Abstract:
A substrate transfer system comprising a cassette table for mounting a cassette which has an opening portion for loading and unloading a substrate and a cover detachably provided to the opening portion, process portion for processing the substrate housed in a cassette on the cassette table, a transfer arm mechanism for taking out the substrate from the cassette table, transferring it to process units G 1  to G 5 , and returning a processed substrate to the cassette on the cassette table, partition members provided between the transfer arm mechanism and the cassette table, for separating an atmosphere on the side of the transfer arm mechanism from that on the side of the cassette table, a passage formed in the partition member so as to face the opening portion of the cassette on the cassette table, for passing the substrate taken out from the cassette on the cassette table by the transfer arm mechanism and returning the substrate to the cassette on the cassette table, cassette moving mechanisms for moving the opening portion of the cassette on the cassette table closer to the passage or to be farther from the passage, and a cover removing mechanism for detaching the cover from the opening portion or attaching the cover to the opening portion of the cassette.

Description:
This application is a divisional of U.S. application Ser. No. 08/908,056, filed Aug. 11, 1997, now U.S. Pat. No. 6,074,154. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a substrate treatment system, a substrate transfer system, and substrate transfer method for use in transferring a substrate such as a semiconductor wafer and an LCD substrate from a cassette station to a process station. 
     2. Discussion of the Background 
     Recently, sizes of semiconductor wafers have been increased. With the size increase, the wafers tend to be processed one by one in semiconductor device manufacturing processes. For example, in a complex process system (resist coating and developing are performed in one process), substrates are taken out from a cassette one by one, processed in a process unit, and returned to the cassette one by one. 
     In a conventionally-used coating and developing process system as shown in FIG. 1, a plurality of cassettes CR are placed on a cassette station  102 . Wafers W are taken out from the cassette CR one by one by means of a wafer transfer mechanism  105 , loaded into a process station  101 , and subjected to a resist coating and developing process. The wafer transfer mechanism  105  comprises a movement unit  103  and an arm  104 . The arm  104  is moved separately by means of the movement unit  103  in the X, Y, and Z axis directions and rotated about the Z axis by a θ angle. The processed wafer W is returned to the cassette CR on the cassette station  102  by the wafer transfer mechanism  105 . 
     To prevent particles from attaching onto the wafer W, the resist coating and developing process system is positioned in a clean room where clean air constantly flows downwardly. 
     Furthermore, to prevent particles from entering the cassette CR during the conveyance of the cassette, a detachable cover is provided to the opening of the cassette CR. However, when the cassette CR is placed in the cassette station  102  with the cover removed, the cover intervenes in the down-flow of clean air in the process system, creating an air flow which will allow invasion of particles into a process station  101 . 
     In the wafer processing step, a washing device (scrubber) is used for washing the front and rear surfaces of the wafer with a brush. The washing device comprises a cassette station  401  and a process station  402 . The process station  402  comprises a center transfer passage  420 , a front-surface washing unit  421 , a rear-surface washing unit  423 , wafer reverse units  427 ,  428 , heating and cooling units  425 ,  426 , and a wafer transfer mechanism  403 . 
     To prevent particles from attaching to the wafer as much as possible in such a washing device, the wafer cassette CR is placed in an airtight chamber (So-called SMIF POD)  413  and the SMIF POD containing the cassette CR is transferred to the cassette station  401 . In the cassette station  401 , the SMIF POD  413  is descended to the wafer transfer portion while the SMIF POD  413  is kept airtight. In the SMIF system, wafers are transferred one by one from the cassette CR of the wafer transfer portion to the process station  402 , washed, and returned to the cassette CR. Thereafter, the wafer cassette CR is ascended and returned to the SMIF POD  413  on the cassette station  401 . 
     However, the conventionally-used device has a problem. If a wafer W protrudes from the cassette CR, the protruding wafer sometimes hits against the upper wall of the wafer transfer portion and causes damages when the wafer cassette CR is returned to the SMIF POD  413 . Furthermore, when the wafer protrudes from the cassette CR, the protruding wafer interferes with a mapping sensor  21   b , inducing misoperation of mapping, as shown in FIG.  9 . 
     BRIEF SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a substrate treatment system, a substrate transfer system, and a substrate transfer method capable of loading and unloading a substrate to a cassette with a cover without disturbing a down-flow of clean air in the cassette station, capable of preventing particles from attaching to the substrate, efficiently, and capable of preventing particles from flowing into the process station side from the cassette station side. 
     Another object of the present invention is to provide a substrate treatment system, a substrate transfer system, and a substrate transfer method producing no substrate breakage when a cassette is returned to a cassette mounting portion from a substrate transfer portion. 
     (1) A substrate treatment system according to the present invention comprises 
     a cassette table for mounting a cassette which has an opening portion for loading and unloading a substrate and a cover detachably provided to the opening portion, 
     a process portion for processing the substrate stored in the cassette on the cassette table, 
     a transfer arm mechanism for taking out the substrate from the cassette on the cassette table, transferring the substrate to the process portion and returning a processed substrate to the cassette on the cassette table, 
     a partition member provided between the transfer arm mechanism and the cassette table, for separating an atmosphere on the transfer arm mechanism side from that on the cassette table side, 
     a passage for passing the substrate taken out from the cassette on the cassette table by the transfer arm mechanism and for passing the substrate to be returned to the cassette on the cassette table, the passage being formed in the partition member so as to face the opening of the cassette on the cassette table, 
     a cassette moving mechanism for moving the cassette placed on the cassette table so as to be closer to the passage or to be farther from the passage, and 
     a cover removing mechanism for attaching or detaching of the cover to the opening portion of the cassette. 
     According to the present invention, a down flow of clean air will not be disturbed by open/shut movement of the cassette cover, in the transfer room. 
     (2) A system according to the present invention comprises 
     a cassette having an opening portion for loading/unloading a plurality of substrates and having a cover detachably provided to the opening portion, 
     substrate transfer means for loading/unloading a substrate from the cassette through the opening portion, 
     a partition member for separating a space on a cassette-side from a space on a substrate-transfer-means side, the partition member having a transfer window for transferring the substrate between the spaces, and 
     a cover transfer mechanism for removing the cover from the cassette and transferring the cover to the lower space on the substrate-transfer-means side through the transfer window. 
     According to the present invention, the down flow of clean air will not be disturbed by the cover itself in the transfer room when the cover is attached to or detached from the cassette. 
     (3) A substrate treatment system according to the present invention comprises: 
     a cassette having an opening portion for loading/unloading a plurality of substrates and having a cover detachably provided to the opening portion, 
     a cassette table on which a cassette is to be mounted, 
     substrate transfer means for loading/unloading a substrate through the opening portion of the cassette mounted on the cassette table, 
     a partition member for separating a space on a cassette side from a space on a substrate-transfer-means side, the partition member having a window for transferring a substrate between the spaces, 
     a cover storage portion formed on a side of the cassette table, facing the space on the substrate-transfer-means side, for storing a cover removed from the cassette, and 
     a cover transfer mechanism for removing the cover from the cassette through the transfer window, transferring the removed cover to the space on the substrate-transfer-means side, and storing the cover in the cover storage portion. 
     According to the present invention, the down flow of clean air will not be disturbed in the transfer room when the cassette is opened and shut. In addition, particles are prevented from attaching to a substrate in the transfer room and the process chamber  31 A. 
     (4) A substrate transfer system according to the present invention comprises, 
     a cassette having an opening portion for loading/unloading a plurality of substrates and having a cover detachably provided to the opening portion, 
     substrate transfer means for loading/unloading a substrate from the cassette, the substrate transfer means being provided in a transfer room whose pressure is set higher than the inner pressure of the cassette, 
     a partition member for separating a space on a cassette side from a space on a substrate-transfer-means side, the partition member having a window for transferring a substrate between the spaces, and 
     a cover transfer mechanism for removing the cover from the cassette through the transfer window and transferring the removed cover to a lower space of the substrate transfer means. 
     According to the present invention, since the inner pressure of the transfer room for transferring the substrate is set higher than the outside pressure, particles can be prevented from entering the transfer room from the outside. 
     (5) A substrate treatment system according to the present invention is provided in a clean room. The substrate treatment system comprises: 
     a cassette having an opening portion for loading/unloading a plurality of substrates and having a cover detachably provided to the opening portion, 
     substrate transfer means for loading/unloading a substrate from the cassette, the substrate transfer means being provided in a transfer room whose pressure is set higher than the inner pressure of the clean room, 
     a partition member for separating a space on a cassette side from a space on a substrate-transfer-means side, the partition member having a window for transferring a substrate between the spaces, and 
     a cover transfer mechanism for removing the cover from the cassette through the transfer window and transferring the removed cover to a lower space of the substrate-transfer-means side. 
     According to the present invention, since the pressure of the transfer room is set higher than an inner pressure of the clean room, particles can be prevented from entering the transfer room from the clean room. 
     (6) A substrate treatment system according to the present invention comprises: 
     a cassette having an opening portion for loading/unloading a plurality of substrates and having a cover detachably provided to the opening portion, 
     a transfer room separated by a partition member having a first transfer window, for transferring a substrate from the cassette, 
     substrate transfer means provided in the transfer room, for transferring a substrate from the cassette, and vice versa, through the first transfer window, 
     a cover removing mechanism provided in the transfer room for removing a cover from the cassette through the first transfer window and transferring the cover to a lower space on a substrate-transfer-means side, 
     a cover transfer mechanism for removing the cover from the cassette through the transfer window and transferring the removed cover to the lower space of a substrate-transfer-means side, and 
     a process chamber  31 A for processing the substrate transferred from a second transfer window. The process chamber  31 A being provided adjacent to the transfer room, which has a second transfer window for transferring a substrate by the substrate transfer means between the process chamber  31 A and the transfer room. 
     (7) A substrate treatment system according to the present invention comprises: 
     a cassette having an opening portion for loading/unloading a plurality of substrates and having a cover detachably provided to the opening portion, 
     a transfer room set at a higher pressure than an inner pressure of the cassette and having a first transfer window for transferring a substrate from the cassette, 
     substrate transfer means provided in the transfer room, for transferring a substrate from the cassette, and vice versa, through the first transfer window, 
     a cover transfer mechanism provided in the transfer room, for removing a cover from the cassette through the first transfer window and transferring the cover to a lower portion of a space of the substrate transfer means side, 
     a process chamber  31 A for processing the substrate transferred through a second transfer window, the process chamber  31 A being set at a higher pressure than an inner pressure of the transfer room, being disposed adjacent to the transfer room, and having the second transfer window for transferring a substrate by the substrate transfer means to the transfer room. 
     According to the present invention, since the pressure of the process chamber  31 A is set higher than the inner pressure of the transfer room, particles can be prevented from entering the process chamber  31 A from the transfer room. 
     (8) A substrate transfer system according to the present invention comprises a process portion for processing a substrate and a transfer portion for transferring the substrate to the process portion, and vice versa. In this substrate transfer system, the transfer portion comprises 
     a mounting portion for mounting a cassette in which a plurality of substrates are horizontally placed, 
     a substrate transfer portion provided below the mounting portion, for transferring the substrate to the process portion, 
     moving means for moving the cassette between the mounting portion and the substrate transfer portion, 
     detection means for detecting a protruding substrate when the cassette moves to the mounting portion from the transfer portion, and 
     pushing means for pushing a protruding substrate detected by the detection means into the cassette. 
     (9) A substrate treatment system comprises a process portion for processing a substrate under airtight conditions and a transfer portion for transferring a substrate to the process portion under the airtight conditions. In this substrate treatment system, 
     the transfer portion comprises 
     a mounting portion for mounting an airtight container containing a cassette in which a plurality of substrates are horizontally placed, 
     a substrate transfer portion provided in an airtight space communicated with the process portion below the mounting portion, 
     moving means for moving the cassette between the table and the substrate transfer portion, 
     detection means for detecting a protruding substrate when the cassette moves from the substrate transfer portion to the mounting portion, and 
     pushing means for pushing the protruding substrate into the cassette when the protruding substrate is detected by the detection means. 
     (10) A substrate transfer system for transferring a substrate comprises: 
     a mounting portion for mounting a cassette in which a plurality of substrates are placed horizontally, 
     a substrate transfer portion provided below the mounting portion, for transferring a substrate to other system, and vice versa, 
     moving means for moving the cassette between the mounting portion and the substrate transfer portion, 
     detection means for detecting a protruding substrate when the cassette is moved from the substrate transfer portion to the mounting portion, and 
     pushing means for pushing a protruding substrate detected by the detection means. 
     (11) A method for transferring a substrate comprises the steps of: 
     (a) mounting a cassette in which a plurality of substrates are placed horizontally, on a mounting portion, 
     (b) moving the cassette to a substrate transfer portion below the mounting portion, 
     (c) transferring the substrate in the cassette to other system at a substrate transfer portion, 
     (d) receiving the substrate from the other system into the cassette positioned at the substrate transfer portion, 
     (e) moving the cassette containing the received substrate to the mounting portion, 
     (f) detecting a protruding substrate when the cassette is moved to the mounting portion, and 
     (g) pushing the protruding substrate detected into the cassette. 
     (12) A method for transferring substrate under airtight conditions, comprises the steps of: 
     (A) mounting an airtight container on a mounting portion, the airtight container containing a cassette in which a plurality of substrates are horizontally placed, 
     (B) moving the cassette to a substrate transferring portion positioned in an airtight space below the mounting portion, 
     (C) transferring the substrate in the cassette to other system at the substrate transfer portion, the other system being provided in an airtight space communicated with the airtight space, 
     (D) receiving a substrate into the cassette positioned at the substrate transfer portion from the other system, 
     (E) moving a cassette containing the received substrate to the mounting portion, 
     (F) detecting a protruding substrate when the cassette is moved to the mounting portion, and 
     (G) pushing the protruding substrate detected into the cassette. 
     Additional object and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The object and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 is a schematic perspective view of a cassette portion of a conventionally-used coating and developing process system; 
     FIG. 2 is a plan view of the entire substrate treatment system according to the present invention; 
     FIG. 3 is a front view of the substrate treatment system; 
     FIG. 4 is a rear view of the substrate treatment system; 
     FIG. 5 is a perspective cross-sectional view of a cassette station, partially broken away, showing a cassette-cover removing mechanism of a first embodiment; 
     FIG. 6 is a perspective view of the casssette-cover removing mechanism of the first embodiment; 
     FIG. 7 is a block diagram of first and second position sensors for detecting the position of the front end protion of a cassette and for detecting a wafer protruding from a cassette, respectively; 
     FIGS. 8A to  8 M respectively show a series of procedures of removing a cassette-cover by the cassette cover removing mechanism of the first embodiment, sequentially; 
     FIG. 9 is a plan view of a protruding wafer and a mapping sensor, showing a case where a wafer protruding from a cassette interfers with the mapping sensor; 
     FIG. 10 is a partial perspective view of a cassette station, partically broken away, showing a cassette-cover removing mechanism of a second embodiment; 
     FIG. 11 is an exploded perspective view of a cassette and a cover; 
     FIG. 12 is a perspective view of a cassette-cover removing mechanism and a cassette cover of a second embodiment; 
     FIG. 13 is a plan view of the cassette-cover removing mechanism of the second embodiment; 
     FIG. 14 is an exploaded perspective view of the cassette-cover removing mechanism; 
     FIG. 15 is a cross-sectional view of a lock key of the cassette-cover removing mechanisms of first and second embodiments; 
     FIG. 16 is a perspective view showing a clean-air flow in the substrate treatment system; 
     FIG. 17 is a perspective view showing a clean-air flow in the substrate treatment system; 
     FIGS. 18A to  18 E show a series of procedures for removing a cassette-cover according to the second embodiment, sequentially; 
     FIG. 19 is a perspective cross-sectional view of a cassette station, partically broken away, showing a cassette-cover removing mechanism of a third embodiment; 
     FIG. 20 is a perspective view of a cassette-cover removing mechanism of a third embodiment; 
     FIG. 21 is a transverse cross sectional view of a lock key of a cassette-cover removing mechanism of a third embodiment; 
     FIG. 22 is a schematic perspective view of a substrate treatment system; 
     FIG. 23 is a longitudinal perspective view of a cassette station when a cassette is positioned on the cassette table; 
     FIG. 24 is a longitudinal perspective view of a cassette station when a cassette is positioned on a wafer transfer portion; 
     FIG. 25 is a schematic plan view of a protruding wafer and a wafer pushing member for explaining procedures of detecting a wafer protruding of a cassette and of pushing the wafer into the cassette; 
     FIG. 26 is a plan view of a gas-supply nozzle for supplying an inert gas into an airtight container placed on the cassette table of the cassette station; and 
     FIG. 27 is a flow chart of a substrate transfer method according to Embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinbelow, preferable embodiments of the present invention will be described with reference to the accompanying drawings. 
     As shown in FIG. 2, the coating and developing process system  1  (provided in a clean room) comprises a cassette station  10 , a process station  11 , an interface portion  12 , first and second sub-arm mechanisms  21  and  24 , and a main arm mechanism  22 . Above the portions  10 ,  11  and  12 , air-conditioning fan filter units (FFU) are provided. The fan filter unit (FFU) is responsible for blowing out clean air downwardly, thereby forming a clean-air down flow. 
     The cassette station  10  has a cassette table  20  designed for placing a plurality of cassettes CR thereon. The cassette CR contains a predetermined number of wafers W (either  25  or  13 ). A wafer W is taken out from the cassette CR by the sub-arm mechanism  21  and loaded into the process station  11 . 
     As shown in FIGS. 3 and 4, the process station  11  has 5 process units G 1  to G 5 . The process units G 1  to G 5  are arranged in a multiple-stage vertical array. Wafers are loaded/unloaded one by one to each of the process units by the main arm mechanism  22 . The interface portion  12  is interposed between the process station  11  and a light-exposure device (not shown). The wafer W is loaded/unloaded into the light-exposure device by the sub-arm mechanism  24 . 
     Four projections  20   a  are provided on the cassette table  20 . The projections  20   a  are responsible for placing the cassette CR at a predetermined position of the table  20 . A cover  44  is provided to the cassette CR to be loaded into the cassette station  10 . The cassette CR is positioned on the cassette table  20  in such a way that the cover faces the process station  11 . 
     The process station  11  has 5 process units G 1 , G 2 , G 3 , G 4  and G 5 . The first and second process units G 1  and G 2  are arranged in the front side of the system. The third process unit G 3  is positioned adjacent to the cassette station  10 . The fourth process unit G 4  is positioned next to the interface portion  12 . The fifth process unit G 5  is positioned in the rear side of the system. 
     The main arm mechanism  22  has moving mechanisms along the X-axis and Z-axis and a rotating mechanism about the Z axis by angle θ. The main arm mechanism  22  receives the wafer W from the first sub-arm mechanism  21  and then transfers the wafer W to an alignment unit (ALIM) and an extension unit (EXT) belonging to the third process unit G 3  in the process station  11 . 
     As shown in FIG. 3, in the first process unit G 1 , two spinner type process units are provided in which predetermined processing is respectively applied to the wafer mounted on a spin chuck in the cup (CP). To be more specific, a resist coating (COT) unit and a developing (DEV) unit are superposed in this order from the bottom. In the same manner, two spinner type process units, COP and DEV units are superposed in the second process unit G 2 . These COT units are preferably arranged in a lower position to facilitate the discharge. 
     As shown in FIG. 4, the third process unit G 3  consists of 8 layers, that is, a cooling (COL) unit, an adhesion unit, an alignment(ALIM) unit, an extension (EXT) unit, prebaking (PREBAKE) units, and post baking (POBAKE) units. They are superposed in this order from the bottom. In the same manner, the fourth process unit G 4  consists of 8 layers, that is,. a cooling (COL) unit, an extension cooling (EXTCOL) unit, an extension (EXT) unit, a cooling unit (COL), prebaking (PREBAKE) units, and postbaking (POBAKE) units. 
     Since the COL and EXTCOL units responsible for low-temperature processing are placed in the lower stage and PREBAKE, POBAKE, and AD units responsible for high temperature processing are placed in the upper stage, thermal interference between the units can be lowered. 
     The size in the X-axis direction of the interface portion  12  is almost equal to that of the process station  11 . However, the size in the Y-axis direction is smaller than that of the process station  11 . In the front portion of the interface portion  12 , an immobile buffer cassette BR is arranged. In the rear portion, a peripheral light exposure device  23  is positioned. In the center portion (in the vertical direction), the second sub-arm mechanism  24  is provided. The second sub-arm mechanism  24  has the same moving mechanisms as those of the first sub-arm mechanism  21 . The second sub-arm mechanism can access to the EXT unit belonging to the forth process unit G 4  and to the adjoining wafer transfer portion (not shown) provided on the light exposure side. 
     In the coating and developing process system  1 , the fifth process unit G 5  may be arranged on the back side of the main wafer transfer mechanism  22 . The fifth process unit G 5  can be moved in the Y-axis direction along a guide rail  25 . If the fifth process unit G 5  is moved, an enough space can be given for performing maintenance and inspection of the main arm mechanism  22  from the back side. 
     As shown in FIG. 5, a transfer chamber  31  of the cassette station  10  is shut out from a clean-room atmosphere by means of a first vertical partition board  32 . In the lower portion of the first vertical partition board  32 , a gate block  60  is provided. In the gate block  60 , an upper opening passage (tunnel)  33   a  and a lower opening (storage room)  33   b  are formed. In the space made of these upper and lower openings  33   a  and  33   b , a cover-removing mechanism  47  is provided. In the passage  33   a , a cover  44  is removed from the cassette CR by means of the cover-removing mechanism  47  and stored in the storage room  33   b  for a while. 
     The cassette station  10  and process station  11  are separated from each other by a second vertical partition board  35 . The second vertical partition board  35  has a communication passage  36  with an open/close shutter  37 . 
     In the space between the first and second partition boards, the first sub-arm mechanism  21  is provided. The first sub-arm mechanism  21  comprises an X-axis moving mechanism  42  for moving the arm  21   a  in the X direction, a Y-axis moving mechanism  39  for moving the arm  21   a  in the Y direction, and a Z-axis moving and rotating mechanism  40  for moving the arm  21   a  in the Z direction and rotating the arm  21   a  about the Z-axis. A wafer W is taken out from the cassette CR by the first sub-arm mechanism  21  through the passage (tunnel)  33   a  of the gate block  60  and loaded into a process station  11  through the passage  36  of the second partition board  35 . 
     Hereinbelow, the cassette table  20  and the cover removing mechanism  47  will be explained with reference to FIGS. 6,  7 ,  8 A to  8 M,  11  and  15 . 
     To the cassette table  20 , a movable base  80  is provided which is connected to a rod  82   a  of a Y-axis cylinder  82 . On the middle of the upper surface of the movable base  80 , the projection  20   a  is provided. When the cassette CR is mounted on the cassette table  20 , the projection  20   a  is engaged with a depression (not shown) formed on the bottom of the cassette CR. In this manner, the cassette CR is positioned at a predetermined position. The projection  20   a  has a touch sensor function. Hence, when the cassette CR is placed on the cassette table  20 , the presence of the cassette CR is detected by the sensor. The detection signal is sent from the touch sensor to a controller  59 . 
     As shown in FIG. 6, the cover-removing mechanism  47  has a shutter board  49  and an elevator mechanism  52 . The elevator mechanism  52  comprises a pair of linear guides  48 , a ball screw  53 , and a motor  55 . The linear guides  48  are provided vertically on sides of both the upper opening (tunnel)  33   a  and the lower opening  33   b . Nuts  49   a  are provided on the left and right end portions of the shutter board  49  and respectively connected to linear guides  48 . The nuts  49   a  are screwed on the ball screw  53 . A gear  54  of the screw  53  is engaged with a movement gear  56  of the motor  55 . The shutter board  49  can be moved through a space in the Z direction from the passage(tunnel)  33   a  to the storage room  33   b  by means of the elevator mechanism  52 . It should be noted that an air cylinder may be employed as the elevator mechanism  52 . 
     The shutter board  49  has a pair of keys  50 . Each of the keys  50  is supported by a θ′ rotation mechanism (not shown). Each of the keys  50  is provided on the shutter board  49  so as to correspond to each of key holes  45  formed in the cassette cover  44  shown in FIG.  11 . As shown in FIG. 15, when the key  50  is inserted in the key hole  45  and rotated by an angle of θ′, a lock piece  249  engaged with a key groove of the key hole  45 . In this manner, the cassette cover  44  is locked on the shutter board  49 . 
     As shown in FIG. 7, first optical sensors  57   a  and  57   b  are provided above and below the gate block  60 , respectively, in such a way that the optical axis formed between the sensors crosses the front portion of the cassette CR set on a second position. The second optical sensors  58   a  and  58   b  are provided above and below the gate block  60 , respectively, in such a way that the optical axis formed between the sensors crosses the front portion of the cassette CR set on a third position. 
     The controller  59  controls the movements of the Y-axis cylinder  82  on the cassette table  20  and the motor  55  of the cover-removing mechanism  47  on the basis of detection data sent from the touch sensor  20   a  and the first and second optical sensors  57   a ,  57   b ,  58   a  and  58   b.    
     As shown in FIG. 8B, the initial position of the cassette CR at which the cassette CR is placed for the first time on the cassette table  20  is defined as “a first position”. As shown in FIG. 8F, the position at which the cassette CR is moved backward from a removed cover  44  is defined as a “second position”. Furthermore, the position of the cassette CR when the cover  44  is removed from the cassette CR (shown in FIGS. 8D and 8L) and the position of the cassette CR with the cover  44  removed (shown in FIGS. 8G to  8 I) when the wafer W is taken out from the cassette CR is defined as “a third position”. 
     The cover  44  of the cassette CR at the first position is located on an entrance (front end portion) of the passage (tunnel)  33   a . The controller  59  detects whether the cassette CR is positioned at the first position or not on the basis of the detection data sent from the touch sensor  20   a  and the first and second optical sensors  57   a ,  57   b ,  58   a  and  58   b.    
     The first sensors  57   a  and  57   b  are responsible for detecting the wafer Wh protruding from the cassette without the cover. The second sensors  58   a  and  58   b  are responsible for detecting the protruding wafer Wh from the cassette CR in order to prevent the interference between the first sub arm mechanism  21  and the wafer Wh. 
     Hereinafter, the operation of the cover-removing mechanism  47  will be explained with reference to FIGS. 8A to  8 M and FIG.  9 . 
     Before the cassette CR is mounted on the cassette table  20 , a shutter board  49  of the cover removing mechanism  47  is positioned on a passage (tunnel)  33   a , as shown in FIG.  8 A. The atmosphere inside the transfer chamber  31  is isolated from that of the clean room. 
     As shown in FIG. 8B, when the cassette CR is mounted on the cassette table  20 , the projection  20   a  is engaged with a depressed portion (not shown) of the cassette bottom. In this manner, the cassette CR is positioned at the first position. 
     As shown in FIG. 8C, the cassette CR is moved forward from the first position to the third position. In this way, the cassette cover  44  is pressed against the shutter board  49 . Then, as shown in FIGS. 8D,  11 , and  15 , the key  50  is inserted into the key hole  45  and turned to lock the shutter board  49  to the cover  44 . In this manner, the cassette cover  44  and the shutter board  49  are made into one body. 
     As shown in FIG. 8E, the cassette CR is moved back from the third position to the second position to remove the cover  44  from the cassette CR. Subsequently, as shown in FIG. 8F, the cover  44  is descended together with the shutter board  49  to house the cover  44  in the storage room (the lower opening)  33   b.    
     In the second position, since the front portion of the cassette CR is within the passage (tunnel)  33   a , the atmosphere in the cassette communicates with that of the process system  1  and the cassette CR cannot be raised from the cassette table  20  during the processing of the wafer W. Therefore, it is possible to prevent an accident in which an operator mistakenly picks up the cassette CR during the processing and interrupts the operation. 
     As shown in FIG. 8G, the cassette CR is then moved forward from the second position to the third position to arrange a front distal end portion of the cassette CR to the place to which the arm  21   a  of the first sub-arm mechanism accesses. By virtue of the presence of the cassette CR, the atmosphere of the transfer chamber  31  is shut out from that of the clean room with the result that particles are prevented from entering the process system  1  through the passage  33   a.    
     As shown in FIGS. 8H and 8I, the arm  21   a  of the first sub-arm mechanism  21  is inserted into the cassette CR and takes out the wafer W from the cassette CR. As shown in FIG. 9, to the arm  21   a  of the first sub arm mechanism  21 , a pair of mapping sensors  21   b  are movably provided. When the mapping operation is made, these sensors  21   b  are designed to move to the distal end of the arm  21   a . Due to these structures, if there is a wafer Wh protruding from the cassette CR, the sensor  21   b  hits against the protruding wafer Wh, causing not only misoperation of the mapping but also damage of the wafer Wh. When the wafer Wh protruding from the cassette CR is detected by the first sensors  57   a  and  57   b , the detection signal is sent to the controller  59 , the mapping operation is immediately stopped with the sound of an alarm to avoid mutual interference between the protruding wafer Wh and the sensor  21   b . The operator checks the wafer Wh in the cassette CR and returns the wafer Wh to a right position. Thereafter, the operator pushes a reset button to restart the processing operation. The protruding wafer Wh may be pushed into the cassette CR by a wafer pushing mechanism which will be described later (see FIGS. 23 to  25 ) instead of manual operation by the operator. 
     The wafer W is loaded from the cassette station  10  into the process station  11 , processed through individual units of the process station  11 , exposed light in the light-exposure device, and returned to the cassette CR of the cassette station  10 , again. 
     After completion of processing all wafers W in the cassette CR, the cassette CR is moved back from the third position to the second position. Since the cassette CR is located at the second position, mutual interference between the cover  44  and the cassette CR can be prevented even if the cover  44  is ascended from the storage room  33   b  to the passage  33   a.    
     As shown in FIG. 8K, the cover  44  is ascended together with the shutter board  49  until the cover  44  comes to the passage  33   a . Subsequently, as shown in FIG. 8L, the cassette CR is moved forward from the second position to the third position. As a result, the opening portion of the cassette CR is pressed to the cover  44 . In this manner, the cover fits into the opening of the cassette CR. 
     Furthermore, as shown in FIGS. 11 and 15, the key  50  is turned to release the lock between the shutter board  49  and the cover  44 . As shown in FIG. 8M, the cassette CR is moved back from the third position to the first position to take the cover away from the shutter board  49 . The cassette CR is then unloaded from the station  10 . 
     According to the aforementioned device, the shutter board  49  shuts up the passage  33   a  when no operation is made and the cassette CR shuts up the passage  33   a  when the operation is made. It is therefore difficult for particles to enter the system from the clean room. 
     Since the cassette CR is moved forward and backward toward the passage  33   a  by the Y-axis cylinder  82 , it is not necessary to provide the Y-axis movement mechanism to the cover removing mechanism  47 . Therefore, the structure of the cover removing mechanism  47  may be simplified, reducing the amount of particles generated. 
     Since the wafer W is loaded to and unloaded from the cassette CR while the front end of the cassette CR is present in the passage  33   a , the trouble that an operator inadvertently picks up the cassette CR from the cassette table  20  during the processing can be fully prevented. 
     Hereinafter, the device and method of the second embodiment will be explained with reference to FIGS. 10-18E. The part of the second embodiment common in the first embodiment will be omitted. 
     As shown in FIG. 10, the transfer chamber  31  of the cassette station  10  is separated from the atmosphere of the clean room by a partition board  32  made of, for example, an acrylic board and a stainless steel board. On the partition board  32 , four passages  33  are formed. The sub arm mechanism  21  is provided in the transfer chamber  31 . The sub arm mechanism  21  is responsible for load/unload of the wafer W to the cassette CR through the passage  33 . The size of the passage  33  is slightly larger than the opening  43  of the cassette CR. Above the passage  33 , an open-close shutter  34  is provided. The shutter  34  is opened when the cassette CR is present on the cassette table  20  and closed when the cassette CR is not on the cassette table  20 . 
     As shown in FIG. 11, the opening  43  is formed in the front portion of the cassette CR. The wafer W is loaded/unloaded to the cassette CR through the opening  43 . The cover  44  is provided to the opening  43  in order to keep the inside of the cassette CR airtight. The cassette CR is charged with a non-oxidative gas such as a N 2  gas. Alternatively, N 2  gas charging means may be provided to the cassette table  20  to supply the N 2  gas or the like into the cassette CR from which the wafer is to be taken out. Inside the cover  44 , lock means  44  (not shown) is provided to fix the cover to the cassette CR. On the surface side of the cover  44 , two key holes  45  are formed. The distance between two key holes is desirably a half or more of a lengthwise side of the cover. 
     As shown FIG. 10, on the transfer chamber  31  side of the cassette table  20 , four cover storage portions  246  are arranged side by side in the X-axis direction. The storage  246  is a portion for storing the cover  44  removed from the cassette CR. 
     On the other hand, four cover removing means  247  are provided to the transfer chamber  31 . The cover removing means  247  are formed in correspondence with the cover storage portions  246 . The cover  44  removed from the cassette CR is stored in the cover storage portions  246  below. 
     As shown in FIGS. 12 to  14 , the cover removing mechanism  247  has Z-axis moving means  251  and Y-axis moving means  252 . The Z-axis moving means  251  has two Z-axis cylinders  254  which synchronously move up and down. A cover transfer member  248  is supported by the Z-axis cylinders  254 . Each of the Z-axis cylinders  254  is supported by the both ends of a supporting member  255 . The supporting member  255  is connected to two Y-axis cylinders  256 . The Y-axis cylinder  256  is provided to the cassette table  20  and designed to move the cover transfer member  248  in the Y-axis direction. 
     The coating and developing process system  1  is placed in the clean room in which a clean-air flows downwardly. As shown in FIGS. 16 and 17, a clean-air downflow is also formed within the system  1  to keep individual units of the process system  1 , clean. In the upper portions of the cassette station  10 , process station  11  and the interface portion  12  of the system  1 , air-supply chambers  61 ,  62  and  63  are provided. In the lower surfaces of the air supply chambers  61 ,  62  and  63 , dustproof ULPA filters  64 ,  65  and  66  are provided. 
     As shown in FIG. 17, an air-conditioning  67  is provided on the outside or the backside of the process system  1 . Air is introduced into the air-supply chambers  61 ,  62  and  63  from the air-conditioning  67  by way of a pipe  68 . The introduced air is converted into clean air by means of the ULPA filters  64 ,  65  and  66  provided in the individual air-supply chambers. The clean air is supplied downwardly to the portions  10 ,  11  and  12 . The down-flow air is collected at a vent  70  through the air holes  69  appropriately provided in the lower portion of the system. The air is returned to the air conditioning  67  from the vent  70  through the pipe  71 . 
     In the ceilings of the resist coating unit (COT),(COT) positioned lower portion of the first and second process units G 1  and G 2  in the process station  11 , an ULPA filter  72  is provided. Air from the air-conditioning  67  is sent to the ULPA filter  72  by way of a pipe  73  branched from the pipe  68 . In the middle of the pipe  73 , a temperature/humidity controller (not shown) is provided for supplying clean air to the resist coating unit (COT) (COT). The controller controls the clean air so as to have a predetermined temperature/humidity suitable for the resist coating step. A temperature/humidity sensor  74  is provided in the proximity of the blow-out port of the ULPA filter  72 . The data obtained by the sensor is fed-back to the control portion of the temperature/humidity controller to control the temperature/humidity of the clean air accurately. 
     In FIG. 16, in the side wall of each of spinner-type process units such as COT and DEV, facing the main wafer transfer mechanism  22 , openings DR are formed trough which the wafer and the transfer arm go in and out. Furthermore, to each of the openings DR, a shutter (not shown) is provided to prevent particles or the like from entering the space on the side of the main arm mechanism  22 . 
     The amounts of air supplied or exhausted to the transfer chamber  31  are controlled by the air conditioning  67 . By this control, the inner pressure of the transfer chamber  31  is set higher than the inner pressure of the clean room. It is therefore possible to prevent the formation of the air flow from the clean room and the cassette CR to the transfer chamber  31 . As a result, particles are successfully prevented from entering the transfer chamber  31 . Since the inner pressure of the process station  11  is set higher than the inner pressure of the transfer chamber  31 , the formation of air flow from the transfer chamber  31  to the process station  11  can be prevented. As a result, particles are successfully prevented from entering the process station  11 . 
     Hereinbelow, movement of the cover removing mechanism  247  will be explained with reference to FIGS. 18A to  18 E. Movement of the cover-removing mechanism  247  is controlled by a controller  59  shown in FIG.  7 . 
     As shown in FIG. 18A, the shutter  34  is opened and the cassette CR is mounted on the cassette table  20 . Then, the cover transfer member  248  is moved forward to the passage  33  by an Y-axis movement mechanism  256 . Thereafter, as shown in FIG. 18B, the key  249  for the cover-transfer member  248  is inserted in the key hole  45  of the cover  44  and locked to each other through an inner lock mechanism. The key  249  is rotated by an angle θ′, thereby releasing the lock between the cover  44  and cassette CR. In this manner, the cover  44  can be removed from the cassette CR. 
     As shown in FIG. 18C, the cover transfer member  248  is then moved back together with the cover  44  in the Y-axis direction to load the cover into the transfer chamber  31  through the passage  33 . The cover transfer member  248  is descended together with the cover  44 , as shown in FIG. 18D, by means of a Z-axis direction moving mechanism  251  to the position facing the storage portion  246 . Then, as shown in FIG. 18E, the cover transfer member  248  is moved forward in the Y-axis direction to store the cover  44  in the storage portion  246 . 
     Thereafter, the wafer W is taken out from the cassette CR by means of the sub-arm mechanism  21  and transferred to the process station  11 . After the wafer W is processed in individual process units, the wafer W is returned to the cassette CR. After the processing of all wafers housed in the cassette CR is completed, the cover  44  is transferred from the storage portion  246  to the passage  33  to put the cover on the opening of the cassette CR. The cassette CR is covered with the cover  44 , locked and transferred outside the system  1 . 
     In the aforementioned process system  1 , the clean air downwardly flowing in the transfer chamber  31  is not disturbed by the attach and detach movement of the cover  44  from the cassette CR. 
     Since the cover is housed in the storage portion  246 , the cover  44  itself does not disturb the down flow of the clean-air in the transfer chamber  31 . Therefore, deficiency of manufactured products due to particles can be reduced. 
     A third embodiment of the present invention will be explained with reference to FIGS. 19-21. 
     In the system of the third embodiment shown in FIGS. 19 and 20, the cover removed from the cassette CR is rotated about a horizontal axis  384  by 180 degrees by means of a rotation mechanism  382  and then housed in a storage portion  346 . The rotation mechanism  382  comprises a U-shape arm member  381 , a key  349 , a horizontal supporting axis  384 , θ′ rotation motor (not shown), and θ″ rotation motor (not shown). The key  349  is provided to one end of the U-shape arm member  381 . The θ′ rotation motor is used for rotating the key  349  by an angle of θ′. The θ″ rotation motor is used for rotating the key  349  by an angle of θ″, together with the horizontal support axis  384  and the U-shape arm member  381 . 
     As shown in FIG. 21, the key  349  is rotatably provided in the arm member  381 . When the key  349  is inserted in the key hole  45 , a lock piece  350  is engaged with a key groove. When the key  349  is rotated by angle θ′, the lock between the cover  44  and the cassette CR is released. In this manner, the cover  44  becomes detachable from the cassette CR. When the horizontal support axis is rotated by angle θ″, a cover  44  is rotated by 180 degrees and housed in the storage portion  346 . As described above, in the system according to the third embodiment, the cover can be housed in a simplified mechanism. 
     The embodiments mentioned above are concerned with a resist coating and developing process system used in the photolithography step of the semiconductor device manufacturing process. The present invention is applicable to other process systems. The substrate to be processed is not limited to a semiconductor wafer. Examples of applicable substrates include an LCD substrate, a glass substrate, a CD substrate, a photomask, a printing substrate, a ceramic substrate and the like. 
     According to one aspect of the present invention, even if the cover is opened or shut at the opening portion of the cassette, the clean-air downflow will not be disturbed by the open/shut movement. Deficiency in manufactured products due to particles can be reduced. 
     According to another aspect of the present invention, even if the cover is opened or shut at the opening portion of the cassette, the clean-air downflow will not be disturbed. In addition, particles can be prevented from attaching to a substrate in a transfer room and in a process chamber. As a result, deficiency in manufactured products due to particles is successfully prevented. 
     According to still another aspect of the present invention, no particles flow out from the cassette side to the device side. 
     According to a further aspect of the present invention, no particles flow out from the clean room and the cassette to the device side. 
     According to a still further aspect of the present invention, no particles flow out from the transfer room to the process chamber  31 A. 
     Hereinbelow, a fourth embodiment of the present invention will be explained with reference to FIGS. 22 to  27 . In the fourth embodiment, the present invention is applied to a substrate washing process system having a scrubber for brush-washing a semiconductor wafer W. 
     The substrate washing process system comprises a cassette station  401  and a washing process station  402  having a plurality of units. The cassette station  401  comprises mounting portions  414  for mounting airtight containers (SMIF POD)  413  having cassettes C. A plurality of wafers W are stored in each cassette C. In the cassette station  401 , the wafer W is transferred to other system, and to the washing process station  402 , and vice versa. 
     On the mounting portion  414 , three mounting boards are provided. Each board has a table  412  for mounting the cassette thereon. Below the mounting portion  414 , a wafer-transfer portion  415  (described later) is formed. On the side of the washing process portion  402  of the cassette station  401 , a passage  410  is provided in the arrangement direction of the table  412 . The cassette station arm  411  is provided to the passage  410  which moves therealong. The wafer W is transferred from cassette C present in the wafer transfer portion  415  to the washing process station  402 , and vice versa, by means of the cassette station arm  411 . The passage  410  is covered with a cover (not shown) and shut out from the atmosphere of the clean room. 
     In the middle of the washing process station  402 , a passage  420  is provided. The passage  420  crosses the passage  410  at a right angle. The washing process station  402  comprises a plurality of units arranged on both sides of the passage  420 . To be more specific, on one side of the passage  420 , two surface washing units  421  and thermal system units  422  are arranged side by side. On the other side of the passage  420 , two rear-surface washing units  423  and reverse-turn units  424  are juxtaposed. The thermal system units  422  consist of four units layered one on top of another. The three units from the above are heating units  425 . The lowermost one is a cooling unit  426 . The reverse-turn units  424  consist of two units. The upper reverse-turn unit  427  plays a part of turning over the wafer W. The lower reverse-turn unit  428  has an alignment mechanism of the wafer W other then the turn-over mechanism of the wafer W. 
     The washing process station  402  has a wafer-transfer mechanism  403  which is movable along the passage  420 . The transfer mechanism  403  has a transfer main arm  403   a , rotatable and movable back and forth and up and down. The main arm  403   a  is responsible for transferring the wafer W to the cassette station arm  411  and to each of units, and vice versa, and further responsible for load/unload of the wafer to each of units. Note that the entire system is covered with a wrapping cover (not shown). 
     Hereinbelow, the cassette station  401  will be explained in detail with reference to FIGS. 23 and 24. 
     The cassette station  401  has a substrate transfer portion  415  in an airtight space  416  below the mounting portion  414 . On the mounting portion  414 , an elevator table  412  is provided for receiving the cassette C accommodated in the airtight container  413 . The airtight container  413  has a lock mechanism (not shown) responsible for maintaining the cassette C under airtight conditions. In the cassette C, wafers W are placed horizontally and arranged vertically. 
     When the airtight container  413  is placed on the mounting portion  414  and then the lock mechanism is released, the cassette C is ready to move together with the elevator table  412  to the wafer transfer portion  415 . In this case, the space between the airtight chamber  413  and the box forming the airtight space  416  is maintained airtight. 
     The elevator table  412  is movably supported by the elevator  430 . The cassette C is designed to move up and down by the elevator  430  between the mounting portion  414  and the wafer transfer portion  415  in the airtight space  416 . The elevator  430  comprises a support member  431  for supporting the elevator table  412 , a ball screw  432  for engaging with the support member  431 , a stepping motor  434  for rotating the ball screw  432 , and a guide member  433 . 
     As shown in FIG. 24, the elevator table  412  is descended by the elevator  430  until the cassette C faces the wafer transfer portion  415 . The wafer W is transferred to the process portion  402  by the cassette station arm  411 . Since the passage  410  is covered with a cover  410   a , the passage  410  is an airtight space communicable with the airtight space  416  and further communicable with airtight space, passage  420 , as mentioned above. Therefore, the wafer W is washed in a series of washing processes without exposed to outer air. 
     Above the airtight space  416 , a pushing member  435  is provided for pushing the wafer into the cassette from the wafer transfer side. In the proximity of the pushing member  435 , a light emitting portion  436   a  and a light receiving portion  436   b  (serving as a first detection device) are provided so as to bridge the surface of the wafer transferred from the cassette C, as shown in FIG.  25 . If there is a wafer Wh protruding from the cassette C, the protruding wafer Wh intervenes in light traveling from the light emitting portion  436   a  to the light receiving portion  436   b . In this manner, the protruding wafer Wh can be detected. To the upper and lower portions on the side of the airtight space  416  from which the cassette is transferred, a light emitting portion  437   a  and a light receiving portion  437 b serving as a second detection device are provided. When the light traveling from the light emitting portion  437   a  to the light receiving portion  437   b  is intervened, subsequent movement is immediately stopped. 
     On the back side of the cassette C in the upper portion of the airtight space  416 , a gas supply nozzle  438  is provided. When the cassette C is not present in the airtight container  413 , the airtight container  413  is purged with a non-oxidative gas such as a nitrogen gas supplied from the nozzle  438  to eliminate particles or the like almost completely. The nozzle  438  comprises a nozzle head  439  having a plurality of gas releasing holes  440  arranged along the X-axis and a supporting portion  441  for supporting the nozzle head  439 , as shown in FIG.  26 . 
     Hereinbelow, the movement of the above-mentioned device will be explained with reference to FIG.  27 . 
     The airtight container  413  housing a cassette C is loaded into the cassette station  401  and placed on the mounting portion  414  (S  1 ). After the lock between the airtight container  413  and the cassette C is released, the cassette C is transferred onto the elevator table  412 . The elevator table  412  is then descended by the elevator  430  to the wafer transfer portion  415  of the airtight space  416  (S  2 ). A nitrogen gas is supplied from the nozzle  438  to purge the airtight container  413  (S  3 ). While the cassette C is being descended, the position of a wafer is detected by using the first and second sensors  436   a ,  436   b ,  437   a  and  437   b . The detection data is input to the process computer and processed therein. This is called “mapping operation”by which the wafer information, such as a wafer pitch and the presence or absence of the wafer in the cassette C are obtained (S  4 ). The mapping operation is performed to determine whether or not the wafer Wh protrudes from the cassette C by CPU (S  5 ). 
     The second sensors  437   a  and  437   b  detect the protruding wafer Wh. When the second sensors determine that it is difficult to push back the protruding wafer by use of the pushing member  435 , an alarm is turned on (S  6 ) and the movement is immediately stopped (S  7 ). When no protruding wafer Wh from the cassette C is detected, cassette C is ascended (S  8 ). 
     During the ascending step (S  8 ), if the first sensors  436   a  and  436   b  detect the protruding wafer Wh (S  9 ), the ascending of the cassette C is stopped and the pushing member  435  is moved toward the cassette C and pushes the protruding wafer Wh into the cassette C (S  10 ). During the ascending step (S  8 ), all protruding wafers Wh are pushed into the cassette C by the pushing member  435  by checking the wafers W one by one. In this manner, the wafer is successfully prevented from hitting against the upper-wall of the air-tight space  416  while the cassette C is being ascended through the airtight container  413 . Therefore, the breakage of the wafer W is successfully prevented. 
     When the cassette C is present in the wafer transfer portion  415 , the wafer W is received by the cassette station arm  411  and transferred to the main arm  403   a  of the transfer mechanism  403 (S  11 ). 
     The wafer W on the main arm  403   a  is then subjected to a series of washing processes according to a predetermined recipe. First, the surface of the wafer W is washed with a brush in the surface washing unit  421  (S  12 ). Subsequently, the wafer W is turned over by the reverse-turn unit  427  or  428 . The rear surface of the wafer W is washed with a brush in the rear surface washing unit  423 . Thereafter, the wafer is turned over by means of the reverse-turn unit  427  or  428 . If necessary, the washed wafer W is dried with heat in the heating unit  425  (S  13 ), and cooled in the cooling unit  426  (S  14 ). 
     After a series of processing are completed, the wafer W is transferred from the main arm  403   a  to the cassette station arm  411  and housed in the cassette C present in the wafer transfer portion  415  by the arm  411  (S  15 ). The same processing is performed with respect to a predetermined number of wafers W. When the predetermined number of wafers W are processed and housed in the cassette C, the cassette C is ascended (S  16 ) and returned to the airtight container  413  on the mounting portion  414 . The cassette C is locked in the airtight container  413 . The airtight container  413  containing the cassette C is transferred outside of the system (S  17 ). 
     The present invention is not limited to the above-mentioned embodiments. Modification of the present invention may be made in various ways. In the above-mentioned embodiments, we explained the example in which the present invention is applied to the washing unit. The present invention may be used in a unit in which other processing other than the washing is made, for example, a resist-coating and developing unit. The present invention may be effective not only when processing carried out in an airtight system but also when a substrate may hit against something or may be caught by something during the movement of a cassette. The present invention may be applied to various transfer units other than the process units. The substrate to be used in the present invention is not limited to the semiconductor wafer. Examples of the substrate include an LCD substrate, glass substrate, CD substrate, photomask, printing substrate and the like. 
     As explained in the foregoing, the present invention makes it possible to prevent the breakage of the substrate when the cassette is returned to the substrate transfer portion to the mounting portion, thereby attaining an extremely high yield of the wafers since a protruding substrate is checked by the detection means while the cassette is being moved from the transfer portion to the mounting portion, and the detected protruding substrate is pushed back by the pushing means. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalent.