Abstract:
An apparatus for electropolishing a conductive layer on a wafer using a solution is disclosed. The apparatus comprises an electrode assembly immersed in the solution configured proximate to the conductive layer having a longitudinal dimension extending to at least a periphery of the wafer, the electrode assembly including an elongated contact electrode configured to receive a potential difference, an isolator adjacent the elongated contact electrode, and an elongated process electrode adjacent the isolator configured to receive the potential difference, a voltage supply is configured to supply the potential difference between the contact electrode and the process electrode to electropolish the conductive layer on the wafer.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. application Ser. No. 10/391,924, filed Mar. 18, 2003, incorporated herein by reference. 
    
    
     FIELD 
     The present invention generally relates to semiconductor integrated circuit technology and, more particularly, to an electroetching or electropolishing process and apparatus. 
     BACKGROUND 
     Conventional semiconductor devices generally include a semiconductor substrate, usually a silicon substrate, and a plurality of sequentially formed dielectric layers such as silicon dioxide and conductive paths or interconnects made of conductive materials. Interconnects are usually formed by filling a conductive material in trenches etched into the dielectric layers. In an integrated circuit multiple levels of interconnect networks laterally extend with respect to the substrate surface. Interconnects formed in different layers can be electrically connected using vias or contacts. 
     The filling of a conductive material into features such as vias, trenches, pads or contacts, can be carried out by electrodeposition. In electrodeposition or electroplating method, a conductive material, such as copper is deposited over the substrate surface including into such features. Then, a material removal technique is employed to planarize and remove the excess metal from the top surface, leaving conductors only in the features or cavities. The standard material removal technique that is most commonly used for this purpose is chemical mechanical polishing (CMP). Chemical etching and electropolishing, which is also referred to as electroetching or electrochemical etching, are also attractive process options that are being evaluated for this application. Copper is the material of choice, at this time, for interconnect applications because of its low resistivity and good electromigration properties. Therefore, the present invention will be described for the electropolishing of copper and copper alloy layers as an example, although electropolishing of other materials such as Pt, Co, Ni etc., can also be achieved using the method and apparatus of this invention. 
     Standard electroplating techniques yield copper layers that deposit conformally over large features, such as features with widths larger than a few micrometers. This results in a plated wafer surface topography that is not flat.  FIG. 1A  shows a workpiece surface  100  with an exemplary via  102  and an exemplary trench  104  coated with conductor  106  using standard electroplating technique. As can be seen from this figure, although the surface of the conductor  106  may be flat over the small via  102 , the surface of the conductor  106  over the larger trench  104  has a step “S”. During the excess conductor or overburden removal process step employing CMP, etching or electroetching, this non-flat surface topography needs to be planarized as the excess conductor is removed from the surface leaving it only within the features. If planarization is not achieved, as the thickness of the conductor is reduced, presence of the step S causes loss of conductor from within the large trench. Dashed lines  110  and  112  schematically show how conductor loss from the trench may increase from an amount “d” to a larger amount “D” as the excess conductor thickness on the surface is reduced from “t” to nearly zero, respectively. As can be appreciated, such conductor loss from within features is not acceptable. 
     CMP techniques have been developed to provide the capability of planarizing and at the same time removing the excess conductor layers. This is shown in  FIG. 1B  as dashed lines of  120  and  122 . After excess conductor removal, the resulting surface is ideally planar as indicated by dashed line  122 , and both the via  102  and the trench  104  are completely filled with the conductor. It should be noted that any remaining part of the excess conductor along with any other conductor layer (such as a barrier layer) are all removed to assure electrical isolation between the conductors within features  102  and  104 . 
     Planarization capability of standard electroetching techniques is not as good as CMP. Therefore, results from these processes may lie somewhere between the cases shown in  FIGS. 1A and 1B . Planarization capability of electroetching may be increased and the ideal result shown as dashed line  122  in  FIG. 1B  may be approached by employing a planarization pad or workpiece surface influencing device (WSID) which introduces mechanical action on the wafer surface as the conductor removal from the workpiece surface is performed. This way it may be possible to planarize the non-planar or non-flat copper surface as the excess copper is removed. Since there is mechanical action in such processes they are referred to as Electrochemical Mechanical Etching (ECME) or Electrochemical Mechanical Polishing. As the name suggest, in such approaches, electroetching is carried out as the wafer surface is contacted by a planarization pad and relative motion is established between the wafer surface and the planarization pad. 
     As described above, standard electroplating techniques yield conformal deposits and non-planar workpiece surfaces that need to be planarized during the excess material removal step. Newly developed electrodeposition techniques, which are collectively called Electrochemical Mechanical Deposition (ECMD) methods, utilize a pad or WSID in close proximity of the wafer surface during conductor deposition. Action of the WSID during plating gives planar deposits with flat surface topography even over the largest features present on the workpiece surface. Such a planar deposit is shown as layer  130  in  FIG. 1C . Removal of excess conductive material, such as copper from such planar deposits does not require further planarization during the material removal step. Therefore, CMP, electroetching, chemical etching, electrochemical mechanical etching and chemical mechanical etching techniques may all be successfully employed for removing the overburden in a planar and uniform manner in this case. 
     There are several patents and patent applications describing the electroetching process carried out with the assistance of the mechanical action provided by a pad or WSID. Details of such processes are given in the following patents and patent applications: U.S. Pat. No. 6,402,925; U.S. application Ser. No. 10/238,665, entitled “Method and Apparatus for Electro-Chemical Mechanical Deposition,” filed Sep. 9, 2002 now U.S. Pat. No. 6,902,659; U.S. application Ser. No. 09/671,800, entitled “Process to Minimize and/or Eliminate Conductive Material Coating over the Top Surface of a Patterned Substrate and Layer Structure Made Thereby,” filed Sep. 28, 2000; U.S. application Ser. No. 09/841,622, entitled “Electroetching Process and System,” filed Apr. 23, 2001, now U.S. Pat. No. 6,852,630; U.S. application Ser. No. 10/201,604, entitled “Multi Step Electrodeposition Process for Reducing Defects and Minimizing Film Thickness,” filed Jul. 22, 2002, now U.S. Pat. No. 6,946,066; and U.S. Provisional Application Ser. No. 60/362,513, filed Sep. 1, 2003, entitled “Method and Apparatus for Planar Material Removal Technique Using Multi-Phase Process Environment,” filed Mar. 6, 2002. 
     During the standard electrodeposition and electroetching processes, workpiece or wafer is typically contacted on its front surface near its edge, all around its circumference. The conventional way of contacting the wafer involves a clamp-ring design where electrical contacts such as spring-loaded metallic fingers are pressed against the edge of the surface along the perimeter of the wafer. Contacts are protected from the process solution using seals such as O-rings or lip seals that are pushed against the wafer surface at the edge. Advance of low-k material usage in wafer processing, however, is bringing new restrictions to the use of such contacts. Low-k materials are relatively soft and mechanically weak. Pressing metallic contacts and seals against conductive films deposited on low-k materials causes damage to such materials and may even cause loss of electrical contact since the conductive film over the damaged low-k layer may itself become discontinuous. To address this challenge, a new method for forming an electrical contact to a wafer edge has been disclosed in U.S. Pat. Nos. 6,471,847 and 6,251,235, which are commonly owned by the assignee of the present invention. In this approach there is no metallic contact touching the wafer. Electrical contact is achieved using a liquid conductor, which is confined within a chamber. 
     Review of the above mentioned art related to Electrochemical Mechanical Etching and Electrochemical Mechanical Deposition techniques will reveal that these methods have the capability to electrotreat, i.e., electrodeposit as well as electropolish, full surface of the wafer without any need to set aside a “contacting region” protected from the process solution, such as the edge surface region that would be under a clamp-ring in an apparatus that uses electrical contacts with a clamp-ring design. 
     Contact designs that allow full-face electrodeposition or electroetching have been described in the following U.S. patent applications: U.S. application Ser. No. 09/685,934, entitled “Device Providing Electrical Contact to the Surface of a Semiconductor Workpiece During Metal Plating,” filed Oct. 11, 2000, now U.S. Pat. No. 6,497,800; U.S. application Ser. No. 09/735,546, entitled “Method of and Apparatus for Making Electrical Contact to Wafer Surface for Full-Face Electroplating or Electropolishing,” filed Dec. 14, 2000, now U.S. Pat. No. 6,482,307; and U.S. application Ser. No. 09/760,757, entitled “Method and Apparatus for Electrodeposition of Uniform Film with Minimal Edge Exclusion on Substrate,” filed Jan. 17, 2001, now U.S. Pat. No. 6,610,190, all commonly owned by the assignee of the present invention. As described in these applications, one method of making electrical contact to the workpiece surface involves physically touching the conductive surface of the workpiece by conductive contact elements, such as wires, fingers, springs, rollers, brushes etc., and establishing a relative motion between the contact elements and the wafer surface so that different sections of the wafer surface is physically and electrically contacted at different times. In another method, electrical contact to the workpiece surface is achieved without physically touching the wafer by the conductive contact elements. Either way, electrical contacts may be made substantially all over the surface of the wafer or only at the edge region of the wafer. 
     Although much progress has been made in electropolishing approaches and apparatus including contacting means of the workpiece during electropolishing, there is still need for alternative contacting means and electroetching techniques that uniformly remove excess conductive films from workpiece surfaces without causing damage and defects especially on advanced wafers with low-k materials. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the identified limitations of conventional electropolishing approaches and provides alternative contacting means and electroetching techniques that uniformly remove conductive films from a workpiece surface. 
     In one or more embodiments of the invention, an apparatus and a method for electropolishing a surface of a conductive layer on a workpiece are disclosed. The method of the present invention includes the steps immersing a contact electrode in a contact solution, contacting a portion of the surface of the conductive layer with the contact solution to define a contact region, immersing a process electrode in a process solution, contacting a portion of the surface of the conductive layer with the process solution to define a process region, and applying an electrical potential between the contact electrode and the process electrode to electropolish the surface of the conductive layer of the process region. 
     According to another aspect of the invention, the method further includes the step of moving at least one of the contact or process region from a first location to a second location on the surface of the conductive layer. In moving at least one of the regions from the first location to another location throughout the process, the entire surface of the conductive layer can be electropolished. 
     In another aspect of the invention, the contact solution and the process solution are the same conductive solution. The conductive solution contacts the surface of the conductive layer. 
     According to another aspect of the invention, a second contact electrode is provided, and the method further includes the steps of immersing the second electrode in the contact solution, contacting a portion of the surface of the conductive layer with the contact solution to define a second contact region, and applying an electrical potential between the contact electrodes and the process electrode to electropolish the second contact region. 
     According to another aspect of the invention, the method further includes the step of contacting the surface of the conductive layer with a top surface of a pad thereby planarizing non-uniformities of the surface of the conductive layer during electropolishing. The top surface of the pad may be abrasive. The pad may intermittently contact the surface of the conductive layer. 
     In another embodiment of the present invention, an apparatus for electropolishing a surface of a conductive layer on a workpiece includes a contact unit containing a contact solution, a contact electrode immersed therein and having an opening through which the contact solution contacts a portion of the surface of the conductive layer to define a contact region, and a process unit containing a process solution, a process electrode immersed therein and having an opening through which the process solution contacts a portion of the surface of the conductive layer to define a process region configured to electropolish the surface of the conductive layer defined by the process region in response to a potential difference applied between the contact electrode and the process electrode. 
     According to other aspects of the invention, the contact electrode and/or the process electrode may be proximate to the surface of the conductive layer. The potential difference includes a DC voltage or a variable voltage. 
     According to yet another aspect of the invention, a mechanism produces relative motion between the process region and the surface of the conductive layer to electropolish substantially the whole surface of the conductive layer on the workpiece. The mechanism may further produce relative motion between the contact region and the surface of the conductive layer. 
     According to additional aspects of the invention, the process unit includes a plurality of process openings through which the process solution contacts portions of the surface of the conductive layer to define a plurality of process regions and the potential difference applied between the contact electrode and the process electrode electropolishes the surface of the conductive layer defined by the plurality of process regions. Moreover, the contact unit includes a plurality of contact openings through which the contact solution contacts portions of the surface of the conductive layer, each contact opening includes a contact electrode disposed therein and the potential difference applied between the contact electrodes and the process electrode electropolishes the surface of the conductive layer defined by the plurality of process regions. 
     In yet other aspects of the invention, a first set of contact units is configured to contact portions of the surface of the conductive layer wherein the potential difference applied between the contact electrodes of the first set of contact units and the process electrode electropolishes the surface of the conductive layer defined by a first set of process regions. Moreover, a second set of contact units is configured to contact portions of the surface of the conductive layer wherein a second potential difference applied between the contact electrodes of the second set of contact units and the process electrode electropolishes the surface of the conductive layer defined by a second set of process regions. 
     In yet another aspect of the invention, a zone switch is configured to select the first contact zone or the second contact zone to apply the potential difference. The potential difference and the second potential difference may be different voltages. 
     The above and additional advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description when taken in conjunction with the accompanying drawings. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the identified limitations of conventional electropolishing approaches and provides alternative contacting means and electroetching techniques that uniformly remove conductive films from a workpiece surface. The present invention achieves electropolishing of the conductive films through the combination of the use of a process solution and electrical contact electrodes that do not make physical contact to the workpiece surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic illustration of a substrate having a non-planar copper overburden layer which has been deposited using a conventional deposition process; 
         FIG. 1B  is a schematic illustration of the substrate shown in  FIG. 1A  wherein a planarization process has been applied to the non-planar copper overburden layer; 
         FIG. 1C  is a schematic illustration of a substrate having a planar copper overburden layer which has been deposited using an electrochemical mechanical deposition process; 
         FIG. 2A  is a schematic cross-sectional view of a portion of a semiconductor wafer having a copper layer formed on it; 
         FIG. 2B  is a schematic cross sectional view of the semiconductor wafer in detail; 
         FIG. 3A  is a schematic illustration of an embodiment of an electropolishing system of the present invention; 
         FIGS. 3B-3D  are schematic illustrations of various embodiments of the contact units for establishing electrical contact with wafer surface through the process solution; 
         FIGS. 3E-3G   3 D are schematic illustrations of various designs of the contact units and process units for establishing electrical contact with and processing a wafer surface; 
         FIG. 4A  is a schematic illustration of another embodiment of an electropolishing system of the present invention including multiple contact and process electrodes; 
         FIG. 4B  is a schematic planar view of the electropolishing system shown in  FIG. 4A ; 
         FIG. 5  is a schematic illustration of yet another embodiment of an electropolishing system of the present invention using multiple contact electrodes with a single process electrode; 
         FIGS. 6A-6B  are schematic illustrations of a holder structure used with the electropolishing system of the present invention; 
         FIGS. 8A-8B  are schematic illustrations of another holder structure used with the electropolishing system of the present invention; 
         FIGS. 9A-9B  are schematic illustrations of yet another holder structure used with the electropolishing system of the present invention; 
         FIG. 10A-10B  are schematic illustrations of other embodiments of an electropolishing system of the present invention using multiple contact electrodes with a single process electrode; 
         FIGS. 11A-11B  are schematic illustrations of stages of an electropolishing process using the electropolishing system shown in  FIG. 10A ; 
         FIG. 12A  is a schematic side view of an electropolishing system of the present invention wherein a holder structure of the system includes and array of contact and process units; 
         FIG. 12B  is a schematic perspective view of the system shown in  FIG. 2A ; 
         FIG. 12C  is a top plan view of the system shown in  FIG. 12B ; 
         FIG. 13  is a schematic illustration of the holder structure shown in  FIGS. 12A-12C , wherein the holder structure has multiple electropolishing zones; 
         FIG. 14A  is a schematic perspective view of an embodiment of an holder structure including an array of electrodes and insulating members; 
         FIG. 14B  is a schematic plan view of the array of an holder structure shown in  FIG. 14B : 
         FIG. 15  is a schematic cross sectional view of the array of the holder structure including a compressible material layer; 
         FIG. 16  is a schematic side view of the holder structure with electrical contact devices; and 
         FIG. 17  is a schematic cross sectional view of a contact device of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     As will be described below, the present invention provides a method and a system to electroetch or electropolish a conductive material layer deposited on a surface of a semiconductor. The invention can be used with Electrochemical Mechanical Etching processes or conventional electroetching systems. The present invention achieves electroetching of the conductive material through the combination of the use of a process solution and electrical contact elements that do not make physical contact to the workpiece surface. 
     Reference will now be made to the drawings wherein like numerals refer to like parts throughout.  FIG. 2A  shows a cross-sectional view of a portion of a workpiece  100   a . The workpiece may be an exemplary portion of a preprocessed semiconductor wafer. As also shown in  FIG. 2B  in detail, a top layer  102   a  of the workpiece  100   a  may include a layer of conductive material such as electroplated copper. A bottom layer  104   a  of the workpiece may include an insulating layer  106   a  such as a low-k dielectric film and substrate  108  of the wafer, preferably silicon. In this embodiment, although the conductive layer  102   a  is a part of the workpiece  100   a , it is within the scope of the present invention that the workpiece  100   a  may be entirely made of a conductive material. 
     The insulating layer is patterned to provide a via feature  110  and a trench feature  112 . The features and surface  114  of the insulating layer may be lined with a barrier layer  116  such as a layer of Ta, TaN, Ti, WCN, WN, TiN or a composite of these materials. The barrier layer may be also coated with a conductive seed layer such as a copper seed layer that is not shown in  FIG. 2B  for the purpose of clarity. Such seed layers are commonly deposited on semiconductor wafers before conductive layer deposition. The workpiece  100   a  may comprise a plurality of via, trench and other features. As illustrated in  FIG. 2B , in order to exemplify one embodiment of the present invention the surface  103   a  of the conductive layer  102   a  may be planar, i.e. may not have a surface topography having high and low regions formed during the deposition of the conductive layer  102   a . It should be appreciated that the invention can also process non-planar wafer surfaces. 
       FIG. 3A  schematically explains how electropolishing of a material on a wafer surface may be achieved using a remote electrical contact to the wafer. The cross-sectional segment in  FIG. 3A  shows a portion of an exemplary electroetching or electropolishing system  200  to electrochemically etch a portion of the copper layer  102   a , off the surface of the workpiece  100   a,  which is held by a wafer carrier (not shown). The electroetching system in this example embodiment has a contact unit  202  and a process unit  204 . As will be described more fully below, the contact unit  202  is able to establish electrical contact with the conductive layer  102   a  through a liquid contact solution. 
     In this respect, the contact unit  202  comprises a contact container  206  or a contact nozzle to contain a contact solution  208 . A contact electrode  209  is placed inside the contact container  206  and thus immersed in the contact solution  208 . The contact electrode does not physically touch the surface  103   a  of the copper layer  102   a . The contact electrode  209  is electrically connected to a positive terminal of a power source  210 . Contact solution  208  fills the container through a contact inlet  212  and leaves the container through contact opening  214 . The inlet  212  may be connected to a contact solution reservoir (not shown). The contact opening  214  is placed in close proximity of a contact region  220   a  of the surface  103   a  of the conductive layer  102   a . As the contact solution  208  flows through the opening  214 , it physically touches the contact area and establishes electrical communication between the electrode  209  and the contact region  220   a  since it is a conductive liquid. For lowest voltage drop, the contact electrode  209  is as close as possible to the contact area  220   a . However, if the resistivity of the contact solution  208  is low and the voltage drop is not a concern the contact electrode  209  may even be placed outside the contact container and placed anywhere as long as it maintains physical contact with the contact solution  208 . 
     The process unit  204  comprises a process container  222  or a process nozzle to contain process solution  224 , which is an electroetching or electropolishing solution. A process electrode  226  is located inside the process container  222  and kept immersed in the process solution  224 . It should be noted that the process electrode does not have to be confined in the process container. It may be outside as long as it physically touches the process solution and therefore establishes electrical contact with it. The process electrode  226  is electrically connected to a negative terminal of the power source  210 . Process solution  224  fills the process container through a process inlet  228  and exits the container through process opening  230 . The process solution  224  can be re-circulated or agitated. The inlet  228  may be connected to a process solution reservoir (not shown). The process opening  230  is placed in close proximity of a process region  220   b  of the surface  103   a  of the conductive layer  102   a . In this embodiment, the process region  220   b  may be approximately equal to the area of the process opening  230 . The process solution  224  flowing through the opening  230  contacts the process region  220   b  and establishes electrical contact between the process electrode  226  and the process region  220   b . Although a specific contact region and process region are illustrated in  FIG. 3A , it is understood that these regions may be located anywhere on the workpiece. Furthermore, a plurality of contact units and process units may be used. The contact solution and the process solution may be different solutions or they may be same. If they are the same solution, they need to be effective electroetching or electropolishing solutions for the material to be removed from the workpiece surface. 
     The contact units and process units may be constructed in different ways using various different materials. For example, it is possible that the contact electrode  209  is on the wall of the container  206  or it actually is the wall of the container  206 . Similar approach may be used for the construction of the process container  222 . The contact or process units may comprise an insulating spongy material within which the conductive electrodes are embedded.  FIG. 3B  shows such a case for the contact unit  202   a , comprising insulating spongy material  250 , which holds and passes through the contact solution  208 . Contact electrode  209  touches the contact solution  208  in the sponge  250 . It should be noted that, as shown in  FIG. 3B , the spongy material may physically touch the copper film  102   a  surface during electropolishing since it is a soft material and does not damage the surface. Similarly, use of an insulating spongy material or insulating soft pad in the construction of the process unit, which may physically contact the wafer surface during processing is within the scope of this invention. 
     Referring to  FIG. 3A , electroetching of the copper layer  102   a  is initiated at the process region  220   b  when a potential is applied between the contact electrode  209 , which is anode, and the process electrode  226 , which is cathode. The electrical current passes from the contact electrode  209  to the contact solution  208  and through the contact solution enters the copper layer  102   a  at the contact region  220   a . The current then flows in the copper layer  102   a  towards the process region  220   b , enters the electroetching solution  224  and flows to the process electrode  226 . In this respect, the contact electrode  209  is more anodic than the copper film at the contact region  220   a  and the copper film at the process region  220   b  is more anodic than the cathode  226 . The anodic voltage on the copper film at the process region causes electropolishing or electroetching of the copper in this particular region. The copper removed from the substrate in this region deposits on the process electrode  226 . If the solution is formulated to contain complexing agents it is possible that copper complexes to stay in the solution rather than deposit on the process electrode  226 . However, in this embodiment the process solution is a standard electroetching solution such as a phosphoric acid solution. The contact electrode  209  is made of an inert material such as Pt or Pt-coated metal, stainless steel, conductive mesh or foam etc., and therefore anodic voltage on this inert electrode cannot remove any material. It may, however, generate bubbles of gas, which can be removed by the flowing solution or by other designs built in the contact unit. One such design is shown in  FIG. 3C  and it includes a permeable barrier  260  placed over the contact electrode  209 . The permeable barrier  260  is porous and it lets the contact solution  208  through. It, however, does not allow the bubbles to go to the substrate surface by guiding them towards a bleed opening  261 , which directs them away from the workpiece surface. Similar structure may be used in the process unit also. Another design shown in  FIG. 3D  is a two-chamber contact container  206   a , which comprises a primary container  206   aa  and a secondary container  206   aaa . The contact electrode  209  is placed in the primary container  206   aa , and therefore any bubble that is generated may be diverted away from the substrate surface through the bleed opening  261   a . More complex designs of contact containers and process containers utilizing multi chambers can be used for bubble minimization or elimination. 
     Referring back to  FIG. 3A , since the copper film at the contact region  220   a  is more cathodic compared to the contact electrode  209 , no copper dissolution is expected in this region. In fact, copper is protected by this cathodic voltage. In this respect, it is important that the contact solution does not contain any ions of materials that can deposit onto the surface of the copper layer and the contact electrode  209  does not contain any material that may be etched or electroetched by the contact solution  208 . Therefore, deposition solutions containing ionic species of metals are not suitable for use as a contact solution. 
     During the process, the process unit is preferably moved between the edge of the workpiece and the center of the workpiece while the workpiece is rotated or otherwise moved. The movement of the process unit along the radius of the wafer can cause electoetching of the entire surface of the wafer as the wafer is rotated. Other motions can also be used. What is important is to make every point on the wafer a process region at some point in time to remove copper from substantially the whole surface. Scanning of the wafer surface by the process unit can be accomplished by moving the wafer, the process unit or both with respect to each other. 
     It is possible to design contact units and process units in different shapes and forms. These designs include but are not limited to circular, oval, pie shape, linear and others and they define the shape of the contact region and the process region. Depending upon the nature of the relative motion established between the workpiece surface and the contact and process units the most appropriate shapes of these units may be selected for the most uniform electroetching. Three of such examples are shown in  FIGS. 3E ,  3 F, and  3 G, which show the top view of process units  270   a ,  270   b  and  270   c , and contact units  280   a ,  280   b  and  280   c . Wafer  290  is placed in close proximity (preferably 0.1 to 5 mm range depending on the conductivity of the solutions used) of the process and contact units so that its copper coated surface (not shown) is wetted by the process and contact solutions. As explained before, when the electroetching process is initiated wafer  290  in  FIG. 3E  may be translated over the contact units  280   a , and the process unit  270   a  in a linear direction  291 . Wafer may also be slowly rotated. The linear motion may or may not be bi-directional. During process, the process unit  270   a  effectively scans the whole surface of the wafer for uniform material removal. Multiple contact units assure electrical contact to wafer at all times. Even more process and contact units may be used in the design (see for example,  FIGS. 6A ,  6 B,  7 A,  7 B,  8 A,  8 B,  9 A,  9 B). A specific design of contact unit  280   b  and process unit  270   b , appropriate for rotational motion of the wafer  290  is shown in  FIG. 3F . The pie-shaped process region in this case scans the wafer surface for uniform material removal from the whole front surface. Contact unit  280   b  maybe placed anywhere at the edge of the wafer. Again, multiple contact and process units may be utilized in this design. In  FIG. 3G , a ring-shaped contact region is provided. The process region, where material removal is carried out constitutes the rest of the wafer surface. In this case copper left in the contact region needs to be removed later using another process such as chemical etching or electrochemical etching. There are many other shapes and forms of the process and contact units that can be optimized for best uniformity of material removal. 
       FIGS. 4A ,  4 B and  5  illustrate two alternative electroetching systems that may include a plurality of contact units and process units. The contact and process units in these embodiments are held by various base structures that allow units to use the same electroetching solution as the contact solution as well as the process solution. In both embodiments, electrical contact to the wafer surface is established through the electroetching solution applied through the contact units. The contact electrodes do not physically contact to the surface of the wafer, however, as described earlier a soft, sponge or pad like material may be placed in the contact or process units and this material may touch the workpiece surface at the contact region and the process region. The electroetching solution provides the conductive path between the contact electrode and the conductive surface of the wafer. 
     Exemplary electroetching or electropolishing system  300  of  FIG. 4A  may be used for processing copper layer  102   b  of the substrate  100   b , which is held by a carrier (not shown). The electroetching system in this example embodiment has also a contact unit  302  and a process unit  304 . Differing from the previous embodiment, the units  302 ,  304  are held by or formed in a holder structure  301 . The holder structure  301  in this embodiment is shaped as a plate having a top surface  303  and a bottom surface  305 . As described in the previous embodiment, the contact unit  302  is able to establish electrical contact with the conductive layer  102   b  through a liquid electrical contact. During the process, the holder structure  301  and the workpiece may be moved relative to one another. The contact unit  302  or a contact nozzle may be comprised of a contact hole  306  formed in the holder structure  301 . A contact electrode  309  inside the contact unit  306  is immersed in an electroetching solution  308 . It should be understood that the contact electrode shown in  FIG. 4A  may totally fill the contact hole  306  in which case the electroetching solution  308  would mainly wet the top surface of the contact electrode  309 . The top surface of the contact electrode may be below the level of the top surface  303  of the holder structure  301  as shown in  FIG. 4A , it may be at the same level as the top surface  303  of the holder structure  301 , it may even be above the top surface  303  of the holder structure  301  as long as it does not touch the surface of the wafer. These embodiments are applicable to all examples herein and any variations thereof. 
     In this embodiment, the electroetching solution  308  is used for both establishing contact and electroetching the conductive layer  102   b . The contact electrode  309  is electrically connected to a positive terminal of a power source  310 . The electroetching solution  308  fills the unit and touches the conductive layer. The contact opening  314  is preferably in the plane of the top surface  303  of the holder structure  301 . The inlet  312  may be connected to a common electroetching solution reservoir (not shown) or the whole structure may be immersed into an electroetching solution that fills all the gaps including the contact unit and the process unit. The contact opening  314  is placed in close proximity of a contact region  320   a  of the surface  103   b  of the conductive layer  102   b . Since the holder structure  301  and the wafer  100   b  is moved relative to one another during the process, the contact region  320   a  may be at any appropriate location on the surface of the wafer and may be at any location at a given instant. As the solution  308  wets the contact region, the solution establishes electrical contact between the electrode  309  and the contact region  320   a  since the solution  308  is selected to be conductive. 
     The process unit  304  may be comprised of a process hole  322 . A process electrode  326  is in physical contact with the solution  308 . The process electrode  326  is electrically connectable to a negative terminal of the power source  310 . The top surface  303  of the holder structure is placed across the surface of the wafer in a substantially parallel fashion during the process. In this respect, the process opening  330  is placed in close proximity of a process region  320   b  of the surface  103   b  of the conductive layer  102   b . In this embodiment, the process region may be approximately equal to the area of the opening  230 . Due to the relative motion between the wafer and the holder structure  301 , the process region  320   b  may be at various locations on the surface  103   b  of the wafer at different times during the process. 
       FIG. 4B  shows the top surface  303  of an exemplary holder structure  301  in plan view. The top surface  303  comprises contact and process openings  314 ,  330  of the units  302  and  304 , which may be distributed in a predetermined pattern. Shapes of the process openings and contact openings shown in  FIG. 4B  are only exemplary, and as discussed in relation to  FIGS. 3A ,  3 B,  3 C,  3 D,  3 E,  3 F,  3 G, various shapes and forms of process or contact units may be employed. The contact electrodes  309  and process electrodes  326 , which are immersed in the electroetching solution may also have any geometrical shape and cross section. They may be in the form of mesh or even conductive foam. 
     During the process, the surface  303  is substantially parallel to the conductive surface of the wafer to perform uniform electroetching. Electroetching solution  308  contacts the process region  320   b  and establishes electrical contact between the electrode  326  and the process region  320   b . The electroetching of the copper layer  102   b  is initiated when a potential is applied between the contact electrode, which becomes an anode and the process electrode, which becomes a cathode. The electrical current passes from contact electrode  309 , into the electroetching solution  308  and enters the copper layer  102   b  at the contact region  320   a . The current then flows in the copper film  102   b  towards the process region  320   b , enters the electroetching solution  308  and flows to the cathode  326 . Although, there may be electroetching solution between the surface  103   b  of the wafer and the top surface  303  of the holder  301 , the resistivity of this electroetching solution is much higher than the resistivity of copper layer. If the distance between the surface of the holder structure and the surface of the wafer is small enough, such as 0.1-5 mm, the total resistance of this section of the etching solution will also be higher. Consequently, the electrical current will substantially follow the path through the copper layer and cause electroetching at the process region  320   b . Any leakage of electrical current through the solution itself will reduce the efficiency of material removal since such leakage current would not result in electropolishing of the copper film. It should be noted that in this embodiment the electroetching solution is the common solution for the contact unit and the process unit and the units are in fluid communication through the electroetching solution that exists between the wafer surface and the top surface of the holder structure. As described before, the anodic voltage on the copper layer at the process region  320   b  causes electropolishing or electroetching of the copper in that region. 
     During the process, the wafer may be rotated and/or linearly moved over the holder structure  301  to accomplish uniform electroetching over the entire surface of the wafer. The process may be performed by bringing the wafer surface  103   b  in close proximity of the surface  303  of the holder  301  or even by contacting the surface  103   b  to the top surface  303  of the holder structure  301 . If wafer surface is physically contacted to the top surface  303 , it is preferable that the top surface comprises a pad material. With the selection of an appropriate pad, an electrochemical mechanical etching or polishing process can be carried out, which can planarize originally non-planar workpiece surfaces as discussed earlier, for electrochemical mechanical etching applications, a soft pad or a pad comprising abrasives on its surface may be employed. 
     The power sources  210  and  310  shown in  FIG. 3A  and  FIG. 4A  provide the power necessary to accomplish electropolishing. It should be understood that the various electrodes described may be all connected to a single power supply or multiple power supplies may be connected groups of electrodes to form zones, which may be controlled independently from each other. For example, a first group of process electrodes may be used to remove copper from the near-edge surface of the wafer and they may be connected to the negative terminal of a first power supply. A second group of process electrodes may scan the central region of the wafer surface to remove copper from this central region. This second group of process electrodes may be connected to the negative terminal of a second power supply. In this case, an electropolishing process may be carried out at the central region of the wafer using the second power supply and the second group of process electrodes. Then copper removal from the near-edge portion may be initiated powering the first group of process electrodes by the first power supply. Ability of independently removing material from multiple different zones on a wafer allows great flexibility in obtaining highly uniform electropolishing. Number of zones and number of electrodes per zone may be as small or large as practical. The contact electrodes may or may not be divided into different zones. 
     When the copper is removed from a certain zone on the wafer, the electrical current passing through that zone is expected to decrease, if voltage is constant. Alternately, if a constant current source is used as the power supply, as copper is removed from the surface, voltage drop is expected to increase. These changes in the current or voltage can be used to monitor the amount of material removed from the wafer surface. By knowing the position of a certain process area on the wafer surface at a certain time and the value of the current and voltage, one can determine the amount of copper left at that process region. If constant voltage supplies are used as power supplies, as the copper is removed by electroetching at a certain process area, the current value drops and therefore the electroetching rate also drops. This way, self-limiting of the electroetching process is achieved at regions of the wafer where copper is removed. This is important to avoid the copper loss from within the features as indicated in  FIG. 1A . 
       FIG. 5  shows another exemplary electroetching or electropolishing system  400  that can be used to electrochemically etch the copper layer  102   c . The system  400  comprises a plurality of contact and process units. In this embodiment, a common cathode, which is immersed in an electroetching solution, is used to electroetch the layer  102   c  through the process units and provides electrical power to the layer  102   c  through the contact units. This design is attractive especially for cases where material is being removed from the surface of the wafer and it gets deposited onto the common cathode. Since cathode is large and away from the wafer surface many wafers such as a few thousand wafers can be processed in this approach before the need to clean or replace the cathode. Referring to  FIG. 5 , a plurality of contact units  402  and process units  404  may be formed in a holder structure  401 . The holder structure  401  in this embodiment is also shaped as a plate having a top surface  403  and a bottom surface  405 . The system  400  is operated the way the system  300  is operated in the previous embodiment. 
     In the example shown in  FIG. 5 , the contact units  402  or contact nozzles are comprised of contact holes  406  formed in the holder  401 . Contact electrodes  409  are placed inside the contact holes  406  and thus immersed in an electroetching solution  408 . As mentioned before, in this embodiment, the electroetching solution  408  is used for both establishing contact with and electroetching the conductive layer  102   c . The contact electrodes  409  are electrically connected to a positive terminal of a power source  410 . In this embodiment, the process units  404  or nozzles are comprised of process holes  430  or process openings formed through the holder structure  401 . The electroetching solution  408  fills the contact holes  406  as well as the process holes  430 . During processing, contact holes are in close proximity of the wafer surface and they define contact regions  420   a  on the surface  103   c  of the conductive layer  102   c . A common process electrode  426 , which is the cathode, is placed in the reservoir and kept in physical contact with the electroetching solution  408 . The process electrode  426  is electrically connected to a negative terminal of the power source  410 . The electroetching solution  408  fills the process holes  430 . In this embodiment, in order to minimize electrical current leakage from the contact electrodes through the electroetching solution to the process electrode, the contact electrodes may be placed very close to the wafer surface and insulating plugs  450  may be used below the contact electrodes. These insulating plugs may or may not be permeable by the solution. Wires connecting the various electrodes to the power supply are preferably isolated from the solution. 
     During processing, the top surface  403  of the holder  401  may or may not physically contact the wafer surface. If there is physical contact, it is preferred that the top surface  403  comprise a pad. It is also possible to use a fixed abrasive pad at the top surface to sweep the surface of the wafer to assist the material removal process, especially if planarization is required during copper electropolishing step. The holder  401  may itself be made of a pad material with process openings  430  and contact openings  406  cut into it. Contact electrodes  409  may then be placed into this pad. Contact electrodes may be placed very close to the top surface  403  to reduce voltage drop, but they should not protrude beyond the surface  403  to avoid physical contact with the surface of the copper layer  102   c . Holder structures having various designs of process openings  430  and contact openings  406  may be employed as explained before. 
       FIGS. 6A through 9B  depict some of these different holder structures having various contact and process unit designs. As in all above embodiments, in the following embodiments, the contact electrodes in the contact units do not physically contact the wafer surface that is electropolished. The electrical conduction between the surface of the wafer under process and the contact electrodes is provided through the process solution that is touches the contact electrodes and the surface. 
     As illustrated in one embodiment, in  FIG. 6A  in a perspective view and in  FIG. 6B  in plan view, a holder structure  460  has a top surface  462  and a bottom surface  464 . A number of contact units  466  are formed in the top surface  462  of the holder structure  460 . Further, a number of process units  468  are formed through the holder structure  460  and between the top surface  462  and bottom surface  464 . In this embodiment, the contact units  466  are channels, preferably near-rectangular in cross-section, having a bottom wall  470  and side walls  472 . Although in this embodiment, the channels are distributed parallel and separated one another equidistantly, they may be distributed in any manner such as non-parallel or radial and the distance between the channels may vary. The contact electrode  474  is placed in the channel  466 , preferably on the bottom wall  470 . The electrodes are shaped as bars or wires extending along the channels. Although it is not necessary, there may be a contact base  476  between the electrode  474  and the bottom wall  470 . The contact electrodes may be directly placed on the bottom wall  470 . If there is, the base  476  may be extended down to the bottom surface of the holder structure  460  and may be made of an insulator. The height of the electrode is at the level of the surface  462  or slightly less than the depth of the channel so that during the process the electrode cannot touch the wafer surface that is electropolished but allow current flow through the process solution. An insulated wire  478  connects the electrode to a terminal of a power supply (not shown). In this embodiment, the process units  468  may be shaped as round holes extending through the holder structure and allowing solution flow to the top surface. Holes  468  may be rectangular or any other geometrical form, including slits. Process units may also be continuous slits in between the channels  466 . 
     It should be noted that the designs of  FIGS. 6A ,  6 B,  7 A,  7 B,  8 A,  8 B,  9 A,  9 B,  10 A and  10 B will be described as applied to the concept shown in  FIG. 5 , namely, a design with one cathode and multiple contact electrodes. It will be appreciated, however, that the designs and concepts shown in  FIGS. 6A-10B  are also directly applicable to the cases shown in  FIGS. 3A ,  3 B,  3 C,  3 D,  3 E,  3 F,  3 G and  4 A. For example, in the embodiment shown in  FIG. 6A , every other channel  466  may be made a contact unit (shown as  302  in  FIG. 4A ) with a contact electrode  474  in it (shown as  309  in  FIG. 4A ). In between these contact units then, every other channel  466  could be a process unit (shown as  304  in  FIG. 4A ), and the electrodes within these process units would be the process electrodes (shown as  326  in  FIG. 4A ). In this case solution would be fed through the openings (shown as  468  in  FIG. 6A ), and power would be applied between contact electrodes and process electrodes as shown in  FIG. 4A . In this case, a single power source can be used if all contact electrodes are connected together and all process electrodes are connected together. Alternately, as discussed earlier, multiple power supplies can be used to power multiple contact electrode-process electrode pairs, or a single power supply may be switched between various pairs of contact electrode-process electrode. 
       FIG. 7A  shows, in plan view and  FIG. 7B  in partial cross section, another embodiment of a holder structure  480 , which is a variation of the holder structure  460  shown in the previous embodiment. The holder structure  480  comprises channels  486  and holes  488 . The channels in this example are placed in diagonal fashion and equidistantly parallel to one another. The channels  486  are in rectangular shape and are defined by a bottom wall  490  and side walls  492 , as shown in  FIG. 7B . Contact electrodes  494  are shaped as beads that are lined along the bottom of the channels  486  and connected a terminal of a power supply (not shown). As described above, the contact electrodes  494  may or may not be placed on an electrode base  496 . 
       FIGS. 8A-8B  illustrate another embodiment of a holder structure  500 . In  FIG. 8A  in a perspective view and in  FIG. 8B  in plan view, the holder structure  500  has a top surface  502  and a bottom surface  504 . A number of contact units  506  are formed in the top surface  502 . Further, a number of process units  508  are formed through the holder structure  500  and between the top and bottom surfaces  502 ,  504 . In this embodiment, the contact units  506  are channels, preferably rectangular cross-section, having a bottom wall  510  and side-walls  512 . As in the previous embodiments, the channels are distributed parallel and separated one another equidistantly, they may also be distributed in any manner such as non-parallel or radial, and the distance between the channels may vary. In this embodiment, contact electrodes  514  are preferably conductive brushes made of thin conductive wires or bristles. The contact electrodes  514  are placed in the channel  506 , preferably on the bottom wall  510 . As in the previous embodiments, there may be a contact base  516  between the conductive brushes  514  and the bottom wall  510 . The height of the conductive brushes  514  is preferably slightly less than the depth of the channel  506  so that during the process brushes  514  cannot touch the wafer surface that is electropolished but allow current to flow through the process solution. As in the previous embodiments, the base  516  may be extended down to the bottom surface of the holder structure  500  and may be made of an insulator. An insulated electrical line  518  connects the conductive brushes to a terminal of a power supply (not shown). In this embodiment, the process units  508  may be shaped as round holes extending through the holder structure and allowing solution flow to the top surface during the process. Holes  502  may be rectangular or any other geometrical form. 
       FIGS. 9A-9B  illustrate another embodiment of the holder structure using conductive brushes that are used in the previous embodiment. Of course, use of conductive brushes is for the purpose of exemplifying subject embodiment. Contact electrodes with any other shape and geometry may be used with the embodiments described in connection to  FIGS. 9A-9B . Similarly, use of different shape, size and geometry of process units and contact units as well as their possible distribution alternatives on the holder structures are within the scope of this invention. 
     As illustrated in  FIG. 9A  in perspective view and in  FIG. 9B  in a partial perspective side view, a holder structure  520  is a variation of the holder structure  500  shown in the previous embodiment. The holder structure  520  comprises contact units  526  and process units  528 . The process units  528  in this example are placed in diagonal fashion and equidistantly parallel to one another. The process units in this embodiment are shaped as slits extending between the top and bottom surfaces  522 ,  524  of the holder structure  520  and allowing process solution to flow. The contact units in this embodiment are shaped as holes in the holder structure. The contact units  526  include a bottom wall  530  and side-wall  532  which is cylindrical in this example. Conductive brushes  534  are placed on the bottom wall  530  of the contact units  526  and connected to a terminal of a power supply (not shown). As described above, the contact electrodes  534  may be placed on an electrode base  536 . 
     Two other designs that employ the buried electrical contact concept of the present invention are shown in  FIGS. 10A and 10B . As shown in  FIG. 10A , contact electrodes  600  are over supports  601  and they are in close proximity of the surface  103   c  of the copper layer  102   c . The supports  601  may be held by a holder structure (not shown), which may be made of an open frame. Supports  601  are made of insulating material and they reduce the electrical current leakage that may flow from the contact electrodes  600  through the electropolishing solution  608 , to the electrode  626  when a voltage rendering the contact electrodes anodic is applied between the electrode and the contact electrodes. In operation, contact electrodes  600  do not touch the surface  103   c . However, close proximity of them to the surface electrically couples the contact electrodes  600  to the copper surface  103   c . As in previous examples, most of the material removal takes place on the wafer surface in the area in between the contact electrodes, i.e., process openings. Reduction of leakage current is important in this design. Such reduction may be achieved by insulating all surfaces of contact electrodes except the surface facing the wafer and by reducing the distance between the wafer and the contact electrodes. A version of the design in  FIG. 10A  that can be used for touchprocessing is shown in  FIG. 10B . In  FIG. 10B , the contact electrodes  600   b  and structures  601   b  are buried in a spongy material  620  or a pad material. The spongy material maybe a porous polymeric pad that allows the electroetching solution  608   b  to wet the wafer surface as well as the contact electrodes  626   b . During electropolishing, the surface of the copper layer  102   c  may or may not touch the surface of the pad material. Again, in this embodiment, most of the material removal takes place on the wafer surface in the area in between the contact electrodes, i.e., process openings, which may contain the spongy material as shown in  FIG. 10B , or spongy material may be removed from these process openings to reduce electrical resistance and resistance to flow of the electrolyte. The surface of the pad material may comprise abrasives to assist material removal process, especially if planarization is required during electropolishing, i.e., the starting copper surface is non-planar. 
       FIGS. 1A and 1B  schematically illustrates exemplary stages of an electropolishing process using the system described in  FIG. 10A . In this example for the purpose of clarification, a system  700  with two contact electrodes, a first contact electrode  701  a and a second contact electrode  701   b . The electrodes are placed on supports  702  and connected to a positive terminal of a power supply. In this respect, a cathode electrode  705  is also connected to a negative terminal of the power supply. Since the electropolishing process is exemplified with two contact electrodes, a portion of cathode electrode  705  is shown in  FIGS. 11A-11B . 
     Electropolishing process is applied to an exemplary substrate  704  having a copper layer  706 . The material removal takes place on the wafer surface in a process opening  707  in between the contact electrodes. The substrate  704  may be a semiconductor substrate including features  708  filled with copper layer. The features  708  and the surface of the substrate  704  may be lined with a barrier layer  710 , which has generally a lesser conductivity than the conductivity of the copper. As described before, Ta, W, WN, WCN or TaN are the typical barrier materials for copper deposition. A copper removal solution such as an electropolishing solution  712  is in contact with the copper layer  706  and the cathode electrode  705  (see also  FIG. 10A ). 
     As shown in  FIG. 11A , during an instant of the electropolishing process the contact electrodes  701   a  and  701   b  are placed in close proximity of the copper layer. As the current from the contact electrodes  701   a  and  701   b  flow through the copper layer  706 , a surface portion  714   a  of the copper layer  706  is removed or electropolished. The surface portion is the portion of the copper layer that is located across the process hole  707  and the contact electrodes. As shown in  FIG. 1A , direction of the current flow from the first contact electrode  701  a and the second contact electrode  701   b  is depicted with the arrows A and B respectively. The electropolishing uniformly reduces the thickness of the copper layer down to the barrier layer level and continues as long as conductive copper remains on the barrier layer. It will be appreciated that during the removal of the portion  714 , resistance against the current flow increases and the current flow chooses the least resistive path where it may still have conductive copper and continues etching the remaining copper until the surface portion  714  is almost entirely removed. This brings the electropolishing of the copper layer to a stop at that location of the surface, i.e., process self-limits, before moving over the neighboring location as shown in  FIG. 11B .  FIG. 11B  shows another instant during the electropolishing process, as the system  700  moves over the remaining portion of the copper layer  706 . As the contact electrode  701   a  moves over the copper layer  706 , current flows through the remaining layer and starts electropolishing process. At this instant, since the second contact electrode is still over the exposed barrier layer, current flow from the second electrode faces resistance. This causes a larger current I 1 to flow through the first electrode  701   a  and through the path A compared to the current I 2  that flows through the second electrode  701   b . The current flow from the first contact electrode causes electropolishing of the remaining copper, whereas the small current or lack of current through the electrode  701   b  arrests further copper removal from the areas where barrier is exposed. Accordingly, the system  700  is able to reduce and increase the current flow from a particular contact electrode depending on the remaining copper across that particular electrode as the process progresses and once the barrier is exposed copper removal is drastically reduced or arrested to avoid copper loss from within the features  708 . 
       FIGS. 12A-12C  illustrate an exemplary non-contact electropolishing system  800  of the present invention. As shown in  FIG. 12A  in a side view, the system  800  comprises a solution container  802 , a holder structure  804  and a wafer carrier  806 . The holder structure  804  is placed on the side walls  808  of the solution container  802  which contains a process solution  810 , preferably an electropolishing or electroetching or electrochemical mechanical polishing solution. The process solution  810  flows through the holder structure and wets front surface  812  of the wafer  814  during the process. The diameter of the wafer, or wafer size, may be any size such as 200 mm or 300 mm or larger. The front surface  812  comprises a conductor, such as copper, tantalum, tungsten and other materials commonly used in electronics industry. The process solution  810  is delivered to the container  802  through a solution inlet  816 . The solution  810  leaves the system from the edges of the holder structure  804  and filtered and pumped back to the container  802  from a recycle unit (not shown). The wafer carrier  806  can rotate and move the wafer laterally or in orbital fashion in proximity of the holder structure  804 . 
     In this embodiment, the holder structure  804  is comprised of a plurality of contact electrodes  818  and process electrodes  820  that are separated and electrically insulated from one another by insulation members  822 . The contact electrodes  818  and the process electrodes  820  have top surfaces  819  and  821  respectively. Except the top surfaces  819  and the  821 , the electrodes  818  and  820  are coated with an insulating film such as a Teflon™ film. In this embodiment, all of the top surfaces  819  and  821  are leveled and substantially coplanar. 
     As shown in  FIGS. 12B and 12C , which are perspective and top plan views of the system  800 , the contact electrodes and the process electrodes are made of conductive rods that are separated by insulating members  822  which are also shaped as rods. The widths of the contact electrodes, process electrodes and the insulating members may be in the range of 1-10 mm. Referring back to  FIG. 12A-12C , a compressible layer  824  is also attached to the holder structure  804 , in between the contact and process electrodes. The compressible layer  824  is preferably made of strips of a compressible and insulating material, such as polyurethane, and attached on top of the insulation members. The compressible material of the strips also has a closed pore structure, which does not allow solution to flow through it and therefore also limits any leakage current that may pass between the contact electrodes and adjacent process electrodes, forcing the current to pass through the conductor on the workpiece surface. The compressible layer will be referred to as compressible layer strips hereinafter. Top surfaces  826  of the compressible layer strips  824  are all substantially coplanar. The top surfaces  826  of the compressible layer strips  824  are above the top surfaces  819  and  821  of the contact and process electrodes by an amount that may be in the range of 1-10 mm, preferably 2-5 millimeters. In this respect, top surfaces of the contact electrode rods between the compressible layer strips define contact units and, top surfaces of the process electrode rods between the strips define process units of the electropolishing system  800 . As described earlier, the contact electrodes do not physically contact the surface of the wafer. However, the surface of the compressible layer strips  824  may touch the surface of the wafer during polishing stage. Due to the closed pore structure of the strips, very little or no electrical current flows between the contact units and the process units, directly through the compressible layer strips. The tops of the strips  824  preferably contain a suitable pad material that efficiently sweeps the substrate surface when a contact is established between the tops of the strips and the substrate surface. 
     As shown in  FIG. 12B , contact electrodes  818  are electrically connected to a positive terminal of a power source  828  through a first electrical contact  830 . The process electrodes  820  are connected to a negative terminal of the power source  828  using a second electrical contact  832 . It should be noted that instead of the polarity stated above, a variable polarity may also be applied to the two terminals. In other words a variable voltage or an AC voltage may be applied between the contact and process electrodes. As will be described below, multiple electropolishing zones may be established on the holder structure (see  FIG. 13 ). 
     The process solution provides the conductive path that allows current to pass between the contact and process electrodes and the conductive surface. As shown in  FIGS. 12A and 12C , the process solution flows through openings  828  provided in the holder structure  804 . The openings are preferably formed through the contact electrodes  818  and process electrodes  820 , although they may also be opened through the insulating members  822  and the compressible strips  826 . As in the above embodiments, the process solution is used for both establishing electrical contact and electrochemical processing of the conductive layer. 
     As described above, electrical contact to the wafer surface is established through the contact electrodes while the electrochemical processing of the surface is performed over the process electrodes. During the process, the surface  812  of the wafer is placed in close proximity of the top surface of the compressible strips  824 . In this position the surface  819  of the contact electrodes are positioned across contact regions  834  of the surface  812  while the process electrodes are positioned across the process regions  836  of the surface  812  (see  FIG. 12A ). Since the holder structure  804  and the wafer  814  are moved relative to one another during the process, the contact regions and process regions may be at any appropriate location on the surface of the wafer and may be at any location at a given instant. As the process solution wets the contact region, the solution establishes electrical contact between the contact electrode and the contact region since the solution is selected to be conductive. In this respect, the current flows from the contact electrodes and through the process solution to the surface, and then through the process solution to the process electrodes. Although both electrodes are coated with an insulating film, except at the top surfaces facing the substrate, an extended isolation member  838  may be used between the contact and process electrodes to further assure minimum leakage current between these electrodes at their bottom ends. This way the pathway for any possible leakage current between the bottom ends of the electrodes is made longer, increasing the resistance. To reduce the overall leakage of the assembly, all isolation members may be made extended type. 
     With predetermined intervals, the top surface of the holder structure  804  may be cleaned and conditioned using a pad cleaner/conditioner or a platen surface cleaner to remove particulates dispersed or formed on the holder structure during the electropolishing or electrochemical mechanical polishing process. Such particulates may be small pieces of conductor deposits formed on the pad surfaces or electrode surfaces. Designs such as those used for cleaning pads employed in ECMD and ECME processes disclosed in our recent patent applications are applicable to the present case. 
     Referring to  FIG. 12A-12C , rods forming the contact and process electrodes as well as the isolation members are all fastened together as an array of alternating process and contact electrodes which are separated by the isolation members. Although many alternative fastening methods may be used, in this embodiment, a pin and nut combination may be used to fasten the electrodes and the isolation members together. 
     As shown in  FIG. 12A , a number of pins  840  are placed through holes  842  extending through the electrode and the isolation member rods and locked in place by tightening nuts  844 . The holes extend perpendicular to the longitudinal axis of the array of rods. End plates  846  may be placed at both ends of the array of electrodes and the isolation members to tighten the array of electrodes and the isolation members more uniformly. 
     It should be noted that ability of placing pad materials at the top surface of compressible strips allow application of uniform and low force on the wafer surface when this surface makes contact with the pad material during process. In this case, the wafer surface is brought in contact with the pad material surface and then pushed against it. Once the pad strips on top of the compressible layer strips are pushed down by the wafer surface by a predetermined distance, force generated by the compressible layer strips pushes the pad material against the wafer surface. The amount of force depends on the spring constant of the compressible material and can be pre-selected by selecting harder or softer compressible materials of the strips. This design assures good and uniform physical contact between the pad material and the wafer surface. 
       FIG. 13  illustrates an embodiment of the holder structure  800  with multiple electropolishing zones. In one embodiment, multiple electropolishing zones may be established by dividing the surface area of the holder structure into different zones such as zones z 1  and z 2 . For example zone z 1  may electropolish a center region  815   a  of the surface of the wafer  814 ′ and the zone z 2  may electropolish an edge region  815   b  of the same surface. As shown in  FIG. 13 , the zone z 1  may cover a center area of the holder structure and extends longitudinally from the beginning to the end of the electrodes. Zone z 2  may be the rest of the surface area located at both sides of the zone z 1 . Electrodes in zone z 1  are connected to terminals of a power source S 1 , and the electrodes in the zone z 2  are connected to terminals of a power source S 2 . In this respect, lines C 1 , C 3 , C 1 ′ and C 3 ′ electrically connect the contact electrodes  818  in zone z 2  to a positive terminal of the power source S 2 . Lines C 2  and C 2 ′ electrically connect the process electrodes  820  in zone z 2  to a negative terminal of the power source S 2 . Similarly, lines C 4  and C 6  electrically connect the process electrodes  820  in zone z 1  to a negative terminal of the power source S 2 . Line C 5  connects the contact electrode  818  in zone z 1  to a negative terminal of the power source S 1 . During the electropolishing process, as the wafer  814 ′ is rotated and moved laterally in the x-direction, uniformity of the electropolishing is controlled by varying the voltage in zones z 1  and z 2 . Varying the voltage in the zones z 1  and z 2 , in turn, vary the electropolishing rate in the center region  815   a  and edge region  815   b  of the wafer. 
     As described above and shown in  FIG. 5 , process units may be established as openings in the holder structure. In such holder designs, a common cathode electrode works through the openings or the process units. The same inventive approach is also applicable to the holder structure  804  that is illustrated in  FIGS. 12A through 12B . In order to form the process openings, the process electrodes  820  are removed from the holder structure and small spacers are placed, preferably, between the insulating members  822  to form the openings. Two spacers maybe used one at each end of the insulating members. Alternately, more spacers may be used at specific intervals. This is possible as long as the spacers are narrow and do not shade the wafer surface from the field going to the process electrode. Similar end result may be obtained by using insulating members having holes or slits and placing them between the contact electrodes  818  after the removal of the process electrodes  820 . In a following step a single process electrode (cathode) is placed in the process solution  810  to complete the system. 
       FIG. 14A  illustrates an embodiment of an array  900  of contact electrodes  902 , process electrodes  904  as well as isolation members  906 . The array  900  can be used to form a holder structure that also includes compressible strips (not shown). In this embodiment, for the purpose of clarity the pad strips  908  are only shown in  FIG. 14 . The array  900  is held between end plates  909  and fastened together by inserting pins  910  through the holes  912  and using nuts  914  ( FIG. 15 ) to hold the pins in place, as described above. 
     For illustration purposes, in  FIG. 14A , one of the electrodes and one of the end plates are separated from the array to demonstrate their design characteristics. As shown in  FIGS. 14A and 14B , a number of grooves  916  extend from bottom surfaces  918  of the electrodes to top surfaces  920 . The grooves are formed on the side walls  922  of the electrodes so that when the side walls  922  of the electrodes placed against the side walls of the insulation members  906  to form the array  900 , openings for solution flow are formed. 
     In this embodiment the contact electrodes  902  and the process electrodes  904  are connected to a power source (not shown) through their contact ends  903  and  905 . Except an electrical contact line  924  and the top surfaces  920  of the electrodes  902  and  904 , all other surfaces are coated with an insulating film such as a Teflon™ film. The contact ends  903  and  905  may be shaped as a step with a recessed surface  926 . Contact ends of the contact electrodes and the contact ends of the process electrodes are positioned along opposite ends of the array  900  as in the manner shown in  FIGS. 14A-16 . The electrical contact line may be placed on the recessed surface  926 . Electrical connection to the power source may be made using a first contact device  928  to connect contact electrodes to the positive terminal and a second contact device  930  to connect the process electrodes to the negative terminal of the power source. 
     As shown in  FIG. 14A-17 , the contact devices  928  and  930  comprise an elongated body  932  having a support portion  934  and an extended portion  936 . Referring to  FIGS. 16 and 17 , a front wall  938  of the support portion  936  and a bottom wall  940  of the extended portion  936  of the contact devices define a corner cavity which engages with the step shaped ends of the electrodes  902  and  904  when the contact devices are attached to the ends of the electrodes. In this respect, the first contact device  928  is attached to the contact ends  903  of the contact electrodes while the second contact device  930  is attached to the contact ends  905  of the process electrodes. In  FIG. 17  the contact device  928  is illustrated in detail. Although the contact device  928  is used to describe details of the contact devices, the same description is applicable to the contact device  930  because of the identical features of the devices. As shown in  FIG. 17 , a contact member  942 , which is attached to the bottom wall  940  of the extended portion  936 , enables electrical connection between the contact line  924  on the recessed surface  926  and the power source. Except the contact member  942 , bodies  932  of the contact devices  928  and  930  are coated with Teflon or made of insulating materials such as polymer base materials. Although the example shown in  FIGS. 14A-17  describes an array for a single zone holder structure, a system with a multiple zone electropolishing holder structure (see FIG.  13 )can also be established using the array  900  and within the scope of this invention. 
     A conductive line  944  connects the contact member  942  to the power source terminal. Referring to  FIG. 16 , the contact devices are attached to top ends  946  of side walls of a solution container (not shown) using bolts  948  or other fastening means. The bolts  948  are inserted through the bolt holes  950  shown in for example  FIG. 14B . In this respect contact devices  928  and  930  secure the array  900  and hence the holder structure on the process solution container (see  FIGS. 12A-12B ). Seal members  952  on top ends  946  of the side walls and seal members  954  on the extended portion of the contact devices  928  and  930  prevents any solution leakage into the area where the contact member  942  touches the contact line  924 . 
     Although various preferred embodiments and the best mode have been described in detail above, those skilled in the art will readily appreciate that many modifications of the exemplary embodiment are possible without materially departing from the novel teachings and advantages of this invention.