WO1988005959A1

WO1988005959A1 - Ceramic substrate with conductively-filled vias and method for producing

Info

Publication number: WO1988005959A1
Application number: PCT/US1988/000334
Authority: WO
Inventors: Lawrence Cochran; Robert E. Mcreynolds; John Barker
Original assignee: Coors Porcelain Company
Priority date: 1987-02-04
Filing date: 1988-01-29
Publication date: 1988-08-11
Also published as: AU1346088A

Abstract

A ceramic substrate (10) having vias (16) filled with a conductive material (18) which forms a hermetic seal for the vias. The conductive material (18) in the vias is the sintered product of a powder mixture comprising a metal, preferably tungsten, and a ceramic powder, preferably alumina. The conductive material (18) in the via has a resistance less than about 0.5 ohms. The conductive material (18) forms a seal sufficient to provide a leakage rate of less than 1.0 x 10-8 STD cc per second of helium at a one atmosphere differential.

Description

CERAMIC SUBSTRATE WITH CONDUCTIVELY-FILLED VIAS AND METHOD FOR PRODUCING

Field of The Invention

The present invention relates to a ceramic sub- strate for electronics use having vias filled with a conductive material and particularly to a substrate for application of circuitry to a surface thereof with vias which are filled with a conductive material in such a manner as to hermetically seal the vias.

Background Information

Substrates for supporting electronic circuitry such as printed circuit boards or chip carriers, are commonly formed of a ceramic material. Ceramics are useful for printed circuit or chip carrier applications because ceramics can be provided with high electrical insulative properties, good resistance to thermal and mechanical stress or chemical corrosion, a surface which provides for good adhesion of circuitry layers, a thermal expansion coeffecient compatible with that of attached circuitry and other electronic components, and high smoothness of surface, as required for production of fine-lined circuitry on the substrate surface. In some applications, it is desired to provide a conduc- tive path which is perpendicular to a circuit-carrying surface of the substrate, for example when it is desired to provide electrical connection between cir¬ cuitry on one surface of the substrate and another sur¬ face of the substrate or when it is desired to provide electrical connection between one surface of the sub¬ strate and circuitry or other conductive material em¬ bedded in the substrate. Such perpendicular conductive pathway can be formed employing a via which is a hole through the substrate providing communication from one surface of the substrate to another surface. A via can be used to form a connective pathway by coating the wall of the via with a conductive material, or by plac¬ ing conductive material in the via hole so that a con¬ ductive "plug" resides in the via.

A number of methods have been proposed for forming conductive paths through vias and the like. U.S. Patent No. 3,561,110, issued February 9, 1971 to Feul- ner, et al. , discloses placing powdered metal into via holes and sintering. U.S. Patent No. 4,521,449, issued June 4, 1985 to Arnold, et al., discloses filling vias with conductive metal paste and sintering. Neither of these references, however, provide a conductive filling for a via which seals the via hermetically, i.e. so as to be leak-free when subjected to a gas pressure dif¬ ferential across the via such as a differential of about one atmosphere or more. Such hermetic sealing is important when the substrate is to form part of an enclosure for maintaining circuitry in a controlled at¬ mosphere such as a vacuum, a noble gas atmosphere, or a dry or dust-free atmosphere. Hermetic sealing is also important when a pressure difference is to be main¬ tained across the substrate, for example when the sub¬ strate is to be placed in a low pressure environment, including high altitude, or space environment or in a high pressure environment, including underwater en- vironments. Further, a hermetic seal provides some in¬ dication of the absence of voids, gaps or pin holes in the conductive.material and thus is indicative of high and uniform conductance in the via.

Summary of The Invention

The present invention involves a ceramic substrate with via holes which are filled with a conductive material in such a manner that the via holes are her¬ metically sealed. Such a hermetically sealed substrate is useful in connection with providing circuitry which is hermetically sealed from the ambient atmosphere, e.g., so as to avoid unwanted contact with humidity or moisture which can lead to corrosion of circuitry ele- ments.

A substrate green body is formed with one or more vias providing communication between two substrate sur¬ faces. The via is filled with a mixture of a metal powder and a powder of a ceramic material. The pow- dered metal is preferably a refractory metal such as tungsten. The ceramic powder is preferably of a ceramic of the same type as the ceramic from which the green substrate is formed. The mixture preferably con¬ tains between about 35 percent and about 42 percent by weight of the ceramic powder. The powder is placed in the holes preferably using a squeegee apparatus. A sheet of resilient material such as latex is placed on the substrate surface and pressure is applied to com¬ pact the fill material. The squeegee and compaction is performed on each side of the substrate and the entire process is preferably repeated one^' or more times to completely fill and compact the mixture into the via holes. The green body substrate with filled vias is sintered to produce a sintered substrate with hermeti- cally sealed vias. The vias have low resistance, preferably less than about 0.5 ohms for a 0.3 mm (0.012 inches) diameter via which is 1.3 mm (0.05 inches) deep. The vias have no pin holes greater than about 0.05 mm (0.002 inches) in diameter and are able to provide a leakage rate of less than 1.0 x 10-^{8 "}STD cc per second at one atmosphere differential. Brief Description of The Drawings

Fig. 1 is a perspective view of the ceramic sub¬ strate with filled vias with a portion broken away to show the interior of a filled via; Fig. 2 is a cross-sectional view of the ceramic substrate taken along line 2-2 of Fig. 1;

Fig. 3 is a schematic elevational view of an ap¬ paratus for filling vias using a squeegee with a por¬ tion broken away to show the interior of the apparatus; Fig. 4 is a schematic elevational view of ap¬ paratus for compacting conductive material in the vias; Fig. 5 is an exploded perspective view of a device for hermatically sealing a circuit-bearing surface of a substrate from the ambient atmosphere; and Fig. 6 is a cross-sectional view of a hermetically sealed device having a circuit-bearing surface of a substrate sealed from the ambient atmosphere.

Detailed Description of The Preferred Embodiment According to the present invention, a ceramic sub¬ strate includes vias which are filled with a conductive material in a hermetically sealed fashion. As depicted in Fig. 1, the finished substrate 10 has an upper sur¬ face 12 and a lower_^surface 1 . A number of vias 16 extend between the upper surface 12 and the lower sur¬ face 14 so as to provide communication between the up¬ per surface 12 and the lower surface 14. The vias 16 are filled with a conductive material 18 in such a way that the vias 16 are hermetically sealed. Because the vias 16 are hermetically sealed, when the fluid pres¬ sure across the upper surface 12 of the substrate 10 is different from the fluid pressure over the lower sur¬ face 14 of the substrate 10, there is no substantial fluid leakage through the vias 16. The vias 16 are substantially without voids, gaps, or pin holes, and particulary there are no pin holes through the conduc¬ tive material 18 in the vias 16 which are larger than about 0.05 millimeters (0.002 inches). Preferably, when the pressure over the upper surface 12 differs from the pressure over the lower surface 14 by about 1 atmosphere (100 KPa), the rate of leakage of, for ex¬ ample, helium is less than 1.0 x 10^~8 STD cc per second. The conductive material 18 has a conductivity such that the resistance of the conductive material 18 is less than about 0.5 ohms, preferably less than about 0.3 ohms, and most preferably approximately 0.2 ohms for, e.g., a 0.3 mm (0.012 inches) diameter via which is 1.3 mm (0.05 inches) deep.

The substrate 10 is formed using any of a number of ceramic materials including alumina, or zirconia. The ceramic material preferably possesses a high dielectric constant, resistance to mechanical or ther- m l shock, and resistance to chemical corrosion, good adhesion of conductive circuitry or other desired layers, a coefficient of thermal expansion similar to that of the circuity or other electronic component, and can be formed into a substrate having a smooth surface 12. Alumina is preferred because it provides the desired characteristics at an economical cost and can be sintered at a moderate sintering temperature, such as about 1600°C.

The conductive material 18 is formed by sintering a mixture of a metallic material and a powdered^"ceramic material. The metallic material preferably provides a high electrical conductivity and a melting point greater than the sintering temperature for the ceramic used in forming the substrate. A preferred metal is tungsten, although other metals such as molybdenum can be used. The ceramic component of the mixture from which the conductive material 18 is formed can be any of a number of ceramics and is preferably the same ceramic composition as the ceramic from which the sub¬ strate 10 is formed.

The metal/ceramic mixture from which the conduc¬ tive material 18 is formed is mixed in a ceramic-to- metal weight proportion which is high enough that the mixture, upon sintering, forms a hermetic seal in the vias 16, but low enough that the conductive material 18 has a low resistance, preferably less than about 0.5 ohms, more preferably less than about 0.3 ohms, and most preferably approximately 0.2 ohms for a cylinder with a 0.3 mm (0.012 inches) diameter, 1.3 mm (0.05 inches) in length. When the metal is tungsten and the ceramic is alumina, the mixture should contain more than about 35 weight percent alumina in order for the conductive material 18 to form a hermetic seal in the via 16, and should be less than about 42 weight percent alumina in order for the conductive material 18 to have a sufficiently low resistance. A mixture of about 40 percent alumina is preferred.

The method of forming a ceramic substrate with hermetically-filled conductive vias will now be described. A green ceramic substrate 30 having one or more vias 32 is formed by any of a number of known methods for producing a green ceramic substrate includ¬ ing dry-pressing, isostatic pressing, injection old- ing, or cast tape processes. The preferred process is a cast tape process. The green substrate is formed with one or more vias 32 providing communication be¬ tween two surfaces of the substrate. The vias 32 are of such a diameter that following the shrinkage which the green body 30 undergoes during sintering, the vias 16 in the final product 10 will have the desired diameter. A final product via 16 with a diameter of between about 0.25 mm (0.010 inches) and about 0.40 mm (0.015 inches) is operable, although larger and smaller via sizes are also operable.

The substrate ^'30 is placed on a vacuum chuck 34 which is connected to a vacuum source by a line 36. A porous membrane 38 is positioned between the green sub- strate 30 and the vacuum chuck 34. The vacuum chuck 34 is provided with a number of holes 40 which are in a pattern matching the pattern of vias 32 in the green substrate 30. The green substrate 30 is positioned so as to align the vias 32 with the holes 40. Fill material 42 is prepared by mixing a metal powder and a ceramic powder. An amount of tungsten is milled to provide a tungsten powder having an average particle size of about 2.5 microns. The tungsten pow¬ der is mixed with a ceramic alumina powder having an average particle size of about 2.5 microns in a ratio of about 40 weight percent alumina to 60 weight percent tungsten. The mixture can be f rther milled to provide a uniformly mixed powder of tungsten and alumina with an average particle size of about 2.5 microns. A quantity of the fill material 42, having the composition described above, is placed on the upper surface 44 of the green substrate 30. The vacuum chuck 34 is provided with a vacuum of about 23 inches of mer¬ cury. A rubber squeegee 46 mounted in a pressure handle 48 is placed in contact with the upper ^"surface 44 of the green substrate 30. The pressure handle 48 is adjusted to provide a squeegee pressure of about 80 psi (0.55 KPa) . The squeegee 46 is drawn across the upper surface 44 of the green substrate 30, preferably at a rate of about three cycles per second. The squeegee process is preferably repeated three times to provide a total of four complete squeegee cycles against the upper surface 44 of the green substrate 30. The green substrate 30 is then removed from the vacuum chuck 34 and turned over so that the lower sur¬ face 50, as depicted in Fig. 3, is uppermost, occupying the position of the upper surface 44, depicted in Fig. 3. The squeegee process is then repeated with respect to the lower surface 50 of the green substrate 30, i.e. fill material 42 is placed on the surface 50, 23 inches of mercury vacuum is created in the vacuum chuck 34, the vias 32 are aligned with the holes 40, and the squeegee 46 is drawn across the surface 50 under a pressure of about 80 psi (0.55 KPa), preferably for four complete cycles of squeegee application. The com¬ bined effect of the squeegee pressure and the vacuum suction is to place an amount of the fill material 42 into each of the vias 32. Following the vacuum/pressure-squeegee procedure described above, the substrate is subjected to a com¬ paction process. As best seen in Fig. 4, the green substrate 30 is sandwiched between two sheets of latex rubber 52. The rubber is preferably "double latex" ^" having a thickness of about 0.5 mm (0.02 inches). A sheet of a smooth polyester material such as MYLAR 54 is placed underneath the bottom sheet of latex. The green substrate 30, latex 52 and MYLAR 54 are then sub¬ jected to pressure, such as by pressing in a flat-plate press 56 or feeding between adjustable pressure^'rollers to provide a pressure of about 270 psi (2.4 KPa). Be¬ cause of the deformability and compressibility of the latex 52, the fill material 42 residing in the vias 32 is compacted within the vias 32. Following application of pressure to the substrate 30, latex 52 and MYLAR 54, the latex layers 52 and MYLAR layer 54 are removed from the green ceramic sub¬ strate 30 and the green substrate 30 is turned over so that the surface which was on the bottom during the ap¬ plication of pressure is now on top. With the green substrate 30 in this orientation, the ceramic substrate 30 is again sandwiched between the sheets of latex 52 and the MYLAR sheet 54 is placed underneath the bottom sheet of latex. The green substrate 30, latex 52 and MYLAR 54 are again subjected to a pressure of about 270 psi (2.4 KPa) .

Following the described compaction process, it is preferred to repeat the vacuum/pressure-squeegee fill process one or more times, each of the applications of the fill process to be followed by a compaction process. In particular, following the first compaction process, the green substrate 30 is subjected to a fill process applied to each of the substrate surfaces 44, 50, as described above in connection with Fig. 3, and following the fill process, the substrate is subjected to a compaction process preferably only once, i.e. without turning over the green substrate 30 and sub¬ jecting the turned-over substrate 30 to a second ap- plication of pressure. The substrate is then subjected to another fill process, but with respect to only the top surface 44 of the green substrate 30, i.e. without turning over the green substrate 30 so that the lower surface 50 is uppermost. After performing the fill process on one side of the green substrate 30, the green substrate 30 is subjected to a final compaction process which is identical to the compaction process described above in connection with Fig. 4. except that the green substrate 30 is sandwiched between two layers of MYLAR 54 and no layers of latex 52 are used.

Following the fill and compaction process, the green substrate 30 with filled vias 32 is cleaned such as with lint-free cloth. The cleaned green substrate 30 is then sintered in a manner well-known such as by placing the substrate 30 in a kiln and heating to the sintering temperature of the ceramic, preferably in a reducing atmosphere. When the green substrate 30 is formed of alumina, a sintering temperature of about 1600°C can be used. During sintering, the fill material 42 which has been compacted in the vias 32 forms a solid mass of conductive material 18 substan¬ tially filling the vias 16 and forming a hermetic seal in the vias 16.

After the substrate with filled, hermetically sealed vias has cooled, the substrate can be further treated to, for example, form conductive films or cir¬ cuitry on one or both surfaces. According to one em- bodiment, the finished substrate is first coated on the upper and lower surfaces with a resistance layer. A preferred material is tantalum nitride. This layer is preferably deposited in a thickness of about 400 to 500 angstroms. Next, both surfaces are provided with an adhesion or barrier layer. This layer is provided for two reasons. It forms a surface to which the final conduc¬ tive layer will adhere. It also acts to prevent diffu¬ sion of the conductive layer into the resistance layer, especially during subsequent heating steps. A chrome or chrome alloy layer about 1250 angstroms thick is preferred, although other alloys such as titanium- tungsten can also be used. On both surfaces, a conductive layer is deposited over the barrier or adhesion layer. The conductive layer is preferably gold, and preferably about 1250 angstroms thick. After all three layers are deposited, the device is aged by heating to produce an oxide layer, changing the resistance of the resistance layer from about 36 ohms per square centimeter to about 50 ohms per square centimeter. The oxide layer protects the tantalum nitride from further reaction and also is useful in avoiding oxidation of the tungsten in the vias.

Circuitry can be provided on one or both of the surfaces by well-known photolithography methods. In one embodiment, circuitry is provided on one surface by photolithography methods, with the other gold-plated surface acting as a ground plane. The circuitry is preferably positioned in a preferred alignment with the conductive vias so that the circuitry can include a conductive pathway to the ground plane as desired. Preferably, a number of distinct and separable circuits are provided on the surface of the substrate. After patterning and placing of the circuitry, the substrate is cut into a plurality of substrates, each bearing a complete desired circuit. The substrate with desired circuitry and hermetically filled vias can be further treated to hermetically seal the circuit-bearing sur¬ face of the substrate in the following manner-.

As best depicted in Figs. 5 and 6, circuitry 100 is placed on one surface 110 of the substrate 120 op- posite the lower or ground plane surface 122 of the substrate 120. Preferably, at least some portion of the circuitry 100 is in electrical contact with one or more vias 123 which are hermetically filled with the conductive material. The conductive material in the vias 123 is preferably in contact with the ground plane 122. One or more leads 124 are connected to portions of the circuitry 100 and extend outward from the sub¬ strate 120 beyond the periphery thereof. A hollow spacer or "window frame" 126 is provided in the shape of and in a size similar to the periphery of the sub¬ strate 120.^" The spacer 126 is preferably in the form of four straight legs, integrally joined to form a square structure. The spacer 126 can be made of any of a number of materials which are capable of forming a hermetic barrier. In the preferred example, the spacer 126 is made of a ceramic, but could also be made of other materials such as metal or glass. The bottom surface 128 of the window frame 126 preferably bears a material 130 which is capable of forming a hermetic seal between the bottom surface 128 of the spacer 126 and the upper surface 110 of the substrate 120. A preferred sealing material is glass, although other sealing materials known in the art can be used. The spacer 126 with the sealing material 130 on its lower surface 128 is positioned with its lower sur¬ face 128 adjacent to the upper surface 110 of the sub¬ strate 120 around the periphery of the substrate 120. The spacer 126 and substrate 120 are then heated to a temperature sufficient to cause softening of the glass 130 so as to form a seal between the spacer 126 and the substrate 120 around the periphery thereof, with the leads 124 sealingly projecting through the glass seal 130. The upper surface 132 of the spacer ^'126 is preferably coated with a material such as gold, which can be used in sealing the upper portion of the spacer 126 as described below. A top plate 134 of a size similar to the outer dimensions of the spacer 126 and the substrate 120 is positioned adjacent to the top surface 132 of the spacer 126 and sealed thereto by, e.g., soldering, welding or the like. The top plate 134 can be of any material capable of forming a her- metic seal with the spacer 126, and is preferably a me¬ tal plate.

The steps of connecting the spacer 126 and the plate 134 to hermetically seal the circuit-bearing sur¬ face 110 of the substrate 120 are preferably conducted such that the hermetically sealed interior environment of the package 138 has desired characteristics such as being dust-free, moisture free, and the like. The steps can be conducted in a desired atmosphere, or the interior space 128 can be filled during the fabrication process with a desired atmosphere such as a noble gas atmosphere or a vacuum.

As will be apparent from the above description, a number of advantages are provided by the present inven¬ tion. The vias provide a short-path, low resistivity connection between the two surfaces of the substrate, particularly for use in connecting circuitry to a ground plane. By providing hermetically-filled vias, the circuitry-bearing surface of the substrate can be hermetically sealed from the outside environment to as- sist in avoiding corrosion or other deterioration of the circuitry, such as occurs when circuitry is exposed to certain elements of the environment such as moisture or corrosive elements.

By providing hermetically-filled vias, the circuitry-bearing surface of the substrate and the at¬ mosphere in contact therewith can be hermetically sealed from undesired environments or influences such as the ambient atmosphere in a cost effective way without requiring that the entire substrate be enclosed in a sealed box.

The conductively-filled vias are also useful be¬ cause of their high heat conductivity, typically at least 10 times the heat conductivity of the ceramic substrate material. The vias thus are useful in providing heat sinks for heat generating elements of the circuitry 100. In order to provide this function, the vias do not necessarily have to be physically in contact with the circuitry or with the ground plate , but can be strategically located so as to be physically proximate to heat generating elements of the circuitry 100 in order that heat can be transmitted through the vias to reach the ground plate or other heat radiating surfaces.

The vias are also useful because conductive vias can act as antennae. By strategically locating the vias, the antenna-like nature of the vias can be used in preventing or reducing unwanted "cross talk" between circuits or circuit elements. This is particularly useful when the circuitry 100 is to be used for high frequency applications, for example, frequencies of about 2 gigahertz or more. By providing strategically located antenna-like conductive vias in the substrate, high frequency or other circuit elements can be located physically closer to each other than would otherwise be the case so that the resulting circuitry package can be provided in a physically smaller and, therefore, more economical size. A number of variations and modifications ^'of the described preferred embodiment can be practiced within the scope of this invention. Although the process produces a substate in which the vias are hermetically sealed, such a substrate can be used in applications where such a hermetic seal is not required. The process can be used to fill only some of the vias in the substrate. The metal/ceramic mixture can be used to fill regions of a substrate other than vias, such as tunnels or recesses. Ceramic materials other than alumina and metals other than tungsten can be used, in which case the proportion of metal-to-ceramic in the fill mixture can be other than between 35 weight per¬ cent and 42 weight percent ceramic, provided a conduc- tive hermetically sealing via fill is produced. The ceramic used in the fill mixture can be a ceramic other than the substrate ceramic, provided it is compatible in the sense that the mixture forms a hermetic seal with the substrate.

Example 1,000 grams of W 6.6 tungsten (dry blended) mixed with acetone was milled for 48 hours and dried to provide a tungsten powder. 240 grams of the tungsten powder was combined with 160 grams of 99.5 percent pure K-237 and K-638 alumina. The mixture was milled for 1 1/2 hours with acetone fill. The milled mixture was dried and lumps removed from the powder. The powder mixture was placed on a surface of a 1.3 mm thick green ceramic alumina substrate with via holes having a diameter of about 0.3 mm. One surface of the substrate was subjected to a vacuum of 23 inches of mercury and the dry powder was placed into the vias using a squeegee under 80 psi pressure drawn across the surface of the substrate four times. The fill process was repeated on the opposite face of the substrate. The filled substrate was placed between two sheets of 0.02 inch latex with MYLAR placed against the bottom sheet of latex. The substrate, latex and MYLAR were sub- jected to pressure in a flat-plate press at 270 psi. The green substrate was turned over and the parts were again subjected to pressure at 270 psi. The fill process was repeated and a compaction process was con¬ ducted without turning over the green substrate. The fill process was again repeated on only one side of the green substrate and the green substrate was sandwiched between two sheets of MYLAR and subjected to pressure at 270 psi cold. The green substrate was cleaned with lint-free cloth, and fired in a reducing atmosphere at a temperature of about 1600°C. The sintered substrate with the filled vias was tested for hermeticity using the MIL-STD-883 method 1014. The leakage rate of helium was found to be less than about 1 x 10-⁸ STD cc per second. Although the present invention has been described with reference to certain embodiments, it should be ap¬ preciated that further modifications can be effected within the spirit and scope of the invention as limited only by the appended claims.

Claims

What Is Claimed Is:

1. A method for making a sintered ceramic sub¬ strate with a conductive via comprisingt forming a substrate green body having at least one via from a ceramic material; providing a mixture comprising a refractory metal powder and a powder of a ceramic material; filling said via with said mixture; and sintering said green body to produce a ceramic substrate having a via filled with a conductive material.

2. A method, as claimed in Claim 1, wherein said providing step comprises: providing a mixture comprising a refractory metal powder and between about 35 weight percent and about 42 weight percent of said powder of a ceramic material.

3. .-A method, as claimed in Claim 1, wherein said providing step comprises: providing a mixture comprising tungsten powder and between about 35 weight percent and about 42 weight percent of alumina powder.

4. A method, as claimed in Claim 1, wherein said sintering step comprises: sintering said green body at a temperature at least equal to the sintering temperature of said green body to produce a hermetic seal between said conductive material and the sintered ceramic substrate.

5. A method, as claimed in Claim 1, wherein said filling step comprises: placing said mixture in said via using a squeegee.

6. A method, as claimed in Claim 1, wherein said filling step comprises: placing an amount of said mixture in said via; placing a sheet of deformable material adjacent to a surface of said substrate; and subjecting said substrate and said sheet of defor- mable material to a pressure sufficient to compact said mixture in said via.

7. A method, as claimed in Claim 1, wherein said conductive material in said via has a resistance less than about 0.5 ohms.

8. The method, as claimed in Claim 1, further comprising: forming a conductive film proximate to one surface of said substrate; and etching portions of said conductive film to produce electrical circuitry on said surface of said substrate.

9. A method, as claimed in Claim 1, further comprising: forming a resistance layer adjacent at least one surface of said substrate; forming an adhesion layer adjacent to said resis¬ tance layer; and forming a conductive layer adjacent to said adhe¬ sion layer.

10. A method, as claimed in Claim 9, wherein: said resistance layer comprises tantalum nitride.

11. A method, as claimed in Claim 9, wherein: said resistance layer comprises a material selected from the group consisting of chrome, a chrome alloy, and a titanium-tungsten alloy.

12. A method, as claimed in Claim 9, wherein: said conductive layer comprises gold.

13. A method for filling a ceramic substrate via with a powdered fill material comprising: placing a portion of said powdered fill material in said via; placing a sheet -of deformable material adjacent a surface of said ceramic substrate; and applying to said said ceramic substrate and said deformable material sufficient pressure to compact said fill material within said via.

14. A method, as claimed in Claim 13, further comprising: sintering said ceramic substrate with compacted fill material at a temperature sufficient to form a hermetic seal between said fill material and said ceramic substrate.

15. A ceramic substrate useful for carrying electronic circuitry comprising: a substrate formed from a ceramic material having at least one via; and a conductive material disposed in said via in in¬ timate contact with said substrate and forming a her¬ metic seal in said via.

16. A ceramic substrate, as claimed in Claim 15, further comprising: electronic circuitry adjacent to at least one sur¬ face of said substrate.

17. A ceramic substrate, as claimed in Claim 16, further comprising: means for hermetically sealing an atmosphere in contact with said circuitry from the ambient atmos¬ phere.

18. A ceramic substrate, as claimed in Claim 17, wherein said means for hermetically sealing comprise: a hollow spacer hermetically sealed to said substrate; and a top plate hermetically sealed to said spacer.

19. A substrate, as claimed in Claim 15, wherein: said conductive material is a sintered powder.

20. A substrate, as claimed in Claim 15, wherein: said conductive material comprises a metal and be¬ tween about 35 weight percent and about 40 weight per¬ cent of a ceramic material.

21. A substrate, as claimed in Claim 20, wherein: said metal comprises tungsten and said ceramic material comprises alumina.

22. A substrate, as claimed in Claim 15, wherein: said conductive material has a resistance of less than about 0.5 ohms.

23. A method for making a sintered ceramic sub¬ strate with a conductive via comprising: forming a substrate green body having at least one via from a first material comprising alumina; filling said via with a second material comprising tungsten; and sintering said green body at a temperature at least equal to 1600°C to produce a hermetic seal be¬ tween said second material and the ceramic substrate.

24. The method of Claim 23 wherein said second material comprises between about 35 weight percent and about 42 weight percent of alumina powder.

25. The method of Claim 23 wherein said sintered second material has a resistance less than about 0.5 ohms.

26. A method for producing a hermetically sealed environment for electric circuitry comprising: forming a ceramic substrate having at least one via; filling said via with a conductive material; forming a hermetic seal between said conductive material and the walls of said via; positioning the circuitry on a first surface of said substrate; positioning a hollow square spacer having upper and lower surfaces with said lower surface in contact with a sealing material such that said sealing material is adjacent to said first surface of said substrate; forming a hermetic seal between said spacer and said substrate; positioning a plate adjacent to said top surface of said spacer; and forming a hermetic seal between said plate and said top surface of said spacer.

27. A method, as claimed in Claim 26, further comprising: positioning electrically conductive leads in con¬ tact with said circuitry extending outward of said substrate; and forming-said hermetic seal between said spacer and said substrate such that said leads extend through said sealing material and are hermetically sealed therewith.