measurement of the pore sizes for anodic aluminum oxide (aao)

Park a a Department of Mechanical Engineering, Pohang University of Science and Technology, San 31, Hyoja Dong, Nam-gu, Pohang, Kyungbuk, 790-784, Republic of Korea b Department of Chemi

Measurement of the pore sizes for anodic aluminum oxide (AAO)

D.H Choi a, P.S Lee b, W Hwang a,*, K.H Lee b, H.C Park a

a Department of Mechanical Engineering, Pohang University of Science and Technology, San 31, Hyoja Dong,

Nam-gu, Pohang, Kyungbuk, 790-784, Republic of Korea

b Department of Chemical Engineering, Pohang University of Science and Technology, San 31, Hyoja Dong, Nam-gu,

Pohang, Kyungbuk, 790-784, Republic of Korea

Received 5 August 2005 Available online 9 March 2006

Abstract

A new method for the measurement of pore size of the porous-type anodic aluminum oxide (AAO) using atomic force microscopy (AFM) is proposed The measurement of the pore size by AFM makes no damage to the specimen AAO film can be referred as a nano-honeycomb structure because the pore shape of AAO is like the nano-honeycomb structure and its pore size is in the nanometer scale To measure the pore sizes from AFM images, it is assumed that the top surface of AAO is not etched much in the pore widening process

It is because the etching rate of the top surface is very slow due to the mechanical packing stress in the middle of the pore wall of AAO The pore sizes from AFM images showed good agreement with those from SEM images

Ó 2006 Elsevier B.V All rights reserved

PACS: 06.30.Bp; 07.79.Lh

Keywords: Anodic aluminum oxide; Atomic force microscopy; Pore size; Pore widening; Mechanical packing stress

1 Introduction

In recent years, anodic aluminum oxide (AAO) film has

attracted considerable attention due to its potential use

in a new field, nanotechnology There are barrier-type and

porous-type films in the AAO film The porous-type AAO

film is generally called as AAO film AAO film is

consid-ered a ‘‘nanohoneycomb’’ structure because the pore shape

of AAO is similar to a honeycomb structure, and its pore

size is typically in a nanometer scale Its extremely high

aspect ratio and self-ordered hexagonal pore structure are

the most attractive features Moreover, it is advantageous

nature of AAO film that pore dimensions such as diameter,

length and density can be controlled by varying the

anod-izing condition[1]

The initial pore size of AAO depends on the acid

solu-tion used in the anodizasolu-tion process as reported in many

previous sources The pore size of AAO is increased by the etching process Because the various properties of the

adjust the pore size of AAO and it is important to measure the pore size SEM is used most widely to measure the pore size[5–7] There are some methods using TEM[8], replica

[9,10], and others [11,12] However, these methods are destructive and material property dependent The specimen has to be conductive to be measured with SEM, and has to

be destroyed to measured with TEM

Atomic force microscopy (AFM) has been given a great deal of attention by researchers, who study surface topog-raphy, frictional properties and magnetic properties in the nanometer scale because of its various applicable modes and its unique advantages AFM measurements are nonde-structive and wide range of materials can be used as spec-imens However, there is the distortion of an image referred to convolution effects in AFM measurements It comes from the geometric shape of a tip Even if a

1567-1739/$ - see front matter Ó 2006 Elsevier B.V All rights reserved.

doi:10.1016/j.cap.2006.01.024

*

Corresponding author Fax: +82 54 279 5899.

E-mail address: whwang@postech.ac.kr (W Hwang).

www.elsevier.com/locate/cap

www.kps.or.kr

AFM becomes the V shape Therefore, all

geo-metrical dimensions of specimens cannot be obtained from

AFM images

In this study, a new method for measuring the pore size

of AAO with AFM is investigated AAO film is fabricated

by perchloric acid, and it is widened by phosphoric acid

Because the etching rate of the top surface of AAO is much

slower compared with the etching rate of the side wall of

AAO, it is assumed that the top surface of AAO is not

etched much To verify the proposed method, the results

from the AFM images are compared with those of SEM

2 Etching rates in AAO

Although the surface of AAO has been flat in the

sche-matic diagram of many researches, the top surface is not

which is fabricated by two step-anodizing at 195 V in a

be considered a crown shape A material in isotropic wet

etchants is generally etched in all directions at nearly the

same rate, but it was found in many tests that the etching

rate of the top surface of AAO was much slower than

the rate of the pore side wall, as shown in dashed lines of

Fig 1(b) It is due to the mechanical packing stress is

applied to the middle of the pore wall of AAO, as shown

in Fig 1(b) [12,13] When the mechanical packing stress

applies to the material, the etching rate becomes slow

because the packed part is denser than the other parts

Unfortunately, the etching rate of the top surface of

AAO could not be obtained because the surface of the

porous-type AAO was very irregular

Fig 2shows the top view and the cross section of AAO

measured by AFM The pore wall is actually straight, but it

appears as a curved line in the AFM image The pore size

cannot then be determined from the AFM image In this

study, it is assumed that the vertical height (seeFig 1(b))

between the top surface and the pore entrance did not

change much during the etching process because the

etch-ing rate of the top surface is much slower compared with

the etching rate of the pore of AAO in the pore widening

process

3 Experiments 3.1 Fabrication of AAO film

A pure Al sheet (99.999%) with a thickness of 1 mm was electropolished in a mixture of perchloric acid and ethanol

sur-face irregularities The specimen was used as an anode while a flat Pt was used as a cathode A constant voltage

of 20 V was applied between the cathode and the anode for 60–90 s and the solution temperature was maintained

the first step anodization was carried out in a 0.3 M oxalic acid solution for more than 2 h The AAO layer was then removed by immersing the specimen in a mixture of 1.8 wt% chromic acid and 6 wt% phosphoric acid at

Fig 1 (a) SEM image of AAO in side view and (b) schematic diagram of the pore widening of AAO The top shape of AAO is like the crown.

Fig 2 (a) A top view and (b) a cross section of AAO by AFM.

65°C for 1–4 h Finally, porous-type AAO films were

obtained by the subsequent second anodization under the

same condition as in the first step After AAO films are

fab-ricated, the pores are widened by etching in a 0.1 M

and 50 min

3.2 Measurement of the pore size of AAO in AFM

Atomic force microscope (AFM, Seiko SPA 400) is used

to measure the pore size In AFM, the surface is scanned

under non-contact mode with a Si cantilever (SEIHR) which has a tip radius less than 10 nm and typical bending stiffness of 10 N/m fabricated by Nanosensors Company

Fig 2shows the top view and the cross section of AAO measured by AFM Because the initial pore size of AAO can be found in many sources, in the AFM image we can find the positions where the initial pore size starts, shown

study, the positions are called as the pore entrance The

Fig 2(b)) from the top surface to the pore entrance in

Fig 3 AFM images of AAO Etching time was (a) 0 min, (b) 30 min and (c) 50 min.

AAO before widening is used to measure the pore size of

the other AAOs because the vertical height is assumed

not changed in the widening process In other words, the

pore entrance in the widened AAO can be defined from

the vertical height which is determined in AAO before

wid-ening The pore size of the widened AAO is determined by

the distance between the pore entrances

4 Results and discussion

Figs 3 and 4 show the AFM images and the SEM

images of AAO fabricated in this study, respectively For

the SEM images, field emission scanning electron micro-scope (FE-SEM, JEOL JSM-6330F) was used and the specimens were coated with Pt In the AFM image, the ver-tical height between the pore entrance and the top surface for AAO before widening was found to be 10.5 nm because the initial pore size of AAO fabricated by perchloric acid is

other AAOs were determined by using this vertical height

InFig 5, the pore sizes measured by the AFM images were compared with those measured by the SEM images The measured pore sizes showed good agreements between with AFM and with SEM However, the variations in measured value in each image were large This is because the surface

of AAO was very irregular

5 Conclusions

In this study, the pore sizes of AAO were measured by AFM without any damages of the specimens To determine the pore size by AFM, it was assumed and verified in our study that the top surface is not etched much

Acknowledgement This work was supported by National R&D Program (M10214000191-02B1500-02910) in Korea Institute of Sci-ence and Technology Evaluation and Planning/Ministry of Science & Technology

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Fig 4 SEM images of AAO Etching time was (a) 0 min, (b) 30 min and

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10 20 30 40 50 60 70

58.8 59.6

46.8 46.2

31.6 31.1

50 30

0

Widening Time (min)

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σ σ

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Fig 5 Measured pore sizes and their standard deviations.

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