Structural, optical, and mechanical properties of TiO₂ nanolaminates

Abstract

The structural, optical, and mechanical properties of TiO 2 nanolaminate films grown by plasmaenhanced atomic layer deposition are discussed. Several TiO 2 /Al 2 O 3 and TiO 2 /SiO 2 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 TiO 2 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 Al 2 O 3 (corresponding to ca. 0.5 nm) between 335 ALD cycles of TiO 2 (ca. 11 nm) form a closed, distinct layer suppressing the crystallization in TiO 2 film. Consequently, the roughness of the pure TiO 2 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 TiO 2 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 TiO 2 /Al 2 O 3 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 TiO 2 /Al 2 O 3 nanolaminates with reduced mechanical stress, a narrow bandpass filter was realized on various substrates, including half-ball and aspherical lenses.

Effect of Plasma-Enhanced Atomic Layer Deposition on Oxygen Overabundance and Its Influence on the Morphological, Optical, Structural, and Mechanical Properties of Al-Doped TiO2 Coating

“…The decrease of refractive index on TiO 2 /Al 2 O 3 nanolaminates is due to the lower value of the refractive index of Al 2 O 3 (1.62). Therefore, these results show that it is possible to adjust the refractive index by incorporating amorphous Al 2 O 3 to TiO 2 with low optical losses (similar results were showed by Ghazaryan et al [ 78 ]).…”

“…Hence, besides ALD process parameters, such as substrate temperature, oxygen gas flow rate, and plasma power [ 68 , 77 ], incorporating thin Al 2 O 3 layers into the TiO 2 film suppress the crystallinity of TiO 2 films without decrease the quality of the films [ 78 ]. Figure 6 shows that with the decrease in crystallinity, the surface roughness of TiO 2 /Al 2 O 3 nanolaminates undergoes a considerable reduction, which was observed in previous works carried out in thermal ALD [ 24 ].…”

“…Figure 8 c shows the optical losses, an essential parameter to design thin-film optical sensors that should have as small losses as possible. Optical losses are calculated from 100%-transmittance-reflectance (100%-T-R) [ 78 ]. Absorption and scattering reduce the intensity of transmitted light, causing these losses, and the spectrophotometry results showed the homogeneity of the nanolaminates and low optical losses (<10%).…”

Impact of Ions on Film Conformality and Crystallinity during Plasma-Assisted Atomic Layer Deposition of TiO2

“…These observations are in good agreement with results reported in the literature. 5,17,19,30,38,39 To probe the crystallinity of the TiO 2 grown with and without exposure to ions, the Raman measurements presented Figure 5. Substrate-corrected Raman spectra of TiO 2 films grown with exposure to ions, at table temperatures of 100, 200, and 300 °C, using a grounded substrate (mean ion energy of 9 ± 1 eV) and plasma steps of 38 s. Additionally, one film is grown at 200 °C using 60 W substrate biasing (mean ion energy of ∼120 eV) and plasma steps of 12 s. All spectra are normalized to the 302 cm −1 peak of the silicon substrate (see the Supporting Information).…”

“…Titanium oxide is a widely studied material and as a thin film has many applications, such as in photocatalysis, photonics, − photovoltaics, and nanoelectronics, where in the latter, TiO 2 primarily functions as high-k dielectric. Especially in the field of nanoelectronics, the miniaturization of device structures has strongly increased the demand for atomic-scale processing techniques such as (plasma-assisted) atomic layer deposition (ALD), which can typically provide atomic-level thickness control. − In the case of TiO 2 , ALD is used, for instance, in self-aligned multiple patterning for the preparation of nanometer-thin TiO 2 sidewall spacers. , Furthermore, plasma ALD of TiO 2 has been reported for gap-filling and encapsulation applications in the fabrication of memory devices such as dynamic random-access memory (DRAM), where a high level of TiO 2 film conformality is required to isolate adjacent memory cells…”