Patent Publication Number: US-2023145815-A1

Title: Multilayer capacitor

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims benefit of priority to Korean Patent Application No. 10-2021-0153889 filed on Nov. 10, 2021 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present disclosure relates to a multilayer capacitor. 
     BACKGROUND 
     A capacitor is a device which may store electricity, and uses a principle that the electricity is accumulated in each of two electrodes of the capacitor generally when a voltage is applied to the capacitor in a state where the two electrodes are disposed opposite to each other. In a case where a direct current (DC) voltage is applied to the capacitor, a current flows in the capacitor while the electricity is accumulated in the capacitor. However, the current does not flow in the capacitor when the accumulation of the electricity is completed. Meanwhile, in a case where an alternating current (AC) voltage is applied to the capacitor, an AC current flows in the capacitor while polarities of the electrodes are alternated with each other. 
     The capacitor may be classified into several types based on a type of an insulator positioned between the electrodes such as an aluminum electrolytic capacitor in which the electrodes are made of aluminum and a thin oxide film is disposed between these aluminum electrodes, a tantalum capacitor in which tantalum is used as an electrode material, a ceramic capacitor in which a dielectric material of a high dielectric constant such as a barium titanate is positioned between the electrodes, a multilayer ceramic capacitor (MLCC) in which ceramic of a high dielectric constant, formed in a multilayer structure, is used as a dielectric material positioned between the electrodes, a film capacitor in which a polystyrene film is used as a dielectric material positioned between the electrodes, etc. 
     Among these capacitors, the multilayer ceramic capacitor has excellent temperature and frequency characteristics and may be implemented in a small size, and has thus been recently used widely in various fields such as a high frequency circuit. In recent years, there have been continuous efforts to make the multilayer ceramic capacitor smaller, and the dielectric layer, the internal electrodes and external electrodes are thinned to this end. 
     In accordance with an increasing demand in recent years for reducing a thickness of an electronic component, researches have been continuously conducted to reduce the thickness of the multilayer ceramic capacitor. However, when having a smaller thickness, the multilayer ceramic capacitor may have difficulty in sufficiently securing its electrical characteristics. In addition, when thinned, the external electrode may have reduced mechanical rigidity or electrical characteristics. 
     SUMMARY 
     An aspect of the present disclosure may provide a multilayer capacitor including an external electrode having improved electrical characteristics. Another aspect of the present disclosure may provide a multilayer capacitor including an external electrode having improved structural stability. 
     According to an aspect of the present disclosure, a multilayer capacitor includes a body including a dielectric layer and a plurality of internal electrodes stacked on each other interposing the dielectric layer therebetween, and external electrodes disposed externally on the body, respectively including a first layer connected to the internal electrode and a second layer covering the first layer, wherein the first layer includes a metal particle including an element A and a Z-A-O phase disposed in the metal particle, and the element Z is an alkali metal. 
     The first layer may further include an oxide of the element A. 
     The oxide of element A may exist on a surface of the metal particle. 
     Some of the metal particles may not include the oxide of the element A. 
     The element A may be at least one element selected from the group consisting of nickel (Ni), silver (Ag), palladium (Pd) and gold (Au). 
     The second layer may be a copper (Cu) plating layer. 
     The second layer may include a diffusion region of the element A. 
     The diffusion region may exist at a grain boundary of a metal component included in the second layer. 
     The second layer may include a diffusion region of the element Z. 
     The diffusion region may exist at a grain boundary of a metal component included in the second layer. 
     The element Z may be at least one element selected from the group consisting of lithium (Li), sodium (Na) and potassium (K). 
     The external electrode may include 0.2 mol or more of the element component Z relative to 100 mol of the element component A. 
     The multilayer capacitor may further include a dummy electrode disposed in the body, and disposed at at least one of uppermost and lowermost portions of the plurality of internal electrodes. 
     A length of one side may have a value between −10% and +10% of (250+n* 350 ) μm based on a direction in which the plurality of internal electrodes are stacked on each other, and “n” may be a natural number. 
     The multilayer capacitor may have a thickness of 70 μm or less when the thickness is defined based on its length measured in a direction in which the plurality of internal electrodes are stacked on each other. 
     According to an aspect of the present disclosure, a method of manufacturing a multilayer capacitor includes coating the metal particle with at least one of a carbonate of the element Z and a hydroxide of the element Z. 
     The element Z may be at least one element selected from the group consisting of lithium (Li), sodium (Na), and potassium (K). 
     The coating the metal particle may include coating the metal particle with the carbonate of the element Z. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The above and other aspects, features and advantages of the present disclosure will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG.  1    is a perspective view schematically showing an appearance of a multilayer capacitor according to an exemplary embodiment of the present disclosure; 
       Each of  FIGS.  2  and  3    is a cross-sectional view of the multilayer capacitor of  FIG.  1   ; 
         FIGS.  4  and  6    are enlarged views of a partial region of an external electrode; 
         FIG.  5    is an enlarged view of a metal particle of the external electrode; 
         FIGS.  7  through  10    are results of testing characteristics of electrode structures according to exemplary embodiments of the present disclosure; and 
         FIGS.  11  through  16    show multilayer capacitors according to modified examples of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, exemplary embodiments of the present disclosure will now be described in detail with reference to the accompanying drawings. 
       FIG.  1    is a perspective view schematically showing an appearance of a multilayer capacitor according to an exemplary embodiment of the present disclosure. Each of  FIGS.  2  and  3    is a cross-sectional view of the multilayer capacitor of  FIG.  1   ;  FIGS.  4  and  6    are enlarged views of a partial region of an external electrode; and  FIG.  5    is an enlarged view of a metal particle of the external electrode. 
     Referring to  FIGS.  1  through  3   , a multilayer capacitor according to an exemplary embodiment of the present disclosure may include a body  110  including a dielectric layer  111  and a plurality of internal electrodes  121  and  122  stacked on each other interposing the dielectric layer  111  therebetween, and external electrodes  131  and  132 . Here, the external electrodes  131  and  132  may include first layers  131   a  and  132   a  and second layers  131   b  and  132   b , respectively, and may additionally include third layers  131   c  and  132   c , respectively. As shown in  FIGS.  4  and  5   , the first layers  131   a  and  132   a  may each include a metal particle  140  and a Z-A-O phase  142  formed in the metal particle. Here, “A” is a metal element included in the metal particle, and the element Z is an alkali metal. 
     The body  110  may have a structure in which the plurality of dielectric layers  111  are stacked on each other in a first direction (i.e. X-direction), and may be obtained by stacking a plurality of green sheets on each other and then sintering the same for example. The plurality of dielectric layers  111  may have an integrated shape by this sintering process, and may include a plurality of grains. In addition, as shown in  FIG.  1   , the body  110  may have a shape similar to a rectangular parallelepiped. The dielectric layer  111  included in the body  110  may include a ceramic material having a high dielectric constant, BT-based ceramics, i.e. barium titanate (BaTiO 3 ) based ceramics, for example, and may also include any other material known in the art as long as the capacitor obtains sufficient capacitance. The dielectric layer  111  may further include an additive, an organic solvent, a plasticizer, a binder, a dispersant and the like, if necessary, in addition to the ceramic material which is a main component. Here, when using the additive, the dielectric layer may include the additive in the form of a metal oxide during its manufacturing process. The metal oxide additive may include, for example, at least one of manganese dioxide (MnO 2 ), dysprosium oxide (Dy 2 O 3 ), barium oxide (BaO), magnesium oxide (MgO), aluminium oxide (Al 2 O 3 ), silicon dioxide (SiO 2 ), chromium(III) oxide (Cr 2 O 3 ) and calcium carbonate (CaCO 3 ). 
     The plurality of internal electrodes  121  and  122  may be obtained by printing a paste, which includes a conductive metal (e.g., nickel (Ni), silver (Ag), copper (Cu), titanium (Ti), palladium (Pd) or the like) having a predetermined thickness, on one surface of a ceramic green sheet and then firing the same. In this case, the plurality of internal electrodes  121  and  121  may respectively be first and second internal electrodes  121  and  122  exposed from the body  110  in directions opposite to each other. The first and second internal electrodes  121  and  122  may respectively be connected to different external electrodes  131  and  132  to have different polarities while being driven, and may be electrically isolated from each other by the dielectric layer  111  disposed therebetween. However, the number of the external electrodes  131  and  132  or a method in which the external electrodes  131  and  132  and the internal electrodes  121  and  121  are respectively connected to each other may depend on exemplary embodiments. 
     The external electrodes  131  and  132  may be formed externally on the body  110  and connected to the internal electrodes  121  and  121 . In detail, the external electrodes  131  and  132  may respectively be first and second external electrodes  131  and  132  disposed on surfaces of the body  110 , opposite to each other. The external electrodes  131  and  132  may respectively include the first layers  131   a  and  132   a  and the second layers  131   b  and  132   b , and here the second layers  131   b  and  132   b  may be plating layers. In more detail, the second layers  131   b  and  132   b  may be copper (Cu) plating layers. In addition, the third layers  131   c  and  132   c  may also be plating layers, and for example, tin (Sn) plating layers each having a thickness of 4.5 μm or more. 
     In this exemplary embodiment, the first layers  131   a  and  132   a  respectively included in the external electrodes  131  and  132  may each be a structure designed to have structural stability and high electrical conductivity. As described above, the first layers  131   a  and  132   a  may each include the metal particle  140  including the element A, the Z-A-O phase  142  formed in the metal particle. Here, the element A may use an element having excellent electrical conductivity and suitable for being diffused into the second layers  131   b  and  132   b , and may be, for example, at least one element selected from the group consisting of Ni, Ag, Pd and gold (Au). In this case, the first layers  131   a  and  132   a  may be fired electrode layers obtained by firing the conductive paste, and may further include a glass component in addition to a metal component. Therefore, unlike shown in  FIG.  4   , boundaries between the metal particles  140  after the firing may not be distinguished from each other. Hereinafter, the element A is described based on Ni as its representative example, and may use another element other than Ni. When the metal particle  140  includes Ni, the Ni component may be diffused into the second layers  131   b  and  132   b , thereby improving adhesion between the first layers  131   a  and  132   a  and the second layers  131   b  and  132   b , which may improve the structural stability of the external electrodes  131  and  132 . 
     This adhesion improvement effect may be reduced when the metal particle  140  is oxidized to make a Ni oxide occur, which is because the diffusion of the Ni component is reduced due to the Ni oxide. In addition, the electrical conductivity of each of the first layers  131   a  and  132   a  may be reduced as the more Ni oxides occur. This exemplary embodiment uses a structure in which the alkali metal component Z is doped to the metal particle  140  to minimize the occurrence of the Ni oxide. In detail, first layers  131   a  and  132   a  may be formed by further having, for example, at least one element selected from the group consisting of lithium (Li), sodium (Na) and potassium (K) in addition to the metal particle  140 . In this case, the first layers  131   a  and  132   a  may be prepared by coating the element component Z on a surface of the metal particle  140  or further having a material containing the element component Z, and in more detail, by using the Ni particle coated by Li 2 CO 3 . For example, the coating process may include mixing, in a solvent (e.g., water), the metal particles and at least one of the element component Z and a material containing the element component Z. The mixing may be performed using a stirrer and the mixture may be heated. The solvent may be removed before the coated metal particles are fired. Other processes, solvents, and equipment appreciated by one of ordinary skill in the art may also be used even if not described in the present disclosure. 
     During a firing process, CO may be separated from the Li element in Li 2 CO 3  coated on the surface of the Ni particle  140  and volatilized, and the Li element may then infiltrate into the Ni particle  140  to form the Z-A-O phase  142 , i.e. Li—Ni—O phase. The Li—Ni—O phase may have the electrical conductivity by causing a small polaron hopping by replacing a portion of Ni with Li having a relatively small size, thereby improving the electrical conductivity of the Ni particle  140  including the Li—Ni—O phase. In addition, oxidation amount of Ni may be reduced as the Li—Ni—O phase is formed in the Ni particle. In addition, CO generated during the firing process may reduce an occurrence amount of NiO by reducing NiO. When the occurrence of NiO is reduced in this way, the electrical conductivity of each of the first layers  131   a  and  132   a  may be improved, and an amount of Ni component, diffused to the second layers  131   b  and  132   b  may be increased, and the adhesion of the first layers  131   a  and  132   a  and the second layers  131   b  and  132   b  may also be enhanced, respectively.  FIG.  5    illustrates one metal particle  140 . Here, the Z-A-O phase  142  may exist in the metal particle  140 , and the Ni oxide  141  may exist on the surface of the metal particle  140 . However, as described above, in this exemplary embodiment, the occurrence of the Ni oxide  141  may be minimized, and some of the metal particles  140  may not include the Ni oxide  141 . For example, 0.001% to 90% of the metal particles may not include the oxide of the element A. 
       FIG.  6    illustrates an example in which a diffusion region  143  is formed when some components are diffused from the first layers  131   a  and  132   a  to the second layers  131   b  and  132   b , respectively. As described above, the second layers  131   b  and  132   b  may be copper (Cu) plating layers, and may each include the diffusion region  143  of the element A, for example, the Ni component. Accordingly, the diffusion region  143  may include an intermetallic compound of Cu—Ni. In addition, the diffusion region  143  may include the element Z, for example, the Li component. Here, the alkali metal element has high diffusivity and may thus easily migrate to the second layers  131   b  and  132   b . Accordingly, the diffusion region  143  may include the intermetallic compound of Cu—Li. Furthermore, the diffusion region  143  may be formed by diffusion of both the element A and the element Z, and may thus include an intermetallic compound of Cu—Ni—Li. When such diffusion is generated, the diffusion region  143  of each of the second layers  131   b  and  132   b  may exist at a grain boundary of a metal component, e.g. Cu, included in the second layers  131   b  and  132   b . The diffusion region may be observed by, for example, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), electron probe microanalyzer (EPMA), and secondary-ion mass spectrometry (SIMS) (e.g., Time-of-Flight SIMS). The grain boundary may be observed by, for example, optical microscopy, electron microscopy, and field-ion microscopy. 
     Meanwhile, the element Z may use another alkali metal element other than Li, and to this end, Na 2 CO 3  or K 2 CO 3  may be coated on the surface of the metal particle  140  or added separately to the first layer. In addition, it is not necessary to use only carbonate as a compound with the element Z, and, for example, the element Z may be coated or added to the surface of the metal particle  140  in the form of LiOH. 
     The characteristics of an inventive example of the present disclosure and the characteristics of a comparative example are described with reference to the experimental results of  FIGS.  7  through  10   . First, a graph of  FIG.  7    illustrates a change in the occurrence amount of NiO depending on a heat treatment temperature, and the results are examined by changing content of Li. Here, the occurrence amount of NiO may be obtained by in-situ x-ray diffraction (XRD), under a specific measurement condition in which a temperature is raised from an air atmosphere and room temperature to 1200° C. for 18 minutes, and maintained at 900° C. for 10 minutes, and at 950° C. for 40 minutes. The measurement is performed by using the XRD and for 9 times in the order of room temperatures (25° C., 300° C., 600° C., 700° C., 750° C., 900° C., 950° C. and 1200° C.) and the room temperature (25° C.). Here, the content of Li indicated in the graph corresponds to a molar ratio of Li to Ni. 
     According to a result shown in the graph of  FIG.  7   , a large amount of NiO occurs in the temperature around 800 to 900° C. In particular, the very large amount of NiO occurs in an electrode (i.e. example indicated by Ni) not including Li, that is, including only the Ni particle. When the Li component is added (Li 2 CO 3  is coated, in the inventive example) to the Ni particle as in this exemplary embodiment, it is possible to significantly reduce the occurrence of NiO. The graph of  FIG.  8    illustrates a result of checking the amount of NiO occurring when Li, Na and K components are added. The experiment is conducted by classifying Li into Li 2 CO 3  and LiOH. In addition, the content of alkali metal in the experimental condition is set to 10.9 mol relative to 100 mol of Ni, that is, 10.9 mol %. As can be seen from the results of this experiment, the occurrence of NiO is significantly suppressed in the external electrode further including an alkali metal in addition to Ni compared to the electrode including only Ni. In addition, an XRD graph of  FIG.  9    illustrates that a peak shifts as the heat treatment temperature is increased, which indicates that the Li—Ni—O phase (generally, a Z-A-O phase) is formed. The Z-A-O phase may also be observed by, for example, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS). 
     Meanwhile, an occurrence degree of NiO may also be found out by performing Raman analysis on a surface of the electrode. According to the experimental results, when Li is added at the level of 10.9 mol %, the intensity of NiO has a level of 18.25, significantly lower than a level of 32.4 in which Li is not added (and only Ni particle is added). In addition,  FIG.  10    illustrates a result of measuring the electrical characteristics (i.e. specific resistance) of the electrode, based on the content of Li. When including 0.2 mol or more of the element component Z, i.e. Li, relative to 100 mol of the element component A, i.e. Ni, the external electrode may have the significantly improved electrical characteristics, compared to a case (indicated as “Ref”) where the external electrode does not include the element component Z. The specific resistance may be measured by, for example, a 2-terminal or 4-terminal test method. 
     Hereinafter, multilayer capacitors according to modified examples are described with reference to  FIGS.  11  through  16   . The multilayer capacitor having the external electrode described above may have the excellent structural stability and electrical characteristics, may thus be suitable to be used in a miniaturized component, and may have a remarkable effect when specifically used in a multilayer capacitor having a small thickness.  FIG.  11    illustrates an exemplary embodiment in which dummy electrodes  123  and  124  are further disposed in addition to the internal electrodes  121  and  122 , in which the multilayer capacitor may secure sufficient rigidity even when having the small thickness. Here, the multilayer capacitor may have a thickness T of about 70 μm or less when the thickness is defined based on a length of the multilayer capacitor, measured in a direction (i.e. X direction) in which the plurality of internal electrodes  121  and  122  are stacked on each other. This thickness condition may be applied to both the previous and subsequent exemplary embodiments. The dummy electrodes  123  and  124  may respectively be formed in the same shape as the adjacent internal electrodes  121  and  122 , and may respectively be connected to the external electrodes  131  and  132  respectively having the same polarities as the dummy electrodes  123  and  124 . However, the dummy electrodes  123  and  124  may have different shapes from the adjacent internal electrodes  121  and  122 , and may be connected to none of the external electrodes  131  and  132 .  FIG.  11    illustrates an example in which one dummy electrode  123  or  124  is disposed in each of upper and lower portions of the body  110 . However, the plurality of dummy electrodes may be disposed in each of the upper and lower portions of the body  110 . In addition, the dummy electrodes  123  and  124  may be applied to the following examples. 
     Next,  FIG.  12    illustrates another modified example in which the external electrode has a different shape from that of the previous example. In this modified example, external electrodes  131 ′ and  132 ′ may each have a so-called bottom electrode structure, and may cover the side and lower surfaces of the body  110 . In this case, the external electrodes  131 ′ and  132 ′ may include the same multi-layer structures as the above-described external electrodes  131  and  132 , i.e. first layers  131   a  and  132   a  and second layers  131   b  and  132   b , respectively. An overall size (or length in the X direction) of each of the external electrodes  131 ′ and  132 ′ may be reduced by using the bottom electrode structure, which may be advantageous for making the multilayer capacitor thinner. In addition, this bottom electrode structure may be applied to the following examples. 
     Next,  FIGS.  13  through  16    show yet another modified examples in which a multilayer capacitor  200  has a four-terminal square structure, which may be suitable for the multilayer capacitor to maintain high rigidity even when having the small thickness. A body  210  may include a dielectric layer  211  and a plurality of internal electrodes  221  and  222 , and may have the square structure based on the direction (i.e. X direction) in which the plurality of internal electrodes  221  and  222  are stacked on each other. Accordingly, the multilayer capacitor  200  may also have an overall shape similar to the square. For example, the multilayer capacitor  200  may have a size in which a length of a side A 1  or A 2  is (250+n*350) μm based on the X direction, where “n” may be a natural number. For example, when “n” is 1, the multilayer capacitor  200  has a size of 600 μm*600 μm. However, when considering an error range, the length of the one side A 1  or A 2  may have a value between −10% and +10% of (250+n*350) μm. Here, the length of the one side may be a multiple of 350 μm in consideration of a pitch value of a solder ball and the like when the multilayer capacitor  200  is mounted on a board. In addition, a thickness T of the multilayer capacitor  200  may be as small as 70 μm or less, based on its length measured in the X direction. Meanwhile, the thickness T and length of the one side A 1  or A 2  of the multilayer capacitor  200  may indicate a maximum value among values measured in a plurality of regions, or may be a value obtained by averaging the plurality of values. The thicknesses discloses herein, including thickness T, and length of the one side A 1  or A 2  may be measured by, for example, an optical microscope or a scanning electron microscope (SEM). 
     In this modified example, external electrodes  231  and  232  may include the same multi-layer structures as the above-described external electrodes  131  and  132 , i.e. first layers  231   a  and  232   a  and second layers  231   b  and  232   b , and may further include third layers  231   c  and  232   c , respectively. 
     Referring to  FIGS.  15  and  16   , the first internal electrode  221  may be connected to the pair of first external electrodes  231 , and may include a first main portion  211   a  and a first lead portion  211   b . The first lead portion  211   b  may be connected to the first external electrode  231  by being extended in a diagonal direction in which each of first and second corners C 1  and C 2  of the body  210  and the first main portion  211   a  are connected to each other. The second internal electrode  212  may be connected to the pair of second external electrodes  232 , and may include a second main portion  212   a  and a second lead portion  212   b . The second lead portion  212   b  may be connected to the second external electrode  232  by being extended in a diagonal direction in which each of third and fourth corners C 3  and C 4  of the body  210  and the second main portion  212   a  are connected to each other. 
     The pair of first external electrodes  231  may respectively be disposed on the first and second corners C 1  and C 2  of the body  210 , which are not adjacent to each other, and may be connected to the first internal electrode  211 . Similarly, the pair of second external electrodes  232  may respectively be disposed on the third and fourth corners C 3  and C 4  of the body  210 , which are not adjacent to each other, and may be connected to the second internal electrode  222 . As shown in the drawings, the first and second external electrodes  231  and  232  may respectively be disposed on the opposite surfaces of the body  210  in the direction (i.e. X direction) in which the first and second internal electrodes  221  and  222  are stacked on each other, and may be extended to partially cover a side surface of the body  210 . 
     As set forth above, the external electrode included in the multilayer capacitor according to an exemplary embodiment of the present disclosure may have at least one improved characteristics of the structural stability and the electrical characteristics. 
     While the exemplary embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present disclosure as defined by the appended claims.