Patent Document

This is a divisional of application Ser. No. 09/296,567, filed Apr. 22, 1999, now U.S. Pat. No. 6,220,945. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for polishing workpieces such as semiconductor wafers, various kinds of hard disks, glass substrates and liquid crystal display panels. 
     2. Description of the Related Art 
     In a conventional chemical mechanical polishing (CMP) apparatus used in fabrication of a semiconductor integrated circuit, a semiconductor wafer is held by a holder called a “top ring” and is rotated and pressed against a polishing cloth mounted on a rotating turntable while being supplied with abrading slurry including free abrading grains at a sliding interface. However, such a CMP apparatus presents a problem that, depending on the type of surface patterns and differences in the heights of fine surface structures fabricated on the wafer, it is not possible to obtain a precisely polished flat surface. 
     Therefore, in place of the above-mentioned CMP process, another CMP technique has been developed, where the wafer is placed in sliding contact with a solid polishing member shaped usually in the form of a plate, in which abrading grains are bound in a matrix, while a polishing liquid or a polishing solution is supplied at the sliding interface. The solid polishing members include variations such as a ring-type member or a cup-type member having abrading pellets distributed in a ring shape. 
     FIG. 11 illustrates basic movements of a cup-type polishing member. A cup-type polishing member  80  has a ring-shaped abrading member  81  attached on the bottom surface of a polishing member holder  83 , and is pressed against a wafer  100  held in a wafer holder  85 . Both are rotated, for example, in the same G, H directions, and the wafer  100  is uniformly polished by moving the polishing member  80  linearly in the radial direction of the wafer  100  (indicated by the arrow I) so that the abrading member  81  polishes the entire surface of wafer  100 . The polishing member holder  83  is connected to the drive shaft  89  through a spherical bearing  87  so as to transmit a pressing force F from the drive shaft  89  through the spherical bearing  87 , and coupling of drive pin  91  passive pin  93  transmits the rotation H from the drive shaft  89 . 
     In general, the polishing member  80  is pressed on the wafer  100  through the drive shaft  89 , therefore, when drive axis k of the drive shaft  89  is projected within the wafer  100 , as shown in FIG. 11, there is no tilting of the polishing member  80 . But, when it is in the position shown in FIG. 12, the rotation axis k projects outside the wafer  100 , and even if a part of the abrading member  81  is on the wafer, a lever action produces tilting of the abrading member  81  about fulcrum at the edge of the wafer  100 . This prevents the abrading member  81  from having a planar contact with the wafer  100 , and polishing becomes impossible. Therefore, to avoid such a situation, conventional abrading member  81  could only move within an area of support for the drive axis k. This problem is the same in a conventional polishing apparatus using a top ring holding the wafer to press it against a polishing table. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a polishing apparatus using a polishing member that has a wide stable polishing range to perform effective polishing, even if the rotation axis moves away from the edge of a workpiece to be polished. 
     The object has been achieved in a polishing apparatus comprised by a polishing member holder for holding a polishing member and a workpiece holder for holding a workpiece to be polished; and a drive device to produce a relative sliding motion between the polishing member and the workpiece; wherein at least one holder of either the polishing member holder or the workpiece holder is rotatable about a rotation axis and is tiltable with respect to the other holder, and the one holder is provided with a mechanism to stabilize the orientation or desired posture of the one holder by applying an adjusting pressure to the one holder at a location away from the rotation axis. 
     The polishing apparatus of such a construction can maintain stable contact of the workpiece to be polished to the polishing member at all times to produce stable polishing, even when a projected line of the rotation axis is outside the workpiece to be polished, thereby widening the relative movable range of the polishing member to the workpiece and providing an increased selection for controlling parameters or controlled systems. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a first embodiment of a polishing apparatus; 
     FIGS.  2 A˜ 2 C are illustrations of the movement of the apparatus shown in FIG. 1; 
     FIGS.  3 A˜ 3 C are graphs to illustrating pressure mechanisms; 
     FIGS.  4 A˜ 4 C are illustrations of a variation of pressing devices in the polishing apparatus; 
     FIG. 5 is a side view of a second embodiment of the polishing apparatus; 
     FIGS. 6A,  6 B are, respectively, a side view and a plan view of a third embodiment; 
     FIG. 7 is a side view of a fourth embodiment of the polishing apparatus; 
     FIG. 8 is a side view of a fifth embodiment of the polishing apparatus; 
     FIG. 9 is an illustration of the contact of a polishing member on a surface of a wafer to be polished; 
     FIG. 10 is a side view of a sixth embodiment of the polishing apparatus; 
     FIG. 11 is an illustration of the action of a conventional polishing apparatus; and 
     FIG. 12 is an illustration of problems associated with the conventional polishing apparatus. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments will be presented with reference to the drawings. 
     FIG. 1 shows a perspective view of an overall polishing apparatus having a solid polishing member according to the first embodiment of the present invention. The apparatus comprises a base plate  30 ; a table  40  moving linearly in the direction C by a drive mechanism (not shown); a wafer holder  45  disposed on the table  40 ; a polishing member  10  disposed at the end of a drive shaft  50  extending from the bottom surface of a support arm  31 . 
     The wafer holder  45  has a wafer holding section for holding the wafer  100 , and is rotated by a drive mechanism provided inside the table  40 . The polishing member  10  has a ring-shaped abrading member  11  (or pellet-like abrading member arranged in a ring shape) on the bottom surface of a polishing member support disk (polishing member holder)  13 , and is rotated by the shaft  50 . Between the drive shaft  50  and the polishing member  10 , a spherical bearing  52  (FIG. 2A) is provided for transmitting a pressing force from the drive shaft  50  to the polishing member  10 . Also, drive pins and passive pins (not shown) are provided for transmitting rotation from the drive shaft  50  to the polishing member  10 , as in the conventional polishing apparatus shown in FIGS. 11,  12 . The pressure against the wafer is mainly applied by the drive shaft. 
     On both sides of the shaft  50 , pressing devices  20  each having a top end fixed to a side surface at the distal end of the support arm  31  are provided. Each pressing device  20  has a pressing cylinder  21 , a rod  23  extending therefrom, and a rotatable roller  25  disposed at the bottom end of the rod  23 . The rollers  25  are on opposite sides of and straddle the rotation axis of the polishing member  10 ; relative to direction C of linear movement of the polishing member  10 , and the rolling surfaces run along the circumferential periphery of the polishing member  10  so as to press on the back surface (top surface in FIG. 1) of the polishing member  10  near its edge. It is permissible to provide one or more than three pressing devices  20 . 
     Pressing cylinders (only one is shown in FIG. 1)  21  have respective pressure control units  27 ,  28 , and share a control section  29  (having CPU and other components) to output control signals for the units  27 ,  28 . Table  40  is provided with position sensors to detect the position of the table  40 . A pressing pressure control section is thus comprised by the control section  29 , pressure control units  27 ,  28  and position sensors disposed on the table  40 . 
     The operation of the apparatus will be explained with reference to FIG.  2 . First, the wafer holder  45  and the polishing member  10  are independently rotated in the respective A, B directions, and the table  40  is linearly and reciprocatingly moved along the direction C to perform uniform polishing of the overall surface of the wafer  100  with the abrading member  11 . 
     The control section  29  detects the positions of the table  40  and the polishing member  10  according to signals output by the position sensors, and outputs control signals to pressure control units  27 ,  28 . As illustrated in FIG. 2A, not only when the polishing member  10  is entirely situated within the wafer  100 , but even when a part of the polishing member is extending out of the wafer  100 , as illustrated in FIG. 2B, there is no danger of the polishing member  10  tilting, so that control signals are output in such a way that the pressure control units  27 ,  28  produce the same pressures. 
     On the other hand, when the control section  29  detects, from the position sensor signals on the table  40 , that the rotation axis of the polishing member  10  is outside the periphery of the wafer  100 , as illustrated in FIG. 2C, the control section  29  outputs control signals to pressure control units  27 ,  28  so that they will be outputting different pressures against the polishing member  10  through the respective cylinders  21 . In other words, pressing pressure of the pressing device  20  for the on-wafer side is made higher relative to that for the off-wafer side. In this manner, the application point of a balancing or leveling pressure will always be projected on the wafer  100 , and there will be no tilting of the polishing member  10 . Rotation of the polishing member  10  is not affected adversely by the pressing device  20  because the pressure of cylinders  21  is applied to the back surface of the polishing member  10  through friction reducing rollers  25 . 
     FIGS.  3 A˜ 3 C show a pressure control methodology using the cylinders  21 . The horizontal axis of all the graphs relates to relative positions of wafer and abrading member, and on the vertical axis, FIG. 3A shows ratios of contact area of abrading member to wafer; FIG. 3B shows ratios of pressures in the pressing cylinders; and FIG. 3C shows respective cylinder pressures. 
     As shown in FIG. 3A, when the rotation axis m of the polishing member  10  is near the central area of the wafer  100 , the total surface area of the abrading member  11  is in contact with the wafer  100 . When the polishing member  10  moves to the left or the right to overhang from the edge of the wafer  100 , the contact area between the abrading member  11  and the wafer changes rapidly. Therefore, in order to maintain the pressure of abrading member  11  on the wafer constant, the pressing force exerted on the polishing member  10  must be reduced accordingly. 
     As shown in FIG. 3B, when the rotation axis m of the polishing member  10  moves away from the edge of the wafer  100 , the off-wafer side pressing device  20  must exert less pressure relative to the on-wafer side pressing device  20 . The two pressing devices  20  are operated in such a way that the further the polishing member  10  is away from the edge of the wafer  100  the higher the ratio of the pressures in the two pressing devices  20  so as to maintain a balancing pressure within the wafer  100 . 
     As shown in FIG. 3C, the magnitude of the pressure is maintained the same in each pressing device  20  when the rotation axis m is located within the wafer  100 , but as the rotation axis m moves away from the edge of the wafer, the pressure in the on-wafer side pressing device  20  is made higher than that in the off-wafer side pressing device  20 . As the rotation axis m moves further away from the edge of the wafer  100 , pressures are altered as shown in FIG. 3C, so that the actual magnitude of the pressure will be adjusted according to the ratios of the pressures as seen in FIG. 3B at corresponding relative locations of the abrading member  11  and the wafer  100 . 
     Accordingly, even when the rotation axis m moves off the edge of the wafer  100 , it is possible to control the orientation or desired posture of the abrading member  11  to abrade on the wafer  100 , thereby expanding the operational range of the polishing member  10 . 
     The same effect can be achieved by using magnetic bearings. FIGS.  4 A˜ 4 C show examples of the use of different types of magnetic bearings. A pair of magnetic bearings  121 ,  121   a,    121   b  are used as shown in FIGS.  4 A˜ 4 C to non-contactingly support abrading member support disk  13   e  to balance the load on polishing member  10   e.  In FIG. 4B, the balancing mechanism is provided on a cylindrical portion of the abrading member support disk  13   e.  Such arrangements of paired magnetic bearings  121 ,  121   a,    121   b  are effective in leveling the abrading member support disk  13  and expand the operational control range of the polishing member  10 . 
     FIG. 5 shows essential parts of a second embodiment of polishing member  10   a  and pressing devices  20   a.  This polishing member  10   a  includes an abrading member support disk  13   a  and a ring-shaped abrading member  11   a  (or pellet-like abrading member arranged in a ring shape) and is provided with an outer edge or brim section  15   a  around the circumference of the disk  13   a  that is outside the abrading member  11   a.  In this case, shaft  50   a  is used only to support the polishing member  10   a  and is not rotated. 
     The pressing devices  20   a  comprises a pair of upper rollers  25   a  and a pair of lower rollers  26   a,  each provided at the end of a rod  23   a  extending from the bottom of a respective pressing cylinder  21   a.  Left and right pairs of upper and lower rollers  25   a,    26   a  are used to clamp the brim section  15   a.  One upper roller  25   a  is rotated by an abrading member drive motor  27   a  provided on the outside of the respective pressing device  20   a.    
     In this polishing member  10   a,  abrading member drive motor  27   a  is operated to rotate the polishing member  10   a,  and concurrently the pressures of the pressing devices  20   a  are individually adjusted to maintain the polishing member  10   a  in a level position or desired posture even if the rotation axis m of the polishing member  10   a  moves away from the edge of the wafer  100 . 
     FIGS. 6A,  6 B show essential parts of a third embodiment of polishing member  10   b  and three pressing devices  20   b  in a side view in FIG. 6A, and in a plan view in FIG.  6 B. The polishing member  10   b  is the same as the polishing member  10   a  shown in FIG. 5, and comprises an abrading member  11   b  attached to the bottom surface of an abrading member support disk  13   b,  and a brim section  15   b  on the edge of the abrading member support disk  13   b.  However, this polishing member  10   b  does not have a shaft  50   a  shown in FIG.  5 . 
     The pressing device  20   b  is also the same as the pressing device  20   a  shown in FIG. 5, and comprises upper and lower rollers  25   b,    26   b  attached to the end of a rod  23   b  so as to clamp the brim section  15   b,  and one of the pressing rollers  20   b  is provided with a drive motor  27   b.  In this embodiment, each pressing device  20   b  is provided, at the end of the respective rod  23   b,  with an edge guide roller  17   b  to guide the abrading member support disk  13   b,  by contacting the outer vertical periphery of the disk  13   b.    
     In effect, the shaft  50   a  for supporting the polishing member  10   a  in the second embodiment is replaced with the edge guide rollers  17   b  in this embodiment. The polishing member  10   b  is rotated by operating the abrading member drive motor  27   b,  and concurrently, individual pressures in the pressing devices  20   b  are adjusted to maintain the polishing member  10   b  in a level position or desired posture even if the rotation axis m of the polishing member  10   b  moves away from the edge of the wafer  100 , as in the second embodiment. 
     FIG. 7 shows a schematic side view of pressing devices  20   c  for leveling a polishing member  10   c  in a fourth embodiment. The polishing member  10   c  is the same as the polishing member  10   a  shown in FIG.  5  and comprises an abrading member  11   c  attached to the bottom surface of an abrading member support disk  13   c,  and a brim section  15   c  on the edge of the abrading member support disk  13   c.  In this case, shaft  50   c  supports and rotates the polishing member  10   c.  Each pressing device  20   c  is provided with only a lower roller  26   c  provided at the end of a rod  23   c,  extending from the bottom of a respective pressing cylinder  21   c,  to contact the bottom surface of the brim section  15   c.    
     In this embodiment, the polishing member  10   c  is rotated by rotating the shaft  50   c,  and concurrently, each of the pressing devices  20   c  is adjusted to vary the lift force exerted through the rod  23   c  to maintain the polishing member  10   c  in a level position or desired posture even if the rotation axis m of the polishing member  10   c  moves away from the edge of the wafer  100 , as in the second embodiment. 
     FIG. 8 shows a schematic side view of pressing devices  20   d  for leveling a polishing member  10   d  in a fifth embodiment. The polishing member  10   d  is the same as the polishing member  10   a  shown in FIG.  5  and comprises an abrading member  11   d  attached to the bottom surface of an abrading member support disk  13   d,  and a brim section  15   d  on the edge of the abrading member support disk  13   d  which is rotated with a shaft  50   d.  The pressing device  20   d  is the same as the pressing device  20   c  shown in FIG. 7, and is provided with only a lower roller  26   d  provided at the end of a rod  23   d,  extending from the bottom of a respective pressing cylinder  21   d,  to contact the bottom surface of the brim section  15   d.    
     In this embodiment, two position sensors  60  are provided near the edge of the top surface of the polishing member  10   d,  and signals output from the position sensors  60  are input in a position sensor signal amplification circuit  63  in a control device  61 , and a pressing cylinder drive circuit  67  outputs control signals to the pressing cylinders  21   d  according to an abrading member tilt computation section  65 . 
     In this embodiment, polishing is performed with the polishing member  10   d  inclined at angle θ to the wafer  100 , as shown in FIG.  8 . Regardless of the location of the rotation axis m of the polishing member  10   d,  pressure values for the pressing cylinders  21   d  are computed and controlled so that, in this case, the vertical distance between the right position sensor  60  and the polishing member  10   d  is longer than the distance between the left position sensor  60  and the polishing member  10   d.    
     By controlling the pressing cylinders  21   d  in this manner, the abrading member  11   d  is tilted at a given angle, and moves over the surface of the wafer  100  while maintaining such tilt or desired posture. The reason for tilting the abrading member  11  is as follows. When the abrading member  11   d  is made to contact the wafer  100  at a given angle, as illustrated in FIGS. 8 and 9, because of a specific elasticity of the abrading member  11   d,  contact occurs not over a line contact but over a contact area S. The contact area S is always a specific constant value, no matter where the abrading member  11  is moved over the wafer  100 . Therefore, uniform polishing of the entire surface of the wafer may be achieved easily, by controlling the feed speed of the abrading member  11   d,  and because the contact area S is always constant, pressure control is simplified. 
     In contrast, when the entire abrading surface of the abrading member  11   d  is in contact with the wafer  100 , the contact area varies depending on where the abrading member  11   d  is on the wafer so that the control parameters (feed speed for abrading member  11   d  and pressing pressure on abrading member  11   d ) to provide uniform polishing become more complex. 
     The control method based on position sensors  60  and the control device  61  can be applied to the foregoing first to fourth embodiments. In other words, the method is equally applicable when it is not desired to tilt the polishing member. Also, the above embodiments each utilizes a cup-type abrading member ( 11 ,  11   a,    11   b,    11   c,    11   d ), but a disc-type abrading member can be used to produce the same effects. 
     Locations for applying balancing pressure and the number of pressing devices are not limited to those demonstrated in the foregoing embodiments, and they can be changed to suit each application, for example, the pressing location may only be one location. In the case of first to third embodiments, the abrading member is pushed towards the workpiece to be polished, therefore, when the rotation axis projects off the wafer, it is necessary to press on any area still remaining on the workpiece by lowering the pressing cylinders. On the other hand, in fourth and fifth embodiments, the abrading member is forced to be lifted away from the workpiece so that, when the rotation axis projects off the workpiece, it is necessary to lift any area that is off the workpiece by raising the pressing cylinders. The important point is to adjust the pressing devices in such a way that even though the rotation axis may be off the workpiece, the point of applying a balancing pressure is always projected within the workpiece. 
     Also, in the fifth embodiment, pressing devices  20   d  were controlled according to position sensors  60 , but the pressures of the pressing devices  20   d  can be controlled by using other sensing means such as to directly detect the tilting angle of the cup-type abrading member  10   d.    
     In some cases, the conventional CMP process may be applied either before or after the polishing process based on the abrading member according to the present invention. 
     FIG. 10 shows a schematic side view of a sixth embodiment of the polishing member used in conjunction with a combination of a turntable and a top ring. The polishing apparatus comprises a rotating turntable  71  and a polishing cloth (polishing tool)  72  mounted on top thereof, and a rotating top ring  73  holding a wafer (workpiece)  74  in the bottom section to press against the polishing cloth  72 . Polishing is performed using a polishing solution including free abrading grains suspended therein. As in the first embodiment, a pair of pressing devices  76  are provided for balancing purposes so as to straddle the rotation axis  0  of the top ring  73 . In this example, they are disposed symmetrically across the rotation axis  0 . The pressing devices  76  can be selected from many choices including hydraulic pressure devices based on water or oil or air, and balance control may be achieved by elasticity, piezoelectric controls and others means. 
     In this case, the top ring  73  is rotated by a rotation shaft  75  and, at the same time, is pressed against the wafer  73  by the two pressing devices  76 . This arrangement is effective in providing balanced polishing or desired posture, even when the rotation axis  0  is off the edge of the table  71 , by adjusting the pressures in the pressing devices  76  so as to maintain the projected point of applying a balancing pressure for the top ring  73  within the turntable  7  to prevent tilting of the top ring  73 . 
     Polishing cloth  72  may be replaced with a polishing member of various types such as an abrasive stone. Locations of the pressing devices  76  and their designs may be changed to suit each application. The number of pressing devices may be varied from a minimum of one device to more than three devices. Also, the pressing devices  76  may be made in the same manner as those in the second to fifth embodiments.

Technology Category: 7