Abstract:
To provide a method for producing a semiconductor substrate able to uniformly and quickly fill through-holes in the semiconductor substrate with conductive material. This method comprises a process for forming through-holes ( 14 ) in a substrate ( 10 ), a process for disposing solder ( 42 ) on one surface of the substrate, and a process for pressing the solder on a side of the substrate by a press ( 40 ) and heat-melting the solder to fill the through-holes in the substrate with the solder.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a method for producing a circuit board on which is mounted a semiconductor element and, particularly, to a method for producing a circuit board improved in the filling of an electro-conductive material in holes provided in substrate material.  
         [0003]     2. Description of Related Invention  
         [0004]     There is a circuit board on which are mounted semiconductor elements, wherein circuit patterns formed on both surfaces of a substrate having the electro-insulation are electrically connected to each other via conductive vias in through-holes provided through the substrate. Such a circuit board may be formed as a multi-layer circuit board by superposing a plurality of circuit patterns on both surfaces thereof via insulation layers, and a semiconductor device is produced by mounting semiconductor elements on the multi-layer circuit board.  
         [0005]     FIGS.  1  ( a ) to ( f ) illustrate a prior art disclosed in Patent document 1, wherein conductive metal is filled in the through-holes provided in an insulation and adhesive substrate by an electrolytic plating, and circuit patterns formed on one surface thereof via the conductive metal are electrically connected to those formed on the other surface in the same manner.  
         [0006]      FIG. 1 ( a ) illustrates a substrate  10  to be a body of the circuit board. The substrate  10  is an insulation material onto which a metallic foil such as copper foil forming a conductive layer for the circuit pattern is bonded by heating and pressing. For example, a non-cured resinous plate of a glass cloth impregnated with thermosettable resin can be used for the substrate  10 .  
         [0007]      FIG. 1 ( b ) illustrates a state wherein through-holes  14  are provided in the substrate  10  by a laser beam machining. The through-holes  14  can be easily formed by the laser beam machining in the substrate  10  formed of the glass cloth impregnated with resin. According to the laser beam machining, even a through-hole  14  as small as less than 100 μm diameter, which is difficult to mechanically drill, can be simply and accurately formed.  
         [0008]      FIG. 1 ( c ) shows a state wherein a copper foil  30   a  is bonded by the heating and pressing as a metallic foil onto one surface of the substrate  10  provided with the through-holes  14 . The substrate  10  is formed of a glass cloth impregnated with non-cured thermosettable resin, and the resin is melted when the copper foil  30   a  is heated and pressed, and bonded to the substrate  10  as a one-piece body. In this regard, a barrier layer  32   a  of nickel is preliminarily provided on one surface of the copper foil  30   a  to be bonded to the substrate  10 , and the copper foil  30   a  is bonded to the substrate  10  while using the barrier layer  32   a  as a surface to be bonded to the substrate  10 . The barrier layer  32   a  prevents solder from being diffused into the copper foil  30   a , to generate voids on the boundary surface, thus increasing the conductive resistance when the circuit patterns are electrically connected between the layers in a post process.  
         [0009]     As shown in  FIG. 1 ( c ), as the copper foil  30   a  is bonded onto the one surface of the substrate  10 , the through-hole  14  is closed on one side to define a recess  14   a .  FIG. 1 ( d ) illustrates a state wherein the electrolytic plating has been carried out while using, as a power supply layer, the copper foil  30   a  bonded to the one surface of the substrate  10  to fill the interior of the recess  14   a  with conductive metal  34 .  
         [0010]      FIG. 1 ( e ) illustrates a state wherein a copper foil  30   b  is bonded onto the other surface of the substrate  10  by the heating and pressing. On one surface of the copper foil  30   b  bonded to the other surface of the substrate  10 , a barrier layer  32   b  of nickel is preliminarily provided in the same manner as described before. The conductive metal  34  is solder because solder melts due to the heating when the conductive metal  34  is heated and pressed onto the other surface of the substrate  10 , as shown in  FIG. 1 ( e ), to weld the copper foil  30   b  with the conductive metal  34 .  
         [0011]     Regarding the copper foil  30   a  bonded to the one surface of the substrate  10 , as the conductive metal  34  is bonded to the barrier layer  32   a  by the electrolytic plating, the conductive metal  34  is assuredly electrically conducted to the copper foil  30   a . On the other hand, regarding the other surface of the substrate  10 , the electric conduction between the conductive metal  34  and the copper foil  30   b  becomes insufficient if the copper foil  30   b  is merely bonded to the substrate  10 . However, as the conductive metal  34  is melted when the copper foil  30   b  is thermally press-bonded to the substrate  10 , the copper foil  30   b  is integral with the substrate  10 , and the copper foil  30   b  is assuredly electrically connected with the conductive metal  34  during the thermal press-bonding of the copper foil  30   b  to the substrate  10 . In this regard, while the resin in the substrate  10  is in a non-cured state upon bonding the copper foil  30   b  to the substrate  10 , the resin is completely cured to bond the substrate  10  to the copper foil  30   b  as a one-piece body by heating the resin to a melting point and then cooling the same to a hardening point. Even if cracks are generated in the resin of the substrate in the preceding laser beam machining process, they are remedied by the process for integrally melt-bonding the copper foil  30   b  to the substrate  10 .  
         [0012]      FIG. 1 ( f ) illustrates a state wherein circuit patterns  36  are formed on opposite surfaces of the substrate  10  by etching the copper foils  30   a ,  30   b  and the barrier layers  32   a ,  32   b  bonded thereto. The circuit patterns  36  formed on both surfaces of the substrate are electrically connected each other through the conductive metal  34  formed as conductive sections. Thus, the substrate shown in  FIG. 1 ( f ) forms a circuit board  38  in which the circuit patterns  36  formed on the both surfaces of the substrate are electrically conducted to each other via the conductive sections.  
         [0013]      FIG. 2  illustrates in detail a process, disclosed in Patent Document 1, for filling the recesses  14   a  with the conductive metal  34  by plating as shown in  FIG. 1 ( d ). Note, in  FIG. 2 , the substrate  10  is a silicon substrate having a thickness in a range from 200 to 250 μm and is provided with the through-holes  14  of 60 μm diameter at a pitch in a range from 100 to 200 μm.  
         [0014]     In this case, the electrolytic plating is carried out while using, as a power supply layer, the copper foil  30   a  bonded to one surface of the substrate  10 . An anode plate  40  of metal to be filled is disposed on a side of the other surface of the substrate  10  at a position slightly apart from the substrate  10 , and the electrolytic plating is carried out in a proper electrolytic plating tank (not shown) by the well-known method. As the copper foil  30   a  is bonded to the one surface of the substrate  10 , one of opposite openings of the through-hole  14  is closed by the copper foil  30   a  which is the power supply layer. On the other hand, as the other of the openings of the through-hole  14  is free, and the anode plate  40  of the filling metal is disposed at the position slightly apart from the substrate  10 , the conductive metal  34  is deposited on the copper foil  30   a  in the interior of the through-hole  14  as the electrolytic plating progresses whereby the conductive metal is growing in the interior of the through-hole  14  to perfectly fill the interior of the through-hole  14  with the conductive metal.  
         [0015]     According to such electrolytic plating using the copper foil  30   a  as a plating power supply layer, it is easy to fill the interior of the through-hole  14  with the conductive metal  34  even if the through-hole has a small diameter. In this regard, when the electrolytic plating is carried out by using the copper foil  30   a  as a plating power supply layer, an exposed surface of the copper foil  30   a  may be covered with a plate-protection film as a whole so that a thickness of the copper foil  30   a  does not increase by the plating during the formation of the conductive metal  34 . An electrolytic plating for filling the recess  14   a  is a solder plating so that the recess  14   a  is filled with solder.  
         [0016]     According to JP-A 10-275966, a bonding pin is inserted into a through-hole connected to a conductor circuit of a printed circuit board which is bonded to a pad of a mated substrate via the bonding pin. The bonding pin is made of material not melted at a temperature at which the bonding pin is heat-bonded to the mated pad, and consists of a bonding head forming a section to be bonded with the mated pad, having a diameter larger than that of the opening of the through-hole, a leg having a size capable of inserting into the interior of the through-hole. The leg is bonded to the through-hole by conductive material such as solder inserted into the through-hole.  
         [0017]     As shown in JP-A 2003-332705 described above, electrolytic plating, as a method for filling the through-hole of the circuit board with conductive material, is problematic particularly when an inner diameter of the through-hole is small in that a long time is necessary for the growth of the conductive material in the through-hole, the growth speed of the conductive material is slow in a central area of the through-hole although that in the wall surface area and the peripheral area in the vicinity thereof is relatively fast whereby the uniform growth of the conductive material is not expected, and air bubbles may occur in the conductive material, during the depositing of the metal in the electrolytic plating, and form voids therein to disturb the formation of the dense growth of the conductive material. To solve such problems, the agitation of plating solution in the electrolytic tank may be carried out, for example. However, a satisfactory result has not been obtained.  
         [0018]     An object of the present invention is to provide a method for manufacturing a semiconductor substrate capable of uniformly and quickly filling the interior of a through-hole with conductive material when the through-hole of the substrate is filled with conductive material.  
         [0019]     To achieve the above-mentioned object, according to the present invention, a method for producing a semiconductor substrate, comprising the following steps of: forming through-holes in a substrate, disposing molten solder on one surface of the substrate, and pressing the solder onto the substrate by a press while heating to melt the solder to be filled into the through-holes in the substrate. Thereby, it is possible to fill the solder which is conductive material in the through-hole of the semiconductor substrate at a low cost, whereby it is possible to complete the filling operation of the conductive material into the through-hole of the substrate in a shorter time.  
         [0020]     To facilitate the wetting of a wall of the through-hole in the substrate by the solder, a wetting power improvement agent is preliminarily coated on the through-hole of the substrate. In this case, as the wetting power improvement agent, a flux having a relatively low viscosity is used. By such a treatment for improving the wetting power between the wall surface of the through-hole in the substrate and the solder, it is possible to densely fill the conductive material into the through-hole of the semiconductor substrate in a short time.  
         [0021]     When the solder is pressed onto the semiconductor substrate by the press, the solder is sucked from the opposite side of the substrate. By applying such a suction from the opposite side, it is possible to assist the pressure caused by the press to shorten a time necessary for filling the conductive material in the through-hole from one surface to the other surface of the substrate.  
         [0022]     The pressing operation of the solder onto the substrate by the press by the press is carried out under a reduced pressure environment. By carrying out the pressing operation under such a reduced pressure environment, a surface tension of the solder decrease when the solder is filled in the through-hole of the semiconductor substrate, whereby it is possible to densely fill the conductive material into the through-hole of the substrate in a shorter time. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     The present invention will be described in more detail below with reference to the attached drawings wherein:  
         [0024]     FIGS.  1 ( a ),  1 ( b ),  1 ( c ),  1 ( d ),  1 ( e ) and  1 ( f ) sequentially illustrate a prior art processes for producing a circuit board;  
         [0025]      FIG. 2  illustrates a process for filling the through-hole by the prior art electrolytic plating; and  
         [0026]      FIG. 3  illustrates a method for producing a semiconductor substrate including a process for filling the through-hole by a press according to the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]     In  FIG. 3 , the semiconductor substrate  10  is preferably made of insulation material such as a ceramic, for example, silicon substrate. A resinous insulating substrate, such as a glass-epoxy substrate, may be used. In such a case, a substrate high in heat resistance is used. This is because the melting point of a tin/silver solder is 260° C.  
         [0028]     The silicon substrate  10  has a thickness, for example, in a range from 200 to 250 μm and is provided with a number of through-holes  14  having a diameter of 60 μm and arranged at a pitch in a range from 100 to 200 μm. Although a process for forming the through-holes  14  is not shown, it is carried out by the laser beam machining or precision drilling in the same manner as in the prior art.  
         [0029]     Prior to the subsequent pressing process, the through-hole  14  of the silicon substrate  10  is subjected to the coating operation of a flux for forming a film for improving the wetting power between the wall of the through-hole  14  and solder. The flux used preferably has as low a viscosity as possible. Preferably, to facilitate the wetting of the silicon material forming the substrate  10 , a thin copper film  14   a  is preliminarily formed on the wall surface of the through-hole  14  by sputtering or plating.  
         [0030]     Then, a plate-like solder  42  is mounted to one surface of the silicon substrate  10 . The solder  42  is preferably tin/silver solder having a relatively high melting point of approximately 260° C. Alternatively, lead-rich tin/lead solder may be used. A size of the plate-like solder  42  is such that it has an area covering a region of the silicon substrate  10  provided with a number of through-holes  14  (for example, a 10 mm square area) and a volume larger than a total volume of all the through-hoes  14  to be filled therewith.  
         [0031]     To easily melt the solder  42  in a shorter time in the subsequent pressing process, the silicon substrate  10  itself is preferably preliminarily heated to approximately 150° C. when the plate-like solder  42  is mounted to the silicon substrate  10 . Thereby, prior to the subsequent pressing process, the solder  42  is heated to a temperature close to 150° C. which is the temperature of the silicon substrate  10 .  
         [0032]     The plate-like solder  42  is pressed by the press  40  from the one surface of the silicon substrate  10  on which is disposed the plate-like solder  42 , and heated. The press  40  is preliminarily heated to a temperature higher than the melting point of the solder  42  (if the solder  42  is tin/silver solder, the melting point thereof is 260° C.). Accordingly, after the plate-like solder  42  is pressed by the press  40 , heat of the press  40  is transferred to the plate-like solder  42  to melt the latter. At the commencement of the melting of the solder  42 , the press  40  is forcibly moved toward the one surface of the silicon substrate  10  to press the melted and fluidized solder  42   a  into the through-holes  14  of the silicon substrate  10 .  
         [0033]     To heat the pate-like solder  40  in a shorter time, in addition to heating the press  40  itself, other heating means such as an electric heater may be provided around the plate-like solder  40 .  
         [0034]     As the film  14   a  for improving the wetting by the solder is preliminarily coated on the wall surface of the through-hole  14  in the silicon substrate  10 , when the fluidized solder  42   a  flows into the through-hole  14 , the adverse effect of the surface tension of the solder is less, whereby the solder is favorably in close contact with the wall surface of the through-hole.  
         [0035]     A suitable suction device not shown is provided for sucking the fluidized solder  42   a  from the other surface side of the silicon substrate  10  to facilitate the flow-in of the solder  42   a  into the through-hole  14  when the plate-like solder  42  is pressed by the press  40  from the one surface side of the silicon substrate  10 , whereby the movement of the fluidized solder  42   a  in the through-hole  14  is accelerated. Alternatively, the devices including the press  40  may be operated under a reduced pressure.  
         [0036]     When the through-hole  42   a  of the silicon substrate  10  is filled with the solder  42   a , the press  40  is moved away from the silicon substrate  10 , and the residual solder adhered onto the surface of the silicon substrate  10  is removed. Thus, the operation is finished.  
         [0037]     The present invention has been described above based on the preferred embodiment thereof with reference to the attached drawings. The present invention, however, is not limited to the above-mentioned embodiment but includes various changes and modifications thereof without departing from the spirit or scope of the present invention.  
         [0038]     As described above, according to the present invention, through-holes are formed in a semiconductor substrate, and a solder is disposed on one surface thereof and heated and melted to fill the through-hole in the substrate therewith, whereby it is possible to uniformly and quickly fill the through-holes with the conductive material, resulting in a dense conductor via.