Sintered ceramic bodies for use as electrical components and their manufacture

This application is an addition to Application No. 3872I.

The invention of the present application and the main Patent No. 38721 relate to the formation of electrodes and/or conductors in ceramic dielectric or insulating bodies and is particularly concerned with the provision of such electrodes and/or conductors by a procedure which obviates the necessity of firing them at the same time that the ceramic bodies, with which they are associated, are fired. Sxamples of products which may be produced in accordance with the invention are monolithic capacitors and multi-layer circuit structures such as are used for hybrid integrated circuits.

In use for Ceramic capacitors have been/many years and for many

a ־

purpose0 have replaced paper, mica, and other types of capacitors beca<j,a^ of tho relatively high dielectric constant of barium titanate and certain other available ceramic materials. This has permitted tho production of highcapacitance, miniaturised bodies. and high-speed pressing procedures have boon developed ¢0 reduce production costs. However, there has still been a demand for oven higher capacities In very 8mall7b0dle's; ’ 2viultllayo־r, monolit hic, ceramic capacitors have been produced to meet this demand.

While there are many variant procooeoo In use for tho production of׳ such monolithic, ceramic capacitors. In a typical process a doctor blade io uoed to cast on a smooth, non-aboorbent surface, a thin layer of a suitable t ceramic dielectric composition mixed with a solution of an organic binder. After tho layer dries, tho resultant shoot may be cut Into small pieces of roctang alar shape to which an elcctroding pacta of a noble metal such 30 platinum or palladium 10 applied by a ollk-ocracnlhg procedure in ouch a way that a margin 10 loft around three oldco of the metal coating, but the electrode paste o״tci;ds to one edge of the small sheet. A plurality of tho sheets with electrode paoto thoraon are then stacked v/lth alternate sheets having the electrode paote extending Ito opposite edges. The stack of shoots to than consolidated and

Ucatad to drive off or decompose tho organic binders of the shoot and the 010ctrading pasta and to sinter tho dielectric composition into a unitary body having electrodes exposed alternately on each end so that those c:rpoccd at each and may bo gonnocted together electrically by metallising the ends of the body. Thus, there is obtained a capacitor which may havo from a few to a great numbar, 50 or more being common, of very thin (often 0. 05 mm or less) ceramic dielectric layers. Such capacitors havo very high capacitance densities and thus the use of extremely small units In many circuits is permitted.

It may bo seen from the foregoing description that considerable expanse is involved in the production of monolithic ceramic capacitors because of the necessity for using noble motal electrodes. Silver electrodes, ouch as

are commonly used with other ceramic capacitors, are generally unsuitable therein because firing to a high temperature is required after the electrodes ' are applied.

K 10, accordingly, one of the objects of the present invention to provide a process by which the coat of monolithic, ceramic capacitors may be reduced by eliminating the use of noble metal electrodes. >

I ' ' ' ־ ’ ג ' ־

Another object of the present invention io to provide a procedure for making ceramic articles having conductive areas therein which does not ,.'.־.,.λ require the firing of the conductive material at the same time the ceramic articlo is formed by firing.

It is also an object of the present invention to produce multilayer circuit structure® for hybrid integrated circuits in which conductors for attachment of components are provided at various levels in a ceramic substrate or matrix

Satv/Trigy of the lavaatioa

The first two of the above-stated objects aro achieved by forming a sintered, monolithic, ceramic body which comprises a plurality of thin strata. The strata are of two types, strata of one type being dense and impervious and being formed of ceramic dielectric material with a relatively high dielectric coaetant, and strata of the other typo being of ceramic material having an open structure characterised by interconnected voids. Strata of one typo alternate with strata of the other type through the thickness of the body. This structure can be achieved by introducing between sheets of a powdered ceramic dielectric composition bonded with a temporary bond, a deposit of a temporarily bonded, powdered ceramic material that on firing develops the desired open structure, consolidating a plurality of such sheets with Intervening deposits, as described, and firing the consolidated mass to sinter it.' Such deposits may be formed in situ, for example by screen printing or painting, or may be preformed leaves or films. Alternate ones of such open-structured strata extend to a pair of different edge regions of the sintered body. but since the deposits of

. ׳׳ '. ' tho sccond-montioned ceramic material, and thus the open-structured strata, ore smaller io area than the dense dielectric strata, the other edge regions

0£ the fired body and the interior thereof immediately adjacent the latter״ msationod regions are composed exclusively of tho dielectric material.

׳\ Tho monolithic ceramic body, after being sintered by firing, ie convaried to a capacitor by introducing a molten metal Into the open-structured strata within tho body to form internal electrodes. When tho terms open structure and opon-otructurod are employed herein with reference to a body or portions thereof, it io meant that tho body or portion so characterised or described has enough voids of sufficiently largo else and sufficient intercosmectioa between such voids as to permit, in most cases, the infiltration of molten metal thereinto with tho use of no more than moderate pressure, e. g. ls hg/cm^ or lower,.׳'

Tho molten metal may be introduced into ouch strata before termination electrodes are applied on the edge faces of the body having Graced internal electrodes and termination electrodes may bo applied in conventional or desired manner. Alternatively, a penetrable barrier, which may bo a terminatlon electrode, can bo applied to each of ouch edge faces prior to introduc. liag molten metal into the.open-structured strata of said body and the metal can then be forced through said penetrable barriers Into the said open-ctructured strata. If said barriers are not termination electrodes, ouch electrodes can then ba applied after. If necessary or desired, removing ail or parts of tho barriers. In any event, the present invention provides a simple, relatively Inexpensive and officiant method for forming monolithic capacitors having a vary high volume capacitance which do not require noble metal internal oloctrodos and which do not require cofiring of metal and ceramic.

. ,A very similar technique can bo employed In producing multilayer circuit structures. For esample, thia sheets of a powdered, ceramic, Inaulatlag material temporarily bonded with a fugitive, temporary bond are provided, by a suitable procedure such as printing, with a desired pattern of lines, pads, and tho like of a ceramic composition (which may be termed a pseudo-conductor) that on. firing develops an open structure having Interconnected voids as with the above-described bodies. Tho shoots are then stacked, compacted and fired to produce sintered bodies with predetermined open-structured areas corresponding to the applied patterns of the pseudo-conductor, which are thon impregnated with a molten metal to provide conductors in place of the pseudoconductor. 7.י . \ . ' ,

The term metal” as used la this specification and the appended claims io employed broadly to include not only pure and substantially pure metals, but .also alloys.

Short Description of the Drawings . Figure 1 i@ an enlarged sectional view of a finished, monolithic, ceramic capacitor in accordance with the present invention«

Figure 2 la a sectional view along the plane of the line 2—2 of Figurel, . Figure 3 is a plan view of a bonded sheet of a ceramic dielectric composition having deposited thereon, in a pattern, a ceramic composition suitable for formation of an open-structured stratum.

Figure 4 is an enlarged perspective view of two sheets of a bonded ceramic, dielectric composition, each sheet having an area thereon covered with a ceramic composition suitable for formation of aa open-structured stratum.

Figure S is a farther enlarged, detail sectional view of a ceramic body according to the present Invention after assembly and sintering of a plurality of sheets such as shown in Figure Ί;

Figure 6 is an enlarged, sectional view of a multilayer ceramic circuit structure according to the present invention;

Figure 7 is an enlarged, es^Ioded view showing the eevoral ceramic sheets forming the structure shown in Figaro 6 with paeudocondactors thereon; and

Figure @ is a fragmentary, enlarged, sectional view similar to Figure 5 of a modified form of ceramic body according to the present invention.

Detailed Description of the Invention

A preferred process for preparing monolithic ceramic capacitor□ according to the present invention Is broadly as follows:

A suitable, finely divided, ceramic, dielectric material is formed into a thia film with the aid of a suitable, fugitive, heat-re movable, film-forming agent. After drying, the film io cut into oheets of suitable; also. On. these sheets is then applied a thin layer, film, or coating, la a desired pattern, of a suitable paste or the like containing a fugitive or heat-removable binder and a powdered ceramic composition which when fired at sintering temperatures will, instead of becoming dense and compact, form an open structure, i. e. a structure,׳ a substantial portion of the volume of which is composed of Interconnected voids. A plurality of the thuo-coated ceramic sheets Is assembled la Stacked relation, consolidated into a block, and cut into, smaller blocks or chips. The latter are heated io remove the film-forming, temporary binding agents and are then further heated to a high temperature in air to produce small, coherent, sintered bodies with dense, ceramic dielectric strata altersating with open-structured strata. In each of the chips th® latter strata extend to an edge face and thus, according to the present Invention, may bo infiltrated or Impregnated with a conductive material such ao a metal or alloy. Upon suitable infiltration or impregnation, there ia obtained a structure la which there are alternate layers of dielectric material and.metal which, when aa end or termination electrode io provided on each end to electrically connect the metal layers exposed thereon, forms a monolithic capacitor.

The drawings depict ouch a structure. Figures I and 2 illustrating on an enlarged and exaggerated scale a monolithic capacitor 11 having thin layore IS of dielectric material with thinner layers 15 of conductive material suchjss & metal or alloy, inlorpcaed between tho layers 13. Ao will bo seen in .Figure 1, th® layers 15 are 00 formed that alternate one ט extend to the oppoaito and faces of the capacitor and are there connected together electrically by metallising the ends in a suitable, known, manner to provide the end or termination electrodes 17 and 19. Where, as shown at 21, there is no intervanlng conductive material, the dielectric layers 13 are united.

In Figure 3 there la shown a film or chaei 25 of temporarily Loaded dielectric material on which a pasta or the like, containing a fugitive binder and a ceramic composition which on firing to sintering temperatures will form an open structure, has been printed in email areas 27 to form a pattern.

In Figure 4 there are shown, enlarged, two small thin sheets 35 of dielectric material bonded with a fugitive bond, each of the sheets 35 having thereon a layer, film, or coating 37 of a temporarily bonded, ceramic composition that on firing will form a sintered open structure. The sheets 35, which may ba formed individually or by appropriate cutting of larger sheets such as the sheet 25 (Figure 3), are arranged so that when superimposed or stacked the ends of the layers 37 that extend to the edges of the shoots will be at opposite ends of the stack. Whan a plurality of such sheets are stacked and fired at sintering temperatures a structure like that shown in Figure 5 is obtained.

In Figure 5 there is shown, further enlarged, a partial sectional view of a sintered body in accordance with tho praoaat invention with alternating dielectric strata 41 and open-structured strata i3, the latter being adapted to receive a conductive material.

Figures 6*3, inclusive, will be hereinafter described in connection with the description of the structure involved.

In tho following examples, details of the production, according to tho present invention, of monolithic ceramic capacitors are set forth.

EXAMPLE 1

An ancalcined ceramic dielectric composition consisting of 93% of barium titanate (BaTlOj) and 7% of bismuth sirconate (HijOj . 3ZrOa) io employed. A mis of 100 g of the dielectric composition in finely divided form (approximately 1. 5|tfm particle else) with 65 ml of toluene, 3 g butyl beaayl phthalate, 10 ml dichloroathane, and 2 ml acetic acid io ball milled for 4 hours. To the ball milled product there is then slowly added with stirring, an additional 20 ml of dichlorothano and 4 g os ethyl cellulose. If necessary to eliminate bubbles, the stirring may be slowly continued for several hours. A film of the mixture approximately 610 mm by 102 mm la area by 0. 051 mm thick 10 formed with a doctor blade on a sheet of smooth plate glace. V/hoa, the film dries, the sheet thus formed U removed and small rectangular sheets or leaves approximately 1Q2 mm by 51 mm are cut therefrom.

The composition for tho מסקס.structured, porous strata io formed from a second ceramic composition consisting of 66. 94% barium carbor— ato (BaCOs), 27. 1% titanium dioxide (TiO2), 3, 32% bismuth oxide (Dl2Os), and 2. 64% alrconium mddo (ZrO2), all in powdered form, blended in a 1:1 weight ratio with a vehicle of the type known as oquoegeo media which is composed of 80 ml pine oil, 14 g acrylic rosin, and

1.5 g lecithin dispersing agent to which 1. 3% (based on the total weight of all other ingredients of the composition) of ethyl cellulose is added to Increase the viscosity. Tho average particle ciao of the TiO2 in the composition is preferably from about 5 to 10 μπ3 and the particle sisaa of the other ceramic ingredients used preferably average from about 1 to 2 j/m.- This composition is screen printed approximately 0. 033 mm thick in a recurring pattern, such as shown in Figure 3, on the small leaves of dielectric composition formed as described above. Tho printed leaves are then indexed and stacked in groups of 10 so that tho

. ״9’ printed patterns oaalternate loaves arc offset. The broken line !29 נ la Figaro 3 indicate the placement of the printed pattern on the sheets above and/or below the shoot 25 whoa the shoots are stacked. The stocked shoots are pressed nt.about S5°C and 28 kg/cm ׳ to form blocks. Th□ blocks arc then cut, by suitable means ouch as kalvoo, to form smaller blocks or chips, the cutting being done along such lines so the broken Haos 31 aud 32, □0 that la oach of the smaller blocks the alter.׳ mto strata of screen printed composition are 'exposed oa opposite ends but are not exposed on the sides.

The omallor blocks are then heated.quite slowly in air to drive off and/or decompose the temporary binding material in the ceramic layoro and are thereafter fired at a high temperature, also la air, to form small, coherent, sintered chips or bodies.

A suitable heating schedule for removal of the temporary binding material io as follows!

100®C - 16 hours 295^C . 2 houre

15O®C -16 hours 325QC - 1. 5 hours

175°C - 3 hours 355 C 1־hour׳

21Θ C - 16 hours * 305 C . 1 hour

2’25°C - 8 hours 420 C - 0. 5 hour

- , 250°C - 16 hours 815°C - 0. 5 hour

The temperature Is then raised to 1260°C and maintained for 2 ׳ hoars to sinter the chips.

Th® sintered chips obtained, after cooling, may be provided with a metal or alloy ia tho porous strata and provided with termination electrodes on their opposite ends to obtain efficient monolithic capacitors.

Ia the foregoing example the porous, open-structured strata of

ע ' ׳ ׳ the monolithic ceramic capacitors are essentially the same chemically as the dense dielectric layers, th© porosity.of the porous strata being produced as a recult'of She decreased volume occupied by the ceramic material used after tha reaction thereof which occurs during heating. In the following two examples the porous strata are chemically different from ths dielectric strata,

EXAMPLE 2

A finely divided iapproximatoly l.5'׳Jtfm particle else) ceramic dielectric composition consisting of 98% BaTiOj and 2% niobium oxide \libaO3) 10 employed. A mis conoisting of 480 g of the powdered dieloctric composition, 4. S g of a. lecithin dlBporsing agent, 12. 6 g of dibutyl phthalate, and 75 ml of toluene is ball milled for 4 hours.Tt©5?Q is then added 156 g of a 40% acrylic rosin - 60% toluene solution. The mixture 10 slowly stirred for a period of time sufficient to increase . tho viscosity by evaporation of solvent and xto remove entrapped.air.

. B Is then cast oa a smooth glass plate in a sheet about 610 mm square and allowed to dry. Tho air-dried cast, sheets are about 0. 07 ram thick and are cut into smaller shoots or leaves approsdmately 102 ram by

9I mm.

The composition for the porcus strata is formed from a second Eijliituro consisting of barium oiaalato (0aCaO4) and TlOj in a Is 1 mol ratio. The T1O2, which comprises 26.17% of the mixture, preferably hao an average particle also of about 2-5 ym. The mixture is blended ta a Is 1 weight ratio with the squeegee medium described in Example 1 and screen printed in a predetermined.recurring pattern on the small leave® of-dielectric material. Tho printed leaves ar® then indexed, stacked 15 high, and compacted. Th® thus formed blocks are cut, as ta Example 1, to form a plurality of smaller blocks or chips, la each ©f which alternate layers of the screoned-on composition ejdend to opposite end faces of the chips, but are otherwise inaccessible.

The chips ar© heated ia accordance with a suitable schedule, .

which may bo the on® sot forth in Example 1, io eliminate the fugitive binders and are then fired for about 2 hours at about 1325 C to sinter ihom.'A0 in Esamplo 1, the strata between the donoo dielectric strata

-I®.

...: -;:׳;-״־:ר-־ v ; .-.:' leave an open atructure comprising a network of Interconnected pores mad,. a a a result of the relatively greater shrinkage when the barium oxaldta and TiOj react to form BaTlOs, the major portion, by volume, of ouch strata is void. After cooling, the fired chips may be, as hero* laafter described, provided.with electrodes lathe porous areas or ׳/':1 ׳G®EOGa formed between the dielectric strata and with termination ' .',־־.' י,. >

©leetrodes by suitable procedure, thereby forming monolithic capacitors. / Evon more widely different ceramic materials ia the dielectric layers and. porous layers, respectively, are used in the following example.

: ' EXAMPUS 3

A mixture is made of 472. 3 g TiOa (average particle else about J. S jt/®0 L 7. 2 g Kaolin, 4. Q g lacUthla dispersing agent, 13. 6 g dihutyl phthalate, and 75 ml toluene and this mixture is ball milled׳ £024־ hours. it io then mixed with 124, 9 g of a 1:1 acrylic resintolaeno solution and, after da-airing, is cast טס a smooth glass plate ' with a doctor blade to a thickness of 0. 2 mm to produce on drying a sheet about 0. 08 mm thick which Is cut into smaller sheets approxtaataly 102 mm by 51 mm.

׳ Using the procedure of Example 2, the smaller sheets ar® 'screen printed in. a predetermined recurring pattern with a composition fessaod by. mixing 27. 58% powdered alumina(AlgO9) having an average particle sisa of 2. 5 jjm, 14. 14% carbon black, and 58. 27% of the squeegee medium described in Example 1. The printed sheets are then indexed, stacked 10 high, compacted, and cat to form a plurality of Mecke or chips in each of which alternate layers of the screened-oa composition extend to opposite end faces of the chips, but are otherwise inaccessible.

The chips are.heated, and then fired in substantially the same way £>e the chips In Example I, a final firing far 2 hours at about 1320°C being employed. As in Example 1, the open-structured strata between fito donso, dielectric, TIO3 strata have a aotwork of interconnecting pas’□□» Those result irons too combustion of th® carbon black and th® largo? particle 0130 02 tho AlaOj. Tho porous strata caabs impregnated with a metal, by oao of tho procedure□ hereinafter disclosed, and provided with suitable termination electrodes, thereby forming monolitoie. capacitor ־.פ

Ja, jjijQ following onamplo another procedure for obtaining bodies vito alternate dielectric and open״ structured strata 10 Hluetratod.

EXAMPLE ύ

Small shoots or loaves of a reoia-boaded dleloctric ceramic • eorapasltion are prepared in the manner sot forth in Enarnplo 2. A screen printing compoeltlon 10 made by blooding Ιώ g of to® squeegee saedhMn doacribad in Esampl© 1 with 12 g BaTlOs (approximately 4 Jim particle oiao) and 4 g carbon black, Stoddard solvent being added aa aecoacary to obtain the desired viscosity. Thia composition 10. then ccrQQnod’oa tho loaves la the same manno? ac in Example 2 and allowed to dry. Blocks and smaller cut blocks or chips ar® formed from tho prisJtod leaves In tho same way^s la Enampie 2 and tho chips are hoatod ppd fiffod, also la the sama way. la the course of tho firing tho carbon M?.ck burn□.out ioaving aa open-structure comprising^ network of Into ?connected pores in the areas betwoon th® donoo dielectric strata. The use of the relatively coaroo DaTlOs in too printing composition tacronoos th® porosity. These porous areas can be filled with a metal la ©no of tho ways described horoinaftor. and.provided with end electrodes . tn form monolithic capacitors.

Etlli another way of forming monolithic ceramic capacitors oeeardiag-to tho, principles of the present invention is, illustrated la the follow Irg .©sample.

• .12EXAMPLE 5 • A sheet about 0. 0 8 mm thick of a ceramic dielectric material such as the one produced in Example 2 is cut into smaller eheeta or loavei) approximately 20 mm by 20 mm. Another sheet of slightly less thickness, for providing porous strata, io formed by casting a comp* ositibn formed from 351 g BaTiOj, 7 g NbjO8, and 115 ;g carbon black, these ingredients being ball milled for aeveral hours with toluene and dibutyl phthalate and then, after admixture with a 1:1 acrylic resintoluene solution, de-aired before casting. The second sheet is cut into leaves approximately 13 mm by 16 mm. The leaves of dielectric material and of the other coramic material are then stacked 11 high. Th© second-mentioned leaves are alternated with the leaves of dielactric material and have their long side cdgeo aligned and equally spaced from the edges of the larger leaves. Alternate leaves of the second composition are laid in place so that the ends thereof cxUnd to opposite edges of the dielectric material leaves. The stack is then consolidated by pressing at about 7 kg/cm and a temperature of about 40°C and the consolidated block ie heated to barn out the temporary binders and the carbon black and to sinter the ceramic materials into a structure in which open-structured, porous, ceramic strata alternate with dense, ceramic, dielectric strata. A heating schedule like that specified in Example 1 is used, the final heating, however, being at 1370°C for 2 hours, firing being in air. The fired block may b© impregnated with a metal in the porous strata, thereby forming internal electrodes, by any of the procedures described hereinafter. Suitable termination electrodes can also bo provided.

Although in Examples 1 - 2׳ the dielectric materials used are modified barium titanate compositions, it will be clear that others of tho

• . '־ ' largo number of ceramic dielectric compositions known may also be used. For oxamplo, TiOa (note Example 3), glass, steatite, and barium strontium nlebaSs, as well as barium titanate alone can be used, suitable changes well knovm in the art being made aa required in firing conditions and the like to achieve proper sintering. Obviously, the capacitance of the resulting capacitorn will vary as a result of using materials with higher or lower dielectric constant□.

It will also be understood that the composition of the openstructured strata in ceramic chips according to the invention may vary widely. Not only may the desired open-structure be achieved by use of a composition which is identical with or similar to the composition of the dielectric strata, although having a greater shrinkage on firing, but also the composition may be quite different, as for example, in Ejtample 3. An open-structure may also be produced or the void volume of such structure be increased by other means, for example by employing a combustible material in the mix aa illustrated in Examples 3 and 5. It is important, however, to employ materials which, at the temperatures reached during heating and sintering, do not by reacting with the dielectric composition used, deleteriously affect the dielectric properties of the latter. Those skilled in the art are familiar with the effects of various materials and can readily make proper choices thereof. It should be mentionad hero that, 0. g. by choice of one or more of tha several means discussed above, tha open-structured areas of ceramic chips can be varied not only in toto but that different areas and portions of areas may be more or less porous than others. This enables the production of chips in which the portions of the porous strata adjacent the exposed end faces thereof are less porous or have finer pores than the portions lying nearer the center of the chip□.

Farther, It will be understood that there are available commorclally many media or vehicles which can be used for forming films and/or

- . ' making screen printing compositions from fine particles according to the present invention and that many more ouch vehicles are known to thone skilled In tha.art. Essentially, tho purpose of ouch a medium or vehicle ia to ouepend the particles and provide a temporary or fugitive bond therefor during formation of leaves and/or layer® and the conoolidation of a. plurality thereof into green bodies prior to sintering. In the sintered bodies tho temporary or fugitive bond, ao well as any combustible particulate material coed, has disappeared. Accordingly, tho medium or vehicle used to a matter of choice or convenience and in most Instances any change in the composition bonded thereby will require some change or modification, e.g; adjustment of viscosity* in any medium or vehicle employed.

Firing of small ceramic units or chips to sinter them into unitary bodies is preferably carried out in a kiln. An electrically heated tunnel kiln or furnace le preferred but other kilns or other heating means may be employed. Ordinarily an oxidising atmosphere is used but, when convenient, other atmospheres can be employed. The temperature, atmosphere, ©nd the time of firing will depend on ths ceramic compositions employed. Those okLIkd in the art are familiar with ouch details, aa pointed out above, and t with the fact that in general the sintering time necessary varies inversely with the temperature and vice versa. As indicated above, a prolonged period ©f heating nt relatively low temperatures is preferred for removal of tho temporary bonds used in the leave® and printed areas and any burn-out particles employed. If too rapid heating ia employed expansion of gases formed in the decomposition or burning of thee© materials may rupture tho cMpo.

Ia Figure 6 there la illustrated a typical ceramic multi layer circuit structure, 50 such as is used for hybrid Integrated circuits. The otructure 50 has a ceramic matrix 52 and a plurality of conductors 54 extendIng Into and through the matrix. The thickness of both conductors and matrix .15.

c3?aggo rated in Figure 6 for convenience in viewing. Hitherto such ί .

atructuros have been expensive to produce and normally would be made by screen printing a metallic paste containing a noble metal auch'as palladium or platinum in the desired conductor patterns on sheets of desired thicknacn of a temporary bonded, electrically Insulating, ceramic material such as alumina powder, .consolidating the several sheets, and sintering the alumina sheets into a unitary body. ״;.'״

As mentioned above, such ceramic multilayer circuit structures may also be produced by techniques essentially similar to the processes disclosed above for producing monolithic capacitors. The necessity for using 93ipan3ivQ noble metals as conductors io thus avoided since firing of metal Gs?.d sintering the ceramic concurrently is not necessary. One method for producing such a structure as that shown in Figure 6 by the technique of the present Invention will be briefly described with reference to Figure 7.

The sheets or films A, B, and C shown in Figure 7 are formed In the desired else, shape, and thickness by casting, molding, or the like a desired electrical insulating, ceramic composition, /or example finely divided alumina, using a resin, ethyl cellulose, or the like as a temporary *

bond therefor. Pseudoconductors following the paths of the desired conductors In and/or on the structure as shown at 60 are then screen printed on the sheets filnae using, for example a ceramic material in a suitable vehicle or squeegoo medium, the ceramic material being one, e. g. coarser alumina powder, which upon firing to sintering temperature will develop an open structure. The sheets are assembled, consolidated, and heated to sinter them into a ffi31tary body, all In the same manner as described above in connection with the production of monolithic capacitors. As with the,latter, the unitary or monolithic body produced by heating comprises a dense matrix of the ceramic insulating composition having therein open-structured areas of ceramic material, which may be the same or different In composition, a substantial partion of the volume of such areas comprising interconnected voids. Each of said areas extends to at least one region on an outer face, e. g. an edge

/.*&’ .

face, of said body.

Conductors in and through said bodies may bo formed by introdiscing into the open-structured areas a suitable conductive metal in accordance with one of the procedures described below. After such impregnation, leads may be attached, by suitable known means, to exposed conductors where desired and small components such as transistors, diodes, etc. may be soldered at predetermined points, loads therefrom extending if desired, to underlying conductors 54 through holes 62 provided originally in one or more of the ceramic sheets. If desired, one or more of the holes 62 may be filled with tho material employed to form the psoudoconductors when such material is applied to the faces of the sheets.

Although other procedures may be employed, a convenient and efficient way to provide conductive metal in the open-structured areas of small, sintered, ceramic bodies or chips or ceramic, multilayer, circuit structures produced as described above is to inject the meta! therein. Typical Infiltration or injection procedures are set forth in the following «xamplas.

EXAMPLE 6

A plurality of sintered chips made In accordance with Example 1 are placed in a bath of a molten metal alloy consisting of 50% Bi, 25% Pb, 12, 5% Sa and 12. 5% Cd. The molten metal is held at a temperature from about 100°C to about 125°C in a suitable closed vessel. After Introduction of the chips the pressure in the veeael is reduced to evacuate the open-structured strata of the chips and the ' 2 pressure is then raised to about 14 kg/cm to force the molten metal into tho interconnecting pores of such strata. The chips after removal from the bath contain electrodes formed by deposit of the alloy in the open-structured strata between the dense dielectric strata and, after the provision of termination electrodes in any desired manner, are

-'.

I- '. :.

. ' י

I satisfactory monolithic capacitors.

It will be understood that other molten metals can be used for impregnation of the open*structured strata areas or strata of ceramic chips of the types described above or of multilayer circuit structures as above ! described. For example, there may conveniently be used instead of the alloy specified in Example 6: tho metals lead, aluminum, copper, zinc, tin and cadmium and alloys containing one or more of these metals. Other ן metals are also usable but because of their higher coot, higher resistivity, greater ease of oxidation and/or high melting points, they are not as deairable for forming electrodes. Examples of the many other alloys that can be conveniently used are: Pb 25%, Sn 10%, Bi 63%, In 2%; Al 4%, Cu 1%, balance Zn; Cu 2fi%, Ag 72%, and various brasses and bronzes. As with the relatively pure metals,, however, the cost, resistivity, ease of oxidation, and melting point of an alloy affect greatly its desirability for carrying out | the present invention.

In general it has been found desirable to employ as internal electrodes or conductors, metals which do not easily wet tho ceramic chips and/or circuit boards Into which they are injected. By avoiding combinations in which the ceramic is readily wet by the metal it is possible to prevent or minimise undesirable surface deposits of the metal which would require removal to preclude possible shorting.

| In the conventional manufacture of monolithic capacitors there is no problem in providing a termination electrode on each end of tho capacitor units to electrically connect the exposed internal metal electrodes thereon since the metal electroding pastes commonly employed for thio purpose do not require heating to a temperature higher than the melting point of the internal electrodes. The same is true of monolithic capacitors produced by

. ׳ ' : ' 1

-IS.

.- -ך-- -^’7- 'ί v V - /'/' יי

-. λ:/׳''- . W׳:. ‘ ., ' י; ., / < י ' ' ./. : deposition of metal in open-structured strata of fired ceramic chips by any of the procedures described in tho said copending application when the melting point of th© metal 00 deposited is higher than the temperature required to bapply the termination electrodes. When, however, as may occur in carrying out the present process, the metal deposited in the open-structured areas or strata of ceramic chips or circuit boards is liquid at a temperature equal to or lower than th© temperature employed in applying the termination electrodes, the provision of the latter may present problems.

In the manufacture of monolithic capacitors by the process of the present invention it has been found useful ia many cases to provide penetrabl® barriers at the ends of the ceramic chips before injecting molten metal into the open-structured strata of the chips. Such barriers should bo easily provided, should be of material having a melting point higher than the temperature at which the metal internal electrodes are Injected, and should be resistant to attack or dissolution in the bath of molten metal used for the internal electrodes. They allow evacuation of air from the openstructured areas or otraia of fired ceramic chips such ao those produced in accordance with any of Examples 1-5 and the injection therein of molten metal to form interior electrodes. They also serve to restrict flow from ouch areas or strata when the pressure in the vessel is released. Similar results aro obtained by the use of ouch barriers on the desired faces of multilayer circuit structures according to the invention.

Suitable penetrable barriers can be formed in several ways. For example, when low-melting metals are uoed for Infiltration of the unitary ceramic bodies a coating of a commercial palladium-silver or palladiumgold electroding paste can be applied to the surfaces of sintered chips at which porous strata are exposed and fired in the conventional manner.

.19. A'.

The! coherent end electrode thus formed io penetrable to the molten metal.

When higher temperatures are required for infiltration of metal into tho porous areas or strata of ceramic bodies, it has been found feasible to apply over ouch areas or strata on faces of the bodies a coating of a ceramic material which is fired to form a penetrable, porous, ceramic barrier. Tho ceramic material may ba applied to the green ceramic chipe and fired at tha same time as the latter or may be applied to the already sintered chips a nd then firsd.

Tho use of an end or termination electrode ao a penetrable barrier i□ described in the following example.

EXAMPLE 7

Porous end termination electrodes are applied to a plurality of sintered ceramic chips substantially like those produced in accordance with Example 1 by coating the end faces (i. e. the faces on which the porous strata are exposed) of tho chips with a commercial palladiumOliver electroding paste (DuPont No. 8198) and firing tho thus-coated chips at about 880°C, the firing cycle being about 1 hour.

Using apparatus similar to that used in Example 6, tho chips

I are placed in a heated pressure vessel above a bath of molten tin held at about 3!5°C. The vessel is closed and through a suitable connection the interior of tho vessel is evacuated to a pressure of about 60 mm of mercury to remove air from the porous strata of the chips. The chips, which have now been sufficiently heated so that no substantial thermal 'shock will result, are then lowered into the tin b&th and the pressure In th® vessel is raised, by supplying a compressed gas, ouch as nitro2 • ne thereto, to about 17, 5kg/cm . Tha chips are then removed from the melt and after cooling in the vessel to below tho molting point of tin tho gas pressure in the vessel 13 released and the chips are withdrawn from the vessel, adhering tin being removed if necessary. Microscopic inspection of broken chips reveals that tin has been forced into the porous strata thereof and the impregnated chipo are very satisfactory monolithic capacitors.

As indicated above, the penetrable barriers employed in injecting or impregnating open-structured areas need not be electrically conductive, termination electrodes. The following three examples illustrate this.

יי EXAMPLES

Unfired ceramic blocks or chips such as ones prepared in accordcuaca with Example 4 are employed. The end faces of a plurality of the chips, I. e. the surfaces at which the alternate layers of acreened-on composition are exposed, are coated, conveniently by painting, with the same screen printing composition as is applied to the leaves of the resin-bonded, dielectric, ceramic composition. The coated chips are then subjected to heating in air to eliminate the combustible mate Hale and fired Ln air at about 1325°C to sinter the ceramic. The resulting fired ceramic chips have alternate dielectric strata and open-structured r.

strata and end barriers which are permeable to molten metal.

The fired chips are impregnated with molten tin In the manner described in Example 7, After removal of the metal-impregnated chips from the pressure vessel and cooling, the ceramic barriers and any undesirod metal adhering to the surfaces of the chips are removed, for example, by sandblasting, and electrically conducting termination electrodes are applied in accordance with any desired procedure. The resultant monolithic capacitors are very satisfactory.

EXAMPLE 9

Unfl red ceramic blocks or chips such as ones produced in . : accordance with E3sampla 5 ar® used. In procedure aimliar to that in

׳ 1; ' '

Example 8, the end faces of a plurality of the chips are coated, by painting or dipping, with the liquid composition used in Example 5 for casting the sheets employed in forming the porous strata In the chips. The coated chips are then heated in air to burn out the combustible materials and sintered in the manner described in Example 5 to obtain small ceramic bodicslwith porous strata that can be injected with metal through the porous,open .structured, ceramic barriers formed during

' ' י' י. י _ ' sintering.

The fired chips are impregnated with a molten metal alloy consistIng of 72% Ag and 28% Cu using essentially the procedure described in Example 7. The temperature of the alloy during impregnation la proforably about 880°C. When the chips are impregnated and cooled, and after sandblasting to remove the ceramic barriers if this la necessary to obtain good electrical contact with the internal electrodes, conductive ond terminations are applied to form monolithic, capacitors.

la the two immediately preceding examples the ceramic barriers t nro applied to the end surfaces of the ceramic chips before th® latter are eimtorod and are thus sintered at the oam® time. In th® following example a penetrable ceramic barrier is applied to th® chips subsequent to sinterin[׳ thorn.

EXAMPLE 10

A readily spreadable past® is prepared by blending finely divided borosilicate glass having a molting point of about 1030 C in about a 1:2 weight ratio with a liquid vehicle formed from 80 ml pin® oil, 14 g acrylic re.flin, 1. 5 g lecithin dispersing agent and sufficient, about 1 to 2 g, ethyl cellulose to impart th® deolrod viscosity. This past® is applied by spreading it on th® end face® {where the porous strata are •22* κ

י י י exposed) of sintered ceramic chips such as are produced by Example 5.

Tha coated chips arc then heated to about 790 C to burn off the vehicle of the applied paste and provide a porous, sintered glass barrier on the ׳׳ .chip end faces.

The thus-prepared chips are impregnated with molten lead by th© same procedure employed in the metal impregnation of chips described In Example 7 the temperature of the lead bath during impregnation being about 450°C. After sandblasting or otherwise removing the glass barriers and any uadeoired surface metal deposits the chips may b® formed into satisfactory monolithic capacitors by providing ®ad termination electrodes by any known or desired procedure.

Extensive experiments have shown that׳by tho process of th® present invention monolithic capacitors with internal electrodes of infiltrated base motal can bo produced with capacitances oubotantially the came as those of monolithic capacitors of the same else and number of layers produced by convontional procasses with noble metal internal electrodes. This has been demonstrated with dielectric compositions having relatively high dielectric י' H constants aa well aa ones with relatively low dielectric constants.

A

Th© structure obtained when a permeable barrier is provided on aa cad face of a ceramic chip preparatory to forming a monolithic capacitor io illustrated in Figure 8 of the accompanying drawings. In thia figure, which Id a fragmentary, enlarged sectional view, tho strata 70 represent sintered ceramic dielectric material, th© numeral 71 designates the open-structured otrata, alternating with th© first-mentioned strata, into which molten metal 10 injected to form internal electrodes, and the numeral 72 designates th© όροη-Qtructured, penetrable barrier through which the molten metal 10 injectod. Th© showing in Figure 8 is somewhat diagrammatic since when a ceramic psnotrablo barrier is used, it is so sintered to tho ceramic chip as to be . -23unitary and frequently there 10 no clear line of demarcation between the barrier and the chip.

As will be clear to those skilled la the art, the permeability of the penetrable barriers employed may be adjusted as desired. This can be ׳ dona, for example, by the inclusion of greater or lesser amounts of burnout material, each as carbon black, in the compositions used for providing such barriers and/or by adjustment of the particle else of the solid materials in ouch compositions. Other procedures for such adjustment may be used if desired. It will be understood that In some instances termination electrodes may be applied to bodies, by any desired method, over penetrable ceramic barriers subsequent to the injection of metal therethrough into! open-structur©d areas of the body. However, to ensure good electrical contact of such ©lectrodea with the infiltrated internal electrodes it is frequently desirable to remove the barriers before applying termination electrodes. This can be, as pointed out above, readily accomplished by sand blasting.

Although Examples 6-10 are directed to the production of monolithic capacitors by the injection of metal into the open-structured strata of sintered ceramic chips, it is evident that similar techniques, including the use of penetrable barriers, if desired, may be used in providing conductors in multilayer circuit structures such as are described herein.

The pressure required for the impregnation of open-structured areas of ceramic chips or ceramic multilayer circuit structures with molten metal according to the invention will vary with the siae of the voids therein end the 0iz© of the interconnections therebetween. Also affecting the required pressure are the viscosity of the molten metal and the surface energy thereof relative to that of the open-structured ceramic. In some cases, it may bo necessary to make preliminary experiments to discover the optimum pressure to use. However, it has been found that in the use of metals of medium . -24and low melting potato, pressures greater than about 17. 5 kg/cm are not necessary, It should be understood that the pore or void size in a penetrable barrier may, If desired, be less than that in the open-structured areas of the body on which the barrier is applied.

' .

End or termination electrodes can bo formed not only with metallic electroding pastes of conventional types, but also, when suitable, by applying to faces of bodies coatings of air-drying conductive metal paints, electroless nickel, indium-gallium alloy, and sprayed metal. It may be noted that flame-or arc-sprayed metal is usually deposited in a rather porous layer. Hence, such deposits can be used, if desired, as conductive pensj '.

trable barriers.

Monolithic capacitors according to the present invention may vary widely in Gino, Not only may the dimensions of the capacitor b® varied, but the number and thickness of the strata therein may also vary. Although In most cases It is preferred to make the dielectric strata thicker than th® conductive layers, this Is subject to variation as desired. Capacitors as email as 2. 0 mm x 3. 0 mm x 0. 9 mm with 20 dielectric strata as thin as about 0. 03 mm and 19 porous strata as thin as about 0. 015 mm can bo readily made, and larger ones are, of course, possible. Capacitors of any desired capacitance may be obtained according to the invention by proper choice of dielectric material and the size, thickness, and number of the strata. It will be understood that one or more extra or additonal dielectric leaves or sheets may be placed at the bottom and/or top of a stack of alternated dielectric leaves or sheets and leaves or sheets containing a ceramic composition adapted to form porous strata. This is often done to give additional mechanical strength to the capacitors and/or to adjust their thickness. An unprinted leaf or leaves of a dielectric ceramic composition can be used. However, the presence of a printed ceramic film on the top dielectric film or leaf of such a stack will ordinarily not bo detrimental since after sintering the resultant

exposed porous deposit will either not hold an electrode material or such material can be easily removed, for ejcamplc by sanding.

In the foregoing description and the examples, ths leaves of diolsctric and/or potentially porous ceramic and the capacitors formed there* from are rectangular. However, the present invention comprehends capacitors of other shapes. Thus, if deaired, monolithic capacitors according to * tho invention may be triangularly shaped. In ouch case, obviously, alternate porous strata and the electrodes formed therein can not be exposed on opposite edge faces. Consequently, it will be understood that in the appended claims the term edge region la used comprehensively to indicate an area oa aa edge face of a body regardless of the geometry of the body and whether it has one or a plurality of edges.

The terms of position or direction, such as upper, lower, left, right, etc., used herein are with reference to th© accompanying drawings and should not be interpreted as limiting the invention or requiring any specific positioning of the capacitors in use.

Except as otherwise indicated, ratios, percentages, and parts referred to herein are ratios, percentages, and parts by weight.

It will be evident from the foregoing description that many variatlono and modifications of the present invention are possible without departIng from the spirit thereof. For example, instead of using leaves of temporarily bonded, powdered dielectric or insulating ceramic material which are formed a© distinct entities, sheet-like films of such material in a suitable medium or vehicle may be formed by screen printing on underlying sheets or layers. Also, for ejmmplo, instead of screen printing the compositions which develop porosity on firing, such compositions can be painted on or applied in other ways. Further, although a soli-sustaining body is desired for firing, the stack of leaves or of leaves and tho layers thereon need not

./ be pressed to consolidate the stack. In some cases, for example, rolling of the stack as it is built up will provide sufficient consolidation. Also, if desired, the open-structured areas or strata can be partially filled with metal by one of the procedures described in the said copending Application No. 3Θ721 and additional metal may be injected into th© partially filled areas or strata by the process of the present invention. In such case, the metal injected or infiltrated may be the same as that first introduced or a different metal.can be used.