Electricity: electrical systems and devices – Electrostatic capacitors – Fixed capacitor
Reexamination Certificate
2000-10-30
2001-11-20
Dinkins, Anthony (Department: 2831)
Electricity: electrical systems and devices
Electrostatic capacitors
Fixed capacitor
C361S306300, C029S025410
Reexamination Certificate
active
06320738
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electronic components and methods for making the same. In particular, the present invention relates to a monolithic ceramic electronic component having a ceramic element including ceramic layers and internal electrode layers and to a method for making the same.
2. Description of the Related Art
Dielectric ceramic materials having perovskite structures, such as barium titanate, strontium titanate and calcium titanate, have been widely used in capacitors due to the high specific dielectric constants thereof. Trends towards miniaturization of electronic components require more compact capacitors having large electrostatic capacitance.
Since conventional monolithic ceramic capacitors using dielectric ceramic materials as dielectric layers are sintered at temperatures as high as approximately 1,300° C., noble metals such as palladium must be used as internal electrode materials. The use of such expensive noble metals inevitably increases the material cost of the capacitors.
The use of base metals in internal electrodes of monolithic ceramic capacitors is progressing for solving the above problem, and various dielectric materials having reduction resistance and capable of sintering in neutral and reducing atmospheres have been developed to prevent oxidation of electrodes during sintering.
A further reduction in size and a further increase in capacitance are required for monolithic ceramic capacitors, and technologies are being developed for achieving higher dielectric constants of dielectric ceramic materials, thinner dielectric ceramic layers and thinner internal electrode layers.
When the thickness of the ceramic layer disposed between the internal electrode layers is reduced to 3 &mgr;m or less, unevenness of the interface between the dielectric ceramic layer and the internal electrode layer increases or defects or pores in the dielectric ceramic increase, resulting in shorter service lives.
A reduction in particle size of the powdered ceramic material is proposed in order to improve smoothness of green ceramic sheets for forming ceramic layers and to increase the density of the green ceramic sheet (Japanese Unexamined Patent Application Publication No. 10-223469). As the particle size decreases, the powdered ceramic readily agglomerates, resulting in poor dispersibility. Thus, the improvement in the surface smoothness and the density of the green ceramic sheet is not sufficient only by the reduction in particle size. Moreover, the dielectric constant of the powdered ceramic decreases as the particle size decreases in the same composition, and the reduction in particle size is not suitable for monolithic ceramic capacitors having higher capacitance.
As the size of the metal particles used in internal electrodes decreases, the initial sintering temperature of the metal particles decreases, and delamination will readily occur. It is difficult to use such metal particles as electrode materials for monolithic capacitors.
When the content of organic binders in ceramic is increased in order to improve the surface smoothness of a green ceramic sheet, the volume fraction of the powdered ceramic in the green ceramic sheet is decreased and the volumetric shrinkage of the ceramic element (chip) increases during sintering. When the volumetric shrinkage of the ceramic element is large, the area of the electrode paste on the green ceramic sheet also decreases in response to the areal shrinkage of the green ceramic sheet. Since the volume of the electrode material, such as nickel, is constant in the internal electrode, the thickness of the internal electrode layer unintentionally increases contrary to the trends toward thinner multilayers.
In a green ceramic sheet containing a large amount of organic binder and having a large areal shrinkage, the thickness of the electrode paste applied thereon can be reduced in consideration of the areal shrinkage of the green ceramic sheet. The reduction in thickness, however, results in the formation of pinholes in the electrode paste layer and an increase in surface roughness of the electrode due to a decreased leveling of the electrode paste. These defects decrease the electrode coverage (effective electrode area) after sintering, resulting in deterioration of electrical characteristics of the product.
The above-described problems occur also in various monolithic ceramic electronic components other than the monolithic ceramic capacitors.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a monolithic ceramic electronic component and a method for making the same, which ensure a prolonged service life due to smoothness of the interface between the internal electrode and the ceramic layer and reduce the formation of structural defects, such as delamination and curvation of the electrode in the thin multilayer configuration.
According to a first aspect of the present invention, a monolithic ceramic electronic component comprises a ceramic element including a plurality of ceramic layers and a plurality of internal electrode layers, each disposed between two adjacent ceramic layers. In the monolithic ceramic electronic component, the roughness of the interface between each internal electrode layer and each ceramic layer is 200 about nm or less, and the incidence of pores in the ceramic layer is about 1% or less by area at a polished cut cross-section.
Such roughness and incidence contribute to a prolonged service life due to improved smoothness of the interface between the internal electrode and the ceramic layer and reduced structural defects, such as delamination and curvation in the thin multilayer configuration. As a result, the monolithic ceramic electronic component can be miniaturized and exhibits superior durability.
When the roughness Ra exceeds about 200 nm, the service life of the monolithic ceramic electronic component is significantly short. When the incidence of pores exceeds about 1%, the service life of the monolithic ceramic electronic component is also significantly short.
In the present invention, the roughness of the interface represents the center-line-average roughness Ra defined by Japanese Industrial Standard (JIS) B-0601.
Examples of the monolithic ceramic electronic components of the present invention include monolithic ceramic capacitors, monolithic ceramic varistors, monolithic ceramic piezoelectric components and monolithic substrates.
In the monolithic ceramic electronic component of the present invention, the thickness of each ceramic layer disposed between the internal electrode layers is preferably about 3 &mgr;m or less.
Since the roughness of the interface is about 200 nm or less in the present invention, the thickness of the ceramic layer can be reduced to about 3 &mgr;m or less, and the monolithic ceramic electronic component can be miniaturized and exhibit superior durability. In conventional monolithic ceramic electronic components, such thin ceramic layers cause significantly short service lives.
Preferably, the thickness of each internal electrode layer is in a range of about 0.2 to 0.7 &mgr;m.
With respect to the internal electrode layer, a thickness of less than about 0.2&mgr; is insufficient to maintain the function as the internal electrode, since this layer partially reacts with the ceramic layer during sintering and the coverage (effective electrode layer) is decreased. A thickness exceeding about 0.7 &mgr;m causes delamination which precludes the functions of the monolithic ceramic electronic component.
When the thickness of the internal electrode layer is in a range of about 0.2 to 0.7 &mgr;m, the electrode paste layer applied in the production process does not have pinholes and has a smooth surface. Moreover, the total thickness of the monolithic ceramic electronic component can be reduced. As a result, the monolithic ceramic electronic component can be miniaturized and exhibit high performance, high reliability and superior durability.
In the monolithic ceramic electronic component of the present inv
Miyazaki Takaharu
Yamana Tsuyoshi
Dinkins Anthony
Murata Manufacturing Co. Ltd.
Ostrolenk Faber Gerb & Soffen, LLP
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