Abstract:
An apparatus for providing an electrical bias to a substrate in a processing system is described. The apparatus generally includes a conductive annular body defining a central opening. The conductive annular body may have a substrate seating surface adapted to receive the substrate and a plurality of scallops formed on a surface opposing the substrate seating surface. A plurality of electrical contacts may be formed on the substrate seating surface opposite the plurality of scallops. The electrical contacts may be adapted to engage a plating surface of the substrate.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    Embodiments of the invention generally relate to electrochemical plating and, more particularly, to a contact ring for providing an electrical bias to a substrate during an electrochemical plating process.  
           [0003]    2. Description of the Related Art  
           [0004]    Metallization of sub-quarter micron sized features is a foundational technology for present and future generations of integrated circuit manufacturing processes. More particularly, in devices such as ultra large scale integration-type devices, i.e., devices having integrated circuits with more than a million logic gates, the multilevel interconnects that lie at the heart of these devices are generally formed by filling high aspect ratio (greater than about 4:1, for example) interconnect features with a conductive material, such as copper or aluminum, for example. Conventionally, deposition techniques such as chemical vapor deposition (CVD) and physical vapor deposition (PVD) have been used to fill these interconnect features. However, as the interconnect sizes decrease and aspect ratios increase, void-free interconnect feature fill via conventional metallization techniques becomes increasingly difficult. As a result thereof, plating techniques, such as electrochemical plating (ECP) and electroless plating, for example, have emerged as promising processes for void free filling of sub-quarter micron sized high aspect ratio interconnect features in integrated circuit manufacturing processes.  
           [0005]    In an ECP process, for example, sub-quarter micron sized high aspect ratio features formed into the surface of a substrate (or a layer deposited thereon) may be efficiently filled with a conductive material, such as copper, for example. ECP processes are generally two stage processes, wherein a seed layer is first formed over the surface features of the substrate, and then the surface features of the substrate are exposed to a plating solution, while an electrical bias is simultaneously applied between the substrate and a copper anode positioned within the plating solution. The plating solution is generally rich in ions to be plated onto the surface of the substrate, and therefore, the application of the electrical bias causes these ions to be urged out of the plating solution and to be plated onto the seed layer.  
           [0006]    The electrical bias is typically applied to the seed layer formed on the substrate via a conductive contact ring. In an effort to provide a uniform electrical bias to the substrate, the contact ring may have a plurality of electrical contacts configured to electrically contact the seed layer along a perimeter edge of the substrate at evenly spaced intervals. The electrical contacts typically apply a negative voltage to the seed layer of the substrate, creating a current density across the seed layer, which has an associated resistance. A current path through the seed layer to the electrical contacts increases at points farther away from the electrical contacts relative to points nearer the electrical contacts. Unfortunately, along with this increased current path comes an increased seed layer resistance, which leads to a decrease in current at points between the contacts relative to points at or near the contacts. In general, this decrease in current leads to decreased plating at points on the seed layer. Consequently, this decrease in current between the contacts may lead to plating nonuniformities along the perimeter edge of the substrate in the form of less plating between the contacts and more plating at or near the contacts.  
           [0007]    Therefore, there is a need for an improved contact ring for use in an electrochemical deposition system that results in improved plating uniformity along a perimeter edge of a substrate.  
         SUMMARY OF THE INVENTION  
         [0008]    One embodiment of the present invention provides an apparatus for providing an electrical bias to a substrate in a processing system. The apparatus generally includes a conductive annular body defining a central opening, the conductive annular body having a substrate seating surface adapted to receive the substrate and a plurality of protrusions formed on a surface opposing the substrate seating surface. A plurality of electrical contacts may be formed on the substrate seating surface opposite the plurality of protrusions, the electrical contacts adapted to engage a plating surface of the substrate.  
           [0009]    Another embodiment provides an apparatus for securing a substrate in a processing system generally including a contact ring including a conductive annular body defining a central opening, the conductive annular body having a substrate seating surface, a plurality of electrical contacts disposed on the substrate seating surface, the electrical contacts adapted to engage a plating surface of the substrate, and a plurality of protrusions formed opposite the electrical contacts on a surface opposing the substrate seating surface. The apparatus may also include a thrust plate assembly including a thrust plate adapted to exert a securing force on the substrate to secure the substrate to the substrate seating surface.  
           [0010]    Another embodiment provides a method of fabricating a contact ring for providing an electrical bias to a substrate in a processing system. The method generally includes providing a conductive annular ring having a substantially flat first surface adapted to receive the substrate and a second surface opposing the first surface, wherein a plurality of protrusions extend from the second surface, and forming a plurality of electrical contacts on the first surface of the conductive annular ring, wherein the plurality of electrical contacts are formed opposite the plurality of protrusions. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are, therefore, not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
         [0012]    [0012]FIG. 1 illustrates an exemplary plating cell according to one embodiment of the present invention.  
         [0013]    [0013]FIG. 2 is a perspective view of a contact ring and thrust plate assembly according to one embodiment of the present invention.  
         [0014]    FIGS.  3 A-D are detailed cross sectional views of contact rings according to embodiments of the present invention.  
         [0015]    FIGS.  4 A-B are graphs illustrating plating uniformity achieved using a conventional contact ring and a contact ring according to an embodiment of the present invention, respectively.  
         [0016]    FIGS.  5 A- 5 F illustrate an exemplary contact ring at different steps of a fabrication process according to another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]    According to some aspects of the present invention, a contact ring having a plurality of electrical contacts is provided to supply an electrical bias to a substrate in a processing system. An average thickness of the contact ring may be increased via protrusions, or “scallops,” formed in the contact ring below the contacts. The scallops may help control variations in current density between the contacts by compensating for increased seed layer resistance that exists between the contacts.  
         [0018]    As used herein, the term scallop generally refers to portions of a contact ring having an increased thickness at or near the contacts relative to (thinner) portions of the contact ring in between the contacts. For example, scallops may be formed on a bottom surface of a contact ring, below electrical contacts. Further, as used herein, top and bottom are relative terms, not limited to any specific orientation, generally applying to portions of a contact ring away from (top) or facing (bottom) a plating bath. In other words, in a processing system where a plating surface of a substrate faces up, what is referred to herein as a top surface of the contact ring may actually face down.  
         [0019]    [0019]FIG. 1 illustrates a partial perspective and sectional view of an exemplary electrochemical plating (ECP) system  100  utilizing a contact ring  150  with scallops  156  according to one embodiment of the present invention. The ECP system  100  generally includes a head assembly  102 , a substrate securing assembly  110  and a plating bath assembly  161 . The head assembly  102  is attached to a base  104  by a support arm  106 . The head assembly  102  is adapted to support the substrate securing assembly  110  at a position above the plating bath assembly  161  in a manner that allows the head assembly  102  to position a substrate  120  (held in the substrate securing assembly  110 ) in a plating bath  165  for processing. The head assembly  102  may also be adapted to provide vertical, rotational, and angular movement to the substrate securing assembly  110  before, during, and after the substrate  120  is placed in the plating bath  165 .  
         [0020]    The plating bath assembly  161  generally includes an inner basin  167 , contained within a larger diameter outer basin  163 . Any suitable technique may be used to supply a plating solution to the plating assembly  160 . For example, a plating solution may be supplied to the inner basin  167  through an inlet  166  at a bottom surface of the inner basin  167 . The inlet  166  may be connected to a supply line, for example, from a reservoir system (not shown). The outer basin  163  may operate to collect fluids from the inner basin  163  and drain the collected fluids via a fluid drain  168 , which may also be connected to the electrolyte reservoir system.  
         [0021]    An anode assembly  170  is generally positioned within a lower region of the inner basin  163 . The anode assembly  170  may be any suitable consumable or non-consumable-type anode. For some embodiments, a membrane (not shown) may be generally positioned across the diameter of inner basin at a position above the anode assembly  170 . The membrane may be any suitable type membrane, such as a cation membrane, an anion membrane, an uncharged-type membrane, or a multi-layer diffusion differentiated permeable membrane. Any suitable method may be used to provide an electrical connection to the anode assembly  170 .  
         [0022]    For example, an electrical connection to the anode assembly  170  may be provided through an anode electrode contact  174 . The anode electrode contact  174  may be made from any suitable conductive material that is insoluble in the plating solution, such as titanium, platinum and platinum-coated stainless steel. As illustrated, the anode electrode contact  174  may extend through a bottom surface of the plating bath assembly  161  and may be connected to an anode connection of a power supply (not shown), for example, through any suitable wiring conduit. A cathode connection of the power supply may be connected to the contact ring  150  to supply an electrical bias between the anode assembly  170  and the substrate  120 . In response to the electrical bias applied between the anode assembly  170  and a plating surface  122  of the substrate  120 , electrical current, represented by current flux lines  180 , generally flows from the anode assembly  170  to the substrate  120 . The current flux lines  180  may tend to gather at a perimeter edge of the substrate  120 . Therefore, the contact ring  150  may include a plurality of scallops  156  generally formed beneath a plurality of contacts  154 . The scallops  156  may serve to control the current flux lines  180  at the perimeter edge of the substrate  120  at or near the contacts  154 , in an effort to control variations in current density along a perimeter edge of the substrate  120 , as will be described in more detail below.  
       SUBSTRATE SECURING ASSEMBLY  
       [0023]    The substrate securing assembly  110  generally includes a mounting member  112  attached to the contact ring  150  via attachment members  116 . The attachment members  116  may be spaced sufficiently to allow insertion of the substrate  120  (i.e., a spacing of the attachment members  116  may be greater than a diameter of the substrate  120 ). The mounting member  112  may allow for attachment of the substrate securing assembly  110  to the head assembly  102 , via a mounting plate  146  of a thrust plate assembly. Other embodiments of the substrate securing assembly  110  may lack the mounting member  112  and may be attached, for example, directly to the mounting plate  146  via the contact ring  150 . The mounting member  112 , contact ring  150 , and the attachment members  116  may each be coated with a plating-resistant material, such as a PTFE material (e.g., Aflon® or Tefzel®) or any other suitable plating-resistant coating material.  
         [0024]    The contact ring  150  may have a substrate seating surface  152  generally adapted to receive the substrate  120  with the plating surface  122  of the substrate facing the plating bath  165 . The substrate securing assembly  110  may also include a thrust plate  144  with an attached seal plate  142  generally adapted to exert a securing force on the substrate  120  for securing the substrate  120  to the substrate seating surface  152 . The securing force applied by the thrust plate  144  may be sufficient to ensure adequate sealing between an annular sealing member  148  disposed on the seal plate  142  and the non-plating surface  124  of the substrate. As illustrated, the annular sealing member  148  may be adapted to contact the non-plating surface  124  of the substrate  120  at a substantially equal location radially inward from an edge of the substrate as the contacts  154  engage the plating surface  122  of the substrate. For some embodiments, the substrate securing assembly  110  may include an inflatable bladder assembly (not shown) adapted to apply a downward force that is evenly distributed along the non-plating surface  124  of the substrate  120 .  
         [0025]    The securing force exerted by the thrust plate  144  may also be sufficient to ensure adequate electrical contact between the plating surface  122  of the substrate and the contacts  154  extending from the substrate seating surface  152  of the contact ring  150 . The contacts  154  are generally adapted to electrically contact the plating surface  122  of the substrate  120  in order to supply an electrical plating bias to the plating surface  122 . The contacts  154  may be made of any suitable conductive material, such as copper (Cu), platinum (Pt), tantalum (Ta), titanium (Ti), gold (Au), silver (Ag), stainless steel, an alloy thereof, or any other suitable conducting material.  
         [0026]    As illustrated in FIG. 2, the contacts  154  may be formed above the scallops  156  in a generally circular pattern around the substrate seating surface  152  of the contact ring  150 . The contacts  154  may vary in number, for example, according to a size of the substrate  120  (not shown in FIG. 2). The contacts  154  may also be flexible to contact non-plating surfaces with non-uniform heights. Power may be supplied to the contacts  154  via a power supply (not shown). The power supply may supply electrical power to all of the electrical contacts  154  cooperatively, banks or groups of the electrical contacts  154  separately, or to the individual contacts  154 . In embodiments where current is supplied to groups or individual contacts  154 , a current control system may be employed to control the current applied to each group or pin.  
         [0027]    For some embodiments, the contact ring  150 , attachment members  116  and mounting member  112  may all be made of an electrically conductive material. As with the contacts  154 , the contact ring  150 , attachment members  116  and mounting member  112  may be made of any suitable electrically conductive material and, for some embodiments, may be made of stainless steel. Accordingly, the attachment members  116  may electrically couple the mounting member  112  and the contact ring  150 . Therefore, power may be supplied to the contacts  154  by one or more electrical connections between the mounting member  112  and a power supply.  
         [0028]    Further, for some embodiments, the mounting member  112  may be physically and electrically coupled with the thrust plate mounting plate  146 , which may also be made of an electrically conductive material and may be attached to a power supply. The mounting member  112  or mounting plate  146  may be connected to the power supply via any suitable attachment means adapted to provide power to the contacts  154  as the substrate securing assembly  110  is moved (i.e., raised, lowered and rotated) by the head assembly  102  of FIG. 1.  
         [0029]    As previously described, the seal plate  142  may be attached to the thrust plate  144 . The thrust plate  144  may be adapted to move (i.e., up and down) independently of the contact ring  150  to exert a securing force with the sealing member  148  on the non-plating surface of a substrate to secure the substrate to the substrate seating surface  152  of the contact ring  150 . The sealing member  148  may be designed to provide a uniform contact force between the contacts  154  and the plating surface of the substrate.  
         [0030]    For example, the sealing member  148  may be made of a pliable material designed to decrease an effective spring constant of the sealing member  148 . In other words, the sealing member  148  may compress to adapt to slight non-uniformities in the non-plating surface of the substrate (or slight non-uniformities in the annular sealing member  148 ). For example, as the sealing member  148  compresses, less force may be needed to seal against the highest point of the non-plating surface before sealing against the lowest point. With less force difference between the highest and lowest points, the local force on the non-plating surface of the substrate, and therefore on the contacts  154  in contact with the plating surface of the substrate, may be more uniform. A more uniform force on the contacts  154  may lead to uniform contact resistance and improved plating uniformity.  
         [0031]    The plurality of scallops  156  may be formed on a bottom surface of the contact ring  150  below the plurality of contacts  154 . The size and shape of the scallops  156  are not limited and may vary according to different applications. For example, as illustrated in FIG. 2, the scallops  156  formed below the contacts  154  may be substantially rectangular in shape. For other embodiments, however, scallops may be other shapes, including, but not limited to rounded shapes (e.g., semi-cylindrical or hemispherical) and triangular shapes (e.g., pyramid or saw-tooth shaped). As illustrated, the scallops  156  may extend from a bottom surface of the contact ring  150  (e.g., opposite the substrate seating surface  152 ). However, for other embodiments, scallops may extend from the substrate seating surface  152 , in effect raising the contacts  154 .  
         [0032]    [0032]FIG. 3A is a detailed cross sectional view of the contact ring  150 . As illustrated, the contact ring  150  may have a thickness t 1  between contacts  154 , and a thickness t 2  at the scallops  156 . The thickness t 1  and t 2  may be measured from the substrate seating surface  152  to a bottom surface  162  of the contact ring  150  between the contacts  154  and a bottom surface  164  of the contact ring  150  below the contacts, respectively. In general, as t 2  increases, an amount of current density at or near the contacts  154  decreases, and an amount of plating at or near the contacts  154  decreases. Similarly, as t 1  decreases, an amount of current density between the contacts increases, and an amount of plating between the contacts  154  increases. By controlling the ratio of thickness t 2  to t 1 , uniform current density and, thus, non-uniformities in plating thickness around a perimeter edge of the substrate  120  may be reduced.  
         [0033]    As illustrated, the contact ring  150  may be formed of an electrically conductive core  160  surrounded by a plating-resistant coating  158 . For some embodiments, the conductive core  160  may be a solid piece of conductive material. The contacts  154  may extend from the plating surface  152  through the plating-resistant coating  158 . In an effort to maximize a surface area of the plating surface  122  exposed to plating solution, the contacts  154  may be adapted to engage the plating surface  122  of the substrate  120  at or near a perimeter edge. For example, for different embodiments, the contacts  154  may be adapted to engage the plating surface  122  less than 5 mm from an edge of the substrate  120  (e.g., 2.5 mm or 4.5 mm). As previously described, a thrust plate assembly may include a sealing member (not shown in FIG. 3A) adapted to exert a securing force against the non-plating surface  124  of the substrate  120 , at a location just opposite the contacts  154 , to secure the substrate  120  against the substrate seating surface  152  of the contact ring  150 . The sealing member may be adapted to provide a uniform sealing force between the contacts  154  and the plating surface  122 , which may help to provide a uniform contact resistance which may help provide a uniform current across the plating surface  122 .  
         [0034]    As illustrated in FIG. 3B, for some embodiments, a sealing member  130  attached to the contact ring  150  may be adapted to engage the plating surface  122  of the substrate  120  radially inward from the electrical contacts  154 . Accordingly, the sealing member  130  may shield the contacts  154  from the flow of plating solution, which may also help to provide a uniform contact resistance, for example, by preventing plating on the contacts  154 .  
         [0035]    The current at any point on the plating surface  122  is generally inversely proportional to a sum of seed layer resistance, contact resistance, and electrolyte resistance. As previously described, points on the plating surface  122  between the contacts  154  may see a larger effective seed layer resistance than points on the plating surface at or near the contacts  154 . This increase in seed layer resistance may result in decreased current and, therefore, less plating between the contacts  154 . However, as illustrated in FIGS. 3C and 3D, the thicker dimension of the scallops  156  may compensate for the increased seed layer resistance between the contacts and, therefore, reduce variations in current along the perimeter of the plating surface  122 .  
         [0036]    [0036]FIG. 3C illustrates current flux lines  180  extending to the plating surface  122  beneath the contacts  154 , while FIG. 3D illustrates flux lines  180  extending to the plating surface  122  between the contacts  154 . As illustrated, in either case, the flux lines  180  tend to squeeze together around the contact ring  150 , which effectively increases an effective resistance of the plating solution. However, due to the increased thickness of the scallops  156 , the flux lines  180  in FIG. 3C are squeezed together for a longer distance than the flux lines  180  in FIG. 3D. Accordingly, in regions between the scallops  156 , there is a lower effective resistance of the plating solution, which may compensate for increased seed layer resistance between the contacts  154 .  
         [0037]    FIGS.  4 A-B are graphs illustrating plating uniformity achieved using a conventional contact ring and a scalloped contact ring, respectively. The graphs each show sampled plating thickness along a perimeter edge for 2 300 mm substrates having a 40 nm seed layer. The plating thickness were sampled along a half quadrant (e.g., 45 degrees) of a perimeter. As illustrated, the half quadrant may include 6 contacts, labeled as pins in the figures (i.e, there may be 48 contacts total). The sample substrates of FIG. 4A were plated using a conventional contact ring having a uniform thickness of approximately 7 mm (i.e., below and between the contacts). The sample substrates of FIG. 4B were plated using a scalloped contact ring having a thickness of 5 mm between the contacts (t 1 ) and 7 mm below the contacts (t 2 ). As illustrated in FIG. 4A, using the conventional contact ring, plating thickness increases at or near the contacts, and decreases between the contacts. For example, the plating thickness may vary from approximately 8000 Angstroms at or near the contacts to less than 6500 Angstroms at points between the contacts. In contrast, as illustrated in FIG. 4B, using the scalloped contact ring, plating thickness varies only slightly. Of course actual plating uniformity may vary for different embodiments and for different applications.  
         [0038]    Accordingly, for different applications, the size and shape of the scallops may be varied to achieve optimal plating uniformity. For example, the thickness of the contact ring between the scallops (t 1 ) and the thickness of the contact ring (t 2 ) may be varied based on different application parameters, such as seed layer thickness, desired plating thickness, substrate size, strength of the electrical bias, material being plated, etc. In other words, t 2  may be increased as necessary to decrease plating thickness at or near the contacts, while t 1  may be decreased as necessary to increase plating thickness between the contacts. As illustrated in the example above, for one embodiment, the thickness t 2  may be approximately 7 mm, while the thickness t 1  may be approximately 5 mm. The thickness t 2  (beneath the contacts) may typically be in a range from 3 mm to 9 mm, while the thickness t 1  (between the contacts) may typically be in a range from 1 to 5 mm.  
       CONTACT RING FABRICATION  
       [0039]    As described above, uniform contact resistance may also promote uniform plating thickness. Therefore, for some embodiments, a contact ring may be fabricated according to a process with operations intended to ensure uniform contact resistance. FIGS.  5 A- 5 F illustrate top views (e.g., looking down at the substrate seating surface) of an exemplary contact ring  550  at different steps of a fabrication process according to still another embodiment of the present invention.  
         [0040]    For example, in FIG. 5A, the contact ring  550  may include a single piece of conductive material  560  (e.g., stainless steel). A contact may be formed on the contact ring  550  by bonding a piece of contact material  570  to the contact ring  550 . The piece of contact material  570  may be bonded to the contact ring  550  by any suitable bonding technique, such as soldering or welding. (Generally, soldering is performed with metals having melting temperatures below 450° C., while brazing is performed with metals having melting temperatures above 450° C.) For some embodiments, the piece of contact material  570  may be bonded via a brazing process. For example, the piece of contact material  570  may be placed in a cavity  562  formed in the contact ring  550 , with a top portion of the contact material  570  protruding above a top surface of the cavity  562 .  
         [0041]    As illustrated in FIG. 5B, one or more pieces of brazing material  572  may be placed in the cavity  562  adjacent the piece of contact material  570 . In general, the brazing material  572  should also have a melting temperature below a melting temperature of the conductive material  560  and the contact material  570 . The brazing material  572  may also be chosen to have a high corrosion resistance, high purity to avoid contamination, a low vapor pressure at a braze temperature, and the ability to wet the contact material  570  and the conductive material  560 . For example, for some embodiments, the contact material  570  may be a platinum-indium alloy (e.g., 85% platinum, 15% indium) having a melting temperature of approximately 2230° C. and the conductive material  560  may be stainless steel having a melting point of approximately 1650° C. One example of a suitable brazing material  572  for brazing platinum-indium alloy contacts to a stainless steel contact ring (e.g., with the properties described above) is a palladium-cobalt alloy (e.g., 65% palladium, 35% cobalt) having a melting temperature of approximately 1220° C. In other words, the contact ring  550  may be heated (e.g., in a furnace), to a temperature above the melting point of the brazing material  572  (e.g., above 1220° C.), causing the brazing material  572  to melt and form a single piece of brazing material  574  that anchors the contact material  570  to the contact ring  550 , as illustrated in FIG. 5C. Advantages of brazing may include increased contact lifetime, more uniform contact height and more uniform contact resistance.  
         [0042]    As previously described, it is generally desirable to maximize an amount of plating surface area of a substrate. Therefore, for some embodiments, an inner annular portion of the contact ring  550 , indicated by dashed lines in FIG. 5D, may be removed (e.g., machined off) to prevent the removed portion from shielding the substrate from plating solution and to allow the contacts being formed to contact a plating surface of the substrate near an edge of the substrate.  
         [0043]    [0043]FIG. 5E illustrates the contact ring  550  after removing the annular inner portion. For some embodiments, prior to applying a coating of plating-resistant material  558  (shown in FIG. 5F), a surface of the conductive material  560  of the contact ring  550  may be treated to improve adhesion of the plating-resistant material  558  to the contact ring  550 . For example, the surface of the conductive material  560  may be grit blasted which may alter a surface finish of the conductive material  560  and improve adhesion of the plating resistant material  558 . Grit blasting may also prevent the plating resistant material  558  from sliding over the top of the contact material  570  over time, which may prevent sufficient electrical contact between the plating surface of the substrate and the contact and, consequently, increase contact resistance. Further, for some embodiments, a coating of primer material may be applied to the surface of the conductive material  560  in addition to, or instead of, grit blasting the surface, to improve adhesion of the plating-resistant material  558 .  
         [0044]    [0044]FIG. 5F illustrates the final contact ring  550  after applying the coat of plating-resistant material  558 . As illustrated, a portion of the contact material  570  is exposed through the coat of plating-resistant material  558 , allowing the contact material  570  to engage a plating surface of a substrate. For some embodiments, the contact material  570  may be masked prior to applying the coat of plating-resistant material  558 , to prevent coating the contact material with plating-resistant material  558 . For other embodiments, the coat of plating-resistant material  558  may be applied to the contact material  570 , and subsequently removed.  
         [0045]    While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.