Abstract:
Polishing machines and methods for mechanical and/or chemical-mechanical polishing of microfeature workpieces are disclosed herein. In one embodiment, a machine includes a table having a support surface, an under-pad carried by the support surface, and a workpiece carrier assembly over the table. The under-pad has a cavity and the carrier assembly is configured to carry a microfeature workpiece. The machine further includes a magnetic field source configured to generate a magnetic field in the cavity and a magnetorheological fluid in the cavity. The magnetorheological fluid changes viscosity within the cavity under the influence of the magnetic field source. It is emphasized that this Abstract is provided to comply with the rules requiring an abstract. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. 37 C.F.R §172 (b).

Description:
TECHNICAL FIELD 
     The present invention relates to polishing machines and methods for polishing microfeature workpieces. In particular, the present invention relates to mechanical and/or chemical-mechanical polishing of microfeature workpieces with polishing machines that include under-pads. 
     BACKGROUND 
     Mechanical and chemical-mechanical planarization (“CMP”) processes remove material from the surface of microfeature workpieces in the production of microelectronic devices and other products.  FIG. 1  schematically illustrates a rotary CMP machine  10  with a platen  20 , a carrier head  30 , and a planarizing pad  40 . The CMP machine  10  may also include an under-pad  50  between an upper surface  22  of the platen  20  and a lower surface of the planarizing pad  40 . The under-pad  50  provides a thermal and mechanical interface between the planarizing pad  40  and the platen  20 . A drive assembly  26  rotates the platen  20  (indicated by arrow F) and/or reciprocates the platen  20  back and forth (indicated by arrow G). Since the planarizing pad  40  is attached to the under-pad  50 , the planarizing pad  40  moves with the platen  20  during planarization. 
     The carrier head  30  has a lower surface  32  to which a microfeature workpiece  12  may be attached, or the workpiece  12  may be attached to a resilient pad  34  under the lower surface  32 . The carrier head  30  may be a weighted, free-floating wafer carrier, or an actuator assembly  31  may be attached to the carrier head  30  to impart rotational motion to the microfeature workpiece  12  (indicated by arrow J) and/or reciprocate the workpiece  12  back and forth (indicated by arrow I). 
     The planarizing pad  40  and a planarizing solution  44  define a planarizing medium that mechanically and/or chemically-mechanically removes material from the surface of the microfeature workpiece  12 . The planarizing solution  44  may be a conventional CMP slurry with abrasive particles and chemicals that etch and/or oxidize the surface of the microfeature workpiece  12 , or the planarizing solution  44  may be a “clean” nonabrasive planarizing solution without abrasive particles. In most CMP applications, abrasive slurries with abrasive particles are used on nonabrasive polishing pads, and clean nonabrasive solutions without abrasive particles are used on fixed-abrasive polishing pads. 
     To planarize the microfeature workpiece  12  with the CMP machine  10 , the carrier head  30  presses the workpiece  12  facedown against the planarizing pad  40 . More specifically, the carrier head  30  generally presses the microfeature workpiece  12  against the planarizing solution  44  on a planarizing surface  42  of the planarizing pad  40 , and the platen  20  and/or the carrier head  30  moves to rub the workpiece  12  against the planarizing surface  42 . As the microfeature workpiece  12  rubs against the planarizing surface  42 , the planarizing medium removes material from the face of the workpiece  12 . The force generated by friction between the microfeature workpiece  12  and the planarizing pad  40  will, at any given instant, be exerted across the surface of the workpiece  12  primarily in the direction of the relative movement between the workpiece  12  and the planarizing pad  40 . A retaining ring  33  can be used to counter this force and hold the microfeature workpiece  12  in position. The frictional force drives the microfeature workpiece  12  against the retaining ring  33 , which exerts a counterbalancing force to maintain the workpiece  12  in position. 
     The CMP process must consistently and accurately produce a uniformly planar surface on workpieces to enable precise fabrication of circuits and photo-patterns. A nonuniform surface can result, for example, when material from one area of a workpiece is removed more quickly than material from another area during CMP processing. In certain applications, the downward pressure of the retaining ring causes the under-pad and the planarizing pad to deform, creating a standing wave inside the retaining ring. Consequently, the planarizing pad removes material more quickly from the region of the workpiece adjacent to the standing wave than from the regions of the workpiece radially outward and inward from the wave. Thus, the CMP process may not produce a planar surface on the workpiece. 
     One approach to improve the planarity of a workpiece surface is to use a carrier head with interior and exterior bladders that modulate the downward forces on selected areas of the workpiece. These bladders can exert pressure on selected areas of the back side of the workpiece to increase the rate at which material is removed from corresponding areas on the front side. These carrier heads, however, have several drawbacks. For example, the typical bladder has a curved edge that makes it difficult to exert a uniform downward force at the perimeter. Moreover, conventional bladders cover a fairly broad area of the workpiece which limits the ability to localize the downward force on the workpiece. Furthermore, conventional bladders are often filled with compressible air that inhibits precise control of the downward force. In addition, carrier heads with multiple bladders form a complex system that is subject to significant downtime for repair and/or maintenance causing a concomitant reduction in throughput. 
     Another approach to improve the planarity of a workpiece surface is to use a hard under-pad to reduce the deformation caused by the retaining ring. Hard under-pads, however, increase the frequency of scratches and other defects on the workpiece because particles in the planarizing solution become trapped between the workpiece and the planarizing pad. Thus, there is a need to improve the polishing process to form uniformly planar surfaces on workpieces. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional side view of a portion of a rotary planarizing machine in accordance with the prior art. 
         FIG. 2  is a schematic cross-sectional view of a portion of a CMP machine for polishing a microfeature workpiece in accordance with one embodiment of the invention. 
         FIG. 3A  is a schematic top planform view of a plurality of magnetic field sources for use in a CMP machine in accordance with an additional embodiment of the invention. 
         FIG. 3B  is a schematic top planform view of a plurality of magnetic field sources for use in a CMP machine in accordance with an additional embodiment of the invention. 
         FIG. 4  is a schematic cross-sectional view of a portion of a CMP machine in accordance with another embodiment of the invention. 
         FIG. 5  is a schematic cross-sectional top view of an under-pad in accordance with yet another embodiment of the invention. 
         FIG. 6  is a schematic cross-sectional view of a portion of a CMP machine in accordance with still another embodiment of the invention. 
         FIG. 7  is a schematic cross-sectional view of a portion of a CMP machine in accordance with yet another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     A. Overview 
     The present invention is directed toward polishing machines and methods for mechanical and/or chemical-mechanical polishing of microfeature workpieces. The term “microfeature workpiece” is used throughout to include substrates in or on which microelectronic devices, micro-mechanical devices, data storage elements, and other features are fabricated. For example, microfeature workpieces can be semiconductor wafers, glass substrates, insulated substrates, or many other types of substrates. Furthermore, the terms “planarization” and “planarizing” mean either forming a planar surface and/or forming a smooth surface (e.g., “polishing”). Several specific details of the invention are set forth in the following description and in  FIGS. 2-7  to provide a thorough understanding of certain embodiments of the invention. One skilled in the art, however, will understand that the present invention may have additional embodiments, or that other embodiments of the invention may be practiced without several of the specific features explained in the following description. 
     One aspect of the invention is directed to a polishing machine for mechanical and/or chemical-mechanical polishing of microfeature workpieces. In one embodiment, the machine includes a table having a support surface, an under-pad carried by the support surface, and a workpiece carrier assembly over the table. The under-pad has a cavity and the carrier assembly is configured to carry a microfeature workpiece. The machine further includes a magnetic field source configured to generate a magnetic field in the cavity and a magnetorheological fluid disposed within the cavity. The magnetorheological fluid changes viscosity within the cavity under the influence of the magnetic field source. The change in the viscosity of the magnetorheological fluid changes the compressibility of the under-pad. In one aspect of this embodiment, the magnetic field source is carried by the under-pad, the workpiece carrier assembly, or the table. In another aspect of this embodiment, the under-pad includes a first surface and a second surface, and the cavity is enclosed between the first surface and the second surface. 
     Another aspect of the invention is directed to an under-pad for use on a polishing machine in the mechanical and/or chemical-mechanical polishing of microfeature workpieces. In one embodiment, the under-pad includes a body having a first surface, a second surface, and a cavity between the first and second surfaces. The first surface is juxtaposed to the second surface. The under-pad further includes a magnetorheological fluid in the cavity. The magnetorheological fluid changes viscosity within the cavity in response to a magnetic field. In one aspect of this embodiment, the cavity includes a plurality of cells arranged generally concentrically, in a grid, or in another pattern. In another aspect of this embodiment, the magnetic field source includes an electrically conductive coil or an electromagnet. 
     Another aspect of the invention is directed to a method of polishing a microfeature workpiece with a polishing machine having a carrier head, a polishing pad, and an under-pad carrying the polishing pad. In one embodiment, the method includes moving at least one of the carrier head and the polishing pad relative to the other to rub the microfeature workpiece against the polishing pad. The under-pad has a cavity and a magnetorheological fluid disposed within the cavity. The method further includes changing the compressibility of the under-pad by generating a magnetic field to change the viscosity of the magnetorheological fluid within the cavity of the under-pad. In one aspect of this embodiment, generating the magnetic field comprises energizing an electromagnet or an electrically conductive coil. 
     B. Polishing Systems 
       FIG. 2  is a schematic cross-sectional view of a CMP machine  110  for polishing a microfeature workpiece  112  in accordance with one embodiment of the invention. The CMP machine  110  includes a platen  120 , a workpiece carrier assembly  130  over the platen  120 , and a planarizing pad  140  coupled to the platen  120 . The workpiece carrier assembly  130  can be coupled to an actuator assembly  131  (shown schematically) to move the workpiece  112  across a planarizing surface  142  of the planarizing pad  140 . In the illustrated embodiment, the workpiece carrier assembly  130  includes a head  132  having a support member  134  and a retaining ring  133  coupled to the support member  134 . The support member  134  can be an annular housing having an upper plate coupled to the actuator assembly  131 . The retaining ring  133  can extend around the support member  134  and project toward the workpiece  112  below a bottom rim of the support member  134 . 
     The CMP machine  110  further includes a dynamic under-pad  150  that dynamically modulates its compressibility to control the polishing rate, defects, planarity, and other characteristics of the polishing process. The under-pad  150  has an upper surface  153  attached to the planarizing pad  140 , a lower surface  154  attached to the platen  120 , and a cavity  152  between the upper surface  153  and the lower surface  154 . The cavity  152  is defined by a first surface  156 , a second surface  157  opposite the first surface  156 , and an outer surface  158 . The cavity  152  is configured to hold a viscosity changing fluid to selectively change the compressibility of the under-pad  150 . The under-pad  150  can be manufactured using polymers, rubbers, coated fabrics, composites, and/or any other suitable materials. In one aspect of this embodiment, the under-pad  150  has a thickness T of between approximately 0.5 mm to approximately 10 mm. In other embodiments, the thickness T of the under-pad  150  can be less than 0.5 mm or greater than 10 mm. 
     In one aspect of this embodiment, the cavity  152  contains a magnetorheological fluid  160  that changes viscosity in response to a magnetic field. For example, the viscosity of the magnetorheological fluid  160  can increase from a viscosity similar to that of motor oil to a viscosity of a nearly solid material depending on the polarity and magnitude of the magnetic field. In additional embodiments, the magnetorheological fluid  160  may experience a smaller change in viscosity in response to the magnetic field and/or the magnetorheological fluid  160  may decrease in viscosity in response to the magnetic field. 
     The CMP machine  110  further includes a magnetic field source  170  that is configured to generate a magnetic field in the cavity  152  of the under-pad  150 . In the illustrated embodiment, the magnetic field source  170  includes an electromagnet that is selectively energized to generate the magnetic field. In other embodiments, such as those described below with reference to  FIG. 4 , the magnetic field source  170  can be an electrically conductive coil, a magnet, or any other suitable device to generate the magnetic field in the cavity  152 . In the illustrated embodiment, the platen  120  includes a depression  122  that receives the magnetic field source  170 . Accordingly, an upper surface  172  of the magnetic field source  170  and an upper surface  124  of the platen  120  carry the under-pad  150 . In other embodiments, such as those described below with reference to  FIGS. 4 and 6 , the platen  120  may not carry the magnetic field source  170 . For example, the workpiece carrier assembly  130 , the planarizing pad  140 , and/or the under-pad  150  can carry the magnetic field source  170 . 
     In one aspect of this embodiment, the CMP machine  110  also includes a controller  190  operably coupled to the magnetic field source  170  to selectively energize the magnetic field source  170 . The controller  190  selectively energizes the magnetic field source  170 , which generates a magnetic field to change the viscosity of the magnetorheological fluid  160  within the cavity  152 . As the viscosity of the magnetorheological fluid  160  increases, the compressibility of the under-pad  150  decreases. For example, when the magnetorheological fluid  160  has a high viscosity, the under-pad  150  is relatively inflexible in a direction D. Accordingly, the controller  190  can dynamically control in real time the compressibility of the under-pad  150  by varying the power applied to the magnetic field source  170  before, during, and/or after polishing workpieces. 
     One embodiment of a process for polishing the workpiece  112  includes a first stage in which the under-pad  150  is generally hard and a second stage in which the under-pad  150  is generally compressible. During the first stage in which the under-pad  150  is hard, the planarizing pad  140  efficiently creates a planar surface on the workpiece  112  without removing excessive amounts of material from the workpiece  112 . The hard under-pad  150 , however, can create a significant number of defects on the surface of the workpiece  112 . For example, the defects can result from particles in the planarizing solution that become trapped between the planarizing pad  140  and the surface of the workpiece  112 . During the second stage in which the under-pad  150  is compressible, the planarizing pad  140  removes the defects from the surface of the workpiece  112 . Typically, in this embodiment, the under-pad  150  is not compressible during the first stage of the polishing process because a compressible under-pad does not efficiently create a planar surface on the workpiece  112  and can cause dishing in low density areas of the workpiece  112 . 
     One feature of the CMP machine  110  of this embodiment is the ability to change the compressibility of the under-pad in real time during the polishing cycle. An advantage of this feature is the ability to obtain the benefits of polishing the workpiece using a hard under-pad and polishing the workpiece using a compressible under-pad at different stages of planarizing a workpiece. More specifically, the under-pad can efficiently create a planar surface on the workpiece and then remove the defects from the planar surface. 
     C. Other Configurations of Magnetic Field Sources and Under-Pads 
       FIGS. 3A and 3B  are schematic top planform views of several configurations of magnetic field sources for use in CMP machines in accordance with additional embodiments of the invention. For example,  FIG. 3A  illustrates a plurality of magnetic field sources  270  arranged in a grid with a plurality of rows R 1 -R 8  and a plurality of columns C 1 -C 8 . The magnetic field sources proximate to the perimeter can have a curved side that corresponds with the curvature of an under-pad. The magnetic field sources  270  can be operably coupled to a controller to generate magnetic fields in corresponding portions of an under-pad. In additional embodiments, the size of the magnetic field sources  270  can decrease to increase the resolution such that a much larger number of rows and columns can be used. 
       FIG. 3B  is a schematic top planform view of a plurality of magnetic field sources  370  (identified individually as  370   a-d ) in accordance with another embodiment of the invention. A first magnetic field source  370   a , a second magnetic field source  370   b , and a third magnetic field source  370   c  have generally annular configurations and are arranged concentrically around a fourth magnetic field source  370   d . In other embodiments, the magnetic field sources  370  can be spaced apart from each other and/or arranged in other configurations such as in quadrants. 
       FIG. 4  is a schematic cross-sectional view of a CMP machine  410  in accordance with another embodiment of the invention. The CMP machine  410  can be similar to the CMP machine  110  discussed above with reference to FIG.  2 . For example, the CMP machine  410  includes a platen  420 , a workpiece carrier assembly  130  over the platen  420 , and a planarizing pad  140  over the platen  420 . The CMP machine  410  further includes an under-pad  450  between the platen  420  and the planarizing pad  140 . The underpad  450  has a cavity  452  with a plurality of cells  452   a-c  and a magnetorheological fluid  160  disposed within the cells  452   a-c . A first cell  452   a  and a second cell  452   b  have generally annular configurations and are arranged concentrically around a third cell  452   c . The cells  452   a-c  are defined by a first surface  456 , a second surface  457  opposite the first surface  456 , a third surface  458 , and a fourth surface  459  opposite the third surface  458 . Discrete volumes of the magnetorheological fluid  160  are disposed within the cells  452   a-c . In other embodiments, such as those described below with reference to  FIG. 5 , an under-pad can include a different number of cells and/or the cells can be arranged in a different configuration. 
     The CMP machine  410  also includes a plurality of magnetic field sources  470  (identified individually as  470   a-c ) carried by the under-pad  450 . The magnetic field sources  470  are positioned to selectively generate magnetic fields in corresponding cells  452   a-c . For example, a first magnetic field source  470   a  is positioned to generate a magnetic field in the first cell  452   a . Accordingly, discrete portions of the under-pad  450  can be compressible while other portions of the under-pad  450  are hard. For example, in the embodiment illustrated in  FIG. 4 , a second magnetic field source  470   b  generates a magnetic field in the second cell  452   b . Consequently, the region of the under-pad  450  defined by the second cell  452   b  is hard while the regions of the under-pad  450  defined by the first and third cells  452   a  and  452   c  are compressible. In one aspect of the illustrated embodiment, the magnetic field sources  470  are electrically conductive coils embedded in the under-pad  450  between a lower surface  454  and the second surface  457 . In other embodiments, a CMP machine may include a different number of magnetic field sources and/or the magnetic field sources may be positioned in other locations in the under-pad. In additional embodiments, the under-pad  450  can be used in conjunction with other configurations and/or types of magnetic field sources, such as magnetic field sources that are carried by the platen as described with reference to  FIGS. 2-3B ,  6  and  7 . 
       FIG. 5  is a schematic cross-sectional top view of an under-pad  550  for use on a CMP machine in accordance with another embodiment of the invention. The under-pad  550  includes a plurality of cells  552  arranged in a grid with a plurality of columns C 1 -C 8  and a plurality of rows R 1 -R 8 . The cells  552  are defined by a first surface  554 , a second surface  555  opposite the first surface  554 , a third surface  558 , and a fourth surface  559  opposite the third surface  558 . The cells  552  proximate to the perimeter have a curved side that corresponds with the curvature of the under-pad  550 . The cells  552  are configured to receive discrete portions of the magnetorheological fluid  160  (FIG.  4 ). In additional embodiments, the size of the cells  552  can decrease to increase the resolution such that a much larger number of rows and columns can be used. 
       FIG. 6  is a schematic cross-sectional view of a CMP machine  610  in accordance with another embodiment of the invention. The CMP machine  610  can be similar to the CMP machine  110  discussed above with reference to FIG.  2 . For example, the CMP machine  610  includes a planarizing pad  140 , an under-pad  150  carrying the planarizing pad  140 , a platen  620  carrying the under-pad  150 , and a workpiece carrier assembly  630  over the planarizing pad  140 . The under-pad  150  has a cavity  152  containing a magnetorheological fluid  160 . The workpiece carrier assembly  630  includes a head  632  having a support member  634  and a retaining ring  633  coupled to the support member  634 . The support member  634  can include a plurality of magnetic field sources  670  that are configured to generate magnetic fields in at least a portion of the cavity  152  proximate to the workpiece carrier assembly  630 . Accordingly, the CMP machine  610  can selectively control the compressibility of the under-pad  150  proximate to the workpiece carrier assembly  630 . 
       FIG. 7  is a schematic cross-sectional view of a CMP machine  710  in accordance with another embodiment of the invention. The CMP machine  710  can be similar to the CMP machine  110  discussed above with reference to FIG.  2 . For example, the CMP machine  710  includes a workpiece carrier assembly  130 , a planarizing pad  140 , an under-pad  750  carrying the planarizing pad  140 , a platen  720  carrying the under-pad  750 , and a magnetic field source  770  carried by the platen  720 . The under-pad  750  has a cavity  752  containing a magnetorheological fluid  160 . The CMP machine  710  further includes a reservoir  762  in fluid communication with the cavity  752  and a pump  764  to transfer the magnetorheological fluid  160  between the cavity  752  and the reservoir  762 . A conduit  768  extending through an aperture  726  in the platen  720  and an aperture  772  in the magnetic field source  770  couples the cavity  752  to the reservoir  762  and the pump  764 . The pump  764  can transfer a portion of the magnetorheological fluid  160  from the reservoir  762  to the cavity  752  to increase the pressure in the cavity  752 . The increased pressure in the cavity  752  accordingly reduces the compressibility of the under-pad  750 . Alternatively, the pump  764  can transfer a portion of the magnetorheological fluid  160  from the cavity  752  to the reservoir  762  to increase the compressibility of the under-pad  750 . 
     From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.