Abstract:
A process and apparatus are provided for electroplating a film onto a substrate having a top side including a plating surface includes the following steps. Provide a plating tank with an electroplating bath. Provide an anode in the bath. Place a substrate having a plating surface to be electroplated into the electroplating bath connecting surfaces to be plated to a first cathode. Support a second cathode including a portion thereof with openings therethrough extending across the plating surface of the substrate and positioned between the substrate and the anode. Connect power to provide a negative voltage to the first cathode and provide a negative voltage to the second cathode, and provide a positive voltage to the anode.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to processes and apparatus for plating metals onto a workpiece and more particularly to processes and apparatus for relatively uniformly, and selectively plating of small features on a workpiece.  
           [0003]    2. Description of Related Art  
           [0004]    A serious problem in electroplating microscopic features non-uniformly dispersed on a large substrate of a workpiece is that the total current required for depositing the material onto these microscopic features is very small. There are no commercially available power supplies that can reliably deliver the required kind of small currents to the workpiece/substrate.  
           [0005]    FIGS.  1 A- 1 C are schematic diagrams which illustrate a prior art type of plating system using a thief electrode ring  22  surrounding a workpiece  26  such as a silicon wafer to enhance the quality of plating of metal onto the workpiece  26  through a photoresist mask formed thereon (not shown) as is widely practiced in the art. In FIG. 1A which is a vertical elevational view, a plating tank  10  has a bottom  12 , left sidewall  14  and right sidewall  15  and a top  16  shown to be open. FIG. 1B shows the thief ring  22  and the substrate  26  isolated from the other elements seen in FIG. 1A. FIG. 1C is a right side view of what is shown in FIG. 1B. In the example shown here, the tank  10 , which is formed of a dielectric material, contains an electroplating bath  17  up to the level shown by line  18 . An anode  33  is located in the plating tank  10  near the right sidewall  15 . A positive voltage V 3  is applied to anode  33  by a connection wire  32 . The workpiece  26 , which is shown on the side of the tank  10  near the left sidewall  14  has a negative voltage (−V 1 ) applied thereto by a connection wire  36 . There is a thief ring  22 , which surrounds the workpiece  26  is coplanar with the workpiece  26 . A second negative voltage (−V 2 ) is applied to the thief ring  22  by a connection wire  34 . A space  24  is provided between the workpiece  26  and the thief ring  22 . The thief ring is adjusted in voltage to adjust the plating current to the workpiece  26 , but it is not possible to maintain an equal current density across the large surface of the substrate  26  which may be several inches wide.  
           [0006]    As a result, the thickness of the material deposited on various features on the workpiece  26  can vary from workpiece to workpiece. This variation creates a very big quality control problem for the plating engineer who is required to deposit a desired thickness on all of the features of the workpieces  26  within narrow tolerances.  
           [0007]    A second and more important problem in dealing with small features dispersed on a large substrate  26  is that the secondary current, and higher order currents, cause tremendous non-uniformity in the thickness of the deposited material from place to place across the workpiece  26 . This non-uniformity will vary depending upon the density of the features and also on the size of the features on the substrate  26 .  
           [0008]    Prior art relating to cathodes in electroplating baths include the following patents.  
           [0009]    U.S. Pat. No. 6,027,631 of Broadbent for “Electroplating System with Shields for Varying Thickness Profile of Deposited Layer”, which is concerned with plating a blanket layer across a substrate, describes an electroplating system where a shield is placed above and adjacent to a workpiece; and the workpiece is rotated, so as to form uniform plating across the workpiece. The process described employs physical obstruction of current by use of the shield(s). However, the shield(s) is electrically inactive and is inserted between cathode (part or substrate) and the anode. The shield is placed in location and the substrate is rotated with respect to the shield. Thus the substrate or wafer is exposed to the anode part of the time. It is believed by the inventors that this method is applicable to plating larger features in inert matrix photoresist mask, but not to plating of small features distributed in a non-uniform distribution across the entire substrate. Also this method does not help when the active area that needs to be plated is small since the shield is not electrically connected to the substrate to increase the ‘apparent size’ of the workpiece or substrate.  
           [0010]    U.S. Pat. No. 6,077,405 of Biggs et al., commonly assigned, for “Method and Apparatus for Making Electrical Contact to a Substrate During Electroplating” also shows a peripheral ring electrode, often referred to as a “thief ring” since it is an auxiliary cathode which diverts cathode current away from the primary cathode. The Biggs et al. patent describes the structure of exemplary substrates and mechanical and electrical connections to the substrates.  
           [0011]    U.S. Pat. No. 5,135,636 of Yee et al. for “Electroplating Method” describes a plating rack for use in electroplating at least one substrate comprising a silicon wafer surrounded by a metal ring with cam assemblies holding the wafer in place and for making electrical contact between the ring and the wafer and passing a current from the ring to the wafer while they are submerged in an electroplating bath.  
           [0012]    U.S. Pat. No. 5,620,581 of Ang for “Apparatus for Electroplating Metal Films Including a Cathode Ring, Insulator Ring, and Thief Ring” describes apparatus for electroplating metal films composed of dual metal, i.e. a permalloy (NiFe) alloy, where a wafer workpiece is set inside a thief ring and coplanar to the ring. The part is connected to a first power supply. The power to thief ring is described by text which is a variance with the drawings which show a second connection line to a common connection to a D.C. voltage source which is referred to as a “dual channel power supply . . . employed to generate separately controlled current densities to the thief ring . . . and the cathode ring . . . ” which is used so that by controlling “the thief current density, the metal composition of the electroplated metal film is controlled.” There is also a stainless steel “cathode ring” which mechanically supports the lower surface of the substrate/wafer which electrically connects the wafer to a power supply. The objectives of Ang include a compositional uniformity as well as thickness uniformity. Essentially, the Ang patent addresses edge effects and the primary current distribution problem.  
           [0013]    U.S. Pat. No. 6,001,235 of Arken et al., commonly assigned, for “Rotary Plater with Radially Distribute Plating Solution” shows a rotating cathode and a rotating segmented ring formed of a set of separated annular thief elements.  
           [0014]    U.S. Pat. No. 6,071,388 of Uzoh for “Electroplating Workpiece Fixture Having Liquid Gap Spacer” shows a peripheral thief ring electrode. Uzoh suggests that the thief ring electrode should comprise of a stainless steel or titanium plate including a metal mesh or screen such as No. 4 or No. 30 metal mesh corresponding to wires per inch. The mesh increases the surface area of the thief ring electrode, but does not solve the problem of non-uniformity of plating of small features.  
           [0015]    U.S. Pat. No. 6,074,544 of Reid et al. for “Method for Electroplating Semiconductor Wafer Using Variable Currents and Mass Transfer to Obtain Uniform Plated Layer” describes forming a metal seed layer and providing electrical contacts at the edge of a wafer which leads to the dish-effect in which the thickness of the layer is less in the center. Reid teaches minimization of the dishing effect by using a low plating current density initially to reduce the resistive (IR) voltage drop followed by increasing the current density to a higher level after reaching a predetermined thickness and resistivity.  
           [0016]    U.S. Pat. No. 4,828,653 of Traini et al. for “Long Lasting Anode for High Current Density Galvanization” relates to anodes in electroplating baths includes the following patent, which is not analogous to this invention since it pertains to cathodes in electroplating baths includes the following patent but does show to employment of a mesh in a plating bath electrode well over a decade ago. Traini et al. describes a long lasting anode formed by several parallel layers of foraminous (i.e. having small openings or perforations) sheets of metallic mesh with different patterns. The sheets of metallic mesh are resistant to the electrolyte such as Ti, Ta, NB or W in electrical contact with each other. The metals used in the mesh are preferably inert to a plating liquid in a electroplating bath presumably to prevent dissolution of the metals in the mesh into the plating solution during plating of the cathode.  
           [0017]    The current electroplating process used in some thin film applications such as semiconductor packaging uses a fixture that holds a workpiece/substrate and an auxiliary electrode also widely known as a thief plate. The auxiliary (thief) electrode surrounds the actual workpiece in such a way that the substrate surface and the auxiliary electrode surface are in a plane. The main workpiece and the auxiliary electrode can be connected to two different power supplies so that the voltages/currents can be controlled independently. This arrangement works fairly well when the active area being plated is relatively large and uniformly distributed across the entire substrate. However, problems occur with the peripheral thief ring process when the active area on the substrate is microscopic and/or when the area is non-uniformly distributed over the substrate surface.  
         SUMMARY OF THE INVENTION  
         [0018]    There are problems with electroplating microscopic features which are often out of the primary current distribution region. There is a lack of sufficient thieving activity to provide the desired secondary current distribution and higher order current distribution in the plating bath which will permit uniform plating. Moreover there is a lack of commercial power supplies that can reliably deliver small currents to electroplate small areas. In addition there has been an inability to provide methods and means for electroplating myriad microscopic features with unknown active areas. In the ever increasing trend towards smaller and smaller microscopic electronic devices it is not possible to provide apparatus and methods which can be tailored to each permutation of distribution of microscopic features on a workpiece.  
           [0019]    In addition there is a limitation of substrate size that can be used with traditional peripheral thieving rings surrounding the workpiece.  
           [0020]    It is an object of this invention to electroplate microscopic features that are non-uniformly dispersed on a large substrate.  
           [0021]    An object of this invention is an electroplating process including plating fixture for electroplating very microscopic features dispersed on a large substrate with a high degree of uniformity.  
           [0022]    A process in accordance with this invention is provided for electroplating a film onto a substrate having a top side including a plating surface includes the following steps. Provide a plating tank with an electroplating bath. Provide an anode in the bath. Place a substrate having a surface to be electroplated into the electroplating bath connecting surfaces to be plated to a first cathode. Support a second cathode including a portion thereof with openings therethrough extending across the plating surface of the substrate and positioned between the substrate and the anode. Connect power to provide a negative voltage to the first cathode and provide a negative voltage to the second cathode, and provide a positive voltage to the anode.  
           [0023]    Preferably, the openings comprise apertures through the second cathode extending across the substrate the openings are formed between parallel wires extending across the substrate, or the openings comprise a wire mesh extending across the substrate, or the openings comprise apertures formed in a plate which is preferably a stainless steel plate located between the anode and the cathode.  
           [0024]    Preferably, the first cathode and the second cathode are connected to an adjustable power supply.  
           [0025]    Preferably, the first cathode and the second cathode are connected to the same power supply.  
           [0026]    Preferably, the first cathode and the second cathode are connected to the same power supply with a resistor in series with the second cathode.  
           [0027]    In accordance with another aspect of this invention, a process for electroplating a film onto a substrate having a top side including a plating surface comprises the following steps. Provide a plating tank with an electroplating bath. Provide an anode in the bath. Place a substrate to be electroplated into the electroplating bath connecting surfaces to be plated to a first cathode. Support a second cathode including a partially open screening electrode selected from a plating mesh and aperture plate and parallel wires proximate to the substrate between the substrate and the anode. Connect a power supply by providing a negative voltage to the first cathode and to the screening electrode and providing a positive voltage to the anode.  
           [0028]    The openings comprise a wire mesh extending across the substrate and the first cathode and the second cathode are connected to power supplied in a manner selected from the group consisting of a power supply with a resistor in series with the second cathode, the first cathode and the second cathode are connected to the same power supply, the first cathode and the second cathode are connected to same power supply with a resistor in series with the second cathode, and the first cathode and the second cathode are connected to an adjustable power supply. process in accordance with this invention is provided for electroplating a film onto a substrate having a top side including a plating surface includes the following steps. Provide a plating tank with an electroplating bath. Provide an anode in the bath. Place a substrate having a plating surface to be electroplated into the electroplating bath connecting surfaces to be plated to a first cathode. Support a second cathode including a portion thereof with openings therethrough extending across the plating surface of the substrate and positioned between the substrate and the anode. Connecting power to provide a negative voltage to the first cathode and provide a negative voltage to the second cathode, and provide a positive voltage to the anode.  
           [0029]    Apparatus in accordance with this invention is provided for electroplating a film onto a substrate having a top side including a plating surface includes the following steps. The apparatus includes a plating tank with an electroplating bath, an anode in the bath. A substrate having a plating surface to be electroplated is placed in the electroplating bath with surfaces to be plated connected to a first cathode. A second cathode including a portion thereof with openings therethrough is supported extending across the plating surface of the substrate and positioned between the substrate and the anode. Power to provide a negative voltage is connected to the first cathode and provide a negative voltage to the second cathode, and provide a positive voltage to the anode.  
           [0030]    Preferably, the openings comprise apertures through the second cathode extending across the substrate the openings are formed between parallel wires extending across the substrate, or the openings comprise a wire mesh extending across the substrate, or the openings comprise apertures formed in a plate which is preferably a stainless steel plate located between the anode and the cathode.  
           [0031]    Preferably, the first cathode and the second cathode are connected to an adjustable power supply, the first cathode and the second cathode are connected to the same power supply, and the first cathode and the second cathode are connected to the same power supply with a resistor in series with the second cathode.  
           [0032]    In accordance with another aspect of this invention, means are provided for electroplating a film onto a substrate having a top side including a plating surface comprises the following steps including a plating tank with an electroplating bath and an anode in the bath. The substrate to be electroplated is placed into the electroplating bath connecting surfaces to be plated to a first cathode. A second cathode is supported in the bath. The second cathode includes a partially open screening electrode selected from a plating mesh and aperture plate and parallel wires proximate to the substrate between the substrate and the anode. A power supply is connected by providing a negative voltage to the first cathode and to the screening electrode and providing a positive voltage to the anode.  
           [0033]    The openings comprise a wire mesh extending across the substrate and the first cathode and the second cathode are connected to power supplied in a manner selected from the group consisting of a power supply with a resistor in series with the second cathode, the first cathode and the second cathode are connected to the same power supply, the first cathode and the second cathode are connected to same power supply with a resistor in means for providing. and the first cathode and the second cathode are connected to an adjustable power supply. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]    The foregoing and other aspects and advantages of this invention are explained and described below with reference to the accompanying drawings, in which:  
         [0035]    FIGS.  1 A- 1 C are schematic diagrams which illustrate a prior art type of plating system using a thief electrode ring surrounding a workpiece such as a silicon wafer of enhance the quality of plating of metal onto the workpiece through a photoresist mask formed thereon.  
         [0036]    FIGS.  2 A- 2 C are schematic diagrams which illustrate an electroplating system including a plating tank, a bath, an anode and a second cathode (thief electrode/mesh) for plating a workpiece that is removably affixed to a cathode fixture in accordance with this invention.  
         [0037]    [0037]FIG. 3 shows a modification of FIG. 2A in which there is an adjustable power supply with a positive voltage on terminal connected to the anode. There is a negative voltage −V 1 ′ connected on line to line to the cathode fixture. A line from the power supply connects voltage −V 2 ′ to the thief electrode.  
         [0038]    [0038]FIG. 4 is a flow chart with steps which show the process flow of the present invention.  
         [0039]    FIGS.  5 A- 5 C show a modification of the embodiment of FIG. 2A wherein the mesh has been replaced by a parallel array of wires which disperse the current to a substantial degree providing easy circulation of ions to all portions of the substrate.  
         [0040]    FIGS.  6 A- 6 C show a modification of the embodiment of FIG. 2A wherein the mesh has been replaced by a stainless steel plate which has through holes machined therethrough in the form of a matrix 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0041]    FIGS.  2 A- 2 C are schematic diagrams which illustrate an electroplating system including a plating tank  50 , a bath  57 , an anode  76  and a second cathode (thief electrode/mesh)  42 / 48  for plating a workpiece  46  that is removably affixed to a cathode fixture  44  in accordance with this invention.  
         [0042]    Referring to FIG. 2A, the plating process of this invention overcomes the above-mentioned limitations by employing a second cathode  42 / 48  comprising the combination of a peripheral, shadow workpiece (thief) electrode  42  which supports a wire mesh  48  of woven wires which provides a screen between the workpiece  46  and the metal cations which are moving from the direction of the anode  76  towards the workpiece  46  where they are to be plated. The mesh  48  (screen) maintains the second cathode voltage at a substantially uniform value across the workpiece  46 , thereby dominating the electropotential in the bath  57  across the total surface area of a first cathode  46  comprising a workpiece  46  being plated. Wire mesh  48  which is bonded or tied to an auxiliary electrode  42  has many electrically conductive wires which are located in close proximity to all of the features on the workpiece  46 . That makes it possible to control the current and the concentration of the cations delivered to all of the various locations and microscopic features on the workpiece  46 .  
         [0043]    The workpiece  46  may be a semiconductor wafer, a ceramic substrate of the kind employed for packaging semiconductor chips or a plastic material suitable for use with printed circuits. The important characteristic of the workpiece  46  is that it must have one or many elements which can be electroplated.  
         [0044]    Electrical contact to the features to be plated may be from the back of the workpiece  46 . In that case, the workpiece  46  may have conductors which extend between the front and the back surfaces of the workpiece  46  such as through holes, vias or studs which can be used to connect a voltage from the front to the back of the workpiece  46 .  
         [0045]    Alternatively, the workpiece  46  can be coated with a seed layer by vacuum techniques such as PVD, sputtering or chemical deposition techniques, such as electroless plating or an electroplating method. Then a mask such as a photoresist mask can be applied to provide a pattern for electroplating of metal onto the seed layer through the holes in mask. After plating the seed layer can be etched away from the surface of the workpiece as will be well understood by those skilled in the art.  
         [0046]    Referring again to FIGS.  2 A- 2 C, they illustrate a type of plating system using a thief electrode  42  (shown in this particular embodiment as having a rectangular shape) surrounding a rectangular workpiece  46  to enhance the quality of plating of metal onto the workpiece  46  such as a semiconductor wafer, a ceramic package, an organic plastic package or a printed circuit board, through a photoresist mask formed thereon (not shown) as is widely practiced in the art. Obviously a semiconductor wafer would be circular in shape but the same general features would be employed.  
         [0047]    In FIG. 2A which is a vertical elevational view, the plating tank  50  has a bottom  52 , a left sidewall  54  and right sidewall  55 , as well as the conventional front and back surface (not shown) which complete the plating tank  50  which is open at the top  56  in the embodiment shown in FIG. 2A. FIG. 2B shows the thief electrode  42  and the cathode-fixture  44  combined with the workpiece  46  and the thief  42  with lead lines  47 / 41  isolated from the other elements seen in FIG. 2A. FIG. 2C is a right side view of what is shown in FIG. 2B.  
         [0048]    The plating tank  50  can be composed of polypropylene or stainless steel coated with an insulating coating if the tank  50  is not connected to the same potential as the ground electrode of the system.  
         [0049]    In the example shown here, the plating tank  50 , which is formed of a dielectric material, contains an electroplating bath  57  up to the level shown by line  58 . The plating anode  76  is shown located in the plating tank  50  near the right sidewall  55 . A positive voltage is applied from an adjustable power supply  60  to the anode  76  by connection through a wire  73 . The workpiece  46  comprising the cathode is shown on the left side of the plating tank  50  near the left sidewall  54 . The workpiece  46  has a negative voltage applied thereto by connection wires  61 , node  62  and  47  from the negative output from the adjustable power supply  60 . The thief electrode  42 , which surrounds the workpiece  46  is coplanar with the workpiece  46 . A more negative voltage is applied to the thief electrode  42  by connection through wire  61 , node  62  and a variable resistor  67  which provides an IR drop as a function of current to the thief electrode  42 . A space  45  is provided between the workpiece  46  and the thief electrode  42 . The thief electrode  42  is adjusted in voltage to adjust the plating current to the workpiece  46 , but it is not possible to maintain an equal current density across the large surface of the workpiece  46  which may be several inches wide.  
         [0050]    The resistor  67  can have a resistance value from 0 ohms to a value selected as a function of the plating conditions required selected by the application on the basis of empirical data. For permalloy or plural metals use of the resistor  67  is necessary but it is possible that it is not necessary in the case of plating of nickel, gold and copper as a function of empirical data. For nickel, gold and copper, plating can be performed with the low and high current values as follows:  
                                                                     Low Current (ma/cm 2 )   High Current (ma/cm 2 )                                        Nickel   5.0   30.0           Gold   0.5   5.0           Copper   5.0   30.0                      
 
         [0051]    Our invention employs a metallic mesh  48  placed between the anode  33  and the workpiece (cathode)  46  so that the mesh  48  is not in contact with the substrate  46  but is connected electrically to the thief plate  42 . In some cases the substrate  46 , the thief plate  42  and the mesh  48  are all electrically connected to the same terminal of the power supply  60 .  
         [0052]    The woven mesh  48  is secured to the thief electrode  42  (which can comprise a stainless steel frame) by drilling holes in the electrode  42  and screwing the mesh preferably composed of stainless steel to the electrode  42  (frame). The woven mesh  48  and the thief electrode  42  can be deplated, i.e. cleaned of plated material, in a mixture of KCl and nitric acid.  
         [0053]    The size of the woven mesh  48  is chosen based on the size of the microscopic features being plated on the substrate  46  as well as the feature density. For example a 40 mesh (40 wires/inch) is made of wires with a diameter of about 0.25 mm and a spacing of about 0.4 mm. The distance between the substrate  46  and the mesh  48  also depends upon the size and density of the features. Thus, the current reaching a substrate  46 , which is a workpiece, is essentially filtered by the mesh  48 . This process has been demonstrated on 127 mm substrates where only the interlayer vias/studs were plated successfully. The invention solves the above-mentioned problems of non-uniformity of plating by effectively controlling the current reaching the microscopic features on the substrate  46 . The mesh  48  acts as a filter for excess current.  
         [0054]    Metals which are commonly plated in the types of applications mentioned above include nickel, copper and gold. In those cases the anode  76  could be composed of a soluble metal such as nickel or copper or an insoluble metal for plating nickel or copper respectively, typically an insoluble anode for plating gold. Alternatively, the anode  76  can be composed of an insoluble metal such as platinum or platinized titanium.  
       Bath I  
       [0055]    A bath for plating copper onto a substrate is an aqueous copper sulphate solution.  
                                                   Chemicals   Concentration                           CuSO 4     ¼ molar solution           H 2 SO 4     10%           HCl   50 ppm-100 ppm           Organic additives                      
 
       Bath II  
       [0056]    A bath for plating nickel onto a substrate is a chloride free nickel sulfamate bath.  
                                   Chemicals   Concentration                   Nickel Sulfamate   300 grams/liter to 400 gram/liter       Boric acid   30 grams/liter       Activation agents and wetting agents                  
 
       Bath III  
       [0057]    A bath, for plating gold onto a substrate is an aqueous solution of gold cyanide.  
                                   Chemicals   Concentration                   Gold (metal conc.)   4 grams/liter to 8 grams/liter       NaCitrate or KCitrate or   20 grams/liter to 50 grams/liter Inorganic       NaPhosphate or KPhosphate   additives, e.g. Pb                  
 
         [0058]    This invention can apply to a ceramic repair process where electroplating of microscopic, variable areas must be plated uniformly to the desired thickness. The variation in the active area of the substrate can be 2% to 300%. However, the arrangement described in our invention nullifies this variation by having the mesh  48  and thief plate  42  dominate the total surface area of the workpiece  46 . Thus, the variation of the plating current in the active area of the workpiece  46  itself becomes insignificant. This invention is also applicable to plating solder onto silicon wafers to create C4 bumps. The C4 bump pattern is not necessarily very uniform on the surface of the wafer and also the effectiveness of the surrounding thief plate (the current practice) decreases as the size of the wafer increases to 300 mm and beyond.  
         [0059]    [0059]FIG. 3 shows a modification of FIG. 2A in which there is an adjustable power supply  80  with a positive voltage on terminal  83  connected to the anode  76 . There is a negative voltage −V 1 ′ connected on line  81  to line  47  to the cathode fixture  44 . A line  82  from power supply  80  connects voltage −V 2 ′ on line  82  to the thief electrode  42 .  
         [0060]    [0060]FIG. 4 is a flow chart with steps  90 - 91  which show the process flow of the present invention. In step  90 , a plating base is formed on the substrate  46  which will consist of either electrical connectors such as through studs from the back of the substrate  46  or pads on the edge or the border of the substrate  46  or a coating of a seed layer formed on the surface of the substrate  46 .  
         [0061]    Then in step  91 , a photoresist mask is formed on the substrate  46  which provides the pattern which is to be plated onto the exposed surface of the substrate  46 .  
         [0062]    In step  92 , the substrate  46  which has been coated with the photoresist mask is placed in or supported by a cathode fixture  44 . At the same time a thief electrode  42 / 48  or the like has been placed around the borders of the substrate  46  or in front of substrate  46  as seen in FIGS. 2A, 3,  5 A and  6 A, etc. between the substrate  46  and the anode  76 .  
         [0063]    In step  93 , negative voltage is applied to the substrate  46  and the thief electrode  42 / 46 .  
         [0064]    FIGS.  5 A- 5 C show a modification of the embodiment of FIG. 2A wherein the mesh  48  has been replaced by a parallel array of wires  48 ′/ 48 H′ which disperse the current to a substantial degree providing easy circulation of ions to all portions of substrate  46 .  
         [0065]    FIGS.  6 A- 6 C show a modification of the embodiment of FIG. 2A wherein the mesh  48  has been replaced by a stainless steel plate  48 P which has through holes  48 A machined therethrough in the form of a matrix. The dimensions and separation of the holes  48 A is a function of the data collected for an empirical approach to optimization of the uniformity of plating.  
       SUMMARY  
       [0066]    This invention applies to microelectronics applications such as flip-chip interconnections and electroplating these bumps through resist masks.  
         [0067]    One can use the same recipe to electroplate substrates with different amounts of active area (change the time proportionately to obtain desired thickness). It is not necessary to have multiple power supplies.  
         [0068]    The uniformity of dispersion of the features does not matter. It is unnecessary to have special custom-designed power supplies to control very small currents, and it is unnecessary to optimize the plating parameters for every new substrate design.  
         [0069]    One can mix and match different substrates without changing the parameters to avoid misprocessing and operator errors (thus avoiding yield loss). There are no limitations on the size of the substrate. It is possible to achieve good plating uniformity without the use of paddle cell plating apparatus.  
         [0070]    Applications of the process include all plating-thru mask plating processes, thin-film repairs performed by plating, ceramic repair processes performed by electroplating, C4 bump plating of wafers (even the chip designs with only peripheral I/O&#39;s); and all thin film wiring and interconnect via/stud plating for microelectronic applications.  
         [0071]    While this invention has been described in terms of the above specific embodiment(s), those skilled in the art will recognize that the invention can be practiced with modifications within the spirit and scope of the appended claims, i.e. that changes can be made in form and detail, without departing from the spirit and scope of the invention. Accordingly all such changes come within the purview of the present invention and the invention encompasses the subject matter of the claims which follow.