Abstract:
An apparatus for holding a workpiece is incorporated in a polishing apparatus which polishes the workpiece to a flat mirror finish. The workpiece holding apparatus has a top ring holding a workpiece and a top ring drive shaft for rotating the top ring and pressing the top ring holding the workpiece against a turntable. A rotary joint is removably provided on the top ring drive shaft for allowing fluid to pass therethrough to thereby communicate with an external fluid source. A passage provided in the top ring drive shaft allows the rotary joint to communicate with through-holes formed in the top ring.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for holding a workpiece, and more particularly to an apparatus for holding a workpiece such as a semiconductor wafer for use in a polishing apparatus which polishes the workpiece to a flat mirror finish. 
     2. Description of the Related Art 
     Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnections is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 μm wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small. 
     It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surfaces of semiconductor wafers is to polish them with a polishing apparatus. 
     Conventionally, a polishing apparatus comprises a turntable having a polishing cloth thereon, a top ring for holding a workpiece such as a semiconductor wafer, a pressing device for pressing the workpiece held by the top ring against the polishing cloth on the turntable, and a driving device for rotating the top ring about its own axis. The top ring is coupled to the pressing device and the driving device through a top ring drive shaft. When the workpiece is transferred to the top ring, it is held by the lower surface of the top ring under vacuum developed in the top ring. When the workpiece is polished, a pressurized fluid such as compressed air is supplied from the top ring to the backside surface of the workpiece, thereby pressing a surface of the workpiece to be polished against a polishing surface comprising the polishing cloth on the turntable. Therefore, the top ring drive shaft coupled to the upper portion of the top ring is provided at its upper part with a rotary joint by which the top ring communicates with an external vacuum source or an external fluid source. 
     In the conventional polishing apparatus, the rotary joint is integrally formed with the top ring drive shaft. To be more specific, a lateral hole communicating with a vertical hole formed in the top ring drive shaft is formed in the upper part of the top ring drive shaft. By fixing the rotary joint incorporating the sealing portion therein to the top ring drive shaft, the lateral hole is caused to communicate with the connecting portion of the rotary joint which is connected to the external fluid source. Therefore, the conventional rotary-joint structure is problematic in that making the ring drive shaft is complicated and the replacement of the rotary joint is extremely troublesome when the sealing portion is damaged or worn. 
     Further, in the conventional rotary joint, the contacting surface between a stationary ring and a rotating ring serves as a sealing surface, and it is necessary to seal against a vacuum, pressurized air and pressurized liquid. In the case where the top ring communicates with the vacuum source through the rotary joint, a slurry-like abrasive liquid containing abrasive particles (or grains) is occasionally sucked up which then reaches, the sealing surface of the rotary joint. In this case, the abrasive liquid enters the sealing surface between the stationary ring and the rotating ring to thereby wear the sealing surface, and hence the sealing surface becomes irregular to cause fluid to leak therefrom. 
     Further, in the conventional rotary joint, fluid is prevented from leaking by a high contact pressure produced by making a spring force pressing the rotating ring and the stationary ring against each other larger. Therefore, the wear of the sealing surface progresses and the temperature rises in the sealing surface occurs, which causes thermal-stress cracking in either the stationary ring or the rotating ring. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an apparatus for holding a workpiece in which a rotary joint can be attached to a top ring drive shaft without requiring a specific manufacturing operation for the top ring drive shaft, and can be easily replaced if the rotary joint is damaged or worn, by making the rotary joint a discrete unit. 
     Another object of the present invention is to provide an apparatus for holding a workpiece in which slurry can be prevented from entering a sealing surface of the rotary joint, and a temperature rise in the sealing surface can also be prevented from occurring to thereby avoid thermal-stress cracking in either a stationary ring or a rotating ring. 
     According to an aspect of the present invention, there is provided an apparatus for holding a workpiece comprising a top ring holding a workpiece, a top ring drive shaft for rotating the top ring and pressing the top ring holding the workpiece against a turntable, a rotary joint removably provided on the top ring drive shaft for allowing fluid to pass therethrough to thereby communicate with an external fluid source, and a passage provided in the top ring drive shaft for allowing the rotary joint to communicate with through-holes formed in the top ring. 
     According to the present invention, since the rotary joint is removably provided on the top ring drive shaft, it can be easily replaced with a new one if it is damaged or worn. Further, since the rotary joint is constructed as a discrete unit removable from the top ring drive shaft, a specific manufacturing operation for the top ring drive shaft, such as boring, is not required, thus reducing the manufacturing cost of the top ring drive shaft. 
     The rotary joint comprises a body, a stationary ring fixed to the body, a rotating ring housed in the body and rotated integrally with the top ring drive shaft, and a liquid supply hole formed in the body for supplying liquid to an outer circumferential portion of a contacting surface between the stationary ring and the rotating ring. 
     With the above structure, by supplying liquid such as pure water to the outer circumferential portion of the contacting surface between the stationary ring and the rotating ring, a sealing film of liquid is formed between the stationary ring and the rotating ring when a vacuum is developed. This sealing liquid prevents slurry such as an abrasive liquid from entering the contacting surface between the stationary ring and the rotating ring. Further, by supplying liquid to the contacting surface between the stationary ring a and the rotating ring, temperature rise generated by relative sliding movement of the stationary ring and the rotating ring can be suppressed to thus prevent thermal-stress cracking in either the rotating ring or the stationary ring. 
     The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate a preferred embodiment of the present invention by way of example. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a vertical cross-sectional view showing an apparatus for holding a workpiece in a polishing apparatus according to an embodiment of the present invention; 
     FIG. 2 is an enlarged fragmentary vertical cross-sectional view showing essential parts of the apparatus for holding the workpiece shown in FIG. 1; 
     FIG. 3 is a vertical cross-sectional view of a rotary joint in the apparatus for holding the workpiece shown in FIG. 1; and 
     FIG. 4 is a cross-sectional view showing the whole structure of the polishing apparatus according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An apparatus for holding a workpiece according to an embodiment of the present invention will be described below with reference to FIGS. 1 through 3. 
     As shown in FIGS. 1 and 2, an apparatus for holding a workpiece (hereinafter referred to as workpiece holding apparatus  1 ) comprises a substantially disk-like top ring  10 , a top ring drive shaft  12  for supporting the top ring  10  and transmitting a rotating force and a pressing force to the top ring  10 , and a universal joint unit  14  for coupling the top ring drive shaft  12  and the top ring  10  in such a manner that the top ring drive shaft  12  and the top ring  10  tilt relative to each other. 
     The top ring  10  comprises a substantially disk-like holding plate  16  for holding a semiconductor wafer (workpiece) to be polished at a lower surface thereof, a substantially disk-like cover plate  18  fixed to the holding plate  16  so as to define a gap S between the holding plate  16  and the cover plate  18 , and an annular plate  20  for covering the holding plate  16  and the cover plate  18 . A guide ring  22  is attached to the lower circumferential portion of the holding plate  16  for retaining a circumferential edge of the workpiece. An elastic pad  15  is attached to the lower surface of the holding plate  16 . 
     The holding plate  16  has a recess  24  at an upper central part thereof, and a step  26  around the recess  24 . The cover plate  18  has at its lower surface a projecting portion  28  which is fitted in the recess  24  of the holding plate  16 . Further, the cover plate  18  has a flange  30 , around the projecting portion  28 , which is fixed to the step  26  of the holding plate  16  by bolts. The cover plate  18  has at its upper surface a recess  32  and an annular shoulder  34  around the recess  32 , and a step outwardly of the shoulder  34  for attachment of the annular plate  20 . The depth of the recess  24  of the holding plate  16  is larger than the height of the projecting portion  28  of the cover plate  18 . Thus, the certain gap S is defined between the recess  24  and the projecting portion  28 . The holding plate  16  has a number of through holes  38  which communicate with holes  40  formed in the annular plate  20  through the gap S between the cover plate  18  and the holding plate  16 . 
     Further, the gap S communicates with the backside surface of the workpiece held by the lower surface of the holding plate  16  through the through holes  38 . In the gap S, negative pressure or positive pressure is developed by allowing the gap S to communicate with a vacuum source or a pressurized fluid source. To be more specific, if the gap S communicates with the vacuum source, an attracting force is applied to the backside surface of the workpiece, and if the gap S communicates with the pressurized fluid source, a pressing force is applied to the backside surface of the workpiece. 
     The top ring drive shaft  12  is rotatably and vertically movably supported by a top ring head  42  which is supported by a frame of the polishing apparatus. That is, the top ring drive shaft  12  is coupled to an output shaft of a driving source (comprising a motor with reduction gears) provided on the top ring head  42  through a pully-belt mechanism  44  so as to be rotatable. 
     Further, the top ring drive shaft  12  is vertically movable by a top ring cylinder  48  provided between the top ring head  42  and a drive shaft holder  46 . The top ring cylinder  48  is actuated by supplying air there to and discharging air there from. The body of the top ring cylinder  48  is fixed to the shoulder of the drive shaft holder  46 , and the forward end of the rod  48   a  is fixed to the upper surface of the top ring head  42 . 
     The top ring drive shaft  12  is a hollow cylindrical member, and has at its central part a vertical hole  50  which communicates with an external fluid source  54  through a rotary joint  70 . In the vertical hole  50 , there is provided a tube  56  made of synthetic resin having corrosion resistance such as Teflon (the trade name of polytetrafluoroethylene) or polypropylene. The upper end of the tube  56  is connected to the rotary joint  70 , and the lower end of the tube  56  is branched into two tubes  56   a,    56   b  through a branch connection  57 . The tubes  56   a,    56   b  communicate with the holes  40  of the annular plate  20 . 
     The rotary joint  70  has at its lower end a screw  70   a  which is threaded into a thread  63   a  of a member  63  fixed to the upper end of the top ring drive shaft  12 . That is, the rotary joint  70  is fixed to the top ring drive shaft  12  by the screw engagement, and hence mounting or dismounting of the rotary joint  70  can be easily performed, and the replacement of the rotary joint  70  can be easily performed if it is damaged or worn. Further, since the rotary joint  70  is a discrete unit which is separable from the top ring drive shaft  12 , a lateral hole communicating with the vertical hole  50  is not required to be formed in the top ring drive shaft  12  and the manufacture of the top ring drive shaft  12  can be easily conducted. 
     The external fluid source  54  comprises a vacuum source  58 , a pressurized air source  60  and a pure water supply source  62  which can selectably communicate with the through holes  38  of the holding plate  16  through selective control valves  64   a  through  64   c,  the rotary joint  70 , the tube  56 , and the tubes  56   a,    56   b.    
     FIG. 3 is a front view partly in section showing the detailed structure of the rotary joint  70 . As shown in FIG. 3, the rotary joint  70  comprises a lower body  71 , an upper body  72  fixed to the lower body  71 , a hollow rotating shaft  74  supported by ball bearings  73 ,  73  provided in the lower body  71 , and a hollow stationary shaft  75  fixed to the upper body  72 . The upper body  72  has a part  72   a  through which fluid is supplied to the rotary joint  70  or discharged from the rotary joint  70 . 
     A rotating ring  76  made of ceramics such as silicon carbide (SiC) is fixed to the upper end of the rotating shaft  74 . The rotating shaft  74  has at its lower end the screw  70   a  which is threaded into the top ring drive shaft  12 . A stationary ring  77  made of ceramics such silicon carbide (SiC) is fixed to the lower end of the stationary shaft  75 . The stationary ring  77  is brought into sliding contact with the rotating ring  76 . The stationary ring  77  is pressed against the rotating ring  76  by a compression coil spring  79  to cause the stationary ring  77  to normally contact the rotary ring  76 . That is, the rotating ring  76  and the stationary ring  77  allow fluid to pass therethrough to thereby supply fluid between the rotating part and the stationary part and constitute a sealing surface which prevents fluid from leaking externally. 
     An oil seal  80  is provided around the upper circumferential portion of the rotating shaft  74 , and pure water is supplied to a space  81  defined above the oil seal  80  through a pure water supply hole  71   a  formed in the lower body  71 . Therefore, it is possible to supply pure water to the outer circumferential portion of the sealing surface between the rotating ring  76  and the stationary ring  77 . A water expulsion mechanism  82  comprising upper and lower rings is provided between the oil seal  80  and the ball bearing  73  to prevent liquid such as pure water from entering the ball bearing  73 . That is, the water expulsion mechanism  82  constitutes a leakage-prevention section which prevents liquid from entering the ball bearing  73 . The reference numeral  83  represents a drain hole which serves to discharge leakage liquid from the oil seal  80  to the exterior of the rotary joint  72 . 
     As shown in FIGS. 1 and 2, a driving plate  68  having a flange  66  extending outwardly is fixed to the lower end of the top ring drive shaft  12 . The universal joint unit  14  is provided between the driving plate  68  and the cover plate  18  of the top ring  10  so that the top ring  10  is tiltably supported by the top ring drive shaft  12  and the pressing force is transmitted from the top ring drive shaft  12  to the top ring  10 . The universal joint unit  14  comprises a spherical bearing mechanism  170  and a rotation transmission mechanism  172  for transmitting the rotation of the top ring drive shaft  12  to the top ring  10 . 
     The spherical bearing mechanism  170  comprises a central spherical concave surface  100  formed in the lower end of the projecting portion  176  of the driving plate  68 , a central spherical concave surface  102  formed in the upper end of the cover plate  18 , and a ball  78  made of high hardness material such as ceramics interposed between the spherical concave surfaces  100  and  102 . A plurality of pins  84  and  86  (six in this embodiment) that extend upwardly are provided at equal angular intervals on the shoulder  34  of the cover plate  18 , and these pins  84  and  86  are inserted into holes  88  and  90  formed in the flange  66  of the driving plate  68 . The pins  84  serve to lift the top ring  10  and these pins  86  serve to transmit rotation of the top ring drive shaft  12  to the top ring  10 . The pin  84  projects from the upper surface of the driving plate  68 , and a compression coil spring  94  is provided between a stopper  92  of the pin  84  and the driving plate  68  to support a part of weight of the top ring  10  by the resilient force of the spring  94 . Two parallel pins  98  extending horizontally are provided in the flange  66  so as to sandwich the pin  86 . In this case, even if the holding plate  16  is inclined, since the driven pin  86  and the driving pins  98  are relatively vertically movable, the torque of the top ring drive shaft  12  can be reliably transmitted to the holding plate  16  with movement of the contacting point of the pins  86  and  98 . 
     FIG. 4 shows the polishing apparatus which incorporates the workpiece holding apparatus  1  shown in FIGS. 1 through 3. As shown in FIG. 4, a turntable  90  is supported on a central shaft  90   a  and is rotatable about the axis of the shaft  90   a.  A polishing cloth  91  is attached to the upper surface of the turntable  90 . The workpiece holding apparatus  1  holding a semiconductor wafer  2  is disposed above the turntable  90 . An abrasive liquid supply nozzle  92  is disposed above the turntable  90  so that an abrasive liquid Q containing abrasive material can be supplied to the polishing cloth  91  on the turntable  90 . 
     Next, the operation of the polishing apparatus incorporating the workpiece holding apparatus  1  shown in FIGS. 1 through 4 will be described. 
     The tube  56  in the top ring drive shaft  12  communicates with the vacuum source  58  of the external fluid source  54  through the rotary joint  70 , and hence the semiconductor wafer  2  is held by the lower surface of the holding plate  16  under vacuum developed in the through holes  38  of the holding plate  16 . The driving source is energized, and the top ring drive shaft  12  is rotated to thus rotate the holding plate  16 . In this case, the tube  56  made of Teflon or polypropylene has a strength such that it is hardly deformed when it communicates with the vacuum source  58 . 
     The semiconductor wafer  2  is held by the top ring  10 , and pressed against the polishing cloth  91  on the turntable  90  by the top ring cylinder  48 . At this time, the turntable  90  is being rotated, and the top ring  10  is also being rotated to thus produce the relative motion between the semiconductor wafer  2  and the polishing cloth  91 . Further, the abrasive liquid Q is supplied from the abrasive liquid supply nozzle  92  onto the polishing cloth  91 . The supplied abrasive liquid Q is retained on the polishing cloth  91 , and the semiconductor wafer  2  is polished in contact with the polishing cloth  91 . During polishing, by allowing the tube  56  in the top ring drive shaft  12  to communicate with the pressurized air source  60  of the fluid source  54  through the rotary joint  70 , the pressurized air is supplied to the backside surface of the semiconductor wafer  2  through the through holes  38 , whereby the semiconductor wafer  2  is pressed against the polishing cloth  91 . 
     In this case, the pressing force of the top ring drive shaft  12  is transmitted to the top ring  10  through the spherical bearing mechanism  170 . When the upper surface of the turntable  90  is slightly tilted during polishing of the semiconductor wafer, the holding plate  16  is tilted about the ball  78  with respect to the top ring drive shaft  12  to thereby bring the en-tire surface of the semiconductor wafer  2  in close contact with the polishing surface of the turntable  90 . 
     After completing polishing of the semiconductor wafer, the top ring drive shaft  12  is lifted to raise the top ring  10 , and then the top ring  10  is moved away from the turntable  90  and positioned above a transfer device for transferring the semiconductor wafer from or to the top ring  10 . At this time, the semiconductor wafer  2  is held by the lower surface of the holding plate  16  under vacuum by causing the tube  56  in the top ring drive shaft  12  to communicate with the vacuum source  58  through the rotary joint  70 . Thereafter, the communication between the tube  56  and the vacuum source  58  is stopped, and the tube  56  communicates with the pure water supply source  62  through the rotary joint  70 . Thus, the semiconductor wafer  2  is easily removed from the holding plate  16  because the pure water supplied through the tube  56 , the gap S and the through holes  38  of the holding plate  16  pushes the backside surface of the semiconductor wafer  2 . 
     According to the embodiment of the present invention, the rotary joint  70  is constructed as a discrete unit removable from the top ring drive shaft  12 , and is fixed to the upper end of the top ring drive shaft  12  through the screw engagement. Therefore, the mounting or dismounting of the rotary joint  70  can be easily performed, and the rotary joint  70  can be easily replaced with a new one if it is damaged or worn. Further, the rotary joint  70  can be mounted or dismounted in an axial direction of the top ring drive shaft  12 , thereby facilitating the replacement work. Further, it is unnecessary to form a lateral hole which communicates with the vertical hole  50  in the top ring drive shaft  12 , and hence the work of the top ring drive shaft  12  is extremely simple. 
     Further, according to the embodiment of the present invention, by supplying pure water to the outer circumferential portion of the sealing surface of the rotary joint  70 , i.e., the outer circumferential portion of the rotating ring  76  and the stationary ring  77 , a sealing film is formed between the rotating ring  76  and the stationary ring  77  when a vacuum is developed in the sealing surface of the both rings  76  and  77 . This sealing film of pure water prevents a slurry such as an abrasive liquid from entering the sealing surface of the rotary joint  70 . Further, by supplying pure water to the outer circumferential portion of the sealing surface, a temperature rise caused by frictional heat between the rotating ring  76  and the stationary ring  77  can be suppressed to thus prevent thermal-stress cracking in either the rotating ring  76  or the stationary ring  77 . By providing the water expulsion mechanism  82  above the ball bearing  73 , liquid leaking from the sealing surface is prevented from entering the interior of the ball bearing  73  to thus prevent the bearing  73  from being damaged. 
     Further, according to the embodiment of the present invention, by suitably arranging the outer diameters of the rotating ring  76  and the stationary ring  77 , depending on the property or pressure of fluid, the sealing surface of the rotary joint  70  can have an optimum pressure balancing diameter, and hence an excessive pressure is not applied by the spring  79  to the sealing surface of the rotary joint  70 . Therefore, the wear of the sealing surface can be reduced to a minimum, and the temperature rise of the sealing surface can also be prevented. Since the tube  56  connected to the rotary joint  70  is made of a corrosion-resistant material, even if air and water are allowed to pass therethrough alternately, the generation of rust is prevented in the tube  56 . Thus, fluid is not contaminated by rust, and hence the polishing performance and quality of the polished semiconductor wafer are improved. 
     Although certain preferred embodiments of the present invention have been shown and described in detail, it should be understood that various changes and modifications may be made therein without departing from the scope of the appended claims.