Patent Publication Number: US-7588677-B2

Title: Methods and apparatus for electrical, mechanical and/or chemical removal of conductive material from a microelectronic substrate

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 09/888,084, now U.S. Pat. No. 7,112,121, filed Jun. 21, 2001, which is a continuation-in-part of U.S. application Ser. No. 09/651,779, now U.S. Pat. No. 7,074,113, filed Aug. 30, 2000, U.S. application Ser. No. 09/887,767, now U.S. Pat. No. 7,094,131, filed Jun. 21, 2001, and U.S. application Ser. No. 09/888,002, now U.S. Pat. No. 7,160,176, filed Jun. 21, 2001, all of which are incorporated herein in their entireties by reference. 
    
    
     TECHNICAL FIELD 
     This invention relates to methods and apparatuses for removing conductive material from microelectronic substrates. 
     BACKGROUND 
     Microelectronic substrates and substrate assemblies typically include a semiconductor material having features, such as memory cells, that are linked with conductive lines. The conductive lines can be formed by first forming trenches or other recesses in the semiconductor material, and then overlaying a conductive material (such as a metal) in the trenches. The conductive material is then selectively removed to leave conductive lines extending from one feature in the semiconductor material to another. 
     Electrolytic techniques have been used to both deposit and remove metallic layers from semiconductor substrates. For example, an alternating current can be applied to a conductive layer via an intermediate electrolyte to remove portions of the layer. In one arrangement, shown in  FIG. 1 , a conventional apparatus  60  includes a first electrode  20   a  and a second electrode  20   b  coupled to a current source  21 . The first electrode  20   a  is attached directly to a metallic layer  11  of a semiconductor substrate  10  and the second electrode  20   b  is at least partially immersed in a liquid electrolyte  31  disposed on the surface of the metallic layer  11  by moving the second electrode downwardly until it contacts the electrolyte  31 . A barrier  22  protects the first electrode  20   a  from direct contact with the electrolyte  31 . The current source  21  applies alternating current to the substrate  10  via the electrodes  20   a  and  20   b  and the electrolyte  31  to remove conductive material from the conductive layer  11 . The alternating current signal can have a variety of wave forms, such as those disclosed by Frankenthal et al. in a publication entitled, “Electroetching of Platinum in the Titanium-Platinum-Gold Metallization on Silicon Integrated Circuits” (Bell Laboratories), incorporated herein in its entirety by reference. 
     One drawback with the arrangement shown in  FIG. 1  is that it may not be possible to remove material from the conductive layer  11  in the region where the first electrode  20   a  is attached because the barrier  22  prevents the electrolyte  31  from contacting the substrate  10  in this region. Alternatively, if the first electrode  20   a  contacts the electrolyte in this region, the electrolytic process can degrade the first electrode  20   a . Still a further drawback is that the electrolytic process may not uniformly remove material from the substrate  10 . For example, “islands” of residual conductive material having no direct electrical connection to the first electrode  20   a  may develop in the conductive layer  11 . The residual conductive material can interfere with the formation and/or operation of the conductive lines, and it may be difficult or impossible to remove with the electrolytic process unless the first electrode  20   a  is repositioned to be coupled to such “islands.” 
     One approach to addressing some of the foregoing drawbacks is to attach a plurality of first electrodes  20   a  around the periphery of the substrate  10  to increase the uniformity with which the conductive material is removed. However, islands of conductive material may still remain despite the additional first electrodes  20   a . Another approach is to form the electrodes  20   a  and  20   b  from an inert material, such as carbon, and remove the barrier  22  to increase the area of the conductive layer  11  in contact with the electrolyte  31 . However, such inert electrodes may not be as effective as more reactive electrodes at removing the conductive material, and the inert electrodes may still leave residual conductive material on the substrate  10 . 
       FIG. 2  shows still another approach to addressing some of the foregoing drawbacks in which two substrates  10  are partially immersed in a vessel  30  containing the electrolyte  31 . The first electrode  20   a  is attached to one substrate  10  and the second electrode  20   b  is attached to the other substrate  10 . An advantage of this approach is that the electrodes  20   a  and  20   b  do not contact the electrolyte. However, islands of conductive material may still remain after the electrolytic process is complete, and it may be difficult to remove conductive material from the points at which the electrodes  20   a  and  20   b  are attached to the substrates  10 . 
     Another method for removing material from a semiconductor substrate is chemical-mechanical planarization (“CMP”). Conventional CMP techniques include engaging the substrate with a polishing pad in a chemically active environment and then moving the polishing pad and/or the substrate relative to each other to chemically and/or mechanically remove material from the face of the substrate. The polishing pad can include fixed abrasive particles to abrade material from the substrate, or abrasive particles can be suspended in a liquid slurry disposed between the polishing pad and the substrate. 
     One drawback with conventional CMP techniques is that it may be extremely difficult or impossible to remove certain materials (such at platinum) from the substrate with such techniques. Alternatively, chemically etching materials, such as platinum, is not appropriate when the material is to be removed in a single direction (i.e., anisotropically) rather than in any direction (isotropically). Another drawback with conventional CMP techniques is that certain hard materials may be difficult to remove without applying a very large normal force to the substrate. Such a force can damage the substrate and can reduce the life expectancy of the CMP equipment. 
     International Application PCT/US00/08336 (published as WO/00/59682) discloses an apparatus having a first chamber for applying a conductive material to a semiconductor wafer, and a second chamber for removing conductive material from the semiconductor wafer by electropolishing or chemical-mechanical polishing. The second chamber includes an anode having a paint roller configuration with a cylindrical mechanical pad that contacts both an electrolyte bath and the face of the wafer as the anode and the wafer rotate about perpendicular axes. A cathode, which can include a conductive liquid isolated from the electrolytic bath, is electrically coupled to an edge of the wafer. One drawback with this device is that it, too, can leave islands of residual conductive material on the wafer. 
     SUMMARY 
     The present invention is directed toward methods and apparatuses for removing conductive materials from microelectronic substrates. A method in accordance with one aspect of the invention includes engaging the microelectronic substrate with the polishing surface of a polishing pad and electrically coupling a conductive material of the microelectronic substrate to a source of electrical potential while the microelectronic substrate is engaged with the polishing surface of the polishing pad. For example, the method can include positioning first and second electrodes proximate to and spaced apart from a face surface of the microelectronic substrate, and disposing an electrolytic fluid between the face surface and the electrodes, with the electrodes in fluid communication with each other and the electrolytic fluid. In a further aspect of the invention, the first and second electrodes can face toward the face surface of the microelectronic substrate, with one electrode defining a cathode and the other electrode defining an anode. The method can further include oxidizing at least a portion of the conductive material by passing an electrical current through the conductive material from the source of electrical potential, and removing the portion of the conductive material from the microelectronic substrate by moving at least one of the microelectronic substrate and the polishing pad relative to the other. The conductive material can include a metal, such as platinum or another noble metal, or a semiconductor material, such as doped polysilicon. 
     In a further aspect of the invention, the method can include selecting characteristics of the electrolytic fluid. For example, the fluid can include a concentration of chlorine ions of from about 50 ppm to about 5,000 ppm. The fluid can include at least one of (NH 4 ) 2 SO 4 , H 2 SO 4 , MgSO 4 , K 2 SO 4  and H 3 PO 4 . The pH of the fluid can be less than about 3 or greater than about 10 when the conductive material includes platinum, less than about 3 or greater than about 4 when the conductive material includes tungsten, and/or less than about 6 or greater than about 8 when the conductive material includes copper. 
     A method in accordance with another aspect of the invention includes providing a microelectronic substrate having a first conductive material disposed adjacent to a second conductive material, with the second conductive material having a different composition than the first conductive material. The first conductive material is engaged with the polishing surface of a polishing pad and is electrically coupled to a source of electrical potential by positioning first and second electrodes apart from the face surface and disposing a first electrolytic fluid between the face surface and the electrodes, with both the electrodes in fluid communication with the first electrolytic fluid. At least a portion of the first conductive material is oxidized by passing an electrical current through the first conductive material while the first conductive material is engaged with the polishing surface. The method can further include removing the portion of the first conductive material from the microelectronic substrate by moving at least one of the microelectronic substrate and the polishing pad relative to the other. The second conductive material is then engaged with the polishing surface, coupled to the first and second electrodes with a second electrolytic fluid, and oxidized by passing an electrical current through the second conductive material. At least a portion of the second conductive material is then removed from the microelectronic substrate by relative movement of the substrate relative to the polishing pad. In a further aspect of this method, further removal of material from the microelectronic substrate can be halted by engaging the polishing surface with an oxide layer positioned beneath one of the conductive materials. 
     The invention is also directed toward an apparatus for removing conductive material from a microelectronic substrate. In one aspect of the invention, the apparatus can include a substrate support configured to engage the microelectronic substrate, and a material removal medium positioned proximate to the substrate support. The material removal medium can include a polishing pad having a polishing surface positioned to engage the microelectronic substrate during operation. The material removal medium can further include a liquid disposed on the polishing pad and at least one electrode positioned at least proximate to the substrate support and coupleable to a source of electrical potential. Neither the polishing pad nor the liquid has discrete abrasive elements. At least one of the material removal medium and the substrate support is movable relative to the other when the substrate support and the material removal medium engage the microelectronic substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partially schematic, side elevational view of an apparatus for removing conductive material from a semiconductor substrate in accordance with the prior art. 
         FIG. 2  is a partially schematic side, elevational view of another apparatus for removing conductive material from two semiconductor substrates in accordance with the prior art. 
         FIG. 3  is a partially schematic, side elevational view of an apparatus having a support member and a pair of electrodes for removing conductive material from a microelectronic substrate in accordance with an embodiment of the invention. 
         FIG. 4  is a partially schematic, side elevational view of an apparatus for removing conductive material and sensing characteristics of the microelectronic substrate from which the material is removed in accordance with another embodiment of the invention. 
         FIG. 5  is a partially schematic, side elevational view of an apparatus that includes two electrolytes in accordance with still another embodiment of the invention. 
         FIG. 6  is a partially schematic, plan view of a substrate adjacent to a plurality of electrodes in accordance with still further embodiments of the invention. 
         FIG. 7  is a cross-sectional, side elevational view of an electrode and a substrate in accordance with yet another embodiment of the invention. 
         FIG. 8A  is a partially schematic, isometric view of a portion of a support for housing electrode pairs in accordance with still another embodiment of the invention. 
         FIGS. 8B and 8C  are isometric views of electrodes in accordance with still further embodiments of the invention. 
         FIG. 9  is a partially schematic, side elevational view of an apparatus for both planarizing and electrolytically processing microelectronic substrates in accordance with yet another embodiment of the invention. 
         FIG. 10  is a partially schematic, partially exploded isometric view of a planarizing pad and a plurality of electrodes in accordance with still another embodiment of the invention. 
         FIG. 11  is a partially schematic, side elevational view of an apparatus for both planarizing and electrolytically processing microelectronic substrates in accordance with still another embodiment of the invention. 
         FIGS. 12A and 12B  schematically illustrate a process for removing semiconductor material from a microelectronic substrate in accordance with an embodiment of the invention. 
         FIGS. 13A-C  schematically illustrate a process for removing two conductive materials from a microelectronic substrate and halting removal on an oxide layer in accordance with an embodiment of the invention. 
         FIGS. 14A and 14B  schematically illustrate a circuit and waveform for electrolytically processing a microelectronic substrate in accordance with yet another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure describes methods and apparatuses for removing conductive materials from a microelectronic substrate and/or substrate assembly used in the fabrication of microelectronic devices. As used herein, the term conductive materials includes, but is not limited to, metals, such as copper, platinum and aluminum, and semiconductor materials, such as doped polysilicon. Many specific details of certain embodiments of the invention are set forth in the following description and in  FIGS. 3-14B  to provide a thorough understanding of these embodiments. One skilled in the art, however, will understand that the present invention may have additional embodiments, or that the invention may be practiced without several of the details described below. 
       FIG. 3  is a partially schematic, side elevational view of an apparatus  160  for removing conductive material from a microelectronic substrate or substrate assembly  110  in accordance with an embodiment of the invention. In one aspect of this embodiment, the apparatus  160  includes a vessel  130  containing an electrolyte  131 , which can be in a liquid or a gel state. As used herein, the terms electrolyte and electrolytic fluid refer generally to electrolytic liquids and gels. Structures in fluid communication with electrolytic fluids are accordingly in fluid communication with electrolytic liquids or gels. 
     The microelectronic substrate  110  has an edge surface  112  and two face surfaces  113 . A support member  140  supports the microelectronic substrate  110  relative to the vessel  130  so that a conductive layer  111  on at least one of the face surfaces  113  of the substrate  110  contacts the electrolyte  131 . The conductive layer  111  can include metals such as platinum, tungsten, tantalum, gold, copper, rhodium, iridium, titanium or other conductive materials, such as doped polysilicon. In another aspect of this embodiment, the support member  140  is coupled to a substrate drive unit  141  that moves the support member  140  and the substrate  110  relative to the vessel  130 . For example, the substrate drive unit  141  can translate the support member  140  (as indicated by arrow “A”) and/or rotate the support member  140  (as indicated by arrow “B”). 
     The apparatus  160  can further include a first electrode  120   a  and a second electrode  120   b  (referred to collectively as electrodes  120 ) supported relative to the microelectronic substrate  110  by a support member  124 . In one aspect of this embodiment, the support arm  124  is coupled to an electrode drive unit  123  for moving the electrodes  120  relative to the microelectronic substrate  110 . For example, the electrode drive unit  123  can move the electrodes toward and away from the conductive layer  111  of the microelectronic substrate  110 , (as indicated by arrow “C”), and/or transversely (as indicated by arrow “D”) in a plane generally parallel to the conductive layer  111 . Alternatively, the electrode drive unit  123  can move the electrodes in other fashions, or the electrode drive unit  123  can be eliminated when the substrate drive unit  141  provides sufficient relative motion between the substrate  110  and the electrodes  120 . 
     In either embodiment described above with reference to  FIG. 3 , the electrodes  120  are coupled to a current source  121  with leads  128  for supplying electrical current to the electrolyte  131  and the conductive layer  111 . In operation, the current source  121  supplies an alternating current (single phase or multiphase) to the electrodes  120 . The current passes through the electrolyte  131  and reacts electrochemically with the conductive layer  111  to remove material (for example, atoms or groups of atoms) from the conductive layer  111 . The electrodes  120  and/or the substrate  110  can be moved relative to each other to remove material from selected portions of the conductive layer  111 , or from the entire conductive layer  111 . 
     In one aspect of an embodiment of the apparatus  160  shown in  FIG. 3 , a distance D 1  between the electrodes  120  and the conductive layer  111  is set to be smaller than a distance D 2  between the first electrode  120   a  and the second electrode  120   b . Furthermore, the electrolyte  131  generally has a higher resistance than the conductive layer  11 . Accordingly, the alternating current follows the path of least resistance from the first electrode  120   a , through the electrolyte  131  to the conductive layer  111  and back through the electrolyte  131  to the second electrode  120   b , rather than from the first electrode  120   a  directly through the electrolyte  131  to the second electrode  120   b . Alternatively, a low dielectric material (not shown) can be positioned between the first electrode  120   a  and the second electrode  120   b  to decouple direct electrical communication between the electrodes  120  that does not first pass through the conductive layer  111 . 
     One feature of an embodiment of the apparatus  160  shown in  FIG. 3  is that the electrodes  120  do not contact the conductive layer  111  of the substrate  110 . An advantage of this arrangement is that it can eliminate the residual conductive material resulting from a direct electrical connection between the electrodes  120  and the conductive layer  111 , described above with reference to  FIGS. 1 and 2 . For example, the apparatus  160  can eliminate residual conductive material adjacent to the contact region between the electrodes and the conductive layer because the electrodes  120  do not contact the conductive layer  111 . 
     Another feature of an embodiment of the apparatus  160  described above with reference to  FIG. 3  is that the substrate  110  and/or the electrodes  120  can move relative to the other to position the electrodes  120  at any point adjacent to the conductive layer  111 . An advantage of this arrangement is that the electrodes  120  can be sequentially positioned adjacent to every portion of the conductive layer to remove material from the entire conductive layer  111 . Alternatively, when it is desired to remove only selected portions of the conductive layer  111 , the electrodes  120  can be moved to those selected portions, leaving the remaining portions of the conductive layer  111  intact. 
       FIG. 4  is a partially schematic, side elevational view of an apparatus  260  that includes a support member  240  positioned to support the substrate  110  in accordance with another embodiment of the invention. In one aspect of this embodiment, the support member  240  supports the substrate  110  with the conductive layer  111  facing upwardly. A substrate drive unit  241  can move the support member  240  and the substrate  110 , as described above with reference to  FIG. 3 . First and second electrodes  220   a  and  220   b  are positioned above the conductive layer  111  and are coupled to a current source  221 . A support member  224  supports the electrodes  220  relative to the substrate  110  and is coupled to an electrode drive unit  223  to move the electrodes  220  over the surface of the support conductive layer  111  in a manner generally similar to that described above with reference to  FIG. 3 . 
     In one aspect of the embodiment shown in  FIG. 4 , the apparatus  260  further includes an electrolyte vessel  230  having a supply conduit  237  with an aperture  238  positioned proximate to the electrodes  220 . Accordingly, an electrolyte  231  can be disposed locally in an interface region  239  between the electrodes  220  and the conductive layer  111 , without necessarily covering the entire conductive layer  111 . The electrolyte  231  and the conductive material removed from the conductive layer  111  flow over the substrate  110  and collect in an electrolyte receptacle  232 . The mixture of electrolyte  231  and conductive material can flow to a reclaimer  233  that removes most of the conductive material from the electrolyte  231 . A filter  234  positioned downstream of the reclaimer  233  provides additional filtration of the electrolyte  231  and a pump  235  returns the reconditioned electrolyte  231  to the electrolyte vessel  230  via a return line  236 . 
     In another aspect of the embodiment shown in  FIG. 4 , the apparatus  260  can include a sensor assembly  250  having a sensor  251  positioned proximate to the conductive layer  111 , and a sensor control unit  252  coupled to the sensor  251  for processing signals generated by the sensor  251 . The control unit  252  can also move the sensor  251  relative to the substrate  110 . In a further aspect of this embodiment, the sensor assembly  250  can be coupled via a feedback path  253  to the electrode drive unit  223  and/or the substrate drive unit  241 . Accordingly, the sensor  251  can determine which areas of the conductive layer  111  require additional material removal and can move the electrodes  220  and/or the substrate  110  relative to each other to position the electrodes  220  over those areas. Alternatively, (for example, when the removal process is highly repeatable), the electrodes  220  and/or the substrate  110  can move relative to each other according to a pre-determined motion schedule. 
     The sensor  251  and the sensor control unit  252  can have any of a number of suitable configurations. For example, in one embodiment, the sensor  251  can be an optical sensor that detects removal of the conductive layer  111  by detecting a change in the intensity, wavelength or phase shift of the light reflected from the substrate  110  when the conductive material is removed. Alternatively, the sensor  251  can emit and detect reflections of radiation having other wavelengths, for example, x-ray radiation. In still another embodiment, the sensor  251  can measure a change in resistance or capacitance of the conductive layer  111  between two selected points. In a further aspect of this embodiment, one or both of the electrodes  220  can perform the function of the sensor  251  (as well as the material removal function described above), eliminating the need for a separate sensor  251 . In still further embodiments, the sensor  251  can detect a change in the voltage and/or current drawn from the current supply  221  as the conductive layer  111  is removed. 
     In any of the embodiments described above with reference to  FIG. 4 , the sensor  251  can be positioned apart from the electrolyte  231  because the electrolyte  231  is concentrated in the interface region  239  between the electrodes  220  and the conductive layer  111 . Accordingly, the accuracy with which the sensor  251  determines the progress of the electrolytic process can be improved because the electrolyte  231  will be less likely to interfere with the operation of the sensor  251 . For example, when the sensor  251  is an optical sensor, the electrolyte  231  will be less likely to distort the radiation reflected from the surface of the substrate  110  because the sensor  251  is positioned away from the interface region  239 . 
     Another feature of an embodiment of the apparatus  260  described above with reference to  FIG. 4  is that the electrolyte  231  supplied to the interface region  239  is continually replenished, either with a reconditioned electrolyte or a fresh electrolyte. An advantage of this feature is that the electrochemical reaction between the electrodes  220  and the conductive layer  111  can be maintained at a high and consistent level. 
       FIG. 5  is a partially schematic, side elevational view of an apparatus  360  that directs alternating current to the substrate  110  through a first electrolyte  331   a  and a second electrolyte  331   b . In one aspect of this embodiment, the first electrolyte  331   a  is disposed in two first electrolyte vessels  330   a , and the second electrolyte  331   b  is disposed in a second electrolyte vessel  330   b . The first electrolyte vessels  330   a  are partially submerged in the second electrolyte  331   b . The apparatus  360  can further include electrodes  320 , shown as a first electrode  320   a  and a second electrode  320   b , each coupled to a current supply  321  and each housed in one of the first electrolyte vessels  330   a . Alternatively, one of the electrodes  320  can be coupled to ground. The electrodes  320  can include materials such as silver, platinum, copper and/or other materials, and the first electrolyte  331   a  can include sodium chloride, potassium chloride, copper sulfate and/or other electrolytes that are compatible with the material forming the electrodes  320 . 
     In one aspect of this embodiment, the first electrolyte vessels  330   a  include a flow restrictor  322 , such as a permeable isolation membrane formed from Teflon™, sintered materials such as sintered glass, quartz or sapphire, or other suitable porous materials that allow ions to pass back and forth between the first electrolyte vessels  330   a  and the second electrolyte vessel  330   b , but do not allow the second electrolyte  330   b  to pass inwardly toward the electrodes  320  (for example, in a manner generally similar to a salt bridge). Alternatively, the first electrolyte  331   a  can be supplied to the electrode vessels  330   a  from a first electrolyte source  339  at a pressure and rate sufficient to direct the first electrolyte  331   a  outwardly through the flow restrictor  322  without allowing the first electrolyte  331   a  or the second electrolyte  330   b  to return through the flow restrictor  322 . In either embodiment, the second electrolyte  331   b  remains electrically coupled to the electrodes  320  by the flow of the first electrolyte  331   a  through the restrictor  322 . 
     In one aspect of this embodiment, the apparatus  360  can also include a support member  340  that supports the substrate  110  with the conductive layer  111  facing toward the electrodes  320 . For example, the support member  340  can be positioned in the second electrolyte vessel  330   b . In a further aspect of this embodiment, the support member  340  and/or the electrodes  320  can be movable relative to each other by one or more drive units (not shown). 
     One feature of an embodiment of the apparatus  360  described above with reference to  FIG. 5  is that the first electrolyte  331   a  can be selected to be compatible with the electrodes  320 . An advantage of this feature is that the first electrolyte  331   a  can be less likely than conventional electrolytes to degrade the electrodes  320 . Conversely, the second electrolyte  331   b  can be selected without regard to the effect it has on the electrodes  320  because it is chemically isolated from the electrodes  320  by the flow restrictor  322 . Accordingly, the second electrolyte  331   b  can include hydrochloric acid or another agent that reacts aggressively with the conductive layer  111  of the substrate  110 . 
       FIG. 6  is a top plan view of the microelectronic substrate  110  positioned beneath a plurality of electrodes having shapes and configurations in accordance with several embodiments of the invention. For purposes of illustration, several different types of electrodes are shown positioned proximate to the same microelectronic substrate  110 ; however, in practice, electrodes of the same type can be positioned relative to a single microelectronic substrate  110 . 
     In one embodiment, electrodes  720   a  and  720   b  can be grouped to form an electrode pair  770   a , with each electrode  720   a  and  720   b  coupled to an opposite terminal of a current supply  121  ( FIG. 3 ). The electrodes  770   a  and  770   b  can have an elongated or strip-type shape and can be arranged to extend parallel to each other over the diameter of the substrate  110 . The spacing between adjacent electrodes of an electrode pair  370   a  can be selected to direct the electrical current into the substrate  110 , as described above with reference to  FIG. 3 . 
     In an alternate embodiment, electrodes  720   c  and  720   d  can be grouped to form an electrode pair  770   b , and each electrode  720   c  and  720   d  can have a wedge or “pie” shape that tapers inwardly toward the center of the microelectronic substrate  110 . In still another embodiment, narrow, strip-type electrodes  720   e  and  720   f  can be grouped to form electrode pairs  770   c , with each electrode  720   e  and  720   f  extending radially outwardly from the center  113  of the microelectronic substrate  110  toward the periphery  112  of the microelectronic substrate  110 . 
     In still another embodiment, a single electrode  720   g  can extend over approximately half the area of the microelectronic substrate  110  and can have a semicircular planform shape. The electrode  720   g  can be grouped with another electrode (not shown) having a shape corresponding to a mirror image of the electrode  720   g , and both electrodes can be coupled to the current source  121  to provide alternating current to the microelectronic substrate in any of the manners described above with reference to  FIGS. 3-5 . 
       FIG. 7  is a partially schematic, cross-sectional side elevational view of a portion of the substrate  110  positioned beneath the electrode  720   c  described above with reference to  FIG. 6 . In one aspect of this embodiment, the electrode  720   c  has an upper surface  771  and a lower surface  772  opposite the upper surface  771  and facing the conductive layer  111  of the substrate  110 . The lower surface  772  can taper downwardly from the center  113  of the substrate  110  toward the perimeter  112  of the substrate  110  in one aspect of this embodiment to give the electrode  720   c  a wedge-shaped profile. Alternatively, the electrode  720   c  can have a plate-type configuration with the lower surface  772  positioned as shown in  FIG. 7  and the upper surface  771  parallel to the lower surface  772 . One feature of either embodiment is that the electrical coupling between the electrode  720   c  and the substrate  110  can be stronger toward the periphery  112  of the substrate  110  than toward the center  113  of the substrate  110 . This feature can be advantageous when the periphery  112  of the substrate  110  moves relative to the electrode  720   c  at a faster rate than does the center  113  of the substrate  110 , for example, when the substrate  110  rotates about its center  113 . Accordingly, the electrode  720   c  can be shaped to account for relative motion between the electrode and the substrate  110 . 
     In other embodiments, the electrode  720   c  can have other shapes. For example, the lower surface  772  can have a curved rather than a flat profile. Alternatively, any of the electrodes described above with reference to  FIG. 6  (or other electrodes having shapes other than those shown in  FIG. 6 ) can have a sloped or curved lower surface. In still further embodiments, the electrodes can have other shapes that account for relative motion between the electrodes and the substrate  110 . 
       FIG. 8A  is a partially schematic view of an electrode support  473  for supporting a plurality of electrodes in accordance with another embodiment of the invention. In one aspect of this embodiment, the electrode support  473  can include a plurality of electrode apertures  474 , each of which houses either a first electrode  420   a  or a second electrode  420   b . The first electrodes  420   a  are coupled through the apertures  474  to a first lead  428   a  and the second electrodes  420   b  are coupled to a second lead  428   b . Both of the leads  428   a  and  428   b  are coupled to a current supply  421 . Accordingly, each pair  470  of first and second electrodes  420   a  and  420   b  defines part of a circuit that is completed by the substrate  110  and the electrolyte(s) described above with reference to  FIGS. 3-5 . 
     In one aspect of this embodiment, the first lead  428   a  can be offset from the second lead  428   b  to reduce the likelihood for short circuits and/or capacitive coupling between the leads. In a further aspect of this embodiment, the electrode support  473  can have a configuration generally similar to any of those described above with reference to  FIGS. 1-7 . For example, any of the individual electrodes (e.g.,  320   a ,  320   c ,  320   e , or  320   g ) described above with reference to  FIG. 6  can be replaced with an electrode support  473  having the same overall shape and including a plurality of apertures  474 , each of which houses one of the first electrodes  420   a  or the second electrodes  420   b.    
     In still a further aspect of this embodiment, the electrode pairs  470  shown in  FIG. 8A  can be arranged in a manner that corresponds to the proximity between the electrodes  420   a ,  420   b  and the microelectronic substrate  110  ( FIG. 7 ), and/or the electrode pairs  470  can be arranged to correspond to the rate of relative motion between the electrodes  420   a ,  420   b  and the microelectronic substrate  110 . For example, the electrode pairs  470  can be more heavily concentrated in the periphery  112  of the substrate  110  or other regions where the relative velocity between the electrode pairs  470  and the substrate  110  is relatively high (see  FIG. 7 ). Accordingly, the increased concentration of electrode pairs  470  can provide an increased electrolytic current to compensate for the high relative velocity. Furthermore, the first electrode  420   a  and the second electrode  420   b  of each electrode pair  470  can be relatively close together in regions (such as the periphery  112  of the substrate  110 ) where the electrodes are close to the conductive layer  111  (see  FIG. 7 ) because the close proximity to the conductive layer  111  reduces the likelihood for direct electrical coupling between the first electrode  420   a  and the second electrode  420   b . In still a further aspect of this embodiment, the amplitude, frequency and/or waveform shape supplied to different electrode pairs  470  can vary depending on factors such as the spacing between the electrode pair  470  and the microelectronic substrate  110 , and the relative velocity between the electrode pair  470  and the microelectronic substrate  110 . 
       FIGS. 8B and 8C  illustrate electrodes  820  (shown as first electrodes  820   a  and second electrodes  820   b ) arranged concentrically in accordance with still further embodiments of the invention. In one embodiment shown in  FIG. 8B , the first electrode  820   a  can be positioned concentrically around the second electrode  820   b , and a dielectric material  829  can be disposed between the first electrode  820   a  and the second electrode  820   b . The first electrode  820   a  can define a complete 360° arc around the second electrode  820   b , as shown in  FIG. 8B , or alternatively, the first electrode  820   a  can define an arc of less than 360°. 
     In another embodiment, shown in  FIG. 8C , the first electrode  820   a  can be concentrically disposed between two second electrodes  820   b , with the dielectric material  829  disposed between neighboring electrodes  820 . In one aspect of this embodiment, current can be supplied to each of the second electrodes  820   b  with no phase shifting. Alternatively, the current supplied to one second electrode  820   b  can be phase-shifted relative to the current supplied to the other second electrode  820   b . In a further aspect of the embodiment, the current supplied to each second electrode  820   b  can differ in characteristics other than phase, for example, amplitude. 
     One feature of the electrodes  820  described above with respect to  FIGS. 8B and 8C  is that the first electrode  820   a  can shield the second electrode(s)  820   b  from interference from other current sources. For example, the first electrode  820   a  can be coupled to ground to shield the second electrodes  820   b . An advantage of this arrangement is that the current applied to the substrate  110  ( FIG. 7 ) via the electrodes  820  can be more accurately controlled. 
       FIG. 9  schematically illustrates an apparatus  560  for chemically, mechanically and/or electrolytically processing the microelectronic substrate  110  in accordance with an embodiment of the invention. In one aspect of this embodiment, the apparatus  560  has a support table  580  with a top-panel  581  at a workstation where an operative portion “W” of a polishing pad  582  is positioned. The top-panel  581  is generally a rigid plate to provide a flat, solid surface to which a particular section of the polishing pad  582  may be secured during material removal processes. 
     The apparatus  560  can also have a plurality of rollers to guide, position and hold the polishing pad  582  over the top-panel  581 . The rollers can include a supply roller  583 , first and second idler rollers  584   a  and  584   b , first and second guide rollers  585   a  and  585   b , and a take-up roller  586 . The supply roller  583  carries an unused or pre-operative portion of the polishing pad  582 , and the take-up roller  583  carries a used or post-operative portion of the polishing pad  582 . Additionally, the first idler roller  584   a  and the first guide roller  585   a  can stretch the polishing pad  582  over the top-panel  581  to hold the polishing pad  582  stationary during operation. A motor (not shown) drives at least one of the supply roller  583  and the take-up roller  586  to sequentially advance the polishing pad  582  across the top-panel  581 . Accordingly, clean pre-operative sections of the polishing pad  582  may be quickly substituted for used sections to provide a consistent surface for polishing and/or cleaning the substrate  110 . 
     The apparatus  560  can also have a carrier assembly  590  that controls and protects the substrate  110  during the material removal processes. The carrier assembly  590  can include a substrate holder  592  to pick up, hold and release the substrate  110  at appropriate stages of the material removal process. The carrier assembly  590  can also have a support gantry  594  carrying a drive assembly  595  that can translate along the gantry  594 . The drive assembly  595  can have an actuator  596 , a drive shaft  597  coupled to the actuator  596 , and an arm  598  projecting from the drive shaft  597 . The arm  598  carries the substrate holder  592  via a terminal shaft  599  such that the drive assembly  595  orbits the substrate holder  592  about an axis E-E (as indicated by arrow “R 1 ”). The terminal shaft  599  may also rotate the substrate holder  592  about its central axis F-F (as indicated by arrow “R 2 ”). 
     In one embodiment, the polishing pad  582  and a planarizing solution  587  define at least a portion of a material removal medium that mechanically and/or chemically-mechanically removes material from the surface of the substrate  110 . The polishing pad  582  used in the apparatus  560  can be a fixed-abrasive polishing pad having abrasive particles that are fixedly bonded to a suspension medium. Accordingly, the planarizing solution  587  can be a “clean solution” without abrasive particles because the abrasive particles are fixedly distributed across a polishing surface  588  of the polishing pad  582 . In other applications, the polishing pad  582  may be a non-abrasive pad without abrasive particles, and the planarizing solution  587  can be a slurry with abrasive particles and chemicals to remove material from the substrate  110 . In still further applications, both the polishing pad  582  and the planarizing solution  587  can be configured without abrasive particles or elements, as described in greater detail below with reference to  FIGS. 9-11 . 
     To remove material from the substrate  110  with the apparatus  560 , the carrier assembly  590  presses the face  113  of the substrate  110  against the polishing surface  588  of the polishing pad  582  in the presence of the planarizing solution  587 . The drive assembly  595  then orbits the substrate holder  592  about the axis E-E and optionally rotates the substrate holder  592  about the axis F-F to translate the substrate  110  across the planarizing surface  588 . As a result, the abrasive particles and/or the chemicals in the material removal medium remove material from the surface of the substrate  110  in a chemical and/or chemical-mechanical planarization (CMP) process. Accordingly, in one embodiment, the polishing pad  582  can smooth the substrate  110  by removing rough features projecting from the conductive layer  111  of the substrate  110 . 
     In a further aspect of this embodiment, the apparatus  560  can include an electrolyte supply vessel  530  that delivers an electrolyte to the planarizing surface  588  of the polishing pad  582  with a conduit  537 , as described in greater detail with reference to  FIG. 10 . The apparatus  560  can further include a current supply  521  coupled to the support table  580  and/or the top-panel  581  to supply an electrical current to electrodes positioned in the support table  580  and/or the top-panel  581 . Accordingly, the apparatus  560  can electrolytically remove material from the conductive layer  111  in a manner similar to that described above with reference to  FIGS. 1-8C . 
     In one aspect of an embodiment of the apparatus  560  described above with reference to  FIG. 9 , material can be sequentially removed from the conductive layer  111  of the substrate  110  first by an electrolytic process and then by a CMP process. For example, the electrolytic process can remove material from the conductive layer  111  in a manner that roughens the conductive layer  111 . After a selected period of electrolytic processing time has elapsed, the electrolytic processing operation can be halted and additional material can be removed via CMP processing. Alternatively, the electrolytic process and the CMP process can be conducted simultaneously. In either of these processing arrangements, one feature of an embodiment of the apparatus  560  described above with reference to  FIG. 9  is that the same apparatus  560  can planarize the substrate  110  via CMP and remove material from the substrate  110  via an electrolytic process. An advantage of this arrangement is that the substrate  110  need not be moved from one apparatus to another to undergo both CMP and electrolytic processing. 
     Another advantage of an embodiment of the apparatus  560  described above with reference to  FIG. 9  is that the processes, when used in conjunction with each other, are expected to remove material from the substrate  110  more quickly and accurately than some conventional processes. For example, as described above, the electrolytic process can remove relatively large amounts of material in a manner that roughens the microelectronic substrate  110 , and the planarizing process can remove material on a finer scale in a manner that smoothes and/or flattens the microelectronic substrate  110 . 
       FIG. 10  is a partially exploded, partially schematic isometric view of a portion of the apparatus  560  described above with reference to  FIG. 9 . In one aspect of an embodiment shown in  FIG. 10 , the top-panel  581  houses a plurality of electrode pairs  570 , each of which includes a first electrode  520   a  and a second electrode  520   b . The first electrodes  520   a  are coupled to a first lead  528   a  and the second electrodes  520   b  are coupled to a second lead  528   b . The first and second leads  528   a  and  528   b  are coupled to the current source  521  ( FIG. 9 ). In one aspect of this embodiment, the first electrode  520   a  can be separated from the second electrodes  520   b  by an electrode dielectric layer  529   a  that includes Teflon™ or another suitable dielectric material. The electrode dielectric layer  529   a  can accordingly control the volume and dielectric constant of the region between the first and second electrodes  520   a  and  520   b  to control electrical coupling between the electrodes. 
     The electrodes  520   a  and  520   b  can be electrically coupled to the microelectronic substrate  110  ( FIG. 9 ) by the polishing pad  582 . In one aspect of this embodiment, the polishing pad  582  is saturated with an electrolyte  531  supplied by the supply conduits  537  through apertures  538  in the top-panel  581  just beneath the polishing pad  582 . Accordingly, the electrodes  520   a  and  520   b  are selected to be compatible with the electrolyte  531 . In an alternate arrangement, the electrolyte  531  can be supplied to the polishing pad  582  from above (for example, by disposing the electrolyte  531  in the planarizing liquid  587 ) rather than through the top-panel  581 . Accordingly, the polishing pad  582  can include a pad dielectric layer  529   b  positioned between the polishing pad  582  and the electrodes  520   a  and  520   b . When the pad dielectric layer  529   b  is in place, the electrodes  520   a  and  520   b  are isolated from physical contact with the electrolyte  531  and can accordingly be selected from materials that are not necessarily compatible with the electrolyte  531 . In either embodiment, the electrodes  520   a  and  520   b  can be in fluid communication with each other and the conductive layer  111  via a common volume of electrolyte  531 . Each electrode  520   a ,  520   b  can be more directly electrically coupled to the conductive layer  111  ( FIG. 9 ) than to the other electrode so that electrical current passes from one electrode through the conductive layer  111  to the other electrode. 
     In one aspect of an embodiment of the apparatus shown in  FIG. 10 , the electrodes  520   a  and  520   b  face toward the face surface  113  ( FIG. 9 ) of the microelectronic substrate  110 , with the polishing pad  582  interposed between the electrodes  520   a  and  520   b  and the face surface  113 . As the microelectronic substrate  110  and the electrodes  520   a  and  520   b  move relative to each other, the electrodes can electrically couple to at least a substantial portion of the face surface  113 . Accordingly, the likelihood for forming electrically isolated “islands” in the conductive layer  111  ( FIG. 9 ) at the face surface  113  can be reduced when compared to conventional devices. Alternatively, if the apparatus includes only two electrodes, each configured to face toward about one-half of the face surface  113  (in a manner generally similar to that described above with reference to electrode  220   g  of  FIG. 6 ), then the electrodes can also electrically coupled to at least a substantial portion of the face surface  113 . 
     In any of the embodiments described above with reference to  FIG. 10 , the polishing pad  582  can provide several additional advantages over some conventional electrolytic arrangements. For example, the polishing pad  582  can uniformly separate the electrodes  520   a  and  520   b  from the microelectronic substrate  110  ( FIG. 9 ), which can increase the uniformity with which the electrolytic process removes material from the conductive layer  111  ( FIG. 9 ). The polishing pad  582  can also have abrasive particles  589  for planarizing the microelectronic substrate  110  in the manner described above with reference to  FIG. 9 . Furthermore, the polishing pad  582  can filter carbon or other material that erodes from the electrodes  520   a  and  520   b  to prevent the electrode material from contacting the microelectronic substrate  110 . Still further, the polishing pad  582  can act as a sponge to retain the electrolyte  531  in close proximity to the microelectronic substrate  110 . 
       FIG. 11  is a partially schematic, cross-sectional side elevational view of a rotary apparatus  660  for mechanically, chemically and/or electrolytically processing the microelectronic substrate  110  in accordance with another embodiment of the invention. In one aspect of this embodiment, the apparatus  660  has a generally circular platen or table  680 , a carrier assembly  690 , a polishing pad  682  positioned on the table  680 , and a planarizing liquid  687  on the polishing pad  682 . The polishing pad  682  can be a fixed abrasive polishing pad or, alternatively, the planarizing liquid  687  can be a slurry having a suspension of abrasive elements and the polishing pad  682  can be a non-abrasive pad. A drive assembly  695  rotates (arrow “G”) and/or reciprocates (arrow “H”) the platen  680  to move the polishing pad  682  during planarization. Accordingly, the motion of the microelectronic substrate  110  relative to the polishing pad  682  can include circular, elliptical, orbital, precessional or non-precessional motions. 
     The carrier assembly  690  controls and protects the microelectronic substrate  110  during the material removal process. The carrier assembly  690  typically has a substrate holder  692  with a pad  694  that holds the microelectronic substrate  110  via suction. A drive assembly  696  of the carrier assembly  690  typically rotates and/or translates the substrate holder  692  (arrows “I” and “J,” respectively). Alternatively, the substrate holder  692  may include a weighted, free-floating disk (not shown) that slides over the polishing pad  682 . 
     To planarize the microelectronic substrate  110  with the apparatus  660  in one embodiment, the carrier assembly  690  presses the microelectronic substrate  110  against a polishing surface  688  of the polishing pad  682 . The platen  680  and/or the substrate holder  692  then move relative to one another to translate the microelectronic substrate  110  across the polishing surface  688 . As a result, the abrasive particles in the polishing pad  682  and/or the chemicals in the planarizing liquid  687  remove material from the surface of the microelectronic substrate  110 . 
     The apparatus  660  can also include a current source  621  coupled with leads  628   a  and  628   b  to one or more electrode pairs  670  (one of which is shown in  FIG. 11 ). The electrode pairs  670  can be integrated with the platen  680  in generally the same manner with which the electrodes  520   a  and  520   b  ( FIG. 10 ) are integrated with the top panel  581  ( FIG. 10 ). Alternatively, the electrode pairs  670  can be integrated with the polishing pad  682 . In either embodiment, the electrode pairs  670  can include electrodes having shapes and configurations generally similar to any of those described above with reference to  FIGS. 3-10  to electrolytically remove conductive material from the microelectronic substrate  110 . The electrolytic process can be carried out before, during or after the CMP process, as described above with reference to  FIG. 9 . 
     In other embodiments of the invention, the apparatuses described above with reference to  FIGS. 3-11  can be used in accordance with other methods. For example, the electrolytic process can be used in addition to or in lieu of direct chemical interactions to oxidize conductive (including semiconductive) portions of the microelectronic substrate  110 . In one aspect of this embodiment, the electrolytic process can oxidize metals (such as platinum, rhodium, iridium, or gold) that are normally difficult or nearly impracticable to oxidize. An advantage of this arrangement is that it can make the use of such metals more practical for microelectronic applications. For example, platinum and other noble metals that resist oxidation are generally difficult to remove from the microelectronic substrate  110  without employing an isotropic etching chemical (i.e., a chemical that etches indiscriminately in all directions) and/or a very high downforce applied to the microelectronic substrate by the polishing pad  682 . The electrolytic process can anisotropically oxidize the platinum (or other conductive material) generally in a direction normal to the polishing surface  688  of the polishing pad  682 . 
     Once the conductive material is oxidized, it can be removed from the microelectronic substrate  110 . For example, it is believed that the electrolytic oxidation process roughens the surface of the conductive material and penetrates only a short distance beneath the surface. The oxidized material can then be removed by chemical and/or mechanical interactions with the polishing pad and/or planarizing solution. Furthermore, the downforce required to remove the oxidized material can be less than the downforce required by techniques that do not include an electrolytic process. In one specific example, it has been determined that a pressure of approximately 0.2 psi will remove 1,000 angstroms of platinum in ten minutes using an embodiment of the invention, whereas it is typically not possible to anisotropically remove platinum at any rate using conventional CMP techniques. Alternatively, the apparatuses described above with reference to  FIGS. 9-11  can oxidize and remove materials other than platinum at higher rates and/or with lower downforces than are typically required with conventional CMP apparatuses. 
     An advantage of increasing the rate with which conductive material can be oxidized and removed from the microelectronic substrates  110  is that the throughput of microelectronic substrates  110  can be increased when compared to conventional techniques. An advantage of anisotropically oxidizing and removing the conductive material from the microelectronic substrates  110  is that this technique can remove over-layers of the conductive material without undercutting adjacent structures in a lateral direction. Accordingly, methods in accordance with embodiments of the invention can more reliably form vias, conductive lines, and other conductive structures in the microelectronic substrate  110 . An advantage of reducing the downforce applied to the microelectronic substrate  110  during processing is that this technique can reduce the likelihood for damaging the microelectronic substrate  110  and can increase the life expectancy of the apparatus applying the downforce. 
     In a method in accordance with another embodiment of the invention, the characteristics of the electrical signal applied to the microelectronic substrate  110  can be selected to control the rate and/or manner with which the material is removed from the microelectronic substrate  110 . For example, the amplitude of the electrical current can be increased to increase the rate at which the conductive material oxidizes, and accordingly, the rate at which the oxidized material is available for removal. Alternatively, the amplitude of the electrical current can be reduced to reduce the oxidation rate. In another embodiment, the current can be halted to control the rate at which conductive material is removed from the microelectronic substrate  110 . For example, if the material is still susceptible to mechanical and/or chemical removal after the electrical current is halted, then halting the electrical current can slow, but not stop, the rate at which the material is removed. Alternatively, when mechanical removal and/or anisotropic chemical removal is not possible (for example, when the material includes platinum), then material removal can cease upon (or shortly after) halting the current applied to the conductive material. In any of these embodiments, the current amplitude can be varied from about 1 amp to about 10 amps, depending upon the desired oxidation and removal rate, and depending upon the type of material removed from the microelectronic substrate  110 . 
     In a further embodiment, other characteristics of the electrical signal can be controlled to control the material oxidation and removal rate. For example, the voltage applied to the material can be increased or decreased to increase or decrease, respectively, the material oxidation and removal rates. In one embodiment, the voltage can be varied up to about 100 volts. In another embodiment, the frequency with which the electrical signal is applied can be varied to control the material oxidation and removal rate. In one specific embodiment, a potential of about 10 volts rms can be applied to a platinum layer of the microelectronic substrate at a frequency of about 60 Hz while the microelectronic substrate  110  is engaged with the polishing pad  582  to anisotropically remove a portion of the platinum from the microelectronic substrate  110 . 
     In any of the foregoing embodiments, the polishing pad  582  can be a conventional pad, such as an IC 1000 polishing pad (available from Rodell, Inc. of Phoenix, Ariz.). In one aspect of this embodiment, the polishing pad  582  can have abrasive elements fixedly distributed in a suspension medium. Alternatively, the abrasive elements can be suspended in a planarizing liquid or slurry disposed between the polishing pad  582  and the microelectronic substrate  110 . In either embodiment, the abrasive elements can include chromium dioxide, aluminum oxide or silicon dioxide, and the planarizing liquid can include an electrolyte to electrically couple the microelectronic substrate to a source of electrical potential. In still a further embodiment, the abrasive elements can be eliminated entirely from the material removal medium, and the material can be removed from the microelectronic substrate  110  as a result of the electrolytic process and contact with the polishing pad  582 . 
     In yet a further embodiment, the electrical-mechanical interaction described above can be supplemented with a chemical interaction by exposing the microelectronic substrate  110  to one or more chemically reactive liquid solutions. In one aspect of this embodiment, the chemical solutions can be generally similar to those typically used for CMP processing. Alternatively, the chemical solutions, the chemical environment, and the chemical interactions can be different than those associated with conventional CMP techniques. For example, the solution can include an electrolytic fluid having (NH 4 ) 2 SO 4 , H 2 SO 4 , K 2 SO 4 , MgSO 4 , and/or H 3 PO 4 . Alternatively, the fluid can have other constituents, such as those described below with reference to  FIGS. 13A-C . The fluid can also include a relatively low concentration of chloride ions (e.g., from about 50 ppm to about 5,000 ppm for copper removal, and from about 100 ppm to about 5,000 ppm for platinum removal). In one specific example, suitable for platinum removal, the liquid can include a mixture of (NH) 2 SO 4  at a concentration of from about 1M (moles/liter) to about 5.5M, H 2 SO 4  at a concentration of up to about 0.5M, and about 500 ppm chloride ions. This is unlike typical planarizing liquids that include chlorine-based substances (such as KCl or HCl) and have much higher concentrations of chloride ions (for example, about 100,000 ppm). 
     An advantage of the chemical solutions described above is that they can more effectively remove materials, such as platinum, that are otherwise difficult to remove from the microelectronic substrate  110 . It is believed that in one aspect of this embodiment, the chloride ions&#39; can adsorb to the metal surface and roughen the exposed surface of the conductive material, making the conductive material easier to remove from the microelectronic substrate. 
     Another feature of the chemical solutions described above is that they can define a material removal environment that has a wider range of pHs than is typical for most conventional CMP operations. In fact, in one aspect of this embodiment, the pH of the environment can have any value from about 1 up to about 14. When the chemical solutions are used to remove platinum, the pH of the environment can be from about 1 to about 14, or, in a specific embodiment, less than about 3 or greater than about 10. While the pH of liquid typically used to planarize tungsten has a range from about 3 to about 4, the liquid in accordance with another aspect of the invention can have a pH of less than about 3 or greater than about 4. Still further, while the pH of a liquid typically used to planarize copper is about 7, the pH of a liquid in accordance with another aspect of the invention can have a pH of less than about 6 or greater than about 8. An advantage of the foregoing embodiments is that the user can select from a broader array of chemicals and chemical compounds to remove conductive material from the microelectronic substrate  110  because, so long as the compounds can electrically couple the conductive material to the adjacent electrodes, the compounds need not be selected on the basis of pH. As a result, the user can select chemicals that are less chemically reactive, easier to handle, and/or easier to dispose of after use than are typical CMP chemicals. 
       FIGS. 12A-B  schematically illustrate applying the foregoing methods and apparatuses to removing semiconductor material  1211  from a microelectronic substrate  1210  in accordance with an embodiment of the invention. In one aspect of this embodiment, the microelectronic substrate  1210  can include a substrate material  1215  having a recess  1212  in which the semiconductor material  1211  is disposed. The substrate material can include a borophosphate silicon glass (BPSG) or another substrate material. In one embodiment, the semiconductor material  1211  can include polysilicon doped with phosphorous or boron, and in other embodiments, the semiconductor material  1211  can include other compositions. In any of these embodiments, the semiconductor material  1211  can have a recessed surface  1214   a  directly over the recess  1212 . A portion of the semiconductor material  1211  can be removed to form a flat surface  1214   b  ( FIG. 12   b ) by electrolytically oxidizing the semiconductor material  1211  and removing the semiconductor material  1211  with chemical and/or mechanical forces, generally as described above. 
     Conventional techniques for removing doped polysilicon include planarizing the polysilicon with a slurry having a pH of from about 10.5 to about 11.5. The conventional slurry typically includes tetramethyl ammonium hydroxide (TMAH) and a suspension of silicon dioxide abrasive particles. An advantage of a method for removing polysilicon and other semiconductor materials in accordance with an embodiment of the invention is that the material can be removed without the use of abrasive elements, and the material can be removed using an electrolytic fluid having a pH less than 10.5 or greater than 11.5. Accordingly, the user can select electrolytic fluids (such as those described above) having a wider variety of pHs than are conventionally used. For example, in one particular embodiment, the electrolytic fluid can include dilute hydrofluoric acid or a combination of ammonium hydroxide and TMAH. The voltage applied to the semiconductor material  1211  can range from about 25 volts rms to about 100 volts rms, for phosphorous-doped polysilicon. For boron-doped polysilicon, the electrolytic fluid can include a mixture of hydrofluoric acid and TMAH, and the voltage applied to the semiconductor material can be approximately the same as that discussed above for phosphorous-doped polysilicon. 
     A further advantage of a method in accordance with an embodiment of the invention is that the electrolytic fluid selected to remove the semiconductor material  1211  from the microelectronic substrate  1210  can be selected to have little or no chemical interaction with the substrate material  1215 . Accordingly, for applications in which the semiconductor material  1211  is removed down to the level of the substrate material  1215 , the removal process can automatically stop (i.e., endpoint) when the substrate material  1215  is exposed. Accordingly, the process can eliminate other more cumbersome and/or less accurate conventional endpointing techniques. 
       FIGS. 13A-C  schematically illustrate methods for applying the foregoing techniques and apparatuses to removing a first conductive material  1311  and a second conductive material  1317  from a microelectronic substrate  1310 . The microelectronic substrate  1310  can include a substrate material  1315  having a dielectric portion  1316  (such as an oxide layer) with recesses  1312  or other features formed in the dielectric portion  1316 . The second conductive material  1317  is disposed in the recesses  1312  and on the dielectric portion  1316  (for example, in the form of a barrier layer), and the first conductive material  1311  is disposed on the second conductive material  1317 . In one embodiment, the first conductive material  1311  can include copper and the second conductive material  1317  can include tantalum, tantalum nitride, tungsten, tungsten nitride, titanium, titanium nitride, titanium silicon nitride, and/or tantalum silicon nitride. In other embodiments, the first and second conductive materials  1311 ,  1317  can include other compositions. 
     Referring now to  FIG. 13B , the first conductive material  1311  can be removed down to the level of the second conductive material  1317  using any of the devices described above with reference to  FIGS. 9-11 . In one aspect of this embodiment, the electrolytic fluid used to remove the first conductive material  1311  can include dilute H 3 PO 4 , or an organic acid, such as ammonium citrate. The electrolytic liquid can include chloride ions in concentrations generally similar to those described above. In one aspect of this embodiment, the concentration of the chloride ions can be used to control the rate at which the first conductive material  1311  is removed. For example, the peak removal rate can be achieved with a selected concentration of chloride ions that depends upon the other constituents of the electrolytic fluid and the composition of the first conductive material  1311 . The material removal rate can decrease with either an increase or a decrease in the concentration of chloride ions from the selected concentration. In a further aspect of this embodiment, an alcohol (such as isopropyl alcohol or acetone) can be added to slow the rate of material removal, either in conjunction with, or in lieu of controlling the concentration of chloride ions. 
     When the first conductive material  1311  includes copper, the downforce applied to the first conductive material  1311  by the polishing pad can vary from less than 1 psi to several psi. Furthermore, the material of the electrode positioned at least proximate to the first conductive material  1311  can include platinum or graphite, and the potential applied to the electrodes can vary from about 1 volt to about 15 volts, depending upon the composition of the electrolytic liquid. Whether the first conductive material  1311  includes copper or another element, compound or mixture, the chemical interaction with the first conductive material  1311  can include an etching process, a complexing process, and/or a chelating process. 
     Referring now to  FIG. 13C , the second conductive material  1317  can be removed down to the level of the dielectric portion  1316  using methods and apparatuses generally similar to those described above. In one particular aspect of this embodiment for which the second conductive material  1317  includes tantalum, the electrolytic fluid disposed on the second conductive material  1317  can include dilute hydrochloric acid, NH 4 Cl, and/or dilute phosphoric acid, or any organic or inorganic acid. In a further aspect of this embodiment, the electrolytic fluid can include a corrosion inhibitor to inhibit corrosion of the exposed first conductive material  1311 . For example, when the first conductive material  1311  includes copper, the corrosion inhibitor can include BTA. In a further aspect of this embodiment, the electrodes positioned proximate to the second conductive material  1317  can include graphite, and the voltage applied to the electrodes can be approximately the same as the voltage applied to the first conductive material  1311 . Alternatively, the voltage applied to the second conductive material  1317  can be different. In one embodiment, the downforce applied to the second conductive material  1317  can be the same as the downforce applied to the first conductive material  1311 , and alternatively, the downforce applied to the second conductive material  1317  can be different than the downforce applied to the first conductive material  1311 . 
     In a further aspect of this embodiment, the process for removing the second conductive material  1317  can automatically stop when the polishing pad engages the initially buried dielectric portion  1316 . Accordingly, an advantage of a method in accordance with an embodiment of the invention is that terminating the process for removing the second conductive material  1317  can be simpler than conventional techniques because a step specifically directed to endpointing is not required. 
     Another feature of a method in accordance with an embodiment of the invention described above is that the downforce applied to the microelectronic substrate  1310  while the first conductive material  1311  and the second conductive material  1317  are removed can be less than the downforces applied during conventional CMP operations (i.e., CMP operations that do not include electrolytically oxidizing the first and second conductive materials). As described above, an advantage of this feature is that the apparatus applying the downforce can have a longer life span. A further advantage is that the lower downforce may be less likely than conventional downforces to damage the substrate material  1315  and/or structures formed in the substrate material  1315  prior to applying the downforce. This feature may be particularly advantageous when the substrate material  1315  has a low dielectric constant, for example, a dielectric constant of from about 1.5 to about 3.0. Such materials can include porous silica. 
       FIG. 14A  is a schematic circuit representation of some of the components described above with reference to  FIG. 10 . The circuit analogy can also apply to any of the arrangements described above with reference to  FIGS. 3-13C . As shown schematically in  FIG. 14A , the current source  521  is coupled to the first electrode  520   a  and the second electrode  520   b  with leads  528   a  and  528   b , respectively. The electrodes  520   a  and  520   b  are coupled to the microelectronic substrate  110  with the electrolyte  531  in an arrangement that can be represented schematically by two sets of parallel capacitors and resistors. A third capacitor and resistor schematically indicates that the microelectronic substrate  110  “floats” relative to ground or another potential. 
     In one aspect of an embodiment shown in  FIG. 14A , the current source  521  can be coupled to an amplitude modulator  522  that modulates the signal produced by the current source  521 , as is shown in  FIG. 14B . Accordingly, the current source  521  can generate a high-frequency wave  1404 , and the amplitude modulator  522  can superimpose a low-frequency wave  1402  on the high-frequency wave  1404 . For example, the high-frequency wave  1404  can include a series of positive or negative voltage spikes contained within a square wave envelope defined by the low-frequency wave  1402 . Each spike of the high-frequency wave  1404  can have a relatively steep rise time slope to transfer charge through the dielectric to the electrolyte, and a more gradual fall time slope. The fall time slope can define a straight line, as indicated by high-frequency wave  1404 , or a curved line, as indicated by high-frequency wave  1404   a . In other embodiments, the high-frequency wave  1404  and the low-frequency wave  1402  can have other shapes depending, for example, on the particular characteristics of the dielectric material and electrolyte adjacent to the electrodes  420 , the characteristics of the substrate  110 , and/or the target rate at which material is to be removed from the substrate  110 . 
     An advantage of this arrangement is that the high frequency signal can transmit the required electrical energy from the electrodes  520   a  and  520   b  to the microelectronic substrate  110 , while the low frequency superimposed signal can more effectively promote the electrochemical reaction between the electrolyte  531  and the conductive layer  111  of the microelectronic substrate  110 . Accordingly, any of the embodiments described above with reference to  FIGS. 3-13C  can include an amplitude modulator in addition to a current source. 
     From the foregoing, it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. For example, some or all of the techniques described above in the context of a web-format apparatus (such as the one shown in  FIG. 9 ) can be applied was well to a rotary apparatus (such as the one shown in  FIG. 11 ) and vice versa. Accordingly, the invention is not limited except as by the appended claims.