Reduction-stable ceramic substances

Details Reduction-stable ceramic substances Reduction-stable ceramic substances

2001-05-21

2003-09-02

Brunsman, David (Department: 1755)

Compositions: ceramic

Ceramic compositions

Glass compositions, compositions containing glass other than...

C501S138000, C501S139000, C361S321400, C361S321500

Reexamination Certificate

active

06613705

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to reduction-stable COG ceramic compounds having a high dielectric constant.
2. Discussion of the Related Art
Such COG ceramic compounds are utilized, for example, in multi-layer capacitors and LC filters with copper electrodes and are suitable for applications in the high-frequency range. What are referred to as COG ceramic are compounds that have a low (<30 ppm/K) temperature coefficient of the capacitance.
In the case of materials with a low dielectric constant DK, technical solutions have already been disclosed that allow the common sintering of a ceramic compound with Cu electrodes under reducing conditions in that the sintering temperature is lowered below the melting temperature of the copper (1083° C.). Specific sintering aids, preferably additives of glass frit, whose material basis is systems containing lead oxide and/or bismuth oxide are applied. A partial oxygen pressure<10
−2
Pa must be applied in order to suppress the oxidation of the copper during the sintering in the region of 1000° C.
At the same time, a lower, critical limit value of the partial oxygen pressure dare not be downwardly transgressed since, otherwise, the ceramic or a constituent of the added glass frit is subjected to reduction, which necessarily leads to a lowering of the insulation resistance and to an unacceptable increase in the dielectric losses. The de-bindering of the green compact must be completely
REDUCTION-STABLE CERAMIC COMPOUNDS
The invention is directed to reduction-stable COG ceramic compounds having a high dielectric constant according to the preamble of patent claim
1
.
Such COG ceramic compounds are utilized, for example, in multi-layer capacitors and LC filters with copper electrodes and are suitable for applications in the high-frequency range. What are referred to as COG ceramic are compounds that have a low (<30 ppm/K) temperature coefficient of the capacitance.
In the case of materials with a low dielectric constant DK, technical solutions have already been disclosed that allow the common sintering of a ceramic compound with Cu electrodes under reducing conditions in that the sintering temperature is lowered below the melting temperature of the copper (1083° C.). Specific sintering aids, preferably additives of glass frit, whose material basis is systems containing lead oxide and/or bismuth oxide are applied. A partial oxygen pressure<10
−2
Pa must be applied in order to suppress the oxidation of the copper during the sintering in the region of 1000° C. At the same time, a lower, critical limit value of the partial oxygen pressure dare not be downwardly transgressed since, otherwise, the ceramic or a constituent of the added glass frit is subjected to reduction, which necessarily leads to a lowering of the insulation resistance and to an unacceptable increase in the dielectric losses. The de-bindering of the green compact must be completely realized before the onset of the sintering in order to avoid a local downward transgression of this lower critical limit value.
The publications EP 0 534 802 A1, U.S. Pat. Nos. 5,264,403, 5,304,521, 5,350,721, 5,479,140, 5,493,262, 5,488,019 and 5,485,132 disclose ceramic compounds of the substance systems BaO—TiO
2
—SE
2
O
3
wherein the oxide of the rare earth metals SE
2
O
3
can be partially replaced by Bi
2
O
3
whose sinter compression already partly succeeds at 900° C. in that glass frit parts that contain CdO, PbO or Bi
2
O
3
or glasses of the system ZnO—B
2
O
3
—SiO
2
are added. A common sintering with silver electrodes in air is thereby enabled. For a common sintering with copper electrodes under inert conditions, for example under nitrogen, the systems prove to be inadequately stable with respect to a partial reduction, which results in a lowering of the insulation resistance and increase in the losses.
The publications EP 0 534 801 A1, U.S. Pat. Nos. 5,458,981 and 5,292,694 have likewise protected BaO—TiO
2
—Se
2
O
3
ceramic compounds in combination with glass additives containing B
2
O
3
and ZnO for the purpose of the common sintering with silver electrodes. In these cases, too, the de-bindering under air access prevents the combination with copper electrodes, so that there must be recourse to silver or silver/palladium alloys as electrode material. The advantage of a cost-beneficial application of silver electrodes is opposed by the disadvantage of great mobility, particularly at high temperature, this leading to migration effects and causing a deterioration of the dielectric properties as a result.
In the earlier German Patent Application 19749858.2, the material system BaO—PbO—Nd
2
O
3
—TiO
2
used for the manufacture of high-DK COG capacitors and microwave resonators in the region of the phase formation of rhombic bronzes (Ba
1−y
Pby[sic])
6−x
Nd
8+2x/3
Ti
18
O
54
with 0.6<x2.1 and 0<y<0.6 is made available for a sintering at temperatures<1030° C. and, thus, for the common sintering with Cu electrodes in that the sintering aid, preferably PbO-free glass frit having a specific composition is added and a complete de-bindering under nitrogen is achieved by the action of water steam at elevated temperature upon utilization of the steam reforming process known from petroleum processing. The matching of the temperature coefficient of the capacitance TKC or, respectively, of the temperature coefficient of the resonant frequency TK&ngr;0 to the tolerance limits valid for COG capacitors TKC<30 ppm/K or, respectively, LC filters with TK&ngr;0<10 ppm/K succeeds in that modifications of the composition of the ceramic compound are undertaken with additives of Nd
2
Ti
2
O
7
or of Ba
4.5
Sm
9
Ti
18
O
54
. It should be noted as a limitation to this technical solution that the stability of the ceramic is limited due to the PbO content, which requires an especially careful de-bindering and the avoidance of too low a partial oxygen pressure in the sintering. The two demands are linked to one another since, in particular, the unacceptable downward transgression of the critical partial oxygen pressure limit, caused, for instance, by slight organic residue constituents, must also be avoided locally. Otherwise, a eutectic Pb/Cu alloy forms which melts at 924° C., i.e. the electrodes dribble out.
Systems free of PbO and Bi
2
O
3
prove suitable for avoiding such a disadvantage. Thus, for example, the ceramic Ba
4
(Sm
0.6
Nd
0.4
)
9.233
Ti
18
O
54
that can be presented by sintering at approximately 1400° C. is characterized by a TK&ngr;0=value of +9 ppm/K and a quality-frequency product Q&ngr;0=9000 (3 GHz) [P. Lafez, G. Desgardin, B. Raveau, J. Mater. Sc. 30 (1995) 267]. Comparable values are recited in the literature for the composition Ba
4.2
(Sm
0.3
Nd
0.7
)
9.2
Ti
18
O
54
[H. Ohsato, H. Kato, M. Mizuta, S. Nishigaki, T. Okuda, Jpn. J. Appl. Phys. 34 (1995) 5413]. For lowering the high sintering temperature required for ceramic compounds of the system BaO—TiO
2
—Nd
2
O
3
—Sm
2
O
3
, EP 0 701 081 A1 proposes glass frit additives that contain PbO and even GeO
2
, which is disadvantageous for the stability with respect to reduction for the reasons that have been presented.
The invention is based on the object of manufacturing COG ceramic compounds with optimally high dielectric constant on the basis of a substance system that is highly stable with respect to reduction and that allows a TKC value<30 ppm/K or a TK&ngr;0 value<ppm/K to be set dependent on the selected composition, and to also specify a sintering temperature that lies under the melting temperature of the copper and enables the sintered compression in the presence of inside copper electrodes upon preservation of the dielectric properties needed for COG multi-layer capacitors and LC filters.
This object is inventively achieved with a reduction-stable COG ceramic compound that comprises the features of patent claim
1
.
Advantageous developments are recited in the subclaims. T

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