Abstract:
This invention pertains to a polishing apparatus for polishing a semiconductor wafer. The apparatus comprises a storage section that is capable of receiving a workpiece to be polished and a polished workpiece. The polishing unit that polishes the workpiece includes a primary polishing table and a secondary polishing table, wherein the polishing surface of the secondary polishing table is constructed to be arranged such that at least a portion of a surface of the workpiece being polished by the polishing surface of the secondary polishing table extends beyond an edge of the polishing surface of the secondary polishing table. Also provided is a film thickness measuring device, which measures the thickness of a film formed on a polished workpiece while the polished workpiece is held by a top ring above a pusher.

Description:
This application is a divisional of U.S. application Ser. No. 09/984,433, filed Oct. 30, 2001, now U.S. Pat. No. 6,413,146, which is a divisional application of U.S. application Ser. No. 09/341,882, filed Sep. 8, 1999, now U.S. Pat. No. 6,332,826, which is a 371 of PCT/JP98/05252, filed Nov. 20, 1998. 
    
    
     TECHNICAL FIELD 
     The present invention relates to polishing apparatus in general, and relates in particular to a polishing apparatus to produce a flat and mirror polished surface on workpieces such as semiconductor wafers. 
     BACKGROUND ART 
     With increasing intensity of circuit integration in semiconductor devices in recent years, circuit lines have become finer and interline spacing has also been drastically reduced. With this trend for finer resolution in circuit fabrication, it is now necessary to provide a precision flat substrate surface because of the extreme shallow depth of focus required in optical photolithography using stepper reproduction of circuit layout. One method of obtaining a flat surface is mechano-chemical polishing carried out by pressing wafers held on a carrier against a polishing cloth mounted on a rotating turntable while dripping a solution containing abrasive powder at the interface of the wafer and the polishing cloth. 
       FIG. 11  shows a polishing apparatus disclosed in a Japanese Patent Laid-Open Publication, H9-117857. The facility is comprised by a pair of polishing units  101   a,    101   b  disposed symmetrically at one end of a rectangular-shaped floor, and a loading/unloading unit including wafer cassettes  102   a,    102   b  disposed on the opposite end of the floor for storing wafers. Transport rails  103  are disposed along a line joining the polishing units  101   a,    101   b  and the loading/unloading unit, and alongside the rails  103 , there are wafer inverters  105 ,  106  surrounded by respective cleaning units  107   a,    107   b  and  108   a,    108   b.    
     Such a polishing apparatus, comprised by a pair of parallel processing lines arranged on both sides of the rails, is able to handle workpieces polished through a single step process in each line of the facility to improve its productivity. For those workpieces requiring a double step polishing, such as compound semiconductor materials requiring polishing steps using different solutions, after completing a first polishing step through one polishing line  101   a,  the workpieces are cleaned next, and then transferred over to the second line  101   b  to carry out a second polishing step. Thus, such a polishing apparatus is able to carry out a series-operation for workpieces processed in double-step polishing, and a parallel-operation for workpieces processed in single-step polishing. 
     Transport of workpieces in the parallel polishing process is carried out as follows. After completing a polishing operation of the polishing units  101   a ,  101   b,  the top ring (workpiece carrier)  110  rotates and moves over to the workpiece pusher (transfer device)  112  to transfer the polished workpiece. A second robot  104   b  transports the workpiece over to the cleaning units  107   a  or  107   b,  and receives an unpolished workpiece from the inverter  105 ,  106 , and transfers it to the workpiece pusher  112 . The top ring  110  receives the unpolished workpieces and moves back to the turntable  109  to begin polishing. A dresser  111  is provided to carry out reconditioning of a polishing cloth. 
     A polishing unit, such as the one shown in  FIG. 12 , is comprised by a turntable  109  having a polishing cloth  115  bonded to its top surface, and a top ring  113  for holding and pressing a wafer W onto the turntable  109 . Polishing action is produced by rotating and pressing the wafer W by the top ring  113  against the rotating turntable  109  while a polishing solution Q is supplied in the interface between the wafer W and the polishing cloth  115 . The polishing solution Q is held between the surface to be polished (bottom surface) of the wafer W and the polishing cloth  115  while the wafer is being polished. 
     In such a polishing unit, the turntable  109  and the top ring  113  are rotated at their own independent speeds, and the top ring  113  is positioned, as shown in  FIG. 12 , so that the inner edge of the wafer W will be off from the center of the turntable  109  at a distance “a”, and the outer edge of the wafer W will be at a distance “b” from the periphery of the turntable  109 , respectively. The wafer W is polished in this condition at high rotational speeds so that the surface of the wafer will be polished uniformly and quickly. Therefore, the diameter “D” of the turntable  109  is chosen to be more than double the radius “d” of the wafer W according to the following expression:
 
 D= 2( d+a+b ).
 
     Polished wafers W are stored in the wafer cassette  102   a,    102   b  after having gone through one or more cleaning and drying steps. Cleaning methods for wafers include scrubbing with brush made of nylon or mohair, and sponges including polyvinyl alcohol (PVA). 
     One of the problems in the existing polishing apparatus is its productivity. To increase the through-put from such a facility, the efficiency-determining processes involving polishing at the turntable  109  must be raised. However, in the existing technology, one robot  104   b  is required to carry out a multiple duty of removing polished wafers and supplying unpolished wafers to and from two workpiece pushers  112 . This is time-consuming, resulting in idle time for the turntable  109 . 
     Therefore, there is a need to provide, as a first objective, a polishing apparatus having two parallel processing lines that carries out efficient parallel processing by minimizing the idle time for the turntable and maximizing the through-put. 
     Furthermore, in the existing polishing apparatus, a high relative speed between the turntable  109  and the top ring  113  is used to achieve effective polishing as well as high flatness of the wafer surface, but this high relative speed may also cause micro-scratch marks on the wafers due to abrasive particles contained in the polishing solution. 
     To prevent fine scratches, it is possible to consider utilizing two sets of turntables  109 , and carry out polishing in two stages, by changing polishing parameters such as the material and abrasive characteristics of the polishing cloth  115 , rotation speed of the turntable  109 , and polishing solution. However, as mentioned above, the large size of the turntable  109  occupying a large installation space and requiring high capital cost are disadvantages of such an approach, and this type of problem is expected to become more serious in the future, as larger diameter wafers become more common. 
     On the other hand, it is also possible to consider using one turntable by switching polishing solutions or by reducing the rotational speed to resolve existing problems, but such approaches are not expected to lead to improved productivity, because mixing of solutions may lead to poor performance and polishing time would be lengthened. 
     Another problem is related to cleaning of the wafers. When the wafers are scrubbed after polishing with abrasive particles, it is difficult to remove particles of sub-micron sizes, and if the adhesion force between the wafer and particles is strong, such cleaning method is sometimes ineffective for removing such particles. 
     Therefore, there is a need to provide, as a second objective, a compact polishing apparatus that can provide excellent flatness and efficient cleaning. 
     DISCLOSURE OF INVENTION 
     These objectives of the present invention are realized in a polishing apparatus comprising: a storage section for storing a workpiece to be polished; at least two processing lines extending substantially in parallel from the storage section, with each line being provided with a cleaning unit and a polishing unit; a temporary storage station disposed between the cleaning unit and the polishing unit and shared by the processing lines; and at least two robotic devices disposed for each of the processing lines for transferring workpieces among the temporary storage station, the polishing unit and the cleaning unit. 
     Accordingly, each of the robotic devices is used to supply an unpolished wafer placed on the temporary storage station to a polishing unit, and a polished wafer in another polishing unit directly to a cleaning unit. Therefore, replacing of wafers between processes is carried out very quickly. Therefore, the productivity-limiting step of idle time for the polishing unit can be minimized, thereby enabling the through-put of the polishing apparatus to be increased. 
     In such a polishing apparatus, the polishing unit may be provided with a turntable, a top ling device, and a workpiece pusher for facilitating transfer of a workpiece to and from the robotic device. 
     In such a polishing apparatus, the top ring device may be comprised by two top rings, which can be positioned to work with the turntable and with the workpiece pusher, and a swing arm for supporting the top rings rotatably in a horizontal plane. In this case, while one top ring is performing polishing, the other top ring is in a position to exchange a polished wafer with an unpolished wafer, so that the idle time for the turntable is reduced, thereby increasing the through-put of the facility. 
     In such a polishing apparatus, the polishing unit may be provided with a film thickness measuring device for remotely measuring thickness of a film formed on a workpiece being held on the top ring. Adopting this arrangement will enable the amount of material removed from the surface of the workpiece to be finely controlled. In addition, the polishing unit may be provided with a buffing table having a buffing disk. 
     In another aspect of the invention, a polishing apparatus comprises: a storage section for storing a workpiece disposed at one end of an installation floor space; two polishing units disposed at an opposite end of the installation floor space, with each polishing unit having a turntable, a top ring device and a workpiece pusher; at least two cleaning units for cleaning polished workpieces polished in the polishing units; and a transport device for transferring workpieces between processing units, wherein a group of polishing and cleaning units and another group of polishing and cleaning units are disposed symmetrically opposite to each other across a center line extending from the one end to the opposite end of the installation floor space, and wherein the transport device comprises a temporary storage station disposed on the center line, and robotic devices disposed on both lateral sides of the temporary storage station. 
     In another aspect of the invention, a polishing apparatus for polishing a circular workpiece attached to a holder device, by rotating and pressing a workpiece surface against a rotating polishing surface of a circular polishing tool, comprises: a primary polishing table whose polishing surface radius is larger than a diameter of the workpiece; and a secondary polishing table whose polishing surface radius is smaller than a diameter but larger than a radius of the workpiece. 
     Such a polishing apparatus is used to carry out a two-step polishing operation. On the first polishing table, high speed polishing is applied to polish a workpiece as in the conventional process, while the second polishing table is used to remove micro-scratches or to carry out preliminary cleaning. On the second polishing table, although not all the workpiece surface is in contact with the polishing surface at all times, because of the oscillating motion of the workpiece, the workpiece itself is rotated so that all areas of the workpiece comes into contact with the polishing surface, and results in uniform material removal. To avoid producing a slanted polished surface, the axis of the workpiece should stay constantly on the polishing surface. The size of the secondary polishing table may be made small in comparison to the very large size of the primary polishing table, thereby providing a compact apparatus even with an additional polishing device. 
     In such a polishing apparatus, it may be arranged that the holder device is able to transport a workpiece to both the primary polishing table and the secondary polishing table. The secondary polishing table should be positioned within the swing trace of the wafer holding device, because it revolves about an axis to transfer the workpiece between the polishing unit and a wafer transfer position. 
     Another aspect of the invention is a polishing apparatus for polishing a circular workpiece attached to a holder device, by rotating and pressing a workpiece surface against a rotating polishing surface of a polishing table, wherein a radius of the polishing surface is smaller than a diameter but larger than a radius of the workpiece surface, a center of the workpiece surface stays on the polishing surface, and a distance between a center of the workpiece surface and an edge portion of the polishing surface is smaller than a radius of the workpiece surface. This arrangement is attractive for making the apparatus compact and economical. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic plan view of a flow of workpieces with respect to polishing stations in the present polishing apparatus; 
         FIG. 2  is a front view of a polishing unit of the present polishing apparatus; 
         FIG. 3  is a plan view of the polishing unit; 
         FIG. 4A  is a side view of a buffing unit; 
         FIG. 4B  is a side view of a dresser elevating device; 
         FIG. 5A  is a plan view of the buffing unit; 
         FIG. 5B  is a side view of the buffing unit; 
         FIG. 6  is a schematic plan view to show relative positions of a buffing table and the workpiece; 
         FIG. 7  is a cross sectional view of a temporary storage station; 
         FIGS. 8A-8D  are plan views to show die actions of the polishing unit; 
         FIG. 9  is a plan view of another example of a flow of workpieces with respect to polishing stations in the present polishing apparatus; 
         FIG. 10  is a front view of another embodiment of the polishing apparatus; 
         FIG. 11  is a schematic plan view of a conventional polishing apparatus; and 
         FIG. 12  is a schematic side view of a conventional polishing apparatus. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, preferred embodiments will be presented with reference to the drawings. 
       FIG. 1  is a schematic illustration of a first embodiment of the present polishing apparatus. The present polishing apparatus is contained in a rectangular-shaped floor space F, and the constituting elements arranged on the left/right sides are disposed in a symmetrical pattern with respect the center line C. Specifically, at one end of the rectangular-shaped floor, a pair of polishing units  10   a,    10   b  are disposed symmetrically on the left and right side, respectively, and a loading/unloading unit  12  mounting a pair of cassettes  12   a,    12   b  for storing wafers are disposed on an opposite end of the floor. Between these two ends, there are disposed, beginning from the loading/unloading unit side, a pair of secondary cleaning units  14   a,    14   b,  a pair of wafer inverters  16   a,    16   b,  a pair of primary cleaning units  18   a,    18   b,  and one temporary storage station  20 . The pairs of primary and secondary cleaning units  18   a,    18   b  and  14   a,    14   b,  and the pair of wafer inverters  16   a,    16   b  are disposed opposite to each other across the center line C, and stationary robots  22 ,  24  having arms with articulating joints are provided on the center line C. On both sides of the temporary storage station  20 , stationary robots  26   a,    26   b  are provided. 
     As shown in  FIGS. 2 and 3 , each of the polishing units  10   a,    10   b  is provided with a set of operational devices, disposed approximately parallel to the center line, and comprised by: a workpiece pusher  30  for transferring a workpiece W; a top ring device  36  having two top rings  32 ,  34 ; a turntable (primary polishing table)  38  having an abrading tool on its top surface; and a dresser  40  for reconditioning the abrading tool. Also, in this embodiment, a buffing table (final polishing table)  42  for performing buffing (final polishing) is disposed next to the top ring device  36 . 
     As shown in  FIG. 2 , the top ring device  36  is comprised by: a vertical support shaft  50  rotatably supported by a base  48  mounted on a bracket  46  laterally protruding from a turntable support base  44 ; a horizontally extending swing arm  52  attached to the top end of the support shaft  50 ; and the pair of top rings  32 ,  34  attached to both ends of the swing arm  52 . A swing arm drive motor  47  for oscillating the swing arm around the support shaft  50  is provided in the bracket  46 . Each of the top rings  32 ,  34  has a suction device on the bottom surface to hold a workpiece by vacuum suction, each is driven by its own drive motor  56  so as to enable each to rotate horizontally, and each can also be raised or lowered by using an air cylinder  58 , independently of the other. 
     Turntable  38  is a rotatable polishing table having a polishing cloth mounted on the top surface, which is basically the same as the turntable shown in  FIG. 12 , and includes a support base  44  for supporting the polishing table, a turntable drive motor  45 , and a polishing solution supply nozzle. 
     As shown in  FIGS. 4A ,  4 B,  5 A and  5 B, buffing table  42  includes a small diameter buffing disk  82  having a buffing cloth  80  on its top surface, and is rotatable by virtue of a driving device  86  contained in a housing  84 . A dresser  94  includes: a rotation driver  88 ; swing device  90 ; and an elevating device  92 , with an air cylinder  93  provided adjacent the buffing table  42 . The size of the buffing table  42  is such that the radius “R” of the polishing surface is smaller than the diameter “2r” of a workpiece but is larger than its radius “r”. 
     Buffing table  42  is used to perform a secondary polishing step on a wafer W which has been through the primary polishing step. The secondary polishing is a finish polishing step carried out by using either a polishing solution containing polishing particles, pure water in case of a “water polish”, or a certain chemical solution. In the example shown in  FIG. 4A , finish polishing is performed by placing the center of the wafer W at a distance “e” from an edge of the buffing disk  82  to carry out polishing and cleaning. The magnitude of the distance “e” is small in comparison to the radius “r” of the workpiece W. Therefore, as shown in  FIG. 6 , the surface to be polished is exposed outside of the buffing disk  82  in a shape resembling a quarter moon with a maximum width “(r−e)”. 
     In such a setup, the outer peripheral area of the polishing surface of the buffing cloth  80  attached on the disk  82  can provide a maximum polishing ability, where the speed of the workpiece surface thereat relative to the speed of the workpiece surface at the inner regions of the disk  82  is larger. This polishing region is termed an effective polishing area Ep, as illustrated in FIG.  6 . Because the workpiece surface is also rotated, each section of the workpiece surface is successively brought into contact with the effective polishing area Ep, and ultimately, the amount of material removed from all sections of the workpiece surface is averaged. 
     To improve the degree of precision of the buffing operation, die distance “e” and rotational speeds, as well as polishing duration of the workpiece, should be adjusted accordingly. Polishing can be performed while adjusting the distance “e” by rotating the swing arm  52  of the top rings  32 ,  34 , or corrective polishing can be carried out in the same manner in addition to the normal polishing operation. 
     With reference to  FIG. 3 , the workpiece pusher  30  is positioned on the opposite side of the support shaft  50  with respect to the turntable  38 , and when one top ring  32  (or  34 ) is on the turntable  38 , the other top ring  34  (or  32 ) is directly above the workpiece pusher  30 . Workpiece pusher  30  has a workpiece table  60  which can be raised or lowered, and serves to transfer workpieces between the top rings  32 ,  34  and robots  26   a,    26   b.  With reference to  FIG. 2 , the bracket  62  extending from the base  44  opposite to the top rings  32 ,  34  rotatably supports a dresser shaft  64  for the dresser  40 . 
     As shown in  FIG. 7 , the temporary storage station  20  is divided into upper and lower levels. The upper level is a dry station  20 A for placing dry workpieces, and the lower level is a wet station  20 B for placing wet workpieces. The dry station  20 A is an open structure, but the wet station  20 B is a closed box structure  68  having spray nozzles  66  disposed above and below the workpiece W. The workpieces W are handled through a gate  70  provided on the side of the box structure  68 . 
     The cleaning units  14   a,    14   b  and  18   a,    18   b  can be selected to suit applications, but in this embodiment, the primary cleaning units  18   a,    18   b  beside the polishing units  10   a ,  10   b  are of the sponge roller type to scrub both front and back surfaces of a wafer, for example, and the secondary cleaning units  14   a,    14   b  are made to rotate the wafer horizontally by holding the edge of the wafer while supplying a cleaning solution thereto. The latter device can also serve as a spin dryer for dewatering the wafer by centrifugal force. 
     The wafer inverters  16   a,    16   b  are needed in this embodiment, because of the wafer storage method using cassettes  12   a,    12   b,  and their working relation to the handling mechanism of the robots, but such inverters are not needed for a system where the polished wafers are transported with the polished surface always facing downward, for example. Also, such inverters  16   a,    16   b  are not needed where the robots comprise inverting facilities. In this embodiment, the two wafer inverters  16   a,    16   b  are assigned separately to handling dry wafers and to handling wet wafers. 
     In this embodiment, four robots  22 ,  24 ,  26   a,    26   b  are provided, and they are of a stationary type operating with articulating arms having a hand at the end of the arms. The first robot  22  handles workpieces for a pair of cassettes  12   a,    12   b,  secondary cleaning units  14   a,    14   b  and the wafer inverters  16   a,    16   b.  The second robot  24  handles workpieces for the pair of wafer inverters  16   a,    16   b,  primary cleaning units  18   a,    18   b,  and temporary storage station  20 . The third and fourth robots  26   a,    26   b  handle workpieces for temporary storage station  20 , either one of the cleaning units  18   a  or  18   b,  and either one of the workpiece pushers  30 . 
     The polishing apparatus can be used for series or parallel operation as explained in the following.  FIG. 1  shows flow of workpieces W in parallel operation using one cassette in the loading/unloading unit. In the following description, the processing line which is in the top section in  FIG. 1  is designated as the “right” processing line, and the processing line which is in the bottom section is designated as the “left” processing line. Here, wafer (workpiece) W is shown by a blank circle when its work surface (polished surface) is directed upwards, by a densely meshed circle when its work surface is directed downwards, and by a sparsely meshed circle when it is inverted. 
     The flow of workpieces (semiconductor wafers) W in the right processing line for parallel processing is as follows: right cassette  12   a→ first robot  22 →dry inverter  16   a →second robot  24 →dry station  20 A→third robot  26   a →workpiece pusher  30  for right polishing unit  10   a →top ring  32  or  34 →polishing on turntable  38 →if necessary, buffing on buffing table  42 →workpiece pusher  30 →third robot  26   a →primary cleaning unit  18   a →second robot  24 →wet inverter  16   b →first robot  22 →secondary cleaning unit  14   a →right cassette  12   a.    
     Processing flow in each polishing unit  10   a,    10   b  will be explained with reference to  FIGS. 8A-8C . Workpiece pusher  30  already is provided with anew unpolished wafer delivered by the third robot  26   a  (or fourth robot  26   b ). As shown in  FIG. 8A , polishing is performed by using the top ring  32  holding the wafer, and during this time, the other top ring  34  is above the workpiece pusher  30  and receives an unpolished wafer. After finishing polishing on the turntable  38 , top ring  32  moves over to the buffing table  42  by the swing action of the swing arm  52 , as shown in  FIG. 8B , to carry out buffing, dual-purpose water polishing for concurrently performing finishing, as well as cleaning. The wafer may also be transferred directly by the workpiece pusher  30  after the primary polishing. 
     When the water polishing is finished, the swing arm  52  is rotated and the top ring  32  is moved directly over the workpiece pusher  30 , as shown in FIG.  8 C. Then, the polished wafer is transferred to the workpiece pusher  30  by either lowering the top ring  32  or raising the workpiece pusher  30 . The polished wafer is replaced with a new unpolished wafer by using third robot  26   a  (or fourth robot  26   b ). During this period, the other top ring  34  is moved over to the turntable  38 , and the wafer is polished on the turntable  38 . Further, as shown in  FIG. 8D , the wafer moves over to the buffing table  42  by the swing action of the swing arm  52 . The polished wafer is water polished for finishing and cleaning, and the process begins all over from the step shown in FIG.  8 A. 
     In the above process, because robots  26   a,    26   b  are provided for each processing line for handling the wafers for polishing units  10   a,    10   b,  the polished wafer on the workpiece pusher  30  is quickly exchanged with a new unpolished wafer. Therefore, there is no waiting time for the top ring  32 ,  34  for the next wafer to be polished, and the idle time for the turntable  38  is reduced. 
     On the contrary, since the wafer exchange is rapidly performed, top rings  32 ,  34  may wait for the turntable  38  to finish polishing while holding an unpolished wafer by vacuum. In this case, if the wafer is clamped by vacuum for a long time, a backing film provided between the wafer and the top ring  32 ,  34  will be deformed. Therefore, in this embodiment, the top rings  32 ,  34  are programmed to release the vacuum when a long term waiting is expected. The wafer is maintained on the lower surface of the top rings  32 ,  34  by remaining adhesion forces of wet backing film. 
     Also, in this embodiment, because the top ring device  36  is provided with two top rings  32 ,  34  disposed on the both ends of the swing arm  52 , while one wafer is being processed by one top ring, the wafer on the other top ring is replaced with a new unpolished wafer. Therefore, there is no need to wait for the top rings  32 ,  34  for the wafer to be transferred for processing. Therefore, the through-put of the turntable  38  is increased, thereby enabling it to perform a high efficiency parallel operation. 
     Through-put by the facility shown in  FIG. 1  will be compared with that by the conventional facility shown in FIG.  11 . Assume that polishing time of a wafer is two minutes, and that cleaning is carried out by primary and secondary cleaning steps. In the conventional setup, forty wafers are polished in one hour while in the present facility, fifty three wafers are polished. Comparing the through-put per unit area of installation space, it is 7.4 wafers/m 2 ·hour for the conventional system, while in the present facility, it is 7.9 wafers/m 2 ·hour. 
       FIG. 9  shows a flow process for two-step polishing, i.e ., a series operation. The process is as follows: right cassette  12   a →first robot  22 →dry inverter  16   a →second robot  24 →dry station  20 A→third robot  26   a →first polishing unit  10   a →third robot  26   a →right primary cleaning unit  18   a →second robot  24 →wet station  20 B→third robot  26   b →secondary polishing unit  10   b →third robot  26   b→ left primary cleaning unit  18   b →second robot  24 →wet inverter  16   b →first robot  22 →left secondary cleaning unit  14   b →first robot  22 →right cassette  12   a.    
     In this series processing operation, because a wet wafer is supplied to polishing unit  10   b,  the dry station  20 A and the wet station  20 B are separately used for placing dry wafers and wet wafers, respectively. In the wet station  20 B, the top and bottom surfaces of the wafer W are rinsed with a rinsing solution to prevent drying of the polished wafer. It should be noted that the wet and dry stations  20 A,  20 B are separately shown in  FIG. 9  for convenience in flow illustration, but they are stacked vertically, as shown in FIG.  7 . 
       FIG. 10  shows another embodiment according to the present invention. In this polishing unit, a film thickness measuring device  72  is provided adjacent the top ring  34  located above the workpiece pusher  30  for measuring the film thickness of a wafer held in the top ring  34 . The film thickness measuring device  72  is comprised by: an optical head  74  attached at the tip of an arm  76  for performing non-contact measurement of film thickness; and a positioning device  78  such as an x-y table for moving the arm  76  along the workpiece surface. 
     Using this arrangement, it is possible to measure film thickness fabricated on a polished wafer held on the top ring  34  when the swing arm  52  is rotated in position shown in FIG.  10 . The thickness measurement provides a basis for determining the amount of material removed so that, if necessary, polishing time for the next wafer may be adjusted by a feedback control device. Or, if the value has not yet reached an allowable range, a control device may rearrange polishing schedule so that it can be repolished. The advantage is that there is no need to provide a separate space for determining the film thickness of a polished wafer, because the thickness can be determined in-place above the workpiece pusher  30 . The time required to exchange the wafers by the third or fourth robots  26   a,    26   b  is shorter than the time required by the turntable  38  to polish a wafer, and therefore, such film measurement can be performed during this time without generating any down time of the line. 
     INDUSTRIAL APPLICABILITY 
     The present invention is useful for polishing workpieces, such as semiconductor wafers, glass plates and liquid crystal display panels which require a high surface flatness.