Patent Publication Number: US-6663470-B2

Title: Method and apparatus for releasably attaching a polishing pad to a chemical-mechanical planarization machine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of pending U.S. patent application Ser. No. 09/539,854, filed Mar. 31, 2000, U.S. Pat. No. 6,482,079 which is a divisional of pending U.S. patent application Ser. No. 09/181,578, filed Oct. 28, 1998. 
    
    
     TECHNICAL FIELD 
     The present invention relates to methods and devices for releasably attaching polishing pads to the platens of chemical-mechanical planarization machines. 
     BACKGROUND OF THE INVENTION 
     Chemical-mechanical planarization (“CMP”) processes remove material from the surface of a semiconductor wafer in the production of integrated circuits. FIG. 1 schematically illustrates a CMP machine  10  with a platen  20 , a wafer carrier  60 , a polishing pad  40 , and a planarizing liquid  41  on the polishing pad  40 . The polishing pad  40  may be a conventional polishing pad made from a continuous phase matrix material (e.g., polyurethane), or it may be a fixed abrasive polishing pad made from abrasive particles fixedly dispersed in a suspension medium. The planarizing liquid  41  may be a conventional CMP slurry with abrasive particles and chemicals that etch and/or oxidize the wafer, or the planarizing liquid  41  may be a planarizing solution without abrasive particles that contains only chemicals to etch and/or oxidize the surface of the wafer. In most CMP applications, conventional CMP slurries are used on conventional polishing pads, and planarizing solutions without abrasive particles are used on fixed abrasive polishing pads. 
     The CMP machine  10  also has an underpad  25  attached to an upper surface  30  of the platen  20  and the lower surface of the polishing pad  40 . In one type of CMP machine, a drive assembly  50  rotates the platen  20  as indicated by arrow A. In another type of CMP machine, the drive assembly reciprocates the platen back and forth as indicated by arrow B. Since the polishing pad  40  is attached to the underpad  25 , the polishing pad  40  moves with the platen  20 . 
     The wafer carrier  60  has a lower surface  63  to which a wafer  12  may be attached, or the wafer  12  may be attached to a resilient pad  64  positioned between the wafer  12  and the lower surface  63 . The wafer carrier  60  may be a weighted, free-floating wafer carrier, or an actuator assembly  61  may be attached to the wafer carrier to impart axial and/or rotational motion (indicated by arrows C and D, respectively). 
     To planarize the wafer  12  with the CMP machine  10 , the wafer carrier  60  presses the wafer  12  face-downward against the polishing pad  40 . While the face of the wafer  12  presses against the polishing pad  40 , at least one of the platen  20  or the wafer carrier  60  moves relative to the other to move the wafer  12  across the planarizing surface  42 . As the face of the wafer  12  moves across the planarizing surface  42 , the polishing pad  40  and the planarizing liquid  41  continually remove material from the face of the wafer  12 . 
     CMP processes must consistently and accurately produce a uniform, planar surface on the wafer to enable precise circuit and device patterns to be formed with photolithography techniques. As the density of integrated circuits increases, it is often necessary to accurately focus the critical dimensions of the photo-patterns to within a tolerance of approximately 0.1 μm. Focusing photo-patterns of such small tolerances, however, is difficult when the planarized surface of the wafer is not uniformly planar. Thus, CMP processes must create a highly uniform, planar surface. 
     One problem with conventional CMP processing techniques is that the planarized surface of the wafer may not be sufficiently uniform due to non-uniformities that may develop in the planarizing surface of the polishing pad during planarization. One conventional approach to addressing this problem is to firmly attach the polishing pad to the platen to decrease the likelihood that the polishing pad will warp or wrinkle as the wafer carrier and substrate move across the planarizing surface. For example, in one conventional approach, the polishing pad may be attached to the platen with a high-strength adhesive. One drawback with this approach is that the planarizing surface of the polishing pad typically wears out during normal use and the polishing pad must therefore be replaced. It may be difficult and time consuming to remove the polishing pad and the high-strength adhesive from the platen, rendering the CMP machine inoperable for extended periods of time. 
     One conventional approach to addressing the foregoing problem is to manufacture a sheet of polishing pad material and stretch it across the platen from one side to the other. As the polishing pad wears, it is incrementally moved across the platen in the manner of a conveyor belt to present an unworn planarizing surface to the wafer. Such a device is manufactured by Obsidian, Inc. of Fremont, Calif. One problem with this approach is that the tension in the sheet may not be sufficient to keep it flat against the platen. Accordingly, the sheet may tend to wrinkle or fold upon itself under the pressure exerted by the wafer carrier and the wafer. 
     SUMMARY OF THE INVENTION 
     The present invention is directed toward a method and apparatus for releasably attaching a planarizing medium to a chemical-mechanical planarization machine. The apparatus can comprise a support and a platen having an engaging surface with one or more vacuum apertures sized and shaped to be coupled to a vacuum source. A planarizing medium can be tightly drawn against the engaging surface of the platen when the vacuum source applies a vacuum to the vacuum apertures. The planarizing medium can include a polishing pad having a generally non-porous surface that seals against the engaging surface of the platen. Alternatively, the planarizing medium can include a porous polishing pad adhesively attached to a pad support. The pad support may have a generally non-porous surface opposite the polishing pad that seals against the platen when the vacuum source is activated. In yet another alternative aspect of the invention, the polishing pad and the pad support can be supported, for example, in a support jig, to condition the polishing pad. In still another alternative aspect of the invention, a signal can be applied to the platen to attract the polishing pad toward the platen via electrostatic or electromagnetic forces. 
     The platen may be movable relative to the support and may include a lip to prevent the planarizing medium from separating from the platen if the vacuum source is deactivated while the platen is still in motion. The platen may also include a releasable stop to further engage the planarizing medium. Alternatively, the platen may be replaced by a base that is fixed relative to the support and the apparatus may further include a supply device and a take-up device that advance an elongated planarizing medium across the base. During planarization, the vacuum source draws the planarizing medium against the base. When the planarizing medium becomes worn (or for other reasons), the vacuum source or charge source may be deactivated and the planarizing medium may be advanced across the base to expose a different portion of the planarizing medium to the semiconductor substrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial cross-sectional elevation view of a chemical-mechanical planarization machine in accordance with the prior art. 
     FIG. 2 is a partial cross-sectional elevation view of an apparatus having a platen with vacuum apertures in accordance with an embodiment of the present invention. 
     FIG. 3 is a top plan view of the platen shown in FIG.  2 . 
     FIG. 4 is a top plan view of a platen having vacuum apertures in accordance with another embodiment of the invention. 
     FIG. 5A is a partial cross-sectional elevation view of a platen having a locking device in accordance with yet another embodiment of the invention. 
     FIG. 5B is a partial cross-sectional elevation view of a jig used to support a platen in accordance with another embodiment of the invention. 
     FIG. 6 is a partial cross-sectional elevation view of a platen having a locking device in accordance with still another embodiment of the invention. 
     FIG. 7A is a partial cross-sectional elevation view of a platen having a plate to attract the pad support disk in accordance with still another embodiment of the invention. 
     FIG. 7B is a partial cross-sectional elevation view of a platen having a plate to attract the polishing pad in accordance with yet another embodiment of the invention. 
     FIG. 8 is a partial cross-sectional elevation view of an apparatus having a supply device and a take-up device in accordance with still another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed toward methods and devices for attaching a polishing pad to a platen of a chemical-mechanical planarization machine. The device may include a vacuum system that releasably attaches the polishing pad to the platen such that the polishing pad may be easily removed and/or replaced, or may be incrementally advanced over the platen. Many specific details of certain embodiments of the invention are set forth in the following description and in FIGS. 2-7 to provide a thorough understanding of such embodiments. One skilled in the art, however, will understand that the present invention may have additional embodiments and that they may be practiced without several of the details described in the following description. 
     FIG. 2 illustrates a CMP apparatus  110  having a platen  120  and a planarizing medium  148 . In the embodiment shown in FIG. 2, the planarizing medium  148  includes polishing pad  140  releasably attached to the platen  120 , and in other embodiments, the planarizing medium  148  may include other components, as is discussed in greater detail below with reference to FIG.  5 . The platen  120  may be movable relative to a support structure  180  by means of a platen drive assembly  150  that may impart rotational motion (indicated by arrow A) and/or translational motion (indicated by arrow B) to the platen  120 . As was discussed above, the CMP apparatus  110  may also include a carrier assembly  160  having a resilient pad  164  that presses a semiconductor substrate  112  against a planarizing surface  142  of the polishing pad  140 . A carrier drive assembly  161  may be coupled to the carrier assembly  160  to move the carrier assembly axially (indicated by arrow C) and/or rotationally (indicated by arrow D) relative to the platen  120 . 
     The platen  120  has an upper surface  130  adjacent the polishing pad  140 . The upper surface  130  includes a plurality of vacuum apertures  122  that are in fluid communication with a vacuum passageway  123 . The vacuum passageway  123  is coupled to a vacuum source  170 , as will be discussed in greater detail below, such that when the vacuum source  170  is activated, it draws a vacuum through the vacuum apertures  122  and draws the polishing pad  140  tightly against the upper surface  130  of the platen  120 . 
     FIG. 3 is a top plan view of the platen  120  and the polishing pad  140  shown in FIG.  2 . Referring to FIGS. 2 and 3, the vacuum apertures  122  of the platen  120  may have a circular cross-sectional shape at the platen upper surface  130  and may have other shapes in other embodiments, as will be discussed below with reference to FIG.  4 . The platen  120  may have twelve vacuum apertures  122 , as shown in FIGS. 2 and 3, and may have a greater or lesser number of vacuum apertures  122  in other embodiments, so long as the force exerted by the vacuum source  170  (FIG. 2) through the vacuum apertures  122  is sufficient to secure the polishing pad  140  to the platen  120 . In one embodiment, the vacuum source  170  may generate a vacuum pressure of 10 lb/in 2  (6.9×10 4  N/m 2 ) below atmospheric pressure, measured at the vacuum apertures  122 . In other embodiments, the vacuum source  170  may generate other pressures sufficient to secure the polishing pad  140  to the platen  120 , depending on the characteristics of the polishing pad  140  and the size, shape, and number of the vacuum apertures  122 . 
     The vacuum apertures  122  extend downwardly through the platen upper surface  130  to the vacuum passageway  123  below. In the embodiment shown in FIGS. 2 and 3, the vacuum passageway  123  may have a plurality of radially extending arms  131  that meet near the center of the platen  120 . In other embodiments, the vacuum passageway  123  may have other configurations that provide fluid communication between the vacuum apertures  122  and the vacuum source  170 . 
     As shown in FIG. 2, each arm  131  of the vacuum passageway  123  may have a liquid trap  124  to separate liquid from the fluid stream that passes through the vacuum passageway  123  when the vacuum source  170  is activated. The fluid stream may include air or other gases adjacent the planarizing surface  142 , as well as liquids, such as a planarizing liquid  141 . In one embodiment, the liquid trap  124  may include a vertical bend in each arm  131  and a vertical collection tube  132  at the low point of each bend. Liquid drawn into the vacuum passageway  123  will tend to settle in the collection tubes  132  under the force of gravity. A valve  125  may be positioned at the base of each of the collection tubes  132  to periodically drain the liquid collected in the liquid trap  124 . 
     In other embodiments, other means may be used to separate liquid from the fluid drawn through the vacuum passageway  123 . For example, the liquid trap  124  may be separate from the platen  120 , as discussed in greater detail below with reference to FIG. 7, and/or the liquid trap may be integral with the vacuum source  170 . In another embodiment (not shown), where the angular velocity of the platen  120  is relatively high, the liquid trap may be positioned toward the outer edge of the platen  120  and may take advantage of centrifugal forces to separate liquid from the fluid stream passing through the vacuum passageway  123 . An advantage of the gravity-driven liquid trap  124  shown in FIG. 2 may be that it will continue to collect liquid when the platen  120  has stopped rotating. 
     A rotary drive  151  may be coupled to the platen  120  with a rotary drive shaft  153  to rotate the platen  120 , as indicated by arrow A. The rotary drive shaft  153  may include a central passage  155  that extends from the vacuum passageway  123  to a non-rotating conduit  128 . The conduit  128  is in turn coupled to the vacuum source  170 . A rotating seal  126  may be coupled between the conduit  128  and the rotating drive shaft  153  to provide a gas-tight seal between the conduit and the drive shaft and maintain vacuum pressures in the vacuum passage  123  when the platen  120  rotates relative to the vacuum source  170 . 
     The platen  120  may also be translated and/or oscillated by a linear drive  152  coupled to the platen with a linear drive shaft  154 . In one embodiment, the linear drive shaft  154  may include telescoping segments  154   a  and  154   b . In other embodiments, splines or other means may be used to transmit lateral motion from the fixed linear drive  152  to the platen  120 . The conduit  128  may include a bellows section  133  that expands and contracts as the platen  120  moves laterally relative to the vacuum source  170 . In other embodiments, other means may be used to couple the vacuum source  170  to the translating platen  120 . For example, in one such embodiment (not shown), the conduit  128  may be coiled in the manner of a telephone cord to account for relative lateral motion between the platen  120  and the vacuum source  170 . 
     The platen  120  may include a lip  121  that extends upwardly from the platen upper surface  130  to engage a side surface  146  of the polishing pad  140  and prevent the polishing pad from sliding off the platen  120  if the vacuum source  170  is deactivated while the platen  120  is in motion. The lip  121  may accordingly engage the entire side surface  146 , as shown in FIG. 2, or a portion of the side surface  146 . For example, the lip  121  may engage less than the fill height of the side surface  146 , or may extend around less than the entire periphery of the polishing pad  140 , so long as it engages enough of the side surface  146  to prevent the polishing pad  140  from sliding laterally off the platen  120 . In other embodiments, other means may be used to restrict motion of the polishing pad  140  relative to the platen  120 , as will be discussed in greater detail with reference to FIGS. 5 and 6. 
     In one embodiment, the polishing pad  140  may comprise a non-porous or nearly non-porous material that provides a gas-tight or nearly gas-tight seal with the platen upper surface  130  when a vacuum is drawn through the vacuum apertures  122 . For example, the polishing pad  140  may comprise polymers such as polyurethane, or may comprise glass or other non-porous materials. In another embodiment, the polishing pad  140  may comprise porous materials, as will be discussed in greater detail below with reference to FIG.  5 . 
     One advantage of the CMP apparatus  110  shown in FIGS. 2-3 is that the polishing pad  140  may be easily removed from the platen  120  when, for example, the polishing pad is replaced due to normal wear or for other reasons. To replace the polishing pad  140 , the vacuum source  170  is deactivated or otherwise decoupled from the platen  120 , the polishing pad  140  is lifted from the platen, and a new polishing pad is positioned in its place. The entire operation may be completed in a relatively short period of time. By contrast, it may take a substantially longer period of time to detach a conventional, adhesively bonded polishing pad from the platen  120 , remove any remaining adhesive from the platen, and adhesively bond a replacement polishing pad to the platen. 
     Another advantage of the CMP apparatus  110  shown in FIGS. 2-3 is that the vacuum source  170  may be deactivated when the polishing pad  140  is not in. use and may be subsequently reactivated without affecting the bonding force between the polishing pad  140  and the platen  120 . By contrast, the adhesives that may be used in conventional installations to bond the polishing pad  140  to the platen  120  may degrade over time, causing the bond between the polishing pad and the platen to fail. 
     FIG. 4 is a top plan view of a platen  220  having concentric, arcuate vacuum apertures  222 . Each vacuum aperture  222  is in fluid communication with the arms  231  of the vacuum passageway  223 , as was discussed above with reference to FIG.  2 . An advantage of the arcuate vacuum apertures  222  when compared with the vacuum apertures  122  shown in FIGS. 2-3 is that the arcuate vacuum apertures may have a greater tendency to prevent the polishing pad  140  from wrinkling in the radial direction. Conversely, an advantage of the platen  120  having the vacuum apertures  122  shown in FIGS. 2-3 is that it may be simpler and less expensive to manufacture. 
     FIG. 5A is a partial cross-sectional side elevation view of a platen  320  having a vacuum source  370  attached thereto. The vacuum source  370  is accordingly coupled to the vacuum passageway  323  without the need for intervening conduits and rotating and/or translating gas-tight seals. In the embodiment shown in FIG. 5A, a power supply  371  is attached to the platen  320  and coupled to the vacuum source  370  to provide power to the vacuum source. The power supply  371  may include a battery, a solar panel, or other known devices that may supply power to the vacuum source  370  during planarization without the need for external connections. In another embodiment (not shown), the power supply  371  may be positioned apart from the platen  320  and may be coupled to the vacuum source  370  with slip rings or other rotating electrical connections. 
     In one embodiment, the vacuum source  370  and the power supply  371  may be relatively light in weight to reduce the power required by the platen drive assembly  150  (FIG. 2) to translate and/or rotate the platen  320 . The platen  320  may also include a counterweight  372  positioned opposite the vacuum source  370  and the power supply  371  to balance the platen and reduce the likelihood that the platen will vibrate when it rotates. The counterweight  372  may comprise a simple dead weight or may comprise a functioning component of the platen  320 , as is discussed in greater detail below with reference to FIG.  6 . 
     An advantage of the vacuum source  370  and the power supply  371  shown in FIG. 5A is that they may eliminate the need for rotating and/or translating seals and electrical connections, as discussed above, and may accordingly simplify the construction and maintenance of the platen  320 . Conversely, an advantage of the stationary vacuum source  170  shown in FIG. 2 is that it may include an existing commercially available device that need not be balanced and/or selected for low weight. 
     As shown in FIG. 5A, the planarizing medium  348  may include a polishing pad  340  attached to a pad support disk  343 . The pad support disk  343  may have a generally non-porous attachment surface  347  that forms a gas-tight or nearly gas-tight seal with the platen upper surface  330 . In the embodiment shown in FIG. 5A, the polishing pad  340  is attached to the pad support disk  343  with an adhesive  344  positioned therebetween. In other embodiments, other means are used to attach the polishing pad  340  to the pad support disk  343 . Should it become necessary to replace the polishing pad  340 , the polishing pad and the pad support disk  343  may be removed as a unit and replaced with a new planarizing medium  348 . 
     In one embodiment, the entire planarizing medium  348  may be disposable. In another embodiment, the support disk  343  may be recycled by removing the old polishing pad  340  from the support disk and attaching a new polishing pad in its place. In either case, it may be advantageous to adhesively attach the polishing pad  340  to the pad support disk  343  rather than to adhesively attach the polishing pad to the platen  320  directly (as may be done conventionally) because the pad support disk  343  may be less costly than the platen. Accordingly, a large number of low-cost pad support disks  343  with polishing pads  340  attached may be kept on hand and available when needed. A further advantage is that the pad support disk  343  may be attached to a porous polishing pad  340 , so that even the porous polishing pad may be releasably attached to the platen  320  by applying a vacuum to the support disk  343 . 
     As shown in FIG. 5A, the platen  320  may include a locking device or stop  334  in addition to the lip  321 , to further resist relative lateral and/or vertical motion between the planarizing medium  348  and the platen  320 . In one embodiment, the stop  334  includes a female thread  329  in the lip  321  that engages a corresponding male thread  345  in the pad support disk  343 . In another embodiment, where the polishing pad  340  is sufficiently rigid, the male thread  345  may be positioned in the polishing pad  340 , rather than in the support disk  343 . Obviously, the positions of the male thread  345  and the female thread  329  may be interchanged without departing from the scope of the invention. In one aspect of the embodiment shown in FIG. 5A, the threads  345  and  329  loosely engage each other so as not to inhibit the action of the vacuum source  370  as it draws the pad assembly  348  against the platen  320 . In another embodiment, the threads  345  and  329  can more tightly engage each other to still further resist relative motion between the planarizing medium  348  and the platen  320 . In one aspect of this embodiment, the mechanical connection between the planarizing medium  348  and the platen  320  can be secure enough to eliminate the need for the vacuum source  370  and the vacuum passageway  323 . An advantage of the stop  334  shown in FIG. 5A is that it may further decrease the likelihood that the polishing pad  340  will separate from the platen  320 , either axially or laterally, if the vacuum source  370  is halted while the platen  320  is moving. 
     FIG. 5B is a partial cross-sectional elevation view of a support jig  350  for supporting the polishing pad  340  and the support disk  343  during conditioning of the polishing pad  340 . In one embodiment, the support jig  350  can include a vacuum passageway  323   a  coupled to a vacuum source  170  (FIG. 2) and/or a female thread  329   a  that engages the corresponding male thread  345  of the support disk  343 . When the support jig  350  includes the vacuum passageway  323   a  to draw the support disk  343  toward the support jig  350 , the support disk  343  can include a non-porous attachment surface  347 . When the support jig  350  includes the female thread  329   a  to engage the support disk  343 , the support disk  343  and male thread  345  can include a relatively rigid material, such as metal or hard plastic to engage the female thread  329   a . In other embodiments, the support jig  350  can include any means for firmly supporting the polishing pad  340  and the support disk  343 . For example, in one embodiment, the support jig  350  can include a planarizing machine, and in a specific aspect of this embodiment, a planarizing machine that is no longer suitable for planarization. 
     The support jig  350  can include a pad conditioner  360  for conditioning the polishing pad  340 . In one embodiment, the pad conditioner  360  can include an end effector  361  coupled to a drive device  362  that moves the end effector in one or more directions relative to the polishing pad  340 . In one aspect of this embodiment, the end effector  361  can have a diamond abrasive surface. Alternatively, the end effector  361  can include any surface or other means for removing material from the planarizing surface or otherwise conditioning the planarizing surface of the polishing pad  340 . 
     An advantage of the support jig  350  and the pad conditioner  360  shown in FIG. 5B is that they allow the pad  340  to be conditioned without requiring a planarization machine. Accordingly, the polishing pad  340  can be conditioned at the same time the planarization machine (with a different polishing pad installed) is used to planarize microelectronic substrates. For example, a new polishing pad  340  typically requires conditioning during an initial “break-in” period to remove extraneous materials that may have been deposited on the polishing pad  340  during manufacture or shipment. The support jig  350  allows the break-in period to be completed without impacting the throughput of planarization machines such as the one shown in FIG.  2 . 
     FIG. 6 is a partial cross-sectional side elevation view of a platen  420  having two stops  434  (shown as  434   a  and  434   b ) in accordance with another embodiment of the invention. Each stop  434  may have a handle  435  that projects from an aperture in the lip  421 , and a tab  436  toward the lower end of the handle  435 . The tab  436  is sized and shaped to be received in a corresponding tab aperture  449  in the polishing pad  440 . The stop  434  may be placed in an engaged position (as shown by the one stop  434   a ) by rotating the handle  435  until the tab  436  is within the corresponding tab aperture  449 . The tab  436  may fit loosely within the tab aperture  449  to permit the vacuum source  470  to draw the planarizing medium  448  toward the platen  420 , substantially as was discussed above with reference to FIG.  5 . The stop  434  may be placed in a disengaged position (as shown by the other stop  434   b ) by rotating the handle  435  until the tab  436  is disengaged from the corresponding tab aperture  449 , allowing the polishing pad  440  to be lifted from the platen  420 . 
     As is also shown in FIG. 6, the vacuum source  470  may be positioned opposite the power supply  471  to balance the platen  420  when the platen rotates. In other embodiments, the power supply  471  may be positioned at other circumferential locations relative to the vacuum source  470 , depending on the relative weights of the power supply and the vacuum source. In still other embodiments, other functional components of the platen  420  may be used in place of, or in addition to the power source  471  to balance the platen  420 . An advantage of this arrangement is that it eliminates the need for the counterweight  372  (FIG.  5 ). 
     FIG. 7A is a partial cross-sectional side elevation view of a platen  320   a  having a conductive plate  390  that draws the support disk  343  (with the polishing pad  340  attached) toward the platen upper surface  330  via electrostatic forces. As shown in FIG. 7A, the conductive plate  390  can be used in place of the vacuum systems discussed above with reference to FIGS. 2-6. In other embodiments, the conductive plate  390  can supplement a vacuum system such as one of the systems shown in FIGS. 2-6. 
     The conductive plate  390  can include any conductive material, such as aluminum or copper and can be charged by applying an electrical voltage to an electrode  391 , which is electrically coupled to the conductive plate  390 . The voltage on the conductive plate  390  can electrostatically attract the support disk  343 , causing the support disk  343  to attach to the platen  320   a . Any charge induced by the voltage can later be removed from the conductive plate  390  to detach the polishing pad  340 . 
     In the embodiment shown in FIG. 7A, the support disk  343  can include the locking device  334  to further resist lateral and/or vertical motion between the polishing pad  340  and the platen  320   a . In other embodiments, the locking device  334  can be eliminated. An advantage of the platen  320   a  shown in FIG. 7A is that it may be simpler to draw the polishing pad  340  and the support disk  343  toward the platen  320   a  with an electrostatic force than with other devices. 
     FIG. 7B is a partial cross-sectional view of a platen  320   b  with the conductive plate  390 , and a polishing pad  340   a  having particles  341  distributed therein. The particles  341  can include a conductive material or any material capable of receiving an attractive force from the conductive plate  390  in a manner generally similar to that discussed above with reference to FIG.  7 A. The particles  341  can also include a ferrous material so as to draw the polishing pad  340   a  toward the platen  320   b  via electromagnetic forces. Accordingly, the conductive plate  390  can include a pair of electrodes  391  for passing a current through the conductive plate  390 . The particles  341  can be distributed in a generally uniform fashion, as shown in FIG. 7B, or the particles  341  can be concentrated near the attachment surface  347  of the polishing pad  340   a  to increase the effect of the force between the polishing pad  340   a  and the platen  320   a.    
     FIG. 8 is a partial cross-sectional side elevation view of a CMP apparatus  510  having a plananzing medium  548  that translates relative to a fixed platen or base  520 . The base  520  is supported by a support table  514  and generally includes a substantially incompressible material to provide a flat, solid surface to which the planarizing medium  548  may be secured during planarization. The CMP apparatus  510  further includes a positioning device  590  that draws the planarizing medium  548  over the base  520 . In the embodiment shown in FIG. 8, the positioning device  590  includes a supply roller  591 , first and second idler rollers  592   a  and  592   b , first and second guide rollers  594   a  and  594   b , and a take-up roller  593 . The supply roller  591  carries an unused part of the planarizing medium  548 , and the take-up roller  593  carries a used part of the planarizing medium  548 . The supply roller  591  and/or the take-up roller  593  may be driven to sequentially advance unused portions of the planarizing medium  548  onto the base  520 . As such, unused portions of the planarizing medium  548  may be quickly substituted for worn or used portions to provide a consistent surface for planarizing the substrate  112 . In one embodiment, the first idler roller  592   a  and the first guide roller  594   a  position the planarizing medium  548  slightly below the base  520  so that the supply and take-up rollers  591  and  593  stretch the planarizing medium  548  across the base during planarization. In other embodiments, the planarizing medium  548  need not be stretched, as is discussed in greater detail below. 
     The base  520  includes a plurality of vacuum apertures  522  in fluid communication with a vacuum passageway  523 . The vacuum apertures  522  may have a circular cross-sectional shape, as shown in FIG. 8, or may comprise slots or have other shapes in other embodiments. The vacuum passageway  523  is connected to a conduit  528  that is in turn coupled to the vacuum source  570 , generally as was discussed above with reference to FIG.  2 . In the embodiment shown in FIG. 8, a liquid trap  524  may be positioned in the conduit  528  and apart from the base  520  to separate liquid from the fluid drawn by the vacuum source  570 . In another embodiment, the liquid trap  524  may form an integral component of the vacuum source  570 . 
     In operation, the planarizing medium  548  is rolled up on the supply roller  591  and one end is stretched over the base  520  and attached to the take-up roller  593 . The vacuum source  570  is activated to draw the planarizing medium  548  tightly against the base  520 . A carrier assembly  560  is moved relative to the planarizing medium  548  to planarize the semiconductor substrate  112 . Periodically, either during the planarization of a single semiconductor substrate  112 , or after a semiconductor substrate has been planarized, the carrier assembly  560  may be halted, the vacuum source  570  deactivated, and the planarizing medium advanced slightly over the base  520  by rotating the take-up roller  593  and the supply roller  591 . Once the planarizing medium  548  has been advanced by a selected amount, the vacuum source  570  may be reactivated, and planarizing may recommence. 
     In an alternative embodiment (not shown), the vacuum source  570  can be replaced with a voltage source to attract the planarizing medium toward the base  520  via electrostatic forces, in a manner generally similar to that discussed above with reference to FIGS. 7A-7B. In still a further alternative embodiment, the base  520  can include a permanent magnet or an electromagnet, as was discussed above with reference to FIG.  7 B. It may be preferable to include an electromagnet rather than a permanent magnet to allow the magnet to be deactivated for advancing the planarizing medium  548  across the base  520 . In either alternative embodiment, the planarizing medium  548  can include a conductive layer adjacent the base  520  in a manner generally similar to that shown in FIG.  7 A. Alternatively, the planarizing medium  548  can include particles capable of receiving an induced electrostatic or electromagnetic force in a manner generally similar to that shown in FIG.  7 B. 
     An advantage of the CMP apparatus  510  shown in FIG. 7 is that the suction force, electrostatic force or electromagnetic force may more securely engage the planarizing medium  548  with the platen  520  and may accordingly prevent the planarizing medium from wrinkling or folding when the semiconductor substrate  112  is planarized. A further advantage of the CMP apparatus  510  shown in FIG. 7 is that the planarizing medium  548  may be releasably attached to the platen  520  without the need for tensioning the planarizing medium. Accordingly, the planarizing medium  548  may be less likely to stretch or otherwise deform. Alternatively, the planarizing medium  548  may comprise a thinner, less costly sheet than is conventionally used because it does not need to withstand high tension forces. 
     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, 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.