Abstract:
There is disclosed an electrostatic attracting method in which a direct-current voltage is applied to an electrode disposed at a table formed of a dielectric material to attract/hold a substrate onto a holding surface of the table with an electrostatic force produced thereby, the method comprising a first step of applying a voltage having a predetermined polarity to the electrode to charge the holding surface with an electric charge having a polarity different from that applied to the electrode, a second step of holding the substrate in contact with the holding surface to prevent electric charges charged on the holding surface from disappearing, and a third step of applying a voltage having a polarity different from that applied in the first step to the electrode in a state in which the substrate contacts the holding surface to produce an electric charge having the same polarity as that charged at the holding surface in the first step on the holding surface of the table, and attracting/holding the substrate with the electric charge together with the electric charge charged on the holding surface in the first step.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2002-352710, filed Dec. 4, 2002; and No. 2003-176608, filed Jun. 20, 2003, the entire contents of both of which are incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to an electrostatic attracting method of attracting/holding a substrate onto a table by an electrostatic force, an electrostatic attracting apparatus, and a bonding apparatus in which the electrostatic attracting apparatus is used to bond two substrates to each other.  
           [0004]    2. Description of the Related Art  
           [0005]    In a manufacturing process of a flat display panel represented by a liquid crystal display panel, a bonding operation is carried out in which two substrates are disposed opposite to each other at a predetermined interval, and a liquid crystal as a fluid is sealed between the substrates to bond the substrates together by a sealing agent.  
           [0006]    In the bonding operation, either of two substrates is coated with the sealing agent in a frame shape, and a predetermined amount of the liquid crystal is dropped/supplied to the substrate or to a portion of the other substrate corresponding to a portion in a frame of the sealing agent.  
           [0007]    Next, the two substrates are held by upper and lower holding tables, and are disposed opposite to each other at a predetermined interval in a vertical direction. In this state, the substrates are positioned in X, Y, and θ directions which are horizontal directions of these substrates, and one substrate is driven downwards to bond the substrates to each other.  
           [0008]    The substrates are bonded to each other in a reduced-pressure chamber. After bonding the substrates, a gas is introduced into the chamber to raise the pressure. Then, one pair of substrates bonded to each other can be pressurized with a load sufficiently larger than a bonding load by a difference pressure between a pressure between the pair of bonded substrates and a pressure in the chamber.  
           [0009]    When two substrates are bonded to each other in the chamber, the substrates are attracted to the pair of tables in vacuum. Then, the pressure in the chamber is reduced, and accordingly a holding force of the substrates is lost. Therefore, the substrate held by the upper holding table drops, or the substrates sometimes shift on the tables, when two substrates are brought in contact with each other and positioned.  
           [0010]    To solve the problem, an electrode is disposed at least on the upper holding table in one pair of holding tables, and a direct-current voltage is applied to the electrode. Accordingly, since an electrostatic force is generated in a holding surface of the holding table, the substrate is attracted/held by the electrostatic force. Such technique is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2000-66163.  
           [0011]    When the substrate is attracted/held onto the holding table by the electrostatic force, the holding force depends on a magnitude of the direct-current voltage applied to the electrode. As described above, when one pair of substrates of a liquid crystal display panel are bonded to each other, a high voltage exceeding ten kilovolts is sometimes required to be applied as the direct-current voltage to the electrode in order to increase the electrostatic force so that the substrates held on the holding tables are prevented from shifting.  
           [0012]    However, when the high voltage exceeding ten kilovolts is applied to the electrode, dielectric breakdown occurs by discharge on the holding tables and in the chamber, the holding tables are sometimes damaged, or an electric charge is insufficiently accumulated into the electrode and a large electrostatic force cannot sometimes be produced on the holding tables.  
           [0013]    An object of the present invention is to provide an electrostatic attracting method in which an electrostatic force produced on a table can be increased without applying a high voltage to an electrode, and a substrate can securely be held; an electrostatic attracting apparatus, and a bonding apparatus.  
         BRIEF SUMMARY OF THE INVENTION  
         [0014]    According to the present invention, there is provided an electrostatic attracting method in which a direct-current voltage is applied to an electrode disposed at a table formed of a dielectric material to attract/hold a substrate onto a holding surface of the table with an electrostatic force produced thereby, the method comprising:  
           [0015]    a first step of applying a voltage having a predetermined polarity to the electrode to charge the holding surface with an electric charge having a polarity different from that applied to the electrode;  
           [0016]    a second step of holding the substrate on the holding surface; and  
           [0017]    a third step of applying a voltage having a polarity different from that applied in the first step to the electrode in a state in which the substrate is held on the holding surface to produce an electric charge having the same polarity as that charged on the holding surface in the first step at the holding surface of the table, and attracting/holding the substrate with the electric charge together with the electric charge charged on the holding surface in the first step.  
           [0018]    According to the present invention, there is provided an electrostatic attracting apparatus which attracts/holds a substrate, comprising:  
           [0019]    a table formed by a dielectric material and including a holding surface which holds the substrate by an electrostatic force;  
           [0020]    an electrode disposed at the table;  
           [0021]    a direct-current power supply which applies a direct-current voltage onto the electrode; and  
           [0022]    switch means for switching a polarity of a direct-current voltage applied to the electrode by the direct-current power supply,  
           [0023]    wherein after applying the direct-current voltage having a predetermined polarity to the electrode, the substrate is held on the holding surface, the switch means is operated to apply a direct-current voltage having a different polarity to the electrode.  
           [0024]    According to the present invention, there is provided a bonding apparatus in which a fluid is disposed between two substrates to bond these substrates to each other by a sealing agent, the apparatus comprising:  
           [0025]    a chamber in which a pressure of an inner space can be reduced;  
           [0026]    a pair of tables which are disposed opposite to each other in the chamber and which include holding surfaces to hold the substrates on the surfaces disposed opposite to each other and at least one of which is formed of a dielectric material;  
           [0027]    an electrode disposed at the table formed of the dielectric material;  
           [0028]    a direct-current power supply which applies a direct-current voltage to the electrode and produces an electrostatic force to hold the substrate on the holding surface of the table;  
           [0029]    switch means for switching a polarity of the direct-current voltage applied to the electrode by the direct-current power supply; and  
           [0030]    driving means for driving the pair of tables with respect to each other in vertical and horizontal directions and positioning the substrates held on the holding surfaces of the pair of tables in the horizontal direction to bond the substrates to each other,  
           [0031]    wherein after applying the direct-current voltage having a predetermined polarity to the electrode, the substrate is held on the holding surface, the switch means is operated to apply a direct-current voltage having a different polarity to the electrode.  
           [0032]    According to the present invention, after charging the holding surface of the table with the electric charge having the predetermined polarity, the electric charge is prevented from disappearing, and the electric charge having the same polarity is further supplied. Therefore, an electric charge amount supplied onto the holding surface increases, and the electrostatic force produced on the holding surface accordingly increases.  
           [0033]    Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0034]    The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention.  
         [0035]    [0035]FIG. 1 is a schematic constitution diagram of a bonding apparatus according to one embodiment of the present invention;  
         [0036]    [0036]FIG. 2 is a control circuit diagram for applying a direct-current voltage having a predetermined polarity to an electrode;  
         [0037]    [0037]FIG. 3 is an explanatory view of a first step of charging a holding surface of an upper holding table with an electric charge having the predetermined polarity;  
         [0038]    [0038]FIG. 4 is an explanatory view of a second step of holding the substrate on the holding surface charged with the electric charge having the predetermined polarity; and  
         [0039]    [0039]FIG. 5 is an explanatory view of a third step of switching a polarity of the direct-current voltage applied to the electrode to a reverse polarity. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0040]    One embodiment of the present invention will hereinafter be described with reference to the drawings.  
         [0041]    [0041]FIG. 1 shows a bonding apparatus according to one embodiment of the present invention. An electrostatic attracting apparatus of the present invention is used in this bonding apparatus. That is, the bonding apparatus includes a chamber  1 . The chamber  1  is airtightly connectably divided into a lower chamber  2  and an upper chamber  3 .  
         [0042]    A lower holding table  4  is disposed in the lower chamber  2  in such a manner that the table can be driven by a first driving source  5  in X, Y, and θ directions, that is, in the horizontal direction. For first and second substrates  7  and  8  formed of glass or resin to constitute liquid crystal display panels, the first substrate  7  is supplied/laid on a holding surface  4   a  which is an upper surface of the lower holding table  4 .  
         [0043]    An elastic sheet (not shown), for example, having a predetermined friction resistance is disposed on the holding surface  4   a . The first substrate  7  is held so as to be immobile in the horizontal direction on the holding surface  4   a  by contact resistance with the elastic sheet. The upper surface of the first substrate  7  held by the holding surface  4   a  is coated with a sealing agent  9  in a rectangular frame shape, and droplet-shaped liquid crystals  10 , which are fluids, are dropped/supplied in a matrix form in the frame.  
         [0044]    The lower chamber  2  is connected to a first vacuum pump  12  which is pressure reducing means. A pressure of an inner space of the chamber  1  constituted of the chambers is reduced by the first vacuum pump  12  in a state in which the upper chamber  3  is connected to the lower chamber  2  in an airtight manner as described later. The reduced pressure in the chamber  1  is measured with a pressure gauge  13  connected to the lower chamber  2 . The lower chamber  2  is further connected to a gas supply tube  14  for supplying a gas into the chamber  1  after bonding two substrates  7 ,  8  to each other. An open/close control valve (not shown) is disposed in the gas supply tube  14 .  
         [0045]    An attached member  15  is inserted through an upper wall of the upper chamber  3  in the airtight manner. A movable shaft  16  is movably inserted through the attached member  15 , an upper holding table  18  is disposed on a projected lower end of the movable shaft  16  in the upper chamber  3 .  
         [0046]    The upper holding table  18  is formed in a plate shape having a predetermined thickness by dielectric materials such as polyimide or ceramic, and a first electrode  19  and a second electrode  20  forming one pair are buried in a middle portion in a thickness direction. A direct-current voltage is applied to these electrodes  19 ,  20  as described later, and the second substrate  8  is attracted/held onto a holding surface  18   a  which is a lower surface of the upper holding table  18  with an electrostatic force produced by the voltage.  
         [0047]    It is to be noted that one end of a suction hole  21  is opened/formed in a holding surface  18   a  of the upper holding table  18 . A second vacuum pump  22  and a pressurized gas supply source (not shown) are switchably connected to the other end of the suction hole  21 . When the second vacuum pump  22  operates, a suction force can be produced in the suction hole  21 . Therefore, the second substrate  8  can be attracted/held on the holding surface  18   a  of the upper holding table  18  by a vacuum attracting force.  
         [0048]    Moreover, when the pump is changed to the pressurized gas supply source, and a gas is spouted from the suction hole  21  constituting an open hole, the attracted/held substrate  8  can be detached from the holding surface  18   a  by the pressure of the spouted gas.  
         [0049]    A second driving source  23 , capable of driving the movable shaft  16  in a vertical direction corresponding to a Z direction and vertically driving only the upper holding table  18  separately from the upper chamber  3 , is disposed on an upper end of the attached member  15  projecting from a top surface of the upper chamber  3 . The second driving source  23  is driven together with the upper chamber  3  by a third driving source  24  in the Z direction. When the upper chamber  3  is driven in a descending direction by the third driving source  24 , a lower end surface of the chamber abuts on an upper end surface of the lower chamber  2  on which a sealing material  25  is disposed in the airtight manner. Accordingly, the inside of the chamber  1  is closed in the airtight manner.  
         [0050]    The upper chamber  3  is connected to an ionizer  26  which is charged particle supply means. The ionizer  26  is capable of spraying the gas containing electrically charged particles toward the holding surface  18   a  of the upper holding table  18 .  
         [0051]    As shown in FIG. 2, the first electrode  19  and the second electrode  20  are connected to a first direct-current power supply  28  and a second direct-current power supply  29  disposed in a power supply section  27  via a switch device  31  and an earth device  32 . The switch device  31  includes a first polarity changing section  33  and a second polarity changing section  34 .  
         [0052]    As shown in FIG. 3, the polarity changing sections  33 ,  34  include first and second changeover switches  33   a ,  33   b ,  34   a ,  34   b . When these changeover switches are operated, the polarity of the direct-current voltage applied to the first and second electrodes  19 ,  20  is reversed.  
         [0053]    The earth device  32  includes a first earth changing section  37  connected to the first polarity changing section  33 , and a second earth changing section  38  connected to the second polarity changing section  34 . The first earth changing section  37  is connected to the first electrode  19  disposed on the upper holding table  18  via a changeover switch  37   a , and the second earth changing section  38  is connected to the second electrode  20  via a changeover switch  38   a.    
         [0054]    The respective changeover switches of the first and second polarity changing sections  33 ,  34  and first and second earth changing sections  37 ,  38  can be operated by a control device  39  which is control means. That is, when the first and second switches  33   a ,  33   b ,  34   a ,  34   b  of the first and second polarity changing sections  33 ,  34  are in a state shown in FIG. 3, an anode voltage of the first direct-current power supply  28  is applied to the first electrode  19  via the first polarity changing section  33  and the first earth changing section  37 . Similarly, a cathode voltage of the second direct-current power supply  29  is applied to the second electrode  20  via the second polarity changing section  34  and the second earth changing section  38 .  
         [0055]    When the first and second switches  33   a ,  33   b ,  34   a ,  34   b  of the first polarity changing section  33  and second polarity changing section  34  are operated by the control device  39  as shown in FIG. 5, the cathode voltage of the second direct-current power supply  29  is applied to the first electrode  19 , and the anode voltage of the first direct-current power supply  28  is applied to the second electrode  20 .  
         [0056]    The changeover switches  37   a  and  38   a  of the first and second earth changing sections  37  and  38  of the earth device  32  are changed/operated in a state shown by a chain line from a state shown by a solid line in FIG. 2 by the control device  39 , the first and second electrodes  19  and  20  are earthed. Accordingly, the charges stored in these electrodes  19 ,  20  are released.  
         [0057]    Next, a procedure for bonding the first and second substrates  7  and  8  together by the bonding apparatus constituted as described above will be described with reference to FIGS.  3  to  5 .  
         [0058]    First, in a first step, the first substrate  7  which is coated with the sealing agent  9  and on which the liquid crystal  10  is dropped is supplied/laid on the lower holding table  4  in a state in which the lower chamber  2  is disposed apart from the upper chamber  3 . Then, as shown in FIG. 3, the first changeover switches  33   a ,  34   a  of the first and second polarity changing sections  33 ,  34  of the switch device  31  are turned on, and the second changeover switches  33   b ,  34   b  are maintained in an off state. Accordingly, the anode voltage of the first direct-current power supply  28  is applied to the first electrode  19  disposed at the upper holding table  18 , and the cathode voltage of the second direct-current power supply  29  is applied to the second electrode  20 .  
         [0059]    The upper holding table  18  at which the first and second electrodes  19  and  20  are provided is disposed in the atmosphere. Therefore, when the voltages having predetermined polarities are applied to the first and second electrodes  19 ,  20 , the charged particles are generated by a polarization phenomenon or a momentary ionization phenomenon of gas components existing in the atmosphere. The holding surface  18   a  of the upper holding table  18  is charged with the charged particles, or a moisture in the atmosphere is polarized and charged.  
         [0060]    Concretely, as shown in FIG. 3, a portion of the holding surface  18   a  of the upper holding table  18  corresponding to the first electrode  19  is charged with a cathode charge −E 1  having a polarity reverse to that of the direct-current voltage applied to the first electrode  19 . A portion of the holding surface  18   a  corresponding to the second electrode  20  is charged with an anode charge +E 1  having a polarity reverse to that of the direct-current voltage applied to the second electrode  20 . At this time, as shown in FIG. 3, cathode and anode are polarized in a portion between the first and second electrodes  19  and  20  and the holding surface  18   a  of the upper holding table  18 .  
         [0061]    It is to be noted that to charge the upper holding table  18 , the lower and upper chambers  2  and  3  are closed, and the inside of the chamber  1  is preferably brought in a pressure atmosphere indicating 10 Pa or more including a pressure which is not less than an atmospheric pressure. With a pressure of 10 Pa or more, it is possible to generate the charged particles by the polarization phenomenon or the momentary ionization phenomenon of the gas components existing in the pressure atmosphere and to satisfactorily charge the holding surface  18   a  with the electric charges. It is to be noted that it is possible to charge the holding surface  18   a  with the electric charge even with a pressure of 10 Pa or less in the chamber  1 . When the pressure is not less than the atmospheric pressure, the ionization phenomenon occurs by the application of the voltage, and it is therefore possible to charge the holding surface.  
         [0062]    When the pressure inside the chamber  1  is set to 10 Pa or more, the pressure is varied in a pressure range of 10 Pa or more. Then, the pressure atmosphere in which discharge easily occurs can surely be produced, and the charged particles can efficiently be generated by the discharge.  
         [0063]    The upper chamber  3  is connected to the ionizer  26 . Therefore, even when the gas containing the charged particles is spouted toward the upper holding table  18  from the ionizer  26 , the holding surface  18   a  can efficiently electrically be charged.  
         [0064]    Moreover, even when the inside of the chamber  1  is brought into the atmosphere containing steam or oxygen, a satisfactory polarization or ionization phenomenon can be obtained. When the inside of the chamber  1  has the atmosphere containing steam or oxygen, instead of the ionizer  26 , a steam generation device or an oxygen supply device may be used to positively supply steam or oxygen to the chamber  1 .  
         [0065]    When the upper holding table  18  is electrically charged as described above, next in a second step, the second substrate  8  is held onto the holding surface  18   a  of the upper holding table  18  by vacuum attraction as shown in FIG. 4. Accordingly, the electric charges on the holding surface  18   a  are prevented from being lost.  
         [0066]    That is, when discharge occurs between the holding surface  18   a  and a conductor around the chamber  1  or the like, the electric charges of the holding surface  18   a  are lost due to the discharge. However, when the substrate  8  constituted of an insulating material is held on the holding surface  18   a , no discharge occurs between the holding surface  18   a  and the conductor around the chamber  1  or the like, and the electric charges can be prevented from being lost.  
         [0067]    Even when the second substrate  8  is held on the holding surface  18   a , the polarities of the electric charges −E 1 , +E 1  on the holding surface  18   a  are different from those of the electric charges positioned on a holding surface  18   a  side among the electric charges polarized between the holding surface  18   a  and the respective electrodes  19 ,  20 , and these electric charges different from one another in polarity attract one another. Therefore, the suction force by the electric charge is hardly generated in the holding surface  18   a . Therefore, a holding force of the second substrate  8  by the electric charges on the holding surface  18   a  is remarkably small. Therefore, the second substrate  8  is held on the holding surface  18   a  by a vacuum suction force by the second vacuum pump  22 .  
         [0068]    It is to be noted that the first step of electrically charging the upper holding table  18  is carried out under the pressure atmosphere of 10 Pa or more while the chamber  1  is hermetically closed. In this case, when the second substrate  8  is supplied into the chamber  1  beforehand, the substrate can be held on the upper holding table  18  without opening the chamber  1  in a second step. In this case, supply holding means for holding the second substrate  8  supplied beforehand into the chamber  1  in the chamber  1  and for supplying/holding the second substrate  8  onto the holding surface  18   a  after electrically charging the holding surface  18   a  needs to be disposed in the chamber  1 .  
         [0069]    The supply holding means may also be used to hold the second substrate  8  in contact with the holding surface  18   a  instead of attracting the second substrate  8  onto the holding surface  18   a  in vacuum.  
         [0070]    When the second substrate  8  is supplied from the outside of the chamber  1  in the second step, the chamber  1  is opened, and the substrate may be supplied onto the holding surface  18   a  of the upper holding table  18  by supply means such as a robot.  
         [0071]    When the second substrate  8  is supplied to the holding surface  18   a  of the upper holding table  18 , the anode voltage and the cathode voltage may continuously be applied to the first and second electrodes  19  and  20 , or may be cut off. A voltage value in continuing to apply the direct-current voltage may be the same as or different from that in electrically charging the holding surface. In short, the polarity applied to each electrode has not to be changed.  
         [0072]    When the second substrate  8  is held on the holding surface  18   a , the changeover switches  37   a ,  38   a  of the first and second earth changing sections  37 ,  38  of the earth device  32  are changed to the state shown by the chain line from that shown by the solid line in FIG. 2 by the control device  39 , and the first and second electrodes  19 ,  20  are earthed for a predetermined time, respectively. An earth step is carried out in this manner. Accordingly, as shown in FIG. 3, the electric charges of the anode accumulated in the first electrode  19  and those of the cathode accumulated in the second electrode  20  are discharged. Therefore, a state in which the electric charges are not accumulated in the electrodes  19 ,  20  is achieved.  
         [0073]    When the earths of the respective electrodes  19 ,  20  are cut off, a third step shown in FIG. 5 is carried out. In the third step, the first changeover switches  33   a ,  34   a  of the first and second polarity changing sections  33 ,  34  of the switch device  31  are turned off, and the second changeover switches  33   b ,  34   b  are turned on. Accordingly, the cathode voltage of the second direct-current power supply  29  is applied to the first electrode  19 , and the anode voltage of the first direct-current power supply  28  is applied to the second electrode  20 . That is, in the third step, the polarities of the direct-current voltages applied to the first and second electrodes  19  and  20  are reversed.  
         [0074]    When the polarities of the direct-current voltages applied to the respective electrodes  19 ,  20  are changed, and the voltage of the cathode is applied to the first electrode  19 , the polarization occurs in the portion between the first electrode  19  and the holding surface  18   a  of the upper holding table  18  to form an anode at the lower surface of the first electrode  19  and a cathode at the holding surface  18   a . The polarity of charge −E 2  generated on the holding surface  18   a  side by the polarization is negative as that of the charge −E 1  held beforehand on the holding surface  18   a.    
         [0075]    On the other hand, dielectric polarization occurs on the second substrate  8  when voltages are applied to the first and second electrodes  19  and  20 . For the portion of the second substrate  8  corresponding to the first electrode  19 , the polarization occurs to form an anode at the upper surface facing the holding surface  18   a , and a cathode at the lower surface. Therefore, the portion of the second substrate  8  corresponding to the first electrode  19  is attracted/held by functions of both the cathode charges −E 1 , −E 2 .  
         [0076]    Similarly, when the voltage of the anode is applied to the second electrode  20 , the polarization occurs in the portion between the second electrode  20  and the holding surface  18   a  of the upper holding table  18  to form a cathode at the second electrode  20  and an anode at the holding surface  18   a . The polarity of charge +E 2  generated at the holding surface  18   a  by the polarization is positive as that of the charge +E 1  held beforehand on the portion of the holding surface  18   a  corresponding to the second electrode  20 .  
         [0077]    On the other hand, the dielectric polarization occurs on the second substrate  8  when voltages are applied to the first and second electrodes  19  and  20 . For the portion of the second substrate  8  corresponding to the second electrode  20 , the polarization occurs to form a cathode at the upper surface facing the holding surface  18   a , and an anode at the lower surface. Therefore, the portion of the second substrate  8  corresponding to the second electrode  20  is attracted/held by functions of both the anode charges +E 1 , +E 2 .  
         [0078]    That is, as compared with a case where the second substrate  8  is simply held on the holding surface  18   a  and the direct-current voltage is applied, the second substrate  8  is attracted/held on the holding surface  18   a  with a strong holding force because the charges −E 1 , +E 1  are held beforehand on the holding surface  18   a  in the first step. In other words, even when the direct-current voltages applied to the first and second electrodes  19  and  20  are not raised, a large electrostatic force can be produced on the holding surface  18   a.    
         [0079]    When the polarities of the direct-current voltages applied to the respective electrodes  19 ,  20  are changed in the third step, the pressure in the chamber  1  is preferably set to 80 kPa or less before the changing. When the pressure in the chamber  1  is set to 80 kPa or less, and the polarities applied to the respective electrodes  19 ,  20  are changed, the discharge can be prevented from occurring even via a gap between the holding surface  18   a  and the second substrate  8 , if any, at the time of the changing. Accordingly, the charges −E 1 , +E 1  supplied to the holding surface  18   a  in the first step and held by the second substrate  8  in the second step can be prevented from being alleviated by the discharge.  
         [0080]    It is to be noted that when slight alleviation of the charge on the holding surface  18   a  is permitted, the polarities of the direct-current voltages applied to the respective electrodes  19 ,  20  may be changed in the atmosphere with a pressure of 80 kPa or more. After the second substrate  8  is held by the third step, the vacuum attraction by the second vacuum pump  22  may be stopped.  
         [0081]    In the third step, after the electric charges accumulated in the electrodes  19 ,  20  in the first step, are discharged the voltage having the polarity different from that of the first step is applied. Therefore, the electric charges having polarities different from those charged in the first step can efficiently and quickly be accumulated in the respective electrodes  19 ,  20 .  
         [0082]    When the second substrate  8  is held on the holding surface  18   a  of the upper holding table  18  in this manner, the first substrate  7  held by the lower holding table  4  is driven in the horizontal direction by the first driving source  5  and positioned with the second substrate  8  in the chamber  1  brought in a predetermined reduced pressure atmosphere. Next, the upper holding table  18  is driven in the descending direction by the second driving source  23 , and the second substrate  8  is bonded to the first substrate  7  via the sealing agent  9  at a predetermined load.  
         [0083]    When the bonding ends, the first changeover switches  33   b ,  34   b  are turned off, and the changeover switches  37   a ,  38   b  of the first and second earth changing sections  37 ,  38  are changed into the state shown by the chain line of FIG. 2. Moreover, the gas is introduced into the chamber  1  via the gas supply tube  14 , and the pressure in the chamber  1  is raised. Accordingly, the first and second substrates  7  and  8  bonded together are pressurized with a pressure larger than that at the time of the bonding, by the difference pressure between the pressures between one pair of substrates  7 ,  8  and in the chamber  1  and therefore an interval between the substrates  7 ,  8  is set to a predetermined value.  
         [0084]    Thereafter, the suction hole  21  is connected to the pressurized gas supply source, and the gas is spouted from the suction hole  21 . The upper holding table  18  is then raised. Accordingly, the second substrate  8  is detached from the holding surface  18   a.    
         [0085]    When the gas is spouted from the suction hole  21 , the second substrate  8  is detached from the holding surface  18   a . Therefore, even when the electric charges remain in the holding surface  18   a  after turning off the switches  33   b ,  34   b , the second substrate  8  can easily and securely be detached from the holding surface  18   a . Thereafter, when the chamber  1  is opened, and the bonded first and second substrates  7  and  8  are taken out, the bonding ends.  
         [0086]    In accordance with the above-described embodiment, the following function/effect is obtained.  
         [0087]    In the first step, the direct-current voltages having the predetermined polarities are applied to the first and second electrodes  19  and  20  to charge the holding surface  18   a  of the upper holding table  18  with the electric charges −E 1 , +E 1 . In the second step, the second substrate  8  is vacuum-attracted and held on the holding surface  18   a . Next, in the third step, the polarity of the direct-current voltage applied to the first and second electrodes  19  and  20  is changed to the polarity reverse to that of the first step.  
         [0088]    Therefore, as compared with a case where the second substrate  8  is simply held on the holding surface  18   a  and the direct-current voltages having the predetermined polarities are applied to the first and second electrodes  19  and  20 , the second substrate  8  can securely be attracted/held on the holding surface  18   a  with a strong holding force because the holding surface  18   a  of the upper holding table  18  is charged beforehand with the charges −E 1 , +E 1  in the first step.  
         [0089]    Therefore, when the present invention is applied to the bonding apparatus as described above, the second substrate  8  held by the holding surface  18   a  can be prevented from dropping, the bonding operation can therefore be prevented from being discontinued by the dropping of the second substrate  8  from the holding surface  18   a , and an operation ratio of the bonding apparatus can be raised.  
         [0090]    Moreover, since the second substrate  8  held on the holding surface  18   a  can be prevented from dropping, the first and second substrates  7 ,  8  can be prevented from being damaged or becoming dirty by the dropping of the second substrate  8  from the holding surface  18   a , thus the yield in the bonding operation can be enhanced.  
         [0091]    Furthermore, the second substrate  8  can be prevented from shifting on the holding surface  18   a  at the time of the bonding operation of two substrates  7 ,  8  by the first driving source  5  or by the second driving source  23 . Accordingly, a positional deviation between two substrates  7 ,  8  can be prevented from being caused by the shifting of the second substrate  8  on the holding surface  18   a . A quality of a liquid crystal display panel manufactured by bonding two substrates  7 ,  8  together can therefore be enhanced.  
         [0092]    Before removing the bonded first and second substrates  7  and  8  from the chamber  1 , that is, when the bonding ends and the second substrate  8  is detached from the upper holding table  18 , the direct-current voltages having the predetermined polarities are applied to the first and second electrodes  19  and  20  of the upper holding table  18  as described above, and the first step of charging the holding surface  18   a  with the electric charges may be started. Accordingly, a tact time required for the bonding can be reduced.  
         [0093]    It is to be noted that the reduction of the tact time is not limited to a case where the first step is started at the time of the detaching of one pair of bonded substrates from the upper holding table  18 . The first step may be started between when the bonded substrates are detached from the table  18  and when the next first substrate  7  is supplied to the lower holding table  4 . That is, when the first step is carried out in parallel with another operation such as a delivery operation of the bonded substrates, the tact time can be reduced as compared with a case where the first is carried out after supplying the substrate  7  to the lower holding table  4 .  
         [0094]    The present invention is not limited to the above-described embodiment, and may also be applied, for example, to a constitution in which the electrode is also disposed on the lower holding table and the substrate is held by the electrode with the electrostatic force in the same manner as in the upper holding table.  
         [0095]    One pair of electrodes are disposed on the upper holding table, but a plurality of pairs of electrodes may also be disposed, or only one electrode may also be disposed. When the electrode is also disposed on the lower holding table, at least one electrode may be disposed in the same manner as in the upper holding table.  
         [0096]    In the first and third steps, the polarities of the direct-current voltages applied to the first and second electrodes may also be changed automatically by the control device, or may manually be changed without using any control device.  
         [0097]    After the second step, the earth step of grounding the first and second electrodes for the predetermined time is carried out, but the third step may also be carried out after the second step without performing the earth step.  
         [0098]    In the second step, the second substrate is held on the holding surface of the upper holding table by the vacuum attraction. However, when it is possible to hold the second substrate with the suction force by the electric charge generated on the holding surface, the second substrate may also be held on the holding surface of the upper holding table only with the suction force.  
         [0099]    The chamber  1  is divided into lower and upper chambers, but a box-shaped chamber including an outlet/inlet port of the substrate in side walls may also be used.  
         [0100]    Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.