Patent Publication Number: US-9844794-B2

Title: Substrate plating apparatus and substrate plating method

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2013-090773 filed Apr. 23, 2013, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     There has been known a plating apparatus that performs plating of a substrate surface by immersing a substrate such as a semiconductor wafer, held by a substrate holder, in a plating solution. In this type of plating apparatus, after the plating, the substrate holder, holding the substrate, is immersed in a cleaning liquid, such as pure water, stored in a cleaning bath to clean the substrate and the substrate holder. By immersing the substrate holder, holding the substrate, in the cleaning liquid in this manner, a liquid chemical such as the plating solution, adhering to the substrate and the substrate holder, is removed from them by basically diffusion due to a difference in the concentration between the liquids. The cleaning liquid in which the liquid chemical, such as the plating solution, has been diffused is discharged from the cleaning bath, so that one cleaning cycle using the cleaning liquid is completed. This cleaning cycle is repeated multiple times. 
     The cleaning bath for use in such post-plating cleaning of a substrate and a substrate holder generally has a shape of an open-top box with a constant opening area. Dimensions (i.e., a thickness, a width, and a depth) of the interior space of the cleaning bath are set to be large enough to prevent contact of the substrate holder with the cleaning bath when the substrate holder is introduced into the cleaning bath and the substrate holder is raised from the cleaning bath, taking into consideration the maximum thickness, the maximum width, and the immersion depth of the substrate holder. 
     An amount of the cleaning liquid, such as pure water, to be used in one cleaning cycle is determined by the volume of the cleaning bath that is determined from the product of the opening area and the depth of the cleaning bath. The total amount of the cleaning liquid to be used in a cleaning process consisting of multiple cleaning cycles, excepting an extra amount of the cleaning liquid to be supplied for the purpose of overflowing and an additional amount of the cleaning liquid to be supplied e.g., from a shower head, is equal to the product of the number of cleaning cycles and a value obtained by subtracting the volume of the substrate holder from the interior volume of the cleaning bath. 
     The applicant has proposed a substrate plating apparatus in which a narrow flow passage (processing chamber) is formed between an openable/closable lid provided in a cleaning bath and a substrate held in a bath body (see Japanese Laid-Open Patent Publication No. 2000-58496). A plating solution is passed through the flow passage while allowing the plating solution to flow along a surface of the substrate. 
     An apparatus has been proposed which, in order to reduce the use of a processing fluid, introduces the processing fluid into a narrow space between two plates which are movable in directions closer to and away from each other (see Japanese Laid-Open Patent Publication No. 2002-535831 (Translation of PCT Application)). A cleaning apparatus has been proposed which introduces a cleaning liquid into a cleaning chamber in which a cleaning object is set, and performs cleaning of the cleaning object while repeating pressurization and depressurization of the cleaning liquid (see Japanese Laid-Open Patent Publication No. 2004-14642). 
     Further, an apparatus has been proposed in which a processing object, which is suspended in a vertical position by a transport device, is surrounded by a surrounding means, and the processing object is cleaned by supplying a processing liquid into the surrounding means (see Japanese Laid-Open Patent Publication No. 2008-223094). 
     A substrate holder is configured to hold a substrate while sealing a gap between a peripheral portion of the substrate and the substrate holder with a sealing member. The substrate holder is immersed in a plating solution while holding the substrate. In general, the front surface of the substrate holder, holding the substrate, is not flat. Irregularities, including a fairly large recess having a diameter which is approximately equal to the diameter of the substrate, are formed in the front surface of the substrate holder holding the substrate. The front surface of the substrate holder thus has an uneven configuration. When such substrate holder holding the substrate is placed in a box-like cleaning bath so that the substrate and the substrate holder are cleaned with a cleaning liquid such as pure water, a large amount of the cleaning liquid flows into the recess formed by the substrate and the substrate holder, resulting in an increase in the amount of the cleaning liquid used. Further, according to this cleaning method, the use of cleaning liquid increases as the size of substrates increases. 
     A substrate holder, including a structure for sealing a gap between a peripheral portion of a substrate and the substrate holder, has a certain degree of thickness. When cleaning such a substrate holder with the use of the above-described box-like cleaning bath which is designed taking into consideration the maximum thickness, the maximum width, and the immersion depth of the substrate holder, it is necessary to use a cleaning liquid in a large amount. This problem will be more significant for substrates of a larger size. In the case of immersion cleaning using such a cleaning bath, it takes some time to store a cleaning liquid into the cleaning bath and to discharge the cleaning liquid from the cleaning bath in one cleaning cycle. Accordingly, when a cleaning process is performed by repeating the cleaning cycle multiple times, it takes a long time to complete the cleaning process. 
     None of the above-referenced patent documents are directed to a cleaning technique for cleaning a substrate holder, holding a substrate, with a cleaning liquid in a cleaning bath, the substrate holder being configured to hold the substrate while sealing a peripheral portion of the substrate with a sealing member, and having been subjected to plating of the substrate by immersing the substrate holder, holding the substrate, in a plating solution. 
     In the apparatus disclosed in the above-referenced Japanese Laid-Open Patent Publication No. 2008-223094, a processing liquid is supplied into the interior space of the surrounding member, having larger dimensions than the maximum width and the maximum thickness of a processing object and the transport device, without taking the uneven exterior configuration of the transport device into consideration. Therefore, the processing apparatus requires the use of a large amount of the processing liquid. In addition, a processing object is cleaned with the processing liquid while circulating the processing liquid. Therefore, the processing liquid can become contaminated gradually, resulting in insufficient cleaning of the processing object. 
     SUMMARY OF THE INVENTION 
     It is therefore an object to provide a substrate plating apparatus and a substrate plating method which can clean a substrate, held by a substrate holder, with use of a smaller amount of cleaning liquid without lowering a throughput. 
     Embodiments, which will be described hereinafter, relate to a substrate plating apparatus and a substrate plating method of dip type which use a substrate holder for holding a substrate, such as a wafer, and immersing the substrate into a plating solution so as to form connection bumps, interconnects, and the like on the substrate surface. 
     In an embodiment, a substrate plating apparatus includes: a substrate holder configured to hold a substrate with a sealing member pressing on a peripheral portion of the substrate; a plating bath configured to plate a surface of the substrate when the substrate, held by the substrate holder, is immersed in a plating solution; a cleaning bath configured to clean the substrate holder and the substrate with a cleaning liquid; an inner shell disposed in the cleaning bath and configured to house therein the substrate holder holding the substrate, the inner shell being configured to be able to be opened and closed and having an inner surface which has an uneven configuration that follows an uneven exterior configuration of the substrate holder holding the substrate; and a cleaning liquid supply conduit configured to supply a cleaning liquid into the inner shell when the inner shell in a closed state to clean the substrate, together with the substrate holder, with the cleaning liquid. 
     In an embodiment, a gap of 1 mm to 5 mm is formed between the inner surface of the inner shell in the closed state and the substrate holder. 
     In an embodiment, the substrate plating apparatus further includes a gas feed line configured to feed a gas into the cleaning liquid to be supplied into the inner shell in the closed state. 
     In an embodiment, the substrate plating apparatus further includes a mechanism configured to cause a surface level of the cleaning liquid in the inner shell to fluctuate vertically when the inner shell is in the closed state. 
     In an embodiment, the mechanism is one of an oscillation mechanism configured to cause walls of the inner shell to oscillate, a diaphragm drive mechanism configured to vibrate a diaphragm in contact with the cleaning liquid in the inner shell, and a syringe mechanism or pump device configured to repeatedly supply and discharge the cleaning liquid into and from the inner shell. 
     In an embodiment, the inner shell has a plurality of peripheral holes and a central hole, the peripheral holes being arranged so as to face a peripheral portion of the substrate, the central hole being arranged so as to face a central portion of the substrate, and the cleaning liquid being supplied into the inner shell through at least one of the central hole and the peripheral holes. 
     In an embodiment, the inner shell has a plurality of peripheral holes and a central hole through which the cleaning liquid is supplied into the inner shell, the peripheral holes being arranged so as to face a peripheral portion of the substrate, and the central hole being arranged so as to face a central portion of the substrate. 
     In an embodiment, the substrate plating apparatus further includes a substrate holder moving mechanism configured to cause the substrate holder to oscillate horizontally. 
     In an embodiment, a substrate plating method includes: holding a substrate by a substrate holder with a sealing member pressing on a peripheral portion of the substrate; plating a surface of the substrate while immersing the substrate, held by the substrate holder, in a plating solution; placing the plated substrate, held by the substrate holder, in an inner shell which is in an opened state; closing the inner shell to bring an inner surface of the inner shell, having an uneven configuration that follows an uneven exterior configuration of the substrate holder holding the substrate, into proximity to the substrate holder and the substrate; and supplying a cleaning liquid into the inner shell in the closed state to clean the substrate, together with the substrate holder, in the inner shell. 
     In an embodiment, a gap of 1 mm to 5 mm is formed between the inner surface of the inner shell in the closed state and the substrate holder. 
     In an embodiment, the substrate plating method further includes feeding a gas into the cleaning liquid to be supplied into the inner shell in the closed state. 
     In an embodiment, the substrate plating method further includes causing a surface level of the cleaning liquid in the inner shell to fluctuate vertically. 
     In an embodiment, supplying the cleaning liquid comprises supplying a cleaning liquid through a plurality of through-holes into the inner shell in the closed state to clean the substrate, together with the substrate holder, in the inner shell, the through-holes being arranged so as to face the surface of the substrate. 
     In an embodiment, supplying the cleaning liquid comprises supplying a cleaning liquid through at least one of a central hole and a plurality of peripheral holes into the inner shell in the closed state to clean the substrate, together with the substrate holder, in the inner shell, the peripheral holes being arranged so as to face a peripheral portion of the substrate, and the central hole being arranged so as to face a central portion of the substrate. 
     In an embodiment, the substrate plating method further includes after supplying the cleaning liquid into the inner shell in the closed state, causing the substrate holder to oscillate horizontally. 
     The inner shell has an interior configuration that is complementary to the exterior configuration of the substrate holder holding a substrate. This configuration can reduce the amount of the cleaning liquid used in one cleaning cycle. This can also reduce the time to supply and discharge the cleaning liquid into and from the inner shell. It therefore becomes possible to increase a time for a plating solution, adhering to the substrate holder and a substrate, to disperse into the cleaning liquid, or to increase the number of cleaning cycles. Thus, it becomes possible to enhance the cleaning effect without lowering the throughput. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an overall layout plan view of a plating apparatus according to an embodiment; 
         FIG. 2  is a perspective view of a substrate holder; 
         FIG. 3  is a plan view of the substrate holder shown in  FIG. 2 ; 
         FIG. 4  is a right side view of the substrate holder shown in  FIG. 2 ; 
         FIG. 5  is an enlarged view of the portion A of  FIG. 4 ; 
         FIG. 6  is a vertical cross-sectional front view of a cleaning bath according to an embodiment, illustrating the cleaning bath when a substrate holder, holding a substrate, is placed at a predetermined position in the cleaning bath; 
         FIG. 7  is a vertical cross-sectional front view of the cleaning bath when cleaning the substrate and the substrate holder set at the predetermined position in the cleaning bath; 
         FIG. 8  is a perspective view of a shell side plate; 
         FIG. 9  is a perspective view of a shell end plate disposed beside the shell side plate; 
         FIG. 10  is a graph showing experimental results of Example 1 which used the cleaning bath shown in  FIGS. 6 to 9  and experimental results of Comparative Example 1 which used a conventional cleaning bath; 
         FIG. 11  is a schematic view of a cleaning bath according to another embodiment; 
         FIG. 12  is a schematic view of a cleaning bath according to yet another embodiment; 
         FIG. 13  is a diagram illustrating varying positions of a shell end plate when it is opened, closed, and oscillating; 
         FIG. 14  is a graph showing a relationship between the position of the shell and plate and time when the shell end plate is opened, closed, and oscillating; 
         FIG. 15  is a schematic view of a cleaning bath according to yet another embodiment; 
         FIG. 16  is a schematic view of a cleaning bath according to yet another embodiment; 
         FIG. 17  is a schematic view of a cleaning bath according to yet another embodiment; 
         FIG. 18  is a schematic view of a cleaning bath according to yet another embodiment; 
         FIG. 19  is a schematic view of a cleaning bath according to yet another embodiment; 
         FIG. 20  is a schematic view of a cleaning bath according to yet another embodiment; 
         FIG. 21  is a schematic view of a cleaning bath according to yet another embodiment; 
         FIG. 22  is a schematic view of a cleaning bath according to yet another embodiment; 
         FIG. 23  is a schematic view of a cleaning bath according to yet another embodiment; 
         FIG. 24  is a schematic view of a cleaning bath according to yet another embodiment, illustrating the cleaning bath when a substrate holder, holding a substrate, is placed at a predetermined position in the cleaning bath; 
         FIG. 25  is a schematic view of the cleaning bath when cleaning the substrate and the substrate holder set at the predetermined position in the cleaning bath; 
         FIG. 26  is a schematic view of a cleaning bath according to yet another embodiment; 
         FIG. 27  is an enlarged view of a main portion of a substrate holder which is cleaned in the cleaning bath shown in  FIG. 26 ; 
         FIG. 28  is a diagram illustrating an exemplary process of cleaning the interior of the cleaning bath by storing a cleaning liquid in the cleaning bath; 
         FIG. 29  is a schematic view of a cleaning bath according to yet another embodiment; and 
         FIG. 30  shows a process sequence of a cleaning process intended to address a problem of an increased amount of cleaning liquid droplets adhering to a substrate or a substrate holder after cleaning. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Embodiments will now be described with reference to the drawings. The same reference numerals are used in the following figures and descriptions to refer to the same or like elements, components, etc., and duplicate descriptions thereof will be omitted. 
       FIG. 1  shows an overall layout plan of a plating apparatus according to an embodiment. As shown in  FIG. 1 , the plating apparatus includes two cassette tables  12  on which substrate cassettes  10 , each storing substrates, such as semiconductor wafers, are placed, an aligner  14  for aligning an orientation flat or a notch of a substrate in a predetermined direction, and a spin rinse drier  16  for drying a plated substrate by rotating it at a high speed. Near the spin rinse drier  16  is provided a substrate loading unit  20  for placing a substrate holder  18  thereon and loading the substrate into the substrate holder  18  and removing the substrate from the substrate holder  18 . Further, in the center of these units  10 ,  14 ,  16 , and  20  is disposed a substrate transport device  22  constituted by a transport robot for transporting the substrate between these units. 
     The substrate loading unit  20 , a stocker  24  for storing (and temporarily storing) substrate holders  18  therein, a pee-wetting bath  26  for immersing the substrate in pure water, a pre-soaking bath  28  for etching away an oxide film formed on a surface of a film (e.g., a seed layer) of the substrate, a first cleaning bath  30   a  for cleaning the surface of the pre-soaked substrate, together with the substrate holder  18 , with a cleaning liquid (e.g., pure water), a blow bath  32  for removing the cleaning liquid from the cleaned substrate, a second cleaning bath  30  for cleaning the plated substrate, together with the substrate holder  18 , with a cleaning liquid (e.g., pure water), and a plating bath  34  are arranged in this order. The plating bath  34  includes an overflow bath  36  and a plurality of plating cells  38  surrounded by the overflow bath  36 . Each plating cell  38  is configured to receive one substrate therein and perform plating, e.g., copper plating, on the surface of the substrate. 
     Located lateral to the above baths, there is provided a substrate holder transport device  40 , driven e.g., by a linear motor, for transporting the substrate holder  18 , together with a substrate, between the baths. The substrate holder transport device  40  has a first transporter  42  for transporting a substrate between the substrate loading unit  20 , the stocker  24 , the pre-wetting bath  26 , the pre-soaking bath  28 , the first cleaning bath  30   a , and the blow bath  32 , and a second transporter  44  for transporting the substrate between the first cleaning baths  30   a , the second cleaning bath  30 , the blow bath  32 , and the plating bath  34 . The substrate holder transport device  40  may be provided with only the first transporter  42  without being provided with the second transporter  44 . 
     Paddle drive devices  46  are provided each for driving a paddle (not shown) disposed in each plating cell  38  as an agitator for agitating a plating solution in the plating cell  38 . The paddle drive devices  46  are located beside the overflow bath  36  at the opposite side of the substrate holder transport device  40 . 
     The substrate loading unit  20  includes a flat stage plate  52  which is laterally slidable along rails  50 . Two substrate holders  18 , parallel to each other, are placed horizontally on the stage plate  52 . One substrate is transferred between one substrate holder  18  and the substrate transport device  22 , and then the stage plate  52  is slid laterally so that the other substrate is transferred between the other substrate holder  18  and the substrate transport device  22 . 
     As shown in  FIGS. 2 through 5 , the substrate holder  18  includes a first holding member (or a base holding member)  54  having a rectangular plate shape and made of e.g., vinyl chloride, and a second holding member (or a movable holding member)  58  rotatably coupled to the first holding member  54  through a hinge  56  which allows the second holding member  58  to open and close with respect to the first holding member  54 . While the second holding member  58  is configured to be openable and closable through the hinge  56  in this embodiment, it is also possible to dispose the second holding member  58  opposite to the first holding member  54  and to move the second holding member  58  away from and toward the first holding member  54  to thereby open and close the second holding member  58 . 
     The second holding member  58  includes a base portion  60  and a ring-shaped seal holder  62 . The seal holder  62  is made of vinyl chloride so as to enable a retaining ring  64 , which will be described later, to slide well. An annular substrate-side sealing member  66  is fixed to an upper portion of the seal holder  62 . This substrate-side sealing member  66  is placed in pressure contact with a periphery of the surface of the substrate W to seal a gap between the substrate W and the second holding member  58  when the substrate W is held by the substrate holder  18 . An annular holder-side sealing member  68  is fixed to a surface, facing the first holding member  54 , of the seal holder  62 . This holder-side sealing member  68  is placed in pressure contact with the first holding member  54  to seal a gap between the first holding member  54  and the second holding member  58 . The holder-side sealing member  68  is located at the outer side of the substrate-side sealing member  66 . 
     As shown in  FIG. 5 , the substrate-side sealing member  66  is sandwiched between the seal holder  62  and a first mounting ring  70   a , which is secured to the seal holder  62  by fastening tools  69   a , such as screws. The holder-side sealing member  68  is sandwiched between the seal holder  62  and a second mounting ring  70   b , which is secured to the seal holder  62  by fastening tools  69   b , such as screws. 
     The seal holder  62  has a stepped portion at a periphery thereof, and the retaining ring  64  is rotatably mounted to the stepped portion through a spacer  65 . The retaining ring  64  is inescapably held by an outer peripheral portion of the first mounting ring  70   a . This retaining ring  64  is made of a material (e.g., titanium) having high rigidity and excellent acid and alkali corrosion resistance and the spacer  65  is made of a material having a low friction coefficient, for example PTFE, so that the retaining ring  64  can rotate smoothly. 
     Inverted L-shaped clampers  74 , each having an inwardly projecting portion and located at the outer side of the retaining ring  64 , are provided on the first holding member  54  at equal intervals along a circumferential direction of the retaining ring  64 . The retaining ring  64  has, on its outer circumferential surface, outwardly projecting portions  64   b  arranged at positions corresponding to positions of the clampers  74 . A lower surface of the inwardly projecting portion of each clamper  74  and an upper surface of each projecting portion  64   b  of the retaining ring  64  are inclined in opposite directions along the rotational direction of the retaining ring  64 . A plurality (e.g., three) of upwardly projecting protrusions  64   a  are provided on the retaining ring  64  at predetermined positions along the circumferential direction of the retaining ring  64 . The retaining ring  64  can be rotated by pushing and moving each protrusion  64   a  from a lateral direction by means of a rotating pin (not shown). 
     With the second holding member  58  open, the substrate W is inserted into the central portion of the first holding member  54 , and the second holding member  58  is then closed through the hinge  56 . Subsequently the retaining ring  64  is rotated clockwise so that each projecting portion  64   b  of the retaining ring  64  slides into the inwardly projecting portion of each clamper  74 . As a result, the first holding member  54  and the second holding member  58  are fastened to each other and locked by engagement between the inclined surfaces of the retaining ring  64  and the inclined surfaces of the clampers  74 . The lock of the second holding member  58  can be released by rotating the retaining ring  64  counterclockwise to disengage the projecting portions  64   b  of the retaining ring  64  from the inverted L-shaped clampers  74 . When the second holding member  58  is locked in the above-described manner, the downwardly-protruding portion of the substrate-side sealing member  66  is placed in pressure contact with the periphery of the surface of the substrate W. The substrate-side sealing member  66  is pressed uniformly against the substrate W to thereby seal the gap between the periphery of the surface of the substrate W and the second holding member  58 . Similarly, when the second holding member  58  is locked, the downwardly-protruding portion of the holder-side sealing member  68  is placed in pressure contact with the surface of the first holding member  54 . The sealing holder-side sealing member  68  is uniformly pressed against the first holding member  54  to thereby seal the gap between the first holding member  54  and the second holding member  58 . 
     When the substrate-side sealing member  66  is pressed against the periphery of the surface of the substrate W to establish the sealed state, a stepped portion in a ring shape is formed between the surface of the substrate W and an inner circumferential surface of the substrate-side sealing member  66 . This stepped portion extends continuously along a contact portion D 1  of the periphery of the surface of the substrate W that contacts the substrate-side sealing member  66 , i.e., along a seal surface of the substrate-side sealing member  66 . With the substrate-side sealing member  66  in contact with the periphery of the surface of the substrate W, the substrate W is immersed together with the substrate holder  18  in the plating solution held in the plating unit  38 , and then the substrate W is plated. After plating of the substrate W, the substrate holder  18  is raised from the plating unit  38 . At this time, the plating solution is liable to remain on the contact portion D 1 . As will be described later, the cleaning bath is configured to clean the substrate holder  18 , holding the substrate W, so that the plating solution remaining on the contact portion D 1  can be removed (or cleaned) efficiently. 
     A protruding portion  82  is formed on the upper surface of the first holding member  54  so as to protrude in a ring shape with a size corresponding to a size of the substrate W. The protruding portion  82  has an annular support surface  80  which contacts a periphery of the substrate W to support the substrate W. The protruding portion  82  has recesses  84  arranged at predetermined positions along a circumferential direction of the protruding portion  82 . 
     A pair of outwardly-projecting holder hangers  90  is provided on the ends of the first holding member  54  of the substrate holder  18 . These holder hangers  90  serve as a support when the substrate holder  18  is transported and when the substrate holder  18  is supported in a suspended state. A hand lever  92  extends between the holder hangers  90  on both sides. The substrate holder transport device  40  is configured to grip the hand lever  92  to thereby hold the substrate holder  18 . In the stocker  24 , the holder hangers  90  are placed on an upper surface of a surrounding wall of the stocker  24 , whereby the substrate holder  18  is suspended in a vertical position. When transporting the substrate holder  18  from the stocker  24 , the holder hangers  90  of the suspended substrate holder  18  are gripped by the first transporter  42  of the substrate holder transport device  40 . Also in the pre-wetting bath  26 , the pre-soaking bath  28 , the cleaning baths  30   a  and  30 , the blow bath  32 , and the plating bath  34 , the substrate holder  18  is held in a suspended state with the holder hangers  90  placed on a surrounding wall of the bath. 
     As shown in  FIG. 3 , a plurality of (e.g.,  12  as illustrated) electrical conductors (electrical contacts)  86  are disposed in the recesses  84 , respectively. These electrical conductors  86  are coupled respectively to wires extending from connecting terminals  91  provided on the holder hanger  90 . The electrical conductors  86  have their end portions, respectively, which are located outwardly of the periphery of the substrate W so that the electrical conductors  86  themselves do not contact the substrate W. When the substrate W is placed on the support surface  80  of the first holding member  54 , the end portions of the electrical conductors  86  spring out around the substrate W to resiliently contact lower portions of electrical contacts  88  shown in  FIG. 5 . 
     The electrical contacts  88 , which are to be electrically connected to the electrical conductors  86 , are secured to the seal holder  62  of the second holding member  58  by fastening tools  89 , such as screws. Each of the electrical contacts  88  has a leaf spring-like contact portion located at the outer side of the substrate-side sealing member  66  and projecting inwardly. This spring-like contact portion is springy and bends easily. When the substrate W is held by the first holding member  54  and the second holding member  58 , the contact portions of the electrical contacts  88  come into elastic contact with the peripheral surface of the substrate W supported on the support surface  80  of the first holding member  54 . 
     The second holding member  58  is opened and closed by a not-shown pneumatic cylinder and by a weight of the second holding member  58  itself. More specifically, the first holding member  54  has a through-hole  54   a , and a pneumatic cylinder is provided so as to face the through-hole  54   a  when the substrate holder  18  is placed on the stage plate  52 . The second holding member  58  is opened by extending a piston rod of the pneumatic cylinder through the through-hole  54   a  to push up the seal holder  62  of the second holding member  58  through a pushing rod. The second holding member  58  is closed by its own weight when the piston rod is retracted. 
     Next, the second cleaning bath  30  for cleaning a plated substrate W, together with the substrate holder  18 , with a cleaning liquid (e.g., pure water) will be described in detail. The first cleaning bath  30   a  for cleaning a pre-soaked substrate, together with the substrate holder  18 , with a cleaning liquid may have the same structure as the second cleaning bath  30 . 
       FIG. 6  is a vertical cross-sectional front view of the second cleaning bath (hereinafter referred to simply as cleaning bath)  30 , illustrating the cleaning bath  30  when the substrate holder  18 , holding a substrate W, is placed at a predetermined position in the cleaning bath  30 .  FIG. 7  is a vertical cross-sectional front view of the cleaning bath  30  when cleaning the substrate W and the substrate holder  18  set at the predetermined position in the cleaning bath  30 . 
     As shown in  FIGS. 6 and 7 , the cleaning bath  30  has a shape of an open-top box. An openable and closable open-top inner shell  100  is installed in the cleaning bath  30 . The inner shell  100  is configured to be transformed selectively into a first state (an opened state shown in  FIG. 6 ), in which the substrate holder  18  holding the substrate W is placed at the predetermined position, and a second state (a closed state shown in  FIG. 7 ) in which the substrate holder  18 , except its top portion, is hermetically enclosed by the inner shell  100 . As shown in  FIG. 7 , the inner shell  100  has an inner surface having a configuration that follows an uneven exterior configuration of the substrate holder  18  holding the substrate W so that when the inner shell  100  is closed, the inner surface of the inner shell  100  is placed in proximity to the substrate holder  18  and the substrate W. 
     The inner shell  100  includes a shell side plate  102  and a pair of flat plate-like shell end plates  10 ,  108  disposed at both sides of the shell side plate  102 . The shell end plates  106 ,  108  are rotatably coupled to the shell side plate  102  by hinges  104 . The one shell end plate  106  is located at a side of the front surface of the substrate holder  18  so as to face the substrate W held by the substrate holder  18 , while the other shell end plate  108  is located at a side of the back surface of the substrate holder  18 . The inner shell  100  can be transformed into the first state when the shell end plates  106 ,  108  are opened, and can be transformed into the second state when the shell end plates  106 ,  108  are closed. 
       FIG. 8  is a perspective view of the shell side plate  102 . As shown in  FIG. 8 , the shell side plate  102  has a U-shape which surrounds both side portions and a bottom portion of the substrate holder  18 , and has its upper ends reaching the top of the cleaning bath  30 . The shell side plate  102  has end surfaces that face the shell end plates  106 ,  108 . To each end surface is attached a sealing member  110  which extends along the end surface. A silicone tube, having an inner diameter of 2 mm and an outer diameter of 3 mm, may be used as the sealing member  110 . The use of such a silicone tube can prevent a lowering of the scaling performance of the sealing member  110  even when the shell end plates  106 ,  108  move toward the shell side plate  102  in the closing direction within a distance of about 0.1 mm, as will be described later. The sealing member  110  may have any different shape and may be made of any different material so long as the intended sealing performance is ensured. 
     Side supports  112  and a lower support  114  are mounted respectively to side portions and a lower portion of the shell side plate  102 . The shell side plate  102  is fixed at a predetermined position in the cleaning bath  30  through the side supports  112  and the lower support  114 . A hinge pin  116 , constituting a hinge  104 , is mounted to the lower support  114 . 
       FIG. 9  is a perspective view of the shell end plate  106  disposed beside the shell side plate  102 . The shell end plate  106  is formed of, for example, a composite material including an inner layer of polyvinyl chloride and an outer reinforcing layer of stainless steel so that the shell end plate  106  can be made thin and can have a sufficient rigidity. The shell end plate  106  has a shape that covers the entire end surface of the U-shaped shell side plate  102 . The shell end plate  106  has a rectangular overflow hole  106   a  at an upper portion thereof. A pair of projecting portions  118 , projecting downward, is formed on a bottom of the shell end plate  106 . The hinge pin  116 , mounted to the lower support  114 , is loosely inserted into the projecting portions  118 , thus constituting the hinge  104  which couples the shell end plate  106  to the shell side plate  102  while allowing the shell end plate  106  to be opened and closed. 
     The above-discussed construction of the hinge  104  is substantially the same as that of the other shell end plate  108 . The shell end plate  108  has a rectangular overflow hole  108   a  at an upper portion thereof (see  FIGS. 6 and 7 ). In order to prevent overflow of a cleaning liquid, the shell end plates  106 ,  108  each have a simple seal  120  (see  FIGS. 6 and 7 ), provided at the upper end of the inner surface of each plate, to seal gaps between the substrate holder  18  and the shell end plates  106 ,  108  when the shell end plates  106 ,  108  are closed. 
     As shown in  FIGS. 6 and 7 , the cleaning bath  30  is provided with a lid  122 . The lid  122  is provided with an opening and closing mechanism  124  for opening and closing the shell end plates  106 ,  108 . The lid  122  has a shape that does not interfere with a vertical movement of the substrate holder  18 . The opening and closing mechanism  124  includes an actuator (e.g., air chuck)  128  for simultaneously moving, in opposite directions, a pair of open-close rods  126  extending approximately horizontally, and a pair of support shafts  132  rotatably supported by brackets  130 . Each support shaft  132  extends approximately horizontally and is perpendicular to the corresponding open-close rod  126 . Upwardly-extending connecting arms  134  are secured to the support shafts  132 , respectively, and upper ends of the connecting arms  134  are rotatably coupled to the open-close rods  126 . Downwardly-extending operating arms  136  are secured to the support shafts  132 , respectively, and lower ends of the operating arms  136  are rotatably coupled to the shell end plates  106 ,  108 , respectively. 
     As the open-close rods  126  are moved in directions away from each other (i.e., outwardly), the inner shell  100  changes from the first state in which the shell end plates  106 ,  108  are opened, shown in  FIG. 6 , to the second state in which the shell end plates  106 ,  108  are closed, shown in  FIG. 7 . In conjunction with the movement of the open-close rods  126 , the support shafts  132  are rotated by the connecting arms  134 , whereby the operating arms  136  pivot on the support shafts  132  to cause the shell end plates  106 ,  108  to pivot (rotate) on the hinge  104  in directions closer to each other. When the open-close rods  126  are moved in directions closer to each other (i.e., inwardly), the shell end plates  106 ,  108  pivot (rotate) on the hinge  104  in directions away from each other. Consequently, the inner shell  100  changes from the second state in which the shell end plates  106 ,  108  are closed, shown in  FIG. 7 , to the first state in which the shell end plates  106 ,  108  are opened, shown in  FIG. 6 . 
     When the inner shell  100  is in the first state in which the shell end plates  106 ,  108  are opened, the substrate holder  18 , holding the substrate W, can be lowered without contact with the inner shell  100  and placed at a predetermined position in the inner shell  100 . When the inner shell  100  is in the second state in which the shell end plates  106 ,  108  are closed, the shell end plates  106 ,  108  are in pressure contact with the sealing members  110  so that portions of the inner shell  100  other than the top portion, i.e., the side and bottom portions of the inner shell  100 , are liquid-tightly sealed with the sealing members  110 . The inner surface of the inner shell  100  has the configuration that follows the uneven exterior configuration of the substrate holder  18  holding the substrate W. 
     As shown in  FIG. 5 , the front surface side of the substrate holder  18 , holding the substrate W, has the uneven exterior configuration defined by the first holding member  54 , the second holding member  58 , the clampers  74 , the substrate W, etc. As shown in  FIG. 7 , an uneven portion  106   b , which follows the corresponding uneven exterior configuration of the substrate holder  18 , is formed on the inner surface of the shell end plate  106  located on the side of the front surface of the substrate holder  18 . Since the back surface side of the substrate holder  18  is approximately flat, the shell end plate  108 , located at the back surface side of the substrate holder  18 , has a flat inner surface. A gap G1 between the substrate holder  18  and the shell end plate  106  is set to 1 mm to 5 mm, preferably 1.5 mm to 2 mm. A gap G2 between the substrate holder  18  and the shell end plate  108  is set to 1 mm to 5 mm, preferably 1 mm to 1.5 mm. 
     The shell end plate  106  rotates about the hinge  104  provided at the bottom thereof. Accordingly, even when the shell end plate  106  is opened, a portion of the shell end plate  106  which lies below the uneven portion  106   b  remains close to the substrate holder  18 . Therefore, in order to avoid contact between the shell end plate  106  and the substrate holder  18 , a cutout (not shown) is locally formed in the inner surface of the shell end plate  106  at a position below the uneven portion  106   b.    
     The substrate W and the substrate holder  18  are cleaned with a cleaning liquid supplied into the inner shell  100  in the second state. The gap G1 between the substrate holder  18  and the shell end plate  106  and the gap G2 between the substrate holder  18  and the shell end plate  108  are determined based on an amount of the cleaning liquid used in one cleaning cycle (or one cleaning step) and a flow velocity of the cleaning liquid. As is known, the higher the flow velocity of the cleaning liquid is, the higher is the cleaning power. In this embodiment, in order to intensively clean the front surfaces of the substrate holder  18  and the substrate W, the cleaning liquid is preferably allowed to flow at a higher flow velocity along the front surfaces of the substrate holder  18  and the substrate W. 
     It is therefore preferred to set the gap G1 between the substrate holder  18  and the shell end plate  106  to be larger than the gap G2 between the substrate holder  18  and the shell end plate  108  (G1&gt;G2). This can minimize the sum of the gap G1 and the gap G2 to thereby reduce the amount of the cleaning liquid used in one cleaning cycle, while reducing the resistance to the cleaning liquid flowing through the gap G1 between the substrate holder  18  and the shell end plate  106 , thereby increasing the flow velocity of the cleaning liquid flowing through the gap G1. An uneven configuration may be provided on the back surface of the substrate holder  18  and/or the inner surface of the shell end plate  108  so as to increase the resistance to the cleaning liquid flowing through the gap G2. 
     The cleaning bath  30 , at its bottom, is provided with a cleaning liquid supply pipe (or a cleaning liquid supply conduit)  142  and a cleaning liquid discharge pipe  146 . The cleaning liquid supply pipe  142  penetrates through the bottom of the shell side plate  102 , and is connected to a cleaning liquid supply line  140 . The cleaning liquid discharge pipe  146  communicates with the interior of the cleaning bath  30 , and is connected to a cleaning liquid discharge line  144 . The cleaning liquid is supplied through the cleaning liquid supply pipe  142  into the inner shell  100  in the second state. Further, by transforming the inner shell  100  from the second state to the first state, the cleaning liquid in the inner shell  100  is introduced to the bottom of the cleaning bath  30  through the gaps formed between the shell side plate  102  and the shell end plates  106 ,  108 , and is discharged out of the cleaning bath  30  through the cleaning liquid discharge pipe  146 . 
     A gap between the cleaning liquid supply pipe  142  and the cleaning bath  30  is sealed with an O-ring (not shown) so that the inner shell  100  can be attached and detached to and from the cleaning bath  30 . 
     An exemplary cleaning process of cleaning a plated substrate W, together with the substrate holder  18 , with use of the above-described cleaning bath  30  will now be described. A plating solution is likely to remain on the uneven surface of the substrate holder  18 , especially on the contact portion D 1  (see  FIG. 5 ) of the surface of the substrate W contacting the substrate-side sealing member  66 . Therefore, it is necessary to remove the plating solution remaining on such stepped portion by the supply of the cleaning liquid. 
     First, when the inner shell  100  is in the first state in which the shell end plates  106 ,  108  are opened as shown in  FIG. 6 , the substrate holder  18  holding the substrate W is moved to a position just above the inner shell  100 . The substrate holder  18  is then lowered to place the substrate W at a predetermined position in the inner shell  100 . The substrate  18  being lowered does not make contact with the inner shell  100  in the first state. Thus, the inner shell  100  does not interfere with the movement (i.e., the downward movement) of the substrate holder  18 . 
     Next, the opening and closing mechanism  124  transforms the inner shell  100  from the first state to the second state in which the shell end plates  106 ,  108  are closed, shown in  FIG. 7 . The inner surface of the inner shell  100  (inner surface of the shell end plate  106 ) in the second state has such a configuration that follows the uneven configuration of the front surfaces of the substrate holder  18  and the substrate W that are housed at the predetermined position in the inner shell  100 . Thus, the inner shell  100  has such an interior configuration as to minimize the gap formed between the inner surface of the inner shell  100  and the substrate holder  18  holding the substrate W. The inner shell  100 , in which the substrate holder  18  is housed, therefore has a fairly small interior volume. The portions of the inner shell  100  other than the top portion are liquid-tightly sealed with the sealing members  110 . The interior volume of the inner shell  100  in which the substrate holder  18  is housed, i.e., the volume of the cleaning liquid to be stored in the inner shell  100 , is e.g., about 1.0 L to 1.5 L in the case of a substrate W having a diameter of 450 mm. 
     Next, supply of the cleaning liquid, such as pure water, into the inner shell  100  in the second state is started. The cleaning liquid is gradually supplied into the interior space of the inner shell  100  and eventually comes to overflow the inner shell  100  through the overflow holes  106   a ,  108   a . In this embodiment the cleaning liquid is discharged out of the cleaning bath  30  through the cleaning liquid discharge pipe  146  and the cleaning liquid discharge line  144 . The cleaning liquid is supplied into the inner shell  100  at a high flow rate, so that the cleaning liquid that has overflown through the overflow holes  106   a ,  108   a  is temporarily collected on the bottom of the cleaning bath  30 . The cleaning bath  30  is provided with a liquid level sensor (not shown). If the liquid level of the cleaning liquid that has accumulated on the bottom of the cleaning bath  30  reaches a level H1, it is determined that the interior space of the inner shell  100  is filled with the cleaning liquid. Then, the supply of the cleaning liquid is stopped. Because of the fairly small interior volume of the inner shell  100 , the supply of the cleaning liquid can be completed in a short time, such as about 5 to 7 seconds. The cleaning liquid temporarily collected on the bottom of the cleaning bath  30  is spontaneously discharged through the cleaning liquid discharge pipe  146  with the elapse of time. 
     The cleaning bath  30  is configured to surround the inner shell  100  in order to prevent scattering of the cleaning liquid that has overflown the inner shell  100  or scattering of the cleaning liquid discharged from the inner shell  100 . Thus, the cleaning liquid need not necessarily be collected on the bottom of the cleaning bath  30 . The cleaning liquid overflowing through the overflow holes  106   a ,  108   a  may be directly detected, or the flow of the cleaning liquid in the cleaning liquid discharge pipe  146  may be detected, so long as it is possible to determine, by any other device, that the inner shell  100  is filled with the cleaning liquid. Alternatively, an integrated value of the flow rate of the cleaning liquid, which is measured by a flow meter (not shown) provided in the cleaning liquid supply line  140 , may be calculated, and the supply of the cleaning liquid may be stopped if the integrated value reaches a predetermined value. 
     The inner shell  100  is kept filled with the cleaning liquid for a predetermined period of time. The plating solution, adhering to the substrate W and the substrate holder  18 , is removed (cleaned off) from them basically by diffusion due to a difference in the concentration between the liquids. 
     After the predetermined time has elapsed, the opening and closing mechanism  124  transforms the inner shell  100  from the second state in which the shell end plates  106 ,  108  are closed, shown in  FIG. 7 , to the first state in which the shell end plates  106 ,  108  are opened, shown in  FIG. 6 . Because the shell end plates  106 ,  108  are separated from the scaling members  110  attached to the shell side plate  102  by this operation, the cleaning liquid stored in the inner shell  100  quickly (in one or two seconds) flows out through the gaps between the shell side plate  102  and the shell end plates  106 ,  108 . The cleaning liquid that has flowed out of the inner shell  100  is temporarily collected on the bottom of the cleaning bath  30 . As a result, the surface level of the cleaning liquid collected on the bottom of the cleaning bath  30  becomes higher than the above-described liquid level H1 set in the liquid level sensor. The cleaning liquid temporarily collected on the bottom of the cleaning bath  30  is spontaneously discharged through the cleaning liquid discharge pipe  146  with the elapse of time, whereby the first cleaning cycle of cleaning the substrate W and the substrate holder  18  with use of the cleaning liquid supplied into the inner shell  100  is terminated. If the surface level of the cleaning liquid collected on the bottom of the cleaning bath  30  does not become lower than the liquid level H1 after a predetermined time has elapsed from the opening of the shell end plates  106 ,  108 , then it is determined that an abnormality has occurred in the discharge of the cleaning liquid from the cleaning bath  30 , and an error signal is issued. 
     After the completion of discharge of the cleaning liquid from the inner shell  100  is detected, i.e., the completion of the first cleaning cycle is detected, the inner shell  100  is transformed from the first state to the second state, and a second cleaning cycle is started by supplying a cleaning liquid into the inner shell  100  in the second state. The cleaning liquid for use in the second cleaning cycle is not the one that has once been discharged in the first cleaning cycle, but a newly-supplied cleaning liquid. 
     The above-described cleaning cycle is repeated multiple times (e.g., three times), until the cleaning process of the substrate W and the substrate holder  18  with use of the cleaning liquid is completed. A point of time when all of the cleaning liquid existing in the cleaning liquid discharge line  144  is discharged is determined to be a point of time when the discharge of the cleaning liquid from the cleaning bath  30  is completed. 
     When the cleaning process is terminated, the inner shell  100  is in the first state in which the shell end plates  106 ,  108  are opened. In this state, the second transporter  44  raises the substrate holder  18 , holding the substrate W, from the cleaning bath  30 , and transports the substrate holder  18  to the next process. 
     The cleaning bath  30  receives the cleaning liquid that has overflowed the inner shell  100  or the cleaning liquid that has been discharged from the inner shell  100 . It is therefore desirable to provide a dedicated nozzle for periodically cleaning the inner wall of the cleaning bath  30  in order to prevent contamination of the inner wall of the cleaning bath  30  due to accumulation of components of the plating solution. As an alternative, the cleaning bath  30  may be configured to be capable of being filled with pure water to perform cleaning of the inner wall. In that case, the cleaning liquid discharge line  144  may be provided with an on-off valve. Such cleaning of the interior of the cleaning bath  30  may be performed either periodically or every time the substrate W and the substrate holder  18  are cleaned in the inner shell  100 . Alternatively, cleaning of the interior of the cleaning bath  30  may be performed independently. 
       FIG. 10  is a graph showing experimental results of Example 1 which used the cleaning bath  30  and experimental results of Comparative Example 1 which used a conventional cleaning bath. In Example 1, cleaning of a substrate and a substrate holder was performed by supplying a predetermined amount of cleaning liquid into the inner shell  100  in the second state, followed by discharge of the cleaning liquid from the inner shell  100 . This cleaning cycle was repeated three times. After each cleaning cycle, a concentration of a component of a plating solution adhering to the substrate and the substrate holder was measured. In  FIG. 10 , the concentration of the component of the plating solution before cleaning is defined as 100%. In Comparative Example 1, cleaning of a substrate and a substrate holder was performed using a conventional cleaning bath (i.e., a cleaning bath which is not provided with an inner shell and which simply stores a cleaning liquid therein). More specifically, cleaning was carried out by supplying the same cleaning liquid as used in Example 1, but in an amount that is twice the amount used in Example 1, into the cleaning bath in which the substrate holder, holding the substrate, was disposed, followed by discharge of the cleaning liquid from the cleaning bath. This cleaning cycle was repeated twice. After each cleaning cycle, the concentration of the same component of the plating solution adhering to the substrate and the substrate holder was measured. The results of the measurement are shown in  FIG. 10  in terms of the relationship between the plating solution component concentration and an amount of the cleaning liquid used. 
     As can be seen in  FIG. 10 , the cleaning performance achieved by one cleaning cycle in Example 1 is low because the amount of the cleaning liquid supplied into the inner shell  100  is smaller than (one-half of) the amount of the cleaning liquid supplied into the cleaning bath in Comparative Example 1; however, the cleaning process of Example 1, which repeats the cleaning cycle three times, can achieve the same level of cleanliness as achieved by the cleaning process of Comparative Example 1 which repeats twice the cleaning cycle using the cleaning liquid in an amount which is twice that of Example 1. The total amount of the cleaning liquid used in Example 1 was ¾ of that of Comparative Example 1. 
     When the substrate holder  18  is lowered into the inner shell  100  in the first state in which the shell end plates  106 ,  108  are opened, a cleaning liquid may be supplied at a low flow rate from shower nozzles  149  shown in  FIG. 6  in order to rinse a plating solution from the surfaces of the substrate W and the substrate holder  18 . With this rinsing operation, the used cleaning liquid, containing the plating solution in a high concentration, can be discharged out of the inner shell  100  before it is transformed into the closed second state. This rinsing operation can therefore increase the cleaning efficiency while reducing the total amount of the cleaning liquid used. 
     According to this embodiment, the interior volume of the inner shell  100  can be made small. It therefore becomes possible to reduce the time required for supplying the cleaning liquid into the inner shell  100 . Because of the small gaps G1, G2 between the substrate holder  18  (and the substrate W) and the shell end plates  106 ,  108 , the cleaning liquid can flow along the substrate holder  18  and the substrate W at a high speed, thus increasing the cleaning effect of the substrate W and the substrate holder  18 . In this embodiment, in order to prevent the cleaning liquid from overflowing from the top opening of the inner shell  100 , labyrinth seals  120  are provided at the tops of the shell end plates  106 ,  108  to narrow the top opening. In addition, the supply of the cleaning liquid is stopped as soon as the inner shell  100  is filled with the cleaning liquid. 
     In order to prevent the cleaning liquid from overflowing from the top opening of the inner shell  100  when supplying the cleaning liquid, the supply of the cleaning liquid may be reduced when the surface level of the cleaning liquid in the inner shell  100  reaches a predetermined high level. By stopping the supply of the cleaning liquid as soon as the inner shell  100  has become filled with the cleaning liquid, the amount of the cleaning liquid used in one cleaning cycle can be minimized. Further, by repeating such a cleaning cycle multiple times, the total amount of the cleaning liquid used can be reduced. 
     Similarly, the use of the inner shell  100  having a small interior volume can reduce the time required for discharging the cleaning liquid from the inner shell  100 . In particular, when the shell end plates  106 ,  108  are opened, the cleaning liquid is discharged from the inner shell  100  through the gaps formed between the shell side plate  102  and the shell end plates  106 ,  108 . Since the cleaning liquid spills into the interior of the cleaning bath  30  in a moment, the discharge time of the cleaning liquid can be further reduced. 
     According to this embodiment, the cleaning liquid supply time and the cleaning liquid discharge time in one cleaning cycle can be reduced with the small interior volume of the inner shell  100 . It therefore becomes possible to increase the number of cleaning cycles without a decrease in the throughput. 
     A sequence of plating process steps performed by the above-described plating apparatus will now be described. First, one substrate is taken by the substrate transport device  22  out of the cassette  10  mounted on the cassette table  12 , and the substrate is placed on the aligner  14 , which aligns an orientation flat or a notch of the substrate in a predetermined direction. After the alignment operation, the substrate is transported to the substrate loading device  20  by the substrate transport device  22 . 
     Two substrate holders  18 , housed in the stocker  24 , are simultaneously gripped by the first transporter  42 , and transported to the substrate loading device  20 . The substrate holders  18  are lowered in a horizontal position simultaneously until the two substrate holders  18  are placed on the stage plate  52  of the substrate loading device  20 , and then two pneumatic cylinders are actuated to open the second holding members  58  of the two substrate holders  18 . 
     The substrate which has been transported by the substrate transport device  22  is inserted into the substrate holder  18  positioned on the center side, and the pneumatic cylinder is reversely actuated to close the second holding member  58 . The second holding member  58  is then locked by means of a locking/unlocking mechanism (not shown). After the substrate holder  18  is loaded with the substrate, the stage plate  52  is slid laterally, and the other substrate holder  18  is loaded with a substrate in the same manner. Thereafter, the stage plate  52  is returned to its original position. 
     The substrate is mounted to the substrate holder  18  with its front surface (to-be-plated surface) exposed in the opening of the substrate holder  18 . To prevent intrusion of the plating solution into the internal space of the substrate holder  18 , the gap between the peripheral portion of the substrate and the second holding member  58  is sealed with the substrate-side sealing member  66 , and the gap between the first holding member  54  and the second holding member  58  is sealed with the holder-side sealing member  68 . The substrate W, at a sealed portion not in contact with the plating solution, electrically connects with the electrical contacts  88 . Electric wires extending from the electrical contacts  88  are connected to the connecting terminal  91  of the substrate holder  18 . Therefore, an electric current can be supplied to e.g., a seed layer of the substrate by connecting a power source to the connecting terminal  91 . The substrate loading device  20  has a sensor for sensing a contact between the substrate W, held by the substrate holder  18 , and the electrical contacts  88 . The sensor, when it detects poor contact between the substrate W and the electrical contacts  88 , outputs a signal to a controller (not shown). 
     The two substrate holders  18 , each holding the substrate, are transported from the substrate loading device  20  to the pre-wetting bath  26  by the first transporter  42  of the substrate holder transport device  40 . The first transporter  42  lowers the substrate holders  18  to immerse the substrates, together with the substrate holders  18 , in a pre-wetting liquid (e.g., pure water) in the pre-wetting bath  26 . 
     Next, the two substrate holders  18  holding the substrates are transported from the pre-wetting bath  26  to the pre-soaking bath  28  by the first transporter  42 . In the pre-soaking bath  28 , a surface oxide film of each substrate is etched away, thereby exposing a clean metal surface. Thereafter, the substrate holders  18  holding the substrates are transported to the first cleaning bath  30   a  by the first transporter  42 . In the first cleaning bath  30   a , the substrates and the substrate holders  18  are cleaned with a cleaning liquid supplied into the first cleaning bath  30   a . Pure water or a chemical liquid can be used as the cleaning liquid. 
     The substrate holders  18 , holding the cleaned substrates, are transported from the first cleaning bath  30   a  to the plating bath  34  by the second transporter  44  of the substrate holder transport device  40 . The substrate holders  18  are lowered by the second transporter  44  into the plating cells  38 , and are suspended from the tops of the plating cells  38 . The second transporter  44  of the substrate holder transport device  40  sequentially repeats the above operation to sequentially transport substrate holders  18 , each holding a substrate, to the plating cells  38  of the plating bath  34 . 
     After setting substrates in all the plating cells  38 , plating of the surface of each substrate is carried out by applying a plating voltage between each substrate and an anode (not shown) in each plating cell  38  while reciprocating the paddle parallel to the surface of the substrate by means of the paddle drive device  46 . Each substrate holder  18  is suspended and fixed with the holder hangers  90  supported on the top of each plating cell  38 . During plating, an electric current is supplied from the plating power source to the seed layer of the substrate through the electrical conductors  86  and the electrical contacts  88 . During the plating, the plating solution overflows the plating cells  38  into the overflow bath  36 , and is returned from the overflow bath  36  to the plating cells  38  through a circulation line (not shown). The plating solution circulates at all times basically during the operation of the apparatus. The plating solution is kept at a constant temperature by means of a not-shown constant-temperature unit provided in the circulation line. 
     After the plating operation is terminated, the application of the plating voltage and the reciprocation of the paddles are stopped. The two substrate holders  18 , each loaded with the plated substrate, are transported from the plating bath  34  to the second cleaning bath  30  by the second transporter  44  of the substrate holder transport device  40 . In the second cleaning bath  30 , the substrates and the substrate holders  18  are cleaned with the cleaning liquid supplied into the inner shell  100 , as described above. The cleaning in the second cleaning bath  30  may preferably be repeated multiple times. 
     The substrate holders  18 , holding the cleaned substrates, are transported from the second cleaning bath  30  to the blow bath  32  by the second transporter  44 . In the blow bath  32 , air or nitrogen gas is ejected onto the surfaces of the substrates, held by the substrate holders  18 , to remove liquid droplets from the substrate surfaces, thereby drying the substrates. 
     The two substrate holders  18  after drying in the blow bath  32  are transported to the substrate loading device  20  by the first transporter  42 , and are placed on the stage plate  52  of the substrate loading device  20 . A substrate holder  18 , in which is housed a substrate whose contact with the electrical contacts  88  has been determined to be poor by the sensor provided in the substrate loading device  20  and which has been stored in the stocker  24 , is also transported to the substrate loading device  20  and placed on the stage plate  52 . 
     The second holding member  58  of the substrate holder  18  positioned on the center side is unlocked by means of the locking/unlocking mechanism, and the pneumatic cylinder is actuated to open the second holding member  58 . The substrate transport device  22  removes the substrate from the substrate holder  18 , and transports the substrate to the spin rinse drier  16 , where the substrate is spin-dried (drained) by high-speed rotation of the spin rinse drier  16 . The dried substrate is returned by the substrate transport device  22  to the cassette  10 . 
     After or in parallel with returning the substrate, which has been removed from the one substrate holder  18 , to the cassette  10 , the stage plate  52  is slid laterally and the other substrate is removed from the other substrate holder  18 . The substrate is then spin-dried by the spin rinse drier  16 , and the dried substrate is returned to the cassette  10  by the substrate transport device  22 . 
       FIG. 11  is a schematic view of a cleaning bath  30  according to another embodiment. The cleaning bath  30  of this embodiment differs from the embodiment shown in  FIGS. 6 through 9  in that a gas feed line  150  is coupled to the cleaning liquid supply line  140  so as to feed a gas, such as air or N 2  gas, into a cleaning liquid, such as pure water, to be supplied into the inner shell  100  in the second state. A very small amount of the gas is fed into the cleaning liquid. It is verified from experiment that the cleaning power of the cleaning liquid, to be used in the cleaning shell  100 , can be enhanced by a gas, such as air or N 2  gas, that has been fed into the cleaning liquid before it is supplied into the inner shell  100 . 
       FIG. 12  is a schematic view of a cleaning bath  30  according to yet another embodiment. The cleaning bath  30  of this embodiment differs from the embodiment shown in  FIGS. 6 through 9  in that instead of the opening and closing mechanism  124 , the cleaning bath  30  is provided with an oscillation mechanism  158  which functions also as an opening and closing mechanism for opening and closing the shell end plates  106 ,  108 . The oscillation mechanism  158  includes a pair of open-close rods  152 , and a pair of servo motors  154  which can control the positions of the open-close rods  152 . A distal end of each open-close rod  152  is rotatably coupled to the upper end of each of operation rods  156  which are secured to the upper ends of the shell end plates  106 ,  108 , respectively. 
     In this embodiment, the oscillation mechanism  158  not only opens and closes the shell end plates  106 ,  108 , but can also force the shell end plates  106 ,  108  to oscillate while they are in the closed state. By causing the shell end plates  106 ,  108  to oscillate on the order of, e.g., 0.1 mm in this manner, the interior volume of the inner shell  100  in the second state is changed and the surface level of the cleaning liquid in the inner shell  100  is moved vertically e.g., on the order of 5 mm. This oscillating operation can enhance the cleaning power of the cleaning liquid. 
       FIG. 13  is a diagram illustrating varying positions of the shell end plate  106  when it is opened, closed, and oscillating. Although not illustrated, the position of the other shell end plate  108  varies in the same way as shown in  FIG. 13 . A symbol A in  FIG. 13  shows a position of the shell end plate  106  when it is in the opened state, a symbol B shows a position of the shell end plate  106  when it is in the closed state, and a symbol C shows a position of the shell end plate  106  when it is oscillating in the closed state. A symbol V represents an amplitude of the shell end plate  106  when it is oscillating. The amplitude V is on the order of 0.1 mm, e.g., in a range of 0.1 mm to 0.2 mm. When the shell end plates  106 ,  108  are oscillating, the surface level of the cleaning liquid in the inner shell  100  fluctuates vertically within the range of about 1 mm to 5 mm, for example. 
       FIG. 14  is a graph showing a relationship between the position of the shell end plate  106  and time when the shell end plate  106  is opened, closed, and oscillating. First, the shell end plate  106  in the opened position indicated by the symbol A shown in  FIG. 13  is moved to the closed position indicated by the symbol B shown in  FIG. 13  (time t1). The inner shell  100  is filled with the cleaning liquid in the above-described manner while the shell end plate  106  (and the shell end plate  108 ) is kept in the closed state (time t1-t2). Next, the shell and plate  106  in the closed position indicated by the symbol B is moved to the position indicated by the symbol C shown in  FIG. 13  (time t2). Thereafter, the shell end plate  106  is returned to the position indicated by the symbol B (time t3). The time interval between t2 and t3 is, for example, 0.5 seconds. Next, the shell end plate  106  in the position indicated by the symbol B is again moved to the position indicated by the symbol C (time t4). The time interval between t3 and t4 is, for example, 0.5 seconds. The movement (i.e., the oscillation) of the shell end plate  106  is repeated n times, and then the shell end plate  106  in the closed position is moved to the opened position indicated by the symbol A shown in  FIG. 13  (time tn). 
       FIG. 15  is a schematic view of a cleaning bath  30  according to yet another embodiment. The cleaning bath  30  of this embodiment differs from the embodiment shown in  FIGS. 6 through 9  in that a diaphragm drive mechanism  162  is incorporated in the shell end plate  106  which is disposed on the front surface side of the substrate holder  18 . The diaphragm drive mechanism  162  includes a diaphragm  160  disposed so as to contact the cleaning liquid supplied into the inner shell  100 . Air is supplied into and discharged from a space formed inside the diaphragm  160  to thereby vibrate the diaphragm  160 . 
     In this embodiment, the cleaning liquid is supplied into the inner shell  100 , and then the diaphragm  160  is vibrated by the diaphragm drive mechanism  162 , thereby causing the surface level of the cleaning liquid to fluctuate vertically in the inner shell  100  in the range of 1 mm to 2 mm, for example. This operation can enhance the cleaning power of the cleaning liquid. 
     Instead of the diaphragm drive mechanism  162 , an ultrasonic oscillator may be used to cause the surface level of the cleaning liquid to fluctuate vertically in the inner shell  100 . 
       FIG. 16  is a schematic view of a cleaning bath  30  according to yet another embodiment. The cleaning bath  30  of this embodiment differs from the embodiment shown in  FIGS. 6 through 9  in that a syringe mechanism  164  is coupled to the cleaning liquid supply line  140 . In this embodiment, after filling the inner shell  100  with the cleaning liquid, the syringe mechanism  164  causes the surface level of the cleaning liquid to fluctuate vertically in the inner shell  100  on the order of 5 mm, for example. A pump device may be used instead of the syringe mechanism  164 . 
       FIG. 17  is a schematic view of a cleaning bath  30  according to yet another embodiment. The cleaning bath  30  of this embodiment differs from the embodiment shown in  FIGS. 6 through 9  in that the shell end plate  106  disposed at the front surface side of the substrate holder  18  has, in its interior, a cleaning liquid storing chamber  166 , and that the cleaning liquid supply pipe  142  communicates with the cleaning liquid storing chamber  166 . The shell end plate  106  has a number of through-holes  168  which are arranged so as to face the entire surface of the substrate W and which communicate with the cleaning liquid storing chamber  166 . 
     According to this embodiment, the cleaning liquid can be supplied into the inner shell  100  in the second state through the through-holes  168  provided in the shell end plate  106 , thereby selectively cleaning the surface of the substrate W in its entirety. 
     In this embodiment the cleaning liquid supply pipe  142  is coupled to the shell end plate  106  which is configured to open and close. It is therefore preferred to use a flexible tube, such as a PFA tube, as the cleaning liquid supply pipe  142 . The cleaning bath  30  of this embodiment may also be provided with a blow line that branches off from the cleaning liquid supply pipe  142 . After the cleaning liquid is discharged from the inner shell  100 , a gas (air or N 2  gas) is ejected toward the substrate W from the through-holes  168  to remove liquid droplets from the substrate holder  18  and the substrate W. In this embodiment, the cleaning bath  30  may preferably be provided with an exhaust duct for recovering the gas. 
       FIG. 18  is a schematic view of a cleaning bath  30  according to yet another embodiment. The cleaning bath  30  of this embodiment differs from the embodiment shown in  FIGS. 6 through 9  in that the shell end plate  106  disposed on the side of the front surface of the substrate holder  18  has, in its interior, a cleaning liquid storing chamber  166 , and that the cleaning liquid supply pipe  142  communicates with the cleaning liquid storing chamber  166 . The shell end plate  106  has a plurality of peripheral holes  170  at positions facing a peripheral portion of the substrate W, and further has a central hole  172  at a position facing a central portion of the substrate W. The peripheral holes  170  and the central hole  172  communicate with the cleaning liquid storing chamber  166 . 
     In this embodiment, when the cleaning liquid is supplied into the inner shell  100  in the second state, the cleaning liquid can be supplied intensively to the peripheral portion of the substrate W through the peripheral holes  170 , thereby efficiently cleaning an area along the contact portion D 1  (see  FIG. 5 ) in the peripheral area of the surface of the substrate W that contacts the substrate-side sealing member  66 . Furthermore, the cleaning liquid is supplied intensively to the central area of the substrate W through the central hole  172 , thus forming flow of the cleaning liquid in the radial direction of the substrate W on the substrate surface. Such radial flow of the cleaning liquid can clean the area along the substrate-side sealing member  66 . In the embodiment shown in  FIG. 18 , the central hole  172  may be omitted, i.e., only the peripheral holes  170  may be provided in the shell end plate  106 . 
       FIG. 19  is a schematic view of a cleaning bath  30  according to yet another embodiment. The cleaning bath  30  of this embodiment differs from the embodiment shown in  FIGS. 6 through 9  in that the cleaning bath  30  is further provided with a substrate holder moving mechanism  174  for horizontally moving the substrate holder  18 . The substrate holder moving mechanism  174  is configured to move the substrate holder  18  back and forth and/or from side to side, i.e., to cause the substrate holder  18  to oscillate horizontally. 
     According to this embodiment, after the cleaning liquid is supplied into the inner shell  100 , the substrate holder  18  is slightly moved back and forth and/or from side to side by the substrate holder moving mechanism  174 , thereby agitating the cleaning liquid in the inner shell  100 . This operation can enhance the cleaning power of the cleaning liquid. 
       FIGS. 20 through 23  shows cleaning baths  30  according to other embodiments as viewed from above. The cleaning bath  30  shown in  FIG. 20  differs from the embodiment shown in  FIGS. 6 through 9  in that the opening and closing mechanism  124  for opening and closing the shell end plates  106 ,  108  includes a pair of connection rods  174  which are movable in synchronization in directions closer to and away from each other and which are kept parallel to each other during the movement. Beside the cleaning bath  30  is disposed an opening and closing chuck  176  which is configured to move the pair of connection rods  174  in directions closer to and away from each other while keeping them parallel to each other. The shell end plates  106 ,  108  are secured to the connection rods  174 , respectively. 
     In this embodiment the shell end plates  106 ,  108  are moved parallel to each other to be opened and closed. A pair of guides  178 , extending horizontally in a direction perpendicular to the connection rods  174 , is provided in the cleaning bath  30  so that the shell end plates  106 ,  108  can move while keeping parallel to each other. 
       FIG. 21  is a schematic view of a cleaning bath  30  according to yet another embodiment. The cleaning bath  30  of this embodiment differs from the embodiment shown in  FIGS. 6 through 9  in that the opening and closing mechanism  124  for opening and closing the shell end plates  106 ,  108  includes a pair of opening and closing chucks  180  disposed at both sides of the cleaning bath  30 , and a pair of connection rods  182  extending between the opening and closing chucks  180 . The connection rods  182  are movable in synchronization in directions closer to and away from each other while they are kept parallel to each other. The shell end plates  106 ,  108  are secured to the connection rods  182 . 
     Also in this embodiment, the pair of guides  178  is provided at a predetermined position in the cleaning bath  30  so that the shell end plates  106 ,  108  can move while keeping parallel to each other. 
       FIG. 22  is a schematic view of a cleaning bath  30  according to yet another embodiment. The cleaning bath  30  of this embodiment differs from the embodiment shown in  FIGS. 6 through 9  in that the opening and closing mechanism  124  for opening and closing the shell end plates  106 ,  108  includes a pair of pneumatic cylinders  184  disposed beside the cleaning bath  30 . Piston rods  186  of the pneumatic cylinders  184  are coupled to the shell end plates  106 ,  108 , respectively. Also in this embodiment, the pair of guides  178  is provided at a predetermined position in the cleaning bath  30 . 
       FIG. 23  is a schematic view of a cleaning bath  30  according to yet another embodiment. The cleaning bath  30  of this embodiment differs from the embodiment shown in  FIGS. 6 through 9  in that the opening and closing mechanism  124  for opening and closing the shell end plates  106 ,  108  includes an opening and closing chuck  190  disposed beside the cleaning bath  30 , and a pair of connection rods  194  which, by the actuation of the opening and closing chuck  190 , pivot on an axis  192  in synchronization with each other in opposite directions. The shell end plates  106 ,  108  are secured to the pair of connection rods  194 , respectively. 
       FIGS. 24 and 25  show a cleaning bath  30  according to yet another embodiment. The inner shell  100  of this embodiment includes an open-top elastic bladder  200  (e.g., made of Viton), a pair of airbags  202  interposed between the bladder  200  and the inner surface of the cleaning bath  30 , and a projection  204  provided at a predetermined position on an inner surface of the bladder  200 . When the airbags  202  are shrunk, the inner shell  100  becomes in the first state in which the substrate holder  18 , holding the substrate W, is placed at a predetermined position in the bladder  200 , as shown in  FIG. 24 . When the airbags  202  are inflated, the inner shell  100  becomes in the second state in which the inner surface of the inner shell  100  has a configuration that follows the exterior configuration of the substrate holder  18  holding the substrate W, as shown in  FIG. 25 . When the inner shell  100  is in the second state, the projection  204  lies close to the substrate W held by the substrate holder  18 . 
     A cleaning liquid delivery pipe (or a cleaning liquid supply conduit)  206 , connected to a cleaning liquid supply line and a cleaning liquid discharge line, is provided at the bottom of the cleaning bath  30 . The cleaning liquid delivery pipe  206  communicates with the interior of the bladder  200 . 
     In operation, as shown in  FIG. 24 , when the inner shell  100  is in the first state in which the airbags  202  are shrunk, the substrate holder  18 , holding the substrate W, is lowered, without interference with the inner shell  100 , to place the substrate W at a predetermined position in the inner shell  100 . The airbags  202  are then inflated to transform the inner shell  100  into the second state in which the inner surface of the inner shell  100  has a configuration that follows the uneven exterior configuration of the substrate holder  18  holding the substrate W, as shown in  FIG. 25 . Thereafter, the cleaning liquid is supplied through the cleaning liquid delivery pipe  206  into the inner shell  100  (i.e., into the bladder  200 ) to clean the substrate W together with the substrate holder  18 . After the completion of the cleaning, the cleaning liquid is discharged out of the inner shell  100  (i.e., out of the bladder  200 ) through the cleaning liquid delivery pipe  206 . The cleaning bath  30  of this embodiment can thus reduce the amount of the cleaning liquid used in one cleaning cycle for cleaning of the substrate W and the substrate holder  18 . 
     The airbags  202  are preferably made of a corrosion-resistant material. Other types of actuators, such as pneumatic cylinders, may be used instead of the airbags  202 . 
       FIG. 26  is a schematic view of a cleaning bath  30  according to yet another embodiment. In this embodiment a substrate W is cleaned together with a substrate holder  210 , with both a first surface (a front surface) and a second surface (a back surface) of the substrate W exposed.  FIG. 27  is an enlarged view of a portion of the substrate holder  210  which is cleaned in the cleaning bath  30 . 
     As shown in  FIG. 27 , the substrate holder  210  includes a plate-like first holding member  212  and a plate-like second holding member  214 , which are made of a resin material (e.g., HTPVC) and are openable and closable relative to each other through a hinge (not shown). The first holding member  212  has an open hole  212   a , and the second holding member  214  has an open hole  214   a . The first holding member  212  and the second holding member  214 , when they are in the closed state (overlapped state), are held by a pair of openable and closable clamps  216  made of a resin material (e.g., HTPVC). 
     A seal ring  218 , extending around the open hole  212   a , is mounted to the first holding member  212  at a position facing the second holding member  214 . A seal ring  220 , extending around the open hole  214   a , is mounted to the second holding member  214  at a position facing the first holding member  212 . The seal rings  218 ,  220  are made of a rubber material (e.g., silicone rubber). An O-ring  222  is mounted on a surface, which faces the first holding member  212 , of the second holding member  214 . The O-ring  222  is arranged at the outer side of the seal ring  220 . 
     The seal rings  218 ,  220  each have a rectangular cross-section and have sealing portions  218   a ,  220   a , respectively, in their inner peripheral ends. When the first holding member  212  and the second holding member  214  are in the overlapped state with the substrate W interposed therebetween, the sealing portions  218   a ,  220   a  press on both surfaces of the substrate W, thus forming a hermetically enclosed space surrounded by the sealing portions  218   a ,  220   a  and the O-ring  222 . This hermetically enclosed space is a sealed space which does not permit intrusion of a plating solution thereinto. 
     A plurality of conductive plates  224  are provided around the open hole  212   a  of the first holding member  212 . Half of these conductive plates  224  are electrically connected via conductive pins  226  to one surface (e.g., the front surface) of the substrate W, while the other half of the conductive plates  224  are electrically connected via conductive pins  226  to the other surface (e.g., the back surface) of the substrate W. The conductive plates  224  are electrically connected to an external terminal provided in a holder hanger (not shown) of the substrate holder  210 . 
     In the substrate holder  210 , the substrate W is placed in a predetermined position on the first holding member  212  when the first holding member  212  and the second holding member  214  are in the opened state. Thereafter, the first holding member  212  and the second holding member  214  are closed through the hinge, and the pair of clamps  216  is rotated until peripheral portions of both the first holding member  212  and the second holding member  214  are inserted into a groove  216   a  of the clamps  216 . The substrate W is thus held by the first holding member  212  and the second holding member  214 . 
     When the substrate W is held by the first holding member  212  and the second holding member  214 , the space surrounded by the sealing portions  218   a ,  220   a  of the seal rings  218 ,  220  and the O-ring  222  is sealed hermetically to be in a liquid-tight state which does not permit intrusion of a plating solution thereinto. The portion of the substrate W, lying on the outer side of the sealing portions  218   a ,  220   a , lies in this sealed space, while the other portion of the substrate W, including the majority of the both surfaces, is exposed in the open holes  212   a ,  214   a.    
     Recesses with fairly large volume are formed on both the front surface side and the back surface side of the substrate holder  210  when holding the substrate W. In view of this, as shown in  FIG. 26 , the shell end plate  106 , located at the side of the front surface of the substrate holder  210 , has on its inner surface an uneven portion  106   b  that follows the uneven external configuration of the front surface of the substrate holder  210 . Further, the shell end plate  108 , located at the side of the back surface of the substrate holder  210 , has on its inner surface an uneven portion  108   b  that follows the uneven external configuration of the back surface of the substrate holder  210 . 
       FIG. 28  shows a diagram illustrating an exemplary process of cleaning the interior of the cleaning bath  30  by storing a cleaning liquid in the cleaning bath  30 . A valve  229  is provided in the cleaning liquid discharge line  144 . With the valve  229  closed, the cleaning liquid is supplied from the cleaning liquid supply line  140  into the inner shell  100 , and the cleaning liquid is further supplied through the overflow holes  106   a ,  108   a  into the cleaning bath  30 . The supply of the cleaning liquid is continued until the liquid level in the cleaning bath  30  reaches a predetermined value H2. The inner shell  100  is preferably kept in the closed state (in the first state) during the cleaning of the cleaning bath  30  so that dirt, dispersed from the inner surface of the cleaning bath  30 , will not enter the inner shell  100 . 
     The cleaning liquid can be quickly discharged from the inner shell  100  by transforming the shell end plates  106 ,  108  from the second state (the closed state) to the first state (the opened state). This is desirable in the light of reduced processing time. However, because of the quick discharge of the cleaning liquid, the cleaning liquid may remain as droplets on the substrate W or the substrate holder  18  and may dilute a processing liquid in the next processing step. Thus, when raising the substrate holder  18  from the inner shell  100  after the completion of cleaning, it is desirable to eject air or N 2  gas from blow nozzles  230  onto the substrate holder  18  so as to force liquid droplets out of the substrate W and the substrate holder  18 , as shown in  FIG. 29 . The supply of air or N 2  gas from the blow nozzles  230  is not to dry the substrate W and the substrate holder  18 , but to reduce the amount of liquid droplets adhering to them. Therefore, the jet of air or N 2  gas is preferably at a low flow rate so that the gas does not scatter around. 
       FIG. 30  shows a process sequence of a cleaning process according to another embodiment, which is intended to address the problem of an increased amount of cleaning liquid droplets adhering to the substrate W or the substrate holder  18  after cleaning. In this cleaning process shown in  FIG. 30 , a cleaning cycle, involving supply and discharge of a cleaning liquid into and from the inner shell  100 , is repeated several times. The cleaning liquid is slowly discharged from the inner shell  100  only in the last cleaning cycle so that the surface level of the cleaning liquid in the inner shell  100  is lowered gradually. This operation can prevent the cleaning liquid from remaining as droplets on the substrate W or the substrate holder  18 . In particular, the shell end plates  106 ,  108  are opened not quickly but slowly so that the sealing members  110  on the shell side plate  102  are separated gradually from the upper portion of the shell end plate  106  or the shell end plate  108 . For this purpose, it may be necessary to use, as an opening and closing mechanism for the shell end plates  106 ,  108 , not an actuator using an pneumatic cylinder but an opening and closing mechanism having a speed control function, e.g., using a servo motor as shown in  FIG. 12 . 
     Although the embodiments have been described above, it should be understood that the present invention is not limited to the above embodiments, but various changes and modifications may be made to the embodiments without departing from the scope of the appended claims.