Patent Publication Number: US-2003221969-A1

Title: Method for filling blind via holes

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Technical Field of the Invention  
       [0002] This invention relates to a method for filling blind via holes with metallic copper by electrically copper-plating inside blind via holes formed in a silicon wafer.  
       [0003] 2. Description of the Related Art  
       [0004] For a multi-layered LSI board, it has been known to fill a blind via hole 1 μm or less in diameter and with an aspect ratio of around 5, preferentially from the bottom of the hole. Further, it has been also known, in the case of forming a printed circuit board of a multi-layered structure by a build-up method, to form a blind via hole 100 μm or more in diameter and with an aspect ratio of 3 or less in the board to be built up, and to copper-plate inside the hole.  
       [0005] However, air spaces are produced in the case of filling the blind via hole having a particularly large aspect ratio with the metallic copper by copper-plating inside the hole, according to a known method. Specifically, when a blind via hole having an opening diameter of 1 μm or less is filled by a known method, the plating speed is increased at a part in a range from the hole opening edge to around 20 μm depth, due to a strong promotion action on the copperization. As the result, the opening part is closed prior to the time the inside of the blind via hole is filled in, leaving voids in the via hole. When the blind via hole having an opening diameter of 100 μm or more is filled in by the known method, the plated film is distributed in conformity with the shape in the blind via hole, although the opening edge is not closed, leaving an elongated void extending from the opening edge to the bottom around the center portion of the blind via hole.  
       [0006] The present invention was made in response to the situation mentioned above, and has an object of providing a method for filling blind via holes with metallic copper excellent in conductivity, without leaving voids in the holes.  
       SUMMARY OF THE INVENTION  
       [0007] The inventors made an extensive study to achieve the above object. They found that the object is accomplished by using a copper sulfate plating bath containing specified ingredients, and by providing an electric copper-plating treatment under specified electrolytic conditions, and created the following invention.  
       [0008] (1) A method for filling blind via holes with metallic copper, comprises etching a substrate to form blind via holes with an inner wall, forming a seed layer for plating on the inner wall of the blind via holes, and conducting an electric copper-plating treatment with the seed layer as one electrode, in a copper sulfate plating bath, to fill the bind via holes with metallic copper, wherein said copper sulfate plating bath contains ingredients (a) and (b) below:  
       [0009] (a) a polyether containing at least five ether oxygen atoms in a molecule; and  
       [0010] (b) a compound represented by formula (I) below: 
       R 1 —S—(CH 2 O) n —R 2 —SO 3 M  (I) 
       [0011] wherein, R 1  is a hydrogen atom, —(S) n —(CH 2 O) n —R 2 —SO 3 M, or —CS n —(CH 2 O) n —R 2 —SO 3 M, R 2  is an alkylene group having 3 to 8 carbon atoms, M is a hydrogen atom or alkaline metal, and n is 0 or 1,  
       [0012] and said electric copper-plating treatment is conducted by reversing the current in a cycle of 1 to 50 msec. for a time of positive electrolysis, 0.2 to 5 msec. for a time of reverse electrolysis, and 1 to 50 msec. for a stop time, between one electrode comprising the ground layer and another electrode immersed in said copper sulfate plating bath.  
       [0013] (2) The method for filling blind via holes according to the above (1), wherein said ingredient (a) comprises one or more among the substances represented by formulae (II) to (IV) below: 
       HO—(CH 2 —CH 2 —O) a —H  (II) 
       [0014] wherein a=5 to 500  
                 
 
       [0015] wherein a=5 to 200  
                 
 
       [0016] (3) The method for filling blind via holes according to above (1), wherein said ingredient (b) comprises one or more among the substances represented by formulae (V) to (X) below: 
       M—SO 3 —(CH 2 ) a —S—(CH 2 ) b —SO 3 —M  (V) 
       [0017] wherein a=3 to 8, b=3 to 8, M is a hydrogen or alkali metal element 
       M—SO 3 —(CH 2 ) a —O—CH 2 —S—CH 2 —O—(CH 2 ) b —SO 3 —M  (VI) 
       [0018] wherein a=3 to 8, b=3 to 8, M is a hydrogen or alkali metal element 
       M—SO 3 —(CH 2 ) a —S—S—(CH 2 ) b —SO 3 —M  (VII) 
       [0019] wherein a=3 to 8, b=3 to 8, M is a hydrogen or alkali metal element 
       M—SO 3 —(CH 2 ) a —O—CH 2 —S—S—CH 2 —O—(CH 2 ) b —SO 3 —M  (VIII) 
       [0020] wherein a=3 to 8, b=3 to 8, M is a hydrogen or alkali metal element  
                 
 
       [0021] (4) The method for filling blind via holes according to any one of the above (1) to (3), wherein the concentration of ingredient (a) in the copper sulfate plating bath is from 0.05 to 10 g/liter, and that of ingredient (b) in the copper sulfate plating bath is from 0.1 to 100 mg/liter.  
       [0022] (5) The method for filling blind via holes according to any one of the above (1) to (4), wherein the blind via hole is from 3 to 50 μm in diameter, is from 30 to 100 μm in depth, and has an aspect ratio of 4 to 20, which is the value of dividing the depth by the diameter.  
       [0023] (6) The method for filling blind via holes according to any one of the above (1) to (5), wherein the ratio of the current density in reverse electrolysis to that in positive electrolysis is from 1 to 10.  
       [0024] (7) The method for filling blind via holes according to the above (6), wherein the current density in positive electrolysis is from 0.1 to 20 A/dm 2  and that in reverse electrolysis is from 0.1 to 200 A/dm 2 .  
       [0025] (8) The method for filling blind via holes according to any one of the above (1) to (7), wherein said substrate is a silicon wafer.  
       [0026] (9) The method for filling blind via holes according to the above (8), further comprising forming an insulating film on the inner wall of the blind via holes prior to said forming of the seed layer for plating, on the inner wall of the via holes formed in the silicon wafer.  
       [0027] (10) A method for forming an electrode penetrating a substrate, comprising filling the blind via holes with metallic copper provided in the substrate by the method according to any one of the above (1) to (9), and polishing the back of the substrate to obtain a substrate having the via holes which are filled with the metallic copper and which penetrate the substrate. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0028]FIGS. 1A to  1 C are schematic views explaining the steps of filing a blind via hole provided in a substrate with copper, and grinding the back of the substrate to form a penetrating electrode.  
     [0029]FIG. 2 is a sectional view along the centerline of a blind via hole, filled with metallic copper, according to Example 1.  
     [0030]FIG. 3 is a sectional view along the centerline of a blind via hole, filled with metallic copper, according to Example 2.  
     [0031]FIG. 4 is a sectional view along the centerline of a blind via hole, filled with metallic copper, according to Comparative Example 1.  
     [0032]FIG. 5 is a sectional view, along the centerline of a blind via hole, filled with metallic copper, according to Comparative Example 2. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0033] The invention is explained in detail below.  
     [0034] The blind via hole is a via hole having a bottom, provided in a substrate. After the blind via hole is filled with a conductive material such as metallic copper, the material is exposed by grinding the back of the substrate to obtain the substrate penetrated by the via hole which is filled with the conductive material.  
     [0035] The copper sulfate plating bath used for filling the blind via hole with metallic copper according to the invention, has a fundamental composition of sulfuric acid, copper sulfate and a water-soluble chlorine compound, plus additives (a) and (b).  
     [0036] Any fundamental composition, which has been used for a copper sulfate plating, can be used without particular restriction.  
     [0037] The suitable concentration of the sulfuric acid is from 30 to 400 g/liter, and preferably from 80 to 120 g/liter. In the case where the concentration is less than 30 g/liter, for example, the plating bath decreases in conductivity, causing difficulty in energizing the plating bath. On the other hand, when the concentration is more than 400 g/liter, the copper sulfate is impeded from dissolving in the plating bath and it can be precipitated, in an extreme case.  
     [0038] The suitable concentration of the copper sulfate is from 20 to 300 g/liter, and preferably from 150 to 250 g/liter. In the case where the concentration is less than 20 g/liter, for example, copper ions are insufficiently supplied to the substance to be plated. As the result, it becomes impossible to form a normal plated film. Further, it is difficult to dissolve the copper sulfate in the case of a concentration over 300 g/liter.  
     [0039] Any water-soluble chloride can be used, without particular restriction, if it has been used for a copper sulfate plating. Examples of water-soluble chlorides include hydrochloric acid, sodium chloride, potassium chloride, and ammonium chloride. The water-soluble chlorides may be used either singly or in the form of a mixture of two or more compounds.  
     [0040] The concentration of the water-soluble chlorides contained in the copper sulfate bath used for the invention is suitably 10 to 200 mg/liter, and preferably 30 to 100 mg/liter, in terms of a chloride ion concentration. When the chlorine ion concentration is less than 10 mg/liter, for example, it becomes difficult for the additives to function properly. In the case where the concentration is over 200 mg/liter, the positive electrode becomes passivated, making it impossible to energize the plating bath.  
     [0041] Additive (a) used for the invention is a substance which acts as a wetting agent, in the plating bath, and contains, in each molecule, preferably at least 5, and more preferably at least 20 ether oxygens.  
     [0042] Additive (a) used for the invention may be used either singly or in the form of a mixture of two or more additives. A preferred additive (a) is polyalkylene glycol which has at least 5, more preferably 50 to 100 ether oxygens.  
     [0043] As preferred additive (a) used for the invention, there may be mentioned those compounds represented by the formulae (II) to (IV) below: 
     HO—(CH 2 —CH 2 —O) a —H  (II) 
     [0044] wherein a=5 to 500  
                 
 
     [0045] The concentration of additive (a) used in the invention is preferably from 0.05 to 10 g/liter and, more preferably, from 0.1 to 2 g/liter. If the concentration of additive (a) in the plating bath is less than 0.05 g/liter, many pinholes are produced in the plated film due to an insufficient wetting effect, resulting in difficulty in depositing a proper plated film. On the other hand, a concentration of over 10 g/liter is not preferable from the economical point, because there is scarcely any improvement in effect corresponding to the increased concentration.  
     [0046] Additive (b) used for the invention is a substance which is positively charged in the plating bath, and which is adsorbed to the surface of the substance to be plated during electrolyzing, and leaves from the surface at the time of reverse electrolysis. When additive (b) is adsorbed to the surface of the substance to be plated, it contributes to the growth of the plated copper film.  
     [0047] Examples of additive (b) used for the invention include compounds which have a structure of —S—CH 2 O—R—SO 3 M in a molecule or those which have a structure of —S—R—SO 3 M in a molecule, wherein M is a hydrogen or an alkali metal atom, R is an alkyl group having 3 to 8 carbon atoms.  
     [0048] As additive (b) used for the invention, there may be mentioned those compounds represented by the formulae (V) to (X) below: 
     M—SO 3 —(CH 2 ) a —S—(CH 2 ) b —SO 3 —M  (V) 
     [0049] wherein a=3 to 8, b=3 to 8, M is a hydrogen or alkali metal element 
     M—SO 3 —(CH 2 ) a —O—CH 2 —S—CH 2 —O—(CH 2 ) b —SO 3 —M  (VI) 
     [0050] wherein a=3 to 8, b=3 to 8, M is a hydrogen or alkali metal element 
     M—SO 3 —(CH 2 ) a —S—S—(CH 2 ) b —SO 3 —M  (VII) 
     [0051] wherein a=3 to 8, b=3 to 8, M is a hydrogen or alkali metal element 
     M—SO 3 —(CH 2 ) a —O—CH 2 —S—S—CH 2 —O—(CH 2 ) b —SO 3 —M  (VIII) 
     [0052] wherein a=3 to 8, b=3 to 8, M is a hydrogen or alkali metal element  
                 
 
     [0053] Particularly preferred additives (b) include compounds represented by formula (XI) below: 
     Na—SO 3 —(CH 2 ) 3 —O—CH 2 —S—CH 2 —O—(CH 2 ) 3 —SO 3 —Na  (XI) 
     [0054] Additive (b) in the invention may be used either singly or in the form of a mixture of two or more additives.  
     [0055] Additive (b) in the invention is preferably used in an amount of 0.1 to 100 mg/liter and, more preferably, 0.2 to 10 mg/liter. If the concentration of additive (a) is less than 0.1 mg/liter in the plating bath, there is obtained no effect for promoting the growth of the plated copper film. On the other hand, a concentration of over 100 mg/liter is not preferable from the economical point, because there is scarcely any improvement in effect corresponding to the increased concentration.  
     [0056] When it is applied a PPR electrolysis, i.e., an electroplating method involving reversing the current direction at a short cycle, additive (b) is adsorbed to the inner side of the blind via hole of the substance to be plated by the electrolysis, and is desorped only around the opening edge of the blind via holes, where current is likely to be concentrated, during the short time for reverse electrolysis.  
     [0057] Therefore, by repeating the reversal of the current direction, the amount of additive (b) adsorbed at the neighborhood of the bottom of the blind via hole is high, and that at around the opening edge is low.  
     [0058] As the result, the action of additive (b) assisting the growth of a plated copper film is high near the bottom of the blind via hole. Consequently, the copper depositing rate, to form a plated film, is higher near the bottom than that at the opening edge, whereby it becomes possible to fully fill the blind via hole with copper deposit, without leaving any air space within the hole.  
     [0059] The PPR electrolysis used for the invention is a method involving a repetition, over a short cycle, of a positive electrolysis (electrolysis for depositing a coat), a reverse electrolysis and a stop time. A preferred cycle of the electrolysis is from 1 to 50 msec. for a time of positive electrolysis, from 0.2 to 5 msec. for a time of reverse electrolysis, and from 1 to 50 msec. for a stop time. It is essential that the time of positive electrolysis is longer than the time of reverse electrolysis.  
     [0060] A time of positive electrolysis of shorter than 1 msec. is not preferable because the electrolysis is stopped prior to the time a normal copper deposit starts. In the case the time of positive electrolysis is longer than 50 msec., additive (b) is increasingly adsorbed at the neighborhood of the opening edge of the bind via hole. Therefore, the rate of depositing copper to form a plated film around the bottom of the blind via hole cannot be made faster than that at the opening edge, negating the effect of the invention.  
     [0061] When the time of reverse electrolysis is shorter than 0.2 msec., additive (b) adsorbed around the blind via hole cannot be desorped. Accordingly, because the rate of depositing copper to form a plated film around the bottom of the blind via hole cannot be made faster than that at the opening edge, the effect of the invention is lost. On the other hand, it is not preferred that the time of reverse electrolysis is longer than 5 msec., because the deposited copper film is dissolved and the time for copper-plating inside the bind via hole becomes longer.  
     [0062] The stop time helps to supply copper ions into the inside of the blind via hole. A preferred stop time is 1 to 50 msec., and more preferably 5 to 10 msec. A time of shorter than 1 msec. is not sufficient for helping to supply copper ions into the inside of the blind via hole. When the stop time is longer than 50 msec., the gradient of the copper ion concentration between the inside of the blind via hole and the plating bath is decreased, and the effect of helping to supply copper ions is not improved. Further, the time for copper-plating inside the blind via hole becomes prolonged.  
     [0063] The ratio of the current density in the electrolysis is 1 to 10, preferably 2 to 5 for reverse electrolysis, based on 1 for positive electrolysis.  
     [0064] When the ratio of the current density in the electrolysis is less than 1 for reverse electrolysis, an additive adsorbed to the neighborhood of the blind via hole cannot be sufficiently desorped. Consequently, the rate of depositing copper to form a plated film around the bottom of the blind via hole cannot be made faster than that at the opening edge, negating the effect of the invention. When the ratio of the current density in the electrolysis is more than 10 for reverse electrolysis, based on 1 for positive electrolysis, the plated copper film once deposited is dissolved and the time for copper-plating inside the blind via hole becomes longer.  
     [0065] The preferred current density for positive electrolysis is, for example, from 0.1 to 20 A/dm 2  and more preferably from 0.2 to 5 A/dm 2 , and that for reverse electrolysis is 0.1 to 200 A/dm 2  and more preferably from 0.2 to 20 A/dm 2 .  
     [0066] Next, preferred examples of the method for filling blind via holes and that for forming a penetrating electrode of the invention are explained with reference to FIGS. 1A to  1 C.  
     [0067] A blind via hole 4, i.e., a bottomed via hole, is formed by a photolithography, for example, in a substrate such as a silicon wafer. The blind via hole has an opening edge having a diameter of 10 μm and a depth of 60 μm. After an insulating film  2  is optionally formed for insulation between the substrate and the copper filled in the hole, a plating seed layer (electrically conductive layer)  3  is formed (FIG. 1A). Then, the blind via hole  4  is electrically plated, with the seed layer  3  as one electrode, to be filled with metallic copper  5 . By using the blind via hole filling method of the invention, the via hole is filled with the copper without leaving significant air space therein (FIG. 1B). Then, the back surface la of the substrate opposite to the opening edge of the via hole is polished to expose the bottom surface  5   a  of the metallic copper filled in the blind via hole, and to obtain the substrate through which the copper filled via hole penetrates (FIG. 1C).  
     [0068] The inside of the blind via hole should have an electrical conductivity prior to electroplating, to carry out the plating of the invention. To provide the conductivity, various methods such as a non-electric plating method, a conductive fine particles adsorption treatment, a vapor phase plating method, etc. can be adopted.  
     [0069] The electrical plating method of the invention is carried out at a temperature of, for example, 10 to 40° C., and preferably 20 to 25° C. If the plating temperature is lower than 10° C., the plating bath has a reduced conductivity. Therefore, because it is not possible to increase the current density at the time of electrolysis, the growth rate of the plated film is reduced and the productivity is decreased. It is not effective to raise the plating temperature above 40° C., because additives (a) and (b) may be decomposed.  
     [0070] According to the electric plating method of the invention, any anode can be used so long as it has been used for a copper sulfate plating. The anode may be a soluble or insoluble electrode.  
     [0071] According to the plating method of the invention, the plating solution may be preferably stirred to uniformly supply copper ions and additives to the surface of the substance to be plated. Further, it is possible to carry out a displacing filtration or a circulating filtration. Particularly, it is preferred to subject the plating solution to a circulating filtration with a filter to make the temperature of the plating solution uniform and to remove dust, sediment and the like from the solution.  
     [0072] After the blind via hole in the silicon wafer is filled with metallic copper, the wafer is ground from the side opposite to the opening edge of the hole to expose the edge of the metallic copper filled in the hole. Thus, a silicon wafer provided with a penetrating electrode is formed.  
     [0073] As has been explained, the blind via hole can be filled with metallic copper without leaving any air space therein, according to the invention, by using a copper sulfate plating bath containing specified ingredients (a) and (b) and by electrically copper-plating inside the blind via hole by repeating a short cycle of a positive electrolysis, a reverse electrolysis and a stop time.  
     EXAMPLES  
     [0074] The invention will be explained in greater detail by referring to the following examples and comparative examples. It is to be understood, however, that the scope of the invention is by no ways limited by them.  
     [0075] Following plating solutions and conditions were employed for the electrolysis.  
     Example 1  
     [0076] Plating solution:  
     [0077] sulfuric acid: 100 g/liter  
     [0078] copper sulfate: 200 g/liter  
     [0079] chlorine ions: 70 mg/liter  
     [0080] compound of the formula below: 0.4 g/liter  
                 
 
     [0081] wherein a+c=45, b=45, and  
     [0082] a compound of the formula below: 1 mg/liter 
     Na—SO 3 —(CH 2 ) 3 —S—S—(CH 2 ) 3 —SO 3 —Na. 
     [0083] Conditions for electrolysis: PPR electrolysis  
     [0084] time of positive electrolysis: 10 msec.  
     [0085] time of reverse electrolysis: 0.5 msec.  
     [0086] stop time: 10 msec.  
     [0087] current density at positive electrolysis: 0.25 A/dm 2    
     [0088] current density at reverse electrolysis: 0.5 A/dm 2    
     [0089] ratio of current density: positive electrolysis vs. reverse electrolysis=1 vs. 2  
     [0090] plating time: 280 min.  
     Example 2  
     [0091] Plating solution:  
     [0092] sulfuric acid: 100 g/liter  
     [0093] copper sulfate: 200 g/liter  
     [0094] chlorine ions: 70 mg/liter  
     [0095] compound of the formula below: 0.4 g/liter  
                 
 
     [0096] wherein a+c=45, b=45, and  
     [0097] a compound of the formula below: 1 mg/liter 
     Na—SO 3 —(CH 2 ) 3 —S—S—(CH 2 ) 3 —SO 3 —Na. 
     [0098] Conditions for electrolysis: PPR electrolysis  
     [0099] time of positive electrolysis: 10 msec.  
     [0100] time of reverse electrolysis: 0.5 msec.  
     [0101] stop time: 5 msec.  
     [0102] current density at positive electrolysis: 0.25 A/dm 2    
     [0103] current density at reverse electrolysis: 0.5 A/dm 2    
     [0104] ratio of current density: positive electrolysis vs. reverse electrolysis=1 vs. 2  
     [0105] plating time: 280 min.  
     Comparative Example 1  
     [0106] Plating solution:  
     [0107] sulfuric acid: 100 g/liter  
     [0108] copper sulfate: 200 g/liter  
     [0109] chlorine ions: 70 mg/liter, and  
     [0110] a compound of the formula below: 0.4 g/liter  
                 
 
     [0111] wherein a+c=45, b=45.  
     [0112] Conditions for electrolysis: PPR electrolysis  
     [0113] time of positive electrolysis: 10 msec.  
     [0114] time of reverse electrolysis: 0.5 msec.  
     [0115] stop time: 5 msec.  
     [0116] current density at positive electrolysis: 0.5 A/dm 2    
     [0117] current density at reverse electrolysis: 1.0A/dm 2    
     [0118] ratio of current density: positive electrolysis vs. reverse electrolysis=1 vs. 2  
     [0119] plating time: 100 min.  
     Comparative Example 2  
     [0120] Plating solution:  
     [0121] sulfuric acid: 100 g/liter  
     [0122] copper sulfate: 200 g/liter  
     [0123] chlorine ions: 70 mg/liter  
     [0124] a compound of the formula below: 0.2 g/liter  
                 
 
     [0125] wherein a+c=40, b=10, and  
     [0126] a compound of the formula below: 2 mg/liter 
     Na—SO 3 —(CH 2 ) 3 —S—S—(CH 2 ) 3 —SO 3 —Na. 
     [0127] Conditions for electrolysis: Pulse electrolyzing method  
     [0128] current density: 0.5 A/dm 2 .  
     [0129] electrolytic time: 10 msec.  
     [0130] stop time: 10 msec.  
     [0131] plating time: 50 min.  
     [0132] The filled state of the copper-plated blind via holes was evaluated as follows:  
     [0133] &lt;Sample Production Method&gt;  
     [0134] A wiring layer is formed on a silicon wafer. Alternatively, a mask for silicon etching is formed directly on a silicon wafer. The mask may be an insulation film, metallic film and the like in which a pattern is formed using a photo resist or photolithography. The silicon at the opening edge of the mask is etched to form blind via holes such that the opening edge of the hole has a diameter of 10 μm and a depth of 60 μm.  
     [0135] To insulate the inside of the via holes from the silicon wafer, an insulation film is formed on the inner wall of the via holes. Subsequently, the inside of the via holes is treated to be conductive.  
     [0136] The samples, thus prepared, were plated by the method of the invention.  
     [0137] &lt;Evaluation Method&gt;  
     [0138] A destructive inspection and a non-destructive inspection were carried out. The destructive inspection was carried out as follows. First, the silicon wafer was cut at the neighborhood of the blind via hole, and a section passing through the center of the via hole was revealed by machine-grinding or polishing the wafer. Then, an inspection was made as to whether there were air spaces in the blind via hole, and to measure the plated copper film thickness, by an electronic scanning microscope. In the case where no air space was found by the destructive inspection, a non-destructive inspection was carried out as follows. An X ray was irradiated to the depth direction of the blind via hole, and an inspection was made to determine if the copper density at the center of the hole was lower than or the same as that at the outer circumference of the hole, to determine the existence of the air space.  
     [0139] According to Comparative Example 1 where electroplating was effected in an electronic copper-plating bath using no ingredient (b), an elongated air space  10  remained finally, from the bottom to the upper part of the blind via hole  1 , as shown in FIG. 4.  
     [0140] In the case of Comparative Example 2 where no reverse electrolysis was effected and the cycle of the positive electrolysis and a stop time only was repeated, an air space  10  was produced at the vicinity of the bottom of the blind via hole  11  due to the insufficient deposit of metallic copper  12  in the hole, as shown in FIG. 5.  
     [0141] Contrary to the above, according to Examples 1 and 2, copperplating was carried out in a copper sulfate bath containing ingredients (a) and (b), with repeating the cycle of positive electrolysis, reverse electrolysis and a stop time. The result was, as shown in the sectional views of FIGS. 2 and 3, that substantially no air space was produced or that the generation of air space  10  was constrained to the least degree while the blind via hole  11  was filled with metallic copper  12 .