Abstract:
A feedthrough filter capacitor assembly is described. The feedthrough filter capacitor assembly comprises an outer ferrule hermetically sealed to an insulator of a dielectric material seated within the ferrule. The insulative material is also hermetically sealed to at least one lead wire. Instead of being made of platinum or platinum/iridium, the lead wire comprises a core of a non-noble metal supporting a functionally graded coating. The metal core has an inner layer of the same the non-noble metal of the core and an outer layer of a noble metal. A gradient transition zone exists between the non-noble metal and the outer noble metal. Consequently, lead wires having all the beneficial attributes of platinum and platinum/iridium wire can be built into hermetic feedthroughs, but at a significantly reduced cost. In a preferred form, a filter capacitor is mounted on the insulator and electrically connected to the lead wires and to the ferrule to prevent unwanted EMI signals from traveling along the wires and entering the interior of the medical device.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from provisional application Ser. No. 60/986,304, filed Nov. 8, 2007. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to hermetic feedthrough assemblies, preferably of the type incorporating a filter capacitor. More specifically, this invention relates to a method for producing lower cost lead wires for use in hermetic feedthrough filter capacitor assemblies. Preferably, the feedthrough filter capacitor assemblies are of the type used in implantable medical devices such as cardiac pacemakers, cardioverter defibrillators, and the like, to decouple and shield internal electronic components of the medical device from undesirable electromagnetic interference (EMI) signals. The feedthrough assemblies provide a hermetic seal that prevents passage or leakage of fluids into the medical device. The lead wires and, consequently the hermetic feedthrough assemblies, are considerably less expensive than those made by the prior art using platinum, platinum/iridium lead wires while still achieving the same benefits of biocompatibility, providing good mechanical strength and achieving a hermetic feedthrough seal. 
     2. Prior Art 
     Implantable hermetic feedthrough assemblies are typically manufactured with lead wires composed of platinum or a combination of platinum and iridium. The platinum or platinum/iridium material is biocompatible and creates a hermetic seal through a gold brazing process that seals any gap between the lead wires and the ceramic substrate. The use of platinum and platinum/iridium lead wires also provides for good mechanical strength, which adds to the durability of the feedthrough. However, platinum is a precious metal that creates a manufacturing cost barrier. 
     Materials other than platinum and platinum/iridium with suitable mechanical properties and biostability can easily be found, but suffer from poor oxidation resistance, which can result in poor electrical conduction, poor weldability and poor solderability. In order to overcome these shortcomings, the prior art has developed several techniques for applying a layer of noble metal to a non-noble metal wire core. In the case of a mechanically clad coating having a typical thickness of from 5,000 nm to 20,000 nm, the coating material is not well adhered to the wire core, which causes problems with brazing and welding. 
     Relatively thick coating layers can also be produced by vacuum deposition techniques. However, they produce coatings that are subjected to relatively high stresses. Therefore, the practical limitation of a vacuum deposition coating is about 2,000 nm to 5,000 nm thick. 
     Another prior art process involves coating a relatively thin layer about 100 nm to 1,000 nm thick on the surface of a non-noble metal wire core. While such relatively thin coatings do not suffer from the stress forces inherent in thicker coatings, regardless how they are adhered to the wire core, these relatively thin coating thicknesses are insufficient to act as a barrier to migration of the non-noble metal to the surface of the wire. Subsequent operations such as brazing, welding and soldering can expose the non-noble material to oxidation, thereby causing a non-wettable surface that cannot be soldered. 
     Accordingly, there is a need for a relatively low cost lead wire for incorporation into hermetic feedthrough assemblies. The lead wire must be significantly lower in cost than those made from platinum and platinum/iridium without sacrificing biocompatibility, mechanical strength and ultimately the hermeticity of the feedthrough assembly into which it is built. 
     SUMMARY OF THE INVENTION 
     A feedthrough filter capacitor assembly comprises an outer ferrule of titanium hermetically sealed to either an alumina insulator or fused glass dielectric material seated within the ferrule. Titanium is a material typically used for the ferrule. The insulative material is also hermetically sealed to at least one lead wire. A gold braze typically accomplishes these seals. That way, the feedthrough assembly prevents leakage of fluid, such as body fluid in a human implant application, past the hermetic seal at the insulator/ferrule and insulator/lead wire interfaces. In a preferred form, a filter capacitor is mounted on the insulator and electrically connected to the lead wires and to the ferrule to prevent unwanted EMI signals from traveling along the wires and entering the interior of the medical device. 
     As an alternative to lead wires made of platinum or platinum/iridium, there is a desire in industry to build hermetic feedthroughs with lead wires of non-noble metal materials. The primary motivation is reduced cost. However, non-noble metals are highly reactive with gold. Over time, the non-noble wire material will diffuse into the braze material and cause severe erosion of the lead wire. For example, it is known that when titanium lead wires are brazed using gold, titanium diffusion into the gold can be significant enough to cause erosion and structural damage to the titanium wire. The result is that strength and, therefore, performance of the titanium lead wire is compromised by this excessive diffusion. 
     As a solution, the present invention provides a functionally graded coating on a non-noble metal wire core. The metal core acts as the mechanical support for the wire and is chosen on the basis of its mechanical properties so that it is suitable for use in a hermetic feedthrough, such as a feedthrough. Additionally, the metal core is chosen for its electrical conductivity, however, that is secondary to the mechanical requirement. Importantly, the core material is processed so that an indiscrete transition from the core to a functionally graded noble metal-containing protective coating is created. The noble metal-containing coating must be thick enough to act as a barrier for material diffusion while eliminating the relatively poor adhesion inherent in a mechanically clad coating and the problems attendant with vacuum deposited materials, and the like. Once the lead wire is incorporated into the feedthrough, the functionally graded noble metal-containing coating acts as a barrier against diffusion of the core material into the braze material. Consequently, lead wires having all the beneficial attributes of platinum and platinum/iridium wire can be built into hermetic feedthroughs, but at a significantly reduced cost. 
     These and other objects and advantages of the present invention will become increasingly more apparent by a reading of the following description in conjunction with the appended drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a hermetic feedthrough assembly embodying the novel features of the present invention. 
         FIG. 2  is an enlarged sectional view taken along line  2 - 2  of  FIG. 1 . 
         FIGS. 3 and 4  are schematic representations of lead wires according to the present invention. 
         FIG. 5  is a schematic representation of a lead wire according to a prior art process. 
         FIG. 6  is a cross-sectional view taken along line  6 - 6  of  FIG. 2 . 
         FIG. 7  is a cross-sectional view taken along line  7 - 7  of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings,  FIGS. 1 and 2  show an internally grounded feedthrough filter capacitor assembly  10  comprising a feedthrough  12  supporting a filter discoidal capacitor  14 . The feedthrough filter capacitor assembly  10  is useful with medical devices, preferably implantable devices such as pacemakers, cardiac defibrillators, cardioverter defibrillators, cochlear implants, neurostimulators, internal drug pumps, deep brain stimulators, hearing assist devices, incontinence devices, obesity treatment devices, Parkinson&#39;s disease therapy devices, bone growth stimulators, and the like. The feedthrough  12  portion of the assembly  10  hermetically seals the interior of the medical device against ingress of patient body fluids that could otherwise disrupt device operation or cause instrument malfunction and includes terminal pins or lead wires  16  that provide for coupling, transmitting and receiving electrical signals to and from a patient&#39;s heart. While not necessary for accomplishing these functions, it is desirable to attach the filter capacitor  14  to the feedthrough  12  for suppressing or decoupling undesirable EMI signals and noise transmission into the interior of the medical device along the lead wires  16 . The filter capacitor  14  will be described in greater detail hereinafter. 
     More particularly, the feedthrough  12  of the feedthrough filter capacitor assembly  10  comprises a ferrule  18  defining a bore surrounding an insulator  20 . The ferrule  18  may be of any geometry, non-limiting examples being round, rectangle, and oblong. Titanium is an electrically conductive material that is preferred for the ferrule  18 . A surrounding flange  22  extends from the ferrule  18  to facilitate attachment of the feedthrough  10  to the casing (not shown) of, for example, one of the previously described implantable medical devices. The method of attachment may be by laser welding or other suitable methods. 
     The insulator  20  is of a ceramic material such as of alumina, zirconia, zirconia toughened alumina, aluminum nitride, boron nitride, silicon carbide, glass or combinations thereof. Preferably, the insulating material is alumina, which is highly purified aluminum oxide, and comprises a sidewall  24  extending to a first upper side  26  and a second lower side  28 . The insulator  20  is also provided with bores  30  that receive the lead wires  16  passing there through. A layer of metal  32 , referred to as metallization, is applied to the insulator sidewall  24  and the sidewall of the lead wire bores  30  to aid a braze material  34  in hermetically sealing between the ferrule  18  and the insulator  24  and between the lead wires  16  and the insulator  24 , respectively. 
     Suitable metallization materials  32  include titanium, titanium nitride, titanium carbide, iridium, iridium oxide, niobium, tantalum, tantalum oxide, ruthenium, ruthenium oxide, zirconium, gold, palladium, molybdenum, silver, platinum, copper, carbon, carbon nitride, and combinations thereof. The metallization layer may be applied by various means including, but not limited to, sputtering, electron-beam deposition, pulsed laser deposition, plating, electroless plating, chemical vapor deposition, vacuum evaporation, thick film application methods, aerosol spray deposition, and thin cladding. Parylene, alumina, silicone, fluoropolymers, and mixtures thereof are also useful metallization materials. 
     Non-limiting examples of braze materials include gold, gold alloys, and silver. Then, if the feedthrough  10  is used where it will contact bodily fluids, the resulting brazes do not need to be covered with a biocompatible coating material. In other embodiments, if the brazes are not biocompatible, for example, if they contain copper, they are coated with a layer/coating of biocompatible/biostable material. Broadly, the biocompatibility requirement is met if contact of the braze/coating with body tissue and blood results in little or no immune response from the body, especially thrombogenicity (clotting) and encapsulation of the electrode with fibrotic tissue. The biostability requirement means that the braze/coating remains physically, electrically, and chemically constant and unchanged over the life of the patient. 
     The lead wires  16  of the present invention comprise a metallic core  36  selected from niobium, tantalum, nickel-titanium (NITINOL®), titanium, particularly beta titanium, titanium alloys, stainless steel, molybdenum, tungsten, palladium, palladium alloys, and combinations thereof. If desired, platinum and iridium can also be useful materials for the core  36 . According to the present invention, a functionally graded protective coating  38 , preferably comprising a noble metal, is provided on the surface  36 A of the core material  36 . This is done prior to assembling the wire  16  into a hermetic feedthrough assembly. 
     The functionally graded coating acts as a barrier with all the beneficial properties of a mechanically clad coating while eliminating the poor adhesion. The invention also eliminates the stresses inherent in the vacuum deposited coating process by gradually transitioning from one material to the next as opposed to discrete layers. This transitioning is done during processing and therefore the resulting coating is strongly adhered to the core wire material without subsequent processing. The preferred method for producing the functionally graded coating of the present invention utilized a high-pressure plasma nozzle operating at &gt;100 mTorr. A suitable high pressure plasma device is described in U.S. Pat. No. 5,571,332 to Halpern, titled “Electron Jet Vapor Deposition System”. This patent is incorporated herein by reference. 
     An electron jet vapor deposition system is preferred because the high gas pressure it generates allows for uniform coating coverage, high throughput and low gas contamination. The equipment necessary for the deposition of the functionally graded material requires multiple sputtering sources. These sources are located inside a single chamber so that they can be simultaneously used. For example, in the case of a system using three materials (A, B and C) such as is described below, the nozzles or jets must be located so that materials A and B are deposited simultaneously on the core wire substrate. Likewise, materials B and C must be capable of being deposited simultaneously. 
     Once vacuum and part cleanliness has been established the process of depositing the coating begins. The first step is to deposit material A, which is substantially the same as the core wire material. Sputtering material A in the plasma and allowing it to condense on the core wire substrate accomplishes this. Once a sufficient thickness of material A has been deposited, the rate of deposition for material A is reduced. In the preferred jet process, this is accomplished by decreasing the plasma power and the rate at which the coating target material is fed into the evaporator. Simultaneously, the deposition rate of material B is increased from zero to an optimum setting through the same methods. This graded coating structure is provided by simultaneously decreasing deposition rate of material A as the deposition rate of material B is increased. Similarly other materials may be added and graded to material B. In addition, other processes may be used to produce the functionally graded coating of the present invention as long as they consist of multiple materials being deposited simultaneously with the deposition rate of the respective materials being changed gradually and independently of each other. 
       FIG. 3  is a schematic drawing showing one embodiment of a lead wire  16  according to the present invention comprising a core material  36  provided with a functionally graded coating  38  comprising a two metal system. The coating  38  comprises a first, inner portion  40  (designated by circles), a second, intermediate or transition portion  42  (designated by circles and squares) and a third, outer portion  44  (designated by squares). 
     The first portion  40  of the coating  38  is located at the core/coating interface and is for all intents and purposes the same material as that of the core  36  (designated by circles), but is in addition to the material comprising the core of the lead wire  16 . In other words, the first portion  40  of the coating  38  is 100% coating material contacted to the outer surface  36 A of the core material  36 . As previously described with respect to materials that are suitable for the wire core  36 , the first portion  40  is selected from the group consisting of niobium, tantalum, nickel-titanium (NITINOL®), titanium, particularly beta titanium, titanium alloys, stainless steel, molybdenum, tungsten, palladium, platinum, iridium, and combinations and alloys thereof, as long as the core  36  and first portion  40  are substantially the same material. The first portion  40  of the coating is preferably a dense, adhesive and defect free layer from about 0.5 μm to about 25 μm thick. 
     The second, intermediate portion  42  is a transition mixture of the material comprising the first portion and a noble metal. Preferably, the intermediate portion  42  is a mixture of the same non-noble metal comprising the wire core  36  and the first portion  40  and a noble metal, such a platinum or iridium. As the thickness of the intermediate portion  42  increases spaced from the outer surface  36 A of the wire core  36 , there is a gradient trending toward increasingly more of the noble metal and less of the non-noble metal. The thickness of the second portion  42  preferably ranges from about 0.5 μm to about 25 μm. Non-limiting examples of suitable noble metals include gold, platinum, palladium, iridium, tantalum, ruthenium, rhodium, and combinations and alloys thereof. Then, the third, outer portion  44  is substantially pure noble metal, such as platinum or iridium. The thickness of the third portion preferably ranges from about 0.5 μm to about 25 μm. 
     A preferred two metal system comprises tantalum as the metal of the wire core  36  and the first portion  40  (circles) and platinum as the metal of the third portion  44  (squares). 
       FIG. 4  is a schematic drawing showing another embodiment of a lead wire  16  according to the present invention comprising a core material  36  provided with a functionally graded coating  46  comprising a three metal system. The coating  46  comprises a first, inner portion  48  (designated by circles), a second, inner intermediate portion  50  (designated by circles and triangles), a third, outer intermediate portion  52  (designated by triangles and squares), and an outer portion  54  (designated by squares). 
     The first portion  48  of the coating  38  is located at the core/coating interface and is for all intents and purposes the same material as that of the core  36  (designated by circles), but is in addition to the material comprising the core of the lead wire  16 . Materials that are suitable for the wire core  36  and the first portion  48  are selected from the group consisting of niobium, tantalum, nickel-titanium (NITINOL®), titanium, particularly beta titanium, titanium alloys, stainless steel, molybdenum, tungsten, palladium, platinum, iridium, and combinations and alloys thereof. The first portion  48  of the coating preferably has a thickness of from about 0.5 μm to about 25 μm thick. 
     The second, inner intermediate portion  50  is a mixture of the material comprising the first portion  48  and either another non-noble metal or a noble metal. Preferably, the second inner intermediate portion  50  is a transition mixture of the same non noble metal comprising the wire core  36  and the first portion  50  and a noble metal, such as titanium. As the thickness of the inner intermediate portion  50  increases spaced from the outer surface  36 A of the wire core  36 , there is a gradient trending toward increasingly more of the noble metal or the other non-noble metal and less of the non-noble metal comprising the core. The second, inner intermediate portion  50  has a thickness that preferably ranges from about 0.5 μm to about 25 μm. Non-limiting examples of suitable noble metals include gold, platinum, palladium, iridium, tantalum, ruthenium, rhodium, and combinations and alloys thereof. 
     The third intermediate portion  52  is substantially of either the non-noble metal or the noble metal comprising the mixture of the second, inner intermediate portion  50 . The third intermediate portion  52  has thickness that preferably ranges from about 0.5 μm to about 25 μm. 
     The fourth outer intermediate portion  54  is a mixture of the non-noble metal or a noble metal material comprising the second, inner intermediate portion  50  and a noble metal. Preferably, the fourth outer intermediate portion  54  is a transition mixture of two noble metals such as titanium and platinum. As the thickness of the forth outer intermediate portion  54  increases spaced from the outer surface  36 A of the wire core  36 , there is a gradient trending toward increasingly more of the second noble metal, for example platinum, and less of the first noble metal, for example titanium. The fourth outer intermediate portion  54  has a thickness that preferably ranges from about 0.5 μm to about 25 μm. 
     In this embodiment, the functionally graded coating  46  is completed by the fifth outer portion  56 , which is substantially of a pure noble metal, such as platinum. The fifth portion  56  preferably has a thickness ranging from about 0.5 μm to about 25 μm. 
     A preferred three metal system comprises niobium as the metal of the core wire  36  and the first portion  48  (circles), titanium as the other metal of the intermediate portions  50 ,  52  and  54  (triangles) and platinum as the metal of the outer portion  56  (squares). 
     In contrast to the present invention,  FIG. 5  is a schematic drawing showing a prior art lead wire comprising the core material  36  provided with a non-functionally graded coating  60  as a two metal system. The coating  60  comprises discrete layers of a first, inner portion  62  (designated by triangles) and a second, outer portion  64  (designated by squares). As with the exemplary embodiments of the present invention schematically illustrated in  FIGS. 3 and 4 , the wire core  36  is selected from the group consisting of niobium, tantalum, nickel-titanium (NITINOL®), titanium, particularly beta titanium, titanium alloys, stainless steel, molybdenum, tungsten, palladium, platinum, iridium, and combinations and alloys thereof. 
     The first, inner portion  62  of the prior art coating  60  is either a non-noble or a noble metal that is different than the non-noble metal of the wire core  36 . There is no gradient transition between the wire core  36  and the first portion  62 . Instead, they have distinct and well defined boundaries. The method of deposition of the inner portion  62  onto the wire core  36  is not necessarily limited and can be done by mechanical cladding, vacuum deposition techniques, and the like, as long as it results in a distinct boundary between the two materials. 
     The second, outer portion  64  of the prior art coating  60  is a noble metal that is different than the material of the inner portion  62 . Again, there is no gradient transition between the material  62  and  64 . Instead, they have distinct and well defined boundaries. As before, the method of deposition of the outer portion  64  onto the inner portion  62  is not necessarily limited and can be done by mechanical cladding, vacuum deposition techniques, and the like, as long as it results in a distinct boundary between the two materials. An exemplary lead wire according to the prior art comprises niobium as the wire core  36  (circles), titanium as the inner coating portion  62  (triangles) and platinum as the outer portion  64  (squares). 
     The problem with the prior art non-functionally graded coating is that the inner and outer portions  62 ,  64  of the system can be prone to delamination, spalling, sloughing, flaking, and the like. These are undesirable in many applications, especially those where the lead wire is subsequently designed into a hermetic feedthrough. In contrast, the functionally graded coating of the present invention does not suffer from these shortcomings. In all cases, the inner portion of the coating is the same material as the wire core, which results in a strong, ionic bond between the two. Then, as the coating transitions from the core material to the outer noble metal, the transitional gradient is an ionic bond that is extremely strong and capable of withstanding severe forces without delamination, spalling, sloughing, flaking and similar types of failures. 
     As further shown in  FIGS. 1 ,  2 ,  6  and  7 , a preferred form of the feedthrough filter capacitor assembly  10  includes a filter capacitor  14  that provides for filtering undesirable EMI signals before they can enter the device housing via the terminal pins  16 . The filter capacitor  14  comprises a ceramic or ceramic-based dielectric monolith  70  having multiple capacitor-forming conductive electrode plates formed therein. The capacitor dielectric  70  preferably has a circular cross-section matching the cross-section of the ferrule  18  and supports a plurality of spaced-apart layers of first or “active” electrode plates  72  in spaced relationship with a plurality of spaced apart layers of second or “ground” electrode plates  74 . The filter capacitor  14  is preferably joined to the feedthrough  12  adjacent to the insulator side  26  by an annular bead  76  of conductive material, such as a solder or braze ring, or a thermal-setting conductive adhesive, and the like. The dielectric  36  includes lead bores  44  provided with an inner surface metallization layer. The terminal pins  16  pass there through and are conductively coupled to the active plates  72  by a conductive braze material  78  contacting between the lead wires  16  and the bore metallization. In a similar manner, the ground plates  78  are electrically connected through an outer surface metallization  80  and the conductive material  76  to the ferrule  18 . 
     It is appreciated that various modifications to the invention concepts described herein may be apparent to those of ordinary skill in the art without departing from the scope of the present invention as defined by the appended claims.