Mechanically robust antireflective moth-eye structures with a tailored coating of dielectric materials

Bioinspired moth-eye surface provides broadband antireflection features, which significantly enhance performances in optical components/devices. However, their practical uses are strictly limited due to poor mechanical stability of nano-patterns. In this study, we artificially engineered moth-eye structures on polycarbonate substrate through a thin-film coating of mechanically stable dielectric materials (i.e., Al2O3, Cr2O3, ZrO2, and TiO2 etc). The geometry of Al2O3-coated moth-eye surface is designed by considering the effective medium theory and confirmed by calculation of diffraction efficiency based on a rigorous coupled-wave analysis method. The tailored Al2O3-coating on moth eye surface exhibit the improved hardness while maintaining high optical transmittance. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Despite of the advantage of AR SWSs, transferring this technology from bench-top status to successful industrial level is limited due to poor mechanical stability of moth-eye nano-patterns. For instance, in practical applications such as display, windows, and eye-glasses, the AR SWSs are exposed to external physical contacts (e.g., scratches and/or pressures), which influences the AR performance. This undesired effect is particularly noticeable in the situation of using polymeric materials, which is commonly used for large area and mass production of nanostructures. As a simple route to improve hardness, dielectric materials with relatively higher hardness, such as Al 2 O 3 , Cr 2 O 3 , ZrO 2 , and TiO 2 , etc., can be additionally coated on nanostructures, however, there is another critical issue on matching of refractive index between dielectrics and polymers, based on the effective medium theory [42][43][44][45][46]. To overcome material limitations and structural properties in nanophotonics, fine tailoring in nanostructures is required [47][48][49].
In our previous work, we designed and optimized each SWSs for several applications with different materials using optical simulation [50][51][52][53][54][55]. Based on these design schemes of SWSs, in this study, we present antireflective moth-eye structures (AMSs) with an oxide coating for hardness enhancement. AMSs are fabricated on PC substrate using hot embossing technique for large area production, and Al 2 O 3 , as a hardness enhancement layer, was coated by atomic layer deposition (ALD). Simulation results show RI profile and transmittance of AMSs for the filling fraction and thickness of Al 2 O 3 coating layer. From these results, we propose the optimal design of Al 2 O 3 coated AMSs. Experimentally, our fabricated sample shows hardness enhancement maintaining improved transmittance. In the process, we conducted the transmittance measurement and the scratch test in fabricated samples. Furthermore, we discuss optical properties for the morphology of AMSs with oxide coating layers.

Optical properties of Al 2 O 3 coated PC films
Figure 1(a) shows the schematic illustration of antireflective moth-eye structures (AMSs) with Al 2 O 3 monolayer coated on polycarbonate (PC) substrate for improvements of transmittance and surface hardness. The geometrical parameters such as period (Λ), height (h), and filling fraction (FF) were set to analyze optical performance depending on the parabolic moth-eye structure as shown in Fig. 1(b). The effective refractive indices were calculated with each FF factors of 60% and 100% with 75 nm Al 2 O 3 thickness. In Fig. 1(c), the effective refractive indices were calculated by the volume weighted averaged refractive index between air and Al 2 O 3 coated PC films depending on the nanostructure height [5,56,57]. The abrupt change of effective refractive index was exhibited in FF 100% with 75 nm of Al 2 O 3 layer, due to that Al 2 O 3 has higher refractive index than PC film. By modifying structural parameter of FF, the effective refractive index was graded as shown in Fig. 1(c) right. In order to observe light propagations in antireflective films, the Al 2 O 3 coated moth-eye structures were calculated at 500 nm wavelengths by rigorous coupled wave analysis (RCWA) with varying FF [58]. In E-field distribution, bare film, Al 2 O 3 coating on bare film, moth-eye film, and Al 2 O 3 coating on Moth-eye film were simulated for four cases. In the absence of the structure, high reflections were obtained due to the high refractive index of Al 2 O 3 . The results, on the other hand, show that the reflection of the AMSs with Al 2 O 3 layer is significantly reduced. Transmittances of fabricated AMSs with different Al 2 O 3 thicknesses were measured by spectrophotometer (Cary 500 Scan UV-Visible Spectrophotometer, Varian) from 400 nm to 800 nm wavelength ranges (Fig. 1e). The averaged transmittance (Fig. 1e inset) exhibited the increased transmittance of AMSs. Enhanced transmittances of AMSs on PC films were maintained with compared to the flat PC film even though of Al 2 O 3 layer was coated.

Fabrication methods
Figure 2(a) shows the schemes of hot-embossing process using nanostructured nickel master stamp (HT-AR-02, Temicon) to achieve AMSs on PC substrate. The nickel mold was cleaned using acetone, isopropyl alcohol (IPA), and deionized (DI) water with blowing N 2 gas for self-assembled monolayer (SAM) treatment. The SAM of octadecyltrichlorosilane (OTS) was treated to nickel stamps for releasing a PC replica from the nickel stamp. The annealing process was subsequently performed for 5 min, at 120°C using the hot plate. AMSs patterned PC samples were fabricated under the pressure of 20 bar, temperature of 120°C at the glass transition temperature. Then, Al 2 O 3 thin film was coated by using atomic layer deposition (Lucida D100 ALD system, NCD) at 80°C. Trimethylaluminum (TMA) and H 2 O were used as the precursor, and oxidant, respectively. The TMA pulse, N 2 purge, H 2 O pulse, and N 2 purge cycle was repeated with base pressure of 500 mTorr and ∼0.1 nm/cycle growth rate. The fabricated samples of photographs were shown in the Fig. 2(b), the glare-less PC films were obtained in all the antireflective films. The field emission scanning electron microscope (FE-SEM, S-4700, Hitachi) image was exhibited to figure out the uniformly coated Al 2 O 3 on AMSs.

Nano-scratch testing of Al 2 O 3 coated PC films
The mechanical property was measured by nano-scratch resistance measurement (NST 3 , Anton Paar) with diamond stylus as illustrated in Fig. 3(a). The optical micrographs of fabricated samples with different Al 2 O 3 thicknesses were shown in Fig. 3(b). The AMSs on PC film without Al 2 O 3 coating (A 0 nm) shows clear scratch marks. Also, as the thicknesses of Al 2 O 3 are increased, gradually reduced scratch marks are observed. The scratch resistance was estimated by friction coefficient versus lateral displacement as shown in Fig. 3(c). The results show that the thicker Al 2 O 3 increases the mechanical resistance. The average friction coefficient was reduced about 20% in A 100 nm samples compared to A 0 nm.

Optical simulations
To verify the optical properties of Al 2 O 3 /AMSs on PC films with varying geometric parameters for wide range applications, the numerical simulation was used. The rigorous coupled-wave analysis (RCWA) was exploited to obtain transmittance spectra for periodic AMSs on PC films (Diffract MOD, RSoft Design Group). In the RCWA calculation, a fifth diffraction order and periodic boundary conditions were set for the diffraction efficiency. The un-polarized light was achieved by averaging the TE/TM polarization. The normal incident plane wave was illuminated on Al 2 O 3 /PC nanostructures. Optical constants of PC and Al 2 O 3 were measured by ellipsometer (UVISEL ER Benchtop AGAS, Horiba) and literature [57]. The material dispersion of optical constants was considered for wavelength dependent numerical simulations. Effective refractive indices were calculated by using MATLAB software (Mathworks, Inc) depending on their structural parameter (i.e., FF of AMSs and thickness of Al 2 O 3 ) with the fixed factors (i.e., height of 300 nm and period of 250 nm), as shown in Fig. 4(a) and (b). Basically, the effective refractive index profile of AMSs is highly dependent on FF. The main reason is that the difference in effective refractive index between the AMSs and the PC substrate is determined by FF. Ideally, without Al 2 O 3 coating, the closely packed AMSs (i.e., FF 100%) is the smoothest in the refractive index profile. However, since the Al 2 O 3 coating causes an increase in the overall FF of the AMSs with Al 2 O 3 , the refractive index profile of the AMSs with Al 2 O 3 were optimized at the Al 2 O 3 thickness of 75 nm with FF 60% of AMSs. Figure 4(c) shows the contour plots of aforementioned geometries. In the case of AMSs without Al 2 O 3 coating, the highest transmittance was obtained with FF 100%. The result shows a tendency for the transmittance to increase with FF of AMSs (Fig. 4(c) left). As shown in Fig. 4(c) right, the highest transmittance was observed in the 75 nm coated Al 2 O 3 . From these results, we note that the gradual profile of effective refractive index causes increased transmittance as suppressing surface reflection. Figure 4(d) shows the contour plot of the average transmittance of AMSs with Al 2 O 3 thickness and FF. The result shows the correlation between Al 2 O 3 thickness and FF in the transmittance of AMSs, which demonstrates the fact that AMSs with the lower FF require the thicker Al 2 O 3 thickness for optimal transmittance.
To analyze geometrical effects, we considered the deformation of AMSs as a parabolic ratio defined by a geometrical change from the cylindrical shape (0%) to parabolic shape (100%), as shown in Fig. 5(a). With varying the parabolic ratio, we calculated the effective refractive index to form gradual profiles of effective refractive indices in Fig. 5(b). The refractive indices of AMSs without Al 2 O 3 were varied by changing parabolic ratio (dot line). Relatively, the gradual profiles of effective refractive indices were achieved at the parabolic ratio from 60 to 100% (rigid line) with the fixed factors (i.e. Λ is 250 nm, h is 300 nm, and FF is 60%). Figure 5(c) shows the contour plots for transmittance of AMSs without and with Al 2 O 3 coating (i.e., 75 nm). As a result, with Al 2 O 3 coating, the improvement in transmittance was confirmed at 60 to 100%. Figure 5(d) shows the angle dependencies of Al 2 O 3 coated moth-eye PC films, the improvement of transmittance was maintained until ∼40 degree. Figure 5 different materials, and our proposed scheme can be varied by designs depending on the various applications with different materials.

Conclusion
We have designed and demonstrated the tailored Al 2 O 3 coating on moth-eye structure for enhancing mechanical and optical performances. We confirmed that increased optical and mechanical properties were observed at uniformly coated Al 2 O 3 monolayer by ALD deposition on the existed moth-eye antireflective structure. We calculate and modify the graded refractive index by varying structural parameter. The polycarbonate film coated with 75 nm Al 2 O 3 thickness exhibits the smoothest graph at 75% of filling fraction than 100%, with the fixed factors (period is 250 nm and height is 300 nm). We notice that the effective refractive index easily calculated by volume weighted averaging method through nano-structure geometric design. Additionally, our proposed structure is applicable to various nanostructures which have different refractive index materials.

Funding
Hyundai Motor Group; National Research Foundation of Korea (2019H1A2A1060954, NRF2018M3D1A1039288); Korea Institute of Energy Technology Evaluation and Planning (20183010014310).