Multilayer ceramic capacitor and manufacturing method thereof

[0002] 1. Field of the Invention

[0003] The present invention relates to methods of manufacturing multilayer ceramic capacitors, and more particularly relates to a multilayer ceramic capacitor formed with a CaTiO₃/CaZrO3 ceramic capable of being fired in a neutral or reducing atmosphere.

[0004] 2. Description of the Related Art

[0005] Internal electrodes of ceramic electronic components, such as a multilayer ceramic capacitor, have been formed of base metals, such as Ni and Cu, to reduce the material cost. Since base metals are readily oxidized, ceramics used for multilayer ceramic capacitors including such base metallic internal electrodes need to be fired in a neutral or reducing atmosphere.

[0006] Japanese Unexamined Patent Application Publication No.63-126117 discloses a dielectric ceramic composition for temperature compensation capable of being fired in a neutral or reducing atmosphere. The major component of the composition is expressed by the formula (Ca_1-xSr_x)_m(Zr_1-yTi_y)O₃, wherein variables x, y and m satisfy 0≦×≦0.6, 0≦y≦0.6, and 0.85≦m≦1.30, respectively. In addition, relative to 100 parts by weight of the major component, 0.5 to 8 parts by weight of MnO2 and 0.5 to 8 parts by weight of glass components containing TiO₂, SiO₂and the like are present.

[0007] According to this publication, the temperature characteristics of multilayer ceramic capacitors can be extensively controlled by controlling the Ti/Zr ratio of the dielectric ceramic composition. The temperature characteristic of the multilayer ceramic capacitor means the rate of change in capacitance with temperature and generally refers to a temperature coefficient represented by ppm/° C. The temperature coefficient is defined by the following equation:

[0008] Temperature coefficient=(C_i−C₀)×10⁶/(T_i=T₀)

[0009] wherein C_iand C₀are capacitances at temperatures t_iand T₀, respectively.

[0010] The temperature characteristic of a temperature-compensating ceramic capacitor is expressed by a combination of two alphabetical characters, one representing the temperature coefficient and the other representing an allowable error of the temperature coefficient. The dielectric ceramic composition according to the foregoing publication achieves a multilayer ceramic capacitor having a desired temperature characteristic in the range of CG to SL.

[0011] CG represents a temperature coefficient of 0 ppm/° C. and a temperature-coefficient allowable error of ±30 ppm/° C., and SL represents a temperature coefficient in the range of +350 to −1000 ppm/° C., as shown in Tables 1 and 2.

[0012] Also, the publication discloses that increasing the Ti content improves the temperature characteristic of the ceramic capacitor.

[0013] However, multilayer ceramic capacitors formed of CaTiO₃/CaZrO₃are likely to cause a short circuit and a degradation of the insulation resistance (IR) at elevated temperature, as shown in a high-temperature loading test.

[0014] Accordingly, it is an object of the present invention to provide a reliable multilayer CaTiO₃/CaZrO₃ceramic capacitor and a method of manufacturing the same. This multilayer ceramic capacitor rarely exhibits a short circuit or degradation of the insulation resistance in a high-temperature loading test, solving the problem described above.

[0015] To this end, according to one aspect of the present invention, there is provided a multilayer ceramic capacitor comprising a sintered ceramic compact comprising primary crystalline phases mainly containing CaTiO₃and CaZrO₃and secondary phases mainly containing Si and Ca. The amount of Ca in the secondary phase is about 30 mol % or less. The multilayer ceramic capacitor also comprises internal electrodes lying in the sintered ceramic compact.

[0016] The internal electrodes may comprise a base metal to reduce the material cost of the multilayer ceramic capacitor.

[0017] Another aspect of the present invention is directed to a method of manufacturing the multilayer ceramic capacitor. The method comprises preparing a green ceramic laminate comprising main components including CaTiO₃and CaZrO₃, sub-components including MnO₂and SiO₂, and a plurality of internal electrodes. The method also comprises a step of firing the green ceramic laminate at a heating rate of at least about 5° C./min up to a maximum temperature. Thus, a multilayer ceramic capacitor having high reliability in a high-temperature loading test can be achieved.

[0018] FIG. 1 is a schematic sectional view of a multilayer ceramic capacitor according to an embodiment of the present invention; and

[0019] FIG. 2 is a fragmentary sectional view showing primary and secondary phases in the multilayer ceramic capacitor shown in FIG. 1.

[0020] Typical embodiments of the present invention will now be described.

[0021] A sintered ceramic compact of a multilayer ceramic capacitor according to the present invention has primary crystalline phases mainly containing CaTiO₃and CaZrO₃and secondary phases mainly containing Ca and Si. Such a sintered ceramic compact having these primary and the secondary phases is, for example as disclosed in Japanese Unexamined Patent Application Publication No. 63-126117, prepared by firing a composition containing about 0.5 to 8 parts by weight of MnO₂and about 0.5 to 8 parts by weight of glass components including TiO₂and SiO2, relative to 100 parts by weight of main components expressed by the formula (Ca_1-xSr_x)_m(Zr_1-yTi_y)O₃The variables x, y and m in the formula satisfy 0≦×≦0.6, 0 ≦y≦0.6, and 0.85≦m≦1.30, respectively, and either x or y is not 0.

[0022] The glass components include about 30 to 70 mol % (total) TiO₂and SiO₂Further, the glass components include Li₂O, RO and Al₂O₃, wherein R is at least one element selected from Ba, Sr, Ca and Mg.

[0023] In the sintered ceramic compact of the present invention, the secondary phases contain about 30 mol % or less of Ca, thereby improving the reliability in a high-temperature loading test. A concrete example of the present invention will now be described.

[0024] A multilayer ceramic capacitor having a length of 2.0 mm, a width of 1.2 mm and a depth of 1.2 mm, was prepared according to the following procedure.

[0025] First, calcined CaZrO₃and CaTiO₃, MnO₃powder and fine powder containing SiO₂were mixed, and an organic binder and a solvent were added to the mixture to prepare a ceramic slurry. The ceramic slurry contained 1.0 weight percent of a Si—Li sintering accelerator, 1.4 weight percent of MnO₂, 10 weight percent of binder and 1.4 weight percent of a plasticizer, relative to 100 weight percent of the calcined CaZrO₃and CaTiO₃.

[0026] The ceramic slurry was formed into green ceramic sheets having a thickness capable of resulting in a thickness of 5.0 μm after firing. Some of the green ceramic sheets were printed with Ni paste to form an electrode pattern. The pattern-printed green ceramic sheets were laminated, and then other plain green ceramic sheets were laid on the top and bottom surfaces of the laminated green sheets to form a mother laminate. The mother laminate was pressed in the thickness direction, and then was cut into a green ceramic laminate for forming multilayer ceramic capacitors. Each green ceramic laminate had 10 ceramic layers lying between internal electrodes, and hence had 11 internal electrodes.

[0027] The green ceramic laminate was fired at a heating rate described below to obtain a sintered ceramic compact. In this instance, the maximum firing temperature was 1300° C.

[0028] The heating rate was selected from the following five:

[0029] (a) 1.0° C./min

[0030] (b) 3.0° C./min

[0031] (c) 5.0° C./min

[0032] (d) 10.0° C./min

[0033] (e) 20.0° C./min

[0034] After barrel polishing was applied, both ends of each sintered ceramic compact were coated with a Cu paste followed by baking to form external electrodes. One external electrode was provided with a Ni-plated layer and the other external electrode was provided with a Sn-plated layer. Thus, a multilayer ceramic capacitor 1 shown in FIG. 1 was completed.

[0035] The multilayer ceramic capacitor 1 comprises a sintered ceramic compact 2 and internal electrodes 3 to 6 lying in the sintered ceramic compact 2. Two sides 2aand 2b, opposing each other, of the sintered ceramic compact 2 have external electrodes 7 and 8, respectively.

[0036] The multilayer ceramic capacitor formed according to the steps described above was subjected to a high-temperature loading test and analyzed for the composition.

[0037] In the high-temperature loading test, the multilayer ceramic capacitor was subjected to a voltage of 100 V at 150° C. for 200 hours and was examined for evidence of a short circuit and degradation of the insulation resistance.

[0038] In the composition analysis, the secondary phase of the multilayer ceramic capacitor was qualitatively analyzed with a scanning electron microscope (SEM) and an energy dispersion X-ray spectrometer (EDX). Results are shown in Table 3.

[0039] Table 3 shows that each multilayer ceramic capacitor formed at a heating rate of 1 or 3° C./min, as in the aforementioned Publication No. 63-126117, exhibited a 5 short circuit and/or degradation of the insulation resistance in the high-temperature loading test. In contrast, each multilayer ceramic capacitor formed at a heating rate of 5 to 20° C./min did not exhibit these defects. Thus, a heating rate of at least about 5° C./min leads to a reliable multilayer ceramic capacitor which rarely exhibiting the defects described above in the high-temperature loading test.

[0040] The results of the composition analysis of the secondary phases show that firing at heating rates of 5 to 20° C./min led to a low Ca content of about 30 mol % or less in the secondary phases. In other words, a multilayer ceramic capacitor having the secondary phases containing about 30 mol % or less of Ca does not exhibit the defects described above.

[0041] FIG. 2 is a fragmentary sectional view showing the state of the secondary phases of the multilayer ceramic capacitor formed at the heating rate of 5° C./min. Each ceramic layer between the internal electrodes 3, 4 and 5 substantially consists of the primary crystalline phase A; however, secondary phases B appear separate F;: from the primary crystalline phase A. The Ca content of these secondary phases B are limited to about 30 mol % in the present invention.

[0042] Table 3 suggests that a heating rate of about 5° C./min or more leads to secondary phases containing about 30 mol % or less of Ca.

[0043] By limiting the Ca contents in secondary phases to about 30 mol %, a multilayer ceramic capacitor capable of preventing short circuits and degradation of the insulation resistance in a high-temperature loading test, the resulting capacitor has primary crystalline phases mainly containing CaTiO₃and CaZrO₃and secondary phases mainly containing Ca and Si.

[0044] Ni was used for internal electrodes in the example described above, but another base metal, such as Cu, may be used to decrease the material cost of the multilayer ceramic capacitor as well. Also, noble metals, such as Ag, Pd, and Ag-Pd, may be used for the internal electrodes in the present invention. Even using such a noble metal in the present invention, short circuit and degradation of the insulation resistance in a high-temperature loading test can be substantially prevented.

[0045] A green ceramic laminate including base metallic internal electrodes needs to be fired in a neutral atmosphere or reducing atmosphere, and this is preferable in the sintered ceramic compact according to the present invention.

[0046] The multilayer ceramic capacitor of the present invention has a primary crystalline phases mainly containing CaTiO₃and CaZrO₃and secondary phases mainly containing Ca and Si. The Ca contents of the secondary phases are limited to about 30 mol %, thus substantially preventing short circuits and degradation of the insulation resistance in the high-temperature loading test. This has been concluded according to the experimental results.

[0047] Accordingly, even when the CaTiO₃/CaZrO₃sintered ceramic compact, which can be fired in a neutral or reducing atmosphere, has a high Ti content to thereby obtain a desired temperature characteristic, it can result in a reliable multilayer ceramic capacitor which rarely exhibits short circuits or degradation of the insulation resistance in the high-temperature loading test. Thus, various multilayer ceramic capacitors having temperature characteristics according to application can be stably supplied.

[0048] In particular, when a base metal is used for the internal electrodes, the green ceramic laminate including the internal electrodes needs to be fired in a neutral or reducing atmosphere and, therefore, the present invention is preferably applied.

[0049] In the method of manufacturing a multilayer ceramic capacitor of the present invention, the green CaTiO₃/CaZrO₃ceramic laminate is fired at a heating rate of about 5° C./min or more to form the multilayer ceramic capacitor, and thus the Ca content of the secondary phases are limited to about 30 mol %, at most. The multilayer ceramic capacitor according to the present invention, therefore, can have high reliability in the high-temperature loading test.