Patent Publication Number: US-8123926-B2

Title: Electrolytic copper process using anion permeable barrier

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. application Ser. No. 11/299,293, filed Dec. 8, 2005, now abandoned, which is a continuation-in-part of U.S. application Ser. No. 11/296,574, filed Dec. 7, 2005, now abandoned priority from the filing dates of which is hereby claimed under 35 U.S.C. §120. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to electrolytic processing of microfeature workpieces and an electrolytic treatment process that utilizes an anion permeable barrier. 
     BACKGROUND OF THE INVENTION 
     Microfeature devices, such as semiconductor devices, imagers, displays, thin film heads, micromechanical components, microelectromechanical systems (MEMS), and large through-wafers vias are generally fabricated on and/or in microfeature workpieces using a number of machines that deposit and/or etch materials from the workpieces. Many current microfeature devices require interconnects and other very small, submicron sized features (e.g., 45-250 nanometers) formed by depositing materials into small trenches or holes. One particularly useful process for depositing materials into small trenches and/or vias is electrolytic processing, e.g., electroplating. Typical electrolytic processing techniques include electroplating processes that deposit copper, nickel, lead, gold, silver, tin, platinum, and other materials onto microfeature workpieces and etching processes that remove metals from microfeature workpiece surfaces. 
     In certain electroplating or etching processes, chelants or complexing agents are used to affect the electric potential at which metal ions are deposited onto or removed from surfaces of microfeature workpieces. Other components that may be present in the processing fluids include accelerators, suppressors, and levelers which can affect the results of the electroplating or electroetching process. Although these types of materials can positively influence the electroplating or electroetching processes, their use is not without drawbacks. For example, it is possible for these components to have an adverse impact on the electrolytic process as a result of reactions or other interactions with electrodes used in the electrolytic process. 
     Another challenge in depositing metals into narrow, deep trenches or vias is that it is difficult to completely fill the small features without creating voids or other nonuniformities in the deposited metal. For example, when depositing metal into a trench having a critical dimension of 45 nanometers to 250 nanometers, an ultrathin seed layer may be used, but care must be taken to ensure sufficient vacant space in the trench for the subsequently deposited bulk metal. In addition, ultrathin seed layers may be problematic because the quality of the deposited seed layer may not be uniform. For example, ultrathin seed layers may have voids or other nonuniform physical properties that can result in nonuniformities in the material deposited onto the seed layer. Such challenges may be overcome by enhancing the seed layers or forming a seed layer directly on a barrier layer to provide competent seed layers that are well suited for depositing metals into trenches or holes with small critical dimensions. One technique for enhancing the seed layer or forming a seed layer directly on a barrier layer is to electroplate a material using a processing solution with a low conductivity. Such low conductivity processing fluids have relatively low hydrogen ion (H + ) concentrations, i.e., relatively high pH. Suitable electrochemical processes for forming competent seed layers using low conductivity processing fluids are disclosed in U.S. Pat. No. 6,197,181, which is herein incorporated by reference. 
     Electroplating onto seed layers or electroplating materials directly onto barrier layers using low conductivity/high pH processing fluids presents additional challenges. For example, inert anodes are generally required when high pH processing fluids are used because the high pH tends to passivate consumable anodes. Such passivation may produce metal hydroxide particles and/or flakes that can create defects in the microfeatures. Use of inert anodes is not without its drawbacks. The present inventors have observed that when inert anodes are used, the resistivity of the deposited material increases significantly over a relatively small number of plating cycles. One way to combat this increase in the resistivity of the deposited material is to frequently change the processing fluid; however, this solution increases the operating cost of the process. 
     As a result, there is a need for electrolytic processes for treating microfeature workpieces that reduce adverse impacts created by the presence of complexing agents and/or other additives and also maintain deposit resistivity within desired ranges. 
     SUMMARY 
     The embodiments described herein relate to processes for electrolytically processing a microfeature workpiece to deposit copper ions or remove copper from surfaces of microfeature workpieces. In certain embodiments, the processes are capable of producing deposits that exhibit resistivity values within desired ranges over an extended number of plating cycles. The embodiments described herein also relate to processes that reduce the adverse impacts created by the presence of complexing agents and/or other additives in processing fluids used to electrolytically process a microfeature workpiece. In some embodiments, the described processes employ low conductivity/high pH processing fluids without suffering from the drawback of defect formation in the deposited material resulting from the presence of metal hydroxide particles or flakes present in processing fluids in contact with the microfeature workpiece. Processors of microfeature workpieces will find certain processes described herein desirable because the processes produce high yields of deposits that exhibit resistivity values within acceptable ranges without requiring costly frequent replacement of processing fluids. Reducing adverse impacts created by the presence of complexing agents and/or other additives in the processing fluids may also be considered desirable by users of the electrolytic processes described herein. 
     In one embodiment, a surface of a microfeature workpiece is contacted with a first processing fluid that includes first processing fluid species, such as a copper ion, an anion, and a complexing agent. A counter electrode is in contact with a second processing fluid and an electrochemical reaction occurs at the counter electrode. The process effectively prevents movement of cationic species between the first processing fluid and the second processing fluid. In certain embodiments, the first processing fluid can be a high pH processing fluid, the copper ion can be deposited onto the surface of the microfeature workpiece, and the counter electrode can be an inert electrode. 
     In another embodiment, a surface of a microfeature workpiece is contacted with a first processing fluid that includes a copper ion to be deposited onto the surface of the microfeature workpiece. In addition, the first processing fluid includes a complexing agent and a counter anion to the copper ion. An inert anode is in contact with a second processing fluid, and an oxidizing agent is produced at the inert anode. The process employs an anion permeable barrier between the first processing fluid and the second processing fluid. The anion permeable barrier allows counter anions to pass from the first processing fluid to the second processing fluid. In this embodiment, copper ions in the first processing fluid are deposited onto the surface of the microelectronic workpiece. In certain embodiments, the first and second processing fluids can be high pH processing fluids. 
     In a further embodiment, a surface of a microfeature workpiece is contacted with a first processing fluid that includes a copper ion to be deposited onto a surface of the microelectronic workpiece. In this embodiment, an inert anode is in contact with a second processing fluid that includes a buffer and pH adjustment agent and an anion permeable barrier is located between the first processing fluid and the second processing fluid. 
     The processes summarized above can be carried out in a system for electrolytically processing a microfeature workpiece. The system includes a chamber that has a processing unit for receiving a first processing fluid and counter electrode unit for receiving a second processing fluid. A counter electrode is located in the counter electrode unit, and an anion permeable barrier is located between the processing unit and the counter electrode unit. The chamber further includes a source of copper ion in fluid communication with the processing unit and a source of a pH adjustment agent in fluid communication with the counter electrode unit. 
     Through the use of processes described above and the system described above, copper can be deposited onto surfaces of a microfeature workpiece. Such surfaces can take the form of seed layers or barrier layers. 
     The process embodiments and system described above can be used to electroplate materials onto a surface of a microfeature workpiece or used to electroetch or deplate materials from a surface of a microfeature workpiece. When the process is used to electroplate materials, the microfeature workpiece will function as a cathode, and the counter electrode will function as an anode. In contrast, when deplating is carried out, the microfeature workpiece will function as an anode, and the counter electrode will function as a cathode. 
     Accordingly, in another embodiment, a surface of a microfeature workpiece is contacted with a first processing fluid that includes a counter ion to copper on the surface. A cathode is contacted with a second processing fluid also containing a counter ion, and an anion permeable barrier is located between the first processing fluid and the second processing fluid. Chemical species in the second processing fluid are reduced, and an acid is introduced to the second processing fluid. Counter ions from the second processing fluid are passed through the anion permeable barrier to the first processing fluid. In accordance with this embodiment, copper from the surface of the microfeature workpiece is electrolytically dissolved, i.e., oxidized and deplated. 
     The process summarized in the previous paragraph can be carried out in a system for electrolytically processing a microfeature workpiece that includes a chamber that has a processing unit for receiving a first processing fluid and a counter electrode unit for receiving a second processing fluid. An anion permeable barrier is positioned between the processing unit and the counter electrode unit. The system further includes a cathode in the counter electrode unit, a source of copper counter ions in fluid communication with the counter electrode unit, and a source of pH adjustment agent in fluid communication with the processing unit. 
     Through the use of the processes and systems described above for removing copper from surfaces of a microfeature workpiece, copper can be deplated from a microfeature workpiece surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and many of the attendant advantages of the processes described herein will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a schematic illustration of a reactor for carrying out processes described herein; 
         FIG. 2  graphically illustrates deposit resistivity as a function of bath age for a deposit formed using processing fluids separated by an anion permeable barrier and a deposit formed using a processing fluid without an anion permeable barrier; 
         FIG. 3  is a schematic illustration of a chamber for carrying out processes described herein; 
         FIG. 4  is a schematic illustration of the chemistry and chemical reactions occurring in one embodiment of the processes for electroplating a metal described herein; 
         FIG. 5  is a schematic illustration of the chemistry and chemical reactions occurring in one embodiment of the processes for electroplating two metals described herein using an inert anode; 
         FIG. 6  is a schematic illustration of the chemistry and chemical reactions occurring in one embodiment of the processes for electroplating two metals described herein using a consumable anode; 
         FIG. 7  is a schematic illustration of the chemistry and chemical reactions occurring in one embodiment of the processes for deplating a metal described herein; 
         FIG. 8  is a schematic illustration of a tool that includes chambers for carrying out processes described herein; 
         FIGS. 9A-9C  are schematic illustrations of one embodiment of the processes described herein for electrolytically treating a seed layer; and 
         FIGS. 10A and 10B  are schematic illustrations of one embodiment of the processes described herein for electrolytically treating a barrier layer. 
     
    
    
     DETAILED DESCRIPTION 
     As used herein, the terms “microfeature workpiece” or “workpiece” refer to substrates on and/or in which micro devices are formed. Such substrates include semiconductive substrates (e.g., silicon wafers and gallium arsenide wafers), nonconductive substrates (e.g., ceramic or glass substrates), and conductive substrates (e.g., doped wafers). Examples of micro devices include microelectronic circuits or components, micromechanical devices, microelectromechanical devices, micro optics, thin film recording heads, data storage elements, microfluidic devices, and other small scale devices. 
     In the description that follows regarding electroplating a metal onto a microfeature workpiece, specific reference is made to copper as an example of a metal ion that can be electroplated onto a microfeature workpiece. The reference to copper ions is for exemplary purposes, and it should be understood that the following description is not limited to copper ions. Examples of other metal ions useful in the processes described herein include gold ions, tin ions, silver ions, platinum ions, lead ions, cobalt ions, zinc ions, nickel ions, ruthenium ions, rhodium ions, iridium ions, osmium ions, rhenium ions, and palladium ions. 
     In the description that follows regarding electroplating more than one metal onto a microfeature workpiece, specific reference is made to a tin-silver solder system as an example of metal ions that can be electroplated onto a microfeature workpiece to form a composite deposit. The reference to deposition of a tin-silver solder is for exemplary purposes, and it should be understood that the description is not limited to tin and silver ions. 
     With respect to the description that follows regarding deplating a metal from a microfeature workpiece, specific reference is made to copper as an example of a metal ion that can be deplated from a microfeature workpiece. The reference to copper is for exemplary purposes, and it should be understood that the description regarding deplating are not limited to the removal of copper. Examples of other metals that can be removed from a microfeature workpiece in accordance with embodiments described herein include gold ions, tin ions, silver ions, platinum ions, lead ions, cobalt ions, zinc ions, nickel ions, ruthenium ions, rhodium ions, iridium ions, osmium ions, rhenium ions, and palladium ions. 
     Processes described herein can be carried out in an electrochemical reactor, e.g., an electroplating or deplating reactor, such as the one described below with reference to  FIG. 1 . Referring to  FIG. 1 , reactor  10  includes an upper processing unit  12  containing a first processing fluid  14  (e.g., a catholyte in an electroplating process or an anolyte in a deplating process) and a counter electrode unit  18  below the processing unit  12  that contains a second processing fluid  20  (e.g., anolyte in an electroplating process or a catholyte in a deplating process) which may be different in composition and/or properties from the first processing fluid  14 . Processing unit  12  receives a working electrode  16  (e.g., a microfeature workpiece) and delivers first processing fluid  14  to the working electrode  16 . Counter electrode unit  18  includes a counter electrode  22  that is in contact with the second processing fluid  20 . When copper is to be deposited onto working electrode  16 , working electrode  16  is the cathode and counter electrode  22  is the anode. Accordingly, in plating applications, first processing fluid  14  is a catholyte, and second processing fluid  20  is an anolyte. In general, the catholyte contains components in the form of ionic species, such as acid ions, hydroxyl ions, and metal ions, and a complexing agent capable of forming a complex with the metal ions. The catholyte may also include organic components, such as accelerators, suppressors, and levelers that improve the results of the electroplating process. In addition, the catholyte may include a pH adjustment agent to affect the pH of the catholyte. The anolyte generally includes ionic species as well, such as acid ions, hydroxyl ions, and metal ions. The catholyte may also include a pH adjustment agent. Additional details regarding the various components in the catholyte and anolyte are provided below. 
     When copper is to be deplated from working electrode  16 , working electrode  16  is the anode, and counter electrode  22  is the cathode. Accordingly, in deplating applications, the first processing fluid  14  is an anolyte, and the second processing fluid  20  is a catholyte. 
     Reactor  10  also includes a nonporous anion permeable barrier  24  between the first processing fluid  14  and the second processing fluid  20 . Nonporous anion permeable barrier  24  allows anions to pass through the barrier while inhibiting or substantially preventing non-anionic components, such as cations, from passing between the first processing fluid  14  and second processing fluid  20 . By inhibiting or substantially preventing nonionic components from passing between the first processing fluid  14  and second processing fluid  20 , adverse effects on the deposited material resulting from the presence of unwanted nonanionic components, such as cations, in the first processing fluid  14  can be avoided. As such, nonporous anion permeable barrier  24  separates first processing fluid  14  and second processing fluid  20  such that first processing fluid  14  can have different chemical characteristics and properties than second processing fluid  20 . For example, the chemical components of first processing fluid  14  and second processing fluid  20  can be different, the pH of first processing fluid  14  and second processing fluid  20  can be different, and concentrations of components common to both first processing fluid  14  and second processing fluid  20  can be different. 
     In the following description of an electroplating process, for consistency, working electrode  16  will be referred to as the cathode, and counter electrode  22  will be referred to as the anode. Likewise, first processing fluid  14  will be referred to as the catholyte, and second processing fluid  20  will be referred to as the anolyte. When reactor  10  is used to electrolytically process a microfeature workpiece to deposit metal ions thereon, an electric potential is applied between anode  22  and cathode  16 . Copper ions in the catholyte are consumed by the deposition of copper ions onto the cathode. Meanwhile, the anode becomes positively charged and attracts negatively charged ions to its surface. For example, hydroxyl ions in the anolyte are attracted to the anode where they react to liberate oxygen and produce water. The foregoing results in a gradient of charge in the anolyte with unbalanced positively charged species in the anolyte solution, and negatively charged species in the catholyte solution. This charge imbalance encourages the transfer of negatively charged anions through the anion permeable barrier  24  from catholyte  14  to the anolyte  20 . An electrochemical reaction (e.g., losing or gaining electrons) occurs at cathode  16 , resulting in metal ions being reduced (i.e., gaining electrons) to metal on surfaces of cathode  16 . 
     Reactor  10  effectively maintains the concentration of metal ions in catholyte  14  during the electroplating process in the following manner. As metal ions are deposited onto the surface of cathode  16 , additional metal ions are introduced to catholyte  14  from a source of metal ions  130 , which is in fluid communication with processing unit  12 . As explained below in more detail, these metal ions can be provided by delivering a metal salt solution to processing unit  12 . Processing unit  12  can also be in fluid communication with sources of other components that need replenishment. In a similar fashion, counter electrode unit  18  may be in fluid communication with sources of components that require replenishment. For example, counter electrode unit  18  can be in fluid communication with a source of pH adjustment agent  132 . Likewise, both processing unit  12  and electrode unit  18  can include conduits or other structures for removing portions of catholyte  14  from processing unit  12  or portions of anolyte  20  from counter electrode unit  18 . 
     Anode  22  may be a consumable anode or an inert anode. Exemplary consumable anodes and inert anodes are described below in more detail. 
     Referring to  FIG. 4 , the chemistry present in processing unit  12  and counter electrode unit  18  are described in more detail along with various chemical reactions that are believed to occur. It should be understood that by describing chemical reactions that are believed to occur within reactor  10 , the processes described herein are not limited to processes wherein these reactions occur. 
       FIG. 4  schematically illustrates an example of the operation of reactor  10  using an anion permeable barrier  24  and an inert anode  22  in combination with a low conductivity/high pH first processing fluid and a low conductivity/high pH second processing fluid suitable for processing copper seed layers or plating directly onto a barrier layer. In the description that follows, high pH first processing fluid  14  in processing unit  12  is a catholyte containing a metal ion (M + ), e.g., copper ions (Cu 2+ ), a counter ion (X − ) for the metal ion, e.g., sulfate ions (SO 4   2− ), a complexing agent CA, as described below, chelated with the metal ions, a pH buffer such as boric acid (H 3 BO 3 ) that dissociates into hydrogen ions (H + ) and H 2 BO 3 −, and a pH adjustment agent, such as tetramethylammonium hydroxide (TMAH) that dissociates into hydroxyl ion (OH − ) and TMA + . The specific hydrogen ion concentration in catholyte  14  can be chosen taking into consideration conventional factors such as complexing ability of the complexing agent, buffering capability of the buffer, metal ion concentrations, volatile organics concentrations, deposition potential of the complex at the particular pH, solubility of the catholyte constituents, stability of the catholyte, desired characteristics of the deposits, and diffusion coefficients of the metal ions. 
     For example, for electroplating embodiments, high pH second processing fluid  20  in counter electrode unit  18  is an anolyte. Anolyte  20  can have a concentration of H +  that is approximately equal to the concentration of H +  in catholyte  14 , although this is not required. By adjusting H +  concentration in anolyte  20  to be approximately equal to the concentration of H +  in catholyte  14 , transfer of negatively charged hydroxyl ions from catholyte  14  to anolyte  20  through anion permeable membrane  24  during electroplating is inhibited. By inhibiting the transfer of negatively charged hydroxyl ions from catholyte  14  to anolyte  20 , a more constant catholyte pH can be maintained. By maintaining pH of the catholyte relatively constant, the need to add pH adjustment agent to the catholyte is reduced or eliminated. This simplifies maintenance of the catholyte and helps to maintain the conductivity of the catholyte relatively stable through repeated plating cycles. In the description that follows with reference to  FIG. 4 , anolyte  20  includes a pH adjustment agent, such as TMAH, and a buffer, such as boric acid. 
     As mentioned above, during a plating cycle, an electric potential is applied between cathode  16  and anode  22 . As copper ions are reduced and electroplated onto cathode  16 , sulfate ions (SO 4   2− ) accumulate in the catholyte near a first surface  32  of anion permeable barrier  24 . Additionally, depending on the pH of the anolyte at positively charged anode  22 , hydroxyl ions (OH − ) are converted to water (H 2 O) and oxygen (O 2 ) and/or water is decomposed to hydrogen ions (H + ) and oxygen. The resulting electrical charge gradient causes the negatively charged sulfate ions to move from first surface  32  of anion permeable barrier  24  to the second surface  34  of anion permeable barrier  24 . The transfer of negatively charged sulfate ions from catholyte  14  to anolyte  20  during the plating cycle maintains the charge balance of reactor  10 . To maintain the concentration of the negatively charged ions resulting from the dissociation of boric acid in catholyte  14  during electroplating, the concentration of boric acid in the anolyte may be set so it is significantly greater than the concentration of boric acid in the catholyte. This concentration differential inhibits the negatively charged ions resulting from the dissociation of boric acid in catholyte  14  from moving through anion permeable membrane  24  to anolyte  20  during electroplating. 
     Continuing to refer to  FIG. 4 , during a plating cycle, as explained above, copper ions in catholyte  14  are reduced at cathode  16  and are deposited as copper. Copper ions that are consumed by the electroplating are replenished by the addition of a solution of copper sulfate to the catholyte. During the plating cycle, sulfate ions which are introduced to catholyte  14  as a result of adding the copper sulfate transfer across anion permeable barrier  24  to anolyte  20 . Portions of the anolyte can be removed from counter electrode unit ( 18  in  FIG. 1 ) in order to avoid the excessive build up of sulfate ions in the anolyte  20 . Non-anionic components in catholyte  14  (e.g., Cu 2+ , H + , TMA + , H 3 BO 3 , and Cu(ED) 2   2+ ) generally do not pass through anion permeable membrane  24  and thus remain in catholyte  14 . As described above, transfer of the hydroxyl (OH − ) ion from the catholyte to the anolyte is minimized by maintaining pH of anolyte  20  at substantially the same level as pH of catholyte  14 . Since hydroxyl ions are consumed at anode  22 , a pH adjustment agent, such as TMAOH, may be added to anolyte  20  as described above to maintain the pH of the anolyte at desired levels. 
     Microfeature workpieces that can be processed using processes described herein can include different structures on their surfaces that can be electrolytically processed to deposit materials thereon. For example, a semiconductor microfeature workpiece can include seed layers or barrier layers. Referring to  FIGS. 9A-9C , one sequence of steps for electrolytically processing a seed layer using a process described herein is provided. 
     Referring to  FIG. 9A , a cross-sectional view of a microstructure, such as trench  105  that is to be filled with bulk metallization is illustrated and will be used to describe use of processes described herein to enhance a seed layer. As shown, a thin barrier layer  110 , for example, titanium nitride or tantalum nitride, is deposited over the surface of a semiconductor device or, as illustrated in  FIG. 9A , over a layer of dielectric  108  such as silicon dioxide. Any known technique such as chemical vapor deposition (CVD) or physical vapor deposition (PVD), can be used to deposit barrier layer  110 . 
     After deposition of barrier layer  110 , an ultrathin copper seed layer  115  is deposited on barrier layer  110 . The resulting structure is illustrated in  FIG. 9B . Seed layer  115  can be formed using a vapor deposition technique also, such as CVD or PVD. Alternatively, seed layer  115  can be formed by direct electroplating onto barrier layer  110  as described below in more detail. Owing to the small dimensions of trench  105 , techniques used to form ultrathin seed layer  115  should be capable of forming the seed layer without closing off small geometry trenches. In order to avoid closing off small geometry trenches, seed layer  115  should be as thin as possible while still providing a suitable substrate upon which to deposit bulk metal. For example, ultrathin seed layer  115  can have a thickness of about 50 to about 500 angstroms, about 100 to about 250 angstroms, or specifically about 200 angstroms. 
     The use of ultrathin seed layer  115  introduces its own set of drawbacks. For example, ultrathin seed layers may not coat the barrier layer in a uniform manner. For example, voids or non-continuous seed layer regions on the sidewalls of the trenches such as at  120 , can be present in ultrathin seed layer  115 . The processes described herein can be used to enhance seed layer  115  to fill the void or non-continuous regions  120  found in ultrathin seed layer  115 . Referring to  FIG. 9C , to achieve this enhancement, the microfeature workpiece is processed as described herein to deposit a further amount of metal  118  onto ultrathin seed layer  115  and/or portions of underlying barrier layer  110  that are exposed at voids or non-continuous portions  120 . 
     Preferably, this seed layer enhancement continues until a sidewall step coverage, i.e., the ratio of seed layer  115  thickness at the bottom sidewall regions to the nominal thickness of seed layer  115  at the exteriorly disposed side of the workpiece, achieves a value of at least 10%. More preferably, the sidewall step coverage is at least about 20%. Preferably, such sidewall step coverage values are present in substantially all of the recessed structures of the microfeature workpiece; however, it will be recognized that certain recessed structures may not reach such sidewall step values. 
     Another type of feature on the surface of a microfeature workpiece that can be electrolytically treated using processes described herein is a barrier layer. Barrier layers are used because of the tendency of certain metals to diffuse into silicon junctions and alter the electrical characteristics of semiconductor devices formed in a substrate. Barrier layers made of materials such as titanium, titanium nitride, tantalum, tantalum nitride, tungsten, and tungsten nitride are often laid over silicon junctions and any intervening layers prior to depositing a layer of metal. Referring to  FIG. 10A , a cross-sectional view of a microstructure, such as trench  205  that is to be filled with bulk metallization is illustrated, and will be used to describe the formation of a metal layer directly onto a barrier layer using processes described herein. As illustrated in  FIG. 10A , thin barrier layer  210  is deposited over the surface of a semiconductor device or, as illustrated in  FIG. 10A , over a layer of dielectric  208 , such as silicon dioxide. Barrier layer  210  can be deposited as described above with reference to  FIG. 9A  using CVD or PVD techniques. After barrier layer  210  is deposited, the microfeature workpiece is processed as described herein to form a metal feature  215  over barrier layer  210 . The resulting structure can then be further processed to deposit bulk metal (not shown) to fill the trench  205 . 
     The pH of processing fluids described herein can vary from alkaline to acidic. The low conductivity/high pH processing fluids described herein are distinct from low pH processing fluids such as acidic electroplating baths. The concentration of H +  useful in high pH processing fluids may vary with those providing pHs above 7, preferably above 8 and most preferably above 9 being examples of useful high pH processing fluids. 
     As noted above, processes described herein are useful to electroplate metals other than copper, for example, gold, silver, platinum, nickel, tin, lead, ruthenium, rhodium, iridium, osmium, rhenium, and palladium. Metal ions useful in the catholyte can be provided from a solution of a metal salt. Exemplary metal salts include gluconates, cyanides, sulfamates, citrates, fluoroborates, pyrophosphates, sulfates, chlorides, sulfides, chlorites, sulfites, nitrates, nitrites, and methane sulfonates. Exemplary concentrations of metal salts in the catholyte used for plating applications range from about 0.03 to about 0.25M. 
     The ability to electroplate metal ions can be affected by chelating the metal ion with a complexing agent. In the context of the electroplating of copper, copper ions chelated with ethylene diamine complexing agent exhibit a higher deposition potential compared to copper ions that have not been chelated. Complexing agents useful for chelating and forming complexes with metal ions include chemical compounds having at least one part with the chemical structure COOR 1 —COHR 2 R 3  where R 1  is an organic group covalently bound to the carboxylate group (COO), R 2  is either hydrogen or an organic group, and R 3  is either hydrogen or an organic group. Specific examples of such type of complexing agents include citric acid and salts thereof, tartaric acid and salts thereof, diethyltartrate, diisoproyltartrate, and dimethyltartrate. Another type of useful complexing agent includes compounds that contain a nitrogen containing chelating group R—NR 2 —R 1 , wherein R is any alkyl or aromatic group, and R 1  and R 2  are H, alkyl, or aryl organic groups or polymer chains. Specific examples of these types of complexing agents include ethylene diamine, ethylene diamine tetraacetic acid and its salts, cyclam, porphrin, bipyridyl, pyrolle, thiophene, and polyamines. In plating embodiments, suitable ratios between the concentration of metal ions and concentrations of complexing agents in the catholyte can range from 1:25 to 25:1; for example, 1:10 to 10:1 or 1:5 to 5:1. 
     Useful pH adjustment agents include materials capable of adjusting the pH of the first processing fluid and the second processing fluid, for example, to above 7 to about 13, and more specifically, above about 9.0. When ethylene diamine or citric acid are used as a complexing agent for copper ions, a pH of about 9.5 is useful. When ethylene diamine tetraacetic acid is used as a complexing agent for copper ions, a pH of about 12.5 is suitable. Examples of suitable pH adjustment agents include alkaline agents such as potassium hydroxide, ammonium hydroxide, tetramethyl ammonium hydroxide, sodium hydroxide, and other alkaline metal hydroxides. A useful amount and concentration of pH adjustment agents will depend upon the level of pH adjustment desired and other factors, such as the volume of processing fluid and the other components in the processing fluid. Useful pH adjustment agents also include materials capable of adjusting the pH of the first and second processing fluid to below 7. 
     Useful buffers include materials that maintain the pH relatively constant, preferably at a level that facilitates complex formation and desirable complexed species. Boric acid was described above as an example of a suitable buffer. Other useful buffers include sodium acetate/acetic acid and phosphates. Exemplary concentrations of buffer range from about 0.01 to about 0.5M in the catholyte for plating applications. 
     The catholyte can include other additives such as those that lower the resistivity of the fluid, e.g., ammonium sulfate; and those that increase the conformality of the deposit, e.g., ethylene glycol. For plating applications, exemplary concentrations of resistivity effecting agents in the catholyte range from about 0.01 to about 0.5M. For conformality affecting agents concentrations ranging from about 0 to 1.0M are exemplary. 
     Useful anion permeable barriers include nonporous barriers, such as semi-permeable anion exchange membranes. A semi-permeable anion exchange membrane allows anions to pass but not non-anionic species, such as cations. The nonporous feature of the barrier inhibits fluid flow between first processing fluid  14  and second processing fluid  20  within reactor  10  in  FIG. 1 . Accordingly, an electric potential, a charge imbalance between the processing fluids, and/or differences in the concentrations of substances in the processing fluids can drive anions across an anion permeable barrier. In comparison to porous barriers, nonporous barriers are characterized by having little or no porosity or open space. In a normal electroplating reactor, nonporous barriers generally do not permit fluid flow when the pressure differential across the barrier is less than about 6 psi. Because the nonporous barriers are substantially free of open area, fluid is inhibited from passing through the nonporous barrier. Water, however, may be transported through the nonporous barrier via osmosis and/or electro-osmosis. Osmosis can occur when the molar concentration in the first and second processing fluids are substantially different. Electro-osmosis occurs as water is carried through the nonporous barrier with current-carrying ions in the form of a hydration sphere. When the first and second processing fluids have similar molar concentrations and no electrical current is passed through the processing fluids, fluid flow between the first and second processing fluids via the nonporous barrier is substantially prevented. 
     A nonporous barrier can be hydrophilic so that bubbles in the processing fluids do not cause portions of the barrier to dry, which reduces conductivity through the barrier. Exemplary nonporous barriers include Ionac® membranes manufactured by Sybron Chemicals, Inc., and NeoSepta® manufactured by Asahi Kasei Company. 
     In addition to the nonporous barriers described above, anion permeable barrier can also be a porous barrier. Porous barriers include substantial amounts of open area or pores that permit fluid to pass through the porous barrier. Both ionic materials and nonionic materials are capable of passing through a porous barrier; however, passage of certain materials may be limited or restricted if the materials are of a size that allows the porous barrier to inhibit their passage. While useful porous barriers may limit the chemical transport (via diffusion and/or convection) of some materials in the first processing fluid and the second processing fluid, they allow migration of anionic species (enhanced passage of current) during application of electric fields associated with electrolytic processing. In the context of electrolytic processing a useful porous barrier enables migration of anionic species across the porous barrier while substantially limiting diffusion or mixing (i.e., transport across the barrier) of larger organic components and other non-anionic components between the anolyte and catholyte. Thus, porous barriers permit maintaining different chemical compositions for the anolyte and the catholyte. The porous barriers should be chemically compatible with the processing fluids over extended operational time periods. Examples of suitable porous barrier layers include porous glasses (e.g., glass frits made by sintering fine glass powder), porous ceramics (e.g., alumina and zirconia), silica aerogel, organic aerogels (e.g., resorcinol formaldehyde aerogel), and porous polymeric materials, such as expanded Teflon® (Gortex®). Suitable porous ceramics include grade P-6-C available from CoorsTek of Golden, Colo. An example of a porous barrier is a suitable porous plastic, such as Kynar™, a sintered polyethylene or polypropylene. Suitable materials can have a porosity (void faction) of about 25%-85% by volume with average pore sizes ranging from about 0.5 to about 20 micrometers. Such porous plastic materials are available from Poretex Corporation of Fairburn, Ga. These porous plastics may be made from three separate layers of material that include a thin, small pore size material sandwiched between two thicker, larger pore-sized sheets. An example of a product useful for the middle layer having a small pore size is CelGard™ 2400, made by CelGard Corporation, a division of Hoechst, of Charlotte, N.C. The outer layers of the sandwich construction can be a material such as ultra-fine grade sintered polyethylene sheet, available from Poretex Corporation. Porous barrier materials allow fluid flow across themselves in response to the application of pressures normally encountered in an electrochemical treatment process, e.g., pressures normally ranging from about 6 psi and below. 
     Inert anodes useful in processes described herein are also referred to as non-consumable anodes and/or dimensionally stable anodes and are of the type that when an electric potential is applied between a cathode and an anode in contact with an electrolyte solution, that there is no dissolution of the chemical species of the inert anode. Exemplary materials for inert anodes include platinum, ruthenium, ruthenium oxide, iridium, and other noble metals. 
     Consumable anodes useful in processes described herein are of the type that when an electric potential is applied between a cathode and an anode in contact with an electrolyte solution, dissolution of the chemical species making up the anode occurs. Exemplary materials for consumable anodes will include those materials that are to be deposited onto the microfeature workpiece, for example, copper, gold, tin, silver, lead, platinum, nickel, cobalt, zinc, and the like. 
     The temperature of the processing fluids can be chosen taking into consideration conventional factors such as complexing ability of the complexing agent, buffering capability of the buffer, metal ion concentration, volatile organics concentration, deposition potential of the complexed metal at the particular pH, solubility of the processing fluid constituents, stability of the processing fluids, desired deposit characteristics, and diffusion coefficients of the metal ions. Generally, temperatures ranging from about 20° C.-35° C. are suitable, although temperatures above or below this range may be useful. 
     As described above in the context of an electroplating process, oxidation of hydroxyl ions or water at the anode produces oxygen capable of oxidizing components in the catholyte. When an anion permeable barrier is absent, oxidation of components in the electrolyte can also occur directly at the anode. Oxidation of components in the electrolyte is undesirable because it is believed that the oxidized components contribute to variability in the properties (e.g. resistivity) of the metal deposits. Through the use of an anion permeable barrier, as described above, transfer of oxygen generated at the anode from the anolyte to the catholyte is minimized and/or prevented, and, thus, such oxygen is not available to oxidize components that are present in the catholyte. As discussed above, one way to address the problem of oxygen generated at the anode oxidizing components in the processing fluid is to frequently replace the processing fluid. Because of the time and cost associated with frequently replacing the processing fluid, the processes described herein provide an attractive alternative by allowing the processing fluids to be used in a large number of plating cycles without replacement. Use of the anion permeable barrier also isolates the anode from non-anionic components in the catholyte, e.g., complexing agent, that may otherwise be oxidized at the anode and adversely affect the ability of the catholyte to deposit features having resistivity properties that fall within acceptable ranges. 
     The resistivity of deposited metals as a function of the age of the processing fluid from which the deposit was formed is illustrated in  FIG. 2 .  FIG. 2  illustrates how the use of processes described herein to deposit copper significantly extends the useful operating life of a catholyte compared to conventional processes using similar chemistries without an anion permeable barrier.  FIG. 2  illustrates test results evaluating the resistivity of several  20  nanometer copper seed layers deposited using the same chemistry in contact with the workpiece. One set of copper seed layers was formed using a process that did not employ an anion permeable barrier and a second set of copper seed layers was formed using a process that employed an anion permeable barrier in accordance with a process described herein. More specifically the resistivity of a deposit was measured using a 4-pt resistivity probe from Creative Design Engineering, Inc. that measures the sheet resistance of a substrate. The resistivity of the copper films was obtained as the product of sheet resistance and thickness of the thin film. Several wafers with the same seed layer resistance were obtained, and their sheet resistance was pre-measured (R PVD ). The wafers were then plated in a chamber with no membranes, inert anodes, and a 9.5 pH electrolyte. A fixed amp time was applied for each wafer (0.7 amp-min for a 300 mm wafer corresponding to 20 nm Cu thickness), and a theoretical amount of Cu film deposited on the seed layer was obtained at the same current density. The wafers were then rinsed and dried in a spin rinse dryer. The wafers were then measured in the 4-pt probe again. This provided a post sheet resistance measurement (R total ). With the two sheet resistance measurements, the sheet resistance of just the film deposited was obtained through the method of parallel subtraction using the formula shown below and the sheet of resistance of the electrodeposited seed layer was obtained.
 
 R   ECD Seed =( R   PVD   *R   total )/( R   PVD   −R   total )
 
     The resistivity of the deposited film was obtained by multiplying the thickness by the sheet resistance calculated above. 
     Resistivity of the electrodeposit=Thickness*R ECD Seed =20 nm*R ECD Sed    
     Several such wafers were plated periodically, as the bath ages in terms of amp-min (dummy wafers were plated to age the bath). 
     Similar sets of wafers were plated using an anionic membrane as described herein. The chamber utilized the same electrolyte as the catholyte used to plate the first set of wafers described above. The anolyte was a fluid consisting of buffer, pH adjustment agent, and of the same pH as the catholyte as described herein. The wafers were plated with the same amp time as before under substantially identical conditions. 
     Similar calculations were performed and the resistivity of the electrodeposit was obtained as a function of bath age. 
     The resistivity of seed layers deposited using a process without an anion permeable barrier (line  38 ), increases rapidly with the age of the bath, increasing more than three times in under 2000 amp minutes. By comparison, the resistivity of copper seed layers deposited using a process that employed an anion permeable membrane as described herein, illustrated by line  40 , increases only gradually over time such that little increase is observed even after 10,000 amp minutes. These results illustrate how a process described herein substantially extends the useful life of processing fluids used to deposit metal features onto a microfeature workpiece. 
     Another advantage of employing an anionic permeable barrier in the processes described herein is that the barrier prevents bubbles from the oxygen gas evolved at the anode from transferring to the catholyte. Bubbles in the catholyte are undesirable because they can cause voids or holes in the deposited features. 
     Another feature of processes described herein is that the pH adjustment agent, e.g., tetramethyl ammonium hydroxide, does not accumulate in the first processing fluid. As a result, pH adjustment agent need not be removed from the first processing fluid. This simplifies the maintenance of the first processing fluid. 
     In the foregoing descriptions, copper has been used as an example of a metal that can be used to enhance a seed layer or to form a metal feature directly onto a barrier layer. However, it should be understood that the basic principles of the processes described herein and their use for enhancement of an ultrathin metal layer prior to the bulk deposition of additional metal or the direct electroplating of a metal onto a barrier layer can be applied to other metals or alloys as well as deposition for other purposes. For example, gold is commonly used on for thin film head and III-V semiconductor applications. Gold ions can be electroplated using chloride or sulfite as the counter ion. As with copper, the chloride or sulfite counter ion would migrate across the anionic permeable barrier as described above in the context of copper. Potassium hydroxide could be used as the pH adjustment agent in a gold electroplating embodiment to counteract a drop in pH in the anolyte resulting from the oxidation of hydroxyl ions at the anode. As with the copper example described above, in the gold embodiment, gold chloride or gold sulfite, in the form of sodium gold sulfite or potassium gold sulfite could be added to the catholyte to replenish the gold deposited. 
     As mentioned previously, processes described above are useful for depositing more than one metal ion onto a microfeature workpiece surface. For example, processes described above are useful for depositing multi-component solders such as tin-silver solders. Other types of multi-component metal systems that can be deposited using processes described above include tin-copper, tin-silver-copper, lead-tin, nickel-iron, and tin-copper-antimony. Unlike certain copper features that are formed on the surfaces of microfeature workpieces, solder features tend to be used in packaging applications and are thus large compared to copper microfeatures. Because of their larger size, e.g., 10-200 microns, solder features are more susceptible to the presence of bubbles in processing fluids that can become entrapped and affect the quality of the solder deposits. A tin-silver solder system is an example of plating of a metal with multiple valence states. Generally, metals with multiple valence states can be plated from most of their stable states. Since the charge required to deposit any metal is directly proportional to the electrons required for the reduction, metals in their valence states closest to their neutral states consume less energy for reduction to metal. Unfortunately, most metals in their state closest to their neutral states are inherently unstable, and therefore production-worthy plating can be unfeasible. Through the use of processes for plating metal ions described above, plating solutions that include metals in this inherently unstable state can be applied in an effective process to deposit the desired metal. Through the use of the processes described above for depositing a metal, less oxidation of the inherently unstable metal species occurs, thus providing a more production-worthy process. 
     By way of illustration, most fin-silver plating solutions prefer Sn(II) as the species for tin plating. For such multi-component plating systems, control of tin and silver ions needs to be precise, and the use of silver or tin as an anode is not feasible. The use of such consumable anodes could cause stability issues resulting from plating/reacting with the anodes, and they also create issues relating to the ability to uniformly replenish metal. On the other hand, the use of inert anodes avoids the foregoing issues, but introduces a new issue associated with the production of oxygen through the oxidation of water or hydroxyl ions at the inert anode. Such oxygen not only may oxidize other components in the plating bath, it may also oxidize the desired Sn(II) species to the more stable Sn(IV) ion, which is more difficult to plate onto a workpiece. 
     Referring to  FIG. 5 , a schematic illustration is provided for the operation of reactor  10  using an anion permeable barrier  24  and an inert anode  22  in combination with a first processing fluid and a second processing fluid suitable for depositing tin-silver solder. In the description that follows, processing fluid  14  in processing unit  12  is a catholyte containing metal ions M 1   +  and M 2   + , e.g., Sn 2+  and Ag +  ions; counter ions X 1 − and X 2 − for the metal ions, e.g., methane sulfonate CH 3 SO 3 −; complexing agents CA 1  and CA 2 , e.g., proprietary organic additives, chelated with the metal ions. As discussed above in the context of the electroplating of copper, the specific hydrogen ion concentration in catholyte  14  can be chosen taking into consideration conventional factors such as complexing ability of the complexing agent, buffering capability of the buffer, metal ion concentrations, volatile organics concentrations, alloy deposition potential of the complex at the particular pH, solubility of the catholyte constituents, stability of the catholyte, desired characteristics of the deposits, and diffusion coefficients of the metal ions. 
     The discussions above regarding the concentration of H +  in the anolyte and catholyte, relative concentrations of the buffer in the anolyte and the catholyte, use of the pH adjustment agent, replenishment of the metal ions, cathodic reduction reactions, and anodic oxidation reactions in the context of the electroplating of copper are equally applicable to a tin-silver system. The particular operating conditions that are most desirable are related to the specific chemistry being used. 
     As with the copper plating process, an electric potential applied between cathode  16  and anode  22  results in tin ions and silver ions being reduced at cathode  16  and deposited thereon. The methane sulfonate (MSA) counter ion (CH 3 SO 3 −) accumulates in the catholyte near a first surface  32  of anion permeable barrier  24 . As with the copper system, at positively charged anode  22 , hydroxyl ions are converted to water and oxygen and/or water is decomposed to hydrogen ions and oxygen. The resulting electrical charge gradient causes negatively charged MSA ions (CH 3 SO 3 −) to move from first surface  32  of anion permeable barrier  24  to the second surface  34  of anion permeable barrier  24 . The transfer of negatively charged MSA ions (CH 3 SO 3 −) from catholyte  14  to anolyte  20  during the plating cycle maintains the charge balance of reactor  10 . Tin and silver ions that are deposited onto cathode  16  can be replenished by the addition of a solution of tin methane sulfonate and silver methane sulfonate to the catholyte. During the plating cycle, MSA ions that are introduced to catholyte  14  as a result of the addition of the tin MSA and silver MSA transfer across anion permeable barrier  24  to anolyte  20 . As with the copper process, portions of the anolyte can be removed from counter electrode unit  18  to avoid the buildup of MSA ions in the anolyte. 
     Referring to  FIG. 6 , in a different embodiment, electroplating of two metals, e.g., tin and silver, can also be achieved using a consumable anode  122 . Referring to  FIG. 6 , the catholyte  14  in processing unit  12  is similar to the catholyte described with reference to  FIG. 5 . In the process depicted in  FIG. 6 , metal ion M 2   +  is introduced into processing unit  12  from source  200 , metal ion M 1   +  is supplied to counter electrode unit  18  through oxidation of metal making up consumable anode  122 . Metal ion M 1   +  combines with counter ion X 1 − to form the metal salt M 1 X 1  in counter electrode unit  18  and is then delivered via line  202  to processing unit  12 . Metal salt M 1 X 1  delivered to processing unit  12  dissociates therein to provide a source of metal ion M 1   +  that can be reduced at cathode  16  and deposited thereon as described above with reference to  FIG. 5 . In accordance with this embodiment, complexing agents CA 1  and CA 2  are present in catholyte  14  where they can complex with metal ions M 1   +  and M 2   + . Suitable pH adjustment agents and pH buffers may be present and/or added to the catholyte and anolyte. The charge balance within reactor  10  can be maintained through the transfer of negatively charged counter ion X 1 − in processing unit  12  across anion permeable membrane  24  into counter electrode unit  18 . 
     Metal can be deplated from a microfeature workpiece by reversing the bias of the electric field created between the microfeature workpiece and the working electrode. Referring to  FIG. 7 , a microfeature workpiece  416  is provided that carries a metal M, e.g., copper, on its surface. Microfeature workpiece  416  is contacted with a first processing fluid  414  in processing unit  412 . Processing fluid  414  includes metal ions M 2+ , e.g., copper ions; a complexing agent CA, e.g., ethylene diamine tetraacetic acid; a metal salt MX, e.g., copper sulfate; a complexed metal ion M(CA) 2+ ; hydroxyl ions; a buffer, and a counter ion X 2− , e.g., sulfate ion. First processing unit  412  is separated from a counter electrode unit  418  by anion permeable membrane  424 . Counter electrode unit  418  includes counter electrode  422  and a second processing fluid  420 . In a deplating process, microfeature workpiece  416 , the working electrode, is an anode, and counter electrode  422  is the cathode. Processing unit  412  is also in fluid communication with a source  430  of complexing agent CA and hydroxyl ions. In addition, processing unit  412  can be provided with a mechanism  440  for removing metal salts therefrom. Counter electrode unit  418  is in fluid communication with a source  450  of acid. Through the application of an electric potential between anode  416  and cathode  422 , hydrogen ions are reduced at the cathode  422  to produce hydrogen gas. Copper on the surface of microfeature workpiece  416  is oxidized resulting in copper ions being removed from the surface of the microfeature workpiece. The charge balance within reactor  410  is maintained through the transfer of counter ion X 2−  from catholyte  420  to anolyte  414  through anion permeable membrane  424 . 
     Catholyte  420  in counter electrode unit  418  includes hydroxyl ions, buffer, and counter ion X 2 −. The acid that is added to counter electrode unit  418  can be both a source of counter ion X 2−  as well as a pH adjustment agent. In order to reduce or eliminate transfer of buffer components across anionic permeable membrane  424 , the buffer concentration in anolyte  414  can be maintained at a level equal to or greater than the concentration of buffer in catholyte  420 . 
     One or more of the reactors for electrolytically treating a microfeature workpiece or systems including such reactors may be integrated into a processing tool that is capable of executing a plurality of methods on a workpiece. One such processing tool is an electroplating apparatus available from Semitool, Inc., of Kalispell, Mont. Referring to  FIG. 5 , such a processing tool may include a plurality of processing stations  510 , one or more of which may be designed to carry out an electrolytic processing of a microfeature workpiece with a high pH first processing fluid and an inert anode as described above. Other suitable processing stations include one or more rinsing/drying stations and other stations for carrying out wet chemical processing. The tool also includes a robotic member  520  that is carried on a central track  525  for delivering workpieces from an input/output location to the various processing stations. 
     Referring to  FIG. 3 , a more detailed schematic illustration of one design of a reactor  100  for electroplating metals onto seed layers, directly electroplating metal onto barrier layers, or otherwise depositing materials onto workpieces is illustrated. Reactor  100  includes a vessel  302 , a processing chamber  310  configured to direct a flow of first processing fluid to a processing zone  312 , and an anode chamber  320  configured to contain a second processing fluid separate from the first processing fluid. An anion permeable barrier  330  separates the first processing fluid in the processing unit  310  from the second processing fluid in the anode chamber  320 . Reactor  100  further includes a workpiece holder  340  having a plurality of electrical contacts  342  for applying an electric potential to a workpiece  344  mounted to workpiece holder  340 . Workpiece holder  340  can be a movable head configured to position workpiece  344  in processing zone  312  of processing unit  310 , and workpiece holder  340  can be configured to rotate workpiece  344  in processing zone  312 . Suitable workpiece holders are described in U.S. Pat. Nos. 6,080,291; 6,527,925; 6,773,560, and 7,294,243 all of which are incorporated herein by reference. 
     Reactor  100  further includes a support member  350  in the processing chamber  310  and a counter electrode  360  in the anode chamber  320 . Support member  350  spaces the anion permeable barrier  330  apart from workpiece processing zone  312  by a controlled distance. This feature provides better control of the electric field at processing zone  312  because the distance between the anion permeable barrier  330  and workpiece processing zone  312  affects the field strength at processing zone  312 . Support member  350  generally contacts first surface  332  of anion permeable barrier  330  such that the distance between first surface  332  and processing zone  312  is substantially the same across processing chamber  310 . Another feature of support member  350  is that it also shapes anion permeable barrier  330  so that bubbles do not collect along a second side  334  of anion permeable barrier  330 . 
     Support member  350  is configured to direct flow F 1  of a first processing fluid laterally across first surface  332  of anion permeable barrier  330  and vertically to processing zone  312 . Support member  350  accordingly controls the flow F 1  of the first processing fluid in processing chamber  310  to provide the desired mass transfer characteristics in processing zone  312 . Support member  350  also shapes the electric field in processing chamber  310 . 
     Counter electrode  360  is spaced apart from second surface  334  of anion permeable barrier  330  such that a flow F 2  of the second processing fluid moves regularly outward across second surface  334  of anion permeable barrier  330  at a relatively high velocity. Flow F 2  of the second processing fluid sweeps oxygen bubbles and/or particles from the anion permeable barrier  330 . Reactor  100  further includes flow restrictor  370  around counter electrode  360 . Flow restrictor  370  is a porous material that creates a back pressure in anode chamber  320  to provide a uniform flow between counter electrode  360  and second surface  334  of the anion permeable barrier  330 . As a result, the electric field can be consistently maintained because flow restrictor  370  mitigates velocity gradients in the second processing fluid where bubbles and/or particles can collect. The configuration of counter electrode  360  and flow restrictor  370  also maintains a pressure in the anode chamber  320  during plating that presses the anion permeable barrier  330  against support member  350  to impart the desired contour to anion permeable barrier  330 . 
     Reactor  100  operates by positioning workpiece  344  in processing zone  312 , directing flow F 1  of the first processing fluid through processing chamber  310 , and directing the flow F 2  of the second processing fluid through anode chamber  320 . As the first and second processing fluids flow through reactor  100 , an electric potential is applied to workpiece  344  via electrical contacts  342  and counter electrode  360  to establish an electric field in processing chamber  310  and anode chamber  320 . 
     Another useful reactor for depositing metals using processes described herein is described in U.S. Patent Application No. 2005/0087439, which is expressly incorporated herein by reference. 
     While a preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.