Flat and roll-type translucent anodic porous alumina molds anodized in oxalic acid for UV nanoimprint lithography

Abstract

There is much interest in UV nanoimprinting as a fabrication method for various functional devices because of its suitability for efficient fine patterning. To form patterns on opaque substrates by UV nanoimprinting, it is essential to use molds through which UV light can pass. In this study, translucent anodic porous alumina (APA) molds for UV nanoimprinting were fabricated by the anodization of an Al substrate. To fabricate a translucent APA mold, an ordered APA film used as a mold for UV nanoimprinting was formed on the surface side of the Al substrate, and then anodization was continued from the back surface of the Al substrate to increase its transparency in the UV spectral range. A gradient change of Al thickness is necessary for the production of a large-area translucent mold, since it lowers the thickness of opaque defects remaining in the mold. The resulting translucent mold was effective for UV nanoimprinting to prepare ordered polymer nanopillar arrays on the surfaces of opaque substrates because the transmittance of the resulting translucent APA mold was 40% at a wavelength of 365 nm, which was confirmed to be sufficiently translucent to polymerize the photocurable monomer used in this study. In addition, it was possible to fabricate roll-type translucent APA molds by using Al pipes as a starting material. A seamless ordered nanopillar array can be effectively formed on a substrate by continuous UV nanoimprinting using the resulting roll-type translucent APA molds. Ordered nanopillar arrays formed on opaque substrates by UV nanoimprinting using translucent APA molds have various potential applications, such as those for forming antireflective and water-repellent surfaces.

Fig. 1. Schematic of preparation process for translucent APA mold and UV nanoimprinting using the resulting translucent mold; (a) formation of mold layer by anodization, (b) formation of masking layer on the surface of mold layer, (c) anodization of the sample to prepare a support layer, (d) removal of masking layer to obtain translucent APA mold, (e) UV nanoimprinting, and (f) detachment of the APA mold to form ordered polymer nanopillar array.

Fig. 2. (a) Photograph of translucent APA mold. (b) Surface and (c) cross-sectional SEM images of the translucent APA mold. (d) Cross-sectional SEM image of the opaque portion of the APA mold. (e) Time–current density curve of the sample during the anodization for fabricating the support layer.

Fig. 3. Transmittance spectra of the translucent APA mold and Al sheet with a thickness of ca. 300 μm.

Fig. 4. (a) Surface and (b) cross-sectional SEM images of polymer nanopillar array formed on the surface of Si substrate by UV nanoimprinting using translucent APA mold.

Fig. 5. Schematic of the preparation process and photograph of samples obtained by anodization of (a) Al plate with uniform thickness and (b) Al plate with thickness gradient.

Fig. 6. (a) Schematic of preparation of roll-type translucent APA mold. (b) Photograph and cross-sectional SEM image of the resulting roll-type translucent APA mold.

Fig. 7. (a) Schematic of continuous UV nanoimprinting using roll-type translucent APA mold. (b) Surface and cross-sectional SEM images of polymer nanopillar array obtained by continuous UV nanoimprinting using roll-type translucent APA mold.