IL45512A - Ceramic capacitors and circuit boards and matrices therefor - Google Patents

Description

im^om ο»*β ρ o» iy» ntnt i a* ap oniay Ceramic capacitors and circuit boards and matrices therefor ustries, Inc. 66 CERA MIC CAPACITORS, CIRCUIT BOARDS AND MATRICES THEREFOR iP* Abstract of the Disclosure Monolithic capacitors and multilaye r ci rc uit structures are produced by I 1) providing, on a plurality of thin leaves or sheets of finely divided ceramic material bonded with a thermally- ugitive temporary bond, coatings comprising thin, selected patterns of pseudo- conductive layers or lines that consist of thermally-fugitive material , ( 2) forming a consolidated stack from a plurality of the coated leaves or sheets . 3) firing the resulting block to remove the thermally-fugitive material and sinter the ceramic material into a monolithic body in which there are substantially unobstructed thin cavities or channels . (4) introducing conductive material, preferably metal, into the cavities or channels to replace the thermally-fugitive pseudo-conductors , and { 5) providing suitable electrical connections to the conductive layers or lines.

Background of the Invention The invention of the present application relate s to ceramic capacitors and ceramic circ uit boa rds. It Is particularly concerned with the production c matrices therefor which have unitary, sintered, ceramic bodies, each compris ing a plurality of superposed thin strata of s uitable ceramic mate rial integral ly joined at a plurality of edge portions thereof and having substantial portions of thei r adjacent s urfaces separated from one another to provide a plurality of s ubstantially unobstructed thin spaces the rebetween, said strata being uns upported except at said joined edge portions and said body having an opening into each of said spaces between adjacent strata. By introducing suitable conductive mate rial, preferably metal, into the spaces between the strata of the ceramic bodies and making suitable electrical connections the reto, capacitors , circuit boards such as are used in hybrid integrated circ uits , and the like can be produced.

In its simplest form a ceramic capacitor consists of a relatively thin wafer of desired shape and size, formed by firing a ceramic position, with electrodes on the opposite faces of the wafer. however, capacitors are desired which comprise a unitary or monolithic body formed of a plurality of dielectric layers and a plurality of conductive layers alternating with said dielectric layers, successive ones of said conductive layers being exposed at different edge faces of the capacitor and being there electrically joined, e. g. by termination electrodes.

In a typical method in use for producing such monolithic ceramic capacitors an electroding paste of a noble metal such as platinum or palladium is applied to the top face of a small, usually cast, thin sheet of a suitable ceramic dielectric composition bonded with an organic temporary binder, the application being carried out In such manner that the deposit of electrode paste extends to one edge of the sheet but a margin is left around three sides thereof. A plurality of the small sheets with the deposit of electrode paste are then stacked, successive sheets being rotated about an axis normal to the plane of the sheet, whereby successive electrode paste deposits extend to opposite edges of the stack. The stack of paste-coated sheets is then consolidated and heated to drive off or decompose the organic binders of the ceramic sheet and the electroding paste and to sinter the ceramic dielectric composition Into a unitary, multilayer body having successive electrodes exposed on opposite ends thereof. The electrodes on each end are then electrically connected with a termination electrode In known manner.

Because of the necessity for using noble metal internal electrodes in the process just described, monolithic ceramic capacitors are expensive. Lower cost silver electrodes such as are commonly used with other ceramic capacitors are generally unsuitable for monolithic capacitors because the silver applied as an electrode paste would be subjected to a high temperature during firing to mature the ceramic and would thereby be deleteriously affected. Accordingly, a method of producing monolithic capacitors that does not require the use of noble or very expensive metals has been desired.

Such a method has been described in U. S. Patent No. 3, 679, ^jjj^ granted July 25, 1972. In that patent a number of procedures are disclosed which Involve the formation of ceramic matrices that have strata of dense dielectric material alternating with strata of porous ceramic material and the deposition of conductive material, which may be low- cost metals. In said porous strata. Termination electrodes that connect alternate ones of the thus formed conductive strata are then provided to produce monolithic capacitors.

While very satisfactory, relatively inexpensive, monolithic, ceramic capacitors have been made by methods disclosed in the above-mentioned patent, maintaining continuity of metal in the internal electrodes has been found to be a problem at times. Also it is desirable, particularly when producing capacitors for use at high frequency, to keep the electrode resistance as low as possible and, therefore, an uninterrupted film of metal is desirable.

Accordingly, it is an object of the present invention to provide an improvement on the procedures disclosed in the above-mentioned patent which will result in the production of ceramic matrices In which internal electrodes can be formed by introduction of conductive material, such as metal, to form capacitors wherein continuity In such electrodes and low electrode resistance can be easily obtained and wherein no porous ceramic strata is required.

Summary of the Invention The foregoing object Is achieved by providing as a matrix ceramic bodies that have a plurality of thin layers or strata of dense dielectric material, adjacent layers having between them thin, substantially unobstructed 8 ace s or cavities, open at one edge region, that are essentially planar.

There is thus minimal Impediment to the entry of conductive material Into the thin or planar cavities to provide a body having continuous conductive strata alternating with dielectric strata and no necessity of a compatible, porous ceramic composition. More specif! cally, the invention of the present application comprises a capacitor, formation of which includes the Introduct- ion of a conductive material, metal being generally preferred, into one or more thin, essentially planar, material in a unitary, sintered joined at a plurality of edge portions and are supported, prior to introduction of said conductive material, only at said edge portions whereby to leave said space or spaces unobstructed. This results in a capacitor in which, because the spaces between the dielectric strata contain essentially only electrode material, the electrodes have low resistance. The invention also comprehend production of similar ceramic devices having internal conductors, such as multilayer circuit structures, by a substantially similar procedure. In production of both capacitors and multilayer circuit structures the procedure includes the use of pseudo-conductors formed of thermally-fugitive material that is removed when the ceramic is fired, thereby providing the cavities or channels into which the conductive material is introduced. The shape, size and location of the conductors and/or electrodes are essentially the same as that of the pseudo- conductive material in the green, unfired body, which they replace.

Short Description of the Drawings Figure 1 is a sectional view of a capacitor produced in accordance with the present invention.

Figure 2 Is a sectional view taken on line 2- -2 of Figure I of such a capacitor.

Figure 3 is an enlarged perspective view of a plurality of leaves of a bonded, ceramic, dielectric composition, each leaf having thereon a layer of thermally-fugitive material* Figure 4 is a fragmentary plan view of a bonded leaf or sheet of a ceramic, dielectric composition having thereon a patterned layer of thermally fugitive material.

Figure 5 is a further enlarged, fragmentary, sectional view of a ceramic body according to the present invention after assembly, consolidation and sintering of a plurality of coated leaves such as shown in Figure 3.

Figure 6 is an enlarged sectional view of a multilayer ceramic circuit structure according to the present invention, and Figure 7 is an enlarged exploded view showing the several ceramic sheets forming the structure illustrated in Figure 6 with pseudo-conductors thereon.

It should be noted that in the drawings certain relative dimensions are exaggerated.

Description of the Invention In accordance with the present invention a capacitor can be produced in the following manner.

A plurality of thin leaves of a finely divided, ceramic composition are made by the use of a suitable thermally-fugitive bonding material, for example, a resin or cellulose derivative, the ceramic composition being such as to form a dense dielectric layer when sintered. Such compositions, many of which are well known, include barium tltanate, with or without admixed m odifiers of the dielectric constant and/or other properties thereof, as well as many other types of ceramic compositions. There is then applied to each of a plurality of said leaves a thin layer consisting of thermally-fugitive material. These layers may be preformed, but preferably are produced by depositing a liquid or pasty composition on the leaves, for example, by painting or screen printing. The thermally-fugitive material of which said layers are composed may be a suitable combustible and/or volatile film-forming material, but is preferably a mixture of fine, combustible and/or volatile particles bonded with such a film-forming material.

The layers of thermally-fugitive material (pseudo-conductors) are smaller in surface area and preferably thinner than the leaves to which they are applied and each layer is of such shape as to allow a margin of the associated leaf to extend around a major portion of the perimeter of the layer while a portion of the latter extends to an edge of the leaf on which it is deposited. Preferably, the layers are equal in sise.

A plurality of the leaves of the bonded ceramic composition are then stacked, with the thermally-fugitive layere Intervening, and consolidated The consolidation can be accomplished by means suitable to the particular materials employed and may Involve pressing, heating, and/or the use of a solvent. The leaves and thermally-fugitive layers in the stack are so arranged that successive layere extend to different edge regions of the consolidated stack, but a major portion of the edges of each of said leaves is In contact with the edges of the adjacent leaves In the stack. The consolidated stack of leaves and Intervening layere is then fired to remove the thermally-fugitive materials and to sinter the ceramic composition. There Is thereby formed an Integral, sintered, ceramic body having a plurality of thin sheets or strata of dense dielectric material, adjacent sheets or strata being separated over substantial portions of their adjacent surfaces and joined only at the edge portions thereof.

At the edge regions of the sintered body to which the layere of heat-fugitive material extended there are openings into the spaces between adjacent etrata. Through such openings a conductive material, such as a metal, may be introduced into said spaces by a suitable method, for example one of the methods disclosed In U. S. Patent No. 3, 679, 950. The result Is a body to which termination electrodes can be applied by any desired procedure to form a capacitor and which may, if desired, be suitably encapsulated after attaching leads to the termination electrodes.

Obviously, various modifications and variations of the procedure set forth above may be made and a number of these will be described hereinafter.

Detailed Description of an Embodiment Although as pointed out herein there are a number of variations and modifications possible, a procedure preferred when producing a few rather large monolithic capacitors, is essentially that above-described. A fully detailed description of the procedure Is as follows: EXAMPLE 1 A dispersion ie made by ball-milling for 4 hours the followia ^ composition: 400 g dielectric powder* 4 g dlethylene glycol laurate g butylbenssyl phthalate 120 ml toluene * 96 parts BaTiOs, 4 parts Ce<¾ * ZrO*. all having an average particle size range of 1 - '2 After such milling, the dispersion Is added to a solution formed by dissolving 37 g of ethyl cellulose in 180 ml of toluene with stirring to obtain thorough mixing. The mixture is then de-aired and a film of the mixture, approximately 100 mm by 1500 mm in size, is formed with a doctor blade on a smooth sheet of glass. The film, which after drying Is approximately 0. 045 mm thick. Is removed and cut Into small, rectangular sheets or leaves, each approximately 10 mm by 20 mm.

A thermally-fugitive composition for depositing pseudo-conductors on the leaves prepared as above described may be made by mixing, for example, on a 3- oll mill, 25 g of finely divided carbon with 50 g of a 50% solution of a phenolic -modified, rosin ester resin (PENTALY ® 858) in a high-boiling, aliphatic petroleum naphtha having a Kaurl-butanol value of 33. 8 {No. 460 Solvent). The viscosity of the composition is made suitable for screen printing by mixing therewith additional naphtha solvent. For use with a 325 mesh screen about 2. 5 ml Is required. This composition or ink, as it is often referred to, is screen printed on one side of each of a plurality of leaves of the dielectric composition in a layer about 0. 01 mm in thickness when dry. It will be observed that care should be taken to use components in these compositions which do not dissolve or unduly soften the bonding material In the leaves of dielectric material. Preferably, the solvents used are aliphatic petroleum naphthas with a low \ about 35) Kaurl-butanol value and an evaporation rate slow enough so that the ink does not blind the printing screen used between printing cycles. The heat -fugitive, pseudo-conductive layer is so applied to each of the leaves or sheets of bonded dielectric material that the layer extends to one edge of the leaf, bat has a substantial it on Its other sides.

The printed leaves are then Indexed and stacked in groups of ten so that on alternate leaves In each group the edges of the leaves to which the printed layers extend are aligned and the Intervening leaves are turned horl- o zontally 180 whereby the printed layers thereon are exposed at the opposite end region of the stack. Non-printed leaves are placed on the top and bottom of the stack. The stack is then consolidated by applying a pressure of about 104 kg/cm2 thereto at a temperature of about 80°C for 1 minute to obtain a coherent green body or chip, as these bodies are frequently referred to. The chips are then heated to remove the thermally-fugitive materials therein and to sinter the ceramic composition.

To avoid possible disruption of the chips during firing, they are first heated slowly in air to remove the thermally-fugitive materials and are subsequently fired at a higher temperature to form small, coherent, sintered matrices or chips, each of which has a plurality of thin strata of dense, dielectric material that are Integrally joined at a plurality of edge portions only so tlhat between the strata there are thin, substantially unobstructed spaces or cavities that are essentially planar, I. e. having the height thereof very small In oomparison with the surface area. Each of the cavities has an opening from one of the edge regions of the chip because of the extension of each of the heat-fugitive printed layers to an edge of a leaf of the dielectric composition when the green chip was formed. Since the printed leaves were stacked with alternate thermally-fugitive layers extending to the same edge region of the stack, the openings to the adjacent cavities in the sintered chip are at opposite edge regions of the sintered chip.

A suitable heating schedule for removal of the thermally-fugitive materials Is as follows, all temperatures being In degrees C: to 160 - 2 hours 310 to 314 - 4 hours 160 to 220 - 10 hours at 400 - 1 hour 220 to 225 - 12 hours at 500 - 1 hour 225 to 310 · 20 hours at 600 - 1 hour M Upon completion of the foregoing schedule, the temperature is raised to¾»70¾ and maintained thereat for 1. 25 hours to sinter the chips.

After cooling of the sintered chips, the cavities therein are filled with conductive material, a metal being preferred, any of the methods disclosed in the above-mentioned U. S. patent being suitable. Termination electrodes are then applied by any suitable means, the application of such electrodes being well known. Alte natively, the end terminations can be applied and the cavities then filled with metal In accordance with the disclosure In copending U. S. patent application. Serial No. 274, 668.

Figures 1 and 2 of the accompanying drawings depict, on an enlarged and exaggerated scale, such a monolithic capacitor as that produced by the above-described procedure. The numeral 11 comprehensively designates the capacitor which has strata 13 of ceramic dielectric material with layers 15 of conductive material therebetween that serve as Internal electrodea The latter are so formed, as a result of the else and placement of the cavities Into which the conductive material was introduced, that alternate ones extend to the same end face of the capacitor, the group of electrodes exposed at each end face being electrically joined by end termination electrodes 17. Where there Is no intervening conductive material, the dielectric layers are united as shown at 19.

In Figure 3 there are shown, enlarged, two thin sheets or leaves 31 and 33 of ceramic, dielectric material bonded with a thermally-fugitive bond, each of which has thereon a layer 35 consisting of thermallyfugltlve material. It will be seen that the layer 35 on the sheet 31 extends to the front edge of the sheet, but has a margin around the sides and the rear thereof while the layer 35 on the sheet 33 extends to the rear edge of the sheet and has a margin around the sides and the front of the sheet. Thus, when a plurality of sheets 31 and 33 with the layers 35 thereon are stacked alternately, consolidated. and fired, the cavities produced in the resultant sintered body by removal of the thermally-fugitive material layers 35 will open alternately at o osj^ends of the body.

Figure 5 depicts, on a farther enlarged scale, the structure of a fired ceramic body or chip, suitable as a matrix for production of a monolithic capacitor, which has been made in accordance with the present invention. The etrata 37 are of dielectric material and the cavities or spaces 39 therebetween resulting from the removal of the thermally-fugitive, pseudo- conductive layers 35 are substantially unobstructed.

It will be understood that monolithic capacitors according to the present Invention may be produced Individually as described In the foregoing example. It is preferred, however, when a considerable number of capacitors are to be produced or when the individual capacitors are very small, to employ a procedure In which a plurality of green chips are produced simultaneously and sintered at the same time. Such a procedure is described In the following example.

EXAMPLE 2 Using the same ceramic dielectric composition and temporary bond therefor as In the preceding example, leaves 50 mm x 75 ram and about 0. 05 ram thick after drying are prepared in the manner therein described. Using the same thermally-fugitive composition or ink as employed In Example 1 for forming the heat-fugitive layers, a recurring pattern Is then deposited on each of said leaves, proferably by screen printing. After the deposit has dried, forming a film about 0. 01 ram thick, the printed leaves are Indexed and stacked in groups of ten with the printed film pattern on each successive leaf offset with respect to the pattern on the preceding one. Blocks are then formed by consolidating the stacked leaves, one or more unprtnted leaves preferably being placed on the top and bottom of the stack, the consolidation being produced by applying a pressure of about 104 kg/cm thereto at a temperature of about 85 C for about a minute. There Is thus obtained a green, solid block which is severed or cut, by suitable me^^ such as knives. Into smaller green blocks or chips.

The manner In which this Is done will be more readily understood b referring to Figure 4 of the accompanying drawings. In that figure, the numeral 51 represents (somewhat enlarged and diagrammatic) a large sheet or leaf of ceramic dielectric material temporarily bonded with a thermally-fugitive bonding material. The spaced, rectangular elements 53 thereon are layers or films of the thermally-fugitive material that have been deposit thereon, e. g. by screen printing. In assembling a stack of such printed wheats for consoldatlon Into a large block, all of the sheets are indexed so that the elements 53 thereon are vertically aligned along two opposite edges ϊ but on successive sheets the elements are offset so that only on alternate sheets are the elements 53 wholly In vertical alignment. This is indicated in Figure 4 by the areas 55 (shown in broken lines) which represent the offset, extending portions of the elements 53 on the leaves 51 located in the stack above and below the leaf 51 illustrated. After consolidation of the printed leaves into a green, large block (not shown), the block is severed, e. g. by cutting, along the lines 5? and 59, to form a plurality of smaller, green ceramic blocks or chips in which the elements 53 are exposed alternately at opposite ends of the chips.

These chips are heated in the same manner as described In Exampl 1 to remove the thermally-fugitive materials and to sinter the dielectric composition of each into a unitary body having thin ceramic dielectric strata with planar spaces therebetween. By suitable procedures conductive material, preferably metal, is then Introduced into said spaces or cavities and termination electrodes are provided on each end to connect electrically the conductive layers exposed at each said end. There are thus produced very satisfactory monolithic capacitors.

A somewhat modified procedure for forming a plurality of chlp^, simultaneously is described below.

EXAMPLE 3 The same materials and procedure are employed as set forth above i Example 2 to form blocks from leaves of a dielectric composition carrying thin films or elements of heat-fugitive materials. Then, instead of severin the block into a plurality of green chips, the whole block is heated to remov the thermally-fugitive material and to sinter the ceramic material. The heating and sintering conditions may be substantially the same as those described above. However, because of the greater mass of the large blocks a somewhat longer soaking time may be necessary to achieve proper sinteri After the blocks are sintered they are severed, for example by a diamond saw, into the desired ceramic matrix chips by cutting along lines correspon ing to the lines S? and 59 in Figure 3.

Although in the foregoing examples the dielectric materlalsused are modified barium titanate compositions, it will be clear that there are many other known ceramic dielectric compositions that may also be used. For example, TiO|( glass, steatite, and barium strontium niobate, as well as barium titanate alone, can be used, suitable changes well known in the art being made as required In firing procedures and the like to achieve proper sintering. Obviously, the capacitance of the resulting capacitors will vary as a result of using materials with higher or lower dielectric constants.

Capacitors according to the present invention may vary widely in sise For example, capacitors as small as 2. 0 mm x 3. 0 mm x 0. 9 mm with 20 dielectric strata, each as thin as about 0. 03 mm and 19 internal electrodes, each as thin as about 0. 01 mm, can be readily made, and larger ones are, of course, possible. Not only may the dimensions of the capacitor be varied. but tbe number and thickness of the strata therein may also vary. Capacitors of any desired capacitance may be obtained according to the Invention bj^ proper choice of dielectric material and the else, thickness, and number of the strata and the Intervening pseudo-conductive layers. In general, it is desirable to form the dielectric strata and electrodes as thin as is feasible since a smaller amount of expensive dielectric material Is used and the capacitance per unit of volume of the capacitors is Increased, thus reducing the space required in circuits. It will be understood that the thinness of the dielectric strata is limited by the necessity of having such strata solid and non-porous and of such thickness as to withstand the voltage applied In use. Although irregularities In the surface or the thickness of the leaves of dielectric material may provide problems in the formation of capacitors where extremely thin layers or films of pseudo-conductive material are applied since one or more cavities between such Irregular leaves may be blocked after firing It Is generally preferred to make the electrodes or conductive layers thinner than the dielectric strata. It will also be understood that one or more extra or additional dielectric leaves or sheets m y be placed at the bottom and/or top of a stack of alternated dielectric leaves or sheets and thermally-fugitive layers. This is often done to give additional mechanical strength to the capacitors and/or to adjust their thickness. Unprlnted leaves of a dielectric ceramic composition can be used. However, the presence of a thermally-fugitive deposit on the top dielectric film or leaf of such a stack will ordinarily not be detrimental.

Firing of green ceramic blocks, units, or chips to sinter them into unitary or monolithic bodies is preferably carried out in a kiln in an oxidising atmosphere, such as air. An electrically heated tunnel kiln or furnace is preferred, but other kilns or other heating means may be employed. The tempera ture and the time of firing will depend on the ceramic compositions employed. Those skilled in the art are familiar with such details, as pointed out above. and with the fact that, in general, the sintering t me necessary varies inversel with the temperature. As the term is used herein, "sintering temperature" refers to the temperature necessary to obtain the desired cerami c properties in the body or bodies. As indicated above, a prolonged period of heating at relatively low temperatures Is preferred for removal of the therm lly -fugitive materials in the leaves and the peeudo- conductors. The removal of the the mally- fugitive materials In the leaves and deposited layers should be sufficiently slow so that expansion of gases formed in the decomposition or vaporisation thereof does not rupture the chips.

In the general description and the oxamplee, the leaves of Insulating o dielectric material, the heat-fugitive layers or deposits, and the capacitors or multilayer circuit structures formed therefrom are assumed to be rectangular. However, the present invention comprehends capacitors and circuit structures of other shapes. In such cases, obviously, alternate thin cavities and the electrodes or conductors introduced therein may not be exposed on opposite edge faces. Consequently, It will be understood that In the appended claims the term "edge region" is uoed comprehensively to indicate an area on a surface of a body of whatever shape, formed as described herein, which surface meets or intersects the plane of one or more planar spaces or cavities In said body.

In rigura 6 there is illustrated a typical ceramic, multilayer circuit structure 81 such as is used for hybrid integrated circuits. The structure or body 81 has a ceramic matrix 83 and a plurality of conductors 85 extending into and for through the matrix. The thickness of both conductors and matrix Is exaggerated in Figure 6 for convenience in viewing. Hitherto, such structures have been expensive to produce and normally would be made by screen printing a metallic electroding type paste containing a nobl metal such as palladium or platinum in the desired conductor patterns on a plurality of temporarily bonded sheets of desired thickness of an electrically Insulating, ceramic material each ae fine alumina powder, stacking and consolidating the several printed sheets with an unprlnted sheet on top, and si tering^* consolidated sheets Into a unitary body.

As mentioned above, such ceramic, multilayer circuit structures ma also be produced by techniques essentially similar to the processes disclosed herein for producing capacitors, thus avoiding the necessity for using expen-slve, noble metals as conductors. The production of such a structure as shown in Figure 6 by the technique of the present invention will be briefly described with reference to Figure 7. It will be understood that the procedure described Is only exemplary and that other procedures may also be used such, for example, as forming large ceramic blocks as in Example 2 which may be cut to produce individual circuit structure bodies.

The sheets or leaves A, B, and C, shown In Figure 7, are formed in the desired siase, shape, and thickness by casting, molding, or the like, a desired ceramic, electrical Insulating composition, for example, finely divided alumina, using a thermally-fugitive material such as a resin, ethyl cellulose, or the like as a temporary bond therefor. Heat-fugltlve pseudo-conductors 87 following the paths of the desired conductors 85 In the structure shown in Figure 6 are then screen printed on the sheets or films B and C using a thermally-fugitive screen printing compositon or Ink. It will be understood that the patterns of pseudo-conductors 87 Illustrated are only exemplary and that any desired patterns may be used. The printed sheets are stacked, covered by one or more unprinted top sheets, and the stack is then consolidated in suitable manner and heated to remove the thermally-fugitive materials and sinter the ceramic material la the sheets Into a unitary body, all In substantially the same manner as described above in the productio of capacitors. As with such capacitors, the unitary or monolithic matrix produced by firing comprises a dense body formed of the ceramic, Insulating composition having therein cavities or channels that are substantially unin- terrupted throughtout their lengths. Each of said channels communicates with at least one region on a face, e. g. an edge face, of said body. Conductors In and through said bodies are formed by introducing into the channels a suitable conductive material, metal being preferred.

It will be evident that except for the fact that the matrix thus produced may contain a number of void channels between two adjacent strata of non-conductive ceramic material instead of a single void cavity, the structure is essentially the same as that of the matrices for capacitors described hereinbefore. In both cases, the bodies when green comprise leaves of non-conductive ceramic material with a thermally-fugitive temporary bond, have Intervening deposits or layers of therroally-fugltlvemater.al serving as pseudo-conductors, and the matrices, after sintering, comprise dense, substantially parallel strata with Intervening, substantially unobstructed, void areas Into which conductive material such as metal may be introduced. Because of the variation possible in the thermally-fugitive materials and ceramic materials used In producing the bodies, the heating and sintering procedures will also vary. It Is believed, however, that those skilled In the art can choose satisfactory times and temperatures.

An appropriate one of the procedures mentioned above for introduction of conductive material may be used. Leads may be attached by suitable known means to selected exposed conductors or end termination electrodes when these are used, and small components such as transistors, diodes, etc. may be soldered at predetermined points, leads therefrom extending, If desired, to underlying conductors 85 through holes 89 provided in desired locations In one or more of the insulating ceramic strata. Such holes also may serve, when eontainlng conductive material, to electrically connect conductors on two or more levels of the circuit board. leave β of the temporarily bonded, ceramic insulating composition may be used with the desired pattern of heat-fugitive pseudo-conductors printed or otherwise applied thereon. Thus, structures with conductors on a number of different levels therein may be obtained. The thickness of the ceramic sheets and the pseudo-conductive coatings may vary within a relatively wide range. In general, however, the sheets will range in thickness from about . 05 mm to about 0. 25 mm and the pseudo-conductors will range in thickness from about 0. 007 mm to about 0. 04 mm. It will be seen, therefore, that relatively thin structures may contain many conductors. The width of the pseudo-conductors, and thus the channels for the conductive material, may vary as desired. However, such channels will in substantially all cases have cross-sections that are small relative to the matrix body and will generally be normal to the thin direction of the body. Because of the relative thinness of the channels relative to the width and length thereof, they can be regarded as planar cavities.

As previously Indicated, there are a number of possible variations and/or modifications of the procedure set forth In Examples 1 and 2> For ejarr le, Instead of screen printing a layer consisting of thermally-fugitive material on the small bonded ceramic leaves such as employed in Example 1 , small pieces of a suitable, preformed, thermally-decomposible, plastic film of appropriate size and shape containing a fine combustible material can be laid In proper position between the leaves as the stack of leaves Is built up. Also, the layers of thermally-fugitive material may be applied by painting or spraying, If desired. As a further alternative procedure, a layer consisting of thermally- fugitive material can be applied by suitable means to both sides of a leaf of bonded dielectric or Insulating ceramic material, thereby eliminating the need for such layers on the leaves above and below it when stacking in the formed stack. Although in forming multilayer circuit structures the conductor patterns may be and usually are different in each of the several levels therein, it is generally desirable to have all the internal electrodes of substantially the same size and shape in capacitors produced in accordance with the present invention. Such uniformity makes production easier and helps to ensure that the resulting products will have uniform capacitance.

It will be understood that the compositions used in forming the dielectric or insulating leaves and the pseudo-conductors used in producing ceramic matrices in accordance with the present invention may vary widely. There have been set out above numerous usable ceramic materials. There are also a great many usable media or vehicles that can be used as heat-fugitive bonding materials for these ceramic materiale. Many of these are commercially available or easily prepared by those skilled in the art. Essentially, the purpose of such media and vehicles is to suspend and disperse the particles used to form the leaves and/or layers and provide a temporary, thermally-fugitive bond therefor during formation of leaves and/or layers therefrom and the production of green ceramic bodies from a plurality of leaves and layers. In the sintered ceramic bodies, the temporary bond has disappeared. Accordingly, the medium and f r vehicle used is largely a matter of choice or convenience.

Since the purpose of the pseudo-conductive layers is to provide support for and separate the ceramic-containing leaves or layers until the latter are self- suppo ting so that the desired cavities or channels will be left In the sintered matrices during the heating cycle used to remove thermally-fugitive materiale, the pseudo- conductors should not adversely affect the temporarily bonded ceramic sheets and should remain until the plasticity of said sheets has decreased to such an extent that the sheets ar rigid and do not deform - to produce the desired result. In choosing such particulate, thermally-fugitive material, however, it is important to avoid those which on combustion leave appreciable ash that contains elements detrimental to the dielectric or insulating composition used in the ceramic leaves or strata. Generally suitable for the purpose are fine particles of carbon or carbonizable material such, for example, as starch and cellulose. Among the large number of thermally-fugitive, film-forming materials suitable for use with such particu late materials in forming the the mally-fugitive layers or deposits are ethyl-cellulose, acryloid resins, and polyvinyl alcohol. A suitable solvent for the film-forming material is employed in such amount as to give the desired viscosity to the composition.

As above indicated, in some cases the cavities or channels between the ceramic layers can be produced by the use of pre-formed, thermally-fugitive films, a thin film of resin containing fine particles of carbon, for example, being usable. Also usable for the purpose is a thin deposit of a mixture of fine, granular, combustible material such as carbon, containing no binder, placed in the desired pattern or design on the ceramic leaves. As used herein, a "thermally-fugitive" or "heat-fugitive" material is one which, under the conditions of the processes herein described, volatilises as such or is wholely converted, with or without oxidation. Into products that volatilize.

As also Indicated above, the conductive material introduced into the thin cavities to form internal electrodes in producing capacitors or into the channels to form c onductors in circuit structures is preferably a metal. Thi term is meant to include single metals as well as alloys and In some cases can Include semi-metals or metalloids , e. g. germanium. Suitable metals include lead, tin, zinc, aluminum, silver, and copper. The metal employed should have a melting point lower than the maximum temperature employed in sintering the ceramic of the matrix and should not react deleterlously with ingredients of the matrix.

A s used herein the term "de nee"means that the material absorbs substantially no water when immersed therein, and "thin" is a relative term, which with reference, for example, to the ceramic strata indicates a thickness of the order of 0. 5 mm or lees. Such strata can, however, for specific purposes, be thicker.

The terms "upper", "lower", "top", "bottom", "right", "left", "above", "below" and similar terms of position and/or direction as used herein refer to the illustrations in the accompanying drawings, but are used only for convenience in description or reference. Such terms should not be so construed ae to imply a necessary positioning of the structures or portions thereof or to limit the scope of this invention.

In the foregoing specification and the appended claims, parts and percentages are by weight. -

Claims (12)

1. Unitary, sintered, ceramic body suitable for use in soaking a capacitor, having a plurality of mutually parallel, superposed thin strata of dense dielectric material, substantially unobstructed hollow spaces separated from each other by strata of a dense, dielectric material, said strata being integrally joined at a plurality of edge portions thereof, and openings into the hollow spaces, whereby said hollow spaces are formed by volatilizing a material corresponding to the shape thereof by firing, characterized by said strata having a thickness of about 0.05 to 0.25 mm, whereby major portions of adjacent surfaces of at least two of said strata are separated f om each other to form a substantially unobstructed hollow space therebetween, having a thickness of about 0.007 to 0.04 mm, and whereby the separated strata axe in contact with each other only at said edge portions thereof.

2. Ceramic body according to Claim 1, characterized in that at least one of said openings is in an edge region of said body.

3. Ceramic body according to Claim 1, characterized in that a plurality of openings are in edge regions of said body, and that adjacent ones of said openings are in different edge regions.

4. Ceramic body according to any one o Claims 1 to 3, characterized in that the areas of said spaces are substantially the same.

5. Ceramic body according to any one of Claims 1 to 4, characterized in that the dielectric strata comprise a titanate.

6. Ceramic body according to any one of Claims 1 to 5, characterized in that the dielectric strata comprise barium titanate.

7. Capacitor including a ceramic body according to any one of Claims 1 to 6, characterized in that at least one Ί&' continuous metal layer is disposed betwee an adjacent pair of said strata, the metal of said layer having a melting point lower than the maximum temperature employed in sintering said dielectric ceramic material, and that electrical connections are attached to said metal layers.

8. Process for manufacture of a ceramic body according to any one of Claims 1 to 6 by providing a plurality of thin leaves of a finely divided ceramic composition bonded with thermally-fugitive bond, said ceramic composition forming an essentially dense, dielectric layer when fired to sintering temperatures, characterised by forming a consolidated stack consisting of a plurality of said leaves having interposed between at least two of said leaves a thin layer that consists of thermally-fugitive material, said layer having a smaller area than that of the adjacent leaves, said leaves being so arranged and placed that major portions of the edges of the leaves adjacent said layer are in contact with each other with said layer extending to an edge region of said consolidated stack, or else on some of said leaves a predetermined design is applied, consisting of thermally-fugitive materials, the number, form and arrangement of said thin layers or designs corresponding to those spaces which are desired to b cobtained in said body and firing said consolidated stack under suitable conditions at temperatures sufficiently high to remove said thermally-fugitive materials and to sinter said ceramic composition, whereby to form an integral, sintered ceramic body.

9. Process for manufacture of a unitary ceramic body according to Claim 8, characterized in that the block after firing is severed into smaller bodies each of which comprises

10. Process for manufacture of unitary ceramic bodies according to Claim 8, characterized by severing in a known 4s» manner the block before firing by vertical cuts therethrough whereby to obtain a plurality of small bodies or chips in each of which at least one of said coated areas is exposed at one of a plurality of edge regions.

11. Process for manufacture of a capacitor according to Claim 7, characterized in that a conductive material is introduced into a thin, hollow space through said opening.

12. Process according to Claim 11, characterized in that said conductive material is a metal. HDsmr