Note: Descriptions are shown in the official language in which they were submitted.
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TANTALUM-SILICON AND NIOBIiIM-SILICON SUBSTRATES FOR
CAPACITOR ANODES
FIELD AND BACgGROUND OF THE INVENTION
The present invention relates to substrates for high dielectric constant
capacitors and more particularly powder substrates based on tantalum and/or
niobium
fabricated into porous masses that are electrolytically "foamed" to establish
a thin
oxide of tantalum and/or niobiura (normally tantalum and/or niobium pentoxide)
as
the dielectric layer. These are utilized with well known solid or wet
electrolyte
systems.
The tantalum/niobium powder substrates (primarily tantalum) have been
utilized for over half a Century as materials of choice for highest
capacitance,
compact capacitors with low leakage, low electrical series resistance and
high'voltage
breakdown levels, standing up well to demanding usage and quality control life
tests
of military, computer and telecommunications markets.
The state of the art capacitance level for electrolytic capacitors has moved
up
in the last decade from under 10,000 micro-farad volts per gram to over 50,000
through shrinkage of powder substrate size (with greater surface area of
formed oxide
in relation to weight and volume of the anodes; anode porosity control for
greater
effective access to the expanded area, sinfer controls, doping of the
substrate with .,
phosphorous and in some instances nitrogen, silicon, or sulfur. Improvements
in lead , '.
wire production, lead wire to anode bonding, forming routines, electrolytic
systems '
and packaging have also been made.
However, these advanced high capacitance systems have produced new
expectations as to leakage, series resistance, bias dependence, thermal
stability
generally, in capacitor production and usage, frequency stability, voltage
breakdown
and overall stability that have not been met or are only met with high yield
losses.
Nitrided Ta, Nb and other forms of Ta, Nb modification have helped with
stability as
well as capacitance but insufficiently in relation to expectations.
It is a principal object of the invention to provide a capacitor substrate
system
affording improved leakage, series resistance, bias dependence, thermal
stability
generally in capacitor production and usage, frequency stability, increased
porosity
leading to lower equivalent series resistance ("ESR") and low dissipation
factor
("DF"), in relation to high CVlgram systems (30,000 and higher).
It is a related object to achieve such stability reliably and in high yields.
AMENDED SHEE ~
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suNnvraRY of Tl~ n~IVENTTON
The objects of the invention are met through new tantalum-silicon and
niobium-silicon systems preferably formed as mixtures of 90-98 wgt-9'o Ta, Nb
and 2-
wgt-°!o of Si powders mixed together. One can also add Si to a reactor
for Na
S reduction of KZTaP~. One can also use Si based wetting agents in suspensions
of Ta
as a means of introducing Si to Ta in appropriate amounts and forms.
Enhancement (lowering) of bias dependence after heat treatment has been
achieved and can be achieved reliably through the Ta-Si substrate system and
such
result is now reasonably projected for similar Ta/Nb-Si substrate systems.
10 Electrolytic porous anode capacitors made with such systems can afford
stable
performance at high voltage formations, and under conditions of high frequency
usage.
The benefits of the present invention can also be realized in Ta/Nb-nitride
systems and in systems of Si with Ta/Nb, Ta/Nb-nitride doped with known
capacitance enhancing impurities such as P, Si, S.
The benefits of silicon addition include pore size control of sintered anodes
y ~~~,~ and optimized porosity with generally larger pores and greater
uniformity of pore size
P.
to enable a more certain effective electrolyte precursor access, effective
electrolyte
conduction paths and less degradation~of capacitor performance associated with
varying porosity.
One method to distribute Si homogeneously throughout produced Ta or Nb is
by use of liquid organo-silicon compounds. Due to the desire for reduced
oxygen and
carbon content, the preferred organo-silicon compound would be in the silicone
family. These compounds which are primarily made up of SiOH bonds will
decompose during the high temperature treatment of the powders to si in a
reducing
atmosphere.
The reducing atmosphere.may be provided in the standard technology of the
field but it is preferred to be Mg or H2, or NH to minimize contamination.
Other objects, features and advantages will be apparent from the following
detailed description of preferred embodiments taken in conjunction with the
accompanying drawings in which:
BRIEF DESCRIPTION OF THE DRAVf~TGS
FIG. 1 is a graph of capacitance Ta-Si vs. high capacitance type of Ta (50K)
capacitor with sintering at various temperatures from 1300 to 1550°C;
FIG. 2 compares similar materials as to bias dependence at various test bias
voltages;
FTGS. 3-4 trace capacitance and leakage vs. sinter temperature (similarly to
FTG. 1 ) comparing Ta-Si with Ta and also with TaN+Si;
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IPEAIU~ 0 5 S E P ZOOZ
FIGS. 5-6 compare (similarly to PIG. 2) bias dependence of Ta, Ta-Si,
Ta+Si3N4, TaN-Si3N,, and TaN-Si; and
FIGS. 7-8 compare incremental volume vs. pore diameter characteristics for
Ta vs. Ta-Si, and TaN vs. TaN-Si.
BRIEF DESCRI~'TION OF PREFERRED EMBODIMENTS
U.S. patent ("USP") 4,432,035 dated February 14,1984 of Hsieh (~11~
discloses Ta9 Siz (in lieu of previously tried TaiSi) in thin film capacitors
but has
never afforded the art a path to useful powder substrates for sintered
electrolytic
capacitor anodes.
The present invention starts from a separate path of recognizing, from the
work of T. Tripp et al. USP 4,957,541 (capacitor grade tantalum powder, see
also,
references cited therein), the proper role of tantalum nitride in affording a
new series
of useful powder substrates.
ple 1
Initial tests showed leakage of Ta-Si powder substrate systems about similar
to
t~;~~: Ta powder substrate systems (no gain) but capacitance was enhanced for
Ta-Si vs. Ts
even at higher sinter temperature for the Ta-Si and slightly lower at lower
sinter
temperatures. It appeared that the Si was acting as a sinter retardant.
The tests involved four-peDet-group averaging for each of Ta, T~Si systems.
The Ta was a standard pmduct SOK-9010 made from sodium reduced potassium . '.
heptafluorotantalate with artifacts of leaching, fine sizing, doping and
deoxidization
well known in the art. The Ta-Si was prepared by blending 0.333 gm of 60 mesh
99.999% pure Si powder with 9.667 gm of the SOK 9010 Ta powder, to approximate
_. _ . _ 25 Ta,Siz.
. v ' ~ . Powders of both systems were pressed into pellets and sintered at
1500°C for
first sets of pellets formed at 16, 30, 40, 50, 80 and 100 volts-formation
Voltage, Vf,
and second sets-sintered at various temperatures from 1,350 to 1,550°C.
Conditions of preparation and experimental results are tabulated as follows:
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. IPEAIUS 0 5 5~i' 2002
Tahle 1 Summary of pellet nrenaratinn_ formation and testing conditinnc
,Condition Values
Pellet Mass 0.14
Press Den$i~ cc 5.0
Sinterin Tem nature C 1350,1450,1550
Sinterin Time minutes 20
Formation Tem erature C 80
Formation Volta a 16, 30, 40, 50, 80,100,120
Formation Current mA/ 100
Hold Time 2 hrs. or 5 minutes
Formation Electrol to ' O.1VIV9'o H PO
DCL Test Vol a %V 70
Bias Volta a v 0-20V
DLC Soak Time minutes 5
Table II: Electrical
Results for
Tantalum Silicon
Blend eld
for 5 minutes
Formation 1500C CVIg 9~.F 1500C LC (nA/~,FV)
Vol V/
a
16 32,400 ' - 0.884 1 ,
30 25,100 0.422
40 24,300 0.385
50 23,8 0.560
80 23,000 ' 3.576
100 22,500 2.326
Tahle III: Cavacitance luF~V/Ql
Sinter 50K-9010 LFS-001 50K-9010 LFS-001
Tem nature 50Vf 50Vf 120Vf 120Vf
1350 41,500 31,400
1450 30,600 24,300 19,000 20,900
1550 16,100 19,300
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Table IV: Ixakaae (nA/uF~V/s)
Sinter 50K-9010 LPS-OOI SOK-9010 LFS-001
Tem rature50Vf SOVf I20Vf I20Vf ,
1350 0.322 0.512
1450 0.275 0.24.9 0.608 0.946 '
1550 0.06? 0.065
Table
V:
140
Volt
formation
Capacitance
(uFV/g)
and
I~eskage
(nAIwFV/g)
Sinter SOK-9010 LFS001 50K 9010 LFS-001
LC LC
Tem erature Ca acitance Ca itance
1450 16,900 16,000 ~ 1.230 0.960
1500 18,600 0.500..
S The results are shown graphically in FIG. 1 where capacitance of the Ta=Si
powder substrate capacitors (LFS) is seen to be in the same range as the Ta
powder
substrate capacitors (50K) but shows lesser decline at increasing sinter
temperatures, a
number of enhanced stability and rates of retardance but ambiguous, given
closeness
of the values.
10~ c,~le 2
Further samples were prepared as in Example I but, extending to Ta-Si, TaN-
Si, and Ta-Si3N4:
-- .333 gm 60M 99.999% Si with 9,667 gm 50K-9010;
-- .3106 gm 60M 99.999%v Si with 9.689 gm TaN-003
1S - .545 gm Si3N, with 9.456 gm SOK-9010
-.507 gm Si3N4 with 9.43 gm TaN-003
Ail mixtures had a TaISi ratio of 9/2.
Also included as controls were:
20 - pure TaN-003
- pure 50K-9010
Conditions of experimental procedure and results are set forth in Tables VI-
VII.
a
i
r
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~,PEAIUS o 5 s~~ 2002
Table VI llet
Su of re
aration,
formation
and
testin
conditions
Condition Values
T
Pellet Mass 0.14
Press Densi cc 5.0
Sinterin Tem tore 1350, 1450,1550
C
Sinterin Time minutes) 20
Formation Tem refute 80
(C
Formatiow Volta a 50,120
Formation Current 100
mA/
Hold Time 2 hrs.
Formation Electrol O.1V/V9lo H O
DCL Test Volta a 1oV 70
)
Bias Volta a v 0-20V
DLC Soak Time (minutes) 5
'--:~~~=
'
TabIeVII:
Capacitance
(mFV/g)
Sinter 50K- TaN-003 Ta+Si TaN+Si TaN+Si3N4Ta+Sx3N,
Tem rature9010
1350 40,959 31,220 31,666 33,985 31,194 30,643
1450 29,260 30,643 23,581 30,608 29,946 25,594
1550 14,910 26,714 17,588 26,828 25,253 19,915 .
1450-100Vf18,564 21,336 18,398 18,060 14,318
Table VI II: Leaka FV
a nA/m
Sinter 50K-9010 TaN-003 Ta+Si TaN+Si TaN+Si3P
'~ T
em erature 50Vf
1350 0.272 0.881 0.565 1.006 41.900
1450 0.064 1.079 0.458 0.434 6.726
1550 0.062 0.954 0.058 0.164 0.157
1450-100Vf 0.701 0.880 1.332 11.413
' The results
are shown
graphically
in FIGS.
3-8.
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WO 02/064858 PCT/US02/04073
FIGS. 3-4 show TaN and TaN-Si with lowest cap' loss within varying sinter
temperature, but with leakage enhancement (lowering) for TaN-Si at increasing
sinter
temperatures. A favorable balance of characteristics of Ta-Si is also shown.
FIGS. 5-6 show (on 1450°C and 1350°C sintered test products)
that at various
bias voltages from 0 to 20 volts capacitance declines most at increasing bias
for Ta,
much less for Ta-Si and still less for Ta-Si3N4 and lease for TaN-Si.
FIGA. 7-8 with porosmetry testing results show incremental volume vs. pore
diameter benefit for Ta-Si vs. Ta (FIG. 7) and TaN-Si (FIG. 8). This can lead
to a
reduction of electrical series resistance and improved performance in high
frequency
usage.
The overall results indicate a need.
Example 3
Niobium Silicon (Nb-Si) (Nb-Si_ systems were also processed as for Ta
above. These behaved differently than the Ta-Si system. There wasn't an
improvement on thermal stability and bias dependence, but something different
was
observed. There was an overall increase in capacitance with the addition of
about 1%
Si. There was also a decrease in leakage. The % increase in capacitance arose
with
increasing sinter temperature, decreased in LlC and remained stable generally.
Table IX
Sinter Tem erature % Increase Ca acitance% Decrease L/C
1100C 1% 41%
1200C 4% 36%
1300C 25% 33%
There was an increase in porosity in Nb as seen in Ta, but the sample used had
very good porosity to begin with so no significant decrease in ESR was seen. X-
Ray
was done on a sintered Ta-Si mixture pellet and the result was that an alloy
was
actually made and there was not just a mixture.
Discussion
The present invention establishes uniquely and surprisingly a distinct change
of Ta-Si (and/or TaN-Si) powder substrate sinter characteristic vs. Ta (or
TaN) that
can be tied to higher quality sinter temperature to emerge with beneficial
high
capacitance, low leakage capacitors with various areas of enhanced stability
as to
voltage bias, ESR frequency, heat treatment.
Example 4
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Silanes were used to add silicon to tantalum as described below in parts (a)
and (b), below, and the resultant silicon doped tantalum tested with results
as
indicated at (c).
(a) APST
Tantalum powders were wet with an aqueous solution of APST - amino propyl
silane, triol, i.e. C3H11N03Si, as a means of adding silicon and nitrogen
dopants to the
powder. The doping was done at a level necessary to generate 500 ppm of
silicon.
The tantalum used was a typical 50,000 CV/gm class powder (50 K). This level
of
doping, theoretically should have generated an additional 249 ppm of nitrogen
to the
powder, a desired result. APST is water soluble, and hence can be added with
conventional phosphorous additive using techniques well known to those skilled
in
the art. In this Example, the powder was in fact simultaneously doped with 100
ppm
phosphorous dissolved in the same solution. After doping addition, the powder
was
dried, and then thermally treated (agglomerated) at 1320°C for 30
minutes under
vacuum.
(b) THSMP
Tantalum powders were wet with an aqueous solution of THSMP - sodium 3-
trihydrosilylmethylphosphonate, i.e. PC4H12NaO6Si, as a means of adding
silicon and
phosphorous dopants, at a level to generate about 500 ppm of silicon. Again,
the
tantalum powder used was a typical 50,000 CV/gm class powder. This level of
dopant would be expected to provide an additional level of 550 ppm phosphorus,
a
relatively high level of phosphorous for this type powder. Hence, no
additional
phosphorous was added. Like APST, THSMP is water soluble, and also can be
added
using the typical methods to add phosphorous known to those skilled in the
art. After
addition and drying, the powder was thermally treated under the same
conditions as
the APST sample.
(c) Test Results
The doped powders of (a) and (b) were tested for surface area (SA, sq.
cm./gm), Scott Bulk Density (SBD, cc/gm), Fisher Average Particle Diameter
(FAPD,
microns), Flow (gm/sec.), carbon (C) content in ppm and similarly content of
nitrogen (N), oxygen (O), phosphorus (P) and silicon (Si) and the results are
shown
in Table X, below for APST and THSMP treated powders, with the base 50K
tantalum powder as a control similarly tested. The pick-up of silicon and
nitrogen
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WO 02/064858 PCT/US02/04073
was very accurate (corresponding closely to calculated) and less so for
phosphorus but
that had been provided in excess in any event, as indicated too by the higher
surface
of the 50K + THSMP sample compared to the others. Sodium added via the THSMP
was substantially dissipated in the thermal agglomeration after-treatment
TABLE X
P~xn,~d~r S~ ~HL~ F~4PD Flo~nrC IV 0 P' Si
~~tt 91'2 2'T.2 2.04 0.384 S2 60 1033092 '~1
6~1~ + 10a9'~2f 2.76 0.390 306 266 ~ 78 406
~,P~T .3 9010
~O~t+THSN1P11423 24.3 1.T 0.211 062 00 ~ 39 600
1800 T
The same powders, as agglomerated, were tested in a Malvern Mastersizer
particle size measuring instrument utilizing laser diffraction measurements of
particles suspended in an aqueous bath and results appear in Table XI, below.
The
tabulated results are shown for each of the 50K control, powder as treated
with APST
and with THSMP, the particle size of agglomerated particles at up to 10, 50,
90 wgt-
% fractions, median value (MV) in microns, calculated surface area (CS) in sq.
m/gm,
and wgt-% of llmicron and under fines (fine particles). It is seen that the
doping
served as a significant sinter retardant in both the APST and THSMP cases.
TABLE XI
Powder IO% 50% 90% MV CS llmicron
s
50K 13.4 53.7 149.26 69.923 0.211 7.51
50K+ 16.9 77.89 214.94 103.4 0.17 5.62
APST
50K + 8.68 58.52 177.28 78.051 0.297 12.73
THSMP
Despite the sodium present in the THSMP after vacuum thermal treatment, the Na
present in the sample 50K + THSMP was comparable to the control. It can also
be noted
that even though the silicon is introduced in a compound form, it is converted
to elemental
form in the course of thermal treatment for agglomeration and alloyed with the
host
tantalum.
It should be understood that similar effects are to be expected if similar
silicon
doping is applied to niobium, alloys of either tantalum or niobium, including
alloying
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WO 02/064858 PCT/US02/04073
with each other, and compounds of one or both of these metals including
nitrides and
sub-nitrides. Still further silicon containing compounds and solutions (e.g.,
water
glass) can be utilized to provide benefits of silicon doping as described
above and if
desired to also provide secondary benefits of other dopants - e.g. nitrogen
and/or
phosphorous doping.
The agglomerated particles (or resultant anode compacts) can be subjected to
known per se deoxidation treatments such as exposure to vapors of alkali or
alkaline
earth metals or aluminum, preferably magnesium or calcium, while heating the
powders at 600-1200°C preferably above 800°C as taught, e.g. in
W.W. Albrecht et
al., LT.S. Patents 4,483,819, granted July 19, 1982, and 4,537,641, granted
August 27,
1985. The deoxidation heating also provides a way of advancing the conversion
of
silicon compounds to elemental silicon and its alloying with the host
refractory metal.
Deoxidation can be applied during the thermal agglomeration (reactive
agglomeration). Often the deoxidation is followed by a treatment with an
inorganic
acid to remove residue of the reduction reaction (e.g. magnesium oxide). It is
also
known per se that other impurities of the host refractory metal can be removed
by the
deoxidation process and that thermal agglomeration temperatures can be reduced
because of such process. The combination of chemical and thermal factors of
the
doping, agglomeration, deoxidation and eventual sintering stops can be
optimized for
each situation of doping with silicon, alone or with other additives, to
improve
physical and electrical properties of the capacitors made with porous anode
compacts
made from such agglomerated powders.
It will now be apparent to those skilled in the art that other embodiments,
improvements, details, and uses can be made consistent with the letter and
spirit of the
foregoing disclosure and within the scope of this patent, which is limited
only by the
following claims, construed in accordance with the patent law, including the
doctrine
of equivalents.
What is claimed is:
