JP3793697B2 - Multilayer ceramic capacitor and manufacturing method thereof - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a multilayer ceramic capacitor in which dielectric layers and conductor layers are alternately laminated, and more particularly to the structure of the dielectric layer.
[0002]
[Prior art]
The multilayer ceramic capacitor is manufactured by the following process. First, a slurry is prepared. Specifically, an additive and a binder are added to a ceramic dielectric powder such as BaTiO ₃ , and this is stirred and mixed for several hours by a ball mill or the like to prepare a slurry having an appropriate viscosity.
[0003]
Next, a ceramic green sheet is prepared from the slurry by, for example, a doctor blade method. In this doctor blade method, a slurry is flowed on a base film, and the thickness is adjusted by a gap with the doctor blade. Thereafter, this is dried to obtain a ceramic green sheet having a predetermined thickness.
[0004]
Next, a conductive paste is applied to the ceramic green sheet in a predetermined pattern. Next, a required number of these ceramic green sheets are laminated and pressure-bonded to form a sheet laminate. Thereafter, the sheet laminate is cut into a size per unit part and fired to prepare a sintered body. By this firing step, the ceramic green sheet is fired to become a ceramic dielectric layer, and the conductive paste is fired to become a conductor layer. Finally, a conductive paste is applied to the sintered body and fired to form an external electrode. A multilayer ceramic capacitor is manufactured through the above steps.
[0005]
[Problems to be solved by the invention]
In such a multilayer ceramic capacitor, there is a problem of preventing the occurrence of delamination and cracks. One of the generation factors such as delamination is a difference in shrinkage behavior between the conductive paste and the ceramic green sheet during firing. In order to solve this problem, a technique in which a ceramic dielectric dielectric used for slurry preparation is mixed into a conductive paste for forming a conductor layer has been generalized. In addition, the material mixed in the conductive paste for such a purpose is called “co-material”.
[0006]
However, as the ceramic dielectric layer becomes thinner with the recent increase in capacity and size of multilayer ceramic capacitors, changes in the physical properties of the ceramic dielectric layer due to the diffusion reaction of the co-materials can no longer be ignored. In addition, when the ceramic dielectric layer is thinned, there is also a problem that current resistance characteristics deteriorate.
[0007]
The present invention has been made in view of the above-described object, and an object of the present invention is to provide a multilayer ceramic capacitor having good withstand voltage characteristics and capable of large capacity and miniaturization, and a method for manufacturing the same. .
[0008]
[Means for Solving the Problems]
To achieve the above object, according to the first aspect of the present invention, in the multilayer ceramic capacitor formed by alternately laminating ceramic dielectric layers and conductor layers, the ceramic dielectric layer includes ceramic particles having a core-shell structure. The ceramic particles are formed such that the closer to the conductor layer, the larger the thickness of the shell portion , and the ceramic particles having no core-shell structure at the interface of the ceramic dielectric layer with the conductor layer or in the vicinity of the conductor layer. We propose what is characterized by being formed .
[0009]
According to the present invention, the ceramic dielectric layer has a core-shell structure in which the ceramic particles in the vicinity of the conductor layer have a shell portion thickness larger than the ceramic particles separated from the conductor layer. improves. Therefore, it is easy to increase the capacity and reduce the size by reducing the thickness of the ceramic dielectric layer. Further , according to the present invention, since the ceramic particles whose sintering is promoted and only the shell portion is excellent in the withstand voltage characteristics, the ceramic particles are present at the interface with the conductor layer or in the vicinity of the conductor layer. The withstand voltage characteristics of the entire layer are improved.
[0010]
Here, the core-shell structure of the ceramic particles refers to a structure in which the core portion that is the center portion of the ceramic particles and the shell portion that is the outer shell portion form physically and chemically different phases.
[0011]
Note that not all ceramic particles in the ceramic dielectric layer have a core-shell structure. Therefore, according to a second aspect of the present invention, in the multilayer ceramic capacitor according to the first aspect, the ceramic dielectric layer has a structure in which the number of ceramic particles having a core-shell structure is less than the number of ceramic particles having the core-shell structure. The present invention is characterized by including the number of ( 9/1 ) or less.
[0012]
According to the present invention, the dielectric constant can be improved together with the withstand voltage characteristics. In addition, in the ceramic particles not having the core-shell structure, there are particles only in the core portion and particles only in the shell portion. The ceramic particles only in the core part contribute to the improvement of the dielectric constant, and the ceramic particles only in the shell part contribute to the improvement of the withstand voltage characteristics. The ceramic particles only in the core part are counted as particles having a core-shell structure, and the ceramic particles only in the shell part are counted as particles having no core-shell structure.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
A multilayer ceramic capacitor according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a partially cutaway perspective view of a multilayer ceramic capacitor, and FIG. 2 is an enlarged sectional view of the multilayer ceramic capacitor. 2 is a cross-sectional view observed with a TEM (Transmission Electron Microscope).
[0019]
As shown in FIG. 1, this multilayer ceramic capacitor 10 includes a substantially rectangular parallelepiped laminate 13 in which ceramic dielectric layers 11 and conductor layers 12 are alternately laminated, and the conductor layers formed at both ends of the laminate 13. 12 and an external electrode 14 connected to the external electrode 14. Here, the conductor layers 12 are alternately connected to the external electrodes 14 at both ends. That is, one external electrode 14 is connected to the conductor layer 12 every other layer, and the other external electrode 14 is connected to the conductor layer 12 not connected to the one external electrode 14.
[0020]
The ceramic dielectric layer 11 is made of, for example, a BaTiO ₃ based ceramic sintered body having ferroelectricity. The conductor layer 12 is made of a noble metal material such as Pd or Ag or a base metal material such as Ni. The laminated body 13 is formed by laminating a plurality of ceramic green sheets on which a conductive paste is printed and sintering them. As a result, the ceramic green sheet is sintered to form the ceramic dielectric layer 11, and the conductive paste is sintered to form the conductor layer 12. The external electrode 14 is made of a metal material such as Ni or Ag.
[0021]
As shown in FIG. 2, the ceramic dielectric layer 11 includes ceramic particles 20 having a core-shell structure and ceramic particles 30 having no core-shell structure. The composition ratio between them is about 9: 1 to 7: 3 for ceramic particles 20 having a core-shell structure and ceramic particles 30 having no core-shell structure, and in this embodiment, about 8: 2. Here, the core-shell structure of the ceramic particles refers to a structure in which the core portion 21 that is the center portion of the ceramic particles and the shell portion 22 that is the outer shell portion form physically and chemically different phases. Further, the ceramic particles 20 having the core-shell structure are formed such that the thickness of the shell portion 22 is larger as it is closer to the conductor layer 12.
[0022]
The ceramic particles 30 having no core-shell structure are roughly classified into ceramic particles 30a having the same composition as the core portion of the ceramic particles having the core-shell structure 30 and ceramic particles 30b having the same composition as the shell portion. Many ceramic particles 30a having the same composition as that of the core portion are present in the central portion of the ceramic dielectric layer 11 in the thickness direction. On the other hand, the ceramic particles 30 b having the same composition as the shell part are present at the interface with the conductor layer 12 or in the vicinity of the conductor layer 12.
[0023]
The ceramic particles 20 having the core-shell structure and the ceramic particles 30 not having the core-shell structure are observed by selecting a large number of particles (for example, 100 particles) at random by TEM. And the number of ceramic particles 30 having no core-shell structure is counted and calculated. The ceramic particles 30a having the same composition as the core portion are counted as ceramic particles 20 having a core-shell structure, and the ceramic particles 30b having the same composition as the shell portion are counted as particles having no core-shell structure.
[0024]
A method of manufacturing this multilayer ceramic capacitor will be described with reference to FIG. FIG. 3 is a process diagram of the multilayer ceramic capacitor.
[0025]
First, a first additive and a binder are mixed with the first ceramic dielectric powder to create a slurry (step S1). Here, as the first ceramic dielectric powder, BaTiO ₃ powder having an average particle size of 0.35 μm was used. As the first additive, rare earth oxides were mainly used. Specifically, a mixed powder of MgO, Ho ₂ O ₃ , Sm ₂ O ₃ and BaSiO ₃ was used as a sintering aid. Next, this slurry is formed into a sheet by a doctor blade method or the like to obtain a ceramic green sheet (step S2).
[0026]
On the other hand, a conductive paste for forming a conductor layer is created (step S3). This conductive paste is prepared by mixing a metal material powder made of a noble metal such as Pd or Ag or a base metal such as Ni, a second ceramic dielectric powder and a second additive, and a binder. Here, the metal material powder is a main material for forming the conductor layer. In this embodiment, Ni is used.
[0027]
The second ceramic dielectric is mixed to match the shrinkage behavior with the ceramic green sheet in the firing process. Accordingly, it is preferable that the shrinkage behavior approximates that of the first ceramic dielectric. In the present embodiment, the first ceramic dielectric powder having the same composition and an average particle size of half or less is used.
[0028]
The second additive promotes the growth of the shell portion in the ceramic particles of the ceramic green sheet. The second additive includes at least one component of the first additive. In the present embodiment, a mixed powder of MgO, Sm ₂ O ₃ and BaSiO ₃ is used as the second additive.
[0029]
Next, a conductive paste is printed in a predetermined pattern on the ceramic green sheet (step S4). And this ceramic green sheet is laminated | stacked and crimped | bonded, and a sheet | seat laminated body is obtained (step S5). Next, this sheet laminate is cut into unit dimensions to obtain a laminated chip (step S6). Next, a conductive paste for external electrodes is applied to the laminated chip (step S7).
[0030]
Next, this multilayer chip is baked under predetermined conditions (step S8). Here, the firing conditions are determined by the composition of the ceramic dielectric or conductor layer. In this embodiment, after firing at 1320 ° C. in a reducing atmosphere, firing was performed by switching to a N ₂ —O ₂ weak oxidizing atmosphere at 1000 ° C.
[0031]
Finally, the external electrode is plated to obtain the multilayer ceramic capacitor 10 (step S9).
[0032]
According to the above steps, the ceramic green sheet is fired to form the ceramic dielectric layer 11 in the firing process in step S8, and the conductive paste printed on the ceramic green sheet is fired to form the conductor layer 12. Form. In this firing process, the second additive contained in the conductive paste diffuses into the ceramic green sheet. And the growth of the shell part of the ceramic particle in a ceramic green sheet is accelerated | stimulated by this 2nd additive. Therefore, in the ceramic dielectric layer 11, the ceramic particles 20 having a thicker shell portion 22 are present in the vicinity of the conductor layer 12. In addition, ceramic particles 30b having no core-shell structure having the same composition as the shell portion exist at the interface of the conductor layer 12 or in the vicinity of the conductor layer 12.
[0033]
As described above, according to the present invention, the ceramic dielectric layer 11 of the multilayer ceramic capacitor 10 has the shell portion 22 in which the ceramic particles 20 in the vicinity of the conductor layer 12 are thicker than the ceramic particles separated from the conductor layer 12. Since it has a large core-shell structure, the withstand voltage characteristics are improved. Thereby, the ceramic dielectric layer 12 can be thinned while maintaining good withstand voltage characteristics. Therefore, it is easy to increase the capacity and reduce the size by reducing the thickness of the ceramic dielectric layer 12. In addition, the presence of the ceramic particles 30b in which the sintering is accelerated and only the shell portion is present in the vicinity of the conductor layer 12 improves the withstand voltage characteristics of the ceramic dielectric layer 12.
[0034]
In the present embodiment, the ceramic dielectric layer 11 and the conductor layer 12 and the external electrode 14 are formed by firing at the same time, but the present invention is not limited to this. That is, after the laminated chip obtained in step S6 is fired to obtain a sintered body, an external electrode may be formed by applying a conductive paste to the sintered body and firing it.
[0035]
【The invention's effect】
As described above in detail, according to the present invention, the ceramic dielectric layer of the multilayer ceramic capacitor has a core-shell structure in which the ceramic particles in the vicinity of the conductor layer have a shell portion thicker than the ceramic particles separated from the conductor layer. Therefore, the withstand voltage characteristic is improved. Thereby, the ceramic dielectric layer can be thinned while maintaining good withstand voltage characteristics. Therefore, it is easy to increase the capacity and reduce the size by reducing the thickness of the ceramic dielectric layer.
[Brief description of the drawings]
1 is a partially cutaway perspective view of a multilayer ceramic capacitor. FIG. 2 is an enlarged cross-sectional view of the multilayer ceramic capacitor. FIG. 3 is a process diagram of the multilayer ceramic capacitor.
DESCRIPTION OF SYMBOLS 10 ... Multilayer ceramic capacitor, 11 ... Ceramic dielectric layer, 12 ... Conductor layer, 13 ... Laminated body, 14 ... External electrode, 20, 30 ... Ceramic particle, 21 ... Core part, 22 ... Shell part

Claims (2)

In a multilayer ceramic capacitor formed by alternately laminating ceramic dielectric layers and conductor layers,
The ceramic dielectric layer includes ceramic particles having a core-shell structure. The closer the ceramic particles are to the conductor layer, the larger the thickness of the shell portion is, and the ceramic dielectric layer is connected to the conductor layer. A multilayer ceramic capacitor characterized in that ceramic particles having no core-shell structure are formed in the vicinity of an interface or a conductor layer . The ceramic dielectric layer includes ceramic particles having a core-shell structure in a number of (7/3) or more and (9/1) or less with respect to the number of ceramic particles having no core-shell structure. Multilayer ceramic capacitor.

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