Abstract:
A metallization that includes a composite of alternating metal and metal oxide layers for incorporation into feedthrough filter capacitor assemblies is described. The feedthrough filter capacitor assemblies are particularly useful for incorporation into 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.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. provisional application Ser. No. 61/419,374, filed on Dec. 3, 2010. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a hermetic feedthrough terminal pin assembly, preferably of the type incorporating a filter capacitor. More specifically, this invention relates to metallization comprising oxidized titanium for incorporation into feedthrough filter capacitor assemblies, particularly 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 metallization provides a surface with which a hermetic seal can be established that prevents passage or leakage of fluids through the feedthrough assembly and into the medical device. 
     2. Prior Art 
     Feedthrough assemblies are generally well known in the art for use in connecting electrical signals through the housing or case of an electronic instrument. For example, in an implantable medical device, such as a cardiac pacemaker, defibrillator, or neurostimulator, the feedthrough assembly comprises one or more conductive terminal pins supported by an insulator structure for passage of electrical signals from the exterior to the interior of the medical device. The conductive terminals are fixed into place using a metallization and gold braze process, which provides a hermetic seal between the pin and insulative material. 
     Conventionally, a metallization is composed of a combination of discrete layers of untreated titanium metal and molybdenum or titanium metal and niobium have been used to facilitate bonding of the gold to the insulative material. Untreated titanium metal is widely used as an adhesion layer to provide bonding between a ceramic material, particularly that of alumina, and a different metal. However, the untreated titanium metal typically reacts with gold to form an intermetallic alloy. Intermetallic alloy metals such as those formed by the combination of titanium and gold, typically result in an undesirable brittle bond which may result in loss of hermeticity. Titanium metal is known to have a high diffusion coefficient in liquid gold which increases its tendency to diffuse within gold and form these intermetallic alloy phases. Typically when such metals are brazed, the titanium metal departs or lifts from the surface of the insulator material and forms an intermetallic alloy with the gold braze material. 
     As a result, a barrier layer comprising molybdenum or niobium is applied to the outer surface of the titanium. This additional layer is designed to act as a barrier layer to prohibit the migration of titanium from the surface of the insulator material and thus prevent the formation of a titanium and gold intermetallic. While materials such as molybdenum and niobium typically provide adequate metallization barrier layers, recent work has been focused on an improved metallization layer through incorporation of an oxidized layer of titanium as a means to facilitate bonding of ceramic with that of a metal with minimized migration of the metallization layer. The diffusion rate for the oxidized titanium in gold is less than that of the untreated titanium. Therefore, the metallization comprising the oxidized metal is less likely to lift from the surface of the insulator and form an intermetallic phase with the gold braze material. 
     SUMMARY OF THE INVENTION 
     In a preferred form, a feedthrough filter capacitor assembly according to the present invention comprises an outer ferrule hermetically sealed to either an alumina insulator or fused glass dielectric material seated within the ferrule. The insulative material is also hermetically sealed to at least one terminal pin. 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/terminal pin interfaces. 
     According to the invention, the metallization used to facilitate the formation of the hermetic seal of a feedthrough assembly preferably comprises a composite of alternating layers of metal and metal oxide. Specifically, a layer of a first metal, particularly titanium, is deposited on the surface of an insulator material, such as alumina. The layer of the first metal is then subsequently subjected to a heat treatment process that transforms a portion of the metal layer into an oxidized metal layer establishing a metallization layer comprised of two distinct layers, one being that of a layer of metal, the other being a layer of oxidized metal. In an alternate embodiment, additional layers of alternating metal and metal oxide may be layered onto the second metal oxide layer. 
     The metal and oxidized layered metallization provides improved bonding to the surface of the insulator which is less susceptible to metallization migration. The metallization is also biocompatible and, therefore, provides a long term bonding interface that is immune to the body. 
     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  illustrates a perspective view of an embodiment of a feedthrough assembly. 
         FIG. 2  shows a cross-sectional view of the feedthrough assembly taken along line  2 - 2  of  FIG. 1 . 
         FIG. 3  illustrates a cross-sectional view of the feedthrough assembly taken along line  3 - 3  of  FIG. 2 . 
         FIG. 4  illustrates an embodiment of the present invention of a metallization layer comprising a first metal layer and a second metal oxide layer. 
         FIG. 5  shows an alternate embodiment of the present invention of a metallization layer comprising a first metal layer, a second metal oxide layer, a third metal layer and a fourth metal oxide layer. 
         FIG. 6  is an illustration of a cross-sectional view of an embodiment of a brazed terminal pin. 
         FIG. 7  is a photograph depicting a cross-sectional view of an embodiment of a brazed terminal pin. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings,  FIGS. 1 through 3  illustrate an internally grounded feedthrough capacitor assembly  10  comprising a feedthrough  12  supporting a filter discoidal capacitor  14 . The feedthrough filter 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  includes terminal pins  16  that provide for coupling, transmitting and receiving electrical signals to and from a patient&#39;s heart, while hermetically sealing the interior of the medical instrument against ingress of patient body fluids that could otherwise disrupt instrument operation or cause instrument malfunction. 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. 
     More particularly, the feedthrough  12  of the feedthrough filter capacitor assembly  10  comprises a ferrule  18  defining an insulator-receiving bore formed by a ferrule sidewall extending from a first ferrule end  18 A to a second ferrule end  18 B, the ferrule sidewall surrounding an insulator  20 . Suitable electrically conductive materials for the ferrule  18  include titanium, tantalum, niobium, stainless steel or combinations of alloys thereof, the former being preferred. The ferrule  18  may be of any geometry, non-limiting examples being round, rectangle, and oblong. 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 or end  26  and a second lower side or end  28 , The insulator  20  is also provided with bores  30  that receive the terminal pins  16  passing therethrough. A layer of metal  32 , referred to as metallization, is applied to the insulator sidewall  24  and to a bore sidewall  33  of the terminal pin bores  30  to aid a braze material  34  in hermetically sealing between the ferrule  18  and the outer sidewall  24  of the insulator  20  and between the terminal pins  16  and the bore sidewall  33  of the insulator  20 , respectively. Specifically, the metallization layer  32  is preferably applied to a portion of the outer surface of the insulator sidewall  24  and to a portion of the surface of the inside sidewall  33  of the terminal pin bores  30 . These surfaces are intended to contact and bond with the ferrule  18  and terminal pins  16 , respectively, of the feedthrough assembly  10 , establishing a hermetic seal therebetween. 
     According to one embodiment of the present invention, as shown in  FIGS. 4 and 5 , the metallization  32  comprises a composite of a first metal layer  50  and a second metal oxide layer  52 . As illustrated, the second metal oxide layer  52  resides on a first metal top surface  54  of the first metal layer  50 , the metal layer  50  being deposited on a surface of the insulator  20 . More specifically, the second metal oxide layer  52  is bonded to the first metal top surface  54  of the first metal layer  50  which is bonded to a portion of a surface of the insulator sidewall  24  and/or a portion of a surface of the bore sidewall  33 . 
     In a preferred embodiment, the first metal layer  50  is composed of titanium and titanium alloys. The second metal oxide layer  52  is preferably composed of oxidized titanium or oxidized titanium alloys thereof. Examples of titanium oxide may comprise Ti 2 O 3  or TiO 2 . Although the use of titanium and its associated alloys are preferred, it is contemplated that other metals such as molybdenum, niobium, tungsten, aluminum, vanadium and their associated alloys may also be used as the first metal layer  50 . Furthermore, the associated oxides of these metals or their associated alloys may comprise the second metal oxide layer  52 . 
     In a preferred embodiment, the metallization  32  has an overall thickness ranging from about 0.01 um to about 25 um. In a more preferred embodiment, the metallization  32  has a thickness ranging from about 0.50 um to about 5.0 um. Most preferably, the metallization  32  has a thickness ranging from 1.0 um to about 2.0 um. The thickness of the second metal oxide layer  52  comprises from about 25 percent to about 50 percent of the total metallization layer thickness. The thickness of the first metal layer  50  comprises from about 50 percent to about 75 percent of the total metallization layer thickness. Therefore, the second metal oxide layer  52  may comprise a thickness ranging from about 0.0025 um to about 12.5 um, more preferably from about 0.25 um to about 1.0 um. 
     In a preferred embodiment, the first metal layer  50  is initially applied to the surface of the insulator  20 . The first metal layer  50  may be applied to the surface of the insulator  20  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, and aerosol spray deposition, and thin cladding. Once the first metal layer  50  is applied to the surface of the insulator  20 , the metalized substrate is subsequently heat treated in an ambient atmosphere. Although the use of an ambient atmosphere is preferred, the metalized insulator may also be heat treated in an oxygen rich atmosphere. Such an oxygen rich atmosphere may be used to control the attributes of the second metal oxide layer  52  such as its thickness, density, molecular oxygen ratio, and/or morphology of the oxide layer. “Morphology” is herein defined as the texture of a surface, such as that of the second oxide layer  52 . The oxide layer  52  may have a surface morphology that is smooth, rough or undulated. A “smooth” morphology is primarily characterized by a planar surface. A “rough” morphology is primarily characterized by a surface comprising jagged edges and an “undulated” morphology is primarily characterized by a surface comprising a series of elevated regions. 
     In a preferred embodiment, the titanium is heat treated at a temperature ranging from about 600° C. to about 1,000° C. for about 30 minutes to about 180 minutes. More preferably, the first metal layer  50  is heat treated at a temperature ranging from about 750° C. to about 850° C. for about 60 to about 120 minutes. This heat treating process preferably diffuses oxygen into the chemical structure of the first metal layer  50 , thereby transforming a portion of the metal into a layer of oxidized metal, such as that of the second metal oxide layer  52 , as shown in  FIG. 4 . It is noted that the heat treatment process may be performed within the metal deposition instrument or outside the metal deposition instrument, such as in a heat treating oven (not shown). For example, the first metal layer  50  may be applied using sputtering and subsequently heat treated within the sputtering chamber of the instrument, thereby eliminating the need to remove the metalized insulator  20 . In a specific example, the surface of an alumina insulator  20  is metalized with about 1.5 um of titanium, forming the first metal layer  50 . The metalized layer of titanium is then heat treated at about 800° C. for about 90 minutes to form the second metal oxide layer  52  of titanium oxide. 
     In a second embodiment, as illustrated in  FIG. 5 , a third metal layer  58  may be applied to the second metal oxide layer  52 . More specifically, an additional layer of metal, such as titanium, may be applied to a surface  56  of the second metal oxide layer  52 . Although titanium and its associated alloys are preferred, the third metal layer  58  may comprise other metals comprising molybdenum, niobium, tungsten, aluminum, vanadium and their associated alloys. 
     In a preferred embodiment, the third metal layer  58  having a thickness ranging from about 0.01 um to about 5.0 um, more preferably a thickness ranging from about 0.10 um to about 2.0 um is deposited on the surface  56  of the second metal oxide layer  52 . The insulator  20 , now comprising an additional third metal layer  58 , is again heat treated in an ambient atmosphere at a temperature ranging from about 200° C. to about 500° C. for about 10 minutes to about 60 minutes. More preferably the insulator  20 , comprising the first metal layer  50 , the second metal oxide layer  52  and the third metal layer  58 , is heat treated at a temperature ranging from about 300° C. to about 400° C. for about 30 minutes. Similarly to the first heat treatment, as previously discussed, an oxygen rich atmosphere may also be used. 
     The second heat treatment process preferably forms a fourth metal oxide layer  62  that resides on a surface  60  of the third metal layer  58 . More specifically, the fourth metal oxide layer  62  is chemically bonded to the surface  60  of the third metal layer  58 . Therefore, as shown in  FIG. 5 , the metallization  32  comprises a four layer composite comprising the first metal layer  50 , the second metal oxide layer  52 , the third metal layer  58  and the fourth metal oxide layer  62 . It is contemplated that the metallization layer  32  could also be constructed with additional alternating layers of similar or dissimilar metals and metal oxides. 
     In a specific example of the second embodiment of the present invention, the surface of an alumina insulator  20  is metalized with a first metal layer  50  of titanium with a thickness of about 0.5 um. The first layer of titanium is then heat treated at about 800° C. for about 90 minutes to form the second metal oxide layer  52 . After the first heat treatment, an additional layer of about 1.0 um of titanium, i.e., the third metal layer  58  is applied to the surface  56  of the second layer of titanium oxide. This third metal layer  58  of titanium is then heat treated a second time at about 350° C. for about 30 minutes. 
     Similar to the application of the first metal layer  50 , the third metal layer  58  and subsequent metal layers may be applied using 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, and aerosol spray deposition, and thin cladding. 
     The composite construction comprising alternating layers of metal and metal oxide establish a metallization layer  32  with improved bonding characteristics, particularly compared to those comprising distinct layers of titanium and molybdenum as well as titanium and niobium. The composite metal and oxide layers are bonded together such that diffusion of the metal layer, particularly that of titanium, into the gold braze material is impeded. In other words, the present invention provides a layered metallization  32  that provides improved boding between the insulator  20  and the first metal layer  50  as well as the oxide metal layers  58 ,  62  comprising the top layer of the metallization  32 , and the braze material. 
     As illustrated in  FIGS. 6 and 7 , the first metal layer  50 , at the bottom side of the metallization, is shown forming a bond between the insulator material. In an embodiment, a bond comprising TiAl, Ti 3 Al or combinations thereof is preferably formed between the first metal layer  50  and the surface of the insulator  20 . Likewise at the opposite side or top layer of the metallization  32 , a bond between the oxidized metal, particularly oxidized titanium and gold is formed. 
     Referring to  FIGS. 1 through 3 , non-limiting examples of terminal pins  16  include platinum, platinum alloys, particularly platinum-iridium alloys, palladium and palladium alloys. Furthermore, it is contemplated that the terminal pin  16  may comprise an exterior outer terminal pin coating or layer of platinum, platinum alloys, gold, silver, palladium and palladium alloys. The core terminal pin material may be selected from the group consisting of niobium, tantalum, nickel-titanium (NITINOL®), titanium, particularly beta titanium, titanium alloys, stainless steel, molybdenum, tungsten, platinum, and combinations thereof. The means of coating may include sputtering, cladding, and or plating. The coating may be applied through a process of 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. 
     In addition, 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. 
     As further shown in  FIGS. 1 through 3 , the feedthrough filter capacitor  10  includes the 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  36  having multiple capacitor-forming conductive electrode plates formed therein. The capacitor dielectric  36  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  38  in spaced relationship with a plurality of spaced apart layers of second or “ground” electrode plates  40 . The filter capacitor  14  is preferably joined to the feedthrough  12  adjacent to the insulator side  26  by an annular bead  42  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  38  by a conductive braze material  46  contacting between the terminal pins  16  and the bore metallization. In a similar manner, the ground plates  40  are electrically connected through an outer surface metallization  48  and the conductive material  42  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.