Abstract:
A ferrite substrate for thin-film inductors is provided by means of blending raw materials to meet a composition of di-iron trioxide (Fe 2 O 3 ): 40 to 55 mol %, nickel oxide (NiO): 5 to 35 mol %, zinc oxide (ZnO): 10 to 40 mol %, and bismuth trioxide (Bi 2 O 3 ): 150 to 750 ppm, or of Fe 2 O 3 : 40 to 55 mol %, NiO: 5 to 35 mol %, ZnO: 10 to 40 mol %, cupric oxide (CuO): 5 to 10 mol %, and manganese dioxide (MnO 2 ): 0.5 to 2 mol %, and then molding and sintering the blended material, and applying hot isostatic pressing to the sintered article. A thin-film common mode filter and a thin-film common mode filter array using the ferrite substrate and the manufacturing method of the substrate are also provided.

Description:
PRIORITY CLAIM 
     This application claims priority from Japanese patent application No. 2003-163563, filed on Jun. 9, 2003, which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a ferrite substrate for thin-film inductors, to a thin-film common mode filter using the substrate, to a thin-film common mode filter array using the substrate, and to a manufacturing method of the substrate. 
     2. Description of the Related Art 
     Common mode filter is a device for suppressing common mode currents that cause electromagnetic interference in parallel transmission lines. The common mode filter has magnetically coupled inductors to remove in-phase noise component. 
     Thin-film common mode filter miniaturized and highly integrated by forming bilayered thin-film coils between ferrite substrates and by constructing in chip form, and thin film common mode filter array on which a plurality of the filters are mounted, are described in for example, Japanese Patent Publications Nos. 08-203737A and 11-054326A. 
     Generally, such a ferrite substrate is produced by hot forming press method where a hot-pressed block is cut out into substrates with a desired shape and the substrates are then lapped and formed, or by sheet manufacturing method where sheeted ferrites are stacked and pressed with heat and the stacked ferrite is then lapped and formed into a desired shape. 
     In the thin-film common mode filter, coils are disposed closely to each other in order to satisfy it&#39;s characteristic request and high voltage is applied to these coils. Thus, such filter is required to have high withstand voltage and high reliability in electrical isolation. Also, required is that terminals of the filter should be electrically isolated with each other and formed finely without causing electrical isolation failure between coils. Furthermore, the filter should have miniaturized coils and ferrite substrates with a permeability of about 100-400 in order to be operable at a high frequency (several GHz) band. 
     Conventional ferrite substrate for the thin-film common mode filter, however, has a porous crystalline structure with voids and such on its surface, which causes low insulation resistance on its surface and large surface-degradation. The ferrite substrate, therefore, has too poor mechanical strength to undergo thin-film process, and moreover, it has been difficult to form precisely the terminals on the substrate surface. 
     BRIEF SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a ferrite substrate for thin-film inductors, with a higher surface insulation resistance and less surface-degradation, a thin-film common mode filter using the substrate, a thin-film common mode filter array using the substrate, and a manufacturing method of the substrate. 
     Another object of the present invention is to provide a ferrite substrate for thin-film inductors with a high mechanical strength, a thin-film common mode filter using the substrate, a thin-film common mode filter array using the substrate, and a manufacturing method of the substrate. 
     A further object of the present invention is to provide a ferrite substrate for thin-film inductors, on a surface of which terminals can be precisely formed without difficulty, a thin-film common mode filter using the substrate, a thin-film common mode filter array using the substrate, and a manufacturing method of the substrate. 
     According to the present invention, a ferrite substrate for thin-film inductors is provided, which contains a ferrite composition of di-iron trioxide (Fe 2 O 3 ): 40 to 55 mol %, nickel oxide (NiO): 5 to 35 mol %, zinc oxide (ZnO): 10 to 40 mol %, and bismuth trioxide (Bi 2 O 3 ): 150 to 750 ppm, or of Fe 2 O 3 : 40 to 55 mol %, NiO: 5 to 35 mol %, ZnO: 10 to 40 mol %, cupric oxide (CuO): 5 to 10 mol %, and manganese dioxide (MnO 2 ): 0.5 to 2 mol %, and which has a densified crystalline structure developed by hot isostatic pressing (HIP). Also, a thin-film common mode filter and a thin-film common mode filter array, which are produced from a part of the substrate, are provided. 
     More preferably, the substrate contains a ferrite composition of Fe 2 O 3 : 40 to 55 mol %, NiO: 15 to 30 mol %, ZnO: 20 to 40 mol %, and Bi 2 O 3 : 150 to 750 ppm, or of Fe 2 O 3 : 40 to 55 mol %, NiO: 15 to 30 mol %, ZnO: 20 to 40 mol %, CuO: 5 to 10 mol %, and MnO 2 : 0.5 to 2 mol %. 
     By being given the crystalline structure densified by HIP with the above-mentioned ferrite composition, the substrate achieves high surface insulation resistance of 2×10 10  Ω-cm or more, and the common mode filter produced from the substrate can acquire enough electrical isolation between the coils. And, there is no change (degradation) in bulk insulation resistance and surface insulation resistance of the substrate after being annealed in the thin film process. Further, mechanical strength (bending strength) of the substrate is enhanced to the value at least 1.5 times larger than that of substrate produced by conventional hot forming press method, which is a enough strength for the substrate to undergo the thin film process. Furthermore, the densified substrate-surface with almost no voids can prevent electrical trouble due to plating on unwanted portion when the terminals and the like are formed by plating. In addition, the terminal patterns are able to being formed precisely because of the densified substrate-surface. 
     Preferably, the substrate is a wafer with diameter of 3 inches or more. 
     According to the present invention, a manufacturing method of a ferrite substrate for thin film inductors is further provided, which includes a step of blending, and adding if needed, raw materials to meet a composition of Fe 2 O 3 : 40 to 55 mol %, NiO: 5 to 35 mol %, ZnO: 10 to 40 mol %, and Bi 2 O 3 : 150 to 750 ppm, or of Fe 2 O 3 : 40 to 55 mol %, NiO: 5 to 35 mol %, ZnO: 10 to 40 mol %, CuO: 5 to 10 mol %, and MnO 2 : 0.5 to 2 mol %, and a step of molding and sintering the blended material, and then applying HIP to the sintered article. 
     More preferably, the method includes a step of blending raw materials to meet a composition of Fe 2 O 3 : 40 to 55 mol %, NiO: 15 to 30 mol %, ZnO: 20 to 40 mol %, and Bi 2 O 3 : 150 to 750 ppm, or of Fe 2 O 3 : 40 to 55 mol %, NiO: 15 to 30 mol %, ZnO: 20 to 40 mol %, CuO: 5 to 10 mol %, and MnO 2 : 0.5 to 2 mol %. 
     By undergoing HIP, after being set into the above-mentioned ferrite composition and sintered, the obtained substrate achieves a high surface insulation resistance value of 2×10 10  Ω-cm or more, and the common mode filter produced from the substrate can acquire enough electrical isolation between the coils. Further, there is no change or degradation in bulk insulation resistance and surface insulation resistance of the substrate after being annealed in the thin film process. Further, mechanical strength or bending strength of the substrate is enhanced to the value at least 1.5 times larger than that of substrate produced by conventional hot forming press method, which is a enough strength for the substrate to undergo the thin film process. Furthermore, the densified substrate-surface with almost no voids can prevent electrical trouble due to plating on unwanted portion when the terminals and like are formed by plating. In addition, the terminal patterns are able to being formed precisely because of the densified substrate-surface. 
     Preferably, the method further includes a step of annealing the obtained article and surface-lapping the annealed article with the amount of lapping at least 5 μm after applying HIP to the article. 
     Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  shows a flow diagram schematically illustrating some steps in a preferred embodiment of a manufacturing method of a ferrite substrate for thin-film inductors according to the present invention; 
         FIGS. 2   a  to  2   j  show perspective views for explanation of a wafer process to produce a thin-film common mode filter array; 
         FIGS. 3   a  to  3   j  show perspective views for explanation of a working process to produce the thin-film common mode filter array; 
         FIG. 4  shows a graph illustrating common mode characteristic of a thin-film common mode filter; 
         FIG. 5  shows a graph illustrating a relationship among Fe 2 O 3 , NiO and ZnO compositions and permeability μ in the ferrite substrates; 
         FIG. 6  shows a graph illustrating the measurement results of surface insulation resistance of the ferrite substrates containing various Fe 2 O 3  contents after being sintered; 
         FIG. 7  shows a graph illustrating the measurement results of surface insulation resistance of the sintered ferrite substrates shown in  FIG. 6  after being surface-lapped; 
         FIG. 8  shows a graph illustrating the measurement results of surface insulation resistance of the ferrite substrates shown in  FIG. 7  after being annealed 5 times at a curing temperature of insulating layers (about 400° C.); 
         FIG. 9  shows a graph illustrating the measurement results of surface insulation resistance of the ferrite substrates shown in  FIG. 8  after being annealed in vacuum at 1000° C.; 
         FIG. 10  shows a graph illustrating the measurement results of surface insulation resistance of the annealed-in-vacuum ferrite substrates shown in  FIG. 9  after being surface-lapped with the amount of lapping of 5 μm or more; 
         FIG. 11  shows a graph illustrating the measurement results of the relationship between the amount of lapping and surface resistance of the substrate shown in  FIG. 10 ; 
         FIG. 12  shows a graph illustrating the measurement results of surface insulation resistance of the surface-lapped ferrite substrates shown in  FIG. 10  after being annealed 5 times in vacuum at about 400° C.; 
         FIG. 13  shows a graph illustrating the measurement results of surface insulation resistance of the ferrite substrates shown in  FIG. 12  and NiZn-ferrite substrates produced with addition of Bi 2 O 3  to their basic composition; 
         FIG. 14  shows a graph illustrating the measurement results of the relationship between the amount of added Bi 2 O 3  and insulation resistance in the ferrite substrate with Fe 2 O 3  55 mol %; 
         FIG. 15  shows a graph illustrating the measurement results of the relationship between the amount of added Bi 2 O 3  and bending strength in the ferrite substrate with Fe 2 O 3  55 mol %; 
         FIG. 16  shows a graph illustrating the measurement results of the relationship between bending strength and the amount of added CuO in the NiZn-ferrite substrates produced with addition of CuO to their basic composition; 
         FIG. 17  shows a graph illustrating the measurement results of the relationship between permeability μ and the amount of added MnO 2  in the substrates produced with addition of MnO 2  to the composition shown in  FIG. 16 ; 
         FIG. 18  shows a graph illustrating the measurement results of the relationship between insulation resistance and the amount of added MnO 2  in the substrates produced with addition of MnO 2  to the composition shown in  FIG. 16 ; 
         FIG. 19  shows a graph illustrating the measurement results of the relationship between insulation resistance and the amount of added Bi 2 O 3  in the substrates produced by HP and those produced by HIP; 
         FIG. 20  shows a graph illustrating the measurement results the relationship between bending strength and the amount of added Bi 2 O 3  in the substrates produced by HP and those produced by HIP; 
         FIG. 21  shows a graph illustrating the measurement results of the relationship between applied pressure in HIP and bending strength of the substrates; 
         FIG. 22  shows a graph illustrating the measurement results the relationship between applied pressure in HP and bending strength of the substrates; 
         FIG. 23  shows a graph illustrating the measurement results the relationship between density and applied pressure in the ferrite substrates produced by HP and the ferrite substrates produced by HIP, both of which consist basically of Fe 2 O 3 , NiO and ZnO; 
         FIG. 24  shows a graph illustrating the measurement results of the relationship between surface roughness and grit number of the used abrasive grain, in the ferrite substrates produced by HP and the ferrite substrates produced by HIP, both of which consist basically of Fe 2 O 3 , NiO and ZnO; 
         FIG. 25  shows an optical microscope photograph of the surface of the ferrite substrate that was produced by HIP; 
         FIG. 26  shows an optical microscope photograph of the surface of the ferrite substrate that was produced by HP; and 
         FIG. 27  shows an optical microscope photograph of the surface of the ferrite substrate that was produced by sheet manufacturing method. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  schematically illustrates some steps in a preferred embodiment of a manufacturing method of a ferrite substrate for thin-film inductors according to the present invention. The manufacturing steps of the ferrite substrate will be described in detail with reference to the figure hereafter. 
     First Example of the Substrate Composition 
     First, raw materials are weighed according to the blend table so that obtained ferrite substrates will have a predetermined composition, and then are blended by adding purified water (step S 1 ). The predetermined composition is Fe 2 O 3 : 40 to 55 mol %, NiO: 15 to 30 mol %, and ZnO: 20 to 40 mol %. 
     Next, the obtained blended slurry is dried (step S 2 ), and presintered (step S 3 ). 
     Then, the obtained presintered material is milled with purified water (step S 4 ). The milling is accompanied by adding 150 to 750 ppm in Bi 2 O 3 . CaCO 3  and such also may be added. 
     Next, the obtained milled material is dried and granulated, and then is molded (step S 5 ). Further, it is sintered (step S 6 ), in the atmospheric air as combustion gas at about 1160° C. 
     Then, the sintered article undergoes HIP (step S 7 ) that is performed for about 2 hours under the pressure of about 1000 kg/cm 2  at about 1200° C. 
     Then, the obtained article is plane-grinded, shaped and cut (step S 8 ). 
     Thereafter, the cut article is heated or annealed (step S 9 ), in the atmospheric air at about 1000° C. 
     Then, the surface of the annealed article is lapped with the amount of lapping of at least 5 μm, by use of abrasive grain of grit number #2000 (step S 10 ). 
     Second Example of the Substrate Composition 
     First, raw materials are weighed according to the blend table so that obtained ferrite substrates will have a predetermined composition, and then are blended by adding purified water (step S 1 ). The predetermined composition is Fe 2 O 3 : 40 to 55 mol %, NiO: 15 to 30 mol %, ZnO: 20 to 40 mol %, CuO: 5 to 10 mol %, and MnO 2 : 0.5 to 2 mol %. 
     Next, the obtained blended slurry is dried (step S 2 ), and presintered (step S 3 ). 
     Then, the obtained presintered material is milled with purified water (step S 4 ). The milling may be accompanied by adding CaCO 3  and such. 
     Next, the obtained milled material is dried and granulated, and then is molded (step S 5 ). Further, it is sintered (step S 6 ), in the atmospheric air as combustion gas at about 1160° C. 
     Then, the sintered article undergoes HIP (step S 7 ) that is performed for about 2 hours under the pressure of about 1000 kg/cm 2  at about 1200° C. 
     Then, the obtained article is plane-grinded, shaped and cut (step S 8 ). 
     Thereafter, the cut article is heated or annealed (step S 9 ), in the atmospheric at 1000° C. 
     Then, the surface of the annealed article is lapped with the amount of lapping of at least 5 μm, by use of abrasive grain #2000 (step S 10 ). 
     By undergoing HIP, after being set into the above-mentioned ferrite composition and sintered, and by being annealed and surface-lapped as mentioned above, the obtained substrate achieves a high surface insulation resistance value of 2×10 10  Ω-cm or more. Further, there is no change (degradation) in bulk insulation resistance and surface insulation resistance in the substrate after being annealed in the thin film process thereafter. Further, mechanical strength (bending strength) of the substrate is enhanced to the value at least 1.5 times larger than that of substrate produced by conventional hot forming press method, which is enough strength for the substrate to undergo the thin film process. Furthermore, the substrate surface becomes densified with almost no voids, as well as the surface in the production process. 
       FIGS. 2   a  to  2   j  and  FIGS. 3   a  to  3   j  show perspective views for explanation of the wafer process and the working process to produce a thin film common mode filter array that consists of two coupled thin film common mode filters, fabricated from the above-mentioned ferrite substrate. In  FIGS. 2   a - 2   j  and  FIGS. 3   a - 3   j , the lower parts of the view show a wafer, and the upper parts show individual chips that are not actually cut to separate. The manufacturing process of the thin film common mode filter array will be detailed by these figures hereafter. 
     First, as shown in  FIG. 2   a , a ferrite wafer that was fabricated by the manufacturing method of  FIG. 1  is prepared, and, as shown in  FIG. 2   b , a first insulating layer  21 , made of such as polyimide resin, is coated on the wafer  20 , and is then patterned. 
     Next, as shown in  FIG. 2   c , first leads and electrodes of a copper layer  22  are formed on the first insulating layer  21 . Then, as shown in  FIG. 2   d , a second insulating layer  23 , made of such as polyimide resin, is coated thereon, and patterned. 
     Then, as shown in  FIG. 2   e , first coils of a copper layer  24  are formed on the second insulating layer  23 . Then, as shown in  FIG. 2   f , a third insulating layer  25 , made of such as polyimide resin, is coated thereon, and patterned. 
     Then, as shown in  FIG. 2   g , second coils of a copper layer  26  are formed on the third insulating layer  25 . Then, as shown in  FIG. 2   h , a fourth insulating layer  27 , made of such as polyimide resin, is coated thereon, and patterned. 
     Then, as shown in  FIG. 2   i , second leads of a copper layer  28  are formed on the fourth insulating layer  27 . Then, as shown in  FIGS. 2   j  and  3   a , a fifth insulating layer  29 , made of such as polyimide resin, is coated thereon, and patterned. 
     After that, as shown in  FIG. 3   b , a silver paste  30  is screen-printed on the leads. Then, as shown in  FIG. 3   c , a ferrite paste  31  for flux return portion is embedded in the core portions. 
     Then, as shown in  FIG. 3   d , a ferrite plate cover  32  is bonded on the processed wafer with adhesive. 
     Then, as shown in  FIG. 3   e , the obtained wafer is cut into bars  33  on each of which a plurality of thin film common mode filter array chips are aligned. 
     Then, as shown in  FIG. 3   f , a mark  34  is printed on the upper side of each of the thin film common mode filter array chips in the bar  33 . Then, as shown in  FIG. 3   g , electrode terminals  35  of Nickel are formed by sputtering on the side of each of the thin film common mode filter array chips in the bar  33 . 
     After that, as shown in  FIG. 3   h , each bar is cut to separate into individual chips  36 . Then, as shown in  FIG. 3   i , the electrode terminals  35  are formed into bilayer structure  37  of a Nickel layer and a tin layer by barrel plating. Further, as shown in  FIG. 3   j , the obtained thin film common mode filter array chips  36  are bonded on a tape  38 . 
     The ferrite substrate is required to have high electrical insulation performance in bulk because, as shown in  FIG. 3   g , the thin film common mode filter and thin film common mode filter array have the electrode terminals formed on the cut surface of the ferrite substrate. And the ferrite substrate is also required to have high surface insulation performance. The thin film common mode filter produced from the substrates is required to have insulation resistance on the order of 10 8 Ω between the coil terminals. Although there is no perfect proportionality relation between the substrate surface resistance and actual terminal-to-terminal resistance, the substrate is required to have 2×10 10 Ω or more of the combined resistance of the bulk resistance and the surface resistance, to guarantee at least 10 8 Ω of the insulation resistance. Furthermore, the ferrite substrate is also required to maintain stably high surface insulation performance during such a heat process that is performed in the atmospheric air or Nitrogen gas at more or less 400° C. for heat cure of the insulating layer in the wafer process for forming the thin film common mode filter. 
     In addition, the ferrite substrate is required not to be cracked or so by mechanical shock or thermal shock in the wafer process because, as shown in  FIGS. 2   a - 2   j , the thin film common mode filter and thin film common mode filter array are formed all together on the ferrite substrate. The resistance to such a crack depends on the bending strength of the substrate, therefore the ferrite substrate is required to have higher bending strength. Particularly, the larger is the size of the substrate, the higher bending strength the substrate must have, to enhance its resistance to crack. 
     Further, as mentioned above, thin film micropatterns are formed on the ferrite substrate in the production process of the thin film common mode filter and thin film common mode filter array, therefore the coated film on the substrate is required not to billow, and the micropatterns are required not to be deformed, due to rough surface of the substrate or so. Usually, smaller degree of surface roughness of the substrate than the thickness of the coated film is required for carrying out the thin film process. For example, in the process of coating the polyimide films, patterning is difficult to be performed on the substrate with surface roughness Rmax of 6 μm or more. 
       FIG. 4  shows a graph illustrating common mode characteristic of a thin film common mode filter fabricated by the above-mentioned process, that is, frequency dependence of intrinsic impedance Z. 
     As understood from  FIG. 4 , the common mode filters, using the substrates made of ferrite materials with various permeability μ about 100 to 1400, acquire almost the same common mode characteristic. 
       FIG. 5  shows a graph illustrating the relationship among Fe 2 O 3 , NiO and ZnO compositions and permeability μ in the ferrite substrates. 
     As clarified from in  FIG. 5 , to meet the common mode impedance characteristic of the thin film common mode filter shown in  FIG. 4 , the ferrite substrate is required to contain a composition in the range of Fe 2 O 3 : 40 to 70 mol %, NiO: 5 to 35 mol %, ZnO: 10 to 40 mol %. 
       FIG. 6  shows a graph illustrating the measurement results of surface insulation resistance of the ferrite substrates just after being sintered, which contain various Fe 2 O 3  contents. And  FIG. 7  shows a graph illustrating the measurement results of surface insulation resistance of the sintered ferrite substrates after being surface-lapped. 
     As understood from  FIG. 6 , the NiZn ferrite substrates just after being sintered indicate greatly high surface insulation resistance of 10 12 Ω or more in the Fe 2 O 3  content range from 30 to 65 mol %. Further, the sintered substrates after being surface-lapped maintain greatly high surface insulation resistance. 
       FIG. 8  shows a graph illustrating the measurement results of surface insulation resistance of the ferrite substrates shown in  FIG. 7 , which contain various Fe 2 O 3  contents, after being annealed (5 times) at a curing temperature of insulating layers (about 400° C.). And  FIG. 9  shows a graph illustrating the measurement results of surface insulation resistance of these ferrite substrates after being annealed in vacuum at 1000° C. 
     As understood from  FIG. 8 , the substrates annealed repeatedly at the curing temperature of insulating layer show degraded surface insulation performance down to the order of 10 9 Ω in surface insulation resistance. Further, as understood from  FIG. 9 , the substrates annealed in vacuum show greatly degraded surface insulation performance down to the order of 10 8 Ω in surface insulation resistance. 
       FIG. 10  shows a graph illustrating the measurement results of surface insulation resistance of the annealed-in-vacuum ferrite substrates shown in  FIG. 9  after being surface-lapped with the amount of 5 μm or more. 
     From  FIG. 10 , it is noticed that the substrates reacquire greatly high surface insulation resistance by being surface-lapped. From this fact, understood is that the decrease in resistance is associated with the surface condition of the ferrite substrate. 
       FIG. 11  shows a graph illustrating the measurement results of the relationship between the amount of lapping and surface resistance. 
     From  FIG. 11 , it is noticed that the surface resistance rises sharply over 5 μm of the amount of lapping. Therefore, preferable is that the amount of surface lapping is set at 5 μm or more. 
       FIG. 12  shows a graph illustrating the measurement results of surface insulation resistance of the surface-lapped ferrite substrates shown in  FIG. 10  after being annealed 5 times in vacuum at a curing temperature of insulating layers (about 400° C.). 
     As evidenced by comparing  FIG. 12  with  FIG. 8 , by being surface-lapped with the amount of 5 μm or more, the ferrite substrates annealed for curing insulating layers show smaller decrease in surface resistance. Especially, the substrates indicate high surface resistance values of 10 10 Ω or more in the Fe 2 O 3  content range of 55 mol % or less. The insulation resistance is dropped sharply over the Fe 2 O 3  content range. 
     Therefore, to guarantee the resistance of at least 2×10 10 Ω, the substrate should have a Fe 2 O 3  content of 55 mol % or less. Further, according to the measurement results of permeability μ shown in  FIG. 5 , more preferable is that the substrates have a composition of Fe 2 O 3 : 40 to 55 mol %, NiO: 15 to 30 mol %, ZnO: 20 to 40 mol %. 
       FIG. 13  shows a graph illustrating the measurement results of surface insulation resistance of the ferrite substrates shown in  FIG. 12  and NiZn-ferrite substrates produced with addition of Bi 2 O 3  to their basic composition of Fe 2 O 3 , NiO and ZnO. Line a corresponds to the Bi 2 O 3 -added substrates, and line b corresponds to the no-Bi 2 O 3 -added substrates, both of which were annealed in vacuum at 1000° C. and then were surface-lapped with the amount of 8 μm. 
     It is noted that the surface insulation resistance increases by adding Bi 2 O 3 . 
       FIG. 14  shows a graph illustrating the measurement results of the relationship between the amount of added Bi 2 O 3  and insulation resistance in the ferrite substrate with Fe 2 O 3  content of 55 mol %. 
     From  FIG. 14 , it is noticed that the insulation resistance is greatly improved by adding 150 ppm or more of Bi 2 O 3 . 
       FIG. 15  shows a graph illustrating the measurement results of the relationship between the amount of added Bi 2 O 3  and bending strength in the ferrite substrate with Fe 2 O 3  content of 55 mol %. The measurement was based on JIS transverse test. The span of the measuring object was 1.4 mm, and the weighing rate was 30 mm/min. 
     From  FIG. 15 , it is noticed that the bending strength falls sharply by adding 750 ppm or more of Bi 2 O 3 . 
     As mentioned above, understood is that the insulation resistance and the bending strength are optimized together by adding 150 to 750 ppm of Bi 2 O 3  as the first example of the substrate composition. 
       FIG. 16  shows a graph illustrating the measurement results of the relationship between bending strength and the amount of added CuO in the NiZn-ferrite substrates produced with addition of CuO to their basic composition of Fe 2 O 3 , NiO and ZnO. 
     From  FIG. 16 , it is noticed that the bending strength increases with the amount of added CuO from 5 to 10 mol %. 
       FIG. 17  shows a graph illustrating the measurement results of the relationship between permeability μ and the amount of added MnO 2  in the substrates produced with addition of MnO 2  to the composition shown in  FIG. 16 . And  FIG. 18  shows a graph illustrating the measurement results of the relationship between insulation resistance and the amount of added MnO 2  in the substrates produced with addition of MnO 2  to the composition shown in  FIG. 16 . 
     As clarified from  FIG. 17 , the permeability μ increases with the amount of added MnO 2  from 0.5 to 5 mol %. However, the substrate insulation resistance falls sharply by adding 2 mol % or more of MnO 2 , as shown in  FIG. 18 . 
     Therefore, the bending strength and the permeability μ can be improved together without the insulation resistance decrease, by adding 5 to 10 mol % of CuO and 0.5 to 2 mol % of MnO 2  as the second example of the substrate composition. 
       FIG. 19  shows a graph illustrating the measurement results of the relationship between insulation resistance and the amount of added Bi 2 O 3 , in the ferrite substrates produced by conventional hot forming press method (HP) and the ferrite substrates produced by HIP according to the invention. And  FIG. 20  shows a graph illustrating the measurement results of the relationship between bending strength and the amount of added Bi 2 O 3 , in the ferrite substrates produced by HP and the ferrite substrates produced by HIP. 
     As shown in  FIG. 19 , there is little difference in the insulation resistance between the ferrite substrates produced by HP and those produced by HIP. However, as shown in  FIG. 20 , the bending strength of the substrate produced by HIP is about time and a half larger than that of the substrate produced by HP. That is, the substrate produced by HIP is harder to crack. The tendency becomes marked as the wafer size becomes larger. 
       FIG. 21  shows a graph illustrating the measurement results of the relationship between applied pressure in HIP and bending strength of the substrates. And  FIG. 22  shows a graph illustrating the measurement results of the relationship between applied pressure in HP and bending strength of the substrates. The processing temperature in both HP and HIP was 1200° C. 
     As shown in  FIG. 21 , it is noticed that the substrates acquire large bending strengths by undergoing HIP under the HIP pressure of 0.5 t/cm 2  or more. On the other hand, the substrate cannot acquire so large bending strengths by undergoing HP under the increased HP pressure, as shown in  FIG. 22 . 
     Table 1 illustrates the observation results of crack occurrence frequency in the 3-inch and 6-inch ferrite substrates (with thickness of 2 mm) produced by HP and the 3-inch and 6-inch ferrite substrates produced by HIP, both of which repeatedly underwent 10 times thermal shocks at 110° C. and 10 times sets of suction and detaching in the carrying process. The number of sample was 20. 
     The 3-inch substrates produced by HIP show less crack occurrence frequency than those produced by HP. In the 6-inch substrates, there is a larger difference of the frequency between the substrates by HP and those by HIP. 
     
       
         
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 1st 
                 2nd 
                 3rd 
                 4th 
                 5th 
                 6th 
                 7th 
                 8th 
                 9th 
                 10th 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 3- 
                 HP 
                 0 
                 0 
                 1 
                 1 
                 1 
                 2 
                 2 
                 2 
                 2 
                 2 
               
               
                 inch 
                 HIP 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 wafer 
               
               
                 6- 
                 HP 
                 1 
                 4 
                 6 
                 6 
                 8 
                 9 
                 12 
                 15 
                 18 
                 19 
               
               
                 inch 
                 HIP 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 1 
               
               
                 wafer 
               
               
                   
               
             
          
         
       
     
       FIG. 23  shows a graph illustrating the measurement results of the relationship between density and applied pressure in the ferrite substrates produced by HP and the ferrite substrates produced by HIP, both of which consist basically of Fe 2 O 3 , NiO and ZnO. 
     As shown in  FIG. 23 , it is noticed that the substrate density is improved by applying HIP to the substrate. 
       FIG. 24  shows a graph illustrating the measurement results of the relationship between surface roughness and grit number of the used abrasive grain, in the ferrite substrates produced by HP and the ferrite substrates produced by HIP, both of which consist basically of Fe 2 O 3 , NiO and ZnO. The crystalline grain size of both ferrite substrates was 5 μm, and #2000 SiC was used as abrasive grain for lapping of both substrates. 
     As shown in  FIG. 24 , it is noticed that the surface roughness decreases greatly by application of HIP to the substrate. Further, the surface roughness of the substrates produced by HP shows less change as the used abrasive grain becomes fine according to the grit number change from #1200 to #2000 and to #4000, whereas the surface roughness of the substrates produced by HIP is improved as the used abrasive grain becomes fine. 
       FIG. 25  shows an optical microscope photograph (×220) of the surface of the substrate that was processed by HIP and was lapped with #6000 diamond, and  FIG. 26  shows an optical microscope photograph (×220) of the surface of the substrate that was processed by HP and lapped with #6000 diamond. Further,  FIG. 27  shows an optical microscope photograph (×220) of the surface of the substrate that was processed by sheet manufacturing method and was lapped with #6000 diamond. The crystalline grain sizes of all the ferrite substrates were 5-6 μm. 
     The surface of the ferrite substrate produced by HIP as shown in  FIG. 25  has almost no pin holes, whereas the surface of the ferrite substrate produced by conventional HP as shown in  FIG. 26  or produced by HP after sheet manufacturing has some voids. Further, the surface of the ferrite substrate produced by sheet manufacturing method as shown in  FIG. 27  has some large vacancies from which the ferrite particles were detached. 
     All the foregoing embodiments are by way of example of the present invention only and not intended to be limiting, and many widely different alternations and modifications of the present invention may be constructed without departing from the spirit and scope of the present invention. Accordingly, the present invention is limited only as defined in the following claims and equivalents thereto.