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Abstract
Optimizing the atomic layer deposition (ALD) process of films is particularly important in preparing multilayer interference films. In this work, a series of Al2O3/TiO2 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 Al2O3 interlayers into TiO2 layers, the crystallization of the TiO2 is reduced and the surface roughness becomes smaller. The TEM images show that excessively dense distribution of Al2O3 intercalation leads to the appearance of TiO2 nodules, which in turn leads to increased roughness. The Al2O3/TiO2 nano-laminate with a cycle ratio 40:400 has relatively small surface roughness. Additionally, oxygen-deficient defects exist at the interface of Al2O3 and TiO2, leading to evident absorption. Using O3 as an oxidant instead of H2O for depositing Al2O3 interlayers was verified to be effective in reducing absorption during broadband antireflective coating experiments.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Titanium dioxide (TiO2) has many excellent properties in optics, electricity, chemistry and structure [1–3]. It has been widely used in optical thin films [4–6], photocatalysis [7,8], transparent electrodes [9–11], sensor applications [12], protective films [13], etc. In the field of optical coating, TiO2 film exhibits low absorption and high refractive index compared to other oxide materials. The applications of TiO2 nanomaterials depend on the optical properties, phase structure, surface morphology and compactness, which are highly related to the deposition technology. TiO2 films can be prepared by numerous methods, including sol-gel method, hydrothermal method, chemical vapor deposition (CVD) method, laser deposition method, electron beam evaporation method, direct oxidation method, magnetron sputtering method, etc [14–16].
Atomic layer deposition (ALD) is a unique CVD technique. The films are grown by means of surface self-limiting gas-solid reaction between the precursors when the pulsed gas precursors are injected into the reaction chamber alternately. Each precursor pulse is followed by a long time inert gas purge for the excessive precursor, ensuring ALD has the characteristics of self-saturating surface adsorption and self-limiting reaction. The films deposited by ALD have the advantages of high uniformity and good conformity [17]. TiO2 layers deposited by ALD have attracted extensive research interest and applied in optics to fabricate protective films, photonic crystals, gratings and optical films [18,19].
ALD TiO2 with high refractive index has been proven to be one of the most potentially promising materials for optical coating. However, studies have also shown that ALD TiO2 films deposited at temperatures below 200 °C have sizeable residual stress due to low migration kinetic energy [20]. The gradual accumulation of stress, along with the thickening of the layers, leads to the deformation of substrate and fracture of layers, which will lead to the failure of photoelectric devices. Meanwhile, TiO2 layers deposited at high temperatures above 200 °C can release the residual stress by means of crystallization, which would increase the surface roughness and make the ALD growth per cycle (GPC) unstable. To fabricate precise optical films by ALD, the stability of GPC and precise thickness control of each layer are essential. The controlled, high-quality ALD process of TiO2 films is an important research topic for further applications of TiO2. In this work, we tried to use the laminated structure of Al2O3 and TiO2 to obtain a stable and controllable film with high refractive index deposited by ALD [21,22]. Studies were focused on examining the optical properties, microstructure and surface morphology of the Al2O3/TiO2 nano-laminates. After a series of comparative experiments, we have given an optimized nano-laminate structure. With the optimized ALD process, stable GPC of ALD film has been obtained, defective absorption of nano-laminate has been eliminated, and broadband anti-reflective (BBAR) films with high transmittance have been successfully fabricated. The results of this work should be very useful in many fields with a demand for ALD TiO2.
2. Experiment details
2.1 Samples preparation
The film samples were prepared by ALD (TFS500, Beneq Oy, Finland), including TiO2 and Al2O3. The metal precursors for TiO2 and Al2O3 were TiCl4 and trimethylaluminum (TMA) (both provided by Nanjing ai mou yuan Scientific equipment Co., Ltd, China) with purity > 99.99%, and the oxidant was H2O (deionized water, resistivity ∼0.5 MΩ*cm) or plasma O. ALD process parameters are listed in Table 1. The films were deposited on silicon and fused quartz substrates.
Table 1. Parameters of the ALD process for TiO2 and Al2O3
Two series of samples were prepared for different validation objectives, as shown in Table 2. One was the temperature series, in which TiO2 layers with same 2000 ALD cycles were deposited at different reaction temperatures from 80 to 300 °C to examine the temperature-dependent growth characteristics. To avoid the effects of high stress and low reliability due to low-temperature thermal ALD, plasma oxygen was chosen as the oxidant. The other one was the nano-laminate structure series, in which the samples were prepared with alternating growth of Al2O3 and TiO2 sub-layers, as Fig. 1 shown. ALD cycle ratio 1:10 for Al2O3 and TiO2 was fixed to maintain high refractive index of nano-laminates. Meanwhile, the total number of cycles was varied to ensure that all samples had similar growth thickness.
Table 2. Samples classify by substrates, temperature and structures
2.2 Sample measurement
The optical properties and thickness of samples on Si wafers were measured by spectroscopic ellipsometry (SE, V-VASE, J.A. Woollam, USA) with a spectral range of 0.8-5.2 eV per 0.2 eV (wavelength range of 238.5-1550 nm) at three incident angles of 65°, 70° and 75°. For the nano-laminates on fused quartz, the crystalline structures were analyzed by X-ray diffraction (XRD, Rigaku D/max 2550 V, Japan) with Cu Kα in grazing incidence XRD scan mode at 40 kV and 200 mA. Atomic Force Microscopy (AFM, FSM-Nanoview 1000 AFM, Fishman, Suzhou, China) was used to characterize the surface topography and roughness in tapping mode. Transmission Electron Microscopy (TEM, Tecnai G2 F20, FEI, USA) was used to measure the cross-section morphology of the samples on Si substrate. Besides, the transmission and reflection of the samples on fused quartz were measured by a spectrometer (Lamda 900, PerkinElmer, USA) in the spectral range of 200-2500 nm.
2.3 Ellipsometry analysis method
As mentioned above, variable angle spectroscopic ellipsometry was used to measure the samples and followed by an ellipsometry fitting to obtain the optical properties and thickness of the film. The establishment of a proper model of the film structure and proper dispersion functions for the materials is a key step. In optical film theory, symmetric film structure can be considered as equivalent single layer. Additionally, composite layer with sub-layer thicknesses much smaller than the wavelength can also be simplified to single layer using the EMA model. It is feasible and reasonable to treat nano-laminates as single layers. Here, a five-phase model of Si substrate/SiO2/ALD layer/surface rough (SR) layer/Air was adopted, as shown in Fig. 2, where SiO2 was the natural oxide layer of silicon with a fixed thickness 2.27 nm, ALD layer was TiO2 or Al2O3/TiO2 nano-laminate and SR layer represented the surface roughness of the sample (optical constants of the Si wafer and the SiO2 layer are offered in Supplement 1, Table S1). To fit the measured ellipsometric parameters ϕ and Δ precisely, the Tauc-Lorentz (TL) dispersion model was adopted as dispersion function of ALD layer in the ellipsometry analysis (more details are offered in Supplement 1, Fig. S1, Fig. S2 and text section). It is customary to minimize the number of non-independent variables in ellipsometry analysis to avoid distortion of the fitted results. Therefore, for the samples of nano-laminates, the Al2O3/TiO2 nano-laminate was considered as a single layer with the TL dispersion model.
3. Results and discussion
3.1 Effect of deposition temperature on TiO2 layers
It has been widely known that high temperatures can cause a transition from amorphous to anatase structure of TiO2, and the phase change temperature is around 200 °C [23,24]. To obtain the optical properties and microstructure of ALD TiO2 deposited at different temperatures, a series of TiO2 samples of 2000 ALD cycles were deposited at 80, 150, 200, 250 and 300 °C, followed by XRD, SE and transmission spectroscopy measurements. The main results are shown in Fig. 3.
Fig. 3. Characterization of ALD TiO2 of different deposition temperatures, (a) XRD spectra, (b) SR layer thicknesses by SE, (c) GPC, and (d) transmission spectra of TiO2 layers deposited at different deposition temperatures.
It can be seen in Fig. 3(a) that the TiO2 layer begins to crystallize above the deposition temperature of 200 °C, and shows higher crystallinity at higher deposition temperatures. All the typical characteristic diffraction peaks of the anatase TiO2 layers become apparent when the deposition temperature reaches 300 °C. As the deposition temperature rises, the TiO2 molecules obtain higher kinetic energy to crystallize and become denser via molecular migration [25]. In Fig. 3(b), the analysis results of SE show that the SR layer thickens slowly as the deposition temperature rises, and when TiO2 begins to crystallize above 200 °C, the surface roughness increases significantly, which is due to the increasing grain size.
A rougher surface will lead to a larger surface area, resulting in an increasing and unstable growth rate during the ALD process for the thick film. It is due to the nature of the excellent conformity of ALD. A rough surface will lead to strong light scattering, while an unstable growth rate will result in an uncontrollable film thickness. And they are both problems that must be solved for the preparation of optical thin films. As shown in Fig. 3(c), the GPC of TiO2 layers decreases from 0.095 nm/cyc to 0.058 nm/cyc with the deposition temperature rising from 80 °C to 300 °C. The change in GPC is due to the dynamic and reversible chemical reaction of TiCl4 and H2O.The high temperature leads to an enhanced inverse reaction, which results in a decrease in GPC with increasing temperature [26,27].
Meanwhile, the transmission spectra of TiO2 layers prepared at different temperatures were measured to verify the damage of layer crystallization to the optical properties, as Fig. 3(d) shows. Before crystallization, the peak transmittance of the film in the visible band is the same as that of the substrate, indicating that there is no optical loss in the film. And the peak transmittance of the samples deposited above 200 °C is significantly lower than that of the substrate, for example, 84% for the Qt300 sample. The transmitted light loss is due to light scattering caused by surface roughness and crystalline grain in the TiO2 layers.
3.2 Al2O3/TiO2 nano-laminates with different laminated structures
The crystallization of TiO2 due to high-temperature growth causes problems such as light scattering loss and unstable growth rate, making it unsuitable for optical coatings. The key to solving this problem lies in controlling the crystallization of TiO2, and solutions include low-temperature growth and the use of laminated structures in high-temperature growth process. Due to the lack of a low-temperature ALD process for low-index materials such as SiO2 and the problem of large stress in low-temperature deposited TiO2 [20], the nano-laminate growth at high-temperature turns out to be a key way to solve the problem of TiO2 in optical thin film applications. In this section, different Al2O3/TiO2 nano-laminates deposited at 300 °C were adopted to confine the crystallization of TiO2, and the optical properties and micro structures were investigated.
The XRD patterns of the Al2O3/TiO2 nano-laminates of different structures are shown in Fig. 4 (more XRD spectra are offered in Supplement 1, Fig. S3 and S4). And the diffraction peaks of nano-laminates, such as the typical diffraction peaks around 2θ = 25.3°, decrease with decreasing cycles of TiO2 sub-layer, which indicates that the distributed Al2O3 interlayers help to reduce the crystalline grain and confine the crystallization of TiO2. According to the GPC value of TiO2 deposited at 300 °C, the expected thickness of single TiO2 sub-layers would be reduced from 63.8 nm to 4.64 nm with the more densely distributed Al2O3 interlayers, which limits the grain growth in geometric space. Also, the grain size D could be calculated by the Scherrer Equation [25,28], as Eq. (1):
where K is the Scherrer constant (0.89), λ is the X-ray wavelength (0.15406 nm), β is the full width at half maximum (FWHM) of the diffraction peak, and θ is the angle of diffraction. Taking the diffraction peak of anatase (101) as an example for analysis, the grain size gradually decreases from 22.3 nm to 10.9 nm with decreasing ALD cycles of TiO2 sub-layers, and finally becomes too small to measure for samples Q110 and Q80, as shown in Table 3. The results indicate that the Al2O3 interlayers limit the growth of TiO2 crystalline grains, and the extremely dense distribution of Al2O3 interlayers seems to be able to eliminate the crystalline grain.Fig. 4. XRD spectra for Al2O3/TiO2 nano-laminates deposited on fused quartz substrates. The number of the sample label denotes the ALD cycle number of the TiO2 sub-layer.
Table 3. XRD parameters for diffraction peak (101) of Al2O3/TiO2 nano-laminate samples
The SE measurement and analysis were adopted to obtain the optical and structural properties of the film, and a five-phase structural model was used for SE analysis as described in Fig. 2. Meanwhile, the TL model was used to characterize the optical properties of the Al2O3/TiO2 nano-laminate, which was considered as a single layer. As Fig. 5 shown, the refractive index of Al2O3/TiO2 nano-laminates decreases slightly with decreasing cycle number of TiO2 sub-layer, indicating a decrease in the fraction of TiO2 components due to the relatively low GPC at the beginning of TiO2 ALD process. Also, the Al2O3 interlayers inhibited the growth of TiO2 crystalline grains, resulting in a smoother surface, which in turn should reduce the ALD GPC of TiO2.
The total thickness of ALD film sample included the nano-laminate thickness and 50% the thickness of SR layer by SE, and the GPC of Al2O3 was considered as 0.1 nm/cycle for simplification. So, the GPC of TiO2 sub-layers of different samples were calculated, as shown in Fig. 6. The GPC decreases from 0.055 to 0.034 nm per cycle with decreasing cycle number of TiO2 sub-layer from 1100 to 160, but increases slightly for samples S110 and S80. The peculiar GPC of samples S110 and S80 could be explained by the thickening SR layer. As shown in Table 4, the SR layer gradually thins from 3.1 nm to 0 nm with decreasing cycle number of TiO2 sub-layer from 1100 to 160. However, it could be found that for the densest laminated structure S80, the SR layer thickens sharply to 4.5 nm, which could be the reason for the increasing GPC of S80 and S110.
Table 4. The SR layer thickness of Al2O3/ TiO2 nano-laminates from SE analysis
To verify the trend of the surface roughness obtained from SE measurement, AFM measurement was used to map the surface topography of nano-laminates. The AFM images are given in Fig. 7 and used to calculate the root mean square value (RMS) of surface roughness.
The RMS roughness obtained by AFM is compared with the SR layer thickness obtained by SE measurement and the results reveal the same trend among different samples as shown in Fig. 8. In the range of 160 to 400 cycle numbers for TiO2 sub-layer, the surface roughness of the samples maintains relatively stable, and it can bring constant GPC of ALD. In the sub-layer cycles below 160 or above 400, the roughness becomes larger, which may lead to unstable growth rates.
Fig. 8. Comparison between the SR layer thickness fitted by SE and RMS roughness calculated by AFM data.
To investigate the causes of the generation and variation of film surface roughness, TEM was used to observe the profile structure of the Al2O3/TiO2 nano-laminates. The TEM images of samples S1100, S400, S160 and S80 are shown in Fig. 9. It is seen that there are significant crystalline grains in the samples S1100 and S400, but almost no crystalline grains in S160 and sS80, which is consistent with the XRD findings. As seen in Fig. 9(a), there are gaps between the grains, making the grains protrude from the surface, which results in a rough surface. Interestingly, nodules appear in laminated layers from the TEM images of samples S160 and S80, as marked by the red circle, which is considered to be diffusion phenomenon of the TiO2 crystalline core caused by localized high stress and ultrathin Al2O3 barrier layer. Obviously, the presence of nodules in the nano-laminate is the reason for the deterioration of the surface roughness of S80. As the nodule crossed the Al2O3 barrier layer, the inhibition of TiO2 crystallization was weakened, which in turn would lead to an increase in surface roughness and a higher growth rate.
Fig. 9. TEM images of S1100, S400, S160 and S80. The Al2O3 interlayers separate TiO2 sub-layers obviously to limit the crystallinity and grain size. In the densely laminated structures, crystalline core diffusion happened to form nodules crossing the Al2O3 interlayers. The nodule gets larger in more densely laminated structures as the red circles mark.
The presence of nodules in the films could also be verified by XRD analysis. As seen in Table 2 and Table 3, the grain size of S160 (10.9 nm) is significantly larger than the thickness of the TiO2 sub-layer (5.2 nm). The grain size calculated from XRD is compared with the thickness of TiO2 sub-layers, as shown in Fig. 10. It could be found that when the laminated layers are not densely distributed, the grain size is much smaller than the thickness of TiO2 sub-layers. When Al2O3 interlayers become denser, the grain size decreases, and tends to be consistent with the thickness of TiO2 sub-layer, which verifies the inhibitory effect of Al2O3 interlayers on the crystallization of the TiO2 sub-layer. However, with the thickness of TiO2 sub-layer decreasing to below 14 nm, the crystalline grain size decreases very slowly to a constant value about 11 nm, which is much larger than the thickness of the TiO2 sub-layer for samples S200 and S160. For samples S110 and S80, since the formed nodules are relatively sparse and the growth of the crystalline grains are partially inhibited by the Al2O3 interlayers, no X-ray diffraction peaks with sufficient intensity are generated to characterize the grain size via XRD analysis
For films applied in optical coating, low optical loss is a very critical factor. The nano-laminates inhibit the crystallization of TiO2 and reduce the surface roughness, thus stabilizing the growth rate and facilitating precise thickness control. In addition, inhibited crystallization should also have a good effect on reducing the light scattering loss. We have measured the transmission spectra of the nano-laminate samples and the results are shown in Fig. 11. As seen, Q400 has the best peak transmittance (91.2%), but Q1100, Q110 and Q80 have poor peak transmittance, which are also the samples with obvious crystalline grains or nodules. These results show that the transmitted light loss could be reduced by adopting suitable laminated structures while the unsuitable laminated structure may also be harmful to the transmittance.
Fig. 11. Transmission spectra of Al2O3/TiO2 nano-laminates on fused quartz substrates. Among all the nano-laminates samples, the Q1100 is a bilayer film, which can be regarded as a layer of pure TiO2 grown on an ultra-thin Al2O3 layer instead of a nano-laminate.
As shown in Fig. 11, the peak transmittances of all the nano-laminate samples are smaller than that of the fused quartz substrate, and even the best sample Q400 has a peak transmittance of only 91.2%, which is slightly less than that of substrate 93%. To explain the decreasing transmittance, sample Q400-18 with a laminated structure 18*(40 + 400) was deposited, whose transmittance was measured and compared with that of sample Q400 also renamed Q400-3, as shown in Fig. 12. With the repeat number N increasing from 3 to 18, the peak transmittance decreases significantly. The sample Q400-18 shows absorption characteristics from visible to infrared region, which are not intrinsic to TiO2 or Al2O3. The results suggest that the laminated structure may bring absorption. The absorption characteristics look like those of grey TiO2 film, suggesting that the laminated structure may lead to oxygen-deficient defects at the Al2O3/TiO2 interface. Since aluminum has stronger metal reducibility than titanium, it is reasonable to assume that Ti-atom loses oxygen atoms covalently bonded with Al-atom in the molecular migration of crystallization.
Furthermore, we obtained the absorption spectra by measuring the total reflection and total transmission of sample Q400-18 with an integrating sphere to remove the effect of light scattering, and the spectra were shown in Fig. 13(a). In Fig. 13(a), sample Q400-18 has significant broadband absorption, with maximum absorption of 22.5% at 735 nm. After tracing the ALD process of nano-laminate samples, it was speculated that O3 replacing H2O as the oxidant for Al2O3 could inhibit Al2O3 interlayers from robbing the oxygen atoms of TiO2 sub-layers by providing excessive oxygen atoms. So, sample Q400-14 with a laminated structure of 14*(40 + 400) was prepared, and the reactants for Al2O3 growth were TMA and O3. The total transmittance spectrum of sample Q400-14 was measured, showing much higher peak transmittance than sample Q400-18 deposited without O3 oxidant, as shown in Fig. 13(b). The absorption of sample Q400-14 was also measured and compared with that of sample Q400-18, as shown in Fig. 13(c) and (d). Obviously, the absorption at 750 nm for sample Q400-18 is 22.0% while only 1.7% for Q400-14, showing that the ALD process with O3 oxidant greatly improves the optical performance of Al2O3/TiO2 nano-laminates.
Fig. 13. Measured spectra of samples Q400-18 and Q400-14; (a) total reflection, total transmission and absorption spectra of sample Q400-18, (b) total transmission spectra of Q400-18 and Q400-14, (c) total reflection, total transmission and absorption spectra of sample Q400-14, (d) absorption spectra of Q400-18 and Q400-14.
Therefore, an optimized Al2O3/TiO2 laminated structure (40 + 400) is selected to control the thickness of TiO2 sub-layers around 16 nm (400 cycles), which can achieve accurate growth of TiO2 layer with low crystallinity. Meanwhile, using O3 as oxidant instead of H2O for Al2O3 during the Al2O3/TiO2 nano-laminates deposition could significantly reduce the absorption caused by oxygen-deficient defects. These optimized ALD process would be very useful for various optical coatings such as anti-reflective (AR) coatings.
3.3 AR coatings design and preparation
To evaluate the practical application effect of Al2O3/TiO2 nano-laminates in optical coating, 400-680 nm AR coatings with classic M2HL @500 nm structure (M: Al2O3, H: Al2O3/TiO2 nano-laminates, L: SiO2) were designed in commercial software (FilmWizard, Scientific Computing International, USA) and prepared by ALD [6]. The designed AR film would be deposited by two separate ALD recipes, and the main difference was the oxidant choice (O3 or H2O) for Al2O3 ALD. Besides, the low-index layer SiO2 was also deposited by ALD at 300 °C with precursors bis(t-butylamino)silane (BTBAS, provided by Nanjing Ai Mou Yuan Scientific Equipment Co., Ltd, China, purity > 99%) and O3. Then the two AR coatings were prepared with the same designed structure, and the transmittance of the two was compared, as shown Fig. 14. The result shows that the selected Al2O3/TiO2 nano-laminate 4*(40 + 400) as the H-layer performed well in double-side AR coatings at high temperature, which means precise multilayer optical films could be prepared by ALD without varying the growth temperature [29]. However, it also shows that the absorption caused by oxygen-deficient defects significantly damages the transmittance of the prepared AR coatings deposited with oxidant H2O. Using O3 as oxidant instead of H2O, the average transmittance of double-sided AR coating was increased from 96.5% to 98.5%, which means that oxygen-deficient defects could be significantly reduced in the ALD process with O3 oxidant.
Fig. 14. Transmittance spectra of the double-side AR coatings on fused quartz with two ALD recipes with oxidants O3 and H2O.
4. Conclusion
The TiO2 layers deposited by ALD at high temperatures tend to crystallize, leading to increased surface roughness, which results in an unstable growth rate of ALD layers. The TiO2 film can be divided into multiple sub-layers by inserting ultra-thin Al2O3 interlayers, which preserves the high refractive index of the film while inhibiting the crystallization of TiO2. As the crystallinity decreases, the surface roughness is reduced, and the growth rate of the ALD film tends to be stable, making for the preparation of high-precision optical films. However, with the laminated structure going too dense, nodules would appear in the Al2O3/TiO2 nano-laminates. It makes surface roughness deteriorate and GPC unstable again. After examining the optical and structural properties of a series of different nano-laminates, an optimized laminated structure (40 + 400) has been adopted to keep the thickness of TiO2 sub-layers around 16 nm to ensure homogeneous crystallinity and smooth surface. Meanwhile, it is found that there are oxygen-deficient defects at the Al2O3/TiO2 interface in the laminated structure, leading to broadband absorption in the films. For optical coating, absorption in the layers would lower the transmittance and damage the film quality. An improved ALD process has been provided by adopting O3 instead of H2O as oxidant, which would inhibit the formation of oxygen-deficient defects and then reduce the absorption. Based on the above findings, high-quality AR coatings are deposited. The work should help to expand the application of the ALD TiO2 layers in optical coatings.
Funding
National Natural Science Foundation of China (61805267, 62275256); Youth Innovation Promotion Association of the Chinese Academy of Sciences (2019241); National Key Research and Development Program of China (2021YFB3701504); Projects funded by the central government to guide local Scientific and Technological Development (YDZX20213100003011); Science and Technology Commission of Shanghai Municipality (18ZR1445400); Innovation Program in SITP, CAS (CX-173).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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