Abstract:
The present invention provides a copper plating solution for embedding fine wiring, wherein it contains copper sulfate at 100 to 300 g/L as copper sulfate pentahydrate, sulfuric acid at 5 to 300 g/L, chlorine at 20 to 200 mg/L, a macromolecular surfactant at 0.05 to 20 g/L for controlling the electrodeposition reaction, sulfur-based saturated organic compound at 1 to 100 mg/L for accelerating the electrodeposition reaction, leveling agent composed of a macromolecular amine compound at 0.01 to 10 mg/L and reductant at 0.025 to 25 g/L for stabilizing the copper plating solution. The copper plating solution of the present invention for embedding fine wiring can plate the wafer surface provided with fine wiring patterns with sub-micron order gaps in-between and coated with copper serving as the metallic seed film, to fill the gaps neither leaving any defect therein nor dissolving the metallic seed film.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a plating treatment technique for the surface of a wafer as an electronic device, more particularly to a plating solution of copper sulfate for embedding copper by plating on the wafer surface on which fine wiring patterns with sub-micron order gaps in-between are formed.  
           [0003]    2. Description of the Related Art  
           [0004]    Recently, the fine processing techniques for a wafer as an electronic device have made rapid progress, and the plating techniques for processing wafers have been also extensively under development. The plating techniques for processing wafers include copper plating based on electrolysis, and the representative electrolytic plating solution includes strongly acidic solutions of copper sulfate, and alkaline solutions of cyan- and pyrophosphoric acid-based ones. Among these plating solutions, a strongly acidic solution of copper sulfate has been widely used, because it can be more easily managed and controlled for electrodeposition rate than the alkaline ones.  
           [0005]    The plating solution of copper sulfate is basically composed of copper sulfate, sulfuric acid and an organic compound, e.g., surfactant. Various plating solutions of copper sulfate are known, in which various additives, e.g., polymer and those known as brightener and leveler, are contained at a given content. Addition of these additives controls an electrodeposition rate of the plating metal, thereby providing a plating solution of copper sulfate which allows uniform copper plating.  
           [0006]    Recently, wafers are frequently treated through a copper plating on the surface thereof which has been provided with fine wiring patterns with sub-micron order gaps in-between. In this case, it is required to completely fill these gaps with deposited copper. In other words, the very narrow gap of the sub-micron order must be filled with copper through a plating treatment.  
           [0007]    Completely filling these fine gaps by deposited copper has been coped with by adjusting various additives contained in the plating solution. More specifically, the additives which affect electrodeposition of copper, e.g., leveler, brightener and polymer, are adequately adjusted and incorporated in the plating solution, to provide a copper solution which can be filled in the fine gaps between the wiring patterns. However, it has been pointed out that use of the additive-containing plating solution of copper sulfate may cause the following problems.  
           [0008]    The wafers of late are coated with a metallic film, referred to as seed, by physical vapor deposition (PVD) before being treated by plating, and copper is frequently deposited to cover the wafer as the metallic seed film. The wafer coated with the copper seed film may not be uniformly plated with the copper sulfate solution, because the solution tends to dissolve the copper seed film. More specifically, filling the fine gaps between the wiring patterns with the copper plating solution may produce defects, e.g., voids and seams, in the gaps. In addition, thickness of the deposited copper was sometimes uneven between the wafer surface portion with the fine wiring pattern and that without them.  
           [0009]    The uneven copper plating is mainly caused by dissolution of the copper seed film, and one countermeasure will be to prevent dissolution of the metallic seed film even when copper comes into contact with the plating solution of copper sulfate. More specifically, use of a plating solution of copper sulfate having a composition of high-concentration copper and low-concentration sulfuric acid will be one countermeasure against dissolution of the metallic seed film.  
           [0010]    However, the plating solution of unbalanced composition will be less stable than the common plating solution of copper sulfate, and deteriorate uniformity of the copper plating treatment, because it tends to decompose the additive used for controlling electrodeposition. As a result, it is difficult to continuously keep the uniform copper plating.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention, developed under these circumstances, provides a copper plating solution for embedding fine wiring which plates, with copper from a plating solution of copper sulfate, the wafer surface provided with fine wiring patterns with sub-micron order gaps in-between and coated with copper serving as the metallic seed film, characterized in that it can control dissolution of the metallic seed film, plate the wafer surface by filling the fine gaps with copper without leaving defects in the gaps and cover the entire surface of the wafer to be plated with copper of uniform thickness, and is very high in solution stability.  
           [0012]    The inventors of the present invention have reached the copper plating solution of the present invention for embedding fine wiring, after having intensively studied copper sulfate plating solutions of unbalanced composition containing copper at a high concentration and sulfuric acid at a low concentration.  
           [0013]    The copper sulfate plating solution of the present invention for embedding fine wiring is characterized in that it contains copper sulfate at 100 to 300 g/L as copper sulfate pentahydrate, sulfuric acid at 5 to 300 g/L, chlorine at 20 to 200 mg/L, a macromolecular surfactant at 0.05 to 20 g/L for controlling the electrodeposition reaction, sulfur-based saturated organic compound at 1 to 100 mg/L for accelerating the electrodeposition reaction, leveling agent composed of a macromolecular amine compound at 0.01 to 10 mg/L and reductant at 0.025 to 25 g/L for stabilizing the copper plating solution.  
           [0014]    The function(s) and effect(s) of each component for the copper plating solution composition of the present invention are described for the plating treatment involving embedding fine wiring. First of all, the copper plating solution of the present invention is basically composed of copper sulfate, sulfuric acid and hydrochloric acid. Of these components, copper sulfate and sulfuric acid are contained at 100 to 300 g/L as copper sulfate pentahydrate and 5 to 300 g/L, respectively, because of necessity for giving the solution of high-concentration copper and low-concentration sulfuric acid. The sulfuric acid concentration by itself may be in a general range for the common solution. Viewed from its relationship with concentration of copper sulfate (more specifically copper), however, it satisfies the unbalanced composition of high-concentration copper sulfate and low-concentration sulfuric acid.  
           [0015]    The plating solution composition containing copper sulfate and sulfuric acid in the above ratio dissolves no copper seed, even when the metallic seed film of copper covers the wafer surface. The solution containing sulfuric acid at below 100 g/L as copper sulfate pentahydrate tends to dissolve the copper seed. At a copper sulfate concentration above 300 g/L, on the other hand, salting-out of copper sulfate in the plating solution tends to occur, which accelerates decomposition of each additive, as described later. A sulfuric acid concentration below 5 g/L is accompanied by decreased current efficiency, and difficulty in the uniform plating treatment. At a sulfuric acid concentration above 300 g/L, on the other hand, the plating solution tends to dissolve the copper seed. Chlorine is preferably contained at a concentration in a range of 20 to 200 mg/L. At below 20 mg/L, uniform electrodeposition will be no longer expected. At above 200 mg/L, on the other hand, copper chloride tends to be precipitated on the copper anode surface. Moreover, the plating solution declines in capacity to fill the gaps between the fine wiring patterns at a chlorine concentration beyond 20 to 200 mg/L, tending to leave voids in the plated gaps.  
           [0016]    The macromolecular surfactant, added to the copper plating solution of the present invention for embedding fine wiring to control the electrodeposition reaction, is adsorbed on the wafer surface where the fine wiring patterns with voids are exposed to the plating solution (i.e., portion other than the gap inside), to perform a function of controlling the electrodeposition reaction there. In particular, plating current tends to be concentrated at the edge, which is exposed to the surface side when the gaps are formed, with the result that the plating electrodeposition reaction tends to proceed faster and deposit copper more thickly at the edge than gap inside. Therefore, the macromolecular surfactant is used to be adsorbed on the portions, e.g., edge, which tend to be plated more thickly, and control the plating electrodeposition reaction there. It is adsorbed in the gap inside to a lesser extent, and allows the plating electrodeposition reaction to proceed, because it is not retarded much.  
           [0017]    The macromolecular surfactants useful for the present invention include 1,3-dioxolan polymer, polypropylene glycol, polypropylene propanol, polyethylene glycol, polyethylene glycol derivative, oxyl alkylene polymer and copolymer of ethylene oxide and propylene oxide, which may be used either individually or in combination.  
           [0018]    The macromolecular surfactant for the copper plating solution of the present invention for embedding fine wiring to control the electrodeposition reaction may not fully exhibit its function of controlling the electrodeposition reaction, when contained at below 0.05 g/L. At above 20 g/L, on the other hand, it may excessively control the reaction, with the result that the reaction may not proceed uniformly, tending to produce seams in the fine gaps. The macromolecular surfactant preferably has a molecular weight (weight-average molecular weight Mw) of 100 to 50,000. The one having a molecular weight of less than 100 may not be effective because of the insufficient capacity of controlling the electrodeposition reaction. One the other hand, the one having a molecular weight above 50,000 may be adsorbed unevenly.  
           [0019]    Next, the sulfur-based saturated organic compound for the copper plating solution of the present invention for embedding fine wiring to accelerate the electrodeposition reaction works to accelerate the electrodeposition rate of copper on the wafer surface to be plated. It has a lower molecular weight than the macromolecular surfactant, penetrating into the fine gaps more smoothly, to accelerate the electrodeposition reaction for copper plating within the gap, where the plating reaction is more difficult to proceed.  
           [0020]    The sulfur-based saturated organic compound capable of accelerating the electrodeposition reaction is preferably a dithiobis-alkane-sulfonic acid or salt thereof. More specifically, these compounds include 4,4-dithiobis-butane-sulfonic acid, 3,3-dithiobis-propane-sulfonic acid, 2,2-dithiobis-ethane-sulfonic acid and salts thereof, which may be used either individually or in combination.  
           [0021]    The sulfur-based saturated organic compound for accelerating the electrodeposition reaction is preferably contained at 1 to 100 mg/L. At below 1 mg/L, it cannot sufficiently accelerate the electrodeposition reaction within the gaps on the wafer surface to be plated. At above 100 mg/L, on the other hand, it will start accelerating the reaction in the area other than the fine gap inside, tending to produce voids in the gaps, and the uniform copper plating treatment may not be realized.  
           [0022]    Each of the macromolecular surfactant for controlling the electrodeposition reaction and sulfur-based saturated organic compound for accelerating the electrodeposition reaction works as the additive which helps the plating solution fill the fine gaps between the fine wiring patterns by deposited copper. These components, although contributing to filling the gaps with deposited copper, little exhibit a function of controlling thickness of deposited copper corresponding to the surface shape in the surface on which fine wiring patterns are provided and that which carries no wiring patterns, i.e., flat surface. In other words, the plating solution cannot plate the whole surface with copper of uniform thickness only with the aid of the macromolecular surfactant for controlling the electrodeposition reaction and sulfur-based saturated organic compound for accelerating the electrodeposition reaction, although capable of plating the wafer surface treated to have fine wiring patterns thereon.  
           [0023]    The inventors of the present invention have found that the plating solution significantly deteriorates, when it contains copper at a high concentration and sulfuric acid at a low concentration to prevent dissolution of the metallic seed film, because of tendency of the components to decomposition, notably the sulfur-based saturated organic compound contained to accelerate the electrodeposition reaction.  
           [0024]    Therefore, the copper plating solution of the present invention for embedding fine wiring contains a leveling agent composed of an amine compound and reductant for stabilizing the copper plating solution, in addition to the macromolecular surfactant and sulfur-based saturated organic compound.  
           [0025]    The leveling agent composed of a macromolecular amine compound for the copper plating solution of the present invention for embedding fine wiring has a function of leveling final thickness of the deposited copper. More specifically, it levels thickness of the deposited copper both in the surface on which fine wiring patterns are provided and that which carries no wiring patterns, i.e., flat surface. The examples of the macromolecular amine compounds having a leveling function include polyethylene imine and polypropylene imine. It preferably has an average molecular weight of 5,000 to 100,000. The one having an average molecular weight below 5,000 easily diffuses into the gaps between fine wiring patterns, tending to diminish the function of the sulfur-based saturated organic compound for accelerating the electrodeposition reaction. On the other hand, the one having an average molecular weight above 100,000 tends to make the plating treatment for embedding wiring unstable, conceivably resulting from uneven adsorption of the compound. The macromolecular amine compound having an average molecular weight of 5,000 to 100,000 is difficult to diffuse into the gaps between fine wiring patterns, preventing to a lesser extent the function of the sulfur-based saturated organic compound for accelerating the electrodeposition reaction.  
           [0026]    For the effect of average molecular weight of the macromolecular amine compound on extent of improving uniformity (leveling) of deposited copper thickness, it is found that the effect is exhibited by a small quantity of the compound, when it has a relatively low average molecular weight (100 to 2,000). However, it will not contribute much to the improved uniformity, unless its content is increased to some extent, when it has a relatively high average molecular weight (10,000 to 100,000). Nevertheless, however, it is preferable to use the amine compound of relatively high molecular weight for the copper plating solution of the present invention for embedding fine wiring, because of easiness in analysis and concentration management of the compound.  
           [0027]    The leveling agent composed of the macromolecular amine compound is preferably contained at 0.01 to 10 mg/L. It is difficult to fully exhibit its function of leveling thickness of the deposited copper at below 0.01 mg/L. At above 10 mg/L, on the other hand, it may excessively exhibit the leveling effect to prevent the effect of the sulfur-based saturated organic compound for accelerating the electrodeposition reaction and to cause the conformal separation in which the deposited copper is deposited in the gaps between fine wiring patterns to the same thickness as is on the surface side, with the result that seams and voids tend to be formed in the gaps.  
           [0028]    The reductant for stabilizing the copper plating solution of the present invention for embedding fine wiring (hereinafter referred to as the plating-solution-stabilizing reductant) works to control decomposition of the sulfur-based saturated organic compound, which is sensitive to changes in concentrations of copper and sulfuric acid. The sulfur-based saturated organic compound is decomposed significantly, especially in the plating solution having a composition of high-concentration copper and low-concentration sulfuric acid, to reduce serviceability of the solution. Therefore, the copper plating solution of the present invention for embedding fine wiring is incorporated with the plating-solution-stabilizing reductant, to control decomposition of the sulfur-based saturated organic compound. This allows the copper plating treatment for embedding wiring to be continuously effected, with the sulfur-based saturated organic compound stably present even in the plating solution of high-concentration copper and low-concentration sulfuric acid composition, and also improves serviceability of the plating solution itself.  
           [0029]    The preferable reductants for stabilizing the plating solution of the present invention include an organic compound containing at least one type of functional group selected from aldehyde, hydroxyl and carboxyl, alcohol, and organocarboxylic acid. The organic compounds having at least one functional group of hydroxyl or carboxyl include glyoxal, benzaldehyde, succindialdehyde, acetoaldehyde and propionaldehyde. The alcohols include ethanol, methanol and propanol. The organic carboxylic acids include citric acid, oxalic acid, glyoxylic acid and ascorbic acid. These reductants have a function of effectively preventing decomposition of the sulfur-based saturated organic compound.  
           [0030]    The plating-solution-stabilizing reductant is preferably contained at 0.025 to 25 g/L. It will have an insufficient function of preventing decomposition of the sulfur-based saturated organic compound to cause deterioration of the plating solution, when contained at below 0.025 g/L. At above 25 g/L, on the other hand, it may cause self-decomposition of the plating solution, conversely tending to make the solution unstable.  
           [0031]    The copper plating solution of the present invention for embedding fine wiring may be further incorporated with an organic dye compound which has a function of controlling leveling of the deposited copper. Such a compound works to promote uniform electrodeposition of copper for plating, irrespective of shape of the surface to be plated, when the electrodeposition of copper proceeds to some extent, although exhibiting its function only to a limited extent during the initial stage of plating treatment, i.e., electrodeposition of copper. It is particularly useful for plating copper on a hole (the so-called via) , where the hole inside is plated with copper in such a way that the via opening is not closed.  
           [0032]    These organic dye compounds useful for the present invention include safranine, phenazine, thioflavin, Dye 300, Dye 1556, Dye 3100, Absorber Dye ADI and Cy5, which may be incorporated either individually or in combination. See Japanese Patent Application Laid-Open No. P2000-248397 for details of these compounds. The organic dye compound is preferably contained at 0.01 to 20 mg/L. It may not fully exhibit its leveling function at below 0.01 mg/L, deteriorating the final appearances of the plated product. At above 20 mg/L, on the other hand, it may not effectively fill the gap insides with copper.  
           [0033]    The copper plating solution of the present invention for embedding fine wiring will be most effective for securely preventing dissolution of the copper seed film, when it contains copper sulfate at 150 to 250 g/L as copper sulfate pentahydrate and sulfuric acid at 10 to 100 g/L. Such a solution with a composition of high-concentration copper and low-concentration sulfuric acid can plate the wafer surface uniformly with copper, while little dissolving the copper seed film on the wafer.  
           [0034]    The plating treatment with the copper plating solution of the present invention for embedding fine wiring is preferably effected at a cathode current density of 0.05 to 5A/dm 2 . The treatment will fill the gap inside completely with copper, even when it is very fine, of the order of sub-micron in size, at a cathode current density in the above range, and gives a very well leveled copper layer on the wafer surface. At a cathode current density below 0.05A/dm 2 , the electrodeposition reaction proceeds almost simultaneously in the gap and other areas, with the result that voids tend to be formed in the gap filled with the deposited copper. At above 5A/dm 2 , on the other hand, hydrogen tends to be generated, to deteriorate current efficiency and plating efficiency by electrodeposition. The plating solution is preferably kept at 5 to 30° C. The salting-out tends to occur at below 5° C., whereas the additive tends to be decomposed at above 30° C. Particularly preferably, it is kept at 10 to 20° C., when the plating solution contains copper sulfate at 150 to 250 g/L as copper sulfate pentahydrate and sulfuric acid at 10 to 100 g/L, because salting-out tends to occur in the solution of such high-concentration copper and low-concentration sulfuric acid composition as solution temperature decreases. The solution stably realizes uniform copper plating, when kept at a temperature in the above range, and is serviceable for extended periods.  
           [0035]    It is preferable, when the plating treatment is effected with the copper solution of the present invention for embedding fine wiring, to adequately add each component consumed for the treatment, e.g., the macromolecular surfactant, sulfur-based saturated organic compound, leveling agent composed of a macromolecular amine compound, plating-solution-stabilizing reductant or organic dye compound, while taking into consideration the quantity consumed to keep it at a desired concentration. Each component for the copper plating solution of the present invention for embedding fine wiring is consumed during the treatment process, and the highly uniform copper plating treatment can be effected for extended periods when each component is adequately added while taking into consideration the quantity consumed to keep it at a concentration in the above-described range. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0036]    [0036]FIG. 1 schematically shows a cross-sectional view of test pieces for evaluation of plating for embedding wiring, provided with gap grooves;  
         [0037]    [0037]FIG. 2 presents a SEM microgram showing cross-sectional view of the test pieces for evaluation of plating for embedding wiring, treated with the copper plating solution prepared in Example 2;  
         [0038]    [0038]FIG. 3 presents a SEM microgram showing cross-sectional view of the test pieces for evaluation of plating for embedding wiring, treated with the copper plating solution prepared in Comparative Example 2;  
         [0039]    [0039]FIG. 4 presents a SEM microgram showing cross-sectional view of the test piece for evaluation of plating for embedding wiring, evaluated for running characteristics with the copper plating solution prepared in Example 7;  
         [0040]    [0040]FIG. 5 presents a SEM microgram showing cross-sectional view of the test piece for evaluation of plating for embedding wiring, evaluated for running characteristics with the copper plating solution prepared in Comparative Example 3; and  
         [0041]    [0041]FIG. 6 shows a relationship between average molecular weight of polyethylene imine and the leveling characteristics. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0042]    Embodiments according to the present invention will be described more particularly by way of Examples and Comparative Examples listed in Table 1. Symbols and Ratings are shown in Table 2.  
                                                                                         TABLE 1                                                       Difference                                       in thickness       Running                               of deposited   Embed-   character-           A   B   C   D   E   copper (μm)   dability   istics                                    Example 1   1.0   20   0.01   2.5   0.5   0.55   ◯   ◯       Example 2   1.0   20   0.1   2.5   0.5   0.05   ◯   ◯       Example 3   1.0   20   1.0   2.5   0.5   0.07   ◯   ◯       Example 4   1.0   20   10.0   2.5   0.5   0.05   Δ   Δ       Example 5   1.0   20   20   2.5   0.5   0.06   X   X       Example 6   1.0   20   0.1   0.025   0.5   0.05   ◯   Δ       Example 7   1.0   20   0.1   2.5   0.5   0.06   ◯   ◯       Example 8   1.0   20   0.1   5.0   0.5   0.06   ◯   ◯       Example 9   1.0   20   0.1   25   0.5   0.05   Δ   Δ       Example 10   1.0   20   0.1   30   0.5   0.05   X   X       Example 11   1.0   20   10.0   2.5   0.5   0.05   ◯   ◯       Comparative   1.0   20   —   —   0.5   0.74   X   X       Example 1       Comparative   1.0   20   —   2.5   0.5   0.79   ◯   ◯       Example 2       Comparative   1.0   20   0.1   —   0.5   0.06   Δ   X       Example 3                  
 
         [0043]    [0043]                   TABLE 2                           A   Concentration [g/L] of a macromolecular, triol type surfactant of           polypropylene glycol (PPG) having an average molecular weight of 5,000       B   Concentration [mg/L] of 3,3-dithiobis-propane-sodium sulfonate       C   Concentration [mg/L] of polyethylene imine having an average           molecular weight of 1,000       D   Concentration [g/L] of glyoxyl       E   Concentration [g/L] of safranine       Difference in   Difference between thickness of deposited copper on the surface with the       thickness of   gap and that on the surface without the gap, determined by observation       deposited copper   of the cross section of the test piece for evaluation of plating for           embedding wiring (μm).       Embeddability   Extent of the deposited copper filled in the groove and via hole formed           on the wafer surface, determined by observation of the cross section of           the test piece for evaluation of plating for embedding wiring. It was           evaluated according to the three-grade system: ◯: the gap is completely           filled with copper without forming any defect, e.g., void or seam; Δ: the           gap is filled with copper to some extent, although some defects, e.g., void           or seam, are observed; and X: the gap is not well filled with deposited           copper, showing voids or seams inside.       Running   The test piece was continuously plated with the copper plating solution       characteristics   at 25 Ahr/L, and extent of embeddability was evaluated by the           observation of the cross section according to the three-grade system,           described above.                    
         [0044]    The copper sulfate plating solution used in each of Examples and Comparative Examples described in Table 1 was basically composed of Cu at 50 g/L (adjusted by copper sulfate pentahydrate), sulfuric acid at 50 g/L and chlorine at 70 mg/L. The basic composition was incorporated with various additives given in Table 1, to prepare the plating solution for each of Examples and Comparative Examples. More specifically, the basic composition bath was prepared, to which the components A to E given in Table 1 were added, to prepare the plating solution.  
         [0045]    Examples 1 to 10 described in Table 1 used the same contents of the additive components A, B and E, and varying contents of the components C and D. Polyethylene imine as the additive C had an average molecular weight of 1,000 for all Examples except Example 11, which used the component C having an average molecular weight of 10,000. The plating solutions prepared in Comparative Examples 1 to 3 contained the components A, B and E each at the same content as Examples, but contained only C or D or none of them.  
         [0046]    The plating treatment conditions were liquid temperature: 20° C., anode: phosphorus-containing copper, and plating current: supplied at a cathode current density of 1A/dm 2 . The object to be plated was the test piece for evaluation of plating for embedding wiring, which was composed of a silicon wafer coated beforehand with a metallic seed film of copper, sputtered to a thickness of approximately 0.1 μm (1000 Å). The surface to be plated consisted of the area provided with a total of 11 grooves (position Nos. 1 to 11) of different gap width (the cross-sectional view of which was similar to that of the test piece for evaluation of plating for embedding wiring, shown in FIG. 1), and the flat surface provided with no groove. Each gap groove had a depth D of 1 μm, and the gap width was narrowed as the position number increased, as shown in Table 3.  
                                                           TABLE 3                       Position                                                   No.   1   2   3   4   5   6   7   8   9   10   11                   Gap width   1.65   0.39   0.31   0.27   0.23   0.22   0.20   0.18   0.16   0.15   0.14       (μm)                  
 
         [0047]    The test piece for evaluation of plating for embedding wiring was embedding-plated with copper, to evaluate each plating solution, where the piece cross section was analyzed by a scanning electron microscope, to determine to what extent the solution filled the gap, and its leveling capacity by measuring thickness of deposited copper on the surface provided with the gap groove and that provided with no groove. Moreover, the running characteristics of each plating solution was determined by plating the test piece with the solution at 25 Ahr/L for embedding wiring to observe to what extent the gap was filled with copper.  
         [0048]    As shown in Table 1, it is confirmed that the surface is plated with copper to a greatly different thickness whether it is provided with the gap or not, when the plating solution is free of the component C (Comparative Examples 1 and 2). By contrast, the surface is plated uniformly with copper whether it is provided with the gap or not, when the plating solution prepared in each of Examples 2 to 10.  
         [0049]    The plating solution prepared in Example 2 was compared with that prepared in Comparative Example 2 by thickness of deposited copper, observed by scanning electron microscopic analysis. FIGS. 2 and 3 present the SEM micrograms showing the thickness of copper deposited in the respective examples. In each microgram, the white portion represents the deposited copper, where copper shown in the left side is deposited on the flat wafer surface with no gap, compared with the one in the right side deposited on the wafer surface with gap.  
         [0050]    [0050]FIG. 2 shows the wafer surface plated with copper in Example 2, where difference between thickness of deposited copper on the flat surface and that on the surface with the gap is around 0.05 μm. On the other hand, the difference is much larger at 0.79 μm on the surface plated in Comparative Example 2, shown in FIG. 3. Comparing the surface plated in Example 4 with that plated in Example 11, the plating solution filled the gap well in Example 11, even when it contained polyethylene imine as the component C at the upper limit content, when its average molecular weight was 10,000, as shown in Table 1. The solution containing the imine having an average molecular weight of 10,000 gave a larger embedding margin.  
         [0051]    The running characteristics results of each plating solution revealed that the copper solutions prepared in Examples 1 to 4 and 7 to 9 show no deterioration in its embeddability even when used for extended periods. On the other hand, the surfaces plated in Examples 6 and 10 and Comparative Examples 1 and 3 showed deteriorated embeddability when used for extended periods. Comparing the microgram shown in FIG. 4 with that shown in FIG. 5, which present the plated surface conditions to be observed for the running characteristics, the surface plated in Example 7 was smooth (FIG. 4) whereas that plated in Comparative Example 3 was fairly roughened. Similarly, the surfaces plated with the solutions prepared in Examples 6 and 10 and Comparative Example 1 were fairly roughened, although not shown, when observed for their running characteristics at 25 Ahr/L.  
         [0052]    Finally, the relationship between average molecular weight of polyethylene imine as the component C and leveling characteristics were investigated. The results are described. A total of four levels of average molecular weights of polyethylene imine (hereinafter referred to as PEI), given in Table 4, were investigated, and the silicon wafer surface was plated with the copper plating solution of varied PEI content, where contents of the additives other than the component C, copper, sulfuric acid and chlorine were set at the same levels as in Example 1. The leveling characteristics of the copper plating treatment were evaluated by difference in thickness of deposited copper, where thickness of deposited copper on the flat surface with no circuit patterns and that on the surface provided with gaps continuously formed at intervals of 0.39 μm were determined by the SEM analysis of the wafer cross-sections, to find the difference. The plating conditions were liquid temperature: 20° C., anode: phosphorus-containing copper, and plating current: supplied at a cathode current density of 1 A/dm 2 , where the target thickness of deposited copper was set at 0.7 μm.  
                                     TABLE 4                                   Average molecular weight of PEI                                        Example 12-1    600           Example 12-2    1800           Example 12-3   10000           Example 12-4   70000                      
 
         [0053]    [0053]FIG. 6 plots difference in thickness of deposited copper against concentration of PEI added for the copper plating solutions prepared in Examples 12-1 to 12-4, given in Table 4. The average film thickness was 0.68 μm. It is found, as shown in FIG. 6, that the leveling characteristics are affected at a low PEI content, when its average molecular weight is low (Examples 12-1 and 12-2). It is also found that the surface will not be uniformly plated when PEI having a high average molecular weight such as that for Examples 12-3 and 12-4 is used, unless its content is increased to some extent. These results indicate that it is preferable to use polyethylene imine having an average molecular weight of 10,000 or more, because its analysis and management become easier as its content in the plating solution increases.  
         [0054]    As discussed above, the copper plating solution of the present invention for embedding fine wiring can plate the wafer surface provided with fine wiring patterns with gaps in-between, to completely fill the gaps even when they are very fine, of the order of sub-micron in width, and also allows copper to be deposited to a uniform thickness over the entire wafer surface. It can also realize uniform copper plating continuously and stably for extended periods, in spite of its composition characterized by high copper concentration and low sulfuric acid concentration.