Patent Publication Number: US-2023133282-A1

Title: Wiring circuit board and method of producing the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority from Japanese Patent Application No. 2021-177287 filed on Oct. 29, 2021, the contents of which are hereby incorporated by reference into this application. 
     BACKGROUND ART 
     The present invention relates to a wiring circuit board and a method of producing the same. 
     Wiring circuit boards each including a metal support board, an insulating layer on the metal support board, and a wiring pattern, i.e., a wiring layer on the insulating layer are known. In the wiring circuit board, for example, a metal thin film for ensuring the adhesion of the insulating layer to the metal support board is provided between the metal support board and the insulating layer. The techniques related to the wiring circuit board are disclosed in, for example, Patent Document 1 described below. 
     Citation List 
     Patent Document 
     Patent Document 1: Japanese Unexamined Patent Publication No. 2019-212659 
     SUMMARY OF THE INVENTION 
     Problem to Be Solved by the Invention 
     The wiring circuit board includes, for example, a via that penetrates the insulating layer in the thickness direction to electrically connect the metal support board and the wiring pattern. Such a wiring circuit board is conventionally produced, for example, as follows. 
      First, a metal thin film, i.e., a first metal thin film is formed on a metal support board. Next, an insulating layer having a via hole is formed on the first metal thin film (insulating layer formation step). The insulating layer formation step includes a heating process. Next, a seed layer is formed on the insulating layer. The seed layer is formed also in the via hole. In the via hole, the seed layer is formed to cover the exposed surface of the first metal thin film and the inner wall surface of the via hole. The seed layer is a metal thin film, i.e., a second metal thin film. Next, a conductive layer is formed on the seed layer. The conductive layer includes a first conductive portion having a predetermined pattern on the insulating layer and a second conductive portion in the via hole. Next, the part of the seed layer that is not covered with the conductive layer is removed. In this manner, a wiring pattern composed of the seed layer and the first conductive portion thereon is formed on the insulating layer. Further, a via composed of the seed layer and the second conductive portion thereon is formed on the first metal thin film in the via hole. 
     In the insulating layer formation step in the conventional method as described above, the first metal thin film has an exposed surface at the via hole, and the exposed surface is oxidized in a heating process of the step. Thus, the via is formed on the first metal thin film having the oxidized film on its surface in the via hole. This means that the via in the finished wiring circuit board is electrically connected with the metal support board through the first metal thin film having the oxidized film. Similarly, when the seed layer is made of a material with a relatively high resistance, the via is electrically connected with the metal support board through the seed layer with such a relatively high resistance. Such configurations are not preferable for reducing the resistance of the electrical connection between the metal support board and the wiring layer. 
     The present invention provides a wiring circuit board suitable for achieving the low-resistance electrical connection between a metal support board and a wiring layer formed on the insulating layer on the board, and a method of producing the wiring circuit board. 
     Means for Solving the Problem 
      The present invention [1] includes a wiring circuit board comprising: a metal support board, a first metal thin film, an insulating layer, a second metal thin film, and a conductive layer in this order in a thickness direction, wherein the insulating layer has a through hole penetrating the insulating layer in the thickness direction, the through hole has a first opening end at the first metal thin film side, a second opening end opposite to the first opening end, and an inner wall surface between the first and second opening ends, the first metal thin film has a first opening portion, the first opening portion overlaps the first opening end in a projection view in the thickness direction, the second metal thin film has a second opening portion, the second opening portion overlaps the first opening portion and the second opening end in a projection view in the thickness direction, and the conductive layer has a via portion disposed in the through hole and connected to the metal support board. 
     The present invention [2] includes the wiring circuit board described in [1], wherein the metal support board includes a metal support layer and a surface metal layer disposed at the insulating layer side of the metal support layer and having a higher conductivity than the metal support layer, and the via portion is connected to the surface metal layer. 
     The present invention [3] includes the wiring circuit board described in [1] or [2], wherein the first opening portion is open along the first opening end. 
     The present invention [4] includes the wiring circuit board described in described in any one of [1] to [3], wherein the second metal thin film has a first covering portion on the inner wall surface. 
     The present invention [5] includes the wiring circuit board described in [4], wherein the second opening portion is open along the first opening portion on the metal support board. 
     The present invention [6] includes the wiring circuit board described in [4], wherein the second metal thin film has a second covering portion on the metal support board, and the second opening portion is disposed inside the first opening portion on the metal support board. 
     The present invention [7] includes the wiring circuit board described in described in any one of [1] to [3], wherein the second opening portion is open along the second opening end on the insulating layer. 
     The present invention [8] includes the wiring circuit board described in described in any one of [1] to [3], wherein the second opening portion is open on the insulating layer, and the second opening end is disposed within the second opening portion in a projection view in the thickness direction. 
     The present invention [9] includes the wiring circuit board described in [1] or [2], wherein the first metal thin film includes a protruding portion protruding in the first opening end in a projection view in the thickness direction to define the first opening portion, the second metal thin film includes a first covering portion on the inner wall surface and a second covering portion on the protruding portion, and the second opening portion is open along the first opening portion. 
     The present invention [10] includes a method of producing a wiring circuit board comprising: a first metal thin film forming step of forming a first metal thin film on a one-side surface in a thickness direction of the metal support board; an insulating layer forming step of forming an insulating layer on a one-side surface in the thickness direction of the first metal thin film, the insulating layer having a through hole having a first opening end at the first metal thin film side, a second opening end opposite to the first opening end, and an inner wall surface between the first and second opening ends; a second metal thin film formation step of forming a second metal thin film over a one-side surface in the thickness direction of the insulating layer and the first metal thin film in the through hole; an opening portion formation step of forming an opening portion in the first metal thin film and the second metal thin film, the opening portion overlapping the first opening end and the second opening end in a projection view in the thickness direction, to expose the metal support board at the through hole; and a conductive layer formation step of forming a conductive layer over a one-side surface in the thickness direction of the second metal thin film and the metal support board in the through hole. 
     The present invention [11] includes a method of producing a wiring circuit board comprising: a first metal thin film forming step of forming a first metal thin film on a one-side surface in a thickness direction of the metal support board; an insulating layer forming step of forming an insulating layer on a one-side surface in the thickness direction of the first metal thin film, the insulating layer having a through hole having a first opening end at the first metal thin film side, a second opening end opposite to the first opening end, and an inner wall surface between the first and second opening ends; a first opening portion formation step of forming a first opening portion in the first metal thin film, the first opening portion being open along the first opening end, to expose the metal support board at the through hole; a second metal thin film formation step of forming a second metal thin film over a one-side surface in the thickness direction of the insulating layer and the metal support board in the through hole; a second opening portion formation step of forming a second opening portion in the second metal thin film, the second opening portion overlapping the first opening end and the second opening end in a projection view in the thickness direction, to expose the metal support board at the through hole; and a conductive layer formation step of forming a conductive layer over a one-side surface in the thickness direction of the second metal thin film and the metal support board in the through hole. 
     Effects of the Invention 
     In a projection view in the thickness direction of the wiring circuit board of the present invention, the first opening portion of the first metal thin film overlaps the first opening end of the through hole of the insulating layer and the second opening portion of the second metal thin film overlaps the first opening portion and the second opening end of the through hole. The wiring circuit board of such is suitable for allowing, in the production process, the first opening portion, the through hole, and the second opening portion to once form a space, space for forming the via portion therein, continuous along the thickness direction. In the wiring circuit board, as described above, the via portion of the conductive layer is directly connected to the metal support board without the intervention of the first metal thin film and the intervention of the second metal thin film. The wiring circuit board is suitable for achieving a low-resistance electrical connection between the metal support board and the wiring layer formed on the insulating layer on the board. 
     The method of producing a wiring circuit board of the present invention is suitable to produce the wiring circuit board described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a partial cross-sectional view of one embodiment of the wiring circuit board of the present invention. 
         FIG.  2    is an enlarged partial cross-sectional view of the wiring circuit board of  FIG.  1   . 
         FIGS.  3 A to  3 D  illustrate some steps of one embodiment of the method of producing the wiring circuit board of the present invention.  FIG.  3 A  illustrates a preparation step.  FIG.  3 B  illustrates a first metal thin film formation step.  FIG.  3 C  illustrates an insulating base layer formation step.  FIG.  3 D  illustrates a second metal thin film formation step. 
         FIGS.  4 A to  4 D  illustrate the steps subsequent to the step of  FIG.  3 D .  FIG.  4 A  illustrates an opening portion formation step.  FIG.  4 B  illustrates a conductive layer formation step.  FIG.  4 C  illustrates an etching step.  FIG.  4 D  illustrates an insulating cover layer formation step. 
         FIGS.  5 A to  5 C  illustrate a variation of the conductive layer formation step and the etching step. 
         FIG.  6    is an enlarged partial cross-sectional view of the first variation of the wiring circuit board of  FIG.  1   . 
         FIG.  7    is an enlarged partial cross-sectional view of the second variation of the wiring circuit board of  FIG.  1   . 
         FIG.  8    is an enlarged partial cross-sectional view of the third variation of the wiring circuit board of  FIG.  1   . 
         FIG.  9    is an enlarged partial cross-sectional view of the fourth variation of the wiring circuit board of  FIG.  1   . 
         FIG.  10    is an enlarged partial cross-sectional view of the fifth variation of the wiring circuit board of  FIG.  1   . 
         FIG.  11    is an enlarged partial cross-sectional view of the sixth variation of the wiring circuit board of  FIG.  1   . 
         FIG.  12    is an enlarged partial cross-sectional view of the seventh variation of the wiring circuit board of  FIG.  1   . 
         FIG.  13    is an enlarged partial cross-sectional view of the eighth variation of the wiring circuit board of  FIG.  1   . 
         FIG.  14    is an enlarged partial cross-sectional view of the ninth variation of the wiring circuit board of  FIG.  1   . 
         FIG.  15    is an enlarged partial cross-sectional view of the tenth variation of the wiring circuit board of  FIG.  1   . 
         FIG.  16    is an enlarged partial cross-sectional view of the eleventh variation of the wiring circuit board of  FIG.  1   . 
         FIG.  17    is an enlarged partial cross-sectional view of the twelfth variation of the wiring circuit board of  FIG.  1   . 
         FIGS.  18 A to  18 C  illustrate some steps of another embodiment of the method of producing the wiring circuit board of the present invention.  FIG.  18 A  illustrates a preparation step.  FIG.  18 B  illustrates a first metal thin film formation step.  FIG.  18 C  illustrates an insulating base layer formation step. 
         FIGS.  19 A to  19 C  illustrate the steps subsequent to the step of  FIG.  18 C .  FIG.  19 A  illustrates a first opening portion formation step.  FIG.  19 B  illustrates a second metal thin film formation step.  FIG.  19 C  illustrates a second opening portion formation step. 
         FIGS.  20 A to  20 C  illustrate the steps subsequent to the step of  FIG.  19 C .  FIG.  20 A  illustrates a conductive layer formation step.  FIG.  20 B  illustrates an etching step.  FIG.  20 C  illustrates an insulating cover layer formation step. 
         FIG.  21    is an enlarged partial cross-sectional view of the thirteenth variation of the wiring circuit board of  FIG.  1   . 
         FIG.  22    is an enlarged partial cross-sectional view of the fourteenth variation of the wiring circuit board of  FIG.  1   . 
         FIG.  23    is an enlarged partial cross-sectional view of the fifteenth variation of the wiring circuit board of  FIG.  1   . 
     
    
    
     DESCRIPTION OF THE EMBODIMENT 
     As shown in  FIG.  1    and  FIG.  2   , a wiring circuit board X that is one embodiment of the wiring circuit board of the present invention includes a metal support board  10 , a metal thin film  20 , i.e., a first metal thin film, an insulating layer  30  as an insulating base layer, a metal thin film  40 , i.e., a second metal thin film, a conductive layer  50 , and an insulating layer  60  as an insulating cover layer in this order toward one side in a thickness direction D. The wiring circuit board X extends in a direction orthogonal to the thickness direction D, i.e., a surface direction, and has a predetermined plan-view shape. 
     The metal support board  10  is a substrate for ensuring the strength of the wiring circuit board X. Examples of the material of the metal support board  10  include stainless steels, copper, copper alloys, aluminum, nickel, titanium, and Alloy  42 . An example of the stainless steel includes SUS 304 in conformity with the standards of the American Iron and Steel Institute (AISI). In view of the strength of the metal support board  10 , the metal support board  10  preferably contains at least one selected from the group consisting of a stainless steel, a copper alloy, aluminum, nickel, and titanium. More preferably, the metal support board  10  consists of at least one selected from the group consisting of a stainless steel, a copper alloy, aluminum, nickel, and titanium. In view of the simultaneous achievement of the strength and conductivity of the metal support board  10 , the metal support board  10  preferably consists of a copper alloy. The metal support board  10  has a thickness of, for example, 15 µm or more. The thickness of the metal support board  10  is, for example, 500 µm or less, preferably 250 µm or less. 
     The metal thin film  20  is disposed on a one-side surface in the thickness direction D of the metal support board  10 . The metal thin film  20  is in contact with the metal support board  10 . The metal thin film  20  is a film for ensuring the adhesion of the insulating layer  30  to the metal support board  10 . Examples of the metal thin film  20  include a film formed in a sputtering method, i.e., a sputtering film, a film formed in a plating method, i.e., a plating film, and a film formed in a vacuum deposition method, i.e., a vacuum-deposited film. 
     Examples of the material of the metal thin film  20  include chromium, nickel, and titanium. The material of the metal thin film  20  may be an alloy containing two or more metals selected from the group consisting of chromium, nickel, and titanium. As the material of the metal thin film  20 , chromium is preferably used. 
     The metal thin film  20  has a thickness of, for example, 1 nm or more, preferably 10 nm or more, more preferably 20 nm or more. The thickness of the metal thin film  20  is, for example, 1000 nm or less, preferably 1000 nm or less, more preferably 500 nm or less. 
     The metal thin film  20  has an opening portion  20 A, i.e., a first opening portion penetrating the metal thin film  20  in the thickness direction D. The opening portion  20 A has, for example, an approximately circular shape in a plan view. The opening portion  20 A has a maximum length in the plan view (a diameter when the opening portion  20 A has a circular shape in the plan view) of, for example, 1 µm or more, and, for example, 1000 µm or less depending on the size of a through hole  30 A of the insulating layer  30  to be described below. The metal support board  10  has a portion  10   a  facing the opening portion  20 A. The portion  10   a  is not covered with the metal thin films  20  and  40 . 
     The insulating layer  30  is disposed on a one-side surface in the thickness direction D of the metal thin film  20 . The insulating layer  30  is in contact with the metal thin film  20 . Examples of the material of the insulating layer  30  include resin materials such as polyimide, polyether nitrile, polyether sulfone, polyethylene terephthalate, polyethylene naphthalate, and polyvinyl chloride. The same examples apply to the material of an insulating layer  60  to be described below. The insulating layer  30  has a thickness of, for example, 1 µm or more, preferably 3 µm or more. The thickness of the insulating layer  30  is, for example, 35 µm or less. 
     The insulating layer  30  has a through hole  30 A penetrating the insulating layer  30  in the thickness direction D. The through hole  30 A has an opening end  31 , i.e., a first opening end at a metal thin film  20  side, an opening end  32 , i.e., a second opening end opposite to the opening end  31 , and an inner wall surface  33  between the opening ends  31  and  32 . 
     The opening end  31  has, for example, an approximately circular shape in a plan view. The opening end  31  has a maximum length in the plan view (a diameter when the opening end  31  has a circular shape in the plan view) of, for example, 1 µm or more, and, for example, 1000 µm or less. In a projection view in the thickness direction D, the opening end  31  overlaps the opening portion  20 A of the metal thin film  20  (the positional relationship between the opening end  31  and the opening portion  20 A in the projection view in the thickness direction D is schematically shown under the cross-sectional view in  FIG.  2   ). In other words, the opening portion  20 A overlaps the opening end  31  in the projection view in the thickness direction D. In the present embodiment, the opening end  31  is open along the opening portion  20 A. In other words, the opening portion  20 A is open along the opening end  31 . Further, in the projection view in the thickness direction D, the opening end  31  includes the opening portion  20 A. 
     The opening end  32  has a maximum length (a diameter when the opening end  32  has a circular shape in the plan view) of, for example, 1 µm or more, and, for example, 1000 µm or less. In the present embodiment, the opening end  32  has a larger area than the opening end  31 . In the projection view in the thickness direction D, the opening end  32  includes the opening end  31 . 
     The inner wall surface  33  is inclined in the present embodiment. The inner wall surface  33  is inclined inward as approaching the metal support board  10 . In other words, the inner wall surface  33  is inclined so that the cross-sectional area of the opening of the through hole  30 A gradually decreases as getting closer to the metal support board  10 . 
     The through hole  30 A and the opening portion  20 A of the metal thin film  20  form a through hole H. 
     The metal thin film  40  is disposed directly on a one-side surface in the thickness direction D of the insulating layer  30  and on the inner wall surface  33  of the through hole  30 A in the present embodiment. The metal thin film  40  is a seed layer for forming a conductive layer  50 . Examples of the metal thin film  40  include a sputtering film, a plating film, and a vacuum-deposited film. 
     The metal thin film  40  includes a metal thin film  41  disposed outside the through hole H and a metal thin film  42  disposed inside the through hole H. The metal thin film  41  and the metal thin film  42  are connected. The metal thin film  41  has a predetermined patter on the insulating layer  30 . The metal thin film  42  covers the whole of the inner wall surface  33 . In other words, in the present embodiment, the metal thin film  42  is a covering portion  42   a , i.e., a first covering portion on the inner wall surface  33 . The metal thin film  42  including the covering portion  42   a  helps the growth of the metal on the covering portion  42   a  and the appropriate formation of the via portion  52  in the conductive layer formation step described below and illustrated in  FIG.  4 B . Further, the metal thin film  42  is connected to the metal thin film  20  in the through hole H. 
     The metal thin film  40  has an opening portion  40 A, i.e., a second opening portion penetrating the metal thin film  40  in the thickness direction D. The opening portion  40 A is in the through hole H in the present embodiment. The opening portion  40 A has, for example, an approximately circular shape in a plan view. The opening portion  40 A has a maximum length in the plan view (a diameter when the opening portion  40 A has a circular shape in the plan view) of, for example, 1 µm or more, and, for example, 1000 µm or less, depending on the size of the through hole  30 A. 
     In the projection view in the thickness direction D, the opening portion  40 A overlaps the opening portion  20 A of the metal thin film  20  and the opening end  32  of the through hole  30 A. The positional relationship between the opening portion  20 A, the opening portion  40 A, and the opening end  32  in the projection view in the thickness direction D is schematically shown under the cross-sectional view in  FIG.  2   . In the present embodiment, the opening portion  40 A is open along the opening portion  20 A on the metal support board  10 . 
      Examples of the material of the metal thin film  40  include chromium, copper, nickel, and titanium. The material of the metal thin film  40  may be an alloy containing two or more metals selected from the group consisting of chromium, copper, nickel, and titanium. As the material of the metal thin film  40 , chromium is preferably used. The metal thin film  40  may have a single-layered structure or a multi-layered structure with two or more layers. When the metal thin film  40  has a single-layered structure, the metal thin film  40  is preferably a chromium layer. When the metal thin film  40  has a multi-layered structure, the metal thin film  40  is preferably composed of a chromium layer as a lower layer and a copper layer on the chromium layer. 
     The metal thin film  40  has a thickness of, for example, 1 nm or more, preferably 10 nm or more. The thickness of the metal thin film  40  is, for example, 500 nm or less, preferably 200 nm or less. 
     The conductive layer  50  is disposed directly on a one-side surface in the thickness direction D of the metal thin film  40  and on the portion  10   a , i.e., a part of the metal support board  10  in the through hole H. The conductive layer  50  includes a wiring portion  51  disposed outside the through hole H and a via portion  52  disposed inside the through hole H. The wiring portion  51  has a predetermined pattern shape. A part of the wiring portion  51  is connected to the via portion  52 . The via portion  52  has a concave shape in the vertical cross section illustrated in  FIG.  2   . The via portion  52  has an inclined peripheral side surface  52   a . The peripheral side surface  52   a  is inclined inward as getting closer to the metal support board  10 . In other words, the peripheral side surface  52   a  is inclined so that the horizontal cross-sectional area of the via portion  52  gets smaller as getting closer to the metal support board  10 . Further, the via portion  52  is connected to the metal support board  10 . Specifically, the via portion  52  is directly connected to the metal support board  10  without the interventions of the metal thin films  20  and  40 . 
     Examples of the conductive layer  50  include copper, nickel, and gold. The material of the conductive layer  50  may be an alloy containing two or more metals selected from the group consisting of copper, nickel, and gold. As the material of the conductive layer  50 , copper is preferably used. 
     The metal thin film  41  and the wiring portion  51  on the metal thin film  41  form a wiring layer  71  having a predetermined pattern shape on the insulating layer  30 . The metal thin film  42  and the via portion  52  form a via  72  in the through hole H. The metal support board  10  is electrically connected with a part of the wiring layer  71  through the via  72 . The wiring layer  71  can be electrically connected with a ground through the via  72  and the metal support board  10 . 
     The wiring layer  71  has a thickness of, for example, 3 µm or more, preferably 5 µm or more. The thickness of the wiring layer  71  is, for example, 50 µm or less, preferably 30 µm or less. The wiring layer  71  has a width (a dimension in a direction orthogonal to a direction in which the wiring layer  71  extends) of, for example, 5 µm or more, preferably 8 µm or more. The width of the wiring layer  71  is, for example, 100 µm or less, preferably 50 µm or less. 
     The insulating layer  60  is disposed directly on the one-side surface in the thickness direction D of the insulating layer  30  to cover the wiring layer  71  and the via  72 . The insulating layer  60  has a thickness of, for example, 4 µm or more, preferably 6 µm or more. The thickness of the insulating layer  60  (the height from the insulating layer  30 ) is, for example, 60 µm or less, preferably 40 µm or less. The insulating layer  60  may have an opening portion at which the wiring layer  71  and/or the via  72  are/is partially exposed. In other words, the insulating layer  60  may have an opening portion, and the wiring layer  71  and/or the via  72  may be exposed at the opening portion. The part(s) of the wiring layer  71  and/or the via  72  exposed at the opening portion can function as, for example, a terminal portion of the wiring circuit board X. 
     In the projection view of the wiring circuit board X in the thickness direction D, as described above, the opening portion  20 A of the metal thin film  20  overlaps the opening end  31  of the through hole  30 A of the insulating layer  30  while the opening portion  40 A of the metal thin film  40  overlaps the opening portion  20 A and the opening end  32  of the through hole  30 A. In the present embodiment, the opening portion  20 A substantively coincides with the opening portion  40 A in the projection view in the thickness direction D. In the schematic projection view of  FIG.  2   , a region R in which all the opening portion  20 A, the opening portion40A, the opening end  31 , and the opening end  32  overlap each other is hatched with crossed lines. 
     The wiring circuit board X of such is suitable for allowing, in the production process, the opening portion  20 A, through hole  30 A, and opening portion  40 A to once form a space, space for forming the via portion therein, continuous along the thickness direction D. The region R continues over the opening portion  20 A, the through hole  30 A, and the opening portion  40 A in the thickness direction D. Further, in the wiring circuit board X as described above, the via portion  52  of the conductive layer  50  is directly connected to the metal support board  10  without the interventions of the metal thin films  20  and  40 . The wiring circuit board X is suitable for achieving a low-resistance electrical connection between the metal support board  10  and the wiring layer  71  on the insulating layer  30 . 
     In the wiring circuit board X, as described above, the metal thin film  40  includes the metal thin film  42  in the through hole  30 A of the insulating layer  30 . Further, the metal thin film  42  is connected to the metal thin film  20  on the metal support board  10 . Furthermore, the opening portion  40 A is open along the opening portion  20 A of the metal thin film  20 . Such a structure helps the growth of the metal on the covering portion  42   a  and the appropriate formation of the via portion  52  in the conductive layer formation step described below and illustrated in  FIG.  4 B . 
       FIGS.  3 A to  3 D  and  FIGS.  4 A to  4 D  show a method of producing the wiring circuit board X as one embodiment of the method of producing the wiring circuit board of the present invention.  FIGS.  3 A to  3 D  and  FIGS.  4 A to  4 D  show the change in the cross-section corresponding to  FIG.  1    as the production method of the wiring circuit board X. 
     In the production method, the metal support board  10  is prepared as shown in  FIG.  3 A  (preparation step). 
     Next, as shown in  FIG.  3 B , the metal thin film  20  is formed on a one-side surface in the thickness direction D of the metal support board  10  (first metal thin film formation step). Examples of the method of forming the metal thin film  20  include a sputtering method, a vacuum deposition method, and a plating method. Examples of the plating method include an electrolytic plating method and an electroless plating method. The metal thin film  20  is preferably formed by a sputtering method. 
     Next, as shown in  FIG.  3 C , the insulating layer  30  is formed on the one-side surface in the thickness direction D of the metal thin film  20  (insulating base layer formation step). In this step, the insulating layer  30  is formed, for example, as follows. First, a solution (varnish) of photosensitive resin is applied on the metal thin film  20  to form a film. Next, the film is dried by heating. Next, the dried film is subjected to an exposure process through a predetermined mask, and subsequently a development process. If needed, a baking process follows the development process. In this exemplary manner, the insulating layer  30  including the through hole  30 A is formed on the metal thin film  20 . The through hole  30 A includes, as described above, the opening end  31 , i.e., the first opening end at the metal thin film  20  side, the opening end  32 , i.e., the second opening end opposite to the opening end  31 , and the inner wall surface  33  between the opening ends  31  and  32 . At the through hole  30 A as described above, the portion  20   a  of the metal thin film  20  is exposed. The exposed surface of the portion  20   a  shown at the upper side of  FIG.  3 C  is oxidized through the heating process included in this step. 
     Next, as shown in  FIG.  3 D , the metal thin film  40  is formed as a seed layer (second metal thin film formation step). In this step, the metal thin film  40  is formed continuously on the one-side surface in the thickness direction D of the insulating layer  30 , on the inner wall surface  33  of the through hole  30 A, and on the portion  20   a  of the metal thin film  20  exposed at the through hole  30 A. This means that the metal thin film  40  includes the metal thin film  41  outside the through hole  30 A and the metal thin film  42  inside the through hole  30 A. Examples of the method of forming the metal thin film  40  include a sputtering method, a vacuum deposition method, and a plating method. Examples of the plating method include an electrolytic plating method and an electroless plating method. The metal thin film  40  is preferably formed by a sputtering method. 
     Next, shown in  FIG.  4 A , the opening portions  20 A and  40 A are formed in the metal thin films  20  and  40 , respectively (opening portion formation step). A specific example of the formation is as follows. 
     An etching mask M is formed on the metal thin film  40 . The etching mask M includes an etching opening portion Ma. In a plan view, the etching opening portion Ma has a shape corresponding to the shape of the opening portion  40 A. In the formation of the etching mask M, first, a photosensitive resist film is bonded onto the metal thin film  40  to form a resist film. Next, the resist film is subjected to an exposure process through a predetermined mask and subsequently a development process. If needed, a baking process follows the development process. In this manner, the etching opening portion Ma is formed, and the etching opening portion Ma corresponds to the opening portions  20 A and  40 A to be formed in the metal thin films  20  and  40 . 
     In this step, next, the metal thin films  20  and  40  are subjected to an etching process through the etching mask M on the metal thin film  40 . This etching process removes the part of the metal thin film  40  facing the etching opening portion Ma. Subsequently, the part of the metal thin film  20  facing the etching opening portion Ma is removed. In this manner, the opening portions  20 A and  40 A are formed. Thereafter, the etching mask M is removed from the metal thin film  40 . Examples of the etching process include wet etching and dry etching. Wet etching is preferred. Examples of the etching solution used in the wet etching include a ceric ammonium nitrate solution, a caustic soda solution, a potassium permanganate solution, and a sodium metasilicate solution. Preferably, a ceric ammonium nitrate solution is used. In the wet etching, the etching solution has a temperature of, for example, 20° C. or more, preferably, 30° C. or more. The temperature of the etching solution is, for example, 80° C. or less, preferably, 65° C. or less. The wet etching is carried out for an etching time of, for example, 1 minute or more. The etching time is, for example, 15 minutes or less, preferably 10 minutes or less. 
     As described above in this step, the opening portions  20 A and  40 A are formed in the metal thin films  20  and  40 , respectively. The opening portions  20 A and  40 A overlap opening ends  31  and  32  of the through hole  30 A in a projection view in the thickness direction D. Thus, this step exposes the metal support board  10  at the through hole  30 A. 
     Next, as shown in  FIG.  4 B , the conductive layer  50  is formed on a one-side surface in the thickness direction D of the metal thin film  40  and on the metal support board  10  in the through hole  30 A (conductive layer formation step). A specific example of the formation is as follows. 
     First, a resist pattern is formed on the metal thin film  40 . The resist pattern includes an opening portion while, in a plan view, the opening portion has a shape corresponding to the shape of the pattern of the conductive layer  50 . In the formation of the resist pattern, first, a photosensitive resist film is bonded onto the metal thin film  40  to form a resist film. Next, the resist film is subjected to an exposure process through a predetermined mask and subsequently a development process. If needed, a baking process follows the development process. In the formation of the conductive layer  50 , next, an electrolytic plating method is carried out to allow the above-describe metal to grow on the metal thin film  40  in the opening portion of the resist pattern. In the electrolytic plating method, the metal thin film  20  and the metal support board  10  are used in combination as a power supplying path member for the electrolytic plating. Next, the resist pattern is removed. In this exemplary manner, the conductive layer  50  having a predetermined pattern including the wiring portion  51  and the via portion  52  is formed on the one-side surface in the thickness direction D of the metal thin film  40 . 
     In this production method, next as shown in  FIG.  4 C , the part of the metal thin film  40  that is not covered with the conductive layer  50  is removed by etching (etching step). In this manner, the wiring layer  71  (the wiring portion  51  and the metal thin film  41 ) and the via  72  (the via portion  52  and the metal thin film  42 ) are formed. After this step, for example, an electroless plating method or an electrolytic plating method may be carried out to form a nickel film on a surface of the wiring layer  71 . 
     Next, as shown in  FIG.  4 D , the insulating layer  60  is formed on the insulating layer  30  to cover the wiring layer  71  and the via  72  (insulating cover layer formation step). In this step, the insulating layer  60  is formed, for example, as follows. First, a solution (varnish) of photosensitive resin is applied on the insulating layer  30  and on the wiring layer  71  and the via  72  to form a film. Next, the film is dried. Next, the dried film is subjected to an exposure process through a predetermined mask and subsequently a development process. If needed, a baking process follows the development process. As exemplified above, the insulating layer  60  is formed as an insulating cover layer. 
     As described above, the wiring circuit board X is produced. 
     In the insulating base layer formation step of the present production method as shown in  FIG.  3 C , the surface of the portion  20   a  of the metal thin film  20  facing the through hole  30 A is oxidized. In the second metal thin film formation step as shown in  FIG.  3 D , the metal thin film  40  is formed as a seed layer continuously on the insulating layer  30  and on the portion  20   a  inside the through hole  30 A. As the material of the metal thin film  40 , chromium is preferably used as described above. Chromium has a higher resistance than, for example, copper and is a conductor with a relatively high resistance. However, in the opening portion formation step as shown in  FIG.  4 A , the portions  20   a  and  40   a  of the metal thin films  20  and  40  are removed. In this manner, a part of the metal support board  10 , i.e., the portion  10   a  is exposed. Accordingly, the via portion  52 , which is formed in the conductive layer formation step as shown in  FIG.  4 B , is directly connected to the portion  10   a  of the metal support board  10 . 
     As described above, the present production method allows the formation of the wiring circuit board X where the via portion  52  is directly connected to the metal support board  10 . This means that the metal thin films  20  and  40  do not intervene in the electrical connection between the via portion  52  and the metal support board  10 . Thus, the present production method is suitable for achieving a low-resistance electrical connection between the metal support board  10  and the wiring layer  71  in the wiring circuit board X. 
     After the opening portion formation step as shown in  FIG.  4 A  in the present production method, the conductive layer formation step shown in  FIGS.  5 A and  5 B  and the subsequent etching step as shown in  FIG.  5 C  may be carried out instead of the conductive layer formation step as shown in  FIG.  4 B  and the etching step as shown in  FIG.  4 C . 
     In the conductive layer formation step, as shown in  FIG.  5 A , a conductive thin film  50   a  is formed first. Examples of the material of the conductive thin film  50   a  include the materials exemplifying the material of the conductive layer  50 . Copper is preferably used. Examples of the method of forming the conductive thin film  50   a  include a sputtering method and a vacuum deposition method. The conductive thin film  50   a  is preferably formed by a sputtering method. The conductive thin film  50   a  has a thickness of, for example, 1 nm or more, preferably 10 nm or more, and, for example, 500 nm or less, preferably 200 nm or less. 
     Next, as shown in  FIG.  5 B , a conductive layer  50   b  is formed on a one-side surface in the thickness direction D of the conductive thin film  50   a . Specifically, the conductive layer  50   b  is formed by an electrolytic plating method in the same manner as the method of forming the conductive layer  50  described above with reference to  FIG.  4 B ,. In this electrolytic plating method, the metal thin film  20  and the conductive thin film  50   a  are used as a power supplying path member for the electrolytic plating. Preferably, the metal thin film  20 , the conductive thin film  50   a , and the metal support board  10  are used in combination. The use in combination facilitates the appropriate formation of the via portion  52 . 
     In the subsequent etching step as shown in  FIG.  5 C , the regions of the metal thin film  40  and the conductive thin film  50   a  on which the conductive layer  50   b  is not formed are removed by etching. In this manner, the wiring layer  71  (the wiring portion  51  and the metal thin film  41 ) and the via  72  (the via portion  52  and the metal thin film  42 ) are formed. The wiring portion  51  and the via portion  52  are each formed from the conductive thin film  50   a  and the conductive layer  50   b  thereon. The dotted line of  FIG.  2    shows the boundary between the conductive thin film  50   a  and the conductive layer  50   b  when the conductive layer  50  is formed as described above. The same applies to the variations described below. 
     In the wiring circuit board X, as shown in  FIG.  6   , the metal thin film  20  may include a protruding portion  22  while the metal thin film  40  may include a covering portion  42   b  in addition to the covering portion  42   a  in the through hole H. The protruding portion  22  protrudes into the opening end  31  of the through hole  30 A to define the opening portion  20 A in a projection view in the thickness direction D. The covering portion  42   b  covers the protruding portion  22 . Then, the opening portion  40 A of the metal thin film  40  is open along the opening portion  20 A. 
     The schematic view under the cross-sectional view in  FIG.  6    shows the positional relationship between the opening portion  20 A of the metal thin film  20 , the opening ends  31  and  32  of the through hole  30 A, and the opening portion  40 A of the metal thin film  40  in a projection view of the variation, i.e., the first variation of the wiring circuit board X in the thickness direction D. In the projection view in the thickness direction D, the opening portion  20 A overlaps the opening end  31  while the opening portion  40 A overlaps the opening portion  20 A and the opening end  32 . In the projection view of the present variation in the thickness direction D, the opening portion  20 A and the opening portion  40 A substantively coincide with each other. In the schematic projection view of  FIG.  6   , the region in which all the opening portion  20 A, the opening portion  40 A, the opening end  31 , and the opening end  32  overlap each other is hatched with crossed lines. This applies to the drawings of the variations described below. 
     The first variation is produced in the same manner as the production method described above except for the following. The etching mask M used in the opening portion formation step shown in  FIG.  4 A  includes, an etching opening portion Ma with a smaller diameter than the opening end  31  of the through hole  30 A, where the etching opening portion Ma is within the opening end  31  in a projection view in the thickness direction D. 
      The first variation as described above is also suitable for allowing, in the production process, the opening portion  20 A, the through hole  30 A, and the opening portion  40 A to once form a space, space for forming the via portion therein, continuous along the thickness direction D. The region R continues over the opening portion  20 A, the through hole  30 A, and the opening portion  40 A in the thickness direction D. Further, the via portion  52  of the conductive layer  50  is directly connected to the metal support board  10  without the interventions of the metal thin films  20  and  40 . Accordingly, the first variation of the wiring circuit board X is also suitable for achieving a low-resistance electrical connection between the metal support board  10  and the wiring layer  71  on the insulating layer  30 . The following applies to the variations described below: the wiring circuit board has a region in which all the opening portion  20 A, the opening portion  40 A, the opening end  31 , and the opening end  32  overlap each other in a projection view in the thickness direction D; thus the variation is also suitable for allowing, in the production process, the opening portion  20 A, the through hole  30 A, and the opening portion  40 A to once form a space, space for forming the via portion therein, continuous along the thickness direction D; and therefore the variation is also suitable for achieving the low-resistance electrical connection between the metal support board  10  and the wiring layer  71  on the insulating layer  30 . 
     In the wiring circuit board X, as shown in  FIG.  7   , the opening portion  40 A of the metal thin film  40  may be open along the opening end  32  of the through hole  30 A on the insulating layer  30  while the opening portion  20 A of the metal thin film  20  may be open along the opening end  31  of the through hole  30 A on the metal support board  10 . In such a case, the metal thin film  40  does not include the metal thin film  42  in the through hole  30 A. 
     The schematic view under the cross-sectional view in  FIG.  7    shows the positional relationship between the opening portion  20 A of the metal thin film  20 , the opening ends  31  and  32  of the through hole  30 A, and the opening portion  40 A of the metal thin film  40  in a projection view of the variation, i.e., the second variation of the wiring circuit board X in the thickness direction D. In the projection view in the thickness direction D, the opening portion  20 A overlaps the opening end  31  while the opening portion  40 A overlaps the opening portion  20 A  and the opening end  32 . In the projection view of the present variation in the thickness direction D, the opening portion  20 A and the opening end  31  substantively coincide with each other while the opening portion  40 A and the opening end  32  substantively coincide with each other. 
     The second variation is produced in the same manner as the production method described above except for the following. The etching mask M used in the opening portion formation step shown in  FIG.  4 A  has an etching opening portion Ma that substantively coincides with the opening end  32  of the through hole  30 A in the projection view in the thickness direction D. 
     In the wiring circuit board X as shown in  FIG.  8   , the opening portion  40 A of the metal thin film  40  may be open on the insulating layer  30 , the opening end  32  may be disposed in the opening portion  40 A in a projection view in the thickness direction D, and the opening portion  20 A of the metal thin film  20  may be open along the opening end  31  of the through hole  30 A in the metal support board  10 . In such a case, the metal thin film  40  does not include the metal thin film  42  in the through hole  30 A. 
     The schematic view under the cross-sectional view in  FIG.  8    shows the positional relationship between the opening portion  20 A of the metal thin film  20 , the opening ends  31  and  32  of the through hole  30 A, and the opening portion  40 A of the metal thin film  40  in the projection view of the variation, i.e., the third variation of the wiring circuit board X in the thickness direction D. In the projection view in the thickness direction D, the opening portion  20 A overlaps the opening end  31  while the opening portion  40 A overlaps the opening portion  20 A and the opening end  32 . In the present variation, the opening portion  20 A and the opening end  31  substantively coincide with each other in the projection view in the thickness direction D. 
     The third variation is produced in the same manner as the production method described above except for the following. The etching mask M used in the opening portion formation step as shown in  FIG.  4 A  includes an etching opening portion Ma having a larger diameter than the opening end  32  of the through hole  30 A in a projection view in the thickness direction D and including the opening end  32 . 
     In the wiring circuit board X as shown in  FIG.  9   , the opening portion  40 A of the metal thin film  40  may be formed to partially overlap the opening ends  31  and  32  of the through hole  30 A of the insulating layers  30  in a projection view in the thickness direction D. In such a case, the inner wall surface  33  of the through hole  30 A includes a portion  33   a  that is covered with the metal thin film  40  and a portion  33   b  that is not covered with the metal thin film  40 . 
     The schematic view under the cross-sectional view in  FIG.  9    shows the positional relationship between the opening portion  20 A of the metal thin film  20 , the opening ends  31  and  32  of the through hole  30 A, and the opening portion  40 A of the metal thin film  40  in the projection view of the variation, i.e., the fourth variation of the wiring circuit board X in the thickness direction D. In the projection view in the thickness direction D, the opening portion  20 A overlaps the opening end  31  while the opening portion  40 A overlaps the opening portion  20 A and the opening end  32 . 
     The fourth variation is produced in the same manner as the production method described above except for the following. The etching mask M used in the opening portion formation step as shown in  FIG.  4 A  includes an etching opening portion Ma that partially overlaps the opening ends  31  and  32  of the through hole  30 A in the projection view in the thickness direction D. 
     In the wiring circuit board X as shown in  FIG.  10   , the opening portion  20 A of the metal thin film  20  is formed to partially overlap the opening end  31  of the through hole  30 A of the insulating layer  30  in the projection view in the thickness direction D. 
     The schematic view under the cross-sectional view in  FIG.  10    shows the positional relationship between the opening portion  20 A of the metal thin film  20 , the opening ends  31  and  32  of the through hole  30 A, and the opening portion  40 A of the metal thin film  40  in the projection view of the variation, i.e., the fifth variation of the wiring circuit board X in the thickness direction D. In the projection view in the thickness direction D, the opening portion  20 A overlaps the opening end  31  while the opening portion  40 A overlaps the opening portion  20 A and the opening end  32 . 
     The fifth variation is produced in the same manner as the production method described above except for the following: the opening portion  20 A of the metal thin film  20  is formed after the first metal thin film formation step shown in  FIG.  3 B ; and the opening portion  20 A is formed at a position where the opening portion  20 A partially overlaps the through hole  30 A of the insulating layer  30  in the insulating base layer formation step shown in  FIG.  3 C . The opening portion  20 A is formed by, for example, the same method as the opening portion formation step shown in  FIG.  4 A . Further, the etching mask M used in the opening portion formation step shown in  FIG.  4 A  includes, in the opening end  31 , an etching opening portion Ma with a smaller diameter than the opening end  31  of the through hole  30 A in the projection view in the thickness direction D. 
     As shown in  FIG.  11   , the wiring circuit board X may include a metal support board  10 ′ instead of the metal support board  10 . The metal support board  10 ′ includes a metal support layer  11  and a surface metal layer  12  disposed at the insulating layer  30  side of the metal support layer  11 . 
     The metal support layer  11  is a substrate for ensuring the strength of the wiring circuit board X. Examples of the material of the metal support layer  11  include the materials exemplifying the material of the metal support board  10 . In view of the strength of the metal support layer  11 , the metal support layer  11  preferably contains at least one selected from the group consisting of a stainless steel, a copper alloy, aluminum, nickel, and titanium, and more preferably consists of at least one selected from the group consisting of a stainless steel, a copper alloy, aluminum, nickel, and titanium. In view of the simultaneous achievement of the strength and conductivity of the metal support layer  11 , the metal support layer  11  preferably consists of a copper alloy. The metal support layer  11  has a thickness of, for example, 15 µm or more. The thickness of the metal support layer  11  is, for example, 500 µm or less, preferably 250 µm or less. 
     The surface metal layer  12  is disposed on a one-side surface in the thickness direction D of the metal support layer  11 . The surface metal layer  12  is in contact with the metal support layer  11 . In the present embodiment, the surface metal layer  12  is disposed on the whole of the one-side surface in the thickness direction D of the metal support layer  11 . Examples of the surface metal layer  12  include sputtering films, plating films, and vacuum-deposited films. The surface metal layer  12  has a higher conductivity than the metal support layer  11 . In view of the conductivity of the surface metal layer  12 , the surface metal layer  12  preferably contains at least one selected from the group consisting of gold, silver, and copper, and more preferably consists of at least one selected from the group consisting of gold, silver, and copper. In view of the film formability of the surface metal layer  12  when the metal support layer  11  is a copper alloy, the surface metal layer  12  preferably consists of copper. The thickness of the surface metal layer  12  is preferably 0.5 µm or more, more preferably 3 µm or more. 
     In the present variation, i.e., the sixth variation, the via portion  52  of the conductive layer  50  is connected to the surface metal layer  12  of the metal support board  10 ′. 
     The sixth variation is produced in the same manner as the production method described above except that the metal support board  10 ′ is prepared instead of the metal support board  10  in the preparation step. 
     In the sixth variation as described above, the via portion  52  is connected to the surface metal layer  12  with a higher conductivity than the metal support layer  11  in the metal support board  10 ′. Such a structure is preferable for achieving the low-resistance electrical connection between the metal support board  10 ′ and the wiring layer  71 . 
       FIG.  12    shows the seventh variation in which the metal support board  10  of the first variation is replaced with the metal support board  10 ′.  FIG.  13    shows the eighth variation in which the metal support board  10  of the second variation is replaced with the metal support board  10 ′.  FIG.  14    shows the ninth variation in which the metal support board  10  of the third variation is replaced with the metal support board  10 ′.  FIG.  15    shows the tenth variation in which the metal support board  10  of the fourth variation is replaced with the metal support board  10 ′.  FIG.  16    shows the eleventh variation in which the metal support board  10  of the fifth variation is replaced with the metal support board  10 ′. 
     In the wiring circuit board X, as shown in  FIG.  17   , the metal thin film  40  may be in contact with the metal support board  10  in the through hole H while the opening portion  40 A of the metal thin film  40  may be open along the opening portion  20 A of the metal thin film  20  on the metal support board  10 . In a projection view of this wiring circuit board X in the thickness direction D, the opening portion  20 A of the metal thin film  20  overlaps the opening end  31  of the through hole  30 A of the insulating layer  30  while the opening portion  40 A of the metal thin film  40  overlaps the opening portion  20 A and the opening end  32  of the through hole  30 A. In the projection view of this variation, i.e., the twelfth variation in the thickness direction D, the opening portion  20 A and the opening end  31  substantively coincide with each other. 
       FIGS.  18 A to  18 C ,  FIGS.  19 A to  19 C , and  FIGS.  20 A to  20 C  show a method of producing the twelfth variation of the wiring circuit board X shown in  FIG.  17    as another embodiment of the method of producing the wiring circuit board of the present invention. 
     In the present production method, first, the metal support board  10  is prepared as shown in  FIG.  18 A  (preparation step). 
     Next, as shown in  FIG.  18 B , the metal thin film  20  is formed on the one-side surface in the thickness direction D of the metal support board  10  (first metal thin film formation step). The details of the formation are the same as described above with reference to  FIG.  3 B . 
     Next, as shown in  FIG.  18 C , the insulating layer  30  is formed on the one-side surface in the thickness direction D of the metal thin film  20  (insulating base layer formation step). The details of the formation are the same as described above with reference to  FIG.  3 C . 
     Next, as shown in  FIG.  19 A , the opening portion  20 A is formed in the metal thin film  20  (first opening portion formation step). The details of the formation are, for example, as follows. 
     First, an etching mask M′ is formed on the insulating layer  30 . The etching mask M′ includes an etching opening portion Mb. In a plan view, the etching opening portion Mb has a shape corresponding to the shape of the opening end  31  of the through hole  30 A of the insulating layer  30 . For the formation of the etching mask M′, first, a photosensitive resist film is bonded onto the insulating layer  30  to form a resist film. Next, the resist film is subjected to an exposure process through a predetermined mask and subsequently a development process. If needed, a baking process follows the development process. In this manner, the etching opening portion Mb corresponding to the opening portion  20 A to be formed on the metal thin film  20  is formed. 
     In this step, next, the metal thin film  20  is subjected to an etching process through the etching mask M′ on the insulating layer  30 . This etching process removes the part of the metal thin film  20  facing the etching opening portion Mb. In this manner, the opening portion  20 A is formed. Thereafter, the etching mask M′ on the insulating layer  30  is removed. Examples of the etching process include wet etching and dry etching. Wet etching is preferred. The etching solution and etching conditions for the etching process are the same as those of the etching process described above with reference to  FIG.  4 A . 
     In this step, the opening portion  20 A is formed on the metal thin film  20  as described above. The opening portion  20 A overlaps and substantively coincides with the opening end  31  of the through hole  30 A in a projection view in the thickness direction D. By this step, the metal support board  10  is exposed at the through hole  30 A. 
     Next, as shown in  FIG.  19 B , the metal thin film  40  is formed as a seed layer (second metal thin film formation step). In this step, the metal thin film  40  is formed continuously on the one-side surface in the thickness direction D of the insulating layer  30 , on the inner wall surface  33  of the through hole  30 A, and on the portion  10   a  of the metal support board  10  exposed at the through hole  30 A. The metal thin film  40  includes the metal thin film  41  outside the through hole  30 A and the metal thin film  42  inside the through hole  30 A. The method of forming the metal thin film  40  is the same as described with reference to  FIG.  3 D . 
     Next, as shown in  FIG.  19 C , the opening portion  40 A is formed on the metal thin film  40  (second opening portion formation step). The details of the formation is the same as described with reference to  FIG.  4 A . In this step, the opening portion  40 A open along the opening portion  20 A on the metal support board  10  is formed, thereby exposing the metal support board  10  at the through hole  30 A. The opening portions  20 A and  40 A overlap the opening ends  31  and  32  of the through hole  30 A in a projection view in the thickness direction D. 
     Next, as shown in  FIG.  20 A , the conductive layer  50  including the wiring portion  51  and the via portion  52  is formed throughout on the one-side surface in the thickness direction D of the metal thin film  40  and the metal support board  10  in the through hole  30 A (conductive layer formation step). The details of the formation are the same as described with reference to  FIG.  4 B . 
     Next, as shown in  FIG.  20 B , the part of the metal thin film  40  that is not covered with the conductive layer  50  is removed by etching (etching step). The details of the formation are the same as described with reference to  FIG.  4 C . By this step, the wiring layer  71  (the wiring portion  51  and the metal thin film  41 ) and the via  72  (the via portion  52  and the metal thin film  42 ) are formed. 
     Next, as shown in  FIG.  20 C , the insulating layer  60  is formed on the insulating layer  30  to cover the wiring layer  71  and the via  72  (insulating cover layer formation step). The details of the formation are the same as described with reference to  FIG.  4 D . 
     As described above, the twelfth variation of the wiring circuit board X is produced. 
     In the insulating base layer formation step of the present production method as shown in  FIG.  18 C , a surface of the portion  20   a  of the metal thin film  20  facing the through hole  30 A is oxidized. However, in the subsequent first opening portion formation step shown in  FIG.  19 A , the portion  20   a  of the metal thin film  20  is removed. Further, in the second metal thin film formation step shown in  FIG.  19 B , the metal thin film  40  is formed as a seed layer continuously on the insulating layer  30  and on the portion  10   a  of the metal support board  10 . As the material of the metal thin film  40 , chromium is preferably used as described above. Chromium has a higher resistance than, for example, copper and is a conductor with a relatively high resistance. However, in the subsequent second opening portion formation step as shown in  FIG.  19 C , the portion  40   a  of the metal thin film  40  is removed. In this manner, a part, i.e., the portion  10   a  of the metal support board  10  is exposed. Consequentially, the via portion  52  formed in the conductive layer formation step as shown in  FIG.  20 A  is directly connected to the portion  10   a  of the metal support board  10 . 
     As described above, the present production method allows the formation of the wiring circuit board X where the via portion  52  is directly connected to the metal support board  10 . This means that the metal thin films  20  and  40  do not intervene in the electrical connection between the via portion  52  and the metal support board  10 . Thus, the present production method is suitable for achieving the low-resistance electrical connection between the metal support board  10  and the wiring layer  71  in the wiring circuit board X. 
     After the second opening portion formation step as shown in  FIG.  19 C  in the present production method, the conductive layer formation step described with reference to  FIG.  5 A  and  FIG.  5 B  and subsequently the etching step described with reference to  FIG.  5 C  may be carried out instead of the conductive layer formation step as shown in  FIG.  20 A  and the etching step as shown in  FIG.  20 B . This method allows the formation of the conductive layer  50  having a laminate structure including the conductive thin film  50   a  and the conductive layer  50   b  as described above with reference to  FIGS.  5 A to  5 C . 
      In the wiring circuit board X, as shown in  FIG.  21   , the metal thin film  40  may include a covering portion  42   b  that is in contact with and covers the metal support board  10  in the through hole H. The covering portion  42   b  protrudes into the opening end  31  of the through hole  30 A to define the opening portion  40 A in a projection view in the thickness direction D. The opening portion  40 A is disposed inside the opening portion  20 A of the metal thin film  20 . 
     The schematic view under the cross-sectional view in  FIG.  21    shows the positional relationship between the opening portion  20 A of the metal thin film  20 , the opening ends  31  and  32  of the through hole  30 A, and the opening portion  40 A of the metal thin film  40  in the projection view of the thirteenth variation of the wiring circuit board X in the thickness direction D. In the projection view in the thickness direction D, the opening portion  20 A overlaps the opening end  31  while the opening portion  40 A overlaps the opening portion  20 A and the opening end  32 . In the projection view of the present variation in the thickness direction D, the opening portion  20 A and the opening end  31  substantively coincide with each other and the opening portion  40 A is disposed in the opening end  31 . 
     The thirteenth variation is produced in the same manner as the production method of the twelfth variation shown in  FIGS.  18 A to  20 C  except for the following. The etching mask M used in the second opening portion formation step shown in  FIG.  19 C  includes, in the opening end  31 , an etching opening portion Ma with a smaller diameter than the opening end  31  of the through hole  30 A in the projection view in the thickness direction D. 
       FIG.  22    shows the fourteenth variation in which the metal support board  10  of the twelfth variation is replaced with the metal support board  10 ′. The fourteenth variation is produced in the same method as the twelfth variation except that the metal support board  10 ′ is prepared instead of the metal support board  10  in the preparation step. 
       FIG.  23    shows the fifteenth variation in which the metal support board  10  of the thirteenth variation is replaced with the metal support board  10 ′. The fifteenth variation is produced in the same method as the thirteenth variation except that the metal support board  10 ′  is prepared instead of the metal support board  10  in the preparation step. 
     While the illustrative embodiments of the present invention are provided in the above description, such is for illustrative purpose only and it is not to be construed as limiting in any manner. Modification and variation of the present invention that will be obvious to those skilled in the art is to be covered by the following claims. 
     Description of Reference Numerals  
     
       
         
           
               
               
            
               
                 X 
                 wiring circuit board 
               
               
                 D 
                 thickness direction 
               
               
                 
                   10 
                 
                 metal support board 
               
               
                 
                   10 
                   a 
                 
                 portion 
               
               
                 
                   11 
                 
                 metal support layer 
               
               
                 
                   12 
                 
                 surface metal layer 
               
               
                 
                   20 
                 
                 metal thin film (first metal thin film) 
               
               
                   20 A 
                 opening portion (first opening portion) 
               
               
                 
                   20 
                   a 
                 
                 portion 
               
               
                 
                   30 
                 
                 insulating layer 
               
               
                   30 A, 
                 H through hole 
               
               
                 
                   31 
                 
                 opening end (first opening end) 
               
               
                 
                   32 
                 
                 opening end (second opening end) 
               
               
                 
                   33 
                 
                 inner wall surface 
               
               
                 
                   40 
                 
                 metal thin film (second metal thin film) 
               
               
                   40 A 
                 opening portion (second opening portion) 
               
               
                 
                   40 
                   a 
                 
                 portion 
               
               
                 
                   50 
                 
                 conductive layer 
               
               
                 
                   51 
                 
                 wiring portion 
               
               
                 
                   52 
                 
                 via portion 
               
               
                 
                   60 
                 
                 insulating layer 
               
               
                 
                   71 
                 
                 wiring layer 
               
               
                 
                   72 
                 
                 via