Dielectric ceramic composition

Details Dielectric ceramic composition Dielectric ceramic composition

2002-04-17

2004-05-25

Group, Karl (Department: 1755)

Compositions: ceramic

Ceramic compositions

Titanate, zirconate, stannate, niobate, or tantalate or...

Reexamination Certificate

active

06740614

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a dielectric ceramic composition widely used in high frequency electronic components and, more particularly, to a low-temperature cofired dielectric ceramic composition with a high dielectric constant and a low dielectric loss.
Low-temperature cofired dielectric compositions (materials) means compositions (materials) which can be fired in the temperature range of 800-950° C., lower than the melting point of silver (Ag) or copper (Cu), in contrast to the conventional ceramic dielectrics which are sintered at 1,300° C. or higher.
2. Description of the Prior Art
In view of the recent trend for miniaturization, lightness and modulization of high frequency electronic devices, it is required to develop dielectric material which can be multi-layered, followed by cofired with the inner electrode.
For multilayering and cofiring with the conventional dielectric materials, it is required to use high melting temperature metals, such as Mo or W, as inner electrode patterns.
Where Mo or W is adopted to form inner electrode patterns, however, an economic disadvantage is incurred because of its high cost. Above all, the skin effect, which causes radio frequency current to concentrate on the surface of a conductor, requires the use of metals with low resistance to reduce the electric loss. Accordingly, the use of metals that are relatively inexpensive, as well as being of high electric conductivity, like Ag or Cu is indispensable.
Therefore, it is very important research project to find a dielectric material which can be sintered at lower temperature than the melting point of Ag (960° C.) or Cu (1083° C.).
Generally, low-temperature cofired dielectrics can be made by mixing high-temperature sintered material with a small amount of a low melting point material, for example, glass powder or an additive such as CuO, PbO and Bi
2
O
3
, V
2
O
5
, etc., or by firing glass ceramics comprising ceramics as a filler.
In the case of the latter, the resulting dielectric substrates based on glass show a dielectric constant of 10 or less.
With advantages of low dielectric constants in speeding-up of signal processing and improvement of signal transmission, materials with a low dielectric constant of 10 or less are extensively used for low temperature cofired ceramics (LTCC).
In some cases, meanwhile, substrates made of dielectrics with low dielectric loss and medium dielectric constant (15-100) may be advantageous in terms of circuit design and function without retardation of signal processing, depending on characteristics of applied it circuits.
In addition, the use of dielectrics with high dielectric constants makes the guided wavelength short, leading to a reduction in circuit dimension. Thus, such dielectrics are very useful in applications which give importance to dimensions of electronic devices, as well as having the advantage of reducing insertion loss or frequency deviation in some circuits.
Larger dielectric constants can lower the ratio between the width of transmission lines and the thickness of dielectrics to greater extents, giving circuit designers an opportunity to design better lamination structures.
BaO—Re
2
O—TiO
2
(Re=rare earth element) dielectric composition with a dielectric constant of 70 or higher can be made low in sintering temperature by the addition of B
2
O
3
—SiO
2
based glass frit as disclosed in Korean Pat. Laid-Open Publication (No.1999-62997). However, this dielectric composition can be fired at temperatures of 1,000° C. or higher which are too high to co-fire Ag electrodes.
SUMMARY OF THE INVENTION
Leading to the present invention, the intensive and thorough research into LTCC, conducted by the present inventors, resulted in the finding that in cooperation with glass frit, CuO can serve as a sintering aid to increase the dielectric constant, and play a role in controlling the temperature coefficient of frequency without a large change in Q value.
Therefore, it is an object of the present invention to overcome the above problems encountered in prior arts and provide a dielectric ceramic composition which exhibits high dielectric constant and low dielectric loss and can be sintered at low temperature.
In accordance with an aspect of the present invention, there is provided a dielectric ceramic composition represented by the following chemical formula 1:
Chemical Formula 1
a wt. % {x BaO-y
1
Nd
2
O
3
-y
2
Sm
2
O
3
-w Bi
2
O
3
-z TiO
2
}+
b wt. % (ZnO—B
2
O
3
—SiO
2
—PbO based glass frit)+
 c wt. % CuO
wherein, 10.0 mol %≦x≦20.0 mol %; 7.0 mol %≦y
1
+y
2
≦20.0 mol %; 0.5 mol %≦w≦5.0 mol %; 60.0 mol %≦z≦80.0 mol %, with the proviso that x+y
1
+y
2
+w+z=100; 80.0 wt. %≦a≦98.0 wt. %; 1.0 wt. %≦b≦10.0 wt. %; 1.0 wt. %≦c≦10.0 wt. %.
DETAILED DESCRIPTION OF THE INVENTION
Based on BaO—Nd
2
O
3
—Sm
2
O
3
—Bi
2
O
3
—TiO
2
, the dielectric ceramic composition of the present invention comprises ZnO—B
2
O
3
—SiO
2
—PbO glass frit and CuO. The composition composed of BaO—Nd
2
O
3
—Sm
2
O
3
—Bi
2
O
3
—TiO
2
alone exhibits a sintering temperature of 1,350° C. or higher, which is too high to co-fire silver (Ag) electrodes, which melt at 961° C. In the present invention, glass frit and CuO are adopted to induce the liquid-phase sintering of the base composition, whereby Ag electrodes can be co-fired.
The driving force for the densification of the liquid-phase sintering that enables the low-temperature sintering of materials with high sintering temperature, is driven by the liquid phase's capillary pressure which is exerted among fine particles of a solid phase. For liquid-phase sintering, the following requirements are required.
First, the base dielectric composition is required to include enough amount of a liquid phase to completely cover primary particles thereof, and have some solubility. Additionally, good wettability of the base dielectric composition in the liquid phase is required. Above all, liquid-phase sintering requires the formation of a liquid phase. In this regard, the additives must react with the base dielectric composition to form a liquid phase. Further, the glass frit must have a suitable softening temperature (Ts). Another requirement is that the liquid phase formed has low viscosity as to flow over all particles, thereby uniformly wetting the base composition.
In addition, smaller primary particles incur larger capillary pressures and thus, show larger driving forces for the compaction. Also, the distribution of the liquid phase among the primary particles is an important factor affecting the densification.
Accordingly, the base dielectric composition is mixed with the glass frit and CuO and the mixture is thermally treated at a temperature somewhat higher than the Ts of the glass frit to form a liquid phase which is uniformly distributed over the base dielectric composition, followed by sintering, in accordance with the present invention. As a result, excellent densification can be obtained in the dielectric composition of the present invention. Therefore, it is important to define the molar ratio among the constituent compounds of the base dielectric composition, as well as the composition and amounts of the additives capable of forming a liquid phase by reaction with the base composition, that is, the amount of CuO and the composition and amount of the glass frit.
Useful in a base composition in the present invention is the composition comprising BaO in an amount of 10.0-20.0 mol %, Nd
2
O
3
and Sm
2
O
3
in an amount of 7.0-20.0 mol %, Bi
2
O
3
in an amount of 0.5-5.0 mol %, and TiO
2
in an amount of 60.0-80.0 mol % with the proviso that the total mol % of individual components is 100.
With the content of any component being out of the range therefor, the base dielectric composition exhibits too low a dielectric constant or too high a temperature coefficient of resonant frequency to use in practice. In detail, when BaO is used in an amount less than 10.0 mol %, TiO

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