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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: [email protected]; 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

778 | Nanoscale, 2010, 2, 778–785 This journal is ª The Royal Society of Chemistry 2010

PAPER www.rsc.org/nanoscale | Nanoscale



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.

This journal is ª The Royal Society of Chemistry 2010 Nanoscale, 2010, 2, 778–785 | 779



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




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.




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.




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.




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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