Patent Publication Number: US-7221250-B2

Title: Coil component and method of manufacturing the same

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a coil component used as a major component of a common mode choke coil or a transformer and a method of manufacturing the same. 
   2. Description of the Related Art 
   Reductions in the size of electronic apparatus such as personal computers and portable phones have resulted in demand for reductions in the size and thickness (height) of electronic components such as coils and capacitors mounted on internal circuits of electronic apparatus. 
   However, a wire-wound coil obtained by winding a copper wire around a ferrite core has a problem in that it is difficult to make compact because of structural limitations. Under the circumstance, research and development is active on chip-type coil components which can be provided with a small size and a small height. Known chip-type coil components include multi-layer type coil components provided by forming coil conductor patterns on surfaces of magnetic sheet made of ferrite and stacking the magnetic sheets and thin film type coil components provided by forming insulation films and coil conductors constituted by metal thin films alternately using thin film forming techniques. 
   Common mode choke coils are known as thin film type coil components.  FIGS. 7A and 7B  are sectional views of common mode choke coils  51  taken along a plane including center axes of coil conductors  59  and  61 .  FIG. 7A  shows a common mode choke coil  51  having coil conductors  59  and  61  which are curved in the form of a convex in a top portion thereof when viewed in the coil section.  FIG. 7B  shows a common mode choke coil  51  having coil conductors  59  and  61  having a rectangular coil section. As shown in  FIGS. 7A and 7B , the common mode choke coils  51  have an insulation layer  57  formed by stacking an insulation film between ferrite substrates (magnetic substrates)  53  and  55  which are provided opposite to each other. The coil conductors  59  and  61 , which are provided opposite to each other with the insulation film interposed between them and formed in a spiral configuration, are embedded in the insulation layer  57 . The insulation layer  57  and the coil conductors  59  and  61  are formed in the order listed using thin film forming techniques. 
   An opening  63  is formed on an inner circumferential side of the spiral coil conductors  59  and  61  by removing the insulation layer  57 . An opening  65  is formed on an outer circumferential side of the spiral coil conductors  59  and  61  by removing the insulation layer  57 . Magnetic layers  67  are formed to fill the openings  63  and  65 . Further, a bonding layer  69  is formed on the magnetic layers  67  and the insulation layer  57  to bond a magnetic substrate  55 . 
   When the coil conductors  59  and  61  are energized, a magnetic path M is formed such that it passes through the magnetic substrate  53 , the magnetic layer  67  in the opening  63 , the bonding layer  69 , the magnetic substrate  55 , the bonding layer  69  again, and the magnetic layer  67  in the opening  65  in the section including the center axes of the coil conductors  59  and  61 . The bonding layer  69  is a film having a thickness on the order of a few μm, although it is non-magnetic. Therefore, substantially no leakage of the magnetic flux occurs in this part, and the magnetic path M may be regarded as a substantially closed path. 
   In order to improve the common mode filtering property of the common mode choke coil  51 , strong magnetic coupling must be achieved between the coil conductors  59  and  61 . To increase the strength of the magnetic coupling between the coil conductors  59  and  61 , it is necessary to increase the numbers of turns of the coil conductors  59  and  61 , to reduce the magnetic path length of the magnetic path M, and to space the layers of the coil conductors  59  and  61  at a small and uniform distance. The numbers of turns of the coil conductors  59  and  61  may be increased in a limited region by reducing the conductor width of the coil conductors  59  and  61  and intervals between adjoining parts of the conductors to reduce the pitches of the conductors. However, a reduction in the conductor width results in an increase in the resistance of the coil conductors  59  and  61 . Under the circumstance, the ratio between the height and width (aspect ratio) of the coil sections of the coil conductors  59  and  61  may be increased to maintain the areas of the coil sections substantially constant, so that the resistance will not increase.
         Patent Document 1: JP-A-2003-133135   Patent Document 2: JP-A-11-54326   Patent Document 3: Japanese Patent Application No. 2003-307372   Patent Document 4: Japanese Patent No. 2011372       

   However, as shown in  FIG. 7A , when coil conductors  59  and  61  having an aspect ratio of 0.5 or more are formed using an electro-plating process, the top surfaces of the coil conductors  59  and  61  are curved in the form of convexes, and the bottom surfaces have a planar shape. Therefore, the inter-layer distance between the coil conductors  59  and  61  is shortest at the convex parts of the top surfaces of the coil conductors  59  and gradually increases toward both sides of the convexes. As a result, a capacitance (stray capacitance) between the coil conductors  59  and  61  decreases to reduce the degree of magnetic coupling between the coil conductors  59  and  61 , which results in a problem in that the common mode filter property is degraded. 
   A method for suppressing the reduction in the degree of magnetic coupling attributable to the shape of the top surfaces of the coils is to planarize the top surfaces of the coil conductors  59  and  61  using a chemical mechanical polishing process (CMP process) to make the coil sections rectangular, as shown in  FIG. 7B . In this case, however, the manufacturing cost is increased because of the need for the step for planarizing the top surfaces of the coil conductors  59  and  61 . 
   As thus described, when it is attempted to improve the degree of magnetic coupling by increasing the numbers of turns of the coil conductors  59  and  61  or decreasing the magnetic path length for the purpose of improving common mode filtering property, the capacitance generated between the coil conductors  59  and  61  decreases to hinder a sufficient improvement of the degree of magnetic coupling. When the top surfaces of the coil conductors  59  and  61  are planarized to increase the capacitance of coupling between the coil conductors  59  and  61 , the number of manufacturing steps increases, and this can result in the problem of an increase in the cost of the common mode choke coil  51  through an increase in the manufacturing cost. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to provide a compact and low-profile coil component having a high common mode filtering property and a method of manufacturing the same. 
   The above-described object is achieved by a coil component characterized in that it has a first coil conductor formed with a curved top portion, an insulation film formed on the first coil conductor so as to follow the shape of the top portion of the first coil conductor, a second coil conductor formed on the insulation film, the second coil conductor having a bottom portion formed so as to follow the shape of a top portion of the insulation film. 
   The above invention provides a coil component, characterized in that the center of the top portion of the first coil conductor has a convex shape in a section of the coil. 
   The above invention provides a coil component, characterized in that the second coil conductor is formed directly above the first coil conductor with the insulation film interposed between them. 
   The above invention provides a coil component, characterized in that at least either of the first or the second coil conductors is formed such that a section of the coil has an aspect ratio of 0.5 or more. 
   The above invention provides a coil component, characterized in that the distance between the first and second coil conductors is substantially constant. 
   The above invention provides a coil component, characterized in that the insulation film is formed of a shrinkable resist material. 
   The above-described object is achieved by a method of manufacturing a coil component, characterized in that it includes the steps of forming a first coil conductor having a curved top portion on a magnetic substrate, forming an insulation film on the first coil conductor such that it follows the shape of the top portion of the first coil conductor, and forming a second coil conductor on the insulation film, the second coil conductor having a bottom portion that follows the shape of a top portion of the insulation film. 
   The above invention provides a method of manufacturing a coil component, characterized in that it includes a step of shrinking and hardening a resist film made of a shrinkable resist material by heating the resist film to form the insulation film. 
   The above invention provides a method of manufacturing a coil component, characterized in that the resist film is formed such that it is higher than the uppermost portion of the first coil conductor by 20 to 50% of the height of the first coil conductor. 
   The above invention provides a method of manufacturing a coil component, characterized in that the first and second coil conductors are formed using a frame plating process. 
   The above invention provides a method of manufacturing a coil component, characterized in that the second coil conductor is formed above a convex portion of the insulation film. 
   The invention provides a method of manufacturing a coil component, characterized in that at least either of the first or the second coil conductors is formed such that a section of the coil has an aspect ratio of 0.5 or more. 
   The invention makes it possible to provide a compact and low-profile coil component having a high common mode filtering property. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectional view of a common mode choke coil  1  according to an embodiment of the invention; 
       FIGS. 2A to 2C  are sectional views of the common mode choke coil  1  according to the embodiment of the invention taken at manufacturing steps; 
       FIGS. 3A to 3C  are sectional views of the common mode choke coil  1  according to the embodiment of the invention taken at manufacturing steps; 
       FIGS. 4A to 4C  are sectional views of the common mode choke coil  1  according to the embodiment of the invention taken at manufacturing steps; 
       FIGS. 5A to 5C  are sectional views of the common mode choke coil  1  according to the embodiment of the invention taken at manufacturing steps; 
       FIG. 6  is a sectional view of the common mode choke coil  1  according to the embodiment of the invention taken at a manufacturing step; and 
       FIGS. 7A and 7B  are sectional views of a common mode choke coil  51  according to the related art. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A coil component and a method of manufacturing the same according to an embodiment of the invention will now be described with reference to  FIGS. 1 to 6 . The present embodiment will be described with reference to a common mode choke coil for suppressing a common mode current that can cause electromagnetic interference during balanced transmission, as an example of a coil component. First, a configuration of a common mode choke coil  1  will be described with reference to  FIG. 1 .  FIG. 1  shows a section of the common mode choke coil  1  taken along a plane including center axes of coil conductors  9  and  11 . 
   As shown in  FIG. 1 , the common mode choke coil  1  of the present embodiment comprises an insulation film  7   a  formed of polyimide resin on a magnetic substrate  3  formed of ferrite, a spiral coil conductor (first coil conductor)  9  formed of a conductive material, another insulation film  7   b  formed of a shrinkable resist material, another spiral coil conductor (second coil conductor)  11  formed of a conductive material, and another insulation film  7   c  formed of polyimide resin, the elements being stacked in the order listed. As will be apparent from above, the coil conductors  9  and  11  are embedded in an insulation layer  7  constituted by the insulation films  7   a  to  7   c.    
   The coil conductor  11  is disposed directly above the coil conductor  9  in a face-to-face relationship therewith with the insulation film  7   b  interposed between them. A plane of the coil conductor  9  orthogonal to the direction of a flow of a current through the conductor (a section of the coil) has a convex configuration in which the top portion of the coil section bulges in the middle thereof. The coil conductor  9  is formed such that the ratio between the height and the width of the section of the coil (aspect ratio=height/width) is 0.5 or more. In the present embodiment, a coil conductor  9  having an aspect ratio of substantially 1 in a section thereof is shown by way of example. Since the insulation film  7   b  formed on the coil conductor  9  is hardened such that it follows the shape of top portions (top surfaces) of the coil conductor  9  as a result of thermal shrinkage, the top portion (top surface) of the insulation film  7   b  has spiral irregularities in a general view of the same. 
   The coil conductor  11  is also formed to have an aspect ratio of 0.5 or more. In the present embodiment, a coil conductor  11  having an aspect ratio of substantially 1 in a section thereof is shown by way of example. The coil conductor  11  is formed on the convexes among the irregularities on the top surface of the insulation film  7   b  formed so as to follow the shape of the top surfaces of the coil conductor  9 . Therefore, bottom portions (bottom surfaces) of the coil conductor  11  are formed in a concave shape that follows the shape of the top surface of the insulation film  7   b . Thus, the bottom portions of the coil conductor  11  are formed such that they follow the shape of the top surfaces of the coil conductor  9  with the insulation film  7   b  interposed between them, and the distance between the coil conductors  9  and  11  is substantially constant. The insulation film  7   b  between the coil conductors  9  and  11  is formed with a thickness which is also substantially constant. 
   An opening  13  is formed on an inner circumferential side of the coil conductors  9  and  11  by removing the insulation layer  7 . An opening  15  is formed on an outer circumferential side of the coil conductors  9  and  11  by removing the insulation layer  7 . A magnetic layer  17  is formed such that it fills the openings  13  and  15  to improve the degree of magnetic coupling between the coil conductors  9  and  11  and to improve impedance characteristics through an increase in common impedance. The magnetic layer  17  is formed of a composite ferrite obtained by mixing magnetic powder made of ferrite in polyimide resin. Further, a bonding layer  19  is formed on the magnetic layer  17  and the insulation film  7   c  to bond a magnetic substrate  5  formed of ferrite. 
   An operation of the common mode choke coil  1  of the present embodiment will now be described. When the coil conductors  9  and  11  are energized, as shown in  FIG. 1 , a magnetic path M is formed in a section including center axes of the coil conductors  9  and  11 , the magnetic path passing through the magnetic substrate  3 , the magnetic layer  17  in the opening  13 , the bonding layer  19 , the magnetic substrate  5 , the bonding layer  19  again, and the magnetic layer  17  in the opening  15  in the order listed (or in the reverse order). The bonding layer  19  is a thin film having a thickness on the order of a few μm, although it is non-magnetic. Therefore, substantially no leakage of the magnetic flux occurs in this part, and the magnetic path M can be regarded as a substantially closed path. 
   The magnetic path length of the magnetic path M can be reduced by decreasing the interval between the coil conductors  9  and  11 . As a result, the degree of magnetic coupling between the coil conductors  9  and  11  is improved, and the common mode filtering property of eliminating a noise component at a predetermined frequency is thereby improved. Since the coil conductors  9  and  11  have a low resistance owing to the sectional shapes of the coils having a high aspect ratio, the common mode choke coil  1  can be used in applications in which a relatively high current will flow through them. 
   Further, the bottom portions of the section of the coil conductor  11  are formed in a concave shape that follow the convex shape of the top portions of the section of the coil conductor  9  with the insulation film  7   b  having a substantially constant thickness interposed between them. Therefore, the distance between the coil conductors  9  and  11  can be kept substantially constant. As a result, a high capacitance can be generated between the coil conductors  9  and  11 , which allows the degree of magnetic coupling between the coil conductors  9  and  11  to be improved to achieve a further improvement of the common mode filtering property. 
   As thus described, in the common mode choke coil  1 , the magnetic path length can be made short by the use of the coil conductors  9  and  11  having a coil section with a high aspect ratio, and the degree of magnetic coupling between the coil conductors  9  and  11  can be improved by forming the bottom surface of the coil conductor  11  such that it follows the top surface of the coil conductor  9  to make the distance between the coil conductors  9  and  11  short and constant. As a result, the common mode choke coil  1  can be provided with a high common mode filtering property, and it can be provided with a small size and a small height. 
   A method of manufacturing a common mode choke coil  1  according to the present embodiment will now be described with reference to  FIGS. 2A to 6 .  FIGS. 2A to 6  are sectional views of the common mode choke coil  1  taken at manufacturing steps along a plane including center axes of the coil conductors  9  and  11 . Elements having effects and functions similar to those of the elements of the common mode choke coil  1  shown in  FIG. 1  are indicated by like reference numerals and will not be described. 
   First, as shown in  FIG. 2A , polyimide resin is applied to a thickness of 7 to 8 μm on a magnetic substrate  3  formed of ferrite, and the resin is patterned to form an insulation film  7   a . The insulation film  7   a  is formed with openings  13  and  15 . Next, a frame plating process is used to form a coil conductor  9 . The frame plating process is a method of forming a plating film using a mold (frame) formed by patterning a resist layer. 
   As shown in  FIG. 2B , an electrode film  9   a  is formed on the entire surface using a sputtering process or evaporation process. A bonding layer constituted by two layers, e.g., a chromium (Cr) film having a thickness of 50 nm and a titanium (Ti) film having a thickness of 100 nm, may be formed under the electrode film  9   a  to improve the tightness of the bonding of the same to the insulation film  7   a . The electrode film  9   a  is preferably made of the same material as the metal material to be plated, although there is no problem as long as the material has conductivity. 
   Next, as shown in  FIG. 2C , a positive resist is applied to the entire surface to form a resist layer  21   a , and a pre-baking process is performed on the resist layer  21   a  as occasion demands. A negative resist may be used for the resist layer  21   a . Next, the resist layer  21   a  is exposed by irradiating it with exposure light through a mask  23  having a pattern for the coil conductor  9  drawn thereon. 
   Then, development is performed using an alkali developing solution after performing a thermal process as occasion demands. For example, a tetramethyl ammonium hydrooxide (TMAH) in a predetermined density is used as the alkali developing solution. The developing step is then followed by a cleaning step. The developing solution in the resist layer  21   a  is cleaned away using a cleaning fluid to stop the developing and dissolving reaction of the resist layer  21   a , thereby forming resist frames  21   b  patterned in the shape of the coil conductor  9  as shown in  FIG. 3A . For example, pure water is used as the cleaning fluid. 
   When the cleaning is completed, the cleaning fluid is scattered away to dry the substrate. The magnetic substrate  3  may be heated to dry and remove the cleaning fluid if necessary. Next, a plating process is carried out by immersing the magnetic substrate  3  in a plating solution in a plating bath and using the resist frames  21   b  as a mold to form a plating film  9   b  between the resist frames  21   b  as shown in  FIG. 3B . The plating film  9   b  is formed to have a convex sectional configuration in which the top surface bulges in the middle thereof. Next, as shown in  FIG. 3C , the resist frames  21   b  are removed from the electrode film  9   a  using an organic solvent after washing with water and drying the same as occasion demands. Next, as shown in  FIG. 4A , the electrode film  9   a  is removed by performing dry etching (ion milling or reactive ion etching (RIE), etc.) or wet etching using the plating film  9   b  as a mask. Thus, a coil conductor  9  constituted by the electrode film  9   a  and the plating film  9   b  having a convex top surface is formed. The magnetic substrate  3  is exposed at the openings  13  and  15  because the electrode film  9   a  is dry-etched. 
   When the coil conductor  9  is formed using a frame plating process, a highly shrinkable resist material is applied to the entire surface and patterned as shown in  FIG. 4B  to form a resist film  6 . The resist film  6  constitutes an insulation film  7   b  which is formed with openings  13  and  15  and which covers the coil conductor  9 . The resist film  6  is applied and formed to such a thickness that the film becomes higher than the uppermost portion of the coil conductor  9  by 20% to 50% of the height (thickness) of the coil conductor  9 . Next, as shown in  FIG. 4C , the resist film  6  is thermally shrunk and hardened by heating it to 190° C. to form an insulation film  7   b . Obviously, irradiation with UV light or the like may be also performed when the resist film  6  is hardened. The insulation film  7   b  has a certain thickness on the coil conductor  9 , and the film is hardened so as to follow the convex shape of the top surface of the coil conductor  9 , the top surface of the film consequently having spiral irregularities in a general view of the same. Thus, the top of the insulation film  7   b  has a wavy shape in a plane in parallel with a section of the coil. 
   Next, a coil conductor  11  is formed on the insulation film  7   b  using a frame plating process. An electrode film  11   a  is formed on the entire surface as shown in  FIG. 5A . A positive resist is then applied to the entire surface and patterned using a mask (not shown) having a pattern for the coil conductor  11  drawn thereon to form resist frames  25  which are patterned in the shape of the coil conductor  11 . The resist frames  25  are formed at concaves of the insulation film  7   b  above the gaps between adjoining conductors of the coil conductor  9  such that the coil conductor  11  will be formed directly above the coil conductor  9  with the insulation film  7   b  interposed between them, the frames also being formed at the openings  13  and  15 . The resist frames  25  may be formed using a negative resist. Next, a plating process is carried out by immersing the magnetic substrate  3  in a plating solution in a plating bath and using the resist frames  25  as a mold to form a plating film  11   b  between the resist frames  25  as shown in  FIG. 5B . The bottom surface of the plating film  11   b  has concaves because it is formed so as to follow the convexes on the top surface of the insulation film  7   b.    
   Next, as shown in  FIG. 5C , the resist frames  25  are removed from the electrode film  11   a  using an organic solvent, and the electrode film  11   a  is removed by performing dry etching or wet etching using the plating film  11   b  as a mask. Thus, a coil conductor  11  constituted by the electrode film  11   a  and the plating film  11   b  having concaves on the bottom surface thereof is formed. The magnetic substrate  3  is exposed at the openings  13  and  15  because the electrode film  11   a  is dry-etched. 
   Next, as shown in  FIG. 6 , polyimide resin is applied to the entire surface and patterned to form an insulation film  7   c  which is then cured. The insulation film  7   c  is formed with the openings  13  and  15 . 
   Next, although not shown, a magnetic layer  17  is formed by filling the openings  13  and  15  with a composite ferrite obtained by mixing magnetic powder made of ferrite in polyimide resin. A bonding agent is then applied to the magnetic layer  17  in the openings  13  and  15  and the insulation film  7   c  to form a bonding layer  19 . Next, a magnetic substrate  5  is secured on the bonding layer  19 . 
   Next, external electrodes (not shown) in connection with the coil conductors  9  and  11  are formed on sides of the magnetic substrates  3  and  5  opposite to each other such that they extend substantially perpendicularly to the substrate surfaces and across the magnetic substrates  3  and  5 . A common mode choke coil  1  as shown in  FIG. 1  is thus completed. 
   As described above, according to the method of manufacturing the common mode choke coil  1  in the present embodiment, since a highly shrinkable resist material is used for the insulation film  7   b  formed between the coil conductors  9  and  11 , the distance between the coil conductors  9  and  11  can be kept short and constant. As a result, magnetic coupling between the coil conductors  9  and  11  is improved to allow the common choke coil  1  to be formed with a high common mode filtering property. Further, sufficiently strong magnetic coupling can be achieved between the coil conductors  9  and  11  without planarizing the convex portions on the top surface of the coil conductor  9  resulting from increase in the aspect ratio of the sectional shape of the coil. Since this makes it possible to reduce the number of steps for manufacturing the common mode choke coil  1 , the manufacturing cost can be reduced to provide the common mode choke coil  1  at a low cost. 
   The invention is not limited to the above-described embodiment and may be modified in various ways. 
   While the coil conductor  9  in the above-described embodiment is formed with a convex configuration in which a top portion of the conductor in a section thereof bulges upward in the middle thereof, this is not limiting the invention. Even if the top portion in the section has a wavy shape or concave shape, since the insulation film  7   b  can be formed so as to follow the shape of the top surface of the coil conductor  9 , the bottom surface of the coil conductor  11  formed on the insulation film  7   b  can be formed so as to follow the top surface of the coil conductor  9 . Since the distance between the coil conductors  9  and  11  can therefore be kept short and constant, the same advantage as that in the above embodiment can be achieved. 
   While the coil conductor  11  in the above-described embodiment is formed with a convex configuration in which a top portion of the conductor in a section thereof bulges upward in the middle thereof, this is not limiting the invention. The same advantage as described in the above embodiment can be achieved even when the top surface of the coil conductor  11  has a wavy, concave or planar shape. 
   While the above-described embodiment includes the magnetic layer  17  which is formed to be embedded in the openings  13  and  15 , this is not limiting the invention. The same advantage as that in the above embodiment can be achieved by a structure in which the openings  13  and  15  and the magnetic layer  17  are not formed.