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 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. Mo reover, 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 anodization process as reported in many previous sources. The pore size of AAO is increased by the etching process. Because the various properties of the AAO film depend on the pore size [2–4] , it is necessary to 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 a re 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 measur ed 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 ran ge 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 speci- men has the vertical step , the image from 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 Current Applied Physics 6S1 (2006) e125–e129 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 AA O has been flat in the sche- matic diagram of many researches, the top surface is not actually flat. Fig. 1(a) shows the cross section of AAO film, which is fabricated by two step-anodizing at 195 V in a 0.1 M phosphoric acid at 0 °C. The surface of AAO can 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 AA O 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. 2 shows 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 imag e. The pore size cannot then be determined from the AFM image. In this study, it is assumed that the vertical height (see Fig. 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 (HClO 4 :C 2 H 5 OH = 1:4 in volumetric ratio) to remove 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 at 7 °C during electropolishing. After electropolishing, 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. e126 D.H. Choi et al. / Current Applied Physics 6S1 (2006) e125–e129 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 phos- phoric acid solution at 30 °C. The widening times are 0, 30 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 Nanosens ors Company. Fig. 2 shows 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 as a horizontal solid arrow line (P) in Fig. 2(b). In this study, the positions are called as the pore entrance. The vertical height (the vertical solid arrow lines, V and V 0 in 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. D.H. Choi et al. / Current Applied Physics 6S1 (2006) e125–e129 e127 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 about 31 nm by Nielsch’s rep ort [11]. The pore sizes of the other AAOs were determined by using this vertical height. In Fig. 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 measur ed 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. References [1] J.W. Diggle, T.C. Downie, C.W. Goulding, Chem. Rev. 69 (1969) 365. [2] G. Horowitz, D. Fichou, X. Peng, F. Garnier, Synth. Met. 41–43 (1991) 1127. [3] S.F. Nelson, Y.Y. Lin, D.J. Dundlach, T.N. Jackson, Appl. Phys. Lett. 72 (1998) 1854. [4] T.W. Whitney, J.S. Jiang, P.C. Searson, C.L. Chien, Science 261 (1993) 261. [5] A.P. Li, F. Mu ¨ ller, A. Birner, K. Nielsch, U. Go ¨ sele, J. Vac. Sci. Technol. A 17 (1999) 1428. [6] H. Masuda, K. Fukuda, Science 268 (1995) 1466. [7] R.C. Furneaux, W.R. Rigby, A.P. Davidson, Nature 337 (1989) 147. [8] K. Nielsch, J. Choi, K. Schwirn, R.B. Wehrspohn, U. Go ¨ sele, Nano Letters 2 (2002) 677. Fig. 4. SEM images of AAO. Etching time was (a) 0 min, (b) 30 min and (c) 50 min. 10 20 30 40 50 60 70 58.8 59.6 46.8 46.2 31.6 31.1 SEM = 3.82 AFM = 5.46 SEM = 1.88 AFM = 2.37 SEM = 2.55 AFM = 2.87 50 30 0 Pore Size (nm) Widening Time (min) SEM AFM σ σ σ σ σ σ Fig. 5. Measured pore sizes and their standard deviations. e128 D.H. Choi et al. / Current Applied Physics 6S1 (2006) e125–e129 [9] D. Al-Mawlawi, C.Z. Liu, M. Moskovits, J. Mater. Res. 9 (1994) 1014. [10] H. Masuda, M. Satoh, Jpn. J. Appl. Phys. 35 (1996) L126. [11] H. Takahashi, M. Nagayama, Corros. Sci. 18 (1978) 911. [12] S. Ono, M. Saito, M. Ishiguro, H. Asoh, J. Electrochem. Soc. 151 (2004) B473. [13] A.P. Li, F. Mu ¨ ller, A. Birner, K. Nielsch, U. Go ¨ sele, J. Appl. Phys. 84 (1998) 6023. D.H. Choi et al. / Current Applied Physics 6S1 (2006) e125–e129 e129 . 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. 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