488
13 Preparation of Nanoporous Oxides from Metals by Electrochemical …
13.4.1 Patterning Methods for Achieving Regular Pore
Formation on Aluminium
Since the first successful production of highly ordered PAA preparation in 1995, the
PAA template has been considered as suitable host material for individual magnetic
entities (nanodots or nanowires), which might establish a new technology for the
production of magnetic recording media. However, the polycrystalline nature of
the pore formation remained a substantial drawback, even though the multiple-step
anodization helped some extent to improve the pore regularity. This problem can be
solved by applying a pre-patterning method prior to the anodization. First, it was
carried out by applying a SiC mould patterned by electron beam lithography, hence
achieving a fully regular area of 3 mm × 3 mm with a hexagonal pattern [65]. The
mechanical impact of the mould creates an initiation spot with slightly thinned oxide
layer at the desired position where the pore formation starts during the anodization
(Fig. 13.4a). As a variation of this indentation-initiated pore formation process, a
layer of self-organized array of silica beads on glass also proved to be suitable
as pressing mould [66]. Instead of applying a mechanical tool for pre-patterning,
direct laser writing combined with chemical etching led to similar high pore ordering
levels [67]. These processes all exhibit the advantage that a fixed interpore distance
can be achieved which may also differ to a limited extent from that obtained with
spontaneous pore formation.
Some attempts have been reported on the trials of overwriting the natural hexagonal arrangement of the pore arrays by applying Nanoimprint lithography using a
mould with square-shaped unit cells [68, 69]. Although the pore initiation in accord
with the square-shaped pattern was successful, the crossing points of the diagonals
always behaved as defect sites even if the natural interpore distance of the anodization matched the period of the imprinted pattern. Since the diagonal distance between
the pores in the square pattern is larger than the natural oxide layer thickness, either
unoxidized aluminium remained at the defect sites or secondary pore formation took
place. Similar structure breaks and non-periodical porous layers were obtained with
a strong mismatch of the imprinted pattern and the spontaneous periodicity of the
PAA formation [27]. A duplicated nanoindentation patterning with rotated mould
positions leads to Moiré pattern in the PAA samples [70].
The above-mentioned methods are common in that the aluminium to be anodized
was patterned from “upward”, e.g., from the electrolyte side. The sample patterning
from “downward” is also possible if a hexagonal array of insulating bumps is
deposited onto a conductive substrate which is then covered with a sputtered
aluminium layer (see Fig. 13.4b) [71]. The electrically insulating nanobumps serve
as field guides, and the spots above the non-covered area will behave as active pore
initiation sites during the anodization.
13 Preparation of Nanoporous Oxides from Metals by Electrochemical …
13.4.1 Patterning Methods for Achieving Regular Pore
Formation on Aluminium
Since the first successful production of highly ordered PAA preparation in 1995, the
PAA template has been considered as suitable host material for individual magnetic
entities (nanodots or nanowires), which might establish a new technology for the
production of magnetic recording media. However, the polycrystalline nature of
the pore formation remained a substantial drawback, even though the multiple-step
anodization helped some extent to improve the pore regularity. This problem can be
solved by applying a pre-patterning method prior to the anodization. First, it was
carried out by applying a SiC mould patterned by electron beam lithography, hence
achieving a fully regular area of 3 mm × 3 mm with a hexagonal pattern [65]. The
mechanical impact of the mould creates an initiation spot with slightly thinned oxide
layer at the desired position where the pore formation starts during the anodization
(Fig. 13.4a). As a variation of this indentation-initiated pore formation process, a
layer of self-organized array of silica beads on glass also proved to be suitable
as pressing mould [66]. Instead of applying a mechanical tool for pre-patterning,
direct laser writing combined with chemical etching led to similar high pore ordering
levels [67]. These processes all exhibit the advantage that a fixed interpore distance
can be achieved which may also differ to a limited extent from that obtained with
spontaneous pore formation.
Some attempts have been reported on the trials of overwriting the natural hexagonal arrangement of the pore arrays by applying Nanoimprint lithography using a
mould with square-shaped unit cells [68, 69]. Although the pore initiation in accord
with the square-shaped pattern was successful, the crossing points of the diagonals
always behaved as defect sites even if the natural interpore distance of the anodization matched the period of the imprinted pattern. Since the diagonal distance between
the pores in the square pattern is larger than the natural oxide layer thickness, either
unoxidized aluminium remained at the defect sites or secondary pore formation took
place. Similar structure breaks and non-periodical porous layers were obtained with
a strong mismatch of the imprinted pattern and the spontaneous periodicity of the
PAA formation [27]. A duplicated nanoindentation patterning with rotated mould
positions leads to Moiré pattern in the PAA samples [70].
The above-mentioned methods are common in that the aluminium to be anodized
was patterned from “upward”, e.g., from the electrolyte side. The sample patterning
from “downward” is also possible if a hexagonal array of insulating bumps is
deposited onto a conductive substrate which is then covered with a sputtered
aluminium layer (see Fig. 13.4b) [71]. The electrically insulating nanobumps serve
as field guides, and the spots above the non-covered area will behave as active pore
initiation sites during the anodization.