Porous Alumina
Abstract
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F. Müller, O. Jessensky, A.-P. Li, K. Nielsch, J. Choi, and R. Ji
Self-organization
Self-assembly by means of a chemical or a physical driving force has attracted considerable interest as an alternative way to build nanostructures with a feature size ranging from a few nm up to a few 100 nm. The formation of arrays of high aspect ratio nanopores during the anodic oxidation of ultra-pure aluminium is a well-known phenomenon. The occurrence of a periodic pore arrangement in porous anodic alumina was first reported by Masuda in 1995.
Fig. 1. Top view SEM pictures of ordered alumina pore arrays.
If the anodizing conditions such as applied potential, electrolyte concentration and temperature match self-ordering conditions obeying the 10 % porosity rule, self-ordered porous alumina structures with an interpore distance of 50, 65, 100, 420 and 500 nm can be obtained. Then, the pores at the formation front will be arranged in a hexagonal lattice.
Fig. 2. Currently accessible lattice constants and pore diameters of self-ordered porous alumina.
Imprint lithography
We have developed an imprint stamp consisting of a hexagonal lattice of pyramids with a height of about 260 nm and a lattice constant of 500 nm (Fig. 3a), being particularly adapted to hard imprint in aluminum.
Fig. 3. (a) Scanning electron micrograph of the imprint master consisting of Si3N4 pyramids with a lattice constant of 500 nm. (b) Scanning electron micrograph of monodomain porous alumina with a pore diameter of 180 nm and an interpore distance of 500 nm. (c) Scanning electron micrograph of porous alumina with 300 nm interpore distance fabricated by using the master stamp with 500 nm lattice constant.
Combination of nanoimprint lithography and self-assembly of porous alumina allows the preparation of defect-free porous alumina arrays, which have a hexagonal pore arrangement (Fig. 3b). Moreover, we demonstrated that monodomain alumina pore arrays with an interpore distance smaller than the lattice constant of the master stamp can be synthesized if the anodizing conditions, in particular the applied potential, are well adjusted (Fig. 3c).
Interference lithography
Laser interference lithography (LIL) is a convenient technique for patterning regular arrays of fine features with considerably large area (typically wafer-scale), without the use of complex optical systems or masks.
In LIL, the intensity profile of the pattern formed by two monochromatic laser beams is recorded in a thin layer of photoresist on a substrate, where the period of the pattern is defined by the wavelength (λ) of the beam and the angle (θ) between the two interfering waves. The minimum period achievable is half of the wavelength (i.e., λ/2).
Figs. 5a and 5b show representative patterns generated by LIL. One can clearly see that the shape and the size of the recorded patterns are variable.