Electricity: electrical systems and devices – Electrostatic capacitors – Fixed capacitor
Reexamination Certificate
2002-10-07
2003-08-12
Dinkins, Anthony (Department: 2831)
Electricity: electrical systems and devices
Electrostatic capacitors
Fixed capacitor
C361S321500, C501S138000
Reexamination Certificate
active
06606238
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electroconductive paste, a method of producing a monolithic ceramic electronic part in which the conductive paste is used to form inner conductor films, and a monolithic ceramic electronic part made from the conductive paste and in particular, to an improvement in that structural defects can be suppressed even if layers contained in a monolithic ceramic electronic part are thinned and the number of the layers is increased.
2. Description of the Related Art
Monolithic ceramic capacitors as examples of monolithic ceramic electronic parts are generally produced by the following method.
First, ceramic green sheets containing a dielectric ceramic raw material and having inner conductor films formed in required patterns on a surface of the sheets by use of a conductive paste containing a conductive component are prepared. For example, a material containing BaTiO
3
as a major component is employed as the dielectric ceramic raw material.
A plurality of ceramic green sheets including the above-described ceramic green sheets having the inner conductor films formed thereon are laminated and hot-press bonded. Thus, an integrated green laminate is produced.
Subsequently, the green laminate is fired. Thereby, a sintered laminate is obtained. The laminate has a lamination structure containing a plurality of the ceramic layers which are made form the above-described ceramic green sheets. The above-described inner conductor films are arranged via the ceramic layers inside the laminate so that an electrostatic capacitance can be generated.
Then, outer electrodes are formed on the surface of the laminate to be electrically connected to specific ones of the inner conductor films for use of the static capacitance.
Thus, a monolithic ceramic capacitor is produced.
In recent years, the ceramic layers of such monolithic ceramic capacitors have been further thinned and the number of the layers has been increased for the purpose of reducing the size and increasing the capacitance.
To thin the ceramic layers and increase the number of the ceramic layers, it is important to sufficiently harmonize the shrinkage behaviors during firing of the ceramic layers and the inner conductor films with each other.
Ordinarily, the shrink-starting temperatures of conductive metallic powders contained in inner conductor films are considerably lower than those of the ceramic layers. In the case in which there are differences between the shrink behaviors of the conductive metallic powders and the ceramic layers, relatively large stresses are generated inside the monolithic ceramic capacitors, so that the thermal impact resistances are reduced, and seriously, cracks and peeling may be caused between the ceramic layers and the inner conductor films.
To solve the above-described problems, for example, Japanese Unexamined Patent Application Publication No. 6-290985 has proposed a method of causing the shrink behavior of the inner conductor films to approach that of the ceramic layers as much as possible. According to that method, different types of ceramic raw material powders of which the compositions are the same as or different from those of ceramic raw materials contained in the ceramic layers are added to conductive pastes for forming the inner conductor films.
According to the above-described Japanese Unexamined Patent Application Publication No. 6-290985, oxides of Zr, rare earth elements and the like are added to a conductive paste to suppress the conductive metallic powder contained in the conductive paste from sintering, so that the shrink behavior of the inner conductor films can approach that of the ceramic layers. Thereby, successfully, cracking and peeling between the ceramic layers and the inner conductor films are suppressed.
In addition to Japanese Unexamined Patent Application Publication No. 6-290985, for example, Japanese Examined Patent Application Publication No. 5-63929, Japanese Unexamined Patent Application Publication Nos. 2001-15375, 2000-269073, and 6-969998 and so forth, describe that ceramic raw material powders are added to conductive pastes for forming inner conductor films. In these Patent Specifications, it is described as advantages of the inventions that the dielectric constants are increased, and the coverage of inner conductor films is enhanced in addition to the prevention of structural defects in monolithic ceramic capacitors.
With the recent advancement of electronics, the size of small electronic parts has been remarkably reduced. Also, for monolithic ceramic capacitors, it has been required to further reduce the size and increase the capacitance. For example, monolithic ceramic capacitors having a ceramic layer thickness of about 2 &mgr;m are about to be provided for practical applications.
Regarding the inner conductor films, film-thicknesses of about 1 to 2 &mgr;m are employed in most cases. Accordingly, the thickness of each of the ceramic layers becomes nearly equal to that of each of the inner conductor films. As a result, the problems caused by the difference between the shrink behaviors at firing of the ceramic layers and the inner conductor films become more serious. Thus, structural defects are readily caused in the monolithic ceramic capacitors.
From the standpoint of the principle of material diffusion, it is supposed that some reaction occurs between ceramic raw materials contained in a conductive paste for forming inner conductor films and components present on the ceramic layer side. For example, according to the method described in Japanese Unexamined Patent Application Publication No. 6-290985, structural defects in the monolithic ceramic capacitor can be suppressed. However, a metal oxide, which is not a major component of the ceramic layers, is added to the conductive paste, and therefore, the metal oxide contained in the conductive paste and a component contained in the ceramic layers react with each other. This may change the electrical characteristic of the ceramic layers.
If the above-described reaction occurs uniformly, the problems will not become serious. However, since the reaction occurs unevenly as a practical matter, the electrical characteristics of the ceramic layers are locally changed. This causes the electrical characteristics of the monolithic ceramic capacitor to disperse.
In particular, as described in Japanese Patent No. 2722457, when an oxide of a rare earth element is added to a conductive paste, the part of the ceramic layers in contact with the rare earth element oxide powder becomes semi-conductive. Thus, the thickness of the part of the ceramic layers which practically function as a dielectric is smaller than the apparent thickness of the ceramic layers. As a result, the reliability of the insulation resistance and the other electrical characteristics of the monolithic ceramic capacitor is deteriorated. Therefore, the method in which the rare earth element oxide is added cannot correspond to the thinning of the ceramic layers.
In the case in which a conductive paste having rare earth element oxides added thereto is used, advantages such as increase of the dielectric constant, enhancement of the reliability and so forth, can be obtained by devising the application method, as described in Japanese Examined Patent Application Publication No. 5-63929 and Japanese Unexamined Patent Application Publication No. 2001-15375. However, a major component of the ceramic layers and this major component of the conductive paste, which are metal oxides different from each other, react with each other at random. Therefore, the electrical characteristics of the ceramic layers are dispersed. This causes the electrical characteristics of the monolithic ceramic capacitor as a product to be disperse.
To cope with the above-described dispersion, products may be selected for shipment so as to comply with the standards for the respective characteristics. In this case, the yield in mass-production is low. The defective proportion is high. The high cost is also a problem.
It is
Nakamura Tomoyuki
Sano Harunobu
Shimizu Motohiro
Dickstein , Shapiro, Morin & Oshinsky, LLP
Dinkins Anthony
Murata Manufacturing Co. Ltd
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