Patent Publication Number: US-2006005920-A1

Title: Method for fabricating bonded substrate

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is a divisional of application Ser. No. 10/453,654, filed Jun. 4, 2003, now pending.  
      This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2002-170007 filed on Jun. 11, 2002 and No. 2003-059075 filed on Mar. 5, 2003, and U.S. patent application Ser. No. 10/453,654, filed Jun. 4, 2003, the contents being incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      The present invention relates to a method and an apparatus for fabricating a bonded substrate, and, more particularly, to a method and an apparatus for fabricating a panel, such as a liquid crystal display (LCD), which is provided by bonding two substrates at a predetermined gap therebetween.  
      Recently, there are demands for apparatuses which manufacture large and thin flat display panels, such as liquid crystal display (LCD) panels with a high productivity and at a low cost. An LCD panel is fabricated by arranging two glass substrates to face each other at an extremely narrow gap (several micrometers) and filling a liquid crystal between the two glass substrates. The two glass substrates are, for example, an array substrate on which a plurality of TFTs (Thin Film Transistors) are formed in a matrix form and a color filter substrate on which color filters (red, green and blue) and a light shielding film are formed. The light shielding film contributes to improving contrast and shields light toward the TFTs to prevent generation of an optical leak current. The array substrate is bonded to the color filter substrate by a seal (adhesive) containing a thermosetting resin.  
      A conventional method of fabricating an LCD panel includes a liquid crystal sealing step of sealing a liquid crystal between two glass substrates. The conventional liquid crystal sealing step is carried out by the following vacuum injection method. First, the TFTs-formed array substrate is bonded to the color filter substrate (opposing substrate) via a seal. The seal is cured. An inlet port is formed in the seal. The bonded substrates and a liquid crystal are placed in a vacuum tank. While the inlet port is immersed in the liquid crystal, the pressure in the tank is set back to atmospheric pressure. This causes the liquid crystal to be sucked from the inlet port. Finally, the inlet port of the seal is sealed.  
      Recently, attention has been paid to the following dropping method instead of the vacuum injection method. First, the frame of a seal is formed in such a way as to enclose the outer periphery of the array substrate. A predetermined dose of a liquid crystal is dropped on the surface of the array substrate within the frame of the seal. Finally, the array substrate is bonded to the color filter substrate in vacuum. The dropping method can reduce the amount of a liquid crystal in use significantly and can shorten the time needed for the liquid crystal sealing step, thus resulting in a reduction in panel fabrication cost. It is therefore expected that mass production will be improved.  
      However, a bonded substrate fabricating apparatus which operate according to the dropping method has the following problems.  
      1. Improper Bonding  
      An LCD panel is manufactured by bonding two substrates at a predetermined gap (cell gap) therebetween. To set the cell gap to a predetermined value, such as about 5 micrometers, the two substrates should be held in parallel to each other accurately.  
      There is a case where the bonded substrates are deformed in the process of bonding the two substrates together in a vacuum process chamber in vacuum, setting the pressure in the vacuum process chamber back to atmospheric pressure and curing the seal. This is caused because the force of pressing the substrates in the direction to bond them together does not work outside the seal where atmospheric pressure works, whereas the force of bonding the substrates together works inside the seal where the liquid crystal is sealed. As the substrates are deformed, the cell gap becomes uneven, resulting in improper bonding.  
      As a solution to this shortcoming, Japanese Laid-open Patent Publication No. Hei 11-326922 discloses an outer seal so provided outside the seal as to surround that seal. Keeping the space between the inner seal and the outer seal in vacuum allows the cell gap to be stable even after both seals are cured.  
      The factors for making the cell gap uneven are the deformation of the substrates and variations in the thicknesses of the substrates and the seal. Due to the variations in the thicknesses of the substrates and the seal, in a case where the substrates are bonded without being held in parallel to each other, the outer seal cannot keep the space between the substrates at a high airtightness. This also leads to improper bonding.  
      2. Influence on Substrates When Bonded  
      The two substrates are bonded in a vacuum process chamber while being respectively held by two holding plates that have a vacuum chuck mechanism or an electrostatic chuck mechanism. In vacuum chuck, the bottom surfaces of the substrates are sucked by the chuck surfaces of the holding plates coupled to a vacuum pump. In electrostatic chuck, a voltage is applied between an electrode formed on each holding plate and a conductive film formed on the associated substrate, generating force according to Coulomb&#39;s law between the glass of the substrate and the electrode, which allows the substrate to be chucked on the holding plate. Because the vacuum chuck does not work as the degree of vacuum in the vacuum process chamber becomes high, the substrates are held by electrostatic chuck, not vacuum chuck, under a high vacuum state.  
      Substrates are bonded as follows. The two substrates are held by two holding plates facing each other. A seal is provided on one substrate. The pressure in the vacuum process chamber is reduced. Both holding plates are placed close to each other until the cell gap reaches a predetermined value, thus causing both substrates to firmly contact the seal.  
      If the substrates are not kept in parallel to each other, the substrates may be damaged. Specifically, spacers (spherical spacers, columnar spacers or the like) are provided on one substrate to adjust the cell gap to a predetermined value, so that if both substrates are bonded not in parallel to each other, high pressure is locally applied to the substrates, thus damaging the substrates.  
      3. Deformation of Vacuum Process Chamber and Reduction in Substrate Position Precision  
      As the pressure in the vacuum process chamber is reduced, the difference between the inner pressure of the vacuum process chamber and the outer pressure (atmospheric pressure) slightly deforms the vacuum process chamber. Therefore, the relative positions of both holding plates slightly differ between when the pressure in the vacuum process chamber is reduced and when the pressure in the vacuum process chamber is not reduced. The positional deviation of the holding plates lowers the accuracy of the bonding position of the substrates. If the outer wall of the vacuum process chamber is made thicker to suppress the deformation of the vacuum process chamber, the vacuum process chamber becomes larger which is not desirable.  
     SUMMARY OF THE INVENTION  
      In one aspect of the present invention, a bonded substrate fabricating apparatus for bonding a first substrate and a second substrate together is provided. The apparatus includes a depressurizable process chamber. A first holding plate is disposed in the process chamber for holding the first substrate, and a second holding plate is disposed facing the first holding plate in the process chamber for holding the second substrate. A pressing mechanism drives the first holding plate to press the first and second substrates. The second holding plate is slid and rotated within a horizontal plane by a drive mechanism. Resilient members are disposed between the process chamber and the pressing mechanism and between the process chamber and the drive mechanism.  
      In a further aspect of the present invention, a method of fabricating a bonded substrate from first and second substrates includes the steps of forming a frame of a seal on a surface of the first substrate, disposing the first and second substrates into a process chamber, depressurizing the process chamber, moving at least one of the first and second substrates in such a way that the first and second substrates approach each other, computing a pressing load acting on the first and second substrates, stopping movement of the at least one of the first and second substrates when the computed pressing load reaches a target load, and setting a pressure in the process chamber back to atmospheric pressure.  
      Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which:  
       FIG. 1  is a block diagram of a substrate bonding apparatus according to a first embodiment of the present invention;  
       FIG. 2  is a schematic front view of a press machine;  
       FIG. 3  is a block diagram of a press control unit;  
       FIG. 4  shows an example of connection between the press control unit and load cells;  
       FIGS. 5 and 6  show examples of the layout of the load cells;  
       FIG. 7  is a diagram for explaining the position of a CCD camera;  
       FIG. 8  is a plan view of a substrate to which a seal and a liquid crystal are applied;  
       FIGS. 9A and 9B  are cross-sectional views of substrates in a process of being bonded;  
       FIGS. 10A and 10B  are respectively a plan view and a cross-sectional view of one substrate to which an outer seal is applied;  
       FIGS. 11A and 11B  are respectively a plan view and a cross-sectional view showing another example of one substrate to which an outer seal is applied;  
       FIG. 12  is an enlarged view of an outer seal applied to a corner of a substrate;  
       FIG. 13  is a graph showing the gap between substrates and the pressing load;  
       FIGS. 14 and 15  are flowcharts for a substrate bonding method; and  
       FIG. 16  shows a schematic front view of a press machine according to a second embodiment of the present invention;  
       FIGS. 17A and 17B  are respectively a bottom view and a side view showing a pressure plate of the press machine of  FIG. 16 ;  
       FIGS. 18A, 18B  and  18 C are cross-sectional views of a pressure plate and a table performing bonding of substrates; and  
       FIG. 19  shows a modification of the press machine. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      A bonded substrate fabricating apparatus  11  according to a first embodiment of the present invention will be described below.  
      The bonded substrate fabricating apparatus  11  fabricates a liquid crystal display by placing liquid crystal between a first substrate W 1  and a second substrate W 2  and then bonding the substrates W 1  and W 2 . The liquid crystal display is, for example, an active matrix type liquid crystal display panel. The first substrate W 1  is an array substrate (TFT substrate) of glass which has an array of TFTs. The second substrate W 2  is a color filter (CF) substrate which has color filters and a light shielding film. The substrates W 1  and W 2  are fabricated separately and are supplied to the bonded substrate fabricating apparatus  11 .  
      As shown in  FIG. 1 , the bonded substrate fabricating apparatus  11  includes a main control unit  12 , a seal patterning system  13 , a liquid crystal dropping device  14 , a bonding device  15  and an inspection device  16 . The bonding device  15  includes a press machine  17  and a curing device  18 . The main control unit  12  controls the seal patterning system  13 , the liquid crystal dropping device  14 , the bonding device  15  (the press machine  17  and curing device  18 ) and the inspection device  16 .  
      The bonded substrate fabricating apparatus  11  includes a first transfer equipment  19   a , a second transfer equipment  19   b , a third transfer equipment  19   c , and a fourth transfer equipment  19   d , which transfer the first substrate W 1  and second substrate W 2 . The main control unit  12  controls the transfer equipments  19   a  to  19   d  to transfer the first substrate W 1  and second substrate W 2  and a bonded substrate.  
      The seal patterning system  13  applies a seal at predetermined locations on the top surface of one of the substrates W 1  and W 2  (the first substrate W 1  (array substrate) in the first embodiment) along the periphery, thereby forming the frame of the seal. The seal includes preferably an adhesive, such as a photo-curing adhesive. The first transfer equipment  19   a  transfers the substrates W 1  and W 2  as a set to the liquid dropping device  14  from the seal patterning system  13 .  
      The liquid dropping device  14  drops liquid crystal at plural predetermined locations in the frame of the seal on the top surface of the first substrate W 1 . After the dropping, the substrates W 1  and W 2  are transferred to the press machine  17  by the second transfer equipment  19   b.    
      The press machine  17  has a vacuum process chamber  32  ( FIG. 2 ). The substrates W 1  and W 2  are chucked and held by a lower chuck and an upper chuck, respectively. The press machine  17  evacuates the vacuum process chamber  32  and feeds a preprocess gas to the vacuum process chamber  32 . The preprocess gas is a substitutional gas including a reactive gas, such as an exciting gas for a plasma display panel (PDP), a nitrogen gas, an inactive gas, or dean dry air. In the preprocess, impurities and products which are adhered to the surfaces of the substrates W 1  and W 2  or the surfaces of display elements are exposed to the preprocess gas for a given time. The preprocess stably maintains the property of the bonded surfaces which cannot be unsealed after bonding. In general, an oxide layer is formed on the surfaces of the substrates W 1  and W 2  and airborne materials in the air are adhered to the surfaces. This may change the states of the surfaces of the substrates W 1  and W 2 . As the degree of a change in the surface state varies between the substrates W 1  and W 2 , the qualities of the panels differ from one panel to another. In this respect, changes in the surfaces of the substrates W 1  and W 2  are suppressed by performing the preprocess which suppresses the formation of an oxide layer and the adhesion of impurities and processes the adhered impurities.  
      While optically detecting an alignment mark, the press machine  17  aligns the first substrate W 1  with the second substrate W 2  in such a way that the seal and liquid crystal on the first substrate W 1  do not contact the bottom surface of the second substrate W 2 . The press machine  17  presses the substrates W 1  and W 2  with a predetermined load. After pressing, the press machine  17  releases the vacuum process chamber  32  to set the pressure in the vacuum process chamber  32  to atmospheric pressure. The difference between atmospheric pressure and the pressure in the space between the substrates W 1  and W 2  compresses both substrates W 1  and W 2  to a predetermined cell gap.  
      While monitoring the time passed from the point when the substrates W 1  and W 2  were transferred to the vacuum process chamber  32 , the main control unit  12  controls the elapsed time from the point of transfer to the point of bonding in such a way that the substrates W 1  and W 2  are exposed to the gas supplied to the vacuum process chamber  32  over a predetermined time. This stabilizes the bonded surfaces of the substrates W 1  and W 2  and allows the bonded surfaces to have a predetermined property.  
      The third transfer equipment  19   c  removes the bonded substrates W 1  and W 2  (liquid crystal panel) from the press machine  17  and transfers it to the curing device  18 . When the time elapsed from the point at which the liquid crystal panel was pressed reaches a given time, the main control unit  12  drives the third transfer equipment  19   c  to supply the liquid crystal panel to the curing device  18 .  
      The liquid crystal that has been sealed in the LCD panel spreads between the substrates W 1  and W 2  due to the load from being pressed and atmospheric pressure.  
      It is necessary to cure the seal before the liquid crystal reaches the frame of the seal. Therefore, the curing device  18  irradiates light having a predetermined wavelength on the LCD panel to cure the seal after a predetermined time passes after pressing. The predetermined time is acquired beforehand through experiments from the spreading time of the liquid crystal and the time needed to release the press stress remaining on the substrates W 1  and W 2 .  
      The press stress remains on the bonded substrates W 1  and W 2 . Because the seal is not cured while the substrates W 1  and W 2  are transferred to the curing device  18 , the press stress is released from the substrates W 1  and W 2 . The stress hardly remains on the substrates W 1  and W 2  when the seal is cured. This reduces the occurrence of positional deviation of the bonded substrates W 1  and W 2  after the seal is cured.  
      After the seal is cured, the fourth transfer equipment  19   d  transfers the bonded substrates W 1  and W 2  (LCD panel) from the curing device  18  to the inspection device  16 . The inspection device  16  inspects for positional deviation of the first substrate W 1  and the second substrate W 2  and supplies the inspection result to the main control unit  12 . Based on the inspection result, the main control unit  12  calibrates the alignment of substrates to be pressed next. That is, the positional deviation of an LCD panel to be manufactured thereafter is prevented by shifting beforehand both substrates W 1  and W 2  of the cured-seal in the LCD panel in a direction opposite to the direction of the positional deviation by the amount of the deviation.  
      The press machine  17  which presses the substrates W 1  and W 2  will be discussed below.  
      As shown in  FIG. 2 , the press machine  17  includes a rigid base plate  21  and a rigid gate  22  fixed to the base plate  21 . The base plate  21  and the gate  22  are formed of materials having a high rigidity. Attached to the two supports of the gate  22  are guide rails  23   a  and  23   b  which guide the movements of linear guides  24   a  and  24   b . First and second support plates  25  and  26  are put between the linear guides  24   a  and  24   b . The first support plate  25  is suspended from a support arm  28  which is moved up and down by a pressure motor  27  attached to the upper portion of the gate  22 .  
      A ball screw  29  is coupled to the output shaft of the pressure motor  27  in such a way as to be rotatable together. A nut  30  provided on the support arm  28  is threaded onto the ball screw  29 . The support arm  28  moves up or down in accordance with the rotational direction (forward or reverse) of the output shaft of the pressure motor  27 .  
      The support arm  28  is formed by a top plate  28   a , a bottom plate  28   b  parallel to the top plate  28   a  and a coupling plate  28   c  which couples the top plate  28   a  to the bottom plate  28   b . A plurality of load cells  31  are mounted on the bottom plate  28   b  and abut on the bottom surface of the first support plate  25 .  
      The vacuum process chamber  32  is defined by an upper container  32   a  and a lower container  32   b  which are separable. A first holding plate or a pressure plate  33   a  is provided in the upper container  32   a . A second holding plate or a table  33   b  is provided in the lower container  32   b . The pressure plate  33   a  faces the upper surface of the table  33   b . The pressure plate  33   a  holds the second substrate W 2  (CF substrate) and the table  33   b  holds the first substrate W 1  (TFT substrate).  
      The pressure plate  33   a  is suspended from the second support plate  26  via four suspension rods  34 . Specifically, the second support plate  26  has plural through holes (e.g., four in the first embodiment) where the respective suspension rods  34  are inserted. The upper end of each suspension rod  34  is widened so that the suspension rod  34  does not come off. The pressure plate  33   a  is coupled to the lower ends of the suspension rods  34 .  
      Each suspension rod  34  is covered with an upper bellows  35  as a resilient member. The upper bellows  35  has flange portions at both ends. Both flange portions are coupled to the second support plate  26  and the upper container  32   a  via O-rings as sealing members. The upper bellows  35  is connected in an airtight manner to the vacuum process chamber  32 . The upper container  32   a  is suspended from the second support plate  26  by the upper bellows  35 .  
      The table  33   b  is secured to a positioning stage  36  via plural (four) legs  37 . The positioning stage  36  is fixed to the base plate  21 . The positioning stage  36  has a slide mechanism which moves the table  33   b  horizontally and a rotary mechanism which rotates the table  33   b  within a horizontal plane.  
      The positioning stage  36  is connected to the lower container  32   b  via plural (four) lower bellows  38 . The lower bellows  38  surround the respective legs  37  and are communicated in an airtight manner with the vacuum process chamber  32 . Each lower bellows  38  has flange portions at both ends. Both flange portions are coupled to the positioning stage  36  and the lower container  32   b  via O-rings as sealing members. A plurality of support rods  39  fixed to the base plate  21  are attached to the bottom of the lower container  32   b . Therefore, the lower container  32   b  is supported on the positioning stage  36  via the lower bellows  38  and is also supported on the base plate  21  via the support rods  39 .  
      A level adjuster  40  is provided between the upper end of each suspension rod  34  and the second support plate  26 . The level adjuster  40  includes, for example, a screw and a nut formed on the associated suspension rod  34 , and moves the suspension rod  34  up or down as it is turned. The level adjuster  40  adjusts the pressure plate  33   a  horizontally. It is preferable that the pressure plate  33   a  relative to the table  33   b  be adjusted to 50 micrometers or less deviation from parallel to one another.  
      As the pressure motor  27  is driven, the support arm  28 , the first support plate  25  and the linear guides  24   a  and  24   b  move up or down along the guide rails  23   a  and  23   b  and the second support plate  26 , the upper bellows  35  and the upper container  32   a  move up or down. Therefore, the pressure motor  27  moves the upper container  32   a  closer to or away from the lower container  32   b . When the upper container  32   a  comes in contact with the lower container  32   b , the vacuum process chamber  32  is closed. As the pressure motor  27  is driven further, the pressure plate  33   a  alone moves downward via the second support plate  26  and the suspension rods  34 . The upper bellows  35  are compressed, causing the substrates W 2  and W 1  to be pressed by the pressure plate  33   a  and the table  33   b . The substrates W 2  and W 1  are bonded in this manner.  
      Each load cell  31  measures the load applied from the first support plate  25  at the time of pressing the substrates W 2  and W 1  and informs a press control unit  41  of the measured value. The press control unit  41  sums the four measured values to calculate the total load that acts on the four load cells  31 . When the substrates W 1  and W 2  are not pressed, the total load is the total (A+B) of the weight “A” of the various members supported on the support arm  28  (the first support plate  25 , the linear guides  24   a  and  24   b , the second support plate  26 , the suspension rods  34 , the level adjusters  40 , the pressure plate  33   a  and the substrate W 2 ) and a load “B” which acts on the pressure plate  33   a  via the suspension rods  34  and is based on the difference between the pressure in the vacuum process chamber  32  and atmospheric pressure. The load B is proportional to the thickness (cross-sectional area) of the suspension rod  34 .  
      When the vacuum process chamber  32  is depressurized (evacuated), the load B of about 1 kg/cm 2  is applied to the pressure plate  33   a  via the suspension rods  34 . The load B is applied to the four load cells  31  via the second support plate  26 , the linear guides  24   a  and  24   b  and the first support plate  25 . Therefore, the four load cells  31  detect the total of the weight A and the load B together.  
      At the time the substrates W 1  and W 2  are bonded, the total load (A+B) is reduced by reaction force D of the substrates W 1  and W 2 . Therefore, the actual pressing load applied to the substrates W 1  and W 2  is calculated from changes in the measured values from the four load cells  31 .  
      The resolution of each load cell  31  is about 0.05%. According to the present embodiment, therefore, the total load is detected with the resolution of about 1 kg in a case where a total load of 2000 kg acts on each load cell  31 .  
      The press control unit  41  computes the pressing load applied to the substrates W 1  and W 2  based on electric measurement signals each representing the measured value from the associated load cell  31 . The press control unit  41  supplies a motor drive signal to a motor driver  42  while monitoring the pressing load. The motor driver  42  generates a predetermined number of pulse signals in accordance with the motor drive signal and sends the pulse signals to the pressure motor  27 . The pressure motor  27  is driven in response to the pulse signals. When the pressure motor  27  receives one pulse signal, the support arm  28  or the pressure plate  33   a  is moved up or down by, for example, 0.2 micrometer.  
      The linear guides  24   a  and  24   b  are respectively provided with linear scales  43   a  and  43   b  for detecting the position of the pressure plate  33   a . The linear scales  43   a  and  43   b  detect the relative position (distance) between the table  33   b  and the pressure plate  33   a  based on the detected positions of the linear guides  24   a  and  24   b  and output the results (positional data) to a display unit  44 .  
      The display unit  44  is connected to a reference level sensor  45  provided on the pressure plate  33   a . The display unit  44  stores the target position of the pressure plate  33   a  beforehand. The target position is the position of the pressure plate  33   a  when the pressure plate  33   a  is separated from the table  33   b  by the distance that is equal to the sum of the thicknesses of both substrates W 1  and W 2  and the target cell gap. The display unit  44  calculates the relative position of the pressure plate  33   a  with respect to the target position from the target position and the computation results from the linear scales  43   a  and  43   b.    
      The press control unit  41  determines whether the gap between the substrates W 1  and W 2  being bonded and the pressing load are adequate or not while monitoring the position of the pressure plate  33   a  based on the relative position. When the relationship between the pressing load and the substrate gap is found to be beyond a predetermined allowable range based on the adequate relationship range between the pressing load and the substrate gap which has been acquired beforehand through experiments, the press control unit  41  determines that a bonding abnormality has occurred and stops the pressing process.  
      Referring to  FIG. 3 . the other control mechanisms of the press machine  17  will be elaborated below. Like or same reference numerals are used to indicate those structural portions which are the same as those explained above in connection with  FIG. 2  and their detailed description will be partly omitted.  
      The press control unit  41  generates the motor drive signal based on the total load from the four load cells  31 , and sends the motor drive signal to the motor driver  42 . The motor driver  42  sends the generated pulse signals to the pressure motor  27  in response to the motor drive signal, causing the pressure motor  27  to rotate in the direction to move the pressure plate  33   a  up or down.  
      The press machine  17  includes CCD cameras  50  which detect image of alignment marks formed on both substrates W 1  and W 2 . At the time the substrates W 1  and W 2  are bonded, the CCD cameras  50  sense the alignment marks on the substrates W 1  and W 2  and output image data thereof to an image processing unit  47 . The press control unit  41  generates a stage drive signal for driving a positioning motor  48  in accordance with the calculation result (calculated data of the amount of positional deviation) from the image processing unit  47  and sends the stage drive signal to a motor driver  49 . The motor driver  49  sends a predetermined number of pulse signals, generated in accordance with the stage drive signal, to the positioning motor  48 . As the positioning motor  48  is driven, the positioning stage  36  and the table  33   b  are moved. Both substrates W 1  and W 2  are aligned in this manner.  
      Instead of directly supplying the measured value from each load cell  31  to the press control unit  41 , the measured value from each load cell  31  may be supplied to an arithmetic operation unit  51  ( FIG. 3 ) which adds the measured values from the individual load cells  31 . Alternatively, as shown in  FIG. 4 , an adder  51   a  may be connected between the four load cells  31  (load cells a to d) and the press control unit  41 . The adder  51   a  informs the press control unit  41  of the total load of the measured values from the load cells  31 . Based on the total load, the press control unit  41  determines whether or not to drive the pressure motor  27  and generates the motor drive signal as needed. In this case, the press control unit  41  does not require a computation based on the measured values from the load cells  31  and can thus avoid a response delay so that the pressure motor  27  is driven accurately with high response.  
      The layout of the load cells  31  will be discussed next.  
       FIG. 5  shows the positions of the load cells  31  (black marks) that are projected on the pressure plate  33   a  and the positions of the suspension rods  34  (white marks). The four suspension rods  34  are provided at equal distances from the center C of the pressure plate  33   a . The four load cells  31  are provided at equal distances from the center C of the pressure plate  33   a  and on diagonal lines that connect the suspension rods  34 . Therefore, the load cells  31  are symmetrical about the XZ plane that passes through the center C of the pressure plate  33   a  and are also symmetrical about the YZ plane that passes through the center C of the pressure plate  33   a . It is most desirable that the projected positions of the load cells  31  are in the vicinity of the projected positions of the suspension rods  34 .  
      The weight A is evenly distributed to the four load cells  31 . Even when the vacuum process chamber  32  is depressurized, the load B that acts on the four suspension rods  34  is evenly distributed among the four load cells  31 . During bonding, the pressure plate  33   a  is kept horizontal with high precision. In a case where the pressure plate  33   a  is tilted due to entry of foreign matter or a mechanical deviation that occurred during bonding, the inclination can be checked with high precision from the sum of the measured values or loads of the load cells  31 .  
      As shown in  FIG. 6 , the load cells  31  may be laid out concentrically and symmetrical with respect to the center C of the pressure plate  33   a.    
      In a case where an odd number of load cells  31  are used, it is preferable that one load cell should be arranged at the center C of the pressure plate  33   a  ( FIGS. 5 and 6 ).  
      Pressure control using image pickup means will be discussed below.  
      As shown in  FIG. 7 , the press machine  17  has a device which monitors the pressing load, i.e., the CCD camera  50 . In this embodiment, the CCD camera  50  is shared with the CCD cameras  50  (see  FIG. 3 ) that are used to sense the alignment marks of the substrates W 1  and W 2  for alignment of the substrates W 1  and W 2 .  
      The CCD camera  50  is located above the upper container  32   a  and an illumination unit  52  is located under the lower container  32   b . The CCD camera  50  picks up the image of the peripheral portions of the substrates W 1  and W 2 , particularly, a seal  55  which is pressed at the time of bonding the substrates W 1  and W 2 , through inspection windows  53   a  and  53   b  respectively provided in the upper container  32   a  and the lower container  32   b . Based on image data of the seal  55  sensed by the CCD camera  50 , the width of the seal  55  is measured and is used as an index representing the degree of flattening of the seal  55 . Accordingly, the estimated value for the pressing load is acquired. Based on the estimated value, it is determined whether or not the pressing load to be applied to both substrates W 1  and W 2  is adequate. The relationship between the flattened width of the seal  55  and the pressing load has been acquired beforehand through experiments in accordance with the sizes of the substrates W 1  and W 2  and the type or the like of a liquid crystal  54  or the seal  55 , and the adequate value for the pressing load is determined based on this relationship.  
      The CCD camera  50  is one of the four CCD cameras  50  which respectively sense the seal  55  at the four comers of the substrates W 1  and W 2 . As the four CCD cameras  50  monitor the degree of flattening of the seal  55  at four locations, it is possible to accurately detect if the frame of the seal  55  is firmly attached to both substrates W 1  and W 2  evenly. It is therefore possible to detect the degree of parallelization of the pressure plate  33   a  and the table  33   b  from the degree of flattening of the seal  55 .  
      By monitoring the degree of flattening of the seal  55 , the timing for curing the seal  55  by irradiation ultraviolet rays on the seal  55  after bonding the substrates W 1  and W 2  can be set to the proper timing. Immediately after bonding, the liquid crystal  54  has not yet diffused entirely between the substrates W 1  and W 2  and the cell gap between both substrates W 1  and W 2  has not reached a predetermined value (target gap). The timing at which ultraviolet rays are to be irradiated on the seal  55  is determined in accordance with the diffusion speed of the liquid crystal  54 . If the irradiation of the ultraviolet rays is early, the seal  55  is cured before the gap between both substrates W 1  and W 2  reaches the predetermined cell gap. If the irradiation of the ultraviolet rays is late, on the other hand, the liquid crystal  54  contacts the uncured seal  55 , which leads to display defects of the peripheral portion of the panel. The optimal illumination timing for the ultraviolet rays is determined from the degree of flattening of the seal  55  that is monitored by the CCD cameras  50  so that the seal  55  can be cured with the proper timing.  
      After the substrates W 1  and W 2  are bonded, the pressure plate  33   a  releases the electrostatic chuck force with respect to the substrate W 2  and separates the substrate W 2 . At this time, the CCD cameras  50  may monitor the shape of the seal  55 . In this case, the positional deviation of the substrates W 1  and W 2  is prevented from occurring due to the electrostatic chuck force remaining on the pressure plate  33   a  and the substrate W 2 .  
      A description will now be given of press control at the time of bonding the substrates W 1  and W 2 .  
      As shown in  FIG. 8 , the seal  55  is applied in the form of a frame to one of the substrates W 1  and W 2  (the substrate W 1  in this embodiment). The liquid crystal  54  is dropped at plural locations in the frame of the seal  55  by the amount of, for example, 5 mg each. Then, as shown in  FIGS. 9A and 9B , the substrates W 1  and W 2  are pressed to have a predetermined cell gap which is restricted by spacers  56  formed on the substrate W 1 .  
      As shown in  FIG. 9A , the liquid crystal  54  is dropped in such a way that the liquid crystal  54  becomes higher than the height of the seal  55 . Therefore, alignment of the substrates W 1  and W 2  during bonding is carried out in such a way that the substrate W 2  contacts only the liquid crystal  54  and does not contact the seal  55 . Specifically, the pressing load when the substrate W 2  would contact only the liquid crystal  54  has been acquired empirically beforehand and the downward movement of the pressure plate  33   a  is stopped when the pressing load computed from the measured values from the load cells  31  reaches the empirically acquired pressing load. At this time, it is preferable that the CCD cameras  50  monitor the contact of the substrate W 2  with the seal  55 . With the substrate W 2  in contact with only the liquid crystal  54 , the alignment of the substrates W 1  and W 2  is executed while the alignment marks of the substrates W 1  and W 2  are being sensed by the CCD cameras  50 . Thereafter, the substrates W 1  and W 2  are pressed until nearly the entire surface of the seal  55  is compressed, after which the vacuum process chamber  32  is released. As a result, the substrates W 1  and W 2  are compressed to the predetermined cell gap that it is restricted to by the spacers  56 .  
      If the substrates W 1  and W 2  are aligned while the substrates W 1  and W 2  are in contact with the seal  55  as shown in  FIG. 9B , shearing force acts on the seal  55 . When the vacuum process chamber  32  is released, the shearing force that is acting on the seal  55  is released, thereby causing positional deviation of the substrates W 1  and W 2 . In this embodiment, the positional deviation of the substrates W 1  and W 2  is prevented during a period from the point of bonding of the substrates to the point at which the seal  55  is cured, by aligning the substrates W 1  and W 2  without causing the substrate W 2  to contact the seal  55 .  
      As the load when the substrate W 2  contacts only the liquid crystal  54  is detected, it is possible to detect the position of the pressure plate  33   a  when the substrate W 2  does not contact the seal  55  and when the gap between the substrates W 1  and W 2  is minimized. Alignment in this state can allow the substrates W 1  and W 2  to be bonded together accurately and can prevent the positional deviation of the substrates W 1  and W 2  after bonding.  
      As shown in  FIG. 10A , the frame of an outer seal  61  which surrounds the seal  55  may be formed on the substrate W 1 . When the substrate W 1  has two cells (the number of panels to be formed is two), two inner seals  55  that define the areas for the liquid crystal  54  to be sealed in the two cells are formed on the substrate W 1 . The outer seal  61  is applied to the substrate W 1  in an annular form in such a way as to enclose the two inner seals  55 . The application position of the outer seal  61  is set at an unnecessary portion outside the inner seals  55 . It is preferable that the height and width of the outer seal  61  should be greater than those of the inner seals  55  as shown in  FIG. 10B .  
      The alignment of the substrates W 1  and W 2  is preferably carried out when the substrate W 2  comes in contact with only the outer seal  61 . This prevents the substrates W 1  and W 2  from being damaged during bonding by the influences of the thickness distribution of the substrates W 1  and W 2  and the bending of the substrate W 2 . That is, in a case where positional deviation of the substrates W 1  and W 2  has occurred or parallelism is lost at the time of bonding, such an abnormality can be detected when the substrate gap is larger (when the pressing force is lower) by detecting the load by using the outer seal  61 . It is therefore possible to stably bond the substrates W 1  and W 2 . As the outer seal  61  has an effect of forming a vacuum area between the inner and outer seals  55  and  61 , it is possible to suppress the positional deviation of the substrates W 1  and W 2  even at the time of curing the seals  55  after bonding the substrates, thereby securing a stable cell gap.  
      If the inner seals  55  are set high, the size of the product increases or the seals  55  may not be flattened to the predetermined cell gap by atmospheric pressure. There is a possibility that the seals  55  are not compressed to the predetermined cell gap due to the pressure of the liquid crystal  54  even after the liquid crystal  54  is diffused. It is therefore preferable to use the outer seal  61  without making the inner seals  55  higher.  
      There may be a case where the inner seals  55  reach a film which does not pass light (the peripheral portion or the like of a black matrix) and which is formed on the substrate W 2 . In this case, the degree of flattening of the outer seal  61  may be monitored by the CCD cameras  50 . As the outer seal  61  is larger than the inner seal  55 , the load at the time of bonding is detected accurately.  
      In a case where there is a certain degree of distance between adjoining cells on the substrate W 1  having a plurality of cells, a plurality of outer seals  62  and  63  may be applied outside plural inner seals  55  which are respectively provided in association with the plural cells, as shown in  FIGS. 11A and 11B .  
      As shown in  FIG. 12 , four outer seals  71  may be applied to outside the inner seals  55  and at the four corners of the substrate W 1 .  
      A description will be given of the gap between the substrates W 1  and W 2  and the pressing load.  
      The pressing load on the substrates W 1  and W 2  should be set to the optimal value in consideration of the gap between the substrates W 1  and W 2 . This is because if the pressing load is too high (the amount of downward movement of the pressure plate  33   a  is large), the substrates W 1  and W 2  may be damaged, whereas if the pressing load is too low (the amount of downward movement of the pressure plate  33   a  is small), the substrates W 1  and W 2  are not compressed to the predetermined cell gap after the vacuum process chamber  32  is released. Before performing substrate bonding, therefore, the correlation between the pressing load on the substrates W 1  and W 2  and the gap between the substrates should be acquired beforehand through experiments.  FIG. 13  is a graph showing the results of the experiments. The horizontal scale represents the substrate gap and the vertical scale represents the pressing load. The pressing load before the liquid crystal  54  starts being flattened is 0 kg. As the liquid crystal  54  and the inner seals  55  are compressed, the pressing load rises. When the substrate gap approximately reaches the target size (5 micrometers), the substrate W 2  contacts the spacers  56  and the pressing load rises abruptly. If the substrates W 1  and W 2  are pressed further, the substrates W 1  and W 2  and the pressure plate  33   a  will be damaged. To bond the substrates W 1  and W 2  without producing bubbles and damages, the substrates W 1  and W 2  should preferably be bonded within the range where the pressing load rises gently (nearly linearly).  
      The pressing load when approximately the entire surface of the seal  55  is compressed while being in contact with the substrate W 2  is preferably acquired empirically. In this embodiment, the pressing load becomes 100 kg when the substrate gap is about 15 micrometers. When the load cells  31  detect the pressing load, the downward movement of the pressure plate  33   a  is stopped, thus stopping pressing of the substrates W 1  and W 2 .  
      It is preferable that the pressing load be increased stepwise in consideration of the positional deviation and the inclination of the substrates W 1  and W 2 . When the pressing load detected by the load cells  31  is lower than the target pressing load of 100 kg (e.g., when the pressing load reaches 20 kg or 50 kg), for example, the downward movement of the pressure plate  33   a  is stopped temporarily to check the pressing load again.  
      The pressing load of 20 kg is the load when the substrate gap is about 50 to 30 micrometers which is slightly larger than the initial height of the seal  55  and at which the substrate W 2  contacts only the liquid crystal  54 . The pressing load of 50 kg is the load immediately before the substrate W 2  contacts the seal  55 , i.e., the load when the substrate gap is about 30 to 15 micrometers. The substrate gap is acquired from the pressing load (20 kg, 50 kg) based on the graph in  FIG. 13 .  
      In a case where the pressing load rapidly increases or the difference among the measured values from the plural load cells  31  becomes large (e.g., in a case where the maximum difference among the measured values reaches about 10%) when the pressing load reaches 20 kg or 50 kg, pressing of the substrates W 1  and W 2  is stopped. In a case where no abnormality has occurred during pressing, on the other hand, the pressure plate  33   a  is lowered until the pressing load reaches the target value (100 kg). After pressing of the substrates W 1  and W 2  is stopped, the vacuum process chamber  32  is released. The substrates W 1  and W 2  are compressed to the target cell gap by atmospheric pressure.  
      In a case where both substrates W 1  and W 2  have a size of 650 mm×830 mm and the inner seals  55  are formed 10 mm inside the edge of the associated substrate, the substrates W 1  and W 2  are pressed by the load of about 5100 kg, which is caused by atmospheric pressure. By way of contrast, the pressing load before the vacuum process chamber  32  is released is about 100 kg. Even if a load is locally applied to the substrates W 1  and W 2  at the time of carrying out pressing under a reduced pressure, therefore, the substrates W 1  and W 2  are not largely influenced.  
      By referring to  FIGS. 14 and 15 , a method of bonding the substrates W 1  and W 2  will be discussed.  
      In step S 81 , the substrates W 2  and W 1  are respectively held on the pressure plate  33   a  and the table  33   b . The press control unit  41  drives the pressure motor  27  to lower the upper container  32   a  to dose the vacuum process chamber  32  and depressurize the vacuum process chamber  32 .  
      In step S 82 , the press control unit  41  moves the pressure plate  33   a  downward to cause the substrates W 1  and W 2  to further approach each other.  
      In step S 83 , the press control unit  41  calculates the pressing load based on the measured values from the load cells  31 . When the calculated pressing load reaches 20 kg, the press control unit  41  stops lowering the pressure plate  33   a . The press control unit  41  monitors the degree of flattening of the seal  55  based on picked-up data from the CCD cameras  50 .  
      In step S 84 , the press control unit  41  calculates the pressing load again based on the measured values from the load cells  31  and checks if the difference between the pressing load and 20 kg lies within a predetermined range. When the difference is greater than the predetermined range (NO in step S 84 ), the press control unit  41  stops lowering the pressure plate  33   a  and stops pressing the substrates W 1  and W 2  (step S 85 ). In this case, there is a possibility that the parallelism of the substrates W 1  and W 2  has been lost due to a variation in the thickness of the substrates W 1  and W 2  or the seal  55  or a problem occurred in the press machine  17 , so that the location of an abnormality is checked.  
      When the decision in step S 84  is YES, the press control unit  41  drives the positioning stage  36  to align the substrates W 1  and W 2  while picking up the images of the alignment marks of the substrates W 1  and W 2  by means of the CCD camera  50  (step S 86 ).  
      In step S 87 , the press control unit  41  moves the pressure plate  33   a  downward. When the computed pressing load reaches 50 kg, the press control unit  41  stops lowering the pressure plate  33   a  (step S 88 ). The press control unit  41  monitors the degree of flattening of the seal  55  from the data picked-up from the CCD cameras  50 .  
      The press control unit  41  computes the pressing load again based on the measured values from the load cells  31  and determines whether or not the difference between the pressing load and 50 kg lies within a predetermined range (step S 89 ). When the difference is greater than the predetermined range (NO in step S 89 ), the press control unit  41  stops lowering the pressure plate  33   a  and stops pressing the substrates W 1  and W 2 . In this case, there is a possibility that parallelism of the substrates W 1  and W 2  has been lost, so that the location of an abnormality is checked (step S 90 ).  
      When the decision in step S 89  is YES, on the other hand, the press control unit  41  checks if the flattened width of the seal  55  based on the picked-up data from the CCD cameras  50  lies within a predetermined range (step S 91 ). When the flattened width of the seal  55  is greater than the predetermined range, the press control unit  41  stops pressing the substrates W 1  and W 2  (step S 92 ). When the decision in step S 91  is YES, on the other hand, the press control unit  41  moves the pressure plate  33   a  downward to cause the substrates W 1  and W 2  to further come doser to each other (step S 93 ). When the calculated pressing load reaches 100 kg, the press control unit  41  stops lowering the pressure plate  33   a  (step S 94 ). The press control unit  41  monitors the degree of flattening of the seal  55  based on picked-up data from the CCD cameras  50 .  
      In step S 95 , the press control unit  41  calculates the pressing load again based on the measured values from the load cells  31 . When the difference between the computed pressing load and the pressure value of 100 kg is greater than the predetermined range (NO in step S 95 ), the press control unit  41  stops lowering the pressure plate  33   a  (step S 96 ). In this case, there is a possibility that parallelism of the substrates W 1  and W 2  has been lost, so that the location of an abnormality is checked.  
      When the decision in step S 95  is YES, on the other hand, the press control unit  41  checks if the flattened width of the seal  55  based on the picked-up data from the CCD cameras  50  lies within a predetermined range (step S 97 ). When the flattened width of the seal  55  is greater than the predetermined range, the press control unit  41  stops pressing the substrates W 1  and W 2  (step S 98 ). When the decision in step S 97  is YES, on the other hand, the press control unit  41  moves the pressure plate  33   a  upward to release the vacuum process chamber  32  (step S 99 ). The substrates W 1  and W 2  are compressed to the predetermined cell gap by the difference between atmospheric pressure and the pressure (vacuum) in the space between the substrates.  
      The image processing unit  47  calculates the flattened width of the seal  55  based on the picked-up data from the CCD cameras  50  and estimates the gap between the substrates W 1  and W 2  from this flattened width. The press control unit  41  reads the estimated value of the gap between the substrates W 1  and W 2  (step S 100 ). The press control unit  41  transfers the bonded substrates W 1  and W 2  to the transfer equipment (step S 101 ).  
      The first embodiment has the following advantages.  
      (1) The pressure plate  33   a  and the table  33   b  are provided facing each other in the vacuum process chamber  32 . The pressure plate  33   a  is suspended from the second support plate  26  via the suspension rods  34 . The table  33   b  is supported on the positioning stage  36  via the legs  37 . The upper container  32   a  is suspended from the second support plate  26  via the upper bellows  35 . The lower container  32   b  is supported on the positioning stage  36  via the lower bellows  38 . The second support plate  26  and the positioning stage  36  are supported on the base plate  21  and the gate  22  which have a high rigidity. Even in a case where the vacuum process chamber  32  is depressurized and deformed, the deformation is absorbed by the bellows  35  and  38 . Therefore, the depressurization-originated influence of deformation of the vacuum process chamber  32  does not act on the pressure plate  33   a  and the table  33   b  and does not therefore influence the relative position and parallelism of the substrates W 1  and W 2 . With vibrations from outside the press machine  17  absorbed by the bellows  35  and  38 , vibrations are prevented from being transmitted to the pressure plate  33   a  and the table  33   b . This suppresses positional deviation of the substrates W 1  and W 2  and keeps the substrates W 1  and W 2  parallel to each other.  
      (2) The substrates W 1  and W 2  are pressed while the measured values from the load cells  31  are monitored until the gap between the substrates W 1  and W 2  reaches the gap at which the substrates W 1  and W 2  contact the entire seal  55 . The vacuum process chamber  32  is released while the relative position and parallelism of the substrates W 1  and W 2  are maintained. Thereafter, the substrates W 1  and W 2  are compressed to the target cell gap due to the difference between atmospheric pressure and the pressure in the space between the substrates. Because the pressing load after the vacuum process chamber  32  is released to atmospheric pressure acts evenly on the entire substrates W 1  and W 2 , both substrates W 1  and W 2  are therefore bonded accurately without being damaged. As the pressing load until both substrates W 1  and W 2  contact the seal  55  is significantly lower than the pressing load after release of the vacuum process chamber  32  to atmospheric pressure, damage on the substrates W 1  and W 2  is relatively small even if the substrates W 1  and W 2  are bonded with a mechanical positional deviation occurring in the press machine  17  or while the substrates W 1  and W 2  are not parallel to each other.  
      (3) The pressing load is monitored based on the measured values from the load cells  31 , the position of the pressure plate  33   a  detected by the linear scales  43   a  and  43   b  and the degree of flattening of the seal  55  sensed by the CCD cameras  50 . In a case where the pressing load on the substrates W 1  and W 2  is detected to be abnormal based on the monitoring result, further pressing is stopped, thus preventing the pressure plate  33   a , the table  33   b  and the substrates W 1  and W 2  from being damaged.  
      (4) The load cells  31  are provided at equal distances from the center C of the pressure plate  33   a  and on diagonal lines that connect the suspension rods  34 . This allows a well-balanced load (weight) to be applied to the plural load cells  31  and allows a well-balanced load (atmospheric pressure) to be applied to the plural load cells  31  in the process of depressurizing the vacuum process chamber  32 . Therefore, the pressure plate  33   a  and the table  33   b  are kept parallel to each other regardless of the pressure in the vacuum process chamber  32 . As the parallelism of the pressure plate  33   a  relative to the table  33   b , which may be lost due to entry of a foreign matter or mechanical deviation of the press machine  17 , is inspected based on the measured values from the plural load cells  31  so that the substrates W 1  and W 2  are bonded with high precision while the parallelism is maintained.  
      (5) Alignment of the substrates W 1  and W 2  is carried out when the pressure plate  33   a  is in the position where the gap between the substrates W 1  and W 2  is at a minimum within the range in which the substrate W 2  contacts the liquid crystal  54  but does not contact the seal  55 . Because shearing force does not act on the seal  55 , the positional deviation of the substrates W 1  and W 2  after release of the vacuum process chamber  32  to atmospheric pressure is prevented. This allows the substrates W 1  and W 2  to be bonded with high precision.  
      (6) As the outer seal  61  ( 62 ,  63 ) which is higher and thicker than the inner seals  55  is provided outside the inner seals  55 , it is possible to detect the pressing load accurately and provide a large margin for the substrate gap (the stop position of the pressure plate  33   a ) at the time pressing is stopped. In a case where pressing is abnormal, therefore, the abnormality can be detected earlier. Even in a case where the inner seals  55  reach the light shielding film of the substrate W 2 , the degree of flattening of the outer seal  61  ( 62 ,  63 ) can be sensed by the CCD cameras  50 .  
      (7) As the gap between the substrates W 1  and W 2  is kept approximately constant based on the measured values from the load cells  31 , the time needed to spread the liquid crystal  54  after the vacuum process chamber  32  is released to atmospheric pressure becomes approximately constant. This can allow the timing for irradiation of ultraviolet rays to be made approximately constant, so that the process of curing the seal  55  can be performed at the optimal timing. It is also possible to prevent adhesion of the seal  55  from becoming insufficient due to inadequate curing. This makes it possible to efficiently activate the bonded substrate fabricating apparatus  11  in case of continuously carrying out bonding of the substrates W 1  and W 2 .  
      (8) Because the measured values from the load cells  31  are not influenced by deformation of the vacuum process chamber  32  because of the action of the bellows  35  and  38 , the reliability of the measured values from the load cells  31  is improved. Further, the press control unit  41  can monitor the pressing load on the substrates W 1  and W 2  with high precision.  
      A description will be given below of a press machine  121  according to a second embodiment of the present invention, mainly on differences from the press machine  17  of the first embodiment and omitting descriptions on the same structures.  
      As shown in  FIG. 16 , the press machine  121  has a main support gate  123  attached with guide rails  125  and an inner support frame  124  attached with linear guides  126 . The inner support frame  124  is movable up and down with respect to the main support gate  123 .  
      Plural (two shown in the drawing) pressure motors  127  are provided at the main support gate  123 . Each pressure motor  127  turns an associated ball screw  128 . A support plate  129  is moved up and down in accordance with rotational direction of the ball screw  128 . The inner support frame  124  is supported on the support plate  129  via plural (four shown in the diagram) load cells  130 .  
      A central support frame  131  is provided in the center of the inner support frame  124 . Attached to the central support frame  131  are linear guides  133  which are movable up and down along guide rails  132  attached to the support plate  129 . That is, the central support frame  131  can move up and down with respect to the support plate  129  and the inner support frame  124 .  
      The support plate  129  is provided with a pressure motor  134  which turns a ball screw  135  coupled to a support member  136 . The rotation of the ball screw  135  causes the support member  136  to move up and down. The central support frame  131  is supported on the support member  136  via plural (two shown in the diagram) load cells  137 . It is preferable that the load cells  130  and  137  be laid out as shown in  FIG. 5  or  FIG. 6 .  
      A vacuum process chamber  140  is provided below the inner and central support frames  124  and  131 . The vacuum process chamber  140  is defined by an upper container  140   a  and a lower container  140   b  which are separable. The lower container  140   b  is supported by a plurality of support rods  140   c  attached to the main support gate  123 .  
      An O-ring  140   d , which keeps the vacuum process chamber  140  airtight, is provided at the periphery of the opening of the lower container  140   b . A positioning pin  140   e  provided at the lower container  140   b  is fitted in a positioning hole  140   f  formed in the upper container  140   a  when the vacuum process chamber  140  is closed. This causes the upper container  140   a  to be positioned with respect to the lower container  140   b.    
      A pressure plate  141  and a table  142  are provided in the vacuum process chamber  140  and face each other. The pressure plate  141  holds the second substrate W 2  (CF substrate) and the table  142  holds the first substrate W 1  (TFT substrate). The pressure plate  141  and the table  142  hold the second substrate W 2  and the first substrate W 1  respectively by at least one of vacuum chuck force and electrostatic chuck force.  
      As shown in  FIG. 17A , the pressure plate  141  has a central pressing portion  141   a  and a peripheral pressing portion  141   b  provided outside and apart from the central pressing portion  141   a . The substrate W 2  is held by the central pressing portion  141   a  and the peripheral pressing portion  141   b  which are indicated by hatching in  FIG. 17A . The peripheral pressing portion  141   b  is supported on plural (two shown in the diagram) supports  143  that extend downward from the inner support frame  124 . The central pressing portion  141   a  is supported on plural (two shown in the diagram) supports  144  that extend downward from the central support frame  131 . The supports  143  are integral with the inner support frame  124 , and the supports  144  with the central support frame  131 .  
      Bellows  145  as an elastic member are provided between the inner support frame  124  and the upper container  140   a  in such a way as to surround the individual supports  143 . Each bellows  145  has a flange portion at either end. Both flange portions are respectively coupled to the inner support frame  124  and the upper container  140   a  via O-rings which serve as sealing members.  
      Bellows  146  as an elastic member are provided between the central support frame  131  and the upper container  140   a  in such a way as to surround the individual supports  144 . Each bellows  146  has a flange portion at either end. Both flange portions are respectively coupled to the central support frame  131  and the upper container  140   a  via O-rings which serve as sealing members. The bellows  145  and  166  are connected to the vacuum process chamber  140  airtightly.  
      The table  142  is provided in the lower container  140   b  and is moved horizontally and turned within the horizontal plane by a positioning stage  147 . The positioning stage  147  is slidable and rotatable within the horizontal plane with respect to a base plate  148  secured to the main support gate  123 , and supports the table  142  via plural supports (not shown). As the positioning stage  147  moves, therefore, the table  142  also moves horizontally and turns. The individual supports are surrounded by a bellows (not shown), which keeps the vacuum process chamber  140  airtight between the positioning stage  147  and the lower container  140   b.    
      The main support gate  123 , the inner support frame  124 , the central support frame  131 , the support plate  129 , the support member  136  and the base plate  148  are formed of material which has sufficiently high rigidity.  
      Ultraviolet-ray irradiating devices  149  and  150  are provided on the table  142 . The ultraviolet-ray irradiating device  149  faces the central pressing portion  141   a  of the pressure plate  141 , and the ultraviolet-ray irradiating device  150  faces the peripheral pressing portion  141   b . The ultraviolet-ray irradiating devices  149  and  150  are moved up and down by unillustrated cylinders. The ultraviolet-ray irradiating devices  149  and  150  irradiate ultraviolet rays onto the seal at the time of bonding the first and second substrates W 1  and W 2 . The irradiation cures the seal to temporarily fix both substrates W 1  and W 2 .  
      A lift plate  153  is provided at the outer periphery of the table  142 . The top surface of the lift plate  153  is level with the top surface of the table  142  (which chucks the substrate W 1 ). The outer edges of the lift plate  153  extend out of the table  142 . The lift plate  153  is lifted above the table  142  by a lift mechanism  154 .  
      The operation of the press machine  121  will be discussed below.  
      When the pressure motors  127  are driven, the support plate  129 , the inner support frame  124  and the central support frame  131  are moved up and down with respect to the main support gate  123 . When the pressure motor  134  is driven, the support member  136  and the central support frame  131  are moved up and down with respect to the support plate  129  and the inner support frame  124 . Therefore, the inner support frame  124  and the central support frame  131  are moved up and down independently with respect to the main support gate  123 . In other words, the central pressing portion  141   a  and the peripheral pressing portion  141   b  are moved up and down independently of each other while holding the substrate W 2 , as shown in  FIG. 17B .  
      Each of the load cells  130  and  137  supplies the detected load to the press control unit (not shown).  
      When the vacuum process chamber  140  is depressurized, the load that is associated with the difference between the pressure in the vacuum process chamber  140  and the atmospheric pressure acts on the load cells  130  via the peripheral pressing portion  141   b  and the supports  143 . The load cells  130  detect the sum of the load associated with the pressure difference and the load that is associated with the weight of the member supported on the support plate  129 . The press control unit calculates the pressing load applied to both substrates W 1  and W 2  from the peripheral pressing portion  141   b  based on the decrease in the total load supplied from the load cells  130 .  
      Likewise, when the vacuum process chamber  140  depressurized, the load that is associated with the difference between the pressure in the vacuum process chamber  140  and the atmospheric pressure acts on the load cells  137  via the central pressing portion  141   a  and the supports  144 . The load cells  137  detect the sum of the load associated with the pressure difference and the load that is associated with the weight of the member supported on the support member  136 . The press control unit calculates the pressing load applied to both substrates W 1  and W 2  from the central pressing portion  141   a  based on the decrease in the total load supplied from the load cells  137 .  
      The press control unit controls the pressing load on both substrates W 1  and W 2  by controlling the motors  127  and  134  in accordance with the detection results from the load cells  130  and  137 , as per the first embodiment. Further, the press control unit aligns both substrates W 1  and W 2  with each other by driving the positioning stage  147  based on image data from the CCD cameras  50  as has been described in the foregoing description referring to  FIG. 3 .  
      The linear guides  126  and  133  may be provided with linear scales which respectively detect the moving positions of the peripheral pressing portion  141   b  and the central pressing portion  141   a . In this case, the press control unit may monitor the relative positions of the central pressing portion  141   a  and the peripheral pressing portion  141   b  with respect to the table  142  and determine whether the relationship between the gap between the substrates W 1  and W 2  and the pressing load is adequate or not.  
      Bonding of both substrates W 1  and W 2  will now be discussed referring to  FIG. 18 . A plurality of inner seals for sealing the liquid crystal inside plural cells formed on the first substrate W 1  and an outer seal which surrounds the inner seals are applied on the top surface (bonding surface) of the first substrate W 1 , as has been discussed in the foregoing description referring to  FIG. 10 .  
      As shown in  FIG. 18A , the pressure plate  141  and the table  142  chuck and hold the second substrate W 2  and the first substrate W 1 , respectively. The vacuum process chamber  140  is evacuated, alignment marks are optically detected, and then the peripheral portions of the substrates W 1  and W 2  are aligned in a non-contact manner.  
      As shown in  FIG. 18B , the peripheral pressing portion  141   b  is moved downward to press the peripheral portion of the second substrate W 2  at a pressing load Fo. The pressing load Fo corresponds to a load when the second substrate W 2  is in tight contact with the outer seal of the first substrate W 1 . In that situation, both substrates W 1  and W 2  are aligned with each other by using a camera C 1 . Ultraviolet rays are irradiated from the ultraviolet-ray irradiating device  149  to cure the outer seal, thereby temporarily fixing the peripheral portions of both substrates W 1  and W 2 .  
      As shown in  FIG. 18C , when the peripheral pressing portion  141   b  is unchucked, the peripheral pressing portion  141   b  is moved upward. Then, the central pressing portion  141   a  is moved downward. The center portion of the second substrate W 2  is pressed at a pressing load Fc while positioning the center portions of the substrates W 1  and W 2  using a camera C 2 . The pressing load Fc corresponds to a load when the second substrate W 2  is in tight contact with the inner seals. Thereafter, ultraviolet rays are irradiated from the ultraviolet-ray irradiating device  150  to cure the inner seals, thereby temporarily fixing the center portions of both substrates W 1  and W 2 .  
      With the central pressing portion  141   a  unchucked, the central pressing portion  141   a  is moved upward. Then, the vacuum process chamber  140  is released. The substrates W 1  and W 2  are bonded to a predetermined cell gap (final substrate gap) by the atmospheric pressure.  
      After temporal fixing of the peripheral portions, the central pressing portion  141   a  may be moved downward, without lifting the peripheral pressing portion  141   b  up, for temporal fixing of the center portions.  
      The second embodiment has the following advantages in addition to those of the first embodiment.  
      (1) The pressure plate  141  comprises the central pressing portion  141   a  which presses the center portions of both substrates W 1  and W 2 , and the peripheral pressing portion  141   b  which presses the peripheral portions of the substrates W 1  and W 2 . The peripheral pressing portion  141   b  and the central pressing portion  141   a  are moved up and down independently of each other. As the peripheral portions and center portions of the substrates W 1  and W 2  can be pressed separately, bonding is carried out at the minimum load required. This can allow the substrates W 1  and W 2  to be bonded together at a predetermined cell gap while preventing the substrate W 2  from sliding sideways and being misaligned with the substrate W 1  by the reaction force generated at the time of bonding.  
      (2) In a case where a plurality of inner seals and an outer seal which surrounds the inner seals are provided, the peripheral portions of both substrates W 1  and W 2  are pressed after which the center portions of the substrates W 1  and W 2  are pressed. First, the outer seal is flattened to temporarily fix the peripheral portions of both substrates W 1  and W 2 , and then the inner seals are flattened to temporarily fix the center portions thereof. This can further suppress the occurrence of positional deviation between the substrates W 1  and W 2 .  
      (3) As the peripheral pressing portion  141   b  and the central pressing portion  141   a  are moved independently of each other, the press machine  121  is useful in adequately bonding large substrates W 1  and W 2 .  
      It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. For example, the above embodiment may be modified as follows.  
      Each of the individual devices  12  to  14 ,  17  and  18  may be plural in quantity.  
      A vacuum process chamber  111  shown in  FIG. 16  may be used in place of the separable vacuum process chamber  32 . The vacuum process chamber  111  has a gate which is closed by a gate valve  112 . The pressure plate  33   a  and the table  33   b  are provided in the vacuum process chamber  111  and the pressure plate  33   a  is suspended from the second support plate  26  via the suspension rods  34 . The table  33   b  is supported on the positioning stage  36  via the legs  37 . The upper bellows  35  provided around the associated suspension rod  34  connect the vacuum process chamber  111  to a support plate  113 . The vacuum process chamber  111  is airtightly communicated with the upper bellows  35 . The lower bellows  38  provided around the associated legs  37  connects the bottom of the vacuum process chamber  111  to the positioning stage  36 . A pressing means  114  includes the pressure motor  27  which presses the pressure plate  33   a . The base plate  21  is connected to the gate  22  similar to the one shown in  FIG. 2 , though not illustrated in  FIG. 16 . This modificaton has advantages similar to those of the above embodiment.  
      In a case where the lower container  32   b  can be supported by the lower bellows  38  alone, the support rods  39  shown in  FIG. 2  may be omitted.  
      While the gate  22  is directly coupled to the base plate  21 , another structure which has a sufficiently high rigidity may be provided between the base plate  21  and the gate  22 .  
      The detection of the pressing load on the substrates W 1  and W 2  is not limited to the calculation from the amount of decrease from the sum of the weight A and the load B, but may be detected by other techniques as well.  
      The number of the load cells  31  is not limited to four.  
      The number of the CCD cameras  50  is not limited to four, but may be greater than four or may be in a range of one to three. To efficiently and accurately detect the pressing load and parallelism of the pressure plate  33   a  and the table  33   b , it is preferable that the CCD cameras  50  should be four in quantity.  
      The pressing load may be detected and controlled without using all of the load cells  31 , the linear scales  43   a  and  43   b  and the CCD cameras  50  but using only some of the components. In case of monitoring the loads detected by the four load cells  31  and the degree of flattening of the seal  55 , an abnormality in the pressing load is detected with high precision and high reliability even if a mechanical deviation occurs in the press machine  17 .  
      The degree of flattening of the seal  55  may be monitored by transparent type sensors instead of the CCD cameras  50 . It is however preferable to use the CCD cameras  50  because a worker can visually check the image of the seal  55  on the monitor screen.  
      In the second embodiment, the central pressing portion  141   a  may be moved downward to press the center portions of both substrates W 1  and W 2  first, followed by unchucking of the central pressing portion  141   a  after which the peripheral pressing portion  141   b  may be moved downward to press the peripheral portions.  
      In the second embodiment, the central pressing portion  141   a  and the peripheral pressing portion  141   b  may be moved downward at a time to press the substrates W 1  and W 2  if pressing the entire surfaces does not cause sideway sliding of the substrate W 2 . That is, pressing by the central pressing portion  141   a  and the peripheral pressing portion  141   b  is controlled in accordance with the sizes of the substrates W 1  and W 2 .  
      The present embodiment and examples are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.