Note: Descriptions are shown in the official language in which they were submitted.
rJ-wp-36~3 - ~lacker Chemie
ELECTRIC INS~LATING POLYCRYST~LLIN~ SILIC~l CA~ID~
AND PROCESS FOR TIIE P~EP~RATION TMEREOF ~Y ISOSTATIC
HOT P~F~SSING
Polycrystalline silicon carbide has been known or a long
time~ It has a combination of valuable properties such as high
strength, resistance to oxidation, resistance to thermal shock, lo~
thermal expansion and high heat conductivity and is useful in many
technological fields.
BACKGROUND OF THE INVENTION
Pure silicon carbide, because of its predominantly covalent
bonding, is difficult to sinter. Dense bodies of polycrystalline
silicon carbide can only be produced by known processes such as hot
pressing and pressureless sintering using sintering aids. One of the
oldest sintering aids for silicon carbide is alumina, which, during
the sintering, combines with the impurities in the silicon carbide to
Eorm a ~luid phase which promotes compaction and which is detectable
as a separate phase in the finished sintered body.
In pressureless sintering, aluminum oxide promotes compaction
of the SiC by combining with impurities in the SiC. Additional
adjuvants such as aluminum, silicon and/or carbon compounds are known
to be formed by interaction with the surrounding atmosphere during
the sintering.
Silicon carbide can be densiEied by isostatic hot pressing to
densities oE almost 100% of the theoretical density (hereinaEter
abbreviated as % TD), without the use of sinterlng aids when a very
pure SiC powder with a total content oE metallic impurities oE at
most 0.1~ by weight is used. Compared to the usual hot pressed or
pressureless sintered materials, the polycrystalline SiC sintered
bodies have, as a result of their high purity, an improved heat
--1--
~1
~25~
conductivity of 220 W/mK at room temperature. ~lowever, pure SiC has
semiconductor properties, and dense sintered articles otained by
isostatic hot pressing pure silicon carbide have a specific electric
resistance of only about 100 Ohm. cm.
To be useful as a substrate material ~or microcircuitry and
structural parts of power electronics, the specific electric
resistance must be at least 107 Ohm. cm.
Tests have been conducted to increase the specific electric
resistance of SiC by addition of adjuvants. Beryllium oxide has
proved especially reuseful since it increases the electric resistance
and acts as a sintering adjuvant in hot pressing or in pressureless
sintering. Electrically insulating substrate materials o~ SiC
containing beryllium oxide and additionally containing up to 0.1% by
weight aluminum, up to 0.1~ by weight boron and up to 0.4% by weight
free carbon, with a heat conductivity of at least 167 W/m.K, are
known. It has been demonstrated, by comparison tests, that using
aluminum oxide instead of beryllium oxide, under otherwise equivalent
conditions (HP at 2000C and 30 MPa), there was produced sintered
bodies having 99% TD and high mechanical strength but a considerablY
lower thermal conductivity (75 W/m.K) and a specific electric
resistance of only 10 Ohm.cm Poor results were also obtained by
using aluminum carbide, aluminum nitride and aluminum phosphate
instead of beryllium oxide.
Instead of beryllium oxide per se, it is also possible to use
beryllium compounds or boron compounds such as boron nitride and
other adjuvants to improve the sinterability oE SiC. It has been
demonstrated, by comparison examples ~HP at 2000C and 27 MPa) that,
when using aluminum oxide and beryllium oxide (1% by weight A12O3 +
1% by weight BeO) to sinter silicon carbide, a sintered body with a
thermal conductivity of only 8~ W/mk was produced when using MgO
alone (2% by weight), the sintered body was not dense (55% 't'D).
~56~5~
By lowering the nitrogen content to not more than 500 ppm in
the powder mixture of SiC + berylliwn cotnpounds beiny sintered, no
decrease in the ~esired properties appeared even in large bodies.
Since beryllium compounds are toxic, attempts were made to
use aluminum nitride alone (equal to or ~ 10~ by weight) or mixtures
of aluminum nitride and boron nitride (5 - 15% by weight). Although
sintered SiC bodies with adequate insulating properties were
obtained, the thermal conductivity values were less than 100W/m~.
Sintered bodies of SiC and AlN with additions of calcium,
barium or strontium oxide (0.1 to 3~ by weight) had only a slightly
improved thermal conductivity.
The course of development of electronics in the last year has
led to a reduction in size of the structural parts that is, the
development of megabit chips. The requirements of substrate
materials have thus increased. The substrate materials must have not
only a high specific electric resistance but also a high thermal
eonductivity whieh, in the case of SiC, had been obtained only by use
of beryllium eompounds as sintering aids. Compare~ to other
insulating materials such as aluminum oxide considered for this
purpose, silicon carbide has the advantage of a high mechanical
strength (bending strength of at least 500 N/mm2 at room temperature)
and a thermal expansion coefficient similar to that of silicon
(approximately 3.3 x 10 6/oC).
The problem is to develop a process to produce electrical
insulating substrate materials of dense polycrystalline silicon
carbide which meet these requirements without use of highly toxic
compounds.
BRIEF SUMM~RY OF THE INVENTION
_
Aecording to the invention, electrical insulating substrate
materials of polycrystalline silicon carbide having a density o~ at
least 99.8% TD calculated on the theoretically possible density oE
pure SiC, are provided which consist essentially o~
at least 95.0~ by weight sil:icon carbide
0.25 - 3.5% by weight aluminuM oxide and/or
magnesium oxide,
up to 0.3% by weight free carbon and
up to 0.03% by weight impurities oF elements o~ groups 3a and 5a
oE the Periodic System (predominantly Al + B ~ N) in total, in which
the silicon carbide is essentially in the form of a homogeneous
isotropic microstructure with grain sizes of 5 um maximum, aluminum
oxide and/or magnesium oxide are present predominantly in the grain
boundary of the silicon carbide and are detectable as separate
phase(s). The silicon carbide substrate materials o~ the present
invention have the following properties:
thermal conductivity at 300 K of at least 170 W/mK
specific electric resistance at 300 K of at least 10 Ohm.cm
a coefficient of thermal expansion up to 3.5 x l~ S/K in the range of
from 20C to 300C and a dielectric strength of more than 20 kV~mmO
DETAILED DESCRIPTION OF THE INVENTION
The substrate materials according to the invention are
prepared by sintering homogeneous powder mixtures of silicon carbide
having a purity of at least 99.97% by weight based on total
impurities of elements of groups 3a and 5a of the Periodic System
(predominantly Al + ~ + N) and 0.25 to 3.5~ hy weight aluminum oxide
and/or magnesium oxide in a hermetically sealed casing by isostatic
hot pressing at a temperature of 1700 to 2200C and a pressure of
100 to ~00 MPa in a high pressure autoclave using an inert gas as a
pressure-transmitting medium.
Since the sintering is accomplished in a hermetically sealed
casing, nothing can escape during the isostatic hot pres.sing opera-
~25~6~5~1
tion, the substrate materials of the invention have a density of atleast 99.g~ TD, preferably 100% TD, and the same chemical composition
as the starting powder mixt~lre. It i,s critical that pure sta~ting
powders be used so that the fin;,shed substrate material does not
contain more than the critical amounts of up to 0.?% by weight free
carbon and up to 0.03~ by weight of the total of elements oE groups
3a and 5a of the Periodic System which are understood to be predom-
inantly Al -~ B + N.
The substrate materials of the invention consist predomin-
antly of polycrystalline silicon carbide which has an isotropic
microstructure in which the SiC grain having a maximum grain size oE
5 ~m are homogeneously distributed independently o~ direction while
aluminum oxide and/or magnesium oxide are predominantly present at
the grain boudaries of the SiC and can be detected as a separate
phase or phases by X-ray diffraction analysis or ceramographically.
The substrate materials according to the invention are
preferably prepared from fine powders of alpha- or beta-SiC or
mixtures of alpha- and beta-SiC with a particle size not larger than
5Jum corresponding to a specific surface of 4 to ~0 m /g, preferably
of 5 - 10 m /g (measured according to the BET method), and a purity
of at least 99.97% by weight based on the total amount of impurities
due to elements of groups 3a and 5a of the Periodic System. These
impurities are to be particularly understood to re~er to the elements
Al, B and N, which altogether must not exceed 0.03% by weight of the
SiC starting powder. The content of Al + B -~ N in the SiC powders
preferably is less than 0.025% by weight. The best results are
obtained with a content of Al + B + N of less than 0.0095% by weight.
It has been demonstrated that when a sintered body is Eormed
from a SiC powder having a content of Al + B -~ N of about 0.0O% by
weight, the specific electric resistance of the finished substrate
material is lowered to 105 Ohm.cm. Adherent carbon which can be
~5~
present in the SiC powders is preEerably not yreater than 0.3% by
weight and most preEerably not yreater than 0.2~ by weight. ~lowever,
the small amount of oxygen present in the 5iC powder, which is
generally predominantly in the form of adherent Si~2 is eormed as a
result of oxidation of the SiC during the comrninuting operation, does
not substantially degrade the electric resistance and thermal
conductivity of the sintered body and therefore can be tolerated to a
maximum of 0.6% by weight, but the oxygen content should preferably
be less than 0.5~ by weight.
The sintering aids useful in the practice of the present
invention are oxygen-containing aluminwn or magnesium compounds or
mixtures thereof wich are either in the forrn o~ the oxide or mixed
oxides such as ~12O3, MgO and spinel or can form oxides in situ like
compounds such as MgCO3. The sintering aids must also be very pure
and should have substantially the same particle size as tHe SiC
poweders used. These sintering aids are mixed homogeneously with the
SiC powders in the defined amounts of 0.25 to 3.5 preferably 1 to 3
by weight, calculated as oxide or oxides. The homogeneous powder
mixture are then compacted by molding to form preformed green bodies.
To facilitate forming the green bodies, the starting powder mixture
can be mixed together with a temporary binder or be dispersed in a
solution of the temporary binder in an organic solvent. Sinterable
organic solvents are non~reactive relatively low boiling materials
such as acetone or lower aliphatic alcohols having 1 to about 6 C
torns. Examples of suitable temporary binders are polyvinyl alcohol,
stearic acid, polyethylene glycol, and camphor which can be used in
amounts oE up to about 5% by weight, preferably up to about 3% hy
weight, based on the total weight of the powder mixture. Use o-E a
~em~orary binder is not required.
The green bodies can be ~ormed by known molding processes,
OJ': example, by forging press, isostatic press, injection molding,
extrusion, slip casting or sheet casting at room temperature or at
elevated temperature. After molding, the green bodies must have a
theoretical density of at least 50%, preEerably at least 60% T~. The
green bodies are porous structures with open porosity. Open porosity
is understood to mean that the green bodies have pores or canals open
at the surface. The green bodies are then preferably subjected to a
thermal treatment by heating to 300 to 1200C before being provided
with a gastight casing to ensure that during the hot isostatic
compression no decomposition products Erom the binders or sintering
aids hinder the densification operation or damage the casing.
The substrate materials according to the invention are
prepared by isostatic hot pressing of the preEormed green bodies, oE
the homogeneous startincJ powder mixture, in a hermetically sealed
casing, at a temperature of from 1700 to 2200C and a pressure of
from 100 to 400 M Pa in a high-pressure autoclave using an inert gas
as the pressure-transmitting medium. For carrying out this process,
the preformed green bodies must be provided with a gas tight casing
prior to being exposed to the high-pressure in the autoclave in order
to prevent the gas used as pressure-transmitting medium from pene-
trating into the green bodies and thereby hindering the compression.
The materials for said casings, which are hermetically
sealed, must neither melt nor react with the green bodies at the
pressing temperature (1700-2200C). They must be inert with respect
to said green bodies. The casing must be suEEiciently plastic at the
pressing temperature used, to adapt to the shape oE the body without
tearing , and ensure that the gas pressure is uniEormly transmitted
through the casing to the bodies. Examples oE useful casing
materials that meet said requirements, are high-melting glasses such
as pure quartz glass or high-melting ceramic materials. These
~naterials can be used in the form oE preEabricated casings or
capsules in which the green bodies are introduced. The casings
~56~
together with the contents are then evacuated and herrnetically
sealed. The casings can also be produced on the green bodies
directly by coating, ~or instance, by applyiny a glass or cerarnic-
like composition which is then melted or sintered under vacuum to
form a gas tight casing. The expression "hermetically sealed casing"
is to be understood to refer to a cas;ng that i5 impervious to the
pressure gas acting from outside an~ that does not contain in the
casing, any substantial amount of residual gases that hinder the
compression operation.
The green bodies provided with hermetically sealed casing are
preferably placed in graphite containers and are then introduced into
the high-pressure autoclaves and heated to the required compression
temperature of at least 1700C. It is preferable to separately
regulate pressure and temperature, that is, to raise the gas pressure
only when the casing material reaches a temperature at which it can
be deformed plastically. Preferably, argon or nitrogen is used as
the inert gas for the transmission of pressure. The gas pressure
applied to the casing is preferably in the range of from 150 to 250
MPa, which is reached by a slow increase of pressure at the ~inal
temperature which is preferably in the range of 1800~ to 1950C. The
optimum temperature used in each case depends on the fineness and
purity of the SiC starting powder and should not be exceeded, since
the danger exists that the substrate materials formed are practically
pore free but have a "secondary re-crystallized microstructure" which
is no longer homogeneous since some grains have become thicker than
the rest. The "presence of a secondary recrystallized microstruc-
ture" adversely affects the thermal conductivity.
After the pressure and temperature are lowered, the cooled
substrate materials are removed from the high-pressure autoclave and
removed from the casings, for instance, by sandblasting the glass or
ceramic casings.
~2~ 9
The substrate materlals produce~ are practically free o~
pores and have a density of at least ~9.~%; and are also practicall~
texture-free as result oE the all-round application of pressure. ~rhe
substrate materials have an isotropic microstructure so that their
properties are not dependent on direction but are substantially the
same in all directions.
Since the isostatic hot pressing takes p]ace at a relatively
low temperature that i5, at a temperature which is generally about
100C lower than the temperature used in a conventional hot pressing
process, only a slight grain growth occurs and the aluminum oxide or
magnesium oxide sintering aids remain predominantly at the grain
boundaries of the SiC. The fine-grain microstructure in the finished
substrate material corresponds substantially to the grain distribu-
tion in the starting powder mixture. ~ue to the high purity of the
starting powder, the inclusion in the SiC grain of impurity atoms
that increase the electric conductivity (Al, B and N) is accordingly
low, the substrate materials of the invention have excellent electric
insulating properties tha~ expressed as speciEic electric resistance
at 300 K reach values of up to 1013 Ohm.cm and thermal conductivity
values at 300 K of up to 260 W/mK.
As a result of the high density and the fine-grain isotropic
microstructure, the substrate materials of the invention have a high
mechanical strength which, expressed as bending strength measured
according to the ~-point method, at room temperature, reaches values
of at least 500 N/mm and thermal expansion coefficients of from 3.
to 3.4 x 10 6K in the range of from room temperature to 300C.
That the combination of properties namely, high density, high
electric insulating capacity and high thermal conductivity, which
characterize the substrate material of the invention, can be obtained
by adding aluminum oxide and/or magnesiurn oxide to high-purity SiC
starting powders and forming the substrate by isostatic hot pressing,
5~5~
must be regarded as ~nexpected in view of the prior art. The prior
art teaches that less pure SiC starting powders could be densiEied by
conventional hot pressing by addition of alurninurn oxide alone but the
materials produced had a relatively hiyh electrical conductivity and
a low thermal conductivity; addition of magnesium oxide alone did not
provide a powder which could be densified.
In the examples that follow the object of the invention is
illustrated in detail.
Examples 1 to 9 (according to the invention):
The starting material was an alpha-SiC powder having a
,specific surface area (measured according to sET) of 12.5 m2/g and
the following analysis:
~ we~
Ctotal 29.97
Cfree 0.18
O 0.3
Al 0 0O~O
B 0.0017
N 0.0035
Fe
Ca 0.0020
ifree not determinable
This SiC powder was homogeneously mixed with Einely dispersed
aluminum oxide powder and/or magnesium carbonate powder in the
indicated amounts by stirring in acetone. Shortly before terminating
the mixing operation, 2.0% by weight camphor was incorporated as a
temporary binder. The solvent was removed and the powder mixture
pressed into cylindrical green bodies. The green bodies were
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~2~5~
introduced into preformed quartz glass casings. The casings together
with the green bodies were then heated under vacuum to remove the
temporary binder and the casings were hermetically sealed by melting.
The encased samples were isostatica]ly hot pressed in a high-pressure
autoclave under an argon pressure of 200 MPa at 1950C for 30
minutes. After decompression and cooling of the sintere~ bodies to
room temperature, the sintered bodies were rernoved from the HIP
equipment and the glass casings removed by dismantling and sand-
blasting The sintered bodies produced had a density of more than
99.8% TD. Test bodies were produced Erom the sintered bodies and
surface grained to determine the heat conductivity, the specific
electric resistance and the bending strength.
The thermal conductivity was determined according to the
comparison bar method up to 927C using Armco Iron as reference
material. The specific electric resistance was determined at room
temperature (25C) with direct current according to the 3-point
measuring method. The bending strength was measured according to the
4-point method with abutment spaces of 15 mm (top) and 30 mm (bottom)
at room temperature. The thermal expansion coefficent was
determined.
The following Table 1 sets forth the kind and amount o-E the
sinterings aids, respectively calculated as oxide, and the
meaSurement of the properties of the sintered bodies.
- ~ 2S6~5~
,, ., _
T~BL--L' 1
Example ~dit.ives in ~ by wt. ~ b ~ ~
~o. A123 MgO Ln ~/mn2 in W/mK in Ol~.an in 10-~/K
1 0,3 _ 6~0 190 1ol2 3 3
~ 0.5 ~ 600 ~0 1013 3.3
3 1.0 - 530 2~0 1013 3.3
3.0 _ S00 205 loll 3 ~
- 0.5 ~00 180 lolO 3 3
6 _ 1.0 500 170 1ol2 3,~
7 - 3.0 ~70 170 109 3.~
8 0.3 0.3 550 190 1013 3.3
9 1.0 0.5 500 170 loll
b = bending strength
= thermal conduc~iYity
~ = specific electric resistiv.ity - -
o~ - coefficien~ O:e thermal exp~nsion
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~5~
Comparison E
The starting materials were an alpha-SiC powder having a high
content of impurities (Al ~ B -~ N) and a specific sur~ace area of
12 - 13m2/g together with 0O5% by weight aluminum oxide. Sintered
bodles were produced from the powder mixtures by
a) isostatic hot pressing (HIP) under the same conditions as given in
example 1 (temperature 1950C; pressure 200 M Pa; holding time 30
minutes) and
b) conventional hot pressing (HP) in graphite molds (temperature
2000C; pressure 30 m Pa; holding time 60 minutes).
The thermal conductivity and the specific electric resistance
of samples from the sintered bodies were determined. Table 2
contains a compilation of the contents of impurities (Al ~ B ~ N) in
the alpha-SiC starting powder and the measurements of the sintered
bodies produced therefrom according to a) and b).
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TABLE 2
_ _
Example No.
1~ 11 12 1
alpha-siC powder
by wt. Al 0.0110 0.0420 0.0830 0.00~8
B 0.0035 0.0065 0.0095 0.0350
" N 0.0250 0.0175 0.0235 0.0065
" Sa:
Al + B + N 0.0395 0 0660 0.1160 ~.0463
Molded bodies
prod. a)
IHP
~ in W/mK 155 165 145 98
el. resist. - - 107 10-5 5 x_l_6 1o6_
Molded bodies
prod. b)
IMP
~ in W/mK 105 95 90 85
el. resist.
in Ohm.cm 103 103 103 105
_ _ . . _
The measurements in Table 2 clearly show the eEfect o~ the
content of impurities (Al + s ~ N) in the SiC starting powder on the
heat conductivity and the specific electric resistance oE the
sintered bodies produced therefrom, which are degraded as the content
oE said impurities increases. The measurements also show the e~Eect
of the preparation process, since the sintered bodies produced Erom
SiC powder with the same content oE impurities by convention hot
pressing (HP) had lower heat conductivity and speci~ic electric
resistance than the sintered bodies produced by isostatic hot
pressing (HIP).
