Abstract:
The invention relates to surface-mount type coil components having a mounting surface for mounting them on a printed circuit board or hybrid IC (HIC) and provides a low-cost coil component having a low direct-current resistance. A common mode filter includes a bridge conductor layer which has a bridge conductor one end of which is connected to one end of a lead wire and another end of which is connected to an inner circumferential end of a coil conductor and another bridge conductor one end of which is connected to one end of another lead wire and another end of which is connected to an inner circumferential end of another coil conductor, the bridge conductor layer being formed between two coil conductor layers with an insulation film interposed between the bridge conductor layer and each of the coil conductor layers.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a surface-mount type coil component having a mount surface for mounting the same on a printed circuit board or a hybrid IC (HIC). 
         [0003]    2. Description of the Related Art 
         [0004]    Known coil components mounted on circuits in electronic apparatus such as personal computers and cellular phones include wire-wound type components provided by winding a copper wire around a ferrite core, multi-layer type components provided by forming a coil conductor pattern on a surface of a magnetic sheet made of ferrite and stacking such magnetic sheets one over another, and thin-film type components provided by forming insulation films and coil conductors constituted by metal thin films alternately using thin film forming techniques. Multi-layer type and thin-film type coil components can be easily provided with a small size. 
         [0005]    Known coil components include common mode filters for suppressing a common mode current which can cause electromagnetic interference in a balanced transmission system. Patent Document 1 discloses a common mode choke coil (common mode filter) provided by stacking a first insulator layer, two lead-out electrodes, a second insulator layer, a first coil conductor, a third insulator layer, a second coil conductor, and a fourth insulator layer in the order listed on a surface of a magnetic substrate. One of the two lead-out electrodes is electrically connected to the first coil conductor through a via hole provided on the second insulator layer, and the other electrode is electrically connected to the second coil conductor through via holes provided on the second and third insulator layers. 
         [0006]    To decrease the direct-current resistance of the common mode filter and to improve electrical characteristics of the same such as impedance characteristics and transmission characteristics in a differential mode, the coil conductors of the filter must be formed with a great width. However, when the coil conductors are formed with a greater width, a greater electrostatic capacity (floating capacity) will be generated between the two coil conductors. In a differential mode, an electrostatic capacity is a parasite in parallel with the inductance of a coil conductor. Therefore, when a relatively great electrostatic capacity is generated at the common mode filter, the electrostatic capacity constitutes the dominant impedance of the filter in high frequency bands. 
         [0007]    In order to suppress any increase in the electrostatic capacity while forming the coil conductors with a greater width, the distance between the two coil conductors must be made greater (e.g., 20 μm or more). The thickness of the insulator layer between the two coil conductors must be increased to increase the distance between the two coil conductors. When the thickness of the insulator layer between the two coil conductors of the common mode filter disclosed in Patent Document 1 is increased, the insulator layer between the two coil conductors must be formed with a thickness greater than that of the insulator layer between the lead-out electrodes and the first coil conductor. 
         [0008]    In order to provide the insulator layer between the two coil conductors with a great thickness, for example, the insulator layer must be provided by performing a plurality of forming steps consecutively. When the insulator layer is formed by performing a plurality of steps consecutively, the number of manufacturing steps increases, which results in an increase in manufacturing time. In order to form the insulator layer between the two coil conductors at a time, for example, it is necessary to form the insulator layer between the two coil conductors from a material different from the material of the insulator layer between the lead-out electrodes and the first coil conductor. When the two insulator layers are formed from different materials, for example, different apparatus must be used to form the insulator layer between the two coil conductors and the insulator layer between the lead-out electrodes and the first coil conductor. A problem therefore arises in that an increase in the distance between the two coil conductors results in an increase in the manufacturing cost of the common mode filter. 
         [0009]    Patent Document 2 discloses a thin-film common mode filter provided by stacking a first insulator layer, a lower lead conductor, a second insulator layer, a lower coil conductor, a third insulator layer, an upper coil conductor, a fourth insulator layer, an upper lead conductor, and a fifth insulator layer in the order listed, on an insulated magnetic substrate. The above-described problem occurs also in the thin-film common mode filter disclosed in Patent Document 2. 
         [0010]    There is demand for further reduction in the direct-current resistance of common mode filters. Direct-current resistance is the dominant common mode impedance in low frequency bands. It is desirable for common mode filters to have low common mode impedance in low frequency bands. Further, there is demand for reduction in power consumption in products such as cellular phones. A common mode filter having a low direct-current resistance allows a reduction in the power consumption of an electronic apparatus employing the common mode filter. It is therefore desirable for a common mode filter to have a low direct-current resistance. 
         [0011]    Patent Document 1: Japanese Patent No. 3601619 
         [0012]    Patent Document 2: JP-A-2005-159223 
       SUMMARY OF THE INVENTION 
       [0013]    It is an object of the invention to provide a low-cost coil component having a low direct-current resistance. 
         [0014]    (1) The above-described object is achieved by a coil component characterized in that it includes: 
         [0015]    a first coil conductor layer having first and second electrode terminals, a first lead-out conductor one end of which is connected to the first electrode terminal, and a first coil conductor connected to the second electrode terminal at an outer circumferential end thereof; 
         [0016]    a second coil conductor layer having third and fourth electrode terminals, a second lead-out conductor one end of which is connected to the third electrode terminal, and a second coil conductor connected to the fourth electrode terminal at an outer circumferential end thereof, the second coil conductor layer being disposed opposite to the first coil conductor layer; and 
         [0017]    a bridge conductor layer having a first bridge conductor one end of which is connected to another end of the first lead-out conductor and another end of which is connected to an inner circumferential end of the first coil conductor and a second bridge conductor one end of which is connected to another end of the second lead-out conductor and another end of which is connected to an inner circumferential end of the second coil conductor, the bridge conductor layer being formed between the first and second coil conductor layers with insulation films disposed between the bridge conductor layer and the first and second coil conductor layers. 
         [0018]    (2) The invention provides a coil component according to the item (1), characterized in that a thickness of the first and second lead-out conductors is greater than a thickness of the first and second bridge conductors. 
         [0019]    (3) The invention provides a coil component according to the item (1), characterized in that the first and second bridge conductors are not exposed on side surfaces of the insulation films. 
         [0020]    The invention makes it possible to provide a low-cost coil component having a low direct-current resistance. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a perspective view of a common mode filter  1  according to an embodiment of the invention; 
           [0022]      FIG. 2  is an exploded perspective view of the common mode filter  1  according to the embodiment of the invention; 
           [0023]      FIG. 3  is a sectional view (section  1 ) of the common mode filter  1  according to the embodiment of the invention; 
           [0024]      FIG. 4  is a sectional view (section  2 ) of the common mode filter  1  according to the embodiment of the invention; 
           [0025]      FIG. 5  is an exploded perspective view of a common mode filter  101  which is an example to be compared with the embodiment of the invention; and 
           [0026]      FIG. 6  is a sectional view of the common mode filter  101  which is an example to be compared with the embodiment of the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]    A coil component according to an embodiment of the invention will now be described with reference to  FIGS. 1 to 6 . A common mode filter for suppressing a common mode current which can cause electrostatic interference in a balanced transmission system will be described as an example of a coil component of the present embodiment.  FIG. 1  is a perspective view of a common mode filter  1 . In  FIG. 1 , hidden lines are indicated by broken lines. 
         [0028]    As shown in  FIG. 1 , the common mode filter  1  has a rectangular parallelepiped outline which is formed by stacking thin films between two magnetic substrates  3  and  5  in the form of thin rectangular parallelepiped plates disposed to face each other. An insulation layer  7  and a bonding layer  11  are formed in the order listed between the magnetic substrates  3  and  5  using thin-film forming techniques. Internal electrode terminals  21 ,  23 ,  25 , and  27  are formed in the vicinity of side surfaces of the insulation layer  7  such that they are exposed on the side surfaces of the insulation layer  7 . The internal electrode terminals  21  and  23  are exposed on one side surface, and the internal electrode terminals  25  and  27  are exposed on the side surface opposite to that side surface. The internal electrode terminal  21  is disposed opposite to the internal electrode terminal  25 , and the internal electrode terminal  23  is disposed opposite to the internal electrode terminal  27 . 
         [0029]    An external electrode  13  is formed to extend on the side surface where the internal electrode terminal  21  is exposed and on mounting surfaces of the magnetic substrates  3  and  5 . External electrodes  15 ,  17 , and  19  are formed in the same shape as the external electrode  13 . The external electrodes  13 ,  15 ,  17 , and  19  are electrically connected to the internal electrode terminals  21 ,  23 ,  25 , and  27  exposed on the side surfaces, respectively, 
         [0030]      FIG. 2  is an exploded perspective view of the common mode filter  1  showing a multi-layer structure of the same.  FIG. 2  omits the external electrodes  13 ,  15 ,  17 , and  19 . As shown in  FIG. 2 , an insulation film  7   a , a coil conductor layer (first coil conductor layer)  83 , an insulation film  7   b , a bridge conductor layer  84 , an insulation film  7   c , a coil conductor layer (second coil conductor layer)  85 , an insulation film  7   d , and a bonding layer  11  are stacked in the order listed between the magnetic substrates  3  and  5 . The insulation layer  7  is constituted by the insulation films  7   a ,  7   b ,  7   c , and  7   d . The coil conductor layer  85  is disposed opposite to the coil conductor layer  83  with the insulation film  7   b , the bridge conductor layer  84 , and the insulation film  7   c  interposed between the coil conductor layers. The bridge conductor layer  84  is disposed between the coil conductor layers  83  and  85  with the insulation film  7   b  interposed between the conductor layers  84  and  83  and the insulation film  7   c  interposed between the conductor layers  84  and  85 . 
         [0031]    The coil conductor layer  83  includes internal electrode terminals  21   a ,  23   a ,  25   a , and  27   a , a lead wire (first lead-out conductor)  29 , and a coil conductor (first coil conductor)  33 . One end of the lead wire  29  is connected to the internal electrode terminal (first electrode terminal)  21   a . The coil conductor  33  is formed in a spiral shape. An outer circumferential end of the coil conductor  33  is connected to the internal electrode terminal (second electrode terminal)  25   a . Another end of the lead wire  29  is exposed at a through hole  31   b  formed in the insulation film  7   b . An inner circumferential end of the coil conductor  33  is exposed at a through hole  31   a  formed in the insulation film  7   b.    
         [0032]    The bridge conductor layer  84  includes internal electrode terminals  21   b ,  23   b ,  25   b , and  27   b , a bridge conductor (first bridge conductor)  73 , and a bridge conductor (second bridge conductor)  75 . One end of the bridge conductor  73  is connected to the other end of the lead wire  29  exposed at the through hole  31   b . Another end of the bridge conductor  73  is connected to the inner circumferential end of the coil conductor  33  exposed at the through hole  31   a . The bridge conductor  73  extends across the coil conductor  33  with the insulation film  7   b  interposed when viewed in the normal direction of a substrate surface. Therefore, the coil conductor  33  is electrically connected to the internal electrode terminal  21   a  formed in the coil conductor layer  83  in which the conductor resides through the bridge conductor  73  formed in the bridge conductor layer  84  and through the lead wire  29  which also resides in the same layer as the coil conductor  33 . Two ends of the bridge conductor  75  are exposed at through holes  37   a  and  37   b  formed in the insulation film  7   c , respectively. The lead wire  29  and the bridge conductor  73  are not exposed on side surfaces of the insulation layer  7  (insulation films  7   a ,  7   b ,  7   c , and  7   d ). 
         [0033]    The coil conductor layer  85  includes internal electrode terminals  21   c ,  23   c ,  25   c , and  27   c , a lead wire (second lead-out conductor)  39 , and a coil conductor (second coil conductor)  35 . One end of the lead wire  39  is connected to the internal electrode terminal (third electrode terminal)  23   c . The coil conductor  35  is formed in a spiral shape which is substantially similar to the shape of the coil conductor  33 , and it faces the coil conductor  33  so as to sandwich the insulation film  7   b , the bridge conductor layer  84 , and the insulation film  7   c . An outer circumferential end of the coil conductor  35  is connected to the internal electrode terminal (fourth electrode terminal)  27   c.    
         [0034]    Another end of the lead wire  39  is connected to one end of the bridge conductor  75  through the through hole  37   b . An inner circumferential end of the coil conductor  35  is connected to another end of the bridge conductor  75  through the through hole  37   a . The bridge conductor  75  extends across the coil conductor  35  with the insulation film  7   c  interposed when viewed in the normal direction of the substrate surface. Therefore, the coil conductor  35  is electrically connected to the internal electrode terminal  23   c  formed in the coil conductor layer  85  in which the conductor resides through the bridge conductor  75  formed in the bridge conductor layer  84  and through the lead wire  39  which also resides in the same layer as the coil conductor  35 . The lead wire  39  and the bridge conductor  75  are not exposed on the side surfaces of the insulation layer  7  (insulation films  7   a ,  7   b ,  7   c , and  7   d ). 
         [0035]    The internal electrode terminal  21  shown in  FIG. 1  is formed by stacking the internal electrode terminals  21   a ,  21   b , and  21   c  in the order listed. The internal electrode terminal  23  is formed by stacking the internal electrode terminals  23   a ,  23   b , and  23   c  in the order listed. The internal electrode terminal  25  is formed by stacking the internal electrode terminals  25   a ,  25   b , and  25   c  in the order listed. The internal electrode terminal  27  is formed by stacking the internal electrode terminals  27   a ,  27   b , and  27   c  in the order listed. 
         [0036]    The coil conductors  33  and  35 , the lead wires  29  and  39 , and the bridge conductors  73  and  75  are embedded in the insulation layer  7  to form one choke coil. The external electrode  13  shown in  FIG. 1  is electrically connected to the external electrode  17  through the internal electrode terminal  21 , the lead wire  29 , the bridge conductor  73 , the coil conductor  33 , and the internal electrode terminal  25 . The external electrode  15  is electrically connected to the external electrode  19  through the internal electrode terminal  23 , the lead wire  39 , the bridge conductor  75 , the coil conductor  35 , and the internal electrode terminal  27 . 
         [0037]    The magnetic substrates  3  and  5  are formed from a magnetic material such as sintered ferrite or composite ferrite. Each of the insulation films  7   a ,  7   b ,  7   c , and  7   d  is formed by applying a material having high insulating properties and high workability such as polyimide resin or epoxy resin and patterning the same into a predetermined shape. The coil conductors  33  and  35 , the bridge conductors  73  and  75 , the lead wires  29  and  39 , and the internal electrode terminals  21 ,  23 ,  25 , and  27  are provided by forming films of Cu, silver (Ag), or aluminum (Al) which have high electrical conductivity and workability and patterning the films into predetermined shapes. The insulation films  7   a ,  7   b ,  7   c , and  7   d  and the bridge conductor layer  84  have a thickness, for example, in the range from 7 to 8 μm. Therefore, the distance between the coil conductors  33  and  35  is, for example, in the range from 21 to 24 μm. The coil conductor layers  83  and  85  have a thickness, for example, in the range from 18 to 20 μm. 
         [0038]    To improve the degree of mutual magnetic coupling between the coil conductor  33  and the coil conductor  35  and to improve impedance characteristics by increasing common mode impedance, the insulation layer  7  may be removed in regions of the coil conductors  33  and  35  on the inner circumferential side thereof to form openings, and magnetic layers may be formed so as to fill the openings. For the same purpose, four parts of the insulation layer  7  at the corners thereof may be removed to form openings in those parts respectively, and magnetic layers may be formed so as to fill the openings. For example, the magnetic layers may be formed from composite ferrite obtained by mixing magnetic particles made of ferrite in a resin. 
         [0039]      FIG. 3  is a sectional view taken along an imaginary line A-O-A which is formed by an imaginary straight line extending through the internal electrode terminal  23  constituted by the internal electrode terminals  23   a ,  23   b , and  23   c  and through the lead wire  39  and an imaginary straight line extending perpendicularly to that straight line through the bridge conductor  75  (see  FIG. 2 ).  FIG. 4  shows a section taken along an imaginary line B-O-B which is formed by an imaginary straight line extending through the bridge conductor  73  and an imaginary straight line extending perpendicularly to that straight line through the internal electrode terminal  21  constituted by the internal electrode terminals  21   a ,  21   b , and  21   c  and through the lead wire  29  (see  FIG. 2 ). 
         [0040]    As shown in  FIG. 3 , the conductor connecting the inner circumferential end of the coil conductor  35  and the internal electrode terminal  23  is constituted by the lead wire  39  and the bridge conductor  75 . The conductor is shown as having three divisions of regions A 1 , A 2 , and A 3 . The region A 1  is a region of the bridge conductor  75  excluding parts thereof where the conductor is connected to the coil conductor  35  and the lead wire  39 , respectively. The region A 2  is a region where the lead wire  39  and the bridge conductor  75  are connected to each other. The region A 3  is a region of the lead wire  39  excluding a part thereof where the wire is connected to the bridge conductor  75 . The lead wire  39  has a thickness t 1 . The bridge conductor  75  has a thickness t 2 . Therefore, the conductor has the thickness t 2  in the region A 1  and has the thickness t 1  in the region A 3 . The thickness of the conductor in the region A 2  is substantially equal to the sum of the thickness t 1  of the lead wire  39  and the thickness t 2  of the bridge conductor  75  (t 1 +t 2 ). The thickness of the coil conductor  35  is substantially the same as the thickness t 1  of the lead wire  39 . The thickness t 1  of the lead wire  39  is greater than the thickness t 2  of the bridge conductor  75 . 
         [0041]    As shown in  FIG. 4 , the conductor connecting the inner circumferential end of the coil conductor  33  and the internal electrode terminal  21  is constituted by the lead wire  29  and the bridge conductor  73 . The conductor is shown as having three divisions of regions A 1 , A 2 , and A 3 . The region A 1  is a region of the bridge conductor  73  excluding parts thereof where the conductor is connected to the coil conductor  33  and the lead wire  29 , respectively. The region A 2  is a region where the lead wire  29  and the bridge conductor  73  are connected to each other. The region A 3  is a region of the lead wire  29  excluding a part thereof where the wire is connected to the bridge conductor  73 . The lead wire  29  has a thickness equal to the thickness t 1  of the lead wire  39 . The bridge conductor  73  has a thickness equal to the thickness t 2  of the bridge conductor  75 . Therefore, the conductor has the thickness t 2  in the region A 1  and has the thickness t 1  in the region A 3 . The thickness of the conductor in the region A 2  is substantially equal to the sum of the thickness t 1  of the lead wire  29  and the thickness t 2  of the bridge conductor  73  (t 1 +t 2 ). The thickness of the coil conductor  33  is substantially the same as the thickness t 1  of the lead wire  29 . The thickness t 1  of the lead wire  29  is greater than the thickness t 2  of the bridge conductor  73 . 
         [0042]      FIG. 5  is an exploded perspective view of a multi-layer structure of a common mode filter  101  according to the related art shown as an example to be compared with the common mode filter  1  of the present embodiment. In  FIGS. 5 and 6 , elements having the same functions and effects as the elements shown in  FIGS. 1 to 4  are indicated by like reference numerals and will not be described in detail. 
         [0043]    As shown in  FIG. 5 , an insulation film  7   a , a coil conductor layer  83 , an insulation film  7   b , a lead wire layer  184 , an insulation film  7   c , a coil conductor layer  85 , an insulation film  7   d , and a bonding layer  11  are stacked in the order listed between magnetic substrates  3  and  5 . The lead wire layer  184  includes internal electrode terminals  21   b ,  23   b ,  25   b , and  27   b  and lead wires  129  and  139 . In the common mode filter  101 , the lead wire  129  is formed instead of the lead wire  29  and the bridge conductor  73  of the common mode filter  1 , and the lead wire  139  is formed instead of the lead wire  39  and the bridge conductor  75 . 
         [0044]    The lead wire  129  is formed such that it has an L-like shape when viewed in the normal direction of a surface of the substrate. One end of the lead wire  129  is connected to the internal electrode terminal  21   b . Another end of the lead wire  129  is connected to an inner circumferential end of a coil conductor  33  exposed at a through hole  31   a . The lead wire  129  extends across the coil conductor  33  with the insulation film  7   b  interposed between them when viewed in the normal direction of the substrate surface. Thus, the coil conductor  33  is electrically connected to the internal electrode terminal  21   b , which is in a different layer, through the lead wire  129  formed on the lead wire layer  184 . 
         [0045]    The lead wire  139  is formed such that it has a shape like an inverted L when viewed in the normal direction of the substrate surface. One end of the lead wire  139  is connected to the internal electrode terminal  23   b . Another end of the lead wire  139  is exposed at a through hole  37   a  and connected to an inner circumferential end of a coil conductor  35  through the through hole  37   a . The lead wire  139  extends across the coil conductor  35  with the insulation film  7   c  interposed between them when viewed in the normal direction of the substrate surface. Thus, the coil conductor  35  is electrically connected to the internal electrode terminal  23   b , which is in a different layer, through the lead wire  139  formed on the lead wire layer  184 . 
         [0046]      FIG. 6  is a sectional view taken along an imaginary line A-O-A extending along an internal electrode terminal  23  constituted by internal electrode terminals  23   a ,  23   b , and  23   c  and the lead wire  139  (see  FIG. 5 ).  FIG. 6  shows only the lead wire  139  of the couple of lead wires  129  and  139  having substantially the same shape, and the following description on the shape of the lead wire  139  equally applies to the lead wire  129 . 
         [0047]    As shown in  FIG. 6 , the conductor connecting the inner circumferential end of the coil conductor  35  and the internal electrode terminal  23  is entirely constituted by the lead wire  139 . The conductor has three divisions of regions A 1 , A 2 , and A 3 . The region A 2  is a region of the lead wire  139  where the lead wire  139  is bent, and this region corresponds to the region A 2  shown in  FIG. 3 . The region A 1  is a region of the lead wire  139  extending from a region of the lead wire connected to the coil conductor  35  up to the bent region, and this region corresponds to the region A 1  shown in  FIG. 3 . The region A 3  is a region of the lead wire  139  extending from the bent region up to the internal electrode terminal  23 , and this region corresponds to the region A 3  shown in  FIG. 3 . 
         [0048]    The lead wire  139  has a thickness t 2  equal to the thickness of the bridge conductors  73  and  75 . Therefore, the conductor has the thickness t 2  in all of the regions A 1 , A 2 , and A 3 . For example, the lead wire  139  has a width which is equal to the width of the lead wires  29  and  39  and the bridge conductors  73  and  75 . The lengths and widths of the conductor in the regions A 1 , A 2 , and A 3  shown in  FIG. 6  are the same as the lengths and widths of the conductor in the regions A 1 , A 2 , and A 3  shown in  FIG. 3 , respectively. 
         [0049]    In order to compare direct-current resistances of transmission paths of the common mode filters  1  and  101 , the inventors carried out a simulation to analyze the direct-current resistance of the transmission paths of each of the common mode filters  1  and  101 . The common mode filters  1  and  101  have two transmission paths, i.e., a transmission path including the coil conductor  33  and a transmission path including the coil conductor  35 , and the simulation and analysis was carried out on the direct-current resistance of the transmission path including the coil conductor  35 . As shown in  FIGS. 1 and 2 , in the common mode filter  1 , the transmission path including the coil conductor  35  is constituted by the internal electrode terminal  23 , the lead wire  39 , the bridge conductor  75 , the coil conductor  35 , and the internal electrode terminal  27 . As shown in  FIG. 5 , in the common mode filter  101 , the transmission path including the coil conductor  35  is constituted by the internal electrode terminal  23 , the lead wire  139 , the coil conductor  35 , and the internal electrode terminal  27 . The direct-current resistance of the transmission path including the coil conductor  33  is substantially equal to the direct-current resistance of the transmission path including the coil conductor  35 . 
         [0050]    Design values used for the simulation and analysis were as follows. The lead wires  39  and  139  and the bridge conductor  75  had a width w of 16 μm. The thickness t 1  of the lead wire  39  was 18 μm. The thickness t 2  of the bridge conductor  75  and the lead wire  139  was 7 μm. The conductor had a length L 1  of 250 μm in the region A 1 . The conductor had a length L 2  of 40 μm in the region A 2 . The conductor had a length L 3  of 224 μm in the region A 3 . The lead wire  39  and  139  and the bridge conductor  75  had an electrical resistivity ρ of 1.67×10 −8  (Ω·m). The direct-current resistance of the transmission paths excluding the regions A 1 , A 2 , and A 3  was 0.42 (Ω). 
         [0000]    
       
         
               
               
             
               
               
               
               
             
               
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Thickness(μm) 
               
             
          
           
               
                 Region 
                 Length(μm) 
                 Filter 101 
                 Filter 1 
               
               
                   
               
             
          
           
               
                 A1 
                 250 
                 7 
                 7 
               
               
                 A2 
                 40 
                 7 
                 18 + 7 
               
               
                 A3 
                 224 
                 7 
                 18 
               
             
          
           
               
                 DC Resistance (Ω) in A1, A2, A3 
                 7.66 × 10 −2   
                 5.19 × 10 −2   
               
               
                 DC Resistance (Ω) of Transmission 
                 0.42 
                 0.42 
               
               
                 Path excluding A1, A2, A3 
               
               
                 DC Resistance (Ω) of Transmission 
                 4.97 × 10 −1   
                 4.72 × 10 −1   
               
               
                 Path 
               
               
                   
               
             
          
         
       
     
         [0051]    Table 1 shows results of the simulation and analysis of direct-current resistances in the form of a comparison between the common mode filter  101  and the common mode filter  1 . Let us assume that the length, width, and thickness of a conductor in a region Ak (k=1, 2, 3) are represented by Lk, wk, and t′k, respectively. Then, the region Ak has a direct-current resistance Rk which is given by Rk=ρ(Lk/(wk·t′k)). Since conductors in the regions A 1 , A 2 , and A 3  are connected in series, a conductor connecting the inner circumferential end of the coil conductor  35  and the internal electrode terminal  23  has a direct-current resistance R which is given by R=R 1 +R 2 +R 3 . 
         [0052]    As shown in Table 1, in the case of the common mode filter  101 , the thickness t′k of the conductor connecting the inner circumferential end of the coil conductor  35  and the inner electrode terminal  23  is t 2  (=7 μm) in all of the regions A 1 , A 2 , and A 3 . The direct-current resistance R of the conductor (i.e., the direct-current resistance in the regions A 1 , A 2 , and A 3 ) is 7.66×10 −2  (Ω). 
         [0053]    In the case of the common mode filter  1 , the thickness t′k of the conductor connecting the inner circumferential end of the coil conductor  35  and the internal electrode terminal  23  is t 2  (=7 μm) in the region A 1 , t 1 +t 2  (=18+7 μm) in the region A 2 , and t 1  (=18 μm) in the region A 3 . The direct-current resistance R of the conductor (i.e., the direct-current resistance in the regions A 1 , A 2 , and A 3 ) is 5.19×10 −2  (Q). 
         [0054]    Therefore, the direct-current resistance R of the conductor connecting the inner circumferential end of the coil conductor  35  and the internal electrode terminal  23  of the common mode filter  1  is 32.2% smaller than that in the common mode filter  101 . The direct-current resistance of a transmission path is given as the sum of the direct-current resistance of the path in the regions A 1 , A 2 , and A 3  and the direct-current resistance of the transmission path excluding the regions A 1 , A 2 , and A 3 . The transmission paths in the common mode filter  1  and the common mode filter  101  have the same direct-current resistance that is 0.42 (Ω) when the regions A 1 , A 2 , and A 3  are excluded. Therefore, the direct-current resistance of the transmission path in the common mode filter  101  is 4.97×10 −1  (Ω). The direct-current resistance of the transmission path in the common mode filter  1  is 4.72×10 −1  (Ω). Therefore, the direct-current resistance of the transmission path in the common mode filter  1  is 5.0% smaller than that in the common mode filter  101 . 
         [0055]    The direct-current resistance of the transmission path in the common mode filter  101  may be decreased by increasing the thickness of the lead wires  129  and  139 . In order to increase the thickness of the lead wires  129  and  139 , the lead wire layer  184  must be formed with a greater thickness. However, when the thickness of the lead wire layer  184  is increased to decrease the direct-current resistance, the flatness of the pattern of the coil conductor  35  located above the layer can be affected. As a result, significant variation can partially occur in the distance between the coil conductors  33  and  35 , which can adversely affect signal transmission. When the insulation films  7   b  and  7   c  are formed with a small thickness, shorting can occur between the coil conductor  33  and the lead wires  129  and  139  and between the coil conductor  35  and the lead wires  129  and  139 , and the reliability of the common mode filter  101  can be consequently reduced. Therefore, in order to obtain a common mode filter  101  having desired electrical characteristics, the distance between the two coil conductors  33  and  35  and the thickness of the insulation layers  7   b  and  7   c  must be kept at predetermined values. 
         [0056]    As described above, in the common mode filter  1  of the present embodiment, the conductor connecting the inner circumferential end of the coil conductor  33  and the internal electrode terminal  21  is constituted by the lead wire  29  formed in the coil conductor layer  83  in which the conductor resides and the bridge conductor  73  formed in the bridge conductor layer  84 . Similarly, the conductor connecting the inner circumferential end of the coil conductor  35  and the internal electrode terminal  23  is constituted by the lead wire  39  formed in the coil conductor layer  85  in which the conductor resides and the bridge conductor  75  formed in the bridge conductor layer  84 . The thickness t 1  of the lead wires  29  and  39  is equal to the thickness of the coil conductors  33  and  35 . The thickness t 1  of the lead wires  29  and  39  is greater than the thickness t 2  of the bridge conductors  73  and  75 . Therefore, the direct-current resistance of the common mode filter  1  of the present embodiment can be smaller than that of the common mode filter  101  in which the conductors connecting the inner circumferential ends of the coil conductors and the internal electrode terminals are entirely formed in a layer (lead wire layer  184 ) that is different from the layers in which the coil conductors reside. Therefore, the present embodiment makes it possible to provide a common mode filter  1  having a low direct-current resistance. 
         [0057]    A description will now be made using  FIG. 2  on a method of manufacturing a coil component according to the present embodiment with reference to the common mode filter  1  by way of example. A multiplicity of common mode filters  1  are simultaneously formed on a wafer, but  FIG. 2  shows an exploded perspective view of a single common mode filter  1  to show the multi-layer structure of the same. 
         [0058]    First, as shown in  FIG. 2 , polyimide resin is applied to the top of a magnetic substrate  3  to form an insulation film  7   a  having a thickness in the range from 7 to 8 μm. The insulation film  7   a  is formed using a spin coat process, a dipping process, a spray process, or a printing process. Then, the insulation film  7   a  is cured. Each of insulation films  7   b ,  7   c , and  7   d  to be described later is formed using a similar process. 
         [0059]    Next, a metal layer such as a Cu layer (not shown) is formed throughout the resultant surface using a vacuum film forming process (such as evaporation or sputtering) or a plating process. Although what is required for the electrode film is that it is formed from a conductive material, it is desirable to use the same material as the material for forming plating films to be described later. A bonding layer such as a Cr (chromium) film or Ti (titanium) film may be formed under the electrode film, the bonding layer serving as a buffer film for improving adhesion between the insulation film  7   a  and the electrode film. 
         [0060]    Next, a resist is applied throughout the resultant surface to form a resist layer, and a pre-baking process is performed on the resist layer as occasion demands. Exposure is then carried out on the resist layer by irradiating the resist layer with exposure light through a mask having patterns drawn thereon representing internal electrode terminals  21   a ,  23   a ,  25   a , and  27   a , a lead wire  29 , and a coil conductor  33 . After performing a heating process as occasion demands, the resist layer is developed using an alkaline developer. For example, Tetra-Methyl-Ammonium-Hydroxide (TMAH) having a predetermined concentration may be used as the alkaline developer. 
         [0061]    The process then proceeds from the developing step to a cleaning step. The developer in the resist layer is cleaned away with a cleaning liquid to stop the developing and dissolving reactions of the resist layer. As a result, the resist layer is patterned in a predetermined shape to form a resist frame. For example, pure water is used as the cleaning liquid. When the cleaning is completed, the substrate is dried by shaking the same to remove the cleaning liquid. If necessary, the magnetic substrate  3  may be heated such that the cleaning liquid will dry up. 
         [0062]    Next, the magnetic substrate  3  is immersed in a plating liquid in a plating bath to perform a plating process using the resist frame as a die, and plating films of Cu are thereby formed in gaps of the resist frame. The magnetic substrate  3  is then washed and dried as occasion demands, and the resist frame is removed from the insulation film  7   a  using an organic solvent. Then, dry etching (such as ion milling or reactive ion etching (RIE)) or wet etching is performed using the plating films as a mask to remove the electrode film which has been exposed as a result of the removal of the resist layer. 
         [0063]    Thus, electrode films and plating films are stacked to form internal electrode terminals  21   a ,  23   a ,  25   a , and  27   a , a lead wire  29 , and a coil conductor  33 . Through the above-described steps, a coil conductor layer  83  constituted by the internal electrode terminals  21   a ,  23   a ,  25   a , and  27   a , the lead wire  29 , and the coil conductor  33  is formed. For example, the coil conductor layer  83  is formed with a thickness in the range from 18 to 20 μm. A bridge conductor layer  84  and a coil conductor layer  85 , which will be described layer, are formed in the same manner as the coil conductor  83 . 
         [0064]    Next, polyimide resin is applied throughput the resultant surface to form an insulation film  7   b  having a thickness in the range from 7 to 8 μm. The insulation film  7   b  is then exposed, developed, and patterned. As a result, the insulation film  7   b  is formed with a through hole  31   a  at which an inner circumferential end of the coil conductor  33  is exposed, a through hole  31   b  at which one end of the lead wire  29  is exposed, and openings at which the electrode terminals  21   a ,  23   a ,  25   a , and  27   a  are exposed. The insulation film  7   b  is then cured. 
         [0065]    Next, a metal layer such as a Cu layer (not shown) is formed throughout the resultant surface using a vacuum film forming process (such as evaporation or sputtering) or a plating process. Next, a resist is applied throughout the resultant surface to form a resist layer, and a pre-baking process is performed on the resist layer as occasion demands. Exposure is then carried out on the resist layer by irradiating the resist layer with exposure light through a mask having patterns drawn thereon representing internal electrode terminals  21   b ,  23   b ,  25   b , and  27   b  and bridge conductors  73  and  75 . After performing a heating process as occasion demands, the resist layer is developed using an alkaline developer. Then, a cleaning step similar to that involved in the step of forming the coil conductor layer  83  is performed. As a result, the resist layer is patterned in a predetermined shape to form a resist frame. 
         [0066]    Next, the magnetic substrate  3  is immersed in a plating liquid in a plating bath to perform a plating process using the resist frame as a die, and plating films of Cu are thereby formed in gaps of the resist frame. The magnetic substrate  3  is then washed and dried as occasion demands, and the resist frame is removed from the insulation film  7   b  using an organic solvent. Then, dry etching (such as ion milling or reactive ion etching (RIE)) or wet etching is performed using the plating films as a mask to remove the electrode film which has been exposed as a result of the removal of the resist layer. 
         [0067]    Thus, electrode films and plating films are stacked to form internal electrode terminals  21   b ,  23   b ,  25   b , and  27   b  and bridge conductors  73  and  75 . The internal electrode terminals  21   b ,  23   b ,  25   b , and  27   b  are formed on the electrode terminals  21   a ,  23   a ,  25   a , and  27   a , respectively. One end of the bridge conductor  73  is connected to one end of the lead wire  29  through the through hole  31   b , and another end of the conductor is connected to the inner circumferential end of the coil conductor  33  through the through hole  31   a . Through the above-described steps, a bridge conductor layer  84  constituted by the internal electrode terminals  21   b ,  23   b ,  25   b , and  27   b  and the bridge conductors  73  and  75  is formed. For example, the bridge conductor layer  84  is formed with a thickness in the range from 7 to 8 μm. 
         [0068]    Next, polyimide resin is applied throughput the resultant surface to form an insulation film  7   c  having a thickness in the range from 7 to 8 μm. The insulation film  7   c  is then exposed, developed, and patterned. As a result, the insulation film  7   c  is formed with through holes  37   a  and  37   b  at which both ends of the bridge conductor  75  are exposed and openings at which the electrode terminals  21   b ,  23   b ,  25   b , and  27   b  are exposed. The insulation film  7   c  is then cured. 
         [0069]    Next, a metal layer such as a Cu layer (not shown) is formed throughout the resultant surface using a vacuum film forming process (such as evaporation or sputtering) or a plating process. Next, a resist is applied throughout the resultant surface to form a resist layer, and a pre-baking process is performed on the resist layer as occasion demands. Exposure is then carried out on the resist layer by irradiating the resist layer with exposure light through a mask having patterns drawn thereon representing internal electrode terminals  21   c ,  23   c ,  25   c , and  27   c , a lead wire  39 , and a coil conductor  35 . After performing a heating process as occasion demands, the resist layer is developed using an alkaline developer. Then, a cleaning step similar to that involved in the step of forming the coil conductor layer  83  is performed. As a result, the resist layer is patterned in a predetermined shape to form a resist frame. 
         [0070]    Next, the magnetic substrate  3  is immersed in a plating liquid in a plating bath to perform a plating process using the resist frame as a die, and plating films of Cu are thereby formed in gaps of the resist frame. The magnetic substrate  3  is then washed and dried as occasion demands, and the resist frame is removed from the insulation film  7   c  using an organic solvent. Then, dry etching (such as ion milling or reactive ion etching (RIE)) or wet etching is performed using the plating films as a mask to remove the electrode film which has been exposed as a result of the removal of the resist layer. 
         [0071]    Thus, electrode films and plating films are stacked to form internal electrode terminals  21   c ,  23   c ,  25   c , and  27   c , a lead wire  39 , and a coil conductor  35 . The internal electrode terminals  21   c ,  23   c ,  25   c , and  27   c  are formed on the internal electrode terminals  21   b ,  23   b ,  25   b , and  27   b , respectively. One end of the lead wire  39  is connected to one end of the bridge conductor  75  through the through hole  37   b . An inner circumferential end of the coil conductor  35  is connected to the other end of the bridge conductor  75  through the through hole  37   a . Through the above-described steps, a coil conductor layer  85  constituted by the internal electrode terminals  21   c ,  23   c ,  25   c , and  27   c , the lead wire  39 , and the coil conductor  35  is formed. For example, the coil conductor layer  85  is formed with a thickness in the range from 18 to 20 μm. Internal electrode terminals  21 ,  23 ,  25 , and  27  are also formed through the above-described steps. 
         [0072]    Next, polyimide resin is applied throughput the resultant surface to form an insulation film  7   d  having a thickness in the range from 7 to 8 μm. The insulation film  7   d  is then cured. An adhesive is then applied on the insulation film  7   d  to form a bonding layer  11 . A magnetic substrate  5  is then secured on the bonding layer  11 . 
         [0073]    The wafer is then cut and separated into individual common mode filters  1  in the form of chips. As a result, the internal electrode terminals  21 ,  23 ,  25 , and  27  are exposed on cut surfaces of a common mode filter  1 . The common mode filter  1  is then polished to chamfer corners thereof. 
         [0074]    Although not shown, backing metal films having the same shape as that of external electrodes  13 ,  15 ,  17 , and  19  are formed on the internal electrode terminals  21 ,  23 ,  25 , and  27  of the common mode filter  1 . The backing metal films are provided by forming a combination of a chromium (Cr) film and a Cu film or a combination of a titanium (Ti) film and a Cu film continuously using a masked sputtering process. 
         [0075]    Next, external electrodes  13 ,  15 ,  17 , and  19  having a double-layer structure made of nickel (Ni) and tin (Sn) on the surface of the backing metal film using electroplating are formed and the common mode filter  1  shown in  FIG. 1  is provided. 
         [0076]    As described above, in the present embodiment, the bridge conductor layer  84  formed with the bridge conductors  73  and  75  for connecting the inner circumferential ends of the coil conductors  33  and  35  and the internal electrode terminals  21  and  23  is formed between the coil conductor layer  83  and the coil conductor layer  85 . Thus, three layers, i.e., the insulation film  7   b , the bridge conductor layer  84 , and the insulation film  7   c  are formed between the coil conductor layer  83  and the coil conductor layer  85 . Therefore, when compared to a common mode filter in which only an insulation film is formed between coil conductor layers  83  and  85 , the distance between the coil conductor layers  83  and  85  of the common mode filter  1  can be easily made longer (e.g., 20 μm or more) without any increase in the number of manufacturing steps which can occur when the insulation film is formed by a plurality of consecutive steps and without a need for switching forming apparatus to form the insulation film between the coil conductor layers  83  and  85  from a material different from the material of other insulation films. The present embodiment therefore makes it possible to manufacture a common mode filter  1  in which coil conductor layers  83  and  85  are spaced at a greater distance at a low cost. Thus, a low-cost common mode filer  1  can be provided. 
         [0077]    The method of manufacturing a common mode filter  1  according to the present embodiment allows the number of steps for forming the conductive layers and the number of steps for forming the insulation films to be reduced by one each when compared to the common mode filter disclosed in Patent document 2. Therefore, the common mode filter  1  can be manufactured at a lower cost when compared to the common mode filter disclosed in Patent Document 2. 
         [0078]    The common mode filter  1  of the present embodiment is more advantageous, the greater the distance between the coil conductor layers  83  and  85 . Thus, the common mode filter  1  can be manufactured with a greater cost reduction, for example, when compared to a thin-film common mode filter as disclosed in Patent Document 2 which satisfies the relationship that “the distance between the two coil conductors&gt;the distance between the lead conductor and the coil conductor×2+the thickness of the thinner of the two lead conductors”, 
         [0079]    The invention is not limited to the above-described embodiment and may be modified in various ways. 
         [0080]    Although a common mode filter  1  having a pair of coil conductors  33  and  35  disposed opposite to each other has been explained by way of example in the above description of the embodiment, the invention is not limited to such a configuration. For example, the invention may be applied to a common mode filer array having two pairs of coil conductors in which each of coil conductors  33  and  35  is accompanied by one coil conductor provided side by side. Further, one or more additional coil conductors may be provided between the two pairs of coil conductors arranged side by side. Such common mode filter arrays provide the same advantages as those of the above-described embodiment.