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Electrical instability of composite aluminum oxide films
~~ccrrochitico ~cra, 1978. Vol. 23. pp. 355-362. Rrgamon
PKS.
Printed in Orcat Baas
ELECTRICAL INSTABILITY OF COMPOSITE ALUMINUM OXIDE FILMS R. S. ALW~TTand C. K. DYER United Chemicon, Inc., 380 Union Street, West Springfield,Mass. 01089, U.S.A. (Received 1 February 1977; infirwrlfirnr 16 Ar.gwt 1977) Akstrsct - Composite fi!ms of hydrous + anodii AI,O, have the peculiar property of losing field stren& after anodization. After some time on open circuit a film that had supported several hundred volts wi!! suddenly sustain no more than 10-M V. A “refomation” to the original voltage brings the film to a stable slate. Properties of this instabilityare describedhere. It is shown to result from diffusion ofwater into internal voids within the barrier oxide layer. The voids become filled with anodic oxide during the reformation.
INTXODUCTION The properties of composite aluminum oxide films, produced by the sequential process of reaction with boiling water followed by anodic oxidation, have been the subioct of several investieationsrl-31. The water react& deposits a film of p&&do&h&&., a hydrous oxide with a structure similar to that of boehmite, yADOH, but containing excess water[4]. During the subsequent anodic oxidation some of this hydrous oxide can undergo a field-assisted dehydration and transformation to y-Af,03[1, 31 which is incorporated into the gro+ng bamer oxide layer. The temperature ofanodic oxrdation appears to play an important role in determining the structure of the barrier oxide. With an electrolyte temperature of 95” the barrier layer had a dqnsity and index of refractionxharacteristic of yAl,O,[SJ ; amorphous oxide could only have been present as a. minor constituent. On the other hand, films produced at 24” contained substantial layers of both amorphous and crystalline oxide[6]. The high temperature films have been used for many years as the dielectric layer in aluminum electrolytic capacitors. Successful commercial application in capacitors has required the ability to overcome a peculiar instability : at some time after an apparently normal film formation process the film suddenly loses field strength. That is, after soriie seconds or minutes on open circuit a fi!m that had supported several hundred volts at a low current density suddenly may sustain no more than U&-20 V, A subsequent anodic oxidation to the original voltage requires an amount of charge equal to about 5% of the initial film formation charge. After this “reformation” the film exhibits stable properties. There has been considerable effort to find ways to produce this phenomenon in a predictable and rapid fashion’s0 that the formation and reformatiAn steps can be incorporated into a single process. In patents and commercial literature one finds recommendations to insert one of the following steps between formation and reformation : application of reduced voltage, application of voltage pulses, open circuit in hot electrolyte, open circuit in hot water, heating in air, and heating in v?cuum. Each of these processes is supposed to “depolarize” the barrier film so that a stabilizing reformation can occur. Recent ellipsometric comparisons of composite barrier films before and after reformation showed that
reformation produced an increase in refractive index and density but there was no significant increase in film thickness[5]. in these cases the reformation must have deposited oxide within voids in the barrier layer. We think that these voids are the primary source of the instability. In this paper some characteristics of the instability and properties of the voids are presented. For many years it has been believed in the capacitor industry that the electrical instability was associated with the entrapment of oxygen gas within the barrier oxide during the film formation. Presumably, subsequent removal of the oxygen and replacement with electrolyte resulted in the loss of field strength, and the electrolyte then supported oxide growth in the voids during reformation. Crevecoeur and deWit have recently reported detecting 0, released from anodic alumina Bms[6]. We found no measureable amount of O2 in the films used in this study so we do not thiik there is a necessary relationship between 0, and the electrical instability. These results will be presented here. EXPERMENTAL
PROCEDURE
Measurements were made of reformation characteristics following a period in hot electrolyte on open circuit. This open circuit interval was termed the relaxation time. The sequence of process steps then was hoi! - formation - relaxation - reformation. The standard procedure was to fvst react an aluminum foil specimen with boiling distilled water for 5 min followed by anodic oxidation in 100 &l H,BO, + 0.9 ml/l 3oD/0NH,OH at 95”. (In some experiments the NH,OH was not added but this seemed to have no effect on these results). A constant cd of 1 mA/cm2 was applied until the desired voltage was reached and then the voltage was held constant until the cd dropped to 25 d/cm’. The specimen was then usually left in the electrolyte for the open circuit relaxation, followed by a reformation at 25 PA/cm* until the formation voltage was reached. Coupons with an area of 20cm’ were cut from 99.99”/, A! foil. Smooth foil was first chemically polished in a mixture of 15 parts 70% HNQ3 + 85 parts 85% H3P04 for 2 min at 85”, followed by a 10 min etch in 1N NaOH at room temperature. Most of the experiments reported here were done using heavily etched foil intended for use in electrolytic 355
R. S. ALWII-T
356
AND
capacitors,*’ in which case there was no polishing or other surface preparation. It was estimated from capacitance measurements that the etched foil had a twenty times surface area increase so the applied currents were increased accordingly. Impedance measurements were made with a conventional UCbridge that measured the equivalent series capacitance and dissipation factor. In some experiments the specimen was periodically removed from solution;rinsed and dried, in order to measure sample weight, accurate to + 1Oflg. All results are reported on a specific area basis so that smooth and etched foil data can be directly compared. For this purpose the capacitance+ weight and apparent cd ofetched specimens have heen divided by 20, the surface gain factor. It was expected that film formation would proceed at near iooO/, current efficiency[1,3] but this was verified for some specific conditions. Using an established technique the current emCiency was calculated from weight change, charge, and water content of comnosite oxiddll. The followina results were obtained with indiGd_-1 etcl& foil $ecimens that had received a 5 min boil : Formation voltage 3oov 4ooV RlmJLls
1. Jnuestigation for trapped 0, gas Crevecoeur and deWit have observed large quantities of 0, to be released during the dissolution in KOH solutions of certain anodized aluminum specimens[6]. Samples with this behavior had either been heated in oxveen at 550” or reacted with boiling water before ano& oxidation at 24” in 2% atr&onium pentaborate. We have observed that large quantities of gas were released from specimens prepared in similar fashion during the first few seconds ofimmersion in 5% H,PO, + 2% CrO, solution at 85”. The gas released from a heated + anodized specimen wtis analysed by gas chromatography and found to be oxygen. (The H,PO,-GO3 reagentdissolves the oxide but does not attack the substrate metal; hence no Hz is produced and the evolved gas is only oxygen.} We found that a volume of gas of the order 01-l ml was released from a 2.5 cm x 4 cm coupon of etched foil that had received a 4 min -boil followed by anod2OOV in 2% ization at 0.5 mA/cm’ to NH,W%Om~ 2H,O ‘at room temperature. This -is like the conditions used by Crevecoeur and deWit[6]. However, no more than lo-* ml was collected from 5 min boil + 33OV specimens formed at the conditions specified in our Experimental section. These samples were immersed in the H,POA-CrO, solution immediately after formation and before relaxation occurred. The same volume of gas, which was at the limit of detectability, was collected from other specimens after relaxation in the formation solution. More gas, about 3-4 x LO-z ml, was collected from speci-
l
C0t-p.
KHHllA
foil available from ChemiCon
‘International
C. K. DYER
mens that had been anodized in a glycol-borate electrolyte to 29OV, and these films do not have the instability found with the composite oxides. These small gas volumes may represent a background level for these measuring conditions. In any event, there appeared to be no correlation between the presence of this small volume and the presence or absence of the electrical instability. It was thought that the difference in results obtained by us and by Crevecoeur and deWit was due to the different formation temperatures used, 24” us 95”. To check this, a 200 V composite oxide was prepared at their conditions but with a formation temperature of 95”, and then it was immersedin thestrippingsolution. The volume of gas collected was less than lo-’ ml, confirming the importance of temperature to this phenomenon. 2. Effect of relaxation conditions There was a minimum relaxation time required to produce a reformation; for times shorter than this induction period the formation field was immediately established upon application of a current. This behavior is illustrated in Fig. 1 for two film thicknesses. For 175 V films the induction time (t,,) waa 10-15 s while for individual 200 V s&ens values oft were between 4 and 8 s. The value oft,, was very depe&lent upon film thickness, increasing to almost an hour at some conditions, in which cases the range ofobserved t, also widened appreciably. These observations are summarized in Table 1 for etched foil sanmles. Values oft, decreased with increasing film thick&s through 300 V, and there was then a remarkable peak in the neighborhood of 330V. This peak was extremely,sensitive to film thickness. When 3OOV films on etched foil were slightly thickened by extending the constant voltage formation to lower final cd the effect on t, was as follows:
Estimated
Final cd 25 PA/cm’ 12 5
thickness increase
to <3OS
>Smin >5min
After the induction period had elapsed there was a very rapid transition to a state that changed little upon extended relaxation. This is illustrated in Fig. 1 for 175 V and 200 V specimens. An examination of much longer relaxation times showed that the reformation charge for 300 V formations increased by about 34% over the time interval of 6-1800 s, as shown in Fig. 2.
Table
1. Range
of induction
times during relaxation
Etched foil specimens
Formation voltage 75 loo
Is0 175 200 300 330 400
1, ran&l
>900-
1800
1200>2700 240-600 10 - 15 4-S <6-330 >540>1200 t30-60
Electrical
instability
of composite
aluminum
oxide films
357
8
2
i Fig. 1. Examples ofelectrical instability of composite oxide films on open circuit. Each point represents an individual
specimen.
I J
Fig. 2. Effect of relaxation time on subsequent reformation charge for 300V specimens. Electrolyte for formation and relaxation was 100 g/l H,BO, + 0.9 ml/l 30% NH,OH (filled circles) or 3Og/l H,BO,+0.27 ml/l 30% NH,OH (open circles).
Note that the specimen relaxed for 0.5 min was still within its induction period. Films on etched foil formed to 400 V had a reformation charge of 17.5 f 2.5 mC/qn2 for relaxatiori times up to 4800 s. The reformation charge was insensitive to relaxation electrolyte, concentration or temperature. The data in Fig. 2 were obtained in either 100 g/l or 30 s/l boric acid solutions but all the results could be described by the same line. Also, the same reformation charge was required when the formation and reformation were in loOg/l boric acid but the relaxation was in log/l boric acid. The reformation charge was the same after relaxations at 70” or 95”. A second relaxation after a normal relaxation and reformation had no effect. This was tried for several formation voltages, with both smooth and etched foil, and in each case application of current after the second relaxation only resulted in a rapid rise of voltage.
3. Reformation characteristics As shown in Fig. 3, the reformation charge increased in a regular fashion with increasing film thickness up to
Fig. 3. Dependence of reformation charge on barrier film thickness. Electrolyte was standard boric acid solution (open circles) except for two measurements using 1 g/l NH,H,PO, (filled circles).
330V. These data were collected over a range of relaxation times, t, -z t 2 30 min. No reformation was ever observed for 5OV formations. At 300 V the reformation charge was about 6% of the formation charge for these etched foil samples; with smooth foil this value was 6-S%(S]. Two data points obtained using an electrolyte of 1 g/l NH,HZPOL for the process sequence are shown in Fig. 3. The results were essentially the same as with the boric acid electrolyte. It was noted above that at 4OOV the reformation charge was 17..5mC/cm2, which is only 40% of the value estimated from extrapolation of the line in Fig. 3. This is only the first indication that there is a sharp change in behavjor above 330 V. Typical voltage-time recorder traces for the constant current reformation of different thickness films are shown in Fis_ 4. In each case the final reformation cell voltage was the same as the formation voltage. The values of dV/dr for the two portions of the curves in
R. S. A~wrrr
358
AND
C.
K. DYER
Fig. 4. Typical voltage-time curves for reformation at 25 aA/cm’
of films formed to 75, 100, 200, 300 V.
Fig. 4 are about 5 V/mm for the period of slow voltage rise and 50 V/min for the finai rapid rise. The transition from slow to fast voltage rise occurred at 30-40% of the formation voltage. For comparison, it is estimated that the voltage rise for a bare metal specimen would have been about 1 V/mm. It was thought that the unusual shape of these curves might be due to variations in current efhciency. This was checked for 300 V films, which required a relaxation of less than 30 s to produce a full reformation. This minimized any possible complications from weight changes during relaxation. For two etched specimens the reformation charge and weight gain were the following: Charge (mC/cm? 29.4 28.0
Wt. gain (r8/cm2) 2.53 2.50
Current efficiency 103.5% 107.8%
The current efficiency was calculated on the basis that the weight gain was due only to deposition of 0’ - in the formation of anodic oxide. There was no evidence of low efficiency so the shape of the reformation curve must arise from some other cause. It might he noted that these specimens were each found to have a total barrier oxide weight of 114 pg/em2, so the reformation increased the oxide weight by 4.9%. A typical reformation curve for a 400 V formation is shown in Fig. 5. This resembles the curves in Fig. 4, but with the region of low dV/dt occurring at much higher voltages, existing for only a short time, and with a slope of 10 V/min. The preceding fast rise was 50 V/min and the final voltage increase was at 30 V/min. 4.
Other
changes
during
relaxation
1. Weight char&e and water conterzt. It was observed that 300 V etched foil specimens gained weight during all but the shortest relaxation periods, as shown in Table 2. To see if this weight gain was due to water entering the fXrn, the oxide films on some etched specimens were isolated by dissolving the metaI in warm Br,-methanol, dried at 85” for 30 min. and then heated for 2 h at 900°C11.This was done for specimens that had received no relaxation as well as for some that were relaxed for 40 min. The weight loss during
I
I
IO
s
5
TIME (min)
Fig. 5. Typical V-r reformation curve at 25’@/cmZ
for 400 V specimen relaxed for 10min on open circuit.
heating, assumed to be equal to the fiIm water content, was as follows: Relaxation Wt loss at 900 (&cm’) 6.85, 7.75 none 4Omin 8.78, 9.33 Table 2. Weight change during relaxation 300 V Etched foil specimens Relaxation time Weight change IPded (mid 1 10 1;
183
-0.10 f0.33 +0.68, + 1.05
+0.55
+o.a3
Electrical
instability
of composite aluminum oxide films
During relaxation the films apparently gained in the neighborhood of 1.0 to 2.5 &cm2 of water that was not removed during subsequent drying at 85”. It is interesting to compare the volume of this water, 1.0 - 2.5 x 10m6 ml/cm2, with an estimate of the void volume in the barrier film. The density of this composite barrier oxide is 3.65 g/cm3[5] and the weight of anodic oxide deposited during reformation, as given in the preceding section, was abobt 5.35 &cm2. With the assumption that this oxide had the same density as the surrounding barrier film, the volume filled b> reformation was 1.5 x 10m6 ml/cm2, about the same as the water volume. It seems that the voids in the oxide became filled with water during relaxation, and this water cannot he removed by conventional drying. The oxygen for 5.35 pg/cm’ anodic oxide must come from 2.83 pg/crn’ water. This is greater than the water in the voids so additional water must enter the oxide structure during reformation to complete that process. It should be noted that the weight gain during the 40 min relaxation (-0.85 pg/cm’) was less than the increase in water (- 1.75 pg/cm’). This indicates that some oxide was dissolved during the relaxation. Further evidence for this was the net weight loss of 0.1 pg/cm2 found after 1 min relaxation (see Table 2). Most likely some of the residual hydrous oxide was dissolved but, if it was barrier film, it was no more than 1% of the barrier film weight (109 &cm’) and would have had only a small effect on the reformation. 2. Impedance changes. The capacitance and dissipation factor increased during the relaxation period. There was generally an induction time followed by rapid change leading to a final period of stable or slowly increasing values. It was generally found that the induction time for capacitance change depended on the same parameters as t, for reformation. The dependence of reformation charge and capacitance change on relaxation time (after to) also had similarities. Both charge and capacitance increased slowly but continuously for 300 V samples while for 400 V samples both parameters exhibited a plateau. Some important evidence concerning the film structure was obtained from measurement of series capacitance and DF in electrolytes of different resistivity. Smooth foils were formed to 250 V and 400 V (10 min boil) and then relaxed in the formation solution to get beyond the period of rapid change. After measuring capacitance and DF each specimen was transferred to a more dilute electrolyte at the same temperature and again measured. This was repeated for a third set of measurements. Oxide series resistance (R,) was obtained by subtracting the calculated electrolyte resistance from the total series resistance. The results are shown in Table 3. It was found that the increase in R, during relaxation was not dependent on the electrolyte resistivity. If electrolyte had penetrated into the voids then R, would have decreased with increasing resistivity[7]. Since it was shown that water enters the voids during relaxation, it seems that the portion of the film between the voids and the bulk electrolyte is permeable to water molecules during relaxation, but at least part of this thickness cannot be penetrated by solvated ammonium or borate ions. 3. Efict of low dc voltage during relaxation. A voltage in the range IO-40 V was applied during
359
Table 3. Effect of measuring electrolyte on oxide resistance Smooth foil, 20 cm’ area, 60 Hz
Specimen 250 V (5 min boil) Before’relaxation’ After relaxation
400 V (10 min boil) Before relaxation After relaxation
Measuringelectrolyte
Oxide film
(R-cm)
R,(Q) CW=‘)
CW~~J
P
100 loo 30 10
480 1216 3584
36 274 259 281
0.695 1.35 1.31 1.13
loo loo 30 10
556 556 1264 3424
139 686 727 643
0.435 0.627 0.589 0.596
relaxation of smooth foil specimens that had been formed to 250 V. In all cases the current increased to a maximum and then decayed. Superposed on this curve was a random series of current spikes whose frequency and intensity generally decreased with time. A portion of a typical recorder trace of current us time is shown in Fig. 6 and the sniwthed curve derived from the full trace is shown in Fig. 7(a). A reformed specimen drew a steady current of only about 0.25 PA/cm’ in this type of experiment so background current was not a major interference. The charge passed was proportional to the test
OL
TINE(mln]
of current vs time recorder trace for 250 v specimen held at 40 V during relaxation.
Fig. 6. Portion
R. S. ALWIIT
360
I
I
I
2
4
,
I ‘IWE
Fig. 7. Smoothed
current-time
AND C. K. DYER
I
I
e
tmFn)
curves for
250
V specimens
held al low voltage during relaxation.
voltage. For example, in Fig. 7(a) the total charge passed before the current decayed to rl~~A/cm” was 5.0mC/cmz at 20V and 9.5 mC/cm* at 4OV. Sometimes the current increase was quite large and occurred rapidly, as shown in Fig. 7(b), but the charge remained about the same; in Fig. 7(b) the charge to 4 &A/cm’ at 40 V was X.5 mC/cm2. No more than a few percent of the total charge ever appeared in the spikes and when the i-l trace was examined with an oscilloscope no additional spikes were found. Thus, suddenopening of voids such as by release of gas bubbles or generation of macroscopic cracks appeared to be a secondary process. 5. Formation
experiments
1. Intermediate reformation. Some experiments were done in which specimens were formed, relaxed and reformed at a voltage I’,, and then formed, relaxed and reformed at a higher voltage V,. Of primary interest were the characteristics of the second reformation. The reformation CUTyeS for an etched foil coupon for which V, = 200 V and L’, = 300 V are shown in Fig. 8. A second reformation did occur and its V-r curve had the same characteristic shape as for the first reformation. The sum of the two reformation times was about that expected for an uninterrupted 3oOV formation (see Fig. 3). Perhaps the most interesting feature is that the second reformation started at about 20 V, indicating that the voids produced during the second formation penetrated into the reformed oxide of the first formation. 2. Efict ofamorphous oxide on the reformation. As seen in Figs 3 and 8, a reformation typically has a small initial voltage jump in the range S-20 V. There is indirect evidence, based on the change of d V/dt during formation[lj. for a thin layer of amorphous oxide between the y-alumina and the metal surface. To see if there was a CoMeCtk between this layer and the voltage jump, some films were produced with a thick
Fig. 8. Reformation curves after a 200 V intermediateformatioa and a final 300 V formation. amorphous layer and the effect on a subsequent reformation was observed. Smooth foil was anodized in an electrolyte of 39% ammonium pentaborate in ethylene glycol at 1 mA/cm* to 200 V, and held at 200 V until the cd decreased to 25 pA/cmd. This deposited an amorphous oxide barrier layer[8]. The samples were then reacted with boiling water for 5 min which converted the outer half of the film to pseudoboehmite, identical to the product of the Al + I-f,0 reactionC4-J These specimens were given the standard 3OOV formation and reformation in aqueous boric auid solution. The initial voltage jump in the boric acid formation indicated that 90-95V of amorphous barrier film remained after the water boil. The reformation curves in Fig. 9 have the usual shape but are displaced upward by about 125 V. The location of the amorphous/crystalline boundary is not known with certainty because a small amount of additional amorphous film may have
Fig. 9. Reformation curvesfor 300 V composite oxide films with thick amorphous oxide layers next to metal surf-.
Electrical
instability
of composite
been produced during the second formation. Although we do not know if the voids started at the amorphous surface or a small distance above it, it is clear that the voids did not penetrate into the amorphous layer. DMIUSSlON
One of the more important characteristics revealed by these experiments is that the voids never havedirect contact with bulk electrolyte. The main evidence for this is the absence of any effect of electrolyte resistivity on the series resistance of the oxide after relaxation (Table 3). The presence of a continuous layer of pseudoboehmite on the surface would be suficient to keep solute ions from the barrier oxide. Two studies[3,9] have shown that phosphate ion cannot penetrate the dense inner portion of this hydrous oxide film. Furthermore, a current efficiency of about 70% would have been expected if the reformation took place with bulk electrolyte in contact with barrier oxide or metal[l]. The looO/~current efficiency for reformation is further evidence that this process occurred under the protective cover of the hydrous oxide layer. The insensitivity of reformation charge to relaxation conditions indicates that voids are present in the film after formation but remain undetected (electrically) *.mtil water enters into the structure to produce the loss of field strength and impedance changes. This water is then consumed during the reformation as the voids become filled with anodic oxide. Water from the electrolyte could easily diffuse across the hydrous film in sumcient quantity to fill the voids. It was calculated that during .hydrous film growth the rate of diffusion of water through the film was about 2.5 Irg/rm*-min after 5 min at 100” (see Fig. 2 in [lo]). This compares with the total quantity of water in the voids of a 300 V film of no more than 2S~g/cm*. Thus, with the relaxation temperature of 95” used in these experiments it would be reasonable to consider that the diffusion of water through the hydrous oxide would be sufficiently rapid so that the rate-limiting step for transport of water to the voids occurs within the barrier film above the voids. The current peaks in Fig. 7 were probably associated with the diffusion of water into the voids. Initially water was absent and the current was very low. As water entered the voids it reacted to form anodic oxide and the current increased. The observation that the current increased to a maximum indicates that the flux of water steadily increased over that time interval. Eventually ionic transport across the growing barrier film became rate-limiting and the usual current decay occurred. Other important details of water entry are not understood. Firstly we do not know why the field must be removed, or greatly reduced, before water can enter the barrier oxide, but this is certainly the case. During the constant voltage portion of a formation an occasional small current spike might be seen superposed on the current decay curve, but never a current surge sufficient to produce a reformation. Yet, within some seconds after removing the field a full reformation would be obtained. Secondly, no explanation can be advanced for the existence of an induction period and its dependence on film thickness. This is presumably
aluminum
oxide films
361
an induction period for entry of water but this has not yet been verified experimentally. Since the reformation proceeded at lO@/, current efficiency the v-t curves (Figs 4 and 5) give an approximate measure of void distribution. That is, a low dv/dt corresponds to depositing oxide over a relatively large area and a high dV/dt results from oxide growth over a smaIl fraction of the sample area. The voltage can be used as a measure of distance from the metal interface. Thus, the curves in Fig. 4 indicate that for films between 100 and 3OOV thick the voids are concentrated in the lower 30-40% of the film but do not extend down to the metal interface. The voids occupy about 20% of the cross-sectional area in this region based on the rate of voltage rise of - 5 V/min US 1 V/tin for bare metal. The constancy of this rate of rise with increasing film thickness suggests that the voids have a uniform cross-section that is maintained as they increase in height. Although the voltage sweeps through the outer portion of the f%n quite rapidly ( - 50 V/min), the rate is still relatively slow cotppared with that for charging the dielectric, so some voids appear to exist in this portion of the film, too. Applying this method of analysis to the 400 V reformation in Fig. 5 results in a picture of a reduced void volume occupying about loO/, of the crosssectional area and existing in the outer portion of the barrier oxide. It seems as if a profound change in oxide structure occurred above 330 V. These thicker films had a number of other unusual properties. During relaxation of 400 V films there was a net weight loss : for example, whereas 300 V etched samples gained about 0.9 pg/cm2 during a 40min relaxation the 400 V samples lost about 0.75 j+$zrn’ in this same period. Cabulations that took into account the likely weight of water gained in each case led to an estimated oxide weight loss for the 400 V film that was double that for the 300V film. Electron micrographs of oxide film cross-sections showed a two layer structure for a 450 V barrier film that was not present in a 240 V film[ 1I]_ The index of refraction and density of 440-480 V films were slightly lower than for 220-300V films[5] indicating that there may be more amorphous oxide in the thicker films. It was hoped that the presence in the barrier oxide of a layer containing lO-20% void volume could be independently verified but this has not yet been successful. Ellipsometric measurements[5] showed that the index of refraction of the btirrier film was increased by the reformation, but it was not possible to determine if this was due to changes occurring in only a portion of the oxide thickness. Transmission electron micrographs of cross-sections of a 26OV oxide film[ 1 l] did not show a layer with increased electron transmission, but a 20% difference in electron intensity would have only a small effect on the photographic film emulsion and might not be easily identified. Crosssections of 450 V films did show very fine cracks about 1 nm wide, but these may have been produced during the sectioning operation. We are not able to offer a model for the source of voids or their mode of growth but some further discussion would be in order. One possibility is that voids result from a densification of the barrier film. Perhaps y-alumina does not form directly during
362
R.S.
ALWITTAND
C.K.DYER
anodic oxidation but is preceded by a less dense, more amorphous structure that subsequently rearranges to the spinel lattice, leaving a network of voids to fill the original volume. The results of the intermediate reformation experiment cause us to reject this model because. the initial reformed 200 V film was presumably in the final stable state, and yet after a subsequent 300 V anodization voids were again present within the 200 V thickness. On the other hand, Crevecoeur and deWit have associated the deposition of 0, within anodic films with the presence of yalumina[6]. If 0, can be deposited only in preexisting voids. there may be a connection between voids and
process. Rather, it is an intrinsic, regular property of the film that arises from the presence of internal voids into which water can diffuse when the formation field is removed. A subsequent reformation fills the voids with barrier oxide and stabilizes the film. The presence of 0, in some composite oxides is a secondary effect. Voids may originate from an amorphous Al,O, + y-A1,OB transformation or from the field-assisted dehydration of pseudoboehmite, but there is no completely satisfactory model for this phenomenon. When the film thickens to about 400 V and beyond there is a change in characteristics indicative of further alterations in the film structure.
this crystalline phase. The fact that voids develop in the interior of the y-
Ackru7wledgements - The authors are grateful to Mrs. Judith
alumina layer duri’tig film growth, even after an intermediate reformation, means that the process may depend upon the net transport of oxide from the interior to the film interfaces. This effect could be produced by the movement into the film of vacancies which condense into voids. A source of vacancies is the pseudoboehmite + y-alumina transformation at the barrier oxide/hydrous oxide interface. Vacancies produced at that surface could diffuse into the barrier layer until they reached the y-aIumina/amorphous oxide surface where further motion would be impeded by the amorphous structure. Voids nucleate here when the vacancy concentration reaches some critical level, and they grow by subsequent deposition of additional diffusing vacancies. However, the vacancies would presumably be oxygen vacancies with a positive charge and this model would require them to diffuse against the field, which is not likely. CONCLUSIONS
The electrical
instability
not due to entrapment
of composite oxide films is of gas or any other random
Clini for assistance here.
in obtaining
many of the results reported
REFERENCES
1. R. S. Alwitt, J. electrochem. Sot. 114. 843 (1967). 2. R. S. Alwitt, A. J. Breen and J. S. L. Leach, in Proc. Symp. Oxide-Electrolyte Interfaces (Edited by R. S. Atwitt). Electrochemical Society, Princeton, 1973, pp. 265-275. 3. R. S. Alwitt and W. J. Bernard, 3. elec&rochem. Sot. 121, 1019 (1974). 4. W. Vedder and D. A. Vermilyea, Trans. FraradaySot. 65,
561 (1969).
5. C. K. Dyer and R. S. Alwitt,
Eiectrochim.
Acta 23, 347
(1978). 6. C. Crevecoeur
and H. J. deWitt, paper presented at 27th I.S.E. meeting, Zurich (1976), extended abstract 132. 7. L. Young, Trans. Faroduy SOC.5% 842 (1959). 8. W. J. Bernard and J. W. Cook, J. electrochem. Sot. 106, 643 (1959).
9. 8. R. Baker and J. D. Baker. Proc. of. Sixth Int7 Lioht Metals Conf, (t975), p. 105-107, LeobenjVienna. U to. R. S. Alwitt, J. electrochem. Sot. 121, t322 (1974). Ii. T. Kudo and R. S. Alwitt, Electrochim. Arta. 23, 341 (1978).
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ELECTRICAL INSTABILITY OF COMPOSITE ALUMINUM OXIDE FILMS R. S. ALW~TTand C. K. DYER United Chemicon, Inc., 380 Union Street, West Springfield,Mass. 01089, U.S.A. (Received 1 February 1977; infirwrlfirnr 16 Ar.gwt 1977) Akstrsct - Composite fi!ms of hydrous + anodii AI,O, have the peculiar property of losing field stren& after anodization. After some time on open circuit a film that had supported several hundred volts wi!! suddenly sustain no more than 10-M V. A “refomation” to the original voltage brings the film to a stable slate. Properties of this instabilityare describedhere. It is shown to result from diffusion ofwater into internal voids within the barrier oxide layer. The voids become filled with anodic oxide during the reformation.
INTXODUCTION The properties of composite aluminum oxide films, produced by the sequential process of reaction with boiling water followed by anodic oxidation, have been the subioct of several investieationsrl-31. The water react& deposits a film of p&&do&h&&., a hydrous oxide with a structure similar to that of boehmite, yADOH, but containing excess water[4]. During the subsequent anodic oxidation some of this hydrous oxide can undergo a field-assisted dehydration and transformation to y-Af,03[1, 31 which is incorporated into the gro+ng bamer oxide layer. The temperature ofanodic oxrdation appears to play an important role in determining the structure of the barrier oxide. With an electrolyte temperature of 95” the barrier layer had a dqnsity and index of refractionxharacteristic of yAl,O,[SJ ; amorphous oxide could only have been present as a. minor constituent. On the other hand, films produced at 24” contained substantial layers of both amorphous and crystalline oxide[6]. The high temperature films have been used for many years as the dielectric layer in aluminum electrolytic capacitors. Successful commercial application in capacitors has required the ability to overcome a peculiar instability : at some time after an apparently normal film formation process the film suddenly loses field strength. That is, after soriie seconds or minutes on open circuit a fi!m that had supported several hundred volts at a low current density suddenly may sustain no more than U&-20 V, A subsequent anodic oxidation to the original voltage requires an amount of charge equal to about 5% of the initial film formation charge. After this “reformation” the film exhibits stable properties. There has been considerable effort to find ways to produce this phenomenon in a predictable and rapid fashion’s0 that the formation and reformatiAn steps can be incorporated into a single process. In patents and commercial literature one finds recommendations to insert one of the following steps between formation and reformation : application of reduced voltage, application of voltage pulses, open circuit in hot electrolyte, open circuit in hot water, heating in air, and heating in v?cuum. Each of these processes is supposed to “depolarize” the barrier film so that a stabilizing reformation can occur. Recent ellipsometric comparisons of composite barrier films before and after reformation showed that
reformation produced an increase in refractive index and density but there was no significant increase in film thickness[5]. in these cases the reformation must have deposited oxide within voids in the barrier layer. We think that these voids are the primary source of the instability. In this paper some characteristics of the instability and properties of the voids are presented. For many years it has been believed in the capacitor industry that the electrical instability was associated with the entrapment of oxygen gas within the barrier oxide during the film formation. Presumably, subsequent removal of the oxygen and replacement with electrolyte resulted in the loss of field strength, and the electrolyte then supported oxide growth in the voids during reformation. Crevecoeur and deWit have recently reported detecting 0, released from anodic alumina Bms[6]. We found no measureable amount of O2 in the films used in this study so we do not thiik there is a necessary relationship between 0, and the electrical instability. These results will be presented here. EXPERMENTAL
PROCEDURE
Measurements were made of reformation characteristics following a period in hot electrolyte on open circuit. This open circuit interval was termed the relaxation time. The sequence of process steps then was hoi! - formation - relaxation - reformation. The standard procedure was to fvst react an aluminum foil specimen with boiling distilled water for 5 min followed by anodic oxidation in 100 &l H,BO, + 0.9 ml/l 3oD/0NH,OH at 95”. (In some experiments the NH,OH was not added but this seemed to have no effect on these results). A constant cd of 1 mA/cm2 was applied until the desired voltage was reached and then the voltage was held constant until the cd dropped to 25 d/cm’. The specimen was then usually left in the electrolyte for the open circuit relaxation, followed by a reformation at 25 PA/cm* until the formation voltage was reached. Coupons with an area of 20cm’ were cut from 99.99”/, A! foil. Smooth foil was first chemically polished in a mixture of 15 parts 70% HNQ3 + 85 parts 85% H3P04 for 2 min at 85”, followed by a 10 min etch in 1N NaOH at room temperature. Most of the experiments reported here were done using heavily etched foil intended for use in electrolytic 355
R. S. ALWII-T
356
AND
capacitors,*’ in which case there was no polishing or other surface preparation. It was estimated from capacitance measurements that the etched foil had a twenty times surface area increase so the applied currents were increased accordingly. Impedance measurements were made with a conventional UCbridge that measured the equivalent series capacitance and dissipation factor. In some experiments the specimen was periodically removed from solution;rinsed and dried, in order to measure sample weight, accurate to + 1Oflg. All results are reported on a specific area basis so that smooth and etched foil data can be directly compared. For this purpose the capacitance+ weight and apparent cd ofetched specimens have heen divided by 20, the surface gain factor. It was expected that film formation would proceed at near iooO/, current efficiency[1,3] but this was verified for some specific conditions. Using an established technique the current emCiency was calculated from weight change, charge, and water content of comnosite oxiddll. The followina results were obtained with indiGd_-1 etcl& foil $ecimens that had received a 5 min boil : Formation voltage 3oov 4ooV RlmJLls
1. Jnuestigation for trapped 0, gas Crevecoeur and deWit have observed large quantities of 0, to be released during the dissolution in KOH solutions of certain anodized aluminum specimens[6]. Samples with this behavior had either been heated in oxveen at 550” or reacted with boiling water before ano& oxidation at 24” in 2% atr&onium pentaborate. We have observed that large quantities of gas were released from specimens prepared in similar fashion during the first few seconds ofimmersion in 5% H,PO, + 2% CrO, solution at 85”. The gas released from a heated + anodized specimen wtis analysed by gas chromatography and found to be oxygen. (The H,PO,-GO3 reagentdissolves the oxide but does not attack the substrate metal; hence no Hz is produced and the evolved gas is only oxygen.} We found that a volume of gas of the order 01-l ml was released from a 2.5 cm x 4 cm coupon of etched foil that had received a 4 min -boil followed by anod2OOV in 2% ization at 0.5 mA/cm’ to NH,W%Om~ 2H,O ‘at room temperature. This -is like the conditions used by Crevecoeur and deWit[6]. However, no more than lo-* ml was collected from 5 min boil + 33OV specimens formed at the conditions specified in our Experimental section. These samples were immersed in the H,POA-CrO, solution immediately after formation and before relaxation occurred. The same volume of gas, which was at the limit of detectability, was collected from other specimens after relaxation in the formation solution. More gas, about 3-4 x LO-z ml, was collected from speci-
l
C0t-p.
KHHllA
foil available from ChemiCon
‘International
C. K. DYER
mens that had been anodized in a glycol-borate electrolyte to 29OV, and these films do not have the instability found with the composite oxides. These small gas volumes may represent a background level for these measuring conditions. In any event, there appeared to be no correlation between the presence of this small volume and the presence or absence of the electrical instability. It was thought that the difference in results obtained by us and by Crevecoeur and deWit was due to the different formation temperatures used, 24” us 95”. To check this, a 200 V composite oxide was prepared at their conditions but with a formation temperature of 95”, and then it was immersedin thestrippingsolution. The volume of gas collected was less than lo-’ ml, confirming the importance of temperature to this phenomenon. 2. Effect of relaxation conditions There was a minimum relaxation time required to produce a reformation; for times shorter than this induction period the formation field was immediately established upon application of a current. This behavior is illustrated in Fig. 1 for two film thicknesses. For 175 V films the induction time (t,,) waa 10-15 s while for individual 200 V s&ens values oft were between 4 and 8 s. The value oft,, was very depe&lent upon film thickness, increasing to almost an hour at some conditions, in which cases the range ofobserved t, also widened appreciably. These observations are summarized in Table 1 for etched foil sanmles. Values oft, decreased with increasing film thick&s through 300 V, and there was then a remarkable peak in the neighborhood of 330V. This peak was extremely,sensitive to film thickness. When 3OOV films on etched foil were slightly thickened by extending the constant voltage formation to lower final cd the effect on t, was as follows:
Estimated
Final cd 25 PA/cm’ 12 5
thickness increase
to <3OS
>Smin >5min
After the induction period had elapsed there was a very rapid transition to a state that changed little upon extended relaxation. This is illustrated in Fig. 1 for 175 V and 200 V specimens. An examination of much longer relaxation times showed that the reformation charge for 300 V formations increased by about 34% over the time interval of 6-1800 s, as shown in Fig. 2.
Table
1. Range
of induction
times during relaxation
Etched foil specimens
Formation voltage 75 loo
Is0 175 200 300 330 400
1, ran&l
>900-
1800
1200>2700 240-600 10 - 15 4-S <6-330 >540>1200 t30-60
Electrical
instability
of composite
aluminum
oxide films
357
8
2
i Fig. 1. Examples ofelectrical instability of composite oxide films on open circuit. Each point represents an individual
specimen.
I J
Fig. 2. Effect of relaxation time on subsequent reformation charge for 300V specimens. Electrolyte for formation and relaxation was 100 g/l H,BO, + 0.9 ml/l 30% NH,OH (filled circles) or 3Og/l H,BO,+0.27 ml/l 30% NH,OH (open circles).
Note that the specimen relaxed for 0.5 min was still within its induction period. Films on etched foil formed to 400 V had a reformation charge of 17.5 f 2.5 mC/qn2 for relaxatiori times up to 4800 s. The reformation charge was insensitive to relaxation electrolyte, concentration or temperature. The data in Fig. 2 were obtained in either 100 g/l or 30 s/l boric acid solutions but all the results could be described by the same line. Also, the same reformation charge was required when the formation and reformation were in loOg/l boric acid but the relaxation was in log/l boric acid. The reformation charge was the same after relaxations at 70” or 95”. A second relaxation after a normal relaxation and reformation had no effect. This was tried for several formation voltages, with both smooth and etched foil, and in each case application of current after the second relaxation only resulted in a rapid rise of voltage.
3. Reformation characteristics As shown in Fig. 3, the reformation charge increased in a regular fashion with increasing film thickness up to
Fig. 3. Dependence of reformation charge on barrier film thickness. Electrolyte was standard boric acid solution (open circles) except for two measurements using 1 g/l NH,H,PO, (filled circles).
330V. These data were collected over a range of relaxation times, t, -z t 2 30 min. No reformation was ever observed for 5OV formations. At 300 V the reformation charge was about 6% of the formation charge for these etched foil samples; with smooth foil this value was 6-S%(S]. Two data points obtained using an electrolyte of 1 g/l NH,HZPOL for the process sequence are shown in Fig. 3. The results were essentially the same as with the boric acid electrolyte. It was noted above that at 4OOV the reformation charge was 17..5mC/cm2, which is only 40% of the value estimated from extrapolation of the line in Fig. 3. This is only the first indication that there is a sharp change in behavjor above 330 V. Typical voltage-time recorder traces for the constant current reformation of different thickness films are shown in Fis_ 4. In each case the final reformation cell voltage was the same as the formation voltage. The values of dV/dr for the two portions of the curves in
R. S. A~wrrr
358
AND
C.
K. DYER
Fig. 4. Typical voltage-time curves for reformation at 25 aA/cm’
of films formed to 75, 100, 200, 300 V.
Fig. 4 are about 5 V/mm for the period of slow voltage rise and 50 V/min for the finai rapid rise. The transition from slow to fast voltage rise occurred at 30-40% of the formation voltage. For comparison, it is estimated that the voltage rise for a bare metal specimen would have been about 1 V/mm. It was thought that the unusual shape of these curves might be due to variations in current efhciency. This was checked for 300 V films, which required a relaxation of less than 30 s to produce a full reformation. This minimized any possible complications from weight changes during relaxation. For two etched specimens the reformation charge and weight gain were the following: Charge (mC/cm? 29.4 28.0
Wt. gain (r8/cm2) 2.53 2.50
Current efficiency 103.5% 107.8%
The current efficiency was calculated on the basis that the weight gain was due only to deposition of 0’ - in the formation of anodic oxide. There was no evidence of low efficiency so the shape of the reformation curve must arise from some other cause. It might he noted that these specimens were each found to have a total barrier oxide weight of 114 pg/em2, so the reformation increased the oxide weight by 4.9%. A typical reformation curve for a 400 V formation is shown in Fig. 5. This resembles the curves in Fig. 4, but with the region of low dV/dt occurring at much higher voltages, existing for only a short time, and with a slope of 10 V/min. The preceding fast rise was 50 V/min and the final voltage increase was at 30 V/min. 4.
Other
changes
during
relaxation
1. Weight char&e and water conterzt. It was observed that 300 V etched foil specimens gained weight during all but the shortest relaxation periods, as shown in Table 2. To see if this weight gain was due to water entering the fXrn, the oxide films on some etched specimens were isolated by dissolving the metaI in warm Br,-methanol, dried at 85” for 30 min. and then heated for 2 h at 900°C11.This was done for specimens that had received no relaxation as well as for some that were relaxed for 40 min. The weight loss during
I
I
IO
s
5
TIME (min)
Fig. 5. Typical V-r reformation curve at 25’@/cmZ
for 400 V specimen relaxed for 10min on open circuit.
heating, assumed to be equal to the fiIm water content, was as follows: Relaxation Wt loss at 900 (&cm’) 6.85, 7.75 none 4Omin 8.78, 9.33 Table 2. Weight change during relaxation 300 V Etched foil specimens Relaxation time Weight change IPded (mid 1 10 1;
183
-0.10 f0.33 +0.68, + 1.05
+0.55
+o.a3
Electrical
instability
of composite aluminum oxide films
During relaxation the films apparently gained in the neighborhood of 1.0 to 2.5 &cm2 of water that was not removed during subsequent drying at 85”. It is interesting to compare the volume of this water, 1.0 - 2.5 x 10m6 ml/cm2, with an estimate of the void volume in the barrier film. The density of this composite barrier oxide is 3.65 g/cm3[5] and the weight of anodic oxide deposited during reformation, as given in the preceding section, was abobt 5.35 &cm2. With the assumption that this oxide had the same density as the surrounding barrier film, the volume filled b> reformation was 1.5 x 10m6 ml/cm2, about the same as the water volume. It seems that the voids in the oxide became filled with water during relaxation, and this water cannot he removed by conventional drying. The oxygen for 5.35 pg/cm’ anodic oxide must come from 2.83 pg/crn’ water. This is greater than the water in the voids so additional water must enter the oxide structure during reformation to complete that process. It should be noted that the weight gain during the 40 min relaxation (-0.85 pg/cm’) was less than the increase in water (- 1.75 pg/cm’). This indicates that some oxide was dissolved during the relaxation. Further evidence for this was the net weight loss of 0.1 pg/cm2 found after 1 min relaxation (see Table 2). Most likely some of the residual hydrous oxide was dissolved but, if it was barrier film, it was no more than 1% of the barrier film weight (109 &cm’) and would have had only a small effect on the reformation. 2. Impedance changes. The capacitance and dissipation factor increased during the relaxation period. There was generally an induction time followed by rapid change leading to a final period of stable or slowly increasing values. It was generally found that the induction time for capacitance change depended on the same parameters as t, for reformation. The dependence of reformation charge and capacitance change on relaxation time (after to) also had similarities. Both charge and capacitance increased slowly but continuously for 300 V samples while for 400 V samples both parameters exhibited a plateau. Some important evidence concerning the film structure was obtained from measurement of series capacitance and DF in electrolytes of different resistivity. Smooth foils were formed to 250 V and 400 V (10 min boil) and then relaxed in the formation solution to get beyond the period of rapid change. After measuring capacitance and DF each specimen was transferred to a more dilute electrolyte at the same temperature and again measured. This was repeated for a third set of measurements. Oxide series resistance (R,) was obtained by subtracting the calculated electrolyte resistance from the total series resistance. The results are shown in Table 3. It was found that the increase in R, during relaxation was not dependent on the electrolyte resistivity. If electrolyte had penetrated into the voids then R, would have decreased with increasing resistivity[7]. Since it was shown that water enters the voids during relaxation, it seems that the portion of the film between the voids and the bulk electrolyte is permeable to water molecules during relaxation, but at least part of this thickness cannot be penetrated by solvated ammonium or borate ions. 3. Efict of low dc voltage during relaxation. A voltage in the range IO-40 V was applied during
359
Table 3. Effect of measuring electrolyte on oxide resistance Smooth foil, 20 cm’ area, 60 Hz
Specimen 250 V (5 min boil) Before’relaxation’ After relaxation
400 V (10 min boil) Before relaxation After relaxation
Measuringelectrolyte
Oxide film
(R-cm)
R,(Q) CW=‘)
CW~~J
P
100 loo 30 10
480 1216 3584
36 274 259 281
0.695 1.35 1.31 1.13
loo loo 30 10
556 556 1264 3424
139 686 727 643
0.435 0.627 0.589 0.596
relaxation of smooth foil specimens that had been formed to 250 V. In all cases the current increased to a maximum and then decayed. Superposed on this curve was a random series of current spikes whose frequency and intensity generally decreased with time. A portion of a typical recorder trace of current us time is shown in Fig. 6 and the sniwthed curve derived from the full trace is shown in Fig. 7(a). A reformed specimen drew a steady current of only about 0.25 PA/cm’ in this type of experiment so background current was not a major interference. The charge passed was proportional to the test
OL
TINE(mln]
of current vs time recorder trace for 250 v specimen held at 40 V during relaxation.
Fig. 6. Portion
R. S. ALWIIT
360
I
I
I
2
4
,
I ‘IWE
Fig. 7. Smoothed
current-time
AND C. K. DYER
I
I
e
tmFn)
curves for
250
V specimens
held al low voltage during relaxation.
voltage. For example, in Fig. 7(a) the total charge passed before the current decayed to rl~~A/cm” was 5.0mC/cmz at 20V and 9.5 mC/cm* at 4OV. Sometimes the current increase was quite large and occurred rapidly, as shown in Fig. 7(b), but the charge remained about the same; in Fig. 7(b) the charge to 4 &A/cm’ at 40 V was X.5 mC/cm2. No more than a few percent of the total charge ever appeared in the spikes and when the i-l trace was examined with an oscilloscope no additional spikes were found. Thus, suddenopening of voids such as by release of gas bubbles or generation of macroscopic cracks appeared to be a secondary process. 5. Formation
experiments
1. Intermediate reformation. Some experiments were done in which specimens were formed, relaxed and reformed at a voltage I’,, and then formed, relaxed and reformed at a higher voltage V,. Of primary interest were the characteristics of the second reformation. The reformation CUTyeS for an etched foil coupon for which V, = 200 V and L’, = 300 V are shown in Fig. 8. A second reformation did occur and its V-r curve had the same characteristic shape as for the first reformation. The sum of the two reformation times was about that expected for an uninterrupted 3oOV formation (see Fig. 3). Perhaps the most interesting feature is that the second reformation started at about 20 V, indicating that the voids produced during the second formation penetrated into the reformed oxide of the first formation. 2. Efict ofamorphous oxide on the reformation. As seen in Figs 3 and 8, a reformation typically has a small initial voltage jump in the range S-20 V. There is indirect evidence, based on the change of d V/dt during formation[lj. for a thin layer of amorphous oxide between the y-alumina and the metal surface. To see if there was a CoMeCtk between this layer and the voltage jump, some films were produced with a thick
Fig. 8. Reformation curves after a 200 V intermediateformatioa and a final 300 V formation. amorphous layer and the effect on a subsequent reformation was observed. Smooth foil was anodized in an electrolyte of 39% ammonium pentaborate in ethylene glycol at 1 mA/cm* to 200 V, and held at 200 V until the cd decreased to 25 pA/cmd. This deposited an amorphous oxide barrier layer[8]. The samples were then reacted with boiling water for 5 min which converted the outer half of the film to pseudoboehmite, identical to the product of the Al + I-f,0 reactionC4-J These specimens were given the standard 3OOV formation and reformation in aqueous boric auid solution. The initial voltage jump in the boric acid formation indicated that 90-95V of amorphous barrier film remained after the water boil. The reformation curves in Fig. 9 have the usual shape but are displaced upward by about 125 V. The location of the amorphous/crystalline boundary is not known with certainty because a small amount of additional amorphous film may have
Fig. 9. Reformation curvesfor 300 V composite oxide films with thick amorphous oxide layers next to metal surf-.
Electrical
instability
of composite
been produced during the second formation. Although we do not know if the voids started at the amorphous surface or a small distance above it, it is clear that the voids did not penetrate into the amorphous layer. DMIUSSlON
One of the more important characteristics revealed by these experiments is that the voids never havedirect contact with bulk electrolyte. The main evidence for this is the absence of any effect of electrolyte resistivity on the series resistance of the oxide after relaxation (Table 3). The presence of a continuous layer of pseudoboehmite on the surface would be suficient to keep solute ions from the barrier oxide. Two studies[3,9] have shown that phosphate ion cannot penetrate the dense inner portion of this hydrous oxide film. Furthermore, a current efficiency of about 70% would have been expected if the reformation took place with bulk electrolyte in contact with barrier oxide or metal[l]. The looO/~current efficiency for reformation is further evidence that this process occurred under the protective cover of the hydrous oxide layer. The insensitivity of reformation charge to relaxation conditions indicates that voids are present in the film after formation but remain undetected (electrically) *.mtil water enters into the structure to produce the loss of field strength and impedance changes. This water is then consumed during the reformation as the voids become filled with anodic oxide. Water from the electrolyte could easily diffuse across the hydrous film in sumcient quantity to fill the voids. It was calculated that during .hydrous film growth the rate of diffusion of water through the film was about 2.5 Irg/rm*-min after 5 min at 100” (see Fig. 2 in [lo]). This compares with the total quantity of water in the voids of a 300 V film of no more than 2S~g/cm*. Thus, with the relaxation temperature of 95” used in these experiments it would be reasonable to consider that the diffusion of water through the hydrous oxide would be sufficiently rapid so that the rate-limiting step for transport of water to the voids occurs within the barrier film above the voids. The current peaks in Fig. 7 were probably associated with the diffusion of water into the voids. Initially water was absent and the current was very low. As water entered the voids it reacted to form anodic oxide and the current increased. The observation that the current increased to a maximum indicates that the flux of water steadily increased over that time interval. Eventually ionic transport across the growing barrier film became rate-limiting and the usual current decay occurred. Other important details of water entry are not understood. Firstly we do not know why the field must be removed, or greatly reduced, before water can enter the barrier oxide, but this is certainly the case. During the constant voltage portion of a formation an occasional small current spike might be seen superposed on the current decay curve, but never a current surge sufficient to produce a reformation. Yet, within some seconds after removing the field a full reformation would be obtained. Secondly, no explanation can be advanced for the existence of an induction period and its dependence on film thickness. This is presumably
aluminum
oxide films
361
an induction period for entry of water but this has not yet been verified experimentally. Since the reformation proceeded at lO@/, current efficiency the v-t curves (Figs 4 and 5) give an approximate measure of void distribution. That is, a low dv/dt corresponds to depositing oxide over a relatively large area and a high dV/dt results from oxide growth over a smaIl fraction of the sample area. The voltage can be used as a measure of distance from the metal interface. Thus, the curves in Fig. 4 indicate that for films between 100 and 3OOV thick the voids are concentrated in the lower 30-40% of the film but do not extend down to the metal interface. The voids occupy about 20% of the cross-sectional area in this region based on the rate of voltage rise of - 5 V/min US 1 V/tin for bare metal. The constancy of this rate of rise with increasing film thickness suggests that the voids have a uniform cross-section that is maintained as they increase in height. Although the voltage sweeps through the outer portion of the f%n quite rapidly ( - 50 V/min), the rate is still relatively slow cotppared with that for charging the dielectric, so some voids appear to exist in this portion of the film, too. Applying this method of analysis to the 400 V reformation in Fig. 5 results in a picture of a reduced void volume occupying about loO/, of the crosssectional area and existing in the outer portion of the barrier oxide. It seems as if a profound change in oxide structure occurred above 330 V. These thicker films had a number of other unusual properties. During relaxation of 400 V films there was a net weight loss : for example, whereas 300 V etched samples gained about 0.9 pg/cm2 during a 40min relaxation the 400 V samples lost about 0.75 j+$zrn’ in this same period. Cabulations that took into account the likely weight of water gained in each case led to an estimated oxide weight loss for the 400 V film that was double that for the 300V film. Electron micrographs of oxide film cross-sections showed a two layer structure for a 450 V barrier film that was not present in a 240 V film[ 1I]_ The index of refraction and density of 440-480 V films were slightly lower than for 220-300V films[5] indicating that there may be more amorphous oxide in the thicker films. It was hoped that the presence in the barrier oxide of a layer containing lO-20% void volume could be independently verified but this has not yet been successful. Ellipsometric measurements[5] showed that the index of refraction of the btirrier film was increased by the reformation, but it was not possible to determine if this was due to changes occurring in only a portion of the oxide thickness. Transmission electron micrographs of cross-sections of a 26OV oxide film[ 1 l] did not show a layer with increased electron transmission, but a 20% difference in electron intensity would have only a small effect on the photographic film emulsion and might not be easily identified. Crosssections of 450 V films did show very fine cracks about 1 nm wide, but these may have been produced during the sectioning operation. We are not able to offer a model for the source of voids or their mode of growth but some further discussion would be in order. One possibility is that voids result from a densification of the barrier film. Perhaps y-alumina does not form directly during
362
R.S.
ALWITTAND
C.K.DYER
anodic oxidation but is preceded by a less dense, more amorphous structure that subsequently rearranges to the spinel lattice, leaving a network of voids to fill the original volume. The results of the intermediate reformation experiment cause us to reject this model because. the initial reformed 200 V film was presumably in the final stable state, and yet after a subsequent 300 V anodization voids were again present within the 200 V thickness. On the other hand, Crevecoeur and deWit have associated the deposition of 0, within anodic films with the presence of yalumina[6]. If 0, can be deposited only in preexisting voids. there may be a connection between voids and
process. Rather, it is an intrinsic, regular property of the film that arises from the presence of internal voids into which water can diffuse when the formation field is removed. A subsequent reformation fills the voids with barrier oxide and stabilizes the film. The presence of 0, in some composite oxides is a secondary effect. Voids may originate from an amorphous Al,O, + y-A1,OB transformation or from the field-assisted dehydration of pseudoboehmite, but there is no completely satisfactory model for this phenomenon. When the film thickens to about 400 V and beyond there is a change in characteristics indicative of further alterations in the film structure.
this crystalline phase. The fact that voids develop in the interior of the y-
Ackru7wledgements - The authors are grateful to Mrs. Judith
alumina layer duri’tig film growth, even after an intermediate reformation, means that the process may depend upon the net transport of oxide from the interior to the film interfaces. This effect could be produced by the movement into the film of vacancies which condense into voids. A source of vacancies is the pseudoboehmite + y-alumina transformation at the barrier oxide/hydrous oxide interface. Vacancies produced at that surface could diffuse into the barrier layer until they reached the y-aIumina/amorphous oxide surface where further motion would be impeded by the amorphous structure. Voids nucleate here when the vacancy concentration reaches some critical level, and they grow by subsequent deposition of additional diffusing vacancies. However, the vacancies would presumably be oxygen vacancies with a positive charge and this model would require them to diffuse against the field, which is not likely. CONCLUSIONS
The electrical
instability
not due to entrapment
of composite oxide films is of gas or any other random
Clini for assistance here.
in obtaining
many of the results reported
REFERENCES
1. R. S. Alwitt, J. electrochem. Sot. 114. 843 (1967). 2. R. S. Alwitt, A. J. Breen and J. S. L. Leach, in Proc. Symp. Oxide-Electrolyte Interfaces (Edited by R. S. Atwitt). Electrochemical Society, Princeton, 1973, pp. 265-275. 3. R. S. Alwitt and W. J. Bernard, 3. elec&rochem. Sot. 121, 1019 (1974). 4. W. Vedder and D. A. Vermilyea, Trans. FraradaySot. 65,
561 (1969).
5. C. K. Dyer and R. S. Alwitt,
Eiectrochim.
Acta 23, 347
(1978). 6. C. Crevecoeur
and H. J. deWitt, paper presented at 27th I.S.E. meeting, Zurich (1976), extended abstract 132. 7. L. Young, Trans. Faroduy SOC.5% 842 (1959). 8. W. J. Bernard and J. W. Cook, J. electrochem. Sot. 106, 643 (1959).
9. 8. R. Baker and J. D. Baker. Proc. of. Sixth Int7 Lioht Metals Conf, (t975), p. 105-107, LeobenjVienna. U to. R. S. Alwitt, J. electrochem. Sot. 121, t322 (1974). Ii. T. Kudo and R. S. Alwitt, Electrochim. Arta. 23, 341 (1978).