Monolithic ceramic capacitor - Patent Review 5852542

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a ceramic capacitor used for electronic instruments, and particularly to a monolithic ceramic capacitor having internal electrodes made up of nickel or a nickel alloy.

Hitherto, the production process for a monolithic ceramic capacitor is generally as follows. First, a sheet-form dielectric ceramic layer having coated on its surface an electrode material for forming an internal electrode is prepared. As the dielectric ceramic layer, a material composed of, for example, BaTiO.sub.3 is used as the main component. Then, a laminate is prepared by laminating the sheet-form ceramic dielectric layers coated with the electrode material followed by being pressed and heated at 1250.degree. to 1350.degree. C. Thereby, a ceramic laminate having internal electrodes is obtained. Also, by plating an external electrode electrically connecting to the internal electrodes, a monolithic ceramic capacitor is obtained.

Accordingly, it is necessary that the material for the internal electrodes to meet the following conditions.

1. Because the ceramic laminate and the internal electrodes are simultaneously calcined, the material has a melting point of at least the calcination temperature of the ceramic laminate.

2. The material is not oxidized in an oxidative high-temperature atmosphere and does not react with the dielectric ceramic layer.

As the electrode meeting such conditions, noble metals such as platinum, gold, palladium or a silver-palladium alloy, etc., have been used. These electrode materials have excellent characteristics but on the other hand, because the material is expensive, the ratio of the electrode material cost to the monolithic ceramic capacitor cost reaches from 30 to 70%, which is the largest factor increasing the production cost.

As other materials having a high melting point, there are base metals such as Ni, Fe, Co, W, Mo, etc., but these base metals are easily oxidized in a high-temperature oxidative atmosphere and becomes useless as the electrode. Thus, to use these base metals as the internal electrodes of a monolithic ceramic capacitor, it is necessary that the base metal is calcined in a neutral or reducing atmosphere together with the dielectric ceramic layer. However, conventional dielectric ceramic materials have the fault that when the dielectric ceramic materials are calcined in such a neutral or reducing atmosphere, the material is greatly reduced and converted to semiconductors.

To overcome this fault, a dielectric ceramic material wherein the ratio of barium site/titanium site in the barium titanate solid solution is in excess of the stoichiometric ratio as shown in Examined Published Japanese Patent Application No. Sho 57-42588 and a dielectric ceramic material obtained by adding the oxide of a rare earth element such as La, Nd, Sm, Dy, Y, etc., to a barium titanate solid solution as shown in Unexamined Published Japanese Patent Application No. Sho 61-101459 are proposed.

Also, as dielectric ceramic materials having a lower temperature change of the dielectric constant, for example, the dielectric ceramic materials of the BaTiO.sub.3 --CaZrO.sub.3 --MnO--MgO series composition shown in Unexamined Published Japanese Patent No. 62-256422 and a BaTiO.sub.3 --(Mg, Zn, Sr, Ca)O--B.sub.2 O.sub.3 --SiO.sub.2 series composition shown in Examined Published Japanese Patent Application No. Sho 61-14611 are proposed.

By using such dielectric ceramic materials, a ceramic laminate which is not converted to a semiconductor by calcination in a reducing atmosphere can be obtained and the production of a monolithic ceramic capacitor using a base metal such as nickel, etc., as the internal electrodes becomes possible.

With the recent progress in electronics, small-sizing of electronic parts has proceeded rapidly and the tendency to small size and increase in capacity of the monolithic ceramic capacitor is remarkable. Accordingly, demand for a dielectric ceramic material having a high dielectric constant, showing a lower temperature change of the dielectric constant, and having excellent reliability has increased.

However, in the dielectric ceramic materials shown in Examined Published Japanese Patent Application No. Sho 57-42588 and Unexamined Published Japanese Patent No. Sho 61-101459, a large dielectric constant is obtained but there is the fault that the crystal particles of the ceramic laminate obtained are large and when the thickness of the dielectric ceramic layer in the monolithic ceramic capacitor becomes as thin as 10 .mu.m or thinner, the number of the crystal particles in each layer is decreased and this lowers reliability. Also, there is a problem that the temperature change of the dielectric constant is large and it cannot be said that these materials sufficiently meet the demand of markets.

On the other hand, in the dielectric ceramic material shown in Unexamined Published Japanese Patent No. Sho 62-256422, the dielectric constant is relatively high, the crystal particles of the ceramic laminate obtained are small, and the temperature change of the dielectric constant is small, but because CaZrO.sub.3 or CaTiO.sub.3 formed during the calcination process is liable to form a secondary phase with MnO, there is a problem with reliability at high temperature.

Also, in the dielectric ceramic material shown in Examined Published Japanese Patent Application No. 61-14611, there is a fault that the dielectric constant obtained is from 2,000 to 2,800 and the material is disadvantageous from the view point of small-sizing and increasing the capacity of the monolithic ceramic capacitor. Furthermore, there is a problem that the material does not satisfy the X7R characteristics of the EIA Standard, that is the change ratio of the electrostatic capacity between the temperature range of from -55.degree. to +125.degree. C. is not within .+-.15%.

To solve the above-described problems, compositions are proposed in Unexamined Published Japanese Patent Application Nos. Hei 5-9066, Hei 5-9067, and Hei 5-9068. However, the market demand for reliability thereafter has become more severe and the demand for a dielectric ceramic material which is excellent in reliability has been increased. Also, at the same time, the demand for thinning the ceramic dielectric layer has become severe.

When in the case of thinning the ceramic dielectric layer, the rated voltage is to be the same as the rated voltage before thinning, the electric field applied per layer becomes large and thus the insulation resistance at room temperature and at a high temperature is lowered, whereby the reliability is greatly lowered. Thus, in conventional dielectric ceramic materials, it is required to lower the rated voltage when thinning the ceramic dielectric layer. Therefore, it is desired to provide a monolithic ceramic capacitor which does not need a lowering of the rated voltage by thinning the ceramic dielectric layer, has an insulation resistance under a high electric field strength, and has excellent reliability.

In a small-sized and large capacity monolithic ceramic capacitor, a plated film such as a soft solder, etc., is formed on a baked external electrode of an electrically conductive metal powder to cope with automatic surface packaging.

As the method of forming a plated film, an electrolytic plating method is general used. Usually, fine voids are formed in a baked electrode of an electrically conductive metal powder. Accordingly, there is a problem that when the monolithic ceramic capacitor is immersed in a plating liquid in the case of forming a plating film on the electrodes, the plating liquid permeates the voids of the baked electrodes and reaches the interface between the internal electrode and the dielectric ceramic layer to lower the reliability.

SUMMARY OF THE INVENTION

Thus, the primary object of the present invention is to provide a small-sized and large capacity monolithic ceramic capacitor at a low cost, wherein the dielectric constant is at least 3,000; when the insulation resistance is expressed by the product with the electrostatic capacity (CR product), the insulation resistances at room temperature and at 125.degree. C. are high as at least 2,000 M.OMEGA..multidot..mu.F and at least 500 M.OMEGA..multidot..mu.F respectively; the temperature characteristics of the electrostatic capacity satisfies the B characteristics of the JIS Standard and the X7R characteristics of the EIA Standard; and the reliability is high regardless of the presence or absence of the plated layer.

The present invention has been made in view of the above-described object.

The present invention provides a monolithic ceramic capacitor comprising plural dielectric ceramic layers, plural internal electrodes between the above-described dielectric ceramic layers such that the end of each of the internal electrodes is alternatively exposed to different side surfaces of the above-described dielectric ceramic layer, and external electrodes formed such that they are electrically connected to the exposed internal electrodes, wherein the dielectric ceramic layers are constituted by a material comprising barium titanate having a content of alkali metal oxides as impurities in an amount of not more than about 0.02% by weight, at least one member selected from scandium oxide and yttrium oxide, at least one member selected from samarium oxide and europium oxide, manganese oxide, cobalt oxide, and nickel oxide; containing magnesium oxide as an accessory constituent in an amount of from about 0.5 to 5.0 mols as MgO to 100 mols of a main constituent shown by the following composition formula;

(1-.alpha.-.beta.-.gamma.){BaO}.sub.m.TiO.sub.2 +.alpha.M.sub.2 O.sub.3 +.beta.Re.sub.2 O.sub.3 +.gamma.(Mn.sub.1-x-y Ni.sub.x Co.sub.y)O

(wherein, M.sub.2 O.sub.3 is at least one of Sc.sub.2 O.sub.3 and Y.sub.2 O.sub.3 ; Re.sub.2 O.sub.3 is at least one of Sm.sub.2 O.sub.3 and Eu.sub.2 O.sub.3 and .alpha., .beta., .gamma., m, x, and y are

0.0025.ltoreq..alpha.+.beta.0.025

0.ltoreq..beta..ltoreq.0.0075

0.0025.ltoreq..gamma..ltoreq.0.05

.gamma./(.alpha.+.beta.).ltoreq.4

0.ltoreq.x<1.0

0.ltoreq.y<1.0

0.ltoreq.x+y<1.0

1.000<m.ltoreq.1.035);

and furthermore, containing an Li.sub.2 O--B.sub.2 O.sub.3 --(Si, Ti)O.sub.2 series oxide glass in an amount of from about 0.2 to 3.0 parts by weight to 100 parts by weight of the sum of the above-described main constituent and above-described magnesium oxide, and the above-described internal electrodes are constituted by nickel or a nickel alloy.

Also, in the monolithic ceramic capacitor, it is preferred that in the triangular diagram of {Li.sub.2 O, B.sub.2 O.sub.3, (Si.sub.w Ti.sub.1-w)O.sub.2 } (wherein 0.3.ltoreq.w<1.0), the above-described Li.sub.2 O--B.sub.2 O.sub.3 --(Si, Ti)O.sub.2 series oxide glass is in the inside of the region surrounded by 6 straight lines or on the lines linking 6 points of

A (0, 20, 80)

B (19, 1, 80)

C (49, 1, 50)

D (45, 50, 5)

E (20, 75, 5)

F (0, 80, 20)

and contains by addition at least one of Al.sub.2 O.sub.3 and ZrO.sub.2 in the sum total of not more than about 20 parts by weight (with a proviso that ZrO.sub.2 is not more than about 10 parts by weight) to 100 parts by weight of the above-described constituents.

Also, in the monolithic ceramic capacitor, it is preferred that the above-described external electrode is constituted by a sintered layer of an electrically conductive metal powder or an electrically conductive metal powder mixed with a glass frit.

Furthermore, it is preferred that the above-described external electrode is composed of a first layer made up of sintered layer of an electrically conductive metal powder or an electrically conductive metal powder mixed with a glass frit and a second layer made up of a plated layer on the first layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing an embodiment of the monolithic ceramic capacitor of the present invention,

FIG. 2 is a schematic plane view showing an embodiment of the dielectric ceramic layer having an internal electrode in the present invention,

FIG. 3 is an exploded slant view showing an embodiment of the ceramic laminate of this invention, and

FIG. 4 is a ternary composition diagram of {Li.sub.2 O, B.sub.2 O.sub.3, (Si.sub.w Ti.sub.1-w)O.sub.2 } showing the composition range of the Li.sub.2 O--B.sub.2 O.sub.3 --(Si, Ti)O.sub.2 series oxide glass.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In the monolithic ceramic capacitor of the present invention, by using the dielectric ceramic material having a composition ratio of barium titanate, at least one of scandium oxide and yttrium oxide, at least one of samarium oxide and europium oxide, manganese oxide, cobalt oxide, and nickel oxide adjusted as described above and containing in addition magnesium oxide and the Li.sub.2 O--B.sub.2 O.sub.3 --(Si, Ti)O.sub.2 series oxide glass as the material for the dielectric ceramic layer, the material can be calcined in a reducing atmosphere without deteriorating its characteristics, and the monolithic ceramic capacitor satisfies the B characteristics of the JIS Standard and the X7R characteristics of the EIA Standard, and has a high insulation resistance and a high reliability at room temperature and at a high temperature under a high electric field strength.

Also, because the crystal grain sizes of the dielectric ceramic laminate obtained are small as 1 .mu.m or smaller, the number of the crystal grains existing in each dielectric layer can be increased and thus lowering of the reliability can be prevented when thinning the thickness of the dielectric ceramic layer of the monolithic ceramic capacitor.

In addition, it has been confirmed that in the present invention, of the alkaline earth metal oxides such as SrO, CaO, etc., the alkali metal oxides such as Na.sub.2 O, K.sub.2 O, etc., and other oxides such as Al.sub.2 O.sub.3, SiO.sub.2, etc., existing as impurities in barium titanate in the above-described main constituents, the content of particularly the alkali metal oxides such as Na.sub.2 O, K.sub.2 O, etc., has a large influence on the electric characteristics. That is, it has been confirmed that by using barium titanate wherein the content of the alkali metal oxides existing as impurities are less than about 0.02% by weight, the dielectric constant of 3,000 or higher is obtained.

Also, it has been confirmed that by adding an oxide glass mainly composed of Li.sub.2 O--B.sub.2 O.sub.3 --(Si, Ti)O.sub.2 into the dielectric ceramic layer, the sintering property becomes good and also plating resistance is improved. Furthermore, by adding Al.sub.2 O.sub.3 and ZrO.sub.2 to the oxide glass mainly composed of Li.sub.2 O--B.sub.2 O.sub.3 --(Si, Ti)O.sub.2, it becomes possible to obtain a higher insulation resistance.

When a dielectric ceramic layer is formed using the dielectric ceramic material as described above, a small-sized and large capacity monolithic ceramic capacitor capable of coping with automatic surface packaging, showing a small temperature change of the electrostatic capacity, and having a high reliability can be realized, and also nickel or a nickel alloy can be used as the internal electrodes. Also, a small amount of a ceramic powder can be added together with nickel or a nickel alloy.

Also, there is no particular restriction on the composition of the external electrodes. Practically, the external electrode may be constituted by, for example, a sintered layer of the powder of each of various electrically conductive metals such as Ag, Pd, Ag--Pd, Cu, Cu alloys, etc., or a sintered layer of the above-described electrically conductive metal powder compounded with various glass frits such as the B.sub.2 O.sub.3 --Li.sub.2 O--SiO.sub.2 --BaO series, B.sub.2 O.sub.3 --SiO.sub.2 --BaO series, Li.sub.2 O--SiO.sub.2 --BaO series, B.sub.2 O.sub.3 --SiO.sub.2 --ZnO series, etc. Also, a small amount of a ceramic powder may be added to the electrically conductive metal powder and the glass frit. More preferably, when a plated layer is formed on the sintered layer, the plated layer may be only a plated layer made up of Ni, Cu, an Ni--Cu alloy, etc., and may further have thereon a plated layer of a soft solder, tin, etc.

The present invention will be practically explained based on the following examples but the present invention is not limited by such examples.

First, an embodiment of the monolithic ceramic capacitor of the present invention is explained by referring to the accompanying drawings. FIG. 1 is a schematic cross-sectional view showing an embodiment of the monolithic ceramic capacitor of the present invention, FIG. 2 is a schematic plane view showing an embodiment of the laminated ceramic layer of the present invention having an internal electrode, and FIG. 3 is an exploded slant view showing an embodiment of the present invention.

As shown in FIG. 1, the monolithic ceramic capacitor 1 of the present invention is a rectangular form chip-type monolithic ceramic capacitor prepared by forming external electrodes 5, a first layer 6 of nickel, copper, etc., and a second layer 7 of a soft solder, tin, etc., at both the side surfaces of a ceramic laminate 3 obtained by laminating plural dielectric ceramic layers 2a, 2b with internal electrodes 4 between the dielectric ceramic layers.

Next, the production method of the monolithic ceramic capacitor 1 of the present invention is explained in the order of the production steps.

First, a ceramic laminate 3 is formed. The ceramic laminate 3 is produced as follows. As shown in FIG. 2, a dielectric ceramic layer 2b (green sheet) is prepared by forming a sheet of a slurry comprising the main constituents composed of barium titanate, at least one of scandium oxide and yttrium oxide, at least one of samarium oxide and europium oxide, manganese oxide, cobalt oxide, and nickel oxide, magnesium oxide, and powders composed of an Li.sub.2 O--B.sub.2 O.sub.3 --(Si, Ti)O.sub.2 series oxide glass, and an internal electrode 4 composed of nickel or a nickel alloy is formed on one surface thereof. In addition, the internal electrode 4 may be formed by a screen printing, etc., or may be formed by a vapor-deposition method or a plating method. A necessary number of the dielectric ceramic layers 2b each having an internal electrode are laminated and as shown in FIG. 3, the laminated layers are placed between dielectric ceramic layers 2a having no internal electrode and pressed together to provide a laminate. Thereafter, the laminated dielectric ceramic layers 2a, 2b . . . 2b, 2a are calcined in a reducing atmosphere to form the ceramic laminate 3.

Then two external electrodes 5 are formed at the side surfaces of the ceramic laminate 3 such that they are connected to the internal electrodes 4. As material for the external electrodes 5, the same material as the internal electrodes 4 can be used. Also, silver, palladium, a silver-palladium alloy, copper, a copper alloy, etc., can be used. Also the foregoing metal powder mixed with a glass frit of a B.sub.2 O.sub.3 --SiO.sub.2 --BaO series glass, a Li.sub.2 O--SiO.sub.2 --BaO series glass, etc., can be used. By considering the use, place of use, etc., of the monolithic ceramic capacitor 1, a proper material is selected. The external electrodes 5 can be formed by coating a metal powder paste, which becomes the electrode, on the ceramic laminate 3 obtained by calcination followed by baking but the paste may also coated before calcination and the external electrodes may be formed simultaneously with the ceramic laminate 3. Thereafter, plating of nickel, copper, etc., is applied onto the external electrodes 5 to form a first layer 6. Finally, a plated second layer 7 of a soft solder, tin, etc., is formed on the plated first layer 6 to produce a chip-type monolithic ceramic capacitor 1.

EXAMPLE 1

First, as the starting materials, TiCl.sub.4 and Ba(NO.sub.3).sub.2 having various purities were prepared and after weighing, they were precipitated as barium titanyl oxalate {BaTiO(C.sub.2 O.sub.4).4H.sub.2 O} with oxalic acid. By heat-decomposing the precipitates at a temperature of 1000.degree. C. or higher, the 4 kinds of barium titanate (BaTiO.sub.3) shown in Table 1 below were synthesized.

TABLE 1 ______________________________________ Content of Impurities (wt %) Mean Alkali Particle Kind of Metal Size BaTiO.sub.3 Oxide SrO CaO SiO.sub.2 Al.sub.2 O.sub.3 (.mu.m) ______________________________________ A 0.003 0.012 0.001 0.010 0.005 0.60 B 0.020 0.010 0.003 0.019 0.008 0.56 C 0.012 0.179 0.018 0.155 0.071 0.72 D 0.062 0.014 0.001 0.019 0.004 0.58 ______________________________________

Also, oxides, carbonates and hydroxides were weighed and combined such that the composition ratio of 0.25Li.sub.2 O--0.30B.sub.2 O.sub.3 --0.03TiO.sub.2 --0.42SiO.sub.2 (mol ratio) was obtained after mixing and grinding and evaporating to dryness to provide a power. The powder was placed in an alumina crucible and after melting by heating at 1300.degree. C., quickly cooled and ground the molten powder ground to produce an oxide glass powder having a mean grain size of 1 .mu.m or smaller.

Then, BaCO.sub.3 for adjusting the Ba/Ti mol ratio (m) of barium titanate and Sc.sub.2 O.sub.3, Y.sub.2 O.sub.3, Sm.sub.2 O.sub.3, Eu.sub.2 O.sub.3, MnCO.sub.3, NiO, Co.sub.2 O.sub.3, and MgO each having a purity of at least 99% were prepared. These raw material powders were compounded with the above-described oxide glass powder such that the composition ratios shown in Table 2 were obtained to provide compounded products. To each of the compounded products were added a polyvinyl butyral series binder and an organic solvent such as ethanol, etc., and the resultant mixture was wet-mixed in a ball mill to provide a ceramic slurry. Using the ceramic slurry, a sheet was formed by the doctor blade method to provide a rectangular green sheet having a thickness of 11 .mu.m. Then, an electrically conductive paste mainly composed of Ni was screen-printed on the ceramic green sheet to form an electrically conductive paste layer for constituting the internal electrode.

TABLE 2 __________________________________________________________________________ Addition (1 - .alpha. - .beta. - .gamma.){BaO}.sub.m.TiO.sub.3 + .alpha.M.sub.2 O.sub.3 + .beta.Re.sub.2 O.sub.3 + .gamma.(Mn.sub.1-x-y Ni.sub.x Co.sub.y) O Amount of Sample Kind of M Re .gamma./ Oxide No. BaTiO.sub.3 Sc Y Sm Eu .alpha. + .beta. .beta. .beta./.alpha. .gamma. (.alpha. + .beta.) x y x + y m MgO Glass __________________________________________________________________________ *1 A 0.0000 0.0000 0.0260 0.20 0.20 0.40 1.015 1.00 1.00 *2 A 0.0050 0.0010 0.0060 0.0010 0.20 0.0000 0.00 0.00 0.00 1.010 1.00 0.80 *3 A 0.0080 0.0080 0.0000 0.0150 1.9 0.10 0.10 0.20 1.010 0.80 1.50 *4 A 0.0050 0.0010 0.0060 0.0010 0.20 0.0200 3.3 0.20 0.20 0.40 0.990 1.00 1.20 *5 A 0.0060 0.0015 0.0075 0.0015 0.25 0.0250 3.3 0.20 0.40 0.60 1.000 1.20 0.80 *6 A 0.0080 0.0020 0.0100 0.0020 0.25 0.0200 2.0 0.10 0.10 0.20 1.015 0.10 1.00 *7 A 0.0075 0.0025 0.0100 0.0025 0.33 0.0200 2.0 0.30 0.10 0.40 1.015 0.80 0.00 8 B 0.0015 0.0010 0.0025 0.0010 0.67 0.0025 1.0 0.30 0.10 0.40 1.015 1.00 0.80 9 A 0.0200 0.0050 0.0250 0.0050 0.25 0.0500 2.0 0.05 0.10 0.15 1.010 1.00 1.20 10 A 0.0075 0.0025 0.0100 0.0025 0.33 0.0400 4.0 0.10 0.30 0.40 1.005 1.20 1.00 11 C 0.0125 0.0075 0.0200 0.0075 0.60 0.0400 2.0 0.00 0.00 0.00 1.015 1.00 1.50 12 A 0.0050 0.0050 0.0100 0.0050 1.00 0.0300 3.0 0.10 0.20 0.30 1.035 0.80 1.50 13 A 0.0010 0.0050 0.0010 0.0070 0.0010 0.17 0.0200 2.9 0.00 0.60 0.60 1.010 1.20 1.50 14 A 0.0060 0.0010 0.0070 0.0010 0.17 0.0150 2.1 0.50 0.00 0.50 1.015 1.00 1.20 15 A 0.0005 0.0055 0.0005 0.0005 0.0070 0.0010 0.17 0.0250 3.6 0.10 0.10 0.20 1.010 1.20 3.00 16 A 0.0060 0.0010 0.0010 0.0080 0.0020 0.33 0.0250 3.1 0.05 0.20 0.25 1.015 0.50 0.20 17 A 0.0050 0.0010 0.0060 0.0010 0.20 0.0200 3.3 0.10 0.30 0.40 1.005 5.00 1.50 18 A 0.0040 0.0050 0.0090 0.0050 1.25 0.0250 2.8 0.10 0.10 0.20 1.015 1.20 1.00 *19 A 0.0240 0.0060 0.0300 0.0060 0.25 0.0150 0.5 0.10 0.10 0.20 1.010 1.20 1.00 *20 A 0.0180 0.0030 0.0210 0.0030 0.17 0.0800 3.8 0.30 0.10 0.40 1.010 1.00 1.00 *21 A 0.0060 0.0015 0.0075 0.0015 0.25 0.0400 5.3 0.20 0.20 0.40 1.015 1.00 0.80 *22 A 0.0050 0.0090 0.0140 0.0090 1.80

0.0300 2.1 0.10 0.30 0.40 1.015 0.80 0.80 *23 A 0.0050 0.0010 0.0060 0.0010 0.20 0.0150 2.5 1.00 0.00 1.00 1.010 0.80 1.00 *24 A 0.0075 0.0025 0.0100 0.0025 0.33 0.0150 1.5 0.00 1.00 1.00 1.010 1.20 1.00 *25 A 0.0060 0.0015 0.0075 0.0015 0.25 0.0120 1.6 0.40 0.60 1.00 1.010