Patent Publication Number: US-2007099426-A1

Title: Polishing method, polishing apparatus, and electrolytic polishing apparatus

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention:  
      The present invention relates to a polishing method and a polishing apparatus, and more particularly to a polishing method and a polishing apparatus for polishing a substrate, such as a semiconductor wafer, having an interconnect material (metal), e.g., copper, embedded in fine interconnect recesses formed on a dielectric (interlevel dielectric) on the substrate to thereby form interconnects in the substrate.  
      The present invention also relates to an electrolytic polishing apparatus suitable for use in the above polishing apparatus.  
      2. Description of the Related Art:  
      A so-called damascene process, which comprises embedding an interconnect metal into interconnect recesses such as trenches and via holes formed on a dielectric, has been used as a process of forming interconnects in a semiconductor device. According to the damascene process, the interconnect recesses are formed on the dielectric (interlevel dielectric), which is composed of SiO 2 , SiOF, SiOC, Low-k material, or the like, of the substrate. Subsequently, a barrier film of titanium, tantalum, tungsten, ruthenium, and/or their alloys is formed on an entire surface of the dielectric including the interconnect recesses. Then, an interconnect metal film of aluminum, copper, silver, gold, or their alloys is formed on a surface of the barrier film to fill the interconnect recesses with the interconnect material. Thereafter, extra interconnect metal film and the barrier film formed on portions other than the interconnect recesses are removed. In current high-speed devices, copper or copper alloy is generally used as the interconnect metal, and so-called Low-k material is increasingly used as the dielectric.  
      In the damascene process, formation of the interconnect recesses is generally performed by dry etching or the like, and formation of the barrier film is generally performed by a dry process such as PVD (physical vapor deposition), CVD (chemical vapor deposition), or ALD (atomic layer deposition). Formation of the interconnect metal film is performed by a wet process such as electroplating or electroless plating, or a dry process such as PVD, CVD, or ALD. Recently, electroplating has been widely used to form an interconnect metal film. In a case where the interconnect metal film is to be formed by electroplating onto a barrier film having low electrical conductivity, a seed film, serving as an electric supply layer, is typically formed in advance on a surface of the barrier film subsequent to formation of the barrier film. Generally, extra interconnect metal and the barrier film are removed by a planarizing method such as chemical mechanical polishing (CMP) or electrolytic polishing (composite electrolytic polishing).  
       FIGS. 1A through 1C  of the accompanying drawings show successive steps of a process of forming copper interconnects in a semiconductor device. As shown in  FIG. 1A , an insulating film (interlevel dielectric)  302  composed of, for example, SiO 2  or Low-k material is deposited on a conductive layer  301   a  formed on a semiconductor base  301  where semiconductor elements have been formed thereon Then, via holes  303  and trenches  304  are formed in the insulating film  302  by performing a lithography/etching technique. A barrier film  305  of Ta, TaN, or the like is formed on the insulating film  302  including the via holes  303  and the trenches  304 , and then a seed film  306 , serving as an electric supply layer for electroplating, is formed on the barrier film  305  by performing sputtering or other techniques.  
      Then, as shown in  FIG. 1B , copper plating is performed on a surface of the semiconductor substrate W so as to fill the via holes  303  and the trenches  304  with copper, and, at the same time, deposit a copper film  307  as an interconnect metal film onto the insulating film  302 . Thereafter, the copper film  307 , the seed film  306 , and the barrier film  305  on the insulating film  302  are removed by chemical mechanical polishing (CMP) or other techniques, so that a surface of the copper film  307  filling the via holes  303  and the trenches  34  is substantially flush with the surface of the insulating film  302  As a result, as shown in FIG  1 C, interconnects  308  comprising the seed film  306  and the copper film  307  are formed in the insulating film  302 .  
      In a process of forming interconnects which are as fine as 65 nm or less, Low-k material is expected to be used as the insulating film (dielectric). The Low-k material has low mechanical strength compared to conventional material such as SiO 2 , SiOF, or SiOC Accordingly, polishing of an interconnect metal film on the insulating film of Low-k material at excessive polishing pressure is not preferable in view of preventing damage to the insulating film (i.e., Low-k material). Even if mechanical strength of the Low-k material is improved, high polishing pressure may cause damage to surfaces of the fine interconnects after polishing. Such damage to these polished surfaces would cause adverse influences such as an increase in interconnect resistance. Therefore, there is the need for lowering polishing pressure.  
     SUMMARY OF THE INVENTION  
      The present invention has been made in view of the above drawbacks. It is therefore an object of the present invention to provide a polishing method, a polishing apparatus, and an electrolytic polishing apparatus which can prevent damage to an insulating film and an interconnect metal film during a process of forming interconnects which are as fine as 65 nm or less, and can thus produce a highly durable and high-speed device.  
      In order to solve the above drawbacks, according to one aspect of the present invention, there is provided a polishing method of polishing a substrate so as to remove an interconnect metal film and a barrier film formed on portions other than interconnect recesses. This method comprises: performing a first polishing process of polishing a surface of the substrate; after performing the first polishing process, cleaning the surface of the substrate; and then, performing a second polishing process of farther polishing the surface of the substrate. At least one of performing the first polishing process and performing the second polishing process comprises performing electrolytic polishing.  
      Because the surface of the substrate is cleaned between the first polishing process and the second polishing process, successive polishing can be performed using polishing liquids having greatly different compositions. Accordingly, a manner of polishing can be diversified and, as a result, formation of interconnects with less damage can be achieved.  
      Further, according to the present invention, electrolytic polishing, which generally has a little effect on device elements such as interconnects, is employed as at least part of a polishing process. For example, electrolytic polishing may constitute most part of polishing, i.e., removing most part of the interconnect metal film formed on portions other than the interconnect recesses. Use of electrolytic polishing in this manner can greatly reduce damage to an interconnect structure. Therefore, it is possible to prevent damage to an insulating film and an interconnect metal film during a process of forming interconnects which are as fine as 65 nm or less, and thus to produce a highly durable and high-speed device.  
      Electrolytic polishing includes general electrolytic polishing using phosphoric acid as an electrolytic solution (polishing liquid) for dissolving an interconnect metal film by anode polarization, and composite electrolytic polishing comprising electrolytic polishing and low-pressure mechanical polishing. General electrolytic polishing performs a polishing process utilizing only electrolytic oxidation and etching, and composite electrolytic polishing performs a polishing process utilizing a combination of electrolytic oxidation, etching, and mechanical polishing.  
      In a preferred aspect of the present invention, the electrolytic polishing is performed using an electrolytic solution having an electrical conductivity of not less than 50 mS/cm.  
      By allowing high current to flow through the electrolytic solution, electrolytic polishing can be efficiently performed, even if voltage is low. Use of high voltage results not only in high electric power expense, but also in high production cost of the apparatus because of a need for a high-capacity rectifier. Use of the electrolytic solution polishing liquid) having an electrical conductivity of not less than 50 mS/cm can lower the voltage required for electrolytic polishing to less than 10 V. Therefore, electrolytic polishing can be efficiently performed.  
      In a preferred aspect of the present invention, at least one of performing the first polishing process and performing the second polishing process comprises performing CMP.  
      In general, electrolytic polishing (composite electrolytic polishing) can perform polishing with a little damage to interconnects, but may be inferior in eliminating level differences on a surface of an interconnect metal film. Further, in general, an electrical conductivity of the barrier film is greatly lower than that of the interconnect metal film Accordingly, electrical resistance increases at a time the barrier film is exposed, and hence, electrolytic polishing may be stopped with part of the interconnect metal film remaining, even if electrolytic polishing is requited to be continued. According to the present invention, electrolytic polishing (composite electrolytic polishing) can be performed so as to remove most of the interconnect metal film, and subsequently CMP, which is excellent in eliminating level differences, can be performed so as to remove a remaining interconnect metal film, thus enhancing a flatness of a polished surface of the substrate. In this case, by switching from electrolytic polishing to CMP at a time the barrier film is exposed, the remaining interconnect metal film and the barrier film underneath the interconnect metal film can be sufficiently removed.  
      In a preferred aspect of the present invention, cleaning the surface of the substrate comprises cleaning and rinsing the surface of the substrate using a cleaning unit.  
      In a preferred aspect of the present invention, cleaning the surface of the substrate comprises performing a water polishing process of polishing the substrate on a polishing table while supplying water to the substrate.  
      In a preferred aspect of the present invention, cleaning the surface of the substrate comprises rinsing the surface of the substrate at a position laterally of a polishing table.  
      In a preferred aspect of the present invention, cleaning of the surface of the substrate is performed until an electrical conductivity of a waste cleaning liquid discharged during cleaning the surface of the substrate is reduced to at most one-third of an electrical conductivity of a polishing liquid used in the second polishing process.  
      In electrolytic polishing, a thick electrolytic solution (polishing liquid) having a high electrical conductivity is preferably used. On the other hand, in CMP, a polishing liquid having a high electrical conductivity may cause aggregation of polishing particles, thus deteriorating a polishing property. Accordingly, a thin polishing liquid having a low electrical conductivity in the range o, for example, 1 to 10 mS/cm is generally used in CMP However, if electrolytic polishing (the first polishing process) is performed using a thick electrolytic solution (polishing liquid) having a high electrical conductivity and CMP (the second polishing process) is subsequently performed without cleaning the substrate to which the electrolytic solution adheres, then the polishing liquid used in CMP becomes thick, and hence, a polishing property is deteriorated. In view of such a drawback, cleaning and rinsing are performed until the electrical conductivity of the waste cleaning liquid is reduced to at most one-third of, preferably one-tenth of, more preferably a level substantially equal to the electrical conductivity of the polishing liquid used in the second polishing process, whereby the second polishing process can be performed without deteriorating the polishing property.  
      In a preferred aspect of the present invention, the polishing method further comprises monitoring an electrical conductivity of a waste cleaning liquid discharged during cleaning the surface of the substrate using an electrical conductivity meter.  
      By monitoring the electrical conductivity of the waste cleaning liquid using the electrical conductivity meter, an effect of cleaning can be confirmed and polishing can be sufficiently performed.  
      In a preferred aspect of the present invention, the polishing method further comprises conditioning a polishing surface before or after performing the electrolytic polishing.  
      The present invention can prevent polishing liquids from being mixed with each other, and therefore, the same polishing surface can be used in the first polishing process and the second polishing process.  
      According to another aspect of the present invention, there is provided a polishing apparatus comprising; a first polishing unit including an electrolytic polishing apparatus for polishing a substrate; at least one cleaning unit for cleaning and rinsing the substrate, a second polishing unit for further polishing the substrate after processed by the electrolytic polishing apparatus and the at least one cleaning unit; and at least one drying unit for drying the substrate.  
      According to the present invention, the first polishing unit, the cleaning unit, the second polishing unit, and the drying unit can perform a series of processes in a single polishing apparatus. Further, the substrate, which is introduced to the polishing apparatus in a dry state, can be processed and removed in a dry state from the polishing apparatus. Therefore, a state of the substrate after polishing can be consistent with a state of the substrate before polishing.  
      In a preferred aspect of the present invention, the second polishing unit includes a CMP apparatus.  
      In a preferred aspect of the present invention, the electrolytic polishing apparatus comprises a conditioning member for conditioning a polishing surface, and also serves as the CMP apparatus.  
      Conditioning of the polishing surface can prevent polishing liquids from being mixed with each other, and therefore, the first polishing process and the second polishing process can be performed in the same electrolytic polishing apparatus. Examples of the conditioning member include an atomizer which supplies pressurized pure water or a chemical liquid, which accelerates removal of the electrolytic solution, onto a polishing surface.  
      In a preferred aspect of the present invention, the polishing apparatus further comprises an electrical conductivity meter provided in a drain passage of the cleaning unit for measuring electrical conductivity of a waste rinsing liquid flowing through the drain passage.  
      According to another aspect of the present invention, there is provided an electrolytic polishing apparatus comprising: a first electrode connected to one of poles of a power source; a polishing table electrically connected to the first electrode; a polishing pad provided on an upper surface of the polishing table and having a polishing surface; a top ring operable to hold a substrate and press the substrate against the polishing surface at a pressure of not more than 7 kPa; a second electrode connected to another of the poles of the power source for supplying electricity to the substrate; a liquid supply unit for supplying a liquid onto the polishing surface; a conditioning member for conditioning the polishing surface; and a relative movement mechanism for providing relative movement between the substrate held by the top ring and the polishing pad.  
      With this structure, by conditioning the polishing surface of the polishing pad with use of the conditioning member, polishing by-products and the like can be removed from the polishing pad, and hence a polishing property of the polishing pad can be maintained. After conditioning using water, e.g., normal dressing or atomizing, is performed, it is preferable that the polishing table with the polishing pad is rotated at a speed of 50 to 100 min −1  for several seconds so as to drain the polishing pad. This operation can prevent a change in concentration of the electrolytic solution.  
      In a preferred aspect of the present invention, the conditioning member comprises one of a dresser having diamond particles electrodeposited thereon and a brush.  
      In a preferred aspect of the present invention, the electrolytic polishing apparatus further comprises a counter electrode disposed so as to face the first electrode for conditioning the first electrode by applying a voltage such that the first electrode has polarity reversed from when electrolytic polishing is performed.  
      Conditioning of the first electrode can remove polishing by-products deposited on the first electrode due to electrolytic polishing. Therefore, electrode potential and electrode resistance, which affect a polishing property, can be prevented from changing.  
      In a preferred aspect of the present invention, the electrolytic polishing apparatus further comprises an atomizer for supplying pure water or a chemical liquid onto the polishing surface.  
      With this structure, the pure water or chemical liquid supplied from the atomizer onto the polishing pad can remove unwanted substances, e.g., polishing by-products adhering to the polishing surface, and the remaining electrolytic solution At the same time, unwanted substances, such as reaction by-products deposited on the surface of the first electrode exposed in openings formed in the polishing pad, can be removed.  
      In a preferred aspect of the present invention, the polishing pad has through-holes extending therethrough in a direction perpendicular to the polishing surface, or the polishing pad is made of material having liquid permeability.  
      With this structure, electricity can be supplied to the substrate contacting the polishing pad through the electrolytic solution in the through-holes, whereby electrolytic polishing can be performed. The polishing pad having the through-holes over the entire surface thereof may have grid-like or annular grooves on the surface thereof. If the polishing pad itself has liquid permeability, it is not necessary to provide the through-holes in the polishing pad.  
      In a preferred aspect of the present invention, the electrolytic polishing apparatus further comprises an electrode conditioner for conditioning the second electrode.  
      With this structure, the electrode conditioner can remove polishing by-products, oxide, and the like deposited on a surface of the second electrode during polishing.  
      In a preferred aspect of the present invention, the electrolytic polishing apparatus further comprises an electrode conditioner cleaning unit for cleaning the electrode conditioner.  
      In a preferred aspect of the present invention, the electrolytic polishing apparatus further comprises a counter electrode conditioner for conditioning the counter electrode.  
      According to another aspect of the present invention, there is provided an electrolytic polishing apparatus comprising: a first electrode connected to one of poles of a power source; a polishing table electrically connected to the first electrode; a polishing pad provided on an upper surface of the polishing table and having a polishing surface; a top ring operable to hold a substrate and press the substrate against the polishing surface; a second electrode connected to another of the poles of the power source for supplying electricity to the substrate; a liquid supply unit for supplying a liquid onto the polishing surface; a conditioning member for conditioning the polishing surface; and a relative movement mechanism for providing relative movement between the substrate held by the top ring and the polishing pad. The liquid supply unit is connected to a polishing liquid supply line and an electrolytic solution supply line.  
      With this structure, electrolytic polishing can be performed while supplying electrolytic solution onto the polishing pad through the electrolytic solution supply line, and CMP can be performed while supplying the polishing liquid onto the polishing pad through the polishing liquid supply line. Further, conditioning of the polishing pad can be performed between electrolytic polishing and CMP, thus preventing mixing of the electrolytic solution and the polishing liquid.  
      According to another aspect of the present invention, there is provided an electrolytic polishing apparatus comprising: a first electrode connected to one of poles of a power source; a polishing table electrically connected to the first electrode; a polishing pad provided on an upper surface of the polishing table and having a polishing surface; a top ring operable to hold a substrate and press the substrate against the polishing surface; a second electrode connected to another of the poles of the power source for supplying electricity to the substrate; a liquid supply unit for supplying a liquid onto the polishing surface; a conditioning member for conditioning the polishing surface; and a relative movement mechanism for providing relative movement between the substrate held by the top ring and the polishing pad. The liquid supply unit is connected to a polishing liquid supply line and a supporting electrolyte supply line.  
      In general, there are several types of electrolytic solutions and polishing liquids. For example, there is an electrolytic solution comprising a supporting electrolyte and a polishing liquid to be used in CMP. Further, in a case of polishing interconnect metal such as copper by electrolytic polishing and then performing CMP to remove copper remaining on a barrier film, a polishing liquid for use in CMP may comprise a base liquid of the electrolytic solution to be used in electrolytic polishing. According to this electrolytic polishing apparatus, in such cases, CMP can be performed while supplying the polishing liquid onto the polishing surface through the polishing liquid supply line, and electrolytic polishing can be performed while supplying the polishing liquid and the supporting electrolyte onto the polishing surface through the polishing liquid supply line and the supporting electrolyte supply line, respectively.  
      In a preferred aspect of the present invention, the liquid supply unit is further connected to an additive supply line.  
      With this structure, CMP can be performed using a polishing liquid containing an additive by supplying the polishing liquid onto the polishing surface through the polishing liquid supply line while supplying an additive such as an oxidizing agent onto the polishing surface through the additive supply line. In this case, the additive can be supplied to the polishing surface as needed.  
      In a preferred aspect of the present invention, the polishing liquid supply line and the supporting electrolyte supply line are connected to the liquid supply unit via a buffer for mixing liquids.  
      With this structure, the polishing liquid supplied through the polishing liquid supply line and the supporting electrolyte supplied through the supporting electrolyte supply line can be mixed with each other in the buffer to thereby produce in advance an electrolytic solution stored in the buffer, so that this electrolytic solution can be supplied from the buffer onto the polishing pad.  
      In a preferred aspect of the present invention, the liquid supply unit is further connected to a pure water supply line.  
      With this structure, after polishing, water polishing can be performed to clean the substrate while supplying pure water onto the polishing surface of the polishing pad.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIGS. 1A through 1C  show successive steps of a process of forming copper interconnects in a semiconductor device;  
       FIG. 2  is a plan view showing a layout of various components of a polishing apparatus according to an embodiment of the present invention;  
       FIG. 3  is a plan view showing an essential part of an electrolytic polishing apparatus according to an embodiment of the present invention, which is incorporated in the polishing apparatus shown in  FIG. 2 ;  
       FIG. 4  is a cross-sectional view showing an essential part of an electrolytic polishing apparatus according to the embodiment of the present invention, which is incorporated in the polishing apparatus shown in  FIG. 2 ;  
       FIG. 5  is a cross-sectional view showing an example of a cleaning unit incorporated in the polishing apparatus shown in  FIG. 2 ;  
       FIG. 6  is a graph showing a relationship between flow rate of an electrolytic solution, electrolytic current, and polishing torque during electrolytic polishing;  
       FIG. 7  shows another example of the cleaning unit;  
       FIG. 8  shows another example of the cleaning unit;  
       FIG. 9  is a plan view showing an essential part of an electrolytic polishing apparatus according to another embodiment of the present invention;  
       FIG. 10  is a cross-sectional view showing an essential part of an electrolytic polishing apparatus according to another embodiment of the present invention;  
       FIG. 11  is a plan view showing an essential part of an electrolytic polishing apparatus according to another embodiment of the present invention;  
       FIG. 12  is a cross-sectional view showing an essential part of an electrolytic polishing apparatus according to another embodiment of the present invention;  
       FIG. 13  is a plan view showing an essential part of an electrolytic polishing apparatus according to another embodiment of the present invention;  
       FIG. 14  is a plan view showing an essential part of an electrolytic polishing apparatus according to another embodiment of the present invention;  
       FIG. 15  is a plan view showing an essential part of an electrolytic polishing apparatus according to another embodiment of the present invention;  
       FIG. 16  is a plan view showing an essential part of an electrolytic polishing apparatus according to another embodiment of the present invention;  
       FIG. 17  is a plan view showing an essential part of an electrolytic polishing apparatus according to another embodiment of the present invention;  
       FIG. 18  is a plan view showing an essential part of an electrolytic polishing apparatus according to another embodiment of the present invention;  
       FIG. 19  is a plan view showing an essential part of an electrolytic polishing apparatus according to another embodiment of the present invention;  
       FIG. 20  is a cross-sectional view showing a slit nozzle shown in  FIG. 19 ; and  
       FIG. 21  is a perspective view showing the slit nozzle shown in  FIG. 19 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Embodiments of the present invention will be described below with reference to the drawings.  
       FIG. 2  is a plan view showing a layout of various components of a polishing apparatus according to an embodiment of the present invention. This polishing apparatus is used for forming interconnects. For example, as shown in  FIG. 1B , copper plating is performed on a substrate W so as to fill via hales  303  and trenches  304  with copper and to deposit a copper film  307  as an interconnect metal film onto an insulating film (dielectric)  302 . Then, the substrate W is introduced into the polishing apparatus, where polishing is performed on a surface of the substrate W to thereby remove the copper film  307 , a seed film  306 , and a barrier film  305  on the insulating film  302 . As a result, as shown in  FIG. 1C , interconnects  308  comprising the seed film  306  and the copper film  307  are formed in the insulating film  302 .  
      The polishing apparatus according to the present embodiment comprises four load-unload stages  2  on which substrate cassettes  1  are placed, respectively. Each of the substrate cassettes  1  stores plural substrates W each having the copper film  307  (see  FIG. 1B ) as an interconnect metal film. A transfer robot  4  having two hands is provided on a traveling mechanism  3  so that the hands of the transfer robot  4  can reach respective substrate cassettes  1  on respective load-unload stages  2 . The traveling mechanism  3  comprises a linear motor, and can thus allow the transfer robot  4  to quickly and stably transfer a substrate with increased weight due to a large diameter.  
      In this embodiment, SMIF (Standard Manufacturing Interface) pod or FOUP (Front Opening Unified Pod) is used as the load-unload stages  2  on which the substrate cassettes  1  are placed. The load-unload stages  2  are disposed outside a housing  46  of the polishing apparatus. SMIF and FOUP are hermetic vessels in which substrates are accommodated, and each of them comprises a partition wall allowing an inner space therein to maintain its conditions independently from an outer space. SMIF or FOUP, which is used as the load-unload stages  2 , has shutters operable to be opened together with shutters  52  of the housing  46 , so that the polishing apparatus and the substrate cassettes  1  are coupled integrally to each other. After polishing, the shutters of SMIF or FOUP are closed, so that the substrate cassettes  1  are separated from the polishing apparatus. The substrate cassettes  1  are automatically or manually transferred to other processes, and therefore, internal atmospheres of the substrate cassettes  1  are required to be kept clean.  
      For this reason, a chemical filter is provided so as to form downflow of a clean air at an upper region of an area A through which a substrate passes right before being returned to the substrate cassette  1 . In this embodiment, the linear motor is used to move the transfer robot  4 . Therefore, dust can be prevented from rising, and an atmosphere of area A can thus be kept clean.  
      In order to keep the substrates in the substrate cassettes  1  clean, each of the substrate cassettes  1  may comprise a hermetic vessel such as SMIF or FOUP in which a chemical filter and a fan are disposed so as to constitute a clean box which can keep itself clean  
      Two drying units  5  and  6  are disposed at an opposite side of the substrate cassettes  1  with respect to the traveling mechanism  3 . The drying units  5  and  6  are disposed at positions where the hands of the transfer robot  4  can reach the drying units  5  and  6 . A substrate station  50  having four substrate supports  7 ,  8 ,  9  and  10  is provided between the two drying units  5  and  6  at a position where the hands of the transfer robot  4  can reach the substrate station  50 .  
      The drying units  5  and  6  and the substrate supports  7 ,  8 ,  9  and  10  are disposed in an area B, and the substrate cassettes  1  and the transfer robot  4  are disposed in area A Area A and area B are partitioned by a partition wall  14  so that cleanliness in area A and area B can be separated. The partition wall  14  has an opening for allowing the substrates to be transferred between area A and area B, and a shutter  11  is provided so as to close the opening. A transfer robot  20  is disposed at a position where hands thereof can reach the drying unit  5  and the three substrate supports  7 ,  9  and  10 , and a transfer robot  21  is disposed at a position where hands thereof can reach the drying unit  6  and the three substrate supports  8 ,  9  and  10 .  
      A cleaning unit  22  is disposed adjacent to the drying unit  5  at a position where the hands of the transfer robot  20  can reach the cleaning unit  22 . A cleaning unit  23  is disposed adjacent to the drying unit  6  at a position where the hands of the transfer robot  21  can reach the cleaning unit  23 .  
      The drying units S and  6 , the cleaning units  22  and  23 , the substrate supports  7 ,  8 ,  9  and  10  of the substrate station  50 , and the transfer robots  20  and  21  are all disposed in area B. Pressure in area B is adjusted so as to be lower than pressure in area A.  
      The polishing apparatus comprises the housing  46  surrounding these respective units, and an internal space of the housing  46  is divided by partition walls  24 A and  24 B into plural chambers including area A and area B.  
      Two areas C and D are defined by the partition walls  24 A and  24 B, respectively, and two polishing chambers, which are separated from area B, are formed in areas C and D, respectively. A first polishing unit comprising an electrolytic polishing apparatus  54  is disposed in area C, and a second polishing unit comprising a CMP apparatus  56  is disposed in area D.  
      Specifically, the electrolytic polishing apparatus (i.e., the first polishing unit)  54  in area C comprises polishing tables  34  and  36 , a top ring  32 , an electrolytic solution supply nozzle  40  serving as a liquid supply unit for supplying an electrolytic solution (polishing liquid) onto the polishing table  34 , a dresser  38  for dressing the polishing table  34 , and a dresser  48  for dressing the polishing table  36 . The CMP apparatus (i.e., the second polishing unit)  56  in area D comprises polishing tables  35  and  37 , a top ring  33 , a polishing liquid supply nozzle  41  for supplying a polishing liquid onto the polishing table  35 , a dresser  39  for dressing the polishing table  35 , and a dresser  49  for dressing the polishing table  37 .  
      Although each of the electrolytic polishing apparatus  54  and the CMP apparatus  56  of this embodiment has two polishing tables so as to enable the apparatuses  54  and  56  to perform multistage polishing, the polishing tables  36  and  37  may be omitted.  
      The electrolytic polishing apparatus (i.e., the first polishing unit)  54  further comprises, in addition to mechanical dresser  38 , an atomizer  44  as a dresser utilizing fluid pressure. The CMP apparatus (i.e., the second polishing unit)  56  also comprises, in addition to mechanical dresser  39 , an atomizer  45  as a dresser utilizing fluid pressure. Generally, an atomizer atomizes a mixed fluid of a liquid (e.g., pure water) and a gas (e.g., nitrogen) and ejects this atomized fluid to a polishing surface through a plurality of nozzles. A main object of the atomizer is to wash away polishing dregs and abrasive particles which are firmly deposited on the polishing surface. Cleaning (i.e., atomizing) of the polishing surface by the atomizers  44  and  45  utilizing fluid pressure, and conditioning (dressing) of the polishing surface by the dressers  38  and  39  utilizing mechanical contact, can achieve desirable conditioning, i.e., regeneration of a polishing surface.  
       FIGS. 3 and 4  show an essential part of the electrolytic polishing apparatus (first polishing unit)  54 . The CMP apparatus (second polishing unit)  56  has substantially the same structures as the electrolytic polishing apparatus  54  except no electrodes (i.e., the CMP apparatus  56  does not have a first electrode  62  and a second electrode  64  shown in  FIGS. 3 and 4 ), and will not be described in detail.  
      The polishing table  34  of the electrolytic polishing apparatus  54  is made of material having low ionization tendency such as platinum, especially material having ionization tendency lower than that of an object to be polished, e.g., copper. A disk-shaped first electrode (cathode)  62  and a rod-like second electrode (anode)  64  are disposed on an upper surface of the polishing table  34 . The first electrode  62  is connected to one of poles of a power source  60 , and the second electrode  64  is connected to another of the poles of the power source  60 . The first electrode  62  and the second electrode  64  are electrically insulated from each other. An entire upper surface of the first electrode  62  is covered with a polishing pad  66  having an upper surface that serves as a polishing surface  66   a.    
      The top ring  32  is operable to hold the substrate W and lower it to bring a surface (lower surface) of the substrate W into contact with the polishing surface  66   a  of the polishing pad  66 . When the substrate W is brought into contact with the polishing surface  66   a , an upper end surface of the second electrode  64  comes into contact with the surface of the substrate W to thereby supply electricity to a conductive material such as a copper film  307  (see  FIG. 1B ) formed on the surface of the substrate W. The top ring  32  is further operable to press the substrate W against the polishing surface  66   a  at, for example, not more than 7 kPa which is much lower than pressure (e.g., between 20 and 50 kPa) applied in a CD process.  
      In order to enhance performance of a device (i.e., to improve RC delay), Low-k material is increasingly used as an insulating film (dielectric). However, use of the Low-k material results in a decrease in mechanical strength of the insulating film itself Consequently, in a polishing process, for example, the Low-k material may be peeled off. Thus, in order to solve such a problem, polishing pressure is required to be lowered. At present, polishing pressure in CMP is about 2 psi (about 14 kPa). In order to cope with a downward trend of mechanical strength of the Low-k material, polishing pressure in a polishing process is required to be further lowered to, for example, at most 1 psi (about 7 kPa).  
      The polishing pad  66  is formed from IC1000 manufactured by Rodel Nitta Company. IC1000 is a material having a number of through-holes over its entire surface. With this structure, electrical communication is established between the surface of the substrate W contacting the second electrode  64  and the first electrode  62  through an electrolytic solution in the through-holes, whereby electrolytic polishing can be performed. The polishing pad  66  having the through-holes over the entire surface thereof may have grid-like or annular grooves on the surface thereof If the polishing pad  66  itself has liquid permeability, it is not necessary to provide the through-holes in the polishing pad  66 .  
      The top ring  32  is coupled to a lower end of a top ring drive shaft  68 , which is rotatable and movable between a predetermined polishing position above the polishing table  34  and a position above a pusher  30  (see  FIG. 2 ). The dresser  38  serves as a conditioning member, and comprises a plurality of ring-shaped brushes  38   a  attached to a peripheral lower surface thereof The dresser  38  is coupled to a lower end of a dresser drive shaft  70 , which is rotatable and movable between a predetermined dressing position above the polishing table  34  and a waiting position located laterally of the dressing position.  
      The electrolytic solution supply nozzle  40 , serving as a liquid supply unit, has plural electrolytic solution supply mouths  40   a  arranged along a longitudinal direction thereof The electrolytic solution supply nozzle  40  is disposed above the polishing table  34  so as to extend in a radial direction of the polishing table  34 . Similarly, the atomizer  44  has plural supply mouths arranged along a longitudinal direction thereof, and is disposed above the polishing table  34  so as to extend in the radial direction of the polishing table  34 .  
      Although not shown in the drawings, a pure water supply nozzle for supplying pure water onto the polishing pad  66 , and a dressing liquid supply nozzle for supplying a dressing liquid onto the polishing pad  66 , may be provided above the polishing table  34  as needed.  
      The electrolytic polishing apparatus  54  operates as follows. The top ring  32  holds the substrate W and is moved to the predetermined polishing position above the polishing table  34 . Thereafter, the top ring  32  is rotated and lowered to press the surface (lower surface) of the substrate W against the polishing surface  66   a  of the rotating polishing pad  66  at predetermined pressure. This pressure is, for example, at most 7 kPa, which is much lower than pressure (e.g., between 20 and 50 kPa) in the CMP process. During pressing, the electrolytic solution is supplied onto the polishing surface  66   a  through the electrolytic solution supply nozzle  40 , and a predetermined voltage is applied between the first electrode  62  and the second electrode  64  by the power source  60 , whereby a conductive film such as a copper film  307  (see  FIG. 1B ) on the surface of the substrate W is polished.  
      When high current flows through the electrolytic solution, electrolytic polishing can be efficiently performed, even if the voltage is low. Accordingly, electrolytic polishing is preferably performed using an electrolytic solution having an electrical conductivity of at least 50 mS/cm. Use of high voltage results not only in high electric power expense, but also in high production cost of the apparatus because of the need for a high-capacity rectifier. Use of the electrolytic solution (polishing liquid) having an electrical conductivity of not less than 50 mS/cm can lower the voltage required for electrolytic polishing to less than 10 V Therefore, electrolytic polishing can be efficiently performed.  
      As shown in  FIG. 6 , during electrolytic polishing, supply of the electrolytic solution onto the polishing pad  66  at high flow rate may cause lowered and non-uniform surface pressure due to hydroplaning. On the other hand, supply of the electrolytic solution at low flow rate may cause lowered electrolyic current because of insufficient supply of the electrolytic solution. Thus, during electrolytic polishing, it is preferable to monitor current value and polishing torque (e.g., top ring torque current value) for determining a suitable flow rate of the electrolytic solution so that a maximal current value can be obtained while maintaining the polishing torque  
      After electrolytic polishing, supply of the electrolytic solution onto the polishing pad  66  is stopped, and the first electrode  62  and the second electrode  64  are disconnected from the power source  60 . Then, the substrate W is pressed against the polishing surface  66   a  at low pressure while the substrate W is rotated, and simultaneously pure water is supplied onto the polishing pad  66  to thereby clean the surface of the substrate W, i.e., perform so-called water polishing. Then, the top ring  32  is elevated, and the cleaned substrate W is transferred to a subsequent process.  
      After cleaning the substrate W. the dresser  38  and the atomizer  44  perform conditioning (dressing) on the polishing surface  66   a  of the polishing pad  66 . Specifically, while a lower surface (dressing surface) of the dresser  38  presses the polishing pad  66  at certain pressure, the dresser  38  and the polishing pad  66  are moved relative to each other, and simultaneously a dressing liquid is supplied onto the polishing surface  66   a  of the polishing pad  66 . Additionally, pressurized pure water or a chemical liquid, which accelerates removal of the electrolytic solution, is supplied onto the polishing surface  66   a  through the atomizer  44  so as to remove (atomize) unwanted substances, e.g., polishing by-products adhering to the polishing surface  66   a , and remaining electrolytic solution. At the same time, unwanted substances, such as reaction by-products deposited on a surface of the first electrode  62  exposed in openings or grooves formed in the polishing pad  66 , are removed. Atomizing by the atomizer  44  is preferably performed simultaneously with or shortly after dressing by the dresser  38 . It is preferable to rotate the polishing table  34  after conditioning for several seconds at a speed in the range of 50 to 100 min −1 , which is higher than during conditioning, so as to drain the substrate W. By draining the substrate W after conditioning, a change in concentration of the electrolytic solution can be suppressed.  
      Conditioning of the polishing surface  66   a  of the polishing pad  66  by the dresser  38  and the atomizer  44  may be performed before polishing of the substrate in a state such that the top ring  32  is elevated.  
      As shown in  FIG. 2 , a reversing device  28  for reversing a substrate is provided in area C separated from area B by the partition wall  24 A. The reversing device  28  is disposed at a position where the hands of the transfer robot  20  can reach the reversing device  28 . A reversing device  28 ′ for reversing a substrate is provided in area D separated from area B by the partition wall  24 B. The reversing device  28 ′ is disposed at a position where the hands of the transfer robot  21  can reach the reversing device  28 ′. The partition walls  24 A and  24 B, which separate areas C and D from area B, have openings, respectively, for allowing the reversing devices  28  and  28 ′ to transfer the substrate therethrough. Shutters  25  and  26  are provided at the openings of the partition walls  24 A and  24 B, respectively.  
      Each of the reversing devices  28  and  28 ′ has a chuck mechanism for chucking the substrate, a reversing mechanism for reversing the substrate upside down, and a substrate detecting sensor for detecting whether the chuck mechanism chucks the substrate. The transfer robot  20  transfers the substrate to the reversing device  28 , and the transfer robot  21  transfers the substrate to the reversing device  28 ′.  
      In area C serving as one of the polishing chambers, a linear transporter (transfer mechanism)  27 A is provided for transferring the substrate between the reversing device  28  and the top ring  32  of the electrolytic polishing apparatus  54 . In area D serving as the other of the polishing chambers, a linear transporter (transfer mechanism)  27 B is provided for transferring the substrate between the reversing device  28 ′ and the top ring  33  of the CMP apparatus  56 . The linear transporter  27 A has two stages which linearly reciprocate between a lifter  29  and the pusher  30 . The linear transporter  27 B also has two stages which linearly reciprocate between a lifter  29 ′ and a pusher  30 ′.  
       FIG. 5  shows an example of the cleaning units  22  and  23 . Each of the cleaning units  22  and  23  comprises a substrate holder  110  for detachably holding the substrate W, to be cleaned, with a chuck mechanism  113  which holds a peripheral portion of the substrate W, a cleaning cup  120  surrounding the substrate holder  110  so as to prevent scattering of liquids such as a rinsing liquid, and a cleaning vessel  130  enclosing the cleaning cup  120 . Further, each of the cleaning units  22  and  23  comprises a chemical liquid supply nozzle  140  disposed inside the cleaning vessel  130  at a predetermined position for supplying a chemical liquid onto a surface of the substrate W, a rinsing liquid supply nozzle  150  disposed inside the cleaning vessel  130  at a predetermined position for supplying a rinsing liquid such as pure water onto the surface of the substrate W, and cleaning liquid supply nozzles  190  for supplying a cleaning liquid to the substrate holder  110 .  
      The substrate holder  110  is coupled to a drive unit  115 , and is rotated by the drive unit  115 . A bottom portion of the cleaning cup  120  is connected to a drain passage  122 , and an electrical conductivity meter  124  is provided in the drain passage  122  for measuring electrical conductivity of a waste rinsing liquid flowing through the drain passage  122 .  
      Operation of the cleaning units  22  and  23  is performed as follows. The chuck mechanism  113  holds the substrate W, and the drive unit  115  rotates the substrate holder  110 . In this state, the chemical liquid (DHF solution) is ejected toward the substrate W through the chemical liquid supply nozzle  140  to thereby clean the surface of the substrate W. Subsequently, supply of the chemical liquid through the chemical liquid supply nozzle  140  is stopped, and then the rinsing liquid (pure water) is ejected toward the substrate W through the rinsing liquid supply nozzle  150  to thereby rinse the surface of the substrate W. At this time, the electrical conductivity meter  124  measures the electrical conductivity of the waste rinsing liquid flowing through the drain passage  122 .  
      When the electrical conductivity of the waste rinsing liquid reaches a predetermined value, or a predetermined period of time has elapsed, supply of the rinsing liquid to the substrate W is stopped, and then the substrate W is rotated by the drive unit  115  at a high speed, so that the substrate W is spin-dried. In this manner, cleaning and rinsing are performed.  
      This cleaning and rinsing process is preferably performed until the electrical conductivity of the waste rinsing liquid discharged from the cleaning and rinsing process is reduced to at most one-third of, preferably one-tenth of, an electrical conductivity of a polishing liquid used in the CMP process.  
      In electrolytic polishing, a thick electrolytic solution (polishing liquid) having a high electrical conductivity is preferably used. On the other hand, in CMP, a polishing liquid having a high electrical conductivity may cause aggregation of polishing particles, thus deteriorating a polishing property. Accordingly, a thin polishing liquid having a low electrical conductivity in the range of, for example, 1 to 10 mS/cm is generally used in CMP. However, if electrolytic polishing is performed using a thick electrolytic solution (polishing liquid) having a high electrical conductivity and CMP is subsequently performed without cleaning and rinsing the substrate to which the electrolytic solution adheres, then the polishing liquid used in CMP becomes thick, and hence, a polishing property is deteriorated. In view of such a drawback, cleaning and rinsing are performed until the electrical conductivity of the waste rinsing liquid is reduced to at most one-third of; preferably one-tenth of, more preferably a level substantially equal to the electrical conductivity of the polishing liquid used in CMP, whereby CMP (second polishing) can be performed without deteriorating the polishing property.  
      Next, operation of the polishing apparatus will be described.  
      Firstly, the substrate cassette(s)  1 , which accommodates plural substrates each having the copper film  307  on the surface thereof (see  FIG. 1B ), is set on the load-unload stage(s)  2 . Then, one of the substrates is removed from the substrate cassette  1  by the transfer robot  4 , and is placed onto the substrate station  50 . The transfer robot  20  receives the substrate from the substrate station  50 , and transfers the substrate to the reversing device  28  in area C, where the substrate is reversed. The lifter  29  receives this reversed substrate from the reversing device  28 , and transfers it to the linear transporter  27 A. The linear transporter  27 A is horizontally moved to place the substrate onto the pusher  30 . In this state, the top ring  32  of the electrolytic polishing apparatus (first polishing unit)  54  is moved to a position above the pusher  30 .  
      The top ring  32  receives the substrate from the pusher  30 , and holds the substrate inside a guide ring (not shown) by vacuum attraction. While holding the substrate, the top ring  32  is moved from the position above the pusher  30  to the polishing position above the polishing table  34 . Then, the top ring  32  is lowered to press the substrate against the polishing surface  66   a  of the polishing pad  66  at a predetermined pressure of not more than 7 kPa. At the same time, an electrolytic solution, which has an electrical conductivity of not less than 50 mS/cm, is supplied onto the polishing pad  66 . As described above, current value and polishing torque (e.g., top ring torque current value) are monitored during electrolytic polishing so that a flow rate of the electrolytic solution suitable for obtaining a maximal current while maintaining the polishing torque is determined. In this manner, the conductive film such as the copper film  307  (see  FIG. 1B ) on the surface of the substrate W is polished. During polishing of the substrate on the polishing pad  66 , the top ring  32  may release the vacuum attraction.  
      The electrolytic polishing apparatus  54  polishes the copper film  307  (and the seed film  306 ) until the barrier film  305  is exposed on the surface of the substrate, as indicated by line A-A in  FIG. 1B . In this manner, electrolytic polishing, which generally has a little effect on device elements such as interconnects, is employed as at least part of the polishing process. For example, electrolytic polishing may constitute most part of polishing, i.e., removing most part of an interconnect metal film formed on portions other than interconnect recesses. Use of electrolytic polishing in this manner can greatly reduce damage to an interconnect structure.  
      After the electrolytic polishing apparatus  54  finishes electrolytic polishing, the dresser  38  and the atomizer  44  perform conditioning of the polishing surface  66   a  of the polishing pad  66 , so that the polishing surface  66   a  can be ready for subsequent polishing.  
      The substrate, which has been polished by the electrolytic polishing apparatus  54 , is transferred again to the position above the pusher  30 . The top ring  32  releases the substrate onto the pusher  30 , and a cleaning nozzle provided on the pusher  30  cleans a polished surface and a rear surface of the substrate. Then, the linear transporter  27 A and the lifter  29  transfer the substrate to the reversing device  28 , where the substrate is reversed. The transfer robot  20  transfers this reversed substrate to the cleaning unit  22 . In this cleaning unit  22 , as described above, the electrical conductivity of the waste rinsing liquid discharged from the cleaning and rinsing process is measured by the electrical conductivity meter  124  so that cleaning and rinsing of the surface of the substrate is performed until the electrical conductivity of the waste rinsing liquid is reduced to at most one-third of, preferably one-tenth of more preferably a level substantially equal to the electrical conductivity of the polishing liquid used in the CMP process. After cleaning and rinsing, the transfer robot  20  transfers the substrate to the substrate station  50 , and places it onto the substrate station  50 .  
      Because the substrate is cleaned and rinsed between the first polishing process and the second polishing process, polishing liquids having greatly different compositions can be used in these respective polishing processes. Accordingly, a manner of polishing can be diversified and, as a result, formation of interconnects with less damage can be achieved.  
      The transfer robot  21  holds the substrate on the substrate station  50 , and transfers it to the reversing device  28 ′ in area D, where the substrate is reversed. The lifter  29 ′ receives this reversed substrate from the reversing device  28 ′, and transfers it to the linear transporter  27 B. The linear transporter  27 B is horizontally moved to place the substrate onto the pusher  30 ′. In this state, the top ring  33  of the CMP apparatus (second polishing unit)  56  is moved to a position above the pusher  30 ′.  
      The top ring  33  receives the substrate from the pusher  30 ′, and holds the substrate inside a guide ring (not shown) by vacuum attraction. While holding the substrate, the top ring  33  is moved from the position above the pusher  30 ′ to the polishing position above the polishing table  35 . Then, the top ring  33  is lowered to press the substrate against a polishing surface of a polishing pad attached to an upper surface of the polishing table  35  at predetermined pressure. At the same time, a polishing liquid is supplied onto the polishing pad through the polishing liquid supply nozzle  41 . In this state, the top ring  33  and the polishing table  35  are rotated to thereby further polish the surface of the substrate. During polishing of the substrate on the polishing pad, the top ring  33  may release vacuum attraction. The polishing pad is formed from, for example, IC1000 manufactured by Rodel Nitta Company, as with the electrolytic polishing apparatus  54 .  
      The CMP apparatus  56  polishes an exposed surface of the barrier film  305  and the copper film  307  (and the seed film  306 ) remaining on the surface of the substrate, which has been polished by the electrolytic polishing apparatus  54 , so as to completely remove unwanted copper film  307  (and the seed film  306 ), and to remove the barrier film  305 . As a result, as shown in  FIG. 1C , the interconnect  308  is formed in the insulating film  302 .  
      In this manner, most of the interconnect metal film is removed by the electrolytic polishing process (composite electrolytic polishing process), and subsequently, a remaining interconnect metal film is removed by the conventional CMP process, which can sufficiently eliminate level differences on the surface of the substrate. According to the polishing method of this embodiment, a highly flat surface of the substrate can be obtained. Further, by switching from electrolytic polishing to CMP at a time the barrier film is exposed, the remaining interconnect metal film and the barrier film underneath the interconnect metal film can be sufficiently removed.  
      After the CMP apparatus  56  finishes polishing, the dresser  39  and the atomizer  45  perform conditioning of the polishing surface of the polishing pad, so that the polishing surface can be ready for subsequent polishing, as with the electrolytic polishing apparatus  54 .  
      According to  FIGS. 1A through 1C , the polishing process of the substrate can be divided into several polishing steps: a step of polishing the copper film  307  (Bulk Cu); a step of polishing the copper film  307  and the seed film  306  until the barrier film  305  is exposed (Cu Clear); a step of polishing the barrier film  305  or a hard mask, i.e., a layer formed between the barrier film and the Low-k material (BM/HM Clear); and a step of finish-polishing the insulating film  302 , i.e., the Low-k material (Low-k T.U.). Accordingly, several combinations of the polishing processes are available as shown in table below.  
                                                           TABLE 1                                   1platen   2platen(1)   2platen(2)   2platen(3)   3platen(1)   3platen(2)   3platen(3)   4platen(1)   4platen(2)   4platen(3)                                                                                Bulk Cu   ECP-C   ECP-C   ECP-C   ECP-C   ECP-C   ECP-C   ECP-C   ECP-C   ECP-C 1   ECP-C 1       Cu Clear       CMP           CMP 1   CMP 1       CMP 1   ECP-C 2   ECP-C 2       BM/HM Clear           CMP       CMP 2       CMP 1   CMP 2   CMP 1   ECP-C 3       Low-k T.U.               CMP       CMP 2   CMP 2   CMP 3   CMP 2   CMP                       (HM Included)                       (HM Included)                  
 
      In Table 1, “1 platen” means that all steps are performed using one polishing table, and “2 platen”, “3 platen”, and “4 platen” mean that polishing is performed using two, three, and four polishing tables, respectively. ECP-C means an electrolytic polishing process in which the substrate contacts the polishing surface during process. In ECP-C, a liquid such as electrolyte or pure water is supplied, and slurry containing abrasive particles can be used, as needed. Although the polishing process is changed from step to step, only processing conditions may be changed while using the same polishing table.  
      The substrate, which has been polished by the CMP apparatus  56 , is transferred again to the position above the pusher  30 ′, and is placed onto the pusher  30 ′. Then, the linear transporter  27 B and the lifter  29 ′ transfer the substrate to the reversing device  28 ′, where the substrate is reversed. The transfer robot  21  transfers this reversed substrate to the cleaning unit  23 . In this cleaning unit  23 , as described above, the surface of the substrate is cleaned and rinsed. After cleaning and rinsing, the transfer robot  21  transfers the substrate to the substrate station  50 , and places it onto the substrate station  50 .  
      The transfer robot  20  (or  21 ) removes this cleaned substrate from the substrate station  50 , and transfers it to the drying unit  5  (or  6 ) which may comprise a pen sponge for cleaning the upper surface of the substrate, and may have a spin dry function. The substrate is cleaned and dried by the drying unit  5  (or  6 ). Then, the transfer robot  4  returns this cleaned and dried substrate to the substrate cassette  1 .  
      Although a semiconductor wafer is used as a substrate to be polished in this embodiment, it should be understood that a substrate to be polished is not limited to a semiconductor wafer. Instead of a polishing cloth, a fixed abrasive pad impregnated with abrasive particles or a polishing pad containing no abrasive particles may be used as the polishing pad  66  of the electrolytic polishing apparatus  54  and/or the polishing pad of the CMP apparatus  56 . The fixed abrasive pad has a relatively hard polishing surface which can be self-regenerated after the polishing surface is destroyed.  
      As shown in  FIG. 7 , each of the cleaning units  22  and  23  may comprise a plurality of (six in  FIG. 7 ) spindles  211  for holding a peripheral portion of the substrate W, two roll-type cleaning members  213  and  215  disposed above and below the substrate W, respectively, drive mechanisms  217  and  218  for moving rotation shafts  213   b  and  215   b , which are disposed in parallel with the surface of the substrate W, toward and away from the substrate W and for rotating the rotation shafts  213   b  and  215   b  in directions indicated by arrows F 1  and F 2 , respectively, and a rinsing liquid supply nozzle  219  for supplying a rinsing liquid such as pure water onto a surface of the substrate W.  
      The rinsing liquid supply nozzle  219  may comprise an ultrasonic nozzle which applies an ultrasonic energy to a rinsing liquid to be ejected, a cavitation nozzle which generates cavitations in a rinsing liquid to be ejected, or an ultrasonic cavitation nozzle which applies an ultrasonic energy to and generates cavitations in a rinsing liquid to be ejected. The rinsing liquid supply nozzle  219  is provided on a swing arm  220 , and is swung by a swing shaft  221  in a direction indicated by arrow A while supplying the rinsing liquid onto the surface of the substrate W. The rinsing liquid supply nozzle  219  is operable to stop its movement at a desired position above the substrate W and at a given waiting position. Although not shown in the drawings, a nozzle for supplying a rinsing liquid onto a lower surface (a rear surface) of the substrate W is also provided.  
      The roll-type cleaning members  213  and  215  comprise cylindrical members  213   a  and  215   a  formed from a porous PVF sponge, and the rotation shafts  213   b  and  215   b  extending through the cylindrical members  213   a  and  215   a , respectively. Test results show that a smaller average diameter of holes of a sponge forming the cylindrical members  213   a  and  215   a  results in a better capability of removing dusts (particles). A preferable average diameter of the holes of the sponge is not more than 110 μm. The cylindrical members  213   a  and  215   a  may be made of urethane foam. The drive mechanisms  217  and  218  are moved respectively by non-illustrated moving mechanisms so as to vertically move away from the substrate W as indicated by arrow B, and to move to waiting positions as indicated by arrow C.  
      Cleaning and rinsing of the substrate W are performed as follows. With the surface, to be cleaned, facing upwardly, a peripheral portion of the substrate W is held and pressed by circumferential grooves formed on tops  212  on upper portions of the spindles  211 . The tops  212  are rotated at an equal high speed to thereby rotate the substrate W at a substantially constant speed in a direction indicated by arrow E. Subsequently, the roll-type cleaning members  213  and  215  are brought into contact with the upper and lower surfaces of the substrate W. respectively, and at the same time, a rinsing liquid with an ultrasonic energy applied thereto is ejected, or a rinsing liquid with cavitations generated therein is ejected, or a rinsing liquid with an ultrasonic energy and cavitations is ejected through the rinsing liquid supply nozzle  219 . At this time, a rinsing liquid is supplied onto the lower surface of the substrate W through the non-illustrated rinsing liquid supply nozzle. In this manner, particles adhering to the upper and lower surfaces of the substrate W are removed and washed away by the rinsing liquid.  
      As shown in  FIG. 8 , each of the cleaning units  22  and  23  may be a pencil-type cleaning unit comprising a rotating chuck mechanism  231  and a pencil-type brush cleaning mechanism  241 . The rotating chuck mechanism  231  has chuck claws  233  at an upper portion thereof for holding a peripheral portion of the disk-shaped substrate W, and is rotated by a rotating drive shaft  235  in a direction indicated by arrow G. The chuck claws  233  of the rotating chuck mechanism  231  have a non-illustrated opening mechanism for allowing the substrate W to be transferred to and removed from the rotating chuck mechanism  231  by a hand of a transfer robot.  
      Each of the cleaning units  22  and  23  has a swing arm  245  having one end fixed to a shaft  243 . A rotating drive shaft  249  extends downwardly from another end of the swing arm  245  toward the surface (to be cleaned) of the substrate W. A pencil-type cleaning member  251  formed from a porous PVF sponge is attached to a lower end of the rotating drive shaft  249 . The pencil-type cleaning member  251  may be made of foamed polyethylene. The pencil-type cleaning member  251  has a substantially column shape having a horizontal bottom surface to be brought into contact with the substrate W. The pencil-type cleaning member  251  has a height of about 5 mm, and a diameter of about 20 mm. An average diameter of fine holes formed in the sponge is about 110 μm. Generally, the smaller the average diameter of the fine holes, the greater cleaning effect of the sponge. Therefore, a preferable diameter of the fine holes is less than 80 μm.  
      The shaft  243  is vertically movable as indicated by arrow H, and the swing arm  245  is swung by the shaft  243  in a direction indicated by arrow I. The pencil-type cleaning member  251  is rotated by the rotating drive shaft  249  in a direction indicated by arrow J. Each of the cleaning units  22  and  23  comprises a rinsing liquid supply nozzle  255  for supplying a rinsing liquid to the substrate W, and a cup-shaped brush storage  253  for storing and cleaning the pencil-type cleaning member  251  while the pencil-type brush cleaning mechanism  241  is not in operation.  
      The cleaning units  22  and  23  operate as follows. The chuck claws  233  hold a peripheral portion of the substrate W, and in this state, the rotating chuck mechanism  231  in its entirety is rotated by the rotating drive shaft  235  at a high speed to thereby rotate the substrate W at a predetermined speed in the range of 500 to 1500 min −1 . A rotational speed of the substrate W rotated by the rotating chuck mechanism  231  during cleaning is controlled by a non-illustrated rotation control unit coupled to the rotating drive shaft  235 , and can be selected within a range of permissible rotational speed, e.g., several thousands min −1 . The bottom surface of the rotating pencil-type cleaning member  251  is brought into contact with the surface (upper surface) of the substrate W. In this state, the rinsing liquid is supplied onto the upper surface of the substrate W through the rinsing liquid supply nozzle  255 , and simultaneously the swing arm  245  is swung to thereby clean and rinse the substrate W.  
       FIGS. 9 and 10  show an essential part of an electrolytic polishing apparatus according to another embodiment of the present invention. The embodiment shown in  FIGS. 9 and 10  is different from the embodiment shown in  FIGS. 3 and 4  in that, instead of the rod-like second electrode (anode)  64 , a ring-shaped second electrode  64   a  is provide around first electrode  62  such that these electrodes  62  and  64   a  are electrically insulated from each other. According to this embodiment, while top ring  32  and polishing table  34  are rotated to polish a substrate held by the top ring  32 , the second electrode  64   a  and a conductive film such as a copper film  307  (see  FIG. 1B ) can be held in contact with each other at all times.  
       FIGS. 11 and 12  show an essential part of an electrolytic polishing apparatus according to another embodiment of the present invention. The embodiment shown in  FIGS. 11 and 12  is different from the embodiment shown in  FIGS. 3 and 4  in that, instead of the rod-like second electrode (anode)  64 , a small disk-shaped second electrode  64   b  is provide at a central portion of first electrode  62  such that these electrodes  62  and  64   b  are electrically insulated from each other. In addition, a central hole is formed in polishing pad  66  at a position corresponding to the second electrode  64   a  so that a surface of the second electrode  64   b  is exposed. In this embodiment also, while top ring  32  and polishing table  34  are rotated to polish a substrate held by the top ring  32 , the second electrode  64   b  and a conductive film such as a copper film  307  (see  FIG. 1B ) can be held in contact with each other at all times.  
       FIG. 13  shows an essential part of an electrolytic polishing apparatus according to another embodiment of the present invention. The embodiment shown in  FIG. 13  is different from the embodiment shown in  FIGS. 3 and 4  in that, instead of the dresser  38 , a small-diameter scan dresser  72  having diamond particles electrodeposited on an entire small-circular lower surface thereof is used as a conditioning member. This small-diameter scan dresser  72  is operable to dress (condition) polishing surface  66   a  of polishing pad  66  in its original place, i.e., in a so-called in-situ manner.  
       FIG. 14  shows an essential part of an electrolytic polishing apparatus according to another embodiment of the present invention. The electrolytic polishing apparatus of this embodiment comprises, in addition to the embodiment shown in  FIGS. 3 and 4 , a counter electrode  74  vertically movable between a predetermined position above polishing pad  66  and a waiting position. The counter electrode  74  is made of, for example, bulk Pt.  
      During electrolytic polishing, polishing by-products may be deposited on a surface of first electrode (cathode)  62  (see  FIG. 4 ). If such polishing by-products remain as they are, electrode potential and electrode resistance would be changed, and a polishing property would be adversely affected. According to this embodiment, at a time of interval, e.g., a time of replacement of a substrate, the counter electrode  74  is periodically brought into contact with the polishing surface  66   a  of the polishing table  66  while the counter electrode  74  and the polishing surface  66   a  are moved relative to each other. In this state, an electrolytic solution is supplied onto the polishing surface  66   a , and simultaneously a voltage is applied between the first electrode  62  and the counter electrode  74  such that the first electrode  62  has polarity reversed from when electrolytic polishing is performed, thereby conditioning the first electrode  62 .  
      In this manner, the polishing by-products deposited on the first electrode  62  due to electrolytic polishing are transferred to the counter electrode  74  and removed from the first electrode  62 . Therefore, electrode potential and electrode resistance, which affect a polishing property, can be prevented from changing.  
      Dresser  38  may be made of material having good conductivity so that the dresser  38  can serve as the counter electrode.  
       FIG. 15  shows an electrolytic polishing apparatus according to another embodiment of the present invention. The electrolytic polishing apparatus of this embodiment comprises, in addition to the embodiment shown in  FIGS. 3 and 4 , an electrode conditioner  76  which is vertically movable and rotatable. The electrode conditioner  76  is movable between a predetermined position above second electrode  64  and a waiting position, and is reciprocated in a horizontal direction. The electrode conditioner  76  is made of PVA sponge, polyester nonwoven fabric, or the like. The electrode conditioner  76  is operable to scrub and clean a surface (upper surface) of the second electrode  64  while a cleaning liquid, water, or dilute acid (e.g, at most 1 wt % of sulfuric acid, hydrochloric acid, nitric acid, or citric acid) is supplied onto the surface of the second electrode  64 . For example, the electrode conditioner  76  is rotated and reciprocated while pressing the surface (upper surface) of the second electrode  64 , and simultaneously water is supplied onto the surface of the second electrode  64 , whereby substances adhering to the second electrode  64  are removed. When an oxide adheres to the second electrode  64 , the dilute acid is supplied instead of water so as to remove the oxide.  
      The second electrode  64  has an exposed surface, and hence, during electrolytic polishing, polishing by-products, the oxide, and the like adhere to the exposed surface. According to this embodiment, at a time of interval, e.g., a time of replacement of a substrate, the electrode conditioner  76  can periodically condition the second electrode  64  to remove such polishing by-products, the oxide, and the like deposited on the surface of the second electrode  64  during polishing.  
      Although not shown in the drawings, it is preferable that the electrolytic polishing apparatus comprises an electrode conditioner cleaning unit for cleaning the electrode conditioner  76  so that the electrode conditioner cleaning unit periodically cleans the electrode conditioner  76 .  
      In the above-mentioned embodiments, electrolytic polishing and CMP are independently performed by the electrolytic polishing apparatus  54  and the CMP apparatus  56 , respectively. However, the electrolytic polishing apparatus  54  shown in  FIGS. 3 and 4  may be designed such that the electrolytic solution supply nozzle  40  as a liquid supply unit can selectively supply an electrolytic solution or a polishing liquid onto the polishing pad  66 , so that the electrolytic polishing apparatus  54  can perform both electrolytic polishing and CMP.  
       FIG. 16  shows an essential part of an electrolytic polishing apparatus according to another embodiment of the present invention, This electrolytic polishing apparatus is operable to perform both electrolytic polishing and CMP, and is different from that shown in  FIGS. 3 and 4  in that, instead of the electrolytic solution supply nozzle  40 , a liquid supply nozzle  82  connected to an electrolytic solution supply line  78  and a polishing liquid supply line  80  is used as a liquid supply unit. With this structure, the liquid supply nozzle  82  can selectively supply an electrolytic solution or a polishing liquid without mixing them with each other. The liquid supply nozzle  82  has electrolytic solution supply mouths  82   a  through which the electrolytic solution is supplied, and polishing liquid supply mouths  82   b  through which the polishing liquid is supplied. The electrolytic solution supply mouths  82   a  and the polishing liquid supply mouths  82   b  are alternately arranged.  
      In this embodiment, a substrate is held by top ring  32  and pressed against polishing surface  66   a  of polishing pad  66  at predetermined pressure, and the top ring  32  and polishing table  34  are rotated together with each other, At the same time, the electrolytic solution is supplied onto the polishing surface  66   a  through the electrolytic solution supply mouths  82   a  of the liquid supply nozzle  82 , and a predetermined voltage is applied between first electrode  62  and second electrode  64  to thereby perform electrolytic polishing.  
      CMP is performed as follows. A substrate is held by the top ring  32  and pressed against the polishing surface  66   a  of the polishing pad  66  at predetermined pressure, and the top ring  32  and the polishing table  34  are rotated together with each other. At the same time, the polishing liquid is supplied onto the polishing surface  66   a  through the polishing liquid supply mouths  82   b  of the liquid supply nozzle  82  without applying a voltage between the first electrode  62  and the second electrode  64  to thereby perform CMP.  
      After performing electrolytic polishing in this electrolytic polishing apparatus, the substrate is transferred to the cleaning unit  22 , where the substrate is cleaned and rinsed, as previously described. After cleaning and rinsing, the top ring  32  of the electrolytic polishing apparatus holds the substrate again to perform CMP.  
      Cleaning and rinsing of the substrate to which the electrolytic solution is attached is preferably performed until electrical conductivity of a waste rinsing liquid discharged from this cleaning and rinsing process is reduced to at most one-tenth of electrical conductivity of the polishing liquid used in the CMP process, as with the above mentioned embodiments. The electrical conductivity of the waste rinsing liquid discharged through discharge passage  122  is monitored by electrical conductivity meter  124  (see  FIG. 5 ) so that an effect of cleaning and rinsing can be confirmed and polishing can be sufficiently performed.  
      Conditioning of the polishing surface  66   a  by dresser  38  and atomizer  44  is performed between electrolytic polishing and CMP. After conditioning, it is preferable that the polishing table  34  is rotated at a speed in the range of 50 to 100 min −1  for several seconds so as to drain the polishing pad  66 .  
      Depending on types of electrolytic solution (polishing liquid), a slight change in concentration (particularly due to mixing of water) may cause a change in physical properties of the solution, i.e., a change in a polishing property. Generally, conditioning such as dressing and atomizing uses water, and such water remaining after conditioning may cause a change in concentration. Therefore, removal of water (i.e., draining) is required. Thus, after conditioning, the polishing table  34  with the polishing pad  66  is rotated at a speed in the range of 50 to 100 min −1  for several seconds so as to drain the polishing pad  66 , whereby a concentration of the electrolytic solution (polishing liquid) can be prevented from changing.  
      In the embodiment shown in  FIG. 16 , a rinsing liquid supply nozzle  84  for upwardly ejecting a rinsing liquid such as pure water is disposed laterally of the polishing table  34 . With this arrangement, the top ring  32  holds and elevates the substrate W after electrolytic polishing, and then moves the substrate W to an overhanging position where part of the substrate W projects from an edge of the polishing table  34  so that the substrate W is positioned above the rinsing liquid supply nozzle  84 . Thereafter, the substrate W is rotated, and a rinsing liquid is supplied toward a surface (lower surface) of the substrate W over an area F through the rinsing liquid supply nozzle  84  to thereby rinse the lower surface of the substrate W.  
      According to this embodiment, electrolytic polishing, cleaning and rinsing, and CMP can be successively performed on the substrate W while the top ring  32  holds the substrate W.  
      In a case of polishing copper by electrolytic polishing and then performing CMP to completely remove copper remaining on a surface of a barrier film, an electrolytic solution for use in electrolytic polishing may comprise a supporting electrolyte and a polishing liquid to be used in CMP .  FIG. 17  shows another embodiment of an essential part of an electrolytic polishing apparatus designed to use such an electrolytic solution comprising a supporting electrolyte and a polishing liquid to be used in CMP so as to perform both electrolytic polishing and CMP.  
      The electrolytic polishing apparatus of this embodiment comprises a liquid supply nozzle  86  as a liquid supply unit located above polishing pad  66 . The liquid supply nozzle  86  is connected to a pure water supply line  88  through which only pure water is delivered, a supporting electrolyte supply line  90  through which a liquid containing only a supporting electrolyte is delivered, a polishing liquid supply line  92  through which only a polishing liquid (complexing agent, anticorrosive, abrasive particles) is delivered, and an additive supply line  94  through which a liquid containing only an additive (oxidizing agent) is delivered. Theses supply lines  88 ,  90 ,  92  and  94  are capable of adjusting supply flow rate within the range of for example, 0.01 to 0.5 L/min.  
      Additionally, the liquid supply nozzle  86  has a pure water supply mouth  96   a , a supporting electrolyte supply mouth  96   b , a polishing liquid supply mouth  96   c , and an additive supply mouth  96   d . Pure water, which is delivered through the pure water supply line  88 , is supplied onto the polishing pad  66  through the pure water supply mouth  96   a . The liquid containing only the supporting electrolyte, which is delivered through the supporting electrolyte supply line  90 , is supplied onto the polishing pad  66  through the supporting electrolyte supply mouth  96   b . The polishing liquid, which is delivered through the polishing liquid supply line  92 , is supplied onto the polishing pad  66  through the polishing liquid supply mouth  96   c . The liquid containing only the additive, which is delivered through the additive supply line  94 , is supplied onto the polishing pad  66  through the additive supply mouth  96   d . The supply mouths  96   a ,  96   b ,  96   c  and  96   d  may comprise plural supply mouths, respectively.  
      According to this embodiment, electrolytic polishing can be performed while the liquid containing only the supporting electrolyte and the polishing liquid are supplied onto the polishing pad  66  through the supporting electrolyte supply mouth  96   b  and the polishing liquid supply mouth  96   c , respectively. Thereafter, CMP can be performed while the polishing liquid and the liquid containing only the additive are supplied onto the polishing pad  66  through the polishing liquid supply mouth  96   c  and the additive supply mouth  96   d , respectively. Further, after polishing (electrolytic polishing and CMP), water polishing can be performed while pure water is supplied onto the polishing pad  66  through the pure water supply mouth  96   a , thus cleaning the substrate that has been polished.  
       FIG. 18  shows an essential part of an electrolytic polishing apparatus according to another embodiment of the present invention- The embodiment shown in  FIG. 18  is different from the embodiment shown in  FIG. 17  in the following points. Supporting electrolyte supply line  90  and polishing liquid supply line  92  are connected to a first buffer  98   a , and the polishing liquid supply line  92  and additive supply line  94  are connected to a second buffer  98   b . The first buffer  98   a  and liquid supply nozzle  86  are connected through an electrolytic solution supply line  100 . The second buffer  98   b  and the liquid supply nozzle  86  are connected through an additive-containing polishing liquid supply line  102 . The liquid supply nozzle  86  has an electrolytic solution supply mouth  96   e  through which an electrolytic solution, which is supplied through the electrolytic solution supply line  100 , is supplied onto the polishing pad  66 . The liquid supply nozzle  86  also has an additive-containing polishing liquid supply mouth  96   f  through which an additive-containing polishing liquid, which is supplied through the additive-containing polishing liquid supply line  102 , is supplied onto the polishing pad  66 .  
      The electrolytic solution supply line  100  and the additive-containing polishing liquid supply line  102  are capable of adjusting supply flow rate within the range of, for example, 0.1 to 1.0 L/min. The electrolytic solution supply mouth  96   e  and the additive-containing polishing liquid supply mouth  96   f  may comprise plural mouths, respectively.  
      According to this embodiment, the liquid, which contains only the supporting electrolyte, and the polishing liquid are supplied to the first buffer  98   a , where these liquids are mixed with each other to produce in advance an electrolytic solution stored in the first buffer  98   a . Then, electrolytic polishing is performed while supplying the electrolytic solution from the first buffer  98   a  onto the polishing pad  66 . Further, the polishing liquid and the additive are supplied to the second buffer  98   b , where the polishing liquid and the additive are mixed with each other to produce in advance an additive-containing polishing liquid stored in the second buffer  98   b . Then, CMP is performed while supplying the additive-containing polishing liquid from the second buffer  98   b  onto the polishing pad  66 . In this case, the additive is supplied to the second buffer  98   b  according to need.  
       FIGS. 19 through 21  show an essential part of an electrolytic polishing apparatus according to another embodiment of the present invention. This embodiment is different from the embodiment shown in  FIG. 17  in that, instead of the liquid supply nozzle  86  having the plural supply mouths, a slit nozzle  104  is used as a liquid supply unit. This slit nozzle  104  has a slit mouth extending in a longitudinal direction thereof and disposed at a lower end thereof. As shown in  FIG. 20 , a baffle  105  for mixing liquids is disposed in the slit nozzle  104 , and an inside space of the slit nozzle  104  is divided by the baffle  105  into a mixing bath  106  and a buffer bath  107 . A dispersing layer  108 , which is formed from a mesh or a porous material, is attached to the slit mouth. The dispersing layer  108  may be held in contact or non-contact with polishing surface  66   a  of polishing pad  66  during polishing.  
      According to this embodiment, a liquid containing only a supporting electrolyte is supplied through supporting electrolyte supply line  90  to the slit nozzle  104 , and a polishing liquid is supplied through polishing liquid supply line  92  to the slit nozzle  104 . In the slit nozzle  104 , the liquid containing only the supporting electrolyte and the polishing liquid are mixed with each other to produce an electrolytic solution right before polishing. This electrolytic solution is supplied onto the polishing pad  66 , and electrolytic polishing is thus performed. Further, a polishing liquid is supplied through the polishing liquid supply line  92  to the slit nozzle  104 , and an additive, if necessary, is also supplied through the additive supply line  94  to the slit nozzle  104 , where the polishing liquid and the additive are mixed with each other to produce an additive-containing polishing liquid right before polishing. This additive-containing polishing liquid is supplied onto the polishing pad  66 , and CMP is thus performed. According to this embodiment, the electrolytic solution and the additive-containing polishing liquid can be uniformly supplied onto the polishing pad  66  through a lower end of the slit nozzle  104 .  
      Although two-step polishing is performed to remove the barrier film and the copper film (and the seed film) on the substrate W in the above-mentioned embodiments, the present invention is not limited to this manner. For example, the copper film (and the seed film) may be polished by two-step polishing comprising electrolytic polishing and CMP, and then remaining copper film (and the seed film) and the barrier film may be removed respectively by different steps of CMP. In this manner, more than two-step polishing can be performed to polish the surface of the substrate.