Patent Publication Number: US-6905398-B2

Title: Chemical mechanical polishing tool, apparatus and method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to chemical mechanical polishing (CMP) used in semiconductor manufacturing. More particularly, it relates to a chemical mechanical polishing tool and to its use. 
     2. Discussion of the Related Art 
     Modern semiconductor manufacturing is a highly competitive industry that requires the ability to fabricate complex semiconductor devices at high speed, with high yields, and at low cost. 
     Semiconductor devices are fabricated on semiconductor wafers. Such wafers are made by carefully growing a large, high purity semiconductor crystal, which is then sliced into individual semiconductor wafers. For storage and protection the sliced semiconductor wafers are usually loaded into wafer cassettes. A wafer cassette individually stacks the sliced semiconductor wafers in slots. Wafer cassettes are beneficial in that the large numbers of semiconductor wafers can be stored and transported in a protected environment. 
     Unfortunately, immediately after slicing a semiconductor wafer is unsuitable for semiconductor device fabrication because the slicing leaves rough surfaces on the semiconductor wafers. Surface roughness is a serious problem because modern fabrication processes require accurate focusing of photolithographic circuit patterns onto the semiconductor wafer. As the density of the circuit patterns increases, focus tolerances better than 0.1 μmeters can be required. Focusing with such small tolerances is not practical if the surface of a semiconductor wafer not highly smooth and planar. 
     A number of techniques for reducing semiconductor wafer surface roughness exist. A semiconductor wafer can be mechanically worked by an abrasive pad to produce a fairly smooth surface. However, as indicated above, modern semiconductor wafer surfaces must be exceptionally smooth and planar. 
     One technique that can suitably finish the surface of a semiconductor is Chemical-Mechanical Polishing (“CMP”). In CMP, a semiconductor wafer is mechanically and chemically worked under carefully controlled conditions. Such work is performed using a special abrasive substance that is rubbed over the surface of the semiconductor wafer. The special abrasive substance is typically a slurry that contains minute particles that abrade, and chemicals that etch, dissolve, and/or oxidize, the surface of the semiconductor wafer. 
     CMP is a well-known and commonly used process. As shown in  FIG. 1 , a conventional chemical mechanical polishing apparatus includes a mount  3  for holding and rotating a semiconductor substrate  4 . That apparatus also includes a rotating disk  1  that retains a polishing pad  2 . As shown, that pad has a diameter that is much larger than that of the semiconductor substrate  4 . Furthermore, a nozzle  6  applies a polishing slurry  7  to the polishing pad  2 . 
     The semiconductor substrate  4  is polished by the applied polishing slurry, by rotating the mount  3  in the direction B, by moving the mount  3  in directions C while pressing the substrate  4  against the polishing pad  2 , and by rotating the polishing pad  2  in the direction A. 
     While the chemical mechanical polishing apparatus illustrated in  FIG. 1  has been generally successful, in practice using a polishing pad  2  with a larger diameter than that of the semiconductor substrate  4  may not be optimal. For example, vibration, which can be detrimental to precise polishing, is a significant problem if a large polishing pad is rotated too fast. Thus, when using a chemical mechanical polishing apparatus similar to that illustrated in  FIG. 1 , the achievable polishing rate is limited. Another problem with using a large polishing pad is that since the semiconductor substrate  4  is polished over its entire surface, it is difficult to efficiently remove localized defects. 
     Another approach to chemical mechanical polishing is provided in U.S. patent application Ser. No. 6,179,695 B1. Referring now to  FIG. 2 , that patent discloses a chemical mechanical polishing apparatus having a polishing station E 1  that holds a semiconductor substrate W. The polishing station E 1  further includes a slider  104  that both rotates and horizontally moves a table  105  on a support  106 . The semiconductor substrate W is placed on and held by the table  105 . The slider  104  itself is on a guide table  103  on a base  101 . 
     Also included in the chemical mechanical polishing apparatus of  FIG. 2  is a polishing head E 2  having a plurality of polishing-tools  110 . Referring now to  FIGS. 2 and 3 , the polishing-tools  110  are circumferentially disposed above the polishing station E 1 . The polishing-tools  110  are mounted such that they can rotate. 
     Still referring to  FIGS. 2 and 3 , the polishing head E 2  also includes a revolution table  108  that is rotatably supported on a lower yoke  102   a , which extends from a supporting member  102  that mounts on the base  101 . The revolution table  108  is attached to an output shaft of a driving mechanism  107 , which is supported on an upper yoke  102   b , which extends from the supporting member  102 . The driving mechanism  107  revolves the revolution table  108  at a predetermined rate, which causes the polishing-tools  110  to revolve. 
     The three polishing-tools  110  are interchangeable. Turning now to  FIGS. 3 and 4 , each polishing-tool  110  includes a plurality of ring-shaped polishing pads  111   a  and  111   b  on the end of shafts  113   a  and  113   b . Beneficially, the polishing pads are made of a nonwoven fabric, foamed polyurethane or the like. 
     Referring now to  FIG. 5 , the outer cylindrical shaft  113   a  is bearing  115   a  mounted and rotatable with respect to a lower supporting member  108   a  (also shown in FIG.  2 ). The inner cylindrical shaft  113   b  is co-axially disposed within the outer cylindrical shaft  113   a . The inner cylindrical shaft is also bearing  115   b  mounted and rotatable. The ring-shaped polishing pads  111   a  and  111   b , which are held in position by holding members  112   a  and  112   b , have surface areas centered at radiuses r 1  and r 2 . 
     Referring now to  FIGS. 2 and 5 , drive mechanisms  114   a  and  114   b  (which are on the revolution table  108 ) connect to the cylindrical shafts  113   a  and  113   b , respectively. Thus, the ring-shaped polishing pads  111   a  and  111   b  can be independently rotated at high speeds. The drive mechanisms  114   a  and  114   b  are controlled such that the linear velocity of the polishing pads are the same. That is, the rotational velocity of the ring-shaped polishing pads  111   a  and  111   b  are used to compensate for the different radiuses r 1  and r 2 . 
     To polish a semiconductor substrate W, the ring-shaped polishing pads  111   a  and  111   b  are moved into contact at a predetermined pressure with the surface of the semiconductor substrate W. Then, the slider  104  is moved such that the semiconductor substrate W is at a polishing position. Then, the driving mechanisms  114   a  and  114   b  rotate the ring-shaped polishing pads  111   a  and  111   b  while a polishing slurry is applied to the surface of the semiconductor substrate W. At the same time, the rotating table  105  is rotated and is moved radially (with short strokes). 
     Since the surface being polished is polished using multiple, small diameter ring-shaped polishing pads it is possible to rotate the polishing pads at high speeds while very precisely polishing the surface irrespective of local defects. Additionally, the ring-shapes reduce vibration over that of a continuous polishing pad. It should also be noted that it is possible to use only one of the ring-shaped polishing pads when polishing. 
     Beneficially, the inner and outer ring-shaped polishing pads  111   a  and  111   b  can move axially with respect to each other. This makes it possible to adjust the relative heights of the polishing pads  111   a  and  111   b , and to independently set the polishing pad pressures against the surface of the semiconductor substrate W. In turn, this enables pressure control such that the optimum processing pressures can be used. 
     While the apparatus illustrated in  FIGS. 2-5  is beneficial, it also may not be optimal. For example, the polishing area is relatively small, even when both polishing pads contact the semiconductor wafer W. This increases the required polishing time. Furthermore, while the apparatus illustrated in  FIGS. 2-5  is believed to be effective in reducing the detrimental effects of vibration, vibration is primarily only a problem after polishing has been performed for some time. Finally, the apparatus illustrated in  FIGS. 2-5  may not be the best for localized polishing as the radiuses of the polishing pads causes relatively widely separated areas to be polished. 
     Therefore, a new semiconductor wafer polishing apparatus, and a method of using such an apparatus, that can reduce the detrimental effects of vibration, that can polish both broad and localized areas, and that can rapidly remove material from a semiconductor wafer would be beneficial. 
     SUMMARY OF THE INVENTION 
     The principles of the present invention provide for a new polishing tool that can polish a semiconductor wafer at high speed, while reducing the detrimental effects of vibration, and while enabling both broad area and localized polishing of a semiconductor wafer. 
     A polishing tool that is in accord with the principles of the present invention includes a central polishing assembly comprised of a central pad mount on a central shaft. That central pad mount is capable of retaining a center polishing pad having a continuous polishing surface. The polishing tool further includes a ring polishing assembly comprised of a ring pad mount with a central aperture on a ring shaft with a central aperture. The ring pad mount is capable of retaining a ring polishing pad having a central aperture. The central polishing assembly and the ring polishing assembly are fabricated such that the central polishing assembly can move in an axial direction relative to said ring polishing assembly, and such that the central shaft is disposed within the apertures of the ring assembly. 
     Beneficially, the polishing assembly and the central polishing assembly are both rotatable and axially movable independent of one another. Furthermore, both pad mounts beneficially retain polishing pads. 
     The principles of the present invention further provide for a new semiconductor wafer polishing apparatus that can polish a semiconductor wafer at high speed, while reducing the detrimental effects of vibration, and while enabling both broad area and localized polishing of a semiconductor wafer. A semiconductor wafer polishing apparatus that is in accord with the principles of the present invention includes a rotating polishing table for retaining a semiconductor wafer having a surface to be polished, and at least one polishing tool having a central polishing assembly comprised of a central pad mount on a central shaft. That central pad mount is capable of retaining a center polishing pad having a continuous polishing surface. The polishing tool further includes a ring polishing assembly comprised of a ring pad mount with a central aperture on a ring shaft with a central aperture. The ring pad mount is capable of retaining a ring polishing pad having a central aperture. The central polishing assembly and the ring polishing assembly are fabricated such that the central polishing assembly can move in an axial direction relative to said ring polishing assembly, and such that the central shaft is axially disposed within the apertures of the ring assembly. 
     Beneficially, the central pad mount holds a center pad, and the ring pad mount retains a ring pad. Also beneficially, the center pad and the ring pad are independently rotatable and axially movable. Furthermore, the center pad and the ring pad are beneficially mounted such that they can move across a surface of semiconductor wafer retained on the rotating polishing table. Also beneficially, a nozzle is provided for supplying a polishing slurry onto a surface of semiconductor wafer retained on the rotating polishing table. Preferably, a ring-shaped rim surrounds the polishing table. The rim provides a reference plane when polishing a semiconductor wafer. 
     The principles of the present invention further for a new method of polishing a semiconductor wafer. That method includes rotating a semiconductor wafer on a rotating polishing table such that a surface to be polished is exposed. Then, selectively and independently moving a solid center polishing pad having an axis of rotation and/or an axially aligned ring-shaped polishing pad into contact with the surface of the semiconductor wafer. Furthermore, the center polishing pad and/or the ring-shaped polishing pad are beneficially swept across a semiconductor wafer being polished. 
     Additional features and advantages of the invention will be set forth in the description and figures that follow, and in part will be apparent from that description and figures, or may be learned by practice of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The accompanying drawings, which are included to provide a further understanding of the invention and which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. 
       In the drawings: 
         FIG. 1  a schematic view illustrating a conventional related art chemical mechanical polishing apparatus; 
         FIG. 2  a schematic view illustrating a related art chemical mechanical polishing apparatus; 
         FIG. 3  illustrates the relationship between a revolution table and the polishing-tools of the chemical mechanical polishing apparatus of  FIG. 2 ; 
         FIG. 4  is a perspective view of the lower end of a polishing-tool of the chemical mechanical polishing apparatus of  FIG. 2 ; 
         FIG. 5  is a schematic cross-sectional view of a polishing-tool of the chemical mechanical polishing apparatus of  FIG. 2 ; 
         FIG. 6  is a schematic cross-sectional view of a chemical mechanical polishing apparatus that is in accord with the principles of the present invention; and 
         FIG. 7  illustrates a method of polishing a semiconductor wafer that is in accord with the principles of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     Reference will now be made in detail to an illustrated embodiment of the present invention, the example of which is shown in the accompanying drawings. The principles of the present invention provide for both rapid, broad area polishing, and for localized area polishing of a semiconductor wafer. Consequently, the polishing rate can be increased, the polishing finish can be improved, and the detrimental effects of vibration can be avoided. 
       FIG. 6  schematically illustrates a simplified chemical mechanical polishing apparatus  300  that is in accord with the principles of the present invention. That apparatus includes a rotatable polishing table  302  capable of retaining, holding, and rotating a semiconductor substrate  304  that is to be polished. The polishing table is mounted on a shaft  306  that turns in the direction  308 . It should be understood that the chemical mechanical polishing apparatus  300  can include any of the features of the chemical mechanical polishing apparatus illustrated in FIG.  5 . 
     Surrounding and adjacent the polishing table  302  is a ring-shaped rim  310 . The relative positions of the ring-shaped rim  310  and the polishing table  302  beneficially can be adjusted along directions  311  such that the surface  350  of the semiconductor substrate  304  is level with the top  312  of the rim  310 . 
     The chemical mechanical polishing apparatus  300  further includes a polishing tool  320 . That polishing tool is distinct from the polishing tools of the chemical mechanical polishing apparatus illustrated in  FIGS. 2 and 5 . The polishing tool  320  includes a central polishing assembly  322  that includes a center polishing pad  324  on a central mount  326  that is on the end of a central shaft  328 . The polishing tool  320  further includes at least one coaxially disposed ring pad  330  on a ring mount  332  of a ring shaft  334 . 
     As shown in  FIG. 6 , the central shaft  328  is centrally disposed within the ring shaft  334 . Further, those shafts share the same axis of rotation. The central shaft  328  and the ring shaft  334  are capable of independent rotation in the direction  308 . Furthermore, the central shaft  328  and the ring shaft  334  are also capable of independent motion in the directions  338 . Motion in the directions  308  and  338  can be provided by any suitable means (which are not shown in FIG.  6 ), including the driving mechanisms  114   a  and  114   b  of  FIG. 5 , and those suggested with regard to  FIGS. 1 , and  2 . Furthermore, a linear driving mechanism (which is also not shown) moves the polishing head  320  relative to the polishing table  302  in the directions  342  such that the polishing pads  324  and  330  can selectively and controllably move across the semiconductor wafer  304 . 
     As provided for above, the chemical mechanical polishing apparatus  300  is capable of multiple degrees of motion. First, the polishing table  302  rotates in the direction  308 . For simplicity, this can be performed at a constant rotational velocity. Furthermore, the center polishing pad  324  and the rim polishing pad  330  can be rotated independently and with different rotational velocities in the direction  308 . Those pads can also be moved independently in the directions  338 . This enables each polishing pad to be brought into contact with the surface  350 . Additionally, the center polishing pad  324  and the rim polishing pad  330  can be moved in the directions  342  relative to the semiconductor wafer  304 . Finally, the relative position of the semiconductor wafer  304  and the top  312  of the rim  310  can be controlled. Thus, the center polishing pad  324  and the rim polishing pad  330  can be independently brought into contact with, and swept across the surface  350  of the semiconductor wafer  304 . Furthermore, the rim  310  can control and even out the pressure applied to the outer perimeter of the semiconductor wafer  304 . 
       FIG. 7  illustrates various methods of using the chemical mechanical polishing apparatus  300 . As shown in FIG.  7 ( a ), a cut-away view, and in FIG.  7 ( b ), a top down view of the polishing pads  330  and  324 , both the center polishing pad  324  and the ring polishing pad  330  can be brought into contact with the surface  350  of a semiconductor wafer  304 . As may be seen with reference to FIGS.  7 ( a ) and  7 ( b ), the polishing pad  324  has a solid polishing surface which extends across a diameter of the polishing pad  324 . The center polishing pad  324  and the ring polishing pad  330  are beneficially aligned horizontally and moved together across the surface  350  in the directions  342 . The rim  310  provides a leveling reference plane for the surface  350 . Since both polishing pads contact the semiconductor wafer, the polishing pads remove the maximum amount of material from the semiconductor wafer. 
     Turn now to FIG.  7 ( c ), a cut-away illustration, and to FIG.  7 ( d ), a top down illustration, for views that depict only the ring polishing pad  330  being brought into contact with the surface  350  of a semiconductor wafer  304 . Such can occur when only localized polishing away from the rim of the semiconductor wafer  304  is desired. Other reasons to use only the ring polishing pad  330  include reducing vibration when polishing at high speed, and when the center polishing pad  324  is defective. As shown in FIGS.  7 ( c ) and  7 ( d ), the ring polishing pad  330  moves across the surface  350  in the directions  342 , while the rim  310  provides a reference plane for the surface  350 . 
     Turn now to FIG.  7 ( e ), a cut-away illustration, and to FIG.  7 ( e ), a top down illustration, for views that depict only the center polishing pad  324  being brought into contact with the surface  350  of a semiconductor wafer  304 . Such is beneficial when localized polishing near the rim of the semiconductor wafer  304  is desired. Another reason to use only the center polishing pad  324  is when the ring polishing pad  330  is defective. As shown in FIGS.  7 ( e ) and  7 ( f ), the center polishing pad  324  moves across the surface  350  of the semiconductor wafer  304  in the directions  342 . The rim  310  provides a leveling reference for the surface  350  when localized polishing near the rim of the semiconductor wafer  304  is being performed. 
     The chemical mechanical polishing apparatus  300  illustrated in FIGS.  6  and  7 ( a )- 7 ( f ) is a simplified depiction of a practical apparatus. In practice, various mechanisms that provide the required motion, and various controllers to control such motion, will be included. Furthermore, a mechanism to supply a polishing slurry and a mechanism to retain the semiconductor wafer on the polishing table  302  should be understood as being included. In fact, the CMP apparatus illustrated in  FIG. 5 , but which includes the inventive polishing tool, is a practical CMP apparatus. In any event, the additional components and mechanisms are well-known in chemical mechanical polishing systems. 
     While the present invention has been described with respect to illustrated embodiments, it is to be understood that the present invention is not limited to those embodiments. Furthermore, it will be apparent to those skilled in the art that various modifications and variation can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.