Abstract:
Aimed at providing an electrolytic Ni plating apparatus capable of suppressing passivation of the surface of a Ni anode, preventing current efficiency and rate of film formation from being degraded, providing a stable Ni plating so as to contribute to improvement in quality, and maintaining a stable production capacity. The electrolytic Ni plating apparatus of the present invention is provided with a nickel (Ni) anode having an average grain size of 10 μm or smaller.

Description:
[0001]    This application is based on Japanese patent application No. 2007-017703 the content of which is incorporated hereinto by reference. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present invention relates to an electrolytic Ni plating apparatus and relates to a method of manufacturing a semiconductor device. 
         [0004]    2. Related Art 
         [0005]    Lead-free solder has rapidly been disseminated as a measure for environmental preservation. There is an ongoing trend of replacement of barrier metal adapted to the lead-free solder which is used for flip-chip products of semiconductor devices, from Cu to Ni. Cu shows a large rate of alloying while Ni shows a small rate of alloying. A Ni film is formed generally by electroless plating or electrolytic plating. Ni plating apparatuses, in particular those used for semiconductor manufacturing processes, are required that they can form a Ni plating film in a stable and high-throughput manner. In conventional electrolytic plating apparatuses applied to semiconductor manufacturing processes, a soluble Ni anode has generally been adopted as an anode. 
         [0006]      FIG. 5  is a drawing showing a configuration of a conventional Ni plating apparatus  200 . As shown in  FIG. 5 , the Ni plating apparatus  200  is composed of an inner bath  101 , an outer bath  102 , a wafer holder  103 , and a reservoir tank  104 . At the bottom of the inner bath  101 , a Ni anode  105  and a plating solution jetting port  106  are provided. On the other hand, at the bottom of the outer bath  102 , a plating solution discharging port  107  is provided. Also a pump  108  and a filter  109  are provided. The Ni anode  105  and a contact portion  110  are connected to a power source  111 . A wafer  112  is brought down to and placed on the contact portion  110 , while being held by a wafer holder  103 . Thus, a plating solution jetted out from the plating solution jetting port  106  is allowed to contact with the wafer  112 . When voltage is applied by the power source  111 , so as to define the wafer  112  as a cathode and the Ni anode  105  as an anode, Ni deposits on the surface of the wafer  112 , and Ni dissolves out from the surface of the Ni anode  105 , thereby the plating proceeds. During the initial film formation in the period up to several seconds to several tens of seconds after the start of the voltage application, the Ni anode  105  is kept under a state of high potential. Therefore, there is a problem that, for the case where the wafers are successively processed under conditions of large current density, or in other words, for the case where the Ni anode  105  is kept under high potential for a long duration of time, the surface of the Ni anode  105  is passivated, and thereby current efficiency degrades, and the rate of film formation decreases. 
         [0007]    The anode used for the electrolytic Ni plating apparatus includes insoluble anodes such as those composed of Pt, Ti and so forth, and soluble Ni anode. The Ni anode, which produces only a small amount of oxygen from the anode, is generally used for plating onto a silicon wafer having fine patterns formed thereon. 
         [0008]    In the course of general film formation based on Ni plating, the reaction expressed as Ni→Ni 2+ +2e −  occurs with respect to the Ni anode used as an anode, thereby the Ni anode gradually dissolves as a consequence. However, if the Ni anode is exposed to a potential higher than a predetermined level, a reaction of producing Ni oxide or Ni hydroxide may proceed, besides the above-described dissolution reaction. Thus, the surface of the Ni anode may be passivated. In electrolytic Ni plating using the Ni anode as an anode, it is generally known that a state of high potential appears in the initial stage of film formation within several seconds to several tens of seconds after the start of the voltage application, as compared with potential in the later stage of the film formation. For this reason, in the process of successive plating of wafers under conditions of large current density, the Ni anode is placed under large potential in a high rate, and is more likely to produce thereon Ni oxide or Ni hydroxide. The present inventor found that the conventional Ni anode has a large average grain size and thus the conventional Ni anode has a small ratio of grain boundary and shows a small rate of dissolution. Such a Ni anode is therefore likely to produce Ni oxide or Ni hydroxide thereon, or in other words, likely to be passivated. Passivation of the surface of the anode creates a problem of the lowering in the current efficiency and the decrease in the rate of film formation. 
         [0009]    Japanese Laid-Open Patent Publication No. 2003-171797 discloses a technique of suppressing generation of particles such as sludge on the anode side, by controlling the grain size of a copper anode. However, it is not aimed at solving the problem of passivation. The copper anode will not raise the problem described in the above. The present invention is to solve the problem specific to the case where the Ni anode is used. 
         [0010]    The present invention is conceived after considering the above-described situation, and is aimed at suppressing passivation of the surface of a Ni anode, and thereby preventing the current efficiency and the rate of film formation from degrading. 
       SUMMARY 
       [0011]    According to the present invention, there is provided an electrolytic Ni plating apparatus provided with a nickel (Ni) anode having an average grain size of 10 μm or smaller. 
         [0012]    By using the Ni anode having an average grain size of 10 μm or smaller, the passivation of the surface of Ni anode is suppressed, and thereby the current efficiency and the rate of film formation are prevented from degrading. 
         [0013]    According to the present invention, there is also provided a Ni anode for electrolytic Ni plating having an average grain size of 10 μm or smaller. 
         [0014]    According to the present invention, there is also provided a method of manufacturing a semiconductor device containing an electrolytic Ni plating process, wherein, in the electrolytic Ni plating process, a Ni anode having an average grain size of 10 μm or smaller is used as an anode. 
         [0015]    According to the present invention, passivation of the surface of a Ni anode is suppressed, and thereby the current efficiency and the rate of film formation are prevented from degrading, by adopting the Ni anode having an average grain size of 10 μm or smaller. Providing a stable Ni plating film contributes to improvement in quality, and to maintenance of stable production capacity. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The above and other objects, advantages and features of the present invention will be more apparent from the following description of certain preferred embodiments taken in conjunction with the accompanying drawings, in which: 
           [0017]      FIG. 1  is a drawing showing one configuration of an electrolytic Ni plating apparatus according to one embodiment of the present invention; 
           [0018]      FIG. 2  is an enlarged sectional view of the wafer shown in  FIG. 1 ; 
           [0019]      FIG. 3  is a SIM image of the Ni anode of Example 1; 
           [0020]      FIG. 4  is a graph showing relations between the number of runs of wafer processing and thickness of Ni plating in Example 1; 
           [0021]      FIG. 5  is a drawing showing one configuration of a conventional Ni plating apparatus; 
           [0022]      FIG. 6  is a SIM image of a Ni anode of Comparative Example 1; and 
           [0023]      FIG. 7  is a graph showing relations between the number of runs of wafer processing and thickness of Ni plating in Comparative Example 1. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    The invention will now be described herein with reference to an illustrative embodiment. Those skilled in the art will recognize that many alternative embodiments can be accomplished using the teachings of the present invention and that the invention is not limited to the embodiment illustrated for explanatory purposes. 
         [0025]    Paragraphs below will detail embodiments of the electrolytic Ni plating apparatus according to the present invention, referring to the attached drawings. It is to be noted that any constituents commonly appear in all drawings will be given with the same reference numerals, and those explanations will not be repeated. 
         [0026]      FIG. 1  is a drawing showing an exemplary electrolytic Ni plating apparatus  100  according to the present invention. The Ni plating apparatus  100  has an inner bath  101 , an outer bath  102 , a wafer holder  103 , and a reservoir tank  104 . At the bottom of the inner bath  101 , a Ni anode  105  having an average grain size of 10 μm or smaller, and a plating solution jetting port  106 , through which plating solution is fed into the inner bath  101 , are provided. At the bottom of the outer bath  102 , a plating solution discharging port  107 , through which the plating solution overflown from the inner bath  101  is returned to the reservoir tank  104 , is provided. There are also provided a pump  108  for jetting the plating solution reserved in the reservoir tank  104  into the inner bath  101 , and a filter  109  for removing dust or particles from the solution to be jetted. The Ni anode  105  and the contact portion  110  are connected to a power source  111  for current supply.  FIG. 2  is an enlarged sectional view of the wafer  112  shown in  FIG. 1 , showing the wafer adapted to formation of lead solder bumps or lead-free solder bumps. A Cu seed film  114  is formed over the surface of a semiconductor substrate  113 , and a photoresist  115  is patterned over the Cu seed film  114 . 
         [0027]    The wafer  112  is held face-down by the wafer holder  103 , so as to downwardly direct the surface having the photoresist  115  patterned thereon. The wafer holder  103  and the wafer  112  are brought down to and placed on the contact portion  110 , and allowed to contact with the plating solution jetted from the plating solution jetting port  106 . When voltage is applied by the power source  111 , so as to define the wafer  112  which is in contact with the contact portion  110 , as a cathode, and the Ni anode  105  as an anode, Ni deposits on the surface of the wafer  112  in the portion where the Cu seed film  114  is exposed, and Ni dissolves out from the surface of the Ni anode  105 . 
         [0028]    In this embodiment, the upper limit of average grain size of the Ni anode is 10 μm or smaller, and more preferably 3 μm or smaller. Although the lower limit of the average grain size of the Ni anode is not specifically limited, it is preferably 0.1 μm or larger. 
         [0029]    The average grain size of the Ni anode may be calculated typically according to the procedures below. First, a clean section of an anode is produced using an FIB (focused ion beam) apparatus. An image of this section is taken by a SIM (scanning ion microscope). The number of grains per unit area is then counted on the SIM image. The counted number of grains is divided by the unit area to obtain the average grain size. 
         [0030]    Since the conventionally used Ni anode has a large average grain size and consequently has only a small ratio of grain boundary, it has the slow rate of dissolution. As a consequence, the Ni anode has been likely to produce Ni oxide or Ni hydroxide, and has been likely to be passivated. In contrast, the Ni anode used for the Ni plating apparatus of the present invention has an average grain size of as small as 10 μm or below. The Ni anode therefore has a large ratio of grain boundary, shows a large rate of dissolution, and is consequently less likely to produce Ni oxide nor Ni hydroxide. In other words, the Ni anode of this embodiment is less likely to be passivated. Thus, the current density and the rate of film formation can be prevented from degrading even when the wafers are successively processed under conditions of large current density. 
         [0031]    The conditions of the large current density referred to herein means that the anode current density is equal to or more than 1.5 A/dm 2 . The Ni anode of this embodiment is less likely to be passivated even under the conditions of such large current density. 
         [0032]    In the method of manufacturing a semiconductor device containing an electrolytic Ni plating process, the Ni anode of this embodiment may be used. By using the Ni anode having an average grain size of 10 μm or smaller as an anode in the plating process, the passivation of the surface of the Ni anode can be suppressed, and thereby a stable Ni plating film may be provided. 
         [0033]    As described in the above, the lowering in the rate of film formation can be prevented, even when the wafers are successively processed under conditions of large current density. As a consequence, a stable Ni plating film can be provided. Thus, this embodiment contributes to improvement in quality, and to maintenance of stable production capacity. 
         [0034]    The present invention will be explained into further details, referring to a specific Example. 
       EXAMPLE 
     Example 1 
       [0035]    Using an electrolytic Ni plating apparatus provided with the Ni anode having an average grain size of 10 μm or smaller, the wafers were successively processed under conditions of large current density. The average grain size of the Ni anode was calculated as follows. First, a clean section of the anode was produced using an FIB (focused ion beam) apparatus, and an image of this section is taken by a SIM (scanning ion microscope).  FIG. 3  shows a SIM image of the Ni anode of this Example. 
         [0036]    Mean grain area S can be obtained by dividing the unit area by the number of grains. In the SIM image of the Example, the number of grains per unit area was counted. The mean grain area S was obtained by dividing the unit area by the counted number of grains. Next, in order to convert the area into length, the grain was approximated by a circle, to thereby calculate the average grain size L. The average grain size L is calculated by the equation below: 
         [0000]        L= 2*√{square root over ( )}( S/n ) 
         [0000]    (where, L represents average grain size, and S represents mean grain area). 
         [0037]    By calculation using the equation above, the average grain size L was 0.8 μm. 
         [0038]      FIG. 4  is a graph showing relations between the number of runs of wafer processing and thickness of the Ni plating film. In the graph, maximum values, minimum values, and average values of the Ni plating film were shown for every number of runs of wafer processing. It was found that the Ni anode was not passivated even under successive processing of wafers under conditions of large current density, or in other words, even for the case where the Ni anode was placed under high potential for a long duration of time. Accordingly, the current efficiency was not degraded, and consequently also the rate of film formation was not degraded. From the graph in  FIG. 4 , the thickness of the film did not change even after the number of runs of wafer processing increased. 
       Comparative Example 1 
       [0039]    The wafers were successively processed under the same conditions with Example 1, except that an electrolytic Ni plating apparatus provided with a Ni anode having an average grain size of 15 μm was used. The average grain size was calculated by the same procedure as in Example 1.  FIG. 6  is a SIM image of the Ni anode of this Comparative Example. 
         [0040]      FIG. 7  is a graph showing relations between the number of runs of wafer processing and thickness of the Ni plating film, when the wafers were successively processed under conditions of large current density. As a result of passivation of the surface of the Ni anode, the current efficiency gradually decreased, and thereby the rate of film formation decreased. It is obvious from  FIG. 7  that film thickness gradually decreased. 
         [0041]    Example 1 was largely improved in the rate of film formation as compared with Comparative Example 1. By reducing the average grain size of the Ni anode of the Ni plating apparatus of Example 1 to equal to or smaller than 10 μm, as described in the above, the Ni anode became less likely to produce Ni oxide or Ni hydroxide. Thus, the passivation was prevented, even when the wafers were successively processed under conditions of large current density. Since the passivation was suppressed, the lowering in the current efficiency and in the rate of film formation was avoided. As a consequence, the Example 1 successfully achieved stable provision of Ni plating film. 
         [0042]    Also for the case where the electrolytic Ni plating apparatus provided with the Ni anode having an average grain size of 8 μm was used, it was confirmed that the rate of film formation was not lowered and the passivation was suppressed when successive processing of the wafers was carried out as described in Example 1. 
         [0043]    Although the present invention has been explained based on the embodiments and Example, they are merely examples of the present invention, allowing adoption of various configurations other than those described in the above. 
         [0044]    It is apparent that the present invention is not limited to the above embodiment, that may be modified and changed without departing from the scope and spirit of the invention.