A ferromagnetic material (18,20) in ink or tape form is sinterable using a same firing profile as and has approximately the same thermal shrinkage characteristics as low-temperature-cofired-ceramic (LTCC) tape, and is chemically non-reactive therewith. The ferromagnetic material (18,20) is applied to the surfaces of LTCC tape sheets (12,14,16) to form desired elements such as cores for inductors (22) and transformers and magnetic shields. Ferromagnetic vertical interconnects (vias) (54) can be formed by punching holes (56) through tape sheets (46) and filling them with ferromagnetic ink. The tape sheets (12,14,16) and ferromagnetic elements (18,20) are laminated together and cofired to form an integral structure (10). Ferromagnetic and non-magnetic components (114) can be fabricated separately and inserted into cavities (104a, 106a,108a) in tape sheets (104,106,108) prior to cofiring. A multi-layer transformer (250) includes primary (254b, 256b,258b,254d,256d,258d) and secondary (254c,256c,258c) coils, each being formed of vertically aligned, arcuate conductors which are printed on separate tape sheets (254,256,258) and vertically interconnected at their ends to form continuous electrical paths therethrough.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to the art of hybrid electronic 
circuit structures fabricated from low-temperature-cofired-ceramic (LTCC) 
tape, and more specifically to cofired LTCC tape structures and 
fabrication methods incorporating ferromagnetic elements, drop-in 
components and transformers. 
2. Description of the Related Art 
Fabrication of multilayer electronic structures for hybrid microcircuit 
technology and other applications includes the "thick film process" in 
which individual conductor and dielectric compositions in paste form are 
sequentially deposited on insulating substrates and then fired, one layer 
of material at a time, to fabricate a thick film, multilayer circuit. 
A disadvantage of the thick film process is that voids or pinholes can be 
formed in the thick film dielectric material during the sequential 
printing and firing process. Another disadvantage is that the requirement 
for building up many multiple thick film layers in the more complex hybrid 
circuits results in an expensive process due to the number of individual 
processing steps involved. A third disadvantage is that a mistake on an 
individual layer requires scrapping the entire unit. 
A second approach to the fabrication of hybrid microcircuits is the cofired 
ceramic process. This technology utilizes dielectric material formed into 
sheets having alumina as a main component. Individual sheets of tape are 
printed with metallization and other circuit patterns, stacked on each 
other, laminated together at a predetermined temperature and pressure, and 
then fired at a desired elevated temperature at which the material fuses 
or sinters. 
Where alumina is generally used as the insulating material, tungsten, 
molybdenum or molymanganese is typically used for metallization, and the 
part is fired to about 1,600.degree. C. in an H.sub.2 reducing atmosphere. 
The undesirable high processing temperature and requisite H.sub.2 
atmosphere, and more importantly the electrical performance of the 
refractory metals has led to the development of 
Low-Temperature-Cofired-Ceramic (LTCC) tape. A preferred LTCC is 
commercially available from the DuPont Company as Green Tape.RTM. no. 
851AT. The tape contains a material formulation including a mixture of 
glass and ceramic fillers which sinter at about 850.degree. C., and 
exhibits thermal expansion similar to alumina. 
The low-temperature processing permits the use of air fired resistors and 
precious metal thick film conductors such as gold, silver, or their 
alloys. In the typical high-temperature process, screen-printed resistors 
cannot be used and only refractory metal pastes are used as conductors. 
A discussion of thick film technology, and high and low temperature cofired 
ceramic tape technology, is found in "DEVELOPMENT OF A LOW TEMPERATURE 
COFIRED MULTILAYER CERAMIC TECHNOLOGY", by William Vitriol et al, ISHM 
Proceedings 1983, pp. 593-598. 
Ferromagnetic inks have been developed for use with the thick film process 
described above. These inks can be screen printed together with other 
paste layers onto a substrate to form cores or enhancers for inductors, 
magnetic shield planes, and other ferromagnetic elements. However, these 
inks are not usable with the LTCC process because they have a dissimilar 
shrinkage profile to LTCC tape. This causes warping or buckling of the 
LTCC tape structure during firing. 
For this reason, magnetic components, including transformers, as well as 
non-magnetic components such as heat sinks and varistors have previously 
been fabricated separately and fixed to the surfaces of LTCC structures. 
This is disadvantageous in that the space on the surfaces of the 
structures is severely limited, and should be utilized for the mounting of 
hybrid microelectronic integrated circuit chips and interconnects. 
SUMMARY OF THE INVENTION 
In accordance with a method of the present invention, a ferromagnetic 
material is provided in ink or tape form which is sinterable using a same 
firing profile as and has approximately the same thermal shrinkage 
characteristics as low-temperature-cofired-ceramic (LTCC) tape, and is 
chemically non-reactive therewith. 
The ferromagnetic material is applied to the surfaces of LTCC tape sheets 
to form desired elements such as cores for inductors and transformers and 
magnetic shields. Ferromagnetic vertical interconnects (vias) can be 
formed by punching holes through tape sheets and filling them with the 
ferromagnetic ink. The tape sheets and ferromagnetic elements are 
laminated together and cofired to form an integral structure. 
The ferromagnetic ink and tape enable magnetic elements to be buried in the 
LTCC tape structure, rather than being mounted on the surface. This 
conserves valuable surface space which can be more advantageously used for 
the mounting of hybrid microelectronic integrated circuit chips and 
interconnects. 
In another embodiment of the invention, ferromagnetic and non-magnetic 
components are fabricated separately and inserted into cavities in the 
tape sheets prior to cofiring. Burying separately formed components in the 
LTCC tape structure further conserves surface space. 
A multi-layer transformer embodying the invention includes primary and 
secondary coils, each being formed of vertically aligned, arcuate 
conductors which are printed on separate tape sheets and interconnected at 
their ends by vias to form continuous electrical paths therethrough. 
These and other features and advantages of the present invention will be 
apparent to those skilled in the art from the following detailed 
description, taken together with the accompanying drawings, in which like 
reference numerals refer to like parts.

DETAILED DESCRIPTION OF THE INVENTION 
FIGS. 1 to 7 illustrate low-temperature-cofired ceramic (LTCC) structures 
which include ferromagnetic elements formed from ferromagnetic ink or 
tape. The ink is used in the same manner as other LTCC inks or pastes, and 
is screen printed onto the surface of LTCC sheets in the desired patterns 
to form flat layers or filled into holes to form vertical interconnects 
(vias). The tape is cut to the desired shape and placed on the surface of 
a sheet of LTCC tape. 
The tape sheets and ferromagnetic elements are then sandwiched together, 
laminated, prefired to bake out the organic vehicle materials and cofired 
at a temperature at which the LTCC tape and ferromagnetic material sinter 
(typically 850.degree. C.) to form an integral cofired ceramic tape 
structure. The conventional LTCC processing technology is applicable 
without modification to fabricate structures in accordance with the 
present invention. 
The ferromagnetic material is formulated to be chemically non-reactive with 
the LTCC tape, and have mechanical and thermal properties which are as 
close to those of LTCC tape as possible. In order to be cofirable, the 
ferromaqnetic material must be sinterable using the LTCC firing profile. 
The ferromagnetic material must also have approximately the same thermal 
shrinkage characteristics, including shrinkage (10-15%) and shrinkage 
rate, as the LTCC tape in order to prevent warpage during firing. 
The ferromagnetic material embodying the present invention may be provided 
in fluid (ink or paste) or flexible tape form. The ink formulation may be 
different for inks designed for printing on the surface of LTCC tape and 
for filing vias. In either form, the ferromagnetic material includes three 
main components; a ferromagnetic oxide powder, a glass powder or "frit" 
and an organic binder or vehicle. 
The ferromagnetic oxide may be selected from either of the three main 
groups of ferrites; spinel, garnet and magnetoplumbite, depending on the 
desired properties. Spinels have the general formula MO.Fe.sub.2 O.sub.3, 
MFe.sub.2 O.sub.4 or MFe.sub.3 0.sub.4, where M is typically nickel (Ni), 
zinc (Zn), manganese (Mn), magnesium (Mg), lithium (Li), copper (Cu), 
cobalt (Co) or another element. Garnets have the general formula 3M.sub.2 
O.sub.3.5Fe.sub.2 O.sub.3 or M.sub.3 Fe.sub.5 O.sub.12, where M is most 
commonly yttrium (Y) or one of the rare earth ions. Magnetoplumbites have 
the general formula MFe.sub.12 O.sub.19 or MO.6Fe.sub.2 O.sub.3, where M 
is typically barium (Ba), gallium (Ga), chromium (Cr) or manganese (Mn). 
These ferromagnetic oxides can also be combined in many ways depending on 
a particular application. 
The glass frit generally includes silicon dioxide (SiO.sub.2), and one or a 
mixture of materials including lead oxide (PbO), bismuth oxide (Bi.sub.2 
O.sub.3), sodium oxide (Na.sub.2 O) lithium oxide (Li.sub.2 O) and 
potassium oxide (K.sub.2 O) 
Examples of preferred embodiments of ferromagnetic materials which have 
been determined to be cofirable with LTCC tape and have ferromagnetic 
properties suitable for magnetic inductor and transformer cores, magnetic 
shields and other applications will be described below. It will be 
understood, however, that these particular examples do not limit the scope 
of the invention. 
The preferred ferromagnetic oxide is a nickel-zinc ferrite powder which is 
commercially available from Krystinel Corp. of Paterson, N.J. as product 
no. K31. The main constituent of Krystinel K31 is NiZnFe.sub.3 O.sub.4. 
The volume percentages of the constituents of the print ink (for forming 
planar surface patterns) are 64% ferrite-glass powder and 36% organic 
vehicle. The ferrite-glass powder includes, by volume, 80% Krystinel K31 
and 20% glass frit. 
The glass frit in the print ink is a lead-silicon-borate glass powder which 
is available from Transene Co, Inc. of Rowley, Mass. as Transene Glass 
Composition no. T90. The weight percentages of the constituents of 
Transene T90 are 44% lead oxide (PbO), 4% aluminum oxide (Al.sub.2 
O.sub.3), 10% boron oxide (B.sub.2 O.sub.2) and 40% silicon dioxide 
(SiO.sub.2). 
The composition, particle size and particle size distribution of the 
ferromagnetic oxide and glass frit mainly determine the thermal and 
shrinkage properties of the material. The lead oxide in the Transene T90 
frit lowers the melting point of the glass frit to approximately 
590.degree. C., aiding in densification of the ferromagnetic ink or tape. 
The weight percentages of the constituents of the organic vehicle in the 
print ink are 45% texanol solvent (2,2,4 trimethyl pentanediol-1,3 
monoisobutyrate), 45% butyl carbitol acetate solvent (2-2(butoxyethoxy) 
ethyl acetate, 10% ethyl cellulose polymer vehicle which is available from 
Hercules, Inc. of Tustin, Calif. as Ethyl Cellulose no. N50 and 0.5% 
thixatrol rheology adjuster which is available from Rheox, Inc. of 
Hightstown, N.J. as Thixatrol ST Rheological Additive no. 32051. 
The via ink has the same general composition as the print ink, except that 
it is preferably includes a lower percentage of solvent so that it will 
have lower viscosity Also, the particle size of the constituents is 
preferably larger than for the print ink. 
The volume percentage of the constituents of the ferromagnetic tape are 
6.69% ferrite-glass powder and 91.31% organic vehicle. The ferrite-glass 
powder includes, by volume, 78% Krystinel K31, 20% glass frit and 2% 
bismuth oxide (Bi.sub.2 O.sub.3). The glass frit is the same as for the 
print ink. 
The organic vehicle for the tape includes, by volume, 9.01% DuPont Evacite 
Acrylic Resin no. 2042, 3.75% DuPont Evacite Acrylic Resin no. 2043, 6.1% 
butyl benzyl pthalate plasticizer which is available from ChemCentral of 
Santa Fe Springs, Calif. as Santicizer 160 and 75.88% trichloroethane. 
The present ferromagnetic tape can be manufactured using the same 
technology as for LTCC tape. 
FIG. 1 illustrates an LTCC tape structure 10 prior to lamination which 
includes sheets of the present ferromagnetic tape for increasing or 
enhancing the inductance of an inductive coil. The structure 10 includes 
LTCC tape sheets 12, 14 and 16, and ferromagnetic tape sheets 18 and 20 
which are sandwiched on opposite sides of the sheet 14. 
An inductor 22 is formed as a spiral coil of electrically conductive ink on 
the upper surface of the sheet 14. Ends 22a and 22b of the inductor 22 are 
connected through vias 24 and 26 which extend through the sheet 18 and 
vias 28 and 30 which extend through the sheet 12 to conductor pads 32 and 
34 respectively which are formed on the surface of the sheet 12. Since the 
ferromagnetic sheets 18 and 20 have very high electrical resistivity and 
can be considered as insulators, the vias 24 and 26 are not shorted out by 
the sheet 18. 
The structure 10 is fabricated by sandwiching the elements illustrated in 
FIG. 1 together, laminating, prefiring and cofiring to produce the 
integral structure 10 as illustrated in FIG. 2. 
The sheets 18 and 20 are vertically aligned with the inductor 22, and 
interact with the magnetic field produced upon flow of current through the 
inductor 22 to increase or enhance the inductance of the inductor 22. If 
desired, only one of the sheets 18 and 20 may be provided with a 
corresponding reduction in inductance enhancement. 
FIGS. 3 and 4 illustrate a structure 36 which is similar to the structure 
10 except that the ferromagnetic tape sheet 20 is replaced by a layer 20' 
of ferromagnetic material formed on the upper surface of the sheet 16. In 
addition, the ferromagnetic tape sheet 18 is replaced by a layer 18' of 
ferromagnetic material formed on the upper surface of an LTCC sheet 37. 
The layers 18' and 20' are formed by screen printing or otherwise applying 
ferromagnetic ink embodying the present invention on the surfaces of the 
sheets 37 and 16 respectively, and allowing the ink to dry to form a solid 
layer. It will be noted that since the ferromagnetic ink is essentially 
electrically insulative, the LTCC sheet 37 could be omitted and the 
ferromagnetic layer 18' printed over the inductor 22 on the sheet 14. 
FIGS. 5 and 6 illustrate how an LTCC structure 40 embodying the invention 
can be fabricated as including a magnetic shield for a component 42. The 
external interconnections of the component 42 are not shown. The component 
42 can be an electrical metallization pattern or any other element which 
must be shielded from external magnetic fields. 
The component 42 is inserted in a cavity 44 formed through an LTCC sheet 
46. A ferromagnetic layer 48 is formed of ferromagnetic ink or tape over 
the component 42 and surrounding portions of the sheet 46. Another 
ferromagnetic layer 50 is formed of ferromagnetic tape or ink on the 
surface of an LTCC sheet 52 in vertical alignment with the layer 48. In 
addition, a plurality of ferromagnetic vias 54 are formed in respective 
holes 56 which extend through the sheet 46. The vias 54 are formed by 
screen printing ferromagnetic ink into the holes 56 to fill or at least 
coat the walls of the holes 56. 
The vias 54 form a magnetic ring or fence around the component 42. The vias 
54 extend between the layers 48 and 50 to form a magnetic circuit which 
provides lateral as well as vertical magnetic shielding. Further 
illustrated is another LTCC sheet 58 which is sandwiched above the sheet 
46. 
FIG. 7 illustrates another magnetic shielding arrangement according to the 
present invention. An LTCC structure 60 includes LTCC layers 62, 64, 66, 
68 and 70. Electrical conductor traces or metallizations 72 and 74 are 
formed on the surface of the layer 66, whereas metallizations 76, 78 and 
80 are formed on the surface of the layer 68. Vertical magnetic shielding 
is provided by ferromagnetic layers 82 and 84 formed on the surfaces of 
the layers 64 and 70 respectively. Lateral magnetic shielding is provided 
by ferromagnetic vias 86, 88 and 90 formed through the layers 64, 66 and 
68 respectively. 
The vias 88 are staggered from the vias 86 and 90 to prevent excessive 
local material thickness and weakness. Although the vias described and 
illustrated thus far have circular cross-sections, it is within the scope 
of the invention to form ferromagnetic vias with linear, arcuate or other 
cross-sections. 
FIGS. 8 and 9 illustrate another LTCC structure 100 embodying the present 
invention prior to lamination and after cofiring respectively. The 
structure 100 includes LTCC sheets 102, 104, 106, 108 and 110. Vertically 
aligned holes 104a, 106a and 108a are formed through the sheets 104, 106 
and 108 respectively to form a cavity 112 as illustrated in FIG. 9. A 
drop-in component 114, which can be an inductor, thermistor, capacitor, 
varistor, ferromagnetic core or other element has a shape corresponding to 
the cavity 112 and is inserted therein during assembly. 
The component 114 is made of a material or materials which are cofirable 
with LTCC tape. The component 114 can be fired prior to assembly in the 
structure 100, or can be cofired (for sintering) with the LTCC tape 
sheets. Further illustrated are conductor layers 116 and 118 which extend 
from the opposite lateral ends of the conductor 114 over the upper edge 
thereof, and vias 120 and 122 which extend through the sheet 102 for 
connection to conductor pads 124 and 126 respectively. 
Whereas the cavity 112 of the structure 100 is buried between the sheets 
102 and 110, FIG. 10 illustrates another LTCC structure 130 in which a 
drop-in component is mounted in a cavity 132 formed through only the upper 
layers of the structure 130. More specifically, the structure 130 includes 
LTCC sheets 134, 136, 138 and 140. The cavity 132 extends through only the 
sheets 134 and 136. 
In this case, the drop-in component is a capacitor 142 including a 
dielectric layer 144 which is sandwiched between conductor layers 146 and 
148. A conductor layer 150 extends from the layer 146 downwardly around 
the left end and wraps around a portion of the lower surface of the 
dielectric layer 144. The conductor 148 is omitted in this area to prevent 
shorting of the layer 150 to the layer 148. The conductor layer 148 is 
connected to a buried metallization 152 formed on the sheet 140 by a via 
154 which extends through the sheet 138 and a solder connection 156. The 
conductor layer 150 is connected to a buried metallization 158 formed on 
the sheet 140 by a via 160 which extends through the sheet 138 and a 
solder connection 162. 
As illustrated in FIG. 11, it is within the scope of the invention to 
incorporate a drop-in component made of a non-magnetic material into an 
LTCC structure 170 which includes LTCC tape sheets 172, 174 and 176. A 
cavity 178 extends through all of the sheets 172, 174 and 176. A 
component, such as a microelectronic integrated circuit chip 180, is 
mounted on the structure 170 above the cavity 178 by a thermally 
conductive adhesive 181. 
An integral heat sink 182, made of aluminum or other thermally conductive 
material, includes a planar base portion 184, and a vertical portion 186 
which extends upwardly from the base portion 184 through the cavity 178 
into thermal contact with the component 180. Heat generated by the 
component 180 is conducted to the base portion 184 of the heat sink 182 
through the vertical portion 186 thereof and dissipated into the 
atmosphere. 
The structure 170 is fabricated by assembling the LTCC sheets 172, 174 and 
176 onto the heat sink 182, laminating, prefiring and cofiring. Although 
not specifically illustrated, it is further within the scope to fully or 
partially bury the component 180 in the sheets 172, 174 and 178. 
Further illustrated in FIG. 11 is a metallization 188 which is formed on 
the sheet 174 and connected to the component 180 by a via 190, bonding pad 
192, wire bond 194 and bonding pad 196. A metallization 198 is formed on 
the sheet 176 and connected to the component 180 by vias 200 and 202, a 
bonding pad 204, wire bond 206 and bonding pad 208. 
FIGS. 12 to 14 illustrate an LTCC transformer structure 250 embodying the 
present invention, including LTCC tape sheets 252, 254, 256, 258 and 260 
which are formed with central circular holes 252a, 254a, 256a, 258a and 
260a respectively. Three electrical conductors in the shape of concentric 
circular arcs are formed on each of the sheets 254, 256 and 258. More 
specifically, radially outer conductors 254b, 256b and 258b, radially 
central conductors 254c, 256c and 258c and radially inner conductors 254d, 
256d and 258d are formed on the sheets 254, 256 and 258 respectively. 
Each of the conductors has a gap as illustrated. The ends of the conductors 
on adjacent sheets are interconnected by vias which are not illustrated 
explicitly, but indicated by arrows. The outer conductors 254b, 256b and 
258b are interconnected by vias to constitute a first section of a primary 
winding through which current flows downwardly. The inner conductors 254d, 
256d and 258d are interconnected by vias to constitute a second section of 
the primary winding through which current flows upwardly. 
The first and second sections of the primary winding are interconnected to 
form a continuous electrical current path which generates additive 
magnetic fields. The central conductors 254c, 256c and 258c are 
interconnected to constitute a secondary winding through which current 
flows downwardly. Current flows counterclockwise through all of the 
conductors. 
The operation of the transformer structure 150 will become clear from a 
description of current flow through the conductors and vias. Current 
enters the primary winding through a conductor pad 262 formed on the sheet 
252, flows downwardly through a via 264, through the conductor 254b, 
downwardly through a via 266, through the conductor 256b, downwardly 
through a via 268, through the conductor 258b and downwardly through a via 
270. 
The via 270 is connected to a via 272 by a metallization 274 formed on the 
sheet 260. Current flows upwardly through the via 272, through the 
conductor 258d, upwardly through a via 276, through the conductor 256d, 
upwardly through a via 278, through the conductor 254d and upwardly 
through a via 280 to a bonding pad 282. 
The magnetic field induced in the secondary winding from the primary 
winding flows into a bonding pad 284 and downwardly through a via 286, 
through the conductor 254c, downwardly through a via 288, through the 
conductor 256c, downwardly through a via 290, through the conductor 258c 
and downwardly through a via 292 to a metallization 294 formed on the 
sheet 260. Current flows upwardly from the metallization 294 through a via 
296 and out through a bonding pad 298. 
The transformer structure 250 provides a voltage step-down ratio of 1:2, 
since the primary winding has twice as many turns as the secondary 
winding. It is of course possible to operate the transformer structure 250 
in reverse, in which case the primary winding would include the central 
conductors and the secondary winding would include the inner and outer 
conductors. In this case, the primary winding would have half as many 
turns as the second winding, and the structure 250 would provide a voltage 
step-up ratio of 2:1. It is further within the scope of the invention, 
although not specifically illustrated, to provide two or more primary 
windings and/or two or more secondary windings. 
The transformer structure 250 further comprises a central core 300 formed 
of a ferromagnetic material which extends through the holes 252a, 254a, 
256a, 258a and 260a. Preferably, the core 300 is assembled and cofired 
with the sheets 252, 254, 256, 258 and 260 as described above. As 
illustrated in FIGS. 13 and 14, the structure 250 may further include a 
C-core 302 which engages with the ends of the central core 300 and forms a 
complete magnetic circuit therewith. 
While several illustrative embodiments of the invention have been shown and 
described, numerous variations and alternate embodiments will occur to 
those skilled in the art, without departing from the spirit and scope of 
the invention. Accordingly, it is intended that the present invention not 
be limited solely to the specifically described illustrative embodiments. 
Various modifications are contemplated and can be made without departing 
from the spirit and scope of the invention as defined by the appended 
claims.