Abstract:
A method and apparatus for polishing a workpiece are set forth which can polish the workpiece at a constant rate at a stable condition even when plural workpieces are continually polished. The method comprises dressing a polishing surface of a polishing table while supplying a dressing solution. After the dressing, the dressing solution remaining on the polishing surface is removed by rotating the polishing table at a dewatering rotation speed while stopping the supply of the dressing solution. Then, the workpiece is polished by making the workpiece slidingly contact with the polishing surface while supplying a polishing solution to the polishing surface.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a method and apparatus for polishing a workpiece, and more particularly, to a method and apparatus for polishing a workpiece such as a semiconductor wafer having a thin film formed thereon to a flat and mirror finished surface.  
         [0003]     2. Description of the Related Art  
         [0004]     As integration of semiconductor devices intensifies, the distance between the interconnects formed in the devices becomes narrower. When forming interconnects of a width not more than 0.5 μm through a photolithography process in particular, the depth of focus becomes shallower and the stepper requires a flatter imaging plane. One prevailing device for flattening or planarizing the surface of the semiconductor wafer is a polishing apparatus for performing chemical mechanical polishing (CMP).  
         [0005]     As shown in  FIG. 5 , such polishing apparatus comprises: a polishing table  302  having a polishing cloth or polishing pad  300  on its upper surface for providing a polishing surface  301 ; a top ring  304  for holding a workpiece such as a semiconductor wafer W so that the surface to be polished confronts the polishing table  302 . The apparatus is operated to polish the semiconductor wafer W by respectively rotating the polishing table  302  and the top ring  304 , and by pressing the semiconductor wafer W against the polishing surface  301  by the top ring  304  at a predetermined pressure while supplying a polishing solution from a polishing solution supply nozzle  306  arranged above the polishing table  302  onto the polishing surface  301 .  
         [0006]     The polishing solution supplied from the polishing solution supply nozzle  306  comprises an alkaline solution containing suspended abrasive grains so that the semiconductor wafer W is flat and mirror polished through a composite process of a chemical polish process by the alkaline solution and a mechanical polish process by the abrasive grains. A fixed abrasive is also used lately, instead of the polishing cloth, in which abrasive grains made of a material such as cerium oxide (CeO 2 ) are fixed by a binder.  
         [0007]     As the polishing apparatus continually processes the substrates, polishing performance of the polishing surface  301  of the polishing cloth  300  is deteriorated. Therefore, in order to recover the polishing performance, a dresser  308  having a dressing member  310  at its lower surface is provided for dressing or resetting the polishing cloth  300  during periods such as for exchanging the semiconductor wafer W to be polished. In this dressing process, a dressing solution such as deionized water is supplied to the polishing surface  301  from the water supply nozzle  307 , and the dresser  308  and the polishing table  302  are respectively rotated. The dressing member  310  of the dresser  308  is pressed against the polishing surface  301  of the polishing cloth  300  to remove the polishing solution and polishing debris remaining on the polishing surface  301  as well as to flatten and dress the polishing surface  301  for resetting the polishing surface  301 . This dressing process is also called a conditioning process.  
         [0008]     A process of polishing a semiconductor wafer and dressing the polishing surface using the above described polishing apparatus will be explained with reference to FIGS.  6 A˜ 6 D and  FIG. 7 . FIGS.  6 A˜ 6 D are schematic views showing the conventional polishing process, and  FIG. 7  is a graph showing the rotation speeds of the polishing table during these processes. Table 2 also shows conditions of the process mentioned above.  
                                                 TABLE 2                                   POLISH WITH   POLISH WITH               POLISHING   DEIONIZED               SOLUTION   WATER   DRESSING                                    PROSESS TIME   60 seconds   15 seconds   17 seconds       TOP RING   POLISHING   POLISHING   STANDBY       POSITION   POSITION   POSITION   POSITION       DRESSER   STANDBY   STANDBY   DRESSING       POSITION   POSITION   POSITION   POSITION       POLISHING   SUPPLY   STOP   STOP       SOLUTION       DEIONIZED   STOP   SUPPLY   SUPPLY       WATER       ROTATION   80 rpm   80 rpm   40 rpm       SPEED OF       POLISHING       TABLE                  
 
         [0009]     The semiconductor wafer to be processed (not shown) is placed on a pusher  312  which is arranged adjacent the polishing table  302 . As shown in  FIG. 6A , during the polishing process using polishing solution, the polishing table  302  and the top ring  304  are rotated independently and the polishing solution is supplied from the polishing solution supply nozzle  306  to the polishing surface  301 . At this time, the polishing table  302  is rotated at a speed of 80 rpm, as shown in  FIG. 7 . The top ring  304  receives semiconductor wafer from the pusher  312  and presses the semiconductor wafer against the polishing surface  301  at a prescribed pressure for 60 seconds to polish the semiconductor wafer.  
         [0010]     After finishing polishing using polishing solution, water polishing using deionized water is performed as shown in  FIG. 6B . In this process, the polishing table  302  and the top ring  304  are rotated at respective constant speeds and deionized water is supplied from the water supply nozzle  307  to the polishing surface  301 . The polishing process using deionized water continues for 15 seconds, as shown in  FIG. 7 .  
         [0011]     After finishing the polishing using deionized water, the polishing cloth  300  is dressed or reset by the dresser  308  for recovering the polishing performance of the polishing surface  301  (see  FIG. 5 ), as shown in  FIG. 6C . In the dressing process, the rotation speed of the polishing table  302  is lowered to 40 rpm, and the dressing member  310  of the dresser  308  is forced to slidingly contact with the polishing surface  301  while deionized water is supplied from the water supply nozzle  307  to the polishing surface  301 . During this period, the top ring  304  is moved to a position above the pusher  312  and the polished semiconductor wafer is transferred to the pusher  312  from the top ring  304 . After finishing the dressing process, deionized water supply is stopped, and the polishing solution is supplied from the polishing solution supply nozzle  306  to the polishing surface  301  to start a next polishing process, as shown in  FIG. 6D .  
         [0012]     In case of continually polishing the semiconductor wafers, at the time the next polishing process is started, the polishing cloth  300  (see  FIG. 5 ) has just finished the dressing process so that the polishing surface  301  of the polishing cloth  300  is filled with the supplied dressing solution (deionized water). If the polishing solution for the next polishing process is supplied to the polishing surface  301  containing abundant dressing solution, the polishing solution is diluted by the dressing solution having a different composition, as shown in  FIG. 6D , so that it is difficult to obtain an expected polishing rate even if the polishing is performed at the same conditions. Also, it is necessary to extend the polishing time to obtain a preferred polishing amount resulting in lowering of the throughput.  
         [0013]     The present invention is accomplished to address the above mentioned problems and aimed to present a method and apparatus for polishing a workpiece which can polish the workpiece at a constant rate in a stable condition even when plural workpieces are continually polished.  
       SUMMARY OF THE INVENTION  
       [0014]     According to the present invention, a method for polishing a workpiece comprises: dressing a polishing surface of a polishing table while supplying a dressing solution; after the dressing, removing the dressing solution remaining on the polishing surface by rotating the polishing table at a dewatering rotation speed while stopping the supply of the dressing solution; and after the removing, polishing the workpiece by making the workpiece slidingly contact with the polishing surface while supplying a polishing solution.  
         [0015]     According to the invention, when the polishing process is started, the dressing solution remaining on the polishing surface at the end of the dressing process is removed so that dilution of the polishing solution is prevented even when a plurality of semiconductor wafers are continually polished, and a stable polishing process with a constant polishing rate can be achieved.  
         [0016]     The removing process removes excessive dressing solution. That is, it is not necessary to remove all the dressing solution remaining on the polishing surface. The dressing solution is removed to an extent to prevent substantial dilution of the polishing solution supplied during the following polishing process so that a constant polishing rate can be obtained.  
         [0017]     The dewatering rotation speed may be larger than a rotation speed of the polishing table during the polishing.  
         [0018]     A rotation speed of the polishing table during the polishing may be larger than a rotation speed of the polishing table during the dressing process.  
         [0019]     The dewatering rotation speed may be between 100˜150 rpm.  
         [0020]     The removing dressing solution may be performed for 5˜15 seconds.  
         [0021]     Acceleration at a periphery of the polishing table during the dewatering may be 32.9˜73.9 m/s 2 .  
         [0022]     The polishing may comprise a first polishing step using a first polishing solution and a second polishing step using a second polishing solution.  
         [0023]     The second polishing solution may be deionized water.  
         [0024]     The dewatering rotation speed may be determined according to a driving ability of the polishing table.  
         [0025]     According to another aspect of the invention, a method for polishing a workpiece comprises: dressing a polishing surface of a polishing table by making a dresser slidingly contact with the polishing surface while rotating the polishing table at a dressing rotation speed and supplying a dressing solution to the polishing surface; after the dressing, dewatering the polishing surface by rotating the polishing table at a dewatering rotation speed; and after the dewatering, polishing the workpiece by making the workpiece slidingly contact with the polishing surface while rotating the polishing table at a polishing rotation speed and supplying a polishing solution to the polishing surface.  
         [0026]     According to another aspect of the invention, an apparatus for polishing a workpiece comprises: a polishing unit having a polishing table having a polishing surface and a workpiece holder for holding the workpiece to press it against the polishing surface; a dressing unit having a dresser for dressing the polishing surface; a solution supplying unit for supplying the polishing surface with a polishing solution or a dressing solution; and a controller for controlling operation of the units, the controller sequentially performs dressing of the polishing surface while supplying a dressing solution, removing the dressing solution remaining on the polishing surface by rotating the polishing table at a dewatering rotation speed while stopping the supply of the dressing solution, and polishing the workpiece by making the workpiece slidingly contact with the polishing surface while supplying a polishing solution. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]      FIG. 1A  is a cross-sectional schematic view of a polishing apparatus according to an embodiment of the present invention;  
         [0028]      FIG. 1B  is a block diagram of the polishing apparatus shown in  FIG. 1A ;  
         [0029]     FIGS.  2 A˜ 2 E show views illustrative of a polishing process carried out by the polishing apparatus shown in  FIG. 1A , and  FIG. 2A  shows a polishing process using a polishing solution,  FIG. 2B  shows a polishing process using deionized water;  FIG. 2C  shows a dressing process;  FIG. 2D  shows a residual deionized water removing process; and  FIG. 2E  shows a following polishing process;  
         [0030]      FIG. 3  is a graph showing rotational speeds of the polishing table in respective processes shown in  FIG. 2 ;  
         [0031]      FIG. 4  is a graph showing rotational speeds of the polishing table in respective processes shown in  FIG. 2 ;  
         [0032]      FIG. 5  is a cross-sectional schematic view of a conventional polishing apparatus;  
         [0033]     FIGS.  6 A˜ 6 D show views illustrative of a polishing process carried out by the conventional polishing apparatus; and  
         [0034]      FIG. 7  is a graph showing rotational speeds of the polishing table in respective processes shown in FIGS.  6 A˜ 6 D. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]     An embodiment of the polishing apparatus and process according to the present invention will be described with reference to the attached drawings.  FIG. 1A  is a schematic view showing an apparatus for performing a polishing process of the present invention, and  FIG. 1B  is a block diagram of the polishing apparatus shown in  FIG. 1A . The polishing apparatus comprises: a polishing table  11  having a polishing surface  10  on the upper surface; a top ring unit  12  for holding a semiconductor wafer W, a workpiece to be polished, and pressing it against the polishing table  11  to polish the same; and a dressing unit  13  for dressing or resetting the polishing surface  10  of the polishing table  11 . The polishing table  11  is connected to a motor, not shown and arranged below the table, via a table shaft  11   a  so that the polishing table  11  is rotatable about the table shaft  11   a  as indicated by an arrow C in  FIG. 1A .  
         [0036]     In the embodiment, the polishing surface  10  for polishing the semiconductor wafer W is comprised of a polishing cloth  9  or polishing pad. Here, the term “polishing cloth” is used for a cloth such as a foamed polyurethane or nonwoven fabric cloth which does not include abrasive grains.  
         [0037]     A polishing solution supply nozzle  15  and a water supply nozzle  16  are arranged above the polishing table  11 , thus the polishing solution supply nozzle  15  supplies a polishing solution or slurry and the water supply nozzle  16  supplies deionized water respectively onto the polishing surface  10  of the polishing table  11 . A cup-like frame member  17  is provided around the polishing table  11  for recovering the polishing solution and deionized water, and a ditch  17   a  is provided at a lower portion of the frame member  17 .  
         [0038]     The top ring unit  12  comprises: a rotatable support shaft  20 ; a swing arm  21  connected to the upper end of the support shaft  20 ; a top ring shaft  22  suspended from a free end of the swing arm  21 ; and a disc-like top ring  23  connected to the lower end of the top ring shaft  22 . The top ring  23  is horizontally movable by the swinging movement of the swing arm  21  rotated by the support shaft  20 , thus the top ring  23  is reciprocatingly movable between a delivery position above the pusher (wafer delivery unit, not shown) adjacent the polishing table  11  and a polishing position above the polishing surface  10 . The top ring  23  is connected to a motor (a rotation drive assembly) and an elevation cylinder, both not shown and provided inside the swing arm  21 , via the top ring shaft  22  so that the top ring  23  is elevatable as well as rotatable about the top ring shaft  22  as shown by the arrows D and E in  FIG. 1A . The semiconductor wafer W, a workpiece to be polished, is supported at the lower surface of the top ring  23  by a vacuum suction force or the like. By these configurations, the top ring  23  can rotatingly support the semiconductor wafer W at the lower surface and press it against the polishing surface  10  at a desirable pressure.  
         [0039]     The dressing unit  13  is for reactivating the polishing surface  10  which is deteriorated through polishing, and is arranged at an opposite side of the center of the polishing table  11  to the top ring unit  12 . The dressing unit  13  comprises, similarly to the top ring unit  12 : a rotatable support shaft  30 ; a swing arm  31  connected to the upper end of the support shaft  30 ; a dresser shaft  32  suspended from a free end of the swing arm  31 ; and a dresser  33  connected to the lower end of the dresser shaft  32 . Thus the dresser  33  is horizontally movable according to the swing movement of the swing arm  31  caused by rotation of the support shaft  30  so that the dresser  33  can move reciprocatingly between a dressing position above the polishing surface  10  and a standby position outside the polishing table  11 . The dresser  33  is connected to a motor (a rotation drive assembly) and an elevation cylinder, both not shown and provided inside the swing arm  31 , via the dresser shaft  32  so that the dresser  33  is elevatable as well as rotatable about the dresser shaft  32  as shown by the arrows F and G in  FIG. 1A .  
         [0040]     The dresser  33  comprises at its lower surface a dressing member  34  which slidingly contacts with the polishing surface  10  to dress the same. The dresser  33  presses the dressing member  34  against the polishing surface  10  at a desired pressure while rotating to dress the polishing surface  10 . The dressing member  34  comprises diamond grains deposited on its lower surface through electrodeposition or welding.  
         [0041]     The polishing table  11 , the top ring unit  12 , and their auxiliary devices construct a polishing unit PU. The polishing solution supply nozzle  15 , the water supply nozzle  16 , and their auxiliary devices such as solution tanks, conduits, pumps or valves construct a solution supply unit SSU. The polishing unit PU, the dressing unit  13  and the solution supply unit SSU are connected to and controlled by a controller unit CU, as shown in  FIG. 1B . The controller unit CU comprises a CPU, for example, installed with a program to control the polishing apparatus in a manner as follows.  
         [0042]     Processes for polishing a semiconductor wafer W and dressing the polishing surface  10  by using the above described polishing apparatus will be described by referring to FIGS.  2 A˜ 2 E,  FIG. 3 , and Table 1.  FIG. 2A  is a schematic view of a polishing process using a polishing solution,  FIG. 2B  is a schematic view showing a polishing process using deionized water.  FIG. 2C  is a schematic view showing a dressing process,  FIG. 2D  is a schematic view showing a process for removing deionized water remaining on the polishing surface  10 , and  FIG. 2E  is a schematic view showing a next polishing process using polishing solution. Table 1 shows respective processing conditions of the steps shown in FIGS.  2 A˜ 2 E.  FIG. 3  is a graph showing rotation speeds of the polishing table  11  in the respective steps of FIGS.  2 A˜ 2 E.  
                                                         TABLE 1                                   POLISH WITH   POLISH WITH                   POLISHING   DEIONIZED                   SOLUTION   WATER   DRESSING   DEWATERING                                    PROCESS TIME   60 seconds   15 seconds   17 seconds   10 seconds       TOP RING   POLISHING   POLISHING   PUSHER   PUSHER       POSITION   POSITION   POSITION           DRESSER   STANDBY   STANDBY   DRESSING   STANDBY       POSITION   POSITION   POSITION   POSITION   POSITION       POLISHING   SUPPLY   STOP   STOP   STOP       SOLUTION           DEIONIZED   STOP   SUPPLY   SUPPLY   STOP       WATER           ROTATION SPEED   80 rpm   80 rpm   40 rpm   100 rpm       OF POLISHING           TABLE                      
 
         [0043]     As shown in  FIG. 2A , a pusher  37  is arranged adjacent the polishing apparatus for delivery of the semiconductor wafer W between the top ring  23 . The semiconductor wafer W (not shown) placed on the pusher  37  is held at the lower surface of the top ring  23  by vacuum suction force or the like and is transferred to the position above the polishing surface  10  of the polishing table  11  by the top ring  23 . The top ring  23  and the polishing table  11  are readily rotated respectively and the polishing solution is supplied onto the polishing surface  10  from the polishing solution supply nozzle  15 . At this time, the rotation speed of the polishing table  11  is controlled at 80 rpm as shown in table 1 and  FIG. 3 . The top ring  23  presses the semiconductor wafer W held at the lower surface thereof against the polishing surface  10  of the polishing table  11  at a prescribed pressure for 60 seconds. Thus the semiconductor wafer W held by the top ring  23  is in a sliding contact with the polishing surface  10  so that polishing is performed using the polishing solution.  
         [0044]     After finishing the polishing process using the polishing solution, the supply of the polishing solution is stopped and deionized water is supplied from the water supply nozzle  16  to the polishing surface  10  to perform a water polishing using deionized water, as shown in  FIG. 2B . In this process, the polishing table  11  and the top ring  23  are rotated at respective constant speeds, and the top ring  23  presses semiconductor wafer W against the polishing surface  10  for 15 seconds. By this water polishing using deionized water, the abrasive grains adhering to the surface of the semiconductor wafer W is cleaned and removed. The rotation speeds of the polishing table  11  and the top ring  23  can be changed from those during polishing using the polishing solution. In this case, the rotation speed of the polishing table  11  can be set within a range slower than that during polishing using the polishing solution, faster than that during polishing using deionized water, and also slower than that during dressing the polishing table  11 , such as 50 rpm, for example.  
         [0045]     Then the polishing surface  10  is subjected to a dressing process using the dressing unit  13  (see  FIG. 1A ) for recovering the polishing performance. As shown in  FIG. 2C , when dressing the polishing surface  10 , the top ring  23  is moved to the position above the pusher  37  and the polished semiconductor wafer W is delivered to the pusher  37 . At the same time, the dresser  33  of the dressing unit  13  is moved to the position above the polishing surface  10 . Then the dressing member  34  is forced to slidingly contact with the polishing surface  10  at a predetermined pressure, while the dresser  33  and a polishing table  11  are independently rotated. When or before the dressing member  34  contacts the polishing surface  10 , deionized water as a dressing solution is supplied to the polishing surface  10  from the water supply nozzle  16 . As to the dressing solution, a solution having a different composition from the polishing solution is normally used. The dressing process continues for 17 seconds in which the rotation speed of the polishing table  11  is lowered to 40 rpm, as shown in  FIG. 3 . After the dressing process, the dresser  33  is returned to the standby position by being driven by the swing arm  31  and, at the same time, deionized water supply from the water supply nozzle  16  is stopped.  
         [0046]     After finishing the dressing process, residual deionized water on the polishing surface  10  of the polishing table  11  will be removed, that is, the polishing table  11  is dewatered. In this process, the rotation speed of the polishing table  11  is raised to 100 rpm. Deionized water remaining on the polishing surface  10  is outwardly scattered from the polishing table  11  due to the centrifugal force caused by the rotation of the polishing table  11  so that the deionized water remaining on the polishing surface  10  is removed. This water removing process continues for 10 seconds as shown in  FIG. 3 . The scattered deionized water from the polishing surface  10  is recovered by the ditch  17   a  provided at the lower portion of the frame member  17  shown in  FIG. 1A . In the present embodiment, it is preferable to perform the removing process for 5˜15 seconds. It is also preferable to set the rotational speed at 100˜150 rpm. In case where the diameter of the polishing table  11  is 600 millimeter, the acceleration at the periphery of the polishing table  11  is preferably in the range of 32.9˜73.9 m/s 2 .  
         [0047]     After removing deionized water, the rotation speed of the polishing table  11  is lowered to a usual polishing speed such as 80 rpm, and the polishing surface  10  of the polishing table  11  is supplied with the polishing solution from polishing solution supply nozzle  15  to start the next polishing process as shown in  FIG. 2E . When the next polishing process is started, deionized water does not remain substantially on the polishing surface so that dilution of the polishing solution, which is supplied to the polishing surface  10 , by the deionized water is prevented so that, even when a plurality of semiconductor wafers are continually polished, a stable polishing process with a desired polishing rate can be achieved. Also, a necessary time for removing the deionized water from the polishing surface is as short as 5˜15 seconds, this dressing solution removing process does not affect substantially the whole processing time. Therefore, the polishing process can be stably achieved without decreasing the throughput.  
         [0048]     In the embodiment, the above described processes are controlled by the controller unit CU, but it is also possible to manually control to perform the same process.  
         [0049]      FIG. 4  is a graph showing the results of the polishing amount in the embodiment of the present invention compared to that in a conventional process. Both polishing processes are performed in the following polishing conditions: the rotational speed of the polishing table is 80 rpm, the axial load is 300 hPa, and the polishing time is 60 seconds. Here, the axial load is the load working on the top ring shaft in an axial direction.  
         [0050]     In the conventional polishing process, the rotation speed of the polishing table is not raised after the dressing process, while, in the polishing process of the present invention, the rotation speed of the polishing table is raised to 100 rpm for 10 seconds after the dressing process. Accordingly, as shown in  FIG. 4 , the polishing rate of the polishing process according to the present invention is twice as much as the conventional polishing process. It is permissible to raise the rotational speed up to 150 rpm after the dressing process, if the polishing apparatus facility allows.