Patent Publication Number: US-2011073483-A1

Title: Apparatus and method for improving uniformity in electroplating

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of the co-pending U.S. patent application Ser. No. 11/362,433, filed Feb. 24, 2006, which claims benefit of U.S. Provisional Patent Application Ser. No. 60/656,161 filed Feb. 25, 2005 and U.S. Provisional Patent Application Ser. No. 60/687,404 filed Jun. 3, 2005, which are herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the invention generally relate to methods and apparatus for electrochemical processing. One embodiment of the invention relates to an electrochemical processing cell configured to have an electric field with different profiles at different locations. 
     2. Description of the Related Art 
     Metallization of high aspect ratio 90 nm and smaller sized features is a foundational technology for future generations of integrated circuit manufacturing processes. Metallization of these features is generally accomplished via an electrochemical plating process. In a typical process scheme, a metal, such as copper, is plated onto a substrate comprising an array of integrated circuit devices with open vias and trenches. The plating process is carried out in such a manner as to fill the vias and trenches on the substrate surface with metal and to further deposit an additional amount of metal known as the overburden on the substrate. The overburden is required to enable subsequent polishing and planarization of the deposit through a process step such as chemical mechanical polishing. The total amount of metal deposited in the electrochemical plating process is typically 0.2 to 1 μm. 
     However, electrochemical plating (ECP) of these features presents several challenges to conventional gap fill methods and apparatuses. One such problem, for example, is that electrochemical plating processes generally require a conductive seed layer to be deposited onto the features to support the subsequent plating process. Conventionally, these seed layers have had a thickness of between about 1000 Å and about 2500 Å; however, as a result of the high aspect ratios of 90 nm features, seed layer thicknesses must be reduced to less than about 500 Å, or even below 100 Å. This reduction in the seed layer thickness increases resistivity of the substrate causing a “terminal effect,” which is an increase in the deposition thickness near the perimeter of the substrate being plated. 
     The terminal effect is most severe at the beginning of an electrochemical plating process, for example, within about the first 10 seconds of the electrochemical plating process, when the substrate resistivity is at the highest level. This stage is also the critical stage when the features on the substrates are being filled. The terminal effect results in a large difference in the plating rate across the substrate, leading to variations in film properties such as film composition and resistivity between the center and edge of the substrate. More importantly, a highly non-uniform plating rate across the substrate during the filling of features forces the features at either the center or edge of the wafer to fill under sub-optimal conditions, resulting in problems, such as incomplete filling and trapped voids inside the features. 
     Additionally, it is often desirable to modulate the plating rate at the edge of the substrate after the features have been filled and while the overburden is being deposited. For example, processes that follow electrochemical plating, such as chemical mechanical polishing, may yield better performance if the plated film is thinner at the edge than at the center of the wafer. This is because certain polishing processes are edge-slow (edge fast), so that a slightly edge-thin (edge thick) deposit profile after plating results in an optimally uniform profile after polishing. 
     Therefore, control of the plating rate at the edge of the substrate is desired to mitigate the terminal effect and adjust the overall plating profile. Attempts have been made modulate the plating profile at the edge of the substrate through various apparatus and methods. For example, Conventional plating cells have been modified to include resistive elements, multi-segment anodes, or passive shield or flange members. These configurations were only partly successful because of their lack of proximity to the perimeter of the substrate or inability to dynamically adjust the profile. For example, resistive elements or passive shield or flange members were only sufficient to generate one type of plating profile, such as uniform or edge-thin, whereas future generations of plating processes may require both types of profiles to be available during the plating process. 
     Therefore, there exists a need for an apparatus and method for minimizing terminal effect and/or adjusting plating profile dynamically. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention provide apparatus and method for minimizing terminal effect and adjusting plating profiles. 
     One embodiment of the present invention provides an apparatus for electroplating a substrate. The apparatus comprises a fluid basin, an anode disposed near a bottom of the fluid basin, a restrictor disposed above the anode, and a substrate support member configured to move the substrate within the fluid basin among different elevations relative to the restrictor. 
     Another embodiment of the present invention provides an apparatus for electroplating a substrate. The apparatus comprises a fluid basin configured to retain a plating solution in a fluid volume therein, an anode disposed on a bottom of the fluid basin, a substrate support member configured to position the substrate in the fluid basin along a trajectory, wherein the fluid volume has a restricted section with an inner diameter smaller than the diameter of the substrate. 
     Yet another embodiment of the present invention provides a method for plating a metal onto a substrate. The method comprises moving the substrate in a plating solution to adjust a plating current distribution across the substrate. The method further comprises providing a plating cell having a plating solution retained in a fluid volume, wherein the fluid volume has a restricted section having a smaller sectional area than a plating surface on the substrate, providing an anode on one side of the restricted section, positioning the substrate in a first position, wherein the substrate and the anode are on opposite sides of the restricted section, and applying a first electric bias between the substrate and the anode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1A  illustrates a partial sectional perspective view of an exemplary electrochemical plating cell of the invention. 
         FIG. 1B  illustrates an enlarged partial perspective view of the groove displayed in  FIG. 1A  in which the encased auxiliary electrode is placed. 
         FIG. 1C  illustrates an enlarged view of the wall of the outer basin of the plating cell displayed in  FIG. 1A  showing how the encased auxiliary electrode assembly enters the basin of the plating cell. 
         FIG. 1D  illustrates an alternate placement of the encased auxiliary electrode assembly. 
         FIG. 2  illustrates a perspective view of an anode base plate of the invention. 
         FIG. 3  illustrates a perspective view of an exemplary anode base plate of the invention having an anode positioned therein. 
         FIG. 4  illustrates an exploded perspective view of an exemplary membrane support member of the invention. 
         FIG. 5  illustrates a partial sectional view of an edge of the plating cell of the invention. 
         FIG. 6  illustrates a plating thickness plot for a plating cell with the encased auxiliary electrode assembly in fluid communication with the catholyte compared to a plating cell without the encased auxiliary electrode assembly. 
         FIG. 7  illustrates a flow diagram of a method for immersing a substrate into a plating solution in accordance with one or more of the embodiments of the invention. 
         FIGS. 8A-8C  schematically illustrate an electrochemical plating cell configured to provide an adjustable plating profile. 
         FIG. 9A  illustrates a sectional view of an electrochemical plating cell having an anode smaller than the substrate. 
         FIG. 9B  illustrates a sectional view of an electrochemical plating cell having a diffuser plate with restricted area for fluid to pass through. 
         FIG. 9C  illustrates a section view of an electrochemical plating cell having a shield and an auxiliary electrode. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention generally provides an electrochemical plating cell, with an encased auxiliary electrode assembly in fluid communication with the cathode compartment, configured to uniformly plate metal onto a substrate. 
       FIG. 1A  illustrates a partial sectional perspective view of an exemplary electrochemical plating cell  100  containing an encased auxiliary electrode assembly  130  of the invention. The encased auxiliary electrode assembly  130  is composed of an auxiliary electrode  132  surrounded by electrolyte in a protective tube  134 . In another embodiment, the electrolyte is absent and the protective tube is attached directly to the surface of the auxiliary electrode. The encased auxiliary electrode assembly  130  is generally configured to be used as a cathode when the substrate contacts (not shown) are configured as cathodes. Also, when configured as a cathode, the encased auxiliary electrode assembly  130  can be used (when the substrate contacts are configured as anodes) as a cathode for deplating copper that accumulates on the substrate contacts. The protective tube  134  of the encased auxiliary electrode assembly  130  prevents catholyte solution organics from entering the encased auxiliary electrode assembly  130  while also preventing copper sludge and plated copper formed at the encased auxiliary electrode assembly  130  from exiting the protective tube  134  and contaminating the catholyte solution. 
     The encased auxiliary electrode assembly  130  includes an auxiliary electrode  132  is preferably platinum but may be manufactured from copper or other metals known to be effective as either a soluble or insoluble electrode in an electrochemical plating cell. Additionally, the auxiliary electrode  132  may be manufactured from a core material, such as copper, stainless steel, titanium, or other cost effective core electrode material, and then the outer surfaces, i.e., the upper surface of the auxiliary electrode  132  that is in fluid contact with the electrolyte, may then be plated with another metal, such as platinum, titanium, or other electrode material. This configuration allows the cost of auxiliary electrode  132  to be reduced, as a more cost effective and electrically conductive material is used to manufacture the core of the auxiliary electrode  132 , while another more costly but desirable electrode material, i.e., platinum, is used for the exposed surfaces of the auxiliary electrode  132 . The auxiliary electrode  132  can be a wire, a ring, a torroid or of other shape. 
     Due to the proximity of the auxiliary electrode  132  to the primary cathode (the contact ring, pins and substrate) a large amount of copper sludge is generated at the auxiliary electrode  132 . The protective tube  134  prevents this copper sludge from exiting the encased auxiliary electrode assembly  130  and contaminating the catholyte solution. Further, the protective tube  134  prevents the auxiliary electrode  132  from consuming and degrading the organic additives (levelers, suppressors and activators) in the catholyte solution. Preferably, the protective tube  134  can be composed of an ion exchange material such as Nafion®, CMX-SB or Vicor membrane. Also, the protective tube  134  can be composed of a porous membrane. One example of a hydrophilic porous membrane is the Durapore Hydrophilic Membrane, available from Millipore Corporation, located in Bedford, Mass. Other examples of conventional membranes include porous glass, porous ceramics, silica aerogels, organic aerogels, porous polymeric materials, and filter membranes. Specific membranes include carbon filter layers, Kynar polymer layers or polypropylene membranes. 
     When in a plating configuration, the protective tube  134  allows copper ions to plate onto the auxiliary electrode  132  but does not allow the copper sludge to exit the protective tube and contaminate the catholyte solution, and as such, protective tube  134  is generally an ionic or ion exchange-type membrane. Ion exchange membranes generally include fixed negatively charged groups, such as SO 3   − , COO − , HPO 2   − , SeO 3   − , PO 3   2− , or other negatively charged groups amenable to plating processes. As such, the protective tube  134  is configured to allow a particular type of ion to travel through the membrane, while preventing other types of ions from traveling or passing through the membrane. More particularly, the protective tube  134  may be a cationic membrane that is configured to allow positively charged copper ions (Cu 2+  and H + ) to pass therethrough, i.e., to allow copper ions to travel from the catholyte solution through the protective tube  134  where the copper ions may then be plated onto the auxiliary electrode  132 . Further, the cationic membrane may be configured to prevent passage of negatively charged ions and electrically neutral species in the solution, such as the ions that make up the plating solution and catholyte additives. It is desirable to prevent these catholyte additives from traveling through the protective tube  134  and contacting the auxiliary electrode  132 , as the additives are known to breakdown upon contacting the auxiliary electrode  132 . More particularly, membranes with negatively charged ion groups like SO 3   −  etc. not only operate to facilitate Cu ion transport from the anolyte to the catholyte, but also to prevent penetration of accelerators to the auxiliary electrode. The accelerators are generally negatively charged organic ions, such as, for example,  − SO 3   − —C 3 H 6 —S—S—C 3 H 6   − SO 3   − , so they can&#39;t penetrate into or through the cation membrane and contact the auxiliary electrode where they are consumed and/or depleted. 
     Protective tube  134 , for example, may be a Nafion® membrane manufactured by Dupont Corporation. Nafion® membrane is an example of a poly (tetrafluoroethylene) based ionomer. Nafion® membrane has several desirable characteristics for electrochemical plating applications, such as its thermal and chemical resistance, ion-exchange properties, selectivity, mechanical strength, and insolubility in water. Nafion® membrane is also a cationic membrane based on a fluorized polymer matrix. Because of its fluorized matrix, Nafion® membrane exhibits excellent chemical stability, even in concentrated basic solutions. More particularly, Nafion® membrane is a perfluorinated polymer that contains small proportions of sulfonic or carboxylic ionic functional groups, and has been shown to be effective in transmitting metal ions (copper ions in the present embodiment) therethrough, even at low plating current densities. Specifically, Nafion® membranes have shown to be effective at transmitting between about 93% and about 97% of copper ions therethrough at plating current densities of between about 5 mA/cm 2  and about 20 mA/cm 2 . Additionally, at current densities of between about 20 mA/cm 2  and about 60 mA/cm 2 , Nafion® membrane transmits between about 97% and about 93% of copper ions therethrough. The above noted transmission percentages were observed using a copper sulfate solution having a pH value of about 3.4. Nafion&#39;s® general chemical structure (illustrated below as Diagram 1), illustrates where X is either a sulfonic or carboxylic functional group and M is either a metal cation in the neutralized form or an H +  in the acid form. 
     
       
         
         
             
             
         
       
     
     As a result of electrostatic interactions, the ionic groups that form Nafion® membrane tend to aggregate to form tightly packed regions referred to as clusters. The presence of these electrostatic interactions between the ions and the ion pairs enhance the intermolecular forces and thereby exert a significant effect on the properties of the parent polymer, which makes Nafion® membrane, or other membranes having similar physical and/or operational characteristics, a desirable ionic membrane for use in electrochemical plating cells having separated anolyte and catholyte chambers. 
     Other membranes that may be used in embodiments of the invention include various cationic and anionic membranes. For example, ionic membranes manufactured by Tokuyama of Japan, i.e., CMX-SB ionic membranes that are based on a polydivinilbenzol matrix, may be used to isolate a catholyte solution from an anolyte solution in an electrochemical plating cell. CMX-SB membranes have been shown to be effective in transmitting copper ions while preventing organic plating additives from transmitting therethrough. 
     Additionally, CMX-SB membranes have shown acceptable resistance to transmission of positive hydrogen ions. More particularly, CMX membranes have been shown to transmit above about 92% of copper ions at a current density of about 10 mA/cm 2 , and above about 98% at a current density of about 60 mA/cm 2 . Ionics CR-type membranes from Ionics Inc. have also shown to be able to transmit above about 92% of copper ions at about 10 mA/cm 2  and above about 88% of copper ions at about 60 mA/cm 2 . 
     With regard to other properties of the above noted membranes (Ionics, CMX, and Nafion® membrane), each exhibit relatively high conductivity, i.e., about 41.2, 35.3, and 24.2 ohm cm 2  at 10 mA/cm 2  for Ionics, Neosepta and Nafion® membrane, respectively. Additionally, water moves through the membranes from the anolyte into the catholyte compartment. This effect essentially dilutes the catholyte and is undesirable. For example, between about 0.5 and about 3 liters of water penetrates into the catholyte per 24 hours (or per 200 wafers) depending on the membrane type and electrolysis conditions. For example, CMX shows the minimal water transport at about 1.5 ml/wafer, the Ionics membrane shows about 5 ml/wafer, and Nafion® membrane shows about 6.5 ml/wafer. The transport properties of the CMX and Nafion® membranes result in the CuSO 4 /H 2 SO 4  concentration ratio remaining relatively constant, even after about 200 substrates are plated. This indicates that copper acid concentration changes will be lower than 2%, if the penetrated water will be removed, e.g., by enforced evaporation. As such, the use of CMX or Nafion® membrane requires only a small device to accelerate the water evaporation to 4-6 liters/day. However, Ionics membranes require an additional device that extracts the excess of H 2 SO 4  coming from the anolyte. Table 1 illustrates the respective properties of the above noted membranes. 
     Vicor membranes may also be used to advantage in the plating cell of the invention. Other membranes that may be used in the plating cell of the invention include Neosepta® membranes (ionic and non-ionic) manufactured by Tokuyama, Aciplex® membranes, Selemlon® membranes, and Flemion membranes (all of which are available as ionic and non-ionic) from Asahi Corporation, Raipare™ membranes from Pall Gellman Sciences Corporation, and C-class membranes from Solvay Corporation. 
     Referring to  FIG. 1A , the auxiliary electrode  132  is in electrical communication with a power supply  180  that is configured to electrically bias the auxiliary electrode  132  anodically or cathodically, i.e., the auxiliary electrode  132  may be biased cathodically to assist in controlling the plating uniformity during the plating process which essentially supplements or adds to the fields generated by the primary cathode of the plating cell or to deplate metal from the contact pins that conduct the plating bias to the substrate in the plating cell when the primary cathode is biased anodically. As such, the auxiliary electrode  132  may be used to either withdraw some of the electrical field directed to the primary cathode or to deplate the contact pins of the substrate contact ring when the primary cathode is biased as an anode. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE I 
               
               
                   
               
               
                   
                   
                 Water 
                   
                 Cu/Acid 
               
               
                   
                   
                 transfer, 
                 Resistance 
                 Ratio 
               
               
                 Membrane 
                 Cu 2+  transfer, % 
                 ml/Amphr 
                 ohm cm2 
                 Deviation, % 
               
               
                   
               
             
            
               
                 Ionics 
                 90-95 
                   8-11.5 
                 53 
                 4% 
               
               
                 Nafion 
                 95-98 
                   4-7.5 
                 36 
                 2% 
               
               
                 CMX 
                 97-98 
                 5.0-3.1 
                 47 
                 1% 
               
               
                   
               
            
           
         
       
     
     In operation the auxiliary electrode  132  may be cathodically biased in order to deplate copper that accumulates on substrate contacts used to communicate a plating bias to a substrate during plating operations. As is known in the art, copper tends to build up on the electrical substrate contacts (as a result of the contacts being in communication with the plating solution during the plating process) and may cause varying resistances between the respective contacts and the substrates being plated, which often results in uniformity variations between plated substrates. As such, it is desirable to periodically remove the accumulated copper from the substrate contacts so that plating uniformity between substrates may be maximized. The removal processes is generally conducted at a time period when no substrates are being plated, i.e., between plating substrates. At this time a substrate contact ring (or other apparatus that includes the elements used to electrically contact the substrate during the plating process) is immersed in the plating solution such that the electrical contact pins are in fluid communication with the plating solution. Once immersed, a deplating bias is applied between the contact pins and the auxiliary electrode  132 . More particularly, the deplating bias is configured such that the auxiliary electrode  132  is the cathode electrode and the substrate contact pins are the anodic electrodes. In this configuration the substrate contact pins supply the copper ions to the reduction reaction, and as such, the copper that was plated onto the contact pins during plating operations is removed from the contact pins, transported through the plating solution, and deposited on the auxiliary electrode  132  in the reduction process resulting from the application of the deplating bias. The deplating bias may be between about 3 volts and about 7 volts, for example, and may have a duration of between about 10 seconds and about 30 seconds. Additionally, the deplating time may be increased to above 30 seconds if the time between deplating processes is long, i.e., if the number of substrates plated has been excessive and the copper buildup on the contact pins is more than can be removed in 30 seconds. In this situation the deplating time or duration may be calculated as 20 seconds multiplied by the number of substrates plated since the last deplating process. Thus, for example, if 20 substrates have been plated since the last contact pin deplating process, then the duration of the deplating process may be about 400 seconds to remove the excessive accumulation of copper on the contact pins. Embodiments of the invention contemplate that the contact pins may be deplated between every substrate that is plated in order to maximize uniformity and throughput. In this configuration the deplating process will likely have a duration of less than about 20 seconds. However, the inventors have found that the deplating process may be extended to between every second, third, or fourth plated substrate without a substantial degradation in the uniformity. In this configuration the deplating time may be between about 20 seconds and about 80 seconds, for example. 
     The copper ions that deposit or accumulate on the auxiliary electrode  132  during the deplating process generally do not have an effect upon plating uniformity. However, copper deposits plated onto the auxiliary electrode  132  as a result of the plating or deplating process may later be reintroduced into the plating bath via application of a forward plating bias to the auxiliary electrode  132  (along with the plating anode  105 ) during a plating process. This configuration essentially configures the auxiliary electrode  132  as a secondary or auxiliary anode to the primary plating anode  105 , and as such, when the forward plating bias is applied to the electrodes (the plating anode  105  and the auxiliary electrode  132 ), the copper ions that have plated onto auxiliary electrode  132  will be removed from the auxiliary electrode  132  via the reduction reaction that supports the plating process and reintroduced into the plating solution. More particularly, the auxiliary electrode  132  may be electrically biased during plating operations to the same polarity as the anode  105 , and as such, the auxiliary electrode  132  may contribute to the plating reaction, i.e., supply copper ions to the plating solution, as well as generating a magnetic and/or electric field in the plating cell. More particularly, since the auxiliary electrode  132  is positioned radially outward of the substrate perimeter being plated, the electric field from the auxiliary electrode  132  may be used to provide an additional element of control over plating uniformity across the surface of the substrate as a result of the field effect of the auxiliary electrode  132 . The minimal current supplied to the auxiliary electrode  132  during the plating process may be calculated to generate an electric field sufficient to interact with the substrate and/or the minimal current may be calculated to generate a shaping field, i.e., the field generated by auxiliary electrode  132  (which has a small magnitude) may primarily be used to shape the field generated by the anode  105  (which has a much larger magnitude, as the majority of the plating current is traveling through the anode  105 ). Alternatively, auxiliary electrode  132  may be electrically biased to the same electrical potential as the primary anode  105 . In this configuration the anode  105  and the auxiliary electrode  132  essentially operate as a unitary anode. 
     Additional embodiments of the invention contemplate that the auxiliary electrode  132  may be active (have a forward or plating bias applied thereto) for either the entire plating process time (the time when the primary anode is active) or for only a portion of the plating process. In embodiments where the auxiliary electrode  132  is active for only a portion of the plating process duration, the auxiliary electrode  132  may be activated for a time period that is calculated to remove the copper deposits therefrom. For example, if the auxiliary electrode  132  is activated for 20 seconds during the deplating process, then the auxiliary electrode  132  may be activated for another 20 seconds during the plating process. The effect of this configuration would be to clean the auxiliary electrode, i.e., to redeposit the copper that was plated onto the auxiliary electrode during the deplating process into the electrolyte solution, assuming that equal power is applied during both the deplating and plating steps. 
     Referring to  FIG. 1A , the plating cell  100  further includes an outer basin  101  and an inner basin  102  positioned within outer basin  101 . The encased auxiliary electrode assembly  130  is positioned in a groove  120  located in the lower portion of the inner basin  102  of the plating cell. The inner basin  102  is generally configured to contain a plating solution that is used to plate a metal, e.g., copper, onto a substrate during an electrochemical plating process. During the plating process, the plating solution is generally continuously supplied to the inner basin  102  (at about 1 gallon per minute for a plating cell having a 10 liter total capacity (capacity generally includes the cell volume and the supply tank), for example), and therefore, the plating solution continually overflows the uppermost point of the inner basin  102  and runs into the outer basin  101 . The overflow plating solution is then collected by the outer basin  101  and drained therefrom for recirculation into the inner basin  102 . As illustrated in  FIG. 1 , plating cell  100  is generally positioned at a tilt angle, i.e., a frame portion  103  of the plating cell  100  is generally elevated on one side such that the components of plating cell  100  are tilted between about 3° and about 30°. Therefore, in order to contain an adequate depth of plating solution within inner basin  102  during plating operations, the uppermost portion of inner basin  102  may be extended upward on one side of plating cell  100 , such that the uppermost point of inner basin  102  is generally horizontal and allows for contiguous overflow of the plating solution supplied thereto around the perimeter of inner basin  102 . 
       FIG. 1B  illustrates an enlarged partial perspective sectional view of the lower portion of the inner basin  102  containing the groove  120  where the encased auxiliary electrode assembly  130  is placed. 
       FIG. 1C  illustrates an enlarged view of the outer basin  101  of the plating cell  100  displayed in  FIG. 1A  showing how the encased auxiliary electrode assembly  130  enters the electrochemical the plating cell  100  through a T-connection  140 . After traversing the outer basin  101  of the plating cell  100 , the encased auxiliary electrode assembly  130  circumnavigates the plating cell within the groove  120  and exits at an outlet  170 . Upon entering T-connection  140 , the auxiliary electrode  132  is combined with electrolyte from an electrolyte source  190  and enters the protective tube  134  to form the encased auxiliary electrode assembly  130 . The T-connection  140  is shown with an electrode inlet  136 , an electrolyte inlet  138  and an electrode outlet  142  where the encased auxiliary electrode assembly  130  traverses the wall of the outer basin  101  of the plating cell. The electrode inlet  136 , electrolyte inlet  138  and electrode assembly outlet  142  are sealed in a well known manner with seals  144   a ,  144   b  and  144   c  respectively. 
     The electrolyte, shown by an arrow  139  entering through the electrolyte inlet  138 , is generally an anolyte and is kept separate from a catholyte in the catholyte compartment. The anolyte in the protective tube  134  can either be flowing or stagnant. There are several advantages to using anolyte in the encased auxiliary electrode assembly  130 . First, anolyte does not contain catholyte organics (levelors, suppresors, and activators) which may be quickly consumed or degraded by the auxiliary electrode  132 . Second, the use of anolyte is more cost effective than catholyte but still provides the desired conduction. In another embodiment (not shown) the protective membrane adheres directly to the upper surface of the auxiliary electrode and functions without electrolyte. 
       FIG. 1D  illustrates an additional embodiment of the enlarged partial perspective sectional view of the lower portion of the inner basin  102  containing the encased auxiliary electrode assembly  130 . The encased auxiliary electrode assembly  130  is positioned between an upper surface  151  of the inner basin  102  and a support member  150 . The support member  150  is fastened to the upper surface  151  of the inner basin  102  thus securing the encased auxiliary electrode assembly  130  to the upper surface  151  of the inner basin  102  while also allowing for expansion of the protective tube  134 . The support member  150  and upper surface  151  of the inner basin are welded together, bolted together or fastened using other conventional means. 
       FIG. 2  illustrates a perspective view of the anode base member  104  without the anode  105 . The upper surface of the anode base member  104  generally includes an annular recessed portion  201  defined by a vertical wall  208  and configured to receive a disk shaped anode  105  therein. Further, the bottom surface of annular recessed portion  201  generally includes a plurality of anode base channels  202  formed therein. Each of anode base channels  202  are generally positioned in parallel orientation with each other, extend across the lower portion of the annular recessed portion  201 , and terminate at the periphery of the annular recessed portion  201 . Additionally, the periphery of recessed region  201  includes an annular drain channel  203  that extends around the perimeter of recessed portion  201 . Each of the plurality of parallel positioned anode base channels  202  terminate at opposing ends into annular drain channel  203 . Therefore, anode base channels  202  may receive dense fluids from anode slots  107  (further discussed herein) and transmit the dense fluids to the annular drain channel  203  via anode base channels  202 . The vertical wall  208  that partially defines recessed portion  201  generally includes a plurality of anode base slots  204  formed into the vertical wall  208 . The anode base slots  204  are generally positioned in parallel orientation with each other, and further, are generally positioned in parallel orientation with the plurality of anode base channels  202  formed into the bottom surface of recessed portion  201 . The anode base member  104  also includes at least one conduit  205  configured to dispense a fluid into an anode region of the plating cell  100 , along with at least one plating solution supply conduit  206  that is configured to dispense a plating solution into the cathode compartment of plating cell  100 . The respective conduits  205  and  206  are generally in fluid communication with at least one fluid supply inlet  109  (illustrated in  FIG. 1A ) positioned on a lower surface of the anode base member  104 . 
       FIG. 3  illustrates a perspective view of the anode base member  104  having the disk shaped anode  105  positioned therein. The anode  105 , which is generally a disk shaped copper member, i.e., a soluble-type copper anode generally used to support copper electrochemical plating operations, generally includes a plurality of parallel positioned anode slots  107  formed therein. The anode slots  107  generally extend through the interior of anode  105  and are in fluid communication with both the upper surface and lower surface of anode  105 , as illustrated in the cross section of anode  105  in  FIG. 1 . As such, anode slots  107  allow fluids to travel through the interior of anode  105  from the upper surface to the lower surface of the anode  105 . However, when anode  105  is positioned within annular recess  201  of the anode base member  104 , the parallel anode slots  107  of the anode  105  are generally positioned orthogonal to both anode base slots  204  and anode base channels  202  of the anode base member  104 , as illustrated cooperatively by  FIGS. 2 and 3 . Further, with regard to positioning, the anode slots  107  are generally positioned such that the tilt angle of the cell positions the slots orthogonal to fluid flow as a result of the tilt, i.e., the anode slots  107  are positioned such that fluid flowing across the surface of the anode  105  as a result of the tilt angle of the cell will intersect the anode slots  107  and be received therein. Although the inventors have illustrated the anode slots  107  being positioned orthogonally to the fluid flow, other fluid intersection angles, such as angles between about 5° and about 89°, are contemplated within the scope of the invention. Additionally, anode slots  107  generally do not continuously extend across the upper surface of anode  105 . Rather, anode slots  107  are broken into a longer segment  303  and a shorter segment  304 , with a conductive spacer  305  between the two segments, which operates to generate a longer current path through anode  105  from one side to the other (when the current path is measured orthogonal to the anode slots  107 ). Further, adjacently positioned anode slots  107  have the conductive spacer  305  positioned on opposite sides of the anode upper surface for each alternating anode slot  107 . As such, the current path from the lower side of the anode  105  to the upper side of the anode  105  (orthogonal to the direction of the anode slots  107 ) generally includes a back and forth type path between the respective channels  107  through the spacer  305 . Further, the positioning of spacers  305  and the anode slots  107  provides for improved concentrated Newtonian fluid removal from the surface of the anode  105 , as the positioning of the anode slots  107  provides a shortest possible distance of travel for the dense fluids to be received in the anode slots  107 . This feature is important, as dense fluids generally travel slowly, and therefore, it is desirable. 
     In another embodiment of the invention anode  105  is manufactured from a bipolar insoluble electrode material. In this embodiment, the auxiliary electrode  132  may also be manufactured from a bipolar insoluble electrode material. In this embodiment, the anode  105  and auxiliary electrode  132  may be manufactured from platinum or other metal that is inert and operable as an anode material in an electrochemical plating solution. In this embodiment of the invention, a copper dosing system, such as a copper hydroxide dosing system, for example, may be used to replenish copper into the plating solution, i.e., the anolyte and catholyte of the plating cell, in place of the copper anode that supplies copper to conventional soluble anode plating cells. The auxiliary electrode  132  may be either consumable or a permanent part of the cell. 
     Referring to  FIG. 1A , the plating cell  100  further includes a membrane support assembly  106  configured to support the membrane  108 . The membrane support assembly  106  is generally secured at an outer periphery thereof to the anode base member  104 , and includes an interior region that is configured to allow fluids to pass therethrough via a sequence of oppositely positioned slots, bores, or other fluid apertures (not shown). The membrane support assembly  106  may include an o-ring type seal (not shown) positioned near a perimeter of the membrane  108 , wherein the seal is configured to prevent fluids from traveling from one side of the membrane  108  secured on the membrane support assembly  106  to the other side of the membrane  108  without passing through the membrane  108  itself. 
     The implementation of the membrane between the anode and the substrate being plated generates substantially different behavior in the plating cell as compared to conventional plating cells, both without membranes and those with the membranes discussed in this application. Specifically, the behavior of a copper anode in an acid-free CuSO 4  solution is different from conventional anode behavior. First, the sludge formation rate is lower at current densities of up to about 60 mA/cm 2  than that in CuSO 4 /H 2 SO 4  electrolyte, especially at concentrations of less than about 0.5M. In more concentrated CuSO 4  solutions both the amount of sludge and the probability of anode passiviation increases, especially at low flow rates through the anode compartment. Further, although Cu +  generally forms on the anode in both conventional tools and the tool of the invention, in the configuration of the present invention it accumulates only into the anolyte, mainly at current densities of greater than about 30 mA/cm 2 , when the oxygen dissolved in electrolyte has no time to convert Cu +  into Cu 2+  again. Further still, the stability of the anolyte and catholyte compositions decreases dramatically because of the small volumes of tanks. 
       FIG. 4  illustrates an exploded perspective view of an exemplary membrane support assembly  106  of the invention. Membrane support assembly  106  generally includes an upper ring shaped support member  401 , an intermediate membrane support member  400 , and a lower support member  402 . Upper and lower support members  401  and  402  are generally configured to provide structural support to intermediate membrane support member  400 , i.e., upper support member  401  operates to secure intermediate membrane support member  400  to lower support member  402 , while lower support member  402  receives intermediate membrane support member  400 . Intermediate membrane support member  400  generally includes a substantially planar upper surface having a plurality of bores (not shown) partially formed therethrough. A lower surface of intermediate membrane support member  400  generally includes a tapered outer portion  403  and a substantially planar inner membrane engaging surface  404 . An upper surface of lower support member  402  may include a corresponding tapered portion configured to receive the tapered outer portion  403  of intermediate membrane support member  400  thereon. The membrane engaging surface  404  generally includes a plurality of parallel positioned/orientated channels (not shown). Each of the channels formed into the lower surface of intermediate membrane support member  400  are in fluid communication with at least one of the plurality of bores partially formed through the planar upper surface. The channels operate to allow a membrane positioned in the membrane support assembly to deform slightly upward in the region of the channels, which provides a flow path for air bubbles and less dense fluids in the cathode chamber to travel to the perimeter of the membrane and be evacuated from the anode chamber. 
     In operation, the plating cell  100  of the invention provides a small volume (electrolyte volume) processing cell that may be used for copper electrochemical plating processes, for example. Plating cell  100  may be horizontally positioned or positioned in a tilted orientation, i.e., where one side of the cell is elevated vertically higher than the opposing side of the cell, as illustrated in  FIG. 1 . If plating cell  100  is implemented in a tilted configuration, then a tilted head assembly and substrate support member may be utilized to immerse the substrate at a constant immersion angle, i.e., immerse the substrate such that the angle between the substrate and the upper surface of the electrolyte does not change during the immersion process, or alternatively, at an angle that varies during the immersion process. Further, the immersion process may include a varying immersion velocity, i.e., an increasing velocity as the substrate becomes immersed in the electrolyte solution and rotation of the substrate during the immersion process. The combination of the constant immersion angle, rotation, and the varying immersion velocity operates to eliminate air bubbles on the substrate surface. 
     Assuming a tilted implementation is utilized, a substrate is first immersed into a plating solution contained within inner basin  102 . The immersion process generally includes positioning the substrate onto a cathode substrate support member or substrate contact ring. The substrate contact ring is generally configured to both support the substrate for electrochemical processing, as well as electrically contact the substrate to facilitate the electrolytic plating reaction. The electrical contact between the contact ring and the substrate is generally made via a plurality of electrically conductive contact pins positioned and configured to electrically engage a perimeter portion of the substrate and supply a plating bias to the substrate sufficient to support plating operations. Exemplary contact rings may be found in commonly assigned U.S. Pat. No. 6,136,163, filed on Mar. 5, 1999 and entitled Apparatus for Electrochemical Deposition with Thermal Anneal, commonly assigned U.S. Pat. No. 6,251,236, filed on Nov. 30, 1998 entitled Cathode Contact Ring for Electrochemical Deposition, and commonly assigned U.S. patent application Ser. No. 10/355,479, filed on Jan. 31, 2003 entitled Contact Ring with Embedded Flexible Contacts. All of the above noted cases illustrating contact rings are incorporated by reference herein in their entirety. 
     Once the substrate is immersed in the plating solution, which generally contains copper sulfate, a chlorine ion source, and one or more of a plurality of organic plating additives (levelers, suppressors, accelerators, etc.) configured to control plating parameters, an electrical plating bias is applied between a seed layer on the substrate and the anode  105  positioned in a lower portion of plating cell  100 . The electrical plating bias generally operates to cause metal ions in the plating solution to deposit on the cathodic substrate surface. The plating solution supplied to inner basin  102  is continually circulated through inner basin  102  via fluid inlet  109  and conduits  206 . More particularly, the plating solution may be introduced in plating cell  100  via a fluid inlet  109 . The solution may travel across the lower surface of the anode base member  104  and upward through one of fluid conduits  206 . The plating solution may then be introduced into the cathode chamber via a channel formed into plating cell  100  that communicates with the cathode chamber at a point above membrane support assembly  106  and in fluid communication with conduits  206 . Similarly, the plating solution may be removed from the cathode chamber via a corresponding conduit  206 . For example, as discussed above with respect to  FIG. 2 , the anode base member  104  may include first and second conduits  206  positioned on opposite sides of the anode base member  204 . The oppositely positioned conduits  206  may operate to individually introduce and drain the plating solution from the cathode chamber in a predetermined direction, which also allows for flow direction control. 
     Once the plating solution is introduced into the cathode chamber, the plating solution travels upward through a diffusion plate  110 . Diffusion plate  110 , which is generally a ceramic or other porous disk shaped member, generally operates as a fluid flow restrictor to even out the flow pattern across the surface of the substrate. Further, the diffusion plate  110  operates to resistively damp electrical variations in the electrochemically active area of the anode or cation membrane surface, which has been shown to reduce plating uniformities. Additionally, embodiments of the invention contemplate that the ceramic diffusion plate  110  may be replaced by a hydrophilic plastic member, i.e., a treated polyethylene (PE) member, a polyvinylidene (PVDF) member, a polypropylene (PP) member, or other material that is known to be porous and provide the electrically resistive damping characteristics provided by ceramics. However, the plating solution introduced into the cathode chamber, which is generally a plating catholyte solution, i.e., a plating solution with additives, is not permitted to travel downward through the membrane (not shown) positioned on the lower surface of membrane support assembly  106  into the anode chamber, as the anode chamber is fluidly isolated from the cathode chamber by the membrane. The anode chamber includes separate individual fluid supply and drain sources configured to supply an anolyte solution to the anode chamber. The solution supplied to the anode chamber, which may generally be copper sulfate in a copper electrochemical plating system, circulates exclusively through the anode chamber and does not diffuse or otherwise travel into the cathode chamber, as the membrane  108  positioned on membrane support assembly  106  is not fluid permeable in either direction. 
     Additionally, the flow of the fluid solution (anolyte, i.e., a plating solution without additives, which may be referred to as a virgin makeup solution) into the anode chamber is also directionally controlled in order to maximize plating parameters. For example, anolyte may be communicated to the anode chamber via an individual fluid inlet  109 . Fluid inlet  109  is in fluid communication with a conduit  205  formed into a lower portion of the anode base member. A seal positioned radially outward of conduits  205 , in conjunction with the surrounding structure, directs the anolyte flowing out of conduits  205  upward and into anode base slots  204 . Thereafter, the anolyte generally travels across the upper surface of the anode  105  towards the opposing side of the anode base member  104 , which in a tilted configuration, is generally the lower side of plating cell  100 . The anolyte travels across the surface of the anode below the membrane positioned immediately above. Once the anolyte reaches the opposing side of anode  105 , it is received into a corresponding conduit  205  and drained from plating cell  100  for recirculation thereafter. 
     During plating operations, the application of the electrical plating bias between the anode and the cathode generally causes a breakdown of the anolyte solution contained within the anode chamber. More particularly, the application of the plating bias operates to generate multiple hydrodynamic or Newtonian layers of the copper sulfate solution within the anode chamber. The hydrodynamic layers generally include a layer of concentrated copper sulfate positioned proximate the anode, an intermediate layer of normal copper sulfate, and a top layer of lighter and depleted copper sulfate proximate the membrane. The depleted layer is generally a less dense and lighter layer of copper sulfate than the copper sulfate originally supplied to the anode compartment, while the concentrated layer is generally a heavier and denser layer of copper sulfate having a very viscous consistency. The dense consistency of the concentrated layer proximate the anode causes electrical conductivity problems (known as anode passiviation) in anodes formed without anode slots  107 . However, anode slots  107 , in conjunction with the tilted orientation of plating cell  100 , operate to receive the concentrated viscous layer of copper sulfate and remove the layer from the surface of the anode, which eliminates conductivity variances. Further, as noted above, plating cell  100  generally includes one side that is tilted upward or vertically positioned above the other side, and therefore, the upper surface of anode  105  is generally a plane that is also tilted. This tilt causes the layer of concentrated copper sulfate generated at the surface of the anode to generally flow downhill as a result of the gravitational force acting thereon. As the concentrated copper sulfate layer flows downward, it is received within one of anode slots  107  and removed from the surface of the anode  105 . As discussed above, anode slots  107  are generally parallel to each other and are orthogonal to anode base slots  204 . As such, each of anode slots  107  intersect several of anode base channels  202  at the lower surface of the anode  105 . This configuration allows the concentrated copper sulfate received within anode slots  107  to be communicated to one or more of anode base channels  202 . Thereafter, the concentrated copper sulfate may be communicated via anode base channels  202  to the annular drain channel  203  positioned within recessed portion  201 . The annular drain channel  203  in communication with anode base channels  202  may generally be communicated through the anode base member  104  and back to a central anolyte supply tank, where the concentrated copper sulfate removed from the anode surface may be recombined with a volume of stored copper sulfate used for the anolyte solution. Similarly, the upper portion of anode chamber generates a diluted layer of copper sulfate proximate the membrane. The diluted layer of copper sulfate may be removed from the anode chamber via an air vent/drain  501 , as illustrated in  FIG. 5 . 
       FIG. 5  shows a partial sectional view of an edge of the plating cell  100  showing a groove  120  where the encased auxiliary electrode assembly  130  ( FIG. 1B ) is generally positioned. The groove  120  is located in the lower portion of the inner basin  102 . Air vent/drain  501 , which may include multiple ports, is generally positioned on the upper side of electrochemical plating cell  100 , and therefore, is positioned to receive both bubbles trapped within the anode chamber, as well as the diluted copper sulfate generated at the membrane surface. Air vents  501  are generally in fluid communication with the anolyte tank discussed above, and therefore, conducts the diluted copper sulfate received therein back to the anolyte tank, where the diluted copper sulfate may combine with the concentrated copper sulfate removed via anode slots  107  to form the desired concentration of copper sulfate within the anolyte tank. Any bubbles trapped by air vent  501  may also be removed from the cathode chamber vented to the atmosphere or simply maintained within the anolyte tank and not recirculated into the cathode chamber. 
     The catholyte solution (the solution used to contact and plate metal/copper onto the substrate) generally includes several constituents. The constituents generally include a virgin makeup plating solution or VMS (a plating solution that does not contain any plating additives, such as levelers, suppressors, or accelerators, such as that provided by Shipley Ronal of Marlborough, Mass. or Enthone, a division of  Cookson Electronics PWB Materials  &amp;  Chemistry  of London), water (generally included as part of the VMS, but is may also be added), and a plurality of plating solution additives configured to provide control over various parameters of the plating process. The catholyte is generally a low acid-type of plating solution, i.e., the catholyte generally has between about 5 g/l of acid and about 50 g/l of acid, or more particularly, between about 5 g/l and about 10 g/l. The acid may be sulfuric acid, sulfonic acid (including alkane sulfonic acids), pyrophosphoric acid, citric acid, and other acids known to support electrochemical plating processes. The desired copper concentration in the catholyte is generally between about 25 g/l and about 70 g/l, preferably between about 30 g/l and about 50 g/l of copper. The copper is generally provided to the solution via copper sulfate, and/or through the electrolytic reaction of the plating process wherein copper ions are provided to the solution via the anolyte from a soluble copper anode positioned in the catholyte solution. More particularly, copper sulfate pentahydrate (CuSO 4 .5H 2 O) may be diluted to obtain a copper concentration of about 40 g/l, for example. A common acid and copper source combination is sulfuric acid and copper sulfate, for example. The catholyte also has chlorine ions, which may be supplied by hydrochloric acid or copper chloride, for example, and the concentration of the chlorine may be between about 30 ppm and about 60 ppm. 
     As noted above, the plating solution (catholyte) generally contains one or more plating additives configured to provide a level of control over the plating process. The additives may include suppressors at a concentration of between about 1.5 ml/l and about 4 ml/l, preferably between about 2 ml/l and 3.0 ml/l. Exemplary suppressors include ethylene oxide and propylene oxide copolymers. Additives may also include accelerators at a concentration of between about 3 ml/l and about 10 ml/l, preferably within the range of between about 4.5 ml/l and 8.5 ml/l. Exemplary accelerators are based on sulfopropyl-disulfide or mercapto-propane-sulphonate and their derivatives. Additionally, another additive that may optionally be added to the catholyte solution is a leveler at a concentration of between about 1 ml/l and about 12 ml/l, or more particularly, in the range of between about 1.5 ml/l and 4 ml/l. 
     The anolyte solution, as noted above, is generally contained in the volume below the membrane and above the anode. The anolyte solution may be simply the catholyte solution without the plating additives, i.e., levelers, suppressors, and/or accelerators. However, the inventors have found that specific anolyte solutions, other than just stripped catholyte solutions, provide a substantial improvement in plating parameters. Specifically, copper transfer through the membrane and prevention of copper sulfate and hydroxide precipitation, i.e., when the Cu ions transport through membrane, copper sulfate accumulates in the anolyte and starts to precipitate on the anode provoking its passiviation, are improved. When pH of the anolyte is maintained above about 4.5 to about 4.8, copper hydroxide starts to deposit from Cu salt solutions, i.e., Cu 2+ +2H 2 O=Cu(OH) 2  (deposit)+2H + . More particularly, the inventors have found that if the anolyte can be configured to supply between about 90% and about 100% of the copper to the catholyte, then the membrane essentially operates as a clean copper anode, i.e., the membrane provides copper to the catholyte without the disadvantages associated with the electrochemical reaction that takes place at the surface of the anode (sludge formation, additive consumption, planarity variations due to erosion, etc.). The anolyte of the invention generally includes a soluble copper II salt (copper ions are not complexed with ligands like NH 3 , or EDTA or phyrophoshoric acid anions, as Cu transports through the membrane together with this ligand, like Cu(NH 3 ) 4   2+  will transport together with NH 3 , such as copper sulfate, copper sulfonate, copper chloride, copper bromide, copper nitrate, or a blend of any combination of these salts in an amount sufficient to provide a concentration of copper ions in the catholyte of between about 0.1M and about 2.5M, or more particularly, between about 0.25M and about 2M. 
     Additionally, the pH of the anolyte solution will generally be between about 1.5 and about 6, or more particularly, between about 2 and 4.8, for example. The pH is maintained in this range, as increasing the pH above this range in conventional plating configurations has been shown to cause copper hydroxide precipitation. Additionally, when the pH is below 2, and particularly if the pH is below 1.5, then the solution supports a substantial increase in the hydrogen ion (H + ) transport through the membrane from the anolyte to the catholyte. In this situation, the bulk of the plating current is carried by the H +  ions and the copper ion transport is reduced. As such, the copper ion concentration in the catholyte decreases, potentially to a critical level that will not support plating, while simultaneously the sulfuric acid concentration in the catholyte increases. The anolyte can generally use any soluble Cu 2+  salt, such as CuSO 4  (solubility 300 g/L), CuBr 2  (solubility more that 2 kg/L), CuCl 2  (solubility 700 g/L), CuF 2  (47 g/L), Cu(NO 3 ) 2  (1300 g/L) etc. The selection of anions depends on their impact to prevent or minimize Cu(I) formation and anode passiviation, on penetration through the membrane etc. For instance, the anolyte can be CuSO 4  (0.5 M) with small additions of Cu(NO 3 ) to activate anode surface and minimize Cu(I) formation. To minimize Cu(I) formation, small additions of Cu(ClO 3 ) 2  (solubility 2 kg/L) or Cu(lO 3 ) 2 —solubility 1 g/L may be used. In similar fashion to the catholyte, the source of copper in the anolyte (aside from the anode) may be copper sulfate pentahydrate (CuSO 4 .5H 2 O) at between about 51 g/L and 70 g/L, or at between about 0.75 M and about 0.95 M. Alternatively, in a preferred embodiment, the copper source may be between about 51 g/L and about 60 g/L, preferably about 54 g/L, and at a molarity of between about 0.8 M and about 0.9 M, preferably about 0.85 M. 
       FIG. 6  illustrates a plating thickness plot for a plating cell with the encased auxiliary electrode assembly in fluid communication with the catholyte compared to a plating cell without the encased auxiliary electrode assembly. The substrate plated had a diameter of 300 mm. This specific example is for 200 Å plated on a 300 Å Cu seed layer. The auxiliary electrode current for the “with auxiliary electrode” data is 0.4 A. The auxiliary electrode will work the same way for other seed layers, current densities, and plating thicknesses, but the auxiliary electrode current will vary. 
     The plating thickness plot of  FIG. 6  results from the auxiliary electrode absorbing electric flux near the perimeter of the substrate. This essentially results in the anode electrically seeing a substrate that has a larger surface area than the substrate being plated and as such, the terminal effect is shifted to the encased auxiliary electrode assembly  130  from the perimeter of the substrate, i.e., the increased thickness near the perimeter is shifted to the auxiliary electrode and off or away from the perimeter of the substrate. Further, since the auxiliary electrode may be controlled, the deposition thickness near the perimeter of the substrate may also be controlled. 
       FIG. 7  illustrates a method for immersing a substrate into a plating solution in accordance with one or more of the embodiments of the invention. This method  700  involves modulating the current density applied to the substrate/wafer in a manner that is synchronized with the wafer&#39;s motion and exerting strict control of the current density uniformity across the wafer surface. This approach targets the first few seconds of plating (or the first 50 to about 100 Å of deposition) when features are filled. 
     The initial steps  710  and  720  involve development of an optimized waveform that is applied to the wafer during the initial seconds of plating including the immersion sequence. Details of the immersion sequence and waveform development are described in U.S. patent application Ser. No. 11/052,443, entitled “Immersion Process For Electroplating Applications,” assigned to Applied Materials, Inc., filed Feb. 7, 2005 and herein incorporated by reference to the extent not inconsistent with the invention. The waveform consists of several steps whose durations and amplitudes are tuned to protect the seed-layer induce bottom-up growth, and achieve void-free fill. The steps are closely synchronized with the trajectory of the wafer as it is immersed and moved into its final plating position. 
     The next step  730  involves dynamic control of the instantaneous current distribution on the wafer. This has been achieved by model-guided design of the plating cell  100  configuration, which includes a diffusion plate  110  and an encased auxiliary electrode  132  to compensate for the terminal effect and produce uniform distribution of the instantaneous current density during the stages of plating. Specifically, the auxiliary electrode  132  current is dynamically varied in tandem with the waveform applied to the wafer and the growing thickness of the deposit. 
     As discussed earlier, “terminal effect” causes difference in deposition rate between different points across a substrate. The severity of the “terminal effect” are generally determined by size of the substrate and thickness of a seed layer, or the deposited metal layer. Because the thickness of the deposited metal layer increases as a plating process goes on, the severity of “terminal effect” changes continuously, which may require the compensation of “terminal effect” to be dynamic or adjustable. For example, when the substrate&#39;s resistivity is high, it may be desirable to plate under “edge-thin” conditions to compensate for the severe terminal effect during the initial stages of plating. As the terminal effect disappears, it may become desirable to plate under uniform plating conditions. As the plating process is continues and the overburden is deposited, it may be desirable to plate under an edge-thin, edge-thick, or uniform condition or with a specific plating profile that is optimum for subsequent processes such as CMP. 
     In this invention, the plating profile on the substrate is adjusted over the course of the plating process by plating in an apparatus where the electric field varies significantly as a function of the position of the substrate. This allows the different plating profiles to be achieved by moving the substrate. In the examples below, the electric field is made to vary within the plating cell as a function of the substrate&#39;s height above a restrictor element, and the wafer is moved to different heights during the plating process to achieve the different desired profiles. It should be noted however that the dynamic adjustment of the substrate position during the plating process to access different plating profiles is a general concept applicable to any plating apparatus where the plating profile on the substrate can be made to vary substantially with the substrate&#39;s position. 
       FIGS. 8A-8C  schematically illustrate an electrochemical plating cell  800  configured to provide an adjustable plating profile. A substrate  801  is retained in a substrate support  802  configured to transfer the substrate  801  vertically. A restrictor  803  is disposed parallel to the substrate  801 . The restrictor  803  has a diameter smaller than the diameter of the substrate  801 . During process, the substrate  801  and the restrictor  803  are both immersed in a plating solution, and an electric field is applied to the substrate  801  via the restrictor  803 . Curves  804  indicates the current density of the electric field. 
       FIGS. 8A-8C  illustrate the substrate  801  has a vertical elevation Z 1 , Z 2  and Z 3  respectively relative to the restrictor  803 , wherein elevation Z 1  is the smallest and elevation Z 3  is the largest. At elevation Z 1 , shown in  FIG. 8A , the current density  804  on the surface of the substrate  801  has an edge thin pattern for the substrate  801  is close to the restrictor  803 . At elevation Z 3 , shown in  FIG. 8C , the current density  804  on the surface of the substrate  801  has an edge thick pattern for the substrate  801  is far away from the restrictor  803 . At elevation Z 2 , shown in  FIG. 8B , the current density  804  is relatively uniformly distributed on the surface of the substrate  801 . Therefore, an edge thin plating profile may be achieved by positioning the substrate  801  at a low elevation, e.g., elevation Z 1 , an edge thick plating profile may be achieved by positioning the substrate  801  at a high elevation, e.g., elevation Z 3 , a uniform plating profile may be achieved by positioning the substrate  801  at a proper elevation, such as elevation Z 2 . 
     In one embodiment, the substrate  801  may be moved among different elevations during the plating process while the plating bias is being applied to the substrate  801  through the restrictor  803  having a smaller area than the substrate  801 . 
     It should be noted that the restrictor  803  may be any means that functions as a means to restrict or “retain” the plating current within an area smaller than the plating surface on the substrate. Exemplary restrictor may be an anode with a smaller surface area than the substrate, or a diffuser plate with a smaller open area then the substrate, or part of a fluid basin having a smaller diameter than the rest of the fluid basin, or any means to restrict the path of plating electric field between the anode and the substrate. 
       FIG. 9A  illustrates an electrochemical plating cell  900 A configured to have an adjustable electric field across a substrate during a plating process. The plating cell  900 A comprises a fluid basin  901  defining a fluid volume  909  configured to contain at least one electrolyte. The plating cell  900 A further comprises a substrate support member  902  configured to support a substrate  903  therein. The substrate support member  902  contacts the substrate  903  via a plurality of conductive contact pins  910 . The substrate support member  902  is further configured to move the substrate  903  and to spin the substrate  903  during process. An anode  908  is disposed in the fluid volume  909 . The anode  908  may be a disk having a smaller diameter than the substrate  903 . The substrate  903  may be moved vertically in the fluid volume  909 , for example, into position  903 A or  903 B, during plating to achieve an edge thin, an edge thick or a uniform plating profile. 
     During different stages of a plating process, the substrate  903  may be moved to different elevations and/or biased by different waveforms to achieve a desired plating profile. For example, a first waveform may be applied to the substrate  903  from the substrate  903  first immersed into the plating solution until the substrate  903  reaches a first plating position. In one embodiment, the first waveform may be configured to prevent a seed layer on the substrate  903  from being deplated. Then a second waveform may be applied to the substrate  903  when the substrate  903  is in the first plating position. In one embodiment, the first plating position may have a low elevation relative to the anode  908 , for example, position  903 B, to generate an edge thin profile and the second waveform may be configured accordingly. The substrate  903  can then be moved to a second plating position and a third waveform may be applied to the substrate  903 . In one embodiment, moving the substrate  903  from the first plating position to the second plating position may be conducted when the plated layer becomes thicker and the terminal effect becomes less severe. The second plating position may have a relative higher elevation compared to the first plating position, such as position  903 A, therefore, providing a relatively uniform plating profile. 
       FIG. 9B  illustrates an electrochemical plating cell  900 B configured to have an adjustable electric field across a substrate during a plating process. An anode  911  is disposed near the bottom of the fluid volume  909  of the fluid basin  901 . A diffuser plate  913  is placed above the anode  911 . The diffuser plate  913  may be secured in place by a ring shaped support  912  extended from the fluid basin  901 . The diffuser plate  913  may have a circular area to allow the plating solution inside the fluid volume  909  to pass through. The circular area may include a plurality of vertical channels to guide the flow of the plating solution in the fluid volume  909 . The circular area is smaller than the surface area of the substrate  903 . In one aspect, the diffuser plate  913  is configured to control the fluid flow in the fluid volume  909 . In another aspect, the diffuser plate  913  also restricts the plating current within the circular area smaller than the substrate  903 , which allows the plating profile to be adjusted by moving the substrate  903  vertically during the plating process. For example, the substrate  903  may be positioned at position  903 D for an edge thin plating profile and be moved to position  903 C for a more even plating profile compared to at position  903 D. 
       FIG. 9C  illustrates an electrochemical plating cell  900 C configured to have an adjustable electric field across a substrate during a plating process. The substrate support member  902  contacts the substrate  903  via a plurality of conductive contact pins  910 . An anode  904  is disposed near the bottom of the fluid basin  901 . A power supply  906  is configured to apply an electric bias between the conductive contact pins  910  and the anode  904 . An auxiliary electrode  905  is disposed in the fluid basin  901 . The auxiliary electrode— 905  is coupled to a power supply  916  which applies a bias to the auxiliary electrode  905  to adjust the electric field between the substrate  903  and the anode  904 . The electrochemical plating cell  900  further comprises a shield  907  configured to shield part of the fluid volume  909  from being affected by the auxiliary electrode  905 . The shield  907  may be a ring having an inner diameter smaller than the outer diameter of the substrate  903 . 
     In one embodiment, the auxiliary electrode  905  may be an encased electrode as described above. The auxiliary electrode  905  may be biased independently from the anode  904  and the substrate  903 . The effect of the auxiliary electrode  905  may be adjusted by controlling the bias applied to the auxiliary electrode  905  and/or adjusting the distance between the substrate  903  and the auxiliary electrode  905 . 
     The shield  907  further serves as a restrictor to retain the plating current between the anode  904  and the substrate  903 . Therefore, the plating profile on the substrate  903  can be adjusted by moving the substrate  903  to change the elevation of the substrate  903  relative to the shield  907 . 
     In this configuration, the plating profile may be changed by applying an appropriate bias to the auxiliary electrode in combination with positioning the substrate  903  at different elevations above the shield  907 . As a result, a set of different plating profiles, which are not otherwise available, can be generated on the substrate  903 , including edge-thin, edge thick or uniform, and other desirable plating profiles. 
     In another embodiment, an auxiliary electrode may be used in combination of a diffuser plate with a restricted area for fluid to pass through. For example, referring to  FIG. 9B , an auxiliary electrode  917  may be disposed over the ring shaped support  912 . 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.