Patent Publication Number: US-2005126919-A1

Title: Plating method, plating apparatus and a method of forming fine circuit wiring

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention relates to a copper plating method and more particularly to a copper plating method employing a dechlorinated leveler and the leveler and plating apparatus employed therefor.  
      This invention relates also to a method of forming fine circuit wiring by plating on a substrate having a fine circuit pattern, such as a semiconductor or a printed wiring board, and particularly to a method of forming fine circuit wiring in which phosphorus-doped copper plating is relied upon for preventing the electromigration of the fine circuit wiring as formed, and a plating solution and a plating apparatus therefor.  
      2. Description of the Related Art  
      Aluminum or an aluminum alloy has hitherto been used as a material for forming a wiring circuit on a semiconductor wafer. An improved degree of integration has, however, created a demand for a material of higher electric conductivity. Copper has drawn attention as a material satisfying such a demand and has come to be used for plating a substrate.  
      Various additives, such as a surface active agent, an unsaturated organic compound and chlorine ions, are used in copper plating to form a uniform plating film. Among these additives, it is said that chlorine ions are necessary for assisting the dissolution of the anode and the action of a brightener ( Handbook of Plating , Society for the Study of Electroplating, The Nikkan Kogyo Shinbun, Ltd., page 76). The additives also include a leveler so called, since it is used for leveling the growth of a plating film, and a nitrogen-containing high molecular compound is usually used as the leveler. The nitrogen-containing high molecular compound is often supplied in the form of a hydrochloride as a tertiary or quaternary nitrogen salt and therefore, the leveler often contains chlorine ions.  
      While the chlorine ions play an important role in a plating solution as stated above, it has been a problem that the chlorine ions incorporated in a copper plating film lower its electric conductivity. Another problem has been due to the corrosiveness of chlorine ions, as they cause the corrosion and deterioration of plated wiring.  
      On the other hand, copper is used as a material for wiring on an LSI or printed circuit board owing to its low electric resistivity. Fine copper wiring is, for example, formed on a semiconductor wafer by forming first a wiring pattern by via or other holes and trenches and then plating the wafer with copper to fill the holes and trenches with the copper to thereby form copper wiring.  
      There has recently been growing a strong demand for still smaller, higher capacity and faster LSI devices and printed circuit boards calling for finer and more highly integrated copper wiring. Accordingly, copper wiring has come to be required to allow a steady flow of an electric current at a higher current density, but has come to present a problem of electromigration.  
      When an electric current is passed through copper wiring, the copper ions (electrons) in the wiring are subjected to a coulomb force from an electrical field and a force of bombardment with the flowing electrons. At a high current, a balance between those forces is lost and the migration (diffusion) of copper ions occurs, which is a phenomenon known as electromigration. Electromigration is likely to cause the formation of voids in copper wiring or its insulation and thereby exert a serious effect on electronic devices. Moreover, electromigration causes stress migration as a secondary phenomenon and is likely to increase the frequency of occurrence of the serious effect as stated above.  
      Accordingly, a copper plating film forming copper wiring is required to have a high degree of electromigration resistance.  
      There are, however, only a few reports made so far in respect of methods of imparting electromigration resistance to copper, including a method in which tin is added to copper. As tin is a metal which is electrochemically baser than copper, it has been difficult to form a deposited copper film containing tin uniformly by electroplating, since copper is deposited more actively than tin. Accordingly, there has been no effective way to form copper wiring having a high level of electromigration resistance by copper plating.  
     SUMMARY OF THE INVENTION  
      It is, therefore, an object of this invention to provide a method of forming a plating film containing less chlorine ions.  
      It is another object of this invention to provide a method of forming circuit wiring having a high level of electromigration resistance by electroplating.  
      We, the inventors of this invention, have discovered that the chlorine ions incorporated in a plating film are not the chlorine ions added intentionally to a plating solution, but a small amount of chlorine ions which a leveler contains. This invention is based on our discovery.  
      According to one aspect of this invention, therefore, there is provided a method of copper plating using a leveler containing a nitrogen-containing high molecular compound, wherein the leveler is dechlorinated prior to its use for plating.  
      According to another aspect of this invention, there is provided a leveler for copper plating containing a nitrogen-containing high molecular compound, the leveler having a chlorine ion content of 0.1 g or less per gram of nitrogen-containing high molecular compound.  
      According to still another aspect of this invention, there is provided a plating apparatus comprising a tank for preparing a plating solution, a device for dechlorinating a leveler, a leveler supply station for supplying the dechlorinated leveler to the tank and a plating station.  
      According to a further aspect of this invention, there is provided a plating apparatus for forming a metal film on a seed layer on a substrate surface by electroplating, the apparatus comprising a loading and unloading station, a substrate conveying device, a cleansing unit, a plating tank, a tank for preparing a plating solution and supplying it to the plating tank, a station for measuring the concentrations of additives, a device for dechlorinating a leveler and a leveler supply station for supplying the dechlorinated leveler to the plating solution, the concentration measuring station measuring the concentration of the leveler in the plating solution and in accordance with the result of the measurement, the leveler supply station adding the dechlorinated leveler to the plating solution.  
      We have studied various possibilities of forming copper wiring having a high level of electromigration resistance by copper plating, and discovered that a phosphorus-doped copper plating film deposited by using a copper plating solution containing phosphorus or phosphoric acid ions in addition to copper ions has a higher level of electromigration resistance than that of an ordinary copper plating film. We have found that the use of such a plating solution makes it possible to form copper wiring having a high level of electromigration resistance on a semiconductor wafer, or the like, and we have made this invention.  
      According to a still further aspect of this invention, therefore, there is provided a method of forming fine circuit wiring, comprising forming a circuit with a layer of phosphorus-doped copper plating on a substrate for an electronic circuit having a fine circuit pattern, a barrier layer and any necessary seed layer formed thereon.  
      According to a still further aspect of this invention, there is provided a phosphorus-doped copper plating solution containing 1×10 −6  to 50% by weight of elemental phosphorus in the form of a phosphorus compound in a copper sulfate plating solution containing copper sulfate, sulfuric acid and chlorine ions.  
      According to a still further aspect of this invention, there is provided a plating apparatus having an anode plate and a substrate to be plated which are positioned opposite each other in a plating tank containing a phosphorus-doped copper plating solution for electroplating the surface of the substrate by supplying an electric current between the anode plate and the substrate from a power source, the apparatus having at least a device for supplying a solution containing a phosphorus compound as a device for supplying a constituent composing the plating solution.  
      The use of any copper plating method, leveler or plating apparatus according to this invention makes it possible to reduce the amount of the chlorine ions incorporated in any copper plating film and thereby the possibility of corrosion and deterioration of any copper-plated wiring.  
      The fine circuit wiring formed by phosphorus-doped copper in accordance with this invention has a higher level of electromigration resistance than that of any wiring formed by copper alone, allows an electric current to pass therethrough at a higher current density and therefore copes with a demand for finer and higher density circuit wiring. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIGS. 1A  to  1 C are a series of sectional views showing an example of plating processes;  
       FIG. 2  is a diagram showing a plating apparatus embodying this invention;  
       FIG. 3  is a diagram showing a dechlorinating device and a raw leveler tank in the apparatus embodying this invention;  
       FIG. 4  is a diagram showing a plating station in the apparatus embodying this invention;  
       FIG. 5  is a diagram showing another form of plating station;  
       FIGS. 6A  to  6 D are a series of views showing the steps of a method embodying this invention;  
       FIG. 7  is a phase diagram of copper and phosphorus; and  
       FIG. 8  is a diagram showing another form of plating apparatus embodying this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      A leveler used in copper plating is usually employed for leveling the growth of a plating film and contains a nitrogen-containing high molecular compound. The nitrogen-containing high molecular compound used herein may be any high molecular compound containing a nitrogen element in its molecule, for example, poly(dialkylaminoethyl acrylate), poly(diallyl dimethyl ammonium), polyethyleneimine, polyvinyl pyridine, polyvinyl amidine, polyallylamine or polyaminesulfonic acid. The nitrogen-containing high molecular compound is used in the form of a hydrochloride as a tertiary or quaternary nitrogen salt so that its solubility in water may be improved. Accordingly, the leveler contains a nitrogen-containing high molecular compound and chlorine ions.  
      The leveler is usually provided in the form of an aqueous solution containing, say, 5 to 20 g of nitrogen-containing high molecular compound and 3 to 10 g of chlorine ions per liter.  
      The leveler is added to a plating solution to the extent that the plating solution usually contains 5 to 20 mg of nitrogen-containing high molecular compound per liter. When the leveler is added to the plating solution, the chlorine ions which the leveler contains are carried over into the plating solution and the plating solution usually contains several milligrams of chlorine ions per liter.  
      Other additives used in a copper plating solution include chlorine ions. The chlorine ions are usually added in the amount of 40 to 80 mg per liter, as it is said that they are necessary for assisting the dissolution of the anode and the action of a brightener.  
      As the amount of the chlorine ions carried over from the leveler into the plating solution is by far smaller than that of the chlorine ions added to the plating solution for assisting the dissolution of the anode and the action of the brightener, it has been considered that the chlorine ions which are carried over from the leveler do not exert any effect on plating.  
      Our study has, however, ascertained that the chlorine ions carried over from the leveler into the plating solution are incorporated predominantly into a plating film, as will become obvious from the description of examples. Accordingly, the chlorine ions contained in the leveler are dechlorinated to reduce the amount of chlorine ions incorporated into a plating film.  
      While any method can be employed for dechlorinating the leveler in accordance with this invention if it can reduce the amount of chlorine ions, a method relying upon sedimentation or electrolysis is preferred.  
      The dechlorination of the leveler by sedimentation may, for example, be carried out by adding to it a substance forming a hardly soluble salt with chlorine ions, e.g. silver nitrate, as shown by expression I: 
 
AgNO 3 +Cl − →AgCl↓+NO 3   −   (I) 
 
 The dechlorination by electrolysis may, for example, be carried out by removing chlorine gas at the anode, as shown by expression II: 
 
2Cl − →Cl 2 ↑+2e −   (II) 
 
      The dechlorinated leveler used in accordance with this invention preferably has a chlorine ion content of 0.1 g or less, more preferably 0.05 g or less and still more preferably 0.01 g or less per gram of nitrogen-containing high molecular compound. If its chlorine ion content exceeds 0.1 g per gram of nitrogen-containing high molecular compound, it is likely that an undesirably large amount of chlorine ions may be incorporated into a plating film.  
      Moreover, the dechlorinated leveler used in accordance with this invention preferably has a chlorine ion content of 1 g or less per liter. If it is exceeded, it is likely that an undesirably large amount of chlorine ions may be incorporated into a plating film.  
      The invention will now be described in further detail with reference to the drawings, though the drawings are not intended for limiting the scope of this invention.  
      The plating method, leveler for plating and plating apparatus according to this invention are mainly used for forming copper layers of wiring by electroplating on the surface of a semiconductor wafer to be plated. Reference is first made to  FIGS. 1A  to  1 C showing an example of plating processes.  
      A semiconductor wafer W has a conductive layer la formed on a substrate  1  having a semiconductor device formed thereon, an insulating film  2  of SiO 2  deposited thereon, a contact hole  3  and a wiring trench  4  formed therein by lithography and etching, a barrier layer  5  formed thereon from e.g. TiN and a seed layer  7  formed thereon as a feed layer for electrolytic plating, as shown in  FIG. 1A .  
      The semiconductor wafer W has its surface plated with copper, so that a copper layer  6  maybe deposited on the insulating film  2 , while the contact hole  3  and trench  4  of the substrate  1  are filled with copper, as shown in  FIG. 1B . Then, the copper layer  6  on the insulating film  2  is removed by chemical mechanical polishing (CMP), so that the copper layer  6  filling the contact hole  3  and wiring trench  4  may have a surface substantially flush with the surface of the insulating film  2 . As a result, wiring is formed by the copper layer  6 , as shown in  FIG. 1C .  
       FIG. 2  is a diagram showing a plating apparatus  10  embodying this invention. The plating apparatus  10  of this invention has a plating solution tank  12  for preparing and holding a plating solution  14 , a concentration measuring station  16 , a carrier supply station  40 , a surface active agent supply station  50  and a leveler supply station  30  connected to the plating solution tank  12  by pipelines  15 ,  48 ,  58  and  38 , respectively, a dechlorinating device  60  connected to the leveler supply station  30  by a pipeline  67 , a raw leveler tank  90  connected to the dechlorinating device  60  by a pipeline  94 , a control station  18  connected to all of the concentration measuring station  16 , leveler supply station  30 , carrier supply station  40  and surface active agent supply station  50 , and a plating station  70  connected to the plating solution tank  12  by a pipeline  100 , a pump  102 , a filter  104 , a pipeline  108  and a pump  106 , as shown in  FIG. 2 .  
      The leveler supply station  30  has a leveler tank  32  holding a dechlorinated leveler  34  and a pump  36  connected to the control station  18 , the carrier supply station  40  has a carrier tank  42  holding a carrier  44  and a pump  46  connected to the control station  18  and the surface active agent supply station  50  has a surface active agent tank  52  holding a surface active agent  54  and a pump  56  connected to the control station  18 , as shown in  FIG. 2 .  
       FIG. 3  is a diagram showing details of the dechlorinating device  60  and raw leveler tank  90  by example. The de-chlorinating device  60  has a dechlorinating tank  62 , a cathode  64 , an anode  65  and a stirrer  66 , as shown in  FIG. 3 . The raw leveler tank  90  holds a raw leveler  92  and is connected to the dechlorinating tank  62  by the pipeline  94  with a pump  96 .  
       FIG. 4  is a diagram showing details of the plating station  70  by example. A plating tank  76  connected to the pipelines  100  and  108  holds a plating solution  14  in which a wafer W mounted on a jig and an anode  72  are disposed opposite each other, while a power source  74  is connected between the wafer W and the anode  72 , as shown in  FIG. 4 .  
       FIG. 5  is a top plan view of another form of plating station  70 . The plating station  70  has four loading and unloading units  80  holding a plurality of substrates W therein, four plating units  82  for performing plating and auxiliary treatment, two conveying robots  84  and  85  for conveying the substrates W between the loading and unloading units  80  and the plating units  82 , two bevel and rear surface cleansing units  86 , a film thickness measuring device  87  and a temporary wafer support  88 , as shown in  FIG. 5 . The plating units  82  are all connected to the pipelines  100  and  108 , though they are only partly shown. In  FIG. 5 , the conveying robots  84  and  85  constitute s substrate conveying device according to this invention.  
      All the equipment shown in  FIG. 2  for preparing the plating solution may be installed within the plating station shown in  FIG. 5  so as to form an integral part thereof.  
      Description will now be made of the operation of the plating apparatus of this invention constructed as described above. The raw leveler  92  held in the raw leveler tank  90  and still to be dechlorinated is drawn by the pump  96  through the pipeline  94  into the dechlorinating device  60 , whereby it is dechlorinated. The raw leveler  92  pumped into the dechlorinating tank  62  is uniformly stirred by the stirrer  66 . Upon application of a voltage to the anode  65  and cathode  64 , the reaction shown by the expression II occurs at the anode  65  and chlorine gas (Cl 2 ) emerges. This indicates the dechlorination of the leveler. The dechlorinated leveler  34  is transferred by the pump  69  through the pipeline  67  into the leveler tank  32  in the leveler supply station  30 . 
 
2Cl − →Cl 2 ↑+2e −   (III) 
 
      The plating solution  14  held in the plating solution tank  12  is transferred to the concentration measuring station  16 , in which the concentrations of the leveler, carrier and surface active agent in the solution are measured. The results of the measurements are sent to the control station  18  and if the concentration of any of those additives is lower than the pre-set control range, the dechlorinated leveler  34 , carrier  44  or surface active agent  54  is supplied by the pump  36 ,  46  or  56  from the leveler supply station  30 , carrier supply station  40 , or surface active agent supply station  50  to the plating solution tank  12  through the pipeline  38 ,  48  or  58 , so that the concentration of each additive may be kept within the control range by the control station  18 .  
      The plating solution  14  having its additives controlled in concentration as described is drawn by the pump  102  through the pipeline  100  and the filter  104  into the plating station  70  and used for plating the substrate or substrates W in the plating tank  76  or the plating units  82 . The plating solution having lowered concentrations of surface active agent, carrier and leveler as a result of their consumption by plating is returned by the pump  106  into the plating solution tank  12  through the pipeline  108 .  
      Description will now be made of the operation of the plating station  70 . Referring first to one form of plating station  70  as shown in  FIG. 4 , the plating solution  14  having its additives controlled in concentration is supplied by the pump  102  through the pipeline  100  and the filter  104  into the plating tank  76 . A voltage is applied between the anode  72  and the wafer W by the power source  74 , whereby the wafer W has its surface plated with copper. After plating, the plating solution is returned by the pump  106  into the plating solution tank  12  through the pipeline  108 .  
      Referring now to another form of plating station  70  as shown in  FIG. 5 , a wafer W to be plated is taken out by the conveying robot  84  from a wafer cassette installed in any of the loading and unloading stations  80  and is conveyed to the film thickness measuring device  87  in which the thickness of a plating film for the wafer W to be plated is measured. Then, the wafer W is taken out by the robot  84  from the film thickness measuring device  87  and mounted on the temporary wafer support  88 . Then, the wafer W on the temporary wafer support  88  is taken by the hands of the other conveying robot  85  and charged into any of the plating units  82  through its wafer charge and discharge opening, while its surface to be plated is held upside. The plating solution  14  having its additives controlled in concentration is supplied from the plating solution tank  12  by the pump  102  through the pipeline  100  and the filter  104  into the plating unit  82  to plate the wafer. After plating, the plating solution is returned by the pump  106  through the pipeline  108  into the plating solution tank  12 .  
      After its plating, the wafer W is discharged from the plating unit  82  by the robot  85 . The wafer W as discharged is conveyed to one of the bevel and rear surface cleansing units  86  and after its cleansing and drying, it is mounted on the temporary wafer support  88  by the robot  85  and is, then, conveyed by the robot  84  to the film thickness measuring device  87 , in which the thickness of the plating film formed on the wafer W is measured, and it is conveyed by the robot  84  into the wafer cassette installed in any of the loading and unloading stations  80 . This is the end of the whole process of plating a single wafer W.  
      The method of this invention is carried out by, for example, employing a substrate for an electronic circuit having a fine circuit pattern formed thereon as shown in  FIG. 6A , forming a barrier layer on the substrate as shown in  FIG. 6B , forming a seed or catalyst layer thereon as shown in  FIG. 6C  and forming a phosphorus-doped copper plating layer thereon to fill the fine holes and trenches defining the fine circuit pattern as shown  FIG. 6D . In  FIGS. 6A  to  6 D,  201  and  203  denote interlayer insulating layers formed on the substrate,  202  a conductive layer,  204  the barrier layer,  205  the seed or catalyst layer, and  206  the phosphorus-doped copper plating layer.  
      The substrate on which fine circuit wiring is formed by the method of this invention is a semiconductor wafer, or printed circuit substrate having a fine circuit pattern formed on its surface. Such a pattern is, for example, formed by fine trenches and holes, such as via holes, and the trenches and holes are filled with phosphorus-doped copper to form circuit wiring.  
      The method of forming fine circuit wiring according to this invention is carried out after the substrate is pre-treated by a customary method. The pretreatment of, for example, a silicon substrate such as a silicon wafer is done to form a barrier layer of, for example, Ta, TaN, TiN, WN, SiTiN, CoWP or CoWB ( FIG. 6B ). If electroplating is thereafter carried out, a copper seed layer serving as a power feed layer is formed by e.g. PVD as pretreatment after the formation of a barrier layer. If electroless plating is carried out, pretreatment is done to form a catalytic layer ( FIG. 6C ).  
      The substrate pretreated as described has a phosphorus-doped copper plating layer formed thereon ( FIG. 6D ). This plating is so done as to fill the whole fine trenches and holes forming a fine circuit pattern. Finally, the phosphorus-doped copper film deposited on any other surface than the area of the circuit wiring is removed by e.g. CMP, leaving a fine circuit wiring formed by the phosphorus-doped copper film.  
      The phosphorus-doped copper plating solution used to form a phosphorus-doped copper film in accordance with this invention, which has been discovered by us, the inventors of this invention, contains, for example, the following constituents:  
                                                      Copper sulfate pentahydrate   150 to 250 g/l           Sulfuric acid    10 to 100 g/l           Chlorine ion    30 to 90 mg/l           Phosphorus compound   100 to 10,000 mg/l           (as phosphoric acid ion)           Polymer component    10 to 40 ml/l           Carrier component    1 to 20 ml/l           Leveler component    1 to 20 ml/l                      
 
      When electrolytic copper sulfate plating is employed to fill wiring trenches and holes in the surface of e.g. a semiconductor wafer, it is often the case to add three kinds of organic additives called the polymer, carrier and leveler components to the basic components, copper sulfate (CuSO 4 .5 H 2 O), sulfuric acid (H 2 SO 4 ) and chlorine (Cl), in order to make it possible to form a plating film of improved quality and fill the trenches and holes in an improved way.  
      Firstly, the polymer component is a component added to suppress the deposition of adsorbed copper ions on the cathode surface to thereby increase activation polarization and improve uniformity of electrodeposition, and also called a suppressor or carrier. A surface active agent, such as polyethylene glycol (PEG) or polypropylene glycol (PPG), is usually employed.  
      Secondly, the carrier component is a component added to improve the density and brightness of a plating film and also called a brightener. A sulfur compound, such as mercapto-alkylsulfonic acid or HS—C n H 2n —SO 3 , is usually employed. It is in the form of anions in the plating solution, inhibits the deposition of copper ions and thereby promotes the formation of a finer deposit.  
      Thirdly, the leveler component is a compound containing nitrogen, such as polyamine. It is in the form of cations in the plating solution. The adsorption of the leveler is more likely to occur in a place having a high current density and in the place where the adsorption of the leveler is more likely to occur, activation overvoltage increases and the deposition of copper is suppressed. At the bottom of any fine trench or hole, on the other hand, the adsorption of the leveler is less likely to occur and the deposition of copper is predominant.  
      Examples of the phosphorus compounds in the phosphorus-doped copper plating solution are, for example, phosphoric acid, copper sulfate and phosphorus oxides such as phosphorus pentoxide.  
      In order to have a phosphorus-doped copper film deposited from the above plating solution, it is desirable to control the deposition potentials of copper and phosphorus so that they may be close to each other. Moreover, it is possible to add to the plating solution any organic additive known as used in any known acidic copper plating solution, such as a deposition inhibitor or accelerator, if required.  
      When the above plating solution is used to form a phosphorus-doped copper plating film, it may have a temperature of, say, 15° C. to 40° C. and a current density of, say, 0.3 to 30 mA/cm 2  may be employed. In order to form a phosphorus-doped copper film which is stable in composition, it is desirable to use an insoluble substance, such as platinum (Pt) or iridium oxide (Ir 2 O 3 ), as the anode instead of metallic copper and add a solution of the component to be deposited.  
      The phosphorus-doped copper plating film formed as described is a film of copper containing a very small amount of phosphorus or a phosphorus compound incorporated therein. Although the phase in which phosphorus or a compound thereof is present in copper is not clearly known, it is obvious from the Cu—P phase diagram in  FIG. 7  that when copper has a temperature of 300° C. or below, for example, 0.6 atm % or less of phosphorus may be incorporated in the crystal grain boundary of copper, or its crystal and form a solid solution with it, and its presence in such a phase is, therefore, possible. It is considered that phosphorus provides an improved electro-migration resistance, as it inhibits the diffusion of copper atoms. The same is true with a phosphorus compound.  
       FIG. 8  shows an example of plating apparatus which can be employed to form fine circuit wiring by a phosphorus-doped copper film in accordance with this invention.  
       FIG. 8  is a diagram showing the layout of the plating apparatus by example, which includes a plating tank  210  holding a phosphorus-doped copper plating solution Q, in which an anode plate  211  and a substrate  212  to be plated are disposed opposite each other, and if a plating current is supplied between the anode  211  and the substrate  212  from a power source E, the substrate  212  has its surface plated electrolytically.  
       213  is a plating solution preparing tank for preparing a phosphorus-doped copper plating solution. A standard copper sulfate plating solution Q 1  (basic solution), an additional solution Q 2  obtained by adding the polymer component to the basic solution, an additional solution Q 3  obtained by adding the carrier component to the basic solution, an additional solution Q 4  obtained by adding the leveler component to the basic solution, sulfuric acid (H 2 SO 4 ) Q 5  and hydrochloric acid (HCl) Q 6  can be supplied to the plating solution preparing tank  213  from a standard solution tank  214  through a pump P 1  and a valve V 1 , from an additional solution tank  215  through a pump P 2  and a valve V 2 , from an additional solution tank  216  through a pump P 3  and a valve V 3 , from an additional solution tank  217  through a pump P 4  and a valve V 4 , from a sulfuric acid tank  218  through a pump P 5  and a valve V 5  and from a hydrochloric acid tank  219  through a pump P 6  and a valve V 6 , respectively. Phosphoric acid (H 3 PO 4 ) Q 7  can be supplied from a phosphoric acid tank  226  through a pump P 7  and a valve V 7 .  
      The phosphorus-doped copper plating solution Q 8  prepared in the plating solution preparing tank  213  is supplied by a pump P 8  to the plating tank  210  through a filter  220 . The plating solution Q exceeding a predetermined surface level in the plating tank  210  is returned to the plating solution preparing tank  213 . Thus, the plating solution is circulated between the plating solution preparing tank  213  and the plating tank  210 .  221  is a sampling device for taking a sample of the phosphorus-doped copper plating solution Q 8  supplied to the plating solution  210  and  222  is an automatic analyzing device for analyzing automatically the composition of the sample of the plating solution Q 8  taken by the sampling device  221 .  223  is a waste solution tank,  224  is a level sensor for measuring the surface level of the plating solution Q 8  in the plating solution preparing tank  213  and  225  is a control unit.  
      The composition of the plating solution Q 8  as analyzed by the automatic analyzing device  222  and the level of the plating solution Q 8  as measured by the level sensor  224  are inputted to the control unit  225 . In accordance with the results of analysis of the plating solution Q 8  by the automatic analyzing device  222 , the control unit  225  controls the pumps P 1  to P 7  and the valves V 1  to V 7  to control the standard solution Q 1  supplied from the standard solution tank  214 , the additional solution Q 2  supplied from the additional solution tank  215 , the additional solution Q 3  supplied from the additional solution tank  216 , the additional solution Q 4  supplied from the additional solution tank  217 , sulfuric acid Q 5  supplied from the sulfuric acid tank  218 , hydrochloric acid Q 6  supplied from the hydrochloric acid tank  219  and phosphoric acid Q 7  and thereby regulate the composition of the plating solution Q 8  in the plating solution preparing tank  213 .  
      The invention will now be described in further detail by examples, though these examples are not supposed at all to limit the scope of this invention.  
     EXAMPLE 1  
      A test was conducted to ascertain that chlorine ions incorporated into a plating film were of a leveler.  
      One liter of a plating solution was so prepared as to contain 200 g of CuSO 4 .5H 2 O, 10 g of H 2 SO 4 , 60 mg of chlorine ions, 200 mg of polyethylene glycol having a molecular weight of about 3000 and 5 mg of bis(3-sulfopropyl)disulfide.  
      A quaternary ammonium hydrochloride salt of polyvinyl pyridine yet to be dechlorinated was employed as a leveler and was so added to the plating solution that the plating solution might contain 10 mg of polyvinyl pyridine per liter. The leveler yet to be dechlorinated contained 16 g of polyvinyl pyridine and 4 g of chlorine ions per liter. The plating solution was used for the copper plating of a silicon wafer. The plating solution to which no leveler had been added was also used for the copper plating of a silicon wafer.  
      The copper-plated silicon wafers were examined by a secondary ion mass spectrometer (SIMS) for the chlorine ions incorporated in their copper plating films. As a result, it was confirmed that the amount of chlorine ions in the copper plating film formed by using the leveler containing chlorine ions was about 10 times larger than in the plating film formed without using any leveler.  
      It is, therefore, obvious that the chlorine ions carried over from the leveler into the plating solution are predominantly incorporated into the plating film.  
     EXAMPLE 2  
      The leveler used in Example 1 was dechlorinated to prepare a leveler having a chlorine ion concentration reduced to 1 g/l.  
      The dechlorinated leveler and the same plating solution as in Example 1 were used for the copper plating of a silicon wafer.  
      The examination of the wafer by SIMS as in Example 1 can confirm a reduction in the amount of chlorine ions incorporated in its copper plating film.  
     EXAMPLE 3  
      A phosphorus-doped copper plating solution was prepared by adding 5 ml of 50% phosphoric acid to one liter of a copper sulfate plating solution (basic solution) having the composition shown below. The plating solution was used for one minute of phosphorus-doped copper plating at a temperature of 25° C. and a current density of 30 mA/cm 2  on a semiconductor wafer in which via holes having a width of 150 nm and an aspect ratio of 5 had been formed. The semiconduct or wafer had a barrier and a seed layer formed by customary methods.  
      Composition of the Copper Sulfate Plating Solution:  
                                                      Copper sulfate pentahydrate   200 g/l           Sulfuric acid    50 g/l           Chlorine ion    50 mg/l           Phosphorus compound   100 mg/l           (as phosphoric acid ion)           Polymer component    30 ml/l           Carrier component    10 ml/l           Leveler component    10 ml/l                      
 
      There was obtained a phosphorus -doped copper plating film having a phosphorus content of 1×10 −6  atom % or more along its depth. Its examination by a scanning electron microscope did not reveal any void in any of the via holes in the substrate. Its electromigration resistance was higher than that of any copper plating film not containing phosphorus.  
      As is obvious from the foregoing, the plating method, plating apparatus and leveler according to this invention make it possible to reduce the amount of chlorine ions incorporated in a plating film.  
      The phosphorus-doped copper plating film formed in accordance with this invention can form copper wiring having a higher level of electromigration resistance than that of any ordinary copper plating film.  
      The phosphorus-doped copper wiring is widely useful as wiring for a smaller and more highly integrated substrate for an electronic circuit, such as a semiconductor wafer.