Low temperature fired ceramics

Partially crystallized ceramic articles is prepared by firing at low temperatures of 800.degree. to 1100.degree. C., a mixture consisting essentially of, (a) 40 to 50 wt. % of powdered, noncrystalline glass consisting essentially of 10 to 55 wt. % of at least one selected from the group consisting of CaO and MgO, 45 to 70 wt. % of SiO.sub.2, 0 to 30 wt. % of Al.sub.2 O.sub.3, 0 to 30% of B.sub.2 O.sub.3 and up to 10% impurities, the powder size of said glass being at least 4.0 m.sup.2 /g in terms of specific surface area measured by the BET Method; and (b) 60 to 50 wt. % of powdered Al.sub.2 O.sub.3 and the ceramic article is composed essentially of a noncrystallized glass phase, alumina and at least one of crystallized glass phase among anorthite, wollastonite, cordierite and mullite formed by partially crystallizing the glass (a). The ceramic articles exhibit a very high flexural strength well comparable with alumina substrate, together with greatly improved dielectric constant and thermal expansion and thus are especially useful in making multi-layer ceramic substrates for electronic applications.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to low temperature fired ceramics especially 
useful in the manufacture of electronic components or parts and further 
useful in other various application, such as heat-resistance industrial 
articles, tablewares, kitchen utensiles and decorative articles. 
2. Description of the Prior Art 
With the increasing trend toward high-speed computers and high-frequency 
devices or equipments, substrates having a low dielectric constant have 
been increasingly demanded in place of alumina substrates in current use 
because the conventional alumina substrates have a large dielectric 
constant (.epsilon.=10) which may cause the delay of signals. Further, the 
conventional alumina substrates are necessary to be fired at high 
temperatures and, thus, conductive materials used on the substrates are 
limited only to high melting point metals, such as Mo or W which may delay 
signals due to their relatively high electrical resistance. 
Under such circumstances, certain ceramic compositions firable at low 
temperatures of 1100.degree. C. or less have been put into practical use 
to provide fired ceramics having a low dielectric constant and a thermal 
expansion coefficient near that of silicon so as to minimize the stress 
caused due to the difference of thermal expansion between the substrate 
and large sized silicon chips in view of a LSI chip. Such low temperature 
ceramic compositions can be co-fired with Au, Ag, Ag-Pd or Cu and thereby 
allow the use of these low melting point metallic materials as conductor 
on the ceramics. 
For example, the inventors have proposed low-temperature firable ceramic 
composition comprising CaO-Al.sub.2 O.sub.3 -SiO.sub.2 (-B.sub.2 O.sub.3) 
glass and alumina in our previous U.S. patent application, Ser. No. 
716,722, filed Mar. 27, 1985, now Patent No. 4,621,066. This ceramic 
composition develops superior electrical and mechanical properties in the 
fired products and can be subjected to high speed firing. Since in the 
course of the firing process, the ceramic composition is partially 
crystallized, it exhibit a superior ability to prevent deformation of the 
patterns of conductors and resistors when firing or re-heating treatment. 
However, since in the above materials developed by the inventors, an 
alumina content is limited to a maximum amount of 50%, they have still 
some problems in their mechanical strength. In order to improve their 
mechanical strength, it is necessary to increase the use of alumina, and 
in this invention even if an alumina percentage is increased, it is 
possible to obtain fully densified fired ceramics. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide low 
temperature fired ceramics having a superior combination of properties, 
particularly mechanical strength, desirable dielectric constant and 
thermal expansion in which alumina can be used in a high content up to 
60%, and thereby a greatly increased mechanical strength can be obtained 
without any detrimental effect on densification. 
According to the present invention, there is provided partially 
crystallized ceramic articles which have been prepared by firing at a low 
temperature of 800.degree. to 1100.degree. C., a mixture consisting 
essentially of, in weight percentages: 
(a) 40 to 50% of powdered, noncrystalline glass consisting essentially of 
10 to 55% of at least one selected from the group consisting of CaO and 
MgO, 45 to 70% of SiO.sub.2, 0 to 30% of Al.sub.2 O.sub.3, 0 to 30% of 
B.sub.2 O.sub.3 and up to 10% impurities, the powder size of said glass 
being at least 4.0 m.sup.2 /g in terms of specific surface area measured 
by the BET Method; and 
(b) 60 to 50% of powdered Al.sub.2 O.sub.3. 
Specific surface area given in this application refers to values measured 
by the BET method. 
In the present invention a high density fired body can be readily obtained 
up to the critical maximum amount of alumina of 60% using glass powder of 
at least 4 m.sup.2 /g in specific surface area. But any further increased 
percentage of alumina can not be acceptable for the intended use even if 
glass powder is more finely divided. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
For a better understanding, the present invention will be described in more 
detail hereinafter. In the specification, percentages refer to percentages 
by weight, unless otherwise specified. 
In the preparation of the ceramics of the present invention, it is 
desirable that alumina powder to be mixed with the glass powder set forth 
above has a size of 0.5 .mu.m or greater, since the use of finer alumina 
powder than 0.5 .mu.m make difficult the densification of the resulting 
fired products. 
Since, in the ceramics of the present invention, alumina powder can be used 
in an increased proportion, strength is significantly increased. For 
instance, a high flexural strength of about 3000 kg/cm.sup.2 which is 
almost the same level as that of alumina substrates can be developed in 
the fired ceramics with an alumina content of 55%. Further, despite of an 
increase in the alumina percentage, firing temperature is not appreciably 
changed and firing at temperatures of not higher than 1100.degree. C. can 
be employed. 
The low temperature fired ceramics according to the present invention is 
basically different from the conventional low temperature fired ceramics 
previously set forth, in behavior or mechanism or process of sintering. 
The CaO-MgO-Al.sub.2 O.sub.3 -SiO.sub.2 or CaO-MgO-Al.sub.2 O.sub.3 
-SiO.sub.2 -B.sub.2 O.sub.3 system noncrystallized glass employed in the 
ceramic composition is partially crystallized during the firing step by 
addition of Al.sub.2 O.sub.3 and precipitates crystals of anorthite, 
wollastonite, cordierite and/or mullite. Such partial crystallization not 
only makes possible firing at a lower temperature of 800.degree. to 
1100.degree. C. but also minimizes the deformation of fine patterns caused 
during firing step and makes possible rapid firing. 
Further, the ceramic compositions of the present invention are retained in 
a porous state up to the firing temperatures of 730.degree. to 850.degree. 
C. without softening and shrinking, even if rapidly heated at a heating 
rate of the order of more than 30.degree. C./min and thus binders used in 
the ceramics are readily removed, without any carbon component remaining 
in the glass phase or the formation of cracks. However, since the ceramics 
are rapidly shrunk and sintered near the temperature range of 800.degree. 
to 1100.degree. C., a large-sized, dense ceramic substrate (for example, 
30 cm.times.30 cm in size) can be readily obtained in a shortened firing 
time. Such rapid sintering ability is believed to be due to the partial 
crystallization of the low temperature fired ceramics of the present 
invention and the fact that any shrinkage does not occur due to firing 
until heated to 730.degree. to 850.degree. C. 
The beneficial combination of partial crystallization and rapid shrinkage 
and sintering at high temperatures are characteristic behaviors in firing 
step of the mixture of CaO-MgO-Al.sub.2 O.sub.3 -SiO.sub.2 (-B.sub.2 
O.sub.3) glass and Al.sub.2 O.sub.3 powder of the present invention. 
Further, since, up to the temperature of 730.degree. to 850.degree. C., 
any shrinkage does not occur but, thereafter, at the final firing stage at 
higher temperature, partial crystallization rapidly takes place, flowing 
of glass is prevented and thereby fine patterns with a high precision can 
be readily obtained without causing any deformation. Therefore, the green 
ceramics multilayered, conductors, resistors, capacitors are 
simultaneously fired with a high precision configuration. Further, in 
addition to the advantage that the simultaneous firing can be performed 
without causing any deformation of patterns, RuO.sub.2 resistor, Cu 
conductors or the like can be applied onto the fired ceramic substrate by 
conventional thick film printing techniques and fired without accompanying 
deformation of the multilayer ceramic structure or the circuit patterns 
previously fired onto the substrate by the simultaneous firing, by virtue 
of the partial crystallization caused by firing at 800.degree. to 
1100.degree. C. 
In the present invention, glass powder size is given in terms of specific 
surface area measured by the BET method. This is ascribable to the fact 
that since glass powder can not be successfully dispersed in a liquid, its 
particle size can not be correctly measured by using a usual measuring 
apparatus and it is difficult to define glass powder size in other terms. 
The reason why the glass powder used as a starting material is limited to 
the composition specified above is as follows. 
When the SiO.sub.2 content is less than 45%, dielectric constant and 
thermal expansion coefficient will be increased to an undesirable level 
and precipitates of anorthite, cordierite, wollastonite and/or mullite 
will not be given in enough amounts from partial crystallization and, 
deformation of conductor or resistor patterns formed on a ceramic 
substrate is apt to occur during the firing or re-heating process. On the 
other hand, an excess use of SiO.sub.2 exceeding 70% make difficult the 
low temperature firing at 1100.degree. C. or lower. 
When Al.sub.2 O.sub.3 is employed in an amount more than 30%, firing at 
1100.degree. C. or lower becomes difficult. 
CaO and MgO is contained in the glass solely or in combination thereof in a 
total amount 10 to 55%. The content of CaO and/or MgO in an amount less 
than 10%, will make impossible firing at temperatures of 1100.degree. C. 
or lower. While an excess amount of CaO and/or MgO exceeding 55% results 
in an unfavorable increase in dielectric constant and thermal expansion 
coefficient. The use of MgO will gives smaller dielectric constant and 
thermal expansion than the use of CaO. The content of CaO and/or MgO 
exceeding 55% can not provide enough amounts of cordierite and mullite 
which may be precipitated from partial crystallization. 
B.sub.2 O.sub.3 not only makes it possible to melt glass-forming materials 
near temperatures of about 1300.degree. to 1450.degree. C., but also has 
an effect in lowering the firing temperature of the ceramics to 
1100.degree. C. or lower without changing their electrical properties and 
mechanical or physical properties. An amount of B.sub.2 O.sub.3 exceeding 
30% leads to a deleterious reduction in flexural strength and 
moisture-resistance, thereby lowering reliability, although increasing a 
B.sub.2 O.sub.3 content tends to cause the lowering of dielectric constant 
and thermal expansion coefficient. 
The glass may contain up to 5% alkali metal oxide(s) of Na.sub.2 O and/or 
K.sub.2 O. These alkali metal oxides may be present as an impurity in a 
glass-forming material or are added for the purpose of promoting the 
melting of the glass-forming material at the step of forming a glass. 
However, an amount of these oxides exceeding 5% is unfavorable, because 
such an excess amount will deteriorate electrical properties and 
moisture-resistance and thereby reliability will be lowered. 
The glass may also contain, as further impurities, BaO, PbO, Fe.sub.2 
O.sub.3, MnO.sub.2, Mn.sub.3 O.sub.4, Cr.sub.2 O.sub.3, NiO, Co.sub.2 
O.sub.3, etc., in a total amount (containing the amount of the foregoing 
alkali metal oxides, if present) up to 10% without impairing the 
properties of the ceramics. Further, although, on the firing process of 
the raw material composed of the glass and alumina, partial 
crystallization can take place without requiring the addition of special 
nucleating agent to the glass, the impurities may accelerate the 
crystallization in certain situations. 
In certain applications where high reliability is required under conditions 
of high temperature and high humidity, it is desirable that Pb be 
excluded. 
The fired ceramics prepared by firing the powder mixture consisting of 40 
to 50% of the glass powder with the composition specified above and 50 to 
60% of alumina powder have the composition consisting of, in weight 
percentages, 50 to 72% of Al.sub.2 O.sub.3, 18 to 35% of SiO.sub.2, 4 - 
27.5% of at least one selected from the group of CaO and MgO, 0 to 15% of 
B.sub.2 O.sub.3 and impurities total up to 10% and is composed essentially 
of a noncrystallized glass phase, alumina and at least one of crystallized 
glass phase among anorthite, wollastonite, cordierite and mullite formed 
by partially crystallizing said glass. In a preferred embodiment, the 
powdered, noncrystalline glass consists essentially of 21.1 to 27.3% of 
CaO, 0 to 0.06% of MgO, 50 to 59.1% of SiO.sub.2 ; 4.5 to 14.3% of 
Al.sub.2 O.sub.3 and 8.2 to 9.1 % of B.sub.2 O.sub.3. 
In preparing the ceramics of the present invention, CaO (and/or MgO), 
SiO.sub.2, Al.sub.2 O.sub.3 and B.sub.2 O.sub.3 or precursors thereof are 
thoroughly admixed as glass-forming materials in so as to yield the glass 
composition set forth above, melted at a temperature of 1300.degree. to 
1450.degree. C. and then quenched to provide the glass. The starting 
materials employed in making the glass may be in any form of carbonates, 
oxides or hydroxide. The heating temperature of 1300.degree. to 
1450.degree. C. is a desirable range from the viewpoint of materials used 
in a furnace, etc. 
Powders of the thus obtained glass and alumina are mixed in the given 
proportion and then formed in a usual ceramic-forming manner, such as cold 
pressing or tape casting operation. Thereafter, the formed material is 
fired at temperatures of 800.degree. to 1100.degree. C. 
In the application where the ceramic composition of the present invention 
is used to make multilayer substrates, for example, Ag conductors are 
printed onto the green ceramic sheets, the required number of the sheets 
are stacked, hot-pressed and then co-fired into a integrated multilayer 
structure. As necessary interconnection holes are formed. 
In other applications, resistor pastes, for example RuO.sub.2 or SiC 
resistor paste, or capacitor pastes, such as BaTiO.sub.3, SrTiO.sub.3, 
Pb(Fe.sub.2/3 W.sub.1/3)O.sub.3 - Pb(Fe.sub.1/2 Nb.sub.1/2)O.sub.3, are 
printed onto the green ceramic sheets and then the sheets are stacked, 
hot-pressed and fired at the same time to form a integrated laminated 
substrate incorporating resistors or capacitors. If desired, green ceramic 
sheets consisting of capacitor compositions may be stacked onto the 
invention green ceramic sheets, hot-pressed and simultaneously fired to 
provide integrated structures incorporating capacitors therein. 
Also, a conductor paste comprising copper powder having a controlled 
particle size which has been subjected to an antioxidation treatment is 
printed onto the green sheets and the green sheets are laminated into a 
multilayer structure. The green sheets thus laminated are co-fired in an 
inert atmosphere consisting mainly of nitrogen gas to provide a 
low-temperature co-fired multilayer substrate having Cu conductors 
therein. Also, in such an inert atmosphere of nitrogen, there can be 
prepared a multilayered substrate assembly containing in addition to the 
Cu conductors, resistors using resistor pastes containing metal(s) or 
metallic compound(s), such as Ni - Cr, molybdenum silicide, W - Ni, etc. 
Now, the present invention will be described in detail with reference to 
the detailed examples which follow.

cl EXAMPLES 
CaCO.sub.3, Mg(OH).sub.2, Al.sub.2 O.sub.3, SiO.sub.2 and H.sub.3 BO.sub.3 
were weighed as starting materials for glass in the percentages given in 
Table 1 below and were thoroughly mixed by using a mill to provide each 
powder mixture. Then, the powder mixture was melted at 1400.degree. C. and 
poured in water to yield a glass. 
The glass thus obtained was placed in an alumina pot, together with water 
and alumina balls, was wetmilled and was dried to yield glass powder 
having a specific surface area of 4.5 m.sup.2 /g. 
The glass powders obtained above were blended with alumina powder in the 
proportions shown in Table 1, i.e., 45% of glass and 55% of alumina or 40% 
of glass and 60% of alumina. Each powder mixture was placed in an alumina 
pot with water and alumina balls, then milled for 3 hours and dried. 
100 g of methacryl type binder, 50 g of plasticizer (dioctyl adipate) and 
400 g of solvent (toluene and xylene) were added to 1000 g of each of the 
above powder mixtures and formed into a slip. The slip was formed into a 
1.0 mm thick green sheet using a doctor blade and fired at a temperature 
of 850.degree. to 1000.degree. C., thereby obtaining a fired ceramic. The 
fired ceramics were each tested on their physical properties. 
Examples 1 to 5 given in Table 1 show the relation between physical 
properties and composition and the fired ceramics thus obtained exhibited 
a very high degree of flexural strength of 2600 to 3200 kg/cm.sup.2 which 
are well comparable with that of an alumina substrate. 
TABLE 1 
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Example No. 
1 2 3 4 5 
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Glass Composition 
(wt. %) 
SiO.sub.2 50 54.1 54.4 51.7 59.1 
Al.sub.2 O.sub.3 
13.61 11.9 14.3 15 4.5 
CaO 27.3 21.1 22.4 -- 27.3 
MgO -- -- 0.6 16.7 -- 
B.sub.2 O.sub.3 
9.1 8.3 8.2 16.7 9.1 
Na.sub.2 O + K.sub.2 O 
&lt;0.1 1.8 0.7 &lt;0.1 &lt;0.1 
Other -- MnO.sub.2 
-- -- -- 
Impurities 2.8 
Glass/Alumina 
40/60 45/55 45/55 45/55 45/55 
Weight Ratio 
Firing Temperature 
900 900 900 1000 900 
(.degree.C.) 
Properties of 
Ceramics 
Bulk Density 3.2 3.2 3.1 3.0 3.2 
(g/cc) 
Flexural 2600 3100 3200 2700 3000 
Strength (kg/cm.sup.2) 
Thermal Exp. 5.4 5.7 5.5 4.0 5.5 
Coeff. .times. 10.sup.-6 /.degree.C.*.sup. 1 
Dielectric 8.6 8.3 8.2 6.5 8.4 
Constant at 1 MHz 
tan .delta. .times. 10.sup.- 4 
2 5 7 6 5 
at 1 MHz 
Volume Resis- 
&gt;10.sup.14 
&gt;10.sup.14 
&gt;10.sup.14 
&gt;10.sup.14 
&gt;10.sup.14 
tivity (cm .multidot. .OMEGA.) 
______________________________________ 
*.sup.1 measured at the temperature range from room temperature to 
250.degree. C. 
COMATIVE EXAMPLES 
Fired ceramics of comparative Examples 1 to 4 given in Table 2 were 
obtained in the same way as described in the foregoing Examples using the 
60% of the glass powders used in Examples and 40% of alumina powder. The 
glasses used in comparative Examples 1 to 4 were the same as those of the 
foregoing Examples 1 to 4, respectively. 
The flexural strength of these comparative ceramics was only the order of 
1800 to 2100 kg/cm.sup.2. 
Also, in comparative Examples 5 to 8 given in Table 2, further comparative 
ceramics were provided in the same procedure described in Examples 1 to 4, 
using 45% of glass powder with a specific surface area of 3 m.sup.2 /g and 
55% of alumina powder and their flexural strength are shown in Table 2. 
The glass employed in Comparative Examples 5 to 8 had the same 
compositions as those of Examples 1 to 4, respectively. The flexural 
strength of these comparative ceramics was only the order of 2000 to 2200 
kg/cm.sup.2 because they were not fully densified. 
TABLE 2 
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Comparative Example No. 
1 2 3 4 
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Glass/Alumina 60/40 60/40 60/40 60/40 
Weight Ratio 
Bulk Density (g/cc) 
3 3 3 2.8 
Flexural Strength 
1900 2100 2000 1800 
(kg/cm.sup.2) 
______________________________________ 
Comparative Example No. 
5 6 7 8 
______________________________________ 
Glass/Alumina 45/55 45/55 45/55 45/55 
Weight Ratio 
Bulk Density (g/cc) 
2.9 2.8 2.9 2.7 
Flexural Strength 
2000 2200 2100 2000 
(kg/cm.sup.2) 
______________________________________ 
As described above, the fired ceramics of the present invention can be 
obtained by firing at the lower temperature order of 1100.degree. C. or 
lower and possess a high degree of flexural strength well comparable with 
that of alumina substrates heretofore used. Therefore, the present 
invention can provide the fired ceramics having improved dielectric 
constant and thermal expansion in comparison with known alumina 
substrates, without lowering their strength. Further, due to such a low 
firing temperature, low electrical resistance metallic materials, such as 
Au, Ag, Ag-Pd, can be used as conductors without causing deformation of 
conductor patterns.