AU2002243956B2 - Tantalum-silicon and niobium-silicon substrates for capacitor anodes - Google Patents

Description

P XWPDOCS\NS mhO7N7S26193pccflacIIon dMc.3/912007 -1- TANTALUM-SILICON AND NIOBIUM-SILICON SUBSTRATES FOR CAPACITOR ANODES FIELD AND BACKGROUND 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 "formed" to establish a thin oxide of tantalum and/or niobium (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, sinter 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.

The present invention seeks 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 P \WPDOCSHSMa7826)I093p.oi~iml /9lfl7 -2and low dissipation factor in relation to high CV/gram systems (30,000 and higher).

In one example, the present invention seeks to achieve such stability reliably and in high yields.

SUMMARY OF THE INVENTION Accordingly, there is provided in one example, new tantalum-silicon and niobiumsilicon systems preferably formed as mixtures of 90-98 wgt-% Ta, Nb and 210 wgt-% of Si powders mixed together. One can also add Si to a reactor for Na reduction of K 2 TaP 7 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. 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 and optimized porosity with generally larger pores and greater uniformity of pore size 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 H 2 or NH to minimize contamination.

P \PDOCSUISVrc hO7N726193qtpefiomdxc-.3iV2OO7 -3- Other, 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 DRAWINGS 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 0

C;

FIG. 2 compares similar materials as to bias dependence at various test bias voltages; FIGS. 3-4 trace capacitance and leakage vs. sinter temperature (similarly to FIG. 1) comparing Ta-Si with Ta and also with TaN+Si; FIGS. 5-6 compare (similarly to FIG. 2) bias dependence of Ta, Ta-Si, Ta+Si 3

N

4 TaN- Si 3

N

4 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 DESCRIPTION OF PREFERRED EMBODIMENTS U.S. patent 4,432,035 dated February 14, 1984 of Hsieh (IBM) discloses Ta9 Si 2 (in lieu of previously tried Ta 2 Si) 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.

Example 1 Initial tests showed leakage of Ta-Si powder substrate systems about similar to Ta powder substrate systems (no gain) but capacitance was enhanced for Ta-Si vs. Ta 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-pellet-group averaging for each of Ta, Ta-Si systems. The Ta was a standard product 50K-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 P:\WPDOCS\IS\Ma rch0778260193speciicafiio doc-1/92007 -4the 50K-9010 Ta powder, to approximate Ta 9 Si 2 Powders of both systems were pressed into pellets and sintered at 1500 0 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 0

C.

Conditions of preparation and experimental results are tabulated as follows: Table I Summary of pellet preparation, formation and testing conditions Condition Value(s) Pellet Mass 0.14 Press Density (g/cc) Sintering Temperature 1350, 1450, 1550 Sintering Time (minutes) Formation Temperature Formation Voltage 16, 30, 40, 50, 80, 100, 120 Formation Current (mA/g) 100 Hold Time (hrs) 2 hrs. or 5 minutes Formation Electrolyte 0.1 V/V% H 3 P0 4 DCL Test Voltage Bias Voltage 0-20V DLC Soak Time (minutes) Table II: Electrical Results for Tantalum Silicon Blend (held for 5 minutes) Formation Voltage 15000 C CV/g 9p 1 F 15000 C L/C (nA/LpFV) 16 32,400 0.884 25,100 0.422 24,300 0.385 23,800 0.560 23,000 3.576 100 22,500 2.326 PP wDOCsk1IS\Marcho7\726193pircatim dm-.3116007 Table III: Capacitance i&oV/g) Sinter 50K-901 0 LFS-001 50K-90l10 LFS-001 Temperature 5OVf 5OVf 12OVf l2OVf 1350 41,500 31,400 1450 30,600 24,300 19,000 20,900 1550 16,100 19,300 Table IV: Leakage Sinter 5OK-901 0 LFS-001 5OK-901 0 LFS-001 Temperature 5OVf 5OVf 12OVf l2OVf 1350 0.322 0.512 1450 0.275 0.249 0.608 0.946 1550 0.067 0.065 Table V: 140 Volt formation Capacitance (VF.V g) and Leakage (nIVF.V/g) ~Sinter [50K -9010 LFSOOI 5OK-901 0 L/C LFS-001 L/C Tfemperature C apacitance Capacitance 1450 16,900 16,000 1.230 0.960 1500 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 but shows lesser decline at increasing sinter temperatures, a number of enhanced stability and rates of retardance but ambiguous, given closeness of the values.

Example 2 Further samples were prepared as in Example 1 but extending to Ta-Si, TaN-Si, and Ta-Si 3

N

4 333 gm. 60M 99.999% Si with 9.667 gm 5OK-9010; .3106 gm 60M 99.999% Si with 9.689 gm TaN-003 .545 gM Si 3

N

4 with 9.456 gmn 50K-901 0 .507 gM Si 3

N

4 with 9.43 gm TaN-003 All mixtures had a Ta/Si ratio of 9/2.

P\WPDOCS\HSM.rch07 17826 191spcciricalondo-3t/IX)7 -6- Also included as controls were: pure TaN-003 pure 50K-9010 Conditions of experimental procedure and results are set forth in Tables VI-VII.

Table VI Summary of pellet preparation, formation and testing conditions Condition Value Pellet Mass 0.14 Press Density (g/cc) Sintering Temperature (oC) 1350, 1450, 1550 Sintering Time (minutes) Formation Temperature Formation Voltage 50, 120 Formation Current (mA/g) 100 Hold Time (hrs) 2 hrs.

Formation Electrolyte 0.1 V/V% H 3

PO

4 DCL Test Voltage Bias Voltage 0-20V DLC Soak Time (minutes) Table VII: Capacitance (mF.V/g) Sinter 50K- TaN-003 Ta+Si TaN+Si TaN+Si 3 N4 Ta+Si 3

N

4 Temperature 9010 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 1450-100Vf 18,564 21,336 18,398 18,060 14,318 P %WPDOCS S\MmhO778260193 prpfint,. d o3t)rRO 7 110 -7- Table VIII: Leakage (nA/mF*V) Sinter 50K-9010 TaN-003 Ta+Si TaN+Si TaN+Si3N4 Temperature 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.

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 0 C and 1350 0 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-Si 3

N

4 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. 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 L/C and remained stable generally.

P \WPDOCSQ{S\MuchO7\7826O193slccalll dm-3/161'2(E ct -8- Table IX Sinter Temperature Increase Capacitance Decrease L/C O1100C 1% 41% 1200 0 C 4% 36% 1300 0 C 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 Silanes were used to add silicon to tantalum as described below in parts and below, and the resultant silicon doped tantalum tested with results as indicated at

APST

Tantalum powders were wet with an aqueous solution of APST amino propyl silane, triol, i.e. C 3 H1 NO 3 Si, 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 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 0

C

for 30 minutes under vacuum.

I

P \WPDOCS\SIrduch 78260193 pifulio d-.31 I 200 -9-

THSMP

Tantalum powders were wet with an aqueous solution of THSMP sodium 3trihydrosilylmethylphosphonate, i.e. PC 4

H

1 2 NaO 6 Si, 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.

Test Results The doped powders of and 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 content in ppm and similarly content of nitrogen oxygen phosphorus 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 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 Powder SA SBD FAPD Flow C N 0 P Si 9172 27.2 2.64 0.384 32 60 10330 92 71 10097 27.3 2.76 0.396 365 256 11010 78 405 11423 24.3 1.7 0.211 652 65 11880 317 500 The same powders, as agglomerated, were tested in a Malvem 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 P %WPDOCS\HSVMA, O7\7826(ii93pwicnion dom-3IMI2()7 11micron 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 10% 50% 90% MV CS 1 micron s 13.4 53.7 149.26 69.923 0.211 7.51 16.9 77.89 214.94 103.4 0.17 5.62

APST

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 with each other, and compounds of one or both of these metals including nitrides and subnitrides. Still further silicon containing compounds and solutions 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 0

C

preferably above 800°C as taught, e.g. in W.W. Albrecht et al., U.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 magnesium oxide). It is also known per P:\WPDOCSHS\M acl778260 19 3slpccijliondoc-3/9/X)7 lse 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.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as, an acknowledgement or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

Claims (12)

1. A composition for producing capacitor anodes consisting of a component a) being IND selected from the group consisting of tantalum powder, tantalum nitride powder, niobium 5 powder, niobium nitride powder or mixtures thereof; and a component b) selected from the group consisting of silicon powder, Si3N4 powder, amiono propyl silane triol, sodium-3- Ci trihydrosilylmethylphosphonate.

2. A composition according to claim 1, wherein component b) is added in a sufficient amount to achieve a molar ratio of silicon to refractory metal of 2:9.

3. A powder composition according to claims 1 or 2, wherein component a) is present in an amount of 90 to 98 weight percent and component b) is present in an amount of 2 to weight percent.

4. A powder composition according to any one of claims 1 to 3, wherein compound a) comprises a dopant.

5. A capacitor anode obtained by pressing and sintering a powder composition according to any one of claims 1 to 4.

6. An electrolytic capacitor comprising an anode according to claim

7. A method for producing a powder composition according to any one of claims 1 to 4 comprising the steps of: providing component a) providing component b)

8. A method for producing an electrolytic capacitor comprising the steps of: providing a powder composition according to any one of claims 1 to 4 pressing the powder composition sintering the powder composition.

9. A composition for producing capacitor anodes, being substantially as hereinbefore described with reference to the accompanying figures.

A capacitor anode, being substantially as hereinbefore described with reference to the accompanying figures.

11. An electrolyte capacitor, being substantially as hereinbefore described with reference to the accompanying figures. P %WPDOCSUIS\Ma )77260 9 spmcflntioiooc-3-)n2lX) 13

12. A method for producing an electrolytic capacitor, being substantially as hereinbefore described with reference to the accompanying figures.