Electrically insulating substrate and a method of making such a substrate

From 0.1 to 3.5% by weight of beryllium oxide powder, calculated as beryllium, is added to silicon carbide powder containing up to 0.1% by weight of aluminum, up to 0.1% by weight of boron and up to 0.4% by weight of free carbon, and the mixed powder is pressure-molded. The resulting molded article is heated to a temperature of 1,850.degree. C. to 2,500.degree. C. till there is obtained a sintered body having at least 90% relative density of silicon carbide. Thus, the sintered body having thermal conductivity of at least 0.4 cal/cm.sec..degree. C. at 25.degree. C., electrical resistivity of at least 10.sup.7 Ohm.cm at 25.degree. C. and coefficient of thermal expansion of 3.3.about.4.times.10.sup.-6 /.degree.C. at 25.degree. C. to 300.degree. C. can be obtained.

BACKGROUND OF THE INVENTION 
This invention relates to an electrically insulating substrate having high 
thermal conductivity and high electric resistivity and also to a method of 
making such a substrate. 
The semiconductor industry has made a remarkable progress in recent years 
and a number of circuit constituents such as semiconductor chips have been 
formed in increasingly higher density on an insulating substrate used in 
large scale integration circuit and the like. Demands for devices having 
greater capacity with smaller size have also become keener and insulating 
substrates having high thermal diffusivity have been required. 
As materials for such an insulating substrate, there has conventionally 
been employed an alumina sintered body. Since the alumina sintered body 
does not have satisfactory thermal diffusivity, however, development of an 
insulating substrate having higher thermal diffusivity has been requested. 
The insulating substrate for such an application must satisfy the 
following requirements. 
(1) high electrically insulating property, 
(2) high thermal conductivity, 
(3) its coefficient of thermal expansion is approximate to that of silicon, 
and 
(4) high mechanical strength. 
A sintered body of silicon carbide has a coefficient of thermal expansion 
of 3.7.times.10.sup.-6 /.degree.C. which is smaller than the coefficient 
of thermal expansion of alumina, i.e., about 8.times.10.sup.-6 /.degree.C. 
and is approximate to that of silicon, i.e., about 3.3.times.10.sup.-6 
/.degree.C. As to the mechanical strength of silicon carbide, its bending 
strength is at least 50 Kg/mm.sup.2, the value being by far higher than 
that of alumina, i.e., ca. 25 to 30 Kg/mm.sup.2. Further, the thermal 
conductivity of silicon carbide sintered body is 0.1 to 0.2 
cal/cm.sec..degree.C. which is at least three times that of alumina. 
Silicon carbide is a semiconductor compound of the Group IV--IV consisting 
of carbon and silicon. For this reason, it has been believed difficult to 
obtain a high density sintered body of silicon carbide with high 
electrical resistivity and as a matter of fact, no such sintered body has 
been found to this date. 
On the other hand, it has been known well that since silicon carbide is a 
compound having high covalent bond, it is hard and tough and is stable 
both in oxidation resistance and corrosion resistance even at a 
temperature of 1,500.degree. C. or above. Due to this strong covalent 
bond, however, it is a material from which a high ensity sintered body can 
not be obtained easily. 
Various sintering aids have been employed in order to obtain a high density 
sintered body of silicon carbide. U.S. Pat. No. 4,172,109, for example, 
discloses a sintered body of silicon carbide which is sintered while Be is 
added as the aid. This prior art relates to a high strength material 
obtained by sintering the raw powder of silicon carbide which contains 0.5 
to 5 wt.% of excessive carbon. However, the sintered body thus formed has 
small electric resistivity and can not be used as an electrically 
insulating material. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an electrically 
insulating substrate consisting of silicon carbide as its principal 
component and having high thermal conductivity, and also to provide a 
method of making such a substrate. 
The electrically insulating substrate in accordance with the present 
invention is formed by adding 0.1 to 3.5 wt.% of beryllium oxide powder, 
calculated as beryllium, to silicon carbide powder as the principal 
component, pressure-molding the mixed powder and then sintering the 
resulting molded article at such a temperature and pressure sufficient to 
provide 90% relative density. Here, the aluminum, boron and free carbon 
components in the silicon carbide powder are restricted to at most 0.1 
wt.%, at most 0.1 wt.% and at most 0.4 wt.%, respectively. The thermal 
conductivity at 25.degree. C. of the electrically insulating substrate 
obtained from this sintered body is at least 0.4 cal./cm.sec..degree.C. 
while its electrical resistivity at 25.degree. C. is at least 10.sup.7 
ohm.cm. Further, its coefficient of heat expansion from 25.degree. C. to 
300.degree. C. is up to 3.7.times.10.sup.-6 /.degree.C. 
In the present invention, the amount of beryllium in the sintered body is 
restricted to from 0.1 to 3.5 wt.% for the following reason. If it is 
below 0.1 wt.%, the electrical resistivity becomes smaller than 10.sup.7 
ohm.cm while if it exceeds 3.5 wt.%, the thermal expansion coefficient of 
the sintered body becomes greater than 4.times.10.sup.-6 .degree.C., thus 
rendering a critical problem when the sintered body is used as an 
insulating substrate especially for silicon semiconductor elements. 
It is advisable to add beryllium as BeO. The addition is made by mixing the 
BeO powder and the silicon carbide powder. If about 0.5 to 14 wt.% of BeO 
is added in this case, Be in the sintered body accounts for about 0.1 to 
3.5 wt.%. However, these values vary to some extents depending upon the 
atmosphere and temperature during the sintering. 
It is another important requirement in the present invention that the 
silicon carbide powder does not contain more than 0.4 wt.% of free carbon. 
The presence of free carbon in such an amount markedly reduces the 
electrical resistivity as one of the objects of the present invention. 
In the present invention, the abovementioned beryllium oxide powder and 
silicon carbide powder are minute powder having an average grain size of 
up to 10 .mu.m, preferably up to 2 .mu.m, and the powder is sintered by 
hot-pressing. Though aluminum and boron are not contained preferably in 
the sintered body, there is no problem if their contents are below 0.1 
wt.%, respectively. If aluminum is contained in a greater amount, the 
electrical resistivity of the sintered body becomes below 10.sup.7 Ohm.cm 
disadvantageously. If boron is contained in a greater amount, the thermal 
conductivity becomes smaller than 0.4 cal/cm.sec..degree.C. 
If it is desired to obtain a sintered body having a thermal conductivity of 
at least 0.5 cal/cm.sec..degree.C., it is recommended to sinter the 
silicon carbide powder whose principal component is alpha-type SiC. 
The sintering condition of BeO-containing silicon carbide powder is also an 
important factor, and specifically, sintering must be carried out in a 
non-oxidizing atmosphere. In an oxidizing atmosphere, the surface of the 
silicon carbide powder is oxidized so that a high density sintered body 
can not be obtained. On the other hand, furnace materials that can be used 
in an oxidizing atmosphere at a temperature of about 2,000.degree. C., are 
not availble at present. 
The sintering temperature is from 1,850.degree. to 2,500.degree. C., 
preferably from 1,900.degree. to 2,300.degree. C. If the temperature is 
lower than 1,850.degree. C., a high density sintered body can not be 
obtained while at a sintering temperature higher than 2,500.degree. C., 
sublimation of silicon carbide becomes so vigorous that the sintered body 
is excessively sintered and hence, is not changed into a compact ceramic. 
In the hot press method which applies a high pressure to the sample during 
sintering, the upper limit of the pressurizing load varies with the 
material of dies to be employed. Generally, the dies are made of graphite 
and in that case, a pressure of up to about 700 Kg/cm.sup.2 may be 
applied. 
Generally, however, it is possible to obtain a high density sintered body 
without applying such a great pressure. Generally, the pressure ranges 
from 100 to 300 Kg/cm.sup.2. If the silicon carbide powder having a 
sub-micron grain size is employed, a compact sintered body (relative 
density of 90%) can be obtained without applying the pressure. The optimum 
sintering time can be determined in accordance with the grain size of the 
raw powder, the sintering temperature and the load applied during the 
sintering. Generally speaking, a high density sintered body can be 
obtained within a short sintering time if the grain size of the raw 
material powder is small, the sintering temperature is high and the load 
applied during the sintering is great.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereinafter, the present invention will be described more definitely with 
reference to embodiments thereof. 
EXAMPLE 1 
Be quantity to be added 
Beryllium oxide powder having a grain size of up to 10 .mu.m was mixed with 
silicon carbide powder of an average grain size of 2 .mu.m in an amount of 
0.1 to 20 wt.%. The mixture was shaped into a mold at a room temperature 
by applying a pressure of 1,000 Kg/cm.sup.2. The mold had a density of 
1.60 to 1.67 g/cm.sup.3 (50 to 52% relative density with respect to the 
theoretical density of silicon carbide). Next, the molded article was 
placed in a graphite die and was sintered by a hot press method at a 
reduced pressure of 1.times.10.sup.-5 to 1.times.10.sup.-3 Torr. The 
sintering pressure was 300 Kg/cm.sup.2 and the temperature was raised from 
room temperature to 2,000.degree. C. in the course of about two hours, was 
held at 2,000.degree. C. for one hour and was then left cooling by cutting 
off a heating power source. The pressure was released after the 
temperature fell below 1,500.degree. C. The relationship between the 
properties of the silicon carbide sintered body thus produced and the Be 
quantity is shown in FIGS. 1 through 4. 
The results of FIGS. 1 through 4 illustrate that if the Be quantity in the 
silicon carbide sintered body falls within the range of 0.1 to 3.5 wt.%, 
there can be obtained a sintered body having high density, high thermal 
conductivity, high electrical resistivity and low coefficient of thermal 
expansion (4.times.10.sup.-6 /.degree.C. or below). 
EXAMPLE 2 
Condition of hot-press 
Mixed powder formed by adding 2 wt.% of BeO powder to silicon carbide 
powder was hot-pressed in the same way as in Example 1, yielding a 
sintered body. The Be content in the sintered body in this instance was 
about 0.4 wt.%. In this example, various sintered bodies were produced by 
changing the hot-press conditions. Table 1 illustrates the relationship 
between the properties of the resulting sintered bodies and the hot-press 
conditions. When sintering was effected at a temperature of 1,850.degree. 
to 2,500.degree. C. and at a pressure of at least 100 Kg/cm.sup.2, there 
could be obtained sintered bodies having at least 90% density relative to 
the theoretical density, thermal conductivity of at least 0.4 
cal/cm.sec..degree.C., electric resistivity of at least 10.sup.11 Ohm.cm 
and coefficient of thermal expansion of about 
4.0.about.3.3.times.10.sup.-6 /.degree.C. 
TABLE 1 
__________________________________________________________________________ 
Thermal Conduc- 
Electrical 
Coefficient of 
Temp. 
Pressure 
Time 
Relative 
tivity (cal/ 
Resistivity 
Thermal Expansion 
(.degree.C.) 
(Kg/cm.sup.2) 
(h) 
Density (%) 
cm .multidot. sec .multidot. .degree.C.) 
(.OMEGA. .multidot. cm) 
(.times. 10.sup.6 /.degree.C.) 
__________________________________________________________________________ 
1800 
300 0.5 
71 0.08 10.sup.5 
3.34 
1850 
300 0.5 
88 0.18 10.sup.9 
3.33 
2000 
300 0.5 
98 0.67 10.sup.13 or more 
3.33 
2200 
300 0.5 
99 0.70 10.sup.13 or more 
3.32 
2400 
300 0.5 
96 0.59 10.sup.13 or more 
3.34 
2500 
300 0.5 
90 0.40 10.sup.13 or more 
3.34 
2000 
50 1.0 
74 0.08 10.sup.6 
3.33 
2000 
100 1.0 
95 0.41 10.sup.12 
3.33 
2000 
700 0.5 
99 0.72 10.sup.13 or more 
3.33 
2000 
300 0.1 
95 0.54 10.sup.12 
3.34 
2000 
300 1.0 
100 0.72 10.sup.13 or more 
3.33 
2000 
300 4.0 
98 0.66 10.sup.13 or more 
3.34 
1850 
300 1.0 
92 0.43 10.sup.11 
3.33 
2500 
100 0.3 
97 0.55 10.sup.13 or more 
3.34 
2500 
300 0.1 
98 0.51 10.sup.13 or more 
3.33 
1850 
700 0.5 
96 0.50 10.sup.13 or more 
3.32 
__________________________________________________________________________ 
Remarks: 
Thermal conductivity and electrical resistivity were measured at 
25.degree. C. 
Coefficient of thermal expansion is a mean value of 25 to 300.degree. C. 
EXAMPLE 3 
Atmosphere 
Sintered bodies were produced in the same way as in Example 1 except that 
the BeO quantity was changed to 3 wt.% and the atmosphere was changed to 
Ar gas, He gas and N.sub.2 gas, respectively. The Be content in the 
resulting sintered bodies was 0.9 wt.%. The properties of each sintered 
body were virtually the same as those of the sintered body of Example 1 
containing 1 wt.% of Be. 
EXAMPLE 4 
Grain size of powder 
After 2 wt.% of BeO was added to silicon carbide powder having average 
grain sizes of 0.2 to 20 .mu.m, sintered bodies were produced in the same 
way as in Example 1 by hot-press method. Table 2 illustrates the 
relationship between the average grain size of the silicon carbide powder 
and the relative density of the sintered body. It was found that if the 
average grain size of the silicon carbide raw powder was below 10 .mu.m, 
the sintered body was rendered compact to relative density of at least 
95%, and the sintered bodies rendered compact to relative density of at 
least 95% exhibited the same characteristics as the sintered body of 
Example 1 containing 0.4 wt.% beryllium. In the sintered bodies in which 
the average grain size was greater than 10 .mu.m and in which compactness 
was not sufficiently accomplished had thermal conductivity of as low as 
0.2 cal/cm.sec..degree.C. or below and the mechanical strength of as low 
as 10 Kg/mm.sup.2 or below. 
TABLE 2 
______________________________________ 
Average grain size (.mu.m) 
Relative density (%) 
______________________________________ 
0.2 100 
0.5 100 
1 99 
2 99 
5 98 
10 95 
13 86 
20 68 
______________________________________ 
EXAMPLE 5 
Restriction of free carbon 
Mixed powder was formed by adding 2 wt.% of BeO powder and 0.3 to 3 wt.%, 
based on silicon carbide, of carbon black (minute powder of grain size of 
0.1 .mu.m or below) as an impurity to the silicon carbide powder. The 
mixed powder was hot-pressed in the same way as in Example 1, yielding a 
sintered body. Thus, various sintered bodies were produced by changing the 
quantity of the carbon black. Table 3 illustrates the relationship between 
the carbon black quantity and the properties of the sintered bodies. When 
the carbon black quantity was 0.5 wt.%, the electrical resistivity of the 
product became 10.sup.6 Ohm.cm. 
TABLE 3 
______________________________________ 
Carbon Thermal con- Coefficient 
black Relative ductivity Electrical 
of thermal 
quantity 
density (cal/cm .multidot. 
resistivity 
expansion 
(wt. %) 
(%) sec .multidot. .degree.C.) 
(.OMEGA. cm) 
(.times. 10.sup.-6 /.degree.C.) 
______________________________________ 
0.3 99 0.70 10.sup.13 or more 
3.33 
0.4 99 0.62 10.sup.13 
3.34 
0.5 99 0.70 10.sup.6 3.34 
1.0 98 0.53 10.sup.5 3.35 
______________________________________ 
Remarks: 
Thermal conductivity and electrical resistivity were measured at 
25.degree. C. 
Coefficient of thermal expansion is a mean value of 25.degree. C. to 
300.degree. C. 
EXAMPLE 6 
Restriction of aluminum quantity 
Sintered bodies of silicon carbide were produced in the same way as in 
Example 5 except that aluminum nitride powder (minute powder of a grain 
size of up to 2 .mu.m) was employed as an impurity instead of carbon black 
of Example 5. Table 4 illustrates the relationship between the aluminum 
quantity and the properties of each sintered body when the former was 
changed. It was found that when the aluminum quantity became smaller than 
0.1 wt.%, the electrical resistivity of the sintered body became extremely 
small. 
TABLE 4 
______________________________________ 
Thermal con- 
Elec- Coefficient 
Al content 
Relative ductivity trical of thermal 
in sintered 
density (cal/cm .multidot. 
resistivity 
expansion 
body (wt. %) 
(%) sec .multidot. C.) 
(.OMEGA. cm) 
(.times. 10.sup.-6 /.degree.C.) 
______________________________________ 
0.1 99 0.63 10.sup.8 
3.34 
0.3 99 0.57 10.sup.5 
3.34 
0.5 99 0.52 10.sup.4 
3.34 
______________________________________ 
Remarks: 
Thermal conductivity and electrical resistivity were measured at room 
temperature. 
Coefficient of thermal expansion is a mean value of room temperature to 
300.degree. C. 
EXAMPLE 7 
Restriction of boron quantity 
Sintered bodies of silicon carbide were produced in the same way as in 
Example 5 except that boron nitride powder (minute powder of a grain size 
of up to 5 .mu.m) was employed instead of the carbon black of Example 5. 
Table 5 illustrates the relationship between the boron quantity and the 
properties of the sintered body. It was found that when the boron content 
was more than 0.1 wt.%, the thermal conductivity of the sintered body was 
remarkably reduced. 
TABLE 5 
______________________________________ 
B content 
Rela- Thermal con- Coefficient 
in sintered 
tive ductivity Electrical 
of thermal 
body density (cal/cm .multidot. 
resistivity 
expansion 
(wt. %) (%) sec .multidot. .degree.C.) 
(.OMEGA. cm) 
(.times. 10.sup.-6 /.degree.C.) 
______________________________________ 
0.1 99 0.68 10.sup.13 or more 
3.33 
0.5 99 0.30 10.sup.13 
3.34 
1.0 98 0.12 10.sup.12 
3.35 
______________________________________ 
Remarks: 
Thermal conductivity and electrical resistivity were measured at room 
temperature. 
Coefficient of thermal expansion is a mean value of room temperature to 
300.degree. C. 
EXAMPLE 8 
Sintering without pressure 
As the silicon carbide powder was used one that was synthesized in high 
frequency heat plasma. The powder was extremely minute powder having a 
grain size of 200 A to 0.2 .mu.m. BeO powder of an average grain size of 1 
.mu.m was added to the silicon carbide powder. Next, after the mixed 
powder was molded at a pressure of 1,000 Kg/cm.sup.2, the resulting molded 
article was sintered in vacuum of 1.times.10.sup.-4 Torr. Heating was made 
from room temperature to 2,100.degree. C. in the course of about 2 hours, 
held at 2,100.degree. C. for 0.5 hour, and was thereafter left cooling by 
cutting off the heating power source. The Be content in the sintered body 
was about 0.4 wt.%. Table 6 illustrates the properties of the resulting 
sintered body. The sintered body was rendered compact and had high thermal 
conductivity, high electrical resistance and low coefficient of thermal 
expansion. 
TABLE 6 
______________________________________ 
Relative density (%) 99 
Thermal conductivity (cal/cm .multidot. sec .multidot. .degree.C.) 
0.71 
Electrical resistivity (Ohm .multidot. cm) 
10.sup.13 or more 
Coefficient of thermal expansion 
3.33 
(.times. 10.sup.-6 /.degree.C.) 
______________________________________ 
Remarks: 
Thermal conductivity and electrical resistivity were measured at room 
temperature. 
Coefficient of thermal expansion is a mean value of room temperature to 
300.degree. C. 
EXAMPLE 9 
Application example 
As a definite example of the electrically insulating substrate in 
accordance with the present invention, a semiconductor power module was 
produced using the silicon carbide sintered body of 0.5 wt.% beryllium 
content obtained in Example 1. 
FIG. 5 shows the section of a semiconductor power module assembly of the 
prior art. An organic insulating material 15 is interposed for insulation 
between a Cu conductor 14 and a Cu heat sink 16 while an aluminum 
substrate 17 is interposed for insulation between the heat sink 16 and a 
Cu metal support 18. A spacer 13 is interposed in order to mitigate strain 
due to the difference of thermal expansion coefficients between a silicon 
element 11 and the Cu heat sink 16. Reference numeral 12 denotes an 
aluminum lead wire while reference numeral 19 denotes a solder. 
FIG. 6 is a sectional view of the module assembly using the insulating 
substrate in accordance with the present invention. The substrate 20 of 
the present invention has an extremely simple construction in which the 
substrate is directly brazed to the silicon element 11 via the solder 19. 
According to the construction shown in FIG. 6, it is possible to replace 
the spacer 13, the organic insulating material 15, the Cu heat sink 16, 
the alumina substrate 17 and the metal support 18, each shown in FIG. 5, 
by the single substrate 20 shown in FIG. 6. 
The abovementioned semiconductor device was held at -60.degree. C. for 30 
minutes, then at room temperature for 5 minutes and thereafter heated to 
125.degree. C. and held at that temperature for 30 minutes. When this heat 
cycle was applied 20 times to the semiconductor device of the prior art 
shown in FIG. 5, crack occurred on the alumina substrate and soldered 
positions peeled off. When the same heat cycle was applied 150 times to 
the semiconductor device of the present invention shown in FIG. 6, no 
abnormality was observed. 
COMATIVE EXAMPLE 1 
A sintered body was hot-pressed in the same way as in Example 1 except that 
no additive was added. The properties of the resulting sintered body were 
shown in Table 7. Since the sintered body was not rendered compact, all of 
the thermal conductivity, electrical resistivity and mechanical strength 
were low. 
TABLE 7 
______________________________________ 
Relative density (%) 56 
Thermal conductivity (cal/cm .multidot. sec .multidot. .degree.C.) 
0.04 
Electrical resistivity (Ohm .multidot. cm) 
10.sup.4 
Coefficient of thermal expansion 
3.66 
(.times. 10.sup.-6 /.degree.C.) 
______________________________________ 
Remarks: 
Thermal conductivity and electrical resistivity were measured at room 
temperature. 
Coefficient of thermal expansion is a mean value of room temperature to 
300.degree. C. 
COMATIVE EXAMPLE 2 
Two percents by weight (2 wt.%) of aluminum oxide was added as an additive 
to the silicon carbide powder. The mixed powder was molded and hot-pressed 
in the same way as in Example 1, yielding a sintered body. The properties 
of the sintered body were shown in Table 8. Though the mechanical strength 
was high, the thermal conductivity as well as electrical resistivity were 
low. Similar properties were obtained when aluminum carbide, aluminum 
nitride and aluminum phosphate were respectively added as the additive to 
the silicon carbide powder. 
TABLE 8 
______________________________________ 
Relative density (%) 99 
Thermal conductivity (cal/cm .multidot. sec .multidot. .degree.C.) 
0.18 
Electrical resistivity (Ohm .multidot. cm) 
10 
Coefficient of thermal expansion 
3.38 
(.times. 10.sup.-6 /.degree.C.) 
______________________________________ 
Remarks: 
Thermal conductivity and electrical resistance were measured at room 
temperature. 
Coefficient of thermal expansion is a mean value of room temperature to 
300.degree. C. 
The silicon carbide sintered body in accordance with the present invention 
is characterized by its high compactness, high thermal conductivity, high 
electrical resistivity and low coefficient of thermal expansion. 
Accordingly, the sintered body of the invention is excellent as the 
afore-mentioned electrically insulating substrate. Further, it can be used 
suitably as a member for which heat resistance and oxidation resistance 
are requisites or as a member for which high strength at high temperature 
is required.