Patent Publication Number: US-6659849-B1

Title: Platen with debris control for chemical mechanical planarization

Description:
CROSS REFERENCE TO OTHER RELATED APPLICATIONS 
     This application is related to U.S. patent application Ser. No. 09/651,657, filed Aug. 29, 2000, which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE DISCLOSURE 
     1. Field of Invention 
     Embodiments of the present invention relate generally to a system and a method for controlling debris under a web in a polishing system. 
     2. Background of Invention 
     In semiconductor wafer processing, the use of chemical mechanical planarization, or CMP, has gained favor due to the enhanced ability to increase device density on a semiconductor workpiece, or substrate, such as a wafer. As the demand for planarization of layers formed on wafers in semiconductor fabrication increases, the requirement for greater system (i.e., process tool) throughput with less wafer damage and enhanced wafer planarization has also increased. 
     An exemplary CMP system that addresses these issues is described in U.S. Pat. No. 5,804,507, issued Apr. 15, 1998 by Tolles et al., which is incorporated by reference in its entirety. Tolles et al. discloses a CMP system having a planarization system that is supplied wafers from cassettes located in an adjacent liquid filled bath. A transfer mechanism, or robot, facilitates the transfer of the wafers from the bath to a transfer station. The transfer station generally contains a load cup that positions wafers into one of four processing heads mounted to a carousel. The carousel moves each processing head sequentially over the load cup to receive a wafer. As the processing heads are loaded, the carousel moves the processing heads and wafers through the planarization stations for polishing. The wafers are planarized by moving the wafers relative to a polishing material in the presence of polishing fluid. The polishing fluid typically contains chemicals that aid in the removal of material from the wafer. The mechanical aspect of the polishing process is generally provided by abrasives disposed either in the polishing fluid (i.e., slurry) or disposed on the polishing material. After completion of the planarization process, the wafer is returned back through the transfer station to the proper cassette located in the bath. 
     One type of polishing material that may be utilized for chemical mechanical polishing is known as a fixed abrasive material. The fixed abrasive material comprises a plurality of abrasive particles suspended in a resin binder that is disposed in discrete elements on a backing sheet. As the abrasive particles are contained in the polishing material itself, systems utilizing fixed abrasive material generally use polishing fluid that do not contain abrasives. 
     Fixed abrasive polishing material is generally available in stick-down form but is often utilized in the form of a web. Generally, the web is supported by a platen having a central or working area where the polishing process is performed. The used and unused portions of the web that are not disposed on the platen are stored in a supply roll and take-up roll coupled to the side of the platen. As the web is consumed over the course of polishing a number of wafers, the web is advanced to place an incremental length of unused web in the working area of the platen. The used portion of the web leaving the platen is generally wound on a take-up roll that is disposed on the side of the platen opposite the supply roll. 
     Depending on the material to be polished, the unused portion of the web can be conditioned before entering the working area. Conditioning exposes the abrasive particles that are disposed in the abrasive elements that comprise the web. Conditioning is essential for scratch defect control when polishing soft materials. The web is typically conditioned by removing a layer of resin disposed at the surface of the abrasive elements to expose some of the abrasive particles disposed therein, and more importantly, to remove pad asperities and flatten the top of the individual abrasive elements while establishing a uniform height between the abrasive elements across the pad. 
     Both debris created during polishing, conditioning and contamination generated by other sources must be controlled to ensure good polishing results. For example, debris such as particulates from the conditioning or polishing process may become disposed on the backside of the web that is exposed between the supply roll and the top of the platen. Particles on the backside of the web are transported by the web as it advances. Some of these particles eventually become disposed between the web and the subpad. Since fixed abrasive webs are typically thin and flexible, particulate under the web may cause a corresponding “high-spot” on the surface of the web. As light pressures used to hold the substrate against the web during polishing and the web and substrate have a low surface contact ratio, a high-spot on the web may create a large local contact force between the substrate and web as the substrate passes over the high-spot. 
     For example, on a system utilizing 2 psi of pressure to hold a patterned substrate against a web having an 18 percent contact ratio, the local pressure may be as high as 24 psi which is typically not great enough to cause a scratch. A particle under the web in such a system creates a high-spot that results in a force concentration that can greatly exceed 24 psi at the high-spot. This local force concentration results in scratching or other defects on the substrate&#39;s surface. In some cases, one or more of the fixed abrasive elements over-lying the particle may be fragmented or sheared from the web due to the force concentration above the particle. Due to the periodic web advancement, a single particle on the web can cause the abrasive elements to be fragmented and/or sheared from multiple locations which forms a pattern of defects on the web. All the sheared and/or fragmented abrasive elements can create additional scratches on the substrate. Therefore, the effect of a single particle on the web can greatly amplify scratching issues during polishing. Additionally, global planarization of the substrate may be compromised due to a high rate of material removal caused by both the high-spot and the portions of the web damaged by the force concentration thereon when that portion of the web was positioned above the particle. 
     Therefore, there is a need for a system that controls debris under a web in a polishing system. 
     SUMMARY OF INVENTION 
     One aspect of the invention generally provides an apparatus for cleaning a backside of a web of polishing material. In one embodiment, the apparatus includes a platen having a support surface adapted to support the backside of the web and a web cleaner disposed on the platen adjacent the backside of the web. In another embodiment, an apparatus comprises one or more webs of polishing material, one or more polishing heads, one or more platens and a web cleaner. Each polishing head is adapted to retain the substrate against a respective web. Each platen has a support surface that supports the web from the web&#39;s backside. The web cleaner is coupled to the platen and disposed against the backside of the web. 
     In another aspect of the invention, method for cleaning a web of polishing material is provided. In one embodiment, the method includes the steps of supporting a portion of the web of polishing media on a platen, advancing a portion of the web on to the platen, and cleaning a portion of a backside of the web. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a plan view of a chemical mechanical planarization system of the invention; 
     FIG. 2 is a sectional view of a polishing station taken along section line  2 — 2  of FIG. 1; 
     FIG. 3 is a sectional view of one embodiment of a cleaner; 
     FIG. 4 is a sectional view of another embodiment of a cleaner; 
     FIG. 5 is a sectional view of another embodiment of a cleaner; 
     FIG. 6 is a sectional view of another embodiment of a cleaner; 
     FIG. 7 is a sectional view of another embodiment of a cleaner; and 
     FIG. 8 is a sectional view of another embodiment of a cleaner. 
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
    
    
     DETAILED DESCRIPTION OF INVENTION 
     FIG. 1 depicts a plan view of one embodiment of a chemical mechanical polisher  100  having one or more web cleaners  108 . One polisher  100  that can be used to advantage with the present invention is a REFLEXIONS™ Chemical Mechanical Polisher, manufactured by Applied Materials, Inc., located in Santa Clara, Calif. Although the web cleaners  108  are described on one configuration of a chemical mechanical polisher, one skilled in the art may adapt web cleaners  108  as taught and described herein to be employed on other chemical mechanical polishers that utilize webs of polishing material. 
     An exemplary polisher  100  in which the invention may be used to advantage is generally described in U.S. patent application Ser. No. 09/244,456, filed Feb. 4, 1999 to Birang et al., which is incorporated by reference in its entirety. The polisher  100  generally comprises a loading robot  104 , a transfer station  136 , a plurality of polishing stations  132 , a base  140  and a carousel  134  that supports a plurality of polishing head assemblies  152 . Generally, the loading robot  104  is disposed proximate the polisher  100  and a factory interface (not shown) to facilitate the transfer of substrates  122  therebetween. 
     The transfer station  136  generally includes a transfer robot  146 , an input buffer  142 , an output buffer  144  and a load cup assembly  148 . The input buffer station  142  receives a substrate  122  from the loading robot  104 . The transfer robot  146  moves the substrate  122  from the input buffer station  142  and to the load cup assembly  148  where it may be transferred between the polishing head assembly  152 . An example of a transfer station that may be used to advantage is described by Tobin in U.S. patent application Ser. No. 09/314,771, filed Oct. 6, 1999, which is hereby incorporated by reference. 
     Generally, the carousel  134  has a plurality of arms  150  that each support one of the polishing head assemblies  152 . Two of the arms  150  depicted in FIG. 1 are shown in phantom such that a polishing surface  131  and the cleaner  108  of one of the polishing stations  132  and the transfer station  136  may be seen. The carousel  134  is indexable such that the polishing head assemblies  152  may be moved between the polishing stations  132  and the load cup assembly  148 . Generally, a chemical mechanical polishing process is performed at each polishing station  132 . 
     Generally, a conditioning device  182  is disposed on the base  140  adjacent each polishing station  132 . The conditioning device  182  periodically conditions the polishing surface  131  to maintain uniform polishing results. 
     FIG. 2 depicts a sectional view of the polishing head assembly  152  supported above the polishing station  132 . 
     The polishing head assembly  152  is generally coupled to the carousel  134  by a drive system  202 . The drive system  202  generally provides motion to the polishing head  204  during processing. 
     In one embodiment, the polishing head  204  is a TITAN HEAD™ wafer carrier manufactured by Applied Materials, Inc., Santa Clara, Calif. Generally, the polishing head  204  comprises a housing  214  which is disposed a bladder  220 . The bladder  220  may be controllably inflated or deflated. The bladder  220 , when in contact with the substrate  122 , retains the substrate  122  within the polishing head  204  by deflating, thus creating a vacuum between the substrate  122  and the bladder  220 . A retaining ring  224  circumscribes the polishing head  204  to retain the substrate  122  within the polishing head  204  while polishing. 
     Disposed between the polishing head assembly  154  and the polishing station  132  is a web of polishing material  252 . The web of polishing material  252  may have a smooth surface, a textured surface, a surface containing a fixed abrasive or a combination thereof. The web of polishing material  252  may be advanced across or releasably fixed to the polishing surface. Typically, the web of polishing material  252  is releasably fixed by adhesives, vacuum, mechanical clamps or by other holding methods to the polishing station  132 . 
     The web of polishing material  252  generally has a polishing side  256  and a backside  258 . The polishing side  256  may include fixed abrasives. Fixed abrasive typically comprises a plurality of abrasive particles suspending in a resin binder that is disposed in discrete elements on a backing sheet. Examples of such fixed abrasive pads are available from Minnesota Manufacturing and Mining Company, of Saint Paul, Minn. The web of polishing material  252  may optionally comprise conventional polishing material without fixed abrasives, for example, polyurethane foam available from Rodel Inc., of Newark, Del. 
     The polishing station  132  generally comprises a platen  230  that is rotatably disposed on the base  140 . The platen  230  is typically comprised of aluminum. The platen  230  may be polygonal (i.e., rectangular) or circular. The platen  230  is supported above the base  140  by a bearing  238  so that the platen  230  may rotate in relation to the base  140 . An area of the base  140  circumscribed by the bearing  238  is open and provides a conduit for the electrical, mechanical, pneumatic, control signals and connections communicating with the platen  230 . 
     Conventional bearings, rotary unions and slip rings (not shown) are provided such that electrical, mechanical, pneumatic, control signals and connections may be coupled between the base  140  and the rotating platen  230 . The platen  230  is typically coupled to a motor  232  that provides the rotational motion to the platen  230 . 
     The platen  230  has an upper portion  236  that supports the web of polishing material  252 . A top surface  260  of the platen  230  contains a center recess  276  extending into the top portion  236 . The top portion  236  may optionally include a plurality of passages  244  disposed adjacent to the recess  276 . The passages  244  are coupled to a fluid source (not shown). Fluid flowing through the passages  244  may be used to control the temperature of the platen  230  and the polishing material  252  disposed thereon. 
     A subpad  278  and a subplate  280  are disposed in the center recess  276 . The subpad  278  is typically a plastic, such as polycarbonate or foamed polyurethane. Generally, the hardness or durometer of the subpad may be chosen to produce a particular polishing result. The subpad  278  generally maintains the polishing material  252  parallel to the plane of the substrate  122  held in the polishing head  204  and promotes global planarization of the substrate  122 . The subplate  280  is positioned between the subpad  278  and the bottom of the recess  276  such that the upper surface of the subpad  278  is coplanar with the top surface  260  of the platen  230 . 
     A vacuum port  284  is provided in the recess  276  and is coupled to an external pump  282 . When a vacuum is drawn through the vacuum port  284 , the air removed between the polishing material  252  and the subpad  278  causes the polishing material  252  to be firmly secured to the subpad  278  during polishing. An example of such a polishing material retention system is disclosed in U.S. patent application Ser. No. 09/258,036, filed Feb. 25, 1999, by Sommer et al., which is hereby incorporated by reference in its entirety. The reader should note that other types of devices may be utilized to releasably fix the polishing material  252  to the platen  230 , for example releasable adhesives, bonding, electrostatic chucks, mechanical clamps and other releasable retention mechanisms. 
     Optionally, to assist in releasing the polishing material  252  from the subpad  278  and platen  230  prior to advancing the polishing material  252 , surface tension caused by fluid that may be disposed between the subpad  278  and.the polishing material  252  is overcome by a blast of fluid (e.g., air) provided through the vacuum port  284  or other port (not shown) into the recess  276  by the pump  282  (or other pump). The fluid pressure within the recess  276  moves through apertures (not shown) disposed in the subpad  278  and subplate  280  and lifts the polishing material  252  from the subpad  278  and the top surface  260  of the platen  230 . The polishing material  252  rides upon the cushion of fluid such that it may be freely indexed across the platen  230 . Alternatively, the subpad  278  may be a porous material that permits gas (e.g., air) to permeate therethrough and lift the polishing material  252  from the platen  230 . Such a method for releasing the web  252  is described in U.S. patent application Ser. No. 60/157,303, filed Oct. 1, 1999, by Butterfield, et al., and is hereby incorporated by reference in its entirety. 
     Mounted to a first side  211  of the platen  230  is a supply roll  240 . The supply roll  240  generally contains a portion of the web of polishing media  252  wound thereon. The web of polishing media  252  is fed over a first member  242  and across the top surface  260  of the platen  230 . The web of polishing media  252  is fed over a second member  246  and a take-up roll  248  disposed on a second side  215  of the platen  230 . The members  242 , 246  may be a roller, a rod, a bar or other member configured to allow the web  252  to move thereover with minimal damage to the web, particulate generation or contamination of the web. 
     The supply roll  240  is removably coupled to the platen  230  to facilitate loading another unwind roll containing unused polishing media once the web of polishing media  252  is consumed over the course of polishing a number of substrates. The supply roll  240  is coupled to a slip clutch  250  or similar device that prevents the web of polishing media  252  from inadvertently unwinding from the supply roll  240 . The supply roll  240  is covered by a housing  254  that protects the supply roll  240  from damage and contamination. 
     The take-up roll  248  generally is removably coupled to the platen  230  to facilitate removal of used polishing media that is wound thereon. The take-up roll  248  is coupled to a tensioning device  262  that keeps the web of polishing media  252  keeps taunt between the supply roll  240  and take-up roll  248 . A housing  264  is disposed over the take-up roll  248  protects the take-up roll  248  from damage and contamination. 
     The web of polishing media  252  is advanced between the supply roll  240  and take-up roll  248  by an indexer  266 . In one embodiment, the indexer  266  comprises a drive roller  268  and an idler  270  that pinches the web of polishing media  252  therebetween. The drive roller  268  generally is coupled to the platen  230 . The drive roller  268  is connected to a control motor such as a stepper motor paired with an encoder (motor and encoder not shown). The indexer  266  enables a predetermined length of polishing web to be pulled off the supply roll  240  by drive roller  268  as the drive roller  268  is controllably rotated. A corresponding length of polishing web is wound on the take-up roll  248  as the web of polishing media  252  is advanced across the platen  230 . 
     The cleaner  108  is generally coupled to the platen  230  proximate the first member  242 . The backside  258  of the web  252  of polishing material is disposed over and typically contacts the cleaner  108  as the web  252  passes from the first member  242  to the top surface  260  of the platen  230 . The cleaner  108  generally removes particulates that may be present on the backside  258  of the web  252  before that section of the web is advanced over the subpad  278 . 
     In one embodiment, the cleaner  108  includes a roller that is coupled to the platen  230  by a bracket  212 . The cleaner  108  has a center shaft  208  that permits the cleaner  108  to rotate in the bracket  212 . The cleaner  108  may be covered by a tacky substance or adhesive. The adhesive disposed on the cleaner  108  to trap particles should remain sticky in wet conditions found on the polisher  100 . The adhesive should also be selective as not impede web movement by capturing the web  252  to the cleaner  108 . 
     In another embodiment, the cleaner  108  comprises a roller having a covering of double sided tape. As the advancing web  252  causes the cleaner  108  to rotate, particulates on the backside  258  the web  252  are adhered to the tape disposed on the cleaner  108 . Once the entire web  252  has been used, the used cleaner  108  is replaced with a “fresh” or particle-free cleaner during the change out of the web  252 . 
     Alternatively, the cleaner  108  can be a plate or roller that is electrostatically charged. The electrostatic charge of the cleaner  108  attracts particulates disposed on the backside  258  of the web  252  as the web passes thereover. 
     FIG. 3 depicts another embodiment of a cleaner  300 . The cleaner  300  comprises a brush  302  having bristles  306  extending therefrom. The brush  302  may be cylindrical (as shown) or flat. The brush  302  may be stationary or may be coupled to a device that provides movement to the brush. The brush  302  is generally coupled to the platen  230  by a bracket  304 . The brush  302  is positioned such that bristles  306  extending from the brush  302  come in contact with the backside  258  of the web  252 . Thus, as the web  252  is advanced across the cleaner  300 , the bristles  306  remove the particles that may be present on the backside  258 . 
     In one embodiment, the cleaner  300  is a cylindrical rush  302  that is rotated by a motor  308 . The motor  308 , for example an electrical or air motor, is generally disposed on the platen  230 . The motor  308  is coupled to the brush  302  by a belt or flexible shaft  310 . Other methods and devices may alternatively be employed to provided rotation to the cleaner  300 , for example, solid shafts, timing belts, v-belts, gear assemblies, direct drives, pneumatic motors, stepper motors, pneumatic cylinders and other motion providing devices. 
     The motor  308  generally provides rotation to the brush  302  that is tangentially opposed to the direction of advancement of the web  252 . Optionally, the brush  302  may be positioned such that the bristles  306  contact the roller  242  to remove particulates or other contamination that may be present on the roller  242 . 
     FIG. 4 depicts another embodiment of a cleaner  400 . The cleaner  400  generally comprises a vacuum block  402 . The vacuum block  402  is typically coupled to the platen  230  by a bracket  408 . The vacuum block  402  generally includes a slot or recess  404  that has an opening  406  that faces the backside  258  of the web  252 . The vacuum block includes a port  410  that communicates with the recess  404 . The port  410  is coupled to a vacuum pump  414  by a vacuum line  412 . 
     The vacuum pump  414  applies a vacuum in the recess  404  when the web  252  is advanced. The vacuum pulls particles that may be disposed on the backside  258  of the web  252  through the opening  406 , into the slot  404  and out the vacuum line  412 . In this manner, particulates are substantially prevented from being disposed between the web  252  and the subpad  278 . 
     FIG. 5 depicts another embodiment of a cleaner  500 . The cleaner  500  is generally a fluid delivery tube  504  having at least one aperture  502  disposed therein. The tube  504  is typically disposed across the platen  230  under the web  252 . The fluid delivery tube  504  includes a port  508  that is coupled to a fluid source  512  via a supply line  510 . The fluid delivery tube  504  is coupled to the platen  230  by a bracket  514 . The bracket  514  positions the fluid delivery tube  504  so that the aperture  502  directs a spray  506  of fluid to the backside  258  of the web  252 . Alternatively, the delivery tube  504  may be mounted proximate the edge of the web  252  or platen  230  in an orientation that projects the spray  506  laterally across the backside  258  of the web  252 . The spray  506  removes particulate that may be disposed on the backside  258  of the web  252 . The fluid comprising the spray  506  may include deionized water, nitrogen, ionized air or other fluids. Alternatively, the tube  504  may be positioned elsewhere (i.e., out from under the web  252 ) as long as the one or more apertures  502  directs the spray  506  across the complete width of the backside  258  of the web  252 . 
     In one embodiment, the spray  506  comprises ionized air. The spray  506  of ionized air may be reversed in polarity such that at one time the spray  506  substantially comprises predominantly positive ions while at another instance, the spray  506  comprises predominantly negative ions. The application of alternating ion charges may be repeated in a cyclical manner during the advancement of the web  252 . Alternating ion charges within the spray  506  enhances the removal of particles from the backside  258  of the web  252  as the static attraction of both negatively and positively charged particles to the web are neutralized by the spray  506 . An ion generator that may be adapted to provide the spray  506  is available from Ion System, Inc. of Berkeley, Calif. 
     FIG. 6 depicts another embodiment of a cleaner  600  that is disposed on a lift assembly  602 . The illustrated cleaner  600  is substantially similar to the cleaner  108  described with reference to FIG.  2 . Alternatively, other configurations of cleaners  600  may be utilized, for example, configurations including brushes, vacuums and fluid jets. 
     The lift assembly  602  generally includes the cleaner  600  that may be actuated into an extended position. In the extended position, the cleaner  600  lifts the web  252  of polishing material into a spaced-apart relation with the top surface  260  of the platen  230  (and subpad  278 ) defining a gap  604 . In the extended position, the web  252  is separated from the top  260  of the platen  230  during advancement of the web  252 . The cleaner  600  disposed on the lift assembly  602  removes particulates from the backside  258  of the web  252  as the web is advanced. Additionally, as the web  252  is suspended above the platen  230  during advancement, the probability of the web  252  being re-contaminated by coming in contact with the top of the platen  230  is reduced. 
     The lift assembly  602  generally includes the cleaner  600  and an actuator  606  for moving the cleaner  600  between the extended position shown in FIG. 6 and a retracted position (not shown) . The actuator  606  may include gas pots, springs, linear actuators, pneumatic cylinders, hydraulic cylinders, ball screws, solenoids, and other motion control devices. 
     In one embodiment, the lift assembly  602  includes a rod  608  and bearing block  610 . The rod  608  is slidably disposed in the bearing block  610  that is coupled to the platen  230  via a plurality of mounting screws  612 . The block  610  may have solid, roller or ball bearings such as a pillow block. 
     The rod  608  has a first end  614  and a second end  616 . The first end  614  of the rod  608  is coupled to the cleaner  600 . A bearing  620  may be disposed between the rod  608  and cleaner  600  to enhance the rotation of the cleaner  600  when in the extended position. 
     In one embodiment, the cleaner  600  is a roller having double sided tape disposed thereon. Alternatively, the cleaner  600  may be a brush, a fluid delivery tube, a vacuum, or an electrostatic member. 
     A spring  618  or other energy storage or motion device provides the force to extend the rod  608 . Such devices include pneumatic cylinders, solenoids, hydraulic cylinders, compression springs, belleville washers, elastomers and the like. In one embodiment, the spring  618  comprises a coil spring. The spring  618  is selected to provide the force and travel needed to move the web  252  into the spaced-apart relation with the platen  230  when the slip-clutch  250  (and indexer  266 ) feeds out a length of web  252  and to maintain the gap  604  while the web is advanced across the platen  230 . 
     FIG. 7 depicts another embodiment of a platen  230  having a cleaner  700 . Generally, the platen  230  includes a first lift assembly  704  and a second lift assembly  702 . The illustrative first lift assembly  704  is substantially similar to the lift assembly  602  described with reference to FIG. 6 except that the cleaner  600  is replaced with a roller  242  that guides a web  252  across the platen  230 . The first lift assembly  704  may be actuated to place the web  252  disposed thereon in a spaced-apart relation to the platen  230 . The second lift assembly  702  may be actuated to place the cleaner  700  in contact with the web  252  to facilitate cleaning of the backside of the web  252 . 
     More specifically, the second lift assembly  702  is disposed adjacent to the first lift assembly  704  on one side of the platen  230 . The second lift assembly  702  is typically coupled between the first lift assembly  704  and the platen  230 . When the first lift assembly  702  is actuated to place the roller  242  in the extended position, a gap is defined between the web  252  and platen  230 . The gap assists in facilitating movement of the web  252  over the platen  230 . 
     The second lift assembly  702  is generally extended and retracted in concert with the first lift assembly  704 . When the web  252  is lifted above the platen  230  by the first lift assembly  704 , the second lift assembly  702  places the cleaner  700  in contact with the backside  258  of the web  252 . Alternatively, the second lift assembly  702  may be actuated independently to have the cleaner  700  contact the web  252  when desired. 
     In one embodiment, the cleaner  700  is substantially similar to the cleaner  300  described with reference to FIG.  3 . Alternatively, other configurations of cleaners  700  may be utilized, for example, configurations including tacky rollers, electrostatic rollers, vacuums and fluid jets. The cleaner  700  generally includes a brush  708  that is coupled to an actuator  710  (i.e., a pneumatic cylinder) of the second lift assembly  702 . The brush  708  generally contacts the backside  258  of the web  252  when the second lift assembly  702  is in the extended position. Optionally, the brush may contact the roller  242  when the first and second lift assemblies  704 ,  702  are in various positions (i.e., both extended, both retracted or one retracted and one extended). 
     The brush  708  is generally driven in a direction tangentially opposed to the direction of web advancement. The brush  708  is typically coupled to a motor  712  by a belt or flexible shaft  714 . The brush  708  is rotated to remove particulate that may be present on the backside of the web  252 . 
     FIG. 8 depicts another embodiment of a cleaner  800 . Generally, a lift assembly  802  that includes a roller  804  is utilized to place the web  252  in a spaced-apart relation to the platen  230  when the web  252  is advanced. The cleaner  800 , illustrated as a vacuum but may comprises other types of cleaners as described herein, is coupled to the lift assembly  802  such that the cleaner  800  travels together with the roller  804  and maintains contact with the backside  258  of the web  252  without need for a second lift assembly. 
     As the cleaners described herein removes contaminants from the backside  258  of the web  252  and optionally the surface of the roller  242 , the amount of particulate contamination under on the backside  258  of the web  252  is minimized. The reduction of particulates between the backside  258  and the subpad  278  enhance the polishing performance, extend the life of the web  252  and contribute towards minimizing substrate defects. 
     Although the teachings of the present invention that have been shown and described in detail herein, those skilled in the art can readily devise other varied embodiments that still incorporate the teachings and do not depart from the scope and spirit of the invention.