1. Introduction

Subwavelength nanostructures firstly found in moth eye [1] serve as antireflective structures (ARS) for diverse optical devices such as polymer lenses [2,3], display panels [4–7], solar cells [8–10], and biosensors [11,12]. Conventional fabrication methods for highly efficient ARS such as electron beam lithography [13] and interference lithography [14,15] have some technical limitations in applying for practical applications due to small area and low throughput. Regardless of materials, area, and surface curvature, advanced fabrication methods for ARS are still in need for effectively employing ARS on an optical component for practical uses. Large-area ARS of nanopillars have also been successfully demonstrated on various substrates by using unconventional fabrication methods such as oblique angle deposition of dielectric [16,17], polymer molding from anodic aluminum oxide (AAO) template [7,18,19], etching with dielectric monolayer [20,21], and metal nanostructures [3,22–24]. Indium tin oxide (ITO) nanocolumns deposited by oblique angle deposition serve as a conductive AR layer for GaAs solar cells [16]. Polymer molding using AAO templates provides large-area ARS on polymer films [7,18] and lenses [19]. Etching with nanoparticle masks such as a colloidal polystyrene (PS) monolayer or metal nanoislands provides ARS on miscellaneous substrates including silicon substrates [22,23], glass substrates [11,20,24–26], OLED [21], and polymer microlens [3]. In particular, nanohole structures have been demonstrated to enhance the light transmittance as well as mechanical stability unlike nanopillars [27–30]. Nanoholes are mechanically more stable than nanopillars as a complementary shape, which easily become collapsed in shear or normal stress. However, direct patterning of large-scale nanostructures on practical optical lenses is still limited because commercialized lenses contain some metal oxides, which are hard to achieve replica molding and dry etching [31,32]. In addition, direct patterning on conventional lens surface is also challenging to find the optimal etching condition due to complicate compositions of commercialized lenses [33].

Here we report an optical lens with large-scale glass nanoholes as mechanically stable antireflective structures. The glass nanoholes simultaneously exhibit both highly enhanced light transmittance and mechanical stability compared to nanopillars. The large-scale glass nanoholes are fabricated on a flat surface as well as a lens surface by using solid-state dewetting of ultrathin silver film, dry etching, and wet etching. The solid-state dewetting of ultrathin silver film at low temperature effectively provides large-scale silver nanoholes (AgNH) on both flat and curved surfaces. The nanoholes on a glass substrate are directly transferred from the AgNH by using reactive ion etching (RIE). A glass substrate with nanoholes delivers a clear optical image without an undesirable specular reflection compared to the flat surface due to the antireflection effect as shown in Fig. 1(a) and 1(b). Light transmission through the glass nanoholes is numerically calculated by using the finite difference time domain (FDTD) method. The effective refractive index of nanoholes mainly depends on a fill factor (FF) and a thickness of nanostructures. The averaged transmittance in the full visible range of spectrum is calculated for different FFs and thicknesses of nanoholes for a constant period of 250 nm as shown in Fig. 1(c). The FF is defined by the area ratio of a single hole to a unit cell, i.e. 1-πw²/4p², where w is the width of nanohole and p is the period of a single unit cell. For the FDTD analysis, cylindrical nanoholes are arranged in rectangular arrays and the refractive index of a glass substrate is set to the value of silicon dioxide (SiO₂) in the visible range. Figure 1(d) shows the calculated average transmittance in the visible region of 400 nm to 700 nm for the FF of 0.25 to 0.95 and the thickness of 50 nm to 200 nm. The calculated average transmittance has the maximum value for the FF of 0.45 and the thickness of 110 nm. Besides, the glass nanoholes with the FF of 0.35 ~0.55 and the nanohole thickness of 90 ~130 nm also show highly enhanced transmittance close to the maximum value, because the nanoholes with the FF and the thickness in the ranges have an optimal effective index to reduce an undesirable specular reflection in the visible region.

 

Fig. 1 Large-scale glass nanoholes for antireflective structures. (a) The optical image of perspective view of “Morpho butterfly” through the glass wafer with a half nanoholes surface and a half flat surface. (b) A schematic illustration of nanoholes with subwavelength dimensions for antireflective structures. (c) A schematic diagram of numerical analysis for light transmission through the glass nanoholes by using the finite different time domain (FDTD) method. (d) Numerical results of average transmittance at the visible region (400 ~700 nm) through the glass nanoholes with different fill factors and thicknesses for the fixed period of 250 nm.

Download Full Size | PDF

2. Nanofabrication of glass nanoholes

A wafer-level nanofabrication of glass nanoholes is done by using solid-state dewetting of an ultrathin silver film at a low temperature, RIE, and wet etching on a borosilicate glass substrate as shown in Fig. 2. The ultrathin silver film transfers to the nanoholes at low annealing temperature below 0.16 T_m, where T_m is melting temperature of silver (T_m = 962 °C for Ag) [34]. The 4-in glass wafer is cleaned in piranha solution (10:1 H₂SO₄/H₂O₂) and rinsed with deionized water. An ultrathin silver film with a thickness of 160 Å is thermally evaporated on the glass wafer with a deposition rate of 1 Å/s. The ultrathin silver film is transformed to silver nanoholes (AgNH), which serve as an etch mask during RIE and wet etching by annealing at 90°C for 10 mins on a hotplate based on the solid-state dewetting. The nanoholes are directly transferred on the glass surface after the RIE process with the AgNH mask. The glass substrate is anisotropically etched by using RIE with tetrafluoromethane (CF₄) of 15 standard cubic centimeters per minute (sccm), and fluoroform (CHF₃) of 45 sccm, argon (Ar) of 150 sccm, a pressure of 200 mTorr, and radio frequency power of 300 W. The nanohole height is precisely controlled by the etching time of RIE process. The fill factor is further controlled for widening the nanohole diameter during wet etching in a 6:1 buffered oxide etcher (BOE) for 30 s. The residual AgNH mask is finally removed in silver etchant of iodine and potassium iodine (etchant TFA, Transene Company, Inc.). The FF is calculated from top-view SEM images of glass nanoholes. The FF of glass nanoholes is 0.79 before the wet etching. The chemical etching solution isotopically and precisely removes the glass region, thereby the FF decreased from 0.79 to 0.56 after widening process. The nanoholes are mechanically connected each other for the FF lower than 0.56 after additional widening process. The glass nanoholes with a broad size distribution shown in Fig. 2(c) effectively achieve the antireflective effect in a broadband visible range.

 

Fig. 2 Nanofabrication of nanoholes on a glass substrate. (a) Nanofabrication procedures of glass nanoholes by using solid-state dewetting of ultrathin silver film, reactive ion etching (RIE), and wet etching. The top-view SEM images and the size distribution of silver nanoholes (AgNHs) mask (b), glass nanoholes without widening (c), and glass nanoholes with widening (d).

Download Full Size | PDF

3. Optical and mechanical characterization of glass nanoholes

Light transmittance is measured through glass nanoholes of different thicknesses by using a spectrometer (SP-2300i, Princeton Instruments) with a collimated white-light LED source. The glass nanohole substrates for the FF of 0.56 and different thickness of 70 nm to 150 nm were prepared by controlling the etching time during the RIE process. The nanoholes on a glass substrate clearly increase optical transmittance with the maximum enhancement of 4% compared to a bare glass substrate at the visible region as shown in Fig. 3(a). In addition, mechanical stability of glass nanoholes is evaluated by the hardness test. Both the glass nanoholes and nanopillars of the same FF are prepared by using the solid-state dewetting and RIE. The hardness of glass substrates is measured by using a nanoindenter with the Belkovich type tip (Nano Indenter XP, MTS). Both the nanostructured surfaces show the hardness of 4 GPa and 1.6 GPa, respectively. The experimental results clearly indicate the glass nanoholes have the hardness 2.5 times higher than that of the nanopillars, while having a similar enhancement of transmittance at the visible region as shown in Fig. 3(b). Patterning the nanoholes on the surface of glass substrate can take advantages both the antireflection effect and mechanical robustness in comparison with the glass nanopillars.

 

Fig. 3 (a) Transmittance of bare and antireflective substrates with different thicknesses of nanoholes at visible region. Nanoholes increase the transmittance by up to 4% compared to the bare glass substrate. (b) The transmittance and hardness of bare glass, glass nanoholes and glass nanopillars.

Download Full Size | PDF

4. Optical lens with glass nanoholes

Antireflective glass nanoholes is further fabricated on a conventional lens surface by using the same method with a thin SiO₂ film. A commercialized optical lens (TaF1, HOYA Corp., n_TaF1 = 1.78 @ 550 nm wavelength), mainly made of tantalum flint [33], is prepared to fabricate the antireflective optical lens. An additional thin SiO₂ layer is formed to fabricate the ARS on the TaF1 lens. The lens surface of TaF1 acts as an etching stop layer due to a low etch rate of tantalum flint in conventional reactive ion etching conditions for SiO₂. A 110-nm-thick SiO₂ film is deposited on the lens surface by using plasma-enhanced chemical vapor deposition (PECVD). The SiO₂ nanoholes are fabricated by using RIE with the AgNH etch mask and wet etching. The SiO₂ nanoholes become widened in wet etching by using a 20-fold diluted 6:1 BOE due to faster etch rate of PECVD SiO₂ than the etch rate of borosilicate glass [35]. The optical lens with ARS clearly demonstrates highly enhanced transmittance at the visible region compared to the bare lens. The optical lens of ARS delivers clear optical images for letters and picture without a specular reflection compared to a bare TaF1 lens as shown in Fig. 4(a). As shown in Fig. 4(b), the FF of SiO₂ nanoholes still satisfies the antireflective condition described in Fig. 1(d) regardless of the geometric difference of nanoholes at the different position of lens. Light transmission through the AR lens and the bare lens is measured by using an integrating sphere with a white-light LED source and a fiber-coupled spectrometer (SM642, Spectral Products) as shown in Fig. 4(c). Figure 4(d) shows the glass nanoholes on the lens surface increase light transmittance by 10.5% at 433 nm, 7.2% at 560 nm, and 7.1% at 633 nm, respectively. This improvement is visibly distinguished in specular reflection from the lens surface as shown in the Fig. 4(a).

 

Fig. 4 The antireflective optical lens with glass nanoholes. (a) The optical images of perspective view of letters and picture through a bare lens and a lens with SiO₂ nanoholes. (b) The SEM images of glass nanoholes at the center and the edge of lens. (c) A schematic illustration of measurement system for light transmission of an optical lens using a white light LED source, an integrating sphere, and a spectrometer. (d) The transmittance of bare lens and AR lens at the visible region.

Download Full Size | PDF

5. Conclusion

In summary, this work successfully demonstrates the antireflective optical lens using large-scale glass nanoholes for high optical transmission and mechanical stability. Large-scale glass nanoholes on a 4-in glass wafer have been successfully nanofabricated on both a flat glass substrate and a commercial lens surface by using the solid-state dewetting, reactive ion etching, and wet etching. The glass nanoholes not only increase light transmittance by 4% at the visible region but also have mechanical hardness 2.5 times higher than conventional nanopillars. Furthermore, a commercialized TaF1 lens of glass nanoholes clearly shows low specular reflection, compared to a bare TaF1 lens. This novel method enables the large-scale nanofabrication of glass nanoholes on the surface of commercial optical components for highly efficient optical imaging, illumination, or display applications.