Hybrid circuits and thick film dielectrics used therein

There is disclosed a hybrid circuit embodying a crossover formed on a low expansion substrate of aluminum nitride, mullite or a cordierite-type glass-ceramic. The crossover is a thick film dielectric composed of a glass-ceramic having a primary crystal phase of willemite and a secondary phase of cordierite.

FIELD OF THE INVENTION 
This invention is concerned with hybrid circuits and with ceramic 
dielectrics useful in building such circuits. It is particularly concerned 
with thermally crystallized glass coatings that are compatible with low 
expansion substrates in producing hybrid circuits involving crossovers. 
BACKGROUND OF THE INVENTION 
Alumina (Al.sub.2 O.sub.3) has been a favored ceramic substrate material 
for electronic circuitry where long life and high reliability are 
required. However, for certain high performance applications, sintered 
alumina substrates have proven less than satisfactory. 
While an alumina body can be ground to a smooth finish, it undergoes high 
shrinkage (about 18%) on firing. Sintered alumina has a relatively high 
dielectric constant of about 10, which restricts line spacing and may lead 
to signal delays and noise in operation. A relatively high coefficient of 
thermal expansion (about 65.times.10.sup.-7 /.degree.C.), as compared to 
that of silicon chips (about 35.times.10.sup.-7 /.degree.C.), can make it 
difficult to seal such chips to an alumina substrate. Finally, high firing 
temperatures (about 1600.degree. C.) are required for co-sintering. This 
limits the metals that may be employed to molybdenum and tungsten, and 
excludes silver, copper and gold. 
Accordingly, a search has been made for substrate materials having more 
compatible properties than sintered alumina. In particular, materials 
having coefficients of thermal expansion not over about 
45.times.10.sup.-7, and hence more closely matched to silicon, and a lower 
dielectric constant than alumina, have been sought. 
Glass-ceramic materials, in particular the cordierite-type, have received 
considerable attention. Some of the history is reviewed in co-pending 
application Ser. No. 07/238,574 filed Aug. 31, 1988 and assigned to the 
assignee of this application. This co-pending application discloses 
modified cordierite compositions that have superior properties to those of 
alumina, and that can be sintered below 1,000.degree. C. The latter 
property permits co-sintering, that is, firing noble metal circuitry and 
sintering the substrate in one heat treatment. 
Aluminum nitride (AlN) and mullite are other ceramic materials that hold 
considerable promise as substrate material. In addition to a compatible 
coefficient of thermal expansion, AlN has an exceptionally high thermal 
conductivity. This permits dissipation of heat from hot spots that tend to 
develop during circuit operation, and that might otherwise damage the 
circuitry. U.S. Pat. No. 4,719,187 (Bardhan et al.) describes AlN and an 
improved method for its production. 
The advent of these lower expansion substrate materials has created a need 
for a new dielectric having compatible expansion characteristics in order 
to build hybrid circuits involving crossovers. 
PURPOSES OF THE INVENTION 
A basic purpose of the invention then is to provide thick film dielectrics 
in hybrid circuits involving crossovers. 
Another purpose is to provide such dielectrics having expansion 
characteristics that are compatible with low expansion substrates, such as 
aluminum nitride, mullite and cordierite-type glass-ceramics. 
A further purpose is to provide improved hybrid circuits having low 
expansion substrates with coefficients of thermal expansion not over about 
45.times.10.sup.-7, such as aluminum nitride, mullite and cordierite-type 
glass-ceramics. 
SUMMARY OF THE INVENTION 
One aspect of the invention is a hybrid circuit comprising at least one 
layer of a substrate material having a coefficient of thermal expansion 
not over about 45.times.10.sup.-7 and selected from the group composed of 
aluminum nitride, mullite and a cordierite-type glass-ceramic, and having 
a thick film dielectric adhered to said substrate, said film composed of a 
glass-ceramic containing willemite as a predominant crystal phase and 
cordierite as a secondary phase. Preferably, the glass-ceramic has a 
composition essentially free of Li.sub.2 O, Na.sub.2 O and K.sub.2 O, and 
consisting essentially of, in weight percent as calculated on an oxide 
basis, 15-45% ZnO, 3-15% MgO, 10-30% Al.sub.2 O.sub.3 and 30-55% 
SiO.sub.2. The dielectric film may take the form of an insulating 
crossover where additional circuitry needs to be applied to form a hybrid 
circuit. 
PRIOR LITERATURE 
In addition to references previously noted in the Background section, the 
following U.S. Patents are noted: 
No. 4,221,047 (Narken et al.) discloses a multi-layered glass-ceramic 
superstructure with a multi-layered distribution of conductors on a 
preformed multi-layered glass-ceramic base. The glass-ceramics employed 
are alpha-cordierite and beta-spodumene type. 
No. 4,714,687 (Holleran et al.) discloses the use of glass-ceramics, 
consisting essentially of 15-45% ZnO, 10-30% Al.sub.2 O.sub.3 and 30-55% 
SiO.sub.2 and containing willemite as the predominant crystal phase, as 
substrates in integrated circuit packaging.

GENERAL DESCRIPTION OF THE INVENTION 
FIG. 1 is a top plan view illustrating a simple hybrid circuit involving an 
overlay. The numeral 10 indicates a basic substrate which may be a fired 
and surfaced body of either aluminum nitride, mullite or a cordierite-type 
glass-ceramic. 
Metallized lines 12 extend between terminals 14, and may be formed with a 
noble metal metallizing paste. Pastes particularly adapted to matching low 
expansion substrates are described in our copending application, Ser. No. 
273,586, filed Nov. 21, 1988 and entitled METALLIZED SUBSTRATE FOR 
ELECTRONIC DEVICE. 
As further illustrated in FIGS. 2 and 3, crossover 16 consists of a band of 
electrically insulating glass-ceramic. It effectively forms a substrate 
for a further circuit element 18 corresponding to lines 12. This circuit 
extends across lines 12 and between terminals 20. 
Terminals 14 and 20 may be electrodes of a metal such as gold. They may be 
soldered, or otherwise connected, to other conductive elements. Formation 
of such contacts is conventional practice. 
The present invention is primarily concerned with provision of crossovers 
in hybrid circuits, as illustrated in FIGS. 1, 2 and 3. Prior glass and 
glass-ceramic pastes have not provided the necessary combination of 
properties to match with aluminum nitride, mullite and cordierite-type 
substrates. This combination of properties includes low coefficients of 
thermal expansion, low dielectric constants, low loss tangents, and the 
capability of being fired below 1,000.degree. C. to permit use of noble 
metal circuitry. 
We have now found that glasses that thermally crystallize to produce 
glass-ceramics with a primary crystal phase of willemite and a secondary 
phase of cordierite meet the various requirements. These glasses may 
consist essentially of, in percent by weight as calculated on an oxide 
basis, 15-45% ZnO, 3-15% MgO, 10-30% Al.sub.2 O.sub.3 and 30-55% 
SiO.sub.2, and are essentially free of the alkali metal oxides Na.sub.2 O, 
Li.sub.2 O and K.sub.2 O. A nucleating agent, selected from 8-12% 
ZrO.sub.2 or 0.001-0.05% of a noble metal selected from Au, Pd, and Pt, 
may also be present. Other optional constituents include up to 5% B.sub.2 
O.sub.3, up to 5% CaO and/or SrO and/or BaO and/or PbO, up to 7% Cs.sub.2 
O and up to 15% MnO, providing the total of such optional oxides does not 
exceed about 15%. 
SPECIFIC EMBODIMENTS 
A series of four (4) thick film, dielectric pastes was prepared by blending 
weighed amounts of each of four (4) different powdered glass frits with a 
screening medium. The screening medium in each case was six (6) percent by 
weight of ethyl cellulose dissolved in an alcohol solvent. The solvent is 
available from Eastman Chemical Corp. under the mark Texanol. 
Four (4) different glass frits were employed. Each was capable of thermal 
crystallization to produce a glass-ceramic characterized by a primary 
crystal phase of willemite and a secondary phase of cordierite. 
The frit compositions, as calculated from the batch in parts by weight on 
an oxide basis, are set forth in TABLE I which follows: 
TABLE I 
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A B C D 
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MgO 6.6 7.2 9.9 13.8 
ZnO 37.7 39.0 26.6 18.5 
Al.sub.2 O.sub.3 
17.8 14.3 19.4 23.2 
SiO.sub.2 37.6 39.5 44.1 44.4 
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Each composition was mixed as a glass batch based on the corresponding 
oxides. The glass batches were melted at 1550.degree. C., chilled and 
granulated, and ball milled with alumina balls to an average particle size 
of about 3.3 microns. Each glass was blended in powdered form with the 
screening medium to produce a pasty mixture. Each mixture was homogenized 
to a smooth paste in a three-roll mill for about five (5) minutes. The 
pastes were in the proportion of 60 wt. % of glass frit to 40 wt. % of 
screening medium. However, the screening medium may vary from as little as 
10 wt. % up to 50 wt. % depending on the viscosity desired. 
The pastes thus produced were applied over the surfaces of two different 
types of low expansion substrates. These were in the form of presintered 
strips of either aluminum nitride (AlN) or a cordierite-type glass-ceramic 
having the following composition in calculated weight percent: 51.0% 
SiO.sub.2, 24.8% Al.sub.2 O.sub.3, 13.1% MgO, 3.1% BaO, 1.4% B.sub.2 
O.sub.3 and 6.6% ZnO. 
The paste-coated sustrates were fired at 950.degree. C. Each substrate 
sample had an adherent, non-porous, glass-ceramic coating formed on its 
surface. The coated samples were thermally cycled several times between 
+125.degree. C. and -55.degree. C. without cracking or spalling being 
observed. 
A paste based on composition B, our preferred composition, provided a 
crack-free coating when fired at 925.degree. C. Accordingly, this paste 
was employed with gold electrodes on a cordierite substrate to produce a 
capacitor-type construction for use in checking electrical properties. 
In the construction, a thick film gold electrode (Englehard A-3360) was 
silk screened on the substrate and fired at 925.degree. C. Four coats of 
the paste prepared from composition B were screened over the gold 
electrode, with each coat being dried at 125.degree. C. before applying 
the succeeding coat. The coated substrate was then fired at 925.degree. 
C., followed by application of a top electrode corresponding to the bottom 
electrode. 
Measurements made on the capacitor structure thus produced provided the 
following data: 
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Fired thickness 47 microns 
Breakdown voltage over 1000 volts/mil. 
Dissipation factor 0.003 from 
1 khz to 1 mhz 
Dielectric constant 
app. 7.5 from 
1 khz to 1 mhz 
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Similar data was obtained with a similar construction on an AlN substrate.