Note: Descriptions are shown in the official language in which they were submitted.
~393r~o
r Back~round of the Invention
This invention relates 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. Examples
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.
Ceramic capacitors have been in use for many years and for
many purposes have replaced paper, mica, and other types of capacitors
because of the relatively high dielectric constant of barium
titanate and certain other available ceramic materials. This
has permitted the production of high-capacitance, miniaturized
bodies; and high-speed pressing producures have been developed
to reduce production costs. However, there has still been a
demand for even higher capacities in very small bodies. ~ulit-
layer, monolithic, ceramic capacitors have been produced to
meet this demand.
While there are many variant processes in use for the
production of such monolithic, ceramic capacitors, in a typical
process a doctor blade is used to cast on a smooth, non-absorbent
surface, a thin layer of a suitable ceramic dielectric composition
mixed with a solution of an organic binder. After the layer
dries, the resultant sheet may be cut into small pieces of rec-
tangular shape to which an electroding paste of a noble metal such
as platinum or palladium is applied by a silh-screening procedure
in such a way that a margin is left around three sides of the
~0 netal coating, but the electrode paste extends to one edge of
the small sheet. A plurality of the sheets with electrode paste
lQ39370
thereon are thell stackcd with al ~crnate shcets having tile
electrode paste extending to opposite cdges. rhe stack of
sheets is tllen consolldated and hcated to drive off or decompose
tlle orgallic blnders of the shcet alld the elcctrodlng paste and
to sinter the dielectric composition into a unitary body having
electrodes exposed alternately on each end so that those exposed
at each end may be connected together electrically by metallizlng
the ends of the body. Thus, there is obtained a capacitor which
may have from a ~cw to a great number, 50 or more being common,
of very thin (often 0.05 mm or less) ceramic dielectric layers.
Such capacitors have very l-igh capacitance densities and thus
the use of extremely small units in many circuits is permitted.
It may be seen from the foregoing description that
considerable expense is involved in the production of monolithic
ceramic capacitors because of the necessity for using noble
metal electrodes. Silver electrodes, such as are commonly used
with other ceramic capacitors, are generally unsuitable therein
because firing to a high temperature is re~uired after the
electrodes are applied.
The present invention consists of a process for pro- -
ducing electrodes or conductors in a sintered, unitary ceramic
body, which comprises: providing sheets of a finely divided
insulating or dielectric ceramic composition bonded with a
heat-fugitive bond, which composition forms a dense layer when
fired to sintering temperature; providing layers of a second
composition having a heat-fugitive bond, said second composition
developing an open structure when fired, and arranging and
consolidating a plurality of said sheets and layers whereby -~
to obtain a heat-fugitive bonded, self-sustaining body in
which said sheets of finely-divided insulating or dielectric
ceramic composition alternate with said layers of a second
_ 3 _
- ~.039370
composition and in which alternate ones of said layers extend
to different edge regions of said body; heating said body to
eliminate said heat-fugitive bonds; firing said body to sin-
tering temperature to produce a sintered monolithic body
having alternating areas of dense ceramic material and open-
structured areas, alternate ones of said open-structured areas
extending to different edge re~gions of said monolithic body;
forming penetrable barriers over each of said edge regions,
said barriers being capable of permitting egress of air r~m
said open-structured areas, permitting molten metal to be
forced by pressure differential into said areas and restricting
flow of molten metal therefrom upon termination of said pres-
sure differential; and providing a conductive material in said
open-structered areas by first evacuating said open-structured
areas and then allowing molten metal to flow therein.
When the terms "open structure" and "open-structured"
are employed herein with reference to a body or portions
thereof, it is meant that the body or portion so characterized
or
- 3a -
1039370
described has enough voids of sulficicntly large size and
sufficient interconnection betwcell such voids as to pcrmit, in
most cases, the infiltration of molten metal thereinto with
the use of no more than moderate prcssure, c.g. 35 kg/cm or
lower.
A penetrable barrier, which may be a termination
A electrode, ~i~P~ applied to each of such edge faces prior tD
introducing 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-structured strata. If said barriers
are not termination electrodes, such electrodes can then be
applied after, if necessary or desired, removing all or par~s of
the barriers. In any event, the present invention provides a
simple, relatively inexpensive and efficient method for forming
monolithic capacitors having a very high volume capacitance which
do nnt require noble metal internal electrodes and which do not
requlre cofiring of metal and ceramic. - -
A very similar techniq-lc can be employcd in prodllcillg
multilayer circuit structures. ~or example, thin sheets of a
powdered, ceramic, insulating 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 -
the 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.
The sheets 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 then impregnated with a molten metal to provide `
conductors in place of the pseudo-conductor.
The term "metal" as used in this specification and the
.
1039370
appcnded clalms is employed broa~ly to include not only pure and
substantially pure metals, but also alloys.
Embodlments of the inventlon are illustrated ln the
drawings. The description and drawings also relate to other
inventions which are claimed in Canadian patent application Serial
No. 167,849 filed April 3, 1973 oE which the present application
is a division, as well as a further divisional application
A Serial No. ~ ~ 2 ~~filed concurrently herewith.
In the drawings:
Figure 1 is an enlarged sectional view of a finished,
monolithic, ceramic capacitor;
Figure 2 is a sectional view along the plane of the
line 2-2 of Figure l;
Figure 3 is a plan view of a bonded sheet of a ceramic
dielectric composition having deposited tllereon, in a pattern,
a ceramic composition suitable Eor 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 an open-structured stratum;
Figure 5 is a further enlarged, detail sectional view of
a ceramic body after assembly and sintering of a plurality
of sheets such as shown in Figure 4;
Figure 6 is an enlarged, sectional view of a multi-
layer ceramic circuit structure;
Figure 7 is an enlarged, exploded view showing the several
ceramLc sheets forming the structure shown in Figure 6 with
pseudoconductors thereon; and
Figure 8 is a fragmentary, enlarged, sectional view
similar to Figure 5 of a modified form of ceramic body.
,. . . ~ . .
.. . .
10393t70
Detailed Description of the Embodiments
A preferred process for preparing monolithic ceramic
capacitors is broadly defined as follows:
A suitablej finely divided, ceramic, dielectric material
is formed into a thin film with the aid of a suitable, fugitive,
heat-removable, film-forming agent. After drying, the film is
cut into sheets of suitable size. On these sheets is then
applied a thin layer, film, or coating, in 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 or which is composed of
interconnected voids. A plurality of the thus-coated ceramic
sheets is assembled in stacked relation, consolidated into a
block, and cut into smaller blocks or chips. The latter are
heated to 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 alternating with open-structured strata.
In each of the chips the latter strata extend to an edge face
and thus, according to the present invention, may be infil-
trated or impregnated with a conductive material such as a
metal or alloy. Upon suitable infiltration or impregnation, - -
there is obtained a structure in which there are alternate layers
of dielectric material and metal which, when an end or ter-
mination electrode is provided on each end to electrically
connect the metal layers exposed thereon, forms a monolithic
capacitor
The drawlngs depict such a structure, Figures 1 and 2
lllu~tratlng on an enlarged and exaggerated scale a monolithic
-- 6 --
lQ39370
capacitor 11 having thin layers 13 of dielectric material
with thinner layers lS of conductive material such as a
metal or alloy, interposed between the layers 13. As will
be seen in Figure l, the layers 15 are so formed that alternate
ones extend to the opposite end faces of the capacitor and
are there connected together electrically by metallizing the
ends in a suitable, known, manner to provide the end or ter-
mination electrodes 17 and 19. Where, as shown at 21, there
is no intervening conductive material, the dielectric layers
lO 13 are united.
In Figure 3 there is shown a film or sheet 25 of tem-
porarily bonded dielectric material on which a paste 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 small 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
20 will form a sintered open structure. The sheets 35, which may
be 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 sheets will be at opposite ends of
the stack. When 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 the pre-
30 sent invention with alternating dielectric strata 41 and open-
structured strata 43, the latter being adapted to receive a
10393qO
conductive material.
Figures 6-8, inclusive, will be hereinafter described
inconnection with the description of the structure involved.
In the following examples, details of the production,
according to the present invention, of monolithic ceramic
capacitors are set forth.
EXAMPLE 1
i An uncalcined ceramic dielectric composition consisting
of 93% of barium titanate (BaTiO3) and 7% of bismuth zirconate
(Bi203 3ZrO2) is employed. A mix of 100 g of the dielectric
composition in finely divided form (approximately 1.5 ~m
particle size) with 65 ml of toluene, 3 g of butyl benzyl
phthalate, 10 ml dichlorenthane, and 2 ml acetic acid is ball
milled for 4 hours. To the ball milled product there is then -
slowly added with stirring, an additional 20 ml of dichlorethane
and 4 g of 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 in area by
0.051 mm thick is formed with a doctor blade on a sheet of
smooth plate glass. ~hen the fllm dries, the sheet thus formed
is removed and small rectangular sheets or leaves approximately
102 mm by 51 mm are cut therefrom.
The composition for the open-structured, porous- strata
is formed from a second ceramic composition consisting of
66.94% barium carbonate (BaC03), 27.1% titanium dioxide (TiO2),
3.32% bismuth oxide (Bi203), and 2.64% zirconium oxide (ZrO2),
all in powdered form, blended in a 1:1 weight ratio with a
vehlcle of the type known as squeegee media which is composed -
of 80 ml p$ne oil, 14 g acrylic resin, and 1.5 g lecithin
dispersing agent to whlch 1.3% (based on the total weight of
all other ingredient~ of the composition) of ethyl cellulose
-- 8 --
~0393qO
is added to increase the viscosity. The average particle size
of the TiO2 in the composition is preferably from about 5 to
10 ~m and the particle sizes of the other ceramic ingredients
used preferably average from about 1 to 2 ~m. This composition
is screen printed approximately 0.038 mm thick in a recurring
pattern, such as shown in Figure 3, on the small leaves of
dielectric composition formed as described above. The printed
leaves are then indexed and stacked in groups of 10 so that the
printed patterns of alternate leaves are offset. The broken
lines 29 in Figure 3 indicate the placement of the printed
pattern on-the sheets above and/or below the sheet 25 when the
sheets are stacked. The stacked sheets are pressed at about
85C. and 28 kg/cm2 to form blocks. The blocks are then cut,
by suitable means such as knives, to form smaller blocks or
chips, the cutting being done along such lines as the broken -~
lines 31 and 32, so that in each of the smaller blocks the
alternate strata of screen printed composition are exposed on
opposite ends but are not exposed on the sides. ~ ~
The smaller blocks are then heated quite slowly in air ~ -
to drive off and/or decompose the temporary binding material
in the ceramic layers and are thereafter fired at a high tem-
perature, also in air, to form small, coherent, sintered chips
or bodies. -
A suitable heating schedule for removal of the temporary
binding material is as follows:
100C. - 16 hours 295C. - 2 hours
150C. - 16 hours 325C. - 1.5 hours
175C. - 8 hours 355C. - 1 hour
210C. - 16 hours 385C. - 1 hour
225C. - 8 hours 420C. - 0.5 hour
250C. - 16 hours 815C. - 0.5 hour
The temperature ls then raised to 1260C. and maintained
for 2 hours to sinter the chips.
The sintered chips obtained, after cooling, may be
10393qO
provided with a metal or alloy in the porous strata and provided
- with termination electrodes on their opposite ends to obtain
efficient monolithic capacitors.
In the foregoing example the porous, open-structured
strata of the monolithic ceramic capacitors are essentially
the same chemically as the dense dielectric layers, the porosity
of the porous strata being produced as a result of the de-
creased volume occupied by the ceramic material used after
the reaction thereof which occurs during heating. In the fol-
lowing two examples the porous strata are chemically differentfrom the dielectric strata.
EXAMPLE 2
A finely divided (approximately 1.5 ~m particle size)
ceramic dielectric composition consisting of 98% BaTiO3 and
2% niobium oxide (Nb205) is employed. A mix consisting of 480
g of the powdered dielectric composition, 4.8 g of a lecithin
dispersing agent, 12.6 g of dibutyl phthalate, and 75 ml of
toluene is ball milled for 4 hours. There~is then added 156 g
of a 40% acrylic resin - 60% toluene solution. The mixture
is slowly stirred for a period of time sufficient to increase
the vis-cosity by evaporation of solvent and to remove entrapped
air. It is then cast on a smooth glass plate in a sheet
about 610 mm square and allowed to dry. The air-dried cast
sheets are about 0.07 mm thick and are cut into smaller sheets
or leaves approximately 102 mm by 51 mm.
The composition for the porous strata is formed from a
second mixture consisting of barium oxalate (BaC204) and TiO2
in a 1:1 mol ratio. The TiO2, which comprise~i 26.17% of the
mixture, preferably has an average particle size of about
2-5 ~m. The mixture is blended in a 1:1 weight ratio wlth the
squeegee medium described in ~xample 1 and screen printed in a
-- 10 --
1039370
predetermined recurring pattern on the small leaves of di-
electric materlal. The printed leaves are then indexed, stacked
lS high, and compacted. The thus formed blocks are cut, as
in Example 1, to form a plurality of smaller blocks or chips,
in each of which alternate layers of the screened-on composi-
tion extend to opposite end faces of the chips, but are otherwise
inaccessible.
The chips are heated in accordance with a suitable sch-
edule, which may be the one set forth in Example 1, to eliminate
the fugitive binders and are then fired for about 2 hours at
about 1325C. to sinter them. As in Example 1, the strata
between the dense dielectric strata have an open structure
comprising a network of interconnected pores and, as a result
of the relatively greater shrinkage when the barium oxalate
and TiO2 react to form BaTiO3, the major portion, by volume,
of such strata is void. After cooling, the fired chips may be,
as hereinafter described, provided with electrodes in the
porous areas or strata formed between the dielectric strata and
with termination electrodes by suitable procedure, thereby
forming monolithic capacitors.
Even more widely different ceramic materials in the
dielectric layers and porous layers, respectively, are used
in the following example.
EXAMPLE 3
A mixture is made of 472.8 g TiO2 (average particle size
about 1.5 ~m), 7.2 g Kaolin, 4.8 g lecithin dispersing agent,
13.6 g dibutyl phthalate, and 75 ml toluene and this mixture
is ball milled for 4 hours. It is then mixed with 124.9 g
of a 1:1 acrylic resin-toluene solution and, after de-airing,
is cas~ on 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 approximately 102 mm
-- 11 --
.. . . .
1039370
by 51 mm.
Using the procedure of Example 2, the smaller sheets
are screen printed in a predetermined recurring pattern with a
composition formed by mixing 27.58% powdered alumina (A1203)
having an average particle size of 2.5 ~m, lh.l4% carbon black,
and 58.27% of the squeegee medium described in Example 1.
The printed sheets are then indexed, stacked 10 high, comparted,
and cut to form a plurality of blocks or chips in each of
which alternate layers of the screened-on composition extend
to opposite end faces of the chips, but are otherwise inac-
cessible.
The chips are heated and then fired in substantially the
same way as the chips in Example 1, a final firing for 2 hours
at about 1320C. being employed. As in Example 1, the open-
structured strata between the dense dielectric, TiO2 strata
have a network of interconnecting pores. These result from
the combustion of the carbon black and the larger particle
size of the A1203. The porous strata can be impregnated with
a metal, by one of the procedures hereinafter disclosed, and
provided with suitable termination electrodes, thereby forming
monolithic capacitors.
In the following example another procedure for obtaining
bodies with alternate dielectric and open-structured strata
is illustrated.
EXAMPLE 4
Small sheets or leaves of a resin-bonded dielectric
ceramic composltion are prepared in the manner set forth in
Example 2. A screen printing composition is made by blending
16 g of the squeegee medium described in Example 1 with 12 g
BaTiO3 (approximately 4 ~m particle size) and 4 g carbon black,
Stoddard solvent being added as necessary to obtain the desired
- 12 -
1()39370
viscosity. This composition is then screened on the leaves in
the same manner as in Example 2 and allowed to dry. Blocks
and smaller cut blocks or chips are formed from the printed
leaves in the same way as in Example 2 and the chips are heated
and fired, also in the same way. In the course of the firing
the carbon black burns out leaving an open-structure comprising
a network of interconnected pores in the areas between the dense
dielectric strata. The use of the relatively coarse BaTiO3
in the printing composition increases the porosity. These
10 porous areas can be filled with a metal in one of the ways ~ -
described hereinafter and provided with end electrodes to
form monolithic capacitors.
Still another way of forming monolithic ceramic capacitors
according to the principles of the present invention is il-
lustrated in the following example.
EXAMPLE 5
A sheet about 0.08 mm thick of a ceramic dielectric
material such as the one produced in Example 2 is cut into
smaller sheets or leaves approximately 20 mm by 20 mm. Another
20 sheet of slightly less thickness, for providing porous strata, ~-
is formed by casting a composition formed from 351 g BaTiO3,
7 g Nb2O5, and 115 g carbon black, these ingredients being ball
milled for several hours with toluene and dibutyl phthalate
and then, after admixture with a 1:1 acrylic resin-toluene
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 ceramic material are then
stacked 11 high. The second-mentioned leaves are alternated
with the leaves of dielectric material and have their long
30 ~$de edge~ allgned and equally spaced from the edges of the ~ ~ -
larger leaves. Alternate leaves of the second composition are
- 13 -
---
la3s~70
laid in place so that the ends thereof extend to opposite edges
of the dielectric material leaves. The stack is then con-
solidated by pressing at about 7 kg/cm and a temperature
of about 40C. and the consolidated block is heated to burn
out the temporary binders and the carbon black and to sinter
the ceramic materials into a structure ln which open-structured
porous, ceramic strata alternate with dense, ceramic, di
electric strata. A heating schedule like that specified in
Example 1 is used, the final heating, however, being at 1370C.
for 2 hours, firing being in air. The fired block may be
impregnated with a metal in the porous strata, thereby forming
internal electrodes, by any of the procedures described
hereinafter. Suitable termination electrodes can also be
provided.
Although in Examples 1-2, the dielectric materials used
are modified barium titanate compositions, it will be clear
that others of the large number of ceramic dielectric com-
positions known may also be used. Por example, TiO2 (note
Example 3), 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 con-
ditions 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.
It will also be understood that the composition of the
open-structured strata in ceramic chips according to the in-
vention may vary widely. Not only may the desired open-
structure be achieved by use of a composition which i8 identical
with or similar to the composition of the dielectric strata,
although having a greater shrinkage on firing, but also the
- 14 -
1039370
composition may be quite different, as for example, in Example
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 as 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 mentioned here that, e.g. by choice
of one or more of the several means discussed above, the 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 chips.
Purther, it will be understood that there are available
commercially 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 such vehicles are known to those skilled in the art.
Essentially, the purpose of such a medium or vehlcle is to
suspend the particles and provide a temporary or fugitive bond
therefor during formation of leaves and/or layers and the con- ;~
solidation of a plurality thereof into green bodies prior to
sintering. In the sintered bodies the temporary or fugitive
bond, as well as any combustible particulate material used, -~
has disappeared. Accordingly, the medium or vehicle used is
a matter of choice or convenlence and in most instances any
- 15 -
~U393qO
change in the composition bonded thereby will require some
change or modification, e.g. ad~ustment 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 is preferred but
other kilns or other heating means may be employed. Ordinarily
an oxidizing atmosphere is used,but, when convenient, other
atmospheres can be employed. The temperature, atmosphere, and
the time of firing will depend on the ceramic compositions
employed. Tho~e skilled in the art are familiar with such
details, as pointed out above, and with the fact that in general
the sintering time necessary varies inversely with the tem-
perature and vice versa. As indicated above, a prolonged period
of heating at relatively low temperatures is preferred for
removal of the temporary bonds used in the leaves and printed
areas and any burn-out particles employed. If too rapid heating
is employed expansion of gases formed in the decomposition or
burning of these materials may rupture the chips.
In Figure 6 there is illustrated a typical ceramic
multilayer circuit structure 50 such as is used for hybrid
integrated circuits. The structure 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 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 paste containing
a noble metal such as palladium or platinum in the desired con-
ductor patterns on sheets of desired thickness of a temporary
bonded, electrically insulating, ceramic material such as
alumlna powder, consolldating the several sheets, and sintering
- 16 -
103g~70
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 mono-
lithic capacitors. The necessity for using expensive noble
metals as conductors is thus avoided since firing of metal and
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 des-
cribed with reference to Figure 7.
The sheets or films A, B, and C shown in Figure 7 areformed in the desired size, shape, and thickness by casting,
molding, or the like a desired electrical insulating, ceramic
composition, for 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 or films using, for example a ceramic -
material in a suitable vehicle or squeegee 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 unitary 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
heatlng comprises a dense matrix of the cerami~ insulating
composition having therein open-structured areas of ceramic
material, which may be the same or different in composition,
a substatial portion of the volume of such areas comprising
interconnected voids. Each of said areas extends to at least one
- 17 -
1~)39~70
region on an outer face, e.g. an edge face of said body.
Conductors in and through said bodies may be formed
by introducing into the open-structured areas a suitable
conductive metal in accordance with one of the procedures des-
cribed 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, leads therefrom extending if
desired, to underlying conductors 54 through holes 62 pro-
vided originally in one or more of the ceramic sheets. Ifdesired, one or more of the holes 62 may be filled with the
material employed to form the pseudoconductors when such
material is applied to the faces of the sheets.
Although other procedures may be employed, a con-
venient and efficient way to provide conductive metal in the
open-structured areas of small, sintered, ceramic bodies or
chips or ceramic, multilayer, circuit structure produced as
described above is to inject the metal therein. Typical
infiltration or in~ection procedures are set forth in the
following examples.
EXAMPLE 6
A plurality of sintered chips made in accordance with
Example 1 are placed in a bath of a molten metal alloy consis-
ting of 50% Bi, 25% Pb, 12.5% Sn and 12.5% Cd. The molten
metal is held at a temperature from about 100C. to about 125C.
in a suitable closed vessel. After introduction of the chips
the pressure in the vessel is reduced to evacuate the open-
structured strata of the chips and the pres~ure is then raised
to about 14 kg/cm2 to force the molten metal into the inter-
connecting pores of such strata. The chips after removal fromthe bath contain electrodes formed by deposit of the alloy in
- 18 -
1039~70
the open-structured strata between the dense dielectric
strata and, after the provision of termination electrodes in
any desired manner, are satisfactory monolithic capacitors.
It will be understood that other molten metals can be
used for impregnation of the open-structured areas or strata
of ceramic chips of the types described above or of multi-
layer circuit structures as above described. For example,
there may conveniently be used instead of the alloy specified
in Example 6: the metals lead, aluminum, copper, zinc, tin
and cadmium and alloys containing one or more of these metals.
Other metals are als~o usable but because of their higher cost,
higher resistivity, greater ease of oxidation and/or high
melting points, they are not as desirable 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 28%, 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 the ceramic chips and/or circuit boards into which they
are in~ected. By avoiding combinations in which the ceramic
is readily wet by the metal it is possible to prevent or
minimize 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 the capacitor units to electrically connect the
exposed internal metal electrodes thereon since the metal
electroding pastes commonly employed for this purpose do not
-- 19 --
.
1039~7~
require heating to a temperature higher than the melting
point of the internal electrodes. The same is true of mono--
lithic capacitors produced by deposition of metal in open-
structured strata of fired ceramic chips by any of the
procedures described in the said copending application when
the melting point of the metal so deposited is higher than the
temperature required to supply 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 boàrds is liquid at a temperature
equal to or lower than the temperature employed in applying
the termination electrodes, the provision of the latter may
present problems.
In the manufacture of monolithic capacitors by the pro-
cess of the present invention it has been found useful in
many cases to provide penetrable barriers at the ends of the
ceramic chips before in~ecting molten metal into the open-
structured strata of the chips. Such barriers should be easily
provided, should be of material having a melting point higher
than the temperature at which the metal internal electrodes
are in~ected, 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 open-structured areas
or strata of fired ceramic chips such as 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 restrlct flow from such areas or strata when the pressure
in the vessel is released. Similar results are obtained by
the use of such barriers on the desired faces of multilayer
circuit structures according to the invention.
Suitable penetrable barriers can be formed in several
- 20 -
~039~0
ways. For example, when low-melting metals are used for
infiltration of the unitary ceramic bodies a coating of a
- commercial palladium-silver or palladium-gold electroding paste
can be applied to the surfaces of sintered chips at which
porous strata are exposed and fired in the conventional
manner. The coherent end electrode thus formed is penetrable
to the molten metal.
When higher temperatures are required for infiltration
of metal into the porous areas or stra~a of ceramic bodies, it
has been found feasible to apply over such areas or strata
on faces of the bodies a coating of a ceramic material which
is fired to form a penetrable, porous, ceramic barrier. The
ceramic material may be applied to the green ceramic chips
and fired at the same time as the latter or may be applied to
the already sintered chips and then fired.
2he use of an end or termination electrode as a penetrable
barrier is 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
the chips with a commercial palladium-silver electroding paste
(DuPont No. 8198) and firing the thus-coated chips at about
880C., the firing cycle being about 1 hour.
Using apparatus similar to that used in Example 6, the
chips are placed in a heated pressure vessel above a bath of
molten tln held at about 315C. The vessel is closed and
through a suitable connection the interior of the 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
':
- 21 -
1(~39;~'70
have now been sufficiently heated so that no substantial
thermal shock will result, are then lowered into the tln
bath and the pressure in the vessel is raised, by supplying
a compressed gas, such as nitrogen, thereto , to about 17.5
kg/cm2. The chips are then removed from the melt and after
cooling in the vessel to below the melting point of tin the
gas pressure in the vessel is released and ~he chips are with-
drawn 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
chips are very satisfactory monolithic capacitors.
As indicated above, the penetrable barriers employed
in in~ecting or impregnating open-structured areas need not
be electrically conductive, termination electrodes. The
following three examples illustrate this.
EXAMPLE 8
Unfired ceramic blocks or chips such as ones prepared
in accordance with Example 4 are employed. The end faces of
a plurality of the chips, i.e. the surfaces at which the
alternate layers of screened-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 sub~ected to heating in air to eliminate the com-
bustible materials and fired in air at about 1325~C. to sinter
the ceramic. The resulting fired ceramic chips have alternate
dielectric strata and open-structured 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-
lmpregnàted chips from the pressure vessel and cooling, the
-22 -
',
1039c~70
ceramic barrlers and any undesired metal adhering to the sur-
faces of the chips are removed, for example, by sandblasting,
and electrically conducting termination electrodes are applied
in accordance with any desired procedure. The resultant mono-
lithic capacitors are very satisfactory.
EXAMPLE 9
Unfired ceramic blocks or chips such as ones produced
in accordance with Example 5 are used. In procedure similar
to that in Example 8, the end faces of a plurality of the chips
sre 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 bodies with porous strata that can be injected with
metal through the porous, open-structured, ceramic barriers
formed during 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 is preferably about 880C. When the chips
are impregnated and cooled, and after sandblasting to remove
the ceramic barriers if this is necessary to obtain good
electrical contact with the internal electrodes, conductive
end terminations are applied to form monolithic capacitors.
In the two immediately preceding examples the ceramic
barriers are applied to the end surfaces of the ceramic chips
before the latter are sintered and are thus slntered at the
- 23 -
1039~70
same time. In the following example a penetrable ceramic
- barrier is applied to the chips subsequent to sintering
them.
EXAMPLE 10
A readily spreadable paste is prepared by blending finely
divided borosilicate glass having a melting point of about
1080C. ln about a 1:2 weight ratio with a liquid vehicle
formed from 80 ml pine oil, 14 g acrylic resin, 1.5 g lecithin
dispersing agent and sufficient, about 1 to 2 g, ethyl cel-
lulose to impart the desired viscosity. This paste is appliedby spreading it on the end faces (where the porous strata are
exposed) of sintered ceramic chips such as are produced by
Example 5. The coated chips are then heated to about 790C.
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 the same procedure employed in the metal imp,regnation
of chips described in Example 7 the temperature of the lead
bath during impregnation being about 450C. After sand-
blasting or otherwise removing the glass barriers and anyundesired surface metal deposits the chips may be formed into
satisfactory monolithic capacitors by providing end term-
ination electrodes by any known or desired procedure.
Extensive experiments have shown that by the process of
the present invention monolithic capacitors with internal
electrodes of infiltrated base metal can be produced with cap- -
acitances substantially the same as those of monolithic
capacitors of the same size and number of layers produced by
conventional processes with noble metal internal electrodes.
This has been demonstrated with dielectric compositions having
relatively high dielectric constants as well as ones with
- 24 -
~039370
relatively low dielectric constants.
The structure obtained when a permeable barrier is pro-
vided on an end face of a ceramic chip preparatory to forming
a monolithic capacitor is illustrated in Figure 8 of the ac-
companying drawings. In this figure, which is a fragmentary,
enlarged sectional view, the strata 70 represent sintered
ceramic dielectric material, the numeral` 71 designates the
open-struct~red strata, alternating with the first-mentioned
strata, into which molten metal is injected to form internal
electrodes, and the numeral 72 designates the open-structured,
penetrable barrier through which the molten metal is injected.
The showing in Figure 8 is somewhat diagrammatic since when
a ceramic penetrable barrier is used, it is so sintered to the
ceramic chip as to be unitary and frequently there is no clear
line of demarcation between the barrier and the chip.
As will be clear to those skilled in the art, the
permeability of the penetrable barriers employed may be ad-
justed as desired. This can be done, for example, by the
inclusion of greater or lesser amounts of burnout material,
such as carbon black, in the compositions used for providing
such barriers and/or by adjustment of the particle size of
the solid materials in such compositions. Other procedures
for such ad~ustment 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 there-
through into open-structured areas of the body. However, to
ensure good electrical contact of such electrodes 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
- 25 -
~(~39~qO
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 evi-
dent 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 size of the voids therein and the size 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 be necessary to make preliminary ex-
periments to discover the optimum pressure to use. However,
it has been found that in the use of metals of medium and low
melting points, pressures greater than about 17.5 kg/cm2 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 be 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, ~uch deposits can be used, if desired, as con-
ductive penetrable barriers.
Monolithic capacitor~ according to the present invention
- 26 -
..: ,
1~39~70
may vary widely in size. Not only may the dimensions of the
capacitor be 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 the conductive
layers, this is subject to variation as desired. Capacitors
as small 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 be 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 additional 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
ad~ust 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 be detrimental
since after sintering the resultant exposed porous deposit
will either not hold an electrode material or such material -
can be easily removed, for example by sanding.
In the foregoing description and the examples, the
leaves of dielectrlc and/or potentially porous ceramic and
the capacitors formed therefrom are rectangular. However, the
present invention comprehends capacitors of other shapes.
Thus, if desired, monolithic capacitors according to the
invention may be triangularly shaped. In such case, obviously,
alternate porous strata and the electrodes formed therein can
- 27 -
I'(~39~qO
not be exposed on opposite edge faces. Consequently, it will
be understood that in the appended claims the term "edge
region" is used comprehensively to indicate an area on an
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
the accompanying drawings and should not be interpreted as
limiting the invention or requirlng 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 willbe evident from the foregoing description that
many variations 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 as 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 example, 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 self-sustaining body is
desired for firing, the stack of leaves or of leaves and the
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 de-
sired, the open-structured areas or strata can be partially
fllled wlth metal and additional
- 28 -
~39;~70
metal may be injected into the 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.
- 29 -
