Structural, optical, and mechanical properties of TiO₂ nanolaminates

The structural, optical, and mechanical properties of TiO₂ nanolaminate films grown by plasma-enhanced atomic layer deposition are discussed. Several TiO₂/Al₂O₃ and TiO₂/SiO₂ compositions have been investigated to study the effect of the relative number of ALD oxide cycles on the film properties to obtain a high refractive index coating with low optical losses, low roughness, and low mechanical stress. The formation of crystalline TiO₂ observed at high deposition temperature, or film thickness was inhibited by periodically introducing ultra-thin amorphous layers into the film. Only 4 ALD cycles of Al₂O₃ (corresponding to ca. 0.5 nm) between 335 ALD cycles of TiO₂ (ca. 11 nm) form a closed, distinct layer suppressing the crystallization in TiO₂ film. Consequently, the roughness of the pure TiO₂ film is reduced from ca. 20 nm rms to 1 nm rms in the 335/4 nanolaminate, with only a slight decrease of the refractive index from 2.46 to 2.44 in 100 nm pure TiO₂ and the nanolaminate, respectively. The refractive indices of the nanolaminates in various compositions vary between 2.38 and 2.50 at 632 nm, and the corresponding optical losses from the films are low. The mechanical stress was reduced to about 140 MPa in several TiO₂/Al₂O₃ nanolaminates; however, lower mechanical stress has not been obtained with the studied compositions. The nanolaminate structure is preserved up to 600 °C annealing temperature. After annealing at 800 °C, the individual layers interdiffuse into each other so that no distinct nanolaminate structure is detected. By using TiO₂/Al₂O₃ nanolaminates with reduced mechanical stress, a narrow bandpass filter was realized on various substrates, including half-ball and aspherical lenses.