Abstract:
A filter capacitor comprising a substrate of at least one layer of a low temperature co-fires ceramic (LTCC) tape supporting alternating active and ground electrode layers segregated by a dielectric layer is described. The substrate is preferably a laminate of three LTCC tapes pieces that are heated under pressure and at a relatively low temperature to become a laminate that maintains its shape and structure dimensions even after undergoing numerous sintering steps. Consequently, relatively thin active and ground electrode layers along with the intermediate dielectric layer can be laid down or deposited on the LTCC substrate by a screen-printing technique. A second laminate of LTCC tapes is positioned on top of the active/dielectric/ground layers to finish the capacitor. Consequently, a significant amount of space is saved in comparison to a comparably rated capacitor or, a capacitor of a higher rating can be provided in the same size as a conventional prior art capacitor.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from provisional application Ser. No. 60/782,120, filed Mar. 14, 2006. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates generally to simplified feedthrough filter capacitor assemblies and related methods of construction. Feedthrough filter capacitor assemblies are used to decouple and shield undesirable electromagnetic interference (EMI) signals from implantable medical devices and other electronic devices. 
         [0004]    More specifically, this invention relates to simplified and reduced cost filter capacitors for ceramic feedthrough terminal pin assemblies. In the present invention, the filter capacitor assembly is mounted to the ceramic feedthrough terminal pin assembly. The ceramic feedthrough is used to connect a terminal pin or electrode through a hermetically sealed housing to internal electronic components of the medical device while the filter capacitor decouples EMI signals against entry into the sealed housing via the terminal pin. This invention is particularly designed for use in cardiac pacemakers (bradycardia devices), cardioverter defibrillators (tachycardia), neurostimulators, internal drug pumps, cochlear implants and other medical implant applications. This invention is also applicable to a wide range of other EMI filter applications, such as military or space electronic modules, where it is desirable to preclude entry of EMI signals into a hermetically sealed housing containing sensitive electronic circuitry. 
         [0005]    In that respect, feedthrough terminal pin assemblies are generally well known in the arts for connecting electrical signals through the housing or case of an electronic instrument. For example, in implantable medical devices such as cardiac pacemakers, defibrillators, and the like, the feedthrough assembly comprises one or more conductive terminal pins supported by an insulator structure for feedthrough passage from the exterior to the interior of the medical device. Many different insulator structures and related mounting methods are known in the art for use in medical devices wherein the insulator structure provides a hermetic seal to prevent entry of body fluids into the housing thereof. However, the feedthrough terminal pins are typically connected to one or more lead wires leading to a body organ such as a heart. The lead wires effectively act as an antenna and tend to collect stray EMI signals for transmission into the interior of the medical device. That is why hermetic feedthrough assemblies are combined with a ceramic feedthrough filter capacitor to decouple interference signals to the medical device housing. However, a primary feature of the simplified feedthrough filter capacitor described herein is volume reduction without compromising effectiveness and reliability. This is accomplished by elimination of some of the volume previously occupied by the capacitor dielectric using new screen printing techniques. The present feedthrough filter capacitor is also less costly to manufacture than a comparably rated prior art capacitor. 
         [0006]    2. Prior Art 
         [0007]    In a typical prior art unipolar construction for a feedthrough filter capacitor, such as described in U.S. Pat. No. 5,333,095 to Stevenson et al., a round/discoidal (or rectangular) ceramic feedthrough filter capacitor is combined with a hermetic feedthrough terminal pin assembly. In use, the coaxial capacitor permits passage of relatively low frequency electrical signals along the terminal pin while shielding and decoupling/attenuating undesired interference signals of relatively high frequency to the conductive housing of the medical device. 
         [0008]    One type of hermetic feedthrough terminal pin subassembly widely used in implantable medical devices employs an alumina ceramic insulator which, after sputtering/metallization procedures, is gold brazed into a titanium ferrule. In addition, there are terminal pins, typically made of platinum, which are also gold brazed to the alumina ceramic insulator to complete the hermetic seal. See for example, the subassemblies disclosed in U.S. Pat. Nos. 3,920,888 to Barr; 4,152,540 to Duncan et al.; 4,421,947 to Kyle and 4,424,551 to Stevenson et al. Separately, the feedthrough filter capacitor is constructed by preassembly of the coaxial capacitor and then mounting it onto or within the cylindrical or rectangular hermetically sealed feedthrough terminal pin subassembly including the conductive pins and ferrule. 
         [0009]    The feedthrough capacitor is of a coaxial construction having two sets of electrode plates embedded in spaced relation within an insulative dielectric substrate or base. The dielectric base is typically formed as a ceramic monolithic structure. One set of “active” electrode plates is electrically connected at an inner diameter cylindrical surface to a terminal pin in a unipolar (one terminal pin) construction. Feedthrough capacitors are also available in bipolar (two), tripolar (three), quadpolar (four), pentapolar (five), hexpolar (six), and additional terminal pin configurations. The inner active plates are coupled in parallel together by a metallized layer which is either glass frit fired or plated onto the ceramic capacitor. This metallized band, in turn, is mechanically and electrically coupled to the terminal pin by a conductive adhesive or soldering, and the like. 
         [0010]    The other or second set of “ground” electrode plates is coupled at an outer diameter surface of the discoidal capacitor to a cylindrical ferrule of conductive material. The outer ground plates are coupled in parallel together by a metallized layer which is fired, sputtered or plated onto the ceramic capacitor. This metallized band, in turn, is coupled to the ferrule by conductive adhesive, soldering, brazing, welding, and the like. The ferrule is electrically connected in turn to the conductive housing of the electronic device. 
         [0011]    The device housing is constructed from a biocompatible metal such as of a titanium alloy, which is electrically and mechanically coupled to the hermetic feedthrough terminal pin subassembly, which is, in turn, electrically coupled to the feedthrough filter capacitor. As a result, the filter capacitor coupled to the feedthrough terminal pin subassembly prevents entrance of interference signals to the interior of the device housing, where such interference signals could otherwise adversely affect the desired device function such as cardiac pacing or defibrillation. 
         [0012]    Although feedthrough filter capacitor assemblies of the type described perform in a generally satisfactory manner, the associated manufacturing and assembly costs are unacceptably high. One area where costs can be reduced is in the manufacture of the feedthrough filter capacitor. 
         [0013]      FIG. 1  illustrates a typical prior art feedthrough filter capacitor assembly  10  comprising a filter capacitor  12  mounted to a feedthrough terminal pin subassembly  14 . The filter feedthrough capacitor assembly  10  is shown in one preferred form comprising a so-called bipolar configuration having two separate conductive terminal pins  16  extending through the discoidal-shaped filter capacitor  12  and feedthrough terminal pin subassembly  14 . 
         [0014]    The feedthrough terminal pin subassembly  14  comprises a ferrule  18  defining an insulator-receiving bore  20  surrounding an insulator  22 . Suitable electrically conductive materials for the ferrule substrate  18  include titanium, tantalum, niobium, stainless steel or combinations of alloys thereof. Titanium is preferred for the ferrule  18 , which may be of any geometry, non-limiting examples being round, rectangle, and oblong. A surrounding inwardly facing annular channel  24  is provided in the ferrule  18  to facilitate attachment of the feedthrough filter capacitor assembly  10  to the casing  26  of, for example, the implantable medical device. The method of attachment may be by laser welding or other suitable methods. 
         [0015]    The insulator  22  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  22  is alumina, which is highly purified aluminum oxide, and comprises a sidewall  22 A extending to a first upper surface  22 B and a second lower surface  22 C. A layer of metal  28 , referred to as metallization, is applied to the sidewall  22 A of the insulating material  22  to aid in the creation of a brazed hermetic seal. Suitable metallization materials  28  include titanium, niobium, tantalum, gold, palladium, molybdenum, silver, platinum, copper, carbon, carbon nitride, titanium nitrides, titanium carbide, iridium, iridium oxide, tantalum, tantalum oxide, ruthenium, ruthenium oxide, zirconium, and mixtures thereof. The metallization layer  28  may be applied by various means including, but not limited to, sputtering, e-beam deposition, pulsed laser deposition, plating, electroless plating, chemical vapor deposition, vacuum evaporation, thick film application methods, aerosol spray deposition, and thin cladding. 
         [0016]    The insulator  22  has a sufficient number of bores  30  to receive the terminal pins  16 . The inner surfaces of these bores  30  are provided with a metallization layer  32  in a similar manner as the previously described insulator sidewall  22 A. The terminal pins  16  are then received in the bores  30 . Preforms (not shown) of a conductive, biostable material, such as gold or gold alloy, are moved over the terminal pins  16  to rest against the upper insulator surface  22 B adjacent to annular notches  34  in the insulator. Similarly, a gold preform (not shown) is positioned at the junction of the ferrule insulator-receiving bore  20  and the upper insulator surface  22 B. The thusly-assembled feedthrough terminal pin assembly  14  is then heated in an oven or furnace to melt the preforms and cause them to form their respective brazes  36  and  38 . Braze  36  hermetically seals the terminal pins  16  to the insulator  22  at the terminal pin bores  30  while braze  38  hermetically seals the insulator  22  to the ferrule  18  at the insulator-receiving bore  20 . 
         [0017]    The feedthrough filter capacitor  12  comprises a dielectric  40  formed from multiple layers of a tape cast ceramic or ceramic-based material containing multiple capacitor-forming conductive first “active” electrode layers  42  and second “ground” electrode layers  44  screen-printed in an alternating manner on top of the tape cast dielectric. This layered assembly is then sintered to provide a monolithic dielectric body containing the electrode layers  42 ,  44 . Although the exemplary drawing shows in exaggerated scale a pair of the active electrode layers  42  in parallel staggered relation with a corresponding pair of the ground electrode layers  44 , it will be understood that a large plurality of typically 5 to 40 conductive layers  42  can be provided in alternating stacked and parallel spaced relation with a corresponding number of the conductive layers  44 . 
         [0018]    Each of the active electrode layers  42  is subdivided into two spaced-apart and generally pie-shaped electrode plates (not shown). Accordingly, the two electrode plates  42  of each layer group are electrically insulated from each other by the dielectric material  40  of the capacitor  12 . The multiple spaced-apart layers of the active electrode plates  42  are formed in stacked alignment with the respective active electrode plates  42  of overlying and underlying layers to define two respective active plate stacks. The two terminal pins  16  pass generally centrally through respective bores  46  formed in these active plate stacks, and are conductively coupled to the associated stacked set of active electrode plates  42  by a suitable conductive surface lining such as a surface metallization layer  48  lining each bore  46 . 
         [0019]    A plurality of spaced-apart layers of the second or “ground” electrode plates  44  are also formed within the capacitor  12 . The ground electrode plates  44  are in stacked relation alternating or interleaved with the layers of active electrode plates  42 . These ground electrode plates  44  include outer perimeter edges which are exposed at the outer periphery of the dielectric body  18  where they are electrically connected in parallel by a suitable conductive surface such as a surface metallization layer  50 . Importantly, however, the outer edges of the active electrode plates  42  terminate in spaced relation with the outer periphery of the capacitor body  12 . In that manner, the active electrode plates  42  are electrically isolated by the dielectric body  40  from the conductive layer  50  coupled to the ground electrode plates  44 . Similarly, the ground electrode plates  44  have inner edges which terminate in spaced relation with the terminal pin bores  46 , whereby the ground electrode plates  44  are electrically isolated by the dielectric body  40  from the terminal pins  16  and the conductive metallization layer  48  lining the pin bores  46 . The number of active and ground electrode plates  42  and  44 , together with the dielectric  18  thicknesses or spacing there between may vary in accordance with the desired capacitance value and voltage rating. 
         [0020]    The feedthrough capacitor assembly  10  is constructed by moving the filter capacitor  12  over the feedthrough terminal pin subassembly  14 . A non-conductive disk-shaped member  52  is positioned about the terminal pins  16  at a location sandwiched between the upper insulator surface  22 B of the feedthrough terminal pin subassembly  14  and the bottom of the capacitor terminal pin bores  46 . In this position, the non-conductive disc  52  supports the capacitor  12  in spaced relation above the insulating material  22 . The metallized surface  48  within the terminal pin bores  46  is then connected electrically to the terminal pins  16  by means of a conductive adhesive bead  54 , or by soldering or brazing or the like. In a preferred form, the conductive adhesive  54  is applied to the annular gap between the pins  16  and the capacitor metallized surface  48 , and allowed to fill a portion (about one-half) of the gap length. Similarly, the metallized surface  50  associated with the ground electrode plates  44  of the capacitor  12  is connected electrically to the ferrule  18  by means of an additional fillet  56  of conductive adhesive or the like. One preferred conductive adhesive comprises a curable polyimide adhesive loaded with conductive particles such as spheres or flakes, as described by way of example in U.S. Pat. No. 4,424,551 to Stevenson et al., which is incorporated by reference herein. However, it will be understood that other conductive connecting means may be used, such as solder, braze or the like. Importantly, the adhesive beads  54 ,  56  establish an electrically conductive mounting of the capacitor  12  in a secured stable manner to the feedthrough terminal pin assembly  14 . 
         [0021]    Mechanically, the barium titanate material typically used as the capacitor dielectric material  40  is relatively weak and prone to fracture. Also, if the dielectric material  40  is not sufficiently thick, it tends to warp into a “potato chip” shape upon being heated during sintering of the tape cast material. That is why in the prior art filter feedthrough capacitor assembly  10 , the thickness of the respective lower and upper dielectric zones  58  and  60  below and above the intermediate zone occupied by the electrode layers  42 ,  44  is from about 0.007 inches to 0.015 inches, preferably about 0.010 inches. In other words, the thickness from the lower surface of the dielectric  18  adjacent to the non-conductive disc  52  to the lowest ground plate  44  is about 0.010 inches and the distance from the upper active plate  42  to the upper surface of the dielectric body  18  also about 0.010 inches. Depending on the number of active and ground electrode plates and the spacing between them, the intermediate zone in the dielectric body supporting the electrode plates  42 ,  44  is typically a minimum of about 0.010 inches, to much more. Therefore, the capacitor  12  is generally of a minimum thickness of about 0.030 inches and in certain applications can be significantly greater. The bulk of this thickness is occupied by the lower and upper dielectric zones  58 ,  60  sandwiching the intermediate zone of the electrode plates and each being about 0.010 inches thick. If these dielectric zones  58 ,  60  could be made thinner without compromising function and reliability including planarity, significant size reductions could be realized. 
         [0022]    Accordingly, there is a need for a novel feedthrough filter capacitor assembly that addresses the drawbacks noted above in connection with the prior art. In particular, a novel capacitor assembly is needed which significantly reduces the volume occupied by this assembly, or is of a comparable volume, but of a significantly higher capacitance rating, without any diminution in filtering performance or reliability and yet that may be utilized in many of the same applications where such subassemblies are now found. Additionally, the improved feedthrough filter capacitor assembly should lend itself to standard manufacturing processes so that cost reductions can be realized immediately. Of course, the new design must be capable of effectively filtering out undesirable electromagnetic interference (EMI) signals from the target device. The present invention fulfills these needs and provides other related advantages. 
       SUMMARY OF THE INVENTION 
       [0023]    In a preferred form, a feedthrough terminal pin filter assembly according to the present invention comprises an outer ferrule hermetically sealed to an alumina insulator or a fused glass dielectric material seated within the ferrule. The insulator or dielectric 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. 
         [0024]    When used in an implanted medical device, the feedthrough terminal pins are connected to one or more lead wires which sense signals from the patient&#39;s heart and also couple electronic pacing pulses from the medical device to the heart. Unfortunately, these lead wires can act as an antenna to collect stray EMI signals for transmission via the terminal pins into the interior of the medical device. Such unwanted EMI signals can disrupt proper operation of the medical device, resulting in malfunction or failure. To prevent unwanted EMI signals from transmitting via the terminal pins into the interior of the medical device, a filter capacitor is mounted on the ferrule. 
         [0025]    According to the present invention, the filter capacitor comprises at least one active electrode layer (plate) and at least one ground electrode layer (plate) physically segregated from each other by a dielectric. The dielectric and the electrode layers are screen printed on a substrate comprising one of more layers of a relatively low temperature co-fired ceramic (LTCC) tape. The filter capacitor is completed with a LTCC cap placed on top of the active electrode/dielectric/ground electrode subassembly. 
         [0026]    The use of a LTCC tape substrate and a LTCC tape cap in conjunction with screen printing of the various layers comprising the capacitor means that the total thickness of the capacitor is significantly less than that of a conventionally constructed filter capacitor. Since these capacitor devices are intended for use in applications where a premium is placed on space, such as in implantable medical devices, and the like, providing a filter capacitor of reduced size represents a significant advancement in the art. This is done without compromising the capacitor&#39;s effectiveness in attenuating unwanted EMI signals or its voltage rating. 
         [0027]    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 
         [0028]      FIG. 1  is a cross-sectional view of a prior art filter feedthrough capacitor assembly  10 . 
           [0029]      FIG. 2  is a plan view of a plurality of capacitors  100  supported on a LTCC substrate  102  according to the present invention. 
           [0030]      FIG. 3  is a cross-sectional view taken along line  3 - 3  of  FIG. 2 . 
           [0031]      FIG. 4  is a partial cross-sectional view of the filter capacitor shown in  FIG. 3  electrically connected to a terminal pin  116  by a conductive epoxy bead  118 . 
           [0032]      FIG. 5  is a partial cross-sectional view of the filter capacitor shown in  FIG. 3  electrically connected to a terminal pin  116  by a metallization layer  117  and conductive epoxy  119 . 
           [0033]      FIG. 6  is a cross-sectional view of another embodiment of a filter capacitor  120  according to the present invention. 
           [0034]      FIGS. 7 and 8  are cross-sectional views of various embodiment of the filter capacitor  120  shown in  FIG. 6  attached to a hermetic feedthrough. 
           [0035]      FIG. 9  is a cross-sectional view of another embodiment of a filter feedthrough capacitor  300  with the ground electrode  302  being the first layer screen-printed on the LTCC substrate  102  instead of the active layer  104  as in  FIG. 3 . 
           [0036]      FIG. 10  is a cross-sectional view of another embodiment of a filter feedthrough capacitor  400  with the dielectric layers extending completely to the terminal pin bore  102  and the outer casing edge  108  to thereby segregate the active layers from each other and the ground layers from each other. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0037]    As used herein, the term “low temperature” is defined as a heating that occurs at less than about 950° C. 
         [0038]    Referring now to the drawings, a preferred manufacturing process for constructing a filter capacitor  100  according to the present invention is shown in  FIGS. 3 ,  4  and  6 . The process includes a screen-printing machine (not shown) that accurately and precisely prints the various layers of the filter capacitor  100  on an electrically non-conductive substrate  102  comprising one or more layers of a relatively low temperature co-fired ceramic (LTCC) tape.  FIG. 2  illustrates one completed capacitor substrate  102  comprising 180 individual filter capacitor structures printed thereon in nine rows of twenty capacitors  100 . 
         [0039]    In the exemplary filter capacitor  100 , the substrate  102  is comprised of three layers  102 A,  102 B and  102 C of the LTCC tape. Low temperature, co-fired ceramics are made of a ceramic oxide powder that is mixed with various organic materials, such as acrylic resins, for example a synthetic resin commercially available from E. I. Du Pont De Nemours &amp; Co. under the designation Elvacite®, and glasses such as borosilicate and, possibly, a dielectric material. This mixture is cast or coated in sheets on a carrier where it is allowed to dry, after which it is cut into tape strips. Such strips may have a thickness of between 4½ M mils to about 12 mils when dry. 
         [0040]    Suitable ceramic materials are selected from the group consisting of alumina, zirconia, aluminum nitride, boron nitride, and silicon carbide. Borosilicate is a sintering aid that lowers the temperature at which a dense ceramic can be achieved. Suitable dielectric materials are of the titanate group, preferably barium strontium titanate and sodium bismuth titanate. Finally, the organic binder is preferably cellulose based. Suitable solvents include terpineol (boiling point=220° C.), butyl carbitol (b.p.=230° C.), cyclohexanone (b.p.=155.60° C.), n-octyl alcohol (b.p.=171° C.), ethylene glycol (b.p.=197° C.), glycerol (b.p.=290° C.) and water. These are relatively high boiling point solvents that do not evaporate at room temperature and maintain rheology or viscosity during the manufacturing process. 
         [0041]    The “green tape” sheets  102 A,  102 B and  102 C are then stacked, aligned and subjected to isostatic pressure of about 3,000 psi for about 10 minutes at from about 65° C. to about 85° C. The thermo-plastic organic materials in the tape sheets soften and, under the pressure, flow together to effectively mechanically weld the individual sheets into a monolithic structure or a single laminated part. The thusly formed laminate is dried in an oven at from about 100° C. to about 250° C. to remove the binder and then cut into the appropriate shape to form the “green” substrate  102  ( FIG. 2 ). The substrate  102  is much like a single thick slab and may have a total thickness of about 0.003 inches to about 0.050 inches. Examples of low temperature co-fired ceramic tape processing can be found in “Development of a Low Temperature Co-fired Multi-layer Ceramic Technology,” by William A. Vitriol et al., 1983 ISHM Proceedings, and pages 593-598. 
         [0042]      FIG. 3  shows that a representative one of the filter capacitors  100  is built by first screen-printing a bottom conductive layer  104  directly on the LTCC substrate  102 . Layer  104  is screen-printed to cover the area that will eventually become the terminal pin bore  106 . This is done for ease of manufacturing. In its final form, layer  104  comprises an inner, proximal edge  104 A extending from the bore  106  to a distal edge  104 B, spaced from an outer edge  108  of the capacitor. The terminal pin bore  106  is shown in phantom because it is preferably cut out of the LTCC substrate  102  later in the manufacturing process although, if desired, the bore  106  can be provided before any screen printing takes place. Similarly, the outer edge  108  of the capacitor  100  is shown in phantom because it is preferably cut into the LTCC substrate  102  at completion of the screen printing process, but that is not necessary. A preferred cutting method is by laser cutting. While shown in cross-section, it should be understood that the bottom active electrode layer extends 360° about the circumference of the terminal pin bore  106 . Suitable active materials include Au—, Pt—, Cu—, Ni—, Ir—, Pd—, Ta-based pastes, the preferred one being an Ag—Pt based paste. 
         [0043]    Preferably, the bottom active electrode layer  104  consists of two sub-layers, although one or more than two can be used if desired. After each active sub-layer is printed, the LTCC substrate  102  is put into a belt fed infrared oven to dry-out or flash-off any remaining solvent in the printed pattern. 
         [0044]    After the requisite number of active sub-layers are printed and dried, a dielectric layer  110  is screen-printed over the bottom active electrode layer  104 . The dielectric layer  110  has a proximal base portion  110 A supported directly on the LTCC substrate  102  immediately adjacent to the outer edge  104 B of the bottom active layer  104 . This base portion  110 A does not extend completely to the outer edge  108  of the capacitor  100 . Instead, the proximal dielectric base portion  110 A leads to a distal planar portion  110 B that is in direct contact with the upper surface of the bottom active layer  104 . The distal dielectric portion  110 B extends to an edge  110 C that ends spaced from the terminal pin bore  106 . The dielectric material can be a BaTiO 3 -based thick film paste with a relatively high dielectric constant of about 10,000 k. Other useful high dielectric materials are barium strontium titanate and sodium bismuth titanate, among others. Preferably, the dielectric layer  110  consists of two sub-layers, although one or more than two can be used if desired. After each dielectric sub-layer is printed, the subassembly is put into the belt fed infrared oven to flash-off any remaining solvent in the printed pattern. 
         [0045]    In an alternate embodiment, the dielectric layer is not screen-printed. Instead it is a laminate of the LTCC tape that is placed on top of the active electrode layer  104 . As is the case with the substrate  102 , if the dielectric layer is a LTCC tape laminate, it preferably has three layers. In this case, the thusly processed subassembly is subjected to isostatic pressure of about 3,000 psi for about 10 minutes at from about 65° C. to about 85° C. 
         [0046]    After the requisite number of dielectric sub-layers is printed and dried, a ground electrode layer  112  is screen printed on the LTCC substrate  102 . The ground electrode layer  112  is of a similar material as the active electrode layer  104  and comprises a base portion  112 A supported directly on the LTCC substrate  102  immediately adjacent to the outer edge  108  of the capacitor  100 . The proximal ground base portion  112 A leads to a distal planar portion  112 B that is in direct contact with the upper surface of the dielectric layer  110 . The distal ground electrode portion  112 B extends to an edge  112 C spaced from the terminal pin bore  106 , but directly vertically above the active electrode layer  104 . The ground electrode  112  is of a similar material as the active electrode layer  104  and is dried after each sub-layer in a similar manner as the active electrode layer  104 . Each active and ground electrode layer has a thickness of about 0.0004 inches to about 0.0008 inches. In a similar manner as the active electrode layer  104 , the dielectric layer  110  and the ground electrode layer  112  each extends 360° about the circumference of the terminal pin bore  106 . 
         [0047]    The capacitor  100  may be finished by a cap layer of an LTCC tape laminate  114  extending to the outer substrate edge  108 , but in the capacitor&#39;s finished form covering from the outer capacitor edge  108  to the terminal pin bore  106 . The cap LTCC layer  114  is of materials similar to the LTCC substrate  102 . The capacitor assembly consisting of the LTCC substrate  102 /active electrode layer  104 /dielectric layer  110 /ground electrode layer  112 /cap LTCC layer  114  is then subjected to an isostatic pressing at about 3,000 psi for about 10 minutes at from about 65° C. to about 85° C. 
         [0048]    The thusly formed plurality of capacitors are individually punched or otherwise cut from the substrate  102  of  FIG. 2  and subjected to a final sintering at about 700° C. to about 950° C. for about 10 to 30 minutes. As is the case with the LTCC substrate  102 , the cap LTCC layer  114  preferably consists of three layer of tape, although more or less can be used, if desired. 
         [0049]    Thus,  FIG. 3  illustrates a filter capacitor  100  comprising one active electrode layer  104  and one ground electrode layer  112  segregated from each other by an intermediate dielectric layer  110  sandwiched between the LTCC substrate layer  102  and the LTCC cap layer  114 . This structure is sufficient to provide a feedthrough filter capacitor  100  according to the present invention. What is noteworthy is that the filter capacitor  100  is from about 30% to about 50% thinner (the distance from the bottom of substrate  102  to the top of cap  114 ) than the prior art capacitor  12  of  FIG. 1  while providing comparable capacitance and being structurally stable. 
         [0050]      FIG. 4  shows the finished capacitor  100  electrically connected to a terminal pin  116  by a conductive epoxy bead  118 . The terminal pin  116  is part of a hermetic feedthrough assembly as previously described with respect to the prior art filter feedthrough capacitor  10 . For the sake of simplicity the feedthrough is not shown in this drawing. Nonetheless, the epoxy bead  118  surrounds the terminal pin  116  of a feedthrough assembly and is in direct electrical contact with the active electrode layer  104  and the terminal pin. 
         [0051]    While the active and ground layers  104 ,  112  have been described as a single layer that is not necessary. As described in U.S. Pat. No. 5,978,204 to Stevenson, each layer  104 ,  112  can comprise two closely spaced apart layers separated from each other by a relatively thin dielectric layer. This patent is assigned to the assignee of the present invention, and incorporated herein by reference. 
         [0052]      FIG. 5  shows the filter capacitor  100  of  FIG. 3 , but provided with a metallization layer  117  applied to the sidewall of the terminal pin bore  106  and the outer edge  108 . As with the prior art capacitor  10 , suitable metallization materials  117  include titanium, niobium, tantalum, gold, palladium, molybdenum, silver, platinum, copper, carbon, iridium, iridium oxide, ruthenium, ruthenium oxide, zirconium, and mixtures thereof. The metallization layer  117  may be applied by various means including, but not limited to, sputtering, e-beam deposition, pulsed laser deposition, plating, electroless plating, chemical vapor deposition, vacuum evaporation, thick film application methods, aerosol spray deposition, and thin cladding. The thusly constructed filter capacitor is electrically connected to the terminal pin  116  by a layer of conductive polyimide  119 , and the like. 
         [0053]    In some applications it may be desirable to increase the capacitor&#39;s filtering frequency range, and this is done by increasing the number of segregated active and ground electrode layers supported on the LTCC substrate  102 . This means that after a bottom electrode layer is printed and dried, a dielectric layer is printed on top of the as-printed and dried bottom electrode pattern. After the dielectric layer is printed and dried, a top electrode layer is printed on the dried dielectric pattern. A similar print-dry procedure can be followed to print additional function layers until the desired capacitor sets are achieved. 
         [0054]    In  FIG. 6 , a capacitor  120  containing three plate sets is shown. This capacitor  120  is built by screen-printing additional active and ground electrode plates sandwiched around an intermediate dielectric layer on top of the filter capacitor structure  100  illustrated in  FIGS. 3 to 5 , but without the cap LTCC layer  114 . In particular, a second dielectric layer  122  is first screen-printed over the bottom ground electrode layer  112 . The second dielectric layer  122  has a proximal base portion  122 A that begins at the distal edge  110 C of the first dielectric layer  110 . Dielectric layer  122  is in direct contact with an exposed portion of the distal planar portion  110 B of the first dielectric layer and continues to a distal planar portion  122 B in direct contact with the upper surface of the distal planar portion  112 B of the first ground electrode layer  112 . However, the second dielectric layer  122  terminates at an edge  122 C spaced from the outer edge  108  of the capacitor  120 . This second dielectric layer  122  is completed by drying as previously described. 
         [0055]    In an alternate embodiment, the dielectric layer  122  is a laminate of the LTCC tape that is placed on top of the ground electrode layer  112 . The dielectric layer  122  is preferably a three layer laminate of LTCC tape. 
         [0056]    A second active electrode layer  124  is then screen-printed on top of the second dielectric layer  122 . In its finished form, the second active electrode layer  124  has a proximal base portion  124 A in direct contact with the first active electrode layer  104  adjacent to the terminal pin bore  106 . The direct contact between the proximal base portion  124 A of the second active electrode layer  124  forms a common active base  126  having an edge adjacent to the terminal pin bore  106 . The proximal base portion  124 A leads to a distal planar portion  124 B of the second active electrode plate  124  that is in direct contact with the upper surface of the second dielectric layer  122 . The distal planar portion  124 B of the second active layer  124  extends to an edge  124 C that is in vertical alignment with the edge  104 B of the first active electrode layer  104 . The second active electrode layer  124  is then subjected to a drying process as previously described. 
         [0057]    A third dielectric layer  128  is next screen-printed over the second active electrode layer  124 . The third dielectric layer  128  has a proximal base portion  128 A that begins at the edge  122 C of the distal portion  122 B of the second dielectric layer  122  and in direct contact therewith, and continues to a distal planar portion  128 B in direct contact with the distal planar portion  124 B of the second active electrode layer  124 . However, the third dielectric layer  128  terminates at an edge  128 C spaced from the terminal pin bore  106 . The third dielectric layer  128  is completed by drying as previously described. Alternatively, the dielectric layer is a laminate of LTCC tape. 
         [0058]    A second ground electrode layer  130  is then screen-printed on top of the third dielectric layer  128 . The second ground electrode layer  130  has a proximal base portion  130 A in direct contact with the proximal base portion  112 A of the first ground electrode layer  112  adjacent to the outer edge  108  of the capacitor  120 . The proximal base portion  130 A leads to a distal planar portion  130 B of the second ground electrode plate  130  that is in direct contact with the upper surface of the third dielectric layer  128 . The distal portion  130 B extends to an edge  130 C that is in vertical alignment with the edge  112 C of the first ground electrode layer  112 . The direct contact between the proximal base portions  112 A,  130 A of the respective first and second ground electrodes  112 ,  130  forms a common ground base  132  having an edge aligned with the capacitor outer edge  108 . The second ground electrode layer  130  is then subjected to a drying step as previously described. 
         [0059]    This alternating pattern of screen-printing an active layer followed by a dielectric layer followed by a ground electrode layer continues until as many active/dielectric/ground layer sets as are needed to obtain a desired capacitance value and voltage rating. In each set, the proximal ends of the active electrode layers are in direct contact with each other immediately adjacent to the terminal pin bore  106  and the proximal ends of the ground electrode layers are in direct contact with each other immediately adjacent to the outer edge  108  of the capacitor. Every other dielectric layer has its proximal end in direct contact with the distal portion of the dielectric layer immediately below it, alternating first adjacent to the terminal pin bore  106 , then adjacent to the outer capacitor edge  108 . 
         [0060]    The filter capacitor  120  is finished by a cap LTCC layer  134 . In its finished form, the cap LTCC layer  134  extends from the outer capacitor edge  108  to the terminal pin bore  106 . The cap LTCC layer is of similar materials as the LTCC substrate  102 . The capacitor assembly consisting of the LTCC substrate  102 /active layers  104 ,  124 /dielectric layers  110 ,  122 ,  128 /ground electrode layers  112 ,  130 /cap LTCC layer  134  is then subjected to an isostatic pressing at about 3,000 psi for about 10 minutes at from about 65° C. to about 85° C. The thusly formed capacitors are individually punched or otherwise cut from the substrate  102  of  FIG. 2  and subjected to a final sintering at about 700° C. to about 950° C. for about 10 to 30 minutes. As is the case with the LTCC substrate  102 , the cap LTCC layer  134  preferably consists of three layer of tape, although more or less can be used, if desired. 
         [0061]      FIG. 7  shows the capacitor  120  of  FIG. 6  attached to a feedthrough terminal pin assembly  200 . The feedthrough terminal pin assembly  200  comprises a ferrule  202  defining an insulator-receiving bore  204  surrounding an insulator  206 . The ferrule includes a surrounding flange  208  to facilitate attachment of the feedthrough capacitor assembly  200  to the casing of, for example, an implantable medical device. The method of attachment may be by laser welding or other suitable methods. The insulator  206  comprises a surrounding sidewall  206 A extending to a first upper surface  206 B and a second lower surface  206 C. A metallization layer  210  is applied to the insulator sidewall  206 A to aid in the creation of a brazed hermetic seal. Suitable materials for the ferrule  202 , insulator  206  and metallization layer  210  are the same as described for these components with respect to the prior art capacitor  10 . 
         [0062]    The insulator  206  has a sufficient number of bores  212 ; in this exemplary feedthrough there is one, to receive the requisite number of terminal pins  214 . The inner bore surface  212 A is provided with a metallization layer  216  in a similar manner as the previously described insulator sidewall  206 A. The terminal pin  214  is hermetically sealed in the bore  212  by a conductive, biostable material  218 , such as gold or gold alloy, contacting the metallization layer  216  and the terminal pin  214 . Similarly, a metallization layer  220  is provided on the insulator sidewall  206 A. A gold braze  222  hermetically seals the insulator metallization  220  to the ferrule  202 . 
         [0063]    In the exemplary embodiment of  FIG. 7 , the filter capacitor  120  is attached to the feedthrough terminal pin sub-assembly  202  with the cap LTCC layer  134  seated against the lower insulator surface  206 C. A conductive adhesive  224  contacts between the active electrode layers  104 ,  124  and the terminal pin  214  and a conductive adhesive  226  contacts between the ground electrodes  112 ,  128  and the ferrule  202 . This means that the substrate  102  faces the interior of the medical device housing. In the exemplary embodiment of  FIG. 8 , the filter capacitor  120  is attached to the feedthrough terminal pin sub-assembly  200  with the LTCC substrate  102  seated against the lower insulator surface  206 C and the cap LTCC layer  134  facing the housing interior. 
         [0064]      FIG. 9  is a cross-sectional view of another embodiment of a filter capacitor  300  according to the present invention. This filter capacitor  300  is similar in construction to the filter capacitor  120  shown in  FIG. 6 , except that the first layer screen-printed on top of the LTCC substrate  102  is a first ground electrode layer  302  instead of an active electrode layer. This is followed by a first dielectric layer  304 , a first active electrode layer  306 , a second dielectric layer  308 , a second ground electrode layer  310 , a third dielectric layer  312 , a second active electrode layer  314  and finally a cap LTCC layer  316 . 
         [0065]      FIG. 10  is a cross-sectional view of another exemplary embodiment of a filter capacitor  400  according to the present invention. Instead of the first and second active electrode layers  402 ,  404  being in direct physical contact with each other immediately adjacent to the terminal pin bore  106 , they are segregated from each other. This is done by extending the first dielectric layer  406  completely to the terminal pin bore  106 . Likewise, the first and second ground electrode layers  408 ,  410  are segregated from each other adjacent to the outer edge  108  of the substrate  102 . This is done by extending the second and third dielectric layers  412 ,  414  to the outer substrate edge  108 . A cap LTCC layer  416  is also shown. 
         [0066]    Thus, the present invention provides a filter capacitor that is readily attachable to a hermetic feedthrough to provide a filter feedthrough capacitor. The LTCC substrate and cap are of ceramic materials that maintain their shape and structure dimensions even after undergoing sintering. Consequently, the active and ground electrode layers along with the intermediate dielectric layer can be laid down or deposited by a screen-printing technique, which means that they can be made relatively thin. The result is a functional filter capacitor that is as robust as a conventional prior art capacitor made using tape cast technology. 
         [0067]    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.