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Tailoring morphology in free-standing anodic aluminium oxide: Control of barrier layer opening down to the sub-10 nm diameter†
Jie Gong,‡a William H. Butlera and Giovanni Zangari*b
Received (in Cambridge, MA, USA) 26th January 2010, Accepted 28th January 2010
First published as an Advance Article on the web 18th March 2010
DOI: 10.1039/c0nr00055h
Free-standing, highly ordered porous aluminium oxide templates were fabricated by three-step
anodization in oxalic, sulfuric or phosphoric acid solutions, followed by dissolution of the aluminium
substrate in HgCl2. Opening of the pore bottoms on the barrier layer side of these templates was carried
out by using chemical or ion beam etching. Chemical etching is capable of achieving full pore opening,
but partial pore opening occurs inhomogeneously. On the contrary, ion beam etching enables
homogeneous and reproducible partial pore opening, with the pore size controlled through the etching
time. By this method, pore openings as small as 5 nm can reliably be obtained.
1. Introduction
Nanoporous anodic aluminium oxide (AAO) is finding wideapplication as a template for the fabrication of one-dimensional
nanostructures1 or two-dimensional patterns,2–6 as a photonic
crystal,7 in nanofluidics8 and filtration,9 in magnetic information
storage,10 sensing,11 and even in biomedical applications such as
drug delivery.12 AAO is formed by the electrochemical oxidation
of aluminium (Al) in an appropriate acidic electrolyte, and
consists of an array of parallel nanochannels perpendicular to the
original Al surface, closed at the bottom by a hemispherical oxide
film known as the barrier layer.13–15
AAO has been known for more than 50 years,16 but only after
1995, when Masuda and Fukuda reported the synthesis of
a highly ordered pore array by two-step anodization in oxalic
acid,2
its potential in nanofabrication was appreciated andrelated research efforts flourished. The self-organized pore
growth process, leading to a densely packed hexagonal pore
structure, is controlled through the electrolyte chemistry, applied
voltage, and processing time. Self-organized growth has been
achieved in oxalic,2,17–19 sulfuric,17–20 and phosphoric
acids,18,19,21,22 and mixed oxalic and sulfuric acid solutions.23
Interpore distance and pore diameter can be varied between
50–500 nm and 5–300 nm, respectively, depending on the elec-
trolyte and anodization voltage employed;18,24 in addition, the
latter can be widened by chemical etching in diluted acids. Long
range ordering of the porous structure can be achieved using
a two or three-step anodization process2,25 or by pre-texturing
the initial aluminium surface via nanoimprint,26,27
nano-indentation,28 or ion beam prepatterning.29
Many applications of AAO in nanofabrication require
opening of the pore bottom; this has been achieved by chemical
etching in acidic solutions,30 ion beam31 or plasma etching.3
Complete removal of the pore bottom results in an opening with
the same size as the channel; however, in some cases, close
control over the diameter of the opening, below the channel
diameter, may be required. This is particularly important, for
example, in the fabrication of nanoconstrictions for magnetic
point contacts32 or for the control of magnetic domain wall
motion33 in spintronic applications, as well as in the synthesis of
molecular membranes.34,35
Partial pore opening of oxalic acid anodized AAO by wet
chemical etching has been recently reported;30 however, no
detailed comparison of this process with ion beam etching has
been carried out. Furthermore, although ion beam etching
methods have demonstrated the uniform engraving of holes onthe hemispherical shaped surface of the barrier layer with
diameter as small as 10 nm using oxalic acid AAO,31 no such
process has been reported for phosphoric or sulfuric acid AAO,
which could produce larger or smaller nanoholes, respectively,
due to the different size of the porous structures. In this work,
free-standing sulfuric, oxalic and phosphoric acid AAO have
been synthesized by a three-step anodization process, and the
effects of chemical and ion beam etching on the morphology of
sulfuric, oxalic and phosphoric acid AAOs with different pore
sizes have been compared. It is demonstrated that uniformly
engraved holes with diameter down to 5 nm can be reliably
obtained using ion beam etching of sulfuric acid AAO. Fig. 1
depicts a schematic diagram of the barrier opening processes
discussed in this paper, and provides a qualitative illustration of
the resulting AAO nanostructures.
2. Experimental
Chemicals
All chemicals used in this work were of analytical grade. All
solutions were prepared, and rinsing as well as cleaning were
performed, using Milli-Q water (resistivity 18.2 MU cm).
aCenter for Materials for Information Technology, University of Alabama,Tuscaloosa, AL, 35487-0209bDepartment of Materials Science and Engineering and CESE, Universityof Virginia, 395 McCormick Rd., P.O. Box 400745, Charlottesville, VA, 22904-4745, USA. E-mail: gz3e@virginia.edu; Tel: +1-434-243-5474
† Electronic supplementary information (ESI) available: Currenttransients during anodization, SEM and AFM images of aluminiumoxide anodized in sulfuric acid and phosphoric acid. See DOI:10.1039/c0nr00055h
‡ Now at Seagate Technology, Research and Technology Development,7801 Computer Avenue South, Bloomington MN 55435
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Aluminium sheet pretreatment
A high purity polycrystalline aluminium (Al) sheet sample
(99.998%, Alfa Aesar) was first degreased in a 5% NaOH solu-
tion at 60 C for 30 s, rinsed, and then neutralized in 1 : 1 HNO3
for 5–10 s. After thorough rinsing in water, the substrate was
electropolished (Buehler Electropolisher-4, 32 V, 45–60 s) in
perchloric acid–ethanol electrolyte (165 mL 65% HClO4, 700 mL
ethanol, 100 mL 2-butoxyethanol ‘‘butylcellusove’’, and 137 mL
H2O). The sheet was then washed, first in warm and then in cold
water. Atomic Force Microscopy AFM was utilized to charac-
terize the topography and roughness of the polished surface.
Aluminium anodization
The pretreated Al sheet, with a polished active area of 3 cm2, was
used as the anode during anodization. A degreased and
neutralized 98.5% pure Al sheet with an area ten times larger
(36 cm2) than the anode was used as the cathode. Anodization
was performed under potentiostatic control, and the current
transient during anodization was recorded by monitoring the
voltage drop across a standard resistor (R ¼ 10 Ohm) connected
in series. The temperature of the electrolyte was maintained at
specific values using a jacketed beaker and a cooling system
(Fisher Scientific, Model ISOTEMP 1016D Circulator). Thesolution was stirred vigorously using a magnetic stirrer in order
to accelerate dispersion of the heat generated by the sample
during anodization. Anodization was performed in one of three
electrolyte systems, i.e. sulfuric (0.3 M, 3 C), oxalic (0.3 M,
15 C) or phosphoric acid (1 M, 3 C), to produce AAO films
with different geometries (different interpore distances and pore
diameters).
Highly ordered AAO templates were prepared using a multi-
step anodization process according to the following scheme:25
1. A polished Al sheet was anodized for 5–15 min to
morphologically ‘‘texture’’ the Al surface.36
2. The resulting oxide film was dissolved in a solution
containing 0.2 M H2CrO4 and 0.4 M H3PO4 at 60–70 C for
5–20 min.
3. The substrate was re-anodized for more than 12 h to create
long-range ordering.
4. Step 2 was repeated, with a duration of more than 30 min to
dissolve the thick oxide film formed in step 3.
5. The substrate was finally anodized for a varying period of
time, depending on the thickness of the AAO film desired.
Post-treatment and template fabrication
To completely dissolve the metallic Al substrate without etching
the AAO membrane, the highly ordered, supported AAO film
was carefully floated on the Al side on a saturated mercuric
chloride (HgCl2) solution for 4–6 h (Fig. 1). Caution! HgCl 2 is
harmful if swallowed, inhaled or absorbed through skin. It should
be handled with extreme care using goggles, lab coat and proper
gloves. The AAO film was gently rinsed and dried in vacuum
over a desiccator (P2O5). The barrier layer was then con-
trollably removed by chemical etching (CE) in 5% H3PO4
solution at room temperature (25 C) or by ion beam etching
(IBE).
A Veeco Microetch RF-1201 Ion Beam Etching System was
employed for IBE. An inductively-coupled RF ion source was
used, together with a Plasma Bridge Neutralizer (PBN) to keep
the ion beam collimated. The ion beam etching chamber was first
pumped down to 7–8 Â 10À7 torr, then Ar gas flowed into the
chamber at 20 sccm (standard cubic centimetre per minutes),
keeping the chamber pressure constant at 1.32–1.34 Â 10À4 torr.
During etching, a 450 V beam voltage and 450 mA beam current
were employed. Suppressor voltage was 300 V, and incident RF
power was 300 W. PBN Ar gas flow rate was 3 sccm, and the
ratio of neutralizer current/beam current (K factor) was 1.1. Theion beam incidence angle was 45; the sample fixture was rotated
at 0.1 revolutions per second and water cooled. The etching rate,
calibrated using a Si wafer, was 40 nm minÀ1. When converted to
Al2O3 using a standard milling rate table, the etching rate was
about 9 nm minÀ1.
Characterization
Scanning electron microscopy (SEM, Philips model XL 30) and
atomic force microscopy (AFM, Digital Instruments, Dimen-
sion 3000) were employed to characterize the surface
morphology and topography, respectively. The analyses usingAFM were performed in tapping mode in air and the scan rate
was kept at 1 Hz or lower. Mikromasch ultrasharp silicon
cantilevers, with radius of curvature <10 nm, tip height of
15–20 mm, full tip cone angle <20, were used for AFM char-
acterization. The regularity of the hexagonally patterned
nanohole arrays was assessed quantitatively by fast Fourier
transformation (FFT) of the AFM images, using the proprie-
tary software (Nanoscope V) available with the AFM. The
barrier side constriction size distribution after ion beam etching
was evaluated by direct measurement of more than 50 openings
in three AFM images taken at different locations.
Fig. 1 Schematic view of the f abrication process of free-standing porous
AAO with the barrier layer perforated by wet chemical or ion beam
etching. (a) AAO after anodization; (b) chemical post treatment to
remove the metallic Al support; (c) free-standing AAO before etching; (d)
uniform controllable openings on the barrier side achieved by ion beam
etching; and (e) non-uniform openings on the barrier side obtained by wet
chemical etching. d 1: constriction/opening size; tb: barrier layer thickness;
d p: pore diameter.
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3. Results and discussion
3.1. Anodization process and sample characterization
The purpose of the surface pretreatment is to clean and eliminate
the main surface defects, while minimizing internal stresses
present at the surface. The AFM surface topography after
pretreatment is shown in Fig. S1 of the ESI.† The surface appears
smoother and glossier than before electropolishing. The root-
mean-square (RMS) roughness as determined by AFM is 1.2 nm
over a 4 Â 4 mm2 area.
Anodization was carried out under potentiostatic conditions
in H2SO4, oxalic acid ((COOH)2) or H3PO4 solutions. Voltages
were chosen in the range of values yielding long range order.18
Typical current density–time (I – t) curves in H2SO4 for the first
and second anodization are shown in Fig. 2; the same plots for
oxalic and phosphoric acid solutions are reported in Fig. S2 of
the ESI.† All curves show four stages of pore formation (Fig. 2,
1st anodization). In stage I, current drops abruptly due to the
formation of the planar barrier oxide layer. In stage II, current
gradually increases and after different time intervals (dependent
on the solution and voltage applied) reaches its maximum
between stages II and III. The increase in current is characteristic
for porous AAO formation, indicating that field-enhanced
dissolution starts to concentrate at locally convex surface
regions, nucleating the pores.25 In stage III, the current decreases
slightly to a constant value, indicating that the first initiated
disordered pores are becoming hexagonally ordered, and
stationary pore growth has been attained (stage IV).25
Comparison of the 1st anodization on the electropolished flat
surface and the 2nd anodization on the ‘‘textured’’ surface in the
three electrolyte systems shows that the pore nucleation process
(stage II) is greatly shortened on the ‘‘textured’’ surface. In
addition, the lowest current on the flat surface is always much
lower than on the ‘‘textured’’ surface. This implies that ‘‘pre-
texturing’’ the surface accelerates the ordered pore initiation
process. For both the 1st and 2nd anodization, the duration of
regime II follows the trend H3PO4 (185 V) > (COOH)2 (40 V) >
H2SO4 (26 V), which corresponds to the difficulty of achieving
ordering and the barrier layer thickness.
The fabrication process and porous structure of AAO were
studied in detail in H2SO4, (COOH)2 and H3PO4 solutions. The
process parameters and extracted geometrical characteristics are
summarized in Table 1. Fig. 3(a) and (b) compare the topog-
raphy, as determined by AFM, of the AAO surface after the 2nd
and 3rd anodization in oxalic acid, respectively. Fast Fourier
transform (FFT) patterns of the images, giving information
about the structural periodicity in the inverse (momentum) space
are shown in the corresponding insets. Both the AFM topog-
raphy and the FFT patterns show that the number of defects
decreases and the pore ordering improves after the 3rd anod-
ization step. The diffraction ring and its broadening observed inthe FFT of Fig. 3(a) indicate that 2D ordering of the structure is
imperfect and that the interpore distance varies across the area of
the sample. The barrier side topographic image (Fig. 3(c)),
obtained by separation of the AAO membrane after the 3rd
anodization step in saturated HgCl2, shows highly ordered,
hexagonal patterns of hemispherical domes. The FFT images of
the front and barrier side of AAO surfaces anodized three times
consist of six equidistant and distinct spots, indicative of
a perfectly ordered hexagonal pore lattice. Additional charac-
terization and images of the AAO obtained in oxalic, sulfuric and
phosphoric acid are available in the ESI.†
Similar to anodization on pre-textured Al substrates either by
lithography22 or by nanoimprinting/nanoindentation,26–28 a porearray pattern with double periodicity can, in principle, be
obtained by first anodizing in oxalic acid, then in sulfuric acid.
The resulting morphology of the pore opening and barrier layer
are shown in Fig. 4. Obviously, the pore-to-pore distance in the
AAO obtained by anodization at a different voltage (25–27 V) in
sulfuric acid does not match the pitch of the pattern obtained in
oxalic acid at 40 V. Therefore, the pores growing in the second
stage will not follow the pre-patterned pits and a distorted pore
arrangement will occur. The anodic pores produced in H2SO4
have a much higher density than the pre-existing pits, and are
mostly located along the pattern ridges, with each pattern cell
containing nearly the same number of pores. It is also interesting
to note that, if the anodization sequence is reversed, i.e. anod-izing in sulfuric acid followed by oxalic acid, the final pattern
shows the same features as after anodization in oxalic acid alone.
3.2. Barrier layer etching processes
The barrier layers present at the bottom of the pores of the AAO
channels were opened by using one of two methods: wet chemical
etching or ion beam etching.
3.2.1. Wet chemical etching (CE). The pore bottoms were
opened by placing the AAO film on the surface of a 5% H3PO4
Fig. 2 Typical anodization curves (current density vs. time, I – t) for two-
step aluminium anodization in a sulfuric acid electrolyte (0.3 M H2SO4,
26 V, 3 C).
Table 1 Process parameters and extracted geometric features fora typical three-step aluminium anodization in different electrolytes
Process stepsH2SO4
(26 V, 3 C)(COOH)2(40 V, 15 C)
H3PO4
(185 V, 3 C)
1st anod. time/min 5 5 15–201st oxide strip time/min 10–15 10–15 >302nd anod. time/h >12 >12 16–202nd oxide strip time/min 30–45 30–45 120Growth rate/mm hÀ1 3.5 6 7.5Interpore distance/nm 60 108 450 Æ 50Pore density/cmÀ2
$3 Â 1010$1 Â 1010
$5 Â 108
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solution, with the open side of the pores facing up, and the
closed barrier layer side facing down and contacting the solu-
tion. Oxalic acid AAO films were floated on the etching solution
for 25–40 min at room temperature (25 C); the SEM
morphologies at different stages are shown in Fig. 5. CE can
conveniently etch the barrier layer and fully expose the opened
channels (Fig. 5(d)). However, it is difficult to controllably
achieve across the entire surface partial opening of the holes on
the barrier side, which would be essential when using this
template to generate nano-constrictions of controlled diameter.
In the intermediate stages of CE in fact (Fig. 5(b) and (c)),
opened pores were distributed non-uniformly and the openings’
diameters varied over a wide range; under these conditions,
a plot of the opening size vs. time would exhibit a large scat-tering and would provide limited information.
As a comparison, the results of CE after various etching times
for H2SO4-anodized AAO (Fig. 6) and H3PO4-anodized AAO
(Fig. 7) are also shown. As expected, due to the different barrier
layer thickness in the three cases, the times necessary to fully
open the barrier caps are quite different. For H2SO4-anodized
AAO, 20–25 min of etching is sufficient (Fig. 6(d)); H3PO4-
anodized AAO on the other hand needs 240 min to fully open the
pores (Fig. 7(b)). Again, CE can conveniently open the barrier
layer to a full extent, but it does not allow accurate control of the
size and location of the openings. As shown by AFM topography
in Fig. 6(a1–d1), CE generates a rough and irregular barrier side,
resulting in inhomogeneous etching, and thus hinderingprospective integration with other components. It is also inter-
esting to note that, if the H3PO4-anodized AAO is over-etched,
the walls of the AAO channels are etched at a much slower rate
than the backbone, probably due to preferential anion incorpo-
ration at walls,15 or to differences in chemical composition27,30 or
crystallinity15 between the inner and outer region. As a result,
alumina fibers can be produced, as also reported in ref. 37.
Inhomogeneous pore etching by CE may be explained in terms
of faster etching at defect sites (grain boundaries of the initial Al
surface or boundaries of the ordered pore domains) or at regions
with a thinner barrier layer. An initial, small difference in etching
Fig. 3 AFM surface topography of oxalic acid anodized AAO and the
fast Fourier transform (FFT) patterns: (a) after 2nd anodization; (b) after
3rd anodization; (c) highly ordered hemispherical domes on the barrier
side after dissolution of non-anodized aluminium.
Fig. 4 Double periodicity obtained by anodizing in oxalic acid (1st and
2nd step), followed by anodization in sulfuric acid (3rd step): (a) front
side and (b) barrier side AFM surface topography, (c) SEM large area
morphology.
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rate at separate locations may cause a local depletion of the
etchant and preferential diffusion of the etchant from the bulk
solutions to these regions, resulting in positive feedback. This
effect may explain the clustering of opened pores observed in
Fig. 6 (b2) (probably a domain boundary), or Fig. 6 (c2)
(probably a region with locally thinner barrier layer).
3.2.2. Ion beam etching (IBE). In order to controllably anduniformly engrave openings on the barrier side, and especially to
decrease the size of the constrictions down to less than 10 nm,
IBE was employed. In this instance, the AAO samples were
carefully placed on a water-cooled rotary sample holder with
steel clips, with the barrier layer side facing the ion source.
During etching, the Ar+ ion incidence angle was tilted 45 from
the surface normal and the sample fixture was rotated, to ensure
uniform etching.
The AFM topography of oxalic acid-anodized AAO barrier
side after different IBE processing times is shown in Fig. 8.
Uniform and round holes (diameter 15–30 nm) can be obtained
Fig. 5 SEM surface morphology of the barrier side of oxalic acid-
anodized AAO after different etching times in 5% H3PO4 at room
temperature (25 C): (a) 25 min; (b) 30 min; (c) 35 min; and (d) 40 min.
The inset in (d) shows an enlarged view of the pore opening.
Fig. 6 AFM (a1–d1) and SEM (a2–d2) images of the barrier side of
sulfuric acid-anodized AAO membranes after different etching times in
5% H3PO4 at room temperature (25 C): (a) 5 min; (b) 10 min; (c) 15 min;
and (d) 20 min. The insets in (b2), (c2) and (d2) show enlarged views of
the corresponding images.
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over a large area when the etching time is limited to 5–10 min
(Fig. 8(b) and (c)). Round hemispherical caps are uniformly
milled from the top due to the uniform flux distribution of the ion
beam. Chemical composition15 or stress38 gradients that may be
present in the barrier layer at the pore bottom are of no conse-
quence, since the energy differences involved due to these
gradients are much smaller than the ion beam energy. Long-time
etching leads to the disappearance of the protruding caps and
results in holes with shape and diameter similar to the nanopores
on the opposite side (Fig. 8(d)). This indicates that the
hemispherical barrier layer domes are completely etched away by
the ion beam.
Much smaller constrictions can be obtained by IBE from the
barrier layer side of H2SO4-anodized AAO in shorter times. This
is due to the fact that the pore size and the barrier thickness of
these AAO samples are smaller. The morphologies resulting
from the etching process are shown in Fig. 9 and 10. As shown in
Fig. 9, very uniform small openings over a large area can be
engraved on the barrier side. The opening size can be accuratelycontrolled down to about 5–8 nm by limiting the etching time to
2–4 min. Increasing etching time results in increased opening size
up to more than 18 nm, as well as a flattened surface topography
(Fig. 10).
IBE of H3PO4-anodized AAO produces larger openings,
ranging from 100 to 200 nm, as shown in Fig. 11. The etching
time necessary to open the hemispherical domes is much longer
now since the barrier layer is thicker in these samples. Although
every dome in the images can be opened, the shape and size of
the openings are not uniform on the same sample. This is
probably a consequence of the fact that pores and barrier domes
on phosphoric acid-anodized AAO are not as ordered as those
obtained using the other acids. As shown in Fig. 11(c) and (d),triangular, trapezoidal, square and round openings can be
found.
The constriction size, defined as the full width at half depth of
the AFM topographic scans across the barrier side openings,
produced by ion beam etching on the three types of AAOs are
extracted and summarized in Fig. 12. A broad range of
constriction sizes can be obtained; in addition, the size distri-
bution is narrow, with the exception of the H3PO4-anodized
AAO. IBE processing of AAO, therefore, provides ideal
templates to grow nanoconstrictions with closely controlled
geometry, or to fabricate molecular membranes with closely
controlled pore size in the deep sub-mm range.
Fig. 7 SEM micrographs of the barrier side of phosphoric acid-anod-
ized AAO membranes after different etching times in 5% H3PO4 at room
temperature (25 C): (a) 220 min (left: tilted angle view; right: planar
view; same scale bar); (b) 240 min (left and right image scans are at
different magnifications); (c) 260 min; and (d) 285 min.
Fig. 8 AFM surface topography of the barrier side of oxalic acid-
anodized AAO membranes after different ion beam etching times. (a) no
etching; (b) 5 min (inset shows the enlarged 3D AFM image of a single
cap); (c) 10 min; and (d) 15 min.
Fig. 9 AFM surface topography of the barrier side of sulfuric acid-
anodized AAO after different ion beam etching times: (a) 2 min; (b)
4 min; (c) 8 min; and (d) 12 min. Insets in (a)–(d) show corresponding
enlarged AFM images.
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channel array templates, however, locations of the opened
regions and their size are difficult to control, particularly when
partial pore opening is sought. These inhomogeneities appear to
be a consequence of pre-existing non-uniformities and/or defects
in the AAO barrier layer. Ion beam etching, on the other hand, is
capable to engrave uniform holes on the barrier side, with pore
size controllable by the etching time. Uniformly engraved holes,
with diameter down to 5 nm, can be obtained using sulfuric acid-
anodized AAO and ion beam etching; this process has potentialapplications in the nanofabrication of constrictions with
controlled size, down to the deep sub-mm range.
Acknowledgements
The support of NSF through grant NSF MRSEC DMR 0213985
is gratefully acknowledged.
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