Abstract:
A capacitive filter feedthrough assembly and method of making the same are disclosed for shielding an implantable medical device such as pacemaker or defibrillator from electromagnetic interference or noise. A ferrule is adapted for mounting onto a conductive device housing by welding, soldering, brazing or gluing, and supports a terminal pin for feedthrough passage to a housing interior. A capacitive filter is mounted at the inboard side of a device housing, with capacitive filter electrode plate sets coupled respectively to the housing and the terminal pin by electrically conductive adhesive or brazing, or a combination of adhesive and brazing. In one embodiment of the invention, multiple capacitive filters are provided in an array within a common base structure, where each capacitive filter is associated with a respective terminal pin.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a Continuation-in-Part of U.S. patent application. Ser. No. 08/852,198 to Seifried et al. for &#34;Capacitive Filter Feedthrough for Implantable Medical Device&#34; filed May 6, 1997, hereby incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to electrical feedthroughs of improved design and to their method of fabrication. 
     BACKGROUND OF THE INVENTION 
     Electrical feedthroughs serve the purpose of providing an electrical circuit path extending from the interior of a hermetically sealed container to an external point outside the container. A conductive path is provided through the feedthrough by a conductor pin which is electrically insulated from the container. Many such feedthroughs are known in the art which provide the electrical path and seal the electrical container from its ambient environment. Such feedthroughs typically include a ferrule, the conductor pin or lead and a hermetic glass or ceramic seal which supports the pin within the ferrule. Such feedthroughs are typically used in electrical medical devices such as implantable pulse generators (IPGs). It has recently been discovered that such electrical devices can, under some circumstances, be susceptible to electromagnetic interference (EMI). At certain frequencies for example, EMI can inhibit pacing in an IPG. This problem has been addressed by incorporating a capacitor structure within the feedthrough ferrule, thus shunting any EMI at the entrance to the IPG for high frequencies. This has been accomplished with the aforementioned capacitor device by combining it with the feedthrough and incorporating it directly into the feedthrough ferrule. Typically, the capacitor electrically contacts the pin lead and the ferrule. 
     Some of the more popular materials employed to form the pin lead include tantalum and niobium. Unfortunately, tantalum and niobium are susceptible to oxide growth which can, depending on its extent, act as an insulator instead of a conductor over the surface of the pin lead. During fabrication of a feedthrough and capacitor combination, the pin is subjected to one or more heat treatments which can encourage oxidation, affecting the conductivity of the pin lead and its ability to make good electrical connections between other elements including the capacitor and so forth. 
     Many different insulator structures and related mounting methods are known in the art for use in medical devices wherein the insulator structure also provides a hermetic seal to prevent entry of body fluids into the housing of the medical device. However, the feedthrough terminal pins are connected to one or more lead wires which effectively act as an antenna and thus tend to collect stray or electromagnetic interference (EMI) signals for transmission to the interior of the medical device. In some prior art devices, ceramic chip capacitors are added to the internal electronics to filter and thus control the effects of such interference signals. This internal, so-called &#34;on-board&#34; filtering technique has potentially serious disadvantages due to intrinsic parasitic resonances of the chip capacitors and EMI radiation entering the interior of the device housing. 
     In another and normally preferred approach, a filter capacitor is combined directly with a terminal pin assembly to decouple interference signals to the housing of the medical device. In a typical construction, a coaxial feedthrough filter capacitor is connected to a feedthrough assembly to suppress and decouple undesired interference or noise transmission along a terminal pin. 
     So-called discoidal capacitors having two sets of electrode plates embedded in spaced relation within an insulative substrate or base typically form a ceramic monolith in such capacitors. One set of the electrode plates is electrically connected at an inner diameter surface of the discoidal structure to the conductive terminal pin utilized to pass the desired electrical signal or signals. The other or second set of electrode plates is coupled at an outer diameter surface of the discoidal capacitor to a cylindrical ferrule of conductive material, wherein the ferrule is electrically connected in turn to the conductive housing or case of the electronic instrument. 
     In operation, the discoidal capacitor permits passage of relatively low frequency electrical signals along the terminal pin, while shunting and shielding undesired interference signals of typically high frequency to the conductive housing. Feedthrough capacitors of this general type are commonly employed in implantable pacemakers, defibrillators and the like, wherein a device housing is constructed from a conductive biocompatible metal such as titanium and is electrically coupled to the feedthrough filter capacitor. The filter capacitor and terminal pin assembly prevent interference signals from entering the interior of the device housing, where such interference signals might otherwise adversely affect a desired function such as pacing or defibrillating. 
     In the past, feedthrough filter capacitors for heart pacemakers and the like have typically been constructed by preassembly of the discoidal capacitor with a terminal pin subassembly which includes the conductive terminal pin and ferrule. More specifically, the terminal pin subassembly is prefabricated to include one or more conductive terminal pins supported within the conductive ferrule by means of a hermetically sealed insulator ring or bead. See, for example, the terminal pin subassemblies disclosed in U.S. Pat. Nos. 3,920,888, 4,152,540; 4,421,947; and 4,424,5511. The terminal pin subassembly thus defines a small annular space or gap disposed radially between the inner terminal pin and the outer ferrule. A small discoidal capacitor of appropriate size and shape is then installed into this annular space or gap, in conductive relation with the terminal pin and ferrule, by means of soldering, conductive adhesive, etc. The thus-constructed feedthrough capacitor assembly is then mounted within an opening in the pacemaker housing, with the conductive ferrule in electrical and hermetically sealed relation in respect of the housing, shield or container of the medical device. 
     Although feedthrough filter capacitor assemblies of the type described above have performed in a generally satisfactory manner, the manufacture and installation of such filter capacitor assemblies has been relatively costly and difficult. For example, installation of the discoidal capacitor into the small annular space between the terminal pin and ferrule can be a difficult and complex multi-step procedure to ensure formation of reliable, high quality electrical connections. Moreover, installation of the capacitor at this location inherently limits the capacitor to a small size and thus also limits the capacitance thereof. Similarly, subsequent attachment of the conductive ferrule to the pacemaker housing, typically by welding or brazing processes or the like, can expose the fragile ceramic discoidal capacitor to temperature variations sufficient to create the risk of capacitor cracking and failure. 
     There exists, therefore, a significant need for improvements in feedthrough filter capacitor assemblies of the type used, for example, in implantable medical devices such as heart pacemakers and the like, wherein the filter capacitor is designed for relatively simplified and economical, yet highly reliable, installation. In addition, there exists a need for an improved feedthrough assembly having a discoidal capacitor which can be designed to provide a significantly increased capacitance for improved filtering. The present invention fulfills these needs and provides further advantages. 
     Disclosures relating to implantable medical devices, feedthroughs and capacitive filtering of EMI include the patents listed below in Table 1. 
     Table 1: Prior Art Patents 
     U.S. Patents 
     U.S. Pat. No. 1,180,614 April/1916 Simpson . . . 428/662 
     U.S. Pat. No. 2,756,375 July/1956 Peck . . . 361/302 
     U.S. Pat. No. 3,266,121 August/1966 Rayburn . . . 29/25.42 
     U.S. Pat. No. 3,235,939 Febuary/1966 Rodriguez et al . . . 29/25.42 
     U.S. Pat. No. 3,304,362 Febuary/1967 August . . . 174/50.61 
     U.S. Pat. No. 3,538,464 November/1970 Walsh . . . 361/302 X 
     U.S. Pat. No. 3,624,460 November/1971 Correll . . . 29/25.03 X 
     U.S. Pat. No. 3,844,921 October/1974 Benedict . . . 204/196 
     U.S. Pat. No. 3,920,888 November/1975 Barr . . . 174/152GM 
     U.S. Pat. No. 4,010,759 March/1977 Boer . . . 174/152GM X 
     U.S. Pat. No. 4,015,175 March/1977 Kendall et al . . . 361/313 
     U.S. Pat. No. 4,041,587 August/1977 Kraus . . . 29/25.42 
     U.S. Pat. No. 4,083,022 April/1978 Nijman . . . 333/185 
     U.S. Pat. No. 4,107,762 August/1978 Shirn et al . . . 29/25.04 X 
     U.S. Pat. No. 4,148,003 April/1979 Colburn et al . . . 361/302 
     U.S. Pat. No. 4,152,540 May/1979 Duncan etal . . . 174/152GM 
     U.S. Pat. No. 4,168,351 September/1979 Taylor . . . 333/182 
     U.S. Pat. No. 4,220,813 September/1980 Kyle . . . 174/152GM 
     U.S. Pat. No. 4,247,881 January/1981 Coleman . . . 361/302 
     U.S. Pat. No. 4,314,213 Febuary/1982 Wakino . . . 361/302 
     U.S. Pat. No. 4,352,951 October/1982 Kyle . . . 174/152GM 
     U.S. Pat. No. 4,362,792 December/1982 Bowsky et al . . . 174/152GM 
     U.S. Pat. No. 4,421,947 December/1983 Kyle . . . 174/152GM 
     U.S. Pat. No. 4,424,551 January/1984 Stevenson . . . 361/302 
     U.S. Pat. No. 4,456,786 June/1984 Kyle . . . 174/152GM 
     U.S. Pat. No. 4,556,613 December/1985 Taylor et al . . . 429/101 
     U.S. Pat. No. 4,683,516 July/1987 Miller . . . 361/328 
     U.S. Pat. No. 4,737,601 April/1988 Gartzke . . . 174/152GM 
     U.S. Pat. No. 4,741,710 May/1988 Hogan et al . . . 333/185 
     U.S. Pat. No. 4,791,391 December/1988 Linnell . . . 361/302 
     U.S. Pat. No. 4,934,366 September/1989 Truex et al . . . 128/419 
     U.S. Pat. No. 5,032,692 July/1991 DeVolder . . . 361/30.2 
     U.S. Pat. No. 5,070,605 December/1991 Daglow et al . . . 29/842 
     U.S. Pat. No. 5,104,755 April/1992 Taylor et al . . . 174/50.61 
     U.S. Pat. No. 5,144,946 September/1992 Weinberg et al . . . 178/419 
     U.S. Pat. No. 5,333,095 July/1994 Stevenson et al . . . 29/25.42 X 
     U.S. Pat. No. 5,406,444 April/1995 Seifried . . . 361/302 
     U.S. Pat. No. 5,440,447 August/1995 Shipman et al . . . 361/302 
     U.S. Pat. No. 5,531,003 July/1996 Seifried . . . 29/25.42 
     U.S. Pat. No. 5,535,097 July/1996 Ruben . . . 361/736 
     Foreign Patents 
     2815118 October/1978 Fed. Rep. of Ger. . . . 361/302 
     0331959 September/1989 E.P.O. 
     892492 Febuary/1981 U.S.S.R. . . . 29/25.42 
     As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, Detailed Description of the Preferred Embodiments and Claims set forth below, many of the devices and methods disclosed in the patents of Table 1 may be modified advantageously by using the teachings of the present invention. 
     SUMMARY OF THE INVENTION 
     The present invention has certain objects. That is, the present invention provides solutions to at least some of the problems existing in the prior art respecting capacitive filters in feedthrough assemblies. 
     The present invention provides solutions to at least some of the problems associated with conventional capacitive filter feedthrough assembly designs where a discoidal capacitor is placed within ferrule walls, such as in U.S. Pat. Nos. 4,424,551 and 5,333,095. At least some aspects of known capacitive filter feedthrough assemblies may be characterized generally as: 
     (a) involving difficult to implement conductive epoxy placement steps; 
     (b) having high electrical resistances at refractory metal interfaces owing to the presence of conductive epoxy and undesirable metal oxides; 
     (c) exhibiting poor or variable electrical performance in respect of EMI signal attenuation; 
     (d) requiring multiple labor intensive manufacturing processing steps; 
     (e) having through pins which cannot be wire bonded to, or are difficult to wire bond to; 
     (f) exhibiting electrical shorts owing to uncontrolled or inaccurate epoxy placement; 
     (g) having capacitors crack owing to differing thermal expansion coefficients of the conductive can, the capacitor or the electrically conductive epoxy commonly employed to attach the capacitor to a ferrule or container; 
     (h) providing no opportunity for visual inspection of the feedthrough assembly once installed in the device; 
     (i) not permitting the use of registration or centering elements during the manufacturing process, or 
     (j) exhibiting poor mechanical joint strength. 
     The present invention provides solutions to at least some of the problems associated with conventional capacitive filter feedthrough assembly designs where a capacitor is placed to one side of a feedthrough such as in U.S. Pat. No. 5,333,095. Capacitive filter feedthrough assemblies disclosed in the &#39;095 patent may be characterized generally as: 
     (a) not permitting the use of registrations or centering elements; 
     (b) having through pins which cannot be wire bonded to, or are difficult to wire bond to; 
     (c) having capacitors crack owing to differing thermal expansion coefficients of the conductive can and the capacitor; 
     (d) exhibiting poor mechanical joint strength. 
     The present invention provides solutions to at least some of the problems associated with conventional capacitive filter feedthrough assembly designs where solder is employed to connect a capacitor to a feedthrough. Capacitive filter feedthrough assemblies of the type employing solder to connect capacitors to feedthroughs are generally characterized in the use of flux to solder a capacitor to a feedthrough. The use of flux increases the number of manufacturing steps required to make a device because of the requisite cleaning attending the use of flux. Cleaning is required when using flux because otherwise degradation of the hermetic seal can occur due to the presence of moisture and corrosive ionic components in flux material. 
     Some embodiments of the present invention provide certain advantages which include, but are not limited to: 
     (a) permitting the attachment of a capacitive filter to gold brazing; 
     (b) increasing the electrical conductivity between a capacitive filter and a feedthrough; 
     (c) increasing the EMI filtering capability provided for an implantable medical device; 
     (d) eliminating the presence of electrically resistive metal oxides between a capacitive filter and a shield or feedthrough; 
     (e) requiring only one method for connecting a capacitive filter to a pin or ferrule; 
     (f) eliminating secondary manufacturing process steps such as epoxy application or additional soldering steps; 
     (g) reducing manufacturing costs; 
     (h) reducing implantable medical device costs; 
     (i) enclosing a capacitive filter at least partially in a ferrule to thereby provide additional mechanical support to the filter; 
     (j) eliminating secondary cleaning steps associated with soldering; 
     (k) permitting the use of a capacitive filter having higher capacitances than chip capacitors, and therefore providing enhanced EMI filtering capability; 
     (l) providing a protruding upper capacitive filter wire bond pad suitable for wire bonding thereto, and 
     (m) preventing chipping or abrasion of a capacitive filter due to pass-through pin bending. 
     Some embodiments of the present invention have certain features, including, but not limited to: 
     (a) a capacitive filter that is at least partially disposed within or surrounded by first sidewalls forming a first aperture in a ferrule; 
     (b) a capacitive filter that is surface mounted or otherwise disposed atop a ferrule, the filter not being disposed within the first aperture, or not being surrounded by the first sidewalls; 
     (c) a pin having an upper portion, the upper portion extending upwardly into a second aperture in an insulator, the pin being electrically and mechanically connected to a contact pad extending downwardly into a third aperture of the capacitive filter; 
     (d) a pin, the upper portion thereof extending through or substantially through the second aperture, the upper portion optionally extending through or substantially through the first aperture; 
     (e) a feedthrough assembly having no contact pad disposed within the first ferrule, where electrical and mechanical connection of internal circuitry to the pin of the assembly is accomplished by attaching an electrical conductor in or to the third aperture of the capacitive filter, the filter being disposed within or atop the first aperture; 
     (f) inner braze joints, intermediate braze joints, and/or outer braze joints formed of: (i) pure gold; (ii) gold alloys comprising gold and at least one of titanium, niobium, vanadium, nickel, molybdenum, platinum, palladium, ruthenium, silver, rhodium, osmium, iridium, and alloys, mixtures and thereof; (iii) copper-silver alloys, including copper-silver eutectic alloys, comprising copper and silver and optionally at least one of indium, titanium, tin, gallium, palladium, platinum, and alloys, mixtures and combinations thereof; and (iv) silver-palladium-gallium alloys; 
     (g) inner adhesive joints and outer adhesive joints electrically and mechanically connected to the inner and outer braze joints, respectively, the adhesive joints being formed of a suitable, preferably epoxy-based, material, and 
     (h) at least one capacitive filter having first and second electrical terminals connected electrically and mechanically through braze and solder joints, respectively, to circuitry internal to an implantable medical device and to the implantable medical device case or shield. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a cross-sectional view of one embodiment of a uni-polar feedthrough assembly of the present invention; 
     FIG. 2 shows an exploded, perspective view of the uni-polar feedthrough assembly of FIG. 1; 
     FIG. 3 shows a cross-sectional view of one embodiment of a multi-polar feedthrough assembly of the present invention; 
     FIG. 4 shows an enlarged view of a portion of the multi-polar feedthrough assembly of FIG. 3; 
     FIG. 5 shows an exploded perspective view of portions of the multi-polar feedthrough assembly of FIGS. 3 and 4; 
     FIG. 6 shows a perspective, cut-away view of the internal components of an implantable medical device of the present invention; 
     FIG. 7 shows a cross-sectional view of the implantable medical device of FIG. 6; 
     FIG. 8 shows a flow chart of one method of the present invention; 
     FIG. 9 shows graphs of EMI insertion loss data obtained with capacitive filter feedthroughs of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the claims and specification hereof, the adjective &#34;upper&#34; refers to those portions of feedthrough assembly 5 having contact pad 60 propinquant thereto, the adjective &#34;lower&#34; refers to those portions of feedthrough assembly 5 having pin 30 propinquant thereto, the adjective &#34;inner&#34; refers to those portions of feedthrough assembly 5 having central vertical axis 100 of pin 30 propinquant thereto, and the adjective &#34;outer&#34; refers to those portions of feedthrough assembly 5 having outer surface 80 of capacitive filter 50 propinquant thereto. 
     We refer to U.S. Pat. No. 4,678,868 to Kraska et al., which discloses brazing techniques suitable for use in feedthrough assemblies in implantable medical devices, at least some of which techniques may be adapted for use in the present invention. 
     FIG. 1 shows a cross-sectional view of one embodiment of a uni-polar feedthrough assembly 5 of the present invention after being subjected to the brazing and adhesive-bonding steps of the present invention. FIG. 2 shows an exploded, perspective view of the uni-polar feedthrough assembly of FIG. 1. 
     Electrically conductive ferrule 10 of FIGS. 1 and 2 is preferably welded to shield or container 20 of hermetically sealed implantable medical device 70, and has first aperture 12 disposed therethrough formed by first sidewalls 14. Electrically insulative insulator 25 is disposed within first aperture 12, provides electrical insulation between electrically conductive feedthrough pin 30 and ferrule 10, and has second aperture 27 disposed therethrough formed by second sidewalls 29. 
     Ferrule 10 is typically laser welded to shield or container 20, and may be formed of niobium, titanium, titanium alloys such as titanium-6Al-4V or titanium-vanadium, platinum, molybdenum, zirconium, tantalum, vanadium, tungsten, iridium, rhodium, rhenium, osmium, ruthenium, palladium, silver, and alloys, mixtures and combinations thereof. Ferrule 10 may be welded by other means to shield or container 20, or even soldered or glued thereto. 
     Upper portion 32 of electrically conductive pin 30 is disposed within or may extend at least partially into second aperture 27. Lower portion 34 of pin 30 is generally connected to electrical circuitry, connectors or a connector block external to container 20 of device 70, but may alternatively be connected directly to a connector on a medical lead. In one embodiment of the present invention, upper portion 32 of pin 30 extends upwardly into second aperture 27, and is electrically and mechanically connected by inner braze joint 65 and inner adhesive joint 57 to contact pad 60, where contact pad 60 extends downwardly into third aperture 35 of capacitive filter 50. 
     In another embodiment of the present invention, upper portion 32 of pin 30 extends through or substantially through second aperture 27, and may optionally extend through or substantially through first aperture 12. Upper portion 32 of pin 30 may also be connected directly to an electrical conductor attached to internal circuitry, with no contact pad 60 being disposed in third aperture 35. 
     Pin 30 may be formed of niobium, titanium, titanium alloys such as titanium-6Al-4V or titanium-vanadium, platinum, molybdenum, zirconium, tantalum, vanadium, tungsten, iridium, rhodium, rhenium, osmium, ruthenium, palladium, silver, and alloys, mixtures and combinations thereof. 
     Electrically conductive intermediate braze joint 15 provides an hermetic braze joint and seal between ferrule 10 and insulator 25, and is disposed between at least outer insulator surface 90 and first sidewalls 14 of first aperture 12. 
     Insulator 25 is most preferably formed of alumina (or aluminum oxide), but may be formed of any suitable electrically insulative, ceramic-containing material comprising, for example, sapphire or zirconium oxide. Under certain circumstances, inner insulator surface 85 and outer insulator surface 90 must have a suitable metal or alloy disposed thereon to permit insulator 25 to be brazed to pin 30 or to ferrule 10. 
     In a preferred embodiment of the present invention, where pure gold is employed to form inner and intermediate braze joints 65 and 15, a 25,000 Angstrom thick layer of niobium is sputtered onto surfaces 85 and 90 by vacuum deposition using a Model No. 2400 PERKIN-ELMER® sputtering system. The niobium layer is most preferably between about 15,000 and about 32,000 Angstroms thick. Metals other than niobium may be sputtered on surfaces 85 and 90, such as titanium or molybdenum. If metals such as: (i) gold alloys comprising gold and at least one of titanium, niobium, vanadium, nickel, molybdenum, platinum, palladium, ruthenium, silver, rhodium, osmium, iridium., and alloys, mixtures and thereof; (ii) copper-silver alloys, including copper-silver eutectic alloys, comprising copper and silver and optionally at least one of indium, titanium, tin, gallium, palladium, platinum; or (iii) alloys, mixtures or combinations of (i) or (ii) are employed, then metallization of surfaces 85 and 90 may not be required. 
     Electrically conductive outer braze joint 40 provides a platform for the attachment of outer adhesive joint 47 thereto. In preferred embodiments of the present invention, braze joint 40 is disposed between sidewalls 14 of first aperture 12 and outer surface 80 of capacitive filter 50. In other embodiments of the present invention, outer braze 40 is disposed atop ferrule 10 along the top peripheral surface thereof. Outer braze joint 40 need not, but may, provide a hermetic seal. 
     Electrically conductive outer adhesive joint 47 is preferably disposed between ferrule 10 and outer braze joint 40 on the one hand, and a second terminal or electrode of capacitive filter 50 on the other hand, and provides an adhesive joint therebetween. Outer adhesive joint 47 need not, but may, provide a hermetic seal. In some embodiments of the present invention, outer adhesive joint 47 permits a second terminal or electrode of capacitor 50 to be mechanically and electrically affixed by adhesive means to outer braze joint 40 and therefore to ferrule 10. 
     Alternatively, and in a manner not shown in the Figures, outer adhesive joint 47 may be connected to a second terminal or outer surface 80 of filter 50 as follows: Insulator 25 may have a wider outer diameter and ferrule 10 may have a lower profile (or lower or non-existent top portion 93) than those shown in FIGS. 1 and 2, such that the second terminal or outer surface 80 of filter 50 may engage, through outer adhesive 47, intermediate braze joint 15. In such a configuration, top portion 93 of ferrule 10 may be eliminated or shortened in height, and outer braze joint 40 may be eliminated altogether. 
     Electrically conductive inner adhesive joint 57 is disposed in third aperture or passageway 35 of capacitor 50 between contact pad 60 and inner braze joint 65, and provides an adhesive joint therebetween. Inner adhesive seal 57 need not, but may, provide a hermetic seal. Inner adhesive joint 57 permits a first terminal or electrode of capacitor 50 to be mechanically and electrically affixed by adhesive means to ferrule 10 through inner braze joint 65. 
     Inner braze joint 65 provides a braze joint and seal between insulator 25 and pin 30, and further forms a portion of an electrically conductive pathway extending between pin 30 and contact pad 60, the pathway comprising, but not necessarily limited to, pin 30, inner braze joint 65, inner adhesive joint 57 and contact pad 60. Inner braze joint 65 is disposed atop or at least partially surrounds upper portion 32 of pin 30. Inner braze joint 65 is also disposed between at least a portion of upper portion 32 of pin 30 and second sidewalls 85 (or inner insulator surface 85) of second aperture 27. 
     Inner braze joint 65, intermediate braze joint 15 and outer braze joint 40 are most preferably formed of the same metal or alloy, but may less preferably be formed of different metals or alloys. Braze joints 65, 15 and 40 of the present invention are most preferably formed of 99.9% or purer gold, but may also be formed of: (a) gold alloys comprising gold and at least one of titanium, niobium, vanadium, nickel, molybdenum, platinum, palladium, ruthenium, silver, rhodium, osmium, indium, and alloys, mixtures and thereof; (b) copper-silver alloys, including copper-silver eutectic alloys, comprising copper and silver and optionally at least one of indium, titanium, tin, gallium, palladium, platinum, and alloys, mixtures and combinations thereof; and (c) silver-palladium-gallium alloys. 
     Inner adhesive joint 57 and outer adhesive joint 47 are most preferably formed of the same adhesive material, but may less preferably be formed of different adhesive materials. In a preferred embodiment of the present invention, inner adhesive joint 57 and outer adhesive joint 47 are formed of ABLEBOND® 8700 electrically conductive silver-filled epoxy adhesive provided by ABLESTIK LABORATORIES of Rancho Dominguez, Calif. Other suitable electrically conductive glue- or epoxy-based adhesives and other suitable materials may also be employed in the present invention to form inner adhesive joint 57 or outer adhesive joint 47. Such materials include gold-or copper-filled epoxies, carbon- or graphite-filled epoxies or even electrically conductive plastics acting effectively as adhesive joints after their application and upon cooling, such as at least some of the electrically conductive plastics or polymers disclosed in U.S. Pat. No. 5,685,632 to Schaller et al. for &#34;Electrically Conductive Plastic Light Source.&#34; 
     In one embodiment of the present invention, contact pad 60 is electrically connected to internal circuitry disposed within container or shield 20 of hermetically sealed implantable medical device 70, and is also electrically and mechanically connected to pin 30 through inner braze joint 65 and inner adhesive joint 57. Electrical connection from internal circuitry to contact pad 60 may be established by wire bonding, soldering, welding, laser welding, brazing, gluing or other suitable means. 
     In another embodiment of the present invention, no contact pad 60 is disposed within third aperture 35, and electrical and mechanical connection to internal circuitry of device 70 is accomplished by attaching an electrical conductor directly to inner adhesive joint 57 or inner braze joint 65 through third aperture 35 by appropriate wire bonding, soldering, welding, laser welding, brazing, gluing or other suitable electrically conductive attachment means. 
     Contact pad 60 is most preferably formed of Kovar (an iron-nickel-cobalt alloy) having electroplated layers of first nickel and then gold disposed on the surface thereof. Contact pad 60 may also be formed of: (a) brass first plated with nickel and then gold; (b) pure gold; (c) suitable gold alloy plated with gold; (d) nickel plated with gold; (e) suitable nickel alloy plated with gold, and (f) pure copper or copper alloy first plated with nickel and then gold. Contact pad 60 must be electrically conductive and have a melting temperature exceeding the melting temperature of the solder employed to form inner adhesive joint 57 or outer adhesive joint 47. Additionally, the metal disposed on the outer surface of contact pad 60 must be compatible with the adhesive employed to form inner adhesive joint 57 or outer adhesive joint 47. 
     Ceramic-containing capacitive filter 50 attenuates and filters EMI to prevent the passage or propagation thereof into the interior of shield or container 20. Filter 50 has a third aperture or pathway 35 disposed through a portion thereof for electrical and mechanical connection of contact pad 60 to inner adhesive joint 57. Capacitive filter 50 is most preferably disposed at least partially in first aperture 12 such that ferrule 10 imparts additional mechanical integrity to the mechanical connection between filter 50 and ferrule 10. 
     Alternatively, capacitive filter 50 is disposed outside first aperture 12 in surface mount fashion such that first sidewalls 14 do not at least partially surround outer capacitive filter surface 80, or such that capacitive filter 50 is disposed atop ferrule 10. In those alternative embodiments of the present invention, however, it is generally required that outer adhesive joint 47 provide a mechanical and electrical bridge between outer braze joint 40 (or intermediate braze joint 15) and the second terminal or electrode of capacitive filter 50 or outer capacitive filter surface 80. 
     In preferred embodiments of the present invention, capacitive filter 50 is a discoidal multi-layer ceramic capacitor having a doughnut-like shape and a central cylindrically-shaped aperture 35 disposed through the center thereof. Capacitive filters forming discoidal multi-layer ceramic capacitors finding particularly efficacious application in the present invention are manufactured by AVX CORPORATION of Myrtle Beach, S.C., MAXWELL LABORATORIES of Carson City, Nev., CERAMIC DEVICE, INC. of Wenatchee, Wash., and SPECTRUM CONTROL, INC. of Erie, Pa. 
     Capacitive filters 50 comprising barium titanate have been discovered to provide particularly good results in the present invention. Examples of suitable barium titanate formulations or types for making capacitive filter 50 include, but are not limited to, X7R, Z5U and other formulations. Other types of ceramic capacitors may be employed for capacitive filter 50 of the present invention, such as single-layer capacitors, rectangular capacitors, square capacitors, elliptical capacitors, oval capacitors and the like. 
     In a preferred embodiment of the present invention, capacitive filter 50 is a discoidal multi-layer ceramic capacitor having silver thick films, silver-palladium alloy thick films, or silver-platinum alloy thick films disposed on inner capacitive filter surface 75 and outer capacitive filter surface 80. Such thick films are typically applied by the capacitive filter manufacturer before shipment. Inner capacitive filter surface 75 forms a first electrical terminal or contact of capacitive filter 50. 
     Outer capacitive filter surface 80 forms a second electrical terminal or contact of capacitive filter 50. When outer capacitive filter surface 80 is electrically connected to shield or container 20 and inner capacitive filter surface is electrically connected to circuitry or connectors external to container 20 of implantable medical device 70 through contact pad 60, capacitive filter 50 is connected in parallel with signals entering device 70, and thereby provides its EMI filtering capability. 
     Optionally, two more metal layers may be disposed on inner and outer surfaces 75 and 80 having silver thick films, silver-palladium alloy thick films, or silver-platinum alloy thick films disposed thereon to permit attachment of capacitive filter 50 to outer adhesive joint 47 and inner adhesive joint 57. First layers of nickel are preferably sputtered onto the thick films overlying inner surface 75 and outer surface 80. Next, second layers of gold are preferably sputtered onto the previously deposited nickel layers. The gold layers provide a means for adhesively attaching capacitive filter 50 to inner adhesive joint 57 and outer adhesive joint 47. Metals and alloys other than pure nickel may be employed for forming the first layers. Pure gold is preferred for forming the second layers, but gold of varying purities may less preferably be employed for forming the second layers. 
     In another embodiment of the present invention, gold, nickel, titanium, titanium-tungsten alloys, tungsten or molybdenum metal layers may be sputtered directly onto inner surface 75 or outer surface 80, with no thick films being disposed thereon. 
     In the sputtering step of the present invention, a DC magnetron sputtering technique is preferred, but RF sputtering techniques may less preferably be employed. A DC magnetron machine that may find application in the present invention is an Model 2011 DC magnetron sputtering device manufactured by ADVANCED ENERGY of Fort Collins Colorado. A preferred thickness for second layers formed of gold is about 10,000 Angstroms. A preferred thickness for first layers formed of nickel is about 25,000 Angstroms. 
     FIG. 3 shows a cross-sectional view of one embodiment of multi-polar feedthrough assembly 5 of the present invention after being subjected to the adhesive application and brazing steps of the present invention. FIG. 4 shows an enlarged view of a portion of multi-polar feedthrough assembly 5 of FIG. 3. FIG. 5 shows an exploded perspective view of portions of multi-polar feedthrough assembly 5 of FIGS. 3 and 4. 
     In FIGS. 3, 4 and 5 a plurality of insulators 25, feedthrough pins 30, capacitive filters 50, contact pads 60 and other components are disposed directly in ferrule 10. Spacers or washers 95 in FIGS. 3 and 4 are optional, and need not, but may, be included in assembly 5 if the head portion of pin 30 is appropriately shortened. Unitary multi-polar ferrule or cover 10 of FIGS. 3, 4 and 5 may be replaced with a plurality of separate ferrules that are disposed in and attached to a corresponding cover, substrate, container or shield. FIG. 4 shows inner braze joint 65, intermediate braze joint 15, outer braze joint 40, inner adhesive joint 57 and outer adhesive joint 47 of the present invention. It will now become apparent to those skilled in the art that many other embodiments and configurations of uni-polar and multi-polar feedthrough assemblies fall within the scope of the present invention. 
     FIG. 6 shows a perspective, cut-away view of the internal components of one embodiment of implantable medical device 70 of the present invention. In FIG. 6, a generic implantable pulse generator (or IPG) 70 is shown. IPG 70 includes battery section 100, hybrid electronics section or internal circuitry 75, and feedthrough assembly 5, all enclosed by can, shield or container 20. Conductor materials for feedthrough assemblies 5 are most preferably selected on the basis of their reported stability when in contact with body fluids. Feedthrough assembly may comprise one or more feedthroughs, and provides a hermetic seal for device 70. FIG. 7 shows a cross-sectional view of the implantable medical device of FIG. 6. 
     In one brazing step of the present invention, the metals or alloys employed to form braze joints 15, 40 and 65 must be heated to a temperature exceeding about 500 degrees Celsius. In the adhesive application step of the present invention, the electrically conductive adhesive materials employed to form adhesive joints 47 and 57 must be heated to a temperature that most preferably does not exceed about 200 degrees Celsius to cure the adhesives after their application. A preferred curing temperature for ABLEBOND 8700E adhesive has been found to be about 175 degrees Celsius for a duration of about 1 hour. Maximum curing temperatures for suitable adhesives of the present invention are less than about 500 degrees Celsius and greater than about 15 degrees Celsius. A preferred range of adhesive curing temperatures of the present invention is between about room temperature and about 250 degrees Celsius. A preferred range of adhesive curing times or durations of the present invention is between about 1 minute and about 24 hours. 
     In a preferred method of the present invention, the brazing step occurs at peak temperatures of about 1,090 degrees Celsius, where feedthrough assembly 5 is held and soaked at that peak temperature for about 40 seconds following a preferred heating ramp-up period of about 1 hour during which time assembly 5 is taken from room temperature to the peak temperature. Additionally, it is preferred that assembly 5 be pre-soaked at a temperature of about 1,050 degrees Celsius for about 2 minutes to stabilize temperatures throughout the brazing furnace and graphite fixture within which assembly 5 is held during the brazing step. 
     A preferred cooling ramp-down period following the peak temperature brazing period is also about one hour. Preferred ramp-up and ramp-down periods of the brazing step of the method of the present invention range between about 20 minutes and about 6 hours. The peak temperature of the brazing step of the method of the present invention is most preferably about 50 degrees Celsius above the melting temperature of the brazing metal or alloy selected, but may range as low as the melting temperature of the brazing metal or alloy selected. 
     A preferred furnace for the brazing step of the present invention is a Model No. 3040 WORKHORSE® furnace manufactured by VACUUM INDUSTRIES® of Sommerville, Mass. It is preferred that the brazing step of the present invention occur in a vacuum or inert atmosphere. If a vacuum is employed in the brazing step, pressures less than about 8×10 -5  Torr are preferred prior to initiating brazing. Much less preferably, and owing to the resultant excessive oxidation of the pin and ferrule, the brazing step of the present invention may occur in air or other non-inert atmosphere. 
     In a preferred method of the present invention, the adhesive application step occurs at room temperature, where feedthrough assembly 5 is held at room temperature while a suitable adhesive is applied to inner braze joint 65 and outer braze joint 45. Assembly 5 is then cured at elevated temperatures of about 175 degrees Celsius for about one hour, most preferably in a Model No. OV-12A oven provided by Blue M Electric Company of Blue Island, Ill. 
     FIG. 8 shows a flow chart of one method of the present invention. In FIG. 8, ferrule or cover 10, pin 30, insulator 25, and braze joint pre-forms corresponding to inner braze joint 65, intermediate braze joint 15 and outer braze joint 40 are provided. The foregoing components are assembled in a braze fixture, and most preferably in a graphite braze fixture. Next, assembled ferrule or cover 10, pin 30, insulator 25, and braze joint pre-forms corresponding to inner braze joint 65, intermediate braze joint 15 and outer braze joint 40 are heated to an appropriate brazing temperature exceeding about 500 degrees Celsius in a brazing step. Following the brazing step, capacitive filter 50 is placed on or in the brazed assembly. A suitable electrically conductive adhesive is applied in third aperture 35, which upon subsequent curing forms inner adhesive joint 57. The same or a different suitable electrically conductive adhesive is then preferably applied between outer braze joint 40 and outer surface 80 of capacitor 50, which upon subsequent curing forms outer adhesive joint 47. (In another method of the present invention, the same or a different suitable electrically conductive adhesive is applied between intermediate braze joint 15 and outer surface 80 of capacitor 50, which upon subsequent curing forms outer adhesive joint 47.) The foregoing components, adhesives and brazed assembly are heated to an appropriate temperature, typically less than or equal to about 200 degrees Celsius in an adhesive curing step. 
     In the present invention, it has been discovered that most circumstances it is difficult to apply a continuous, unbroken bead of adhesive between outer braze joint 40 and outer surface 80. Moreover, leaktightness testing of intermediate braze joint 15 may be compromised if a continuous bead of adhesive is employed to form adhesive joint 47. (Such a continuous bead might indicate leaktightness even though braze joint 15 is not leaktight.) For the foregoing reasons, and as illustrated in FIG. 2 hereof, adhesive joint 47 is most preferably formed by a plurality of adjoining separated strips or portions of adhesive. 
     FIG. 9 shows a graph of EMI insertion loss data obtained with capacitive filter feedthroughs of the present invention disposed within conventional pacemakers. FIG. 9 shows test results obtained employing one embodiment of the present invention, where a capacitive filter feedthrough contained gold braze joints or pads 15, 40 and 65 in combination with adhesive joints 47 and 57; test results for this embodiment of the present invention are labeled &#34;gold pad 40&#34; in FIG. 9. FIG. 9 shows test results for another less preferred embodiment of the present invention, where a capacitive filter feedthrough contained gold braze joints or pads 15 and 65 in combination with adhesive joints 47 and 57, but had no gold pad or joint 40 disposed thereon; test results for that embodiment of the present invention are labeled &#34;no gold pad 40&#34; in FIG. 9. 
     Insertion loss is a measurement of the attenuation of unwanted signals such as EMI. Insertion loss was measured using a spectrum analyzer that generated ac signals having frequencies ranging between 0 and 2.9 Gigahertz. Analyzer output signals were applied to feedthrough pins 30 by a first cable. The analyzer received input signals through a second cable connected to contact pad 60. We define insertion loss here as: ##EQU1## where: E 1  =output voltage with feedthrough in the circuit 
     E 2  =output voltage with feedthrough not in the circuit 
     The insertion loss curves of FIG. 9 were generated by sweeping test frequencies between 0 and 2.9 Gigahertz and simultaneously measuring insertion loss. FIG. 9 shows that feedthrough assemblies of the present invention attenuate EMI significantly. 
     Although only a few exemplary embodiments of the present invention have been described in detail above, those skilled in the art will appreciate readily that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of the invention. Accordingly, all such modifications are intended to be included within the scope of the present invention as defined in the following claims. For example, any one of inner adhesive joint 57, outer adhesive joint 47, intermediate braze joint 15, inner braze joint 65, or outer braze joint 40 of the present invention may be replaced with a suitable electrically conductive plastic or polymer containing or blended with silver flakes or a suitable electrically conductive epoxy or filler. 
     The scope of the present invention is not limited to pacing, monitoring or sensing applications, but extends to defibrillation, cardiac mapping and other medical and medical device applications and methods. The scope of the present invention is not limited to applications where a human heart is sensed, monitored, paced, or defibrillated, but includes similar applications in other mammalians and mammalian organs. 
     The preceding specific embodiments are illustrative of the practice of the invention. It is to be understood, therefore, that other expedients known to those skilled in the art or disclosed herein, may be employed without departing from the invention or the scope of the appended claims. 
     In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts a nail and a screw are equivalent structures. 
     All patents listed in Table 1 or elsewhere hereinabove are hereby incorporated by reference into the specification hereof, each in its respective entirety.