Low temperature firing glass-ceramic substrate and production process thereof

A low temperature firing glass-ceramic substrate comprising a borosilicate glass, .alpha.-alumina, .gamma.-alumina, and mullite crystals crystallized out in a dispersed form from the borosilicate glass and the .gamma.-alumina, and a production process of the substrate. It is an object of this invention to provide a glass-ceramic substrate, which is low in dielectric constant and dielectric loss and high in strength and is useful for multilayer wiring.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a low temperature firing glass-ceramic substrate 
permitting co-firing with a low-resistance conductor such as Au, Ag or Cu, 
especially a glass-ceramic substrate having a low dielectric constant, a 
small dielectric loss and high strength and also to a production process 
thereof. 
2. Description of the Related Art 
Ceramic substrates have been used as high-density wiring substrates because 
they easily allow high-degree multilayer wiring by the green sheet 
lamination process. Although aluimna is most common as a ceramic material 
for them, it is accompanied by problems that (1) for its firing 
temperature as high as about 1,600.degree. C., high-resistance W or Mo has 
to be chosen as a wiring material for internal layers to be formed by 
co-firing and (2) for its relative dielectric constant as high as about 
10, a reduction in dielectric constant has to be achieved desirably from 
the viewpoint of achieving high-speed transmission. 
On the other hand, glass-ceramic materials, which are composite materials 
of glass of relatively low softening point and ceramics, have been 
developed as multilayer wiring substrate materials capable of overcoming 
the above-mentioned problems, because (1) they can be fired at 900 to 
1,000.degree. C. and can hence choose a low-resistance conductor such as 
Au, Ag or Cu as a conductor metal for internal layers and (2) they can 
attain a lower dielectric constant by using glass of low dielectric 
constant. 
Nonetheless, low-dielectric-constant materials have a problem in 
reliability as the dielectric constant and strength of a material are 
generally in a substantially proportional relationship. In addition to 
this trade-off relationship between dielectric constant and material 
strength, there is an increasing demand for a reduction in the dielectric 
loss of a substrate material to meet the requirement for lower 
transmission losses in high frequency circuits employed in a communication 
network. 
SUMMARY OF THE INVENTION 
With the above-mentioned various problems in view, the present invention 
has as objects thereof the provision of a glass-ceramic substrate low in 
dielectric constant and dielectric loss and high in strength and also the 
provision of its production process. 
The present invention relates to a low temperature firing glass-ceramic 
substrate comprising a borosilicate glass, .alpha.-alumina, 
.gamma.-alumina, and mullite crystals crystallized out in a dispersed form 
from the borosilicate glass and the .gamma.-alumina. 
In addition, the present invention also relates to a process for producing 
a low temperature firing glass-ceramic substrate by firing at 850 to 
1,100.degree. C. a mixture comprising 50 to 80 wt. % of a borosilicate 
glass and 10 to 50 wt. % of .gamma.-alumina having a particle size not 
greater than 50 nm, which compises causing a mullite crystal phase to 
crystalize out in a dispersed form from the borosilicate glass and the 
.gamma.-alumina. 
According to the present invention, a glass-ceramic low in dielectric 
constant and dielectric loss and high in strength is obtained. Use of this 
material makes it possible to provide a low temperature firing 
glass-ceramic substrate having excellent dielectric characteristics and 
high strength.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Based on embodiments, the present invention will hereinafter be described 
in detail. 
The low temperature firing glass-ceramic substrate according to the present 
invention has a structure that .alpha.-alumina crystal grains and mullite 
crystal grains have been crystallized out in a dispersed form in a 
borosilicate glass matrix. Selection of a composition having a 
significantly low softening point as the borosilicate glass permits low 
temperature firing, so that a low-resistance conductor such as Au, Ag or 
Cu can be chosen as a wiring material for internal layers. Further, owing 
to the possession of the structure that .alpha.-alumina grains and mullite 
grains have crystallized out in a dispersed form, the low temperature 
firing glass-ceramic substrate according to the present invention has 
another characteristic feature that high strengthening can be attained. In 
particular, the reliability has been improved owing to the formation of 
mullite crystals which are needle-like crystals and are relatively low in 
dielectric constant. 
The production process of this invention for a low temperature firing 
glass-ceramic substrate includes the step that a mixture containing a 
borosilicate glass and .gamma.-alumina particles and, if necessary, 
.alpha.-alumina particles is fired to cause a mullite crystal phase to 
crystallize out in a dispersed form from the borosilicate glass and 
.gamma.-alumina particles. 
Preferably, the production process of this invention for the low 
temperature firing glass-ceramic substrate is characterized in that a 
mixture, which contains 50 to 80 wt. % of a borosilicate glass, 0 to 30 
wt. % (preferably 10 to 30 wt. %) of .alpha.-alumina of 0.5 to 5 .mu.m in 
particle size and 10 to 50 wt. % of .gamma.-alumina, is fired at 850 to 
1,100.degree. C. to cause a mullite crystal phase to crystallize out in a 
dispersed form from the borosilicate glass and the .gamma.-alumina in the 
glass matrix. 
The proportion of the borosilicate glass in the above-described mixture 
affects the firing temperature and the strength of the resulting 
substrate. The setting of the proportion of the borosilicate glass at 50 
wt. % or higher has made it possible to satisfactorily employ a firing 
temperature as low as 1,100.degree. C. or lower and also to facilitate 
co-firing with a conductor such as Cu. On the other hand, the setting of 
the proportion of the borosilicate glass at 80 wt. % or lower has made it 
possible to bring about sufficient strength. From the foregoing, it is 
preferable to set the proportion of the borosilicate glass at 50 to 80 wt. 
%. Further, a proportion of from 60 to 75 wt. % is desired because it 
allows to use a firing temperature as low as 900 to 1,000.degree. C. and 
also to bring about flexural strength as high as 200 MPa or higher. 
In the substrate production process of this invention, the above-described 
borosilicate glass may preferably contains 75 to 85 wt. % of SiO.sub.2 and 
15 to 25 wt. % of B.sub.2 O.sub.3, at least. More preferably, the 
borosilicate glass may also contain, in addition to the above-described 
oxides, at least one oxide selected from up to 2.5 wt. % of K.sub.2 O, up 
to 2.5 wt. % of Na.sub.2 O, up to 2 wt. % of Al.sub.2 O.sub.3, up to 1 wt. 
% of MgO, up to 1 wt. % of CaO or up to 1 wt. % of TiO.sub.2. Especially 
preferably, the borosilicate glass may also contain, in addition to the 
above-described oxides, at least one oxide selected from 1.0 to 2.5 wt. % 
of K.sub.2 O, 0.01 to 2.5 wt. % of Na.sub.2 O, 0.01 to 2 wt. % of Al.sub.2 
O.sub.3, 0.01 to 1 wt. % of MgO, 0.01 to 1 wt. % of CaO or 0.01 to 1 wt. % 
of TiO.sub.2. 
In the substrate production process according to the present invention, 
addition of .alpha.-alumina particles of 0.5 to 5 .mu.m in particle size 
is desired as they can improve the strength. A particle size of 5 .mu.m or 
smaller is desired, since dispersion of .alpha.-alumina particles having a 
particle size greater than 5 .mu.m leads to reduced sinterability. On the 
other hand, addition of .alpha.-alumina having a particle size smaller 
than 0.5 .mu.m leads to lowered dispersibility, so that .alpha.-alumina of 
0.5 .mu.m or greater is desired. Further, addition of .alpha.-alumina of 1 
to 2 .mu.m or so is desired from the standpoint of improvements in 
dispersibility and strength. 
In the substrate production process of this invention, addition of 
.gamma.-alumina having a particle size of from 5 to 50 nm has an effect 
that, in the course of the firing at 850 to 1,100.degree. C., it reacts 
with the borosilicate glass to cause a mullite crystal phase to 
crystallize out. Production of .gamma.-alumina particles having a particle 
size smaller than 5 nm is difficult at present. Further, .gamma.-alumina 
of 50 nm or smaller in particle size makes it easier to cause a mullite 
phase to crystallize out during the firing at 1,100.degree. C. or lower. 
It is therefore desired to add .gamma.-alumina of 5 to 50 nm in particle 
size. 
Concerning the proportion of .gamma.-alumina to be added, a proportion of 
10 wt. % or higher leads to a further improvement in strength, and a 
proportion of 50 wt. % or lower leads to a further improvement in 
sinterability so that sintering at 1,100.degree. C. or lower is 
facilitated still further. Accordingly, a proportion of from 10 to 50 wt. 
% is desired. 
Without addition of .alpha.-alumina, it is possible to improve the strength 
by .alpha.-alumina and mullite which are caused to crystallize out from 
the borosilicate glass and .gamma.-alumina in the course of firing. 
Addition of .alpha.-alumina is however desired, because its addition makes 
it possible to improve the strength further and, when fired at a 
temperature of 1,000.degree. C. or higher, is also effective in preventing 
a cristobalite phase from crystallizing out from the borosilicate glass. 
Addition of .alpha.-alumina in any proportion greater than 30 wt. % is 
however not preferred, because such a high proportion of .alpha.-alumina 
lowers the proportion of .gamma.-alumina, which permits sintering at 
1,100.degree. C. or lower, to less than 10 wt. %, thereby resulting in a 
loss of the strength-enhancing effect of .gamma.-alumina. 
A description will next be made of the overall construction of the 
substrate according to the one embodiment of the present invention. As is 
illustrated in FIG. 1, the substrate is a glass-ceramic substrate having a 
structure that .alpha.-alumina crystals 2 and mullite crystals 3 are 
dispersed in a borosilicate glass 1 as a matrix. Internal-layer conductors 
4 and via conductors 6 are desirably made of Au, Ag or Cu. Designated at 
numeral 5 in the drawing are surface-layer conductors. 
In a substrate production process according to one embodiment of the 
present invention, a glass-ceramic substrate having the structure that 
.alpha.-alumina crystals and mullite crystals have crystallized out in a 
matrix of a borosilicate glass is produced by fining a mixture composed of 
the borosilicate glass, .alpha.-alumina and .gamma.-alumina. 
Owing to the crystallized mullite crystals, the thus-obtained glass-ceramic 
substrate is low in dielectric constant and dielectric loss and high in 
strength. 
The present invention will hereinafter be described further by examples. It 
should however be borne in mind that the present invention is not limited 
to the following examples. 
EXAMPLES 1 & 2 
In each example, borosilicate glass powder the composition of which was 
82.0 wt. % SiO.sub.2, 16.0 wt. % B.sub.2 O.sub.3, 0.2 wt. % Al.sub.2 
O.sub.3, 0.2 wt. % MgO, 0.2 wt. % CaO, 0.2 wt. % TiO.sub.2, 0.2 wt. % 
Na.sub.2 O and 2.0 wt. % K.sub.2 O in terms of oxides, .alpha.-alumina 
powder of 1 .mu.m in average particle size and .gamma.-alumina powder of 
10 nm in average particle size were weighed in proportions of 75, 0 and 25 
wt. % (Example 1) or 60, 10 and 30 wt. % (Example 2), respectively. After 
the thus-weighed materials were mixed, green sheets were prepared by a 
usual method. Some of the thus-prepared green sheets were laminated and 
then compression-bonded under heat, followed by firing under the 
corresponding conditions shown in Table 1. 
The dielectric constants of the resultant sintered bodies were 5.0 (Example 
1) and 5.5 (Example 2), respectively. Nonetheless, measuremments of their 
flexural strength indicated strength sufficient to assure high reliability 
as shown in Table 1. Further, pellets were cut out from those sintered 
bodies, and their dielectric losses were measured by the cavity resonator 
method. They were found to have small dielectric losses as shown in Table 
1. 
Those sintered bodies were also subjected to an identification of phases by 
X-ray diffraction. .alpha.-Alumina, mullite and some .gamma.-alumina were 
identified from the sintered body of Example 1, whereas .alpha.-alumina 
and mullite were identified form the sintered body of Example 2. 
In each of the examples, via holes were formed through the remaining green 
sheets. Those green sheets were each subjected to via filling with an Ag 
paste and also to pattern printing. Several green sheets so obtained were 
laminated and compression-bonded under heat into a laminate. The laminate 
was fired under firing conditions similar to the corresponding ones shown 
in Table 1. The green sheets of the respective examples were confirmed to 
be able to provide low temperature firing glass-ceramic substrates. 
TABLE 1 
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Example 1 
Example 2 
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Composition 
Borosilicate glass 
75 wt. % 60 wt. % 
.alpha.-Alumina 
0 wt. % 10 wt. % 
.gamma.-Alumina 
25 wt. % 30 wt. % 
Firing conditions 900.degree. C. 
950.degree. C. 
Held for 1 hr 
Held for 1 hr 
in air in air 
Dielectric constant 
5.0 5.5 
(1 MHz) 
Dielectric loss 0.002 0.002 
tangent (10 GHz) 
Flexural strength 180 MPa 210 MPa 
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