GB2597106A - Feedthrough comprising interconnect pads - Google Patents

Abstract

The assembly comprises a ceramic body 2, a conductive feedthrough element 5 extending through the ceramic body 2 and a conductive pad 6. The conductive pad 6 comprises a multi-layered arrangement comprising: a bonding layer 7 comprising one or more elements selected from the group consisting of Ti, Zr, Nb, Ta, V, Mo, Mn, W, Y and Hf, said bonding layer in bonding contact with an end of the conductive element and the first or second side of the ceramic body; and at least one of a diffusion barrier layer 8 directly disposed upon said bonding layer, comprising one or more elements selected from the group consisting of Nb, Ta, W, Mo and nitrides thereof, and at least one of (i) said diffusion layer having a different composition than the bonding layer; and (ii) one or more sealing layers (9, 9a, 9b), disposed upon said diffusion barrier layer. The sealing layers may comprise noble metals such as Au, Pt, Ag for example. The feedthrough assembly is suitable for use in electronic biomedical applications.

Description

FEEDTHROUGH COMPRISING INTERCONNECT PADS

Field

The present invention relates to feedthroughs comprising interconnect pads and methods of producing thereof. In particular, the present invention relates to implantable medical devices comprising said feedthroughs spaced in close proximity.

Backqround Assemblies comprising metal and ceramic components are used in a wide range of applications. Ceramic-metal assemblies have found particular use in feedthroughs, where one or more electrical conductors are required to pass through a ceramic insulator to provide one or more electrically conductive connections from one surface of the ceramic insulator to another surface of the ceramic insulator. Such arrangements are widely used, for example, in aerospace, transportation, communication and power tube (e.g. x-ray, radio frequency) and medical applications; the present disclosure is not limited to any one application.

Electronic biomedical implants are being used increasingly to diagnose, prevent and treat diseases and other medical conditions. Implantable electronic devices must necessarily comply with safety standards before being approved for clinical use; for example, such implanted devices are needed to be housed in hermetic packages that incorporate electrical feedthroughs for signal transfer between the housed electronic device and the environment. By encapsulating electronically active components hermetically, the human or animal body is protected from toxicity of conventional electronic components and the device is also protected from the relatively harsh environment of the body that may otherwise cause the device to fail prematurely. Such implantable devices, especially those that interface with the human nervous system or organs in the human body such as the cochlea or the retina require a multiplex of electrical leads in the small confined space of a miniature feedthrough. Ceramic materials such as alumina or metals such as titanium have a long history of success in bionic feedthroughs in devices including pace-makers and cochlear implants.

Biocompatible ceramic-metal feedthrough systems may be considered to be the most reliable choice of materials for such devices owing to their chemical inertness (e.g. biocompatibility) and longevity (e.g. bio-stability).

The application of electronic biomedical implants in interacting with the human nervous system is becoming increasingly complex, particularly in neural prosthesis where high resolution stimulating or recording arrays are positioned near peripheral nerves or in the brain. Densely packed electrical feedthroughs are needed to carry input/output (I/O) signals to and from these implanted devices. For certain therapies, it is desirable to increase the number of electrical conductors (which have many names in the art of feedthroughs including: leads, pathways, pins, wires, and vias) in the feedthroughs to increase the overall number of I/O signals to meet the demands of these critical applications.

The challenge to provide densely packed electrical feedthroughs is met with the dimensional constraints placed on reducing the overall size of the feedthrough since it is undesirable to implant large devices (including a large feedthrough) in the human or animal body. In particular, it is also desirable to reduce the invasiveness of the implantation surgeries and/or io the nature of the placement of the device for the target therapies such as in retinal implants where the nature of the application necessitates only those devices that are suitably small. When the device design requires both a large number of conductors (i.e. high pin count) and a small-sized feedthrough, conventional feedthrough manufacturing techniques are inadequate and no longer viable. Existing technologies have limits as to the spacing of conductors within the feedthrough which inhibits the ability to increase the density of conductors in the feedthroughs. Therefore, until now, it has been necessary to opt either to make larger feedthroughs thereby increasing the size of the overall device comprising the feedthrough in order to accommodate a higher density of conductors or to reduce the density of conductors thereby limiting number of I/O signals in favour of a smaller-sized feedthrough, zo both of which fail to meet industrial demands.

The compressive force imparted onto conductors embedded in the ceramic matrix of a feedthrough during co-sintering is often relied upon for hermeticity. The interfaces between the conductors and the ceramic body may lack the hermeticity requirements demanded for suitable feedthroughs in critical and high performance applications.

As active implants are miniaturized to reduce trauma of invasive surgeries, the pin to pin distance between the conductors also decreases. When the pin to pin distance is very small, it becomes impossible to assemble the feedthroughs by gold brazing. Therefore, many investigators have proposed alternative methods to manufacture metal feedthroughs (US8755887 B2; 7988507; 7,989,080 B2; 8,698,006). In such cases the ceramic shrinks directly into the conductor, the bonding may not be as strong as traditional gold brazing which may lead to lack of hermeticity due to slippage at the surface. EP 2 651 510 B1 and US8894914 disclose a method where they use ceramic and metal powders to create the conductor path in the feedthrough by screen printing or filling holes in green ceramic, then co-firing the whole body to get sintered feedthrough. In such method the metal ceramic powders in the ceramic-metal composite (CMC) paste intimately bond with walls of the ceramic vias and give good hermeticity. The metal powders in the composite sinter to give the conductive path for the feedthrough. However, unlike solid pins in traditional feedthrough they do not have high mass-density and electrical conductivity. High conductivity is desired in electrical feedthroughs in order to reduce the resistive losses of signal transfer, which lead to longer battery life. The other problem with the co-firing route is that the CMS based via forms spill over patterns between laminated green tapes or may even percolated into the ceramic insulation and can cause shorting of adjacent conductors. US Patents 7988507, 7,989,080 B2 and 8,698,006 teach us a technique where ceramic insulation is formed in green stage around solid metallic pins and sintered. The ceramic io during sintering shrinks around the solid metal conductor and the compressive force mechanically bonds the ceramics to the conductor. In such methods the solid metal conductors has the desired low electrical resistivity (-1.1x10-8 Om). However, unlike the ceramic metal composite method and the traditional gold braze method, there is no chemical or metal bond between the conductor and the insulator, hermeticity of the joint maybe compromised. Hence there exists a need to increase the hermetic reliability for such feedthroughs. The current invention address this problem.

Another problem with miniaturized feedthroughs is that, the conductors are flush with the insulator surface. In such cases traditional wire bonding or welding of the leads to the conductors become very difficult. US Patent 8,670,829 B2 teaches a method of co-firing the feedthrough with ceramic-metal composites on either end of the conduit in green state. After sintering the ceramic-metal composites create a pad where the leads can be joined. However, this method cannot be adopted for feedthroughs made from direct bonding technology where green ceramic body shrinks around the metal pins during sintering.

Co-pending application PCT/EP2019/060196 provides a feedthrough comprising a higher density of conductors. As a consequence of providing a higher density of conductors, the hermeticity of the feedthrough may be compromised, which for some critical applications such as those described herein may be insufficient. A higher density of conductors may result in micro-cracking adjacent to the conductors which results in reduced hermeticity and thereby a feedthrough that is deficient against performance criteria.

Hermeficity and performance of feedthroughs may be monitored using routine quality control testing leading to removal of the feedthrough if a reduction in the hermeticity or performance is detected. In order to avoid any unnecessary complications, such as repeat surgeries, it is desirable to produce a feedthrough device that provides improved hermeticity and overall performance more reliably.

The biocompatibility of implantable ceramic feedthroughs is provided by the chemical inertness of ceramic materials. However, the conductors in a feedthrough are often exposed outside of the chemically inert ceramic body, which is not electrically conductive, in order to enable further electrical connections to be made, for example, as wire bonding sites on the feedthrough. The inventors have found these regions of the feedthrough and interfaces between the conductors and the body to be particularly susceptible to leakages. Hence, feedthroughs are one of the most common failure points of high performance implantable devices that are required to be hermetic. It is a non-exclusive aim of the present disclosure to provide a hermetic feedthrough which is biocompatible and biostable, particularly for high density feedthroughs. It is also a non-exclusive aim of the present disclosure to meet the demanding package requirement for smaller-sized, high-density and hermetic feedthroughs.

Summary of the Invention

It is the object of the present invention to provide an improved feedthrough device.

In a first aspect of the present invention, there is provided a feedthrough assembly (1) zo comprising: * a ceramic body (2) having a first side (3) and a second side (4); * a conductive element (5) extending through said ceramic body (2) between said first side (3) and said second side (4); * a conductive pad (6) electrically connected to said conductive element (5); wherein the conductive pad (6) comprises a multi-layered arrangement comprising: a bonding layer (7) comprising one or more elements selected from the group consisting of Ti, Zr, Nb, Ta, V, Mo, Mn, W, Y and Hf, said bonding layer in bonding contact with an end of the conductive element and the first or second side of the ceramic body; and at least one of: (i) a diffusion barrier layer (8) directly disposed upon said bonding layer, comprising one or more elements selected from the group consisting of Nb, Ta, W, Mo and nitrides thereof, said diffusion barrier layer having a different composition than the bonding layer; and (ii) one or more sealing layers (9), disposed upon said diffusion barrier layer.

The feedthrough assembly of the present invention provides a conductive pad (also known as an interconnect pad) which is securely bonded to the at least one end of the conductive element with the bonding layer forming a bond with the end of the conductive element and a portion of an end of the ceramic body. The bonding layer preferably forms a reaction bond with the surface of the ceramic body, such that gaseous pathways emanating from the ceramic body, particularly proximal to the conductive element, are not able to escape through the conductive pad, which functions as a hermetic cap. The reliability and longevity of the bonding layer may be enhanced through the addition of a diffusion barrier layer, which functions to prevent the diffusion of components of bonding layer away from a bonding surface comprising an end of the conductive element and an end of the ceramic body.

The diffusion barrier layer enables the sintering of the multi-layered assembly to occur without the unwanted diffusion of components between layers, which may compromise the functionality of the multi-layered assembly. Furthermore, by using thin film deposition techniques, the conductive pad does not significantly contribute to the resistivity of the feedthrough. In a preferred embodiment, the conductive elements are solid (e.g. wire or pin), thereby increasing the conductivity of the conductive pathway relative to assemblies comprising porous conductive elements, such as cermet.

Whilst there may be a degree of diffusion between the bonding layer and the diffusion barrier layer, the diffusion barrier layer preferably provides a continuous layer of one or more of Nb, Ta, W, Mo and nitrides thereof over the portion of the bonding layer that the diffusion barrier layer covers. The bonding layer and/or the diffusion barrier layer may comprise one or more sub-layers. The sub-layers may function to improve the adhesion between adjacent layers (e.g. improve the adhesion between the bonding layer and the diffusion barrier layer).

Other metals may be added to the bonding layer to form an alloy, however the proportion of Ti, Zr, Nb, Ta, V, Mo, Mn, W, Y and/or Hf is preferably at least 10 wt% or at least 20 wt% or at least 30 wt% or in an amount sufficient to form a reaction bond with the surface of the ceramic body. The diffusion layer and sealing layer(s) may also comprise metal alloys.

In one embodiment, the bonding layer extends beyond the periphery of an end of the conductive element circumferentially, such that the minimum distance from the periphery of the bonding layer and the periphery of the conductive element is at least 1.0 pm or at least 2.0 pm or at least 5.0 pm or at least 10.0 pm or at least 20.0 pm or at least 40.0 pm or at least 80.0 pm. In some embodiments the minimum distance from the periphery of the bonding layer and the periphery of the conductive element is no more than 1 mm or no more than 400 pm or no more than 200 pm or no more than 100 pm or no more than 50 pm. The adjacent surface of the ceramic body is preferably substantially flush with the end of the conductive element. However, in some embodiments, the adjacent surface of the ceramic body may be configured to be at an offset level, above or below the height of the end of the conductive element.

The bonding layer preferably reacts with the ceramic body to form a strong reaction bond (i.e. a stronger adhesive bond to the ceramic body than without the formation of the reaction bond). For example, a titanium bonding layer may react with a ceramic to form a reduced fitania (TiO2.). Within limitations, the further the bonding layer extends beyond the periphery of the conductive element, the greater the bonding strength and hermeticity associated with the bonding layer as the overlap enables a hermetic bond to form between components and prevents the formation of gaseous pathways along the interface between the conductive pathway (5) and the ceramic body (2).

The extent that the bonding layer extends beyond the periphery of the conductive element may be limited by the proximity of neighbouring conductive elements. For high density feedthrough configurations, the extent at which the bonding layer extends beyond the periphery of the conductive element is preferably such that the distance between conductive pads is at least 10 pm or at least 20 pm or at least 30 pm or at least 50 pm. This distance provides a sufficient gap for the each conductive pad to be electrically isolated from each other. In general, the greater the reaction bond area the greater gaseous resistance provided.

In another embodiment, the conductive pad extends no more than a distance equivalent to twice or thrice the diameter of the conductive element (5) and preferably no more than the diameter (or half the diameter) of the conductive element.

In one embodiment, the feedthrough comprises a plurality of conductive elements (5). The conductive element preferably have a density of conductive elements exceeding 1 conductor per 200,000 pm2 or exceeding 1 conductor per 100,000 pm2 or exceeding 1 conductor per 50,000 pm2 or exceeding 1 conductor per 20,000 pm2 or exceeding 1 conductor per 14,839 pm2 (23 thou2) through a planar cross-section of the ceramic body. The present disclosure has been found to be particularly beneficial in maintaining hermeticity when applied to feedthroughs having a high density of conductive elements.

In embodiments in which the further electrical connections are made to the conductive pad through mechanical connections, such as clamping, the conducting surface preferably comprises a hard surface. Such hard surfaces may be obtained directly from the diffusion barrier layer or through the selection of an outer layer with the required hardness.

In other embodiments, the conductive pad further comprises one or more sealing layers, either disposed upon (i) said diffusion barrier layer or (h) bonding layer. The one or more sealing layers may provide a number of functional properties to the conductive pad, including passivation, anti-corrosive, wear resistance, fie layer (i.e. to enhance interlayer adhesion); to gaseous barrier etc. However, a central focus of the one or more sealing layers is to enable the conductive pad to function as an interconnect and facilitate connection to other components within the electrical pathways of the device that the feedthrough assembly forms part of.

The first sealing layer preferably comprises one or more elements selected from the group consisting of Co, Ni, Al, Si, Cu, Ag, In, Cr, Ti, Ta, W, Mo. The first sealing layer is preferably in contact with the diffusion barrier layer or the bonding layer.

The second sealing layer preferably comprises one or more elements selected from the group consisting of Au, Pt, Ni, Cr, Cu and Al, said second sealing layer having a different composition than said first sealing layer. The second sealing layer is preferably the top layer of the conductive pad.

The number and combination of sealing layers will be dictated by the specific application.

The one or more sealing layers may comprise a passivation layer to prevent electrical corrosion. In one embodiment, the first layer passivation layer comprises aluminium. Aluminium has a self-passivating surface and the ability to form an intermetallic phase with wire bonding metals (top/second layer) such as gold, copper and silver. In another embodiment, the first layer is a nickel layer which provides mechanical backing for a top gold layer, thereby improving wear resistance of the sealing layers.

It will be appreciated that the total number of layers in the multilayer arrangement may vary between at least 2 layers to typically no more than 10 layers and preferably no more than 6 layers or no more than 4 layers.

The skilled artisan will understand the various combinations of metallic layers which can be used to provide the required bonding to wire or other conductive pathways, having the required mechanical, corrosive resistance and conductive properties.

The ceramic body (2) may comprise advanced ceramic materials including but not limited to oxide or carbide or nitride or silicide ceramic materials. The ceramic body (2) may comprise ceramic-matrix composite materials. The ceramic body (2) may comprise alumina ceramics. The ceramic body (2) may comprise zirconia toughened alumina (ZTA) ceramics. The ceramic body (2) may comprise yttria-stabilized zirconia (YSZ) ceramics The conductive element (5) may comprise Pt, Ir or combinations thereof. The conductive element (5) may comprise any other suitable conductive elements or materials. The conductive element (5) may be solid or porous and may comprise, including but not limited to, a solid rod, wire, lead, pathway, pin, metallic ink, cermet or via or another form of a conductor.

The conductive element (5) may comprise a plurality of conductive sub-elements (5a). The conductive pad (6) may be electrically connected to at least one of the conductive sub-elements (5a). The maximum linear length of the conductive pad (6) may be in the range of about 2 to about 100 times the diameter of each of the conductive sub-elements (5a) or conductive element (5).

The conductive element (5) may comprise at least a first end (14) proximal to said first side (3) of said ceramic body (2) and a second end (15) proximal to said second side (4) of said ceramic body (2). The first end (14) and the second end (15) of said conductive element (5) may be substantially parallel or flush with said first side (3) and said second side (4) of the ceramic body (2) respectively. The first end (14) and the second end (15) of said conductive element (5) may protrude out of said first side (3) and said second side (4) of the ceramic body (2) respectively. The first end (14) and the second end (15) of said conductive element (5) may be sunken into said first side (3) and said second side of the ceramic body (2) respectively.

The conductive pad (6) may provide a conductive pathway to said conductive element (5). The sub-elements (5a) may be in the form of a bundle of conductive elements which are housed within a single channel through the ceramic body (2) or plurality of conductive elements (5), with each conductive element housed within its own channel through the ceramic body (2).

The conductive pad (6) may act as an "interconnect" for further electrical connections to said conductive element (5). It will be understood that a second conductive pad (6b) may be provided on the second side (4) of the ceramic body (2) which is electrically connected to the conductive pad (6) via the conductive element (5).

The conductive element (5) may be brazed with the ceramic body (2) between the first side (3) and second side (4) forming a brazed interface (12a). The brazed interface (12a) may comprise a braze filler alloy comprising one or more elements selected from the list io consisting of Au, Cu, Ag, Ti, Ni or combinations or alloys thereof. The brazed interface (12a) may further comprise of one or more elements originating from the ceramic body (2). The conductive element (5) may be in braze-less contact with the ceramic body (2) between the first side (3) and second side (4) forming a braze-less interface (12b). The braze-less interface (12b) may enable tighter spacing between said conductive element (5) and said ceramic body (2) due to the lack of a braze filler alloy.

The conductive pad (6) bonded to said first side (3) of said ceramic body (2) may provide a hermetic barrier over said first side (3) of said ceramic body (2). The conductive pad (6) bonded to said first side (3) of said ceramic body (2) may provide a hermetic barrier over the conductive element (5). The conductive pad (6) bonded to said first side (3) of the ceramic body (2) may provide a hermetic barrier over said first end (14). The conductive pad (6) bonded to said first side (3) of said ceramic body (2) may provide a hermetic barrier over said brazed interface (12a) or said braze-less interface (12b) between said conductive element (5) and said ceramic body (2).

The bonding layer (7) may have a density of at least 95% or at least 96% or at least 97% or at least 98% of the theoretical density of said bonding layer (7).

The feedthrough assembly (1) may comprise a He permeability of less than 1.0 x 10-7 cc.atm/s. The feedthrough (1) may have a He permeability of less than 1.0 x 10-8 cc-atm/s.

The feedthrough (1) may have a He permeability of less than 1.0 x 10-9 cc-atm/s. The conductive pad (6) may provide the feedthrough (1) with a hermetic seal or a sintered seal over said first side (3) of the ceramic body (2). Reference to an increase in He hermeficity denotes a decrease in the permeability rate of He through the feedthrough assembly. The higher the hermeticity is, lower is the permeability.

In one embodiment a conductive pad (6) may be located between two adjacent components in the ceramic body. The component are preferably layers, although it will be appreciated that the ceramic body may be formed from other geometric configurations. The conductive pad may also extend between the adjacent components and function as a hermetic seal between conductive pathways in opposing components. Conductive pads may be located between several or all adjacent ceramic components in addition to, or as an alternative to, being located on an external surface of the ceramic body. Within this embodiment, holes are made in each of the green ceramic component and the holes filled with a conductive paste (e.g. metallic ink) to form a conductive pathway through the ceramic component.

The conductive paste preferably comprises a metallic conductor, such as a biocompatible metal (e.g. platinum group metal and alloys thereof). The paste may also comprise a binder (preferably a fugitive binder) and/or a ceramic filler to assist in matching the co-efficient of thermal expansion between the conductor and the ceramic body. A metal or metal alloy layer may then be coated over at least one end of the conductive pathway. The process may be repeated with one or more further metal / metal alloy components. The components may then be stacked or otherwise arranged such that the conductive pathway (5) extends from the first side (3) to the second side (4) of the ceramic body (2). In some embodiments, the conductive pathway may extend between the ceramic components as well as through the components. The assembly may be co-fired together such that the one or more conductive pads (6) form a bond between the ceramic components adjacent the conductive pathway. The formation of a feedthrough comprising a plurality of conductive pads between each side of the ceramic body is expected to provide even further enhancements in hermeticity. Further details of the formation of a feedthrough from a plurality of ceramic sheets is provided in US8,872,035.

The bonding layer metal element may react with the ceramic forming a chemical bond between the first side (3) of the ceramic body (2) and the bonding layer, at the joint interface (16). The bonding layer metal element may react with the first side (3) of the ceramic body (2) resulting in the formation of a reaction product. The reaction product may form as a continuous reaction layer at the joint interface (16) The bonding layer (7) may comprise a reaction layer (17) proximal to the first side (3) of the ceramic body (2). The bonding layer metal element may be present in the reaction layer (17) in an amount ranging from about 70 %wt to about 99.5 %wt based on the total weight of the active metal component (i.e. metal component that reacts with the ceramic body to form the reaction layer) in the bonding layer. The reaction product may comprise but not be limited to an oxide, carbide, nitride, or suicide reaction product depending on the ceramic material selected and the reactions between the active alloy and the first side (3) of the ceramic body (2). The reaction layer (17) may comprise one or more elements originating from the bonding layer. The reaction layer (17) may comprise one or more elements originating from the ceramic body (2).

The reaction layer (17) may comprise one or more layers. The one or more layers may comprise a polycrystalline structure. The one or more layers may comprise one or more compounds.

The formation of the reaction layer (17) may depend on the chemical activity of the metal element in the metal or alloy used in the bonding layer. The chemical activity of the metal element may depend on the relative amounts of the metal element; the alloying elements (if used) and the chemical affinity between them. The chemical activity of the metal element may depend on the sintering temperature which provides a thermodynamic driving force for diffusion. The chemical activity of the metal element may depend on the sintering time which provides the time for diffusion to occur at the sintering temperature.

The reaction layer (17) may be continuous layer along the interface (16). The reaction layer (17) may add a higher degree of metallic character to the first side (3) of the ceramic body (2) enabling the active metal/alloy to wet and spread effectively over said first side (3) of the ceramic body (2). The chemical bond between the first side (3) of the ceramic body (2) and the bonding layer at the interface (16) may provide a hermetic seal or a sintered seal. The reaction layer (17) at the interface may provide a hermetic seal or a sintered seal.

The reaction layer (17) may be less than 10 pm thick or less than 5 pm thick or less than 3 pm thick. In one embodiment the thickness of the reaction layer (17) ranges from about 0.05 or 0.1 pm to about 3 pm.

The conductive pad (6) may comprise one or more sealing layer(s) bonded to the diffusion barrier layer (8). The sealing layer(s) (9) may comprise one or more elements selected from the list consisting of Au, Pt, Ni, Cr, V, Cu, Ta, Ti, Nb, Al, Ag and Sn or combinations or alloys thereof.

The sealing layer(s) may function as a passivation barrier (e.g. when comprised of Au or Pt) and/ or as a further bonding layer to connect the conductive pad to further conductive elements, such as wires or other components of an electrical circuit.

The first side of the ceramic body may be provided with a two or more precursor layers. The precursor layers are transformed into the bonding layer during the sintering step.

In one embodiment, the conductive pad (6, 6a) is derived from three or more layers (7, 8, 9a, 9b) comprising a first layer (7) bonded to the first side (3) of the ceramic body (2), a second layer (8) bonded on top of the first layer (7), a third layer (9a) bonded on top of the second layer (8); and a further layer (9b) bonded on top of the third layer.

to The first layer (7) may comprise Ti. The second layer (8) may comprise Nb. The third layer (9a) may comprise Ni; and the fourth layer may comprise Au.

The first layer (7) may have a thickness in the range of about 0.05 micron to 4 micron or 0.1 micron to about 2 micron, or about 0.2 micron to about 1.75 micron, or about 0.3 micron to about 1.5 micron. The second layer (8) may have a thickness in the range of about 0.1 micron to about 1 micron, or about 0.2 micron to about 0.9 micron, or about 0.3 micron to about 0.8 micron. The third layer (9a) may have a thickness in the range of about 0.1 micron to about 1.6 micron, or about 0.2 micron to about 1.4 micron, or about 0.3 micron to about 1.2 micron. The fourth layer (9b) may have a thickness in the range of about 0.1 micron to about 1.6 micron, or about 0.2 micron to about 1.4 micron, or about 0.3 micron to about 1.2 micron.

Depending upon the sealing/bonding technique employed to bond a wire to the conductive pad, the one or more sealing layers may an increased thickness to those embodiments stated above. An increased sealing layer(s) thickness may be preferred for some wire welding applications.

The outer layer (9, 9b) may comprise a coating to provide a passivation layer over said bonding layer (7). The passivation layer may protect the conductive pad (6) thereby contributing to the hermeticity of the feedthrough (1).

The outer layer (9b) may fully encompass the preceding conductive pad layers (7, 8, 9a) so as to provide a protective shell to the conductive pad (6) that is hermetic to further enhance the hermetic seal. The outer layer (9b) may comprise Au and/or Pt to provide said passivation layer.

The outer layer (9b) may provide a conductive pathway to the conductive element (5) through the preceding layers in the conductive pad (6).

The outer layer (9b) may provide further electrical connections to be made, for example, the outer layer (9b) may provide a wire bonding site on the first side (3) of the ceramic body (2) for further electrical connections via said conductive pathway to the conductive element (5). As such the outer layer is conducive to be connected to further electrical connections through soldering, welding or other connection means.

io The feedthrough (1) may comprise a second conductive pad (6a) electrically connected to said conductive element (5) wherein said second conductive pad (6) is bonded to said second side (4) of said ceramic body (2) through a bonding layer (7), said bonding layer (7) comprising a metal or alloy.

The second conductive pad (6a) may be electrically connected to the conductive pad (6) through said conductive element (5) thereby providing an electrical feedthrough with hermetic seals or sintered seals at both ends (14,15) of the conductive element (5).

The second conductive pad (6a) may comprise all embodiments of the conductive pad (6) as zo described herein.

The feedthrough assembly (1) of the present invention may form part of an implantable medical device.

In a second aspect of the present invention, there is provided a method of producing a feedthrough assembly comprising a conductive pad comprising the steps of: A. Providing a feedthrough (1) comprising: * a ceramic body (2) having a first side (3) and a second side (4); * a conductive element (5) extending through said ceramic body (2) between said first side (3) and said second side (4); B. If required, machining an end of the conductive element and/or ceramic body, such that the end of the conductive element is substantially flush or otherwise offset with respect to with an adjacent surface of the ceramic body; C. Optionally masking the area around the end of the conductive element, such that there is an unmasked area exposing the end of the conductive element and a portion of the adjacent surface; D. Depositing a bonding layer to the unmasked area comprising one or more elements selected from the group consisting of Ti, Zr, Nb, Ta, V, Mo, Mn, W, Y and Hf; E. Depositing a diffusion barrier layer comprising one or more elements selected from the group consisting of Nb, Ta, W, Mo and nitrides thereof; and F. Sintering said layers at sufficient temperature for the bonding layer to form a reaction bond with the adjacent surface of the ceramic body.

The said invention increases the hermetic reliability of the feedthrough as well as acting as a pad for a stronger wire termination. The hermetic reliability is increased by creating an added o barrier to the leak path (between the metal pin and the ceramic matrix). This layer is dense and bonds to the ceramic around the pin surface as well as the pin. Thus acting like a cap at both ends of the pin. Also because the pads are dense (because of the heat treatment post deposition) they present a sturdy surface for wire termination. Wire bonding technologies especially ultrasonic welding requires such sturdy interconnect pads for bond reliability and life.

Creating pads especially the gold and nickel layers through electroplating is also limiting. When the pads are too close to each other and the feature resolution is fine, electroplating leads to two issues. Firstly, the fine features are not well defined and may lead to shorting between pads, and two, there is an increase in defects and the plating peels off. Thus even when heat treated, some defects still remain added to some features that are not well defined. Therefore, the pads in the present disclose preferably created by a RF sputter method. This not only creates well defined features but also they are defect free and dense after heat treatment.

In one embodiment, the sealing layers comprise layers of gold and nickel which can be bonded to lead wires to connect both the circuitry in the can or leads to the nervous system, which can be made from platinum. Gold and nickel plating are very difficult to electroplate to feedthroughs with such close pin-to-pin spacing. The current invention overcomes this problem.

In embodiments wherein the conductive pad further comprising one or more sealing layers, these additionally layers are applied on top of the diffusion barrier layer. In some embodiments, the one or more sealing layers may be deposited after the sintering step, with an additional sintering step performed after the application of the one or more sealing layers.

In other embodiments, a single sintering step is performed after the application of the bonding, diffusion barrier and one or more sealing layers. The sintering step assists in bonding the layers together and to the end of the conductive element and adjacent ceramic surface. In addition, the sintering step may densify the layers, thereby further enhancing the conductive cap's gas barrier properties.

In some embodiments, the machining of the conductive element and/or ceramic body results in the conductive element being counter-sunk into the ceramic body. Within this embodiment, the bonding layer may extend below a surface plane of the ceramic body and into a counter-sunk cavity. This configuration may result in a higher hermeficity due to the io more tortuous gaseous pathway.

The unmasked area adjacent surface of the ceramic body is preferably an annular shape, with the layer extending by substantially even distance from the periphery of the conductive element.

The thickness of the bonding layer is in the range of 0.01 pm or 200 pm or 0.05 pm to 100 pm or 0.1 pm to 50 pm or 0.2 pm to 20 pm or 0.3 pm to 10 pm or 04 pm to 2 pm. The thickness of the diffusion barrier layer is in the range 0.05 pm to 200 pm or 0.10 pm to 100 pm or 0.1 pm to 50 pm or 0.2 pm to 20 pm or 0.3 pm to 10 pm or OA pm to 2 pm. The thickness of the one of more sealing layers is in the range 0.1 pm to 200 pm or 0.05 pm to pm or 0.1 pm to 50 pm or 0.2 pm to 20 pm or 0.3 pm to 10 pm or 0.4 pm to 2 pm. The thinner the layer the lower the resistivity the layers contributes to the conductive pad. However, the thickness of the layers have to be sufficient to enable a strong reaction bond with the ceramic surface and for the diffusion barrier layer to impede the mitigation of bonding layer components away from the ceramic surface.

In a one embodiment, the feedthrough may be formed from a larger co-fired monolithic block, multiple feedthroughs machined or sliced off the block to produce a plurality of feedthrough in which the conductive element is flush with the ceramic body at both ends of the feedthrough. Further details of this manufacturing technique is by provided in EP2437850, which is incorporated therein by reference.

The conductive pads of the present invention are particularly advantageous applied to co-fired feedthroughs which have been sliced into smaller feedthrough modules as the machining process can compromise the hermeticity of the co-fired feedthroughs through the generation of micro-cracks within the ceramic body. The use of the conductive pads of the present invention can not only restore the hermeticity of the feedthrough but further enhance the hermeticity as well as extending its durability.

In an alternative embodiment, the a plurality of feedthrough sheets have conductive elements extending there through have conductive pads applied preferably one end, with the conductive pads extending beyond the peripheral of the conducive pads.

The thickness of the individual layers will depend upon the specific application, with thinner thicknesses favoured for feedthroughs used in implantable medical devices, whilst thicker io layers may be favoured for industrial uses high mechanical resilience is required.

The thinner layers may be applied with any suitable technique, such as a thin film deposition techniques, such as sputtering. These techniques are advantageously used with masking to enable the positioning of the layers to be tightly controlled, thereby enabling the conductive pads to the applied to high density feedthrough configurations. Greater layer thicknesses may be achieved using screen printing techniques or the like.

The application of the layers may be achieved using a thin film deposition technique such as sputter coating. The method of providing the bonding layer (7) to the first side (3) of the ceramic body (2) may comprise other thin film deposition techniques including but not limited to chemical vapour deposition, physical vapour deposition or screen printing or other thin film deposition techniques known in the art.

Sintering may be performed in a vacuum furnace at pressures ranging from about 4.0 x 10-4 to about 1.0 x 10-7 mbar. Sintering may be performed in a vacuum furnace at a pressure of less than about 1.0 x 10-5 mbar. Sintering may be performed in other chemically inert environments such as those comprising Ar or He or H gases or other chemically inert gases. The evacuation of oxygen in the chemically inert environment may promote diffusion of a metal element of the bonding layer (7) to the joint interface (16) to form a reaction bond (17).

The assembly may be heated at a heating rate ranging from about 1 °C/min to about 15 °C/min. The assembly may be heated to a sintering temperature for a predetermined time period or sintering time. The assembly may be first heated to a temperature below the sintering temperature for a predetermined time period in the range of between about 2 minutes to about 15 minutes to enable thermal homogenization of all components of the assembly. The sintering temperature may be at least 50°C above the liquidus temperature of the metal or alloy of the bonding layer. The sintering temperature may be selected to at least melt a portion of the bonding layer metal or alloy. The sintering temperature may be selected to enable the diffusion of a metal element to the joint interface (16). The sintering time may be in the range of about 1 minute to about 30 minutes, or about 2 minutes to about 25 minutes, or about 3 minutes to about 20 minutes. The sintering time may provide the time available at the sintering temperature for the metal element to diffuse to the joint interface (16). The sintering time may be selected to control the thickness of the reaction layer (17). The assembly may be cooled at a cooling rate ranging from about 1 °C/min to about 10 °C/min. A slow cooling rate is preferred to minimise thermally induced residual stresses that may be generated as a result of a coefficient of thermal expansion mismatch at the joint interface (16).

The method of producing a feedthrough assembly (1) may comprise a heat treatment comprising the steps of heating said feedthrough assembly (1). The heat treatment may be applied following sintering said bonding layer to said first side (3) of said ceramic body (2).

The heat treatment may further densify the conductive pad (6). The heat treatment may further improve hermeficity of the feedthrough (1).

The method of producing a feedthrough assembly (1) may include pre-placing or depositing the bonding layer on the first side (3) of the ceramic body (2) to form an "assembly". In some embodiments, the metal/alloy may be brushed or painted onto the first side (3) of the ceramic body (2), for example, in embodiments where the metal/alloy is in the form of a paste. The assembly may be subsequently mounted in a vacuum furnace for sintering. As will be appreciated by those skilled in the art, fixtures or fittings may be used to support the assembly during sintering and a load may be applied to secure said sintered assembly during sintering.

A method for connecting the conductive pad to a further electrical pathway may be achieved using a variety of possible bonding techniques including but not limited to welding, soldering, brazing, diffusion bonding, laser assisted diffusion bonding, laser welding, thermo-sonic bonding, ultrasonic bonding, soldering or flip chip bonding or other known joining techniques known in the art as will be appreciated by the skilled person.

Brief Description of Drawings

Embodiments will now be described, by way of example only and with reference to the accompanying drawings having like-reference numerals, in which: Figure 1 shows a schematic cross-sectional representation of the feedthrough assembly of the present invention in a first possible embodiment.

Figure 2a shows a schematic cross-sectional representation of the feedthrough assembly of the present invention in a second possible embodiment.

Figure 2b shows a schematic cross-sectional representation of the feedthrough assembly of the present invention in a third possible embodiment.

Figure 3 shows a schematic cross-sectional representation of the feedthrough assembly of the present invention in a fourth possible embodiment.

to Figure 4 shows a schematic cross-sectional representation of the feedthrough assembly of the present invention in a fifth possible embodiment.

Figure 5 shows a sectional SEM micrograph of a portion of the feedthrough assembly of the present invention corresponding to the fifth possible embodiment.

Figure 6 shows a magnified portion of the sectional SEM micrograph of Figure 5.

Figure 7 shows a photograph of a plurality of conductive pads according to a preferred embodiment of the present invention.

Figure 8 shows an EDS line-scan taken form the feedthrough assembly of Figure 6.

Detailed Description of a Preferred Embodiment

The present invention provides an improved feedthrough device. The feedthrough may comprise assemblies comprising metal and ceramic components. The feedthrough may be used to transmit signals, high voltages, high currents, gases or fluids. The feedthrough may provide electrical insulation and high mechanical strength. The feedthrough may be hermetic and maintain ultra-high levels of vacuum and joint integrity that are maintained even at elevated temperatures, in cryogenic conditions, or in harsh environments such as in the human or animal body.

Sintering is one of the industrially preferred methods for coating ceramics whereby a metal/ alloy is sintered at above 450°C on a ceramic surface. The use of metal/alloys may result in the poor wetting of chemically inert ceramic surfaces and the generation of thermally induced residual stresses upon cooling due to a coefficient of thermal expansion mismatch at the ceramic-bonding layer interface which can cause the sintered coating to fail prematurely. As will be appreciated by the skilled person, the coating -ceramic interface comprises the interfacial region along the surfaces of two or more materials that are in contact or bonded together.

The present disclosure employs the use of a multi-layered conductive pad to overcome the abovementioned problems. Sintering using a multi-layered conductive pad structure enhances the capability of providing a durable and long lasting hermetic seal.

to In accordance with embodiments of the invention, Figure 1 shows a schematic cross-sectional representation of the feedthrough assembly (1) of the present invention in a first possible embodiment. The feedthrough assembly (1) comprises a ceramic body (2) having a first side (3) and a second side (4) and a conductive element (5) extending through said ceramic body (2) between said first side (3) and said second side (4). A conductive pad (6) is electrically connected to said conductive element (5) wherein the conductive pad (6) is bonded to said first side (3) of said ceramic body (2) through a bonding layer (7), with a diffusion barrier layer (8) provided to prevent the diffusion of components of the bonding layer from the joint interface (16). A further sealing layer (9) is provided to facilitate bonding to further electrical pathways that the feedthrough assembly may be connected to. An optional second conductive pad (6a) is similarly bonded on the second side (4).

In one embodiment, the ceramic body (2) comprises alumina, a cost-effective ceramic material with excellent refractoriness, electrical insulation, wear-and corrosion-resistance making it suitable for use in vacuum feedthroughs and high voltage insulation applications.

In another embodiment, the ceramic body (2) comprises ZTA, providing excellent mechanical strength, wear-resistance, and toughness. In another embodiment, the ceramic body (2) comprises YSZ.

The ceramic material selected may depend on the application. For example, alumina may be selected for ultra-high vacuum coaxial feedthroughs used in signal transmission, particle physics, thin film deposition or ion beam applications due to excellent dielectric properties which provides high-voltage insulation with little signal attenuation. Optionally, the ceramic body (2) may comprise a polycrystalline or monocrystalline alumina.

The conductive pad (6) electrically connected to the conductive element (5) and bonded to the first side (3) of the ceramic body (2) has been found to improve hermeticity of the feedthrough (1). The conductive pad (6) is bonded to the first side (3) of the ceramic body (2) through a bonding layer (7). The bonding layer (7) comprises a metal or alloy that is capable for forming a reaction bond with the ceramic body. The overlaid diffusion barrier layer further enhances the hermetic seal through reducing gas permeability through the conductive pad (6) as well as improving the durability of the reaction bond through inhibiting diffusion of bonding layer components. The multi-layered arrangement of the provided by the conductive pad (6) provides a feedthrough assembly with improved hermeticity and performance while acting as an "interconnect" for further electrical connections to the conductive element (5).

io The conductive element (5) may comprise any suitable conductive material such as Pt or Pt/Ir alloy. The conductive element (5) may comprise other conductive elements or materials. The conductive element (5) extends through the ceramic body (2) between said first side (3) and said second side (4).

Referring to Figures 2a and 2b, in other embodiments, the conductive element (5) comprises a plurality of conductive sub-elements (5a). The plurality of conductive sub-elements (5a) may provide a densely packed feedthrough. The plurality of conductive sub-elements (5a) may provide a feedthrough (1) with one or more electrical conductors to increase the overall number of I/O signals as required for certain applications. The conductive pad (6) may be electrically connected to at least one of the conductive sub-elements (5a). Each of the plurality of conductive sub-elements (5a) may comprise one or more conductors with different properties, for example, a first pin comprising Pt, a second pin comprising Ir, and a wire comprising Pt and Ir.

Referring to Figures 1 to 2b, the conductive element (5) or plurality of conductive sub-elements (5a) extending through said ceramic body (2) between said first side (3) and said second side may comprise at least a first end (14, 14a) proximal to said first side (3) of said ceramic body (2) and a second end (15, 15a) proximal to said second side (4) of said ceramic body (2). In one embodiment, the first end (14, 14a) and the second end (15, 15a) of said conductive element (5) or plurality of conductive sub-elements (5a) is configured to be substantially parallel or flush with said first side (3) and said second side (4) of the ceramic body (2) respectively. The first end (14, 14a) and the second end (15, 15a) of said conductive element (5) or plurality of conductive sub-elements (5a) may be ground flat to be flush with said first side (3) and said second side (4) of the ceramic body (2) respectively.

Optionally, the first end (14, 14a) and the second end (15, 15a) of said conductive element (5) or plurality of conductive sub-elements (5a) may protrude out of said first side (3) and said second side (4) of the ceramic body (2) respectively. Optionally, the first end (14, 14a) and the second end (15, 15a) of said conductive element (5) or plurality of conductive sub-elements (5a) may be sunken into said first side (3) and said second side of the ceramic body (2) respectively.

As illustrated in Figure 2b, the feedthrough may comprise the plurality of conductive elements (5), with each conductive element (5) extending from a first side (3) to a second side (4) and being encompassed by said ceramic body (2).

In one embodiment, the conductive pad (6) provides a conductive pathway to the conductive io element (5). In another embodiment, the conductive pad (6) provides a conductive pathway to a plurality of conductive sub-elements (5a). In a further embodiment, as will be discussed hereinafter, the feedthrough (1) may further comprise a second conductive pad (6a) electrically connected to said conductive element (5) wherein said second conductive pad (6b) is bonded to said second side (4) of said ceramic body (2). The conductive pad (6) may be electrically connected to the second conductive pad (6a) through said conductive element (5).

The conductive pad (6) acts as an "interconnect" for further electrical connections to said conductive element (5). In another embodiment, the conductive pad provides a first wire bonding site and a second conductive pad (6a) provides a second wire bonding site for further electrical connections to be connected to the feedthrough (1). The conductive pad (6) and the second conductive pad (6a) may each provide "interconnects" for further electrical connections to said conductive element (5).

In embodiments in which the further electrical connections are made to the conductive pad through mechanical connections, such as clamping, the bonding site preferably comprises a hard surface. Such hard surfaces may be obtained directly from the bonding layer or through the selection of an outer layer with the required hardness. In a particular, embodiment, the hard surface is formed from a multi-layered structure comprising a bonding layer and a diffusion barrier layer.

As will be appreciated by the skilled person, the conductive element (5) or the plurality of conductive sub-elements (5a) may be embedded in a ceramic matrix and compacted to form a green body that may subsequently be co-sintered to densify and impart mechanical strength to said green body compact forming a feedthrough (1) comprising the conductive element (5) or the plurality of conductive sub-elements (5a). The conductive pads (6) corresponding to respective conductive sub-elements (5a) are spaced apart by a gap (X) which corresponding to the location and size of the mask used when the conductive pad layers (6) were deposited.

In one embodiment, the conductive element (5) or the plurality of conductive sub-elements (5a) is brazed to the ceramic body (2) between the first side (3) and second side (4) forming a brazed interface (12a). The brazed interface (12a) may comprise a braze filler alloy comprising one or more elements selected from the list consisting of Au, Cu, Ag, Ti, Ni or combinations or alloys thereof. The brazed interface (12a) may further comprise of one or more elements originating from the ceramic body (2). In another embodiment, the conductive element (5) or the plurality of conductive sub-elements (5a) is in braze-less contact with the ceramic body (2) between the first side (3) and second side (4) forming a braze-less interface (12b). The braze-less interface (12b) may enable tighter spacing between said conductive element (5) and said ceramic body (2) due to the lack of a braze filler alloy. Optionally, the braze-less interface (12b) may enable tighter pin-to-pin spacing between said plurality of conductive sub-elements (5a) due to the lack of a braze filler alloy.

The conductive pads (6, 6a) provide a hermetic barrier or hermetic seal, an airtight seal that may prevent the passage of air, oxygen, or other gases. The hermeticity, or leak-tightness, of a component may be tested using a variety of methods known in the art including leak testing. Leak testing is a non-destructive method used to locate and measure the size of leaks into or out of a component under vacuum or pressure. A tracer gas is introduced to the component connected to a leak detector. Helium leak testing is an effective test method for hermeticity due to the relatively small atomic size of helium atoms which may easily pass through any leaks in the component. Leak rates with a He hermeticity as low as 1.0 x 10-10 cc.atm/s may be detected. For example, for a component required to be watertight, a leak rate with a He hermeticity of 1.0 x 10-4 cc.atm/s would be sufficient. During a helium leak test, a pressure difference between an inner side and an outer side of a component under examination is produced.

In some embodiments, the conductive pad (6) has a Mohs hardness of at least 2.5 or at least 3.0 or at least 3.5 or at least 4.0 or at least 4.5. A high hardness value enables mechanical connections to be made, such as further electrical connections mechanically clamped to the first wire bonding site provided by the top surface of the conductive pad (6). In applications requiring mechanical connections, the properties of the diffusion barrier layer (8) including hardness and strength may be sufficient without the need of a separate outer sealing layer(s) (9), as will be described hereinafter.

The bonding layer may comprise alloying elements may host an active metal element in an active alloy. The alloying elements may facilitate or promote the diffusion of an active metal element to the first side (3) of the ceramic body (2) in the formation of a hermetic seal. The alloying elements may facilitate or promote the diffusion of the active metal element to the joint interface (16) in the formation of a hermetic seal.

The alloying elements may comprise one or more elements with a low "chemical affinity" towards the active metal element. As will be appreciated by the skilled person, the low chemical affinity may comprise a low solubility to form phases or a low tendency to form io compounds between the active metal element and the alloying elements.

The active metal element in the bonding layer may be selected depending on the ceramic material to be sintered, for example, Ti may be selected for an alumina ceramic body (2). The active metal element selected may depend on the metal or alloying elements in the bonding layer and the chemical affinity between said active metal element(s) so as not to inhibit the diffusion of said active metal element to the joint interface (16) in the formation of a hermetic seal or active sintered seal.

The active metal element or the alloying elements selected in forming a suitable active bonding layer may further depend on the physical properties of the active alloy desired, such as strength, hardness, coefficient of thermal expansion, liquidus temperature, corrosion resistance, biocompatibility and electrical conductivity.

The bonding layer may comprise one or more active metal elements or one or more alloying elements to provide an alloy having a eutectic temperature so as to enable a reduced sintering temperature. The alloying elements may form an alloy having a eutectic temperature thereby enabling a reduced sintering temperature. A reduced sintering temperature may help to minimise the generation of thermally induced residual stresses due to a coefficient of thermal expansion mismatch at the joint interface (16).

In some embodiments, the bonding layer may be derived from a layered structure having one or more layers. Each layer may comprise different metals that have a eutectic temperature when formed into an alloy during the sintering process.

Referring to Figure 3, in another embodiment, the bonding layer (7) comprising an active braze alloy comprises a reaction layer (17) proximal to the first side (3) of the ceramic body (2) having one or more layers (18).

In one embodiment, the one or more layers (18) comprises a first layer (18a) and a second layer (18b), the first layer (18a) is proximal to the first side (3) of the ceramic (2) body and the second layer (18b) is bonded on top of the first layer (18a). In another embodiment, the reaction layer (17) comprises the first layer (18a). In another embodiment, the reaction layer (17) comprises the second layer (18b). For example, in some embodiments, the ceramic body (2) comprises an alumina ceramic and the bonding layer comprises an active metal element and alloying elements. The alloying elements comprises an Ag-Cu eutectic alloy with around 72 %wt Ag and around 28 %wt Cu. In one embodiment, the active metal element comprises Ti in the range of about 1.75 to about 4.5 %wt. The reaction layer (17) comprises the first layer (18a) comprising a thin (e.g. nanometer(s) thick) TiO layer and the second layer (18b) comprising a Ti3Cu30. In another embodiment, the active metal element comprises Ti in the range of less than 1.75 %wt. The reaction layer (17) comprises the first layer (18a) comprising a thin TiO layer. In another embodiment, the active metal element comprises Ti in the range of at least 4.5 %wt. The reaction layer (17) comprises the second layer (18b) comprising Ti3Cu30.

Example 1

A co-fired alumina Pt/Ir (diameter 50.8pm) feedthrough was diced (1 mm thickness) from a larger block and subsequently ground and lapped flat to at least a Ra 32 micro-inches finish.

1. Mask the feedthrough such that only the area of the proposed conductive padding is exposed over the pin for sputtering.

2. Deposit a titanium layer of approximately 400 nm thickness on top a pin and extending radially approximately at least 100 pm onto the top of the ceramic substrate.

3. Deposit a niobium layer of approximately 2.0 pm thickness by sputtering.

4. Deposit a nickel/chrome (80/20) layer of approximately 1 pm thickness by sputtering.

5. Sputter coat a final layer of gold of approximately 0.5 pm thickness.

6. Sinter the assembly at 1100°C for approximately 30 minutes.

A variation of the above methodology is to first sinter the niobium and titanium layers at 1100°C for approximately 30 minutes, prior to sputter coating the third and fourth layers after which the assembly is sintered at 950°C for approximately 10 minutes.

A schematic diagram of the layer structure of the abovementioned example is provided in Figure 4, the first side (3) of the ceramic body (2) is provided with a multi-layered conductive pad (6). The bonding layer (7) comprises Ti; the barrier diffusion layer (8) comprises Nb; the first sealing layer (9a) comprises Ni and the second (top) sealing layer (9b) comprises Au.

Figure 5 is a sectional scanning electron microscope (SEM) micrograph showing a cross section of the feedthrough, including the conductive pad according to the configuration illustrated in Figure 4 after the sintering step.

Figures 6 illustrates a portion of the conductive pad (6) reaction bonded to the surface of the first side (3) of the ceramic body (2). An EDS line-scan (50; Figure 8) revealed that the Ti bonding layer (7) was approximately 400 nm thick and the Nb diffusion barrier layer (8) was about 2 pm thick. The line-scan also reveals that there was a small amount of diffusion of titanium into the diffusion barrier layer (e.g. < less than about 500 nm) before the titanium intensity levels reached a background noise level, signifying no detectable titanium levels. Without the diffusion barrier layer, the titanium bonding layer and sealing layers are likely to have diffused into each other, weakening the bond or the longevity thereof, between the bonding layer and the ceramic substrate.

The line-scan (Figure 8) also reveals that the first sealing layer (9a) and the second sealing layer (9b) have diffused into each other to form a single Ni-Au alloy sealing layer having a thickness of about 1.5 pm. The line-scan also reveals a degree of diffusion of nickel and gold into the niobium diffusion barrier layer.

Hermeticity The hermeticity tests were performed on nine samples of the feedthrough with and without a conductive pad. The conductive pad was derived from a four layer assembly structure as represented in Figure 4 which was sintered to produce the feedthrough assemble of Figure 5. The feedthroughs were tested for hermeticity using the protocol of MIL-STD-883 test method 1014 and test condition.

Table 1 shows the results of hermeticity testing performed on nine samples of this embodiment, according to the method discussed herein.

Helium leak rate (cc.atm/s) Sample Without conductive pad With conductive pad 1 6.4x101° 8.2x101 2 5.2x10-9 3.1x10-1° 3 1.3x10-6 6.1x10-11 4 1.9x10-16 2.2x10-16 4.2x10-6 3.1x10-8 6 3.9x10-7 1.6x10-6 7 8.2x10-6 3.3x10-9 8 7.1x10-6 2.4x10-6 9 4.8x10-6 3.1x10-6 Average 2.7x10-6 9.4x10-9

Table 1

The hermeticity tests were subsequently repeated after the conductive pad was bonded to the first side of said ceramic body. The results showed that the conductive pad provided the feedthrough with an improved hermetic seal or a sintered seal over said first side of the ceramic body.

For each sample, an increase in the He hermeticity (reduction in He permeability) was observed. The average He hermeticity increased from 2.7 x 10-6 cc-atmis to 9.4 x 1a9 cc.atm/s for the nine samples.

Resistivity The resistivity (at room temperature) of the feedthrough of Example 1 was measured with and without the conductive pad, with the results (Table 2), confirming that the conductive pad is able to maintain a high conductivity of the feedthrough assembly.

Pt/Ir (90/10) + conductive pad % change Average Resistivity (mom) 2.78 x las 3.79 x 10-5 36 Standard Deviation (mem) 3.65 x 10-6 9.10 x 10-6

Table 2

Effect of the sintering step As illustrated in Figure 7, a feedthrough assembly was formed similar to the procedure of Example 1, with a co-fired zirconia toughened alumina substrate (100) with five 50 pm diameter Pt/Ir pins with a centre to centre spacing of approximately 620 pm. The ceramic substrate was approximately 1 mm thick and machined from a monolithic feedthrough block.

Each of the pins had an oblong conductive pad sputtered coated and sintered.

Each oblong shaped conductive pad had a width of approximate 420 pm (radial overlap of approximately 185 pm) and a length of approximately 800 pm (i.e. 375 pm radial overlap). The gap "A" between adjacent conductive pads was approximately 200 pm.

The second side was sputter coated and sintered with the oblong shaped conductive pad comprising the same thickness and diameter layers of Ti and Nb, followed by a Ni/V alloy coating layer (75 nm) and a 450 nm Au top coating.

A hermeticity test was performed on the feedthrough before and after the sintering step, with the results provided in Table 3. The results indicate that sintering significantly reduces the amount of helium which leaks through the feedthrough. The decrease in the helium leakage may be attributable to the reaction bond layer created at the ceramic -Ti interface, in addition to the sintering step densifying the layers of the conductive pad.

Helium leak rate (cc.atm/s) Sample No sintering First side sintered 1 1.6x10-8 1.7x1011 2 6.4x10-9 8.8x10-11 3 2.4x10-1° 3.6x10-1° Average 1.0x109 3.6x10-11

Table 3

The conductive pads were also evaluated for adhesion to the ceramic surface. When adhesive tape was applied and removed from the unsintered first side of the feedthrough a substantial proportion of the conductive pads were observed to be removed with the adhesive tape. However, there was no removal of the conductive pads when the adhesive tape was applied to the sintered first side of the feedthough. The sintered conductive pad were then resistance welded to gold wires. Tweezers were used to assess the strength of the bond, with the bond strength deemed excellent. The test results confirms the presence of a reaction bond between the bonding layer and the ceramic substrate.

Claims (25)

1. CLAIMS: 1. A feedthrough assembly (1) comprising: a ceramic body (2) having a first side (3) and a second side (4); a conductive element (5) extending through said ceramic body (2) between said first side (3) and said second side (4); a conductive pad (6) electrically connected to said conductive element (5); wherein the conductive pad (6) comprises a multi-layered arrangement comprising: a bonding layer (7) comprising one or more elements selected from the group consisting of Ti, Zr, Nb, Ta, V, Mo, Mn, W, Y and Hf, said bonding layer in bonding contact with an end of the conductive element and the first or second side of the ceramic body; and at least one of: (i) a diffusion barrier layer (8) directly disposed upon said bonding layer, comprising one or more elements selected from the group consisting of Nb, Ta, W, Mo and nitrides thereof, said diffusion layer having a different composition than the bonding layer; and (ii) one or more sealing layers (9, 9a, 9b), disposed upon said diffusion barrier layer.
2. 2. The feedthrough assembly accord to claim 1, wherein the multi-layered arrangement comprises the diffusion barrier layer (8) and one or more sealing layers (9, 9a, 9b), disposed upon said diffusion barrier layer.
3. 3. The feedthrough assembly accord to claim 1, wherein the multi-layered arrangement comprises one or more sealing layers (9, 9a, 9b), disposed upon said bonding layer (7).
4. 4. The feedthrough assembly according to any one of the preceding claims, wherein the one of more sealing layers comprise a first sealing layer (9a) comprising one or more elements selected from the group consisting of Co, Ni, V, Al, Si, Cu, Ag, In, Ti, Cr, Ta, W, Mo.
5. 5. The feedthrough assembly according to claim 4, wherein the one or more sealing layers comprises a second sealing layer (9b) disposed upon the first sealing layer comprising one of more elements selected from the group consisting of Au, Pt, Ni, Cr, Cu and Al, said second sealing layer having a different composition than said first sealing layer.
6. 6. The feedthrough assembly according to any one of the preceding claims, wherein in the thickness of the bonding layer (7) is in the range of 0.01 pm to 10 pm.
7. 7 The feedthrough assembly according to any one of claims 1, 2, 4 to 6, wherein in the thickness of the diffusion barrier layer (8) is in the range 0.05 pm to 10 pm.
8. 8. The feedthrough assembly according to any one of the preceding claims, wherein in the thickness of the each of the one of more sealing layers (9, 9a, 9b) is in the range 0.1 pm to 10 pm.
9. 9. The feedthrough assembly according to any one of the preceding claims, wherein the bonding layer bonds to a bonding area, said bonding area encompassing the end of a conductive element and extending circumferentially from a periphery of the end of the conductive element over an adjacent surface of the ceramic body such that the minimum distance between the periphery of the bonding layer and the periphery of the conductive element is at least 1.0 pm.
10. 10. The feedthrough assembly according to claim 9, wherein the bonding area extends from a periphery of the end of the conductive element such that the distance between the bonding area and an adjacent conductive pad is at least 10 pm.
11. 11. A feedthrough assembly (1) according to any one of the preceding claims comprising a plurality of conductive elements (5).
12. 12. The feedthrough assembly according to claim 11, wherein the density of the conductive elements (5) exceeds 1 conductor per 100,000 pm2 through a planar cross-section of the ceramic body (2).
13. 13. A feedthrough assembly (1) as claimed in any one of the preceding claims, wherein said conductive element (5) is in braze-less contact with said ceramic body (2) between said first side (3) and said second side (4) forming a braze-less interface (12a). 14. 15. 16. io 17. 18. 19.
14. A feedthrough assembly (1) as claimed in claims 1 to 4 wherein said conductive element (5) is brazed to said ceramic body (2) between said first side (3) and said second side (4) forming a brazed interface (12b).
15. A feedthrough assembly (1) as claimed in claim 12 wherein said brazed interface (12b) comprises one or more elements selected from the list consisting of Au, Cu, Ag, Ni or combinations or alloys thereof.
16. A feedthrough assembly (1) according to any one of the preceding claims further comprising a second conductive pad (6a) electrically connected to said conductive element (5) wherein said second conductive pad (6a) is bonded to said opposing side of said ceramic body (2) through said bonding layer (7).
17. A feedthrough assembly (1) as claimed in any of the preceding claims wherein said feedthrough (1) has a He permeability of less than 1.0 x 10' cc.atm/s.
18. A feedthrough assembly (1) as claimed in any of the preceding claims wherein said feedthrough (1) is an implantable medical device.
19. A method of producing a feedthrough assembly accordingly to any one of the preceding claims comprising the steps of A Providing a feedthrough (1) comprising: a ceramic body (2) having a first side (3) and a second side (4); a conductive element (5) extending through said ceramic body (2) between said first side (3) and said second side (4); B If required, machining an end of the conductive element, such that the end of the conductive element is substantially flush or otherwise offset with respect to an adjacent surface of the ceramic body; C Optionally masking the area around the end of the conductive element, such that there is an unmasked area exposing the end of the conductive element and a portion of the adjacent surface; D Depositing a bonding layer to the unmasked area comprising one or more elements selected from the group consisting of Ti, Zr, Nb, Ta, V, Mo, Mn, W, Y and Hf; E. Depositing at least one of: * a diffusion barrier layer to the bonding layer comprising one or more elements selected from the group consisting of Nb, Ta, W, Mo and nitrides thereof; and * one or more sealing layers on the diffusion barrier layer; and F. Sintering said layers at sufficient temperature and time for the bonding layer to form a reaction bond with the adjacent surface of the ceramic body.
20. 20. The method according to claim 19, further comprising the step of depositing one or more sealing layers on top of the diffusion barrier layer.
21. 21. The method according to claim 20, wherein the one or more sealing layers are deposited after Step F, and the one or more sealing layers are sintered at sufficient temperature and time for the one or more sealing layers to bond to the adjacent layers.
22. 22. The method according to any one of claims 19 to 21, wherein the unmasked area defining the surface of the ceramic body is an annular shape.
23. 23. The method according to claim 22, wherein the distance that the unmasked area extends from a periphery of end of the conductive element is greater than 1.0 pm and no more than a distance equivalent to twice the diameter of the conductive element (5).
24. 24. The method according to any one of claims 19 to 23, wherein the feedthrough has been machined from a co-fired monolithic feedthrough block.
25. 25. A feedthrough assembly produced by the method according to any one of claims 19 to 24.