Manufacturing of highly ordered porous anodic alumina with conical pore shape and tunable interpore distance in the range of 550 nm to 650 nm

In this work, highly ordered porous anodic alumina (PAA) with tapered pore structure and interpore distance (D_c) in the range of 550 nm to 650 nm were fabricated. To produce hexagonal close-packed pore structure a two-step process, combining anodization in etidronic acid electrolyte in the first step and high-concentration, high-temperature anodization in citric acid electrolyte in the second step, was applied. The Al pre-patterned surface obtained in the first anodization was used to produce regular tapered pore arrays by subsequent and alternating anodization in 20 wt.% citric acid solution and pore wall etching in 10 wt.% phosphoric acid solution. The height of the tapered pores was ranging between 2.5 μm and 8.0 μm for the PAA with D_c = 550 nm and D_c = 650 nm, respectively. The geometry of the obtained graded structure can be used for a production of efficient antireflective coatings operating in IR spectral region.