Dielectric Aluminum Oxides: Nano-Structural Features and ...

Dielectric Aluminum Oxides: Nano-Structural Features and
Composites
J.L. Stevens 1 , A.C. Geiculescu 2 , T.F. Strange 2
1 Technology Partners, s.p. 2820 Kennerly Rd., Irmo, SC 29063 PH: 1-803-781-3972
jstevens@tec-partner.com or jlstevens@sjm.com
2 St. Jude Medical, Inc. – CRMD 253 Financial Blvd., Liberty, SC 29657 PH: 1-864-843-
8200
cgeiculescu@sjm.com or tstrange@sjm.com
INTRODUCTION
Barrier layer aluminum oxide dielectrics formed in aqueous solutions can be produced in
either an amorphous or a crystalline form by numerous processes 1-4 which have been
reported upon over the last 100 years. Generally, the amorphous oxide variety yields higher
physical stability and lower leakage current and dissipation at the cost of 40% lower
capacitance than crystalline alumina. The crystalline variety therefore is sought in all
applications except those requiring the highest oxide quality. Over the last 40 or 50 years,
much effort has been expended to close the gap in stability between these two oxide types
while maximizing the capacitance obtained.
The authors have developed a nano-composite dielectric aluminum oxide which combines
amorphous oxide’s extreme oxide stability toward the standard boiling water test with most
of the capacitance of crystalline alumina. This oxide possesses a number of unique nanostructural
characteristics which have been studied and are believed to contribute to this blend
of high boiling water test stability and high capacitance.
As an extension of work 7 reported earlier, further study of the amorphous oxide produced in
an ethylene glycol forming environment, such as occurs at cut edges during the traditional
aging process, has led to the discovery of new nano-structural features there as well. These
structures are shown to depend upon process variables and seem to have their origins in the
nano-environment present at the oxide surface during their production.
EXPERIMENTAL
Any capacitors tested consisted of standard production ICD capacitor stacks made with
specially prepared anodes. First charge rise time measurements were performed with a
Keithley Model 237 HV Source/Measurement Unit and a Keithley Model 2001 DMM. ICD
capacitors were charged at a rate of 5mA while foil coupons were charged at a rate of 20
uA/cm 2 . Anode foil coupons were made from very high gain etched, 99.98%+ purity
aluminum anode foil sheets each measuring approximately 1 uF/cm 2 after being formed to a
voltage of 475 volts as 10 cm 2 coupons on a laboratory formation robot. Studies on
formation/aging of raw, cut edges on plain 99.99% formed aluminum tabstrips were done as
a scintillation voltage test by application of 1.25 mA/cm 2 at 37C in an ethylene glycol
electrolyte containing boric acid and azelaic acid salts.
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The surfaces of various films on Al substrates were observed with a Hitachi FE-4800. An
electron beam with 1.5 keV acceleration voltage and 7-10 µA current was used for imaging.
Working distance was kept at ~ 1.5 mm while both SE detectors were mixed for better detail.
Prior to the FE-SEM analysis, all films were coated with a 2 nm Pt/Pd layer from a 208
HR/MTM20 Cressington Sputter unit.
Specimens to be examined by TEM were thinned using a Hitachi FB2000A focused ion
beam (FIB). A beam of Ga ions radiating parallel to the plane of the sample face was used to
cut sections of ~ 50 nm thickness. These thin sections of the oxide films on Al substrate were
examined with a Hitachi HD-2300 TEM. A beam with 200 kV acceleration voltage and 20-
30 µA current was used for imaging. Electron diffraction patterns were generated using a
beam diameter of 200-300 nm at 200 kV and 80-85 nA current.
Stability Testing was accomplished by using a ramp-build-time test in 85 °C citric acid
solution with 0.5 mA/cm2 current density after boiling for two hours in deionized (DI)-
water. The rise times to 95% of V f were measured and used to assess the stability of the
grown oxides, and, in turn, the level of defects/voids exposed by boiling.
Anodization of Etched Foil:
RESULTS
One of the accepted methods of producing amorphous aluminum oxide for high voltage use
involves initially anodizing the etched foil at low temperature in an oxidizing acid such as
sulfuric acid to deposit a thick amorphous, porous layer of alumina inside the etch structure.
Figures 1 and 2 show a plan view and a cross-sectional view of such porous oxides inside of
etch tunnels. While this oxide has little in the way of barrier properties, it serves as the base
layer for a formation step which fills in the pores with additional amorphous oxide to leave a
barrier layer of the desired forming voltage. This oxide has the ability to withstand high
duty cycles of charge – discharge with minimal increases in leakage current and dissipation
factor. Unfortunately, amorphous oxide has a higher dielectric ratio [~1.4 nm/V] as
compared to crystalline aluminum oxide [~1.0nm/V] which leads to 40% lower capacitance.
The disadvantage of crystalline oxide is the presence of appreciable tensile stress which may
lead to relaxation and cracking when subjected to thermal, chemical, electrical or mechanical
stressors. The 40% capacitance advantage of crystalline oxide explains why so much work
has gone into stabilization of the crystalline oxide on capacitor foils.
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Figure 1.Interior Surface SEM: Anodized Etched Foil Tunnel With Pores of ~10nm
Diameter.
Figure 2. Tunnel Cross-sections: Anodized Etched Foil Oxide With Pores of ~10nm
Diameter.
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Standard Crystalline Etched and Formed Foil:
Figure 3 shows a “ramp” build curve of a high stability crystalline oxide on etched foil after
being subjected to a 2 hour boil stress test. To obtain this “ramp” curve, the foil was
immersed in 500 uS/cm citric acid at 85C followed by application of 0.5mA/cm 2 to grow
new oxide in any damaged areas. The graph shows a plot of the required charge/V to repair
any damage as the voltage builds up to the original forming voltage. Therefore the curve
serves as a probe of the degree of cracking or other damage as a function of the distance in
volts from the metal surface [with an indeterminate offset due to numerous series
resistances]. We see that there is a large amount of damage repair deep within the barrier
layer followed by a relatively smaller amount of repair in the 200V to 450V range. Work by
Uchi 5 , Kanno, and Alwitt as well as by Stevens 6 and Shaffer and others has shown that this
damage is due to cracking and delamination of the oxide plus some exposure of voids found
in the upper half of the crystalline oxide layer. While the damage appears severe in this
typical ICD foil curve, the rise time was 20.4 seconds as compared to world-class
commercial foils with crystalline alumina dielectrics which typically exhibit rise times of 40
to 80 seconds. Nevertheless, such tiny residual stresses have small, but finite, probabilities
of relaxing in the capacitor and producing longer charge times and higher leakage currents
until the damage is repaired.
Figure 3.: 2 Hour Boil Test “Ramp” Curve: High Stability Oxide on ICD-Style Etched
Foil. Rise Time = 20.4 sec.
The reason for the residual tensile stress in crystalline anodic oxide is believed to be
associated with the lattice shrinkage that occurs when the pseudoboehmite produced during
hydration of the etched foil is electrolytically converted to gamma´ alumina. Figure 4 shows
a cross-section of a formed etch tunnel in ICD foil with residual hydrate in the tunnel
interior. The textured crystalline barrier alumina is found on the tunnel surface next to the
smooth aluminum metal shown on the left side of the photo.
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Figure 4. Cross-Section View of Etch Tunnel in ICD Foil With Hydrate Present on
Tunnel Interior.
The textured appearance of the crystalline oxide is due to the nano-crystals produced when
the initial hydrate layer is electrolytically converted to barrier oxide. The residual stress is
believed to exist primarily between these nano-crystals. Normal formation processes use
multiple relaxation [depolarization or degassing] steps plus reformation to relax this stress
and repair the damage prior to capacitor assembly. Unfortunately, it is usually not possible
to completely remove all the stress with a finite number of reformation steps.
Nano-Composite Etched and Formed Foil:
Anodization is known to lead to compressive stress in the porous oxide like that found in
simple barrier layer amorphous oxides. Furthermore, the composite amorphous oxide
produced by pore-filling of anodized oxides with amorphous barrier forming techniques also
leads to compressive residual stress, which is believed to contribute to its high degree of
mechanical and electrical stability for high repetition photo-flash and heavy duty motor start
applications. In this spirit, the authors undertook to convert amorphous porous anodized
oxide into crystalline barrier oxide by exposure of the porous oxide to nearly boiling water
to seed the porous structure with hydrate in the hope that the high electric field present
during pore filling would lead to growth of nano-crystals and conversion of the residual
amorphous material to gamma´ alumina.
Figure 5 shows the cross-sectional view of a tunnel with anodizing, hydration and formation
in citric acid to minimize the production of amorphous oxide. Note the highly nano-textured
appearance of the upper oxide layer, a small amount of residual hydrate in the tunnel
opening, and the thin band of amorphous oxide remaining near the tunnel wall after the
majority of the porous oxide has been converted to crystalline alumina. Totally amorphous
oxide would have a capacitance of about 60% of totally crystalline oxide. For the oxide
shown in this photograph, we were able to obtain about 90% of the capacitance expected of
totally crystalline alumina, in line with relative thicknesses of the different layers obtained.
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Figure 5. Cross-Section of ICD Foil Tunnel: Nano-Composite Oxide Produced by
Anodizing, Hydrating, and Forming.
Further study of nano-composite oxide using Focused Ion Beam [FIB] thinning techniques
prior to TEM examination in Figure 6 shows greater detail in the layered structures obtained.
This sample was deliberately produced with excess porous oxide prior to hydration to
illustrate the nearly total conversion of the porous oxide incorporated into the barrier layer
during the pore filling steps of hydration and formation in the prior example. Porous oxide
above the barrier forming voltage is not converted. The greater sensitivity of the TEM to
subtle differences in density and crystallinity reveal that the crystalline layer is not
monolithic but is itself a composite of at least two layers. Electron diffraction patterns of
each region confirm the relative crystallinity in a qualitative sense.
6
5
4
3
1
2
Figure 6. TEM Photo of Nano-Composite Oxide Cross-section:
1: Residual Hydrate; 2: Residual Porous Oxide; 3: Mostly Crystalline Oxide; 4: Highly
Crystalline Oxide; 5: Amorphous Oxide; 6: Base Aluminum Metal
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The degree of success in converting the porous alumina to largely crystalline alumina begs
the question of how much residual tensile stress is found in the nano-composite oxide due to
this conversion. The results of a standard 2-hour boil test are shown in the “ramp” curve in
Figure 7. In this case, no appreciable coulombs are devoted to repair of the oxide at the
lower voltages as in the standard crystalline oxide of Figure 3. The absence of a large peak
low in the curve may be interpreted as a lack of delamination of the amorphous layer from
the base metal or of delamination of the crystalline layer from the amorphous oxide. The
lack of an appreciable “baseline” of charge during the build-up to the effective formation
voltage [EFV], implies a lack of vertical cracks in the oxide typical of stress relaxation and a
rarity of exposed voids normally found in the upper layer of high voltage crystalline
alumina. In short, this boil test ramp is typical of those expected from totally amorphous
anodic oxides used in heavy duty photoflash and garage door opener capacitors. In this case,
however, the capacitance sacrifice versus crystalline oxide is much less. The authors feel
that this high degree of anodic oxide stability will eventually find acceptance in some severe
duty commercial applications.
400
Charge/Volt [uC/V]
300
200
100
0
0 100 200 300 400 500 600
Volts
Figure 7. 2 Hour Boil Test “Ramp” Curve: Nano-Composite Oxide on ICD-Style
Etched Foil. Rise Time = 1.8 sec.
Nano-Structures in E.G. Formation/Aging:
Another fascinating area of study of nano-structures of anodic alumina was introduced by
Stevens 7 , et.al. in a study of the effects of non-aqueous fill [or working] electrolytes on
formation and deformation. Long, convoluted plumes of anodic residue were observed to be
produced on raw cut surfaces of formed aluminum tabstrip when subjected to a scintillation
voltage test in ethylene glycol electrolyte. Other researchers 8-11 have noticed odd deposits on
aluminum formed in various organic solvents with low water content, but this report has
some of the first microscopic evidence connecting nano-scale anomalies and larger plume or
network structures formed in an ethylene glycol electrolyte. Figure 8 shows these plume
structures as formed on the cut edge of formed high purity aluminum tabstrip at 1.25
mA/cm2 at 37C in an ethylene glycol electrolyte containing boric acid and azelaic acid salts
and having less than 3% water. Local electrochemical activity of a parasitic nature appears
to be taking place on these cut edges. Speculation has centered on the presence of impurities
or contaminants on the aluminum substrate.
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Figure 8. FE-SEM view of cut edge surface of a tab-strip after scintillation testing in
ethylene glycol electrolyte
The present work extends the study of these plumes to longer times and other current
densities in search of microscopic evidence of the origin of the plumes. We have seen that
brief scintillation voltage testing in ethylene glycol electrolytes can lead to plume growth on
cut edges and possibly any surface where the original formed oxide is ruptured. In order to
understand the implications of this mechanism for aluminum raw edges during aging of
capacitors, the photograph in figure 9 depicts the corner between the formed surface and the
raw, cut edge on a tabstrip which has been aged at 65C overnight at 390V in the same
ethylene glycol electrolyte. The uncontrolled current density and the long application time
of the current has produced a thick mat of plumes on the cut edge that is visible as a orangebrown
deposit. Clearly, much charge is wasted in this parasitic reaction while the underlying
oxide quality is suspect.
Figure 9. Surface View of Formed Tabstrip Corner between Raw Edge [top] and
Formed Surface [bottom] after Aging at 390V / 65C Overnight.
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Figure 10. Surface View of Cut Edge of 1199 Tabstrip formed in EG electrolyte To
100V at 37C and 1.2mA/cm 2 .
In order to elucidate a mechanism for the formation of these plumes, we examine whether
their formation is associated with contaminants on the cut edges as well as the voltage
dependence of their appearance. Figure 10 shows a view of the cut edge of a tabstrip formed
in the same ethylene glycol electrolyte, but only to 100V.
The anodic oxide exhibits a semi-regular porous structure, especially along cutting lines and
apparent grain boundaries. A number of particles are embedded in this porous oxide, but no
plumes could be found associated with them or anywhere on the cut edges. EDXA studies
of these particles and the surrounding area detected only aluminum and oxygen - no
appreciable level of contaminant elements such as iron, copper or silicon.
An important clue as to the lack of the plume structures in Figure 10 can be found in the
scintillation curve of Figure 11. At constant current, the voltage increases linearly to about
150V where another mechanism reduces the formation efficiency markedly until about
200V. Above 200V, the efficiency improves but does not return to the earlier level. Above
400V, scintillation sets in, preventing further oxide growth.
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800
700
600
500
Volts
400
300
200
100
0
0 100 200 300 400 500 600 700 800
Seconds
Figure 11. Scintillation Curve for 1199 Formed Tabstrip in EG Electrolyte at 37C and
1.2 mA/cm 2 [single-sided].
Examination of the cut edges after scintillation testing to higher voltages in Figure 12 reveals
a further development of the porous oxide structure, but no plumes are found in the vicinity
of any of the embedded oxide particles. However, further extensive searching of the 200V
scintillation-tested tabstrip does reveal a few plume-like structures, but no metallic
contamination is found near the plume mouths.
Figure 12. General Surface View of Porous Oxide On Cut Edge of 1199 Tabstrip
Formed In EG Electrolyte to 200V.
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Closer examination of a short plume on the 200V tested tabstrip edge in Figure 13 reveals
one intricate structure of braided or twisted filaments. This plume is quite large compared to
the sub-micron pores in the surface oxide and is not associated with any particular elemental
surface anomaly. Edge films formed at higher voltages up to the scintillation voltage show
that the plumes become longer and more numerous.
Figure 13. Surface View of Cut Edge of Tabstrip Formed to 200V in EG Showing a
Rare Short Plume.
The cut edge surface of a tabstrip tested up to the scintillation voltage is shown in figure 14.
The plumes are quite wide, long and braided in appearance. A plume “mouth” is seen
clearly at the bottom of this photo with a large plume and several smaller ones originating
from it. The porous oxide surface appears to be ruptured and almost “splattered” in
appearance. This texture is sometimes associated with sparking, breakdown or scintillation
events suggesting that the plumes are correlated with the sites of repeated scintillations. In
addition, the oxide surface now appears less classically porous and more glassy with folds
and fissures.
Figure 14. Surface View of Cut Edge of Tabstrip Formed to Vsc in EG Showing a
Large Plume Base and “Mouth” Area.
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It has been known for many years that growth of oxide in a glycol forming environment can
be current density sensitive. Therefore, an exploration of the effects of current density on
the nano-structures formed on these cut edges is warranted. Figure 15 reveals that the plume
formation is already well underway at 100V when the current density is increased to
12mA/cm 2 whereas none are found at the standard scintillation test current density of
1.2mA/cm 2 .
Figure 15.
Surface View of Cut Edge of 1199
Tabstrip formed in EG electrolyte
To 100V at 37C and 12.0mA/cm 2 .
When the current density is lowered by a factor of 10 to 0.12mA/cm2, large plumes are very
rare up to the scintillation voltage. Instead, there is a tendency for the porous structure to be
more well-developed with small structures sometimes appearing in the pore itself as seen in
Figure 16. The edges of the pore frequently have a small disc lifted off of the surface similar
to a ruptured blister.
Figure 16.
Surface View of Cut Edge of 1199
Tabstrip Formed in EG electrolyte
To 400V at 37C and 0.12mA/cm 2 .
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Careful searching of the cut edge surface at higher magnification reveals a few pores which
still retain delicate “flower-like” filament structures like those seen in Figure 17. The
existence of these structures imply that the great majority of the pores form them during the
build up to scintillation voltage and that the missing oxide film surrounding the pore is
carried away by the growing filament, part of which is retained at the filament tip. The
implication of these photographs is that once pores begin growing, a parasitic reaction which
depends upon voltage and current density can lead to filament growth from the pore. At
higher current density, these filaments probably develop quickly and become entangled with
nearby filaments to merge into the braided appearance of the larger plumes seen earlier.
Hence, these delicate flower-like structures are likely to be the incipient stage of plume
formation. They can still be seen in figure 17 due to the low current density favoring barrier
growth over filament formation.
Figure 17.
High Magnification Surface View
of Cut Edge of 1199 Tabstrip
Formed in EG electrolyte
To 400V at 37C and 0.12mA/cm 2 .
CONCLUSIONS
Despite the commercial exploitation of anodic aluminum oxide films as dielectrics for nearly
a century, we continue to find novel features in these films. We have shown here that nanocomposite
barrier oxides can be produced by a novel combination of porous anodizing and
crystalline barrier layer formation techniques. This oxide is a nano-composite of amorphous
oxide and crystalline oxide that shows tremendous resistance toward the standard boiling
water stability test without resort to the heavy phosphate doping characteristic of standard
amorphous oxides. Furthermore, the nano-composite film can be produced with capacitance
values approaching standard crystalline alumina thereby avoiding most of the capacitance
penalty associated with amorphous alumina barrier layers.
We have also demonstrated that the inflection seen in the scintillation and formation curves
of anodic alumina produced in ethylene glycol based electrolytes is associated with the
development of porosity and nano-structural filaments on the growing oxide layer leading to
microscopic plumes of anodic material under some conditions. These plume-like
agglomerates have been seen macroscopically for many years as orange-brown deposits on
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cut edges, air-electrolyte interfaces and other damaged areas of aluminum capacitor anodes.
The formation of these plumes has been shown to depend upon voltage, current density and
the time of application of the current and represents a parasitic reaction in parallel with the
desirable formation of barrier oxide on these raw aluminum surfaces.
ACKNOWLEDGEMENTS
The authors would like to thank Hitachi – HTA for their assistance in the FIB and TEM
work for this study. In addition, the encouragement and appreciation of the management of
St. Jude Medical, Inc. has made this work possible.
REFERENCES
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211.
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3
“Improved Dielectric Properties fo Anodic Aluminum Oxide Films by Soft/Hard Two-
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4
“Method of Manufacturing Aluminum Foil for Aluminum Electrolytic Capacitor.”; K.
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10
“Cellular Porous Anodic Alumina Grown in Neutral Organic Electrolyte: I. Structure,
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