Abstract:
A method for producing a capacitor includes: a dielectric body composed of basic alumina powder mixed with tungsten powder, interposed between a pair of electrodes disposed opposite to each other. The method includes the steps of: after a conductive alumina ceramic body has been formed by mixing the tungsten powder with the alumina powder, providing a pair of electrodes while interposing the conductive alumina ceramic body therebetween, and intermittently applying an electric current or voltage to the electrodes, high enough to break conductive paths formed in the conductive alumina ceramic body by the tungsten powder, so that the conductive ceramic body is converted to the dielectric body.

Description:
This application is a divisional of application Ser. No. 08/907,915, filed Aug. 11, 1997, now U.S. Pat. No. 5,903,429, which is a continuation of application Ser. No. 08/615,435, filed Mar. 14, 1996, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a capacitor, a method for producing the same and method for producing a dielectric body. Particularly, this invention relates to a capacitor wherein a dielectric body formed of a basic dielectric substance mixed with conductive substance is interposed between a pair of electrodes, a method for producing the same and a method for producing a dielectric body obtained by mixing conductive substance in a basic dielectric substance. 
     2. Description of the Related Art 
     A capacitance of a capacitor is given by the following formula: 
     
       
           C= (ε 0 ) (ε 1 ) ( S/d ) 
       
     
     wherein C: capacitance of capacitor 
     ε 0 : relative dielectric constant in vacuum 
     ε 1 : relative dielectric constant of dielectric body 
     S: area of dielectric body 
     d: thickness of dielectric body 
     To increase the capacitance (C) of the capacitor, it is necessary to enlarge the area (S) of the dielectric body or reduce the thickness (d) of the dielectric body. 
     In Japanese Unexamined Patent Publication (Kokai) No. 3-87091, a method for producing an alumina ceramic circuit board is proposed wherein a green sheet formed of aluminum powder as a ceramic material added with molybdenum powder and/or tungsten powder in a range between 5 and 50% by weight is fired at a predetermined temperature. 
     According to the method disclosed in this Patent Publication, it is possible to easily form a capacitor within the alumina ceramic circuit board. 
     In this production method, as an amount of molybdenum powder and/or tungsten powder gradually increases, the relative dielectric constant of the resultant ceramic dielectric body also increases. 
     However, if the added amount of conductive substance exceeds 50% by weight, a resistance of the obtained ceramic body suddenly decreases to exhibit electrical conductivity. 
     Accordingly, in the method disclosed in the Patent Publication, it is possible to increase the mixed amount of a conductive substance by more than 50% by weight, whereby the relative dielectric constant of the resultant dielectric body is at most 17 or so. 
     On the other hand, there has been a great demand for a dielectric body having a higher dielectric constant in a circuit board of alumina ceramic or the like. 
     If the dielectric body having a higher dielectric constant could be formed by mixing the conductive substance in the dielectric substance, an effective means would be obtained for forming a capacitor having a high capacitance in the circuit board. 
     SUMMARY OF THE INVENTION 
     Thus, an object of the present invention is to provide a capacitor formed of a dielectric body having a high dielectric constant, obtained by the mixture of a dielectric substance and a conductive substance. 
     Inventors of the present invention studied to achieve the above object. As a result, a conductive ceramic body was molded by firing, at a predetermined temperature, a green sheet formed of alumina powder mixed with tungsten powder of 60% by weight. Then, a pair of electrodes were provided opposite to each other while interposing the molded ceramic body. It was found that the ceramic body is converted to a dielectric body by applying a high current or voltage in a pulsed manner to the electrodes. Thus, the present invention was completed. 
     That is, the present invention is a capacitor comprising a dielectric body interposed between a pair of electrodes disposed opposite to each other; said dielectric body being formed of a basic dielectric substance mixed with a conductive substance, characterized in that the dielectric body is one wherein conductive paths provided by the conductive substance mixed with the dielectric substance are electrically broken. 
     Also, the present invention is a method for producing a capacitor comprising a dielectric body composed of a basic dielectric substance mixed with a conductive substance, interposed between a pair of electrodes disposed opposite to each other, characterized in that the method comprises the following steps of: 
     after a conductive body has been formed by mixing the conductive substance with the dielectric substance, providing a pair of electrodes while interposing the conductive body therebetween, and 
     intermittently applying an electric current or voltage to the electrodes, high enough to electrically break conductive paths formed in the conductive body by the conductive substance, so that the conductive body is converted to the dielectric body. 
     In addition, the present invention is a method for producing a dielectric body wherein a conductive substance is mixed with a basic conductive substance, comprising the steps of: 
     after a conductive body has been formed by mixing the conductive substance with the dielectric substance, interposing the conductive body between a pair of electrodes, and 
     intermittently applying an electric current or voltage to the electrodes, high enough to electrically break conductive paths formed in the conductive body by the conductive substance, so that the conductive body is converted to the dielectric body. 
     According to the present invention having such a structure, it is possible to obtain a ceramic or plastic body exhibiting the conductivity by mixing a conductive substance exceeding 50% by weight or 15.9% by volume with a ceramic material exhibiting the dielectric property. 
     It is also possible to form a conductive body capable of being converted to a dielectric body in a circuit board of alumina ceramic at the same time that the circuit board of alumina ceramic is fired, by using, as ceramic material, alumina powder and, as a conductive substance, tungsten powder, tungsten oxide powder, molybdenum powder or molybdenum oxide powder. 
     According to the present invention, the conductive body is formed by mixing a large amount of a conductive substance with the dielectric substance. Conductive paths are formed within this conductive body. 
     Then, the conductive body is converted to the dielectric body by intermittently applying a high current or a high voltage to the electrodes while interposing the conductive substance between them to electrically break the conductive paths. 
     The dielectric body converted to the conductive body as mentioned above has a higher relative dielectric constant compared with that of a dielectric body formed by mixing therewith a conductive substance of an amount capable of substantially maintaining the insulating property of the dielectric body. 
     It is surmised that the reason therefor is that an extremely thin dielectric layer, compared with a dielectric layer in a dielectric body wherein a smaller amount of conductive substance is mixed with the dielectric substance, is formed on the surface of a particle of the conductive substance forming a conductive path. 
     Thus, according to the present invention, it is possible to obtain the dielectric body having a higher relative dielectric constant even though a dielectric substance having a lower relative dielectric constant is used. 
     A dielectric body forming a capacitor according to the present invention can be obtained by mixing a conductive substance with a dielectric substance. 
     Ceramic materials or organic insulating materials such as plastic or others may be used as the dielectric substance. Particularly, alumina powder, aluminum nitride powder or glass powder can be suitably used as the ceramic material. 
     The conductive substance suitably mixed with such dielectric substances is preferably selected while taking into account the relationship with the dielectric substance. For example, when a dielectric substance capable of forming a ceramic body through a high temperature firing is used, a ceramic material such as alumina powder, or a powder of a metal having a melting point in the vicinity of that of the ceramic material is favorably used, such as tungsten powder, tungsten oxide powder, molybdenum powder or molybdenum oxide powder. 
     On the other hand, when an organic insulating body such as plastic or glass powder is used as the dielectric substance, powder of Au, Cu or Ag is preferably used, which has a lower melting point than that of tungsten powder or the like but is excellent in the electrical conductivity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG.  1 ( a ) is a graph showing the relationship between an amount of tungsten (W) to be mixed and the volume resistivity of the resultant ceramic body; 
     FIG.  1 ( b ) is a graph representing the relationship between the amount of tungsten, and the relative dielectric constant (ε) and the dielectric loss tangent (tan δ) of the resultant alumina ceramic body; 
     FIG. 2 is a side sectional view of the resultant conductive body; 
     FIG. 3 is an illustration for explaining the internal structure of a dielectric body obtained by the intermittent application of a high current or voltage to electrodes disposed opposite to each other while interposing a conductive body between them; 
     FIG. 4 is an illustration for explaining a method for forming a capacitor in a circuit board; 
     FIG. 5 is a cross-sectional view showing an example of conductive substance; 
     FIG. 6 is a graph representing the relationship between a frequency of the direct current applied to the ceramic body,.and the relative dielectric constant (ε) and tan δ (dielectric loss tangent) of the resultant alumina ceramic body; and 
     FIG. 7 is a graph representing the relationship between a frequency of the direct current, and the relative dielectric constant (ε) and tan δ of the capacitor in another embodiment, 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to the present invention, it is important that the conductive substance of such an amount that the conductivity can be exhibited is mixed in the dielectric substance. 
     The relationship between the amount of the conductive substance and the resultant volume resistivity of the mixture is shown in FIG.  1 ( a ). FIG.  1 ( a ) is a graph representing the relationship between an amount of tungsten (W) powder used as a conductive substance to be mixed with alumina powder used as dielectric substance and the volume resistivity of the resultant alumina ceramic body obtained by firing a green sheet formed of the tungsten powder mixed with the alumina powder. 
     As present from FIG.  1 ( a ), when an amount of the tungsten powder to be mixed with the alumina powder is 50% by weight or less, the resultant alumina ceramic body is a dielectric body exhibiting a high volume resistivity. 
     While, if an amount of the tungsten powder to be mixed with the alumina powder exceeds 50% by weight, the volume resistivity of the resultant alumina ceramic body is suddenly reduced. Particularly, when an amount of the tungsten powder to be mixed with the alumina powder is 55% by weight or more (especially 60% or more), the resultant alumina ceramic body is a conductive body exhibiting a good conductivity. 
     FIG.  1 ( b ) is a graph representing the relationship between the amount of tungsten (W) % by weight used as the conductive substance to be mixed with alumina ceramic powder used as dielectric substance, and the relative dielectric constant (ε) of and the dielectric loss tangent (δ) of the resultant alumina ceramic body obtained by firing a green sheet formed of the tungsten mixed with the alumina powder. 
     As can be seen from FIG.  1 ( b ), when the amount of tungsten powder is increased, the relative dielectric constant (ε) of the alumina ceramic body is increased. However, when the amount of tungsten powder exceeds 50% by weight, an electrical short circuit may possibly occur in this alumina ceramic body.. This means that the alumina ceramic body will be changed to a conductive substance from the insulation body when an amount of tungsten (W) exceeds that value. Also, during the intermediate region of the amount of tungsten from 15.9% by volume (50% by weight) to 22.1% by volume (60% by weight), i.e., the amount of alumina ceramic from 84.1% by volume (50% by weight) to 77.9% by volume (40 by weight), the difference between the conductive substance and the insulation body will be remarkably represented in the resultant alumina ceramic body. 
     As shown in FIG. 2, the ceramic body has conductive paths consisting of tungsten particles  14 , in a dielectric substance forming the alumina-ceramic body  10  consisting of alumina particles  12 , Thereby, the alumina ceramic body  10  exhibits the electrical conductivity. 
     Electrodes  16  and  18 , shown in FIG. 2, are provided while interposing the alumina ceramic body  10  between them both and are formed of metalized layers which are obtained by firing tungsten paste or the like coated on the surface of a green sheet, which firing is carried out simultaneously with the firing of the green sheet to form the alumina ceramic body  10 . 
     In this regard, instead of forming the electrodes  16  and  18  as the metalized layers, it is possible to provide the electrodes  16  and  18  by metallic layers after the alumina ceramic body  10  has been fired. Alternatively, metallic plates may themselves be used as electrodes  16  and  18 . 
     According to the present invention, the conductive alumina ceramic body  10  can be converted to a dielectric body by electrically breaking conductive paths formed in the interior of the alumina ceramic body  10 . 
     This electrical breakage of the conductive paths is carried out by intermittently applying a current or voltage to the electrodes  16  and  18 , high enough to break the conductivity of the alumina ceramic body  10  exhibiting the conductivity. 
     The high current or voltage to be intermittently applied to the electrodes  16  and  18  can be generated by a spot welder generally used for the production of a semiconductor device or the like. 
     In this regard, the continuous application of the high current or voltage to the electrodes  16  and  18  is not favorable because of heat generation in the ceramic body  10 . Particularly, if an organic insulating body such as plastic is used, voids may be formed due to partial melting of the organic insulating body. 
     It is preferable to preliminarily determine, through experiments, a magnitude of current or voltage to be applied in accordance with kinds, shapes, sizes, mixing ratios or others of the conductive substance to be mixed. 
     The alumina ceramic body  10 , wherein the conductivity has been broken in such a manner, is capable of providing a capacitor having a larger capacitance, because it is converted to a dielectric body having a higher relative dielectric constant compared to that described in the above-described Patent Publication wherein a conductive substance of 50% by weight or less is mixed in a dielectric substance. 
     Also, it is possible to obtain a dielectric body having a high relative dielectric constant by using, as a dielectric substance, ceramic powder having a low relative dielectric constant. 
     It is surmised that the reason why such an alumina ceramic body  10  exhibits a high relative dielectric constant is that, when the high current or voltage is applied to tungsten particles  14 , forming conductive paths, parts of the tungsten particles  14 , forming the conductive paths sequentially fuse while remaining extremely thin dielectric layers  20 , on boundary surfaces of the respective particles, as shown in FIG.  3 . 
     The capacitor described above can be easily formed in a circuit board. For example, as shown in FIG. 4, a conductive ceramic layer  36 , with which conductive substance of an amount enough to exhibit the conductivity is mixed is provided between ceramic layers  32  and  34  forming a circuit board  30 . 
     The conductive ceramic layer  36  is sandwiched between conductive layers  38  and  40  which, in turn, are connected to electrodes  46  and  48  through via-holes  42  and  44 , respectively. 
     If a current or voltage high enough to break the conductivity of the conductive paths formed in the conductive ceramic layer  36  is intermittently applied between the electrodes  46  and  48 , a portion of the conductive ceramic layer  36  between the electrodes  46  and  48  is converted to a dielectric body having a high relative dielectric constant which provides a capacitor having a high capacitance in the circuit board  30 . 
     In FIG. 4, since the conductive ceramic layer  36  is sandwiched between the conductive layers  38  and  40 , the smaller an area of the conductive layer  38  or  40  is, the narrower the portion of the conductive ceramic layer  36  forming the capacitors is. In such a manner, according to the present invention, it is possible to adjust the size of the capacitor. 
     Although metallic powders such as tungsten powder, used as a conductive substance in the above description, it is also possible, as shown in FIG. 5, to use dielectric particles  50 , such as alumina or glass, coated with a metallic layer  52  or with a metallic powder. 
     An example of the present invention will be described in more detail as follows. 
     EXAMPLE 1 
     (1) Production of a Conductive Body 
     A green sheet was prepared by a conventional method from ceramic powder containing alumina powder and a firing agent or others mixed with tungsten powder having a particle size of 1.1 μm of 60% by weight (relative to the ceramic powder). 
     Then, the green sheet was cut into plate-like pieces having a predetermined dimension. A tungsten paste was then coated on both sides of the plate-like piece. 
     Thereafter, the piece was fired at 1550° C. to form a rectangular ceramic body of 4 mm long, 4 mm wide and 0.65 mm thick, which has metalized layers on both sides thereof, as shown in FIG.  2 . The ceramic body has the volume resistivity of 12×10 −3  Ω·cm and is an electrically conductive body. 
     (2) Conversion to a Dielectric Body 
     A direct current of 50 W/sec (DC 20 V) was applied once to electrodes of the resultant conductive body by a spot welder. By the application of DC current, the ceramic body was converted to a dielectric body having the volume resistivity of 1.2×10 9  Ω·cm. 
     Measurements of the volume resistivity were carried out while repeating the intermittent application of DC current of 50 W/sec (DC 20 V) one hundred times. The volume resistivity varied in a range between 1.2×10 9  Ω·cm and 1.6×10 9  Ω·cm. 
     Then, the apparent relative dielectric constant (ε) was measured on a capacitor formed of a portion of the dielectric body thus obtained between the electrodes. The measured values were in a range between 400 through 500. 
     This relative dielectric constant (ε) is much higher than that of the alumina forming the dielectric body. The apparent relative dielectric constant (ε) of alumina is about 8 to 10. 
     FIG. 6 is a graph representing the relationship between a frequency (Hz) of the direct current applied to the ceramic body and the relative dielectric constant (ε) and the dielectric loss tangent (tan δ) of the resultant alumina ceramic body. In the experiment a DC current of 50 W/sec (DC 20 V) was applied to the resultant alumina ceramic body one hundred times and fifty times, respectively, and such an experiment was measured twice. This graph shows that the alumina ceramic body obtained in accordance with the present invention has favorable characteristics as a capacitor. 
     EXAMPLE 2 
     A capacitor was obtained in the same manner as Example 1, except that an epoxy resin (ECON-1045, Nihonkayaku, Kabushiki Kaisha), having a softening point of 90° C. and dielectric constant (ε) of about 4, was used as a dielectric material and copper (Cu) powder was used as a conductive material. Thus, the Cu powder and the epoxy resin were mixed in a water bath as known in the prior art. 
     FIG. 7 is a graph representing the relationship between the frequency (Hz) of the direct current applied to the resultant capacitor obtained in Example 2 and the relative dielectric constant (ε) and the dielectric loss tangent (tan δ) thereof. In the experiment a DC current of 50 W/sec (DC 20 V) was applied to the resultant capacitor one hundred times. The other conditions were the same as the above-mentioned Example 1. FIG. 7 shows that the capacitor obtained from epoxy resin mixed with Cu powder in accordance with Example 2 also has favorable characteristics as a capacitor. 
     Comparative Example 
     A ceramic body was obtained in the same manner as in Example 1, except that an added amount of tungsten powder is 50% by weight. The resultant ceramic body was a dielectric body having the volume resistivity of 1.0×10 14  Ω·cm. 
     Accordingly, without the application of DC current by a spot welder, the apparent relative dielectric constant (ε) of this dielectric body was measured, as about 17. 
     According to the present invention, since a dielectric body is obtained having a higher relative dielectric constant than that of a dielectric substance used therefor, it is possible to form a high capacitance capacitor within a circuit board without changing kinds, molding conditions or others of a dielectric substance conventionally used.