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Film-formation oxygen yield in the anodic oxidation of niobium
Electrochitttica
Acta,
1971. Vol.
16, pp.
1727 to 1737.
Persamoa
Pre!is.
Printed
in Northern
Lrclmd
FILM-FORMATION OXYGEN YIELD IN THE OXIDATION OF NIOBIUM * C.
ANODIC
CHERICI
Groupe de Physique des Solides de l’l?cole Normale Sup&ieure, Tour 23, 9 quai Saint-Bernard, Paris 5e. France Abstract--We have studied the influence of the nature of the electrolyte on the film-formation oxygen yield during the anodic oxidation of niobium. This yield R is delined as the ratio of the amount of oxygen contained in the films (measured by nuclear microanalysis) to the amount of bivalent oxygen equivalent to the oxidation charge. It has been shown that niobium behaves qualitatively like tantalum: R is greater than 1 for films formed in concentrated acids, implying a large incorporation of anions, characteristic of the acids, of the type A$,,n-. The quantitative estimations of the anion incorporation rate deduced from determination of oxygen are in good agreement with those obtained from the direct analysis of nitrogen for films formed in concentrated nitric acid. The influence on the oxygen yield of the formation field and the temperature is also studied and discussed. R&un&Nous avons 6tudi6 l’influence de la nature de l’&ctrolyte sur le rendement en oxyg&ne de formation des iilms pendant l’oxydation anodique du niobium. Ce rendement R est d&ini comrne le rapport de la qua&t6 d’oxyg&nes contenue darts les films (mesun% par microanalyse nucl&ire).iL la qua&t& d’oxygties bivalents, Bquivalente P la charge d’oxydation. 11 a Qt6 month? que le niobium se comporte qualitativement comme le tantale. R est sup&ieur ZIl’unitt5pour des 6lms form& dans des acides concentr&s, ce qui implique une grande incorporation d’anions du type A,O=- caract&istique de l’acide. Les estimations quantitatives du taux d’incorporation d’anions ddduites de la d&err& nation de l’oxyg&ne sont en bon accord avec celles obtenues de l’analyse dire&e de l’azote dans le cas des films form& dans HNO. concent&. L’influence sur le rendement en oxyg&ne du champ de formation et de la temp&-ature a Bt6 aussi &udi& et discut& Zusammenfassung-Wir untersuchten den Einfluss der Elektrolytart auf die Sauerstoffausbeute bei der anodischen Oxvdschichtbildune an Niob. Diese Ausbeute R wird definiert als das Verhsltnis des in der Schicht enihaltenen Sauer&of% (durch AktivierunEsanalyse bestimmt) zurn zweiwertigen Sauerstoff, welcher der Ladungsmenge fi.ir die Oxydation ~&sp&ht. Es wurde gezeigt, dass &h Niob aualitativ wie Tan-1 verhsllt: R ist eriisser als 1 ftir Schichten. welche in konzentrierten S&en gebild& wurden, was besagt, dass in grossem Masse fiir &e Stiure~charakteristische Anionen vom Typ AzOYn- eingebaut werden. Die aus der Sauerstoffbestimmung abgeleitete quantitative Absctitzung der Einbaugeschwindigkeit der Anionen stimmt gut mit der aus der direkten Bestimmung von Stickstoff in Schichten, welche in konzentrierter Salpeterslure gebildet wurden, hergeleiteten iiberein. Der Eintluss des elektrischen Feldes bei der Formierung sowie der Temperatur wurde ebenfalls untersucht und diskutiert. CHEMICALLY niobium is similar to tantalum. However, from the point of view of anodic oxidation, numerous differences appear and these have prevented niobium from advantageously replacing tantalum in electrolytic condensers. In fact, the leakage current of anodic oxides of niobium is usually greater than that for anodic oxides of tantalum, recrystallization under high fields is faster, and the maximum formation potential lower. Is2 In a preceding article,3 which we shall refer to as TA, the influence of the electrolyte on the composition of anodic films on tantalum was studied, and evidence for the incorporation of anions of the type ASOy”- was found in the case of films formed in concentrated acids. The principal method used was the study of the film-formation oxygen yield, ie comparison of the absolute oxygen content of the film, determined by microanalysis of 0 l*, to that equivalent to the ionic charge Q,,, passed during
* Manuscript received4 July 1970.
Work supported by the Centre National DGRST and the DRME.
de la Recherche Scientifique (RCP 1727
no 157),
the
C.
1728
CHERKI
formation. Until now the composition of anodic films on niobium had been studied using radioactive tracing. 4*5 The aim of the present work is to study the oxygen yield and incorporation of anions in niobium and to compare the results with those already obtained on tantalum. Besides this comparison, we have particularly studied the influence of temperature and formation field on the oxygen yield and incorporation rate. The principle of the experimental methods and their use are described in the article TA.s Briefly, the nuclear microanalysis employed to analyse the light nucleii contained in the films (oxygen and nitrogen) consists of direct observation of the charged particles arising from nuclear reactions induced by a beam from a particle accelerator. A detailed account of the method is available.6-10 1. EXPERIMENTAL
A.
Sample
TECHNIQUE
preparation
The niobium used was of 99.95 % purity and O-2 mm thick, The samples were rectangles (2.32 cm2 for each face) to which a tantalum lead O-5 mm in diameter was welded to insure electric contact. Traces of copper deposited by the weld electrodes were later eliminated by chemical polishing. The standard samples were usually degreased with acetone before being chemically polished for 15 s in a mixture of 5 vol 95 % H,SO,, 2 vol 53 % HNOs, and 2 vol 40 % HF. All the acids used for polishing and oxidation were analytically pure products whose maximum water content was known. During preparation of the electrolyte solutions, the composition of which will be expressed by weight, particular care was taken to avoid contact with the ambient atmosphere and absorption of water. Oxidation was carried out at constant cd, adjustable between 0.05 and 10 mA/cm2, with a platinum cathode. The total charge Q,, was measured by a digital current integrator (BNI 1000) linked with a scaler. The current supplied by the programmed Harrison 6209 B was stable to within 1.5 per cent for a variation of output voltage of 100 V. The variation of the voltage at the cell terminals was continuously recorded. Measurement of the potential to within 1% was by Keithley voltmeter of input impedance 1Ol4 Cl_ B. Experimental
conditions
of analysis
For our experiments the 2 MeV Van de Graaff accelerator of the Solid State Physics Group of the I?cole Normale Superieure was used. In most cases the beam intensity was 1 ,uA on a 1 mm diameter spot. This does not produce any appreciable heating of the targets. The beam current was integrated by a BNI 1000 integrator with a precision of about O-1 per cent. The samples were placed in high vacuum, perpendicular to the beam. Detection of charged particles arising from the observed nuclear reactions was by 3 cm2 Ortec semiconductor detectors, placed at 5 cm from the target (solid angle O-12 steradian) and at an angle of 150” with respect to the beam. Backscattered particles were absorbed by a Mylar film 19 pm thick. Table 1 gives precise experimental details for the analysis of the 016 and N14 nuclei. The counting rate indicated allows us to evaluate the minimum measurement time required to obtain a given accuracy, knowing that N counts are necessary for a relative statistical precision of l/%6_
The duration of measurements
was in general
Film-formation oxygen yield in the anodic oxidation of niobium TABLE
1. EXPERIMENTAL
CONDJX-IONS OF NUCLEAR
Nucleus Natural
0’.
abundance
Bombarding MeV
99.635 %
N”(d,
O”(d, p)o”*
a)Cla*
energy,
Energy of particles at detector, MeV Counting
MICROANALYSIS W’
99.759 %
Nuclear reaction
1729
rate*
O-900
l-300
1 1600
6.76
4.70
l The counting rates are given for a target containing lo’@ atom/cm*, for a 1 PA beam bombardment during 1 min, and a O-1 steradian solid angle at 150“ of the beam.
of the order of a minute for an accuracy of 1 per cent, except for targets containing very little of the studied nucleus. Comparison of the samples with 016 and N1* standards, the same as those used before, S allows determination of the number of atoms per cm2 contained in the film, denoted by No and NN respectively. 2.
RESULTS
A. Growth kinetics In Fig. 1 the variation of [ V(t) - V(O)]/t is shown as a function of time for oxidation at constant cd (J,,= = l-06 mA/cm*) for various electrolyte solutions. It appears that for aqueous salt solutions (between O-5 and 5 per cent) the growth of the potential 40
30
l.-.-._
-2.2
l
3 2 -*------
20 -.-.-.P.-.-.-.-.
kP I 1
.
-.-•-
-0-0-.-•
.5 E 3
-C.P.
*I
IC
Time.
v-
FIG. 1. Variation of 7
v,
t
I 2
I I
C
3 min
as a function of time in various solutions;
jo,,
1.06
mA/cmB; 23°C. 1, aqueous salt solutions O-5 % KNO,, 5 % KNO., 5 % ammonium citrate, O-1 N H.SOc; 3, 90% HCOCH; 4, 95% H.SOa. 2, 85 y0 H,PO,; B
c.
1730
CxmRKI
is a linear function of time [in accordance with equation (6) of TA] and at given cd of oxidation, the slope dV/di is constant at 16-7 V/min. The optical thickness, capacitance per unit surface and oxygen content of the oxide obtained at given j,,, for a given limiting potential, remain independent of the solutes used and their concentration. In the case of concentrated acids (H,PO,, H,S04, HCOOH) the potential v(t) is no longer a strictly linear function of time. The deviation from linearity is nevertheless even in this case the variation in especially noticeable for 95 oA HsSO,; dV/dr between the start and end of oxidation is never greater than 10 per cent. The average slope (defined as the average of the slopes at the beginning and the end of The slope in very dilute acid (O-l oxidation) increases with the acid concentration. N HsSO,) is equal to that obtained in aqueous salt solutions. Analogous observations were made for tantalum. s The fact that for the same formation current, the mean slope dV/dt increases with acid concentration is due [following equation (3) of TA] to an increase, either of molar volume of the film formed, or of the oxidation field E, with increasing acid concentration. As with tantalum, the first hypothesis can be explained by the large incorporation of polyatomic anions (SOa2-, POa8-, COOH-) characteristic of the acid. B. Fiim-formation
oxygen yield
(a) 1njruence of the electrolyte and its concentration on the oxygen yield. In this study the films were formed at a current density joX of I-06 mA/cms and up to a predetermined limitary oxidation potential K The variations of QO= and No with V are shown in Figs. 2 and 3 for various types of electrolyte. The following statements may be made. The variation of Q,, and No with Vis linear in all the electrolytes studied with the In the last case the relation between QoX and Vis no longer exception of 95 oA HsSO,. strictly linear. This is a direct consequence of the fact that d V/dr varies during film formation at constant current. For films formed in aqueous salt solutions, the values of the slope of Q&V) and No(V) are independent of the nature and concentration of the salts. But when the electrolyte is a concentrated acid, the value of the slopes depend on the nature and concentration of the acid. For very dilute acids the slopes are the same as for aqueous salt solutions. The oxygen yields can be deduced by combining the results in Figs. 2 and 3. It was assumed that both the electronic and dissolution currents were negligible compared with the ionic current in all the solutions, as in the case of tantalum. This hypothesis is in good agreement with the results in aqueous or dilute solutions, in which a current efficiency of 100% is indeed found. Table 2 gives the oxygen yields for films formed in various solutions (concentrated and dilute acids, aqueous solution of salts). As mentioned in TA,3 the absolute accuracy of the measurement of the yield can be estimated as 2 to 3 %. In Fig. 4 the variation of the oxygen yield with the acid concentration is shown for films formed at 40 V and 22OC in HsSO,. It may be seen that R decreases with acid concentration and tends towards unity at low concentrations. (b) Infruence on the field of formation. In Fig. 5, R is plotted as a function of cd The effect j0,, and hence of the formation field, for 95 o/oHaSO, and 98 oA HCOOH. of cd was studied only down to 0.06 mA because of the particularly long periods of
1731
Film-formation oxygen yield in the anodic oxidation of niobium
0
20
40
60
100
80
V
FIG. 2. Charge QoX required for film formation as a function of the limiting oxidation potential in various solutions; jO,, l-06 mA/cm’. 0. 85% H,PO,; A, aqueous salt solutions 0, 95% HoSOl; +, 90% HCOOH; (5 % ICNOS, 0.5 % KNO,). TABLE 2. DETERMINATION Electrolyte solution 95% H&O. 85 % H.m+ 95 % HNO. 98 % HCOOH 80% HCOOH Aqueous solutions of KNO, and ammonium citrate O*,N H.SOd
OF
OXYGEN
mm,
21°C R
V
10s mA/cnP
40 55 97 40 55
1.06 I-06 4 1.06 1.06
132 I16 127 149 114
70 55
1.06 l-06
101 100
%
oxidation necessary for formation of films at low fields. For both the above cases R decreases as the cd of formation decreases. Figure 6 shows the dependence on oxidation temper(c) InJlt(ence of temperature. It can be seen that ature at j,, = 1.06 mA/cm2 for 95 oA H,SO, and 98 oA HCOOH. R decreases with temperature, the influence of which on R seems less important for HCOOH than for H,SO,. C. Direct
analysis
of nitrogen
Advantage was taken of the possibilities of nuclear microanalysis of N14 to measure directly the amount of incorporated elements other than oxygen in cases of films ExperiThe oxidation was carried out at j,, = 4 mA/cm2. formed in 93 % HN09. ments were limited to this cd because of the difficulty of starting oxidation at lower
C. CHERKI
1732
v FIG. 3. Number of oxygen atoms NO containedin films formed in variouselectrolytesas a function of the limiting oxidation potential; j0=, 1.06 mA/cm2. + , 90% HCOOH; 0, 95 % H,SO,; 0,85x H.PO,; A, aqueous salt solutions. cds. In that case the number of atoms of nitrogen NN increases proportionally to the quantity of oxygen (Fig. 7). Comparing the counting rates with those obtained on tantalum nitride, whose nitrogen content is known to within 10 per cent, we have found an incorporation of 10.4 f 1 atoms of nitrogen for every 100 atoms of oxygen. This corresponds to an incorporation of (10.3 f 1) x 101* atoms per cm2 per V for oxidation at 4 mA/cmz. 3. INTERPRETATION
A.
Oxygen yield and incorporation
AND
DISCUSSION
of anions
The anomaly in the oxygen yield for oxides formed in concentrated acid solution confirms the hypothesis deduced from the variations of dV/dt, of an incorporation of polyatomic anions. This anomaly wouId seem to be better explained by the incorporation of anions AzOyn-, characteristic i>f the acid of formation, than by incorporation of hydroxyl ions, because the incorporation increases with the acid concentration. For various concentrated acids in which the oxygen yield is considerably higher than 100 per cent, Table 3 gives the number of oxygen atoms No from analysis by nuclear reaction, the number of 02- ions deduced from Q,, measurement, the number of oxygen atoms incorporated at polyatomic anions obtained by calculation and the percentage of polyatomic anions with respect to the total number of anions in the films. It was assumed that in the case of H,SO,, H,PO,, HNO, and HCOOH, the only ions other than 0% present in the film are SO**-, P04s-, NO3 and COOH-.
1733
Film-formation oxygen yield in the anodic oxidation of niobium
2-
/ .
.
_.-a R
.-*-*-*
I-
I 0
I 20
1
I 40 %
I
I 60
I
I 80
f I
0
HeSO,
FIG. 4. Variation of the oxygen yield with concentration by weight of H,SO,; jO,, 1.06 mA/cnP, 22°C.
1.7 -
I.6
-
I.5 -
R I.4
-
I.3
-
I.2
-
I I* IO.01
I
I
I
01
I
10
JOrZ
FIG.
mA/cm
5. Variation of the oxygen yield with cd of oxidation, j.,=; a, 95 % H&SO,. + ,98 % HCOOH;
21°C.
1734
C.
,-
2-c
a
I
CHERKI
.
\
,-
IE
. + . “;:\;
.
t
R
+
I .4
.
I .2
1I-
I -10
I 0
20
IO
30
50
40
FIG. 6. Variation of oxygen yield with temperature; j,,=, 1 -06 mA/cm’. + ,98 % HCOOH; 0.95 % HISO,.
I
.o
08
0
I
I 2
I 3
I 4
N0. lOI
I 5
I 6
I 7
I 8
I 9
at/cm2
FIG. 7. Nitrogen content as a function of oxygen content for films formed in 93 y0 HNO,; j0=, 4 mA/cnP.
Film-formation oxygen yield in the anodic oxidation of niobium
173s
The quantitative results obtained for anodic oxidation in 93 % HNOS are important since they confirm that the hypothesis of incorporation in the form of polyatomic anions is valid in this case. In fact the number of nitrogen atoms determined directly by nuclear reaction, (10.3 f. 1) x 1014 atom/cm*/V, is close to the number of nitrogen atoms calculated from Q,,, and N, on supposing incorporation of impurities from the solution in the form of NOa-, 9.5 x 1Ol4 atm/cmx/V. As in the case of tantalum the incorporation of impurities in the form of polyatomic anions during oxidation in concentrated acid takes place to such an extent that they can no longer be considered as trace impurities. The films formed at the surface of the metal therefore seem to be oxysalts whose global chemical composition can be written Nb,O,(, - yj(ApOY)lOylnrwhere y depends on the acid concentration. B. Comparison
with previous work
The first conclusion that can be drawn from our results is the analogy between the behaviour of niobium and tantalum as far as the influence of the nature of the electrolyte on the growth rate and oxygen yield is concerned. However, incorporation in concentrated H,SO, and H,PO, is lower at the same cd and temperature for niobium than for tantalum. The percentage of polyatomic anions incorporated is 30 per cent for Ta and I 1 per cent for Nb for oxidations in 95 % H,S04, and 12.2 per cent for Ta and 6.7 per cent for Nb in 85% H,PO,. The influence of the water content of the solutions of the incorporation rates, and their influence on the physical properties of the films (dielectric constant, refractive index, formation field, erc) have been discussed in article TA,S and the main conclusions drawn there can be applied to niobium. In our experiments we have determined the over-all impurity content of the film without looking to the distribution of this impurity throughout the thickness of the film. Other authors have used the dissolution of the films,ll the localization of radioactive tracers and ellipsometric measurements l2 to show that films formed in acids are composed of two distinct layers. The incorporated matter characteristic of the electrolyte (identified here as polyatomic anions) is contained in the external layer, thickness x1, and is distributed uniformly throughout this layer, whereas the interior portion of the film consists of an oxide, thickness x2, whose properties are close to those of the stoichiometric oxide. In our experiments, the linear relations found between the quantity of incorporated impurity such as nitrogen and the thickness of the film (total number of oxygen atoms) do not imply that the entire film contains anions of the electrolyte, but that the respective proportions of the two layers do not change as the film grows. Several authors have tried to interpret the inhomogeneity of the incorporation, by supposing that the anions are fixed and that the two layers grow at the film/solution and film/metal interfaces, as caused by the transport of metal and oxygen ions. Dell’Oca and Young9 suppose that the transport number of the metal in the film is proportional to x1 and that of oxygen to x2. This hypothesis assumes implicitly that the mechanism of migration of the oxygen and metal ions is the same in the two chemical phases composing the film. In the light of studies on isotopic exchange of films formed on tantalumla in various solutions, which have shown that the self-diffusion coefficient of oxygen is several orders of magnitude greater for films formed in concentrated acid than in stoichiometric films, the implicit hypothesis of identical transport phenomena of oxygen and metal in the two phases appears to be doubtful, Indeed the thicknesses x1 and x, are proportional to the transport number
HtSQ 95% H,PQ 85% HNOl 93% HCOOH 98% HCOOH 80%
Electrolyte
40 55 97 40 55
_
_ _
__
394 540 944 183 475
1.06 1.06 4 1.06 I.06
_ -
298 465 714 123 415
_.. -
______
128 120 276 53 80
___
__,_
._-
32 30 92 26 40
-
._
.--
11 6.7 12.1 16.7 9.2
132 116 127 149 114
R
.-----
No. of bivalent No. of oxygen Forming TotalNo. of oxygenatoms atomsin the anions No. of atoms Polyatomic voltage j,, oxygenatoms deducedfromjo, SOd2-,NOI-,PO:-, COOH- S, N, P and C anions mA/cm2 10” atoms/cm* 10” atoms/cm* - 10” atoms/w+ V 1016atoms/cma %
TABLE3. PERCENTAGE OF POLYATOMIC ANIONS WITH RESPECT TOTHETOTAL NUMBER OF ANIONS IN FILMS POIWED IN VARIOUS CONCENTRATED ACID, 21°C
-
Film-formation oxygen yield in the anodic oxidation of niobium
1737
of oxygen and tantalum ions at the interface of the stoichiometric layer and the layer containing polyatomic anions. If it is assumed that the growth of the stoichiometric layer can occur either at the film/metal or the oxide/oxysalt interface, then it can be seen that the thicknesses of the two layers are linked only to the transport numbers of oxygen and metal in the external oxysalt layer and no quantitative formation can be drawn from them on the transport phenomena in the stoichiometric layer. The increase of R with oxidation current corresponds to an increase in the total number of anionic impurities in the film. Randall et ~1.~ have shown similarly on tantalum oxidized in 0.01 N H3PS204 that the quantity of phosphorus incorporAccording to the work of Randall et al. ated increases with oxidation current. in relatively dilute acids, the increase seems to be due in part to an increase in the concentration of impurity in the external layer and in part to an increase in the thickness of this layer with respect to the total thickness of the film. In contrast, the temperature of oxidation would have no effect on the impurities content in the layer, and the decrease in R as temperature increases would be due to a decrease in the thickness of the oxysalt layer with respect to the total thickness of the film. Taking into account the above remarks about the relationship between the relative thicknesses of the Iayers of different chemical natures, some information can be drawn on the influence of field and temperature on ion transport in the oxysalt if we assume that the chemical dissolution during the growth of the external layer of the films is negligible. It seems that, at constant temperature, the transport number of Nb ions in the oxysalt layers increases when the oxidation field, and consequently the polyatomic anion content, increases. For constant oxidation field the transport number of Nb ions seems to decrease when the temperature rises. However, direct studies of the transport mechanism using 0 l8 tracers and nuclear microanalysis appear necessary to give a precise model of ion migration during growth in these inhomogeneous films. is a pleasure to acknowledge the constant help and interest of Dr. G. Amsel, and to thank Dr. J. Seijka for very helpful discussions. Acknowledgements-It
REFERENCES 1. L. YOUNG, Anodic Oxide Films. Academic Press, New York (1961). 2. A. SHTA&_. and H. T. KNIGHT, J. eiectrochem. SC&. 108, 343 (1961). 3. G. AMSEL. C. CHERKI. G. FE-LADE and J. P. NADAI. J. ohvs. Chem. Solids 30.2117 Cl . 969). _ 4. P. H. G. DRAPER, A& Met. 11,106l (1963). . * * 5. J. J. RANDALLJR., W.J.BERNARD and R. R. WILKINSON, Elecfrochim. Actu 10, 183 (1965). 6. G. AMSEL and D. SAMUEL, Analyt. Chem. 39, 1689 (1967). 7. J. P. NADAI, Thesis, Orsay (1967). 8. G. AMSEL and D. DAVID,*R~U. Phys. appl. 4,383 (1969). 9. G. AMSEL and D. DAVJD. International Conference on Properties and Use of MIS Structures. Grenoble, 1969. 10. G. AMSEL, C. CHERKI, J. P. NADAI and J. SIEJKA,Electrochemicai Society Meeting, New York, May
1969.
11. D. A. VERMILYEA, Acta Met. 2,482 (1954). 12. C. J. DELL'OCA and L. YOUNG, ElectrochemicalSociety 13. C. CEIERKI,Thesis, Paris (1969).
Meeting,
Nerv
York,
May,
1969, p. 28.
Acta,
1971. Vol.
16, pp.
1727 to 1737.
Persamoa
Pre!is.
Printed
in Northern
Lrclmd
FILM-FORMATION OXYGEN YIELD IN THE OXIDATION OF NIOBIUM * C.
ANODIC
CHERICI
Groupe de Physique des Solides de l’l?cole Normale Sup&ieure, Tour 23, 9 quai Saint-Bernard, Paris 5e. France Abstract--We have studied the influence of the nature of the electrolyte on the film-formation oxygen yield during the anodic oxidation of niobium. This yield R is delined as the ratio of the amount of oxygen contained in the films (measured by nuclear microanalysis) to the amount of bivalent oxygen equivalent to the oxidation charge. It has been shown that niobium behaves qualitatively like tantalum: R is greater than 1 for films formed in concentrated acids, implying a large incorporation of anions, characteristic of the acids, of the type A$,,n-. The quantitative estimations of the anion incorporation rate deduced from determination of oxygen are in good agreement with those obtained from the direct analysis of nitrogen for films formed in concentrated nitric acid. The influence on the oxygen yield of the formation field and the temperature is also studied and discussed. R&un&Nous avons 6tudi6 l’influence de la nature de l’&ctrolyte sur le rendement en oxyg&ne de formation des iilms pendant l’oxydation anodique du niobium. Ce rendement R est d&ini comrne le rapport de la qua&t6 d’oxyg&nes contenue darts les films (mesun% par microanalyse nucl&ire).iL la qua&t& d’oxygties bivalents, Bquivalente P la charge d’oxydation. 11 a Qt6 month? que le niobium se comporte qualitativement comme le tantale. R est sup&ieur ZIl’unitt5pour des 6lms form& dans des acides concentr&s, ce qui implique une grande incorporation d’anions du type A,O=- caract&istique de l’acide. Les estimations quantitatives du taux d’incorporation d’anions ddduites de la d&err& nation de l’oxyg&ne sont en bon accord avec celles obtenues de l’analyse dire&e de l’azote dans le cas des films form& dans HNO. concent&. L’influence sur le rendement en oxyg&ne du champ de formation et de la temp&-ature a Bt6 aussi &udi& et discut& Zusammenfassung-Wir untersuchten den Einfluss der Elektrolytart auf die Sauerstoffausbeute bei der anodischen Oxvdschichtbildune an Niob. Diese Ausbeute R wird definiert als das Verhsltnis des in der Schicht enihaltenen Sauer&of% (durch AktivierunEsanalyse bestimmt) zurn zweiwertigen Sauerstoff, welcher der Ladungsmenge fi.ir die Oxydation ~&sp&ht. Es wurde gezeigt, dass &h Niob aualitativ wie Tan-1 verhsllt: R ist eriisser als 1 ftir Schichten. welche in konzentrierten S&en gebild& wurden, was besagt, dass in grossem Masse fiir &e Stiure~charakteristische Anionen vom Typ AzOYn- eingebaut werden. Die aus der Sauerstoffbestimmung abgeleitete quantitative Absctitzung der Einbaugeschwindigkeit der Anionen stimmt gut mit der aus der direkten Bestimmung von Stickstoff in Schichten, welche in konzentrierter Salpeterslure gebildet wurden, hergeleiteten iiberein. Der Eintluss des elektrischen Feldes bei der Formierung sowie der Temperatur wurde ebenfalls untersucht und diskutiert. CHEMICALLY niobium is similar to tantalum. However, from the point of view of anodic oxidation, numerous differences appear and these have prevented niobium from advantageously replacing tantalum in electrolytic condensers. In fact, the leakage current of anodic oxides of niobium is usually greater than that for anodic oxides of tantalum, recrystallization under high fields is faster, and the maximum formation potential lower. Is2 In a preceding article,3 which we shall refer to as TA, the influence of the electrolyte on the composition of anodic films on tantalum was studied, and evidence for the incorporation of anions of the type ASOy”- was found in the case of films formed in concentrated acids. The principal method used was the study of the film-formation oxygen yield, ie comparison of the absolute oxygen content of the film, determined by microanalysis of 0 l*, to that equivalent to the ionic charge Q,,, passed during
* Manuscript received4 July 1970.
Work supported by the Centre National DGRST and the DRME.
de la Recherche Scientifique (RCP 1727
no 157),
the
C.
1728
CHERKI
formation. Until now the composition of anodic films on niobium had been studied using radioactive tracing. 4*5 The aim of the present work is to study the oxygen yield and incorporation of anions in niobium and to compare the results with those already obtained on tantalum. Besides this comparison, we have particularly studied the influence of temperature and formation field on the oxygen yield and incorporation rate. The principle of the experimental methods and their use are described in the article TA.s Briefly, the nuclear microanalysis employed to analyse the light nucleii contained in the films (oxygen and nitrogen) consists of direct observation of the charged particles arising from nuclear reactions induced by a beam from a particle accelerator. A detailed account of the method is available.6-10 1. EXPERIMENTAL
A.
Sample
TECHNIQUE
preparation
The niobium used was of 99.95 % purity and O-2 mm thick, The samples were rectangles (2.32 cm2 for each face) to which a tantalum lead O-5 mm in diameter was welded to insure electric contact. Traces of copper deposited by the weld electrodes were later eliminated by chemical polishing. The standard samples were usually degreased with acetone before being chemically polished for 15 s in a mixture of 5 vol 95 % H,SO,, 2 vol 53 % HNOs, and 2 vol 40 % HF. All the acids used for polishing and oxidation were analytically pure products whose maximum water content was known. During preparation of the electrolyte solutions, the composition of which will be expressed by weight, particular care was taken to avoid contact with the ambient atmosphere and absorption of water. Oxidation was carried out at constant cd, adjustable between 0.05 and 10 mA/cm2, with a platinum cathode. The total charge Q,, was measured by a digital current integrator (BNI 1000) linked with a scaler. The current supplied by the programmed Harrison 6209 B was stable to within 1.5 per cent for a variation of output voltage of 100 V. The variation of the voltage at the cell terminals was continuously recorded. Measurement of the potential to within 1% was by Keithley voltmeter of input impedance 1Ol4 Cl_ B. Experimental
conditions
of analysis
For our experiments the 2 MeV Van de Graaff accelerator of the Solid State Physics Group of the I?cole Normale Superieure was used. In most cases the beam intensity was 1 ,uA on a 1 mm diameter spot. This does not produce any appreciable heating of the targets. The beam current was integrated by a BNI 1000 integrator with a precision of about O-1 per cent. The samples were placed in high vacuum, perpendicular to the beam. Detection of charged particles arising from the observed nuclear reactions was by 3 cm2 Ortec semiconductor detectors, placed at 5 cm from the target (solid angle O-12 steradian) and at an angle of 150” with respect to the beam. Backscattered particles were absorbed by a Mylar film 19 pm thick. Table 1 gives precise experimental details for the analysis of the 016 and N14 nuclei. The counting rate indicated allows us to evaluate the minimum measurement time required to obtain a given accuracy, knowing that N counts are necessary for a relative statistical precision of l/%6_
The duration of measurements
was in general
Film-formation oxygen yield in the anodic oxidation of niobium TABLE
1. EXPERIMENTAL
CONDJX-IONS OF NUCLEAR
Nucleus Natural
0’.
abundance
Bombarding MeV
99.635 %
N”(d,
O”(d, p)o”*
a)Cla*
energy,
Energy of particles at detector, MeV Counting
MICROANALYSIS W’
99.759 %
Nuclear reaction
1729
rate*
O-900
l-300
1 1600
6.76
4.70
l The counting rates are given for a target containing lo’@ atom/cm*, for a 1 PA beam bombardment during 1 min, and a O-1 steradian solid angle at 150“ of the beam.
of the order of a minute for an accuracy of 1 per cent, except for targets containing very little of the studied nucleus. Comparison of the samples with 016 and N1* standards, the same as those used before, S allows determination of the number of atoms per cm2 contained in the film, denoted by No and NN respectively. 2.
RESULTS
A. Growth kinetics In Fig. 1 the variation of [ V(t) - V(O)]/t is shown as a function of time for oxidation at constant cd (J,,= = l-06 mA/cm*) for various electrolyte solutions. It appears that for aqueous salt solutions (between O-5 and 5 per cent) the growth of the potential 40
30
l.-.-._
-2.2
l
3 2 -*------
20 -.-.-.P.-.-.-.-.
kP I 1
.
-.-•-
-0-0-.-•
.5 E 3
-C.P.
*I
IC
Time.
v-
FIG. 1. Variation of 7
v,
t
I 2
I I
C
3 min
as a function of time in various solutions;
jo,,
1.06
mA/cmB; 23°C. 1, aqueous salt solutions O-5 % KNO,, 5 % KNO., 5 % ammonium citrate, O-1 N H.SOc; 3, 90% HCOCH; 4, 95% H.SOa. 2, 85 y0 H,PO,; B
c.
1730
CxmRKI
is a linear function of time [in accordance with equation (6) of TA] and at given cd of oxidation, the slope dV/di is constant at 16-7 V/min. The optical thickness, capacitance per unit surface and oxygen content of the oxide obtained at given j,,, for a given limiting potential, remain independent of the solutes used and their concentration. In the case of concentrated acids (H,PO,, H,S04, HCOOH) the potential v(t) is no longer a strictly linear function of time. The deviation from linearity is nevertheless even in this case the variation in especially noticeable for 95 oA HsSO,; dV/dr between the start and end of oxidation is never greater than 10 per cent. The average slope (defined as the average of the slopes at the beginning and the end of The slope in very dilute acid (O-l oxidation) increases with the acid concentration. N HsSO,) is equal to that obtained in aqueous salt solutions. Analogous observations were made for tantalum. s The fact that for the same formation current, the mean slope dV/dt increases with acid concentration is due [following equation (3) of TA] to an increase, either of molar volume of the film formed, or of the oxidation field E, with increasing acid concentration. As with tantalum, the first hypothesis can be explained by the large incorporation of polyatomic anions (SOa2-, POa8-, COOH-) characteristic of the acid. B. Fiim-formation
oxygen yield
(a) 1njruence of the electrolyte and its concentration on the oxygen yield. In this study the films were formed at a current density joX of I-06 mA/cms and up to a predetermined limitary oxidation potential K The variations of QO= and No with V are shown in Figs. 2 and 3 for various types of electrolyte. The following statements may be made. The variation of Q,, and No with Vis linear in all the electrolytes studied with the In the last case the relation between QoX and Vis no longer exception of 95 oA HsSO,. strictly linear. This is a direct consequence of the fact that d V/dr varies during film formation at constant current. For films formed in aqueous salt solutions, the values of the slope of Q&V) and No(V) are independent of the nature and concentration of the salts. But when the electrolyte is a concentrated acid, the value of the slopes depend on the nature and concentration of the acid. For very dilute acids the slopes are the same as for aqueous salt solutions. The oxygen yields can be deduced by combining the results in Figs. 2 and 3. It was assumed that both the electronic and dissolution currents were negligible compared with the ionic current in all the solutions, as in the case of tantalum. This hypothesis is in good agreement with the results in aqueous or dilute solutions, in which a current efficiency of 100% is indeed found. Table 2 gives the oxygen yields for films formed in various solutions (concentrated and dilute acids, aqueous solution of salts). As mentioned in TA,3 the absolute accuracy of the measurement of the yield can be estimated as 2 to 3 %. In Fig. 4 the variation of the oxygen yield with the acid concentration is shown for films formed at 40 V and 22OC in HsSO,. It may be seen that R decreases with acid concentration and tends towards unity at low concentrations. (b) Infruence on the field of formation. In Fig. 5, R is plotted as a function of cd The effect j0,, and hence of the formation field, for 95 o/oHaSO, and 98 oA HCOOH. of cd was studied only down to 0.06 mA because of the particularly long periods of
1731
Film-formation oxygen yield in the anodic oxidation of niobium
0
20
40
60
100
80
V
FIG. 2. Charge QoX required for film formation as a function of the limiting oxidation potential in various solutions; jO,, l-06 mA/cm’. 0. 85% H,PO,; A, aqueous salt solutions 0, 95% HoSOl; +, 90% HCOOH; (5 % ICNOS, 0.5 % KNO,). TABLE 2. DETERMINATION Electrolyte solution 95% H&O. 85 % H.m+ 95 % HNO. 98 % HCOOH 80% HCOOH Aqueous solutions of KNO, and ammonium citrate O*,N H.SOd
OF
OXYGEN
mm,
21°C R
V
10s mA/cnP
40 55 97 40 55
1.06 I-06 4 1.06 1.06
132 I16 127 149 114
70 55
1.06 l-06
101 100
%
oxidation necessary for formation of films at low fields. For both the above cases R decreases as the cd of formation decreases. Figure 6 shows the dependence on oxidation temper(c) InJlt(ence of temperature. It can be seen that ature at j,, = 1.06 mA/cm2 for 95 oA H,SO, and 98 oA HCOOH. R decreases with temperature, the influence of which on R seems less important for HCOOH than for H,SO,. C. Direct
analysis
of nitrogen
Advantage was taken of the possibilities of nuclear microanalysis of N14 to measure directly the amount of incorporated elements other than oxygen in cases of films ExperiThe oxidation was carried out at j,, = 4 mA/cm2. formed in 93 % HN09. ments were limited to this cd because of the difficulty of starting oxidation at lower
C. CHERKI
1732
v FIG. 3. Number of oxygen atoms NO containedin films formed in variouselectrolytesas a function of the limiting oxidation potential; j0=, 1.06 mA/cm2. + , 90% HCOOH; 0, 95 % H,SO,; 0,85x H.PO,; A, aqueous salt solutions. cds. In that case the number of atoms of nitrogen NN increases proportionally to the quantity of oxygen (Fig. 7). Comparing the counting rates with those obtained on tantalum nitride, whose nitrogen content is known to within 10 per cent, we have found an incorporation of 10.4 f 1 atoms of nitrogen for every 100 atoms of oxygen. This corresponds to an incorporation of (10.3 f 1) x 101* atoms per cm2 per V for oxidation at 4 mA/cmz. 3. INTERPRETATION
A.
Oxygen yield and incorporation
AND
DISCUSSION
of anions
The anomaly in the oxygen yield for oxides formed in concentrated acid solution confirms the hypothesis deduced from the variations of dV/dt, of an incorporation of polyatomic anions. This anomaly wouId seem to be better explained by the incorporation of anions AzOyn-, characteristic i>f the acid of formation, than by incorporation of hydroxyl ions, because the incorporation increases with the acid concentration. For various concentrated acids in which the oxygen yield is considerably higher than 100 per cent, Table 3 gives the number of oxygen atoms No from analysis by nuclear reaction, the number of 02- ions deduced from Q,, measurement, the number of oxygen atoms incorporated at polyatomic anions obtained by calculation and the percentage of polyatomic anions with respect to the total number of anions in the films. It was assumed that in the case of H,SO,, H,PO,, HNO, and HCOOH, the only ions other than 0% present in the film are SO**-, P04s-, NO3 and COOH-.
1733
Film-formation oxygen yield in the anodic oxidation of niobium
2-
/ .
.
_.-a R
.-*-*-*
I-
I 0
I 20
1
I 40 %
I
I 60
I
I 80
f I
0
HeSO,
FIG. 4. Variation of the oxygen yield with concentration by weight of H,SO,; jO,, 1.06 mA/cnP, 22°C.
1.7 -
I.6
-
I.5 -
R I.4
-
I.3
-
I.2
-
I I* IO.01
I
I
I
01
I
10
JOrZ
FIG.
mA/cm
5. Variation of the oxygen yield with cd of oxidation, j.,=; a, 95 % H&SO,. + ,98 % HCOOH;
21°C.
1734
C.
,-
2-c
a
I
CHERKI
.
\
,-
IE
. + . “;:\;
.
t
R
+
I .4
.
I .2
1I-
I -10
I 0
20
IO
30
50
40
FIG. 6. Variation of oxygen yield with temperature; j,,=, 1 -06 mA/cm’. + ,98 % HCOOH; 0.95 % HISO,.
I
.o
08
0
I
I 2
I 3
I 4
N0. lOI
I 5
I 6
I 7
I 8
I 9
at/cm2
FIG. 7. Nitrogen content as a function of oxygen content for films formed in 93 y0 HNO,; j0=, 4 mA/cnP.
Film-formation oxygen yield in the anodic oxidation of niobium
173s
The quantitative results obtained for anodic oxidation in 93 % HNOS are important since they confirm that the hypothesis of incorporation in the form of polyatomic anions is valid in this case. In fact the number of nitrogen atoms determined directly by nuclear reaction, (10.3 f. 1) x 1014 atom/cm*/V, is close to the number of nitrogen atoms calculated from Q,,, and N, on supposing incorporation of impurities from the solution in the form of NOa-, 9.5 x 1Ol4 atm/cmx/V. As in the case of tantalum the incorporation of impurities in the form of polyatomic anions during oxidation in concentrated acid takes place to such an extent that they can no longer be considered as trace impurities. The films formed at the surface of the metal therefore seem to be oxysalts whose global chemical composition can be written Nb,O,(, - yj(ApOY)lOylnrwhere y depends on the acid concentration. B. Comparison
with previous work
The first conclusion that can be drawn from our results is the analogy between the behaviour of niobium and tantalum as far as the influence of the nature of the electrolyte on the growth rate and oxygen yield is concerned. However, incorporation in concentrated H,SO, and H,PO, is lower at the same cd and temperature for niobium than for tantalum. The percentage of polyatomic anions incorporated is 30 per cent for Ta and I 1 per cent for Nb for oxidations in 95 % H,S04, and 12.2 per cent for Ta and 6.7 per cent for Nb in 85% H,PO,. The influence of the water content of the solutions of the incorporation rates, and their influence on the physical properties of the films (dielectric constant, refractive index, formation field, erc) have been discussed in article TA,S and the main conclusions drawn there can be applied to niobium. In our experiments we have determined the over-all impurity content of the film without looking to the distribution of this impurity throughout the thickness of the film. Other authors have used the dissolution of the films,ll the localization of radioactive tracers and ellipsometric measurements l2 to show that films formed in acids are composed of two distinct layers. The incorporated matter characteristic of the electrolyte (identified here as polyatomic anions) is contained in the external layer, thickness x1, and is distributed uniformly throughout this layer, whereas the interior portion of the film consists of an oxide, thickness x2, whose properties are close to those of the stoichiometric oxide. In our experiments, the linear relations found between the quantity of incorporated impurity such as nitrogen and the thickness of the film (total number of oxygen atoms) do not imply that the entire film contains anions of the electrolyte, but that the respective proportions of the two layers do not change as the film grows. Several authors have tried to interpret the inhomogeneity of the incorporation, by supposing that the anions are fixed and that the two layers grow at the film/solution and film/metal interfaces, as caused by the transport of metal and oxygen ions. Dell’Oca and Young9 suppose that the transport number of the metal in the film is proportional to x1 and that of oxygen to x2. This hypothesis assumes implicitly that the mechanism of migration of the oxygen and metal ions is the same in the two chemical phases composing the film. In the light of studies on isotopic exchange of films formed on tantalumla in various solutions, which have shown that the self-diffusion coefficient of oxygen is several orders of magnitude greater for films formed in concentrated acid than in stoichiometric films, the implicit hypothesis of identical transport phenomena of oxygen and metal in the two phases appears to be doubtful, Indeed the thicknesses x1 and x, are proportional to the transport number
HtSQ 95% H,PQ 85% HNOl 93% HCOOH 98% HCOOH 80%
Electrolyte
40 55 97 40 55
_
_ _
__
394 540 944 183 475
1.06 1.06 4 1.06 I.06
_ -
298 465 714 123 415
_.. -
______
128 120 276 53 80
___
__,_
._-
32 30 92 26 40
-
._
.--
11 6.7 12.1 16.7 9.2
132 116 127 149 114
R
.-----
No. of bivalent No. of oxygen Forming TotalNo. of oxygenatoms atomsin the anions No. of atoms Polyatomic voltage j,, oxygenatoms deducedfromjo, SOd2-,NOI-,PO:-, COOH- S, N, P and C anions mA/cm2 10” atoms/cm* 10” atoms/cm* - 10” atoms/w+ V 1016atoms/cma %
TABLE3. PERCENTAGE OF POLYATOMIC ANIONS WITH RESPECT TOTHETOTAL NUMBER OF ANIONS IN FILMS POIWED IN VARIOUS CONCENTRATED ACID, 21°C
-
Film-formation oxygen yield in the anodic oxidation of niobium
1737
of oxygen and tantalum ions at the interface of the stoichiometric layer and the layer containing polyatomic anions. If it is assumed that the growth of the stoichiometric layer can occur either at the film/metal or the oxide/oxysalt interface, then it can be seen that the thicknesses of the two layers are linked only to the transport numbers of oxygen and metal in the external oxysalt layer and no quantitative formation can be drawn from them on the transport phenomena in the stoichiometric layer. The increase of R with oxidation current corresponds to an increase in the total number of anionic impurities in the film. Randall et ~1.~ have shown similarly on tantalum oxidized in 0.01 N H3PS204 that the quantity of phosphorus incorporAccording to the work of Randall et al. ated increases with oxidation current. in relatively dilute acids, the increase seems to be due in part to an increase in the concentration of impurity in the external layer and in part to an increase in the thickness of this layer with respect to the total thickness of the film. In contrast, the temperature of oxidation would have no effect on the impurities content in the layer, and the decrease in R as temperature increases would be due to a decrease in the thickness of the oxysalt layer with respect to the total thickness of the film. Taking into account the above remarks about the relationship between the relative thicknesses of the Iayers of different chemical natures, some information can be drawn on the influence of field and temperature on ion transport in the oxysalt if we assume that the chemical dissolution during the growth of the external layer of the films is negligible. It seems that, at constant temperature, the transport number of Nb ions in the oxysalt layers increases when the oxidation field, and consequently the polyatomic anion content, increases. For constant oxidation field the transport number of Nb ions seems to decrease when the temperature rises. However, direct studies of the transport mechanism using 0 l8 tracers and nuclear microanalysis appear necessary to give a precise model of ion migration during growth in these inhomogeneous films. is a pleasure to acknowledge the constant help and interest of Dr. G. Amsel, and to thank Dr. J. Seijka for very helpful discussions. Acknowledgements-It
REFERENCES 1. L. YOUNG, Anodic Oxide Films. Academic Press, New York (1961). 2. A. SHTA&_. and H. T. KNIGHT, J. eiectrochem. SC&. 108, 343 (1961). 3. G. AMSEL. C. CHERKI. G. FE-LADE and J. P. NADAI. J. ohvs. Chem. Solids 30.2117 Cl . 969). _ 4. P. H. G. DRAPER, A& Met. 11,106l (1963). . * * 5. J. J. RANDALLJR., W.J.BERNARD and R. R. WILKINSON, Elecfrochim. Actu 10, 183 (1965). 6. G. AMSEL and D. SAMUEL, Analyt. Chem. 39, 1689 (1967). 7. J. P. NADAI, Thesis, Orsay (1967). 8. G. AMSEL and D. DAVID,*R~U. Phys. appl. 4,383 (1969). 9. G. AMSEL and D. DAVJD. International Conference on Properties and Use of MIS Structures. Grenoble, 1969. 10. G. AMSEL, C. CHERKI, J. P. NADAI and J. SIEJKA,Electrochemicai Society Meeting, New York, May
1969.
11. D. A. VERMILYEA, Acta Met. 2,482 (1954). 12. C. J. DELL'OCA and L. YOUNG, ElectrochemicalSociety 13. C. CEIERKI,Thesis, Paris (1969).
Meeting,
Nerv
York,
May,
1969, p. 28.