2 O 3 intercalation leads to the appearance of TiO 2 nodules, which in turn leads to increased roughness. The Al 2 O 3 /TiO 2 nano-laminate with a cycle ratio 40:400 has relatively small surface roughness. Additionally, oxygen-deficient defects exist at the interface of Al 2 O 3 and TiO 2 , leading to evident absorption. Using O 3 as an oxidant instead of H 2 O for depositing Al 2 O 3 interlayers was verified to be effective in reducing absorption during broadband antireflective coating experiments.