Patent Publication Number: US-2009229988-A1

Title: Methods For Providing Composite Asperities

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present invention claims priority to U.S. Provisional Patent Application No. 60/973,511, filed Sep. 19, 2007, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Integrated circuits are the mainstream products of the semiconductor industry, and require testing for new designs, reliability and statistical process quality control. They are the main memory and processing devices in computers and all electronic devices that are currently found in the home, industry, automobiles and every other walk of life. Integrated circuits can be individually packaged, for example in a DIP or BGA, or can be combined with other integrated circuits and components into multi-layer circuit panel assemblies. These packages and assemblies are usually mounted on substrates or circuit boards that provide mechanical support, and generally require the use of interposers to provide electrical connections and routing within the package or assembly. The interposers are electronic devices that provide a solder-less connection, for example between an integrated circuit and a printed circuit board. 
     The interposers connect to electrical contact pads on a surface of a microelectronic element, such as a circuit panel, a semiconductor chip or other element having a contact bearing surface. The electrical contact pads, which are made of conductive material, such as copper, aluminum, silver, platinum, tungsten, or nickel, are subject to the buildup of oxidation or other compounds due to reactions with the environment. The oxidation interferes with the ability of the interposers to form good electrical contact with the electrical contact pads, and thus interferes with the electrical properties of the device. 
     A technique to overcome this oxidation problem is the use of a sharp abrasive (asperated) surface on the interposer contact points, for example an array of asperities. The asperities on the interposer are able to penetrate through the buildup of oxides and other contaminants on the electrical contact pads and enable a conductive contact (e.g., a metal-to-metal contact). One method for making such asperities on an interposer having copper contact points, is to laser-cut into the copper to form a first layer of multiple asperities. The contact points are then plated in nickel and then gold to form copper/nickel/gold pyramid-like asperities. With the advent of new alloys used in contact point creation, however, such laser-cut asperities do not provide suitable penetration over the life requirement of interposers in a testing environment. Another known method is to bond or plate semiconductor particles such as diamond onto a circuit board, so that the particles form asperities. This technique provides additional structure for the plated material, but if the plated material wears through, the semiconductor particles with their high electrical resistance can cause poor performance or failure in the device. 
     As technology advances the trend in integrated circuits is smaller and smaller. The limitations in the known methods for forming asperities creates a formidable obstacle in providing an abrasive surface on the contact pads of interposers. Semiconductor particles do not provide electrical performance adequate for the high frequency testing of today&#39;s integrated circuits. The lack of performance in both of the known approaches described above results in poor long term electrical performance, and more frequent replacement of interposers in testing environments. The down time with attendant loss of productivity in changing interposers creates a substantial increased cost burden on testing. 
     Thus, there is a need to provide improved asperities used in integrated circuits, which provide improved electrical performance and have an extended useful lifetime. 
     SUMMARY OF THE INVENTION 
     The present invention relates to methods for providing asperities on a conductive contact region of a circuit panel, comprising providing a circuit panel having at least one conductive contact region, exposing the circuit panel to a first electroplating solution comprising a first conductive metal and a plurality of metal ceramic particles having a low electrical resistance, and supplying a low density current to the first electroplating solution for a period of time sufficient to co-deposit the first conductive metal and the metal ceramic particles as a first conductive layer on the surface of the at least one conductive contact region. Particular methods are those where the first conductive metal is selected from the group consisting of nickel, aluminum, copper, gold, iridium, palladium, platinum, rhodium, ruthenium, silver, titanium, and alloys of these metals, where the metal ceramic particles have an electrical resistance less than about 10 milliohms, and where the low density current is less than 8 amps per square foot (ASF). 
     Also provided are methods for providing asperities on a conductive contact region of a substrate for forming an electrical connection with a contact location on a semiconductor die, comprising providing a substrate having at least one conductive contact region, exposing the at least one conductive contact region of the substrate to a first electroplating solution comprising a first conductive metal and a plurality of metal ceramic particles having a low electrical resistance, and supplying a low density current to the first electroplating solution for a period of time sufficient to co-deposit a layer of the first conductive metal and the metal ceramic particles as a first conductive layer on the surface of the at least one conductive contact region, where the first electroplating solution is vigorously agitated during the period of time in which the current is supplied. Particular methods are those where the first conductive metal is selected from the group consisting of nickel, silver, copper, gold, and alloys of these metals, where the metal ceramic particles are ceramics of a Group IV metal, a Group V metal, or a Group VI metal and have an electrical resistance less than about 25 milliohms, and where the low density current is less than 8 amps per square foot. 
     Further provided are methods for providing asperities on a conductive contact region of a substrate for forming an electrical connection with a contact location on a semiconductor die, comprising providing a substrate having at least one conductive contact region, exposing the at least one conductive contact region of the substrate to a first electroplating solution comprising a first conductive metal and a plurality of metal ceramic particles, supplying a low density current to the first electroplating solution for a period of time sufficient to co-deposit a layer of the first conductive metal and the metal ceramic particles as a first conductive layer on the surface of the at least one conductive contact region, where the first electroplating solution is vigorously agitated during the period of time in which the current is supplied, exposing the at least one conductive contact region of the substrate to a second electroplating solution comprising the first conductive metal, and supplying a current to the second electroplating solution for a period of time sufficient to induce growth of the first conductive layer. Particular methods are those where the first conductive metal is selected from the group consisting of nickel, silver, copper, gold, and alloys of these metals, where the metal ceramic particles are ceramics of a Group IV metal, a Group V metal, or a Group VI metal and have an electrical resistance less than about 15 milliohms, and where the low density current is less than 5 amps per square foot (ASF). 
     Additional advantages and features of the present invention will be apparent from the following detailed description, drawings and examples, which illustrate preferred embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts an integrated circuit having an asperated layer formed thereon according to an embodiment of the present invention. 
         FIG. 2  is a top view of a contact region having an asperated layer formed thereon according to an embodiment of the present invention. 
         FIG. 3  is a cross-section of the contact region of  FIG. 2 , according to a first embodiment of the present invention. 
         FIG. 4  is a cross-section of the contact region of  FIG. 2 , according to a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the presently preferred embodiments of the invention, which, together with the drawings and the following examples, serve to explain the principles of the invention. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized, and that structural, chemical, and electrical changes may be made without departing from the spirit and scope of the present invention. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods, devices, and materials are now described. 
     The present invention concerns methods for providing composite asperities on the surface of interposers or other electrical contact pads, using low current density electroplating. The asperities comprise low electrical resistance particles such as titanium carbide that are bonded or plated in conjunction with a conductive metal such as nickel onto the interposers or other electrical contact pads. An electroplating bath that has the low electrical resistant particles dispersed in solution is used at low current densities to electrolytically plate a composite electrically low resistant abrasive surface. The composite bond between the particles and the substrate may then be further reinforced with standard electroplating of the same conductive metal, if desired, or with a different conductive metal, or with a layer of the same metal and a layer of a different metal, such as gold or rhodium. The result of the methods is the formation of an asperated conductive layer comprising a plurality of asperities, each asperity formed about a particle core. These asperities are useful to enable interconnections in a variety of devices, such as contactor sockets in load boards, testers, programmers, and other devices, ultra-high frequency sockets, printed circuit boards, and semiconductor packages. An especially valuable use of the asperities is on interposer contact surfaces. 
     For example,  FIG. 1  depicts a portion of an integrated circuit  10  (not drawn to scale) with the plated composite asperities according to an embodiment of the present invention. The circuit  10  comprises a substrate  20  having at least one contact region  30  (two are shown in  FIG. 1 ) comprised of a conductive substance, such as a conductive metal, filled silicone, or the like. In a preferred embodiment, the contact regions  30  are formed from a conductive metal such as aluminum, copper, nickel, platinum, silver or tungsten, preferably copper. In another preferred embodiment, the contact regions  30  are formed from conductive silicone, for example as described in U.S. Pat. No. 6,734,250. On the surface of each contact region  30  is an asperated layer  40 , which is adapted to make contact with an electrical contact pad or lead  50 . The electrical contact pad or lead  50  is also formed from a conductive metal, such as aluminum, copper, nickel, platinum, silver or tungsten, preferably copper. 
     In a preferred embodiment, the contact regions  30  have a pitch of less than 500 μm (19.7 mil), 450 μm, 400 μm, 350 μm, 300 μm, 250 μm, 200 μm, 150 μm, 100 μm, 50 μm or 10 μm. The pitch is measured between similar features of adjacent contacts, such as a center-to-center distance. In another preferred embodiment, the contact regions  30  have a pitch of less than 50 μm (1.97 mil), 45 μm, 40 μm, 35 μm, 30 μm, 25 μm, 20 μm, 15 μm, 10 μm, 5 μm or 1 μm, or less than 10 μm, 9 μm, 8 μm, 7 μm, 6 μm, 5 μm, 4 μm, 3 μm, 2 μm, 1 μm, 0.75 μm, 0.5 μm, 0.25 μm, or 0.1 μm, or smaller pitches. A smaller pitch is desirable for certain applications, because it permits the integrated circuit to accommodate microelectronic components having greater contact densities. 
     The top view of the asperated layer  40  of an individual contact region  30  is shown in  FIG. 2 , where it can be seen that the asperated layer  40  comprises a plurality of asperities  45 . The asperities  45  may be arranged randomly as shown in  FIG. 2 , or may be regularly arranged for example in a rectilinear grid or array, or in any other regular pattern. The distribution and arrangement of the asperities  45  can be controlled by adjusting the parameters of the deposition methods of the present embodiments, and through the use of masking layers, as is further explained below. The desired size and the density of the asperities  45  formed on any individual contact region  30  will vary depending on the design of the integrated circuit, and according to such parameters as the contact region size and shape, the configuration of the contact regions, the composition of the contact regions, the number of contact regions, the current conducted by the contact regions, and the like. 
     It is preferred that the asperities have a size substantially smaller than the size of the contact region, such as less than 1/10th the diameter of the contact region, and more preferably less than 1/15th, 1/20th, or 1/30th the diameter of the contact region. In a preferred embodiment, the asperities are less than 100, 90, 80, 70, 60, 50, 40, 30, 20, 10 or 5 microns in size, and more preferably less than 85, 80, 75, 70, 65, 60, 55, 50, 45, 40, 35, 30, 25, 20, 15, 10 or 5 microns in size. In one embodiment, the particles are less than 75 microns in size, preferably less than 45 microns in size. In another preferred embodiment, the asperities are between 5 to 100 microns in size, more preferably between 10 and 75 microns in size, and even more preferably between 45 and 60 microns in size. The asperities may be of a similar size to each other, for example by carrying out the methods using particles having a narrow particle size distribution (e.g., 90% of the particles are between 45 and 55 microns), or may differ greatly in sizes, for example by carrying out the methods using particles having a broad particle size distribution (e.g., 90% of the particles are between 5 to 100 microns in size). The desired particle size distribution may vary for a number of reasons such as the type of connection that is being made, the contact region size and shape, the composition of the contact regions, etc. 
     Further details of the asperated layer  40  are depicted in  FIGS. 3 and 4 , which are cross-sectional views of two different embodiments of an exemplary contact region  30  taken along the line A-A in  FIG. 2 .  FIG. 3  depicts an embodiment in which the contact region  30  has a plurality of asperities  45  embedded within a first conductive layer  42 .  FIG. 4  depicts a different embodiment, in which the asperated layer  40  comprises a second conductive layer  44  on top of the first conductive layer  42  in which the asperities  45  are embedded. The composition of the first and second conductive layers  42 ,  44  may be any suitable conductive substance that can be electroplated according to the embodiments of the present invention. It should be noted that while the asperities  45  are depicted with generally polygonal shapes and equal sizes in these Figures, the asperities are by no means limited to regular shapes or sizes, and they may be irregularly shaped and of varying sizes, diameters and heights. For example, the particles forming the asperities may aggregate to form column-like asperities that are taller than the height of a single particle. 
     The composition of the asperities may be any suitable low electrical resistance substance having a small particle size, high hardness and tensile strength, compatibility with plating conditions, and electrical conductivity. The necessary hardness of the particles is best understood by considering that they must be hard enough to penetrate the oxides and contaminants on the surface of electrical contact pad or lead  50 , without significant wear or flattening over time. Preferably, the particles are also very sharp but less brittle than diamond. The edges of the asperities should be 1 mil (0.0254 mm) or less in thickness. 
     The asperated layers are provided by low current density electroplating methods, in which the low electrical resistance particles and the first conductive metal are co-deposited on the surface of the contact region  30  to produce a composite asperated layer  40 . To achieve this co-deposition, the low electrical resistance particles are mixed with a plating solution containing the first conductive metal, and as current is passed through the plating solution, the particles become embedded throughout a matrix of the first conductive metal as the metal is plated onto the contact regions  30 . The co-deposited particles lodged at or near the plated surface provide the surface with hard edges to cut through the oxides and contaminants on the electrical contact pad or lead  50 , allowing the first conductive layer  42  to make electrical contact with the metal of the electrical contact pad or lead  50 . The particles forming the asperities are also electrically conductive, and therefore do not interfere with the electrical connection between the contact region  30  and the electrical contact pad or lead  50 . 
     The device to be plated, which may be an integrated circuit, semiconductor substrate, or interposer flex panel, can be prepared for plating by overlaying areas not to be plated with a protective layer, for example a photoresist or protective masking layer, so that only the contact regions  30  are exposed. Either one or both sides of the device may be plated at the same time, for example in an interposer flex panel that interconnects on both sides of the panel, plating on both sides may be desired in order to provide all contact regions with an asperated layer. If desired, the exposed surface of the contact regions  30 , e.g., the contact pads of individual interposers, may be laser etched to roughen the surface of the contact regions. Such roughening results in a more even particle distribution, because the roughening creates high current density points that increase the attraction of conductive particles to the roughened areas during electroplating. The roughening of specific areas can thus be used to create particular geometries or patterns of asperities, for example rectilinear arrays, spirals, pinwheels, circular patterns and the like, or can be used to create a random pattern as desired. 
     The circuit or substrate is placed or suspended in a first electroplating solution contained in an electroplating bath. The device may be oriented in any desired orientation in the bath, and may vary from vertical to horizontal and otherwise depending on the dispersed particle concentration and the number of asperities desired in the asperated layer. The bath is also connected to a rectifier which supplies the current for the electroplating process, so that the charge differential between the cathode (the device) and the anode (the solution) results in plating the particles and the first conductive metal onto the contact regions. 
     Typically, low electrical resistance particles are not soluble in electroplating solutions. The particles are rendered soluble in the present solutions by vigorous agitation, which mechanically disperses the particles throughout the solution. In order to accomplish this, the bath is provided with means for agitating the electroplating solution, such as by a slurry pump, air agitation equipment (e.g., turbine blower unit, water washed air unit, etc.), paddle or impeller systems (e.g., a reciprocating paddle system), vibratory agitator (e.g., vibromixer), gas sparging, ultrasonic agitating equipment, a device to move the cathode during plating (e.g., an oscillator attached to the work bar), and the like. In a preferred embodiment, the means for agitating is a slurry pump or air agitation equipment. In a preferred embodiment, vigorous agitation, e.g., an agitation rate of 0.5 to 5 meters per second, is used. 
     The first electroplating solution comprises a first conductive metal, which is used to deposit the first conductive layer, and the low electrical resistance particles. In a preferred embodiment, the first conductive metal is a conductive metal such as nickel, aluminum, copper, gold, iridium, palladium, platinum, rhodium, ruthenium, silver, titanium, or alloys or combinations of one or more of these metals. In another preferred embodiment, the first conductive metal is nickel, silver, copper or gold or an alloy of one or more of these metals, and in a more preferred embodiment, the first conductive layer is nickel or copper, or an alloy of one or both of these metals. For example, nickel may be alloyed with one or more metals such as aluminum, chromium, copper, iron, molybdenum, niobium, tantalum, titanium or tungsten. 
     The low electrical resistance particles are particles having the necessary characteristics suitable for use to form asperities and co-deposit with the conductive metal, and can include nitrides, borides, silicides and carbides, as well as iron, steel, and carbon (e.g., graphite, etc.). In a preferred embodiment, the particles are metal ceramics, e.g., a metal boride, borocarbide, boronitride, borosilicide, carbide, carbonitride, carbosilicide, nitride, nitrosilicide, or silicide, wherein the metal is preferably a Group IV metal (titanium, zirconium, or hafnium), a Group V metal (vanadium, niobium, or tantalum), or a Group VI metal (chromium, molybdenum, or tungsten). In a preferred embodiment, the particles are a Group IV metal ceramic, where the ceramic is of a type selected from the group consisting of boride, borocarbide, boronitride, borosilicide, carbide, carbonitride, carbosilicide, nitride, nitrosilicide, and silicide, and in a more preferred embodiment, the particles are a Group IV or Group V metal carbide, such as titanium carbide, vanadium carbide, and hafnium carbide. In another embodiment, the particles are calcium boride. 
     The particles may comprise a mixture of suitable particles, for example a mixture of titanium carbide and calcium boride particles. These particles are generally commercially available, e.g., from commercial suppliers such as American Elements, Inc. (Los Angeles, Calif.) and Fujimi Corporation (Elmhurst, Ill.) even in very small sizes, such that particle size is not a limiting factor in forming an asperated layer even on the smallest contact regions. Regardless of their composition, the particles must have a low electrical resistance, i.e., an electrical resistance less than about 100 milliohms, preferably less than 50, 40, 30, 20, 10 or 5 milliohms, and more preferably less than 10, 9, 8, 7, 6, 5, 4, 3, 2 or 1 milliohms. In a preferred embodiment, the electrical resistance of the particles is less than about 25 milliohms, more preferably less than about 15 milliohms, and even more preferably less than about 10 or about 5 milliohms. 
     The first conductive metal may be mixed with a suitable starting solution, and supplemented with desirable additives such as surfactants, levelers, buffers, brighteners, grain refiners, wetting agents, and the like, as is known to those skilled in the art. Starting solutions for plating various metals are well known in the art. For example, if the first conductive metal is nickel, a nickel sulfamate solution may be used, or if copper, a copper cyanide solution may be used. Commercially available electroplating solutions may also be used, for example for nickel, the Barrett Sulfamate Nickel Plating solution sold by MacDermid Inc. (Waterbury, Conn.) or the Techni Nickel S or High-Speed Nickel Sulfamate FFP solutions sold by Technic Inc. (Cranston, R.I.); for copper, the Techni FB Bright Acid Copper or Technic CU-330 solutions sold by Technic Inc.; for gold, the Technispeed G or Techni Gold 25 ES solutions sold by Technic Inc.; for platinum, the Platinum TP solution sold by Technic Inc.; and for silver, the Techni Cyless Silver II solution sold by Technic Inc. 
     In a preferred embodiment, a nickel sulfamate bath is used to deposit a first conductive layer of nickel. A low stress nickel sulfamate bath suitable for use in the present embodiments comprises the following: 
                                 Ingredient   Amount per L                                            Nickel sulfamate   327.6   g       Equivalent nickel metal   76.3   g       Boric acid   29.9   g       Dissolution agent (e.g., Barrett Additive “A” or “B”)   2.99   g       Wetting/anti-pitting agent (e.g., Barrett SNAP A/M)   2.2   g                     Water (e.g., ultra pure water)   Balance                    
Barrett Additive “A” is a chloride-bearing corrosion acid and Additive “B” is a bromide bearing acid, both of which are useful to assist in dissolving the anode in the bath. These Additives, as well as Barrett SNAP A/M, which is a low foaming anti-pitting and wetting (surfactant) agent that lowers surface tension and allows the plating solution to spread uniformly over the device being plated, are available from MacDermid Inc. SNAP A/M is designed for use with either air or mechanical agitation. Boric acid acts to increase conductivity and as a buffer in the solution. If the electroplating process is run at a temperature above 32° C., the amount of boric acid used per liter must be increased, for example at 32° C. about 31.7 g should be used, at 43° C. about 37.5 g should be used, at 49° C. about 44.9 g should be used, at 54° C. about 46.8 g should be used, and at 60° C. about 48.6 g should be used.
 
     The low electrical resistance particles are added to the solution, in an amount suitable for the desired density of asperities plated onto the contact regions. Generally, the amount of particles added is within the range of 1 to 250 grams per liter of plating solution, although this amount will vary depending on the composition of the particles, the composition of the bath, the desired density of the asperities, and the like. For example, in a solution such as the nickel sulfamate bath specified above, about 10 to 100 grams per liter of titanium carbide particles can be used to achieve an appropriate asperity density for plating the contact pads of an interposer flex panel, but the amount may be as low as 2.5 to 7 grams per liter. Other particles may be used in larger or smaller amounts, for example it may be desirable to use anywhere from 1 to 50 grams per liter of particles, depending on the particle composition and the desired plating density. In a preferred embodiment, the amount of low electrical resistance particles added to each liter of the electroplating solution is about 1 to 250 grams, about 2 to 225 grams, about 3 to 200 grams, about 4 to 175 grams, or about 5 to 150 grams. 
     Once the electroplating bath has been prepared, the rectifier is used to supply a low density current to the electroplating solution for a period of time sufficient to deposit a layer of the conductive metal and the low electrical resistance particles on the plurality of conductive contact regions. The current density is supplied within the range of 0.01 to 10 Amps per square foot (ASF), which is low density and produces a very uniform plating. Although prior electroplating methods using diamond or silicon particles have required 10 to 25 ASF to achieve a satisfactory particle density when co-plated with metal, it has surprisingly been found that the present combination of low electrical resistance particles and high levels of agitation in the electroplating bath allows the usage of low current densities to produce satisfactory particle density and grain size in the plated asperated layer. Previously it had been thought that such low current densities would result in unacceptable plating results, for example an unsatisfactorily large grain size. 
     In a preferred embodiment, the low current density is within the range of 0.05 to 8 ASF, preferably 0.1 to 5 ASF, more preferably within the range of 0.5 to 3 ASF, and even more preferably within the range of 0.8 to 2 ASF. In another embodiment, the low current density is less than 8, 7, 6, 5, 4, 3, 2, 1, or 0.1 ASF, preferably less than 10, 9.5, 9, 8.5, 8, 7.5, 7, 6.5, 6, 5.5, 5, 4.5, 4, 3.5, 3, 2.5, 2, 1.5, 1, 0.5, 0.25, or 0.01 ASF, and more preferably is less than 2.75, 2.5, 2.25, 2, 1.75, 1.5, 1.25, 1, 0.75, 0.5, 0.25, or 0.01 ASF. The amount of time required to plate the asperated layer varies according to the size of the low electrical resistance particles, the size of the contact regions, the desired thickness of the asperated layer, and the like. Generally, for the formation of an asperated interposer layer, the time varies between 1 and 30 minutes, more typically between 2 and 20 minutes. The temperature of the bath during the plating process may be at room temperature (˜20° C.) or at an elevated temperature, for example a temperature between about 20° C. and 80° C., more preferably from about 30° C. to 70° C., and even more preferably between about 40° C. to 60° C. 
     If desired, the asperated layer may be further reinforced (e.g., made thicker) by placing or suspending the device in a conventional metallic electroplating bath containing a solution of the first conductive metal, and running the plating process at a low current density for an additional period of time as needed to produce the desired thickness for the asperated layer, for example between 5 and 25 minutes, and more preferably between 10 and 20 minutes. The conventional electroplating bath is preferably of the same type as the first electroplating bath, for example, if a low stress nickel sulfamate bath is used to co-deposit the particles and the first conductive metal, then the conventional bath is preferably also a nickel sulfamate bath. 
     In those embodiments in which a second conductive layer is desired to be deposited on top of the first conductive layer, a second electroplating solution is used, which comprises a second conductive metal used to deposit the second conductive layer. In a preferred embodiment, the second conductive layer is also a conductive metal, which may be selected from the same metals as the first conductive layer, e.g., nickel, aluminum, copper, gold, iridium, palladium, platinum, rhodium, ruthenium, silver, titanium, or an alloy of one or more of these metals. In another preferred embodiment, the second conductive layer is a conductive noble metal selected from the group consisting of gold, iridium, osmium, palladium, platinum, rhodium, ruthenium, silver, and alloys and combinations thereof. In a particularly preferred embodiment, the second conductive layer is gold or rhodium, or an alloy of one or both of these metals. To plate the second conductive layer, the device is placed or suspended in a second electroplating bath which comprises a second electroplating solution comprising the second conductive metal. For example, the device is placed in a rhodium plating bath, and current is applied to plate a layer of rhodium to the desired thickness, e.g., a current density of about 2 to 30 ASF is used to plate the rhodium to a thickness of about 0.25 to 4.0 microns. 
     All publications and patents mentioned in the above specification are herein incorporated by reference. The above description, drawings and examples are only illustrative of preferred embodiments which achieve the objects, features and advantages of the present invention. It is not intended that the present invention be limited to the illustrative embodiments. Any modification of the present invention which comes within the spirit and scope of the following claims should be considered part of the present invention.