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Abstract
This work presents a low-cost, simple, convenient, advanced technology to prepare large-area defect-free subwavelength structures (SWSs). SWSs were obtained by a metal-induced one-step self-masking RIE process on a fused-silica surface, in which metal-fluoride (mainly ferrous-fluoride) nanodots were used to induce and gather stable fluorocarbon polymer etching inhibitors in the RIE polymers as masks. The SWS growth processes are visible with an increase in etching time and some exhibit prominent broadband antireflective properties from the visible to the near-infrared wavelength range. Transmission in the 600-900-nm range increased from approximately 93% for the polished fused silica to above 99% for the double-side SWSs on fused silica. A theoretical simulation by a finite-difference time-domain method agreed well with the experiments. Moreover, the surface of the SWSs exhibits excellent superhydrophilic properties.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
There has been a drive to improve the performance of optical components in recent years. An increasing amount of research has been conducted on the broadband antireflective (AR) technique because of its potential application to improve the performance of many light functional components, such as superluminescent diodes [1], flat-panel displays, solar cells [2], camera lenses [3], television screens [4], high-power laser damage [5], and optical sensors [6]. Two effective methods to achieve broadband antireflection include coating porous or multilayered films [7, 8] and subwavelength structures (SWSs) (moth’s eye structures) [9]. Multilayered films can achieve fine broadband AR properties, and perform well in practical applications. However, problems arise as associated with limitations in coating materials, mechanical stability, and durability [10]. Problems also exist in plating films, such as thermal mismatch, layer adhesion instability, and a low laser-damage threshold [11]. Subwavelength structures (SWSs) are emerging in the development of broadband AR technology. The advantages of SWSs are well-known, and include their high laser-damage threshold [12], their high mechanical stability and durability, no external impurities, and no complex experimental processes as layered films [13]. SWSs can be used to tune many factors, such as the period, depth, and cross-sectional geometry, to achieve broadband antireflection [13]. SWSs have been fabricated by mask technique and RIE technique [14, 15]. There are many valuable mask techniques are exploited, such as interference lithography [16], nanoimprint lithography [17], electron-beam lithography [18], and the colloidal lithography [19]. These nanotemplate preparation methods have achieved great success, but some problems exist. For ultraviolet and visible light applications, SWS feature sizes below 200 nm are always necessary. Preparing SWS with such a feature size is beyond the interference and nanoimprint capability [19]. The electron-beam lithography achieve the resolution down to nanometer scale, the costs are always very high because of the sophisticated equipment and the complex preparation procedures [13]. Large scale SWSs fabricated by colloidal lithography is a challenge [20].
Because of its discovery during etching, interest has been expressed on how to use the grassland phenomenon to prepare SWSs. Chen et al. prepared SWSs on different materials, such as silicon, polysilicon, gallium nitride, gallium phoshpide, sapphire, and aluminum, but not on fused silica by an electron cyclotron resonance plasma system [21]. Silicon-carbide protecting caps that are generated from reactant gases were used as masks. Aluminum fluoride (AlF3) nanostructures were formed by the sputtering of a metal aluminum sheet holder on fused silica via a Sentech SI 500 ICP system by Schulze et al. [22]. They contend that the only reason for the AlF3 nanostructure AR properties is that AlF3 has an intermediate refractive index from air to the fused-silica substrate. However, some limitations exist in practical applications, and contamination by aluminum impurities during SWS fabrication results in a low laser-damage threshold [23]. The mechanism of self-mask SWSs that are induced by metal nanodots has not been investigated previously.
Reaction gases that are used in this experiment include trifluoromethane (CHF3) and argon (Ar). The cleaned samples are transferred directly to the etching chamber for etching. Thus, SWSs with a wide application and excellent AR performance are obtained. This paper reports on a cost-effective, time-effective, scalable, metal-induced one-step self-masking approach to fabricate SWSs on polished planar fused silica. The detailed formation mechanism is obtained and the SWS formation is visible as the experiment progresses. SWS surfaces have excellent broadband AR properties in the visible to the near-infrared wavelength region. Simulation results by the FDTD method confirms the experimental results. Multi-functional AR surfaces with antifogging properties have attracted significant recent attention [24]. This added functionality significantly extend the use of AR surfaces for applications, such as goggles, vehicle windshields, and optical devices [25]. The SWSs obtained in this research have excellent superhydrophilic properties, and therefore have potential antifogging applications.
2. Experimental details
The fused silica SWSs were obtained by fluorocarbon radical plasma etching in a RIE-3 system with radio frequency (RF)-plasma 13.56 MHZ and stainless steel sample table. Before etching the RIE chamber is cleaned for 20 min with argon and oxygen plasma to provide for the same initial conditions of each dry-etch run. Firstly, prior to etching, the fused silica samples were cleaned by using the mixed solution (2:1 concentrated 65% HNO3 and 30% H2O2), for 2 h at 80 °C, followed by repeated rinsing with deionized water (18 MΩcm−1). Then the substrates were dried with a nitrogen stream and transferred directly to the RIE chamber. The gaseous reactants used for reaction are trifluoromethane (CHF3) and argon (Ar). The ratio frequency power was set at 500 W during etching, and the chamber pressure was kept at 3 Pa. In order to remove all of the impurity by-product of etching (fluorocarbon polymers, metal fluorides, et al.), we cleaned the samples with alcohol, acetone and the mixed solution (2:1 concentrated 65% HNO3 and 30% H2O2) prior to the light performance and water contact angle tests (etching parameters show in Table 1). The morphology of the SWSs was analyzed by scanning electron microscopy (SEM, ZEISS FESEM ULTRA 55). The planar fused silica substrate with the SWSs was sputtered with a thin layer of Au before imaging. The wavelength-dependent transmission measurements for wavelength between 380 and 2000 nm were measured by perkinElmer lambda 950 spectro-photometer. Elements of sample surface and relative contents were measured by the X-ray photoelectron spectroscopy (XPS, KRATOS XSAM800). The water contact angles (WCAs) were measured using a contact angle goniometer (KRUSS DSA30). The polished fused silica substrate (corning 7980) was provided by Chengdu Berry with size of 20*20*1 mm and roughness less than 0.1 nm. The experiments of this research could be divided into two parts. One is S1-S6, the samples are directly placed on the stainless steel sample table during the etching process. The other one is S7. To prevent sputtering of the sample table, a double polished 8-inch silicon wafer holder was placed on the sample table. Sample S7 was placed on the silicon wafer holder for etching.
Table 1. Experimental conditions for the self-masking RIE process (S6 and S7 are the same experimental parameters, but S7 has an 8-inch silicon wafer holder).
3. Results and discussion
3.1 Formation principle of SWSs
The reaction gases were a mixture of CHF3 and Ar. Discharge products of CHF3 include F, CF, CHF, CF2, CHF2, CF3, and other ions and radicals [26, 27]. Argon plasma has a strong sputtering effect in an RF electric field, as does CFx+ [28]. Therefore, the glass surface and sample table are subject to intense ion bombardment during etching. Consequently, a thin layer of metal atoms or ions is distributed randomly at a slightly higher level than the sample template surface. The ratio of metal atoms or ions to total particles depends on parameters, such as the etching power, gas pressure, gas species, and flowrates. During RIE, some highly-volatile species (mainly SiF4), less-volatile species (mainly fluorocarbon polymer etching inhibitors), and non-volatile species (metal-fluoride etching inhibitors) are generated. Metal-fluoride etching inhibitors (mainly ferrous fluoride, FeF2) originate from the ion-enhanced chemical reaction of metal atoms [29, 30]. These steady non-volatile metal-fluoride etching inhibitors (mainly FeF2) serve as a chain initiator to aggregate fluorocarbon polymer etching inhibitors [31]. Here, fluorocarbon polymer etching inhibitors originate from CHF3 glow-discharge products. As the etching continues, more fluorocarbon polymer etching inhibitors assemble, and thus, masks form as shown in Fig. 1(a).
These non-volatile metal-fluoride etching-inhibitor seeds (mainly FeF2) act as polymerization-induced initiators of less-volatile fluorocarbon polymer etching inhibitors [32–34]. The metal-fluoride etching inhibitors (mainly FeF2) aggregate the fluorocarbon polymer etching inhibitors. Less-volatile fluorocarbon polymer etching inhibitors were deposited on the sample surface where the metal fluoride has been distributed throughout the etching process. The masks and fused silica substrate are etched during the RIE process, but the etching speed of the masks is much slower than that of the fused-silica substrate. Some rough peak surfaces are formed. The surface peaks, where the metal fluoride has been distributed, receive more deposited fluorocarbon polymer etching inhibitors than the valleys, which increases the roughness. Metal fluoride may also deposit in the area where the metal-fluoride etching inhibitors have not been distributed in the subsequent etching process. As a result, the shape of the AR structures forms gradually as shown in Fig. 1(b).
Processes (a) and (b) have been repeated. This iterative process leads to a uniform distribution of SWSs on the sample surface. As the etching process continues, the height and the diameter of nanostructures which were already generated on the sample surface would grow up. The nanostructures which were formed former may swallow up the nanostructures which were formed latter, as the etching process continues. Nanostructures of similar size, height and close proximity tend to combined with each other to form a larger structure. Because of the metal-fluoride seeds that are deposited onto the sample surface at different times, the height of each SWSs is different. Processes (a) and (b) continue to the end of the experiment, and the SWSs are formed as shown in Fig. 1(c).
3.2 Effects of process parameters on SWS formation
The self-masking generation processes are shown in Figs. 2(a)-2(f). Some nanodots were deposited on the sample surface as shown in Fig. 2(a). The nanostructures are distributed randomly and appear to be even. The formation mechanism indicates that nanostructures are formed at all times. The nanostructures that are formed initially grow and some new nanostructures emerge at bare places during the process. Therefore, nanostructures that are formed initially are larger than those formed later. The elementals distributed on the surface of these samples are shown in Figs. 3(a)-3(f). For sample S1, some expected elements, such as carbon (C), oxygen (O), Si, fluorine (F), and uninvited guest iron (Fe), existed on the sample surface, and their relative content could not be ignored, as shown in Fig. 3(a). By combining the spectrum of iron in Fig. 4(a) with special conditions in the reaction process, iron compounds are found to be FeF2. Previous articles have mentioned that the non-volatile metal-fluoride etching inhibitors serve as chain initiators to induce and gather less-volatile fluorocarbon polymer etch-inhibitors as masks [31]. Considerable amounts of fluorocarbon polymers and metal fluorides have been detected, as shown in Fig. 2(a).
Fig. 2 Morphology of fused-silica nanostructures (titled-top view, scale bar 500 nm) prepared using different etching conditions: (a) sample S1, (b) sample S2, (c) sample S3, (d) sample S4, (e) sample S5, (f) sample S6.
Fig. 3 X-ray photoelectron spectroscopy and relative content (insert table) of samples using different etching conditions: (a) sample S1, (b) sample S2, (c) sample S3, (d) sample S4, (e) sample S5, (f) sample S6 (the more detailed XPS data are shown in Image 2).
The distribution of nanostructure in sample S2 has been investigated in this part. The top-view SEM image of the sample S2 is shown in Fig. 5(a). In Fig. 5(b), most nanostructures have been counted and the results are shown in Fig. 5(c). With a gradual increase in nanostructure size, the number of nanostructures decreases. Based on the nanostructure formation mechanism, the number of nanostructures would increase as the etching continues. Nanostructures that form initially grow gradually, and adjacent nanostructures combine to form a larger nanostructure as the etching continues. These nanostructures combine an increasing number of nanostructures to form greater nanostructures. Therefore, all nanostructures grow simultaneously. The scale of nanostructures formed earlier is larger than that of the nanostructures that are formed later. The number of nanostructures that are formed earlier is smaller than that of the nanostructures that are formed later. This result is consistent with the SWS formation principle from the entire formation process. The change in SWS morphology agrees with the mechanism that was described earlier.
Fig. 5 (a) Morphology of fused-silica nanostructures, (b) statistical marks of morphology, (c) statistical results of morgraphy (scale bar 500 nm).
Elements on the S2 sample surface are distributed as shown in Fig. 3(b). Compared with S1, two new elements exist, namely, nitrogen (N) and chromium (Cr). Elemental N and Cr could not be introduced during the experiment, but were provided by the stainless-steel table. Alloying elements are added to stainless steel to enhance its performance. The stainless steel was sputtered and chromium and nitrogen compounds were deposited on the sample surface through the etching process. Compared with the S1 sample, the S2 surface morphology changed significantly. More nanostructures are located closer together. The three-dimensional height increases as shown in Fig. 2(b). Many nanostructures are generated at the structure gaps. The total amount of metal fluoride for S3 sample in Fig. 3(c) increases compared with the S2 condition. This total amount of metal fluoride increase is consistent with the increase in number of SWSs. Figure 2(d) shows the SEM results for the S4 sample and that the nanostructures are closer with an increase in height. The total amount of metal fluoride also increases, as shown in Fig. 3(d). After 10 min of etching, the relative content of metal fluoride continues to increase, as shown in Fig. 3(e). The density and height of the SWSs also increases, as shown in Fig. 2(e). According to a previous theoretical analysis and the laws of the experimental results, the relative content of metal fluoride should increase gradually with an increase in etching time. However, the results are inconsistent with the theoretical analysis for a 20-min etching time, as shown in Fig. 3(f). With an increase in etching time, the number of nanostructures increases. When the density of nanostructures reaches a certain level, the density of nanostructures would not continue to increase. This is because two or more close nanostructures may become a new bigger nanostructure with the etching progress continues. That’s why content of FeF2 in sample S6 is smaller than S5 in Fig. 4(b).
The formation mechanism of the SWSs is reasonable from an analysis of the above materials. The last experiment is used to further demonstrate the mechanism in this paper. To prevent sputtering of the sample table, a double polished 8-inch silicon wafer holder was placed on the sample table. A bare fused-silica sample was placed onto the silicon wafer holder for etching. The sample table was 8 inches, and the silicon wafer holder was sufficiently large to prevent metal-sample table sputtering and deposition. A double-sided polished silicon wafer holder could be contacted with the sample and the sample table. The silicon wafer holder has a good thermal conductivity, which ensures that the sample heat dissipation is as good as that of the samples without the silicon wafer holder. The good conductive property of the silicon wafer ensures that the RF power output of this parameter is the same as the other parameters because all samples have almost the same bias voltage. Compared with the 20-min etching without a silicon wafer holder, there is a distribution of fluorocarbon polymer on the 20-min etching sample surface, but no distribution of metal fluoride as shown in Image 1(a). No SWSs exist on the S7 sample surface, as shown in Image 1(b). The absence of metal-fluoride distribution on the sample surface leads to an absence of SWSs on the sample surface. Therefore, the formation of SWS depends on the presence or absence of metal fluoride on the sample surface. In summary, no metal fluoride results in no SWS formation.
In order to further verify the SWS growth mechanism, the effects of etching power on nanostructure generation are investigated. Experimental conditions for RIE process are shown in Table 2. It can be clearly found that the average diameter of each SWSs shows a gradual increase trend, with the increase of etching power, as shown in Figs. 6(a)-6(c) (top view images). And the average height of SWSs also increased, as shown in Figs. 6(d)-6(f) (cross-section view images). It means etching rate increases with the increase of etching power. According to the nanostructure formation mechanism, the increase of etching rate would lead to two main effects on nanostructure generation. First, the average height of nanostructures would increase. Second, the average diameter of nanostructures would increase. These experimental results just further confirmed the nanostructures growth mechanism. The advantage of the method is there is no need of mask technology. Obviously, large scale SWSs can be fabricated by this self-masking technology.
Table 2. Experimental conditions for the self-masking RIE process
Fig. 6 SEM images of the samples used in this work (scale bar 500 nm), parameters are shown in Table 2. Each column represents a different sample. (a) - (c) Collects top views of the surfaces, (d) - (f) are fractured cross-section views.
3.3 Influence of process parameters on optical properties
Because of the large refractive-index discontinuity, incident-light Fresnel reflection occurred at the interface between the fused silica and air. The refractive index from air to the silica substrate changed gradually and could suppress the Fresnel reflection from the fused-silica substrate. Therefore, the regulation of the refractive-index gradient from air to the substrate becomes important for the reflection. The effective-medium theory shows that the SWSs on the substrate surface seem to be an inserted layer.
In this work, the effect of etching duration on the structure generation is systematic. Therefore, the effect of etching duration on nanostructure optical properties is mainly discussed. The transmittance of the sample after 2 min of etching (sample S1, violet line) and 20 min of etching (sample S7, orange line) with an 8-inch silicon wafer holder are almost the same as the bare fused-silica substrate (bare fused silica, black line), as shown in Fig. 7(a). This result agrees with the SEM results in Fig. 2(a) and Image 1(b). With an increase in etching time from 4 min to 10 min, the AR performance increases gradually. With an increase in the etching time to 20 min, the transmission in the 600-900 nm wavelength range increased from approximately 93% for the polished fused silica to above 96% for SWSs on fused silica. The excellent broadband AR properties are demonstrated from the visible to the near-infrared bands. Almost the same pattern is also shown in reflectance tests in Fig. 7(b).
Fig. 7 (a) (b) Transmittance and reflectance of each parameter in Table 1 S1-S7. (c) Transmittance of fused-silica double-sided SWS and bare fused silica. (d) Reflectance of fused-silica double-sided SWS and bare fused silica. (e) Sum of transmittance and reflectance of fused-silica double-sided SWSs and bare fused silica. Note the OH- absorption at ~1360 nm and the detector change at ~860 nm. (f) Side-view and tilted-view SEM image of fused-silica double-sided SWS (scale bar 500 nm). (g) Photograph of SWS under yellow light illumination. The left, which is indicated by the red arrow, is double-sided SWS, and the right is the polished fused silica. The size of these two substrates is 50 × 50 × 1 mm. Figure (f) was obtained by Wu and Ye.
A double-sided SWS was fabricated by RIE as described in the S6 condition. The morphologies of SWS were nearly identical in Fig. 7(f). The transmittance of fused silica with double-sided SWSs (red line) and the bare fused silica (black line) is presented in Fig. 7(c) with a normal incident source. The transmittance of the SWSs sample increases significantly from 380 to 2000 nm compared with the bare fused-silica sample. The maximum transmittance of the double-sided surfaces was 99.4% at ~700 nm, which is 6% higher than the bare fused silica. The transmittance in the 600-900-nm range increased from ~93% for the polished fused silica to above 99% for the double-sided SWSs on fused silica. Figure 7(d) presents the hemispherical total reflectance (diffuse + specular) as a function of wavelength for the double-sided SWSs (red line) and the bare fused silica (black line) using the integrating sphere. The average hemispherical total reflectance is less than 1% for the 380-550 nm wavelength range. Because of a certain scattering for the short wavelength, the sum of the transmittance and reflectance was less than 100% for the SWSs surfaces and approximately 100% at all measured wavelengths for the bare fused-silica substrate as shown in Fig. 7(e). Figure 7(g) shows a photograph of the SWS under yellow light illumination. The left as indicated by the red arrow is double-sided SWS, and the right is bare fused silica. The non-SWS sample of the fused silica reflects the image of the fluorescent light, but the double-sided SWS sample (the left fused silica that is below the red arrow) provides a clear vision.
3.4 Theoretical simulation by finite-difference time-domain method
Rigorous Coupled Wave Analysis (RCWA) and Fresnel equations often have been used to calculate optical performance of SWSs [35, 36]. Here in this case, FDTD was used to test and verify the accuracy of our experimental results by using the FDTD Solutions program (Lumerical Solutions, Inc., Vancouver, Canada). The incident optical source was set for plane waves. Periodic boundary conditions were set around the unit cell, and the top and bottom boundaries of the cell were perfectly matched layer-absorbing boundary conditions. The unit cell was designed in rectangle lattices. The nanostructure shapes were described using profile functions with parameters D and H. D is the nanostructure diameter, which was derived from an average period of 80 nm. H is the nanostructure height. The transmittance of the paraboloid profile, tip cone, truncated cone, and cylinder were calculated. For the paraboloid profile, the apex radius of the curvature was set to 60 nm. For the tip cone, the tip radius is 30 nm and the cone angle is 1°. For the truncated cone, the bottom radius is 30 nm, and the top radius is 28 nm. For the cylinder, the radius is 30 nm. The substrate (Fused silica Corning 7980, Corning Inc., Corning, USA) was taken into account in this calculation.
Image 3 presents the contour plots of the calculated transmittance-result variations as a function of wavelength for the height of the SWSs with (a) paraboloid profiles, (b) a tip cone, (c) a truncated cone, and (d) a cylinder. The three-dimensional simulation models of the structures that were used in this calculation are shown in the insets of Image 3. As the height increased, the transmittance tended to increase, and the high transmittance region was broadened toward a longer wavelengths for each model. It is almost impossible for parabolic profiles to exhibit a similar trend to the experimental results as shown in Image 3(a). The calculated transmittance-result of paraboloid profile agrees with the experiment is shown in Fig. 8(a). The height is 260 nm, the average period is 64 nm. According to the experiment results in Fig. 6(f), the average height of experiment is below 200 nm. This calculated result is inconsistent with experiment. Image 3(b) shows that the height of the tip-cone nanostructures is ~170-185 nm, which agrees with the experimental results. The height of 180 nm is matched best with the experiments, as shown by the red line in Fig. 8(b). Image 3(c) shows that the height of the truncated cone nanostructures of ~160-180 nm agrees with the experimental results profile. The height of 175 nm may best match the experimental results, as shown by the blue line in Fig. 8(b). Image 3(d) shows that the cylinder nanostructure heights are ~150-170 nm and match the experimental results. The heights of ~160 nm match the experiment best, as shown by the magenta line of Fig. 8(b). A comparison of the experimental and simulation results shows that the nanostructure type is mainly a tip cone, truncated cone, and cylinder, and that the height is mainly between 160 nm and 180 nm. The previous works have published similar result [35, 36].
Fig. 8 (a) Transmittance of double-side SWSs in experiment (black line), the paraboloid simulation result agree well with the experiment; (b) Simulation results agree well with experiment result: (red line, tip cone), (blue line, truncated cone, (magenta line, cylinder).
3.5 Wetting properties
Early theoretical works by Wenzel et al. [37], Cassie et al. [38] and Bico et al. [39] pointed out that the roughness of surface has significant effects on its wettability. For hydrophobic solids, the solid roughness would dramatically enhance its hydrophobicity [40]. For hydrophilic solids, the solid roughness would dramatically enhance its hydrophilicity [41]. In our research, the wettability of the surface covered SWSs fabricated in S6 condition and bare fused silica substrate surface was investigated by measuring their water contact angles (WCAs) using a 3-μL water droplet at room temperature. The water droplet profile of the bare fused silica substrate and the fabricated fused silica SWS surfaces is shown in Figs. 9(a)-9(f). The WCAs for bare fused silica was 46.14° as shown in Fig. 9(a). When the water droplet contacted the fused silica SWS surface, water droplet spreads flat within 0.18 s, and the apparent contact angle was ~0°. Therefore, these results suggest that the SWS surface obtained in this research is superhydrophilic. Obviously, the enhancement of the hydrophilicity of SWS surface is due to the SWSs increased the roughness of its surface. Superhydrophilicity also means antifogging property [41]. Therefore, the SWSs obtained in this research also have potential antifogging functions.
Fig. 9 Superhydrophilic SWS surfaces on the fused silica substrate. (a) Water drop profile on a bare fused silica substrate. (b) - (f) Water drop profile on an SWS surface. The volume of water droplets used was 3 μL.
4. Conclusion
We demonstrated an advanced, rapid, metal-induced, one-step, self-masking RIE process to create SWSs on fused silica. The growth of AR structures was obtained and analyzed through a gradual increase in etching time. The fabricated SWSs on fused silica with an average structural period of ~80 nm exhibited excellent broadband AR and superhydrophilic properties. The emergence of this technology is an improvement for micro- and nanofabrication, and we believe that this advanced method could be applied in many more areas.
Funding
National Natural Science Foundation of China (No. 61705204, 61705206, 51606158, 5160229, 11174258); Development Foundation of China Academy of Engineering Physics (grant number 2015B0403095); Laser Fusion Research Center Funds for Young Talents (No: LFRC-PD011).
References and links
1. Z. Zang, K. Mukai, P. Navaretti, M. Duelk, C. Velez, and K. Hamamoto, “Thermal resistance reduction in high power superluminescent diodes by using active multi-mode interferometer,” Appl. Phys. Lett. 100(3), 031108 (2012). [CrossRef]
2. K. Forberich, G. Dennler, M. Scharber, C. Hingerl, K. Fromherz, and C. Brabec, “Performance improvement of organic solar cells with moth eye anti-reflection coating,” Thin Solid Films 516(20), 7167–7170 (2008). [CrossRef]
3. T. Yanagishita, K. Nishio, and H. Masuda, “Antireflection structures on lenses by nanoimprinting using ordered anodic porous alumina,” Appl. Phys. Express 2(2), 022001 (2009). [CrossRef]
4. G. C. Park, Y. M. Song, J. H. Ha, and Y. T. Lee, “Broadband antireflective glasses with subwavelength structures using randomly distributed Ag nanoparticles,” J. Nanosci. Nanotechnol. 11(7), 6152–6156 (2011). [CrossRef] [PubMed]
5. D. S. Hobbs and B. D. Macleod, “High laser damage threshold surface relief micro-structures for anti-reflection applications,” Proc. SPIE 6720, 67200L (2007). [CrossRef]
6. C. Lee, S. Y. Bae, S. Mobasser, and H. Manohara, “A novel silicon nanotips antireflection surface for the micro sun sensor,” Nano Lett. 5(12), 2438–2442 (2005). [CrossRef] [PubMed]
7. J. Hiller, J. D. Mendelsohn, and M. F. Rubner, “Reversibly erasable nanoporous anti-reflection coatings from polyelectrolyte multilayers,” Nat. Mater. 1(1), 59–63 (2002). [CrossRef] [PubMed]
8. J. Xi, M. Schubert, J. Kim, E. Schubert, M. Chen, and S. Lin, “Optical thin-film materials with low refractive index for broadband elimination of Fresnel reflection,” Nat. Photonics 1(3), 176–179 (2007).
9. Y. Li, J. Zhang, S. Zhu, H. Dong, F. Jia, and Z. Wang, “Biomimetic surfaces for high-performance optics,” Adv. Mater. 21(46), 4731–4734 (2010).
10. H. Kikuta, H. Toyota, and W. Yu, “Optical elements with subwavelength structured surfaces,” Opt. Rev. 10(2), 63–73 (2003). [CrossRef]
11. J. Xi, M. Schubert, J. Kim, E. Schubert, M. Chen, S. Lin, W. Liu, and J. Smart, “Optical thin-film materials with low refractive index for broadband elimination of Fresnel reflection,” Nat. Photonics 1(3), 176–179 (2007).
12. D. Hobbs, “Laser damage threshold measurements of anti-reflection microstructures operating in the near UV and mid-infrared,” Proc. SPIE 7842(1), 104 (2010).
13. Y. Li, J. Zhang, and B. Yang, “Antireflective surfaces based on biomimetic nanopillared arrays,” Nano Today 5(2), 117–127 (2010). [CrossRef]
14. B. J. Kim and J. Kim, “Fabrication of GaAs subwavelength structure (SWS) for solar cell applications,” Opt. Express 19(10), A326 (2011).
15. Y. Ou, I. Aijaz, V. Jokubavicius, R. Yakimova, M. Syväjärvi, and H. Ou, “Broadband antireflection silicon carbide surface by self-assembled nanopatterned reactive-ion etching,” Opt. Mater. Express 3(3), 86–94 (2013). [CrossRef]
16. P. Baroni, B. Päivänranta, T. Scharf, W. Nakagawa, M. Roussey, M. Kuittinen, and H. Herzig, “Nanostructured surface fabricated by laser interference lithography to attenuate the reflectivity of microlens arrays,” J. Eur. Opt. Soc-Rapid 5(1), 138 (2010).
17. T. Yanagishita, T. Endo, K. Nishio, and H. Masuda, “Preparation of antireflection SiO2 structures based on nanoimprinting using anodic porous alumina molds,” Jpn. J. Appl. Phys. 49(6), 065202 (2010). [CrossRef]
18. Y. Kanamori, M. Sasaki, and K. Hane, “Broadband antireflection gratings fabricated upon silicon substrates,” Opt. Lett. 24(20), 1422–1424 (1999). [CrossRef] [PubMed]
19. X. Ye, X. D. Jiang, J. Huang, L. X. Sun, F. Geng, Z. Yi, X. T. Zu, W. D. Wu, and W. G. Zheng, “Subwavelength structures for high power laser antireflection application on fused silica by one-step reactive ion etching,” Opt. Lasers Eng. 78, 48–54 (2016). [CrossRef]
20. J. Son, M. Sakhuja, A. J. Danner, C. S. Bhatia, and H. Yang, “Large scale antireflective glass texturing using grid contacts in anodization methods,” Sol. Energy Mater. Sol. Cells 116, 9–13 (2013). [CrossRef]
21. K. Chen, C. Hsu, H. Lo, S. Chattopadhyay, C. Wu, J. Hwang, D. Das, and L. Chen, “Generally applicable self-masking technique for nanotips array fabrication,” Nanosci. Nanotechnol. 7(3), 129–134 (2004).
22. M. Schulze, H. J. Fuchs, E. B. Kley, and A. Tunnermann, “New approach for antireflective fused silica surfaces by statistical nanostructures,” Proc. SPIE 29(1), 13–20 (2008).
23. L. Hongjie, H. Jin, W. Fengrui, Z. Xinda, Y. Xin, Z. Xiaoyan, S. Laixi, J. Xiaodong, S. Zhan, and Z. Wanguo, “Subsurface defects of fused silica optics and laser induced damage at 351 nm,” Opt. Express 21(10), 12204–12217 (2013). [CrossRef] [PubMed]
24. X. J. Feng and L. Jiang, “Design and creation of superwetting/antiwetting surfaces,” Adv. Mater. 8(23), 3063–3078 (2006). [CrossRef]
25. K. C. Park, H. J. Choi, C. H. Chang, R. E. Cohen, G. H. McKinley, and G. Barbastathis, “Nanotextured silica surfaces with robust superhydrophobicity and omnidirectional broadband supertransmissivity,” ACS Nano 6(5), 3789–3799 (2012). [CrossRef] [PubMed]
26. J. Barz, C. Oehr, and A. Lunk, “Analysis and modeling of gas-phase processes in a CHF3/Ar Discharge,” Plasma Process. Polym. 8(5), 409–423 (2011). [CrossRef]
27. D. Bose, M. Rao, T. Govindan, and M. Meyyappan, “Uncertainty and sensitivity analysis of gas-phase chemistry in a CHF3 plasma,” Plasma Sources Sci. Technol. 12(2), 225–234 (2003). [CrossRef]
28. Z. Cui, Micro-Nanofabrication: Technologies and Applications (Springer, 2005).
29. E. Metwalli and C. Pantano, “Reactive ion etching of glasses: composition dependence,” Nucl. Instrum. Meth. B. 207(1), 21–27 (2003). [CrossRef]
30. H. Craighead, R. Howard, and D. Tennant, “Textured thin-film Si solar selective absorbers using reactive ion etching,” Appl. Phys. Lett. 37(7), 653–655 (1980). [CrossRef]
31. A. J. Van Roosmalen, “Review: dry etching of silicon oxide,” Vacuum 34(3), 429–436 (1984). [CrossRef]
32. G. Kokkoris, V. Constantoudis, P. Angelikopoulos, G. Boulousis, and E. Gogolides, “Dual nanoscale roughness on plasma-etched Si surfaces: Role of etch inhibitors,” Phys. Rev. B 76(19), 3405 (2007). [CrossRef]
33. S. P. Zimin, E. S. Gorlachev, I. I. Amirov, and H. Zogg, “Micromasking effect and nanostructure self-formation on the surface of lead chalcogenide epitaxial films on Si substrates during argon plasma treatment,” J. Phys. D Appl. Phys. 42(16), 165205 (2009). [CrossRef]
34. E. Hein, D. Fox, and H. Fouckhardt, “Self-masking controlled by metallic seed layer during glass dry-etching for optically scattering surfaces,” J. Appl. Phys. 107(3), 033301 (2010). [CrossRef]
35. S. A. Boden and D. M. Bagnall, “Tunable reflection minima of nanostructured antireflective surfaces,” Appl. Phys. Lett. 93(13), 133108 (2008). [CrossRef]
36. A. Asadollahbaik, S. A. Boden, M. D. Charlton, D. N. Payne, S. Cox, and D. M. Bagnall, “Reflectance properties of silicon moth-eyes in response to variations in angle of incidence, polarisation and azimuth orientation,” Opt. Express 22(2), A402 (2014).
37. R. N. Wenzel, “Resistance of solid surfaces to wetting by water,” Ind. Eng. Chem. Res. 28(8), 988–994 (1936). [CrossRef]
38. A. B. D. Cassie and S. Baxter, “Wettability of porous surfaces,” Phys. Chem. Chem. Phys. 40, 546–551 (1944).
39. J. Bico, U. Thiele, and D. Quéré, “Wetting of textured surfaces,” Colloid Surface A. 206(1), 41–46 (2002). [CrossRef]
40. J. Drelich, E. Chibowski, D. D. Meng, and K. Terpilowski, “Hydrophilic and superhydrophilic surfaces and materials,” Soft Matter 7(21), 9804–9828 (2011). [CrossRef]
41. X. Du and J. He, “Structurally colored surfaces with antireflective, self-cleaning, and antifogging properties,” J. Colloid Interface Sci. 381(1), 189–197 (2012). [CrossRef] [PubMed]
