Patent Publication Number: US-2007096315-A1

Title: Ball contact cover for copper loss reduction and spike reduction

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims benefit of U.S. provisional patent application Ser. No. 60/732,447 (APPM/010698L), filed Nov. 1, 2005, which is herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      Embodiments of the present invention generally relate to a method and apparatus for electrochemical mechanical processing, and more specifically, to a contact cover assembly and method for copper loss reduction and voltage spike reduction during an electrochemical mechanical process.  
      2. Description of the Related Art  
      Electrochemical mechanical planarizing (Ecmp) is a technique used to remove conductive materials from a substrate surface by electrochemical dissolution while concurrently polishing the substrate with reduced mechanical abrasion compared to conventional planarization processes. Ecmp systems may generally be adapted for deposition of conductive material on the substrate by reversing the polarity of the bias. Electrochemical dissolution is performed by applying a bias between a cathode and a substrate surface to remove conductive material from the substrate surface into a surrounding electrolyte. Typically, the bias is applied to the substrate surface by a conductive surface part of or passing through a polishing material on which the substrate is processed. A mechanical component of the polishing process is performed by providing relative motion between the substrate and the polishing material that enhances the removal of the conductive material from the substrate.  
      During Ecmp processing, the conductive material on the substrate surface is electrically biased by one or more contact elements. A thin passivation layer builds up on the contact elements during Ecmp processing. This passivation layer leads to a slow but steady increase in polishing time. If the passivation layer becomes too thick, voltage spikes leading to hollow metal defects at the edge of the wafer may occur. Rinsing the contact elements with deionized water is one way to eliminate the passivation layer and improve electrical conduction. However, elimination of the passivation layer exposes the contact elements to oxidation, corrosion, and attack by processing chemistries, thereby resulting in faster wear of the contact elements and diminished electrical conduction to substrates over a period of processing cycles. This metal wear results in voltage spikes leading to hollow metal defects at the edge of the wafer. Moreover, sludge and/or other deposits may accumulate around the electrical contact, further obstructing the maintenance of good electrical biasing of the substrate through the contact element. Good electrical connections for biasing the substrate must be preserved in order to maintain robust process performance.  
      Thus, there is a need for an improved method and apparatus for electrochemical processing which maintains robust process performance.  
     SUMMARY OF THE INVENTION  
      Embodiments of the invention generally provide a method and apparatus for processing a substrate in an electrochemical mechanical planarizing system. In one embodiment, a contact assembly for electrochemically processing a substrate is provided. The contact assembly includes a housing having at least one passage formed therethrough, a conductive ball having a processing position partially extending beyond a first end of the housing is disposed in the passage, and a retaining feature comprising a conductive material, wherein the retaining ring prevents the ball from exiting the first end of the housing.  
      In another embodiment a pad assembly for processing a substrate is provided. The pad assembly includes an upper layer having a dielectric working surface and a lower surface, the dielectric working surface adapted to contact the substrate and having at least one aperture formed through the center of the upper layer, a conductive material coupled to the lower surface of the upper layer, and a contact assembly disposed through the at least one aperture to contact the substrate when the substrate is disposed on the working surface. The contact assembly comprises a housing having at least one passage formed therethrough, a conductive contact element disposed in the passage and having a processing position partially extending beyond a first end of the housing, and a retaining feature comprising a conductive material, wherein the retaining feature prevents the contact element from exiting the first end of the housing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      So that the manner in which the above recited embodiments of the invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
       FIG. 1  is a plan view of an electrochemical mechanical processing system;  
       FIG. 2  is a sectional view of one embodiment of a bulk electrochemical mechanical processing (Ecmp) station of the system of  FIG. 1 ;  
       FIG. 3  is a partial sectional view of one embodiment of a platen assembly of the bulk Ecmp station of  FIG. 2 ;  
       FIG. 4A  is a partial sectional view of the bulk Ecmp station through two contact assemblies;  
      FIGS.  4 B-C are sectional views of plugs;  
      FIGS.  5 A-C are side, exploded and sectional views of one embodiment of a contact assembly;  
       FIG. 5D  is a sectional view of alternative embodiment of the housing of FIGS.  5 A-C;  
       FIG. 6  is one embodiment of a contact element;  
       FIG. 7  is a perspective view of another embodiment of a bulk Ecmp station;  
       FIGS. 8-9  are perspective and partial sectional views of a contact assembly;  
       FIG. 10  is a sectional view of one embodiment of a residual Ecmp station;  
       FIG. 11  is graph depicting wafer thickness (Å) versus radial scan (mm) for electroprocessing a substrate using a PPS contact cover and electroprocessing a substrate using a stainless steel contact cover;  
       FIG. 12   a  is a graph depicting voltage traces (V) versus polishing time (s) for electopolishing with a PPS contact cover; and  
       FIG. 12   b  is a graph depicting voltage traces (V) versus polishing time (s) for electroprocessing a substrate with a stainless steel contact cover. 
    
    
      To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that embodiments present in one embodiment may be beneficially incorporated in other embodiments with out further recitation.  
     DETAILED DESCRIPTION  
      Embodiments for a system and method for removal of conductive material from a substrate are provided. Although the embodiments disclosed below focus primarily on removing material from, e.g., planarizing, a substrate, it is contemplated that the teachings disclosed herein may be used to deposit material on a substrate by reversing the polarity of an electrical bias applied between the substrate and an electrode of the system.  
       FIG. 1  is a plan view of one embodiment of a planarization system  100  having an apparatus for electrochemically processing a substrate. The exemplary system  100  generally comprises a factory interface  102 , a loading robot  104 , and a planarizing module  106 . The loading robot  104  is disposed proximate the factory interface  102  and the planarizing module  106  to facilitate the transfer of substrates  122  therebetween.  
      A controller  108  is provided to facilitate control and integration of the modules of the system  100 . The controller  108  comprises a central processing unit (CPU)  110 , a memory  112 , and support circuits  114 . The controller  108  is coupled to the various components of the system  100  to facilitate control of, for example, the planarizing, cleaning, and transfer processes.  
      The factory interface  102  generally includes a cleaning module  116  and one or more wafer cassettes  118 . An interface robot  120  is employed to transfer substrates  122  between the wafer cassettes  118 , the cleaning module  116  and an input module  124 . The input module  124  is positioned to facilitate transfer of substrates  122  between the planarizing module  106  and the factory interface  102  by grippers, for example vacuum grippers or mechanical clamps.  
      The planarizing module  106  includes at least a first electrochemical mechanical planarizing (Ecmp) station  128 , and optionally, at least one conventional chemical mechanical planarizing (CMP) stations  132  disposed in an environmentally controlled enclosure  188 . Examples of planarizing modules  106  that can be adapted to benefit from the invention include MIRRA®, MIRRA MESA™, REFLEXION®, REFLEXION® LK, and REFLEXION LK Ecmp™ Chemical Mechanical Planarizing Systems, all available from Applied Materials, Inc. of Santa Clara, Calif. Other planarizing modules, including those that use processing pads, planarizing webs, or a combination thereof, and those that move a substrate relative to a planarizing surface in a rotational, linear or other planar motion may also be adapted to benefit from the invention.  
      In the embodiment depicted in  FIG. 1 , the planarizing module  106  includes the first Ecmp station  128 , a second Ecmp station  130  and one CMP station  132 . Bulk removal of conductive material from the substrate is performed through an electrochemical dissolution process at the first Ecmp station  128 . After the bulk material removal at the first Ecmp station  128 , residual conductive material is removed from the substrate at the second Ecmp station  130  through a second electrochemical mechanical process. It is contemplated that more than one residual Ecmp station  130  may be utilized in the planarizing module  106 .  
      A conventional chemical mechanical planarizing process is performed at the planarizing station  132  after processing at the second Ecmp station  130 . An example of a conventional CMP process for the removal of copper is described in U.S. Pat. No. 6,451,697, issued Sep. 17, 2002, which is incorporated by reference in its entirety. An example of a conventional CMP process for the barrier removal is described in U.S. patent application Ser. No. 10/187,857, filed Jun. 27, 2002, which is incorporated by reference in its entirety. It is contemplated that other CMP processes may be alternatively performed. As the CMP stations  132  are conventional in nature, further description thereof has been omitted for the sake of brevity.  
      The exemplary planarizing module  106  also includes a transfer station  136  and a carousel  134  that are disposed on an upper or first side  138  of a machine base  140 . In one embodiment, the transfer station  136  includes an input buffer station  142 , an output buffer station  144 , a transfer robot  146 , and a load cup assembly  148 . The input buffer station  142  receives substrates from the factory interface  102  by the loading robot  104 . The loading robot  104  is also utilized to return polished substrates from the output buffer station  144  to the factory interface  102 . The transfer robot  146  is utilized to move substrates between the buffer stations  142 ,  144  and the load cup assembly  148 .  
      In one embodiment, the transfer robot  146  includes two gripper assemblies, each having pneumatic gripper fingers that hold the substrate by the substrate&#39;s edge. The transfer robot  146  may simultaneously transfer a substrate to be processed from the input buffer station  142  to the load cup assembly  148  while transferring a processed substrate from the load cup assembly  148  to the output buffer station  144 . An example of a transfer station that may be used to advantage is described in U.S. Pat. No. 6,156,124, issued Dec. 5, 2000 to Tobin, which is herein incorporated by reference in its entirety.  
      The carousel  134  is centrally disposed on the base  140 . The carousel  134  typically includes a plurality of arms  150 , each supporting a planarizing head assembly  152 . Two of the arms  150  depicted in  FIG. 1  are shown in phantom such that a planarizing surface  126  of the first Ecmp station  128  and the transfer station  136  may be seen. The carousel  134  is indexable such that the planarizing head assemblies  152  may be moved between the planarizing stations  128 ,  132  and the transfer station  136 . One carousel that may be utilized to advantage is described in U.S. Pat. No. 5,804,507, issued Sep. 8, 1998 to Perlov, et al., which is hereby incorporated by reference in its entirety.  
      A conditioning device  182  is disposed on the base  140  adjacent each of the planarizing stations  128 ,  132 . The conditioning device  182  periodically conditions the planarizing material disposed in the stations  128 ,  132  to maintain uniform planarizing results.  
       FIG. 2  depicts a sectional view of one of the planarizing head assemblies  152  positioned over one embodiment of the first Ecmp station  128 . The planarizing head assembly  152  generally comprises a drive system  202  coupled to a planarizing head  204 . The drive system  202  generally provides at least rotational motion to the planarizing head  204 . The planarizing head  204  additionally may be actuated toward the first Ecmp station  128  such that the substrate  122  retained in the planarizing head  204  may be disposed against the planarizing surface  126  of the first Ecmp station  128  during processing. The drive system  202  is coupled to the controller  108  that provides a signal to the drive system  202  for controlling the rotational speed and direction of the planarizing head  204 .  
      In one embodiment, the planarizing head may be a TITAN HEAD™ or TITAN PROFILER™ wafer carrier manufactured by Applied Materials, Inc. Generally, the planarizing head  204  comprises a housing  214  and retaining ring  224  that defines a center recess in which the substrate  122  is retained. The retaining ring  224  circumscribes the substrate  122  disposed within the planarizing head  204  to prevent the substrate from slipping out from under the planarizing head  204  while processing. The retaining ring  224  can be made of plastic materials such as PPS, PEEK™, and the like, or conductive materials such as stainless steel, Cu, Au, Pd, and the like, or some combination thereof. It is further contemplated that a conductive retaining ring  224  may be electrically biased to control the electric field during Ecmp. It is contemplated that other planarizing heads may be utilized.  
      The first Ecmp station  128  generally includes a platen assembly  230  that is rotationally disposed on the base  140 . The platen assembly  230  is supported above the base  140  by a bearing  238  so that the platen assembly  230  may be rotated relative 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 assembly  230 .  
      Conventional bearings, rotary unions and slip rings, collectively referred to as rotary coupler  276 , are provided such that electrical, mechanical, fluid, pneumatic, control signals and connections may be coupled between the base  140  and the rotating platen assembly  230 . The platen assembly  230  is typically coupled to a motor  232  that provides the rotational motion to the platen assembly  230 . The motor  232  is coupled to the controller  108  that provides a signal for controlling for the rotational speed and direction of the platen assembly  230 .  
      The platen assembly  230  has an upper plate  236  and a lower plate  234 . The upper plate  236  may be fabricated from a rigid material, such as a metal or rigid plastic, and in one embodiment, is fabricated from or coated with a dielectric material, such as CPVC. The upper plate  236  may have a circular, rectangular or other plane form. A top surface  260  of the upper plate  236  supports a processing pad assembly  222  thereon. The processing pad assembly may be retained to the upper plate  236  by magnetic attraction, vacuum, clamps, adhesives and the like.  
      The lower plate  234  is generally fabricated from a rigid material, such as aluminum. In the embodiment depicted in  FIG. 2 , the upper and lower plates  236 ,  234  are coupled by a plurality of fasteners  228 . Generally, a plurality of locating pins  220  (one is shown in  FIG. 2 ) are disposed between the upper and lower plates  236 ,  234  to ensure alignment therebetween. The upper plate  236  and the lower plate  234  may optionally be fabricated from a single, unitary member.  
      A plenum  206  is defined in the platen assembly  230 . The plenum  206  may be partially formed in at least one of the upper or lower plates  236 ,  234 . In the embodiment depicted in  FIG. 2 , the plenum  206  is defined in a recess  208  partially formed in the lower surface of the upper plate  236 . A plurality of holes  210  are formed in the upper plate  236  to allow electrolyte, provided to the plenum  206  from an electrolyte source  248 , to flow uniformly though the platen assembly  230  and into contact with the substrate  122  during processing. The plenum  206  is partially bounded by a cover  212  coupled to the upper plate  236  enclosing the recess  208 .  
       FIG. 3  is a partial sectional view of the platen assembly  230  showing one embodiment of the cover  212  in greater detail. The cover  212  is sealingly coupled to the upper plate  236  by a plurality of fasteners  312 . A plenum seal  314  is disposed between the cover  212  and upper plate  236 .  
      The cover  212  includes a first aperture  302 , a second aperture  304  and a third aperture  306 . The first and second apertures  302 ,  304  provide an inlet and outlet that couple the plenum  206  through the cover  212  to the electrolyte source  248 . In one embodiment, the first and second apertures  302 ,  304  engage male fittings  308  that mate with holes  340  formed in the lower plate  234 . A radial seal  310 , for example, an o-ring or lobed seal, is disposed between the fittings  308  and bore of the holes  340  to provide a fluid seal that prevents electrolyte from leaking out of the plenum  206  through the cover  212 .  
      The third aperture  306  is circumscribed by a seal  316  that isolates the third aperture  306  from electrolyte disposed within the plenum  206 . In one embodiment, the seal  316  is positioned outward of second plenum seal  344  to provide an additional barrier between the first bayonet fitting  318  and the electrolyte disposed in the plenum  206 .  
      A first bayonet fitting  318  is disposed through the third aperture  306  and couples a contact plate  320 , disposed in the plenum  206  and coupled to the upper plate  236 , to a socket  322  disposed in the lower plate  234 . The socket  322  is coupled by a first power line  324  disposed in a passage  326  formed in the lower plate  234  to the power source  242  through the rotary coupler  276  (as shown in  FIG. 2 ).  
      A second line  328  is disposed through the lower plate  234  coupling a socket  334  disposed proximate the perimeter of the lower plate  234  to the power source  242 . A second bayonet fitting  332  is coupled to a contact member  336  disposed in the upper plate  236 . The contact member  336  includes a threaded hole  338  or other element exposed to the top surface  260  of the upper plate  236  that is suitable for electrically coupling the contact member  336  to the processing pad assembly  222 . In the embodiment depicted in  FIG. 3 , the processing pad assembly  222  is coupled by the second bayonet fitting  332  to the power source  242 .  
      The bayonet fittings  318 ,  332  and locating pins  220  facilitate alignment of the plates  234 ,  236  while fluid and electrical connection are made as the upper plate  236  is disposed on the lower plate  234 . This advantageously provides both ease of assembly with robust electrical and fluid coupling between the plates  234 ,  236 .  
      Referring additionally to  FIG. 2 , the processing pad assembly  222  includes an electrode  292  and at least a planarizing portion  290 . At least one contact assembly  250  extends above the processing pad assembly  222  and is adapted to electrically couple the substrate being processing on the processing pad assembly  222  to the power source  242 .  
      The electrode  292  is also coupled to the power source  242  so that an electrical potential may be established between the substrate and electrode  292 . In one embodiment the electrode  292  is electrically coupled to the power source  242  by a fastener  380  disposed through the electrode  292  and engaging the threaded hole  338  of the contact member  336  (as shown in  FIG. 3 ).  
      The electrode  292  is typically comprised of a conductive material, such as stainless steel, copper, aluminum, gold, silver, titanium, tin, nickel, and tungsten, among others. The electrode  292  may be solid, impermeable to electrolyte, permeable to electrolyte or perforated. In the embodiment depicted in  FIG. 3 , the electrode  292  is configured to allow electrolyte therethrough. The electrode  292  may be permeable, have holes formed therethrough or a combination thereof. The electrode  292  is disposed on the top surface  260  of the platen assembly  230  and is coupled to the power source  242  through the platen assembly  230 .  
      Embodiments of the processing pad assembly  222  suitable for bulk removal of material from the substrate  122  may generally include a planarizing surface that is substantially dielectric. As the conductive material to be removed from the substrate  122  substantially covers the substrate  122 , fewer contacts for biasing the substrate  122  are required. Embodiments of the processing pad assembly  222  suitable for residual removal of material from the substrate  122  may generally include a planarizing surface that is substantially conductive. As the conductive material to be removed from the substrate  122  comprises isolated islands of material disposed on the substrate  122 , more contacts for biasing the substrate  122  are required.  
      In one embodiment, the planarizing layer  290  of the processing pad assembly  222  may include a planarizing surface  364  that is dielectric, such as a polyurethane pad. Apertures  390  are formed through the planarizing surface  364  to expose the electrode  292  such that electrolyte may create a conductive path (or cell) between the substrate and electrode. Examples of processing pad assemblies that may be adapted to benefit from the invention are described in U.S. patent application Ser. No. 10/455,941, filed Jun. 6, 2003 by Y. Hu et al., entitled “CONDUCTIVE PLANARIZING ARTICLE FOR ELECTROCHEMICAL MECHANICAL PLANARIZING” and U.S. Pat. No. 6,991,528, issued Jan. 31, 2006 to Y. Hu et al., entitled “CONDUCTIVE PLANARIZING ARTICLE FOR ELECTROCHEMICAL MECHANICAL PLANARIZING”, both of which are hereby incorporated by reference in their entireties.  
       FIG. 4A  is a partial sectional view of the first Ecmp station  128  through two contact assemblies  250 , and FIGS.  5 A-C are side, exploded and sectional views of one of the contact assemblies  250  shown in  FIG. 4A . The platen assembly  230  includes at least one contact assembly  250  projecting therefrom and coupled to the power source  242  that is adapted to bias a surface of the substrate  122  during processing. The contact assemblies  250  may be coupled to the platen assembly  230 , part of the processing pad assembly  222 , or a separate element. Although two contact assemblies  250  are shown in  FIG. 4A , any number of contact assemblies may be utilized and may be distributed in any number of configurations relative to the centerline of the upper plate  236 .  
      The contact assemblies  250  are generally electrically coupled to the contact plate  320  through the upper plate  236  and extend at least partially through respective apertures  468  formed in the processing pad assembly  222 . The position of the contact assemblies  250  may be chosen to have a predetermined configuration across the platen assembly  230 . For predefined processes, individual contact assemblies  250  may be repositioned in different apertures  468 , while apertures not containing contact assemblies may be plugged with a stopper  492  or filled with a nozzle  494  that allows flow of electrolyte from the plenum  206  to the substrate as shown in FIGS.  4 B-C. One contact assembly that may be adapted to benefit from the invention is described in U.S. Pat. No. 6,884,153, issued Apr. 26, 2005, to Butterfield, et al., and is hereby incorporated by reference in its entirety.  
      Although the embodiments of the contact assembly  250  described below with respect to  FIG. 4A  depicts a rolling ball contact, the contact assembly  250  may alternatively comprise a structure or assembly having a conductive upper layer or surface suitable for electrically biasing the substrate  122 . For example, the contact assembly  250  may include a structure having an upper layer made from a conductive material or a conductive composite (i.e., the conductive elements are dispersed integrally with or comprise the material comprising the upper surface), such as a polymer matrix having conductive particles dispersed therein or a conductive coated fabric, among others. Other examples of suitable contact assemblies are described in U.S. Provisional Patent Application Ser. No. 60/516,680, filed Nov. 3, 2003, by Hu, et al., which is hereby incorporated by reference in its entirety.  
      In one embodiment, each of the contact assemblies  250  includes a hollow housing  402 , an adapter  404 , a ball  406 , a contact element  414  and a clamp bushing  416 . The ball  406  has a conductive outer surface and is movably disposed in the housing  402 . The ball  406  may be disposed in a first position having at least a portion of the ball  406  extending above the planarizing surface  364  and at least a second position where the ball  406  is flush with the planarizing surface  364 . The ball  406  is generally suitable for electrically coupling the substrate  122  to the power source  242  through the contact plate  320 .  
      The power source  242  generally provides a positive electrical bias to the ball  406  during processing. Between planarizing substrates, the power source  242  may optionally apply a negative bias to the ball  406  to minimize attack on the ball  406  by process chemistries.  
      The housing  402  is configured to provide a flow of electrolyte from the source  248  to the substrate during processing. The housing  402  is fabricated from a dielectric material compatible with process chemistries. In one embodiment, the housing  402  is made of PEEK™. In another embodiment the housing comprises a conductive material selected from the group consisting of stainless steel, copper, gold, silver, tungsten, palladium, bronze, brass, conductive polymers and the like or some other combinations thereof. The housing  402  has a first end  408  and a second end  410 . A drive feature  412  is formed in and/or on the first end  408  to facilitate installation of the contact assembly  250  to the contact plate  320 . The drive feature  412  may be holes for a spanner wrench, a slot or slots, a recessed drive feature (such as for a TORX® or hex drive, and the like) or a projecting drive feature (such as wrench flats or a hex head, and the like), among others. The first end  408  additionally includes a seat  426  that prevents the ball  406  from passing out of the first end  408  of the housing  402 . The seat  426  optionally may include one or more grooves  448  formed therein that allow fluid flow to exit the housing  402  between the ball  406  and seat  426 . Maintaining fluid past the ball  406  may minimize the propensity of process chemistries to attack the ball  406 .  
      In one embodiment, a plurality of grooves  448  is formed around the seat  426  in a spaced apart relation. The spaced apart relation of the grooves  448  provides a more uniform electrolyte lead flow distribution around the ball  406 , thereby enhancing corrosion protection of the ball. Moreover, the bleed flow allows the force applied to the balls to be the same with or without the substrate presence, compared to conventional housings without bleed flows where the ball force is dramatically different in the up and down position. In the embodiment depicted in  FIG. 5B , six grooves  448  are shown spaced equidistant around the seat  428 .  
      Alternatively as shown in  FIG. 5D , the grooves  448  may be replaced or augmented by one or more spacers  454  extending from the seat  426  (or housing  402 ). The spacers  454  prevent the ball  406  from contacting the seat  426  in a manner that prevents fluid from bleeding past the ball  406  when the ball  406  is urged against (or towards) the seat  426 .  
      In another embodiment, one or more relief holes  446  may be formed through the housing  402  to allow fluid to exit the housing  402  while the ball  406  is disposed against the seat  426 . The relief holes  446  prevent fluid from residing in the housing  402  for extended periods, thereby minimizing accumulation of sludge or other contaminants that may stick to the ball  406  and degrade electrical conductance, obstruct flow through the housing  406  while processing, cause ball stiction or otherwise degrade processing performance.  
      The contact element  414  is coupled between the clamp bushing  416  and adapter  404 . The contact element  414  is generally configured to electrically connect the adapter  404  and ball  406  substantially or completely through the range of ball positions within the housing  402 . In one embodiment, the contact element  414  may be configured as a spring form.  
      In the embodiment depicted in  FIGS. 4 and 5 A-C and detailed in  FIG. 6 , the contact element  414  includes an annular base  442  having a plurality of flexures  444  extending therefrom in a polar array. The flexure  444  includes two support elements  602  extending from the base  442  to a distal end  608 . The support elements  602  are coupled by a plurality of rungs  604  to define apertures  610  that facilitate flow past the contact element  416  with little pressure drop as discussed further below. A contact pad  606  adapted to contact the ball  406  couples the support elements  602  at the distal end  608  of each flexure  444 . Optionally, the contact pad  606  may includes a feature  612  formed thereon that defines the contact point between the pad  606  and the ball  406 . In one embodiment, the feature  612  is a formed round element extending from the pad  606  towards the center on the element  414 .  
      The flexure  444  is generally fabricated from a resilient and conductive material suitable for use with process chemistries. In one embodiment, the flexure  444  is fabricated from gold plated beryllium copper.  
      Returning to FIGS.  4 A and  5 A-B, the clamp bushing  416  includes a flared head  524  having a threaded post  522  extending therefrom. The clamp bushing may be fabricated from either a dielectric or conductive material, or a combination thereof, and in one embodiment, is fabricated from the same material as the housing  402 . The flared head  524  includes a flared flat  592  that maintains the flexures  444  at an acute angle relative to the centerline of the contact assembly  250  so that the contact pads  606  of the contact elements  414  are positioned to spread around the surface of the ball  406  to prevent bending, binding and/or damage to the flexures  444  during assembly of the contact assembly  250  and through the range of motion of the ball  406 .  
      The post  522  of the clamp bushing  416  is disposed through a hole  546  in the base  442  and threads into a threaded portion  440  of a passage  436  formed through the adapter  404 . A passage  418  formed through the clamp bushing  416  includes a drive feature  420  at an end disposed in the flared head  524 . Similarly, the passage  436  includes a drive feature  438  in an end opposite the threaded portion  440 . The drive features  420 ,  438  may be similar to those described above, and in one embodiment, are hexagonal holes suitable for use with a hex driver. The clamp bushing  416  is tightened to a torque that ensures good electrical contact between the contact element  414  and the adapter  404  without damaging the contact element  414  or other component.  
      One or more slots or cross holes  590  are formed through the head  524  to the passage  418 . The cross hole  590  routes at least a portion of the flow of electrolyte through the housing  402  so that the volume within the housing  402  is swept (i.e., the flow is routed so no areas within the housing experience a stagnant or no flow condition), thereby removing sludge or other contaminates that may otherwise accumulate within the housing  402  and eventually lead to poor electrical conduction to the substrate through the ball  406 . In one embodiment, the cross holes  590  exit the clamp bushing  416  through the flats  492 , thereby directing flow directly on the flexures  444  to ensure contaminants do not accumulate on the contact element  414  or cause the flexure  444  to adhere to the ball  406 . Optionally, the passage  418  may be blind and the cross hole  590  coupled to the passage  436 , such that the entire flow enters the housing through the cross hole  590  and is swept at a greater rate through the housing  402 . Since the fluid inlet to the housing  402  (e.g., the cross hole  590 ) is opposite the outlet (e.g., the center opening of the seat  426 ), the entire volume of the housing  402  retaining the ball  406  is swept by electrolyte flow, thereby ensuring that sludge and/or other contaminants do not accumulate within the housing  402 , resulting in extended robust electrical performance of the contact assembly  250 .  
      The adapter  404  is generally fabricated from an electrically conductive material compatible with process chemistries, and in one embodiment, is fabricated from stainless steel. The adapter  404  includes an annular flange  432  having a threaded post  430  extending from one side and a boss  434  extending from the opposite side. The threaded post  430  is adapted to mate with the contact plate  320  disposed in recess  208  of the upper plate  236  which couples the respective balls  406  in the contact assemblies  250  to the power source  242 .  
      The boss  434  is received in the second end  410  of the housing  402  and provides a surface for clamping the contact element  414  thereto. The boss  434  additionally includes at least one threaded hole  506  disposed on the side of the boss  434  that engages a fastener  502  disposed through a hole  504  formed in the housing  402 , thereby securing the housing  402  to the adapter  404  and capturing the ball  406  therein. In the embodiment depicted in  FIG. 5A , three fasteners are shown for coupling the housing  402  to the adapter  404  through counter-sunk holes  504 . It is contemplated that the housing  402  and adapter  404  may be fastened by alternative methods or devices, such as staking, adhering, bonding, press fit, dowel pins, spring pins, rivets and retaining rings, among others.  
      The ball  406  may be solid or hollow and is typically fabricated from a conductive material. For example, the ball  406  may be fabricated from a metal, conductive polymer or a polymeric material filled with conductive material, such as metals, conductive carbon or graphite, among other conductive materials. Alternatively, the ball  406  may be formed from a solid or hollow core that is coated with a conductive material. The core may be non-conductive and at least partially coated with a conductive covering. Examples of suitable core materials include acrylonitrile butadiene styrene (ABS), polypropylene (PP), polyethylene (PE), polystyrene (PS), or polyamide-imide (PAI) (such as TORLON®), and the like. In one embodiment, the ball  406  has a TORLON® or other polymer core coated with a layer of copper or other conductive material.  
      The ball  406  is generally actuated toward the planarizing surface  364  by at least one of spring, buoyant or flow forces. In the embodiment depicted in  FIG. 4 , the passages  436 ,  418  formed through the adapter  404  and clamp bushing  416  are coupled through the upper plate  236  to the electrolyte source  248 . The electrolyte source  248  provides electrolyte through the passages  436  and  418  into the interior of the hollow housing  402 . The electrolyte exits the housing  402  between the seat  426  and ball  406 , thus causing the ball  406  to be biased toward the planarizing surface  364  and into contact with the substrate  122  during processing.  
      So that the force upon the ball  406  is consistent across the different elevations of the ball  406  within the housing  402 , a relief or groove  428  is formed in the interior wall of the housing  402  to accept the distal ends ( 608  in  FIG. 6 ) of the flexures  444  to prevent restricting the flow of electrolyte passing the ball  406 . An end of the groove  428  disposed away from the seat  426  is generally configured to being at or below the diameter of the ball  406  when the ball  406  is in the lowered position.  
      In one embodiment, electrochemical attack on the contact assembly  250  and/or balls  406  by processing chemistries and contaminant accumulation within the housing  402  may be minimized by keeping a bleeding flow of processing chemistry around the balls all the time substantially prevents self catalytic reaction of the balls in the process chemistry (by removing the catalyst byproduct and other contaminants away from the ball), thus minimizing chemical attack on the balls by eliminating the presence of static process chemistry. Flow is maintained past the ball  406  and out the housing  402  by the path provided by the groove  448  and/or relief hole  446 .  
      In another embodiment, minimizing electrochemical attack and cleaning of the electrical contacts within the housing  402  are facilitated by rinsing the contact assembly  250  and/or balls  406  after processing. For example, a rinsing fluid source  450  may be coupled through a selector valve  452  between the electrolyte source  248  and the contact assembly  250 . The selector valve  452  allows a rinsing fluid, such as de-ionized water, to be flowed past the ball  406  during idle periods (when no substrates are being polished on the platen assembly  230 ) to prevent the ball  406  from being attacked by processing chemistries. It is contemplated that other configurations may be utilized to selectively couple the electrolyte source  248  and the rinsing fluid source  450  to the plenum  206 , or that the electrolyte source  248  and the rinsing fluid source  450  may comprise a single fluid delivery system. Keeping a bleeding flow of processing chemistry around the balls all the time substantially prevents self catalytic reaction of the balls in the process chemistry (by removing the catalyst byproduct away from the ball), thus minimizing chemical attack on the balls due by eliminating the presence of static process chemistry.  
       FIG. 7  is a perspective view of another embodiment of an Ecmp station  790  having another embodiment of a contact assembly  700  disposed therein, and  FIGS. 8-9  are perspective and partial sectional views of the contact assembly  700 .  FIG. 8A  is a perspective view of another embodiment of the contact assembly  700 . The Ecmp station  790  includes a platen assembly  750  that supports a processing pad assembly  760  (partially shown in  FIG. 7 ). The platen assembly  750  includes at least one contact assembly  700  projecting therefrom that is coupled to a power source  242 . The contact assembly  700  is adapted to electrically bias a surface of the substrate  122  (shown in  FIG. 9 ) during processing. Although one contact assembly  700  is shown coupled to the center of the platen assembly  750  in  FIG. 7 , any number of contact assemblies may be utilized and may be distributed in any number of configurations relative to the centerline of the platen assembly  750 . The contact assembly  700  may also comprise a structure having a conductive upper surface suitable for biasing the substrate  122 , as discussed above with respect to  FIG. 4 .  
      The processing pad assembly  760  may be any pad assembly suitable for processing the substrate, including any of the embodiments described above. The processing pad assembly  760  may include an electrode  962  and a planarizing layer  966 . In one embodiment, the planarizing layer  966  of the processing pad assembly  760  may include a planarizing surface  964  that is dielectric, such as a polyurethane pad. In another embodiment, the planarizing layer  966  of the processing pad assembly  760  may include a planarizing surface  964  that is conductive or made from a conductive composite (i.e., the conduct elements are dispersed integrally with or comprise the material comprising the planarizing surface), such as a polymer matrix having conductive particles dispersed therein or a conductive coated fabric, among others. In the embodiment wherein the planarizing surface  964  is conductive, the planarizing surface  964  and electrode  962  may be coupled to the power source  242  (shown by the dashed lines) via a switch  996  that allows power to be selectively switched between the contact assembly  700  and the conductive planarizing surface  964  to respectively facilitate bulk metal removal and residual metal removal from the substrate  122  without lifting the substrate  122  from the processing pad assembly  760 . It is contemplated that the Ecmp station  128  may also be similarly configured with a conductive processing pad assembly.  
      The contact assembly  700  is generally coupled to a conductive contact terminal  910  disposed in the platen assembly  750  and extends at least partially through an aperture  968  formed in the processing pad assembly  760 . The contact assembly  700  includes a housing  800  that retains a plurality of balls  406 . The balls  406  are movably disposed in the housing  800 , and may be disposed in a first position having at least a portion of the balls  406  extending above the planarizing surface  964  and at least a second position where the balls  406  are flush with the planarizing surface  964 . The balls  406  are generally suitable for electrically biasing the substrate  122 .  
      The housing  800  is removably coupled to the platen assembly  750  to facilitate replacement of the contact assembly  700  after a number of planarizing cycles. In one embodiment, the housing  800  is coupled to the platen assembly  750  by a plurality of screws  808 . The housing  800  includes a contact cover  804  coupled to a lower housing  806  that retains the balls  406  therebetween. The contact cover  804  is fabricated from a conductive material compatible with process chemistries. In one embodiment, the contact cover  804  is made of a conductive material selected from the group consisting of stainless steel, copper, gold, silver, palladium, tungsten, bronze, brass, titanium, tin, nickel, palladium-tin alloys, lead, conductive polymers, and the like or some other combination thereof. The lower housing  806  is fabricated from a conductive material compatible with process chemistries. In one embodiment, the lower housing  806  is made of stainless steel or other electrically conductive material. In another embodiment, the lower housing  806  is made of the same material as the contact cover  804 . The lower housing  806  is coupled to by a bayonet fitting  912  to the contact terminal  910  which is in turn coupled to the power source  242 . The contact cover  804  and lower housing  806  may be coupled in any number of methods, including but not limited to, screwing, bolting, riveting, bonding, staking and clamping, among others. In the embodiment depicted in  FIGS. 7-9 , the contact cover  804  and lower housing  806  are coupled by a plurality of screws  808 .  
      The balls  406  are disposed in a plurality of apertures  902  formed through the contact cover  804  and lower housing  806 . An upper portion of each of the apertures  902  includes a seat  904  that extends into the aperture  902  from the contact cover  804 . The seat  904  is configured to prevent the ball  406  from exiting the top end of the aperture  902 .  
      A contact element  414  is disposed in each aperture  902  to electrically couple the ball  406  to the lower housing  806 . Each of the contact elements  414  is coupled to the lower housing  806  by a respective clamp bushing  416 . In one embodiment, a post  522  of the clamp bushing  416  is threaded into a threaded portion  914  of the aperture  902  formed through the housing  800 .  
      During processing, the balls  406  disposed within the housing  800  are actuated toward the planarizing surface  760  by at least one of spring, buoyant or flow forces. The balls  406  electrically couple the substrate  122  to the power source  242  and contact terminal  910  through the contact elements  414  and lower housing  806 . Electrolyte, flowing through the housing  800  provides a conductive path between the electrode  962  and biased substrate  122 , thereby driving an electrochemical mechanical planarizing process.  
      In the embodiment depicted in  FIG. 9 , a plenum  940  may be formed in a lower plate  942  of the platen assembly  750 . An electrolyte source  248  is coupled to the plenum  940  and flows electrolyte to the planarizing surface  760  through the apertures  902  of the contact assembly  700 . In this configuration, a top plate  944  may optionally be a unitary component with the lower plate  942 . The plenum  940  may alternatively be disposed in the top plate  944  as described above.  
      To prevent electrochemical attack and prevent accumulation of sludge or other contaminants from degrading the performance of the balls  406  within the housing  800 , the contact assembly  700  is configured to maintain a bleed flow of electrolyte out of the housing  800  past the ball  406  and to sweep the interior of the housing  800  with electrolyte flow. For example, one or more grooves  950  and/or relief holes  952  may be formed through the housing  800  allowing flow to exit the housing  800  during conditions where the ball  406  is in contact with the seat  904 . Additionally, the clamp bushing  416  may include a cross hole  590  to sweep the portion of the housing  800  as described above with reference to the contact assembly  250 . Optionally, the lower housing  806  may include holes  954  formed therethrough to allow electrolyte to sweep alongside the clamp bushing  416 , thereby ensuring the entire volume of the housing  800  retaining each ball  406  has no unswept regions. In another embodiment, the groove  950  is not present.  
      A portion of an exemplary mode of operation of the processing system  100  is described primarily with reference to  FIG. 2 . In operation, the substrate  122  is retained in the planarizing head  204  and moved over the processing pad assembly  222  disposed on the platen assembly  230  of the first Ecmp station  128 . The planarizing head  204  is lowered toward the platen assembly  230  to place the substrate  122  in contact with the planarizing material. Electrolyte is supplied to the processing pad assembly  222  through the outlet and flows into the processing pad assembly  222 .  
      A bias voltage is applied from the power source  242  between the contact assemblies  250  and the electrode  292  of the pad assembly  222 . The contact assemblies  250  are in contact with the substrate and apply a bias thereto. The electrolyte filling the apertures  390  between the electrode  292  and the substrate  122  provides a conductive path between the power source  242  and substrate  122  to drive an electrochemical mechanical planarizing process that results in the removal of conductive material, such as copper, disposed on the surface of the substrate  122 , by an anodic dissolution method.  
      Once the substrate  122  has been adequately planarized by removal of conductive material at the first Ecmp station  128 , the planarizing head  204  is raised to remove the substrate  122  from contact with the platen assembly  230  and the processing pad assembly  222 . The substrate  122  may be transferred to one of another Ecmp station  128 , the second Ecmp station  130  or the CMP station  132  for further processing before removal from the planarizing module  106 .  
       FIG. 10  is a sectional view of one embodiment of the second Ecmp station  130 . The second Ecmp station  130  generally includes a platen  1002  that supports a fully conductive processing pad assembly  1004 . The platen  1002  may be configured similar to the platen assembly  230  described above to deliver electrolyte through the processing pad assembly  1004 , or the platen  1002  may have a fluid delivery arm (not shown) disposed adjacent thereto configured to supply electrolyte to a planarizing surface of the processing pad assembly  1004 .  
      In one embodiment, the processing pad assembly  1004  includes interposed pad  1012  sandwiched between a conductive pad  1010  and an electrode  1014 . The conductive pad  1010  is substantially conductive across its top processing surface and is generally made from a conductive material or a conductive composite (i.e., the conductive elements are dispersed integrally with or comprise the material comprising the planarizing surface), such as a polymer matrix having conductive particles dispersed therein or a conductive coated fabric, among others. The conductive pad  1010  and the electrode  1014  may be fabricated like the conductive pad  966  and the electrode  292  described above. The processing pad assembly  1004  is generally permeable or perforated to allow electrolyte to pass between the electrode  1014  and top surface  1020  of the conductive pad  1010 . In the embodiment depicted in  FIG. 10 , the processing pad assembly  1004  is perforated by apertures  1022  to allow electrolyte to flow therethrough. In one embodiment, the conductive pad  1010  is comprised of a conductive material disposed on a polymer matrix disposed on a conductive fiber, for example, tin particles in a polymer matrix disposed on a woven copper coated polymer. The conductive pad  1010  may also be utilized for the contact assembly  700  in the embodiment of  FIG. 7 .  
      A conductive foil  1016  may additionally be disposed between the conductive pad  1010  and the subpad  1012 . The foil  1016  is coupled to a power source  242  and provides uniform distribution of voltage applied by the source  242  across the conductive pad  1010 . Additionally, the pad assembly  1004  may include an interposed pad  1018 , which, along with the foil  1016 , provides mechanical strength to the overlying conductive pad  1010 . The foil  1016  and interposed pad  1018  may be configured similar to the interposed layer and conductive backing described above.  
      Another portion of an exemplary mode of operation of the processing system  100  is described primarily with reference to  FIG. 10 . In operation, the substrate  122  retained in the planarizing head  204  is moved over the processing pad assembly  1004  disposed on the platen assembly  1002  of the second Ecmp station  130 . The planarizing head  204  is lowered toward the platen assembly  1002  to place the substrate  122  in contact with the top surface  1020  of the conductive pad  1010 . Electrolyte is supplied to the processing pad assembly  222  through the delivery arm (not shown) and flows into the processing pad assembly  1004 .  
      A bias voltage is applied from the power source  242  between the top surface  1020  of the conductive pad  1010  and the electrode  1014  of the pad assembly  1004 . The top surface  1020  of the conductive pad  1010  is in contact with the substrate and applies an electrical bias thereto. The electrolyte filling the apertures  1022  between the electrode  1014  and the substrate  122  provides a conductive path between the power source  242  and substrate  122  to drive an electrochemical mechanical planarizing process that results in the removal of conductive material, such as copper, disposed on the surface of the substrate  122 , by an anodic dissolution method. As the top surface  1020  of the conductive pad  1010  is fully conductive, residual material, such as discrete islands of copper not completely removed through processing at the bulk Ecmp station  128 , may be efficiently removed.  
      Once the substrate  122  has been adequately planarized by removal of residual conductive material at the second Ecmp station  130 , the planarizing head  204  is raised to remove the substrate  122  from contact with the platen assembly  1002  and the processing pad assembly  1004 . The substrate  122  may be transferred to another residual Ecmp station or one of the CMP station  132  for further processing before removal from the planarizing module  106 .  
       FIG. 11  is graph  1100  depicting wafer thickness (Å) versus radial scan (mm) for electroprocessing a substrate using a PPS contact cover, represented by line  1102 , versus electroprocessing a substrate using a stainless steel contact cover, represented by line  1104 . The x-axis represents thickness of the substrate (Å) and the y-axis represents radial scan (mm) from the center of the substrate (0 mm) to the edge of the substrate (150 mm). As shown by Table I, the polishing time, endpoint, and charge were similar for the substrate and defects between the PPS contact cover and the stainless steel contact cover.  
               TABLE I                          Data for electroprocessing of substrates on platen 1 using a PPS contact       cover versus electroprocessing a substrate on platen 1 using a stainless       steel contact cover.                             Polishing Time (sec)   Acc. Charge (Amp. Min)                                             ID       Z1   Z2   Z3   Z1   Z2   Z3                                                     PPS Cover   Voltage   4.3/2.8 V   3.1/2.3 V   3.3/2.5 V                       P1 hi Rate   74.9   82.9   79.9   4.1   9.95   8.15           P1 lo Rate   27.8   26.8   20.9   0.55   1.33   1.03       SST Cover   Voltage   4.3/2.8 V   3.1/2.3 V   3.3/2.5 V           P1 hi Rate   71.6   84.8   79.9   4.09   9.96   8.15           P1 lo Rate   28.2   27.5   21.2   0.57   1.32   1.02                    
      Thus,  FIG. 1  and the data in Table 1 demonstrate that similar polishing profiles are obtained using the PPS and stainless steel covers.  
       FIG. 12   a  is a graph  1200  depicting voltage traces (V) versus polishing time (s) for electroprocessing a substrate using a PPS contact cover. The x-axis represents polish time (seconds) and the y-axis represents voltage (V). Line  1202  represents voltage Z 1 . Line  1204  represents voltage Z 2 . Line  1206  represents Z 3  voltage. Twenty-five wafers were polished with the PPS contact cover resulting in several spikes less than one volt.  
       FIG. 12   b  is a graph  1300  depicting voltage traces (V) versus polishing time (s) for electroprocessing a substrate using a stainless steel contact cover. The x-axis represents polish time (seconds) and the y-axis represents voltage (V). Line  1302  represents voltage Z 1 . Line  1304  represents voltage Z 2 . Line  1306  represents Z 3  voltage. Twenty-five wafers were polished using the stainless steel cover resulting in one small spike.  
      A comparison of  FIG. 12   a  with  FIG. 12   b  demonstrates that the stainless steel contact cover, used in  FIG. 12   b , produces fewer voltage spikes and smaller voltage spikes than the PPS cover used in  FIG. 12   a.    
      Although not wishing to be bound by theory, it is believed that the contact cover comprising a conductive material reduces the wear on the contact elements by functioning as a sacrificial anode thus the contact cover wears out and the wear on the ball is minimized. In the case of stainless steel, the cover does not wear out but the wear on the ball is still greatly reduced. In the case of all conductive materials, it is believed that the conductive cover locally modifies the electric potential of the electrolyte thus protecting the contact elements. It is further believed that the conductive contact cover reduces the number of voltage spikes and the number of defects on the contact elements by electrically shielding the contact elements.  
      Thus, the present invention provides an improved apparatus and method for electrochemically planarizing a substrate. The apparatus advantageously facilitates efficient bulk and residual material removal from a substrate while protecting process components from damage during processing periods. It is also contemplated that an apparatus arranged as described by the teachings herein, may be configured with solely the bulk Ecmp stations  128 , with solely the residual Ecmp stations  130 , with one or more bulk and/or residual Ecmp stations  130  arranged in cooperation with a conventional CMP station  132 , or in any combination thereof.  
      While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.