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Abstract
Wide-angle and broadband antireflection (AR) coating is of the essence in modern optical systems in many fields, which has a great influence on the stray light and imaging quality. A simple and convenient manufacturing method is proposed to address this issue based on a composite coating combining the nano-tapered Al2O3•xH2O (AH) structure and high-low index thin film stack. The optical properties of nano-tapered AH structure at various thickness are first studied and modeled in optics by several homogeneous sub-layers with the graded equivalent index. The designed composite AR coatings are manufactured by vacuum deposition and wet etching subsequently in the hot deionized water. Compared to the common dielectric multilayer antireflection stack, the composite coating presents excellent AR performance. The measured average reflectance values of the double-side coated BK7 glass are as low as 0.40%, 0.41%, 0.56% and 3.13% in 400-1100nm band at angles of incidence (AOI) of 6°, 20°, 40°, and 60°, respectively, while the measured average transmittance at normal incidence increases up to 99.3%. Finally, the process reproductivity, environmental reliability test including long term storage, high temperature annealing and 85°C-85% relative humidity storage of the composite coatings are evaluated. The proposed AR scheme provides a low-cost, efficient, wide-angle and broadband AR coating for kinds of large-curvature components and complex surfaces in fields of consumer electronics, automotive, security, etc.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Large-curvature optical components, especially the lenses with different radii of curvature, have been increasingly applied in modern optical systems in fields of consumer electronics, automotive, security and precision measurement to reduce the number of optical components and compact the systems. Therefore, antireflection (AR) coating for large incidence angles and wide working band becomes one of the key factors to improve the image quality and measurement accuracy of the systems by reducing the loss and the stray light. Two significant challenges must be faced to achieve the large-angle and wide-band AR coating: an increase of the average residual reflectance caused by the broadening of the working band and the deterioration of the AR effect resulted from the admittance mismatch between the Air and substrate with the increasing incidence angles.
For the common dielectric multilayer AR coating, the refractive index of the outermost layer has a great impact on the residual reflectance [1–4]. In recent decades, the bionic structures like moth-eye structures, surface-relief grating structures, etc. have been proved the ultra-low refractive index materials and introduced into the design of large-angle and wide-band AR coatings. A series of fabrication methods have been employed to prepare these broadband AR coatings. The reactive ion etching [5–8] and glancing-angle deposition [9–11] can achieve excellent AR characteristics, but the process requires patterning or special deposition equipment which is difficult to be practically applied. The processes of silica ball [12–13] or porous nanomaterials [14] and sol-gel technology [15–18] are simple by spin coating or pulling methods. However, limited to the processes of patterning and self-assembly methods, nano-structured AR coatings could not be effectively deployed on curved and irregular surfaces. The wet chemical etching technique and plasma etching [19–21] are effective means to provide a low refractive index material and can be easily grown on any irregular or large surface. The plasma etching can produce AR coatings in convenient and rapid mass production, but it is only suitable on plastic material. The method which selectively removes Al2O3 through wet etching [22–23] from an atomic layer deposited SiO2/Al2O3 composite film has an excellent antireflection effect but is limited to atomic layer deposition (ALD) [24–25] technology. A simple one-step wet-etching process [26] that produces graded-index nano-porous SiO2 on borosilicate glass (BK7 and its derivatives) surfaces. The method is cost-effective and environmentally friendly but is not suitable for combining with multilayer film. Another wet chemical etching technique is based on the corrosion properties of alumina with hot deionized water to construct ultra-low refractive index nano-structured membrane [27–29]. The Grass-like alumina [28] combines the advantages of nano-porous SiO2 and plasma-prepared nanostructures, which is not limited to a single deposition method. However, it is still difficult to achieve large-angle and wide-band antireflection with a single-layer nanostructure.
Here, we propose a composite coating based on the combination of the hydrated alumina nanostructure and the high-low index thin film stack to achieve the excellent broadband AR effect for a wide range of incidence angles. The homogeneous sub-layers with the graded equivalent index of the Al2O3·xH2O (AH) taper structures are constructed by exploring the surface morphology, porosity, roughness and optical properties. Based on vacuum deposition and wet etching technology, we explore the optimal design combining a multilayer antireflection stack and AH taper structure for 400-1100 nm band across a wide range of incidence angles. The measured average reflectance values of the double-side coated BK7 glass are as low as 0.40% and 3.13% at angles of incidence (AOI) of 6° and 60°, which presents an excellent AR performance of the composite coating. Thereby, a universal method and process for obtaining low-cost, wide-angle and broadband AR coatings in given bands is provided.
2. Experimental
The single-layer alumina film was deposited by e-beam evaporation (Balzers BAK600). The deposition temperature was 230°C and the deposition rate of Al2O3 was 0.3 nm/s. The thickness of the single alumina film was controlled by quartz crystal monitoring. Then, the deposited sample was immersed into the deionized water at 90°C for time periods on the order of minutes to two hours with reactions after the prior cleaning by ethanol/ether mixed solution. Subsequently, it was washed by soaking into the deionized water for 30 minutes and dried with nitrogen gas (99.5%) at room temperature. Finally, the nano-taper hydrated alumina film was obtained.
The composition of the alumina film before and after the reaction with deionized water was measured by the ESCALAB 250Xi X-ray Photoelectron Spectroscopy from Thermo Scientific with the pass energy of 30 eV and the energy step of 0.1 eV. In the process of testing, the photon energy of incident radiation is 1486.6 eV, and the beamline energy resolution is 0.45 eV. The reflectance/transmittance spectra of the samples at different angles were measured by the Cary 7000 with angle-resolved universal measurement accessory (UMA) from Agilent Technologies Inc. The SEM images were captured by Ultra 55 field emission scanning electron microscope from Carl Zeiss AG.
The influence of reaction parameters on the AR characteristics of AH taper structure was assessed by evaluating reflectance, transmittance, and surface topography. The homogeneous sub-layers stack (layer number >6) was employed to describe the AH taper structure with graded refractive indexes. The software OptiLayer was applied to fit the measured reflectance spectra at different incidence angles by the transfer matrix method and the material dispersion in 400-1100 nm is ignored. The gradient refractive index, ranging from 1.7 (index of Al2O3) to 1.0 (index of Air) and variable thickness of each layer were adjusted during the fitting procedure. Eventually, the equivalent multilayer stack with the graded refractive index profile and the corresponding thickness could be obtained. Due to the small dispersion of AH taper structure in the 400-1100 nm working band, the effect of dispersion on the antireflection and model fitting can be ignored.
The design of the composite AR coating was performed by the transfer matrix method and the AH taper nanostructure was placed as the outermost layer with the determined index profile. The homogeneous layers of the composite AR coating were prepared by e-beam vacuum evaporation (Balzers BAK600). The deposition temperature was 230°C, the deposition rate of TiO2 was 0.2 nm/s, the deposition rate of SiO2 was 0.4 nm/s, and the deposition rate of Al2O3 was 0.3 nm/s. Then, the composite coating was processed with heated deionized water to form AH taper structure as mentioned above. After that, the wide-angle and broadband composite AR coating could be obtained.
3. Results and analysis
3.1 Single layer investigation
Figures 1(a-b) show the measured X-ray Photoelectron Spectroscopy (XPS) spectra of the two samples which are deposited with alumina film before/after the water treatment for O1s and Al2p, respectively. The O1s peak is located in 531.6 eV and the Al2p peak is located in 74.7 eV for the sample before treatment, which is in agreement with the binding energy reported for Al2O3 [30–32]. After reaction, the Al2p peak of the sample moves to 73.6 eV and the O1s peak moves to 531.4 eV. The shift of Al2P peak to lower binding energy is observed after treatment, which could be attritubed to the partial occupation of the coordination bond of oxygen by H+ from the hot deionized water and the competitive relationship between Al3+ and H+. And the binding energy of the O1s peak shows a slight decrease of 0.2 eV, which could be connected with the breaking and recombination of chemical bonds between the Al2O3 and H2O atomic sublayers [32–33]. From the XPS analysis and the change of the mole ratio in Table 1, the chemical properties and composition content of the alumina film significantly change after the treatment with the hot deionized water. This process of partial oxygen loss and molecular recombination of Al2O3 and H2O leads to the modification of the film surface to form AH taper structure.
Fig. 1. High resolution XPS spectra before and after the reaction of alumina film (a) O1s and (b) Al 2P.
Table 1. The mole ratio of oxygen and aluminum before and after the reaction of alumina film.
Based on the above reaction principle of alumina and deionized water [29], we study the influence of different reaction time and initial thickness of the alumina film on the AR effect of the AH taper structure. As shown in Figs. 2(a-b), different residual reflectance could be observed by varying the reaction time with a fixed thickness of the alumina film to estimate the complete reaction process or not. When the reaction time exceeds 5 minutes, the change of average reflectance is less than 0.3%. It can be seen, the increase of the treatment time has no effect on its residual reflectance when the alumina and deionized water completely reacted. The alumina films of 30 nm, 40 nm, 50 nm and 60 nm reacted with deionized water for one hour are labeled as AHS-30, AHS-40, AHS-50 and AHS-60, respectively. The reflectance spectra of the AHS-30 and AHS-60 are shown in Figs. 2(c-d). The thickness of the deposited alumina film has a great impact on the center wavelength of the AR coating, i. e., the thickness of the nanostructured alumina film, and the residual reflectance of the AH taper structure as well. The AH taper structure exhibits improved characteristic up to 60°, by comparison with that of the bare substrate in Figs. 2(c-d). When the film thickness is 30 nm, the AH nanostructure has a better AR effect in the visible light band (400-700 nm). The residual refection within the visible light range is greatly suppressed with the average reflectance of 0.7% and 3% at AOI of 6° and 60° for double-side coated BK7 glass substrate. As the thickness of the initial alumina coating increases, the reflectance and transmittance curves move to the longer wavelengths, shown in Figs. 2(e-f). The minimum reflectance values of AHS-30 and AHS-60 samples are 0.5% and 0.15% around the center wavelength of ∼600 nm and ∼1100 nm, respectively, which demonstrates the large variation range of the center wavelength with the thickness of the initial alumina film. Moreover, the minimum reflectance in the entire spectral band gradually decreases as the deposited thickness increases, while the maximum transmittance gradually increases accordingly.
Fig. 2. (a-b) Reflectance spectra of the double-side coated BK7 glass substrate with the AH structure for different reaction time. (c-d) Reflectance spectra of AHS-30 and AHS-60 at different incidence angles for double-side coated BK7 glass substrate with the AH taper structure. (e-f) Comparison of reflectance/transmittance spectra of alumina films with different thicknesses after reacting with deionized water.
The surface topography is investigated to explore the origin of the differences of these AH taper structures through scanning electron microscope (SEM) images. Figures 3(a-d) display the cross-sectional view images of the as-prepared AH taper structures of different initial thicknesses on silicon wafers. After the alumina film reacts with water, the nanostructure with a gradually increasing porosity from the substrate to the Air is formed, which is AH taper structure with a gradually decreasing equivalent index. The results show that the reaction with deionized water reveals a significant change in morphology of the Al2O3 films with different thicknesses. It can be seen that as the initial film thickness increases, the total physical thickness of the AH taper structure increases and forms a graded refractive index coating. Compared with the AHS-30, the sample AHS-60 has a smoother structure after reacting.
Fig. 3. (a-d) The SEM images of the samples AHS-30, AHS-40, AHS-50 and AHS-60. (e-h) The equivalent index profile of the samples AHS-30, AHS-40, AHS-50 and AHS-60 with graded index profile. (f) The simulated (solid line) and measured (dotted line) reflectance spectra of the sample AHS-30.
To expand the application in practical multilayer stacks, the equivalent refractive index profile of the as-prepared AH taper structure is studied. The AH taper structure is divided into several (≥6) homogeneous sub-layers with different equivalent indexes and physical thicknesses. Thus, the optical performance, i.e., reflectance & transmittance of the multilayer can be simulated by transfer matrix method. A reverse engineering procedure is performed to fit the measured reflectance curves at various incident angles up to 60°. The equivalent index and thickness for every homogeneous sub-layer are determined as shown in Figs. 3(e-h). All the samples demonstrate a graded index profile and the index becomes gradually small from the substrate to Air. The total thickness of the equivalent multilayer coating grows gradually when the initial thickness of the deposited alumina film increases, which is basically consistent with the thickness of the corresponding SEM images shown in Figs. 3(a-d). The refractive index of the top layer near the Air, ranging from 1.03 to 1.1, is very low and close to the Air’s refractive index due to the high porosity. While the refractive index of the bottom layer next to the substrate increases from 1.28 to 1.55 with the increasing thickness of the initial alumina film, bringing out the more effective AR effect for thicker alumina sample with smaller reflection loss at the substrate/layer interface. Figure 3(i) presents the fitting results of the reflectance curves at AOI of 6°, 20°, 40°, and 60° for the sample AHS-30 modeled by 6 homogeneous sub-layers with graded indexes (the mean-square error is 0.125). The thicknesses of the fitting model for AHS-30 are 12 nm (n = 1.274), 47 nm (n = 1.234), 40 nm (n = 1.173), 34 nm (n = 1.132), 54 nm (n = 1.072), and 42 nm (n = 1.032), respectively. Good agreement of the measured curves with those of the simulated could be observed at the small incidence angles. When the incident angle increases to 60°, ∼0.5% reflection deviation arises between the two reflectance curves, which could be attributed to mismatch between the practical AH taper structure and the fitting model of the multilayer stack at large incidence angle. The fitting results exactly reflect the characteristics of the thickness of the AH taper structure and the equivalent refractive index in optical applications. Therefore, the obtained index profiles can be used as the outermost layer to design a composite multilayer AR coating for the wide-angle and broadband applications.
3.2 Multilayer design and characterization
Based on the obtained index profile of the AH taper structure constructed above, we combine the AHS-30 with the alternating high/low refractive index coatings to design the wide-angle and broadband AR composite coating. TiO2 is used as a high refractive index material, and SiO2 is used as a low refractive index material. Figure 4(a) depicts the refractive index profile of the AR coating design @550 nm with the optimized thicknesses of the nine-layer structure (BK7/TiO2 12 nm/SiO2 50 nm/ TiO2 33 nm/ SiO2 22 nm/ TiO2 149 nm/ SiO2 20 nm/ TiO2 21 nm / SiO2 88 nm/ Al2O3 30 nm). SiO2 layer serves as an etch stop layer because it has a high resistance against high temperature water. The theoretical average reflectance values of the designed AR multilayer coating are 0.44% and 2.88% in the 400nm-1100 nm wavelength range at AOI of 0° and 60° for double-side BK7 glass. The SEM cross-sectional view image of the multilayers is depicted in Fig. 4(b), which reveals the composite coating and the AH taper structure clearly.
Fig. 4. (a)Refractive index profile (at 550 nm wavelength) of the AR composite coating design, combining an 8-layer TiO2/SiO2 interference multilayers and AH taper structure. (b) Cross-sectional SEM image of the AR composite coating prepared by wet chemical etching.
We deposited a composite AR coating by electron beam evaporation on double-side BK7 glass and corroded the outermost alumina film with heated deionized water. Figure 5(a) shows the designed and measured reflectance spectra of the large-angle and wide-band AR coating at AOI of 6°, 20°, 40°, and 60°. The experimental results match well with the simulated results, verifying the feasibility of the fitting model of the AH taper structure. In the visible spectral range from 420 to 680 nm, the average reflectance of the BK7 reduces to 0.37% at normal incidence, and the residual average reflectance is still limited to only 0.57% up to 40°. In the visible-near infrared spectral range from 400 to 1100 nm, the measured average reflectance values are 0.40%, 0.41%, 0.56% and 3.13% at AOI of 6°, 20°, 40°, and 60°, respectively. It is difficult to achieve such a low residual reflectance with only high and low refractive index multilayers at large incidence angles for such a broad wavelength range. For comparison, a conventional broadband AR coating based on TiO2/SiO2 film stack and the outermost MgF2 layer is designed for 400-1100 nm band. The measured reflectance of our AR composite coating and the theoretical spectra of the conventional multilayer coating are plotted in Fig. 5(b). The average reflectance of our AR composite coating decreases by 0.42% and 6% at AOI of 6° and 60°. The AH taper structure with graded equivalent index makes a significant contribution to the AR effort on glass substrates. Figure 5(c) presents the transmittance spectra at different incidence angles. The average transmittance measured at normal incidence increases to 99.3% in a broad spectral range from 400 to 1100 nm. We can evaluate the optical loss through the sum of the specular reflectance and transmittance, shown in Fig. 5(d), where the larger loss is observed at the shorter wavelength. The reason accounting for the optical loss of this composite coating could be analyzed from two aspects. The absorption of the TiO2 material introduces a large proportion of the loss at the short wavelengths (400-480nm). And the optical scattering of the rough AH taper structure, involving the Rayleigh scattering and Mie scattering, brings about the slight loss over the entire band, which is dependent on the wavelength as well.
Fig. 5. (a) Simulated (dotted line) and measured (solid line) reflectance for double-side coated BK7 glass substrate with the composite AR coating at AOI of 6°, 20°, 40°, and 60°. (b) The reflectance of a conventional broadband AR coating (the optimal design) and the composite AR coating for double-side coated BK7 glass substrate at AOI of 6°, 40°, and 60°. (c)Measured transmittance for double-side coated BK7 glass substrate at AOI of 6°, 20°, 40°, and 60°. (d) Sum of transmittance and reflectance (T + R) of double-side coated BK7 glass substrate with the composite AR coating.
3.3 Repeatability and environmental studies
The process reproducibility and environmental reliability of this AH taper structure are studied for the practical applications. The reproducibility through repeated experiments could be verified via the reflectance under the identical initial thickness and reaction conditions, shown in Figs. 6(a-b). The difference between the measured average reflectance and transmittance is within 0.2%, which shows a good reproducibility for the fabrication process of AH structure. Then, the stability evaluation of the as-prepared film is carried out, including the long-term storage, high-temperature annealing and damp heat test. The AHS-30 is taken in the daily environmental (temperature 15°C-30°C, humidity 30%-60%) stability test. Figure 6(c) shows the transmittance curves of the AHS-30 at room temperature for 7 days and 14 days. The change of average transmittance in the 400-1100 nm band is less than 0.14% under room temperature/humidity conditions. Figures 6(d-e) show the change in transmittance of AH taper structure which is annealed at 150°C and 400°C for 3 hours and then stored at room temperature for a period. After the alumina film reacting completely with deionized water, it no longer continues to react with water vapor in the Air. When the AH taper structure is annealed at 150°C, it temporarily loses the adsorbed water, but quickly recovers after contact with Air. Thus, the average transmittance varies less than 0.3% during the test. The overlapping transmission curves before/after annealing at 400°C shows that the water molecules reacting with the alumina does not precipitate out at that temperature. The change of average transmittance is less than 0.2% after being placed in the environment for a period. Figure 6(f) show the damp heat test (temperature 85 °C, humidity 85%) results of the AH taper structure. A small variation of the transmission (<0.15%) and the stability is presented as well. As a result, under different environmental tests, the average transmittance of the AH taper structure merely changes 0.15% ∼ 0.3% (the test error of Cary 7000 is ∼ 0.1%), which demonstrates the good environmental reliability of this AH taper structure.
Fig. 6. (a-b) The results of multiple experiments on the reaction of 30 nm-thick alumina film and deionized water at 90°C for the same minutes. (c) Measured transmittance of the sample AHS-30 after preparation, one week later, and two weeks later. (d-e) Measured transmittance of the sample AHS-30 with or without annealing at 150°C and 400°C for 3 hours. (f) Measured transmittance of the sample AHS-30 at 85 °C under a relative humidity of 85% for 3 hours. The samples AHS-30 used in these tests were prepared through the same fabrication process and present the identical optical property.
4. Conclusion
A composite AR coating comprised of the high-low index thin film stack and an ultra-low index AH taper structure is proposed with its simple and convenient preparation process. We first investigate the surface morphology, porosity and optical properties of nano-tapered AH structure formed by the reaction of alumina film with deionized water. The several homogeneous sub-layers with the graded index profile are configured to simulate the optical property of the AH taper structure. The designed composite AR coatings are manufactured by vacuum deposition and wet etching subsequently. For the double-side coated BK7 glass substrate, the measured average reflectance values of the fabricated samples for 400-1100nm band are as low as 0.40%, 0.41%, 0.56% and 3.13% at AOI of 6°, 20°, 40°, and 60°, respectively. The measured average transmittance at normal incidence increases up to 99.3%. Good environmental reliability of the AH taper structure is verified by comparison over the long-term storage as well as the damp heat test. The proposed AR scheme provides a low-cost, efficient, wide-angle and broadband AR coating for kinds of large-curvature components and complex surfaces in fields of consumer electronics, automotive, security and precision measurement.
Funding
Consolidation Program for Fundamental Research (2019JCJQZDXX00); Chinese Aeronautical Establishment (201908076001).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper may be available from the corresponding author upon reasonable request.
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