13.5 Pore Shaping with Modulated Anodization Conditions
491
13.5.2 Monotonous Pore Diameter Variation
Since the barrier oxide layer cannot be chosen arbitrarily for a particular interpore
distance, one cannot grow funnel-like nanopores by simply decreasing the anodization voltage. Funnel-shaped nanopores can be produced with a periodic manufacturing technique involving two consecutive steps: pore growth with anodization and
pore widening with chemical etching [96, 97]. The pore growth takes place with
identical voltage in each cycle. Hence, the interpore distance and the barrier layer
thickness correspond to the ideal self-ordering conditions in every anodization step
and no pore branching takes place. The chemical etching steps between the anodization periods is carried out with another suitable solution and impacts the section of
the pores already produced; this is why the pore diameter will monotonously increase
from the barrier layer to the top of the sample. With a careful design of the lengths
of the anodization and chemical etching sections, nearly arbitrary pore shape with
monotonous pore diameter variation can be designed.
13.5.3 Periodic Pore Diameter Modulation
The prerequisite of the periodic pore diameter modulation carried out solely by the
variation of the anodization voltage is that no pore branching/merging takes place
at the “unfavourable” anodization conditions, but the system can relax the stress
occurring due to the voltage–interpore distance mismatch. The above criterion can be
fulfilled by various means. First, if the cell voltage remains in the anodization regime
of pore formation but changes to a limited extent only [98, 99]. When the voltage
remains in the same regime, rectangular, sawtooth-like and even more complicated
voltage modulations wave forms are possible. Another possibility is that the voltage
applied is changed stepwise between the mild and hard anodization regimes (pulse
anodization) [100–102]. Obviously, the continuous voltage change is not feasible
here since the two voltage domains are separated by a burning region. In such structures, the sections of the normal 10% porosity (mild anodization) alternate with a
section of different porosity. The porosity of the hard anodized section may be either
higher or lower if the solution contains oxalic or sulphuric acid, respectively.
Hard anodization yields other opportunities for the pore diameter modulation.
Since this anodization mode exhibits a regime with spontaneous current density
oscillation, this can be used for modulating the pore diameter [103]. Another means
is the temperature modulation by periodically chilling the back side of the sample
being anodized [104].
Finally, we mention that laterally interconnected, longitudinally periodical pore
arrays can be produced in various ways. Sidewise connecting channels can be
produced between pores of modulated diameters followed by a chemical etching
adjusted to the length so that the narrower pore wall sections can be penetrated while
the wider wall sections remain continuous [105]. However, lateral (or transversal)
491
13.5.2 Monotonous Pore Diameter Variation
Since the barrier oxide layer cannot be chosen arbitrarily for a particular interpore
distance, one cannot grow funnel-like nanopores by simply decreasing the anodization voltage. Funnel-shaped nanopores can be produced with a periodic manufacturing technique involving two consecutive steps: pore growth with anodization and
pore widening with chemical etching [96, 97]. The pore growth takes place with
identical voltage in each cycle. Hence, the interpore distance and the barrier layer
thickness correspond to the ideal self-ordering conditions in every anodization step
and no pore branching takes place. The chemical etching steps between the anodization periods is carried out with another suitable solution and impacts the section of
the pores already produced; this is why the pore diameter will monotonously increase
from the barrier layer to the top of the sample. With a careful design of the lengths
of the anodization and chemical etching sections, nearly arbitrary pore shape with
monotonous pore diameter variation can be designed.
13.5.3 Periodic Pore Diameter Modulation
The prerequisite of the periodic pore diameter modulation carried out solely by the
variation of the anodization voltage is that no pore branching/merging takes place
at the “unfavourable” anodization conditions, but the system can relax the stress
occurring due to the voltage–interpore distance mismatch. The above criterion can be
fulfilled by various means. First, if the cell voltage remains in the anodization regime
of pore formation but changes to a limited extent only [98, 99]. When the voltage
remains in the same regime, rectangular, sawtooth-like and even more complicated
voltage modulations wave forms are possible. Another possibility is that the voltage
applied is changed stepwise between the mild and hard anodization regimes (pulse
anodization) [100–102]. Obviously, the continuous voltage change is not feasible
here since the two voltage domains are separated by a burning region. In such structures, the sections of the normal 10% porosity (mild anodization) alternate with a
section of different porosity. The porosity of the hard anodized section may be either
higher or lower if the solution contains oxalic or sulphuric acid, respectively.
Hard anodization yields other opportunities for the pore diameter modulation.
Since this anodization mode exhibits a regime with spontaneous current density
oscillation, this can be used for modulating the pore diameter [103]. Another means
is the temperature modulation by periodically chilling the back side of the sample
being anodized [104].
Finally, we mention that laterally interconnected, longitudinally periodical pore
arrays can be produced in various ways. Sidewise connecting channels can be
produced between pores of modulated diameters followed by a chemical etching
adjusted to the length so that the narrower pore wall sections can be penetrated while
the wider wall sections remain continuous [105]. However, lateral (or transversal)