Abstract:
A substrate transporting method including inputting process data and determining whether a number of units required for processing a wafer is an odd number or an even number. Depending on the number of units required for processing the wafer, steps of transporting the wafer, taking out the wafer from a cassette section, loading the wafer, unloading the wafer and loading the wafer into a cassette section are performed via predetermined arms and with predetermined processing units.

Description:
This is a Division of Application Ser. No. 09/266,576, filed Mar. 11, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a substrate transport method and apparatus for transporting a substrate to a processing section to clean the surface of the substrate, and also relates to a substrate processing system having the substrate transport apparatus. 
     In a photolithographic process of manufacturing the semiconductor devices, it is very important to maintain the surface of the wafer clean. This is because if contaminants, such as particles, organic substances, and metallic ions, are attached to the surface of the semiconductor wafer, they significantly defect a circuit pattern of the semiconductor device. Therefore, in the photolithographic process, the wafer surface is usually washed when necessary. The contaminants are removed from the wafer surface, for example, by rubbing the wafer surface by a brush while a chemical washing solution is poured thereon. Such a washing treatment is usually carried out in a washing apparatus equipped with a spin chuck and a rotatory brush. 
     As shown in FIG. 1, a conventionally used substrate processing system  100  has a processing section  104  for washing the surface of the wafer with a chemical solution and drying it, and a substrate transporting arm mechanism  105  for transporting the wafer W to the processing section  104 . The processing section  104  has three processing units  101 ,  102 ,  103 . The substrate transport apparatus  105  has three arms  106   a ,  106   b ,  106   c . The processing units  101 ,  102 ,  103  have loading/unloading ports  101   a ,  102   a ,  103   a , respectively. The wafer W is loaded/unloaded into/from units  101 ,  102 ,  103  through the loading/unloading ports  101   a ,  102   a ,  103   a , respectively. 
     The substrate transport apparatus.  105  has an X-axis driving mechanism for moving an arm portion  106  in an X-axis direction, a Z-axis driving mechanism  107  for moving the arm portion  106  in a Z-axis direction, a θ-axis driving mechanism for rotating the arm portion  106  around the Z-axis, and a back-and-forth moving mechanism for moving each of arms  106   a ,  106   b ,  106   c  back and forth. The Z-axis driving mechanism  107  has a single ball screw  110  whose rotation movement is driven by a motor  109 . The motor  109  and the ball screw  110  are surrounded by a cover  108  in the expandable bellows form. 
     However, when the Z-axis driving mechanism  107  is used for a long time, particles are sometimes generated from the cover  108  in the form of bellows, attaching onto the wafer W. 
     Furthermore, as shown in FIG. 2, the vertical opening length of the loading/unloading port  101   a  in the conventional apparatus is larger than the vertical size of an assembly of three arms  106   a ,  106   b ,  106   c . Thus, the particles are likely to enter the processing unit  101  when the wafer W is loaded/unloaded. In addition, a shutter  130  is moved in a long distance and thus long time is required to open/shut the loading/unloading port  101   a . As a result, throughput of the treatment decreases. Furthermore, it take long time to exchange the first arm  106   a  arranged at the uppermost stage and the third arm  106   c  arranged at the lowermost stage, with the result that the throughput decreases. 
     BRIEF SUMMARY OF THE INVENTION. 
     An object of the present invention is to provide a substrate transporting method and apparatus, and a substrate processing system capable of reducing time required,for loading and unloading a substrate into/from a processing unit, thereby improving the throughput. 
     Another object of the present invention is to provide a substrate transport apparatus having a driving mechanism which does not allow the particles to leak outside. 
     According to the present invention, there is provided a method of transporting a substrate by using a substrate transport apparatus which comprises: 
     first, second, and third arms arranged vertically in multiple stages, 
     a plurality of processing units each having a load/unload port for loading/unloading the substrate, 
     the method comprising: 
     (a) inputting data of processing conditions for processing the substrate; 
     (b) determining whether a number of processing units required for processing the substrate is an odd number or an even number; 
     (c1) when a determination result of the step (b) is an odd number, 
     transporting the substrate, in accordance with the following steps (d1) to (i1): 
     (d1) taking out the substrate by the second arm from a substrate loading/unloading section; 
     (e1) loading the substrate by the second arm to an odd-numbered processing unit; 
     (f1) unloading the substrate by the third arm from an odd-numbered processing unit; except for a final processing unit; 
     (g1) loading the substrate by the third arm to an even-numbered processing unit; 
     (h1) unloading the substrate by the second arm from an even-numbered processing unit; and 
     (i1) unloading the substrate from the final processing unit by the first arm and loading the substrate by the first arm into the substrate loading/unloading section; and 
     (c2) when a determination result of the step (b) is an even number, 
     transporting the substrate, in accordance with the following steps (d2) to (i2); 
     (d2) taking out the substrate by the third arm from the substrate loading/unloading section; 
     (e2) loading the substrate by the third arm into an odd-numbered processing unit; 
     (f2) unloading the substrate by the second arm from an odd-numbered processing unit; 
     (g2) loading the substrate by the second arm into an even-numbered processing unit; 
     (h2) unloading the substrate by the third arm from an even-numbered processing unit except for a final processing unit; and 
     (i2) unloading the substrate by the first arm from the final processing unit and loading the substrate by the first arm into the substrate loading/unloading section. 
     Note that a second substrate to be used next may be taken out from the substrate loading/unloading section in advance by the second arm during a period from the step (e1) to (f1). 
     Furthermore, the second substrate may be unloaded from an odd-numbered processing unit in advance by the third arm during a period from the step (g1) to (h1). 
     Moreover, the second substrate may be taken out from the substrate loading/unloading section in advance by the third arm during a period from the step (e2) to (f2). 
     Still further, the second substrate may be unloaded from an odd-numbered processing unit in advance during a period from the step (g2) to (h2). 
     According to the present invention, there is provided a substrate processing system comprising: 
     a substrate loading/unloading section for receiving a plurality of substrates and sending out the substrates sequentially one by one; 
     a processing section having a plurality of processing units each having a loading/unloading port for loading and unloading the substrates; 
     a substrate transport apparatus which has first, second, and third arms arranged movable between the substrate loading/unloading section and the processing section and set vertically in multiple stages, and which has an arm back-and-forth moving mechanism for moving each of the first, second, and third arms, back and forth; 
     a control section for controlling an operation of the substrate transport apparatus; and 
     data input means for inputting data of processing conditions for processing the substrate into the control section. 
     The control section controls the substrate transport apparatus by 
     determining whether a number of processing units required for processing the substrate is an odd number or an even number on the basis of the data of processing conditions; 
     when a determination result is an odd number, 
     taking out the substrate, by the second arm, from the substrate loading/unloading section; 
     loading the substrate by the second arm to an odd-numbered processing unit, 
     unloading the substrate by the third arm from an odd-numbered processing unit except for a final processing unit; 
     loading the substrate by the third arm to an even-numbered processing unit; 
     unloading the substrate by the second arm from an even-numbered processing unit; 
     unloading the substrate by the first arm from the final processing unit; and further 
     loading the substrate by the first arm into the substrate loading/unloading section; and 
     when the determination result is an even number, 
     taking out the substrate, by the third arm, from the substrate loading/unloading section; 
     loading the substrate by the third arm to an odd-numbered processing unit; 
     unloading the substrate by the second arm from an odd-numbered processing unit; 
     loading the substrate by the second arm from an even-numbered processing unit; 
     unloading the substrate by the third arm from an even-numbered processing unit except for a final processing unit; 
     unloading the substrate by the first arm from the final processing unit; and further 
     loading the substrate by the first arm into the substrate loading/unloading section. 
     The substrate transport apparatus comprises: 
     a θ rotation driving mechanism for rotating by an angle of θ around each of vertical axes of the first, second, and third arms; 
     an arm back-and-forth moving mechanism for moving each of the first, second, and third arms, back and forth; 
     a Z-axis driving mechanism for moving the first, second, and third arms in the Z-axis direction; and 
     a cover assembly consisting of a plurality of slide covers surrounding the Z-axis driving mechanism and slidably assembled with each other, a diameter of an upper slide cover being larger than a diameter of a lower slide. 
     In this case, it is preferable that the cover assembly be formed by concentrically assembling a plurality of cylindrical slide covers. 
     Further in this case, it is preferable that the loading/unloading port of the processing unit be sufficiently large to load and unload two arms of the first, second, third arms. 
     According to the present invention, there is provided a substrate transport apparatus for transporting substrates from a substrate loading/unloading section to a processing section sequentially one by one, comprising: 
     a plurality of arms for holding a substrate; 
     a θ rotation driving mechanism for rotating the plurality of arms simultaneously around a vertical axis by an angle of θ; 
     an arm back-and-forth moving mechanism for moving each of the plurality of arms, back and forth; 
     a Z-axis driving mechanism for moving the plurality of arms, simultaneously up and down; and 
     a cover assembly surrounding the Z-axis driving mechanism in order to shut out the Z-axis driving mechanism from an outer atmosphere, the cover assembly consisting of a plurality of cylindrical slide covers slidably assembled concentrically, 
     in which, when the plurality of arms are moved up by the Z-axis driving mechanism, an outer slide cover slidably moves to an inner slide cover, with the result that the outer slide cover is located above the inner slide cover. 
     The cover assembly comprises: 
     an unmovable slide cover member which is not driven by up-and-down movement of the Z-axis driving mechanism and thus maintained unmoved; 
     a first movable slide cover slidably driven by the up-and-down movement of the Z-axis driving mechanism, relative to the unmovable slide cover; and 
     a second movable slide cover slidably driven by the up-and down movement of the Z-axis driving mechanism, relative to the first movable slide cover. 
     It is preferable that the Z-axis driving mechanism comprise: 
     a first ball screw mechanism for moving the first movable slide cover up and down to the unmovable slide cover; and 
     a second ball screw mechanism for moving the second movable slide cover up and down to the first movable slide cover. 
     In this case, it is preferable that the first ball screw mechanism comprise: 
     an unmovable support member connected to the unmovable slide cover; 
     a first ball screw fixed to the unmovable support member; 
     a first nut engaged with the first ball screw; 
     a belt hanging around the first nut; and 
     a common motor for transmitting rotatory driving force to the belt; and 
     the second ball screw mechanism comprises: 
     a first movable support member connected to the first movable support member; 
     a second movable support member connected to the second movable slide cover; 
     a second nut engaged with the second ball screw and fixed to the second movable support member; and 
     a pulley having the belt hung thereon and fitted to a lower portion of the second ball screw. 
     Furthermore, the Z-axis driving mechanism may comprise: 
     a first rack/pinion mechanism for moving the first movable slide cover up and down to the unmovable slide cover; and 
     a second rack/pinion mechanism for moving the second movable slide cover up and down to the first movable slide cover. 
     In this case, it is preferable that the first rack/pinion mechanism comprise: 
     a first rack fixed on the unmovable slide cover; 
     a common pinion engaged with the first rack; and 
     a common motor for transmitting rotatory driving force to the common pinion, and 
     the second rack/pinion mechanism comprises: 
     a second movable support member connected to the second movable slide cover; and 
     a second rack fitted to the second movable support member and engaged with the common pinion. 
     According to the present invention, it is possible to reduce time for loading/unloading the substrate into/from the processing unit. The opening area of the loading/unloading port is reduced. It is therefore possible to prevent an inner atmosphere of the process unit containing chemical substances from being dispersed into an outer atmosphere and to suppress particles from being introduced into the inside the process unit to minimum. 
     Regardless the number of the processing units, namely, odd number or even number, the arm portion may be moved only by the distance corresponding to two steps in the final processing unit. Therefore, the throughput is increased and the durability of the substrate transport apparatus is improved. As a result, energy saving of the substrate processing system is successfully attained. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     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 conventional substrate transport apparatus; 
     FIG. 2 is a schematic side view of the conventional substrate transport apparatus; 
     FIG. 3 is a schematic plan view of a substrate processing system for washing a semiconductor wafer; 
     FIG. 4 is a perspective view of the substrate processing system for washing a semiconductor wafer; 
     FIG. 5 is a perspective view of a substrate transport apparatus according to an embodiment of the present invention; 
     FIG. 6 is a cross sectional view of an arm back-and-forth moving mechanism as viewed from a back-and-forth moving direction; 
     FIG. 7 is a view showing a gist portion of the arm back-and-forth moving mechanism; 
     FIG. 8 is a cross sectional view of a substrate transport apparatus according to an embodiment of the present invention accompanying a block diagram of peripheral elements; 
     FIG. 9 is a cross sectional view of the substrate transport apparatus according to an embodiment of the present invention accompanying a block diagram of peripheral elements; 
     FIG. 10 is a perspective view of a multiple-step cover concentrically arranged, for covering a liftable mechanism; 
     FIG. 11 is a cross sectional view of a washing unit for washing a surface of a semiconductor wafer with a chemical solution, accompanying a block diagram of peripheral elements; 
     FIG. 12 is a plan view of an arm and an spin chuck; 
     FIG. 13 is a cross sectional view of a substrate transfer apparatus according to another embodiment of the present invention, accompanying a block diagram of peripheral elements; 
     FIG. 14 is a perspective view of the substrate transport apparatus and the washing unit; 
     FIG. 15 is a schematic view of a substrate transport apparatus facing the washing unit; 
     FIG. 16 is a flow chart showing the steps of a method for transporting a substrate according to an embodiment of the present invention; 
     FIGS. 17A to  17 F are perspective views showing the substrate transfer apparatus and the washing unit for explaining how to transfer a wafer when an odd number of the steps is required for washing process; 
     FIG. 18 is a flow chart showing the steps of a method for transporting a substrate according to another embodiment of the present invention; 
     FIGS. 19A to  19 D are perspective views showing the substrate transfer apparatus and the washing unit for explaining how to transfer a wafer when an even number of the steps is required for washing process; and 
     FIG. 20 is a plan view of a substrate processing system according to another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Now, various preferred embodiments of the present invention will be explained with reference to the accompanying drawings. 
     As shown in FIGS. 3 and 4, a substrate processing system  1  has a loader/unloader section  2 , a process section  3  and a transfer section  5 . The loader/unloader section  2  serving as a substrate loading/unloading section has a table  2   a  extending in the X-axis direction. In a front surface side of the table  2   a , a cassette transport passage (not shown) is arranged. A cassette C is transported along the transport passage by a transport robot (not shown) and set on the table  2   a . For example, two or three cassettes C are mounted on the table  2   a.  25 sheets of semiconductor wafers W constituting one lot, are stored in each cassette. 
     First, second, and third processing units  6 ,  7 ,  8  are arranged side by side in the order mentioned in the processing section  3 . Fourth, fifth, and sixth units  9 ,  10 ,  11  are arranged below the first, second, third units  6 ,  7 ,  8 , correspondingly. Since the units  9 - 11  are substantially the same as the units  6 - 8 , detailed explanation will be omitted. 
     The cassette  2   a  of the loader/unloader section  2  is divided into a loading region  12  and an unloading region  13 . Unwashed wafers W are stored in a cassette C placed in the loading region  12 . Washed wafers W are stored in a cassette C placed in the unloading region  13 . When the cassette C is filled with the washed wafers W, the cassette C is loaded out of the washing system  1  by the transport robot (not shown). 
     The transport section  5  is provided at a rear surface side of the table  2   a . The transport section  5  houses a substrate transport apparatus  4  therein. As shown in FIG. 8, the substrate transport apparatus  4  has an arm portion  20 , an arm back-and-forth moving mechanism  21 , a cover assembly  22 , a base table  23 , a θ rotation driving mechanism  24 , a Z-axis driving mechanism  47 , an X-axis driving mechanism  60 , and a controller  70 . The arm portion  20  has a first arm  20   a , a second arm  20   b , and a third arm  20   c  in the order mentioned from the above. 
     The controller  70  controls each operation of driving mechanisms  21 ,  24 ,  47 ,  60  on the basis of data initially input thereto. A pair of rails  15  extending in the X-direction is laid on the floor of the transport section  5 . The substrate transport apparatus  4  is moved along the pair of rails  15  in the X-axis direction by the X-axis driving mechanism  60  shown in FIG.  8 . 
     A loading/buffer mechanism (not shown) is arranged at one of the sides of the transfer section  5  and an unloading/buffer mechanism (not shown) at the other side thereof. The unwashed wafer W is taken out from the cassette C by means of the second arm  20   b  and/or third arm  20   c  of the substrate transfer apparatus  4  and temporarily stored in the loading/buffer mechanism. Furthermore, the washed wafer W is taken out from the unloading/buffer mechanism by the first arm  20   a  of the substrate transfer apparatus  4  and stored in the cassette C. 
     As shown in FIG. 5, the arm portion  20  has three arms  20   a ,  20   b ,  20   c  for holding the wafer W. Each of the arms  20   a ,  20   b ,  20   c  is supported by the arm back-and-forth moving mechanism  21  so as to move back and forth individually. 
     As shown in FIGS. 6 and 7, the arm back-and-forth moving mechanism  21  has support members  21   a ,  21   b ,  21   c , an endless timing belt  21   d , a pair of pulleys  21   e ,  21   f , and a stepping motor (not shown). One end of the first support member  21   a  is horizontally connected to a proximal end of the arm  20   a  ( 20   b ,  20   c ). The upper end of the support member  21   b  is vertically connected to the other end of the first support member  21   a . One end of the third support member  21   c  is horizontally connected to the lower end of the second support member  21   b . Furthermore, the other end of the third support member  21   c  is inserted into the base table  23  through an opening  23   a  and connected to an endless belt  21   d . The endless belt  21   d  is stretched between the driving pulley  21   e  and the follower pulley  21   f . The driving pulley  21   e  is connected to a rotatory driving axis (not shown) of the stepping motor. 
     The first arm  20   a , second arm  20   b , and third arm  20   c  of the substrate transport apparatus  4  are selectively used to maintain cleanliness of the wafer W which has been applied to a series of treatments in the first, second, and third processing units  6 ,  7 ,  8 . To describe more specifically, the wafer W is taken out from the cassette C of the loading region  12 , loaded into the first processing unit  6  and unloaded therefrom, loaded into the second processing unit  7  and unloaded therefrom, and loaded into the third processing unit  8  by the second arm  20   b  and the third arm  20   c . On the other hand, the wafer W is unloaded from the third processing unit  8  and stored in the cassette C of the unloading region  13  only by the first arm  20   a . This is because there is the smallest possibility for the particles to fall on the wafer W held by the first arm  20   a.    
     In the meantime, the procedure for transporting the wafer varies depending upon whether the wafer W is processed in either an odd number or an even number of processing units. When the washing is carried out in an odd-numbered processing units, the controller  70  controls the substrate transport apparatus  4  so as to first take out the wafer W from the cassette C of the loading region  12  by the second arm  20   b . In this case, the wafer W is loaded by the second arm  20   b  into odd-numbered (first, third) processing units  6 ,  8 . The wafer W is then unloaded by the third arm  20   c  from an odd-numbered first processing unit  6  except for the final processing unit  8 . The wafer W is loaded into and unloaded from an even-numbered (second) processing unit  7 , by the third arm  20   c.    
     On the other hand, when the number of washing units is an even number, the controller  70  controls the substrate transfer apparatus  4  so as to first take out the wafer W from the cassette C of the loading region  12  by the third arm  20   c . In this case, the wafer W is loaded by the third arm  20   c  into an odd-numbered (first) the processing unit  7  (or  6 ). The wafer W is unloaded from the processing unit  7  (or  6 ) by the second arm  20   b  and loaded into an even-numbered (second) processing unit  8 . 
     As shown in FIG. 8, the θ rotation driving mechanism  24  is fitted right below the arm portion  20  with the base table  23  interposed between them. The θ rotatory driving mechanism  24  has a rotation shaft  24   a  connected to the base table  23  and a stepping motor  24   b  for rotating the rotation shaft  24   a . The arm portion  20  is rotated by an angle of θ around the Z-axis by the mechanism  24 . 
     The X-axis moving mechanism  60  has a motor  61 , a shaft  62 , wheels  63  and brackets  64 . The shaft  62  is connected to a rotatory driving shaft of the motor  61  and rotatably supported by the brackets  64  via bearings. Note that the brackets  64  are fitted to the lower surface of the base plate  16 . The wheels  63  are fitted to the shaft  62  and movably provided on the rails  15 . 
     Next, the Z-axis driving mechanism  47  will be explained with reference to FIGS. 8 to  10 . 
     The Z-axis driving mechanism  47  has a cover assembly  22  consisting of first, second, third slide covers  22   a ,  22   b ,  22   c , an unmovable support member  30 , first and second movable support members  31 ,  32  and the first and second ball screw mechanisms  33 ,  34 . The unmovable support member  30  is fixed on the base plate  16  together with the first slide cover  22   a . The unmovable support member  30  is formed cylindrically and houses a first ball screw mechanism  33  and a part of a second ball screw mechanism  34  therein. Incidentally, the unmovable support member  30  is covered with the first slide cover  22   a  in contact with the outer periphery. At the bottom of the unmovable support member  30 , an exhaust port  30   b  is formed which communicates with a factory exhaust passage assembly (not shown). An inner space  48  of the cover assembly  22  is evacuated through the exhaust port  30   b.    
     The first movable support member  31  is a hollow cylinder closed with a bottom portion  31   a  and houses a part of the first ball screw mechanism  33  and a second ball screw mechanism  34  therein. A slide seal  31   c  is provided in the lower outer periphery of the first movable support member  31 . The first movable support member  31  and the unmovable support member  30  are slidably fit to each other with the slide seal  31   c  interposed between them. The upper end portion of the first movable support member  31  is connected to the second slide cover  22   b . The second slide cover  22   b  is moved up and down together with the first movable support member  31  by the first ball screw mechanism  33 . Note that the first movable support member  31  is covered with a second slide cover  22   b  in contact with the outer periphery thereof. 
     The second movable support member  32  is a hollow cylinder closed with a bottom portion  32   a  and houses a part of the second ball screw mechanism  34  and a part of the θ rotation driving mechanism  24 . A slide seal  32   c  is provided in the lower outer periphery of the second movable support member  32 . The first and second movable support members  31  and  32  are slidably fitted to each other with the slide seal  32   c  interposed between them. Furthermore, the upper end portion of the second movable support member  32  is connected to the third slide cover  22   c  and moved up and down together with the second movable support member  32  by the second ball screw mechanism  34 . Note that the second movable support member  32  is covered with the third slide cover  22   c  in contact with the outer periphery thereof. Note that the upper end portion of the third slide cover  22   c  is connected to the lower portion of the frame of the θ rotation driving mechanism  24 . 
     As shown in FIG. 10, the first, second, third slide covers  22   a ,  22   b ,  22   c  constituting the cover assembly  22  are hollow cylinders concentrically arranged. These slide covers are formed of stainless steel having a fluorine resin coated thereon. The first slide cover  22   a  is arranged within the second slide cover  22   b . The second slide cover  22   b  is arranged within the third slide cover  22   c . In other words, the outer diameter of the first slide cover  22   a  is smaller than the inner diameter of the second slide cover  22   b . The outer diameter of the second slide cover  22   b  is smaller than the inner diameter of the third slide cover  22   c . On the other hand, the unmovable support member  30  is arranged outside the first movable support member  31 . The first movable support member  31  is arranged outside the second movable support member  32 . In other words, the inner diameter of the unmovable support member  30  is larger than the outer diameter of the first movable support member  31 . The inner diameter of the first movable support member  31  is larger than the outer diameter of the second movable support member  32 . For example, when the wafer W of 12 inch diameter is used, the unmovable support member  30  has an inner diameter of 280 mm and a length of 400-500 mm. The first movable support member  31  has an inner diameter of 260 mm and a length of 400-500 mm. The second movable support member  32  has an inner diameter of 240 mm and a length of 400-500 mm. 
     Furthermore, the first and second ball screw mechanisms  33 ,  34  will be explained more specifically with reference to FIGS. 8 and 9. 
     The first and second ball screw mechanisms  33 ,  34  have screws  35 ,  40  and nuts  36 ,  45 , respectively and a common motor  42 . The common motor  42  is fixed at the bottom portion  31   a  of the first movable support member  31 . Its rotation shaft  43  protrudes downward from the bottom portion  31   a . The first screw  35  is engaged with a first nut  36 . The second screw  40  is equipped with a follower pulley  45 . The rotation shaft  43  of the common motor  42  is equipped with the driving pulley  44 . A belt  46  is stretched between the nut  36  and the pulleys  44 ,  45 . When the common motor  42  is driven, the rotatory driving force is transmitted to the nut  36  and the pulley  45 , individually. It follows that the nut  36  is moved up and down relatively to the screw  35 . Synchronously with this, the pulley  45  is moved up and down together with the screw  40 . 
     The first screw  35  is fixed on the base plate  16  and the first slide cover  22   a  at the lower end. The upper portion thereof passes through the bottom portion  31   a  and reaches within the first movable support member  31 . The second screw  40  is connected to the follower pulley  45  at the lower end. The upper portion passes through the bottom portion  32   a  and reaches within the second movable support member  32 . The upper portion of the second screw  40  is fitted to a second nut  41  within the second movable support member  32 . Note that the first screw  35  passes through guide holes formed in the bottom portions  31   a ,  32   a . Bearing (not shown) are provided at the bottom portions  31   a ,  32   a  through which the second screw  40  passes. 
     FIG. 8 shows a substrate transport apparatus  4  whose a cover assembly  22  is most extended. 
     In the state shown in FIG. 8, when the wafer W of 12 inch diameter is used, the height L 1  from the base plate  16  to the arm portion  20  ranges from about 1000 to 1300 mm. In this case, the third slide cover  22   c  which is positioned at the outermost side, is located at the uppermost position. FIG. 9 shows the substrate transport apparatus  4  whose cover assembly  22  is most shrunk. In the state shown in FIG. 9, when the wafer W of 12 inch diameter, the height L 2  from the base plate  16  to the arm portion  20  ranges from about 300 to 500 mm. In this case, the Z-axis driving mechanism  47  is sufficiently protected by multiple slide covers, namely, first, second, and third slide covers  22   a ,  22   b ,  22   c , with the result that leakage of particles never occurs. 
     In addition, since the distance between each of the driving mechanisms  23 ,  24 ,  47  and the bottom exhaust port  30   b  is reduced, the efficiency in evacuating the inner space  48  of the cover assembly is tremendously increased. As a result, the amount of the particles generated in the cover assembly of the present invention is significantly reduced as particularly compared to the conventional cover  108  (see FIG. 1) in the bellows form. 
     Furthermore, since the slide cover  22   c  of a larger diameter is placed upper than the slide covers  22   a ,  22   b  of a small diameter, the chemical washing solution does not enter the inner space  48  even if leakage of the solution occurs. The driving mechanism  47  within the inner space can be protected. 
     Furthermore, since the second and third slide covers  22   b ,  22   c  are synchronously moved up and down, the arm portion  20  is quickly allowed to arrive at the load/unload port  101   a  ( 102   a ,  103   a ) of the processing unit, increasing the throughput. Moreover, since the single motor  42  is commonly used by two ball screws  33 ,  34 , the substrate transport apparatus  4  can be reduced in size. 
     The plurality of processing units can be used in various combination depending upon the washing conditions. For example, a certain unit can be withdrawn from the processing units and conversely another unit may be added. In the substrate processing system, the number of processing units required for processing the wafer W may be used alone or in combination of two or three. 
     Now, the processing unit will be explained with reference to FIGS. 11 and 12. The processing units  6 - 11  have substantially the same structure, so that the first and second units  6 ,  7  will be representatively explained. 
     In this embodiment, a mechanical chuck  80  is used as a substrate holding portion. As shown in FIG. 11, the mechanical chuck  80  is provided within a drain cup  90 . The drain cup  90  has a movable cup portion  90   a  and unmovable cup portion  90   b . The movable cup portion  90   a  is connected to a rod  98   a  of a cylinder  98  through openings  90   c ,  90   f . When the rod  98   a  is allowed to protrude from the cylinder  98 , the movable cup portion  90   a  moves up. Conversely, when the rod  98   a  is withdrawn within the cylinder  98 , the movable cup portion  90   a  moves down. 
     A rotatory driving shaft  83   a  of a motor  83  is passed through the center protruding portion  90   g  of the unmovable cup portion  90   b  and connected to a bottom plate  81  of the mechanical chuck  80 . A seal bearing  90   h  is interposed between the rotatory driving shaft  83   s  and the center protruding portion  90   g . Drain holes  90   d  are appropriately formed in the unmovable cup  90   b . The washing drainage is discarded outside of the cup  80  through the drain holes  90   d.    
     The bottom plate  81  of the mechanical chuck  80  has the same size as the diameter of the wafer W. Six elected portions  84  are provided in the periphery of the bottom plate  81 . A wafer holding portion  85  is provided to each of the elected portions  84 . The lower half inner peripheral portion of the wafer holding portion  85  is inclined inwardly to form an inclined surface. The outer periphery of the wafer is in contact with the upper side of the inclined surface of the wafer holding portion  85 . The wafer holding portion  85  is attached to the elected portion  84  via a horizontal axis  86 . In addition, a weight (not shown) is housed in the wafer holding portion  85 . 
     The wafer W is transferred to the mechanical chuck  80  by the transfer arm  20   a  ( 20   b ,  20   c ) shown in FIG.  12 . The transfer arm  20   a  ( 20   b ,  20   c ) is a ring member (partly cut-away), whose inner diameter is larger than that of the bottom plate  81 . A wafer mounting portion  88  extending inwardly is provided at three portions inside the ring member. The wafer W is mounted on the protruding portion  88   a  formed on the tip portion of the wafer mounting portion  88 . 
     A cut-away portion  81   a  is formed at the bottom plate  81  corresponding to the wafer mounting portion  88  so as to pass the wafer mounting portion  88 . The wafer held by the transfer arm  20   a  ( 20   b ,  20   c ) is transferred to the wafer holding portion  85  of the mechanical chuck  80  by moving down the transfer arm  20   a  ( 20   b ,  20   c ) from a predetermined position above the mechanical chuck  80  and passing through the cut away portion  81   a.    
     A disk brush  91  is movably supported by a moving mechanism (not shown) via the arm  91   a . A first nozzle  92  is movably supported by a moving mechanism (not shown) via an arm  92   a . Furthermore, a second nozzle  93  is movably supported by a moving mechanism (not shown) via an arm  93   a . The brush  91  and the nozzles  92 ,  93  are moved from home positions to operation positions by respective moving mechanisms so as to be faced to the wafer W. 
     The first nozzle  92  is independently communicated with chemical solution supply sources  71 ,  72 , by way of a switching valve  77 . For example, the first chemical solution supply source  71  of the first processing unit  6  contains an ammonia/hydrogen peroxide solution mixture (hereinafter, referred to “APM solution”). The second chemical washing solution supply source  72  thereof contains a hydrochloric acid/hydrogen peroxide solution mixture (hereinafter, referred to as “HPM solution”). The power supply circuit of the switching valve  77  is connected to the controller  70 . The controller  70  controls operation of the switching valve  77  on the basis of input data of processing conditions to switch a processing solution to be supplied to the first nozzle  92  between the APM solution and the HPM solution. 
     On the other hand, the first chemical washing supply source  71  of the second processing unit  7 , contains ammonia/hydrogen peroxide solution mixture (hereinafter, referred to “APM solution”). The second chemical washing supply source  72  thereof contains a hydrofluoric acid solution (hereinafter, referred to as “DHF solution”). The controller controls operation of the switching valve  77  on the basis of input data of processing conditions so as to switch the processing solution to be supplied to the first nozzle  92  between the APM solution and the DHF solution. 
     The second nozzle  93  is communicated with a rinse solution supply source  73  serving as a third solution supply source. The rinse solution supply source  73  contains pure wafer. Each of the solution supply sources  71 ,  72 ,  73  houses a flow rate controller. The controller  70  controls each of operations of the solution supply sources  71 ,  72 ,  73  and adjusts the flow rate of supplying the processing solution. 
     Note that the solution supply sources  71 ,  72  of other processing units  8 ,  9 ,  10 ,  11  contain any one of the APM solution, HPM solution, and DHF solution. 
     Next, a substrate transport apparatus  4 A of another embodiment will be explained with reference to FIG.  13 . 
     The substrate transport apparatus  4 A has a Z-axis driving mechanism  50  as a means for moving the arm portion  20  up and down. The Z-axis driving mechanism  50  has a first and second rack/pinion mechanisms  51 ,  52 . The first rack/pinion mechanism  51  moves the second slide cover  22   b  up and down with respect to the slide cover  22   a . The second rack/pinion mechanism  52  moves the third slide cover  22   c  up and down with respect to the second slide cover  22   b.    
     The first rack/pinion mechanism  51  has a first rack  53  and common pinion  54  which are engaged with each other. The second rack/pinion mechanism  52  has a second rack  55  and a common pinion  54  which are engaged with each other. The shaft of the common pinion  54  is connected to a rotatory driving shaft (not shown) of a motor (not shown) via a speed-reducing mechanism (not shown). The first rack  53  is fixed on the base plate  16  and the first slide cover  22   a  at the lower end. The upper portion of the first rack  53  passes through the bottom portion  31   a  and reaches within the first movable support member  31 . The second rack  55  is fixed to the bottom portion  32   a  of the second movable support member at the upper end. The lower portion of the second rack  55  passes through the bottom portion  31   a  and reaches within the unmovable support member  30 . Note that the first and second racks  53 ,  55  pass through guide holes (not shown) formed in the bottom portion  31   a.    
     As shown in FIGS. 14 and 15, the first, second, third units  6 ,  7 ,  8  respectively have load/unload ports  6   a ,  7   a ,  8   a  in the front surfaces. Longitudinal size L 3  of the load/unload port  6   a  ( 7   a ,  8   a ) is sufficiently large to load at least two arms  20   a ,  20   c  simultaneously into the processing unit  6  ( 7 ,  8 ). In other words, it is desirable that L 3  correspond the height of two arms  20   a  and  20   b  (alternatively,  20   b  and  20   c ). For example, when the wafer w of 12 inches is used, the longitudinal size L 3  of the load/unload port  6   a  ( 7   a ,  8   a ) ranges from about 50 to 80 mm. With this structure, even when the wafer is loaded/unloaded, it is difficult for particles to enter the processing unit  6  from the outside. 
     A shutter  30  is attached to each of the load/unload ports  6   a ,  7   a ,  8   a . Each of the shutters is moved up and down by a cylinder mechanism (not shown) provided thereunder. The moving stroke of the shutter  30  corresponds to the distance between adjacent arms  20   a  and  20   b  vertically arranged (alternatively,  20   b  and  20   c ). 
     Now, we will explain the case where the wafer W is subjected to washing treatment odd number of times by using all of three processing units  6 ,  7 ,  8 , with reference to FIGS. 16,  17 A to  17 F. 
     First, a cassette C is loaded into the loader/unloader section  2  by a transport robot (not shown) (Step S 1 ). The cassette C stores 25 sheets of unprocessed semiconductor wafers W (8 inch or 12 inch in diameter). An identification code having data of wafer processing conditions recorded thereon, is displayed in appropriate portion of the cassette C. The optical sensor (not shown) reads the identification code and input the read data into the controller  70  (Step S 2 ). The controller determines that the number of the processing units applied to treat the lot is odd number on the basis of input data of the processing conditions. Based on the determination results, and then, sends instruction signals to the substrate transport apparatus  4  and washing unit  3 . 
     As shown in FIG. 9, while the cover  22  is shrunk most, the tip portion of the arm portion  20  is faced toward the loader/unloader portion  2 . The second slide cover  22   b  and the third cover  22   c  are simultaneously moved up until the height of the arm portion  20  becomes equal to the level of the load/unload port  6   a  ( 7   a ,  8   a ) of the processing unit. In this manner, the cover  22  is quickly extended. As shown in FIG. 8, the arm portion  20  is moved up to the level of the cassette C to introduce the second arm  20   b  into the cassette C. Subsequently, the second arm  20   b  is moved forward to take out a first wafer W 1  from the cassette C by the second arm  20   b  (Step S 3 ). 
     When the arm portion  20  is moved down, the second slide cover  22   b  and the third slide cover  22   c  are simultaneously moved down by rotating the motor  42  backward. In this way, the cover  22  is quickly shrunk as shown in FIG.  9 . 
     As shown in FIG. 17A, the shutter  30  of the first unit  6  is moved down to load the wafer W 1  into the first unit  6  through the load/unload port  6   a  (Step S 4 ). When the first wafer W 1  is transferred onto the spin chuck  80 , the second arm  20   b  is withdrawn and then, the load/unload port  6   a  is closed. Subsequently, while the wafer W 1  is rotated by the spin chuck  80  and the ammonia/hydrogen peroxide solution mixture is supplied to the wafer W 1  from the first nozzle  92 , the wafer W 1  is rubbed by the rotation brush  91  to wash the surface of the wafer W 1  with the APM solution (Step S 5 ). After completion of the APM washing, pure wafer serving as a rinse solution is supplied from the second nozzle  93  to the wafer W 1  to rinse the wafer W 1 . Furthermore, the wafer W 1  is rotated at a high speed by the spin chuck  80  to separate and remove the attached solution from the wafer W 1  (Step S 6 ). In this manner, the surface of the first wafer W 1  is dried. 
     Subsequently, the load/unload port  6   a  is opened. Then, the third arm  20   c  is inserted into the first unit  6  to unload the first wafer W 1  from the first unit  6  by the third arm  20   c , as shown in FIG. 17B (Step S 7 ). During this period, the wafer W 2  is taken out from the cassette C in advance by the second arm  20   b  and the wafer W 1  is loaded into the second unit  7  by the third arm  20   c.    
     Furthermore, as shown in FIG. 17C, the load/unload port  7   a  of the second unit  7  is opened to load the first wafer W 1  into the second unit  7  by the third arm  20   c  (Step S 8 ). When the first wafer W 1  is transferred onto the spin chuck  80 , the third arm  20   c  is withdrawn and the load/unload port  7   a  is closed. Subsequently, while the wafer W 1  is rotated by the spin chuck  80  and a hydrochloric acid/hydrogen peroxide solution mixture is supplied to the wafer W 1  from the first nozzle  92 , the wafer W 1  is rubbed by the rotation brush  91  to wash the surface of the wafer W 1  with the HPM solution (Step  9 ). 
     After completion of the HPM washing solution, pure water serving as a rinse solution is supplied to the wafer W 1  from the second nozzle  93  to rinse the wafer W 1 . Furthermore, the wafer W 1  is rotated at a high. speed by the spin chuck  80  to separate and remove the attached solution from the wafer W 1  (Step S 10 ). In this manner, the surface of the first wafer W 1  is dried. 
     On the other hand, the wafer W 2  to be used next is taken out in advance by the second arm  20   b  from the loader/unloader section  2  during the period between the step S 5  and S 6  (Step S 21 ). As shown in FIG. 17B, the shutter  30  of the first unit  6  is moved down to load the second wafer W 2  into the first unit  6  through the load/unload port  6   a  (Step S 22 ). When the second wafer W 2  is transferred onto the spin chuck  80 , the second arm  20   b  is withdrawn and the shutter  30  is moved up and then, the load/unload port  6   a  is closed. Subsequently, while the wafer W 2  is rotated by the spin chuck  80  and the ammonia/hydrogen peroxide solution mixture is supplied to the wafer W 2  from the first nozzle  92 , the wafer W 2  is rubbed by the rotation brush  91  to wash the surface of the wafer W 2  with an APM solution (Step S 23 ). After completion of the APM washing, pure wafer serving as a rinse solution is supplied to the wafer W 2  to rinse the wafer W 2 . Furthermore, the wafer W 2  is rotated at a high speed by the spin chuck  80  to separate and remove the attached solution from the wafer W 2  (Step S  24 ). In this manner, the surface of the second wafer W 2  is dried. 
     Then, the load/unload  6   a  of the first unit  6  is opened. Subsequently, the third arm  20   a  is inserted into the first unit  6  to unload the second wafer W 2  from the first unit  6  by the third arm  20   c , as shown in FIG. 17B (Step S 25 ). Furthermore, as shown in FIG. 17C, the load/unload port  7   a  of the second unit  7  is opened to load the second wafer W 2  into the second unit  7  by the third arm  20   c  (Step S 26 ). When the second wafer W 2  is transferred onto the spin chuck  80 , the third arm  20   c  is withdrawn and the shutter  30  is moved up to close the load/unload port  7   a . While the wafer W 2  is rotated by the spin chuck  80  and the hydrochloric acid/hydrogen peroxide solution mixture is supplied from the first nozzle  92  to the wafer W 2 , the wafer W 2  is rubbed by the rotation brush  91  to wash the surface of the wafer W 2  with the HPM solution (Step S 27 ). After completion of the HPM washing, pure water serving as a rinse solution is supplied to the wafer W 2  to rinse the wafer W 2 . Furthermore, the wafer W 2  is rotated at a high speed by the spin chuck  80  to separate and remove the attached solution from the wafer W 2  (Step S 28 ). In this manner, the surface of the wafer W 2  is dried. 
     Before the step S 26 , the load/unload port  7   a  of the second unit  7  is opened. Subsequently, the second arm  20   b  is inserted into the second unit  7  to unload the first wafer W 1  from the second unit  7  by the second arm  20   b , as shown in FIG. 17D (Step S 11 ). Furthermore, as shown in FIG. 17E, the load/unload port  8   a  of the third unit  8  is opened to load the first wafer W 1  into the third unit  8  by the second arm  20   b  (Step S 12 ). When the first wafer W 1  is transferred onto the spin chuck  80 , the second arm  20   b  is withdrawn and the load/unload port  8   a  is closed. While the wafer W 1  is rotated by the spin chuck  80  and the hydrofluoric acid solution is supplied to the wafer W 1  from the first nozzle  92 , the wafer W 1  is rubbed by the rotation brush  91  to wash the surface of the wafer W 1  with the DHF solution (Step S 13 ). After completion of the DHF washing, pure wafer serving as a rinse solution is supplied from the second,nozzle  93  to the wafer W 1  to rinse the wafer W 1 . Furthermore, the wafer W 1  is rotated at a high speed by the spin chuck  80  to separate and remove the attached solution from the wafer W 1  (Step S 14 ). In this manner, the surface of the first wafer W 1  is dried. 
     The load/unload port  8   a  of the third unit  8  is opened. Subsequently, the first arm  20   a  is inserted into the third unit  8  to unload the first wafer W 1  from the third unit  8  by the first arm  20   a  (Step S  15 ), as shown in FIG.  17 F. The substrate transport apparatus  4  is again faced to the loader/unloader section  2  from the washing unit  3 . Then, the first arm  20   a  is moved forward to store the first wafer W 1  in the cassette C (Step S 16 ). 
     After the step S 15 , the load/unload port  7   a  of the second unit  7  is opened. Subsequently, the second arm  20   b  is inserted into the second unit  7  to unload the second wafer W 2  from the second unit  7  by the second arm  20   b , as shown in FIG. 17D (Step S 29 ). Furthermore, as shown in FIG. 17E, the load/unload port  8   a  of the third unit  8  is opened to load the second wafer W 2  into the third unit  8  by the second arm  20   b  (Step S 30 ). When the second wafer W 2  is transferred onto the spin chuck  80 , the second arm  20   b  is withdrawn and the load/unload port  8   a  is closed. While the wafer W 2  is rotated by the spin chuck  80  and the hydrofluoric acid solution is supplied to the wafer W 2  from the first nozzle  92 , the wafer W 2  is rubbed by the rotation brush  91  to wash the surface of the second wafer W 2  with the DHF solution (Step S 31 ). After the DHF washing, pure wafer serving as a rinse solution is supplied to the wafer W 2  from the second nozzle  93  to rinse the wafer W 2 . Furthermore, the wafer W 2  is rotated at a high speed by the spin chuck  80  to separate and removed the attached solution from the wafer W 2  (Step S 32 ). In this manner, the surface of the second wafer W 2  is dried. 
     The load/unload port  8   a  of the third unit  8  is opened. Subsequently, the first arm  20   a  is inserted into the third unit  8  to unload the second wafer W 2  from the third unit  8  by the first arm  20   a , as shown in FIG. 17F (Step S 33 ). Then, the substrate transport apparatus  4  is again faced to the loader/unloader section  2  from the washing section  3 . The first arm  20  a is moved forward to store the second wafer W 2  in the cassette C (Step S 34 ). While up-and-down movement of the arm portion  20  of the substrate transport apparatus  4  is repeated, the wafer W is transferred to the processing units  9 ,  10 ,  11  in the lower stage of the substrate transport apparatus  4  and the wafer W is transferred to the processing units  6 ,  7 ,  8  in the upper stage thereof. 
     After the washed first wafer W 1  to 25th wafer W 25  are continuously stored in the cassette C, the cassette C is unloaded from the system  1  through the loader/unloader portion  2  by the transport robot (not shown) and load into a next process (Step S 35 ). 
     Now, we will explain the case in which the wafer W is washed even-number of times (twice) by using two processing units  7 ,  8 , with reference to FIGS. 18 and 19A to  19 D. 
     The cassette C is loaded into the loader/unloader section  2  by the transport robot (not shown) (Step S 41 ). The cassette C stores 25 sheets of the semiconductor wafers W. An identification code having processing data of the wafer W recorded thereon, is displayed at an appropriate portion of the cassette C. The identification code is read by an optical sensor (not shown) and the data read out is input into the controller (Step S 42 ). The controller  70  determines that the processing units required for treating the lot is even times (twice) on the basis of the input data. Based on the determination results, the controller  70  sends instruction signals to the substrate transport apparatus  4  and the washing unit  3 , respectively. 
     As shown in FIG. 9, while the cover  22  is most shrunk, the tip portion of the arm section  20  is faced to the loader/unloader section  2 . Furthermore, the second slide cover  22   b  and the third slide cover  22   c  are simultaneously moved up until the height of the arm portion  20  becomes equal to the level of the load/unload port  6   a  ( 7   a  and  8   a ) of the processing units. In this way, the cover  22  is quickly extended. As shown in FIG. 8, the arm portion  20  is moved up to the level of the first processing unit  6  at one stroke to allow the third arm  20   c  to enter the second processing unit  7 . Subsequently, the third arm  20   c  is moved forward to take out the first wafer W 1  from the cassette C by the third arm  20   c  (Step S 43 ). When the arm portion  20  is moved down, the motor  42  is rotated backward to move the second and third slide covers  22   b ,  22   c  simultaneously. In this manner, the cover  22  is quickly shrunk as shown in FIG.  9 . 
     As shown in FIG. 19A, the shutter  30  of the second unit  7  is moved down to load the first wafer W 1  into the second unit  7  through the load/unload port  7   a  by the third arm  20   c  (Step S 44 ). The first wafer W 1  is transferred onto the spin chuck  80 , the third arm  20   c  is withdrawn, and then, the load/unload port  7   a  is closed. Subsequently, while the wafer W 1  is rotated by the spin chuck  80  and the ammonia/hydrogen peroxide solution mixture is supplied to the wafer W 1  from the first nozzle  92 , the wafer W 1  is rubbed by the rotation brush  91  to wash the surface of the wafer W 1  with the APM solution (Step S 45 ). After completion of the APM washing, pure wafer serving as a rinse solution is supplied from the second nozzle  93  to the wafer W 1  to rinse the wafer W 1 . Furthermore, the wafer W 1  is rotated at a high speed by the spin chuck  80  to separate and remove the attached solution from the wafer W 1  (Step S 46 ). In this manner, the surface of the first wafer W 1  is dried. 
     Subsequently, as shown in FIG. 19B, the load/unload port  7   a  is opened. Then, the second arm  20   b  is inserted into the second unit  7  to unload the first wafer W 1  from the second unit  7  by the second arm  20   b , (Step S 47 ). 
     Furthermore, as shown in FIG. 19C, the load/unload port  8   a  of the third unit  8  is opened to load the first wafer W 1  into the third unit  8  by the second arm  20   b  (Step S 48 ). When the first wafer W 1  is transferred onto the spin chuck  80 , the second arm  20   b  is withdrawn and the load/unload port  8   a  is closed. Subsequently, while the wafer W 1  is rotated by the spin chuck  80  and the hydrofluoric acid solution is supplied to the wafer W 1  from the first nozzle  92 , the wafer W 1  is rubbed by the rotation brush  91  to wash the surface of the wafer W 1  with the DHF solution (Step  49 ). After completion of the DHF washing solution, pure water serving as a rinse solution is supplied to the wafer W 1  from the second nozzle  93  to rinse the wafer W 1 . Furthermore, the wafer W 1  is rotated at a high speed by the spin chuck  80  to separate and remove the attached solution from the wafer W 1  (Step S 50 ). In this manner, the surface of the first wafer W 1  is dried. 
     As shown in FIG. 19D, the load/unload port  8   a  of the third unit  8  is opened. Subsequently, the first arm  20   a  is inserted into the third unit  8  to unload the first wafer W 1  from the third unit  8  by the first arm  20   a  (Step S 51 ). The substrate transfer apparatus  4  is faced again to the loader/unloader section  2  from the washing unit  3 . Subsequently, the first arm  20   a  is moved forward to store the first wafer W into the cassette C (Step S 52 ). 
     On the other hand, the third arm  20   c  takes out the wafer W 2  to be used next in advance from the loader/unloader section  2  during the period between the step S 45  and S 46  (Step S 61 ). As shown in FIG. 19A, the shutter  30  of the second unit  7  is moved down to load the second wafer W 2  into the second unit  7  through the load/unload port  7   a  (Step S 62 ). When the second wafer W 2  is transferred onto the spin chuck  80 , the third arm  20   c  is withdrawn and the shutter  30  is moved up and then, the load/unload port  7   a  is closed. Subsequently, while the wafer W 2  is rotated by the spin chuck  80  and ammonia/hydrogen peroxide solution mixture is supplied to the wafer W 2  from the first nozzle  92 , the wafer W 2  is rubbed by the rotation brush  91  to wash the surface of the wafer W 2  with an APM solution (Step S 63 ). After completion of the APM washing, pure wafer serving as a rinse solution is supplied to the wafer W 2  to rinse the wafer W 2 . Furthermore, the wafer W 2  is rotated at a high speed by the spin chuck  80  to separate and remove the attached solution from the wafer W 2  (Step S 64 ). In this manner, the surface of the second wafer W 2  is dried. 
     Then, as shown in FIG,  19 B, the load/unload  7   a  of the second unit  7  is opened. Subsequently, the second arm  20   b  is inserted into the second unit  7  to unloaded the second wafer W 2  from the second unit  7  by the second arm  20   b  (Step S 65 ). 
     Furthermore, as shown in FIG. 19C, the load/unload port  8   a  of the third unit  8  is opened to load the second wafer W 2  into the third unit  8  by the second arm  20   b  (Step S 66 ). When the second wafer W 2  is transferred onto the spin chuck  80 , the second arm  20   b  is withdrawn and the shutter  30  is moved up to close the load/unload port  8   a . While the wafer W 2  is rotated by the spin chuck  80  and the hydrofluoric acid solution is supplied from the first nozzle  92  to the wafer W 2 , the wafer W 2  is rubbed by the rotation brush  91  to wash the surface of the wafer W 2  with the DHF solution (Step S 67 ). After completion of the DHF washing, pure water serving as a rinse solution is supplied to the wafer W 2  to rinse the wafer W 2 . Furthermore, the wafer W 2  is rotated at a high speed by the spin chuck  80  to separate and remove the attached solution from the wafer W 2  (Step S 68 ). In this manner, the surface of the wafer W 2  is dried. 
     As shown in FIG. 19D, the load/unload port  8   a  of the third unit  8  is opened. Subsequently, the first arm  20   a  is inserted into the third unit  8  to unload the second wafer W 2  from the third unit  8  by the first arm  20   a  (Step S 69 ). Furthermore, the substrate transport apparatus  4  is faced again to the loader/unloader section  2  from the washing unit  3 . Then, the first arm  20   a  is moved forward to store the second wafer W 2  into the cassette C (Step S 70 ). While up-and-down movement of the arm portion  20  is repeated, the wafer W is transferred to the processing units  9 ,  10 ,  11  in the lower stage of the substrate transport apparatus  4  and the wafer W is transferred to the processing units  6 ,  7 ,  8  in the upper stage thereof. 
     After the washed first wafer W 1  to 25th wafer W 25  are continuously stored in the cassette C, the cassette C is unloaded from the system  1  through the loader/unloader portion  2  by the transport robot (not shown) and load into a next process (Step S 71 ). 
     In the aforementioned embodiments, the number of processing units are two or three. However, the present invention is not limited to thereto. The present invention may be applied to the case where the wafer is treated in four, six or five or seven processing units. 
     According to the embodiments, the first, second, third arms  20   a ,  20   b ,  20   c  are used separately depending upon its function. Therefore, contaminants such as particles rarely attach to a cleaned wafer. As a result, the surface of the wafer W can be maintained clean. 
     The moving distance of the shutter is reduced when the load/unload port is opened and closed as compared to the conventional case. Therefore, the throughput can be improved. Since the moving distance of the arm portion is reduced when the arm for use in loading/unloading is exchanged (since the arm is moved by the distance corresponding to two stages of the arm), the throughput is improved. 
     Furthermore, since the opening areas of the load/unload ports  6   a  to  11   a  are reduced, mutual interference between the inner atmosphere of the processing units  6  to  11  and outer atmosphere can be reduced. 
     Furthermore, the arm portion  20  is quickly moved up by simultaneously moving up the second and third slide covers  22   b ,  22   c . Even if the substrate transport apparatus is used for a long time, airtightness of the cover  22  does not deteriorate at all and substantially no particles are generated. 
     Moreover, by virtue of the structure, liquid does not enter the inside of the cover even if liquid leakage takes place, with the result that inner driving mechanisms are not broken down with rust. In addition, a plurality of driving mechanisms are operated by a common motor and a common pinion, so that the substrate transport apparatus is reduced in size. 
     Now, the substrate processing system of another embodiment will be explained with reference to FIG.  20 . 
     A substrate processing system  1 A has a cassette section  2 , first and second substrate transport arms mechanisms  4 ,  12 , a processing section  3 A and a buffer mechanism  13  serving as the substrate loading/unloading section. The cassette section  2  and the first substrate transporting arm mechanism  4  are the same as those in the aforementioned embodiment. The processing section  3 A has a plurality of processing unit  6 A- 11 A and the second substrate transporting arm mechanism  12 , and the buffer mechanism  13 . The processing units  6 A- 11 A are substantially the same as the processing units  6 - 11  in the aforementioned embodiment. The substrate transporting arm mechanism  12  is arranged in a transport area  5 A of the processing section  3 A. The second substrate transporting arm mechanism  12  has three arms  12   a ,  12   b ,  12   c  each holding the wafer W, a back-and-forth moving mechanism for moving each of the arms  12   a ,  12   b ,  12   c  back and forth, a Z-axis driving mechanism for moving an arm portion up and down, and a θ rotation mechanism for rotating the arm portion around the Z-axis. Such a substrate transport apparatus  12  is disclosed in U.S. Pat. No. 5,664,254. 
     The processing unit  6 A- 11 A and the buffer mechanism  13  are arranged so as to surround the second substrate transporting arm mechanism  12 . The buffer mechanism  13  is interposed between the first transport area  5  and the second transport area  5 A. The wafer W is transferred between the substrate transporting arm mechanism  4  and the second substrate transporting arm mechanism  12  via the buffer mechanism  13 . 
     In the aforementioned embodiments, the buffer mechanism  13  is used as the substrate loading/unloading section. The present invention is not limited to this. The first and second substrate transfer arm mechanisms  4 ,  12  may be used in combination as the substrate loading/unloading section. More specifically, the wafer W may be directly transported between the first substrate transporting arm mechanism  4  and the second substrate transporting arm mechanism  12  without passing through the buffer mechanism  13 . If so, the time required for transferring the wafer W can be reduced, increasing the throughput. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.