Abstract:
A double-sided exposure system ( 1 ) has a first work holding device ( 21 L) for holding a substrate (P) opposite to an exposure mask ( 55 ) to expose a first surface of the substrate (P) through the exposure mask ( 55 ) to light, and a second work holding device ( 21 R) for holding the substrate (P) opposite to another exposure mask ( 55 ) to exposure a second surface of the substrate (P) through the exposure mask ( 55 ) to light. Phases of operations for receiving, transferring and pretreating an unexposed substrate, and those of operations for transferring, exposing and pretreating the substrate having one surface processed by an exposure operation can be staggered to prevent time loss due to waiting during an exposure operation can be prevented even if the double-sided exposure system is provided with a single light source.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a double-sided exposure system. More specifically, the present invention relates to a double-sided exposure system capable of sequentially exposing the first and second surfaces of a plate, such as a substrate for a printed wiring board or a sheet for lead frames, through an exposure mask provided with a predetermined exposure pattern to light, and, particularly, provided with a single exposure light source. 
     2. Description of the Related Art 
     Generally, an exposure system employed in, for example, a process for fabricating a high-density printed wiring board is provided with an extra-high pressure mercury lamp as an exposure light source for high resolution. The mercury lamp of such a type is a very expensive lamp costing as much as about ¥500,000. The process requires the mercury lamp to be kept continuously lighted and the life of the mercury lamp is only about 500 hr. Therefore, the exposure system of this type must be provided, if possible, with a single light source and the light source must be used at the highest possible economic efficiency; that is, it is desired to reduce the exposure cost of each substrate for a printed wiring board (the running cost of the light source) to the lowest possible extent by using the light source for as many exposure cycles as possible in its life. 
     To enhance the economic efficiency of the light source, it is important to enhance the operating speeds of mechanical units including a carrying mechanism for carrying substrates and alignment adjusting mechanism for aligning the substrates with an exposure mask and to reduce waiting time, such as exposure waiting time, near to naught. 
     Referring to FIG. 13 showing a double-sided exposure system  100  disclosed in Japanese Patent Application No. 343971/1997 (JP-A No. 333337/1998), a work holding base  101  for detachably holding a substrate P has opposite surfaces respectively provided with vacuum pads  103 , and the work holding base  101  is moved repeatedly between a home position, i.e., a position indicated by chain lines, between a work receiving unit  105  and a work delivering unit  107 , and an exposure position, i.e., a position indicated by solid lines, between two mask holding mechanisms  109 L and  109 R respectively holding exposure masks  113 . 
     Light emitted by a mercury-short-arc lamp  111 , i.e., a light source, is transmitted through a left optical path and falls on the back surface of the exposure mask  113  held by the left mask holding mechanism  109 L, or the light is transmitted through a right optical path and falls on the back surface of the exposure mask  113  held by the right mask holding mechanism  109 R. An optical path selecting unit, not shown, connects the mercury-short-arc lamp selectively to the left optical path or the right optical path. The work receiving unit  105  adjusts the position of an unexposed substrate P fed thereto for preparatory positioning and transfers the substrate P to the vacuum pad  103  on the left surface, as viewed in FIG. 13, of the work holding base  101  located at the home position. Upon the arrival of the work holding base  101  holding the unexposed substrate P on its left surface at the exposure position, the left mask holding mechanism  109 L advances to bring the exposure mask  113  into contact with the substrate P so that the exposure mask  113  is aligned with the substrate P. After the exposure mask  113  has been exactly aligned with the substrate P, the exposure mask  113  is contacted fixedly to the substrate P. Then an exposure cycle is executed to expose a first surface, i.e., one of the surfaces, of the substrate P through the exposure mask  113  to light. 
     After the exposure cycle has been completed, the left mask holding mechanism  109 L is removed from the substrate P, the left mask holding mechanism  109 L is retracted, a left transfer hand  115 L holds the substrate P and transfers the same to a right transfer hand  115 R. Meanwhile, the work holding base  101  is returned to the home position, and then moved again to the exposure position. Subsequently, the right transfer hand  115 R transfers the substrate P to the right vacuum pad  103 , the right mask holding mechanism  109 R is advanced into contact with the substrate P so that the exposure mask  113  is aligned with the substrate P. After the exposure mask  113  has been exactly aligned with the substrate P, the exposure mask  113  is contacted fixedly to the substrate P. Then an exposure cycle is executed to expose a second surface, i.e., the other surface, of the substrate P through the exposure mask  113  to light. Thus, the exposure of both the surfaces of the substrate P is completed. 
     After the completion of the exposure cycle, the right mask holding mechanism  109 R is removed from the substrate P, the right mask holding mechanism  109 R is retracted and the work holding base  101  is returned to the home position. Then, the left vacuum pad  103  receives an unexposed substrate P from the work receiving unit  105 , and the work delivering unit  107  receives the two-side exposed substrate from the right vacuum pad  103  and delivers the same. 
     FIG. 14A is a time chart representing sequential operations of the prior art double-sided exposure system  100 . In FIG. 14A, time is measured in seconds on the horizontal axis. Generally, exposure time is about 3 s or below. In this time chart, the exposure time is 3 s, and cycle time, i.e., time need to carry out a sequence of operations from the start of reception of an unexposed substrate P to the start of reception of the next unexposed substrate P, is 22 s. As obvious from FIG. 14A, there is a scarce waiting time between the completion of the exposure of the first surface of the substrate P and the start of the exposure of the second surface of the substrate P when the exposure time of the double-sided exposure system  100  is on the order of 3 s. 
     Some resist that is applied to the substrate P requires a longer exposure time. When a long exposure time is necessary, an exposure operation for exposing the second surface of the substrate P to light cannot be started before the completion of an exposure operation for exposing the first surface of the substrate P to light even if preparatory operations of the mechanical units for exposure are completed, which produces a time loss. 
     FIG. 14B is a time chart representing sequential operations of the prior art double-sided exposure system  100 , in which exposure time is 8 s for both the surfaces of the substrate P. In this case, there is a waiting time of 5.5 s between the completion of preparations for the exposure of the second surface of the substrate P to light and the start of the exposure operation for exposing the second surface to light, and there is a waiting time of 5.5 s between the completion of transfer of the substrate P from the left side to the right side of the work holding base  101  and the start of shifting the work holding base  101  to the home position, i.e., during operations for exposing the second surface of the substrate P to light and post-operations. Consequently, the cycle time is as long as 32 s, which is about 1.5 times the cycle time needed by the operations shown in FIG.  14 A and hence the running cost of the lamp  111  increases sharply near to 1.5 times the running cost needed by the operations shown in FIG.  14 A. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the foregoing problems and it is therefore an object of the present invention to provide a double-sided exposure system capable of suppressing significant increase in time loss even if exposure time increases. 
     With the foregoing object in view, the present invention provides a double-sided exposure system for exposing opposite side surfaces of a plate to light through a pair of exposure masks respectively provided with necessary exposure patterns, comprising a first mask holding mechanism holding a first exposure mask to be used in exposing a first surface of the plate to light; a first work holding base capable of moving between a first exposure position in front of the first mask holding mechanism and a first home position where an unexposed plate is mounted thereon; a second mask holding mechanism holding a second exposure mask to be used in exposing a second surface of the plate to light; a second work holding base capable of moving between a second exposure position in front of the second mask holding mechanism and a second home position where the two-side exposed plate is removed therefrom; a single light source; an optical path selecting optical system for selectively connecting the light source to a first optical path leading to the first exposure position or a second optical path leading to the second exposure position; and a work transfer mechanism capable of receiving the plate having the exposed first surface exposed to light at the first exposure position from the first work holding base and of transferring the plate to the second work holding base so that the second surface of the plate can be exposed to light. 
     According to the present invention, the double-sided exposure system is provided with the two work holding bases, i.e., the first work holding base specially for exposing the first surface of the plate to light and the second work holding base specially for exposing the second surface of the plate to light. Therefore, phases of operations for receiving and carrying an unexposed plate, exposing one of the surfaces of the plate to light and preparatory processes can be shifted relative to phases of operations for carrying a plate having an exposed surface, exposing the other surface of the plate to light and preparatory processes. Therefore, the phases of operations of the first processing unit (the first mask holding mechanism and the first work holding base) and those of the second processing unit (the second mask holding mechanism and the second work holding base) can be shifted relative to each other so that the second processing unit carries out an exposure operation while the first processing unit is carrying out an operation other than an exposure operation. Thus, time loss due to waiting for an exposure operation can be surely prevented even if a single light source is employed. 
     Time loss due to a time interval for the movement of the work holding base as well as the time loss due to waiting for exposure can be prevented by timing the operations of the components so that the work transfer mechanism is able to mount the plate having one exposed surface on the second work holding base before the second work holding base reaches the second exposure position after the operations for exposure of the first processing unit has been completed. 
     In the double-sided exposure system according to the present invention , it is preferable that the first work holding base is able to hold the plate in a vertical position at least during an exposure operation for exposing one of the surfaces of the plate to light, and the second work holding base is able to hold the plate in a vertical position at least during an exposure operation for exposing the other surface of the plate to light. 
     When the first and the second work holding base are able to thus hold the plate in a vertical position, the exposure operation can be carried out with both the exposure mask and the plate held in a vertical position, which is effective in preventing faulty exposure attributable to the adhesion of dust to the surfaces of the plate. 
     In the double-sided exposure system according to the present invention, it is preferable that the first and the second mask holding mechanism are disposed opposite to each other, and the work transfer mechanism transfers the plate from the first work carrying device to the second work carrying device in a space between the first and the second mask holding mechanism disposed opposite to each other with at least one of the first and the second work holding base disposed respectively at their home positions. 
     Thus, any special space for transferring the plate is scarcely necessary and hence the double-sided exposure system can be formed in compact construction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following description taken in connection with the accompanying drawings, in which: 
     FIG. 1 is a cutaway front elevation of a double-sided exposure system in a preferred embodiment of the present invention, in which a case is partly cut away; 
     FIG. 2 is a plan view of an essential part of the double-sided exposure system shown in FIG. 1; 
     FIG. 3 is a perspective view of an essential part of the double-sided exposure system shown in FIG. 1; 
     FIG. 4 is a schematic front elevation of assistance in explaining an unexposed substrate receiving operation of a left work elevator in the double-sided exposure system shown in FIG. 1; 
     FIG. 5 is a schematic front elevation of assistance in explaining an exposure operation of a first processing unit in the double-sided exposure system shown in FIG. 1; 
     FIG. 6 is a schematic front elevation of assistance in explaining a substrate receiving operation of a left transfer hand in the double-sided exposure system shown in FIG. 1; 
     FIG. 7 is a schematic front elevation of assistance in explaining a work transfer operation for transferring a substrate between right and left transfer hands in the double-sided exposure system shown in FIG. 1; 
     FIG. 8 is a schematic front elevation of assistance in explaining an exposure operation of the first processing unit and a substrate mounting operation of a right work elevator in the double-sided exposure system shown in FIG. 1; 
     FIG. 9 is a schematic front elevation of assistance in explaining a substrate receiving operation of the left transfer hand and an exposure operation of a second processing unit in the double-sided exposure system shown in FIG. 1; 
     FIG. 10 is a schematic front elevation of assistance in explaining the simultaneous exposure operation of the second processing unit and a transfer operation between the right and the left transfer hand in the double-sided exposure system shown in FIG. 1; 
     FIG. 11 is a schematic front elevation of assistance in explaining the exposure operation of the first processing unit and a two-side exposed substrate removing operation in the double-sided exposure system shown in FIG. 1; 
     FIG. 12 is a time chart showing sequential operations to be carried out by the double-sided exposure system shown in FIG. 1; 
     FIG. 13 is a schematic front elevation of an essential part of a prior art double-sided exposure system; and 
     FIGS. 14A and 14B are time charts representing sequential operations of the prior art double-sided exposure system shown in FIG.  13 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The construction of a double-sided exposure system  1  in a preferred embodiment according to the present invention for processing substrates for forming printed wiring boards by an exposure process will be described with reference to FIGS. 1 to  3 . 
     In FIG. 1, it is supposed that this side of the paper is the front side, the other side of the paper is the back side, the left side of the drawing is the left side and the right side of the drawing is the right side. Referring to FIGS. 1 to  3 , a base plate  5  similar to a wall is set up vertically in a middle region of the interior of a case  3  to define an optical system chamber  7  behind the base  5 . A horizontal partition plate  9  is disposed in a space in front of the base plate  5  on a level at the middle of the height of the space. A space under the partition plate  9  serves as a receiving/delivering chamber  11 , and a space over the partition plate  9  serves as an exposure chamber  13 . An opening  9   a  is formed in a middle part of the partition plate  9 . The receiving/delivering chamber  11  and the exposure chamber  13  communicates with each other by means of the opening  9   a.  A work entrance  15  and a work exit  17  are formed in the left and the right side wall of the case  3 , respectively. A left work elevator (first work holding device)  21 L and a right work elevator (second work holding device)  21 R are disposed in the case  3  so as to move vertically through the opening  9   a.  A left mask holding mechanism  23 L and a right mask holding mechanism  23 R are disposed in the exposure chamber  13  on the outer side of the paths of the left work elevator  21 L and the right work elevator  21 R, respectively. The left work elevator  21 L and the left holding mechanism  23 L constitute a first processing unit  25 L. The right work elevator  21 R and the right mask holding mechanism  23 R constitute a second processing unit  25 R. The first processing unit  25 L is used mainly for exposing to light a first surface of a substrate P for forming a printed wiring board, and the second processing unit  25 R is used mainly for exposing to light a second surface of the substrate P. 
     A loading device  27  is placed in a left region of the receiving/delivering chamber  11  on the left side of the work elevator  21 L. A receiving conveyor  29  has a left end facing the work entrance  15 . A substrate P fed by a feed conveyor, not shown, extended on the left side of the case  3  is delivered through the work entrance  15  onto the receiving conveyor  29 . The substrate P has opposite surfaces respectively coated with ultraviolet curable resist films. The substrate P is transferred by a driving roller, not shown, included in the receiving conveyor  29  from the receiving conveyor  29  onto the loading device  27 . The loading device  27  has a holding function to hold the substrate P by suction on the loading table, and an aligning function to adjust the position of the forward edge of the substrate P in cooperation with a prealignment sensor  31  (FIG.  1 ). After the substrate P has been correctly set on the loading device  27  by prealignment, the loading device  27  is turned through an angle of 90° to a position indicated by two-dot chain lines in FIG. 1, and is moved horizontally. The loading device  27  is supported on a shaft  33  projecting forward from the base plate  5 . The shaft  33  is supported on a moving means, not shown, disposed on the back side of the base plate  5  and capable of moving in lateral directions. 
     An unloading device  35  is disposed on the right side of the path of the work elevator  31 R, and a delivery conveyor  37  is disposed on the right side of the unloading device  35 . The unloading device  35  has an upper surface in which a vacuum can be created when necessary by a vacuum means, not shown. The unloading device  35  can be set in a horizontal position indicated by solid lines in FIG. 1, can be turned through an angle of 90° from the horizontal position to a vertical position in which its upper surface is faced to the left. The unloading device  35  is supported on a shaft  33 ′ projecting forward from the base plate  5 . The shaft  33 ′ is supported on a moving means, not shown, disposed on the back side of the base plate  5  and capable of moving in lateral directions. 
     The delivery conveyor  37  has a right end facing the work exit  17 . When the right work elevator  21 R holding the substrate P came down to the home position, the unloading device  35  set in a vertical position receives the substrate P from the right work elevator  21 R and then the unloading device  35  is turned to a horizontal position. The substrate P is transferred from the unloading device  35  to the delivery conveyor  37  by a driving roller, not shown, included in the unloading device  35 . The delivery conveyor  37  transfers the substrate P through the work exit  17  to a conveyor, not shown, for conveying the substrate P to the next process, such as a developing process. 
     As shown in FIG. 2, a pair of parallel, right vertical guide rails  36  and a pair of parallel, left vertical guide rails  36  are attached to a middle part of the back surface of the base plate  5  with respect to lateral directions. Sliding bases  39  slide along the right and the left vertical guide rails  36 , respectively. Horizontal arms  41  project forward from the sliding bases  39  through slots  5   a  formed in the base plate  5 , respectively. The work elevators  21 L and  21 R are fixed to the forward ends of the horizontal arms  41 , respectively. 
     The work elevators  21 L and  21 R are capable of holding a substrate P in a vertical position by suction and of vertical movement. Although the work elevators  21 L and  21 R are shown typically in simple rectangular flat plates in the drawings, actually, each of the work elevators  21 L and  21 R has a moving base having the shape of a vertical wall, a vacuum pad provided with a plurality of suction holes and attached to one side surface, i.e., the left side surface for the left work elevator  21 L or the right side surface for the right work elevator  21 R, of the moving base, and a vacuum system connected to the vacuum pad. A substrate P can be detachably held by suction on the vacuum pad. 
     The sliding bases  39  are moved by ball screws driven for rotation by servomotor, not shown, respectively, to move the work elevators  21 L and  21 R vertically through the opening  9   a  of the partition plate  9 . The work elevators  21 L and  21 R move between their home positions indicated by solid lines in FIG. 1 between the loading device  27  and the unloading device  35 , and their exposure positions indicated by two-dot chain lines in FIG. 1 between the mask holding mechanisms  23 L and  23 R, respectively. 
     As shown in FIG. 2, horizontal guide rails  45  are attached to parts of the back surface of the base plate  5  on the left and the right side of the vertical guide rails  36  so as to extend laterally. Sliding bases  47  slides along the horizontal guide rails  45 . Arms  49  are projected horizontally forward from the sliding bases  47  through openings  5   b  formed in the base plate  5 , respectively. Each sliding base  47  is provided with a ball screw, i. e., a threaded nut, not shown, and a threaded rod  53  is linked to the threaded nut by ball bearings. The threaded rod  53  is driven for rotation by a servomotor  51 . The threaded rod  53  is rotated to move the sliding base  47  laterally. The mask holding mechanism  23 L ( 23 R) is moved between a back position spaced apart from the path of the work elevator  21 L ( 21 R) and an forward position where a mask held by the mask holding mechanism  23 L ( 23 R) is in contact with the substrate P held on the work elevator  21 L ( 21 R). Although the mask holding mechanisms  23 L and  23 R are shown typically in simple rectangular flat plates in the drawings, actually, each of the mask holding mechanism  23 L and  23 R has a rectangular base, an alignment unit attached to one side surface, i.e., the right side surface for the left mask holding mechanism  23 L or the left side surface for the right mask holding mechanism  23 R, of the base, an exposure mask  55  indicated by dots in FIG. 3, and a camera for detecting errors in alignment. The position of the alignment unit in a vertical plane is adjusted for the fine positional adjustment of the exposure mask  55 . 
     The exposure mask  55  is provided with a predetermined transparent pattern, and photomask marks, not shown, formed at predetermined positions. The degree of alignment of the photomask marks with reference holes formed in a substrate P is measured by the camera to align the exposure mask  55  with the substrate P. 
     A single lamp  61  is disposed in a lower middle region of the optical system chamber  7  of the case  3 . The lamp  61  is a mercury-short-arc lamp. The lamp  61  emits light straight upward. A semitransparent turnable mirror  63  is disposed above the lamp  61 . The turnable mirror  63  can be set in a first position indicated by broken lines in FIG. 1 to reflect the light emitted by the lamp  61  to the left, a second position indicated by two-dot chain lines in FIG. 1 to reflect the light emitted by the lamp  61  to the right or a neutral position, i.e., a horizontal position. 
     An illuminating mirror  65 L is disposed on the left side of the left mask holding mechanism  23 L, and an illuminating mirror  65 R is disposed on the right side of the mask holding mechanism  23 R. Reflecting mirrors  67  are disposed at the left and the right end of the optical system chamber  7 . Compound-eye (fry-eye) lenses  69  are disposed between one of the reflecting mirrors  67  and the turnable mirror  63  and between the other reflecting mirror  67  and the turnable mirror  63 , respectively. In a state where the turnable mirror  63  is set in the first position, the light emitted by the lamp  61  is reflected by the left reflecting mirror  67  and the left illuminating mirror  65 L toward the exposure mask  55  held by the left mask holding mechanism  23 L. In a state where the turnable mirror  63  is set in the second position, the light emitted by the lamp  61  is reflected by the right reflecting mirror  67  and the right illuminating mirror  65 R toward the exposure mask  55  held by the right mask holding mechanism  23 R. 
     When the left work elevator  21 L holding a substrate P is located at the exposure position and the turnable mirror  63  is turned and set in the first position, the left exposure mask  55  is brought into contact with a first surface of the substrate P, the first surface of the substrate P is exposed through the left exposure mask  55  to the light emitted by the lamp  61  to form a latent image of the exposure pattern of the left exposure mask  55  on the first surface of the substrate P. When the right work elevator  21 R holding the substrate P is located at the exposure position and the turnable mirror  63  is turned and set in the second position, the right mask  55  is brought into contact with a second surface of the substrate P, the second surface of the substrate P is exposed through the right exposure mask  55  to the light emitted by the lamp  61  to form a latent image of the exposure pattern of the right exposure mask  55  on the second surface of the substrate P. 
     Referring to FIG. 3, transfer mechanisms  75 L and  75 R have transfer hands  77 L and  77 R, and sliding bases  79 , respectively. A top base plate  71  is placed on the base plate  5 , and guide rails  73  are extended laterally on the top base plate  71 . The sliding bases  79  having the shape of an elongate flat plate, are extended horizontally forward and have rear end parts slidably supported on the guide rails  73 . The left sliding base  79  is moved laterally through a relatively short distance by a pneumatic actuator  81  (FIG.  1 ), and the right sliding base  79  is moved laterally through a relatively long distance by a ball screw  85  driven for rotation by a servomotor  83 . Transfer hand driving units  87  are attached to forward end parts of the sliding bases  79 , respectively. The transfer hand driving units  87  are provided with parallel linkages  89 , respectively. Transfer hands  77 L and  77 R are attached to the operative ends of the parallel linkages  89 , respectively. The transfer hand  77 L ( 77 R) is a hollow structure having opposite side members provided with suction projections. Hollow spaces in the transfer hands  77 L and  77 R are connected to a suction means, not shown. When the hollow spaces are evacuated, a suction is produced on the tip surfaces of the suction projections. 
     The parallel linkages  89  can be turned through an angle of about 90° between horizontal positions at which the parallel linkages  89  extend horizontally forward and vertical positions where the parallel linkages  89  extend vertically downward. When the parallel linkages  89  are turned, the transfer hands  77 L and  77 R held in a horizontal position are moved vertically between a waiting position on a level above the mask holding mechanisms  23 L and  23 R as shown in FIG. 3, and a lower position on a level somewhat higher than the middle parts of the mask holding mechanisms  23 L and  23 R as shown in FIG.  1 . 
     The operation of the double-sided exposure system  1  will be described with reference to FIGS. 4 to  12 . FIGS. 4 to  11  are views of assistance in explaining time-series operations of the double-sided exposure system  1 , and FIG. 12 is a time chart showing the sequential operations of the double-sided exposure system  1 . In an initial stage of operation, only the first processing unit  25 L operates, and operations indicated by asterisks () in FIG. 12 are not carried out until the first processing unit  25 L accomplishes the first exposure cycle. In FIG. 12, intervals between vertical broken lines correspond to 1 s. 
     In an initial state, the work elevators  21 L and  21 R are located at their home positions, respectively, the mask holding mechanisms  23 L and  23 R are held at the back positions, respectively, the loading table device  27  and the unloading device  35  are held in a horizontal position with their upper surfaces facing up, and the transfer hands  77 L and  77 R are at their waiting positions, respectively. The turnable mirror  63  is set in the neutral position. Upon the start of the double-sided exposure system  1 , the lamp  61  is turned on. The lamp  61  is left on until the double-sided exposure system is stopped. Then, a substrate P is delivered to the receiving conveyor  29 , the substrate P is transferred from the receiving conveyor  29  to the loading device  27 , the position of the substrate P on the loading device  27  is adjusted for prealignment. Subsequently, as shown in FIG. 4, the loading device  27  is set in the vertical position and is moved toward the work elevator  21 L to make the work elevator  21 L hold the substrate P by suction (“reception” in FIG.  12 ). 
     The left work elevator  21 L holding the substrate P is raised to the exposure position indicated by two-dots chain lines in FIG. 4 (“ascent” in FIG.  12 ). Then, the loading device  27  is returned to its original position, carries out processes for receiving the next substrate P, and is kept waiting until the left work elevator  21 L is lowered to its waiting position. 
     When the left work elevator  21 L is located at the exposure position, a first surface of the substrate P is placed opposite to the left mask holding mechanism  23 L. Then, the left mask holding mechanism  23 L is moved to the forward position (“advancement” in FIG. 12) so that the exposure mask  55  is brought into light contact with the substrate P and is aligned with the substrate P (“alignment” in FIG.  12 ). Then, the exposure mask  55  is set in close contact with the substrate P (“contact” in FIG.  12 ). Then, the turnable mirror  63  is turned and set in the first position (“first position” in FIG.  12 ). Consequently, ultraviolet radiation emitted by the lamp  61  falls through the exposure mask  55  held by the mask holding mechanism  23 L on the first surface of the substrate P as shown in FIG. 5 (“exposure” in FIG.  12 ). After the first surface of the substrate P has been irradiated with the ultraviolet radiation for a necessary exposure time of, for example about 8 s, the turnable mirror  63  is returned and set in the neutral position, the exposure mask  55  is removed from the substrate P (“release” in FIG.  12 ), and then the left mask holding mechanism  23 L is retracted to the back position (“retraction” in FIG.  12 ). 
     Subsequently, the left transfer hand  77 L is moved via the lower position to the right, holds the substrate P by suction and moves to the lower position as shown in FIG. 6 (“transfer” in FIG.  12 ). The substrate P having the exposed first surface is thus transferred from the left work elevator  21 L to the left transfer hand  77 L. Then, the left work elevator  21 L is lowered to the home position indicated by two-dot chain lines in FIG. 6 (“descent” in FIG.  12 ). The left work elevator  21 L receives the next substrate P from the loading device  27  at the home position. 
     Meanwhile, after the left work elevator  21 L has reached the home position, the right transfer hand  77 R is moved to the left on the level of the waiting position, and is lowered to the lower position so as to face the substrate P held by the left transfer hand  77 L from the right side. Then, the substrate P is transferred from the left transfer hand  77 L to the right transfer hand  77 R (“transfer” in FIG.  12 ). Then, the right transfer hand  77 R holding the substrate P is moved horizontally to a position indicated by two-dot chain lines in FIG. 7 between the paths of the left work elevator  21 L and the right work elevator  21 R. The right transfer hand  77 R is kept waiting at the position (“reception and waiting” in FIG.  12 ). 
     Subsequently, the first processing unit  25 L repeats the raising operation, the advancing operation, the alignment operation and the exposure operation. Meanwhile, the right transfer hand  77 R moves to a position between the right work elevator  21 R and the mask holding mechanism  23 R (“rightward movement” in FIG. 12) immediately after the first processing unit  25 L has started the alignment operation. The time when the alignment operation is started corresponds to the time when the right work elevator  21 R completes moving from the exposure position to the home position in the second and the following exposure cycles. Then, the right work elevator  21 R rises to the exposure position (“ascent” in FIG.  12 ). 
     Subsequently, the right transfer hand  77 R moves leftward to transfer the substrate P having the exposed first surface to the right work elevator  21 R and the right work elevator  21 R holds the substrate P by suction as shown in FIG. 8 (“fixation” in FIG.  12 ). The substrate P is held by suction on the right work elevator  21 R with a second surface thereof faced to the right; that is, the substrate P is inverted in a process for transferring the substrate P from the left work elevator  21 L to the right work elevator  21 R. Subsequently, the right transfer hand  77 R is returned to the waiting position. 
     While the first processing unit  25 L is in an exposure operation, the right mask holding mechanism  23 R of the second processing unit  25 R moves to the forward position (“advancement” in FIG. 12) to bring the exposure mask  55  into light contact with the second surface of the substrate P, and the mask  55  is aligned with the substrate P (“alignment” in FIG.  12 ). The first processing unit  25 L completes the exposure operation just before the completion of the alignment operation, and the turnable mirror  63  is turned and set in the neutral position. 
     In the second processing unit  25 R, the exposure mask  55  held by the right mask holding mechanism  23 R is set in close contact with the substrate P (“close contact” in FIG. 12) after the completion of the alignment operation. Then, the turnable mirror  63  is turned and set in the second position (“second position” in FIG. 12) to irradiate the second surface of the substrate P through the exposure mask  55  held by the right mask holding mechanism  23 R with ultraviolet radiation emitted by the lamp  61  (“exposure” in FIG. 12) as shown in FIG.  9 . In the first processing unit  25 L, the substrate P having the first surface processed by the exposure operation is transferred from the work elevator  21 L to the transfer hand  77 L, and the work elevator  21 L moves down to the home position to receive the next substrate P. 
     While the second processing unit  25 R is in the exposure operation, the right transfer hand  77 R receives the substrate P having the first surface processed by the exposure operation from the left transfer hand  77 L as shown in FIG.  10 . 
     Subsequently, the work elevator  21 L of the first processing unit  25 L is raised for “ascent”, the mask holding mechanism  23 L is operated for “advancement” and “alignment”, the second processing unit  25 R completes the exposure operation, the mask holding mechanism  23 R is retracted for “retraction”, the work elevator  21 R is moved to the home position, and the unloading device  35  receives the substrate P having the opposite surfaces processed by the exposure operation from the work elevator  21 R as shown in FIG.  11 . The thus processed substrate P is delivered by the delivery conveyor  37  outside the case  3 . Thereafter, operations described above in connection with FIGS. 8,  9 ,  10  and  11  are repeated. 
     As obvious from the time chart shown in FIG. 12, there is not any idle time in which any operations are not executed. The cycle time of the exposure operation shown in FIG. 12 is 26.5 s, which is shorter by about 17% than the cycle time of 32 s of the exposure operation shown in FIG. 14B of the prior art exposure system  100  in which the exposure time is 8 s. In other words, although the exposure time for one surface is increased by 5 s from 3 s and the exposure time for both the surfaces is increased by 10 s, increase in time for both the surfaces can be limited to 4.5 s. (The time loss of 0.5 s in FIG. 14A is absorbed.) 
     Although the present invention has been described as applied to the double-sided exposure system for the exposure of the resist films formed on both the surfaces of the substrate, the present invention is not limited there to in its practical application and may be applied to various types of double-sided exposure systems for exposing both the surfaces of plate-shaped work to be exposed through exposure masks provided with predetermined exposure patterns to light. 
     Although the foregoing double-sided exposure system according to the present invention is provided with the work elevators that move vertically, the present invention is applicable to a double-sided exposure system provided with work holder corresponding to the work elevators and designed to move in horizontal directions. 
     Although the invention has been described in its preferred embodiment with a certain degree of particularity, obviously many changes and variations are possible therein. It is therefore to be understood that the present invention may be practiced otherwise than as specifically described herein without departing from the scope and spirit thereof.