Abstract:
A multilayer ceramic electronic component achieves a high electrostatic capacitance and includes an Al inner electrode superior in smoothness and conductivity. The multilayer ceramic electronic component includes a multilayer body including a plurality of stacked ceramic layers and a plurality of inner electrodes arranged along certain interfaces between the ceramic layers and containing Al as a main component, and an outer electrode located on an outer surface of the multilayer body. A surface of the inner electrode is covered with a layer including a noble metal or Ti as a main component.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a multilayer ceramic electronic component such as a multilayer ceramic capacitor, for example, and more particularly to a multilayer ceramic electronic component including an inner electrode including Al as a main component. 
         [0003]    2. Description of the Related Art 
         [0004]    Firstly, referring to  FIG. 1 , a multilayer ceramic capacitor  1  as a typical example of a multilayer ceramic electronic component will be described. 
         [0005]    Multilayer ceramic capacitor  1  includes a multilayer body  2  constituted by stacked dielectric ceramic layers  3  and a plurality of inner electrodes  4  and  5  formed along certain interfaces between dielectric ceramic layers  3 . 
         [0006]    In multilayer ceramic capacitor  1  shown in  FIG. 1 , a first outer electrode  8  and a second outer electrode  9  are formed respectively on opposing end surfaces of multilayer body  2 . Inner electrodes  4  and  5  include a plurality of first inner electrodes  4  electrically connected to first outer electrode  8  and a plurality of second inner electrodes  5  electrically connected to second outer electrode  9 . These first inner electrodes  4  and second inner electrodes  5  are arranged alternately in the layered direction. 
         [0007]    Since miniaturization of a multilayer ceramic capacitor is particularly demanded, a method of layering a green sheet of dielectric ceramic and an inner electrode layer and thereafter firing those concurrently is employed in the manufacturing process. In recent years, a base metal such as Ni is employed in an inner electrode of a multilayer ceramic capacitor for cost reduction. 
         [0008]    However, since Ni is very liable to be oxidized when co-sintered with ceramic, it was necessary to precisely control a temperature condition and an oxygen partial pressure under a reductive atmosphere as an atmosphere in the stage of firing. As a result, the material design was greatly limited. Additionally, the problems of delaminations, cracks, and the like due to an uneven stress along with co-firing occurred. 
         [0009]    Therefore, inner electrodes of various metal species were considered to try to improve a degree of freedom in designing a multilayer ceramic electronic component. 
         [0010]    For example, Japanese Patent Laying-Open No. 2011-97016 describes a multilayer ceramic body employing Al in place of Ni as an inner electrode material, and mechanical characteristics and electrical characteristics are improved by forming an Al 2 O 3  film on a surface portion of the Al inner electrode. 
         [0011]    A larger capacitance is desired increasingly for a multilayer ceramic capacitor as a typical multilayer ceramic electronic component. In such a case, the multilayer ceramic electronic component disclosed in Japanese Patent Laying-Open No. 2011-97016 had a problem that an electrostatic capacitance of the multilayer ceramic electronic component becomes scant due to low permittivity of the Al 2 O 3  film present in the interface between the ceramic layer and the Al inner electrode. 
       SUMMARY OF THE INVENTION 
       [0012]    In view of the above, preferred embodiments of the present invention provide a multilayer ceramic electronic component exhibiting a high electrostatic capacitance while having an Al inner electrode superior in smoothness and conductivity. 
         [0013]    In other words, various preferred embodiments of the present invention relate to a multilayer ceramic electronic component including a multilayer body including stacked ceramic layers and a plurality of inner electrodes arranged along certain interfaces between the ceramic layers and containing Al as a main component, and an outer electrode located on an outer surface of the multilayer body, and a surface of the inner electrode is covered with a layer containing a noble metal or Ti as a main component. 
         [0014]    Preferably, the noble metal is Ag, for example. 
         [0015]    Further, preferably, the main component of the ceramic layer is a titanate barium-based perovskite compound, and the multilayer ceramic electronic component is a multilayer ceramic capacitor. In this case, a permittivity of the ceramic layer becomes high and contributes to the improvement of an electrostatic capacitance. 
         [0016]    Further, preferably, the ceramic layer further includes oxide containing Bi such as Bi 2 O 3  as an accessory component. In this case, low-temperature sintering can be performed, so that coverage of the inner electrode is improved to contribute to the improvement of the electrostatic capacitance. 
         [0017]    According to various preferred embodiments of the present invention, since a thickness of an Al 2 O 3  layer located on a surface portion of the Al inner electrode is significantly reduced to a very low thickness or a minimum thickness required to maintain the mechanical characteristics, an influence exerted to the electrostatic capacitance by low permittivity of the Al 2 O 3  layer is reduced. Therefore, a multilayer ceramic electronic component having a high electrostatic capacitance is provided. 
         [0018]    The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  represents a cross section of a multilayer ceramic capacitor as an example of a multilayer ceramic electronic component of the present invention. 
           [0020]      FIG. 2  represents an enlarged view of a cross section of the multilayer ceramic capacitor as an example of the multilayer ceramic electronic component according to a preferred embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    In a multilayer ceramic electronic component according to a preferred embodiment of the present invention, an inner electrode thereof contains Al as a main component. This inner electrode may be made of either Al alone or an Al alloy, for example. In the case of an Al alloy, a content ratio of Al is preferably about 70% by mole or higher, and more preferably about 90% by mole or higher, for example. 
         [0022]      FIG. 2  represents an enlarged view of a cross section of the multilayer ceramic capacitor as an example according to a preferred embodiment of the present invention. As to the inner electrodes, a periphery of inner electrode  4  is illustrated as a representative among inner electrodes  4  and  5 . 
         [0023]    On the surfaces of inner electrode  4  containing Al as a main component, Al 2 O 3  layers  21  are often present. This is mainly because the surfaces of Al inner electrode  4  are oxidized. Al 2 O 3  layers  21  serve to smooth Al inner electrode  4 , thus preventing or significantly reducing delamination between ceramic layers  3  and Al inner electrode  4 . In view of the above, the thickness of Al 2 O 3  layer  21  is preferably about 0.25% or greater of the thickness of the inner electrode, for example. However, as previously described, it is preferable to significantly reduce the thickness of Al 2 O 3  layer  21  to be as small as possible to secure an electrostatic capacitance. 
         [0024]    Further, at interfaces between the Al inner electrode  4  and ceramic layers  3 , layers  22  including a noble metal or Ti as a main component are preferably provided. Layers  22  containing a noble metal or Ti as a main component prevent or significantly reduce oxidization of Al inner electrode  4  and prevent or significantly reduce an increase in the thickness of Al 2 O 3  layer  21 . Although Ag is known as a typical noble metal, other metals such as Pd, Au, Pt, or an alloy of these metals may be used, for example. 
         [0025]    Next, an example of a method for manufacturing a multilayer ceramic electronic component according to a preferred embodiment of the present invention will be described referring to a multilayer ceramic capacitor as an example. 
         [0026]    Firstly, ceramic raw material is prepared. This ceramic raw material is mixed with an organic binder component in a solvent as needed to obtain a ceramic slurry. This ceramic slurry is shaped into a sheet to obtain a ceramic green sheet. 
         [0027]    Next, layer  22  containing a noble metal or Ti as a main component is formed on the ceramic green sheet. Various methods may be used for this procedure, and the method of performing screen-printing to allow paste containing a noble metal powders or Ti powders and organic vehicle to have a desired pattern is convenient. Methods other than those described above may be used, such as the method of transferring foil prepared in advance or the method of forming a film with use of the vacuum thin film forming method. 
         [0028]    Next, a layer of Al inner electrode  4  is formed on layer  22  containing a noble metal or Ti as a main component. This procedure may also include the method of applying Al paste, the method of transfer, the vacuum thin film forming method, and the like. 
         [0029]    Then, layer  22  containing a noble metal or Ti as a main component is formed again on the layer of Al inner electrode  4 . 
         [0030]    As described above, a plurality of layers are stacked in the order of the ceramic green sheet, layer  22  containing noble metal or Ti as a main component, layer  4  of the Al inner electrode, and layer  22  containing a noble metal or Ti as a main component, and then the layers are press-bonded, so that a non-fired raw multilayer body can be obtained. 
         [0031]    This raw multilayer body is fired in a furnace under a predetermined atmosphere and temperature. For example, when an oxygen partial pressure of 1×10 −4  MPa or higher and a firing temperature of 600° C. or higher are provided during the firing, oxidization on the surface of Al inner electrode  4  progresses, so that Al 2 O 3  layer  21  having an appropriate thickness is created. Further, for example, when the firing temperature of 1000° C. or lower is provided, spheroidizing of Al inner electrodes  4  is effectively prevented. With regard to the oxygen partial pressure, an atmospheric pressure is the most preferable, taking in consideration the convenience of the procedures. 
         [0032]    In this stage, layer  22  containing a noble metal or Ti as a main component reduces contact between Al inner electrodes and oxygen, so that oxidization of Al inner electrode  4  is suppressed to a desired extent or smaller. When Ti is used for layers  22  containing a noble metal or Ti as a main component, the TiO 2  layer may be formed on the surface of Ti by sintering. However, it does not exert any influence on lowering of the electrostatic capacitance since this TiO 2  layer has a low insulation property. 
         [0033]    The ceramic composition in the multilayer ceramic electronic component according to a preferred embodiment of the present invention is not particularly limited. Various materials such as a titanate barium-based material (including those substituted by Ca, Sr, Zr, and the like), a lead titanate-based material, a lead titanate zirconate-based material, or an alumina-based glass ceramic material, a ferrite, a transition-metal oxide-based semiconductor ceramic material, and the like may be used within a range that still achieves the advantages of preferred embodiments of the present invention. 
         [0034]    The multilayer ceramic electronic component of the present invention is not limited to a multilayer ceramic capacitor, and it is also applicable to various electronic components such as a multilayer piezoelectric element, a multilayer thermistor element, a multilayer chip coil, a ceramic multilayer substrate, and the like, for example. 
       EXAMPLES 
       [0035]    The present example shows influence of a metal species and thickness of a layer covering the Al inner electrode in an example of a multilayer ceramic capacitor including titanate barium-based ceramic and Al inner electrode according to a preferred embodiment of the present invention. 
         [0036]    Firstly, BaTiO 3  powder was prepared as a main component of ceramic, and powders of Bi 2 O 3  and BaCO 3  were prepared as auxiliary components. These powders were mixed to have the composition of 100BaTiO 3 +3Bi 2 O 3 +2BaCO 3  to obtain ceramic raw material. 
         [0037]    An ethanol-based organic solvent and a polyvinyl butyral-based binder were added to the ceramic raw material and wet-mixed in a ball mill to obtain a ceramic slurry. This ceramic slurry was shaped into a sheet, so that a ceramic green sheet was obtained. 
         [0038]    Next, films of metals shown in Table 1 were formed on the ceramic green sheet by the vacuum vapor deposition method so as to have the thicknesses shown in Table 1. Then, the layers of the Al inner electrode were formed by the vacuum vapor deposition to have a thickness of 0.6 μm. Further, the films of metals shown in Table 1 were formed thereon similarly by the vacuum vapor deposition method to have the thicknesses shown in Table 1. 
         [0039]    The green sheets obtained in this manner, having the Al inner electrode covered with the metal film shown in Table 1, were layered so that the drawn sides of the Al inner electrodes are alternated, and then press-bonded. Accordingly, a raw multilayer body was obtained. 
         [0040]    This raw multilayer body was heated in the atmosphere at 270° C. to remove the binder. Thereafter, the temperature was raised at a temperature rising rate of 100° C./min, and firing was performed at 650° C. for 1 hour. Ag paste containing a low melting point glass frit was applied to both end surfaces of the obtained multilayer body, and baked at 600° C. in the atmosphere to have outer electrodes connected with the inner electrodes. 
         [0041]    The multilayer ceramic capacitor obtained in the manner described above had a length of about 2.0 mm, a width of about 1.0 mm, and a thickness of about 0.5 mm, for example. A ceramic layer thickness was about 50 μm, for example. A thickness of the Al inner electrode layer was about 0.6 μm, for example. The number of effective layers was 5, for example. 
         [0042]    As to the obtained test samples, an electrostatic capacitance was measured with use of an automatic bridge-type measuring instrument. The result is shown in Table 1. Further, the cross section obtained by the FIB processing is analyzed using the μ-SAM, and the Al 2 O 3  layer at the cross section of the inner electrode was identified. The thickness of the Al 2 O 3  layer was measured at appropriate ten points, and the average thickness was calculated. The results are shown together in Table 1. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Metal 
                 Covering 
                   
                   
               
               
                   
                 Species of 
                 Thickness 
                 Thickness 
                 Electrostatic 
               
               
                 Test 
                 Covering 
                 of Covering 
                 of Al 2 O 3   
                 Capacitance 
               
               
                 Sample No. 
                 Layer 
                 Layer 
                 Layer 
                 (pF) 
               
               
                   
               
             
             
               
                 1 
                 None 
                 — 
                 60 nm 
                 2318 
               
             
          
           
               
                 2 
                 Ag 
                 20 
                 nm 
                 20 nm 
                 2738 
               
               
                 3 
                 Ag 
                 10 
                 nm 
                 20 nm 
                 2725 
               
               
                 4 
                 Ag 
                 5 
                 nm 
                 20 nm 
                 2713 
               
               
                 5 
                 Ti 
                 20 
                 nm 
                 20 nm 
                 2732 
               
               
                 6 
                 Ti 
                 5 
                 nm 
                 20 nm 
                 2701 
               
               
                 7 
                 Ni 
                 20 
                 nm 
                 60 nm 
                 2327 
               
               
                   
               
             
          
         
       
     
         [0043]    According to the result of Table 1, the Al inner electrode having a high electrostatic capacitance was obtained in the test samples of Test Sample Nos. 2-6 covered with Ag and Ti. 
         [0044]    As to the test sample of Test Sample No. 1 having the Al inner electrode not covered with metal, expected electrostatic capacitance could not be obtained due to the influence of large thickness of the Al 2 O 3  layer. 
         [0045]    As to the test sample of Test Sample No. 7 having the Al inner electrode covered with Ni, expected electrostatic capacitance could not be obtained. It seems that such result was obtained because all of the Ni as the covering metal was oxidized, and oxidization of the Al inner electrode progressed therefrom, so that the thickness of the Al 2 O 3  layer became greater as a result. 
         [0046]    A multilayer ceramic electronic component according to various preferred embodiments of the present invention is applicable to a multilayer ceramic capacitor, a multilayer piezoelectric element, a multilayer thermistor, a multilayer chip coil, a ceramic multilayer substrate, and the like, for example. 
         [0047]    While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.