13.4 Patterning Methods in the PAA Preparation Process
489
13.4.2 Anodized Porous Aluminium as a Pattern Transfer
Tool
The more or less regular structure of the PAA membranes makes them suitable
for applications as non-lithographic templates [72–75]. They can be used in nonelectrochemical processes such as chemical or reactive ion etching and electron
beam evaporation. The PAA template can be formed either in situ at the surface
of the specimen to be patterned or produced ex situ and then applied as a selfsupporting mask. Below, the attention will be paid, however, to the processes in
which the masking functionality of the PAA structure is coupled with the anodization
of the underlying material on which the PAA was produced. In such cases, a direct
contact between Al and the substrate must prevail, which can be achieved with the
coating of the substrate with Al by a physical deposition method. The composition
of the interlayer between the substrate and Al may substantially impact the process
[76–78]. The advantage of the direct anodization of the substrate through the PAA
structure is that the periodicity of the nanostructure being formed underneath is fully
determined by the PAA periodicity, totally independently of whether the substrate
material is capable of periodic structure formation or not.
When Al is sputtered (or evaporated) onto the surface of a conducting material
that is less reactive under the anodization conditions then aluminium, the termination
of the Al anodization results in a different barrier layer structure than that obtained
for bulk Al. Since the Al layer is fully consumed, the last stage of the anodization
proceeds on the residue of Al just under the pore walls when the barrier layer under
the pores already reached the substrate material. This situation leads to a different
stress field than during steady-state anodization, and a nanocavity is formed just
under the pore-covering barrier layers (see the inset in Fig. 13.4c).
A peculiar characteristics of the PAA-based electrochemical pattern transfer is
that the resulting pattern caused by the anodization of the substrate strongly depends
on the substrate material. When the substrate is etched without the formation of
a solid reaction product, cavities may form (concave pattern, as it was shown in
Fig. 13.4c). For a pattern formation, the modulation of the anodization voltage upon
achieving the substrate may be necessary. Such nanocavity structure can be produced
on Si when the substrate is etched [79, 80]. However, when the anodization of the
substrate leads to the formation of a solid (mostly oxide), the reaction takes place
with volume expansion. Hence, nanodots, nanopillars and even nanowires can grow
directly into the pores of the alumina (convex pattern). This happens during the
oxidation of various substrates: in TiO 2 formation from either TiN [81] or Ti [82–
85], Ta 2 O 5 formation from Ta [86, 87], Nb 2 O 5 formation from Nb [86, 88], ZrO 2
formation from Zr [89] and SiO 2 formation from Si (if etching does not take place).
489
13.4.2 Anodized Porous Aluminium as a Pattern Transfer
Tool
The more or less regular structure of the PAA membranes makes them suitable
for applications as non-lithographic templates [72–75]. They can be used in nonelectrochemical processes such as chemical or reactive ion etching and electron
beam evaporation. The PAA template can be formed either in situ at the surface
of the specimen to be patterned or produced ex situ and then applied as a selfsupporting mask. Below, the attention will be paid, however, to the processes in
which the masking functionality of the PAA structure is coupled with the anodization
of the underlying material on which the PAA was produced. In such cases, a direct
contact between Al and the substrate must prevail, which can be achieved with the
coating of the substrate with Al by a physical deposition method. The composition
of the interlayer between the substrate and Al may substantially impact the process
[76–78]. The advantage of the direct anodization of the substrate through the PAA
structure is that the periodicity of the nanostructure being formed underneath is fully
determined by the PAA periodicity, totally independently of whether the substrate
material is capable of periodic structure formation or not.
When Al is sputtered (or evaporated) onto the surface of a conducting material
that is less reactive under the anodization conditions then aluminium, the termination
of the Al anodization results in a different barrier layer structure than that obtained
for bulk Al. Since the Al layer is fully consumed, the last stage of the anodization
proceeds on the residue of Al just under the pore walls when the barrier layer under
the pores already reached the substrate material. This situation leads to a different
stress field than during steady-state anodization, and a nanocavity is formed just
under the pore-covering barrier layers (see the inset in Fig. 13.4c).
A peculiar characteristics of the PAA-based electrochemical pattern transfer is
that the resulting pattern caused by the anodization of the substrate strongly depends
on the substrate material. When the substrate is etched without the formation of
a solid reaction product, cavities may form (concave pattern, as it was shown in
Fig. 13.4c). For a pattern formation, the modulation of the anodization voltage upon
achieving the substrate may be necessary. Such nanocavity structure can be produced
on Si when the substrate is etched [79, 80]. However, when the anodization of the
substrate leads to the formation of a solid (mostly oxide), the reaction takes place
with volume expansion. Hence, nanodots, nanopillars and even nanowires can grow
directly into the pores of the alumina (convex pattern). This happens during the
oxidation of various substrates: in TiO 2 formation from either TiN [81] or Ti [82–
85], Ta 2 O 5 formation from Ta [86, 87], Nb 2 O 5 formation from Nb [86, 88], ZrO 2
formation from Zr [89] and SiO 2 formation from Si (if etching does not take place).