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Toward a quantitative description of the anodic oxide films on aluminum
Electrochemistry Communications 3 (2001) 737±741 www.elsevier.com/locate/elecom
Toward a quantitative description of the anodic oxide ®lms on aluminum C.A. Melendres *,1, S. Van Gils, H. Terryn Department of Metallurgy, Electrochemistry and Materials Science, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussel, Belgium Received 16 August 2001; received in revised form 19 September 2001; accepted 24 September 2001
Abstract A method to quantify the composition of anodic oxide ®lms on aluminum using Infrared Spectroscopic Ellipsometry (IRSE) is proposed. It consists of obtaining the absorption coecient of the ®lm as a function of wavelength. Using values of the absorption coecients for the pure components of the ®lm, the percentages (mole or wt%) of each component in the sample can be calculated.The method is demonstrated in a study of the structure of the oxide ®lm on electropolished aluminum and the anodically formed barrier layer ®lm. Both surface oxides were found to be initially a form of amorphous Al2 O3 . While the barrier ®lm is essentially free of water as prepared, the ®lm on electropolished aluminum contained about 25 wt% water. Hydration of both types of ®lms by immersion in boiling water results in the formation of pseudoboehmite (AlOOH). The technique may have more general applicability to the quantitative determination of the composition of corrosion ®lms and other surface layers on metals. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Infrared spectroscopic ellipsometry; Aluminum; Anodic ®lms; Infrared spectroscopy; Surface oxides; Hydration of aluminum oxide ®lms
1. Introduction The structure and composition of surface ®lms on metals is of great interest from both theoretical and practical standpoints. Aluminum has been extensively studied because its corrosion resistance has resulted in its wide use as construction and packaging material, as well as in the fabrication of electrical capacitors [1]. However, a method to determine the composition of the ®lm quantitatively in a non-vacuum environment has been lacking. Infrared Spectroscopic Ellipsometry (IRSE) is a non-destructive technique that provides information on the optical properties of thin ®lms and their thickness [2]. In this communication, we describe the use of IRSE to obtain the absorption coecient of the ®lm and how, with knowledge of the absorption coecient of the pure components of the ®lm, it allows *
Corresponding author. Tel.: +1-530-750-1991; fax: +1-530-7501991. E-mail address: [email protected] (C.A. Melendres). 1 On sabbatical leave of absence from Argonne National Laboratory, Argonne, IL 60439, USA.
the calculation of the ®lm composition. Along with the measured thickness, we have obtained a good quantitative description of the nature of the oxide ®lms on electropolished aluminum as well as the hydrated barrier layer ®lm. 2. Experimental The samples consisted of commercial grade AA 1050 aluminum plates measuring 0.25 mm thick 4 cm wide 8 cm long. They were electropolished in a solution containing 20% (by volume) perchloric acid in ethanol at 20 °C. An anodic current density of 3000 A/ cm2 was applied for 120 s. Anodization to form the barrier ®lm was performed in a 3% solution of diammonium tartrate whose pH was adjusted to 5.6 by addition of L (+)tartaric acid. Samples of various ®lm thicknesses were prepared; they were stored in a dessicator prior to the ellipsometric measurement. Hydration of the ®lms was carried out by immersion of the samples in boiling deionized water for a predetermined length of time. The IRSE measurements were generally made at
1388-2481/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 8 - 2 4 8 1 ( 0 1 ) 0 0 2 5 0 - 8
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C.A. Melendres et al. / Electrochemistry Communications 3 (2001) 737±741
three dierent angles of incidence, (75°, 80°, and 85°) using a J.A. Woollam IR VASE system. The equipment has been described before and data analyses were carried out in a similar manner [3]. 3. Results Figs. 1(a) and (b) show plots of the IRSE parameter W for a sample of electropolished aluminum (EP) and one with a barrier layer oxide ®lm (BL) formed at 120 V, respectively. It is evident from a comparison of the spectra that the two samples are dierent. There is a signi®cant amount of water in the EP sample as evidenced by the broad peak at about 3500 cm 1 ; the BL ®lm does not show such a feature. There is also an unresolved triplet at about 950 cm 1 for the EP sample while we observe a single peak for the 120 V sample. The ellipsometric data (W and D) were ®tted using a model consisting of Lorentzian oscillators with various energies, intensities and broadening. Details of the data analyses will be described in another paper [4]. Film thicknesses of about 12 and 150 nm were obtained for the EP and BL samples, respectively. The broken line in Fig. 1(a) shows the best ®t to the experimental data for
Fig. 1. (a) Plot of the ellipsometric parameter W as a function of frequency at various angles of incidence (upper ± 75°; middle ± 80°; lowest ± 85°) for an electropolished aluminum oxide ®lm (±±); theoretical ®t (- - -). (b) Plot of ellipsometric parameter W as a function of frequency at various angles of incidence (upper ± 75°; middle ± 80°; lowest ± 85°) for a barrier layer ®lm on Al anodized at 120 V (- - - -); theoretical ®t (±±).
the EP sample. The optical constants can be derived from the theoretical ®t. The results are presented here in the form of a plot of the absorption coecients (a) vs. frequency (cm 1 ). Figs. 2(a) and (b) show the absorption coecients for the EP and the BL samples, respectively. Also plotted in Fig. 2(a) (broken line) is the absorption coecient for pure water obtained from Palik's handbook [5]. Fig. 3(a) shows the BL sample after immersion in boiling water for 10 min in order to eect hydration.The thickness of the hydrated oxide ®lm has increased to about 267 nm. We also observe a drastic change in the spectrum of the sample in the spectral region below 1200 cm 1 (compared to the unboiled sample). The BL sample shows a strong band at about 1050 cm 1 , as well as four other bands at lower frequencies. There is also a broad band now at about 3500 cm 1 due to uptake of water. Data ®tting was done over the whole range, although we only show the ®t in the most interesting parts of the spectrum. From the best ®t (broken line shown in Fig. 3(a)), values of absorption coecient were obtained and these are shown in Figs. 3(b) and (c). The peak at about 1050 cm 1 (Fig. 3(b)) is due to pseudoboehmite (PB) (AlOOH), while that at about 930 cm 1 is due to Al2 O3 that was not converted to PB [1]. The broad peak in the OH stretching region around 3400 cm 1 (Fig. 3(c)) has contributions from the PB OH group and water. Deconvolution of the OH band was carried out
Fig. 2. (a) Absorption coecient for electropolished sample (±±); for pure water obtained from Palik [5], (- - - -). (b) Absorption coecient for barrier layer sample.
C.A. Melendres et al. / Electrochemistry Communications 3 (2001) 737±741
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Fig. 3. (a) Ellipsometric parameter W for 120 V anodized sample after immersion in boiling water for 10 min (- - - -); theoretical ®t (±±). (b) Absorption coecient for 120 V anodized sample derived from theoretical ®t in (a). (c) Absorption coecient in the OH stretching region derived from theoretical ®t in (a) (- - - -); deconvolution in terms of contributions from water (±±) and OH from PB ( )
and the spectral contributions from the PB (peak at about 3200 cm 1 ) and water (peak at 3450 cm 1 ) are shown by the broken and dotted lines. We have also made hydration measurements by immersion of the EP and BL samples in boiling water at dierent times. Detailed results will be presented in another paper [4]. 4. Discussion The optical constants derived from IRSE measurements are often presented in the form of plots of the real and imaginary components of the dielectric function vs. frequency. Much valuable chemical information is contained in the equivalent plot of absorption coecient vs. frequency. As with conventional FTIR spectroscopy, it is often possible to identify the components of the material from the peaks in the absorption spectra. Here, we show how the absorption coecient for an oxide ®lm derived from IRSE can be used in combination with the absorption coecient of the pure compounds comprising the material (derived from conventional FTIR absorption or transmission measurements) in order to provide quantitative information on the ®lm composition. The absorption coecient, a, derived from IRSE is related to the real absorption index, k, and the frequency, m, by the equation [6]
a 4pmk:
1
k is in turn related to the real (e0 ) and complex (e00 ) components of the dielectric function by the equations e 0 n2
k2;
2
e00 2nk;
3
where n is the refractive index. The absorbance of a sample, A, is given by the equation A
log T 0:4342at;
4
where T is the transmittance and t is the thickness of the sample. Moreover a 2:303C;
5 1
where is the molar absorptivity (l mol cm 1 ) and C, the sample concentration. is a property of a material and therefore Eq. (5) provides a way of determining the concentration of a component of a ®lm from the `a' derived from IRSE, and `a' for the pure components, i.e., Ccomponent
in film
acomponent in film =apure Cpure component :
component
6
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C.A. Melendres et al. / Electrochemistry Communications 3 (2001) 737±741
4.1. Composition and structure of electropolished and barrier layer ®lms A comparison of the absorption spectra of the BL and EP samples in the aluminum oxide region (below 1200 cm 1 ) with those of crystalline alumina [7,8] shows them to have much broader line widths and lower intensities which are indicative of a disordered or amorphous structure (Fig. 4). The results of Chu et al. [9] for amorphous aluminum oxide lend credence to this classi®cation.The works of Chatelet et al. [10], as well as those of Ortiz et al. [11], also duly support our description as to the amorphous nature of our anodic oxide ®lms. While the BL sample is essentially pure (without water), the EP sample inherently contains water, as prepared. (Fig. 1). The bands at about 1100 and 1430 cm 1 in the latter sample indicates contamination by ClO4 ions from the electrolyte solution and by carbonate (from reaction of the oxide ®lm with CO2 in the air). We will neglect these contaminants in calculating the amount of amorphous Al2 O3 and water in the EP sample. We use the absorption coecient of pure amorphous Al2 O3 from the measured transmission spectrum by Ortiz et al. [11] and that of water from Palik [5]. We found the EP sample to contain 25.4 wt% water and 74.6 wt% amorphous Al2 O3 . 4.2. Eect of hydration Hydration of the BL sample by immersion in boiling water for 10 min resulted in the spectrum shown in Fig. 3. Instead of the narrow Al2 O3 band previously observed in the oxide region, a number of bands are observed over the range from about 350 to 1200 cm 1 . As has been stated before, these bands are attributed to PB on the basis of our own data analysis and information available from the literature [1]. Thus the amorphous
Al2 O3 was converted to PB by immersion in boiling water. There is some Al2 O3 remaining in the ®lm as evidenced by the peak at about 930 cm 1 in Fig. 3(b). This peak decreases in intensity while that of PB increases in intensity with increasing immersion time [4]. Again, one can calculate the composition of the ®lm using values of the absorption coecients for the pure components. In the absence of data on the absorption coecient of pure PB, we used the absorption coecient for boehmite as derived from the work of Bertling [12]. We also used our own measured absorption coecient for pure Al2 O3 (Fig. 2(b)) and that of water from Palik's handbook [5]. The composition of the BL sample hydrated by immersion in boiling water for 10 min calculates out to be: 24.6 wt% Al2 O3 , 40.3% PB, and 35.1% water. Hydration of the EP samples was also carried out and conversion of the ®lm to PB was observed after immersion for 1 min [4]. Opsomer [13] has obtained similar results after immersion for 30 s.
5. Conclusion We have proposed a method using IR spectroscopic ellipsometry and IR absorption spectroscopy in order to obtain quantitative information on the structure, thickness and composition of surface oxide ®lms. The technique has been demonstrated using the anodic oxide ®lms formed on EP and BL samples of aluminum. Hydration of the ®lms was found to result in the conversion of the amorphous oxides to PB. The calculated composition should be considered as an order of magnitude estimate at this time because of the use of the absorption coecient of boehmite for PB. An accurate determination of this parameter should be the object of further work. Extension of the technique to the study of the composition of corrosion ®lms on metals using not only IR but also UV±visible spectroscopic ellipsometry appears attractive.
Acknowledgements
Fig. 4. Absorption coecients for various forms of Al2 O3 ; single crystal Al2 O3 from [12] (±±); Al2 O3 polycrystalline ceramic from [8] (- - -); amorphous Al2 O3 from [9] ( ); anodized 120 V sample from the present work (- - -); electropolished sample from present work (±±).
Financial support for this work was provided by the Research Council of the Vrije Universiteit Brussel (VUB) and the Institute for the Promotion of Innovation by Science and Technology in the Flanders. The authors are grateful to M. Raes for preparing the electropolished aluminum and barrier layer samples. One of us (CAM) thanks Prof. J. Vereecken for accomodating his visit to the VUB and thus making possible the work described herein. We also thank Dr. R. Alwitt of Boundary Technologies Inc. for providing us with a spectrum of PB.
C.A. Melendres et al. / Electrochemistry Communications 3 (2001) 737±741
References [1] R.S. Alwitt, in: J.W. Diggle, A.K. Vijh (Eds.), Oxides and Oxide Films, vol. 4, Marcel Dekker, New York, 1976 (Chapter 3). [2] R.W. Collins, D.E. Aspnes, E.A. Irene (Eds.), Spectroscopic Ellipsometry, Elsevier, Lausanne, 1998. [3] T. Schram, H. Terryn, J. Electrochem. Soc. 148 (2001) F12. [4] S. Van Gils, C.A. Melendres, H.Terryn, in preparation. [5] E.D. Palik (Ed.), Handbook of Optical Constants of Solids II, Academic Press, Boston, MA, 1991. [6] B. Schrader, in: Infrared and Raman Spectroscopy, VCH, Weinheim, 1995, p. 576.
741
[7] F. Gervais, in: E.D. Palik (Ed.), Handbook of Optical Constants of Solids II, Academic Press, Boston, MA, 1991. [8] C.A. Worrell, J. Mater. Sci. 21 (1986) 781. [9] Y.T. Chu, J.B. Bates, C.W. White, G.C. Farlow, J. Appl. Phys. 64 (1988) 3727. [10] J. Chatelet, H.H. Claasen, D.M. Gruen, I. Sheft, R.B. Wright, Appl. Spectrosc. 29 (1975) 185. [11] A. Ortiz, J.C. Alonso, V. Pankov, A. Huanosta, E. Andrade, Thin Solid Films 368 (2000) 74. [12] S. Bertling, Thesis, Swedish Institute for Metals Research, NTIS Report PB96-126552, 1996. [13] O. Opsomer, Ph.D. Thesis, Vrije Universiteit Brussel, 1999.
Toward a quantitative description of the anodic oxide ®lms on aluminum C.A. Melendres *,1, S. Van Gils, H. Terryn Department of Metallurgy, Electrochemistry and Materials Science, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussel, Belgium Received 16 August 2001; received in revised form 19 September 2001; accepted 24 September 2001
Abstract A method to quantify the composition of anodic oxide ®lms on aluminum using Infrared Spectroscopic Ellipsometry (IRSE) is proposed. It consists of obtaining the absorption coecient of the ®lm as a function of wavelength. Using values of the absorption coecients for the pure components of the ®lm, the percentages (mole or wt%) of each component in the sample can be calculated.The method is demonstrated in a study of the structure of the oxide ®lm on electropolished aluminum and the anodically formed barrier layer ®lm. Both surface oxides were found to be initially a form of amorphous Al2 O3 . While the barrier ®lm is essentially free of water as prepared, the ®lm on electropolished aluminum contained about 25 wt% water. Hydration of both types of ®lms by immersion in boiling water results in the formation of pseudoboehmite (AlOOH). The technique may have more general applicability to the quantitative determination of the composition of corrosion ®lms and other surface layers on metals. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Infrared spectroscopic ellipsometry; Aluminum; Anodic ®lms; Infrared spectroscopy; Surface oxides; Hydration of aluminum oxide ®lms
1. Introduction The structure and composition of surface ®lms on metals is of great interest from both theoretical and practical standpoints. Aluminum has been extensively studied because its corrosion resistance has resulted in its wide use as construction and packaging material, as well as in the fabrication of electrical capacitors [1]. However, a method to determine the composition of the ®lm quantitatively in a non-vacuum environment has been lacking. Infrared Spectroscopic Ellipsometry (IRSE) is a non-destructive technique that provides information on the optical properties of thin ®lms and their thickness [2]. In this communication, we describe the use of IRSE to obtain the absorption coecient of the ®lm and how, with knowledge of the absorption coecient of the pure components of the ®lm, it allows *
Corresponding author. Tel.: +1-530-750-1991; fax: +1-530-7501991. E-mail address: [email protected] (C.A. Melendres). 1 On sabbatical leave of absence from Argonne National Laboratory, Argonne, IL 60439, USA.
the calculation of the ®lm composition. Along with the measured thickness, we have obtained a good quantitative description of the nature of the oxide ®lms on electropolished aluminum as well as the hydrated barrier layer ®lm. 2. Experimental The samples consisted of commercial grade AA 1050 aluminum plates measuring 0.25 mm thick 4 cm wide 8 cm long. They were electropolished in a solution containing 20% (by volume) perchloric acid in ethanol at 20 °C. An anodic current density of 3000 A/ cm2 was applied for 120 s. Anodization to form the barrier ®lm was performed in a 3% solution of diammonium tartrate whose pH was adjusted to 5.6 by addition of L (+)tartaric acid. Samples of various ®lm thicknesses were prepared; they were stored in a dessicator prior to the ellipsometric measurement. Hydration of the ®lms was carried out by immersion of the samples in boiling deionized water for a predetermined length of time. The IRSE measurements were generally made at
1388-2481/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 8 - 2 4 8 1 ( 0 1 ) 0 0 2 5 0 - 8
738
C.A. Melendres et al. / Electrochemistry Communications 3 (2001) 737±741
three dierent angles of incidence, (75°, 80°, and 85°) using a J.A. Woollam IR VASE system. The equipment has been described before and data analyses were carried out in a similar manner [3]. 3. Results Figs. 1(a) and (b) show plots of the IRSE parameter W for a sample of electropolished aluminum (EP) and one with a barrier layer oxide ®lm (BL) formed at 120 V, respectively. It is evident from a comparison of the spectra that the two samples are dierent. There is a signi®cant amount of water in the EP sample as evidenced by the broad peak at about 3500 cm 1 ; the BL ®lm does not show such a feature. There is also an unresolved triplet at about 950 cm 1 for the EP sample while we observe a single peak for the 120 V sample. The ellipsometric data (W and D) were ®tted using a model consisting of Lorentzian oscillators with various energies, intensities and broadening. Details of the data analyses will be described in another paper [4]. Film thicknesses of about 12 and 150 nm were obtained for the EP and BL samples, respectively. The broken line in Fig. 1(a) shows the best ®t to the experimental data for
Fig. 1. (a) Plot of the ellipsometric parameter W as a function of frequency at various angles of incidence (upper ± 75°; middle ± 80°; lowest ± 85°) for an electropolished aluminum oxide ®lm (±±); theoretical ®t (- - -). (b) Plot of ellipsometric parameter W as a function of frequency at various angles of incidence (upper ± 75°; middle ± 80°; lowest ± 85°) for a barrier layer ®lm on Al anodized at 120 V (- - - -); theoretical ®t (±±).
the EP sample. The optical constants can be derived from the theoretical ®t. The results are presented here in the form of a plot of the absorption coecients (a) vs. frequency (cm 1 ). Figs. 2(a) and (b) show the absorption coecients for the EP and the BL samples, respectively. Also plotted in Fig. 2(a) (broken line) is the absorption coecient for pure water obtained from Palik's handbook [5]. Fig. 3(a) shows the BL sample after immersion in boiling water for 10 min in order to eect hydration.The thickness of the hydrated oxide ®lm has increased to about 267 nm. We also observe a drastic change in the spectrum of the sample in the spectral region below 1200 cm 1 (compared to the unboiled sample). The BL sample shows a strong band at about 1050 cm 1 , as well as four other bands at lower frequencies. There is also a broad band now at about 3500 cm 1 due to uptake of water. Data ®tting was done over the whole range, although we only show the ®t in the most interesting parts of the spectrum. From the best ®t (broken line shown in Fig. 3(a)), values of absorption coecient were obtained and these are shown in Figs. 3(b) and (c). The peak at about 1050 cm 1 (Fig. 3(b)) is due to pseudoboehmite (PB) (AlOOH), while that at about 930 cm 1 is due to Al2 O3 that was not converted to PB [1]. The broad peak in the OH stretching region around 3400 cm 1 (Fig. 3(c)) has contributions from the PB OH group and water. Deconvolution of the OH band was carried out
Fig. 2. (a) Absorption coecient for electropolished sample (±±); for pure water obtained from Palik [5], (- - - -). (b) Absorption coecient for barrier layer sample.
C.A. Melendres et al. / Electrochemistry Communications 3 (2001) 737±741
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Fig. 3. (a) Ellipsometric parameter W for 120 V anodized sample after immersion in boiling water for 10 min (- - - -); theoretical ®t (±±). (b) Absorption coecient for 120 V anodized sample derived from theoretical ®t in (a). (c) Absorption coecient in the OH stretching region derived from theoretical ®t in (a) (- - - -); deconvolution in terms of contributions from water (±±) and OH from PB ( )
and the spectral contributions from the PB (peak at about 3200 cm 1 ) and water (peak at 3450 cm 1 ) are shown by the broken and dotted lines. We have also made hydration measurements by immersion of the EP and BL samples in boiling water at dierent times. Detailed results will be presented in another paper [4]. 4. Discussion The optical constants derived from IRSE measurements are often presented in the form of plots of the real and imaginary components of the dielectric function vs. frequency. Much valuable chemical information is contained in the equivalent plot of absorption coecient vs. frequency. As with conventional FTIR spectroscopy, it is often possible to identify the components of the material from the peaks in the absorption spectra. Here, we show how the absorption coecient for an oxide ®lm derived from IRSE can be used in combination with the absorption coecient of the pure compounds comprising the material (derived from conventional FTIR absorption or transmission measurements) in order to provide quantitative information on the ®lm composition. The absorption coecient, a, derived from IRSE is related to the real absorption index, k, and the frequency, m, by the equation [6]
a 4pmk:
1
k is in turn related to the real (e0 ) and complex (e00 ) components of the dielectric function by the equations e 0 n2
k2;
2
e00 2nk;
3
where n is the refractive index. The absorbance of a sample, A, is given by the equation A
log T 0:4342at;
4
where T is the transmittance and t is the thickness of the sample. Moreover a 2:303C;
5 1
where is the molar absorptivity (l mol cm 1 ) and C, the sample concentration. is a property of a material and therefore Eq. (5) provides a way of determining the concentration of a component of a ®lm from the `a' derived from IRSE, and `a' for the pure components, i.e., Ccomponent
in film
acomponent in film =apure Cpure component :
component
6
740
C.A. Melendres et al. / Electrochemistry Communications 3 (2001) 737±741
4.1. Composition and structure of electropolished and barrier layer ®lms A comparison of the absorption spectra of the BL and EP samples in the aluminum oxide region (below 1200 cm 1 ) with those of crystalline alumina [7,8] shows them to have much broader line widths and lower intensities which are indicative of a disordered or amorphous structure (Fig. 4). The results of Chu et al. [9] for amorphous aluminum oxide lend credence to this classi®cation.The works of Chatelet et al. [10], as well as those of Ortiz et al. [11], also duly support our description as to the amorphous nature of our anodic oxide ®lms. While the BL sample is essentially pure (without water), the EP sample inherently contains water, as prepared. (Fig. 1). The bands at about 1100 and 1430 cm 1 in the latter sample indicates contamination by ClO4 ions from the electrolyte solution and by carbonate (from reaction of the oxide ®lm with CO2 in the air). We will neglect these contaminants in calculating the amount of amorphous Al2 O3 and water in the EP sample. We use the absorption coecient of pure amorphous Al2 O3 from the measured transmission spectrum by Ortiz et al. [11] and that of water from Palik [5]. We found the EP sample to contain 25.4 wt% water and 74.6 wt% amorphous Al2 O3 . 4.2. Eect of hydration Hydration of the BL sample by immersion in boiling water for 10 min resulted in the spectrum shown in Fig. 3. Instead of the narrow Al2 O3 band previously observed in the oxide region, a number of bands are observed over the range from about 350 to 1200 cm 1 . As has been stated before, these bands are attributed to PB on the basis of our own data analysis and information available from the literature [1]. Thus the amorphous
Al2 O3 was converted to PB by immersion in boiling water. There is some Al2 O3 remaining in the ®lm as evidenced by the peak at about 930 cm 1 in Fig. 3(b). This peak decreases in intensity while that of PB increases in intensity with increasing immersion time [4]. Again, one can calculate the composition of the ®lm using values of the absorption coecients for the pure components. In the absence of data on the absorption coecient of pure PB, we used the absorption coecient for boehmite as derived from the work of Bertling [12]. We also used our own measured absorption coecient for pure Al2 O3 (Fig. 2(b)) and that of water from Palik's handbook [5]. The composition of the BL sample hydrated by immersion in boiling water for 10 min calculates out to be: 24.6 wt% Al2 O3 , 40.3% PB, and 35.1% water. Hydration of the EP samples was also carried out and conversion of the ®lm to PB was observed after immersion for 1 min [4]. Opsomer [13] has obtained similar results after immersion for 30 s.
5. Conclusion We have proposed a method using IR spectroscopic ellipsometry and IR absorption spectroscopy in order to obtain quantitative information on the structure, thickness and composition of surface oxide ®lms. The technique has been demonstrated using the anodic oxide ®lms formed on EP and BL samples of aluminum. Hydration of the ®lms was found to result in the conversion of the amorphous oxides to PB. The calculated composition should be considered as an order of magnitude estimate at this time because of the use of the absorption coecient of boehmite for PB. An accurate determination of this parameter should be the object of further work. Extension of the technique to the study of the composition of corrosion ®lms on metals using not only IR but also UV±visible spectroscopic ellipsometry appears attractive.
Acknowledgements
Fig. 4. Absorption coecients for various forms of Al2 O3 ; single crystal Al2 O3 from [12] (±±); Al2 O3 polycrystalline ceramic from [8] (- - -); amorphous Al2 O3 from [9] ( ); anodized 120 V sample from the present work (- - -); electropolished sample from present work (±±).
Financial support for this work was provided by the Research Council of the Vrije Universiteit Brussel (VUB) and the Institute for the Promotion of Innovation by Science and Technology in the Flanders. The authors are grateful to M. Raes for preparing the electropolished aluminum and barrier layer samples. One of us (CAM) thanks Prof. J. Vereecken for accomodating his visit to the VUB and thus making possible the work described herein. We also thank Dr. R. Alwitt of Boundary Technologies Inc. for providing us with a spectrum of PB.
C.A. Melendres et al. / Electrochemistry Communications 3 (2001) 737±741
References [1] R.S. Alwitt, in: J.W. Diggle, A.K. Vijh (Eds.), Oxides and Oxide Films, vol. 4, Marcel Dekker, New York, 1976 (Chapter 3). [2] R.W. Collins, D.E. Aspnes, E.A. Irene (Eds.), Spectroscopic Ellipsometry, Elsevier, Lausanne, 1998. [3] T. Schram, H. Terryn, J. Electrochem. Soc. 148 (2001) F12. [4] S. Van Gils, C.A. Melendres, H.Terryn, in preparation. [5] E.D. Palik (Ed.), Handbook of Optical Constants of Solids II, Academic Press, Boston, MA, 1991. [6] B. Schrader, in: Infrared and Raman Spectroscopy, VCH, Weinheim, 1995, p. 576.
741
[7] F. Gervais, in: E.D. Palik (Ed.), Handbook of Optical Constants of Solids II, Academic Press, Boston, MA, 1991. [8] C.A. Worrell, J. Mater. Sci. 21 (1986) 781. [9] Y.T. Chu, J.B. Bates, C.W. White, G.C. Farlow, J. Appl. Phys. 64 (1988) 3727. [10] J. Chatelet, H.H. Claasen, D.M. Gruen, I. Sheft, R.B. Wright, Appl. Spectrosc. 29 (1975) 185. [11] A. Ortiz, J.C. Alonso, V. Pankov, A. Huanosta, E. Andrade, Thin Solid Films 368 (2000) 74. [12] S. Bertling, Thesis, Swedish Institute for Metals Research, NTIS Report PB96-126552, 1996. [13] O. Opsomer, Ph.D. Thesis, Vrije Universiteit Brussel, 1999.