Abstract:
Disclosed is an electromagnetic wave shielding apparatus for shielding electromagnetic waves generated from a circuit so as to prevent the electromagnetic waves from being leaked to the outside of the circuit. The apparatus includes a metal member disposed in such a manner as to be opposed to the circuit, and an electromagnetic wave absorbing member disposed on the metal member, wherein the electromagnetic absorbing member is composed of a carbon layer and coil-like carbon fiber structures. The apparatus is allowed to simply obtain an electromagnetic wave absorbing function without increasing the weight of the apparatus.

Description:
RELATED APPLICATION DATA 
     The present application claims priority to Japanese Application No. P10-116623 filed Apr. 27, 1998 which application is incorporated herein by reference to the extent permitted by law. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to an electromagnetic wave shielding apparatus and an electromagnetic wave shielding method used for electric circuits, electronic circuits and the like. 
     In electric circuit arrangements, electronic circuit arrangements and the like, it is necessary to prevent electromagnetic waves generated from electronic parts and the like installed in the circuit arrangement from being leaked to the outside. For example, a circuit board  100  of a related art electronic circuit arrangement shown in FIG. 7 has electronic parts  101  and  102  which generate electromagnetic waves  103  in the directions shown by the arrows, and which circuit board  100  is covered with a case  104 . In this case, the electromagnetic waves  103  are reflected from the inner surfaces of the case  104  and leaked to the outside through a small gap  105  and/or wiring. The case  104 , if being made from a metal, produces a high-frequency current when it is exposed to the electromagnetic waves, and therefore, the case  104  becomes a re-generating source of electromagnetic wave noise. 
     To cope with such an inconvenience, in the related art electronic circuit arrangement, as shown in FIG. 8, an electromagnetic wave absorbing member  110  is fixed on a surface, facing to the electronic parts, of the inner wall of the case  104 . The electromagnetic wave absorbing member  110  functions to absorb or attenuate the electromagnetic waves  103  and prevent leakage of the electromagnetic waves  103  to the outside as much as possible, to reduce the degree of reflection of the electromagnetic waves  103  and decay the electromagnetic waves  103 , and to prevent re-generating of electromagnetic wave noise from the case  104 . 
     The related art electromagnetic wave absorbing member  110 , however, has disadvantages that it takes a lot of time to lay out the member  110  because the member  110  must be stuck on the inner surface of the shield case  104  with an adhesive double coated tape or the like by an operator in such a manner as to ensure the optimum absorption of electromagnetic waves, and that the weight of the circuit board  100  becomes very large because the member  110  having a large thickness is stuck on the inner surface of the case  104  with an adhesive double coated tape or the like. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an electromagnetic wave shielding apparatus and an electromagnetic wave shielding method, which are capable of simply obtaining an electromagnetic wave shielding function without increasing the weight of the apparatus. 
     To achieve the above object, according to a first aspect of the present invention, there is provided an electromagnetic wave shielding apparatus for shielding electromagnetic waves generated from a circuit so as to prevent the electromagnetic waves from being leaked to the outside of the circuit, including: a metal member disposed in such a manner as to be opposed to the circuit; and an electromagnetic wave absorbing member disposed on the metal member; wherein the electromagnetic absorbing member is composed of a carbon layer and coil-like carbon fiber structures. 
     With this configuration, since the electromagnetic wave absorbing member is composed of the carbon layer and the coil-like carbon fiber structures which are formed on the metal member, it can be made lighter than that of the related art electromagnetic wave absorbing member, and since the coil-like carbon fiber structures are electrically conductive, the electromagnetic wave absorbing member is capable of efficiently absorb electromagnetic waves generated from an electromagnetic wave generating source through the coil-like carbon fiber structures. 
     In the above apparatus, preferably, the carbon layer is formed on a surface, facing to the circuit, of the metal members; one-end portions of the coil-like carbon fiber structures are connected to the carbon layer; and the axial directions of the coil-like carbon fiber structures are substantially perpendicular to an electromagnetic wave generating source. With this configuration, electromagnetic waves generated from an electromagnetic wave generating source can be efficiently absorbed in the carbon layer through the coil-like carbon fiber structures. 
     According to a second aspect of the present invention, there is provided an electromagnetic wave shielding method for shielding electromagnetic waves generated from a circuit so as to prevent the electromagnetic waves from being leaked to the outside of the circuit, including the steps of: forming an electromagnetic wave absorbing member on a metal member; and disposing the metal member in such a manner that the metal member is opposed to the circuit; wherein the electromagnetic wave member is composed of a carbon layer and coil-like carbon fiber structures produced by a chemical vapor deposition process based on thermal decomposition. 
     With this configuration, the electromagnetic wave absorbing member can be simply formed on the metal member by chemical vapor deposition, being made lightweight, and is capable of efficiently absorb electromagnetic waves. 
     In the above method, preferably, the carbon layer is formed on a surface, facing to the circuit, of the metal members; one-end portions of the coil-like carbon fiber structures are connected to the carbon layer; and the axial directions of the coil-like carbon fiber structures are substantially perpendicular to an electromagnetic wave generating source. 
     With this configuration, since the electromagnetic wave absorbing member is composed of the carbon layer and the coil-like carbon fiber structures which are formed on the metal member, it can be made lighter than that of the related art electromagnetic wave absorbing member, and since the coil-like carbon fiber structures are electrically conductive, the electromagnetic wave absorbing member is capable of efficiently absorb electromagnetic waves generated from an electromagnetic wave generating source through the coil-like carbon fiber structures. 
     According to the present invention, a shield case for shielding a circuit can be simply prepared by working the metal member on which the electromagnetic wave absorbing member has been formed, into a desired shape. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view showing one example of a circuit board including an electromagnetic wave shielding apparatus according to the present invention; 
     FIG. 2 is a perspective view, with parts partially cutaway, showing the circuit board including the electromagnetic wave shielding apparatus shown in FIG. 1; 
     FIG. 3 is an enlarged sectional view showing a shield case and an electromagnetic wave absorbing member; 
     FIG. 4 is a schematic view showing one example of a method of forming a carbon layer and coil-like carbon fiber structures on a plate-like raw material of the shield case by chemical vapor deposition; 
     FIG. 5 is a perspective view showing one example of the coil-like carbon fiber structure; 
     FIG. 6 is a flow chart showing one example of a method of forming coil-like carbon fiber structures on a plate-like raw material; 
     FIG. 7 is a schematic view showing a circuit board including a related art shield case; and 
     FIG. 8 is a schematic view showing a circuit board including a related art shield case having an electromagnetic wave absorbing member. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Hereinafter, one preferred embodiment of the present invention will be described with reference to the accompanying drawings. 
     FIG. 1 shows a circuit board  12  on which an electromagnetic wave shielding apparatus  10  of the present invention is mounted. The circuit board  12  has a conductor pattern of an electric or electronic circuit, and for example, electronic parts  14 ,  16  and  18  are electrically connected to the conductor pattern. 
     A box-shaped shield case  20  is, having at least one wall, mounted on the circuit board  12  in such a manner as to cover the electronic parts  14 ,  16  and  18 . 
     An electromagnetic wave absorbing member  30  is formed over an inner surface  22  of the shield case  20 . The electromagnetic wave absorbing member  30  is formed, as shown in FIGS. 1 and 2, into a sheet composed of, as shown by the partial enlarged view of FIG. 3, a carbon layer  60  and numberless coil-like carbon fiber structures (may be called carbon coils)  40 . The coil-like carbon fiber structures  40 , substantially parallel to each other, are formed on the carbon layer  60  in the direction substantially perpendicular thereto. To be more specific, one-end sides  41  of the coil-like carbon fiber structures  40  are connected to the carbon layer  60  and the other end sides thereof are taken as free ends or entangled with each other. The coil-like carbon fiber structures are also called helical coil-like carbon fiber structures, which are basically composed of carbon fibers produced by thermal decomposition of a hydrocarbon gas. One example of the coil-like carbon fiber structure  40  will be described with reference to FIG.  5 . 
     The coil-like carbon fiber structure  40  shown in FIG. 5 is electrically conductive. The fiber diameter (fiber thickness) L 1  of the fiber structure  40  is in a range of 0.05 to 5 μm. The coil outside diameter L 2  of the coil structure  40  is about 2 to 10 times greater than the fiber diameter L 1 , that is, in a range of 0.1 to 50 μm. The axial length of the fiber structure  40  is in a range of 3 to 30 μm. The number of turns of the fiber structure  40  is in a range of about 1 to 500. Further, the number of turns×(unit length (10 μm)/coil outside diameter L 2 ) is in a range of 5 to 50. 
     The coil-like carbon fiber structures  40  having the above configuration, which are essentially made from carbon, can be obtained by vapor-phase thermal decomposition of a gas containing a hydrocarbon based gas, particularly, acetylene gas, in a system in which a transition metal is present, at a temperature ranging from 700 to 800° C. 
     Examples of the above hydrocarbon based gases may include an unsaturated hydrocarbon gas such as acetylene, ethylene, or propylene gas and a saturated hydrocarbon gas such as ethane, propane, or butane gas. In particular, acetylene gas is most preferably used from the viewpoint of the catalytic action of a transition metal. 
     The above hydrocarbon gas may be mixed with hydrogen. In addition to this, a diluting gas such as argon, nitrogen, or helium can be of course used for controlling the shape of the coil-like carbon fiber structure  40 . 
     One example of a method of forming the electromagnetic wave absorbing member  30 , which is composed of the carbon layer  60  and the coil-like carbon fiber structures  40  as shown in FIG. 3, on the inner surface  22  of the shield case  20  will be described with reference to FIGS. 4 and 6. 
     A plate-like raw material  74 , which will be taken as a flat-plate shield case  20 , is first prepared. The plate-like raw material  74  is made from a conductive material such as iron, nickel, copper or permalloy and has a thickness ranging from 0.1 to 0.5 mm. 
     Next, at step SP 1  in FIG. 6, the plate-like raw material  74  for forming the shield case  20  is coated with powder  76  of nickel as nuclei for growth of coil-like carbon fiber structures  40  on the conductive plate-like raw material  74 . The powder  76  of nickel has an average particle size of about 5 μm. The plate-like raw material  74  coated with the powder  76  of nickel is mounted on a susceptor  72  in a reactor  70  shown in FIG.  4 . At step SP 2 , the plate-like raw material  74  coated with the powder  76  of nickel is heated in the reactor  70  at a temperature ranging from 700 to 800° C., and at the same time, a reaction gas  80  is uniformly supplied to the plate-like raw material  74  from a gas inlet  78 . To be more specific, a mixed gas of acetylene, hydrogen and chiophene as the reaction gas is allowed to flow to the powder  76  of nickel on the plate-like raw material  74  through a special shower head. 
     The mixed gas (reaction gas) thus supplied is decomposed on the surface of the plate-like raw material  74 . Thus, at step SP 3 , a carbon component is deposited as a carbon layer  60  shown in FIG.  3 . At step SP 4 , part of the carbon component is grown in vapor-phase, with crystal grains of nickel taken as nuclei, on the carbon layer  60  formed on the plate-like raw material  74  in the direction substantially perpendicular thereto, to form coil-like carbon fiber structures  40 . To be more specific, the coil-like carbon fiber structures  40  grow toward the flow-in direction of the reaction gas  80 . 
     At step SP 5 , the numberless coil-like carbon fiber structures  40  grow in such a manner as to be arranged along a specific direction as shown in FIGS. 3 and 4. In this case, it is important that the reaction gas  80  uniformly flows in the direction R, that is, in the direction perpendicular, or substantially perpendicular to the plate-like metal raw material  74 . 
     At step SP 6 , the plate-like raw material  74  formed as shown in FIG. 4, which is removed from the reactor  70 , is cut into a specific dimension and is bent to form a shield case  20  shown in FIGS. 1 and 2. In this way, the electromagnetic wave absorbing member  30  is formed over the inner surface  22  of the shield case  20 . 
     Electromagnetic waves generated from the electronic parts  14 ,  16  and  18  shown in FIG. 1, which are the electromagnetic wave generating sources, is made incident on each of the coil-like carbon fiber structures  40  shown in FIG. 3 in the axial direction thereof. At this time, the coil-like carbon fiber structure  40  produces an induction current “i”. Since the coil-like carbon fiber structure  40  is positioned substantially perpendicularly to the electronic parts  14 ,  16  and  18  as the electromagnetic wave generating sources, it receives the electromagnetic waves (variable magnetic field) in the axial direction and introduces these electromagnetic waves to the carbon layer  60 . The carbon layer  60  absorbs the electromagnetic waves thus introduced by the coil-like carbon fiber structures  40 . 
     Since the electromagnetic wave absorbing member  30  can be formed over the inner surface  22  of the shield case  20 , it can absorb the electromagnetic waves  90  without leakage thereof to the outside. That is to say, the electromagnetic wave absorbing member  30  is desirable to be formed on a widest first surface  22   a  and four side surfaces  22   b  of the inner surface  22  as shown in FIGS. 1 and 2. 
     Since the electromagnetic wave absorbing member  30  can be formed over the inner surface of the shield case  20  by chemical vapor deposition, the present invention has the following merits: 
     (1) Unlike the related art electromagnetic wave absorbing member stuck on the metal member with an adhesive double coated tape, the electromagnetic wave absorbing member  30  is formed over the inner surface  22  of the shield case  20  by chemical vapor deposition, and accordingly, it is possible to thinly form the electromagnetic wave absorbing member  30  and to eliminate the fear that the member  30  is peeled from the inner surface of the shield case  20 . 
     (2) The period of time required for layout of the electromagnetic wave absorbing member  30  can be shortened. That is to say, since the leakage level of electromagnetic waves to the outside is lowered, the margin of the circuit design is increased and thereby the final adjustment of the circuit can be omitted. 
     (3) Since the electromagnetic wave absorbing member  30  is stuck on the metal member by chemical vapor deposition (CVD) based on thermal decomposition, it is possible to thinly form the member  30 , to eliminate the sticking work using an adhesive double coated tape, and to reduce the weight of the member  30 . 
     (4) Since the electromagnetic wave absorbing member  30  can be formed at a time over the inner surface  22  of the shield case  20 , it is possible to increase the ability of preventing the leakage of the electromagnetic waves  90  to the outside. 
     In the above embodiment, description is made by way of the example in which the box-shaped shield case  20  is disposed on an electric or electronic circuit board; however, the shape of the shield case  20  and the shape of the electromagnetic wave absorbing member  30  formed on the shield case  20  are not limited to those shown in the figures. 
     In the example shown in the figures, the coil-like carbon fiber structures  40  are formed in the directions perpendicular or substantially perpendicular to the inner surface of the shield case  20 ; however, the present invention is not limited thereto. For example, the coil-like carbon fiber structures  40  may be slightly tilted with respect to the inner surface of the shield case  20 . In this case, the same effect can be obtained. 
     The electromagnetic wave absorbing member  30  may be formed by tightening the coil-like carbon fiber structures by means of a non-conductive material such as rubber or plastic. 
     While the preferred embodiment has been described using specific terms, such description is illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.