Highly ordered nanochannel-array architecture in anodic alumina

Article abstract of DOI:10.1063/1.120128

The development of the ordered channel array in the anodic porous alumina was initiated by the textured pattern of the surface made by the molding process, and growth of an almost defect-free channel array can be achieved throughout the textured area. The long-range-ordered channel array with dimensions on the order of millimeters with a channel density of 10¹⁰ cm^-2 was obtained, and the aspect ratio was over 150. The master for molding could be used many times, which makes it posSible to overcome problems in the conventional nonolithographic technique, such as low through-put and high cost.

Full text of DOI:10.1063/1.120128

Highly ordered nanochannel-array architecture in anodic alumina
Hideki Masuda, Haruki Yamada, Masahiro Satoh, Hidetaka Asoh, Masashi Nakao et al.
Citation: Appl. Phys. Lett. 71, 2770 (1997); doi: 10.1063/1.120128
View online: http://dx.doi.org/10.1063/1.120128
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Highly ordered nanochannel-array architecture in anodic alumina
Hideki Masuda,a) Haruki Yamada, Masahiro Satoh, and Hidetaka Asoh
Department of Industrial Chemistry, Faculty of Engineering, Tokyo Metropolitan University,
1-1 Minamiosawa, Hachioji, Tokyo 192-03, Japan
Masashi Nakao and Toshiaki Tamamura
NTT Opto-electronics Laboratory, 3-1 Morinosato-Wakamiya, Atugi, Kanagwa 243-01, Japan
͑Received 14 July 1997; accepted for publication 12 September 1997͒
The development of the ordered channel array in the anodic porous alumina was initiated by the
textured pattern of the surface made by the molding process, and growth of an almost defect-free
channel array can be achieved throughout the textured area. The long-range-ordered channel array
with dimensions on the order of millimeters with a channel density of 1010 cmϪ2 was obtained, and
the aspect ratio was over 150. The master for molding could be used many times, which makes it
possible to overcome problems in the conventional nanolithographic technique, such as low
through-put and high cost. © 1997 American Institute of Physics. ͓S0003-6951͑97͒03145-8͔
Nanochannel-array materials, which have fine, uniform
channels of nanometer dimension, have stimulated consider-
able interest in recent years due to their utilization as a host
or template structure for nanometer devices, such as mag-
netic, electronic, and optoelectronic devices.1,2 The most
characteristic feature of these materials has been emphasized
with the extremely high aspect ratio of their channels ͑depth
divided by width͒ which is difficult to achieve with conven-
tional lithographic techniques. Anodic porous alumina, a
typical self-ordered nanochannel material formed by anod-
ization of Al in an appropriate acid solution, has recently
attracted increasing interest as a key material for the fabrica-
tion of nanometer-scale structures.3,4 The structure of anodic
porous alumina is described as a close-packed array of
columnar cells, each containing a central pore of which the
size and interval can be controlled by changing the forming
conditions.5 We reported previously that anodic porous alu-
mina with relatively long-range order can be obtained in ox-
alic acid6 and sulfuric acid7 under an appropriate anodization
condition, in which the pores are organized spontaneously
into close-packed hexagonal arrays. However, the defect-free
area of naturally occurring ordering appears in a domain
structure and is limited to the size of several micrometers.
Here, we propose a novel approach for precise control of
the growth of the channel array in the anodic porous alu-
mina, which enables us to fabricate the long-range-ordered
channel-array architecture on the millimeter scale. Our ap-
proach was motivated by the idea that if the development of
the hole is guided by the appropriate texture of the surface at
the initial stage of anodization, and an appropriate condition
is maintained for the self-ordering, the long-range-ordered
channel-array architecture can be expected to grow sponta-
neously. This process has two points of interest:
introduced for texturing the surface to offer the high
through-put mass production which can overcome the
bottle neck in the conventional nanolithographic process.
The molding process is the simplest way to produce a
textured surface but is rarely applicable in the nanometer
scale. Very recently, a process for the fabrication of micro-
structures of polymer material was reported.9 However, to
our knowledge, the application to a metal substrate has not
been reported in nanometer dimensions. Introduction of the
molding process is favorable for texturing the Al surface,
because Al has sufficient plasticity to be deformed by me-
chanical molding. In contrast to the orientation-dependent
anisotropic wet etching of Si,10 the anodizing process of Al
is independent of the restriction of the orientation of the
crystal, because the main driving force in the formation of
the channel in the anodic alumina is the electric field rather
than the crystal direction. This allows the fabrication of the
initiation point by a mechanical deformation process and
continuous growth of the ordered channels without being
affected by crystal orientation.
The fabrication procedure of the ordered channel-array
structure is schematically shown in Fig. 1. An Al sheet
͑99.99% purity͒ was annealed at 400 °C for 1 h to facilitate
deformation in the molding process, and was polished elec-
trochemically. The master which has a hexagonally arranged
array of convexes was fabricated with the use of conven-
tional electron beam ͑EB͒ lithography. The substrate for the
master requires enough mechanical strength and suitably for
EB lithography on the nanometer scale. We examined sev-
eral kinds of substrates and found that the SiC single-crystal
wafer satisfies these requirements. The size of the SiC master
was typically 4ϫ5 mm, on which the array of convexes was
fabricated in the area of 3ϫ0.6 mm. The largest SiC mold
fabricated was 3 mmϫ3 mm in textured area.
͑1͒ shallow ordered textures ͑array of concaves͒ which can
be easily prepared with a standard lithographic process
can introduce the development of pores8 and can guide
the growth of channels with an extremely high aspect
ratio, and
The master was placed on an Al sheet and pressing was
carried out using an oil press at room temperature. This pro-
cess generated the array of concaves on the surface of Al,
which was the replicated negative of the convexes of the
master. The appropriate pressure, at which the pattern was
fully transferred to the Al surface, was approximately 5
͑2͒ a molding process using an appropriate master has been
a͒
Electronic mail: masuda-hideki@c.metro-u.ac.jp
2770
Appl. Phys. Lett. 71 (19), 10 November 1997
0003-6951/97/71(19)/2770/3/$10.00
© 1997 American Institute of Physics
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FIG. 1. Process for the ordered channel array; SiC mold with hexagonally
ordered array of convexes ͑a͒, molding on the Al ͑b͒, textured Al ͑c͒, anod-
ization and growth of channel architecture ͑d͒, removal of Al and barrier
layer ͑e͒.
FIG. 3. Cross-sectional view of channel array of anodic alumina; pretextur-
ing and anodization conditions are the same as in Fig. 2. Pore widening was
carried out in 5 wt % phosphoric acid at 30 °C for 60 min. The SEM obser-
vation was carried out without removal of Al and barrier layer of the oxide
film.
ton cmϪ2. Under that pressure, the master could be detached
from the patterned Al and reused.
The anodization of Al was conducted in an oxalic acid
solution under the constant-voltage condition. To clarify the
cell configuration, the specimen was dipped in a 5 wt %
phospholic acid solution at 30 °C for pore-widening treat-
ment after the anodization. For the observation of the cell
configuration from the bottom part of the oxide film, called
the barrier layer, the Al layer was removed in a saturated
HgCl2 solution and then the barrier layer was subsequently
removed by etching treatment in a phosphoric acid solution.
Figure 2 illustrates the surface view of the anodic porous
alumina by a scanning electron microscope ͑SEM͒, half of
which ͑right-hand side͒ was pretextured by molding. Chan-
nels arranged in a two-dimensional hexagonal configuration
were observed only in the pretextured area, while the ar-
rangement of the channels was disordered in the untreated
area. The interval between channels, in other word, the size
of the cells, corresponded to that of the pretextured pattern
͑150 nm in the case of the array shown in Fig. 2͒. The result
shown in Fig. 2 confirms that the predetermined pattern
formed by mechanical deformation can act as initiation
points and guide the growth of channels in the oxide film.
The cross-sectional view in Fig. 3 demonstrates the
high-aspect-ratio appearance of the channel in the anodic
alumina. From this photograph, the growth of straight paral-
lel channels perpendicular to the substrate can be confirmed.
The most characteristic feature of the present method is that
shallow predetermined concaves can initiate the pores and
guide the growth of the long channel ͑16 ␮m in the case of
Fig. 3͒. The depth of each concave formed by molding was
ϳ20 nm, as determined by the profile analysis using an
atomic force microscope. This shallow pattern generated the
long-channel architecture with an almost ideal configuration
over 15 ␮m in depth ͑aspect ratio of as much 150͒. The cell
size of the anodic porous alumina is proportional to the
forming voltage5 ͑2.5 nm for a specific applied voltage͒. If
the anodization condition did not satisfy the requirement dic-
tated by the relationship between the cell size and forming
voltage, ordered growth could not be maintained.
The ordered configuration of the channel can be con-
firmed from the view of the barrier layer of the film ͑Fig. 4͒.
The almost defect-free configuration in the area shown in
Fig. 4 verifies that all channels grew straight, parallel to each
other. When the whole specimen is viewed, some defects
could be observed in the cell configuration at the present
stage. The development of defects was affected by the sur-
face condition of the Al after molding. Defects in the texture,
which were due to imperfections in the pattern of the master
or dust at the surface, may generate the defects in the cell
configuration. The ratio of the defects was also dependent on
the depth of the channel in the oxide film. The number of the
defects tended to increase as the film grew.
Figure 5 summarizes the obtained channel-array archi-
tectures of the anodic alumina with different intervals. These
architectures were formed using the predetermined pattern of
concaves with periodicity identical to that of formed cells,
FIG. 2. SEM micrograph of surface view of anodized Al; pretexturing in-
terval was 150 nm. Anodization was conducted in 0.3 M oxalic acid of
17 °C at 60 V for 36 min. Pore widening was carried out in 5 wt % phos-
phoric acid at 30 °C for 70 min.
Appl. Phys. Lett., Vol. 71, No. 19, 10 November 1997
Masuda et al.
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tion in the oxalic acid solution, the most appropriate voltage
for the ordering was ϳ40 V; the cell configurations were less
ordered above 60 V. However, with the aid of surface tex-
turing, no distinct difference in the degree of ordering of the
cell configuration could be observed within the range of an-
odizing voltage from 40 to 80 V used in the present experi-
ment, in so far as the anodization condition obeyed the rela-
tionship ͑2.5 nm/V͒ described above. It is thought that this
difference in the voltage dependence of the ordering between
these two growth modes, that is—the growth with and with-
out pretexturing, originates in the difference in the readiness
of the ordering of cell configuration. The naturally occurring
ordering is presumably strongly dependent on the applied
voltage compared to the growth using pretexturing, because
the rearrangement of the cell configuration through growth
must take place, in contrast to the ordering with texturing in
which the pore can develop orderly in the initial stage of
anodization.
FIG. 4. SEM micrograph of cell configuration in barrier layer; anodization
condition was the same as that for Fig. 2. Al was removed in saturated
HgCl2 solution after pore-widening treatment in 5 wt % phosphoric acid at
30 °C for 30 min, and the barrier layer was removed by dipping the sample
in the phosphoric acid solution for 75 min.
Our experiment shows that the present process will be
suitable for the mass production of restrictedly ordered
nanochannel array architectures with a high aspect ratio. The
nanochannel array with long-range order will be a powerful
tool in the development of nanodevices. For example, long-
range periodicity is promising for the fabrication of huge
data storage systems in which the long-range ordering is es-
sential for tracking in read and write operations.
The ordered channel array obtained with the present pro-
cess can produce the highly ordered channel array in a vari-
ety of materials, such as metals and semiconductors using a
two-step replication process.8,11–14 Air-hole array architec-
tures replicated with high-refractive-index materials will be a
candidate for realizing the photonic band structure in the
visible wavelength region.
with subsequent anodization at the corresponding voltages.
In the smallest architecture formed at 40 V, the channel di-
ameter was 70 nm, and the interval was 100 nm, which cor-
responds to the packing density of 1010/cm2.
In the previous studies of naturally occurring ordering of
the cell configuration without the assistance of the predeter-
mined texture, the degree of self-ordering was strongly de-
pendent on the applied voltage which was determined by the
solution used for the anodization.6,7 In the case of anodiza-
This work was partially supported by a Grant-in-Aid on
Priority Area ͑No. 225͒ from the Ministry of Education, Sci-
ence, Sports, and Culture, Japan.
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Masuda et al.
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