Feedthrough Comprising Interconnect Pads

Abstract

A feedthrough assembly (1) comprising a feedthrough body (10) 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). 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 and V, 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.

Claims

1. A feedthrough assembly comprising: a feedthrough body comprising: a ceramic body having a first side and a second side; a conductive element extending through said ceramic body between said first side and said second side; and a conductive pad electrically connected to said conductive element; wherein the conductive pad comprises a multi-layered arrangement comprising: (i) a bonding layer comprising one or more elements selected from the group consisting of Ti, Zr, Nb and V, said bonding layer in bonding contact with an end of the conductive element and the first side or second side of the ceramic body; and (ii) a diffusion barrier 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 compared to the bonding layer; and/or (iii) one or more sealing layers disposed upon said bonding layer or said diffusion barrier layer.

2. The assembly according to claim 1, wherein the multi-layered arrangement comprises the one or more sealing layers, disposed upon said diffusion barrier layer, said one or more sealing layers each having a different composition compared to the diffusion barrier layer.

3. The assembly according to claim 1, wherein the multi-layered arrangement comprises one or more sealing layers, disposed upon said bonding layer, said one or more sealing layers each having a different composition compared to the bonding layer.

4. The assembly according to claim 1, wherein the one of more sealing layers comprises one or more elements selected from the group consisting of Pt, Au, Ni, Pd, Cr, V, and Co.

5. The assembly according to claim 1, wherein the bonding layer further comprises one or more elements selected from the group consisting of Mo, Ta, W and Hf.

6. The assembly according to claim 1, wherein the bonding layer comprises Ti.

7. The assembly according to claim 1, wherein the diffusion barrier layer comprises one or more elements selected from the group consisting of Nb, Ta, W and nitrides thereof.

8. The assembly according to claim 7, wherein the diffusion barrier layer comprises Nb or nitrides thereof.

9. The assembly according to claim 1, wherein the bonding layer comprises Ti; the diffusion barrier layer comprises Nb; and the one or more sealing layers comprises Ni and Au.

10. The assembly according to claim 9, wherein the assembly comprises a second conductive pad electrically connected to the conductive element on the opposing side of the ceramic body, said second conductive pad comprising a bonding layer comprising Ti; a diffusion barrier layer comprising Nb; and a sealing layer comprising Ni.

11. The assembly according to claim 1, wherein the resistivity of the conductivity element and conductive pad is no more than 5.0×10.sup.−5 Ω.Math.cm.

12. The assembly according to claim 1, wherein the bonding layer and/or the diffusion barrier layer has a thickness in the range of 0.01 μm to 10 μm.

13. The assembly according to claim 1, wherein the one or more sealing layers have a thickness between 1.5 to 100 times greater thickness than the combined thickness of the bonding layer and the diffusion barrier layer.

14. The assembly according to claim 1, wherein the density of the conductive elements exceeds 1 conductor per 100,000 μm.sup.2 through a planar cross-section of the ceramic body.

15. The assembly according to claim 1, wherein said feedthrough upon sintering has a He permeability of less than 1.0×10.sup.−7 cc.Math.atm/s.

16. A medical device feedthrough comprising the assembly according to claim 1.

17. A method of producing a feedthrough assembly comprising: providing a feedthrough body comprising: a ceramic body having a first side and a second side; a conductive element extending through said ceramic body between said first side and said second side; optionally, 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; 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; depositing a bonding layer to the an end of the conductive element and a portion of the adjacent surface of the ceramic body, said bonding layer comprising one or more elements selected from the group consisting of Ti, Zr, Nb, Ta, V, Mo, Mn, W, Y and Hf and combinations thereof; depositing a diffusion barrier layer on the bonding layer comprising one or more elements selected from the group consisting of Nb, Ta, W, Mo and nitrides thereof; and/or one or more sealing layers on the diffusion barrier layer or on the bonding layer; and sintering at least the bonding layer to the ceramic body at a sufficient temperature for the bonding layer to form a reaction bond with a surface of the ceramic body.

18. The method according to claim 17, wherein the feedthrough body has been fired prior the depositing of the bonding layer.

19. The method according to claim 17, 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 layer(s).

20. A feedthrough assembly produced by the method of claim 17.

21. A feedthrough precursor comprising: a feedthrough body comprising: a ceramic body having a first side and a second side; a conductive element extending through said ceramic body between said first side and said second side; and a conductive pad electrically connected to said conductive element; wherein the conductive pad comprises a multi-layered arrangement comprising: (i) a bonding layer comprising one or more elements selected from the group consisting of Ti, Zr, Nb and V, said bonding layer in bonding contact with an end of the conductive element and the first side or second side of the ceramic body; and (ii) a diffusion barrier 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 compared to the bonding layer; and/or (iii) one or more sealing layers disposed upon said bonding layer or said diffusion barrier layer, wherein upon sintering to form a feedthrough assembly, a helium leak rate of the feedthrough assembly decreases relative to the feedthrough precursor.

22. The feedthrough precursor of claim 21, wherein upon sintering to form the feedthrough assembly, the helium leak rate decreases by at least a factor of 10.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0134] Embodiments will now be described, by way of example only and with reference to the accompanying drawings having like-reference numerals, in which:

[0135] FIG. 1 shows a schematic cross-sectional representation of the feedthrough assembly of the present disclosure in a first possible embodiment.

[0136] FIG. 2a shows a schematic cross-sectional representation of the feedthrough assembly of the present disclosure in a second possible embodiment.

[0137] FIG. 2b shows a schematic cross-sectional representation of the feedthrough assembly of the present disclosure in a third possible embodiment.

[0138] FIG. 3 shows a schematic cross-sectional representation of the feedthrough assembly of the present disclosure in a fourth possible embodiment.

[0139] FIG. 4 shows a schematic cross-sectional representation of the feedthrough assembly (1.1) of the present disclosure in a fifth possible embodiment.

[0140] FIG. 5 shows a sectional SEM micrograph of a portion of the feedthrough assembly (1.2) of the present disclosure corresponding to the fifth possible embodiment upon sintering.

[0141] FIG. 6 shows a magnified portion of the sectional SEM micrograph of FIG. 5.

[0142] FIG. 7a shows a photograph of a plurality of conductive pads comprising a Ti bonding layer and a Nb diffusion barrier layer according to a preferred embodiment of the present disclosure.

[0143] FIG. 7b shows a photograph of a plurality of conductive pads of FIG. 7a, with the further addition of Ni and Au sealing layers according to Example 1 of the present disclosure.

[0144] FIG. 7c shows a photograph of a cross-sectional view of the feedthrough assembly of Example 1.

[0145] FIG. 8 shows an EDS line-scan taken form the feedthrough assembly of FIG. 6.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0146] The present disclosure 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.

[0147] 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.

[0148] 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.

[0149] In accordance with embodiments of the disclosure, FIG. 1 shows a schematic cross-sectional representation of the feedthrough assembly (1) of the present disclosure in a first possible embodiment. The feedthrough assembly (1) comprises a feedthrough body (10) and a conductive pad (6). The feedthrough body (10) 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). The 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) or the reactive layer (17), thereby weakening the adhesion of the conductive pad to the ceramic body. 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).

[0150] 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.

[0151] 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.

[0152] 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).

[0153] 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).

[0154] Referring to FIGS. 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.

[0155] Referring to FIGS. 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.

[0156] As illustrated in FIG. 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).

[0157] In one embodiment, the conductive pad (6) provides a conductive pathway to the conductive 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).

[0158] 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).

[0159] 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.

[0160] 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.

[0161] 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.

[0162] 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 1.0×10.sup.−10 cc.Math.atm/s may be detected. For example, for a component required to be watertight, a leak rate with a He hermeticity of 1.0×10.sup.−4 cc.Math.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.

[0163] 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.

[0164] 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.

[0165] 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 compounds between the active metal element and the alloying elements.

[0166] 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.

[0167] 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.

[0168] 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).

[0169] 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.

[0170] Referring to FIG. 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).

[0171] 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 Ti.sub.3Cu.sub.3O. 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 Ti.sub.3Cu.sub.3O.

Example 1

[0172] A co-fired alumina Pt/Ir (diameter 50.8 μm) feedthrough was diced (1 mm thickness) from a larger block and subsequently ground and lapped flat with R.sub.a being less than 10 μm finish. [0173] 1. Mask the feedthrough such that only the area of the proposed conductive padding is exposed over the pin for sputtering. [0174] 2. Deposit a titanium layer of approximately 400 nm thickness on top a pin and extending radially approximately at least 100 μm onto the top of the ceramic substrate. [0175] 3. Deposit a niobium layer of approximately 2.0 μm thickness by sputtering. [0176] 4. Deposit a nickel/chrome (80/20) layer of approximately 1 μm thickness by sputtering. [0177] 5. Sputter coat a final layer of gold of approximately 0.5 μm thickness. [0178] 6. Sinter the assembly at 1100° C. for approximately 30 minutes.

[0179] 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.

[0180] A schematic diagram of the layer structure of the feedthrough precursor (1.1) to the feedthrough in the above mentioned example is provided in FIG. 4, the first side (3) of the ceramic body (2) is provided with a multi-layered conductive pad (6) prior to sintering. 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. After sintering, the first and top sealing layers (9a and 9b, respectively) may disperse into one another to form a single Au—Ni alloy layer. After sintering, there is partial diffusion of the bonding layer (7) into the barrier diffusion layer (8).

[0181] FIG. 5 is a sectional scanning electron microscope (SEM) micrograph showing a cross section of the finished feedthrough (1.2) according to the configuration illustrated in FIG. 4 after the sintering step. The finished feedthrough (1.2) comprises the conductive element (5), the conductive pad (6), and the braze-less interface (12b). The Roughness (R.sub.max) of the conductive pad is estimated to be less than 1.0 μm.

[0182] FIG. 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; FIG. 8) revealed that the Ti bonding layer (7) was approximately 400 nm thick and the Nb diffusion barrier layer (8) was about 2 μm 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.

[0183] FIG. 7a illustrates the top view of the feedthrough assembly (110) after the Ti and Nb has been deposited over the conductive element and adjacent ceramic body (100). FIG. 7b illustrates the top view of the completed sintered feedthrough assembly 110. FIG. 7c is a cross-section view of one of the conductive elements (120) with the conductive pad (110) covering both the top of the conductive element and the surface of the ceramic body (100). The interface 130 between the conductive element and the ceramic body comprises void spaces, formed during the co-firing process, which may enable gas to leak through the feedthrough. The conductive pad, with its secure bond to the surface of the ceramic body provides additional protection against gaseous leaks.

[0184] The line-scan (FIG. 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 μm. The line-scan also reveals a degree of diffusion of nickel and gold into the niobium diffusion barrier layer.

Hermeticity

[0185] 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 FIG. 4 which was sintered to produce the feedthrough assembly of FIG. 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.

TABLE-US-00001 TABLE 1 Helium leak rate (cc .Math. atm/s) Sample Without conductive pad With conductive pad 1 .sup. 6.4 × 10.sup.−10 .sup. 8.2 × 10.sup.−11 2 5.2 × 10.sup.−9 .sup. 3.1 × 10.sup.−10 3 1.3 × 10.sup.−9 .sup. 6.1 × 10.sup.−11 4 .sup. 1.9 × 10.sup.−10 .sup. 2.2 × 10.sup.−10 5 4.2 × 10.sup.−6 3.1 × 10.sup.−8 6 3.9 × 10.sup.−7 1.6 × 10.sup.−8 7 8.2 × 10.sup.−6 3.3 × 10.sup.−9 8 7.1 × 10.sup.−6 2.4 × 10.sup.−9 9 4.8 × 10.sup.−6 3.1 × 10.sup.−8 Average 2.7 × 10.sup.−6 9.4 × 10.sup.−9

[0186] 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 2.7×10.sup.−6 cc.Math.atm/s to 9.4×10.sup.−6 cc.Math.atm/s for the nine samples.

Resistivity

[0187] 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.

TABLE-US-00002 TABLE 2 % Pt/Ir (90/10) +conductive pad change Average Resistivity (Ω .Math. cm) 2.78 × 10.sup.−5 3.79 × 10.sup.−5 36 Standard Deviation (Ω .Math. cm) 3.65 × 10.sup.−6 9.10 × 10.sup.−6 —

Effect of the Sintering Step

[0188] As illustrated in FIGS. 7a b & c, a feedthrough assembly was formed according to the procedure of Example 1, with a co-fired zirconia toughened alumina substrate (100) with five 50 μm diameter Pt/Ir pins with a centre to centre spacing of approximately 620 μm. 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. The estimated roughness (R.sub.max) of the conductive pad is estimated to be less than 1.0 μm.

[0189] Each oblong shaped conductive pad had a width of approximate 420 μm (radial overlap of approximately 185 μm) and a length of approximately 800 μm (i.e. 375 μm radial overlap). The gap “A” between adjacent conductive pads was approximately 200 μm.

[0190] 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.

[0191] 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.

TABLE-US-00003 TABLE 3 Helium leak rate (cc .Math. atm/s) Sample No sintering First side sintered 1 1.6 × 10.sup.−8 1.7 × 10.sup.−11 2 6.4 × 10.sup.−9 8.8 × 10.sup.−11 3 .sup. 2.4 × 10.sup.−10 3.6 × 10.sup.−10 Average 1.0 × 10.sup.−9 3.6 × 10.sup.−11

[0192] 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 feedthrough. 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 confirm the presence of a reaction bond between the bonding layer and the ceramic substrate.