Abstract:
A polishing apparatus applicable to a dry-in/dry-out polishing system is capable of increasing the capacity of processing substrates to be polished, e.g. semiconductor wafers, per unit of time and per unit area of installation. A polishing unit includes a turntable having a polishing surface, and at least two wafer carriers each adapted to hold a wafer and to press the wafer against the polishing surface. Each wafer carrier is supported by a pivot shaft and movable between a polishing position where the wafer is polished on the turntable and a transfer position for transferring the wafer. The at least two wafer carriers can be situated at the polishing position simultaneously and also alternately.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a polishing apparatus and method for polishing a surface of a work-piece such as a semiconductor wafer to a high degree of fineness. 
     In a semiconductor device manufacturing process, a polishing apparatus is used to polish surfaces of semiconductor wafers. However, during polishing particles become detached from the wafer and scattered around the polishing apparatus, and hence it has not been possible to use a conventional polishing apparatus in a clean room environment. In addition, in the conventional system, following polishing, the wafer is placed in a mobile water tank for transportation to a cleaning apparatus, which is inefficient in terms of both the time and space required to complete a polishing and cleaning operation. 
     To solve the above-mentioned problems, a so-called dry-in/dry-out system has been developed in which polishing and cleaning processes are carried out in an apparatus enclosed within a housing. In the dry-in/dry-out system, a semiconductor wafer is loaded into the apparatus in a dry state and, following polishing and cleaning, is unloaded in a dry state. 
     Aside from the development of the dry in/dry out system, a polishing apparatus per se has been modified such that it is able to be used in a clean room. Further, improvements in the processing efficiencies of a polishing apparatus and a cleaning apparatus have enabled their installation in a space equal to or smaller than that required for the installation of a dry-in/dry-out system. However, there remains a difficulty in the automation of the wafer transportation means incorporating a mobile water tank. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a polishing apparatus which is applicable to the above-described dry-in/dry-out polishing system whereby the efficiency with which work-pieces are polished, such as semiconductor wafers, can be increased both in terms of the time required and space utilized. 
     In accordance with the present invention, there is provided a polishing apparatus comprising a polishing unit including a turntable provided on one side surface with a polishing surface and first and second substrate carriers each adapted to hold a substrate and to bring the substrate into contact with the polishing surface. Each of the first and second substrate carriers is movable between a first position where the substrate carrier receives a substrate and a second position where the substrate carrier is positioned over the polishing surface and is able to bring the substrate into contact with the polishing surface of the turntable. The first substrate carrier is movable between its first and second positions stated above while the second substrate carrier is positioned at its second position and vice versa. 
     The polishing apparatus may include first and second pivotal arms which support the first and second substrate carriers, respectively, to enable the first and second substrate carriers to move between their first and second positions stated above. 
     The second position stated above may include a polishing position where the substrate held by the substrate carrier is kept in contact with the polishing surface of the turntable and a waiting position where the substrate held by the substrate carrier is spaced away from the polishing surface. 
     The stated polishing unit may comprise a substrate transfer device for receiving substrates to be polished from the outside and positioning the substrates to enable the first and second substrate carriers to respectively pick up and hold the substrates. The wafer transfer device may be adapted to receive substrates which have been polished from the first and second substrate carriers positioned at their first positions. 
     The polishing unit may comprise first and second wafer transfer devices for receiving substrates to be polished from the outside and positioning the substrates to enable the first and second substrate carriers to pick up and hold the substrates, respectively. The wafer transfer devices may be adapted to receive substrates which have been polished from the first and second substrate carriers positioned at their first positions stated above. 
     In accordance with another aspect of the present invention, there is provided a polishing apparatus comprising a polishing unit including a turntable provided on one side surface with a polishing surface, and two substrate carriers each including a pivotal shaft, an arm having a center portion supported by the pivotal arm and opposite ends, and first and second substrate carriers provided on the opposite ends of the arm which are adapted to hold substrates, respectively. Each of the arms of the substrate carrier are pivotable about the pivotal shafts in such a manner that, in a first pivotal position, one of the first and second substrate carriers receives a substrate to be polished, while the other of the first and second substrate carriers is positioned over the polishing surface to enable the other substrate carrier to bring the substrate held thereby into contact with the polishing surface of the turntable, and in a second pivotal position, the other substrate carrier stated above receives a substrate to be polished, while the one substrate carrier stated above is positioned over the polishing surface to enable the one substrate carrier to bring the substrate held thereby into contact with the polishing surface of the turntable. The arms are movable between their first and second positions stated above without contacting each other. 
     The polishing unit may include a substrate transfer device for receiving substrates to be polished from the outside and positioning the substrates to enable the first and second substrate carriers to pick up and hold the substrates. The wafer transfer device may be adapted to receive substrates which have been polished from the first and second substrate carriers. The polishing unit may comprise two substrate transfer devices for respectively receiving substrates to be polished from the outside and positioning the substrates to enable the first and second substrate carriers to pick up and hold the substrates, respectively. The wafer transfer devices may be adapted to receive substrates which have been polished from the first and second substrate carriers. 
     According to a further aspect of the present invention, there is provided a polishing method comprising the steps of providing a turntable provided on its one side surface with a polishing surface, providing first and second substrate carriers each adapted to hold a substrate and to bring the substrate into contact with the polishing surface, turning the turntable, holding a substrate with the first substrate carrier, moving the first substrate carrier to bring the substrate held thereby into contact with the polishing surface to subject the substrate to polishing, and holding a substrate and then positioning the substrate over the polishing surface with the second substrate carrier, while the first substrate carrier keeps the substrate held thereby in contact with the polishing surface. Following completion of the polishing of the substrate held by the first substrate carrier, the substrate held by the second substrate carrier can be brought into contact with the polishing surface to subject the substrate to polishing without delay. 
     In accordance with the other aspect of the present invention, there is provided a polishing method comprising the steps of providing a turntable provided on its one side surface with a polishing surface, providing first and second substrate carriers each adapted to hold a substrate and to bring the substrate into contact with said polishing surface, turning the turntable, holding substrates by the first substrate carrier, moving the first substrate carrier to bring the substrate held thereby into contact with the polishing surface to subject the substrate to polishing, holding a substrate by the second substrate carrier and then bringing the substrate held by the second substrate carrier into contact with the polishing surface, while the first substrate carrier keeps the substrate held thereby in contact with the polishing surface, and moving the first substrate carrier to separate the substrate held thereby from the polishing surface following completion of polishing thereof, then exchanging the polished substrate with another substrate and bringing it into contact with the polishing surface, while the second substrate carrier keeps the substrate held thereby in contact with the polishing surface. 
     The above and other features and advantages of the present invention will become apparent from the following description and the appended claims taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view showing an arrangement of each part of a first embodiment of a polishing apparatus according to the present invention. 
     FIG. 2 is a view as taken in the direction of the arrow  2 — 2  in FIG. 1, which shows the relationship between a wafer carrier and a turntable for polishing. 
     FIG. 3 shows polishing start timing charts, in which: FIG.  3 ( a ) is a timing chart showing polishing start timing in a typical conventional polishing apparatus; FIG.  3 ( b ) is a timing chart showing polishing start timing in one example of the first embodiment of the polishing apparatus according to the present invention; FIG.  3 ( c ) is a timing chart showing polishing start timing in another example of the first embodiment of the present invention; and FIG.  3 ( d ) is a timing chart showing polishing start timing in still another example of the first embodiment of the present invention. 
     FIG. 4 is a plan view showing the arrangement of each part of a second embodiment of the polishing apparatus according to the present invention. 
     FIG. 5 is a plan view showing the arrangement of each part of a third embodiment of the polishing apparatus according to the present invention. 
     FIG. 6 is a plan view showing the arrangement of each part of a fourth embodiment of the polishing apparatus according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the polishing apparatus according to the present invention will now be described below with reference to the accompanying drawings. 
     FIG. 1 is a plan view of a first embodiment of a polishing apparatus according to the present invention. 
     The polishing apparatus shown in FIG. 1 has two rotatable loading/unloading stages L/UL for placing wafer cassettes  40 - 1  and  40 - 2  each stocked with a large number of semiconductor wafers. A transfer robot RB 1  having two hands is disposed at a position where the robot hands can reach the wafer cassettes  40 - 1  and  40 - 2  on the loading/unloading stages L/UL. 
     Of the two hands of the transfer robot RB 1 , the upper hand holds a dry semiconductor wafer, and the lower hand holds only a wet wafer. Cleaning machine CL 1 - 2  and CL 2 - 2  for cleaning and drying a polished semiconductor wafer are disposed on either side of the transfer robot RB 1  respectively, within reach of the robot hands. Turning-over machines TO 1  and TO 2  for turning over a semiconductor wafer are disposed within reach of the robot hands RB 1 . 
     The turning-over machine TO 1  has a chucking mechanism for chucking a semiconductor wafer and a turning-over mechanism for turning a semiconductor wafer upside down. The turning-over machine TO 1  handles only clean semiconductor wafers, while the turning-over machine TO 2 , in addition to a chucking mechanism and a turning-over mechanism, includes a rinsing mechanism for washing semiconductor wafers. The turning-over machine TO 2  handles only contaminated semiconductor wafers. 
     A second transfer robot RB 2  provided with two hands is positioned in substantially symmetric relation to the transfer robot RB 1 , as viewed along an imaginary line between the turning-over machines TO 1  and TO 2 , respectively. The transfer robot RB 2  is disposed at a position where its hands can reach the turning-over machines TO 1  and TO 2 . Of the two hands of the transfer robot RB 2 , the upper hand is employed for holding dry semiconductor wafers, while the lower hand holds wet wafers. 
     Two cleaning machines CL 1 - 1  and CL 2 - 1  for cleaning a polished semiconductor wafer are disposed on either side of the transfer robot RB 2 . These cleaning machines are different from CL 1 - 2  and CL 2 - 2  in the cleaning operation conducted thereby. Cleaning machines CL 1 - 1  and CL 2 - 1  are installed for access by the hands of the transfer robot RB 2 . Two transfer robots RB 3  and RB 4 , each provided with two hands, are disposed at opposite sides of the cleaning machines CL 1 - 1  and CL 2 - 1  relative to the turning-over machines TO 1  and TO 2 . RB 3  and RB 4  can travel in directions indicated by the double-headed arrow A. The hands of RB 3  are designed to access CL 1 - 1 . The hands of RB 4  are designed to access CL 2 - 1 . A wafer station  20  is installed between RB 3  and RB 4 . Wafer station  20  comprises both a dry and a wet station, and further is provided with a rinsing mechanism. Wafer station  20  is enclosed in a waterproof housing to contain splashes of rinsing water, and is provided on three sides with shuttered openings operative at the time of loading and unloading wafers. 
     The entire polishing apparatus is housed in housing  30  divided by a partition  31  into a cleaning chamber  30 A, containing transfer robots RB 3  and RB 4  and the other above-stated devices, and a polishing chamber  30 B. 
     Polishing unit PU 1  is provided with a turntable TT 1  and two wafer carriers WC 1 - 1  and WC 1 - 2 , each of which are adapted to hold and bring a semiconductor wafer into engagement with a polishing surface on the turntable TT 1 . Loading/unloading pushers P 1 - 1  and P 1 - 2  are provided to transfer semiconductor wafers to wafer carriers WC 1 - 2  and WC 1 - 1 , respectively. Polishing unit PU 1  further includes a dresser DR 1  for dressing or conditioning the polishing surface of the turntable TT 1 . 
     Polishing liquid nozzles N 1 - 1  and N 1 - 2  are provided to supply polishing liquid to the wafer carriers WC 1 - 1  and WC 1 - 2 . A dressing liquid nozzle N 1 - 3  is provided for the dresser DR 1 , and extends to a position approximately at the center of rotation of the turntable TT 1 . 
     The polishing unit PU 2  includes a turntable TT 2 , two wafer carriers WC 2 - 1  and WC 2 - 2 , a loading/unloading pusher P 2 - 1 , a loading/unloading pusher P 2 - 2 , a dresser DR 2  and polishing liquid nozzles N 2 - 1  and N 2 - 2 , which are equivalent to the turntable TT 1 , two wafer carriers WC 1 - 1  and WC 1 - 2 , loading/unloading pusher P 1 - 1 , loading/unloading pusher P 1 - 2 , dresser DR 1  and polishing liquid nozzles N 1 - 1  and N 1 - 2  of the polishing unit PU 1 , respectively, and arranged in the same manner as the latter elements. 
     FIG. 2 is a view taken in the direction of the arrow  2 — 2  in FIG. 1, and shows the relationship between a wafer carrier WC 1 - 1  and the turntable TT 1 . As shown in FIG. 2, the wafer carrier WC 1 - 1  is suspended from a wafer carrier arm  11  through a rotatable shaft  32 . The wafer carrier arm  11  is supported through a pivot shaft  22 , whereby the wafer carrier WC 1 - 1  accesses the turntable TT 1 . An pneumatic cylinder provided in the wafer carrier arm  11  is operated to urge a semiconductor wafer held on the wafer carrier WC 1 - 1  against the turntable TT 1  under a desired load. The dresser DR 1  is suspended from a dresser arm  21  through a rotatable shaft  42 . The dresser arm  21  is supported by a pivot shaft  43  for positioning the dresser arm  21  with respect to the turntable TT 1 . A pneumatic cylinder is also provided in the dresser arm  21  to urge the dresser DR 1  against the turntable TT 1  under a desired load. The turntable TT 1  has a polishing cloth or abrasive plate fitted to the upper surface thereof to provide a polishing surface. Although it is not shown, the arrangement of the wafer carrier WC 1 - 2  is the same as that of the wafer carrier WC 1 - 1 . The wafer carriers WC 2 - 1  and WC 2 - 2  and the dresser DR 2  are arranged in the same manner relative to the turntable TT 2 . 
     In the above-described arrangement, the two wafer carriers WC 1 - 1  and WC 1 - 2  in the polishing unit PU 1  are supported by the respective pivot shafts  33 , with each being movable between a polishing position where a semiconductor wafer is brought into contact with the turntable TT 1  and a transfer position where semiconductor wafers are transferred between the wafer carriers WC 1 - 1  and WC 1 - 2  and the corresponding loading/unloading pushers P 1 - 1  and P 1 - 2 . The two wafer carriers WC 1 - 1  and WC 1 - 2  can be positioned at the respective polishing positions either simultaneously or alternately. 
     As described previously, two polishing modes are available: one where a first wafer carrier WC 1 - 1  is situated at a polishing position over the turntable TT 1  to effect polishing of a wafer while the second wafer carrier WC 1 - 2  loaded with a wafer transferred from the pusher P 1 - 2  is situated at a transfer position ready for movement to the polishing position once polishing of a first wafer is complete; and a second mode where both first and second wafer carriers WC 1 - 1  and WC 1 - 2  are situated at polishing positions over the turntable TT 1  to effect simultaneous polishing of two wafers. In the second polishing mode, differing time periods for a polishing operation can be set with respect to the wafer carriers WC 1 - 1  and WC 1 - 2  depending on the condition of the surface of the semiconductor wafer held thereby for polishing. 
     Although in the foregoing only the polishing unit PU 1  has been described, it should be noted that the polishing unit PU 2  is operated in the same manner as the polishing unit PU 1 . 
     An operation of the entire polishing apparatus arranged as stated above will be described below. 
     Semiconductor wafers to be polished are placed in the wafer cassettes  40 - 1  and  40 - 2 . The wafer cassettes  40 - 1  and  40 - 2  are placed on the loading/unloading stages L/UL, respectively. Following input of data corresponding to processing conditions, the apparatus starts an automated operation as stated below. 
     1. The transfer robot RB 1  picks up and holds a wafer  1  in the wafer cassette  40 - 1  with the dry hand, i.e. the upper one of its two hands and, then, takes out the wafer  1  from the wafer cassette  40 - 1 . Thereafter, the transfer robot RB 1  adjusts the angle and height of the hand holding the wafer  1  and, then, transfers the wafer  1  to the turning-over machine TO 1 . 
     2. The turning-over machine TO 1  actuates its chucks to hold the wafer  1  transferred thereto by the transfer robot RB 1 . After it has been confirmed that the wafer  1  is securely held, the turning-over machine TO 1  turns the wafer  1  through 180° so that a surface of the wafer  1  that is to be processed faces downward. 
     3. After it has been confirmed that the turning-over machine TO 1  has turned the wafer  1  through 180°, the transfer robot RB 2  adjusts the angle and height of the hands thereof. Then, the transfer robot RB 2  opens the chucks of the turning-over machine TO 1  and receives the wafer  1  with the upper one of its two hands. Thereafter, the transfer robot RB 2  adjusts the angle and height of the upper hand and transfers the wafer  1  to the wafer station  20 . 
     4. The wafer station  20  has a mechanism for sensing whether or not the wafer  1  has been accurately transferred thereto. After it has been confirmed that the wafer  1  has been accurately transferred to the wafer station  20 , the transfer robot RB 3  takes the wafer  1  out from the wafer station  20  and transfers the wafer  1  to the pusher P 1 - 1 . 
     5. At this time, the wafer carrier WC 1 - 1  lies at a position above the pusher P, while the wafer carrier WC 1 - 2  remains at a position outside the pivoting locus of the wafer carrier WC 1 - 1 . It is preferable that the wafer carrier WC 1 - 2  stays at a polishing position over the turntable TT 1 . 
     6. After it has been confirmed that the wafer  1  has been transferred to the pusher P 1 - 1 , the pusher P 1 - 1  moves toward the wafer carrier WC 1 - 1  until the wafer  1  comes into contact with the wafer carrier WC 1 - 1 . 
     7. Next, the wafer  1  is transferred to the wafer carrier WC 1 - 1  under the influence of a vacuum. After it has been confirmed that the wafer  1  has been transferred, the pusher P 1 - 1  moves back to the previous position. 
     8. The wafer carrier WC 1 - 1  pivots about its pivot shaft  33  to move to the polishing position and commences polishing of the wafer. Meanwhile, the above-described steps 1 to 4 are conducted to supply the pusher P 1 - 2  with a wafer  2 . 
     9. The commencement of the polishing operation by the wafer carrier WC 1 - 1  triggers the wafer carrier WC 1 - 2  to start pivoting towards the pusher P 1 - 2 . The wafer carrier WC 1 - 2  stops pivoting at a position above the pusher P 1 - 2  where a wafer can be transferred between the pusher P 1 - 2  and the wafer carrier WC 1 - 2 . 
     10. The pusher P 1 - 2  thereafter moves upwards until the wafer  2  held by the pusher comes into contact with the wafer carrier WC 1 - 2 . 
     11. Next, the wafer  2  is transferred to the wafer carrier WC 1 - 2  by the action of a vacuum. After it has been confirmed that the wafer  2  has been transferred, the pusher p 1 - 2  moves back to the previous position. 
     12. The wafer carrier WC 1 - 2  pivots about its pivot shaft  33  to move to the polishing position and waits in this position until the polishing operation of the wafer carrier WC 1 - 1  is completed. After the completion of the polishing of the wafer  1  held by the wafer carrier WC 1 - 1 , polishing of the wafer  2  by the wafer carrier WC 1 - 2  is commenced. Meanwhile, the above-described operations at steps 1 to 4 are conducted, whereby a wafer  3  is held by the dry hand of the transfer robot RB 3 . The transfer robot RB 3  stands by at a position where the wafer  3  can be transferred between the transfer robot RB 3  and the pusher P 1 - 1 . 
     13. Regarding the timing at which the wafer carrier WC 1 - 2  commences polishing at the above-described step  12 , the polishing may be commenced immediately after the completion of the polishing by the wafer carrier WC 1 - 1 . It is also possible to carry out dressing for a desired period of time between the polishing processes. Dressing may be carried out for a desired period of time during the polishing operation. 
     14. After the completion of polishing, the wafer carrier WC 1 - 1  holds the polished wafer  1  under the influence of vacuum and is pivoted towards the pusher P 1 - 1  to finally separate the wafer  1  from the polishing surface and reaches a position above the pusher P 1 - 1 . 
     15. The pusher P 1 - 1  moves toward the wafer carrier WC 1 - 1  and stops at a position where a predetermined spacing is left between the pusher P 1 - 1  and the wafer carrier WC 1 - 1 . Thereafter, the wafer carrier WC 1 - 1  performs a wafer unloading operation to transfer the wafer  1  to the pusher P 1 - 1 . The wafer unloading operation is carried out by cutting off the vacuum applied to the wafer carrier WC 1 - 1  and blowing a combination of air or nitrogen and pure water over the wafer carrier WC 1 - 1 . The pusher P 1 - 1  having the wafer  1  transferred thereto moves down to a transfer position where the wafer is transferred from the pusher P 1 - 1  to the transfer robot RB 3 . 
     16. In this state, rinsing of the wafer  1  and rinsing of the wafer carrier WC 1 - 1  are carried out. The rinsing periods for the wafer  1  and the wafer carrier WC 1 - 1  can be set as desired. However, it is desirable that the wafer rinsing time be not shorter than the wafer carrier rinsing time (wafer rinsing time≧wafer carrier rinsing time). The reason being that since the wafer carrier WC 1 - 1  lies above the pusher P 1 - 1 , the rinsing liquid used for the wafer carrier WC 1 - 1  falls onto the pusher P 1 - 1 . Therefore, it is necessary for liquid on the pusher P 1 - 1  to be washed away from the pusher P 1 - 1 . 
     17. The transfer robot RB 3  receives the wafer  1  from the pusher P 1 - 1  with its lower, wet hand. In sequence, the transfer robot RB 3  transfers another wafer  3  to the pusher P 1 - 1  with its upper dry hand. The wafer transferred to the pusher is, then, subjected to the polishing operation as described above. 
     18. The transfer robot RB 3  places the wafer  1  on the wet station in the wafer station  20 . Subsequently, the transfer robot RB 3  receives a wafer  4  from the dry station in the wafer station  20  and moves to a position where the transfer robot RB 3  can transfer the wafer  4  to the pusher P 1 - 2 . The transfer robot RB 3  repeats the series of operations as stated above. 
     19. The wafer  1  transferred to the wet station in the wafer station  20  may be rinsed again on the wet station. This reduces the amount of slurry and abrasive grains or particles which might be otherwise brought into the subsequent steps. 
     20. After the completion of rinsing on the wet station in the wafer station  20 , the transfer robot RB 2  takes out the rinsed wafer  1  with the lower, wet hand and transfers it to the cleaning machine CL 1 - 1 . 
     21. The cleaning machine CL 1 - 1  is capable of cleaning both sides of a wafer simultaneously and can use an appropriate cleaning liquid (chemical liquid) at need. 
     22. The cleaned wafer  1  is taken out with the upper hand of the transfer robot RB 2  and transferred to the turning-over machine TO 2 . The turning-over machine TO 2  has a rinsing function to prevent the wafer from drying and is therefore capable of rinsing the wafer when it is turned over or during the period of time when the wafer waits for the transfer robot RB 1  to come. It is possible to select and set a desired rinsing quantity and rinsing time and also possible to decide whether to perform rinsing continuously or intermittently. 
     23. The wafer  1  is turned over through 180° by the turning-over machine so that the surface thereof to be processed faces upward. The transfer robot RB 1  receives the turned-over wafer  1  with the lower, wet hand and transfers it to the cleaning machine CL 1 - 2 . The cleaning machine CL 1 - 2  may be a contact-type cleaning machine using a pencil-type sponge or the like, or a non-contact cleaning machine represented by a megasonic jet cleaning machine which may be selected in conformity with the preceding and/or subsequent steps and the kind of film formed on the wafer. The cleaning machine is provided with a spin-drying function. 
     24. The wafer  1  cleaned and dried by the cleaning machine CL 1 - 2  is held under the influence of vacuum by the dry hand of the transfer robot RB 1  and returned to the same slot in the original cassette. 
     25. The above-described series of operations are repeated, thereby allowing wafers to be processed continuously. 
     26. In the foregoing only the polishing unit PU 1  has been described. As shown in FIG. 1, the polishing apparatus has two polishing units, and the operation sequences of the two polishing units are almost the same. In the foregoing description, all the wafers  1 ,  2  and  3  are transferred to the polishing unit PU 1  and processed therein. When the throughput need not be very high, only either the polishing unit PU 1  or PU 2  may be used. 
     27. The polishing apparatus may be operated such that, of wafers taken out from a single cassette and transferred to the dry station in the wafer station  20 , odd-numbered wafers are processed in the polishing unit PU 1 , and even-numbered wafers are processed in the polishing unit PU 2 . In such a case, odd-numbered wafers in the dry station of the wafer station  20  are taken out by the transfer robot RB 3 , and the even-numbered wafers are taken out by the transfer robot RB 4 . 
     28. Although in the above paragraph  27  a transfer pattern of wafers from one cassette has been discussed, limitations may be added such that wafers taken out from a cassette placed on one loading/unloading stage L/UL are processed in the polishing unit PU 1 , and wafers taken out from a cassette placed on the other loading/unloading stage L/UL are processed in the polishing unit PU 2 . 
     29. Although in the example shown in FIG. 1 the number of cassettes is 2, it may be a desired number, e.g. 4. In FIG. 1, the number of wafer carriers is 4. Therefore, the number of cassettes may also be 4. In that case, four cassettes can be associated with the four wafer carriers in one-to-one correspondence to each other for wafer processing history management. 
     In the polishing apparatus according to the present invention, during polishing by the wafer carrier WC 1 - 1 , the wafer carrier WC 1 - 2  is in a position where it is freely movable between the pusher P 1 - 2  and a position over the polishing surface of the turntable TT 1  without interfering with the wafer carrier WC 1 - 1 . Accordingly, during polishing of a wafer by the wafer carrier WC 1 - 1 , the wafer carrier WC 1 - 2  can transfer a polished wafer to the pusher P 1 - 2  and receive a subsequent wafer to be polished from the pusher P 1 - 2  and further move to a place directly above its polishing position to stand by for commencement of polishing. 
     With this arrangement, in which, during polishing by one wafer carrier, another wafer carrier can exchange wafers and move to a position above the polishing surface to stand by, it is possible to changeably set the polishing start timing of each of the wafer carriers WC 1 - 1  and WC 1 - 2  and the process menu to operate the apparatus. In the conventional polishing apparatus, during exchange of wafers, the polishing surface of the turntable is left unused. However, the above-described arrangement minimizes or eliminates idle time of the polishing surface while maintaining the merits of the single-wafer polishing apparatus, such as high polishing performance and ease of maintaining the wafer carriers. Accordingly, the overall throughput in the apparatus also increases. 
     FIG.  3 ( a ) is a timing chart showing the polishing start timing in a typical conventional polishing apparatus having one wafer carrier for one turntable. FIG.  3 ( b ) is a timing chart showing the polishing start timing when wafers are polished as stated above in the polishing apparatus according to the first embodiment of the present invention. As shown in FIG.  3 ( a ), if, in the conventional polishing apparatus, the polishing time is 120 seconds, for example, and the period of time required for other preparatory operations is 70 seconds, for example, 70 seconds is idle time of the turntable. The ratio of the period of time when one wafer is actually polished to the total processing time for the wafer becomes 120:190. 
     In contrast, the polishing apparatus according to the present invention can eliminate the idle time of the turntable. That is, as shown in FIG.  3 ( b ), while one wafer carrier WC 1 - 1  is polishing a wafer, another wafer carrier WC 1 - 2  performs preparatory operations and stands by. Thus, upon completion of the polishing of the wafer by the wafer carrier WC 1 - 1 , polishing by the wafer carrier WC 1 - 2  can commence. Accordingly, the ratio of the period of time when one wafer is actually polished to the total processing time for the wafer is 120:120. Thus, the idle time of the turntable can be reduced to zero. 
     It should be noted, however, that it is possible to perform a polishing operation with respect to two wafers without any stand-by time. FIG.  3 ( c ) is a timing chart showing the polishing start timing in that case. That is, in this example, the periods of time of the polishing operations wafer carriers WC- 1  and WC- 2  overlap each other. As such, the polishing rate becomes higher than 120:120. 
     It is also possible to perform wafer polishing operations by the wafer carriers WC 1 - 1  and WC 1 - 2  in parallel. FIG.  3 ( d ) is a timing chart showing the polishing start timing in the parallel polishing operation. That is, in this example, the wafer polishing operations by the wafer carriers WC 1 - 1  and WC 1 - 2  are simultaneously carried out in parallel, and thus polishing can be performed at a rate twice as high as that in the prior art. 
     FIG. 4 is a plan view showing the arrangement of each part of a second embodiment of the polishing apparatus according to the present invention. 
     In the embodiment shown in FIG. 4, the polishing unit PU 1  has a single pusher P 1 - 1 , and the polishing unit PU 2  has a single pusher P 2 - 1 . Accordingly, in the polishing unit PU 1 , the pusher P 1 - 1  functions as a transfer device common to the wafer carriers WC 1 -l and WC 1 - 2 . In the polishing unit PU 2 , the pusher P 2 - 1  functions as a transfer device common to the wafer carriers WC 2 - 1  and WC 2 - 2 . 
     In the embodiment shown in FIG. 4, each of the transfer robots RB 3  and RB 4  is of the type that does not travel. The arrangement of the rest of the embodiment is the same as that in the embodiment shown in FIG.  1 . In this embodiment, transfer of a work-piece to be polished between the wafer carrier WC 1 - 1  and the pusher P 1 - 1  and the work-piece transfer between the wafer carrier WC 1 - 2  and the pusher P 1 - 1  are performed alternately. The relationship between the wafer carriers WC 2 - 1  and WC 2 - 2  on the one hand and the pusher P 2 - 1  on the other is the same as in the case of the wafer carriers WC 1 - 1  and WC 1 - 2  and the pusher P 1 - 1 . 
     The operation of the entire polishing apparatus arranged as stated above will be described below. 
     Semiconductor wafers to be polished are put in the wafer cassettes  40 - 1  and  40 - 2 . The wafer cassettes  40 - 1  and  40 - 2  are placed on the loading/unloading stages L/UL, respectively. After all processing conditions in the apparatus have been input, the apparatus starts an automated operation. 
     1. The transfer robot RB 1  picks up and holds a wafer  1  in the wafer cassette  40 - 1  with the dry hand, i.e. the upper one of its two hands and, then, takes out the wafer  1  from the wafer cassette  40 - 1 . Thereafter, the transfer robot RB 1  adjusts the angle and height of the hand holding the wafer  1  and, then, transfers the wafer  1  to the turning-over machine TO 1 . 
     2. The turning-over machine TO 1  actuates its chucks to hold the wafer  1  transferred thereto by the transfer robot RB 1 . After it has been confirmed that the wafer  1  is securely held, the turning-over machine TO 1  turns the wafer  1  through 180° so that a surface of the wafer  1  that is to be processed faces downward. 
     3. After it has been confirmed that the turning-over machine TO 1  has turned the wafer  1  through 180°, the transfer robot RB 2  adjusts the angle and height of the hands thereof. Then, the transfer robot RB 2  opens the chucks of the turning-over machine TO 1  and receives the wafer  1  with the upper one of its two hands. Thereafter, the transfer robot RB 2  adjusts the angle and height of the upper hand and transfers the wafer  1  to the wafer station  20 . 
     4. The wafer station  20  has a mechanism for sensing whether or not the wafer  1  has been accurately transferred thereto. After it has been confirmed that the wafer  1  has been accurately transferred to the wafer station  20 , the transfer robot RB 3  takes the wafer  1  out from the wafer station  20  and transfers the wafer  1  to the pusher P 1 - 1 . 
     5. At this time, the wafer carrier WC 1 - 1  lies at a position above the pusher P, while the wafer carrier WC 1 - 2  remains at a position outside the pivoting locus of the wafer carrier WC 1 - 1 . It is preferable that the wafer carrier WC 1 - 2  stays at a polishing position over the turntable TT 1 . 
     6. After it has been confirmed that the wafer  1  has been transferred to the pusher P 1 - 1 , the pusher P 1 - 1  moves toward the wafer carrier WC 1 - 1  until the wafer  1  comes into contact with the wafer carrier WC 1 - 1 . 
     7. Next, the wafer  1  is transferred to the wafer carrier WC 1 - 1  under the influence of a vacuum. After it has been confirmed that the wafer  1  has been transferred, the pusher P 1 - 1  moves back to the previous position. 
     8. The wafer carrier WC 1 - 1  pivots about the pivot shaft  33  to move to the polishing position and a polishing operation commences. Meanwhile, the above-described operations at steps 1 to 4 are repeated to supply the pusher P 1 - 1  with a wafer  2 . 
     9. The fact that polishing with respect to the wafer carrier WC 1 - 1  has commenced triggers the wafer carrier WC 1 - 2  to start pivoting. The wafer carrier WC 1 - 2  stops pivoting at a position above the pusher P 1 - 1  where a wafer can be transferred between the pusher P 1 - 1  and the wafer carrier WC 1 - 2 . 
     10. When the wafer  2  is on the pusher P 1 - 1  and the wafer carrier WC 1 - 2  is at a position above the pusher P 1 - 1  where the wafer  2  can be transferred between the pusher P 1 - 1  and the wafer carrier WC 1 - 2 , the pusher P 1 - 1  moves toward the wafer carrier WC 1 - 2  until the wafer  2  comes into contact with wafer carrier WC 1 - 2 . 
     11. Next, the wafer  2  is transferred to the wafer carrier WC 1 - 2  by the action of a vacuum. After it has been confirmed that the wafer  2  has normally been transferred, the pusher p 1 - 1  moves back to the previous position. 
     12. The wafer carrier WC 1 - 2  pivots about its pivot shaft  33  to move to the polishing position and waits in this position until the polishing operation of the wafer carrier WC 1 - 1  is completed. After the completion of the polishing of the wafer  1  held by the wafer carrier WC 1 - 1 , polishing of the wafer  2  by the wafer carrier WC 1 - 2  is commenced. Meanwhile, the above-described operations at steps 1 to 4 are conducted, whereby a wafer  3  is held by the dry hand of the transfer robot RB 3 . The transfer robot RB 3  stands by at a position where the wafer  3  can be transferred between the transfer robot RB 3  and the pusher P 1 - 1 . 
     13. Regarding the timing at which the wafer carrier WC 1 - 2  commences polishing at the above-described step  12 , the polishing may be commenced immediately after the completion of the polishing by the wafer carrier WC 1 - 1 . It is also possible to carry out dressing for a desired period of time between the polishing processes. Dressing may be carried out for a desired period of time during the polishing operation. 
     14. After the completion of polishing, the wafer carrier WC 1 - 1  holds the polished wafer  1  under the influence of vacuum, is pivoted towards the pusher P 1 - 1  to finally separate the wafer  1  from the polishing surface and reaches a position above the pusher P 1 - 1 . 
     15. The pusher P 1 - 1  moves toward the wafer carrier WC 1 - 1  and stops at a position where a predetermined spacing is left between the pusher P 1 - 1  and the wafer carrier WC 1 - 1 . Thereafter, the wafer carrier WC 1 - 1  performs a wafer unloading operation to transfer the wafer  1  to the pusher P 1 - 1 . The wafer unloading operation is carried out by cutting off the vacuum applied to the wafer carrier WC 1 - 1  and blowing a combination of air or nitrogen and pure water over the wafer carrier WC 1 - 1 . The pusher P 1 - 1  having the wafer  1  transferred thereto moves down to a transfer position where the wafer is transferred from the pusher P 1 - 1  to the transfer robot RB 3 . 
     16. In this state, rinsing of the wafer  1  and rinsing of the wafer carrier WC 1 - 1  are carried out. The rinsing periods for the wafer  1  and the wafer carrier WC 1 - 1  can be set as desired. However, it is desirable that the wafer rinsing time be not shorter than the wafer carrier rinsing time (wafer rinsing time≧wafer carrier rinsing time). The reason being that since the wafer carrier WC 1 - 1  lies above the pusher P 1 - 1 , the rinsing liquid used for the wafer carrier WC 1 - 1  falls onto the pusher P 1 - 1 . Therefore, it is necessary for liquid on the pusher P 1 - 1  to be washed away from the pusher P 1 - 1 . 
     17. The transfer robot RB 3  receives the wafer  1  from the pusher P 1 - 1  with its lower, wet hand. In sequence, the transfer robot RB 3  transfers another wafer  3  to the pusher P 1 - 1  with its upper dry hand. The wafer transferred to the pusher is, then, subjected to the polishing operation as described above. 
     18. The transfer robot RB 3  places the wafer  1  on the wet station in the wafer station  20 . Subsequently, the transfer robot RB 3  receives a wafer  4  from the dry station in the wafer station  20  and moves to the position where the transfer robot RB 3  can transfer the wafer  4  to the pusher P 1 - 1 . The transfer robot RB 3  repeats this series of operations. 
     19. The wafer  1  transferred to the wet station in the wafer station  20  may be rinsed again on the wet station. This reduces the amount of slurry and abrasive grains or particles which might be otherwise brought into the subsequent steps. 
     20. After the completion of rinsing on the wet station in the wafer station  20 , the transfer robot RB 2  takes out the rinsed wafer  1  with the lower, wet hand and transfers it to the cleaning machine CL 1 - 1 . 
     21. The cleaning machine CL 1 - 1  is capable of cleaning both sides of a wafer simultaneously and can use an appropriate cleaning liquid (chemical liquid) at need. 
     22. The cleaned wafer  1  is taken out with the upper hand of the transfer robot RB 2  and transferred to the turning-over machine TO 2 . The turning-over machine TO 2  has a rinsing function to prevent the wafer from drying and is therefore capable of rinsing the wafer when it is turned over or during the period of time when the wafer waits for the transfer robot RB 1  to come. It is possible to select and set a desired rinsing quantity and rinsing time and also possible to decide whether to perform rinsing continuously or intermittently. 
     23. The wafer  1  is turned over through 180° by the turning-over machine so that the surface thereof to be processed faces upward. The transfer robot RB 1  receives the turned-over wafer  1  with the lower, wet hand and transfers it to the cleaning machine CL 1 - 2 . The cleaning machine CL 1 - 2  may be a contact-type cleaning machine using a pencil-type sponge or the like, or a non-contact cleaning machine represented by a megasonic jet cleaning machine which may be selected in conformity with the preceding and/or subsequent steps and the kind of film formed on the wafer. The cleaning machine is provided with a spin-drying function. 
     24. The wafer  1  cleaned and dried by the cleaning machine CL 1 - 2  is held under the influence of vacuum by the dry hand of the transfer robot RB 1  and returned to the same slot in the original cassette. 
     25. The above-described series of operations are repeated, thereby allowing wafers to be processed continuously. 
     26. In the foregoing only the polishing unit PU 1  has been described. As shown in FIG. 1, the polishing apparatus has two polishing units, and the operation sequences of the two polishing units are almost the same. In the foregoing description, all the wafers  1 ,  2  and  3  are transferred to the polishing unit PU 1  and processed therein. When the throughput need not be very high, only one polishing unit, PU 1  or PU 2 , may be used. 
     27. The polishing apparatus may be operated such that, of wafers taken out from a single cassette and transferred to the dry station in the wafer station  20 , odd-numbered wafers are processed in the polishing unit PU 1 , and even-numbered wafers are processed in the polishing unit PU 2 . In such a case, odd-numbered wafers in the dry station of the wafer station  20  are taken out by the transfer robot RB 3 , and the even-numbered wafers are taken out by the transfer robot RB 4 . 
     28. Although in the above paragraph  27  a transfer pattern of wafers from one cassette has been discussed, limitations may be added such that wafers taken out from a cassette placed on one loading/unloading stage L/UL are processed in the polishing unit PU 1 , and wafers taken out from a cassette placed on the other loading/unloading stage L/UL are processed in the polishing unit PU 2 . 
     29. Although in the example shown in FIG. 4 the number of cassettes is 2, it may be some other number, e.g. 4. In FIG. 4, the number of wafer carriers is 4. Therefore, the number of cassettes may also be 4. In that case, four cassettes can be associated with the four wafer carriers in one-to-one correspondence to each other for wafer processing history management. 
     FIG. 5 is a plan view showing the arrangement of each part of a third embodiment of the polishing apparatus according to the present invention. 
     In the embodiment shown in FIG. 5, the polishing unit PU 1  has two wafer carrier arms  11 . One wafer carrier arm  11  has wafer carriers WC 1 - 1   a  and WC 1 - 1   b  at both ends thereof. The other wafer carrier arm  11  has wafer carriers WC 1 - 2   a  and WC 1 - 2   b  at both ends thereof. Similarly, the polishing unit PU 2  has two wafer carrier arms  11 . One wafer carrier arm  11  has wafer carriers WC 2 - 1   a  and WC 2 - 1   b  at both ends thereof. The other wafer carrier arm  11  has wafer carriers WC 2 - 2   a  and WC 2 - 2   b  at both ends thereof. Each wafer carrier arm  11  is supported at the center in the longitudinal direction thereof by a pivot shaft  33 . With this arrangement, when the wafer carriers WC 1 - 1   a  and WC 1 - 2   a  in the polishing unit PU 1  are at respective polishing positions, the wafer carriers WC 1 - 1   b  and WC 1 - 2   b  lie at wafer transfer positions above the pushers P 1 - 1  and P 1 - 2 , respectively. In the polishing unit PU 2 , when the wafer carriers WC 2 - 1   a  and WC 2 - 2   a  are at respective polishing positions, the wafer carriers WC 2 - 1   b  and WC 2 - 2   b  lie at wafer transfer positions above the pushers P 2 - 1  and P 2 - 2 , respectively. The positions of the wafer carriers at both ends of each wafer carrier arm  11  are interchanged with each other by turning the pivot shaft  33  back and forth intermittently through 180° in the horizontal direction. 
     In the embodiment shown in FIG. 5, a fifth transfer robot RB 5  having two hands is provided at a position between the pusher P 1 - 2  of the polishing unit PU 1  and the pusher P 2 - 1  of the polishing unit PU 2 . The transfer robot RB 5  transfers wafers between the wafer station  20  and each of the pushers P 1 - 2  and P 2 - 1 . The third transfer robot RB 3  transfers wafers between the wafer station  20 , the pusher P 1 - 1  and the cleaning machine CL 1 - 1 . The fourth transfer robot RB 4  transfers wafers between the wafer station  20 , the pusher P 2 - 2  and the cleaning machine CL 2 - 1 . It should be noted that, in the illustrated example, dresser cleaning tanks DC 1  and DC 2  are disposed at respective positions close to the dressers DR 1  and DR 2 , and only one loading/unloading stage L/UL is provided. The arrangement of the rest of this embodiment is the same as that in the embodiment shown in FIG.  1 . 
     The operation of the entire polishing apparatus arranged as stated above will now be described below. In this embodiment, operations carried out until a wafer has been transferred to the dry station in the wafer station  20  and operations carried out after the polished wafer has been transferred to the cleaning machine CL 1 - 1  or CL 2 - 1  are similar to those in the foregoing embodiments. Therefore, operations carried out after the wafer has been transferred to the wafer station  20  until the polished wafer has been transferred to the cleaning machine CL 1 - 1  or CL 2 - 1  will be described for each of three different types of parallel processing [parallel operations (1) to (3)] and two different types of serial processing [serial operations (1) and (2)]. 
     A. Parallel Operation (1) 
     1. After it has been confirmed that the wafer  1  has been accurately transferred to the dry station in the wafer station  20 , the transfer robot RB 3  takes out the wafer  1  from the wafer station  20  and transfers the wafer  1  to the pusher P 1 - 1 . 
     2. At this time, the wafer carrier WC 1 - 1   a  is at a position above the pusher P 1 - 1  where a wafer can be transferred between the pusher P 1 - 1  and the wafer carrier WC 1 - 1   a , while the wafer carrier WC 1 - 1   b  is at the polishing position above the turntable TT 1 . 
     3. After it has been confirmed that the wafer  1  has been transferred to the pusher P 1 - 1 , the pusher P 1 - 1  moves toward the wafer carrier WC 1 - 1   a  until the wafer  1  comes into contact with wafer carrier WC 1 - 1   a.    
     4. Next, the wafer  1  is transferred to the wafer carrier WC 1 - 1   a  by the action of a vacuum. After it has been confirmed that the wafer  1  has been transferred, the pusher P 1 - 1  moves back to the previous position. 
     5. The wafer carrier WC 1 - 1   a  pivots about the pivot shaft  33  through 180° in the horizontal direction to move to the polishing position and commences polishing. Meanwhile, as the pivot shaft  33  rotates, the wafer carrier WC 1 - 1   b  moves to a position above the pusher P 1 - 1  where a wafer can be transferred between the pusher P 1 - 1  and the wafer carrier WC 1 - 1   b . A wafer  2  transferred to the pusher P 1 - 1  in the same way as the above is transferred to the wafer carrier WC 1 - 1   b  and held thereon under vacuum. The wafer carrier WC 1 - 1   b  holding the wafer  2  stands by in this state. 
     6. Upon completion of the polishing of the wafer  1  held by the wafer carrier WC 1 - 1   a , the wafer carrier WC 1 - 1   a  suction-holds the polished wafer  1  and is pivoted towards the pusher P 1 - 1  to finally separate the wafer  1  from the polishing surface and reaches a position above the pusher P 1 - 1 . 
     7. The pusher P 1 - 1  moves toward the wafer carrier WC 1 - 1   a  and stops at a position where a predetermined spacing is left between the pusher P 1 - 1  and the wafer carrier WC 1 - 1   a . Thereafter, the wafer carrier WC 1 - 1   a  performs a wafer unloading operation to transfer the wafer  1  to the pusher P 1 - 1 . In this state, rinsing of the wafer  1  and rinsing of the wafer carrier WC 1 - 1   a  are carried out. Meanwhile, as the pivot shaft  33  rotates, the wafer carrier WC 1 - 1   b  moves to the polishing position, and polishing commences. 
     8. The transfer robot RB 3  receives the wafer  1  from the pusher P 1 - 1  by using the lower, wet hand. Next, the transfer robot RB 3  transfers the wafer  1  to the cleaning machine CL 1 - 1 . The transfer robot RB 3  repeats this series of operations. 
     B. Parallel Operation (2) 
     1. After it has been confirmed that the wafer  1  has been accurately transferred to the dry station in the wafer station  20 , the transfer robot RB 5  takes out the wafer  1  from the wafer station  20  and transfers the wafer  1  to the pusher P 1 - 2  or P 2 - 1 . 
     2. At this time, the wafer carrier WC 1 - 2   a  lies at a position above the pusher P 1 - 2  where a wafer can be transferred between the pusher P 1 - 2  and the wafer carrier WC 1 - 2   a , while the wafer carrier WC 1 - 2   b  is at the polishing position above the turntable TT 1 . On the other hand, the wafer carrier WC 2 - 1   a  is at a position above the pusher P 2 - 1  where a wafer can be transferred between the pusher P 2 - 1  and the wafer carrier WC 2 - 1   a , while the wafer carrier WC 2 - 1   b  is at the polishing position above the turntable TT 2 . 
     3. After it has been confirmed that the wafer  1  has been transferred to the pusher P 1 - 2  or P 2 - 1 , the pusher P 1 - 2  or P 2 - 1  moves toward the wafer carrier WC 1 - 2   a  or WC 2 - 1   a  until the wafer  1  comes into contact with wafer carrier WC 1 - 2   a  or WC 2 - 1   a.    
     4. Next, the wafer  1  is transferred to the wafer carrier WC 1 - 2   a  or WC 2 - 1   a  by the action of a vacuum. After it has been confirmed that the wafer  1  has been transferred, the pusher P 1 - 2  or P 2 - 1  moves back to the previous position. 
     5. The wafer carrier WC 1 - 2   a  or WC 2 - 1   a  pivots about the pivot shaft  33  through 180° in the horizontal direction to move to the polishing position and commences polishing. Meanwhile, as the pivot shaft  33  rotates, the wafer carrier WC 1 - 2   b  or WC 2 - 1   b  moves to a position above the pusher P 1 - 2  or P 2 - 1  where a wafer can be transferred between the pusher P 1 - 2  or P 2 - 1  and the wafer carrier WC 1 - 2   b  or WC 2 - 1   b . A wafer  2  transferred to the pusher P 1 - 2  or P 2 - 1  in the same way as the above is transferred to the wafer carrier WC 1 - 2   b  or WC 2 - 1   b  and held thereon under vacuum. The wafer carrier WC 1 - 2   b  or WC 2 - 1   b  holding the wafer  2  stands by in this state. 
     6. Upon completion of the polishing of the wafer  1  held by the wafer carrier WC 1 - 2   a  or WC 2 - 1   a , the wafer carrier WC 1 - 2   a  or WC 2 - 1   a  suction-holds the polished wafer  1  and is pivoted towards the pusher P 1 - 2  or P 2 - 1  to finally separate the wafer  1  from the polishing surface and reach a position above the pusher P 1 - 2  or P 2 - 1  where the wafer  1  can be transferred between the wafer carrier WC 1 - 2   a  or WC 2 - 1   a  and the pusher P 1 - 2  or P 2 - 1 . 
     7. The pusher P 1 - 2  or P 2 - 1  moves toward the wafer carrier WC 1 - 2   a  or WC 2 - 1   a  and stops at a position where a predetermined spacing is left between the pusher P 1 - 2  or P 2 - 1  and the wafer carrier WC 1 - 2   a  or WC 2 - 1   a . Thereafter, the wafer carrier WC 1 - 2   a  or WC 2 - 1   a  performs a wafer unloading operation to transfer the wafer  1  to the pusher P 1 - 2  or P 2 - 1 . In this state, rinsing of the wafer  1  and rinsing of the wafer carrier WC 1 - 2   a  or WC 2 - 1   a  are carried out. Meanwhile, as the pivot shaft  33  rotates, the wafer carrier WC 1 - 2   b  or WC 2 - 1   b  moves to the polishing position, and polishing is commenced. 
     8. The transfer robot RB 5  receives the wafer  1  from the pusher P 1 - 2  or P 2 - 1  by using the lower, wet hand. Next, the transfer robot RB 5  transfers the wafer  1  to the wet station in the wafer station  20 . The transfer robot RB 5  repeats this series of operations. 
     9. The wafer  1  transferred to the wet station in the wafer station  20  is transferred to the cleaning machine CL 1 - 1  or CL 2 - 1  by the transfer robot RB 3  or RB 4 . 
     C. Parallel Operation (3) 
     1. After it has been confirmed that the wafer  1  has been accurately transferred to the dry station in the wafer station  20 , the transfer robot RB 4  takes out the wafer  1  from the wafer station  20  and transfers the wafer  1  to the pusher P 2 - 2 . 
     2. At this time, the wafer carrier WC 2 - 2   a  is at a position above the pusher P 2 - 2  where a wafer can be transferred between the pusher P 2 - 2  and the wafer carrier WC 2 - 2   a , while the wafer carrier WC 2 - 2   b  is at the polishing position above the turntable TT 2 . 
     3. After it has been confirmed that the wafer  1  has been transferred to the pusher P 2 - 2 , the pusher P 2 - 2  moves toward the wafer carrier WC 2 - 2   a  until the wafer  1  comes into contact with wafer carrier WC 2 - 2   a.    
     4. Next, the wafer  1  is transferred to the wafer carrier WC 2 - 2   a  by the action of a vacuum. After it has been confirmed that the wafer  1  has been transferred, the pusher P 2 - 2  moves back to the previous position. 
     5. The wafer carrier WC 2 - 2   a  pivots about the pivot shaft  33  through 180° in the horizontal direction to move to the polishing position and commences polishing. Meanwhile, as the pivot shaft  33  rotates, the wafer carrier WC 2 - 2   b  moves to a position above the pusher P 2 - 2  where a wafer can be transferred between the pusher P 2 - 2  and the wafer carrier WC 2 - 2   b . A wafer  2  transferred to the pusher P 2 - 2  in the same way as the above is transferred to the wafer carrier WC 2 - 2   b  and held thereon under vacuum. The wafer carrier WC 2 - 2   b  holding the wafer  2  stands by in this state. 
     6. Upon completion of the polishing of the wafer  1  held by the wafer carrier WC 2 - 2   a , the wafer carrier WC 2 - 2   a  suction-holds the polished wafer  1  and is pivoted towards the pusher P 2 - 2  to finally separate the wafer  1  from the polishing surface and reaches a position above the pusher P 2 - 2 . 
     7. The pusher P 2 - 2  moves toward the wafer carrier WC 2 - 2   a  and stops at a position where a predetermined spacing is left between the pusher P 2 - 2  and the wafer carrier WC 2 - 2   a . Thereafter, the wafer carrier WC 2 - 2   a  performs a wafer unloading operation to transfer the wafer  1  to the pusher P 2 - 2 . In this state, rinsing of the wafer  1  and rinsing of the wafer carrier WC 2 - 2   a  are carried out. Meanwhile, as the pivot shaft  33  rotates, the wafer carrier WC 2 - 2   b  moves to the polishing position, and polishing commences. 
     8. The transfer robot RB 4  receives the wafer  1  from the pusher P 2 - 2  by using the lower, wet hand. Next, the transfer robot RB 3  transfers the wafer  1  to the cleaning machine CL 2 - 1 . The transfer robot RB 4  repeats this series of operations. 
     D. Serial Operation (1) 
     1. After it has been confirmed that the wafer  1  has been. accurately transferred to the dry station in the wafer station  20 , the transfer robot RB 5  takes out the wafer  1  from the wafer station  20  and transfers the wafer  1  to the pusher P 1 - 2 . 
     2. At this time, the wafer carrier WC 1 - 2   a  is at a position above the pusher P 1 - 2  where a wafer can be transferred between the pusher P 1 - 2  and the wafer carrier WC 1 - 2   a , while the wafer carrier WC 1 - 2   b  is at the polishing position above the turntable TT 1 . 
     3. After it has been confirmed that the wafer  1  has been transferred to the pusher P 1 - 2 , the pusher P 1 - 2  moves toward the wafer carrier WC 1 - 2   a  until the wafer  1  comes into contact with wafer carrier WC 1 - 2   a.    
     4. Next, the wafer  1  is transferred to the wafer carrier WC 1 - 2   a  by the action of a vacuum. After it has been confirmed that the wafer  1  has been transferred, the pusher P 1 - 2  moves back to the previous position. 
     5. The wafer carrier WC 1 - 2   a  pivots about the pivot shaft  33  through 180° in the horizontal direction to move to the polishing position and commences polishing. Meanwhile, as the pivot shaft  33  rotates, the wafer carrier WC 1 - 2   b  moves to a position above the pusher P 1 - 2  where a wafer can be transferred between the pusher P 1 - 2  and the wafer carrier WC 1 - 2   b . A wafer  2  transferred to the pusher P 1 - 2  in the same way as the above is transferred to the wafer carrier WC 1 - 2   b  and held thereon under vacuum. The wafer carrier WC 1 - 2   b  holding the wafer  2  stands by in this state. 
     6. Upon completion of the polishing of the wafer  1  held by the wafer carrier WC 1 - 2   a , the wafer carrier WC 1 - 2   a  suction-holds the polished wafer  1  and is pivoted towards the pusher P 1 - 2  to finally separate the wafer  1  from the polishing surface and reaches a position above the pusher P 1 - 2 . 
     7. The pusher P 1 - 2  moves toward the wafer carrier WC 1 - 2   a  and stops at a position where a predetermined spacing is left between the pusher P 1 - 2  and the wafer carrier WC 1 - 2   a . Thereafter, the wafer carrier WC 1 - 2   a  performs a wafer unloading operation to transfer the wafer  1  to the pusher P 1 - 2 . In this state, rinsing of the wafer  1  and rinsing of the wafer carrier WC 1 - 2   a  are carried out. Meanwhile, as the pivot shaft  33  rotates, the wafer carrier WC 1 - 2   b  moves to the polishing position, and polishing commences. 
     8. The transfer robot RB 5  receive s the wafer  1  from the pusher P- 2  by using the lower, wet hand. Next, the transfer robot RB 5  transfers the wafer  1  to the wet station in the wafer station  20 . The transfer robot RB 5  repeats this series of operations. 
     9. After it has been confirmed that the wafer  1  has been accurately transferred to the wet station in the wafer station  20 , the transfer robot RB 3  takes out the wafer  1  from the wafer station  20  and transfers the wafer  1  to the pusher P 1 - 1 . 
     10. At this time, the wafer carrier WC 1 - 1   a  lies at a position above the pusher P 1 - 1  where a wafer can be transferred between the pusher P 1 - 1  and the wafer carrier WC 1 - 1   a , while the wafer carrier WC 1 - 1   b  lies at the polishing position above the turntable TT 1 . 
     11. After it has been confirmed that the wafer  1  has been transferred to the pusher P 1 - 1 , the pusher P 1 - 1  moves toward the wafer carrier WC 1 - 1   a  until the wafer  1  comes into contact with wafer carrier WC 1 - 1   a.    
     12. Next, the wafer  1  is transferred to the wafer carrier WC 1 - 1   a  by the action of a vacuum. After it has been confirmed that the wafer  1  has normally been transferred, the pusher P 1 - 1  moves back to the previous position. 
     13. The wafer carrier WC 1 - 1   a  pivots about the pivot shaft  33  through 180° in the horizontal direction to move to the polishing position and commences polishing. Meanwhile, as the pivot shaft  33  rotates, the wafer carrier WC 1 - 1   b  moves to a position above the pusher P 1 - 1  where a wafer can be transferred between the pusher P 1 - 1  and the wafer carrier WC 1 - 1   b . A wafer  2  transferred to the pusher P 1 - 1  in the same way as the above is transferred to the wafer carrier WC 1 - 1   b  and held thereon under vacuum. The wafer carrier WC 1 - 1   b  holding the wafer  2  stands by in this state. 
     14. Upon completion of the polishing of the wafer  1  held by the wafer carrier WC 1 - 1   a , the wafer carrier WC 1 - 1   a  suction-holds the polished wafer  1  and is pivoted towards the pusher P 1 - 1  to finally separate the wafer  1  from the polishing surface and reaches a position above the pusher P 1 - 1  where the wafer  1  can be transferred between the wafer carrier WC 1 - 1   a  and the pusher P 1 - 1 . 
     15. The pusher P 1 - 1  moves toward the wafer carrier WC 1 - 1   a  and stops at a position where a predetermined spacing is left between the pusher P 1 - 1  and the wafer carrier WC 1 - 1   a . Thereafter, the wafer carrier WC 1 - 1   a  performs a wafer unloading operation to transfer the wafer  1  to the pusher P 1 - 1 . In this state, rinsing of the wafer  1  and rinsing of the wafer carrier WC 1 - 1   a  are carried out. Meanwhile, as the pivot shaft  33  rotates, the wafer carrier WC 1 - 1   b  moves to the polishing position, and polishing is commenced. 
     16. The transfer robot RB 3  receives the wafer  1  from the pusher P 1 - 1  by using the lower, wet hand. Next, the transfer robot RB 3  transfers the wafer  1  to the cleaning machine CL 1 - 1 . The transfer robot RB 3  repeats this series of operations. 
     Although only the polishing unit PU 1  has been explained in the foregoing description, the operation sequence in the polishing unit PU 2  is almost the same as the above. In the polishing unit PU 2 , a wafer to be polished is transferred to the pusher P 2 - 1  from the transfer robot RB 5 , and after being polished, the wafer is transferred from the pusher P 2 - 2  to the cleaning machine CL 2 - 1  via the transfer robot RB 4 . 
     E. Serial Operation (2) 
     1. After it has been confirmed that the wafer  1  has been accurately transferred to the dry station in the wafer station  20 , the transfer robot RB 5  takes out the wafer  1  from the wafer station  20  and transfers the wafer  1  to the pusher P 1 - 2 . 
     2. At this time, the wafer carrier WC 1 - 2   a  is at a position above the pusher P 1 - 2  where a wafer can be transferred between the pusher P 1 - 2  and the wafer carrier WC 1 - 2   a , while the wafer carrier WC 1 - 2   b  is at the polishing position above the turntable TT 1 . 
     3. After it has been confirmed that the wafer  1  has been transferred to the pusher P 1 - 2 , the pusher P 1 - 2  moves toward the wafer carrier WC 1 - 2   a  until the wafer  1  comes into contact with wafer carrier WC 1 - 2   a.    
     4. Next, the wafer  1  is transferred to the wafer carrier WC 1 - 2   a  by the action of a vacuum. After it has been confirmed that the wafer  1  has been transferred, the pusher P 1 - 2  moves back to the previous position. 
     5. The wafer carrier WC 1 - 2   a  pivots about the pivot shaft  33  through 180° in the horizontal direction to move to the polishing position and commences polishing. Meanwhile, as the pivot shaft  33  rotates, the wafer carrier WC 1 - 2   b  moves to a position above the pusher P 1 - 2  where a wafer can be transferred between the pusher P 1 - 2  and the wafer carrier WC 1 - 2   b . A wafer  2  transferred to the pusher P 1 - 2  in the same way as the above is transferred to the wafer carrier WC 1 - 2   b  and held thereon under vacuum. The wafer carrier WC 1 - 2   b  holding the wafer  2  stands by in this state. 
     6. Upon completion of the polishing of the wafer  1  held by the wafer carrier WC 1 - 2   a , the wafer carrier WC 1 - 2   a  suction-holds the polished wafer  1  and is pivoted towards the pusher P 1 - 2  to finally separate the wafer  1  from the polishing surface and reaches a position above the pusher P 1 - 2 . 
     7. The pusher P 1 - 2  moves toward the wafer carrier WC 1 - 2   a  and stops at a position where a predetermined spacing is left between the pusher P 1 - 2  and the wafer carrier WC 1 - 2   a . Thereafter, the wafer carrier WC 1 - 2   a  performs a wafer unloading operation to transfer the wafer  1  to the pusher P 1 - 2 . In this state, rinsing of the wafer  1  and rinsing of the wafer carrier WC 1 - 2   a  are carried out. Meanwhile, as the pivot shaft  33  rotates, the wafer carrier WC 1 - 2   b  moves to the polishing position, and polishing commences. 
     8. The transfer robot RB 5  receives the wafer  1  from the pusher P 1 - 2  by using the lower, wet hand. Next, the transfer robot RB 5  transfers the wafer  1  to the wet station in the wafer station  20 . After it has been confirmed that the wafer  1  has been accurately transferred to the wet station in the wafer station, the transfer robot RB 5  takes out the wafer  1  from the wafer station  20  and transfers the wafer  1  to the pusher P 2 - 1 . 
     9. At this time, the wafer carrier WC 2 - 1   a  lies at a position above the pusher P 2 - 1  where a wafer can be transferred between the pusher P 2 - 1  and the wafer carrier WC 2 - 1   a , while the wafer carrier WC 2 - 1   b  lies at the polishing position above the turntable TT 1 . 
     10. After it has been confirmed that the wafer  1  has been transferred to the pusher P 2 - 1 , the pusher P 2 - 1  moves toward the wafer carrier WC 2 - 1   a  until the wafer  1  comes into contact with wafer carrier WC 2 - 1   a.    
     11. Next, the wafer  1  is transferred to the wafer carrier WC 2 - 1   a  by the action of a vacuum. After it has been confirmed that the wafer  1  has normally been transferred, the pusher P 2 - 1  moves back to the previous position. 
     12. The wafer carrier WC 2 - 1   a  pivots about the pivot shaft  33  through 180° in the horizontal direction to move to the polishing position and commences polishing. Meanwhile, as the pivot shaft  33  rotates, the wafer carrier WC 2 - 1   b  moves to a position above the pusher P 2 - 1  where a wafer can be transferred between the pusher P 2 - 1  and the wafer carrier WC 2 - 1   b . A wafer  2  transferred to the pusher P 2 - 1  in the same way as the above is transferred to the wafer carrier WC 2 - 1   b  and held thereon under vacuum. The wafer carrier WC 2 - 1   b  holding the wafer  2  stands by in this state. 
     13. Upon completion of the polishing of the wafer  1  held by the wafer carrier WC 2 - 1   a , the wafer carrier WC 2 - 1   a  suction-holds the polished wafer  1  and is pivoted towards the pusher P 2 - 1  to finally separate the wafer  1  from the polishing surface and reaches a position above the pusher P 2 - 1  where the wafer  1  can be transferred between the wafer carrier WC 2 - 1   a  and the pusher P 2 - 1 . 
     14. The pusher P 2 - 1  moves toward the wafer carrier WC 2 - 1   a  and stops at a position where a predetermined spacing is left between the pusher P 2 - 1  and the wafer carrier WC 2 - 1   a . Thereafter, the wafer carrier WC 2 - 1   a  performs a wafer unloading operation to transfer the wafer  1  to the pusher P 2 - 1 . In this state, rinsing of the wafer  1  and rinsing of the wafer carrier WC 2 - 1   a  are carried out. Meanwhile, as the pivot shaft  33  rotates, the wafer carrier WC 2 - 1   b  moves to the polishing position, and polishing is commenced. 
     15. The transfer robot RB 5  receives the wafer  1  from the pusher P 2 - 1  by using the lower, wet hand. The transfer robot RB 5  repeats this series of operations. Next, the transfer robot RB 3  or RB 4  transfers the wafer  1  to the cleaning machine CL 1 - 1  or CL 2 - 1 . 
     Thus, in the above-described operation, a single wafer is polished sequentially in the two polishing units PU 1  and PU 2 . Accordingly, it is possible to subject a single wafer to two different kinds of polishing successively in the two polishing units PU 1  and PU 2 , for example, rough polishing in the polishing unit PU 1  and finish polishing in the polishing unit PU 2 , by making the polishing units PU 1  and PU 2  different from each other in the kind of polishing cloth or abrasive fitted to the upper surfaces of the turntables TT 1  and TT 2  or supplying different polishing liquids to the upper surfaces of the turntables TT 1  and TT 2 . 
     FIG. 6 is a plan view showing the arrangement of each part of a fourth embodiment of the polishing apparatus according to the present invention. 
     The embodiment shown in FIG. 6 has only one of the two polishing units PU 1  and PU 2  in the embodiment shown in FIG. 5, that is, the polishing unit PU 1 . The turntable TT 1  of the polishing unit PU 1  is disposed at a position facing the wafer station  20  across the partition  31 . Further, transfer robots RB 3  and RB 4 , which do not travel, are placed in between the pushers P 1 - 1  and P 1 - 2  on the one hand, which are provided in the polishing unit PU 1 , and the cleaning machines CL 1 - 1  and CL 2 - 1  on the other, which are placed in the cleaning chamber  30 A. The arrangement of the rest of this embodiment is the same as that in the embodiment shown in FIG.  5 . 
     The operation of the entire polishing apparatus according to the embodiment shown in FIG. 6 is substantially the same as the operation of one side of the polishing apparatus according to the embodiment shown in FIG.  5 . Therefore, a description thereof is omitted. 
     As has been stated above, the present invention makes it possible to polish work-pieces, e.g. semiconductor wafers, at all times by using a single polishing turntable and hence possible to markedly increase the number of work-pieces polished per unit of time (i.e. throughput). 
     It is also possible according to the present invention to choose between a first method wherein work-pieces are alternately polished one by one on one turntable and a second method wherein two work-pieces are simultaneously polished on one turntable. In a case where the simultaneous polishing of two work-pieces by the second method causes the process to become unstable and thus brings about adverse effects, e.g. unfavorably low yield, the adverse effects produced by the simultaneous polishing can be eliminated by employing the first polishing method. 
     It should be noted that the present invention is not limited to the foregoing embodiments but can be modified in a variety of ways.