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CRYSTALLINE ALUMINUM OXIDE FILMS
Passivity of Metals and Semiconductors, edited by M. Froment Elsevier Science Publishers B.V., Amsterdam — Printed in The Netherlands
CRYSTALLINE ALUMINUM OXIDE FILMS R. S. ALWITT and H. TAKEI United Chemi-Con, Inc., 3000 Dundee Rd., Northbrook, IL 60062 (USA)
ABSTRACT Thermal oxidation of Al at 525°C for short times develops an amorphous oxide with no detectable crystallites. Subsequent anodic oxidation in citrate electrolyte at 70°C produces a barrier oxide containing γ' alumina. At low voltages the film is amorphous + crystalline but at higher voltages the amorphous oxide transforms to γ' alumina so that the film is essentially a uniform crystalline layer. The growth and properties of this layer are described. INTRODUCTION Crystalline oxide is frequently a component of oxide films on aluminum. Thermal oxidation above 450°C for suitable times produces Ύ alumina crystals at the metal interface under a continuous layer of amorphous oxide (ref.l). Amorphous anodic barrier oxide may have crystalline islands of Ύ' alumina in the vicinity of flaws (ref.2).
Anodization in borate solution after
thermal oxidation at 550°C results in a barrier layer of crystalline oxide sandwiched between amorphous oxide (ref.3).
If Al is reacted with boiling
water to deposit a thick hydrous oxide, subsequent anodization at 90°-95°C creates a crystalline barrier layer next to the metal (ref.4) that has been identified as γ (ref.5) and as γ' (ref.6) alumina. (These oxides have a similar spinel structure, but γ' has more disorder in the cation distribution over the lattice sites (ref.7)). We report here on the properties and growth of barrier oxide films that consist essentially only of a layer of γ' alumina. METHODS Smooth 4N Al foil was chempolished in HNO^+H-PO, and then cleaned in NaOH (ref.4).
Etched 4N capacitor foil (20X surface area increase (ref.4)) was
cleaned in 7% ΗΝ0- at 60°C for 3 min.
This foil was used to get more sensi-
tivity in weight measurements and easier handling of flakes of isolated oxide. Etched foil results are reported on a specific area basis, i.e., nominal value divided by 20. Specimens were heated in air for 20 min. at 525°C and then anodized in 5 g/1 NH- citrate at 70°C, 0.5 mA/cm2.
When the desired voltage (V f ) was
741
742 reached, V f was held constant for 5-7 min. (etched) or 10 min. (smooth). Crystalline films have an electrical instability that is removed by a relaxation and reformation (ref.4).
Etched specimens were relaxed for 2 min. on 2 open circuit in the citrate bath and then reformed at 0.5 mA/cm to V f ; after 2 1-2 minutes at V f the cd had dropped to 25" μΑ/cm
and formation was stopped.
Smooth specimens were relaxed by heating in air for 2 min. at 550°C followed by reformation.
In both cases the reformation charge was 5-10% of the initial
anodization charge.
The electrical conductivity and dielectric properties are
stabilized by these steps. Films on smooth foils were stripped in Hg C U solution and examined in the TEM.
Thermal films were reinforced with carbon before separation.
Cross-
sections were prepared with an ultramicrotone and the oxygen content was measured by nuclear microanalysis. several film properties vs V f .
Etched specimens were used to study
In some cases the film was collected after
dissolving the Al in the Br 2 -methanol.
With other specimens the oxide dissolu-
tion rate was measured in 5% H 3 P0-+2%Cr0 3 stripping solution (ref.5). RESULTS TEM examination of thermal film gave no evidence of crystallites, which could have been detected if 3-5 nm or larger. Low beam intensities were used because of the fragile nature of the film so this observation cannot be taken as certain verification of the absence of crystallites. The film weighed 2.5 yg/cm2.
Fig. 1. TEM appearance of 140V crystalline film (a) Plan view (b) Cross-section
Fig. 2. Voids (white spots) in 140V film, 1.5 ym underfocus
743
Fig. 3. Oxide weight vs. V f . Specimens heated (o) and not heated (·). Fig. 4. Reciprocal capacitance (120Hz) vs V f . Same symbols as Fig. 3.
Without heat treatment the citrate anodization produces an amorphous oxide layer.
With thermal oxide present, anodization above 60V produces a layer of
γ' Alp0 3 .
Fig. 1 shows a 140V film.
It consists of fine crystallites and the
structure appears uniform in both lateral and normal directions.
The film is
126nm thick, equivalent to 0.9 nm/V inverse field strength at 70°C. Nuclear 17 -2 microanalysis gave a total oxygen coverage of 8.05x 10 atoms.cm (ref.8), so 3 the film density is 3.61 g/cm . The film contains a high density of discrete, microscopic voids that can be revealed by the phase contrast caused by defocusing the TEM (ref.9).
Fig. 2 shows the film at 1.5 ym underfocus. The 11 12 "2 voids are several nm in diameter and at a density of 10 -10 cm . The relaxation and reformation have no effect on the appearance of the void distribution, nor any other structural features. Figs. 3 and 4 show the film weight and capacitance (120Hz) vs V f for etched specimens.
The solid line in Fig. 3 is drawn to intersect the abcissa at E°
for Al/AUO- and with a slope calculated from the properties of the 140V crystalline film.
The solid line in Fig. 4 is a best fit through E° and the
higher voltage points; the slope corresponds to a dielectric constant of 8.6 for the crystalline oxide.
The dashed lines are drawn for typical amorphous
barrier oxide properties of 3.2 g/cm , 1.4nm/V, and diel const = 8.4 (ref.10).
744
Without heat treatment the dielectric properties are those of amorphous barrier oxide but the film weight is greater.
There may be an additional porous layer
at the solution side, similar to the structure after borate anodization at elevated temperature (ref.11).
The heat treatment has little effect on the
weight and capacitance of 10V films but at higher V f there is a shift towards the crystalline values.
The transition from amorphous to crystalline structure
is complete at 60-70V. Without heat treatment the dissolution rate in Η~Ρ0 Δ -Ο0~ solution was 0.4 yg/cm s, typical of amorphous oxide (Fig. 5 ) . After rapid removal of 1 yg/cm , the dissolution rate of 60 and 90V films with heat treatment was 5X10" 3 2 yg/cm s, 100 times slower than amorphous film and typical of γ -alumina (ref.5). The 10V heated film dissolved at half the rate of the unheated 10V film, and the 20V and 40V films dissolved progressively more slowly, but still much faster than for crystalline oxide.
For each of these films the rate eventually
dropped to a low value like the 60V and 90V films.
With the 40V film this
occurred beyond 300 s (not shown). The metal was dissolved from these specimens in Br 2 ~ methanol and in each case additional oxide flakes were collected. is shown in Fig. 6.
The weight of this "inner" oxide
TEM examination in dark field and selected area electron
diffraction of the 20 and 40V "inner" oxides showed them to be substantially crystalline with the γ' -structure, (Fig. 7 ) . Films formed through 40V are a combination of amorphous and crystalline oxide.
Above 60V the films are essentially all crystalline.
Between 40 and
60V there must be a transformation of amorphous to γ' oxide. Characteristics of the amorphous oxide before the transformation are of great interest.
With less intense heat conditions the formation of crystalline
Time (sec.)
Fig. 5. Weight loss in hLPO.-OOat 85°C. Heated (open symbols) and not heated (filled symbols).
ttH
1
1
1
L_
0
20
40
60
80
Vf (vs SCE)
Fig. 6. "Inner" oxide weight (Δ) and total oxide weight (o) vs. V f . Total weight same as in Fig. 3.
745 oxide at low voltages can be reduced, so properties related to the amorphous film can be more clearly distinguished. thermal film appears free of crystallites
After heating at 500°C/ 15 min. the by TEM.
Thirty volt anodic film
formed on this substrate appeared amorphous by TEM and SAED.
An x-ray powder
scan using point counting did detect a trace of γ' alumina, and the 2° peak width indicated a 5-8nm particle size (ref.12).
This oxide film dissolved
completely in the stripping solution, at a rate similar to that for the 40V film in Fig. 5.
There was no detectable amount of inner oxide, yet with
further growth such films fully transformed to γ' alumina. DISCUSSION The 60-140V films appear to be the first examples of anodic alumina films consisting soley of crystalline barrier oxide.
This oxide has the same pro-
perties as the barrier layer in composite hydrous + anodic oxide (Refs. 4, 9, 10) but its overall structure and growth process appear to be simpler and more accessible to analysis. The inner oxide may grow either by transformation or by direct crystalline deposition, there is no specific evidence favoring one process.
The
decreasing dissolution rate with increasing film thickness (Fig. 5) suggests that the amorphous structure may become more ordered during growth. occurs across the film and is not restricted to an interface.
This
When the
amorphous oxide becomes sufficiently ordered it appears to transform easily to γ' oxide, as occurred between 40 and 60V.
This effect does not depend
upon the presence of the inner oxide, evidenced by the results with the specimen heated 500°C/ 15 min. Amorphous thermal oxide is thought to be identical with amorphous anodic oxide (ref.13) so it is not clear how it could cause the observed effect. The thermal film is presumably located in the interior of the anodic oxide at the boundary between oxide grown by anion transport and oxide grown by cation transport.
Its only interaction with the bulk film is that the ion
flux passes through this region before depositing at the interfaces, which seems insufficient to cause a structural change. One may argue that some seed crystals are created during thermal oxidation that serve as centers for subsequent crystallization.
The heat conditions
used here appear to be at the threshold for γ alumina nucleation.
Crystallite
growth induction times of about 30 min. at 505°C and 8 min. at 530°C are reported (ref.14).
According to a proposed rate equation (ref.l), heating
at 525°C/ 20 min. deposits crystallites at the metal surface 7nm in diameter 9-2 at a density of 3X10 cm . Such a deposit would have been seen in the TEM. It may be significant that the x-ray technique, which detected crystallites
746 that were not revealed by electron microscopy, requires a much larger sample size.
Perhaps the crystallites were
present at a much lower density or in a non-uniform distribution.
Even if the
presence of γ- alumina nuclei are admitted, it is not clear how they would cause the transformation.
During
thermal oxidation these crystals cause no change in the amorphous oxide layer Fig. 7. Selected area electron
and grow as an independent phase (ref.l).
diffraction from inner oxide of
Pehaps the inner oxide nucleates on
20V (left) and 40V (right) films.
thermal oxide seed crystals.
We have observed that the amorphous transformation depends upon bath composition, e.g., the process is more sluggish in phosphate then in citrate. It is known that anion impurities
in amorphous alumina decrease the ease of
crystallization in an electron beam (ref.15).
Perhaps the thermal oxide in
some way inhibits anion entry into the growing oxide. It may be that the critical role of the thermal oxide will not be explained by a single phenomenon but by some combination of events. REFERENCES 1 A. F. Beck, M. A. Heine, E. J. Caule and M. J. Pryor, Corr.Sci. 7(1962)1-22. 2 K. Shimizu, S. Tajima, G. E. Thompson and G. C. Wood, Electrochim Acta, 25 (1980) 1481-1486. 3 C. Crevecoeur and H. J. deWit, 27th I.S.E. Meeting, Zurich, l976,Abstr. 132 4 R. S. Alwitt and C. K. Dyer, Electrochim. Acta, 23 (1978) 355-362. 5 R. S. Alwitt and W. J. Bernard, J. Electrochem, S o c , 121 (1974) 1019-1022. 6 N. F. Jackson and P.D.S. Waddell, J. Appl. Electrochem., 2 (1972) 345. 7 B. C. Lippens and J. J. Steggerda in B. G. Linsen (Ed.), Physical and Chemical Aspects of Adsorbents and Catalysts, Academic, New York, (1970) pp. 171-211. 8 J. Siejka, private communication. 9 R. S. Alwitt, C. K. Dyer and B. Noble, J. Electrochem S o c , 129 (1982) pp. 711-717. 10 C. K. Dyer and R. S. Alwitt, Electrochim. Acta, 23 (1978) 347-354. 11 G. A. Dorsey, J. Electrochem S o c , 116 (1969) 466-471. 12 H. Chen, private communication. 13 M. J. Dignam, J. Electrochem. S o c , 109 (1962) 183-191 14 M. J. Dignam, W. Fawcett and H. Bohni, ibid, 113 (1966) 656-662. 15 K. Shimizu, G. E. Thompson and G. C. Wood, Thin Solid Films, 77 (1981) pp. 313-318.
CRYSTALLINE ALUMINUM OXIDE FILMS R. S. ALWITT and H. TAKEI United Chemi-Con, Inc., 3000 Dundee Rd., Northbrook, IL 60062 (USA)
ABSTRACT Thermal oxidation of Al at 525°C for short times develops an amorphous oxide with no detectable crystallites. Subsequent anodic oxidation in citrate electrolyte at 70°C produces a barrier oxide containing γ' alumina. At low voltages the film is amorphous + crystalline but at higher voltages the amorphous oxide transforms to γ' alumina so that the film is essentially a uniform crystalline layer. The growth and properties of this layer are described. INTRODUCTION Crystalline oxide is frequently a component of oxide films on aluminum. Thermal oxidation above 450°C for suitable times produces Ύ alumina crystals at the metal interface under a continuous layer of amorphous oxide (ref.l). Amorphous anodic barrier oxide may have crystalline islands of Ύ' alumina in the vicinity of flaws (ref.2).
Anodization in borate solution after
thermal oxidation at 550°C results in a barrier layer of crystalline oxide sandwiched between amorphous oxide (ref.3).
If Al is reacted with boiling
water to deposit a thick hydrous oxide, subsequent anodization at 90°-95°C creates a crystalline barrier layer next to the metal (ref.4) that has been identified as γ (ref.5) and as γ' (ref.6) alumina. (These oxides have a similar spinel structure, but γ' has more disorder in the cation distribution over the lattice sites (ref.7)). We report here on the properties and growth of barrier oxide films that consist essentially only of a layer of γ' alumina. METHODS Smooth 4N Al foil was chempolished in HNO^+H-PO, and then cleaned in NaOH (ref.4).
Etched 4N capacitor foil (20X surface area increase (ref.4)) was
cleaned in 7% ΗΝ0- at 60°C for 3 min.
This foil was used to get more sensi-
tivity in weight measurements and easier handling of flakes of isolated oxide. Etched foil results are reported on a specific area basis, i.e., nominal value divided by 20. Specimens were heated in air for 20 min. at 525°C and then anodized in 5 g/1 NH- citrate at 70°C, 0.5 mA/cm2.
When the desired voltage (V f ) was
741
742 reached, V f was held constant for 5-7 min. (etched) or 10 min. (smooth). Crystalline films have an electrical instability that is removed by a relaxation and reformation (ref.4).
Etched specimens were relaxed for 2 min. on 2 open circuit in the citrate bath and then reformed at 0.5 mA/cm to V f ; after 2 1-2 minutes at V f the cd had dropped to 25" μΑ/cm
and formation was stopped.
Smooth specimens were relaxed by heating in air for 2 min. at 550°C followed by reformation.
In both cases the reformation charge was 5-10% of the initial
anodization charge.
The electrical conductivity and dielectric properties are
stabilized by these steps. Films on smooth foils were stripped in Hg C U solution and examined in the TEM.
Thermal films were reinforced with carbon before separation.
Cross-
sections were prepared with an ultramicrotone and the oxygen content was measured by nuclear microanalysis. several film properties vs V f .
Etched specimens were used to study
In some cases the film was collected after
dissolving the Al in the Br 2 -methanol.
With other specimens the oxide dissolu-
tion rate was measured in 5% H 3 P0-+2%Cr0 3 stripping solution (ref.5). RESULTS TEM examination of thermal film gave no evidence of crystallites, which could have been detected if 3-5 nm or larger. Low beam intensities were used because of the fragile nature of the film so this observation cannot be taken as certain verification of the absence of crystallites. The film weighed 2.5 yg/cm2.
Fig. 1. TEM appearance of 140V crystalline film (a) Plan view (b) Cross-section
Fig. 2. Voids (white spots) in 140V film, 1.5 ym underfocus
743
Fig. 3. Oxide weight vs. V f . Specimens heated (o) and not heated (·). Fig. 4. Reciprocal capacitance (120Hz) vs V f . Same symbols as Fig. 3.
Without heat treatment the citrate anodization produces an amorphous oxide layer.
With thermal oxide present, anodization above 60V produces a layer of
γ' Alp0 3 .
Fig. 1 shows a 140V film.
It consists of fine crystallites and the
structure appears uniform in both lateral and normal directions.
The film is
126nm thick, equivalent to 0.9 nm/V inverse field strength at 70°C. Nuclear 17 -2 microanalysis gave a total oxygen coverage of 8.05x 10 atoms.cm (ref.8), so 3 the film density is 3.61 g/cm . The film contains a high density of discrete, microscopic voids that can be revealed by the phase contrast caused by defocusing the TEM (ref.9).
Fig. 2 shows the film at 1.5 ym underfocus. The 11 12 "2 voids are several nm in diameter and at a density of 10 -10 cm . The relaxation and reformation have no effect on the appearance of the void distribution, nor any other structural features. Figs. 3 and 4 show the film weight and capacitance (120Hz) vs V f for etched specimens.
The solid line in Fig. 3 is drawn to intersect the abcissa at E°
for Al/AUO- and with a slope calculated from the properties of the 140V crystalline film.
The solid line in Fig. 4 is a best fit through E° and the
higher voltage points; the slope corresponds to a dielectric constant of 8.6 for the crystalline oxide.
The dashed lines are drawn for typical amorphous
barrier oxide properties of 3.2 g/cm , 1.4nm/V, and diel const = 8.4 (ref.10).
744
Without heat treatment the dielectric properties are those of amorphous barrier oxide but the film weight is greater.
There may be an additional porous layer
at the solution side, similar to the structure after borate anodization at elevated temperature (ref.11).
The heat treatment has little effect on the
weight and capacitance of 10V films but at higher V f there is a shift towards the crystalline values.
The transition from amorphous to crystalline structure
is complete at 60-70V. Without heat treatment the dissolution rate in Η~Ρ0 Δ -Ο0~ solution was 0.4 yg/cm s, typical of amorphous oxide (Fig. 5 ) . After rapid removal of 1 yg/cm , the dissolution rate of 60 and 90V films with heat treatment was 5X10" 3 2 yg/cm s, 100 times slower than amorphous film and typical of γ -alumina (ref.5). The 10V heated film dissolved at half the rate of the unheated 10V film, and the 20V and 40V films dissolved progressively more slowly, but still much faster than for crystalline oxide.
For each of these films the rate eventually
dropped to a low value like the 60V and 90V films.
With the 40V film this
occurred beyond 300 s (not shown). The metal was dissolved from these specimens in Br 2 ~ methanol and in each case additional oxide flakes were collected. is shown in Fig. 6.
The weight of this "inner" oxide
TEM examination in dark field and selected area electron
diffraction of the 20 and 40V "inner" oxides showed them to be substantially crystalline with the γ' -structure, (Fig. 7 ) . Films formed through 40V are a combination of amorphous and crystalline oxide.
Above 60V the films are essentially all crystalline.
Between 40 and
60V there must be a transformation of amorphous to γ' oxide. Characteristics of the amorphous oxide before the transformation are of great interest.
With less intense heat conditions the formation of crystalline
Time (sec.)
Fig. 5. Weight loss in hLPO.-OOat 85°C. Heated (open symbols) and not heated (filled symbols).
ttH
1
1
1
L_
0
20
40
60
80
Vf (vs SCE)
Fig. 6. "Inner" oxide weight (Δ) and total oxide weight (o) vs. V f . Total weight same as in Fig. 3.
745 oxide at low voltages can be reduced, so properties related to the amorphous film can be more clearly distinguished. thermal film appears free of crystallites
After heating at 500°C/ 15 min. the by TEM.
Thirty volt anodic film
formed on this substrate appeared amorphous by TEM and SAED.
An x-ray powder
scan using point counting did detect a trace of γ' alumina, and the 2° peak width indicated a 5-8nm particle size (ref.12).
This oxide film dissolved
completely in the stripping solution, at a rate similar to that for the 40V film in Fig. 5.
There was no detectable amount of inner oxide, yet with
further growth such films fully transformed to γ' alumina. DISCUSSION The 60-140V films appear to be the first examples of anodic alumina films consisting soley of crystalline barrier oxide.
This oxide has the same pro-
perties as the barrier layer in composite hydrous + anodic oxide (Refs. 4, 9, 10) but its overall structure and growth process appear to be simpler and more accessible to analysis. The inner oxide may grow either by transformation or by direct crystalline deposition, there is no specific evidence favoring one process.
The
decreasing dissolution rate with increasing film thickness (Fig. 5) suggests that the amorphous structure may become more ordered during growth. occurs across the film and is not restricted to an interface.
This
When the
amorphous oxide becomes sufficiently ordered it appears to transform easily to γ' oxide, as occurred between 40 and 60V.
This effect does not depend
upon the presence of the inner oxide, evidenced by the results with the specimen heated 500°C/ 15 min. Amorphous thermal oxide is thought to be identical with amorphous anodic oxide (ref.13) so it is not clear how it could cause the observed effect. The thermal film is presumably located in the interior of the anodic oxide at the boundary between oxide grown by anion transport and oxide grown by cation transport.
Its only interaction with the bulk film is that the ion
flux passes through this region before depositing at the interfaces, which seems insufficient to cause a structural change. One may argue that some seed crystals are created during thermal oxidation that serve as centers for subsequent crystallization.
The heat conditions
used here appear to be at the threshold for γ alumina nucleation.
Crystallite
growth induction times of about 30 min. at 505°C and 8 min. at 530°C are reported (ref.14).
According to a proposed rate equation (ref.l), heating
at 525°C/ 20 min. deposits crystallites at the metal surface 7nm in diameter 9-2 at a density of 3X10 cm . Such a deposit would have been seen in the TEM. It may be significant that the x-ray technique, which detected crystallites
746 that were not revealed by electron microscopy, requires a much larger sample size.
Perhaps the crystallites were
present at a much lower density or in a non-uniform distribution.
Even if the
presence of γ- alumina nuclei are admitted, it is not clear how they would cause the transformation.
During
thermal oxidation these crystals cause no change in the amorphous oxide layer Fig. 7. Selected area electron
and grow as an independent phase (ref.l).
diffraction from inner oxide of
Pehaps the inner oxide nucleates on
20V (left) and 40V (right) films.
thermal oxide seed crystals.
We have observed that the amorphous transformation depends upon bath composition, e.g., the process is more sluggish in phosphate then in citrate. It is known that anion impurities
in amorphous alumina decrease the ease of
crystallization in an electron beam (ref.15).
Perhaps the thermal oxide in
some way inhibits anion entry into the growing oxide. It may be that the critical role of the thermal oxide will not be explained by a single phenomenon but by some combination of events. REFERENCES 1 A. F. Beck, M. A. Heine, E. J. Caule and M. J. Pryor, Corr.Sci. 7(1962)1-22. 2 K. Shimizu, S. Tajima, G. E. Thompson and G. C. Wood, Electrochim Acta, 25 (1980) 1481-1486. 3 C. Crevecoeur and H. J. deWit, 27th I.S.E. Meeting, Zurich, l976,Abstr. 132 4 R. S. Alwitt and C. K. Dyer, Electrochim. Acta, 23 (1978) 355-362. 5 R. S. Alwitt and W. J. Bernard, J. Electrochem, S o c , 121 (1974) 1019-1022. 6 N. F. Jackson and P.D.S. Waddell, J. Appl. Electrochem., 2 (1972) 345. 7 B. C. Lippens and J. J. Steggerda in B. G. Linsen (Ed.), Physical and Chemical Aspects of Adsorbents and Catalysts, Academic, New York, (1970) pp. 171-211. 8 J. Siejka, private communication. 9 R. S. Alwitt, C. K. Dyer and B. Noble, J. Electrochem S o c , 129 (1982) pp. 711-717. 10 C. K. Dyer and R. S. Alwitt, Electrochim. Acta, 23 (1978) 347-354. 11 G. A. Dorsey, J. Electrochem S o c , 116 (1969) 466-471. 12 H. Chen, private communication. 13 M. J. Dignam, J. Electrochem. S o c , 109 (1962) 183-191 14 M. J. Dignam, W. Fawcett and H. Bohni, ibid, 113 (1966) 656-662. 15 K. Shimizu, G. E. Thompson and G. C. Wood, Thin Solid Films, 77 (1981) pp. 313-318.