Fabrication of Moth-Eye Structures with Precisely Controlled Shapes by Nanoimprinting Using Anodic Porous Alumina Molds

Abstract

An anodic porous alumina mold with tapered pores, which can be used to form moth-eye structures, was formed by repetitions of anodization and etching. It was shown that the controllability of the pore shape of the anodic porous alumina mold improved with the number of repetitions of anodization and etching. However, it was found that even when the total anodization time or anodic charge (electrical current × time) was kept constant, the thickness of anodic films was not constant because the total etching time varied. This is because the etching of the anodic porous alumina mold not only increases the pore size but also reduces the thickness of the barrier layer so that pore growth proceeds after the barrier layer is re-formed during re-anodization. Therefore, it was found that if anodization is performed with the additional anodic charge required to re-form the barrier layer, an anodic porous alumina mold with tapered pores and uniform film thickness can be produced even if the etching time is varied. Nanoimprinting using the resulting anodic porous alumina mold was shown to form a moth-eye structure with a reflectance of less than 0.1% over the entire visible light range.