Pushing Tantalum capacitors to the limit: A powder manufacturers view to 300 V
anodizations and beyond.
3rd Space Passive Component Days (SPCD), International Symposium
9-12 October
ESA/ESTEC, Noordwijk, The Netherlands
Marcel Hagymási(1), Ralph Dietmar Otterstedt(2), Christoph Schnitter(3), Oliver Thomas(4), Rich Reifenheiser(5)H.C. Starck Tantalum and Niobium GmbHIm Schleeke 78-91, 38642 Goslar, Germany(1)

marcel.hagymasi@hcstarck.com(2)

ralphdietmar.otterstedt@hcstarck.com(3)

christoph.schnitter@hcstarck.com
(4)

(5)

oliver.thomas@hcstarck.com

rich.reifenheiser@hcstarck.com

Ta capacitors are the best choice when applications require highest capacitance (energy density) on miniature scale
combined with high reliability, low ESR, and excellent stability over wide ranges of time and application temperature.
Recently, the demand of high reliability capacitors withstanding harsher conditions especially with respect to higher
application voltage has been increased. Main driver for such applications are besides aviation and aerospace also
automotive (driver assistance systems, e-vehicles) and transportation, medicine (defibrillators), defense but also consumer
electronics like for data storage (solid state drive-SSD).
H.C. Starck continuously improves its portfolio of high voltage powders allowing to expand the tantalum capacitor
technology to new fields of application. The “High Voltage Medium Capacitance” (HVMC) powders have a unique
combination of structural homogeneity, high purity and tailored pore structure to provide the highest capacitance of Ta
powder for the formation voltage (Vf) range >80 Vf. We will show the latest status of new developed powders that extend
the range of forming voltage to 300 V and even up to 400 V.
Moreover, new options to further improve the pore structure resulting in increased capacitance at a given forming voltage
will be presented as well. Beyond powder design, the tremendous impact of anode processing to master such highanodization voltages is highlighted.

Introduction
Moore’s law predicts that the number of transistors in a dense integrated circuit doubles about every two years [1]. Thislaw describes the impressive speed of miniaturization but also the performance increase of electronic parts that has until1now never stopped. Likewise, the performance of tantalum capacitors is continuously improved but with a slower pace.
For high voltage applications an unceasing and increasing demand for powders optimized for applications voltages >35
V is observed. They are used for different fields of application, often connected with high reliability: consumer electronics(SSD, charging devices, etc.), transportation and automotive, signal transmission for railway, medical devices (e.g.
implantable cardioverter defibrillators), defense and aerospace (planes, satellites). There are in general different trends
that we have realized in recent years: 1. Demand for more stored charge in a given volume at a given voltage, e.g. morecapacitance at 150 V forming voltage not reachable with standard Ta powders to create new capacitor types for new
applications. 2. Demand to further increase the forming voltage above 300 V since the energy W that can be stored by acapacitor is correlated with the voltage U squared:𝑈

𝑊 = 𝐶 ∫0 𝑢 d𝑢 =

𝐶𝑈 2
2

(1)

HVMC powders provide the required capacitance at forming voltages in the range of 100-350 V and are compared tostandard tantalum capacitor powders, as shown in fig.1. At any voltage in this range, there is at least one powder type thathas more capacitance and energy density than comparable standard powder types. It was already shown that the key forthe excellent high voltage ability of HVMC powders is the improved microstructural homogeneity with respect to poreand primary particle sizes [2, 3]. The tighter primary particle distribution allowing higher capacitance is the result ofextensive research to optimize the parameters of our production processes. We have developed different HVMC powder
types, since each anodization voltage has its own optimum concerning primary particle and pore size distribution. To
simplify the powder naming we have decided to put the classification (“HV” = High Voltage) and the recommended
optimum forming voltage in its name (like HV200 – developed for 200 Vf) in contrast to the conventional naming where
the capacitance (mostly measured at lower voltages) is included.
35000
HV100

32500

Capacitance [µFV/g]

30000+ 25 %
27500
25000

HV200

22500
20000
30k µFV/g

17500

HV300
+ 25 %

15000
12500

+ 22 %

17k µFV/g

10000

12k µFV/g
7500
75

100

125

150

175

200
225
250
275
Anodization Voltage [V]

300

325

350

375

Fig.1 Capacitance as a function of forming voltage of HV100-HV300 in comparison to standard Na reduced powders.

2The purpose of this paper is to show our general approach how to improve the microstructure regarding higher capacitanceat a given formation voltage exemplified by our newly developed HV150 powder and how to extend the range ofanodization up to 400 V by referring to our latest development, the HV400 powder. The impact of the anodization
technique when reaching 400 V is significant and has, like the powder itself, to be optimized to realize new types ofcapacitors in the future.

How to provide more capacitance by tailoring the microstructure?
The microstructure of the tantalum powder is the key for realization of higher volumetric capacitances and must be tailoredfor each anodization voltage range to its optimum [2]. Currently, new capacitors with anodization voltage of ~150 Vf arehighly requested. At 150 V anodization our products HV100 and HV200 show nearly comparable capacitance in electrical
wet tests (see fig.2). However, due to its smaller pores as shown in fig. 3, capacitors made from HV100 will show worse
impregnation and ESR behavior compared to those made from HV200 having larger pores. That’s why HV200 would
have been the powder of choice for 150Vf in the past.

Fig. 2 Capacitance of different HV powder for the anodization voltage range of 100-200 V (@60°C forming bathtemperature).

Our task was to develop a powder with distinctly higher capacitance than the HV100 and HV200 products in the range
Vf=100-200 V by improving the particle and pore size distribution of the powders. To achieve this the Mg reduction
process, proprietary to H.C. Starck and key step of the HVMC powder production route, needed to be enhanced. The poresize distribution of sintered anodes is considered as a suitable evaluation method because it demonstrates the influence ofthe pressing and sintering step on the powder structure.

3

HV150

HV100

HV200

HV150

HV100

HV200

Cumulative Pore Area (m²/g)

Log Differential Intrusion (mL/g)

0.3

0.2

secondary
0.1

primary pores

pores

0.4

0.2

a)

b)

0.0

0.0
5

1
0.5
Pore size Diameter (µm)

0.1

5

1
0.5
Pore size Diameter (µm)

0.1

Fig. 3 Microstructure of sintered anodes made from HVMC powders (press density 6.0 g/cm³, 10 % shrinkage) a) poresize distribution b) pore area distribution (determined via Hg porosimetry).

Anodes made from HV100 and HV200 have both a bimodal pore size distribution. The smaller “primary” pores (maxima
at 580 nm for HV100 and at~720 nm for HV200) are responsible for the surface area of the porous anode body andtherefore correlating to the capacitance. The larger “secondary” pores (maxima at 1540 nm for HV100 and at 1310 nm
for HV200) representing the empty space between adjacent agglomerates. By adjustments in our processes it was possible
to modify the pore size distribution for the new HV150 powder significantly. Anodes made from this new powder show
a nearly monomodal pore size distribution and the maximum of the primary pore size is adjusted to 660 nm, i.e. between
those of the maxima for HV100 and HV200 anodes. Moreover, the pore peak height (differential intrusion) is significantly
higher than those for anodes made from HV100 and HV200 powders. This is caused by the more homogenous
microstructure of the new HV150 powder which is intrinsically connected to a more uniform pore size distribution. As aresult, the total pore area of HV150 anodes is even higher than that of HV100 anodes having smaller primary pore andparticles.
The improved microstructure has a direct impact on the capacitance behavior (see fig. 2)! At 150 Vf this powder provides
10 % more cap than the HV100 powder and 15 % more cap than HV200 powder. It is interesting that also at 100 Vf andeven 200 Vf this powder resulting in the highest capacitance – this reflects its enhanced uniformity that causes such stablecapacitance slope. This is supported by the fact that the slope of the capacitance as a function of formation voltage for the
HV150 anodes is not steeper than that for the HV100 anodes, but it is parallel.
This shows the general approach how to tailor capacitor powders to higher and higher capacitances in the future! To
provide even more capacitance at 150 Vf the pores of future powders and anodes have to be as narrow as possible. In fig.
3 this is suggested by the green curve – the smaller and higher the pore peak the higher the feasible capacitance at a given
voltage! It was already shown by a simple mathematical calculation [2] that for a perfect homogeneous anode structure
consisting of uniform cylinders of the same size a maximum capacitance of 52kµFV/g @ 100 V and 34 kµFV/g @150 V
can be achieved. This is still far away from today’s available capacitors and shows the potential that could be realizedwith a powder having an even more improved microstructure in the future.

4

The next generation of high(est) voltage powder: HV400
Our HV300 product is mainly used for anodization voltages in the range of 250-300 Vf and is approved as a powder for
highest reliability applications like medical and aerospace. It was proven by capacitor manufacturers that this is thetantalum powder enabling capacitors with the highest energy density currently available. The target of further increasing
the energy density according to equation (1) was the driving force for the development of an HV400 that can be anodized
up to 400 V. Therefore, again the powder microstructure must be adapted: both, primary particle size as well as themedium pore size must be increased. The higher anodization voltage will “consume” more tantalum that is converted to
Ta2O5 film which in turn needs more space than a film formed at 300 Vf. A direct comparison of the pore structure ofanodes made from HV300 and HV400 powders with identical pressing and shrinkage conditions can be seen in fig 4. The
maximum of the primary pore peak has been shifted from 1200 nm to 1600 nm while the secondary pore peak position
has no significantly change. Main difference is that the pore size distribution of anodes made from the new HV400 powder
is narrower than that of anodes made from HV300 powder – primary and secondary peak position are closer, thedistribution is “less” bimodal.

Fig. 4 Pore size distributions of anodes made from HV300 (green) and HV400 (red) powder (Press density 5,5 g/cm³,
10 % shrinkage)

Anodes of both HVMC powders were used for anodization from 250 to 400 V with the same formation program. At 250
Vf, the HV300 powder has a slightly higher capacitance, but for voltages above 260 V the HV400 has a higher capacitance(fig. 5). At 400 Vf anodes made from HV400 powder show 9550 CV/g, an increase by 20 % when compared to thosemade from HV300 powder. The slope of the HV400 capacitance curve is less steep than that of the HV300 – meaning it
is more stable and could be used for even higher anodization voltages.

5Fig. 5 Capacitance of new HV400 vs. HV300 anodes (Press density 5,5 g/cm³, shrinkage 10 %)The microstructure of an HV400 anode formed at 400 V still show open pores, and an oxide film with an impressive
thickness of ~760 nm suggesting that such a powder could be capable to reach even higher voltages (fig. 6). However, an
increase of the leakage current to values >10 nA/µC at higher voltages limits a further anodization to 450 V due to theoccurrence of oxide film breakdown. Since we could not find any breakdowns or oxide defects after 400 V anodization
by intensive SEM investigations, it is assumed that the quality of the oxide film anodized in our current standard
electrolyte recipe (glycol/water/H3PO4) is not good enough to withstand such high voltages. Therefore, we also havestarted to investigate the anodization process more deeper and its influence on the oxide structure.

Fig 6. Cross section of HV400 anodes after 400 V formation.

6

Impact of anodization
The capacitance of an anode body is strongly depending on pressing and sintering conditions. It was shown that for an
HV300 powder the capacitance, leakage current as well as breakdown stability can be varied over a wide range by using
different pressing and sintering conditions. Each anodization voltage has thereby its own optimum, in case of HV300
powder it was found that a press density of ~6.0 g/cm³ with a shrinkage ~ 10 vol.-% will provide the highest volumetriccapacitance and low leakage [3].
But also the conditions of the next processing step – the anodic oxidation of the sintered anodes – have a strong impact
on the leakage current as well as the breakdown stability and decide if it is finally possible to reach very high voltages of
400 V and higher. According to our observations, for voltages >200 Vf a pure aqueous electrolyte system is not appropriate
since the probability of oxide film breakdown and anode destruction is ever increasing with increasing formation voltage.
To improve the breakdown stability, organic additives are used. Commonly used additives found in literature and patents
are e.g. ethylene glycol [4] and phosphoric [5,6], boric [7] or citric acid [8, 9]. However, “classical” electrochemistry andanode investigations of the interaction of electrolyte and Ta anodes during formation in a range of 200 – 400 V are scarce[8] or regarded as “company secrets”.
As a leading Ta powder manufacturer H.C. Starck is continuously developing and testing new tantalum powder types
under various processing conditions to find the ideal microstructure. The capacitance-voltage curves as shown in fig. 1,
2 and 5, as well as the leakage current behavior are very important parameters to select the most suitable powder.
Therefore, the successful development of an HV400 powder is not only depending on physical and chemical powder
properties. Without a proper anodization program and electrolyte system, a 400 V anodization without oxide film
breakdown and low leakage is hard to realize. It is essential but difficult to separate the impact of anodization from powder
modifications.
Thorough investigations have been started to better understand the processes taking place during anodization to highvoltages. In our tests we used large cylindrical anodes ( 8 mm) with a weight of 2.2 g (press density 6.0 g/cm³, shrinkage~ 11 vol.-%) made from HV200 powder to come closer to the typical size of large anodes for high reliability applications.
These anodes were formed to 150 V-250 V in a glycol-water bath (ratio 3:2) at 60°C with a conductivity of 900 µS/cm
adjusted by adding H3PO4. The “constant rate” anodization profile as well as the capacitance and leakage current behavior
is shown in fig 7.

7

b)

a)

Fig. 7 Anodization of anodes made from HV200 powder a) constant rate program for 250 V b) C-LC-U curve for 150V250 V

The term “constant rate anodization” refers to the way of anodization. Usually, a fixed current value is used there, suchthat the formation rate (V/min) is a function of the surface area of the anode. However, since the surface area of a porous
anodes is decreasing with increasing voltage by consumption of tantalum during anodic oxidation, the real current density(A/m²) is continuously increasing and therefore also the speed of anodization (“V/min”) is increased. It is common sensethat the higher the speed of anodization, the higher the risk of oxide breakdown. In contrast, the “constant rate anodization”
provides a constant forming speed (V/min) by continuous adjustment of the current required to match the desired voltage
ramp. Thus, the considerable reduction of the tantalum surface available for further oxide growth during high voltage
formation is compensated by a corresponding reduction of the current, resulting in a reduced breakdown probability.
At the beginning of our formation program, i.e. up to 100 V, a fast forming rate of 0.5 V/min is typically used since theprobability of breakdown of our anodes turned out to be very low in this range. Then, with increasing voltage, the rate iscontinuously reduced to 0.3 V/min and later to 0.2 V/min when the anode body is more susceptible to oxide breakdown
due to a) smaller and smaller remaining tantalum cores; b) increasing oxide thickness and therefore lower heat
conductivity of the overall anode; and c) decrease of the remaining pore volume. The red current curve in fig. 7a is
automatically adjusted and correlates with the surface and capacitance loss with increasing voltage. Intermediate dwells
at 100, 150 and 200 V reduce the temperature increase during forming and thereby decrease the breakdown probability.
After formation, all anodes were leached for 12 h in de-ionized water to remove all components of the electrolyte bathand were subsequently dried at 90°C. Several elements were then analyzed, the contents of oxygen (carrier gas reduced
fusion), carbon (non-dispersive infrared adsorption following combustion in oxygen) and phosphorus (ICP-OES) as afunction of forming voltage are presented in fig. 8. Please note, that all concentration data refer to weight. The sintered
unformed anodes had a phosphorous content <20 ppm, 0,33 % oxygen and 33 ppm carbon.

8

Fig. 8 Contents of P, C and O after anodization in a range of 100-250 V @ 60°C of anodes made from HV200 powders.

As a consequence of anodization, the oxygen content is increasing with increasing oxide thickness andtherefore decreasing capacitance. It is known by several former publications that phosphorus is incorporated
in the outer regions of the oxide film during anodization, probably as PO43- ions while the inner regions of theanodic Ta2O5 layer do not contain any phosphorus [6, 10-14]. However, a lot of these investigations have been
only done on sputtered films under forming conditions not typically used today, i.e. mostly in water and not
glycol-water solvent systems and mostly at lower voltages. To our knowledge, this is the first time that suchinvestigations on phosphorus contents of high voltage anodes with nodular structure anodized under typicalhigh voltage conditions are reported.
The increased incorporation of phosphorus with increasing anodization voltage is in accordance to former
findings. However, the phosphorus content of 970 ppm for anodes anodized to 150 V increases significantly
to 1320 ppm for those anodized to 250 V. The incorporation of phosphorus is assumed to occur as phosphate.
In accordance with literature [6, 10-14], we found a double layer oxide structure, i.e. a PO43- containing outeroxide layer and a presumably phosphate-free inner layer. This was detected in anodized HV200 anodes after
polishing by using a special orientation contrast (in backscattering mode) with a FE-SEM (fig. 9). The darker
scale of grey color corresponds to a lower relative atomic weight caused by the incorporated phosphate ionsand the uniform appearance of this layer implies that this layer has a uniform PO43- distribution.

9

P rich outer oxide layer

Inner oxide layer

Tantalum

200 Vf

250 VfFig. 9 Impact of electrolyte on oxide structure: Anodes made from HV200 powder anodized in H3PO4/glycol/water
solution to 200 V and 250 V (backscattering mode) show an outer layer with lower density. The P rich outer oxide layer
was partial red colored.
By anodization of similar anodes in glycol-water based solution using HNO3 to adjust to same conductivity of 900 µS/cm
it could be shown that this is not an artifact of sample preparation. These anodes, shown in fig. 10, do not show any twolayer structure as observed for those anodized in H3PO4 before but the oxide film consists of only one layer as indicatedby one scale of gray color using the same microscope adjustments.

Fig. 10 Impact of electrolyte on oxide structure: Anodes made from HV200 powder anodized to 200 Vf in HNO3
/glycol/water solution do not show a double but a single layer oxide structure.
The total oxide thickness of anodes made from HV200 powders anodized to 200Vf was determined to 370 nm, the outerphosphate rich layer being 85 nm. Shimizu et al. [10] investigated the incorporation of phosphorous into tantalum foils

10via SIMS depth profiling. They formed the foil at 25°C with a constant current up to 100 Vf. The total oxide layer thickness
was about ~ 162 nm, while the P rich outer layer had a thickness of ~80 nm while in the following region between 80 to
110 nm the phosphorus content decreases constantly until zero level. Our anodes made from HV200 powders show athinner outer layer which might be a result of totally different formation conditions: higher anodization temperature (60°C
vs. 25°C), different anodization program (constant rate vs. constant current) and different size and shape of specimen(anode with nodular structure vs. foil).
The phosphorus content of 1100 ppm observed for anodes anodized to 200 Vs is corresponding to 3370 ppm Phosphate
while the oxide content of 117000 ppm is corresponding to 637280 ppm Ta2O5 (oxide content of phosphate was alreadyexcluded in this calculation). This means that the total phosphate content of the oxide layer is 0.53 % what might beenough to have a negative impact on the breakdown stability under the assumption the phosphate weakens the amorphousoxide structure.
But it must be further considered that only the outer area comprises the phosphate. This means, that the phosphorus
content in the outer layer is according to the ratio of the layer thicknesses (85 nm of 370 nm) as high as 2,3 wt.-%. Taking
the large difference of the molar weights into account (441,9 g/mol for Ta2O5 to 95,0 g/mol for PO43-) this is equal to 9,9mol% phosphate. On the other hand, it is still an open question if this double layer structure created during anodizationin phosphoric acid is beneficial or detrimental at very high formation voltages. Anodes anodized to 250 V and higher (fig.
11) show at some places non-destructive defects (these anodes had good leakage current!), which means that these anodes
suffered no breakdown during anodization: cavities and holes have formed exactly at the interface between the outerphosphorus-rich and the pure Ta2O5 inner layer. These pores could contain gas (e.g. oxygen formed during anodization)or electrolyte or other unknown products of decomposition formed at high voltages. It will need further investigation tounderstand this observation. It should be mentioned, that these defects were also found in anodes formed to these voltages
made from standard Na reduced powders so that it can be excluded that the Mg reduction process used for the HVMC
powder production is responsible for these cavities. The coincidence of the location of the cavities with the interface
between the outer, phosphorus-rich oxide layer and the phosphorus-free inner oxide layer suggests that phosphorous may
play a role for the formation of the cavities.

Fig. 11 Defects observed after 250 V anodization in glycol-water based H3PO4 electrolyte: cavities and holes formed at
the interface from PO43- rich outer oxide layer and inner oxide layer.

11Surprisingly, not only phosphorus but also a remarkably increased carbon amount above the 33 ppm before anodization
was analyzed. At 150 V, the anode has already picked up 390 ppm of carbon and towards 250V this is continuously
increasing to 510 ppm. It is not known how carbon is incorporated into the anodic oxide film and how it is distributed but
it is assumed that the main source might be the glycol that may decompose during anodization to these high voltages. In[15], it is mentioned that ethylene glycol-based electrolytes used for anodizing aluminum, must be almost totally esterified
to be suitable for anodization voltages >400 Vf. Otherwise oxidation products from free ethylene glycol like oxalic or
formic acid will be generated and will attack the oxide film of the Al capacitor.
According to our own anodization experience, it is very hard or nearly impossible to form a Ta anode to 150 V or even
250 V without breakdown or bad leakage current when the powder used would already have a carbon level of > 60 ppm.
However, the much higher carbon content after anodization did not result in an immediate breakdown during anodization
or a bad leakage current in the case of anodes made from our HV200 powder. This implies that it is of great importancefor the manufacturing of tantalum capacitors in which production step/and how carbon is “introduced”. Carbon rich areasin the powder (e.g. caused by insufficient debindering) could result in the local formation of tantalum carbide during
sintering. Later, during anodization the carbide will cause oxide breakdown or crystallization. In contrast to that, thecarbon that is, howsoever and as whatsoever compound incorporated during anodization seems to be less dangerous.
However, the acceptable limit of C pick-up during anodization is unclear since surpassing a critical carbon level might
be a reason for the increased leakage current observed at 250 V. The decomposition of forming electrolyte that contains
organic components is a limiting factor for the realization of a new kind of Ta capacitor that is anodized to 400 V or
higher. Oxide defects, as illustrated in fig. 10 could also be a consequence of the carbon pick-up (and interaction with
PO43-?) during anodization. Further investigations are necessary to clarify the impact of bath temperature, electrolyte typeand concentration but also of the anodization program and total forming time.
How should a perfect anodization concerning bath composition and forming program look like? Additives are neededthat are stable during the whole formation process. The entrapment or incorporation of additives in which form whatsoever
should be prevented so that a perfect oxide film can be generated otherwise the breakdown strength might be reduced.
Also, the thermal conduction of such electrolyte should be sufficient to dissipate the heat that is generated during
anodization of large size anodes to high voltages.

Conclusion
We have presented the latest development of our HVMC high voltage tantalum powders that provide highest capacitancedue to their improved microstructural homogeneity when compared to current standard powders. The newly developed
HV150 powder provides a further capacitance increase up to 15 % at 150 V f anodization by optimization of the primaryparticle and anode pore size structure. The primary pore peak is much narrower and higher compared to that of our HV100
and HV200 products. Moreover, the secondary pore peak is very close to the primary pore peak. Improved homogeneity
is the key for future powder developments enabling even higher capacitance at high formation voltages.

12

We were also successful to further increase homogenously the primary pore and particle size distribution by adjusting ourprocess resulting in our new HV400 powder that extends the anodization range for tantalum powders from currently
300 V to 400 V allowing to store even more energy in a given size. We found that besides adjustment of the powder
structure the anodization process must be adapted as well. We have shown that not only phosphorous, which was alreadyknown, but also carbon is incorporated significantly into the formed anodic oxide film. Both components are introducedby the used electrolyte system. In contrast to former investigations reported in literature, this was to our knowledge thefirst time that such investigations on phosphorus contents of high voltage anodes with nodular structure anodized undertypical high voltage up to 250 Vf conditions are reported. Nevertheless, finding a “perfect” anodization program in
combination with a “perfect” anodization bath preventing the incorporation of critical amounts of foreign ions remains atask to be followed up in the future.

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