A soft lithographic approach using a modified polyurethane acrylate (PUA) mold for the fabrication of sub-wavelength antireflective structure on polymer film is reported. By introducing an intermediate transferring PUA mold generated by an anodized aluminum oxide membrane, there is no need either to heat nor to deposit metal as a seed layer. Therefore, the most costly and time-consuming master preparation step in the conventional process chain is not a necessity. The soft PUA mold provides a high resolution of 100 nm with an aspect ratio of 1.7 and a conformal contact with the substrate and reduces the pressure needed during the imprinting steps. It is numerically verified that the antireflective film with nanopores has a similar fascinating broadband antireflective effect compared with that of its complementary nanonipple one. In our experiment, the average transmission efficiency of the PET film with dual-side nanopores can be enhanced to 98.7% at normal incidence and 92.5% at an incident angle of 60° over a range of 400~800 nm of the spectrum. The proposed method is simple and cost-effective and the fabricated antireflective polymer film can be mounted on the surfaces of various optical devices for the reduction of Fresnel reflections.
© 2014 Optical Society of America
When light is incident on the interface between media characterized by different refractive indices, a fraction of light is reflected, which imposes severe limits on many optoelectronic devices such as flat panel displays, photodetectors and solar cells. A systematical approach for the construction of high-performance antireflective devices was inspired by the idea of bionics. In the 1960’s, Bernhard & Miller discovered that the outer surface of the facet lenses in moth-eyes consists of an array of cuticular protuberances termed corneal nipples . The optical action of the corneal nipple array is creating a graded refractive index layers which leads to a major substantial reduction of the reflectance of the facet lens surface, and therefore helps to enhance the light sensitivity of the light-craving moths. Over the past several decades, the optical properties of moth-eyes have received considerable biological as well as physical interest. The development of AR surfaces which mimic moth eye sub-wavelength structures (SWS) has shown several advantages over the conventional dielectric antireflective (AR) coatings. These advantages include, amongst others, broad angular and spectral responses, polarization insensitivity, and reliability in harsh and abrasive environments.
Sub-wavelength AR structures have been realized on the surfaces of diverse materials for different applications, such as the following: Si [2–4], amorphous Si (a-Si) , GaAs , GaP , fused silica [8, 9], glass [10–12] and polymers [13–17]. Polymer films with AR surfaces would have great potentials to attach to vehicle dashboards, safeguards monitors or any other optical surface when anti-reñection or anti-glaring is mandatory, as well as in various types and sizes of organic light-emitting and organic photo-voltage devices. However, the fabrication of a uniform and large-area nanoscale pattern on polymer substrates is still a challenge due to the employed time consuming and expensive top-down techniques, such as ebeam and interference lithography. Another methodology is accomplished by bottom-up self-assembly combined with a subsequent etching process or by using the surface topography of the natural insect’s wing as a biotemplate [11, 18–21]. But they are, unfortunately, unsuitable for efficient mass-production. Hence, developing fast and low-priced replica methods is highly desirable for the production of polymer films with anti-reflective nanostructures. Nanoimprinting lithography is on the one hand suitable for roll-to-roll processing and on the other hand it provides an excellent spatial resolution, which to date is limited only by tool-making capabilities [22–24]. It allows a higher aspect ratio, a larger number of compatible materials, a shorter process times which is inherent to parallel structuring techniques.
This article tackles the challenge of taking the shine off polymer materials with the help of a soft lithographic approach, which is evolved from classical nanoimprinting lithography . An anodized aluminum oxide (AAO) membrane is used as the original mother-template, which can offer a high density long-range ordered SWS architecture with hexagonal symmetry and a high aspect ratio. The idea of using an AAO membrane as a mold for SWS-based antireflection purpose is not novel and has been investigated previously. Nanopillars were obtained by protruding soften polymer material, such as polymethylemthacrylate (PMMA) or polycarbonate, to the nanopore array in AAO by casting or by hot embossing [16, 26]. T. Yanogishita et al. employed metal molds from AAO as an intermediate transferring template to fabricate an ordered array of holes on photopolymerizable polymer . However, the above methods are unsuitable for mass production of AR structures on a film surface because of the high cost and complex replication route. In this paper, an intermediate transferring ultraviolet (UV) curable PUA mold is introduced. There is no need neither to heat nror to deposit metal as a seed layer in the process as described in [16, 26, 27]. The modified PUA mold provides not only a higher resolution (~100 nm) than that of using h-PDMS , but also a better conformal contact with the substrate. Another technical benefit is that de-molding can be achieved by pealing the soft mold from the imprinted substrate with an effectively smaller de-molding area, which is extremely beneficial for the attainment of a high aspect-ratio (>1) SWS’s replication . The mechanical strength of the special PUA guarantees that no nanonipple can collapse during the process. The proposed approach has the potential advantage of being a low-cost large area SWS-based AR polymer film manufacturing process.
2. The process
The process can be described by the steps suggested in Figs. 1(a)-1(f). It comprises the fabrication of an AAO template with high order pore array, replication of SWS and ultraviolet curable molding with a soft transparent mold. The AAO template is fabricated by using a well-known two-step anodization process [30,31], as shown in Figs. 1(a)-1(c). In the ðrst anodization process, the pre-treated high pure (99.999% purity, 0.25 mm thickness) aluminum foil is immersed to oxalic acid (0.3 M) at a constant-voltage of 40V and temperature of the solution is maintained at 4 °C for 4 h. During the anodization, the solution is stirred by a pump circulation system. And then, the anodic alumina layer is removed using wet chemical etching in a phosphoric acid solution (6 wt%) at 60 °C for 4 h. The second anodization process is processed under the same conditions as the ðrst anodization process, and the anodization time is 10 min. After that, the pore diameter of the anodic alumina is widened by immersing the sample into a phosphoric acid solution (6 wt%) at 32 °C for 25 min. After the two-step anodization process, the fabricated porous AAO template is obtained. In our experiments, the dimension of rectangular-area AAO template is as large as 10 cm x15 cm with excellent uniformity and the depth of the nanopores array can be changed by adjusting the anodization duration and widening time.
The fabrication procedure of the soft PUA mold, which can be broken down into three stages: molding, exposure and demolding, is outlined in Figs. 1(d)-1(e). Six-functional urethane acrylate is mixed with siloxane epoxy (with mixing ratio 1:1) to enhance the PUA’s rigidity while maintaining flexible. In the molding step, the modified PUA is drop-dispensed on a ñexible poly(ethylene terephthalate) (PET) ðlm with a thickness of 100 microns. In order to increase the liquid mobility, the PUA is pre-heated to 50 °C and then spontaneously moves into the cavity of the AAO by extruding action and then solidiðes when exposed to UV radiation. The imprinting pressure was typically less than 0.3 MPa due to the low viscosity of the PUA. It takes about 20 sec under a UV light-emitting diode with an intensity of 1000mW/cm2 at a distance of 1cm. Then, the PET film with the cured PUA is mechanically peeled off from the AAO membrane. It is used as a soft patterned mold composed of a nanonipple array on the surface and is placed and extruded on the dispersed UV-curable resin (D10, PhiChem) on a substrate (such as PET). The exposure time in the second curing is less than 2 sec, thus it is an efficient replication process. The AR structure of a nanopore array can be formed on both sides of the substrate by using an UV curable imprinting process, as shown in Fig. 1(f).
The obtained surface morphologies were observed using a field emission scanning electron microscope (FE-SEM; Quanta 400 FEG). The contact angles of water were measured using a contact angle meter (DataPhysics Instruments Gmbh) at room temperature. The optical properties of the PET film composed of AR nanostructures were evaluated using a spectrophotometer (PerkinElmer Lambda 750).
3. Experimental results and discussions
Prior to the imprinting, the AAO template is pretreated using a fluoroalkylsilane solution (NOVEC 7100, 3M) to form a releasing layer on its surface. Figures 2(a)-2(b) shows the corresponding goniometer images for 1 μL droplets PUA with apparent contact angles of 49° and 114°, respectively. The larger contact angle means weaker adhesion of the modified PUA to the fluorosilane-treated AAO mold and an easier demolding process, which is a critical issue in high aspect ratio nanostructure replication.
The side and top view images of the AAO template, the PUA mold and a replicated nanopore array on PET film are shown in Figs. 3(a)-3(c) respectively. The structure of AAO can be described as a closely packed array of columnar cells. Each cell contains an elongated cylindrical nanopore which is normal to the aluminum surface, extending from the surface of the oxide to the oxide/metal interface, where it is sealed by a thin barrier oxide layer with approximately parabolic geometry (as shown in Fig. 3(a) and its inset) . By controlling the repetition time of the anodization, the depth of self-porous AAO can be controlled and the pore diameter is constant about 80nm with a uniform interval of 100 nm. Figure 3(b) shows that the fabricated PUA mold contains an array of nanonipples with an average height of 170 nm and an aspect ratio of 1.7. It can be seen that the height of the nanonipples on the PUA mold is less than the depth of the pores in the original AAO. The phenomenon can be attributed to the high contrast on the surface tension between the modified PUA and the surface-treated AAO, which hinders the protruded nano-columns from being completely formed. In addition to a slight volume shrinkage of the PUA during UV exposure, the formed nanostructure is shortened and tapered along the direction of axis . Fortunately, the typical size of the pores of alumina are in the nanoscale region and their depth can be extended to several hundreds of micrometers, which is deep enough for being used as a master for the fabrication of a PUA mold. Moreover, the parabola-like nanostructure array can be considered to have a more realistic geometry for AR structures. The measured contact angle of the PUA mold with the nanonipple arrays is 135°, which also exhibits hydrophilicity. The modified PUA has the advantages of low surface energy, chemical inertness and polymerization characteristics. Figure 3(c) shows the oblique and surface images of the AR structures on PET substrate with an array of nanopores imprinted by the PUA mold.
The soft PUA mold not only provides a resolution down to a several nanometer scale free from cracks, but also requires very little pressure as opposed to a rigid mold. The resolution is limited only by van der Waals contact and the inherent atomic and molecular granularity of matter. Figures 4(a)-4(b) shows the proof of the presumption that physical contact of the soft mold is the key mediator of pattern transfer. This feature is especially useful for a non-flat surface. Nanopore arrays are well duplicated to the particle- and line-related defect area on the surface because of the flexibility of the mold.
An AR structure composed of nipple arrays with sub-wavelength dimension is usually believed to perform better than its complementary one. However, with our finite-difference time-domain (FDTD) simulation results (Lumerical Solutions Inc.) shown in Figs. 5(a)-5(b), both the nanopore and nanonipple arrays with hexagonally packed pattern exhibit the enhanced transmission effect. In our calculations, a layer of 100 μm-thick PET film with dual-side SWSs is chosen to be the substrate and the material dispersion is taken into account. The dispersion curve of the PET can be found in Fig. 6. The structural parameters of the nanonipple and nanopore arrays are as follows: the separation is 100 nm; the top (for nanopores) or base diameter (for nanonipples) is 80 nm. The depth for a typical nanopore and the height for a typical nanonipple varies from zero to 500nm, respectively. The computational geometries of the nanonipples are parabola and a six-fold hexagonal symmetry is used. The nanopores, similarly, are figured to a tapered shape, in order to take into account the gradual change of the refractive index. Unpolarized light is used as incident source. It can be seen from Figs. 5(a)-5(b) that the transmission efficiency is greater than 94% when the nanopore depth in Fig. 5(a) or the nanonipple height in Fig. 5(b) is larger than 100 nm. This suggests that a high aspect ratio greater than 1.0 is to be preferred for AR application in both cases at a fixed separation.
Generally speaking, the AR structure composed of nipple arrays performs a little better than that of pore arrays. One can observe that periodic ripples occur caused by the modulation of the pore depth in the transmission contour plot at short wavelength spectrum. SWS-based AR film composed of pore arrays has several advantages compared with those composed of the conventional nipple arrays. In the case of the nipples array structure, a number of nanonipples may locally adhere to each other to form bundles, therefore it is difficult to keep the upright structures with high aspect ratios. In contrast, a high aspect ratio can be achieved easily in pore-shaped structures.
Figure 7 illustrates the measured angle-dependence transmission spectra of the PET film with bare flat surface, single-side nanopores or nanonipples and dual-side nanopores or nanonipples. The samples with a nipple array are fabricated by direct replication of SWSs from AAO template. The incident angle is in the range of 0 to 60° with an increased step of 15° from the normal to the film. The performances of the samples with SWSs are much better than that of the bare flat PET. The samples with dual-side SWSs demonstrate the highest average transmission efficiency, i.e., 98.7% around normal incidence and 92.5% at the incident angle of 60°, which offer a significant improvement over the single-side samples. The measured result at normal incidence (red line) is well consistent with the simulation result shown in Fig. 5(a), which is denoted by the white dotted line. The morphology of SWSs by using the soft lithography approach generates a superior graded refractive index profile between the air and the surface, which exhibits an optimum AR property over a wide range of angles and wavelengths. It is found that the transmission efficiency of the nanopore array (red line) is about 2% less than that of the SWSs with complementary shape (orange line) at large incident angle in the shorter-wavelength range, but the former is more economic and has a more flat transmission efficiency over the whole spectrum.
The AR effect of the fabricated polymer film can be visually observed, as shown in Fig. 8(a). The UV curable polymer-coated PET with dual-side nanopores are illuminated with white light. The surrounding of the film is left blank and there are no AR nanostructures. With the excellent omnidirectional broadband AR effect, the reduction in reflection off the surface of the patterned area is visually transparent in contrast with the surrounding area. Figure 8(b) shows the picture of the soft PUA mold with a nanonipple array on one side and the fabricated PET film with a nanopore array on both sides. Both of them are flexible and transparent, except the single-side PUA mold (bottom) reflecting more light than that of the dual-side PET film (top). To test the durability issue of the modified PUA mold, experiments have been done repeatedly. It is shown from Figs. 9(a)-9(d) that there is no any deformation or differences between the imprinted nanopores on the samples after the PUA mold used for 10, 20, 30 and 50 times. The Young’s modulus of the modified PUA is measured as 200 MPa, by which it can be expected that the PUA mold might lead to a solution for industrial manufacturing of AR film with high density nanostructures.
In summary, we have demonstrated a simple and economic soft lithographic approach for fabricating large-area sub-wavelength antireflective structure on a polymer film. A modified soft PUA mold is introduced to avoid the costly and time-consuming step of the preparation of the master AAO templates in the conventional process chain. The proposed method, by using a flexible and robust PUA mold as an immediate template, cannot only overcome the difficulty of mold preparation, but also combine the spatial resolution of UV curable nanoimprinting and the close contact in soft lithography. It is numerically verified that the antireflective film with nanopores has a similar fascinating broadband antireflective effect compared with that of its complementary nanonipple one. The average transmission efficiency of a PET film with dual-side nanopores can be enhanced to 98.7% at normal incidence and 92.5% at 60° of incident angle in our experiment. The method also has a potential for patterning antireflective structures on a curved surface for mimicking natural moth eye and might lead to a solution for industrial fabrication. The replicated sub-wavelength structure antireflective polymer film can be mounted on the surfaces of various optical devices for the reduction of Fresnel reflections.
We gratefully acknowledge the staff and facility support from SVG Optronics Corp. and Institute of Functional Nano & soft Materials in Soochow University. We thank Prof. B. J. Hoenders from Soochow University for polishing language. This work was supported by the key Natural Science Foundation of the Higher Education Institutions of Jiangsu Province (grant No. 10KJA140048), by the National Natural Science Foundation of China (NFSC) Major Research Program on Nanomanufacturing (grant No. 91323303), by the NFSC (grant No. 60907010, 91023044) and by a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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