Ceramic green sheet, method for manufacturing multilayer ceramic capacitor, and multilayer ceramic capacitor

Abstract

A ceramic green sheet where the proportion of a Si-containing constituent coating the surface of barium titanate-based ceramic particles is 95% or higher, and the proportion of a rare-earth element-containing constituent coating the surface of the barium titanate-based ceramic particle is 85% or higher.

Claims

1. A ceramic green sheet comprising: a barium titanate-based ceramic particle as a main inorganic component; a Si-containing constituent having, when measured after removal of a binder, a coverage of 95% or greater along a surface of the barium titanate-based ceramic particle; Dy.sub.2O.sub.3 having, when measured after removal of the binder, a coverage of 85% or greater along the surface of the barium titanate-based ceramic particle; a Ba constituent; a Mg constituent; a Mn constituent; and a Si constituent, wherein, when a total content of Ti is 100 parts by mol: a content of Dy is 4.0 parts by mol; a content of Mg is 0.25 parts by mol; a content of Mn is 0.25 parts by mol; and a content of Si is 1.5 parts by mol.

2. The ceramic green sheet according to claim 1, wherein the ceramic green sheet has a Ba/Ti molar ratio of 1.01.

3. A method for manufacturing a multilayer ceramic capacitor, the method comprising: forming ceramic green sheets containing: a barium titanate-based ceramic particle as a main inorganic component, a Si-containing constituent having, when measured after removal of a binder, a coverage of 95% or greater along a surface of the barium titanate-based ceramic particle, and Dy.sub.2O.sub.3 having, when measured after removal of the binder, a coverage of 85% or greater along the surface of the barium titanate-based ceramic particle; a Ba constituent, a Mg constituent, a Mn constituent, and a Si constituent; forming electrode-pattern sheets by applying a conductive paste to some of the ceramic green sheets in a predetermined pattern; forming an unfired stacked structure by stacking the electrode pattern sheets with the ceramic green sheets interposed therebetween; firing the unfired stacked structure to form a multilayer ceramic element having a plurality of dielectric layers and a plurality of internal electrodes opposed to each other with the dielectric layers interposed therebetween; and forming an external electrode electrically connected to the internal electrodes on the multilayer ceramic element, wherein, when a total content of Ti in the ceramic green sheets is 100 parts by mol: a content of Dy is 4.0 parts by mol; a content of Mg is 0.25 parts by mol; a content of Mn is 0.25 parts by mol; and a content of Si is 1.5 parts by mol.

4. The method for manufacturing a multilayer ceramic capacitor according to claim 3, wherein the plurality of dielectric layers have a Ba/Ti molar ratio of 1.01.

5. A multilayer ceramic capacitor comprising: a multilayer ceramic element comprising a plurality of dielectric layers comprising a barium titanate-based ceramic, Dy, Ba, Mg, Mn and Si, and a plurality of internal electrodes opposed to each other with the dielectric layers interposed therebetween; and an external electrode on a surface of the multilayer ceramic element and electrically connected to the internal electrodes, wherein the Dy is present at 98% or greater of all grain boundaries in the barium titanate-based ceramic of the dielectric layers, and wherein, when a total content of Ti is 100 parts by mol in the plurality of dielectric layers: a content of Dy is 4.0 parts by mol; a content of Mg is 0.25 parts by mol; a content of Mn is 0.25 parts by mol; and a content of Si is 1.5 parts by mol.

6. The multilayer ceramic capacitor according to claim 5, wherein the plurality of dielectric layers have a Ba/Ti molar ratio of 1.01.

Description

BRIEF EXPLANATION OF THE DRAWINGS

(1) FIG. 1 is a front cross-sectional view illustrating the configuration of a multilayer ceramic capacitor according to an embodiment of the present invention.

(2) FIG. 2 is a diagram for explaining locations subjected to point analysis on a raw material particle (barium titanate-based ceramic) constituting a ceramic green sheet according to an embodiment of the present invention.

(3) FIG. 3 is a diagram illustrating an example of a conventional multilayer ceramic capacitor.

DETAILED DESCRIPTION OF THE INVENTION

(4) Features of the present invention will be described in more detail below with reference to an embodiment of the present invention.

Embodiment

(5) The multilayer ceramic capacitor according to an embodiment of the present invention has, as shown in FIG. 1, a structure with external electrodes 4 (4a, 4b) provided so as to be electrically connected to a plurality of internal electrodes 2 (2a, 2b), on both end surfaces 3 (3a, 3b) of a multilayer ceramic element (ceramic body) 10 that has the internal electrodes 2 (2a, 2b) laminated with ceramic layers 1 as dielectric layers interposed therebetween.

(6) The internal electrodes 2 (2a, 2b) are preferably non-precious metal electrodes containing Ni as their conductive component.

(7) In addition, the external electrodes 4 (4a, 4b) preferably have a multilayer structure including an external electrode body 11 obtained by baking a conductive paste, a Ni plated film layer 12 formed on the surface of the external electrode body 11, and a Sn plated film layer 13 formed on the surface of the Ni plated film layer 12.

(8) In addition, the dielectric layers (ceramic layer dielectric layers) 1 constituting the multilayer ceramic element (ceramic body) 10 of the multilayer ceramic capacitor are formed from a dielectric ceramic that has a perovskite structure (a barium titanate-based ceramic in this embodiment).

(9) Next, a method for manufacturing this multilayer ceramic electronic component (multilayer ceramic capacitor) will be described.

(10) <1> Preparation of Dielectric Raw Material (Ceramic Raw Material)

(11) First, a barium titanate-based ceramic powder as a dielectric main constituent raw material was prepared in accordance with the following procedure.

(12) BaCO.sub.3 and TiO.sub.2 powders were prepared, and weighed so that the molar ratio between Ba and Ti was 1:1.

(13) Then, the powders were, with the addition of pure water and a dispersant thereto, subjected to grinding/crushing treatment with a forced-circulation wet grinding mill using PSZ media to create a main raw material slurry.

(14) Next, the main raw material slurry subjected to the grinding/crushing treatment was dried in an oven, and subjected to heat treatment at a temperature of 950° C. or higher to obtain a barium titanate-based ceramic powder of 0.20 μm in average grain size.

(15) In addition, besides the barium titanate-based ceramic powder of respective particle sizes obtained as described above, powders of BaCO.sub.3, Dy.sub.2O.sub.3 (SSA (specific surface area): 30 m.sup.2/g), MgCO.sub.3, and MnCO.sub.3 were prepared.

(16) In addition, multiple types of SiO.sub.2 powders varied in SSA (specific surface area) were prepared as additive SiO.sub.2.

(17) Then, the barium titanate-based ceramic powder and the respective additive components (BaCO.sub.3, Dy.sub.2O.sub.3, MgCO.sub.3, MnCO.sub.3, SiO.sub.2) were weighed and sampled in predetermined amounts, and with the addition of pure water and a dispersant thereto, subjected to grinding/crushing treatment with the use of a forced-circulation wet grinding mill (using PSZ media), thereby preparing a blended raw material slurry.

(18) It is to be noted that for preparing the blended raw material slurry, as shown in Table 1, the combination of the types of SiO.sub.2 varied in SSA (specific surface area) and the grinding/crushing treatment time were varied to prepare the blended raw material slurry.

(19) In addition, in this regard, among the additive components, the additive amounts of Dy.sub.2O.sub.3, MgCO.sub.3, MnCO.sub.3, and SiO.sub.2 were adjusted so that:

(20) (a) the total content (parts by mol) of Dy was 4.0;

(21) (b) the content (parts by mol) of Mg was 0.25;

(22) (c) the content (parts by mol) of Mn was 0.25; and

(23) (d) the content (parts by mol) of Si was 1.5;

(24) when the total content of Ti was regarded as 100 parts by mol.

(25) Furthermore, the BaCO.sub.3 was added in a proportion such that the ratio of Ba to Ti (Ba/Ti (molar ratio)) after firing was 1.01 after firing.

(26) Then, the slurry subjected to the grinding/crushing treatment was dried in an oven to obtain respective dielectric raw material powders.

(27) <2> Preparation of Ceramic Green Sheet

(28) The respective dielectric raw material powders prepared in the way described above was then, with the addition to a polyvinyl butyral-based binder and an organic solvent such as ethanol, subjected to wet mixing in a ball mill, thereby preparing ceramic slurry.

(29) This ceramic slurry was subjected to sheet forming so that the fired dielectric element thickness was 5.0 μm, thereby providing rectangular ceramic green sheets.

(30) It is to be noted that while the sheet forming was carried by a doctor blade method in this embodiment, the method for the sheet forming is not to be considered limited thereto, but it is possible to use other various known methods.

(31) <3> Evaluation of Ceramic Green Sheet

(32) First, the ceramic green sheets prepared in the way described above were heated for 2 hours under the condition of 400° C. in the air atmosphere to carry out binder removal treatment, thereby providing raw material particles as a barium titanate-based ceramic powder.

(33) Then, the surface of the raw material particles were observed with a scanning transmission electron microscope (STEM) to confirm the amounts of Dy and Si present by point analysis with the use of EDX.

(34) Further, in this regard, locations subjected to the point analysis on the raw material particles (barium titanate-based ceramic particles) will be described with reference to FIG. 2 which is a pattern diagram of a planar view of the raw material particle when the raw material particle has a spherical shape.

(35) More specifically, thirteen particles were subjected to the point analysis on eight points (the distance between the points was 50 nm or more) per particle, among points at 10 nm inside (toward a central direction) a spherical raw material particle 50 from an outer edge 50a of the barium titanate-based ceramic particle in FIG. 2 (points on the surface of the raw material particle (for example, P.sub.1, P.sub.2, P.sub.3, etc. in FIG. 2)).

(36) Therefore, the total number of point analyses in this case is 104 (8 points×13 particles=104).

(37) Then, the Si-containing constituent coverage was obtained from the following formula (1), and the Dy (rare-earth element)-containing constituent coverage was obtained from the formula (2).
Si-containing Constituent Coverage (%)=(Number of Points with Si Element/Number of Measurement Points)×100  (1)
Dy (rare-earth element)-containing Constituent Coverage (%)=(Number of Points with Rare-earth Element/Number of Measurement Points)×100  (2)

(38) In addition, the points with the detected concentrations of Dy (rare-earth element) and Si of 0.5 atom % or higher were determined to be points with Dy (rare-earth element) and Si.

(39) It is to be noted that JEM-2200FS (from JEOL) was used for the STEM (Scanning Transmission Electron Microscope) in the STEM analysis mentioned above. The acceleration voltage is 200 kV. For the detector EDS (energy dispersive X-ray analyzer), an SDD detector (silicon drift detector) of JED-2300T (from JEOL) with an aperture of 60 mm.sup.2 was used, and Noran System 7 was used for the EDS system.

(40) In addition, the concentration measurement in the STEM point analysis was made for 30 seconds per point, and the concentration for each element was obtained by a Cliff-Lorimer method.

(41) The values of the Si-containing constituent coverage (%) and Dy (rare-earth element)-containing constituent coverage (%) of the raw material particles are shown in Table 1 for each sample (ceramic green sheet) obtained in the way described above.

(42) <4> Preparation of Multilayer Ceramic Element

(43) 1) First, the ceramic green sheets prepared in the way described above were stacked for a predetermined number of sheets so as to form an outer layer part with a predetermined thickness (for example, 100 μm), thereby forming a lower outer layer part.

(44) 2) Next, on the lower outer layer part formed in the step 1) mentioned above, electrode-pattern formed ceramic green sheets with internal electrode patterns formed by screen printing with a conductive paste containing a Ni powder as a conductive component onto the ceramic green sheets prepared in the way described above were stacked for a predetermined number of sheets (170 sheets in this embodiment) so that the internal electrode patterns extended to ends opposed to each other.

(45) 3) Then, on the stacked electrode-pattern formed ceramic green sheets, the ceramic green sheets were stacked for a predetermined number of sheets so as to form an outer layer part with a predetermined thickness (for example, 100 μm), thereby forming an upper outer layer part, and thus forming an unfired stacked block.

(46) 4) The unfired stacked block prepared in the way described above was cut in predetermined locations, thereby providing an unfired stacked structure to serve as the multilayer ceramic element 10 (FIG. 1) after firing.

(47) 5) Then, the unfired stacked structure obtained in the step 4) mentioned above was heated to 250° C. in a N.sub.2 atmosphere to carry out binder removal treatment. Then, the structure was subjected to firing under the condition of a top temperature of 1240 to 1300° C. (1270° C. in this embodiment) with an oxygen partial pressure of 10.sup.−9 to 10.sup.−10 MPa (10.sup.−9 MPa in this embodiment) in a reducing atmosphere composed of a H.sub.2—N.sub.2—H.sub.2O gas, thereby providing a fired multilayer ceramic element.

(48) <5> Formation of External Electrode

(49) To end surfaces of the obtained multilayer ceramic element, a conductive paste (external electrode paste) containing a Cu powder and containing B.sub.2O.sub.3—Li.sub.2O.sub.3—SiO.sub.2—BaO-based glass frit was applied as a conductive component, and baked at a temperature of 850° C. in a N.sub.2 atmosphere to form external electrodes (Cu electrodes) electrically connected to the internal electrodes.

(50) Furthermore, Ni plated layers were formed so as to cover the Cu electrodes formed, and Sn plated layers were further formed so as to cover the Ni plated layers, thereby providing a multilayer ceramic capacitor structured as shown in FIG. 1.

(51) It is to be noted that the external dimensions of the obtained multilayer ceramic capacitor were 2.0 mm in width, 1.3 mm in length, and 1.3 mm in thickness.

(52) In addition, the ceramic layer (dielectric layer) 1 interposed between the internal electrodes 2 was 5.0 μm in thickness.

(53) <6> Evaluation of Ceramic Layer (Porcelain) Constituting Multilayer Ceramic Capacitor

(54) For each of the multilayer ceramic capacitors (samples) prepared in the way described above, five samples were prepared, a part near the center in each of the length direction, width direction, and thickness direction was exposed by polishing for each of the five samples, and the ceramic layers (dielectric layers) near the center were processed into a thin piece.

(55) Then, the sample processed into the thin piece (thin sample) was analyzed by STEM at ten grain boundaries (measurement at one point per grain). In this regard, one thin sample was taken from each of the five multilayer ceramic capacitors (samples), and ten grain boundaries were analyzed for the thin sample. Thus, fifty results from the analysis are obtained for one type of multilayer ceramic capacitor (sample).

(56) Grain boundaries (crystal grain boundaries) nearly perpendicular to the thin film surface with a clear crystal interface between crystal grains adjacent to each other were selected as the grain boundaries analyzed.

(57) It is to be noted that JEM-2200FS (from JEOL) was used for the STEM in the STEM analysis. The acceleration voltage was adjusted to 200 kV.

(58) For the detector EDS, an SDD detector of JED-2300T (from JEOL) with an aperture of 60 mm.sup.2 was used, and Noran System 7 was used for the EDS system.

(59) In addition, the thin sample was approximately 100 nm in thickness.

(60) For the concentration measurement in the STEM analysis, the point analysis was made for 30 seconds per point, and the concentration for each element was obtained by a Cliff-Lorimer method.

(61) Central parts of the selected grain boundaries were subjected to point analysis, and the grain boundaries with a Dy detection concentration of 0.5 atom % or higher with respect to the total of the detected elements excluding C and O were determined to be grain boundaries with Dy present.

(62) Then, the proportion of the number of grain boundaries with Dy present to the number of grain boundaries (the number of grain boundaries with Dy present/the number of grain boundaries analyzed×100) was obtained.

(63) The results are shown as the Dy presence ratio in Table 1.

(64) <7> Evaluation of Multilayer Ceramic Capacitor

(65) For the multilayer ceramic capacitors prepared in the way mentioned above, the electrostatic capacitance was measured at 1 kHz-1 Vac, and the capacitors with the electrostatic capacitance between a 25% value and a 75% value were extracted as multilayer ceramic capacitors to be evaluated.

(66) It is to be noted that the term “electrostatic capacitance between a 25% value and a 75% value” refers to samples excluding samples up to the 25th sample in the ascending order of the electrostatic capacitance and samples up to the 25th sample in the descending order of the electrostatic capacitance, that is, the fifty samples in total from the 26th sample to 75th sample in the ascending order of the electrostatic capacitance, for example, in the case of measuring the electrostatic capacitance for one hundred multilayer ceramic capacitors (samples).

(67) In an environment at 125° C., a DC voltage of 150 V was applied for 2000 hours to the fifty multilayer ceramic capacitors (samples) extracted depending on the electrostatic capacitance value. Then, the insulation resistance values of the multilayer ceramic capacitors were measured while applying the voltage, and the capacitors with an insulation resistance value of 1 MΩ or less were considered to be defective (defective insulation resistance).

(68) For the fifty samples subjected to the test, the number of samples with defective insulation resistance generated and the percent insulation defective are shown together in Table 1.

(69) It is to be noted that in Table 1, the samples with sample numbers marked with * (the samples of sample numbers 1 to 5) refer to comparative samples that fail to meet the requirement of the present invention, whereas the other samples (the samples of sample numbers 6 to 9) refer to samples that meet the requirement of the present invention.

(70) TABLE-US-00001 TABLE 1 The Number of Samples with Defective SiO.sub.2 Powder Insulation Resistance Specific Fired Ceramic Generated and Ratio Surface Layer thereof (Defective Area Grinding/Crushing Ceramic Green Sheet Dy Presence sample number/50 Sample (SSA) Treatment Time Si Coverage Dy Coverage Ratio samples) (Generation Number (m.sup.2/g) (min) (%) (%) (%) Ratio) 1* 31 100 67 81 84% 7/50 (14%) 2* 31 300 82 84 90% 3/50 (6%) 3* 31 600 91 91 92% 1/50 (2%) 4* 63 100 84 76 88% 5/50 (10%) 5* 63 300 96 84 96% 1/50 (2%) 6 63 600 99 96 100% 0/50 (0%) 7 135 100 95 85 98% 0/50 (0%) 8 135 300 100 89 100% 0/50 (0%) 9 135 600 100 94 100% 0/50 (0%)

(71) From Table 1, in the case of sample numbers 6 to 9 that meet the requirements of the present invention prepared by the use of the ceramic green sheets with the Si-containing constituent coverage of 95% or higher obtained from the above formula (1) and the Dy (rare-earth element)-containing constituent coverage of 85% or higher obtained from the above formula (2), where the presence ratio of Dy (rare-earth element) in the dielectric layers is 98% or higher, it has been confirmed that high reliable multilayer ceramic capacitors are achieved which have no defective insulation caused in the above-described test for insulation resistance.

(72) On the other hand, in the case of sample numbers 1 to 5 that fail to meet the requirements of the present invention prepared by the use of the ceramic green sheets with the Si-containing constituent coverage of less than 95% obtained from the above formula (1) or the Dy (rare-earth element)-containing constituent coverage of less than 85% obtained from the above formula (2), where the presence ratio of Dy (rare-earth element) in the ceramic dielectric layers is less than 98%, defective insulation resistance has been confirmed to be unfavorably caused in the above-mentioned test for insulation resistance.

(73) From the results mentioned above, it is determined that the use of the ceramic green sheets with the Si-containing constituent coverage of 95% or higher and the Dy (rare-earth element)-containing constituent coverage of 85% or higher achieves highly reliable multilayer ceramic capacitors where the presence ratio of Dy (rare-earth element) in the ceramic dielectric layers is 98% or higher, without any defective insulation resistance caused.

(74) It is to be noted that while a case of Dy as the rare-earth element has been described as an example in the embodiment described above, similar effects can be achieved even when other rare-earth element (for example, yttrium (Y), gadolinium (Gd), terbium (Tb), holmium (Ho), etc.) is used besides Dy as the rare-earth element.

(75) In addition, while ceramic green sheets have been described where the ratio of Ba to Ti (Ba/Ti (molar ratio)) is 1.01 after firing, the Ba/Ti (molar ratio) is not to be considered limited thereto.

(76) The present invention is further not to be considered limited to the embodiment described above even in other respects, various applications and modifications can be made within the scope of the invention.

DESCRIPTION OF REFERENCE SYMBOLS

(77) 1 ceramic layer 2(2a, 2b) internal electrode 3(3a, 3b) end surface of ceramic body 4(4a, 4b) external electrode 10 ceramic body 11 external electrode body 12 Ni plated film layer 13 Sn plated film layer 50 raw material particle 50a outer edge of raw material particle P.sub.1, P.sub.2, P.sub.3 analyzed point of raw material particle