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Anodic film growth on tantalum in dilute phosphoric acid solution at 20 and 85 °C
Electrochimica Acta 47 (2002) 2761 /2767 www.elsevier.com/locate/electacta
Anodic film growth on tantalum in dilute phosphoric acid solution at 20 and 85 8C Q. Lu a, S. Mato a, P. Skeldon a,*, G.E. Thompson a, D. Masheder b, H. Habazaki c, K. Shimizu d a
Corrosion and Protection Centre, University of Manchester Institute of Science and Technology, P.O. Box 88, Manchester M60 1QD, UK b AVX Limited, Tantalum Division, Long Road, Paignton, Devon TQ4 7ER, UK c Graduate Engineering School, Hokkaido University, N13 W8, Kita-ku, Sapporo 0608628, Japan d University Chemical Laboratory, Keio University, 4-1-1 Hiyoshi, Yokohama 223, Japan Received 21 January 2002; received in revised form 8 March 2002
Abstract The effects of current density and temperature on the anodic films formed on tantalum in dilute H3PO4 (0.06%wt) solution have been studied by transmission electron microscopy, using ultramicrotomed sections, and Rutherford backscattering spectroscopy. Two-layered films have been identified, comprising an inner relatively pure Ta2O5 layer, adjacent to the metal/film interface, and an anions. The total amount and depth of incorporated phosphorus species increase with outer layer containing incorporated PO3 4 increasing current density and decreasing temperature, in correspondence with the enhancement of the electric field. The formation conditions for the films include those relevant to the commercial anodising of tantalum for capacitors for which the extent of phosphorus incorporation is shown to be comparatively low. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Tantalum; Anodizing; Anodic films; Transmission electron microscopy; Rutherford backscattering spectroscopy
1. Introduction It is well known that tantalum can be anodised in almost all electrolytes except concentrated hydrofluoric acid. The role played by the electrolyte in determining the mechanism of formation, the chemical composition and the properties of anodic films on tantalum has also been studied. The films thicken uniformly with increase of electrode potential; the growth involves high field ionic transport processes, associated with the migration of tantalum and oxygen ions [1 /4]. Further, tracer studies [5] have revealed that the outer part of the film formed in phosphoric acid contains incorporated phosphorus species, derived from electrolyte anions, which decrease the permittivity and ionic conductivity of the oxide. The two-layer model for the anodic film has also been confirmed by ellipsometric studies [6,7]. Recent studies [8 /10] have shown that incorporated electrolyte-
* Corresponding author. Tel.: /44-161-200-4872; fax: /44-161200-4865.
derived species are involved in the ionic transport processes necessary for oxide thickening. Such species may be immobile (i.e. silicon species), mobile inward (i.e. phosphate and sulphate anions) and mobile outward (i.e. boron species). The mobility of incorporated species has been explained with reference to a ‘liquid’ model of film growth, and comparison of single metal / oxygen bond energies of the incorporated species in the anodic tantalum oxide [10]. The variations in the distributions of the phosphorus species are indicated by the ratios of the thicknesses of the outer film layers to the total film thicknesses, which are 0.51, 0.48 and 0.11 for films formed at 1 mA cm 2 in 0.1 M phosphoric and sulphuric acids, and 0.1 M ammonium pentaborate electrolyte; at 25 8C, respectively [10]. The influences of current density [7,11], the nature of the electrolyte [10,12] and the electrolyte concentration [13] on film growth have also been examined. Although the incorporation of electrolyte species has been observed and confirmed, and their involvement in ionic transport processes has been studied, the experiments were mainly carried out at
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room temperature. However, there are commercial interests in anodising powder metallurgy tantalum anodes, used to produce tantalum capacitors, at elevated temperatures. Thus, the present study examines the influence of current density on film growth and, in particular, incorporation of electrolyte species into the film for tantalum anodised at 20 and 85 8C, the latter being a temperature that is used in the manufacture of capacitors. Dielectric properties of the films [14] have been reported separately.
2. Experiments 2.1. Specimen preparation A sputtered tantalum layer was used in order to facilitate production of ultramicrotomed sections of anodic films for transmission electron microscopy (TEM). The tantalum layer was deposited on to the surface of electropolished and anodised aluminium specimens by magnetron sputtering using an Atom Tech Ltd System with a 99.9% tantalum target. The thickness of the tantalum layer was about 225 nm. A high temperature tape (Green Tree Ltd.) was used to mask the electrode to define the working area. 2.2. Anodising Tantalum specimens were anodised at different constant current densities of 0.045, 0.1, 1, 5, 10 and 100 mA cm 2, typically to 150 V. A glass electrochemical cell was used for the anodising process. The cathode was made of high purity tantalum sheet, in order to avoid contamination, and the electrolyte was stirred 0.06%wt H3PO4 solution at 859/0.5 8C. For comparison purposes, specimens were also anodised under similar conditions at 20 8C. 2.3. Analysis The thicknesses of the anodic films were measured from ultramicrotomed sections that were generated with a LEICA Ultracut UCT ultramicrotome [15] and examined in a JEOL 2000 FX II transmission electron microscope. In order to determine concentrations and distributions of electrolyte-derived species in the films, Rutherford backscattering spectroscopy (RBS) was carried out using 2.01 MeV He ions, at a current of 60 nA and with a beam diameter of 1 mm. The RBS spectra were interpreted by the RUMP program [16]. Lateral homogeneity was assumed for the interpretation of the spectra. Absolute amounts of tantalum and phosphorus atoms in the oxide, expressed in atoms cm 2, were obtained to an accuracy of /5%. Film thicknesses were derived, assuming an atomic density of
7.67 /1022 atoms cm 3 for the film [17], and compared with results of TEM.
3. Results 3.1. Anodic film growth The anodising behaviour of tantalum in 0.06%wt H3PO4 solution closely follows the expected linear growth mechanism (Fig. 1). The rate of increase of the forming voltage is strongly dependent upon current density, and increases approximately linearly with the latter (Table 1 and Fig. 2). Table 1 also shows that the charge passed during anodising to a selected voltage decreases as the current density increases. This indicates that the thickness of the film decreases with increasing current density, and is confirmed by the results given in Table 2. From comparison of the charge of cations in the film, determined using the results of RBS, with the charge passed during anodising, determined from the time of anodising and the current density, the efficiency of film growth was close to 100%. The full line (Fig. 2) is determined from the equation given by Li and Young [11], which was derived from data at lower temperatures, and assuming formation of stoichiometric Ta2O5 of density r /8.2 g cm 3. The experimental and calculated results are in good agreement. Further, the electric field in the anodic film increases with increase of current density, which also follows the Li and Young relationship (Fig. 3). The film formation ratio (accuracy /9/0.07), i.e. the inverse of the field strength, determined by TEM, therefore decreases with increasing current density, as confirmed by the results of Table 2. The effect of temperature on film formation can be seen in Table 3, where the film formation ratio increases with increasing temperature due to a reduced electric field. Li and Young obtained a value of 1.62 nm V 1 at 1 mA cm 2 and 25 8C,
Fig. 1. Experimental voltage /time response during the anodising of sputtering-deposited tantalum at 1 mA cm 2 to 150 V in 0.06%wt H3PO4 solution at 85 8C.
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Table 1 Slopes of voltage /time responses and charges passed during anodising of sputtering-deposited tantalum at various current densities to 150 V in 0.06%wt H3PO4 solution at 85 8C Current density (mA cm 2)
Slope of V /t response (V s 1)
Charge(V t ) (C cm 2)
Charge(RBS) (C cm 2)
100 10 1 0.1
38 3.7 0.32 0.027
0.40 0.43 0.46 0.56
0.45 0.45 0.48 0.51
Fig. 2. Slope of voltage /time response vs. current density for anodic films formed on sputtering-deposited tantalum in 0.06%wt H3PO4 solution at 85 8C. The points are the experimental data. The line is determined from the equation of Li and Young [11].
compared with 1.87 nm V 1 at 1 mA cm 2 and 85 8C from the present work. 3.2. Nature of the anodic films A two-layer film was evident from RBS and TEM (Figs. 4 and 5). Fig. 4 presents the RBS spectra of tantalum and phosphorus for a specimen anodised at 5 mA cm 2 to 150 V in 0.06%wt H3PO4 solution at 85 8C. The yield for tantalum reveals features in accordance with an expected film consisting of an inner layer of relatively pure Ta2O5, adjacent to the metal, and an outer layer of Ta2O5 containing incorporated PO3 4 anions [5] (Fig. 4(a)). The yield of phosphorus is constant through the outer layer, consistent with a uniform distribution of PO3 anions (Fig. 4(b)). 4
Fig. 3. Field strength vs. current density for anodic films formed on sputtering-deposited tantalum in 0.06%wt H3PO4 solution at 85 8C. The points are the experimental data. The line is determined from the equation of Li and Young [11].
Further, the depth of the boundary between the layers corresponds to an inner layer of thickness 162 nm, and an outer layer of thickness 115 nm, with a ratio of the outer layer thickness to the total film thickness, dout =dtotal ; of 0.42. Fig. 5 shows the alumina and tantalum substrates, at the bottom of the transmission electron micrograph, and the anodic tantala film located above the tantalum. The absence of diffracting regions indicates that the anodic film is amorphous. Direct observation reveals that the anodic tantala film displays two layers of differing contrast, with an inner, darker layer, about 161 nm thick, adjacent to the metal, and an outer, lighter layer, about 114 nm thick. The light and dark layers evidently correspond to the PO3 4 /-containing tantala layer and
Table 2 RBS and TEM results, including the thickness of anodic films and formation ratios (nm V 1) for sputtering-deposited tantalum anodised at various current densities to 150 V in 0.06%wt H3PO4 solution at 85 8C Current density (mA cm 2)
100 10 1 0.1
RBS
TEM
Total thickness (nm)
nm V 1
Total thickness (nm)
nm V 1
267 262 277 290
1.78 1.75 1.84 1.93
267 264 280 286
1.78 1.76 1.87 1.91
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Table 3 Effect of temperature and current density on anodising of sputtering-deposited tantalum in 0.06%wt H3PO4 solution Current density Temperature (8C) 1
Slope of V /t response (V s ) Film formation ratio (nm V 1) Total film thickness (nm) Total amount of phosphorus species in the film ( 1016 atoms cm 2)
45 mA cm 2 (100 V)
5 mA cm 2 (150 V)
20
85
20
85
0.014 2.21 166 1.63
0.012 2.45 184 0.26
2.14 1.67 250 2.69
1.83 1.84 277 1.19
Fig. 5. Transmission electron micrograph of an ultramicrotomed section of the anodic film formed on sputtering-deposited tantalum at 5 mA cm 2 to 150 V in 0.06%wt H3PO4 solution at 85 8C.
anodic film, as determined by RBS to an accuracy of about 5%, are presented in Fig. 6(a) and (b), revealing that both increase linearly with the logarithm of current density, hence, and field strength. If the composition of the outer layer is expressed in the form Ta2O5 ×/ x Ta3(PO4)5, the value of x also increases linearly with increase in the logarithm of current density (Fig. 7).
4. Discussion 4.1. General observations
Fig. 4. Rutherford backscattering spectra of 2.01 MeV 4He ions for sputtering-deposited tantalum anodised at 5 mA cm 2 to 150 V in 0.06% H3PO4 solution at 85 8C: (a) yield for tantalum and (b) yield for phosphorus.
relatively pure tantala layer, respectively. dout/dtotal is about 0.42, which is in agreement with the result from RBS. The effects of current density on the total amount and depth of incorporation of phosphorus species into the
Previous study of anodic tantala films formed on tantalum in phosphoric acid, using a radioactive tracer technique combined with chemical sectioning, has demonstrated clearly that films consist of two layers. The outer layer contains phosphorus species and the inner layer is free of phosphorus species [5]. The incorporation of species derived from the electrolyte into the outer layer can be explained by recent studies [8 /10], which showed that phosphate and sulphate anions are mobile, and involved in the film growth through inward migration during the ionic transport processes. The current work has confirmed the twolayered nature of the anodic films formed on tantalum in dilute phosphoric acid by direct observation of ultramicrotomed sections in the transmission electron microscope as well as by using RBS analysis. The outer
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Fig. 6. (a) Total amount of phosphorus species vs. current density and (b) ratio of the thickness of outer layer to total film thickness vs. current density for anodic films formed on sputtering-deposited tantalum to 150 V in 0.06%wt H3PO4 solution at 85 8C. The points are the experimental data. The lines are the best fits to the data.
Fig. 7. The composition of the outer layer expressed as Ta2O5 ×/ x Ta3(PO4)5 vs. current density for anodic films formed on sputtering-deposited tantalum to 150 V in 0.06%wt H3PO4 solution at 85 8C. The points are the experimental data. The line is the best fit to the data.
PO3 4 /-containing layer can be distinguished from the inner layer of relatively pure tantala by its lighter contrast in micrographs, explained by the lower density of Ta5 ions in the outer film region [18], allowing direct determination of the ratio of the outer layer thickness to the total film thickness, dout/dtotal.
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Fig. 8. (a) Total amount of phosphorus species vs. electric field and (b) ratio of outer layer thickness to total film thickness vs. electric field for anodic films formed on sputtering-deposited tantalum to 150 V in 0.06%wt H3PO4 solution at 85 8C. The points are the experimental data. The lines are the best fits to the data.
dout/dtotal increases with increasing current density, and hence, also electric field (Fig. 8). Similar trends have been reported before: dout/dtotal /0.51 /0.52 at 1 mA cm 2 [5,7,10] and dout/dtotal /0.56 at 10 mA cm 2 [7] for films formed on tantalum in 0.1 M H3PO4 at room temperature. These values compare with 0.40 and 0.46 for films formed at 1 and 10 mA cm 2 to 150 V in 0.06% H3PO4 solution at 85 8C. The reduced values indicate effects of temperature and/or the concentration of the electrolyte, which was more dilute than that used in previous work [5,7,10]. From an earlier study, the concentration of incorporated electrolyte species in the anodic film increases with increasing solution concentration, due to the available number of the appropriate species at the film/electrolyte interface [5]. The electric field strength decreases with increasing temperature and decreasing current density [11], which is confirmed by the present results (Table 3 and Fig. 3). Consequently, the total amount and depth of incorporated phosphate in the film decrease with decrease in electric field (Fig. 8). The result for the film of lowest phosphorus content, formed to 100 V at 45 mA cm 2 and 85 8C, after correction for the lower anodising voltage, deviates significantly from the linear trend of other data and is omitted from Fig. 8(a) and (b). The deviation may arise due to the low level of phosphorus in the film, which is
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approaching the limit of detection by RBS, with the possibility of a dominant signal from phosphorus species adsorbed at the film surface. Thus, the amount of phosphorus in the film may be overestimated, while the depth of the phosphorus species in the film may be underestimated. 4.2. Mechanism of film formation Regarding the mechanism of formation of the films, approximately 76% of the film material forms at the metal/film interface, associated with the inward migration of oxygen species [19], while the remainder forms at the film/electrolyte interface, associated with outward migration of metal species. The metal and oxygen species are generally assumed to be Ta5 and O2 ions, although there may be some contribution to the oxygen transport of OH ions. From observations of conservation of order of tracer species [3,4], the migration mechanism involves movements of practically all of the ions in the film by small distances, rather than shortcircuit processes. The phosphorus species migrate inward during the growth of the film, but more slowly than the oxygen species, and hence, they are distributed throughout the film material developed by migration of tantalum species, and part way through the inner layer of material developed by migration of oxygen species [10]. The controlling factors in the migration of polyatomic anions within anodic films are generally understood rather poorly, with no strong correlations of migration with size, charge or shape apparent readily [10]. The amount of phosphorus species in the film increases with the current density, varying approximately linearly with the field. A possible reason for such variation is that H2PO4 ions, presumed to be the dominant ion in the electrolyte, contribute significantly to the double layer charge density. A previous model of the anodic alumina/electrolyte interface, now applied to tantala films, predicts a linear dependence of the amount of phosphorus species in the film upon the electric field [20]. In the model, the electrolyte anions provide the double layer charge and are incorporated into freshly added monolayers of film. Further, the model indicates an atomic ratio of phosphorus to tantalum, averaged for the whole film, of about 0.14 for a film formed at 5 mA cm 2. This level is about four times greater than the measured value. However, the difference is not unreasonable given the various simplifications and uncertainties of the model and neglect of factors such as the reduced dielectric constant in the outer region of the film due to the presence of incorporated phosphorus species. Limited evidence on the effect of anodising conditions on ion migration in anodic tantala films suggests that the transport number of metal species increases with increase in current density [21,22], i.e. following a trend
of increasing transport number with increasing field. Clearly, the situation is reversed with respect to the transport number of anions, and thus, relatively less film material develops at the metal/film interface with decreasing field. On the other hand, the present findings indicate an enhancement in the relative depth of phosphorus species in the film with increasing field. This suggests that an increase in the field leads to a faster migration of phosphorus species, measured with respect to oxygen species, possibly due to the higher driving force from the field. However, the increased depth of phosphorus species may also be associated with the reduced density of tantalum species in the phosphorus-containing layer compared with the inner, phosphorus-free layer [10]. Thus, for a given number of tantalum species, the thickness of the phosphorus-containing layer is enhanced by increasing levels of phosphorus incorporation and hence, by an increased field. In order to differentiate between the possible factors determining the depth of phosphorus species in the films, marker experiments are needed for a wide range of current densities. Returning to the interest in capacitor production, anodising of sintered tantalum typically employs a temperature of 85 8C and current densities of 5/50 mA cm 2. Inspection of Figs. 6 and 8 indicates that under these conditions incorporation of phosphorus species into the film is at a low level, /3 /1015 P cm 2 for a film formed to 150 V, which is toward the upper end of the voltage range of commercial anodising. Correspondingly, the ratio of Ta3(PO4)5 to Ta2O5 in the outer layer of the film is 5/0.007.
5. Conclusions (i) Anodic films formed on tantalum in dilute 0.06%wt H3PO4 solution at 85 8C consist of two distinct layers: an inner relatively pure Ta2O5 layer, adjacent to the metal, and an outer tantala layer containing incorporated PO3 anions. The inner and outer layers 4 can be distinguished directly by TEM, due to atomic number contrast effects associated with the reduced density of tantalum species in the outer film material. (ii) The forming voltage increases linearly with time, at a rate that increases with increasing current density and decreases with increasing temperature. The formation ratio (nm V 1) increases with decreasing current density, and increasing temperature as expected for a high-field ionic migration mechanism. (iii) The total amount and depth of incorporated phosphorus species increase with increasing current density and decreasing temperature, varying approximately linearly with the electric field. (iv) Anodic tantala films, with reduced amounts of incorporated phosphorus species in the film, can be
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formed at higher temperatures and at lower current densities.
Acknowledgements The authors are grateful to the Engineering and Physical Sciences Research Council (UK) and AVX Ltd. for support of this work. They also wish to thank Dr C. Ortega of the Group de Physique des Solides, Universite´s Paris 7 et 6, for provision of time on the Van de Graaff accelerator (work partially funded by Centre Nationale de la Recherche Scientifique (GDR86)).
References [1] L. Young, Anodic Oxide Films, Academic Press, New York, 1960. [2] J.P.S. Pringle, J. Electrochem. Soc. 120 (1973) 398. [3] J.P.S. Pringle, J. Electrochem. Soc. 120 (1973) 1391. [4] J.P.S. Pringle, Electrochem. Soc. Ext. Abstracts 78-1, Abstract No.95, Seattle, Washington, 1978. [5] J.J. Randall, W.J. Bernard, R.R. Wilkinson, Electrochim. Acta 10 (1965) 183. [6] C.J. Dell’Oca, L. Young, Surf. Sci. 16 (1969) 331.
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[7] C.J. Dell’Oca, L. Young, J. Electrochem. Soc. 117 (1970) 1545. [8] P. Skeldon, K. Shimizu, G.E. Thompson, G.C. Wood, Phil. Trans. R. Soc. 348 (1994) 295. [9] G.E. Thompson, P. Skeldon, K. Shimizu, G.C. Wood, Phil. Trans. R. Soc. 350 (1995) 143. [10] K. Shimizu, K. Kobayashi, G.E. Thompson, P. Skeldon, G.C. Wood, Phil. Mag. B 73 (1996) 461. [11] Y.-M. Li, L. Young, Proc. Roy. Soc. A 454 (1998) 239. [12] G.E. Cavigliasso, M.J. Esplandiu, V. Macagno, J. Appl. Electrochem. 28 (1998) 1213. [13] G. Amsel, C. Cherki, G. Feuillade, J.P. Nadai, J. Phys. Chem. Solids 30 (1969) 2117. [14] Q. Lu, S. Mato, P. Skeldon, G.E. Thompson, D. Masheder, Thin Solid Films, in press. [15] R.C. Furneaux, G.E. Thompson, G.C. Wood, Corros. Sci. 18 (1978) 853. [16] L.R. Doolittle, Nucl. Instrum. Meth. B9 (1985) 344. [17] W.H. Knausenberger, R.N. Tauber, J. Electrochem. Soc. 120 (1964) 927. [18] K. Shimizu, G.M. Brown, H. Habazaki, K. Kobayashi, P. Skeldon, G.E. Thompson, G.C. Wood, Corros. Sci. 40 (1998) 963. [19] J.P.S. Pringle, J. Electrochem. Soc. 120 (1974) 865. [20] G.C. Wood, P. Skeldon, G.E. Thompson, K. Shimizu, J. Electrochem. Soc. 143 (1996) 74. [21] N. Khalil, A. Bowen, J.S.L. Leach, Electrochim. Acta 31 (1986) 1279. [22] N. Khalil, A. Bowen, J.S.L. Leach, Electrochim. Acta 33 (1988) 1721.
Anodic film growth on tantalum in dilute phosphoric acid solution at 20 and 85 8C Q. Lu a, S. Mato a, P. Skeldon a,*, G.E. Thompson a, D. Masheder b, H. Habazaki c, K. Shimizu d a
Corrosion and Protection Centre, University of Manchester Institute of Science and Technology, P.O. Box 88, Manchester M60 1QD, UK b AVX Limited, Tantalum Division, Long Road, Paignton, Devon TQ4 7ER, UK c Graduate Engineering School, Hokkaido University, N13 W8, Kita-ku, Sapporo 0608628, Japan d University Chemical Laboratory, Keio University, 4-1-1 Hiyoshi, Yokohama 223, Japan Received 21 January 2002; received in revised form 8 March 2002
Abstract The effects of current density and temperature on the anodic films formed on tantalum in dilute H3PO4 (0.06%wt) solution have been studied by transmission electron microscopy, using ultramicrotomed sections, and Rutherford backscattering spectroscopy. Two-layered films have been identified, comprising an inner relatively pure Ta2O5 layer, adjacent to the metal/film interface, and an anions. The total amount and depth of incorporated phosphorus species increase with outer layer containing incorporated PO3 4 increasing current density and decreasing temperature, in correspondence with the enhancement of the electric field. The formation conditions for the films include those relevant to the commercial anodising of tantalum for capacitors for which the extent of phosphorus incorporation is shown to be comparatively low. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Tantalum; Anodizing; Anodic films; Transmission electron microscopy; Rutherford backscattering spectroscopy
1. Introduction It is well known that tantalum can be anodised in almost all electrolytes except concentrated hydrofluoric acid. The role played by the electrolyte in determining the mechanism of formation, the chemical composition and the properties of anodic films on tantalum has also been studied. The films thicken uniformly with increase of electrode potential; the growth involves high field ionic transport processes, associated with the migration of tantalum and oxygen ions [1 /4]. Further, tracer studies [5] have revealed that the outer part of the film formed in phosphoric acid contains incorporated phosphorus species, derived from electrolyte anions, which decrease the permittivity and ionic conductivity of the oxide. The two-layer model for the anodic film has also been confirmed by ellipsometric studies [6,7]. Recent studies [8 /10] have shown that incorporated electrolyte-
* Corresponding author. Tel.: /44-161-200-4872; fax: /44-161200-4865.
derived species are involved in the ionic transport processes necessary for oxide thickening. Such species may be immobile (i.e. silicon species), mobile inward (i.e. phosphate and sulphate anions) and mobile outward (i.e. boron species). The mobility of incorporated species has been explained with reference to a ‘liquid’ model of film growth, and comparison of single metal / oxygen bond energies of the incorporated species in the anodic tantalum oxide [10]. The variations in the distributions of the phosphorus species are indicated by the ratios of the thicknesses of the outer film layers to the total film thicknesses, which are 0.51, 0.48 and 0.11 for films formed at 1 mA cm 2 in 0.1 M phosphoric and sulphuric acids, and 0.1 M ammonium pentaborate electrolyte; at 25 8C, respectively [10]. The influences of current density [7,11], the nature of the electrolyte [10,12] and the electrolyte concentration [13] on film growth have also been examined. Although the incorporation of electrolyte species has been observed and confirmed, and their involvement in ionic transport processes has been studied, the experiments were mainly carried out at
0013-4686/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 2 ) 0 0 1 4 1 - X
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room temperature. However, there are commercial interests in anodising powder metallurgy tantalum anodes, used to produce tantalum capacitors, at elevated temperatures. Thus, the present study examines the influence of current density on film growth and, in particular, incorporation of electrolyte species into the film for tantalum anodised at 20 and 85 8C, the latter being a temperature that is used in the manufacture of capacitors. Dielectric properties of the films [14] have been reported separately.
2. Experiments 2.1. Specimen preparation A sputtered tantalum layer was used in order to facilitate production of ultramicrotomed sections of anodic films for transmission electron microscopy (TEM). The tantalum layer was deposited on to the surface of electropolished and anodised aluminium specimens by magnetron sputtering using an Atom Tech Ltd System with a 99.9% tantalum target. The thickness of the tantalum layer was about 225 nm. A high temperature tape (Green Tree Ltd.) was used to mask the electrode to define the working area. 2.2. Anodising Tantalum specimens were anodised at different constant current densities of 0.045, 0.1, 1, 5, 10 and 100 mA cm 2, typically to 150 V. A glass electrochemical cell was used for the anodising process. The cathode was made of high purity tantalum sheet, in order to avoid contamination, and the electrolyte was stirred 0.06%wt H3PO4 solution at 859/0.5 8C. For comparison purposes, specimens were also anodised under similar conditions at 20 8C. 2.3. Analysis The thicknesses of the anodic films were measured from ultramicrotomed sections that were generated with a LEICA Ultracut UCT ultramicrotome [15] and examined in a JEOL 2000 FX II transmission electron microscope. In order to determine concentrations and distributions of electrolyte-derived species in the films, Rutherford backscattering spectroscopy (RBS) was carried out using 2.01 MeV He ions, at a current of 60 nA and with a beam diameter of 1 mm. The RBS spectra were interpreted by the RUMP program [16]. Lateral homogeneity was assumed for the interpretation of the spectra. Absolute amounts of tantalum and phosphorus atoms in the oxide, expressed in atoms cm 2, were obtained to an accuracy of /5%. Film thicknesses were derived, assuming an atomic density of
7.67 /1022 atoms cm 3 for the film [17], and compared with results of TEM.
3. Results 3.1. Anodic film growth The anodising behaviour of tantalum in 0.06%wt H3PO4 solution closely follows the expected linear growth mechanism (Fig. 1). The rate of increase of the forming voltage is strongly dependent upon current density, and increases approximately linearly with the latter (Table 1 and Fig. 2). Table 1 also shows that the charge passed during anodising to a selected voltage decreases as the current density increases. This indicates that the thickness of the film decreases with increasing current density, and is confirmed by the results given in Table 2. From comparison of the charge of cations in the film, determined using the results of RBS, with the charge passed during anodising, determined from the time of anodising and the current density, the efficiency of film growth was close to 100%. The full line (Fig. 2) is determined from the equation given by Li and Young [11], which was derived from data at lower temperatures, and assuming formation of stoichiometric Ta2O5 of density r /8.2 g cm 3. The experimental and calculated results are in good agreement. Further, the electric field in the anodic film increases with increase of current density, which also follows the Li and Young relationship (Fig. 3). The film formation ratio (accuracy /9/0.07), i.e. the inverse of the field strength, determined by TEM, therefore decreases with increasing current density, as confirmed by the results of Table 2. The effect of temperature on film formation can be seen in Table 3, where the film formation ratio increases with increasing temperature due to a reduced electric field. Li and Young obtained a value of 1.62 nm V 1 at 1 mA cm 2 and 25 8C,
Fig. 1. Experimental voltage /time response during the anodising of sputtering-deposited tantalum at 1 mA cm 2 to 150 V in 0.06%wt H3PO4 solution at 85 8C.
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Table 1 Slopes of voltage /time responses and charges passed during anodising of sputtering-deposited tantalum at various current densities to 150 V in 0.06%wt H3PO4 solution at 85 8C Current density (mA cm 2)
Slope of V /t response (V s 1)
Charge(V t ) (C cm 2)
Charge(RBS) (C cm 2)
100 10 1 0.1
38 3.7 0.32 0.027
0.40 0.43 0.46 0.56
0.45 0.45 0.48 0.51
Fig. 2. Slope of voltage /time response vs. current density for anodic films formed on sputtering-deposited tantalum in 0.06%wt H3PO4 solution at 85 8C. The points are the experimental data. The line is determined from the equation of Li and Young [11].
compared with 1.87 nm V 1 at 1 mA cm 2 and 85 8C from the present work. 3.2. Nature of the anodic films A two-layer film was evident from RBS and TEM (Figs. 4 and 5). Fig. 4 presents the RBS spectra of tantalum and phosphorus for a specimen anodised at 5 mA cm 2 to 150 V in 0.06%wt H3PO4 solution at 85 8C. The yield for tantalum reveals features in accordance with an expected film consisting of an inner layer of relatively pure Ta2O5, adjacent to the metal, and an outer layer of Ta2O5 containing incorporated PO3 4 anions [5] (Fig. 4(a)). The yield of phosphorus is constant through the outer layer, consistent with a uniform distribution of PO3 anions (Fig. 4(b)). 4
Fig. 3. Field strength vs. current density for anodic films formed on sputtering-deposited tantalum in 0.06%wt H3PO4 solution at 85 8C. The points are the experimental data. The line is determined from the equation of Li and Young [11].
Further, the depth of the boundary between the layers corresponds to an inner layer of thickness 162 nm, and an outer layer of thickness 115 nm, with a ratio of the outer layer thickness to the total film thickness, dout =dtotal ; of 0.42. Fig. 5 shows the alumina and tantalum substrates, at the bottom of the transmission electron micrograph, and the anodic tantala film located above the tantalum. The absence of diffracting regions indicates that the anodic film is amorphous. Direct observation reveals that the anodic tantala film displays two layers of differing contrast, with an inner, darker layer, about 161 nm thick, adjacent to the metal, and an outer, lighter layer, about 114 nm thick. The light and dark layers evidently correspond to the PO3 4 /-containing tantala layer and
Table 2 RBS and TEM results, including the thickness of anodic films and formation ratios (nm V 1) for sputtering-deposited tantalum anodised at various current densities to 150 V in 0.06%wt H3PO4 solution at 85 8C Current density (mA cm 2)
100 10 1 0.1
RBS
TEM
Total thickness (nm)
nm V 1
Total thickness (nm)
nm V 1
267 262 277 290
1.78 1.75 1.84 1.93
267 264 280 286
1.78 1.76 1.87 1.91
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Table 3 Effect of temperature and current density on anodising of sputtering-deposited tantalum in 0.06%wt H3PO4 solution Current density Temperature (8C) 1
Slope of V /t response (V s ) Film formation ratio (nm V 1) Total film thickness (nm) Total amount of phosphorus species in the film ( 1016 atoms cm 2)
45 mA cm 2 (100 V)
5 mA cm 2 (150 V)
20
85
20
85
0.014 2.21 166 1.63
0.012 2.45 184 0.26
2.14 1.67 250 2.69
1.83 1.84 277 1.19
Fig. 5. Transmission electron micrograph of an ultramicrotomed section of the anodic film formed on sputtering-deposited tantalum at 5 mA cm 2 to 150 V in 0.06%wt H3PO4 solution at 85 8C.
anodic film, as determined by RBS to an accuracy of about 5%, are presented in Fig. 6(a) and (b), revealing that both increase linearly with the logarithm of current density, hence, and field strength. If the composition of the outer layer is expressed in the form Ta2O5 ×/ x Ta3(PO4)5, the value of x also increases linearly with increase in the logarithm of current density (Fig. 7).
4. Discussion 4.1. General observations
Fig. 4. Rutherford backscattering spectra of 2.01 MeV 4He ions for sputtering-deposited tantalum anodised at 5 mA cm 2 to 150 V in 0.06% H3PO4 solution at 85 8C: (a) yield for tantalum and (b) yield for phosphorus.
relatively pure tantala layer, respectively. dout/dtotal is about 0.42, which is in agreement with the result from RBS. The effects of current density on the total amount and depth of incorporation of phosphorus species into the
Previous study of anodic tantala films formed on tantalum in phosphoric acid, using a radioactive tracer technique combined with chemical sectioning, has demonstrated clearly that films consist of two layers. The outer layer contains phosphorus species and the inner layer is free of phosphorus species [5]. The incorporation of species derived from the electrolyte into the outer layer can be explained by recent studies [8 /10], which showed that phosphate and sulphate anions are mobile, and involved in the film growth through inward migration during the ionic transport processes. The current work has confirmed the twolayered nature of the anodic films formed on tantalum in dilute phosphoric acid by direct observation of ultramicrotomed sections in the transmission electron microscope as well as by using RBS analysis. The outer
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Fig. 6. (a) Total amount of phosphorus species vs. current density and (b) ratio of the thickness of outer layer to total film thickness vs. current density for anodic films formed on sputtering-deposited tantalum to 150 V in 0.06%wt H3PO4 solution at 85 8C. The points are the experimental data. The lines are the best fits to the data.
Fig. 7. The composition of the outer layer expressed as Ta2O5 ×/ x Ta3(PO4)5 vs. current density for anodic films formed on sputtering-deposited tantalum to 150 V in 0.06%wt H3PO4 solution at 85 8C. The points are the experimental data. The line is the best fit to the data.
PO3 4 /-containing layer can be distinguished from the inner layer of relatively pure tantala by its lighter contrast in micrographs, explained by the lower density of Ta5 ions in the outer film region [18], allowing direct determination of the ratio of the outer layer thickness to the total film thickness, dout/dtotal.
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Fig. 8. (a) Total amount of phosphorus species vs. electric field and (b) ratio of outer layer thickness to total film thickness vs. electric field for anodic films formed on sputtering-deposited tantalum to 150 V in 0.06%wt H3PO4 solution at 85 8C. The points are the experimental data. The lines are the best fits to the data.
dout/dtotal increases with increasing current density, and hence, also electric field (Fig. 8). Similar trends have been reported before: dout/dtotal /0.51 /0.52 at 1 mA cm 2 [5,7,10] and dout/dtotal /0.56 at 10 mA cm 2 [7] for films formed on tantalum in 0.1 M H3PO4 at room temperature. These values compare with 0.40 and 0.46 for films formed at 1 and 10 mA cm 2 to 150 V in 0.06% H3PO4 solution at 85 8C. The reduced values indicate effects of temperature and/or the concentration of the electrolyte, which was more dilute than that used in previous work [5,7,10]. From an earlier study, the concentration of incorporated electrolyte species in the anodic film increases with increasing solution concentration, due to the available number of the appropriate species at the film/electrolyte interface [5]. The electric field strength decreases with increasing temperature and decreasing current density [11], which is confirmed by the present results (Table 3 and Fig. 3). Consequently, the total amount and depth of incorporated phosphate in the film decrease with decrease in electric field (Fig. 8). The result for the film of lowest phosphorus content, formed to 100 V at 45 mA cm 2 and 85 8C, after correction for the lower anodising voltage, deviates significantly from the linear trend of other data and is omitted from Fig. 8(a) and (b). The deviation may arise due to the low level of phosphorus in the film, which is
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approaching the limit of detection by RBS, with the possibility of a dominant signal from phosphorus species adsorbed at the film surface. Thus, the amount of phosphorus in the film may be overestimated, while the depth of the phosphorus species in the film may be underestimated. 4.2. Mechanism of film formation Regarding the mechanism of formation of the films, approximately 76% of the film material forms at the metal/film interface, associated with the inward migration of oxygen species [19], while the remainder forms at the film/electrolyte interface, associated with outward migration of metal species. The metal and oxygen species are generally assumed to be Ta5 and O2 ions, although there may be some contribution to the oxygen transport of OH ions. From observations of conservation of order of tracer species [3,4], the migration mechanism involves movements of practically all of the ions in the film by small distances, rather than shortcircuit processes. The phosphorus species migrate inward during the growth of the film, but more slowly than the oxygen species, and hence, they are distributed throughout the film material developed by migration of tantalum species, and part way through the inner layer of material developed by migration of oxygen species [10]. The controlling factors in the migration of polyatomic anions within anodic films are generally understood rather poorly, with no strong correlations of migration with size, charge or shape apparent readily [10]. The amount of phosphorus species in the film increases with the current density, varying approximately linearly with the field. A possible reason for such variation is that H2PO4 ions, presumed to be the dominant ion in the electrolyte, contribute significantly to the double layer charge density. A previous model of the anodic alumina/electrolyte interface, now applied to tantala films, predicts a linear dependence of the amount of phosphorus species in the film upon the electric field [20]. In the model, the electrolyte anions provide the double layer charge and are incorporated into freshly added monolayers of film. Further, the model indicates an atomic ratio of phosphorus to tantalum, averaged for the whole film, of about 0.14 for a film formed at 5 mA cm 2. This level is about four times greater than the measured value. However, the difference is not unreasonable given the various simplifications and uncertainties of the model and neglect of factors such as the reduced dielectric constant in the outer region of the film due to the presence of incorporated phosphorus species. Limited evidence on the effect of anodising conditions on ion migration in anodic tantala films suggests that the transport number of metal species increases with increase in current density [21,22], i.e. following a trend
of increasing transport number with increasing field. Clearly, the situation is reversed with respect to the transport number of anions, and thus, relatively less film material develops at the metal/film interface with decreasing field. On the other hand, the present findings indicate an enhancement in the relative depth of phosphorus species in the film with increasing field. This suggests that an increase in the field leads to a faster migration of phosphorus species, measured with respect to oxygen species, possibly due to the higher driving force from the field. However, the increased depth of phosphorus species may also be associated with the reduced density of tantalum species in the phosphorus-containing layer compared with the inner, phosphorus-free layer [10]. Thus, for a given number of tantalum species, the thickness of the phosphorus-containing layer is enhanced by increasing levels of phosphorus incorporation and hence, by an increased field. In order to differentiate between the possible factors determining the depth of phosphorus species in the films, marker experiments are needed for a wide range of current densities. Returning to the interest in capacitor production, anodising of sintered tantalum typically employs a temperature of 85 8C and current densities of 5/50 mA cm 2. Inspection of Figs. 6 and 8 indicates that under these conditions incorporation of phosphorus species into the film is at a low level, /3 /1015 P cm 2 for a film formed to 150 V, which is toward the upper end of the voltage range of commercial anodising. Correspondingly, the ratio of Ta3(PO4)5 to Ta2O5 in the outer layer of the film is 5/0.007.
5. Conclusions (i) Anodic films formed on tantalum in dilute 0.06%wt H3PO4 solution at 85 8C consist of two distinct layers: an inner relatively pure Ta2O5 layer, adjacent to the metal, and an outer tantala layer containing incorporated PO3 anions. The inner and outer layers 4 can be distinguished directly by TEM, due to atomic number contrast effects associated with the reduced density of tantalum species in the outer film material. (ii) The forming voltage increases linearly with time, at a rate that increases with increasing current density and decreases with increasing temperature. The formation ratio (nm V 1) increases with decreasing current density, and increasing temperature as expected for a high-field ionic migration mechanism. (iii) The total amount and depth of incorporated phosphorus species increase with increasing current density and decreasing temperature, varying approximately linearly with the electric field. (iv) Anodic tantala films, with reduced amounts of incorporated phosphorus species in the film, can be
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formed at higher temperatures and at lower current densities.
Acknowledgements The authors are grateful to the Engineering and Physical Sciences Research Council (UK) and AVX Ltd. for support of this work. They also wish to thank Dr C. Ortega of the Group de Physique des Solides, Universite´s Paris 7 et 6, for provision of time on the Van de Graaff accelerator (work partially funded by Centre Nationale de la Recherche Scientifique (GDR86)).
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[7] C.J. Dell’Oca, L. Young, J. Electrochem. Soc. 117 (1970) 1545. [8] P. Skeldon, K. Shimizu, G.E. Thompson, G.C. Wood, Phil. Trans. R. Soc. 348 (1994) 295. [9] G.E. Thompson, P. Skeldon, K. Shimizu, G.C. Wood, Phil. Trans. R. Soc. 350 (1995) 143. [10] K. Shimizu, K. Kobayashi, G.E. Thompson, P. Skeldon, G.C. Wood, Phil. Mag. B 73 (1996) 461. [11] Y.-M. Li, L. Young, Proc. Roy. Soc. A 454 (1998) 239. [12] G.E. Cavigliasso, M.J. Esplandiu, V. Macagno, J. Appl. Electrochem. 28 (1998) 1213. [13] G. Amsel, C. Cherki, G. Feuillade, J.P. Nadai, J. Phys. Chem. Solids 30 (1969) 2117. [14] Q. Lu, S. Mato, P. Skeldon, G.E. Thompson, D. Masheder, Thin Solid Films, in press. [15] R.C. Furneaux, G.E. Thompson, G.C. Wood, Corros. Sci. 18 (1978) 853. [16] L.R. Doolittle, Nucl. Instrum. Meth. B9 (1985) 344. [17] W.H. Knausenberger, R.N. Tauber, J. Electrochem. Soc. 120 (1964) 927. [18] K. Shimizu, G.M. Brown, H. Habazaki, K. Kobayashi, P. Skeldon, G.E. Thompson, G.C. Wood, Corros. Sci. 40 (1998) 963. [19] J.P.S. Pringle, J. Electrochem. Soc. 120 (1974) 865. [20] G.C. Wood, P. Skeldon, G.E. Thompson, K. Shimizu, J. Electrochem. Soc. 143 (1996) 74. [21] N. Khalil, A. Bowen, J.S.L. Leach, Electrochim. Acta 31 (1986) 1279. [22] N. Khalil, A. Bowen, J.S.L. Leach, Electrochim. Acta 33 (1988) 1721.