Dielectric ceramic composition and multilayer ceramic part using the same

The priority Japanese Patent Application Number 2004-76052 upon which this patent application is based is hereby incorporated by reference.

1. Field of the Invention

The present invention relates to a dielectric ceramic composition for use in multilayer ceramic parts and the like, and also to multilayer ceramic parts using the composition.

2. Description of Related Art

With the recent reduction in size and thickness of electronic parts, there is an increasing need for multilayer ceramic parts. A typical multilayer ceramic part includes, in each layer thereof, an inductor or capacitor circuit formed by using a low temperature co-fired ceramic (LTCC) which is co-firable with a conductive material such as Ag. Generally, a dielectric ceramic composition containing alumina or other ceramic filler and a glass is used as the low temperature co-fired ceramic for use in multilayer ceramic parts. However, such a composition has a low dielectric constant of 10 or below and, when applied to an LC filer, shows insufficient dielectric characteristics.

In order for a dielectric ceramic composition to be applicable to an LC filter, it must exhibit a high dielectric constant, a low dielectric loss and a temperature coefficient τf of approximately 0. As a composition which meets such characteristics, a dielectric ceramic composition having a composition of Li 2 O—CaO—Sm 2 O 3 —TiO 2 is disclosed in Japanese Patent Laying-Open No. Hei 5-211007.

Also, Japanese Patent Laying-Open No. 2003-146742 discloses a dielectric ceramic composition containing xCaO—y 1 Sm 2 O 3 -y 2 Nd 2 O 3 -wLi 2 O 3 -zTiO 2 and 3–15% by weight of a ZnO—B 2 O 3 —SiO 2 based glass frit or an Li 2 O—B 2 O 3 —SiO 2 based glass frit.

However, the dielectric ceramic composition disclosed in Japanese Patent Laying-Open No. Hei 5-211007 is fired at a high temperature of about 1,300° C. and its original composition has made it difficult to be applied to multilayer ceramic parts which require firing at a low temperature of about 900° C.–about 1,000° C.

The dielectric ceramic composition disclosed in Japanese Patent Laying-Open No. 2003-146742 needs to increase its glass loading in order to improve sinterability at a low temperature of about 900° C.–about 1,000° C. The higher glass loading results in deterioration of dielectric characteristics, which has been a problem.

It is an object of the present invention to provide a dielectric ceramic composition which can be fired at a low temperature of about 900° C.–about 1,000° C. without marked deterioration of dielectric characteristics, as well as providing multilayer ceramic parts using the dielectric ceramic composition.

The dielectric ceramic composition of the present invention is characterized as containing a dielectric material which contains a dielectric composition represented by the compositional formula a.Li 2 O-b.(CaO 1−x —SrO x )-c.R 2 O 3 -d.TiO 2 (wherein x satisfies 0≦x<1; R is at least one selected from La, Y and other rare-earth metals; and a, b, c and d satisfy 0≦a≦20 mol %, 0≦b≦45 mol %, 0<c≦20 mol % and 40≦d≦80 mol %) and at least one of oxides of Group 4 and Group 14 metallic elements of the Periodic Table.

The dielectric material of the present invention contains the dielectric composition represented by the above-specified compositional formula and at least one of oxides of Group 4 and Group 14 metallic elements of the Periodic Table. Addition of such metallic oxide to the dielectric composition improves sinterability even at a low temperature of about 1,000° C. and enables firing without marked deterioration of dielectric characteristics.

An example of the dielectric composition represented by the above-specified compositional formula can be found in Japanese Patent Laying-Open No. Hei 5-211007. Therefore, addition of an oxide of any metallic element from the Periodic Group 4 and 14 to the dielectric composition disclosed in Japanese Patent Laying-Open No. Hei 5-211007 as having the above-specified compositional formula results in a composition useful as the dielectric material of the present invention.

In the dielectric material of the present invention, the metallic element may substitute for a Ti site in the dielectric composition. The metallic oxide content e of the dielectric material is preferably within the range 0<e≦80 mol %, more preferably within the range 10≦e≦50 mol %, further preferably within the range 10≦e≦30 mol %. If the metallic oxide content is excessively low, the effect of the present invention that renders the composition firable at a low temperature may not be obtained sufficiently. On the other hand, if the metallic oxide content is excessively high, dielectric characteristics may deteriorate. In the case where, other than the dielectric composition, the dielectric material of the present invention contains the metallic oxide alone, a, b, c, d and e are selected such that they make a total of 100 mol %.

As described above, the metallic element belongs to Group 4 and Group 14 elements of the Periodic Table. Examples of Group 4 metallic elements are Zr and Hf. Examples of Group 14 metallic elements are Si, Ge, Sn and Pb. Among them, Si, Ge and Sn are preferably used. Accordingly, examples of preferred metallic oxides in the present invention are SiO 2 , GeO 2 , SnO 2 , ZrO 2 and HfO 2 .

Preferably, the dielectric material of the present invention further contains Bi 2 O 3 . Further inclusion of Bi 2 O 3 in the dielectric material improves sinterability at a low temperature and ameliorates dielectric characteristics. The Bi 2 O 3 content is preferably up to 30 mol %, more preferably 2–10 mol %. In the case where the dielectric material of the present invention further contains Bi 2 O 3 , the values of a, b, c, d and (Bi 2 O 3 content) are suitably selected such that they make a total of 100 mol %. If the Bi 2 O 3 content is excessively low, the effect of adding Bi 2 O 3 , i.e., the improvement of sinterability and dielectric characteristics may not be obtained sufficiently. If the Bi 2 O 3 content is excessively high, dielectric characteristics may deteriorate. Bi may substitute for an R site in the dielectric material.

Besides the aforementioned dielectric material, the dielectric ceramic composition of the present invention may further contain a glass component. Inclusion of the glass component further improves sinterability at a low temperature.

Examples of the glass component for use in the present invention include bismuth based glass comprised mainly of Bi 2 O 3 and B 2 O 3 , borosilicate based glass comprised mainly of B 2 O 3 and SiO 2 , and zinc borosilicate based glass comprised mainly of ZnO, B 2 O 3 and SiO 2 .

The dielectric ceramic composition preferably contains the glass component in the range of 0–10% by weight, more preferably in the range 1–7% by weight, based on a total weight of the composition. If the content of the glass component is excessively low, the effect of adding the glass component, i.e., a sinterability improvement may not be obtained sufficiently. For example, there encounters an occasion where firing can not be accomplished at 900° C. On the other hand, if the content of the glass component is excessively high, dielectric characteristics may deteriorate.

The multilayer ceramic part of the present invention is characterized in that it is obtained by laminating a dielectric layer formed from a slurry containing the dielectric ceramic composition of the present invention with a conductive layer.

In accordance with the present invention, a dielectric ceramic composition can be provided which is firable at a low temperature of about 900° C.– about 1,000° C. without marked deterioration of dielectric characteristics. Therefore, in accordance with the present invention, a low temperature co-fired ceramic can be provided which is co-firable with a conductive material such as Ag.

FIG. 1 is a perspective view which shows an embodiment of a multilayer ceramic part in accordance with the present invention; and

FIG. 2 is an exploded perspective view which shows an embodiment of a multilayer ceramic part in accordance with the present invention.

The present invention is now described in detail with reference to examples. The following examples illustrate the practice of the present invention but are not intended to be limiting thereof. Suitable changes and modifications can be effected without departing from the scope of the present invention.

Li 2 CO 3 , CaCO 3 , SrCO 3 , Sm 2 O 3 and TiO 2 , and Bi 2 O 3 , SiO 2 , GeO 2 , SnO 2 , ZrO 2 and HfO 2 were weighed following the compositional ratio specified in Table 1, in terms of oxide, and then mixed. After addition of isopropanol, the mixture was wet mixed for 5–24 hours using a ball mill consisting of a zirconia pot and a ball, and thereafter calcined at 700–1,200° C. for 1–5 hours to obtain a calcined product. This calcined product was pulverized using the ball mill for 24 hours to obtain a dielectric material.

The obtained dielectric material was granulated with the addition of a binder such as polyvinyl alcohol, classified and then pressed under a pressure of 2,000 kg/cm 2 into a product having a predetermined size and shape. This product was heated at 500° C. for 2 hours for a debindering treatment and then fired at 1,000° C. for 5 hours to obtain a sample.

Sample Nos. 1–12 are dielectric materials in accordance with the present invention. Sample No. 13 is a comparative dielectric material.

Each sample (Sample Nos. 1–13) was subjected to measurement of dielectric constant and Qf value by a dielectric resonator method (Hakki-Coleman method).

As can be clearly seen from the results shown in Table 2, the comparative sample No. 13 shows a high shrinkage and good electric characteristics when it is fired at 1,200° C. However, firing at 1,000° C. lowers its shrinkage and deteriorates its dielectric characteristics. In contrast, each of the sample Nos. 1–12 in accordance with the present invention shows a high shrinkage and good dielectric characteristics even when it is fired at a low temperature of 1,000° C. This demonstrates that addition of an oxide of any metallic element from the Periodic Group 4 and 14 in the dielectric material, in accordance with the present invention, improves sinterability and results in obtaining good dielectric characteristics.

From comparison between the Bi 2 O 3 -containing samples (e.g., sample Nos. 4–8) and the Bi 2 O 3 -excluding sample Nos. 1–3, it is clear that the Bi 2 O 3 -containing samples, when fired, exhibit higher shrinkage and superior dielectric characteristics. This therefore demonstrates that inclusion of Bi 2 O 3 in the dielectric material improves sinterability and results in obtaining better dielectric characteristics.

Subsequently, a glass component was added to the dielectric material sample No. 5 to prepare a dielectric ceramic composition and its sinterability and dielectric characteristics were evaluated. As the glass component, the following substances (G1) and (G2) were used:

(G1) Bi 2 O 3 (55% by weight)—B 2 O 3 (35% by weight)—ZnO (10% by weight)

(G2) Bi 2 O 3 (75% by weight)—B 2 O 3 (15% by weight)—ZnO (10% by weight).

The glass component was added when the calcined product was pulverized, so that they were pulverized and mixed together. For a comparative purpose, the glass component was also added to the comparative sample No. 13 to prepare a dielectric porcelain for evaluation. Other samples without the glass component were also evaluated in a similar manner. The firing temperature was set at 0.900°. The evaluation results are shown in Table 3.

As can be seen from the results shown in Table 3, without addition of the glass component, the samples resulted in poor sinterability and failure of firing at 900° C. Addition of the glass component enabled firing at 900° C. As can also be seen, the dielectric ceramic compositions containing the sample No. 5 and the glass component, in accordance with the present invention, exhibit higher shrinkage and improved sinterability when fired at a low temperature of 900° C., compared to those containing the comparative sample No. 13 and the glass component. They are also found to exhibit superior dielectric characteristics.

FIG. 1 is a perspective view showing one embodiment of a multilayer ceramic part of the present invention and FIG. 2 is an exploded perspective view thereof. As shown in FIGS. 1 and 2 , a conductive layer 2 is formed on a dielectric layer 1 . The conductive layer 2 constitutes a circuit such as an inductor or a capacitor. The dielectric layer 1 may or may not have a via hole 3 . Such dielectric layers 1 are stacked in a pile to constitute the multilayer ceramic part.

As described earlier, the multilayer ceramic part of the present invention can be obtained by firing multilayers of dielectric green sheets each consisting of a dielectric layer comprising the dielectric ceramic composition of the present invention and a conductive layer formed on a surface of the dielectric layer. For example, a dielectric material is first obtained in the same manner as in the preceding Example. A glass component and other additives, if necessary, are added to the dielectric material, followed by mixing in a ball mill. After addition of a polyvinyl butyral (PVB) based binder, the resultant is mixed in a ball mill to prepare a slurry. The slurry is then formed into a 50–100 μm thick sheet using a doctor blade equipment. The obtained sheet is cut into a desired size. An Ag paste is printed thereon in a desired pattern to provide a dielectric green sheet. 8–20 layers of such green sheets, as shown in FIGS. 1 and 2 , are stacked in a pile, bonded compressively, debindered at 400° C. and then fired at 900° C.–1,000° C. for 2 hours, resulting in a multilayer ceramic part.