Plasma-Induced, Self-Masking, One-Step Approach to an

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Functional Nanostructured Materials (including low-D carbon)

Plasma-induced self-masking one-step approach to ultra-broadband antireflective and superhydrophilic subwavelength nanostructured fused silica surface Xin Ye, Ting Shao, Laixi Sun, Jingjun Wu, Fengrui Wang, Junhui He, Xiaodong Jiang, Wei-Dong Wu, and Wanguo Zheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01762 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 6, 2018

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ACS Applied Materials & Interfaces

Self-masking Antireflective Subwavelength Structure 352x148mm (72 x 72 DPI)

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Plasma-induced self-masking one-step approach to ultra-broadband antireflective and superhydrophilic subwavelength nanostructured fused silica surface Xin Ye1,2, Ting Shao1, Laixi Sun1, Jingjun Wu1, Fengrui Wang1, Junhui He3*, Xiaodong Jiang2*, Wei-Dong Wu2, Wanguo Zheng1, 4* 1

Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang,

Sichuan 621900, P.R. China 2

Science and Technology on Plasma Physics Laboratory, Research Center of Laser Fusion,

China Academy of Engineering Physics, Mianyang, 621900, P.R. China 3

Functional Nanomaterials Laboratory, Center for Micro/Nanomaterials and Technology and

Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Zhongguancundonglu 29, Haidianqu, Beijing 100190, China

4

IFSA Collaborative Innovation Center, Shanghai Jiao Tong University, Shanghai, 200240,

P.R. China

Abstract In this work, antireflective and superhydrophilic subwavelength nanostructured fused silica surfaces have been created by one-step self-masking reactive ion etching (RIE). Bare fused silica substrates with no mask were placed in a RIE vacuum chamber, then nanoscale fluorocarbons mask and subwavelength nanostructures (SWS) automatically formed on these substrate after appropriate RIE plasma process. The 1


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mechanism of plasma-induced self-masking SWS has been proposed in this paper. Plasma parameters effects on the morphology of SWS have been investigated to achieve perfect nanocone-like SWS for excellent antireflection, including process time, reactive gas and pressure of chamber. Optical properties, i.e. antireflection and optical scattering, were simulated by the Finite Difference Time Domainin (FDTD) method. Calculated data agree well with the experiment results. The optimized SWS show ultra-broadband antireflective property (up to 99% from 500–1360 nm).

An

excellent improvement of transmission was achieved for the Deep-ultraviolet (DUV) range. The proposed low cost, highly efficient, and maskless method was applied to achieve ultra-broadband antireflective and superhydrophilic SWS structures on 100 mm optical window, which promises great potential for applications in automotive industry, goggles, and optical devices. Keywords: Self-masking etching, Subwavelength structures, Ultra-broadband antireflection, Deep-ultraviolet antireflection, Superhydrophilic

1

Introduction

Fused silica is a technologically important dielectric that has been widely used in numerous optical applications 1. A number of optical elements, e.g. gratings, lenses and optical windows, are commonly manufactured from fused silica and widely used in optical systems. However, more than 7% of incident light is reflected from the substrate surface due to Fresnel reflection. Fresnel reflection occurs at the air-fused silica interface due to the sudden change of refractive index (the refractive index for silica, nsilica~1.46 for ultraviolet (UV) through visible wavelengths) 1. Various antireflection coatings (ARCs) have been developed to suppress Fresnel reflection losses2, but are limited by their high cost of fabrication due to material 2



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requirements and multilayer thickness control. Thermal mismatch also limits ARCs applications, inducing lamination and material diffusion of multilayer ARCs for high power laser applications, such as the National Ignition Facility (NIF)

3-5

, USA; Laser

MegaJoule, France; and SG series laser facility, China 6. Broadband antireflective (AR) nanostructures have attracted attention recently due to their advantages of simple fabrication process, mechanical & environmental 7-9

durability, laser damage resistance, etc.

. If the nanostructure surface pattern

dimension is smaller than the light wavelength, they are called SWSs. Light reflection is avoided by a graded refractive index profile between air and substrate, where the effective refractive index of the nanostructures increases gradually from air to the substrate. Many techniques based on top-down lithography 10, such as electron-beam etching

11-12

lithography

, fast atom beam

13-14

, interference lithography

15-17

, and nanoimprint

18-20

, Colloidal microsphere lithography 21, have been applied to fabricate

SWSs, and ordered arrays structure profiles are often fabricated using these technologies. Disordered SWSs are often prepared by different processes, where nanoparticles are deposited on the substrate surface by thermal dewetting

22-24

, self-organization 25,

or magnetron sputter deposition 26, and act as the etching mask. Top-down lithography requires two steps: mask fabrication and etching, and the structure scale is dictated by the mask scale. For ultraviolet (UV) and deep ultraviolet (DUV) applications, very small feature sizes, below 80 nm, are required, which are very difficult to achieve due to the difficulty of fabricating masks at that scale. Therefore, it is very important to develop a new fabrication method for UV or DUV AR and broadband AR tunable SWSs. Fluorine radical etching and fluorocarbon deposition produce surface roughness, and have been utilized to control surface roughness to precise requirements 3


27-28

.

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There are two types of mechanism to explain how roughness happens on the substrate. The key difference is the origin of micro-masking. One is fluorocarbon deposition, the other is sputtered aluminum.

29

The concept has been developed into an inexpensive

approach to fabricate SWSs on various semiconductor substrates, such as silicon, poly-silicon, GaN, GaP, sapphire, etc.

30

. However, the method has not been widely

employed for fused silica. The authors recently developed a simple fabrication method for AR SWSs in the visible (VIS) and infrared (IR) ranges on fused silica substrates. These AR SWSs exhibit reduced reflectance from Vis to IR, but their broadband performance and DUV AR is limited because the desired SWS profile and dimension have not been achieved. Broadband performance requires a gradient refractive index, which is often achieved by taper profile. It is difficult to fabricate SWSs at the tens of nanometer scale for DUV AR. This paper proposes a one-step, maskless, scalable, and inexpensive method to fabricate disordered SWSs over large areas for developing broadband AR structures on fused silica substrates. Fluorocarbon self-masking mechanisms were investigated in detail. We detail the AR properties of a disordered SWS fabricated by controlling several etching conditions, including gas proportion, etching time and pressure. Finite difference time domain (FDTD) simulations were used to simulate optical performance of SWSs. The effects of SWS dimension on optical scattering for different wavelength were analyzed by simulation and experiment. We tuned the AR performance of a fused silica disordered SWS from DUV to near-infrared (NIR) wavelength regions simply by controlling the etching parameters.

2 2.1

Results and Discussion Formation mechanisms of SWS

In the plasma etching process with SF6/He/CHF3, there is constant competition between active fluorine ions (e.g. CFx+, SFx+) and passivating fluorocarbon radicals 4



(e.g. CFx). Two states occur simultaneously during RIE: fluorocarbon deposition and etching 31. If the degree of etching is larger than deposition, no nanostructure results on the substrate surface, but there is a fluorocarbon coating. The balance of fluorocarbon deposition and etching can be used to fabricate nanostructures, where fluorocarbon deposition on the surface acts as an etching mask. Etching was increased directionally using active fluorine ions (CFx+), and fluorocarbon re-deposition formations were well correlated with high concentrations of CFx radical. Specifically, SFx+ ions were able to remove oxyfluoride by way of the volatile gases. Thus, CHF3 gas is a nearly independent source of fluorocarbon re-deposition. Under appropriate plasma conditions, fluorocarbon deposition forms on the substrate surface randomly during the RIE process, although there is also continuous etching competition. Fluorocarbon deposition will be etched due to ion bombardment during the etching process. In the meantime, fluorocarbon deposition also forms all the etching process 32. Crests and valleys appear on the substrate surface after fluorocarbon deposition. Crests form because fluorocarbon deposition protects the region beneath them, whereas regions without fluorocarbon deposition become valleys. Thus, fluorocarbon deposition increases crest and valley effects with increasing time, which is another competition in the plasma status. Roughness increases when fluorocarbon deposition forms on a crest, increasing its height. If fluorocarbon deposition forms on valley of surface, crest will be etched. There are not SWS appearing on the surface. When deposition is slightly larger than etching, rough surfaces occur. Fused silica pillar-like profiles with fluorocarbon deposition tips are formed by the etching of reactive radicals on the substrate surface, as shown in Fig. 1A. Island fluorocarbon deposition is also removed by the RIE process, but the etching speed is much slower than the fused silica. Therefore, fluorocarbon deposition formed on the fused silica 5


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acts as a mask. Fluorocarbons will deposit on the surface throughout the etching process, as shown in Figs. 1B, C, and D. Fluorocarbon depositions are correlated with the surface roughness topography. Surface peaks will receive more fluorocarbon deposition than the valleys, and fluorocarbon deposition islands formed on nanopillar tips induce more polymer deposition there, as shown in Figs. 1B and C. Since etching and deposition processes produce surface grass, they can be utilized to control the SWS morphology. Subwavelength structures increase with increasing the etching duration, as shown in Figs. 1C and D. Some smaller structures between larger structures are lost due to the increasing larger structures around them. After the oxygen plasma etching process fluorocarbon is removed (Fig.1E). This generation can be observed in our experiment next. Figure 1 Figure 2 shows the SWS generation over time for samples detailed in Table 1. In each subfigure, the left panel is a top view SEM image and the right panel is a tilt view SEM image. The density, nanograss diameter, and average peak-to-peak distance were measured by image processing, morphology, and watershed segmentation, respectively, as shown in Fig. 2F. Rough morphology emerged from the etching process as fluorocarbon formed on the substrate surface. Figure 2A shows the morphology for a 5-min processed sample, with a nanograss density of 588 µm-2, an average peak-to-peak distance is 40.57 nm, and an average nanograss diameter is 13.3 nm. The fluorocarbon structures are almost uniformly spread over the whole surface, with the average value of peak-to-peak distance being greater than the average nanograss diameter. With increasing the etching duration, new fluorocarbon nanodots appear. After 10 min processing, the nanograss density is greater than that after 5 min, and a cone-like profile (Fig. 2B, right panel) becomes apparent, while the average 6



nanograss diameter remains less than the average peak-to-peak distance. As the processing time increased up to 10 min, the nanograss density increased (see Fig. 2F), as discussed above. However, as the processing time increased beyond 10 min, the density decreased because neighboring nanograss peaks tended to clump together. Figure 2C shows that with a longer processing time (15 min), the space between nanograss peaks is reduced, new nanodots are no longer created in the valleys, and the cone-like profile becomes more prominent. Figure 2D and E show samples processed for 20 and 30 min, respectively. The tilt view SEM images show high aspect ratio nanograss fabricated on the substrate surfaces. In a word, the average diameter of nanograss and the average peak-peak value increase with increase of etching duration. Figure 2

2.2

Reactive gas effects on SWS morphology

We investigated the effects of CHF3 and SF6 on the nanograss formation mechanism of nanograss. High etching rate can be achieved with SF6 plasmas due to their high fluorine proportion (6 fluorine ions), so CHF3 is the main source of the fluorocarbon polymer. In a word, the more CHF3, the larger the size of fluorocarbon polymer. Therefore, we varied the CHF3:SF6 ratio to control etching and deposition. Table 2 shows the sample processing parameters and Fig. 3 shows the average nanograss diameter increases with increasing the CHF3 ratio. Figure 3

2.3

Pressure effect on SWS morphology

Vacuum chamber pressure could have a significant effect on the RIE process. At low pressure, the active ions have long free paths and bombard the surface with higher 7


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kinetic energies than at higher pressures. The island-like fluorocarbon depositions would be easily stripped by these high kinetic energy ions. Different pressures are set to prepare series of SWS. Detailed parameters are shown in Table 3. Fig. 4 A and D are processed in 100 Mtorr; The average height of structure is 250nm. Fig.4 B and E are prepared in 150 Mtorr; The average height of structure is 350nm. Fig.4C and F are fabricated in 300Mtorr; The average height of structure is 550nm. Figs.4 A-C are top-view SEM images and Figs.4D-F are side-view SEM images. Figures 4A–C show that the pressures produced distinct nanograss diameters, with smaller diameter for lower pressure. Thus, modifying the chamber pressure could be a useful method for tuning critical nanograss dimensions. Anisotropic etching was also evident with low pressure because of the high energy bombardment. Figures 4D–F (SEM side views) show the morphological effects of pressure for SWS formation. The morphology of SWS in Fig. 4D and E is pillar-like because of anisotropic etching; and Fig. 4 F is taper-like because of isotropic etching. The reactive gas amount of higher pressure in chamber is more than that of low pressure. Therefore, the height of SWS etched under high pressure is higher than that under low pressure. Figure 4

2.4

Optical performance of SWS

Fresnel reflection occurs at the interface of two media with different refractive indices, but may be reduced or cancelled by interposing layer(s) with more closely matched indices. Thus, a suitably graded refractive index along the incidence direction is crucial to provide perfect antireflection. The effective index for normal incidence can be expressed as neff=[nair2▪(1-f)+nsilica2▪f]1/2 8


(1)


where neff, nair, and nsilica are the effective, air, and silica refractive indices, and f is the nanostructure filling factor. Equation (1) is accurate when the nanostructure size is much smaller than the wavelength, i.e., SWS. The SWSs shown in Figs. 2–4 have a conical profile, and so we can calculate the effective reflective index using the close packed cone array model, as shown in Fig. 5. We assume the cone diameter and height are 200 nm and 500 nm, respectively, divide the cone into 500 horizontal layers, and calculate f for every layer, as shown in Fig. 5B, where nsilica = 1.456 for the VIS region. Thus, a gradual refractive index emerges with increasing the cone height. Figure 5 Figure 6 To investigate the optical properties of SWS, The SWS AR properties were investigated using a finite difference time domain (FDTD) model with hexagonal periodic structure, as shown in Fig. 6. The detailed simulation parameters is shown in Experiment Section. The cone height and diameter are important AR performance parameters, as shown in Fig. 7, with sample processing details in Table 4. Effective AR performance depends strongly on the refractive index profile steepness, hence the cone height, and light scattering occurs at the base of the cone nanostructure. To the best of our knowledge, this has not been reported previously. Figure 7A shows the cone array total transmittance as a function of cone height (0–600 nm) and wavelength (300–1400 nm) at the normal incidence for cone bottom diameter = 200 nm. AR performance improves significantly with increasing the cone height. Figure 7D clarifies how SWS transmittance in the long wavelength region 9


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increases with increasing the cone height. Figures 7B and C show the effect of cone diameter (60–500 nm) over wavelengths (300–1400 nm) at a fixed cone height (500 nm) for transmittance and light scattering, respectively, and Fig. 7E shows SWS transmittance for several specific samples. As discussed above, SWS height increases for longer etching time. Thus, while there is good similarity between the experiment and simulation, there are also significant contrasts in the short wavelength region. In the actual samples, the maximum transmittance shifts into to the NIR range as the cone height increases. Figure 7F shows that the minimum SWS reflectance occurs when transmittance is maximum, and reflectivity follows a similar profile to transmittance with increasing height. However, the transmittance + reflectance ≠ 100% in the short wavelength region. The substrate is fused silica, which has no absorption at 300–1400 nm. Therefore, the light deficiency in the short wavelength region is scattered due to the rough SWS structure. Figure 7G shows light scattering (S=1-T-R), where T is transmittance and R is reflectivity) for specific samples. Similar data have been reported in some previous work.33 Comparing these sample parameters and Fig. 7A, there is no correlation between light scattering and cone height. As discussed above, the cone diameter increases with increasing the etching duration, because the adjacent structures clump together. Thus, the cone diameter is critical for light scattering Comparing Figs. 7B and C, when the cone diameter < 200 nm, transmittance > 99% with negligible scattering from 300 to 1400 nm. However, transmittance decreases rapidly in the short wavelength region with increasing diameter (Fig. 7B), with corresponding increasing scattering (Fig. 7C). Figure 7E shows that transmittance decreases for larger structures as light scattering increases, because the features move out of the subwavelength regime for shorter wavelengths. Light scattering increases monotonically as the cone diameter increases, which agrees well with our experimental results (Fig. 7G). Figure 7 shows significant AR performance is 10



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tunable using only etching duration to produce excellent AR for UV and DUV regions as well as broadband AR from VIS to NIR. The angle-transmittance was also investigated in our work. As shown in Fig. S1, SWS still exhibits good antireflective performance for 60o incidence. As the incident angle increases to 60o, the highest transmittance for SWS decreases from >99% to 98%, while the transmittance of bare fused silica decreases from ~93% to 88%. Figure 7 Table 4. Sample processing parameters for Fig. 7 Sample

Process time

Pressure

Power

Gas ratio

(min)

(mTorr)

(W)

(CHF3:SF6:He sccm)

04

20

300

10

25

300

05

30

300

100

20:10:150

11

35

300

08

30

150

Ultra-broadband antireflective property and antireflection for DUV and superhydrophibic performance of SWSs were investigated in this case. High depth-to-width ratio is the key to broadband antireflection. It means that the higher the height of SWS is for the same diameter of SWS, the better broadband antireflection SWS exhibits. Therefore, small diameter and high height of SWS are necessary for broadband antireflective property. According to the discussion about the mechanism of SWS fabrication above, low pressure and long process time and decreasing CHF3 gas are in favour of achieving broadband antireflection. Figure 8A shows that transmittance of double-sided SWS samples prepared by the proposed method achieved broadband AR over 300–1500 nm, with transmittance > 99% at 510–1360 nm. The morphology of SWS is shown in Fig.8B. And this sample was fabricated by following the optimized conditions. Process time is 30 min; pressure is 100 mtorr; gas ratio (CHF3:SF6:He sccm) is 10:10:150. Figure 8B shows that the SWS had uniform 11


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cone-like structures. That’s why the SWS exhibits ultra-broadband antireflection from 300-1500 nm. As discussed above, large diameter of SWS results in light scattering at short wavelength. Therefore, in order to achieve antireflection in DUV, the diameter of SWS must be as small as possible. The deep-UV antireflective SWS was prepared at the same parameters as Fig. 8B. But the etching time is 10 min. Figure 8C shows AR performance for the single-sided SWS structure in the UV-VIS range (200–800 nm). Transmission of single-side SWS improved over fused silica alone by 4.9%, 3.67%, and 3.23% for 193, 248, and 355 nm, respectively. From Figure 8C, excellent DUV antireflection was achieved. Figure 8D shows the corresponding SEM image, with large scale SWS evident uniformly across the sample. Figure 8E shows a digital camera image of a 100 mm fused silica sample, partially covered by broadband antireflective SWS structures, under fluorescent light illumination. The untreated glass area reflects the fluorescent light significantly, whereas the SWS area provides clear vision of the underlying text. Thus, the proposed method can be directly applied to real and large-scale applications. Superdrophilic surface is often applied to anti-fogging. As we know, nanostructures can magnify the intrinsic wetting characteristics of surface. SWSs on fused silica result in superhudrophilic performance because fused silica is a hydrophilic material. The SWSs in Figure 8F shows superhydrophilicity. And a drop of water spreads on the surface immediately. The contact angle reached as low as 0o. Figure 8

3

Conclusions

We developed a simple one-step, maskless, scalable, and inexpensive approach to prepare structured random cone-like antireflective superhydrophbilic SWS. The mechanism for plasma-induced self-masked nanostructures was first proposed. In this plasma process, the balance between etching and re-deposition, and the re-deposition 12



competition between crests and valleys were considered. Self-masking was realized to fabricate SWS on fused silica because of tuning the balance between etching and re-deposition. Re-deposition competition between crests and valleys affects the nanostructure depth-to-width ratio. The process of SWS growth was first observed by SEM with the increase of etching time. A series of process parameters effect on the morphology of SWS have been investigated in detail, i.e. etching time, pressure and reactive gas. Apparently, as process time went by, both height and diameter of SWS increased. The lower the pressure is, the smaller the diameter of SWS is. CHF3 is important origin of fluorocarbon polymer deposition, and it is the key to self-masking technology. The fabricated samples showed excellent AR performance across the DUV, VIS, and NIR wavelength regions. The maximum transmittance exceeded 99.5%, and AR bandwidth was up to 860 nm, with transmittance exceeding 99% from 500 to 1360 nm. The FDTD method was used to analyze antireflection and light scattering. From the result of simulation and experiment, light scattering of SWS was considered to be the main point to affect transmittance of DUV region. Optimized DUV SWS exhibit excellent antireflection performance at 190−300nm. The proposed method was successfully applied to large-scale optics, fabricating SWSs on a 100-mm optical glass. We believe the improved transmittance and reflection properties provided by the proposed method will be helpful for many optical applications, and will help increase optical system efficiencies.

4

Experimental Section 4.1

Materials. Fused silica (Corning® 7980) substrates were polished to

roughness Rq < 1 nm by Z&Z Optoelectronics Tech. Co., Ltd. Sulphuric acid(H2SO4, 98%) and Aquae hydrogenii dioxide (H2O2) were purchased from Aladdin IndustrialCorporation. Deionized water (18.2 MΩ cm−1) was obtained from a Millipore Milli-Q Advantage A10 system. Purified gases of etching, i.e. trifluoromethane (CHF3), sulfur hexafluoride (SF6), and helium (He) were purchased 13


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from Mianyang Changjun Gas Company. 4.2

Fabrication of SWS. Firstly, fused silica substrates were cleaned by

immersion in piranha solution (7:3 concentrated H2SO4:30% H2O2) for 2 h at 90°C, rinsed repeatedly with deionized water (18.2 MΩ cm−1), and then dried in a nitrogen stream.

Then SWSs were constructed by fluorine RIE etcher (Oxford 80plus-RIE,

13.56 MHz RF).

Prepared substrates were placed in vacuum chamber and processed

using the following etching parameters. Etch gases were a mixture of trifluoromethane (CHF3), sulfur hexafluoride (SF6), and helium (He); RF power = 100 W. Disordered SWSs were fabricated using several etching conditions, such as etching time, gas proportion, and pressure. The RIE gas reactants comprised CHF3 (10–30 sccm), SF6 (5–15 sccm) and He (100 sccm). During etching, chamber pressure was kept between 50 and 300 mTorr, and etching duration was 1–40 min. The specific parameters are as follow Table1 to Table 3. Table 1. Sample processed using different time. Sample

Process time

Pressure

Power

Gas ratio

(min)

(mTorr)

(W)

(CHF3:SF6:He sccm)

300

100

20:10:150

01

5

02

10

03

15

04

20

05

30

Table 2. Sample processed using different gas ratio. Sample

Process time

Pressure

Power

Gas ratio

(min)

(mTorr)

(W)

(CHF3:SF6:He sccm)

30

300

100

20:10:150

05 06

10:10:150 30:10:150

07 14



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Table 3. Sample processed using different pressure. Sample

Process time

Pressure

Power

Gas ratio

(min)

(mTorr)

(W)

(CHF3:SF6:He sccm)

300

05 08 09

4.3

30

100

150

20:10:150

100

Characterization. The SWS morphology was analyzed using a scanning

electron microscope (SEM, ZEISS FESEM ULTRA55). Samples were sputtered with a thin Pt layer before SEM imaging. Statistic of SWS dimension and average nanograss diameter and average peak-to-peak distance were obtained by image process method, morphological operation in Image Pro software. Reflectance and transmittance measurements 190–1400 nm were measured using a PerkinElmer Lambda 950 UV-VIS-NIR spectrophotometer, where transmittance was measured at 0° incidence. Light scattering were mearsured using a 150 mm integrating spheres of PerkinElmer Lambda 950 UV-VIS-NIR spectrophotometer. Simulation.

The SWS AR properties were investigated using a finite

difference time domain (FDTD) model with hexagonal periodic nanocone, where the rectangle lattice had width D (cone diameter) and height H (cone height). The schematic of simulation is shown in Fig. 6. Grid size is 1 nm×1 nm×1 nm. –Z axis incident light source, X and Y direction periodic boundary condition, Z direction Perfectly Matched Layer (PML) boundary condition were set in the simulation. The bottom diameter was set from 50 nm−500 nm. The height was set from 0 nm−500 nm. Height effect on transmittance was calculated. Simulated transmittance and light scattering as a function of cone diameter also were calculated in this work. The detailed model of simulation is described in result and discussion part.

15


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Author information * Corresponding authors Wanguo Zheng, [email protected] Xiaodong Jiang, [email protected] Junhui He, [email protected] Notes: The authors declare no competing financial interest.

Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 61705204, 61705206, 51606158, 11174258, 51602296), Development Foundation of China Academy of Engineering Physics (No. 2015B0403095) and Laser Fusion Research Center Funds for Young Talents (No. LFRC-PD011).

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Figure 1. Mechanism for self-mask formation for subwavelength structures. (A) rough surface in the beginning; (B) The density of nanostructure is higher; (C) the nanostructure is bigger; (D) the nanostructure is bigger and the density of nanostructure is lower; (E) fluorocarbon is removed by oxygen ion plasma.

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Figure 2. Typical fused silica SEM images processed for (A) 5 min, (B) 10 min, (C) 15 min, (D) 20 min, and (E) 30 min, where the left pane is the top view and the right pane is the tilt view (sample 01–05, respectively); and (F) average nanograss parameters. Scale bars 500 nm.

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Figure 3. SEM images of subwavelength structures fabricated at (A) 10:10, (B) 20:10, and (C) 30:10 sccm by different CHF3:SF6 ratios (sample 06, 05, and 07, respectively). Scale bars 500 nm.

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Fig.4 SEM images of subwavelength structures fabricated at (A, D) 100, (B, E) 150, and (C, F) 300 mTorr (sample 09, 08 and 05, respectively). Scale bars 500 nm.

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Figure 5. From (A) the nominal conical nanograss model we obtain (B) the effective refractive index profile with subwavelength structure height.

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Figure 6. Finite difference time domain model for subwavelength structure (SWS) antireflective performance. (A) XY view (vertical view), (B) 3D simulation model.

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Figure 7. Subwavelength structure optical properties: (A) simulated total transmittance as a function of cone height, (B) simulated 0th order transmittance as a function of cone diameter, and (C) simulated scattering as a function of cone diameter over a wavelength range at normal incidence; (D) simulated total transmittance for specific cone height (Fig. 8A); experimental (E) scattering, (F) reflectance, and (G) scattering of various specific fabricated samples, as detailed in Table 4. Note. The measurement error is smaller than ±0.5% form manual of PE Lambda 950.

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Figure 8. (A) Broadband antireflection performance of two-side SWS; (B) tilt-SEM image of SWS for broadband antireflection; (C) deep-UV antireflection; the improved transmission values for the DUV laser wavelengths of 193 nm and 248 nm. (D) SEM image of SWS for deep-UV antireflection; (E) 100 mm fused silica substrate partially covered with two-side subwavelength (SWS) structures (right hand side of the sample, showing the reflection, was not SWS treated); and (F) superhydrophilic SWS performance. Note. The measurement error 27


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is smaller than ±0.5% form manual of PE Lambda 950.

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Plasma-Induced, Self-Masking, One-Step Approach to an

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