U.S. patent application number 09/449764 was filed with the patent office on 2002-01-17 for sintered silicon carbide and method for cleaning the same in wet condition.
Invention is credited to OTSUKI, MASASHI, WADA, HIROAKI.
Application Number | 20020005213 09/449764 |
Document ID | / |
Family ID | 26578812 |
Filed Date | 2002-01-17 |
United States Patent
Application |
20020005213 |
Kind Code |
A1 |
OTSUKI, MASASHI ; et
al. |
January 17, 2002 |
SINTERED SILICON CARBIDE AND METHOD FOR CLEANING THE SAME IN WET
CONDITION
Abstract
A sintered silicon carbide has a high density and only small
amounts of organic and inorganic impurities on the surface and in
the vicinity of the surface, i.e., a density of 2.9 g/cm.sup.2 or
more and an amount of each impurity smaller than
1.0.times.10.sup.11 atoms/cm.sup.2 on the surface and in the
vicinity of the surface. A method for cleaning sintered silicon
carbide in a wet condition comprises treating sintered silicon
carbide in a step of dipping into a quasi-aqueous organic solvent,
a step of dipping into an aqueous solution of an ammonium compound,
a step of dipping into an aqueous solution of an inorganic acid and
a step of dipping into pure water. Organic and inorganic impurities
present on the surface and in the vicinity of the surface of the
sintered silicon carbide are removed easily in accordance with the
method.
Inventors: |
OTSUKI, MASASHI;
(MUSASHIMURAYAMA-SHI, JP) ; WADA, HIROAKI;
(KAWASAKI-SHI, JP) |
Correspondence
Address: |
OLIFF & BERRIDGE PLC
P.O. BOX 19928
ALEXANDRIA
VA
22320
|
Family ID: |
26578812 |
Appl. No.: |
09/449764 |
Filed: |
November 26, 1999 |
Current U.S.
Class: |
134/26 ; 134/1;
134/1.3; 134/2; 134/27; 134/28; 134/29; 134/3; 501/87; 501/88;
501/89; 501/90 |
Current CPC
Class: |
B08B 3/12 20130101; C04B
35/575 20130101; C04B 41/5315 20130101; C04B 2111/00844 20130101;
C04B 41/009 20130101; C04B 35/565 20130101; C04B 41/009 20130101;
C04B 41/91 20130101 |
Class at
Publication: |
134/26 ; 501/88;
501/89; 501/90; 501/87; 134/1; 134/1.3; 134/2; 134/3; 134/27;
134/28; 134/29 |
International
Class: |
B08B 007/04; B08B
003/12; C04B 035/56 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 8, 1998 |
JP |
10-348700 |
Dec 8, 1998 |
JP |
10-348701 |
Claims
What is claimed is that:
1. A sintered silicon carbide having a total amount of impurity
present on a surface and in a vicinity of the surface of the
sintered silicon carbide less than 1.0.times.10.sup.11
atoms/cm.sup.2 and a density greater than 2.9 g/cm.sup.2.
2. A sintered silicon carbide according to claim 1, wherein a total
content of impurity elements in the sintered silicon carbide is 10
ppm or less.
3. A sintered silicon carbide having a total amount of impurity
present on a surface and in a vicinity of the surface of the
sintered silicon carbide less than 1.0.times.10.sup.11
atoms/cm.sup.2 and a density greater than 2.9 g/cm.sup.2, formed by
a process comprising the steps of: sintering in which a mixture of
silicon carbide powder and a nonmetallic auxiliary sintering agent
is hot pressed at a temperature of 2,000 to 2,400.degree. C. and at
a pressure of 300 to 700 kg/cm.sup.2 in a nonoxidizing atmosphere;
and a step of cleaning in which the sintered silicon carbide
obtained after the step of sintering is cleaned in a wet
condition.
4. A sintered silicon carbide according to claim 1, wherein the
step of cleaning includes: dipping the sintered silicon carbide
into a quasi-aqueous organic solvent; and then dipping the sintered
silicon carbide into an aqueous solution of an inorganic acid; and
thereafter dipping the sintered silicon carbide into pure
water.
5. A sintered silicon carbide according to claim 1, wherein the
step of cleaning includes: dipping the sintered silicon carbide
into a quasi-aqueous aqueous organic solvent; and then dipping the
sintered silicon carbide into an aqueous solution of an ammonium
compound; followed by dipping the sintered silicon carbide into an
aqueous solution of an inorganic acid; and thereafter dipping the
sintered silicon carbide into pure water, successively.
6. A sintered silicon carbide according to claim 1, wherein the
step of cleaning includes: dipping the sintered silicon carbide
into a quasi-aqueous organic solvent; and then dipping the sintered
silicon carbide into an aqueous solution of an inorganic acid;
followed by dipping the sintered silicon carbide into an aqueous
solution of an ammonium compound; and thereafter dipping the
sintered silicon carbide into pure water.
7. A method for cleaning a sintered silicon carbide in a wet
condition, comprising the steps of: dipping the sintered silicon
carbide into a quasi-aqueous organic solvent; and then dipping the
sintered silicon carbide into an aqueous solution of an inorganic
acid; and thereafter dipping the sintered silicon carbide into pure
water.
8. A method for cleaning a sintered silicon carbide in a wet
condition, comprising the steps of: dipping the sintered silicon
carbide into a quasi-aqueous aqueous organic solvent; and then
dipping into an aqueous solution of an ammonium compound; followed
by dipping the sintered silicon carbide into an aqueous solution of
an inorganic acid; and thereafter dipping into pure water.
9. A method for cleaning a sintered silicon carbide in a wet
condition, comprising the steps of: dipping the sintered silicon
carbide into a quasi-aqueous organic solvent; and then dipping the
sintered silicon carbide into an aqueous solution of an inorganic
acid; followed by dipping the sintered silicon carbide into an
aqueous solution of an ammonium compound; and thereafter dipping
the sintered silicon carbide into pure water.
10. A method for cleaning a sintered silicon carbide in a wet
condition according to claim 7, wherein at least one of the steps
is conducted while ultrasonic vibration is applied to a liquid.
11. A method for cleaning a sintered silicon carbide in a wet
condition according to claim 8, wherein at least one of the steps
is conducted while ultrasonic vibration is applied to a liquid.
12. A method for cleaning a sintered silicon carbide in a wet
condition according to claim 9, wherein at least one of the steps
is conducted while ultrasonic vibration is applied to a liquid.
13. A method for cleaning a sintered silicon carbide in a wet
condition according to claim 7, wherein a temperature of a liquid
in at least one of the steps is 30.degree. C. or more.
14. A method for cleaning a sintered silicon carbide in a wet
condition according to claim 8, wherein a temperature of a liquid
in at least one of the steps is 30.degree. C. or more.
15. A method for cleaning a sintered silicon carbide in a wet
condition according to claim 9, wherein a temperature of a liquid
in at least one of the steps is 30.degree. C. or more.
16. A method for cleaning a sintered silicon carbide in a wet
condition according to claim 7, wherein the quasi-aqueous organic
solvent is selected from the group consisting of: petroleum
hydrocarbons, esters of an organic acid, glycol ethers, a mixed
solvent of these solvents, a mixture of a surfactant and the
solvent and a mixture of the mixed solvent and a surfactant.
17. A method for cleaning a sintered silicon carbide in a wet
condition according to claim 8, wherein the quasi-aqueous organic
solvent is selected from the group consisting of: petroleum
hydrocarbons, esters of an organic acid, glycol ethers, a mixed
solvent of these solvents, a mixture of a surfactant and the
solvent and a mixture of the mixed solvent and a surfactant.
18. A method for cleaning a sintered silicon carbide in a wet
condition according to claim 9, wherein the quasi-aqueous organic
solvent is selected from the group consisting of: petroleum
hydrocarbons, esters of an organic acid, glycol ethers, a mixed
solvent of these solvents, a mixture of a surfactant and the
solvent and a mixture of the mixed solvent and a surfactant.
19. A method for cleaning a sintered silicon carbide in a wet
condition according to claim 7, wherein the aqueous solution of an
inorganic acid is selected from the group consisting of: an aqueous
solution of hydrofluoric acid, an aqueous solution of nitric acid,
an aqueous solution of sulfuric acid, an aqueous solution of
hydrochloric acid, an aqueous solution of hydrogen peroxide, an
aqueous solution of ozone or an aqueous solution of a mixture
thereof.
20. A method for cleaning a sintered silicon carbide in a wet
condition according to claim 8, wherein the aqueous solution of an
inorganic acid is selected from the group consisting of: an aqueous
solution of hydrofluoric acid, an aqueous solution of nitric acid,
an aqueous solution of sulfuric acid, an aqueous solution of
hydrochloric acid, an aqueous solution of hydrogen peroxide, an
aqueous solution of ozone or an aqueous solution of a mixture
thereof.
21. A method for cleaning a sintered silicon carbide in a wet
condition according to claim 9, wherein the aqueous solution of an
inorganic acid is selected from the group consisting of: an aqueous
solution of hydrofluoric acid, an aqueous solution of nitric acid,
an aqueous solution of sulfuric acid, an aqueous solution of
hydrochloric acid, an aqueous solution of hydrogen peroxide, an
aqueous solution of ozone or an aqueous solution of a mixture
thereof.
22. A method for cleaning a sintered silicon carbide in a wet
condition according to claim 8, wherein the aqueous solution of an
ammonium compound is selected from the group consisting of: an
aqueous solution of a tetraalkylammonium halide, an aqueous
solution of a tetraalkylammonium perchlorate, aqueous ammonia and a
mixture of the aqueous solution with an aqueous solution of
hydrogen peroxide.
23. A method for cleaning a sintered silicon carbide in a wet
condition according to claim 9, wherein the aqueous solution of an
ammonium compound is selected from the group consisting of: an
aqueous solution of a tetraalkylammonium halide, an aqueous
solution of a tetraalkylammonium perchlorate, aqueous ammonia and a
mixture of the aqueous solution with an aqueous solution of
hydrogen peroxide.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a sintered silicon carbide
which can be applied to various parts of semiconductors and to
electronic parts. More particularly, the present invention relates
to a sintered silicon carbide which has a high purity, a high
density and to whose surface and vicinity thereof only small
amounts of organic contaminants, metal element contaminants and
particle contaminants attach, and which can be used for dummy
wafers, targets, heating elements and the like. The present
invention also relates to a method for removing organic
contaminants, metal element contaminants and particle contaminants
from a sintered silicon carbide which is used for dummy wafers,
targets, heating elements, which is parts of semiconductors or
electronic parts, and which is required to have a high density and
a high purity.
[0003] 2. Description of the Related Art
[0004] Silicon carbide, particularly sintered silicon carbide, is a
strongly covalently bonded compound and has heretofore been used in
various fields by taking advantage of excellent properties such as
excellent strength at high temperatures, heat resistance, wear
resistance, chemical resistance and the like. These advantageous
properties have attracted attention, and recently expectations have
been placed on application in the fields of electronics,
information and semiconductors.
[0005] As the degree of integration in semiconductor integrated
circuits using silicon substrate increases and the line width of
the circuits becomes smaller, various parts of semiconductors and
electronic parts used in those fields are required to have a higher
purity and a higher density. Therefore, methods of hot press
sintering using nonmetallic auxiliary sintering agents and methods
of reaction sintering have been intensively researched. However,
the surface and the vicinity of the surface of sintered silicon
carbide obtained in accordance with these sintering methods become
contaminated during processes before, during and after the
sintering such as sintering, working and handling, although the
sintered silicon carbide has a high purity and a high density.
[0006] Therefore, to apply a sintered silicon carbide to various
parts of semiconductors and electronic parts, i.e., to prevent
contaminations of surfaces including particle, it is essential that
the surface be cleaned and a purity of the surface as high as that
of silicon wafers be achieved. However, no sintered silicon carbide
having a satisfactory purity on the surface have actually been
obtained.
[0007] Disclosed methods of cleaning a sintered silicon carbide are
as follows: (1) in a method disclosed in Registered Japanese Patent
No. 181841, a sintered silicon carbide is cleaned with an acid,
treated by oxidation at 1,200.degree. C. or higher and is
thereafter surface-treated in an atmosphere of nitrogen; (2) in a
method disclosed in Japanese Patent Application Laid-Open (JP-A)
No. 5-17229, a sintered silicon carbide is cleaned by blasting with
a silica abrasive grain and is then cleaned in a wet condition with
a mixed acid containing hydrofluoric acid and nitric acid; (3) in a
method disclosed in JP-A No. 6-77310, a sintered silicon carbide is
cleaned by dipping into an aqueous solution of hydrofluoric acid,
rinsed with ultrapure water, cleaned with oxygen and a halogen gas
in a dry condition and is then treated with oxygen; and (4) in
methods disclosed in JP-A Nos. 55-158622, 60-138913 and 64-72964,
porous silicon carbide is cleaned with a gas of a hydrogen halide
and an inorganic acid to increase the purity and then the purified
silicon carbide is subjected to secondary sintering because
purification to a high degree is very difficult after the sintering
has been conducted.
[0008] The above methods have a drawback in that additional
treatments such as oxidation, blast cleaning and secondary
sintering is required in addition to simple cleaning in the wet
condition and therefore the processes become complicated. These
methods cannot be considered to be satisfactory as cleaning
methods.
SUMMARY OF THE INVENTION
[0009] The present invention is based on the above circumstances
and an object of the present invention is to provide a sintered
silicon carbide having a high density and containing only small
amounts of organic and inorganic impurities on the surface and in
the vicinity of the surface.
[0010] Another object of the present invention is to provide a
method for easily cleaning a sintered silicon carbide in a wet
condition to remove organic and inorganic impurities present on the
surface and in the vicinity of the surface of the sintered silicon
carbide so that the sintered silicon carbide can be applied to
various parts of semiconductors and electronic parts.
[0011] In the intensive research carried out by the present
inventors to achieve the above objects, the present inventors
focused their attention on the fact that, even when a sintered
silicon carbide having a high density and a high purity which can
be applied to various parts of semiconductors and electronic parts
is obtained, the concentrations of organic and inorganic impurities
on the surface and in oxide layers increase by contamination of the
obtained sintered silicon carbide in succeeding steps, and the
present invention was achieved thereby.
[0012] In the first aspect of the present invention, a sintered
silicon carbide having a total amount of impurity present on a
surface and in a vicinity of the surface of the sintered silicon
carbide less than 1.0.times.10.sup.11 atoms/cm.sup.2 and a density
greater than 2.9 g/cm.sup.2, is provided.
[0013] In a second aspect of the present invention, a sintered
silicon carbide having a total amount of impurity present on a
surface and in a vicinity of the surface of the sintered silicon
carbide less than 1.0.times.10.sup.11 atoms/cm.sup.2 and a density
greater than 2.9 g/cm.sup.2, formed by a process comprising the
steps of: sintering in which a mixture of silicon carbide powder
and a nonmetallic auxiliary sintering agent is hot pressed at a
temperature of 2,000 to 2,400.degree. C. and at a pressure of 300
to 700 kg/cm.sup.2 in a nonoxidizing atmosphere; and a step of
cleaning in which the sintered silicon carbide obtained after the
step of sintering is cleaned in a wet condition, is provided.
[0014] In a third aspect of the present invention, a method for
cleaning a sintered silicon carbide in a wet condition, comprising
the steps of: dipping the sintered silicon carbide into a
quasi-aqueous organic solvent; and then dipping the sintered
silicon carbide into an aqueous solution of an inorganic acid; and
thereafter dipping the sintered silicon carbide into pure water, is
provided.
[0015] In a fourth aspect of the present invention, a method for
cleaning a sintered silicon carbide in a wet condition, comprising
the steps of: dipping the sintered silicon carbide into a
quasi-aqueous aqueous organic solvent; and then dipping into an
aqueous solution of an ammonium compound; followed by dipping the
sintered silicon carbide into an aqueous solution of an inorganic
acid; and thereafter dipping into pure water, is provided.
[0016] In a fifth aspect of the present invention, a method for
cleaning a sintered silicon carbide in a wet condition, comprising
the steps of: dipping the sintered silicon carbide into a
quasi-aqueous organic solvent; and then dipping the sintered
silicon carbide into an aqueous solution of an inorganic acid;
followed by dipping the sintered silicon carbide into an aqueous
solution of an ammonium compound; and thereafter dipping the
sintered silicon carbide into pure water, is provided.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0017] In a sintered silicon carbide of the present invention, a
total amount of an impurity present on a surface and in the
vicinity of the surface (occasionally referred to as the surface
cleanliness, hereinafter) is less than 1.0.times.10.sup.11
atoms/cm.sup.2, preferably less than 5.0.times.10.sup.10
atoms/cm.sup.2 and more preferably less than 1.0.times.10.sup.10
atoms/cm.sup.2. When the amount of an impurity exceeds
1.0.times.10.sup.11 atoms/cm.sup.2, contamination such as
contamination, particle and the like take place due to impurities
present on the surface and in the vicinity of the surface. In the
present invention, "the amount of an impurity" present on the
surface and in the vicinity of the surface means the amount of each
impurity element. "An impurity" means substantially any element
other than Si, C, O, N, halogens and rare gases. "The surface and
the vicinity of the surface" means the outermost surface of a
substrate of a sintered silicon carbide and a spontaneous oxide
layer formed on the outermost surface. The spontaneous oxide layer
formed on the outermost surface is a layer of SiO.sub.2 and
generally has a thickness of about 40 to 60 nm (TAKEO SASAKI, Yogyo
Kyokaishi, Volume 95, page 84 (1987)).
[0018] The amount of an impurity present on the surface and in the
vicinity of the surface described above is measured in accordance
with the analysis using an ICP-MS (Inductively Coupled Plasma Mass
Spectrometer) or a TXRF (Total Reflection X-ray Fluorescencemeter).
The analyses using an ICP-MS and a TXRF give approximately the same
results.
[0019] The sintered silicon carbide of the present invention has a
density of 2.9 g/cm.sup.3 or more and preferably 3.0 g/cm.sup.3 or
more. When the density is less than 2.9 g/cm.sup.3, mechanical
properties, such as bending strength and breaking strength, and
electrical properties deteriorate and particle contamination is
caused since the amount of particles increases.
[0020] In the sintered silicon carbide of the present invention,
the total content of impurities is preferably 10 ppm or less and
more preferably 5 ppm or less. When the total content of impurities
exceeds 10 ppm, impurities tend to diffuse during use at a high
temperature and there is the possibility of the surface of
semiconductive silicon becoming contaminated to cause invention
failure due to leaking at the p-n junction, for example. In the
present invention, the total content of impurities in the sintered
silicon carbide does not include the amounts of impurities on the
surface and in the vicinity of the surface.
[0021] The sintered silicon carbide of the present invention and
the method for producing the sintered silicon carbide of the
present invention will be described in the following.
[0022] The sintered silicon carbide of the present invention may be
obtained in accordance with a method comprising a step of producing
a sintered silicon carbide and a step of cleaning the sintered
silicon carbide in a wet condition.
[0023] The step of producing a sintered silicon carbide will be
described in more detail hereinafter.
[0024] The step of producing a sintered silicon carbide comprises a
step of sintering in which a mixture of silicon carbide powder and
a nonmetallic auxiliary sintering agent is hot pressed at a
temperature of 2,000 to 2,400.degree. C. at a pressure of 300 to
700 kg/cm.sup.2 in a nonoxidizing atmosphere (occasionally referred
to as a step of sintering, hereinafter).
[0025] It is preferable that the silicon carbide powder is obtained
in a step in which a silicon source containing one or more types of
liquid silicon compound, a carbon source containing one or more
types of liquid organic compound and a polymerization catalyst or a
crosslinking catalyst are uniformly mixed together and the obtained
solid material is baked in a nonoxidizing atmosphere to produce
silicon carbide powder (occasionally referred to as a step of
preparation of silicon carbide powder, hereinafter).
[0026] The sintered silicon carbide of the present invention may
contain nitrogen. Nitrogen can be introduced into the sintered
silicon carbide in accordance with a method in which one or more
types of nitrogen source (occasionally referred to as a nitrogen
source, hereinafter) are added in combination with a carbon source
and a silicon source in the step of preparation of silicon carbide
powder described above or in accordance with a method in which the
nitrogen source is added in combination with the nonmetallic
auxiliary sintering agent in the step of sintering.
[0027] As a substance used as the nitrogen source, substances
generating nitrogen in the presence of heat are preferable.
Examples of such substances include polyimide resins, precursors of
polyimide resins and various types of amines such as
hexamethylenetetramine, ammonia and triethylamine.
[0028] When the nitrogen source is added in combination with the
silicon source in the step of preparation of the silicon carbide
powder described above, the amount of the nitrogen source is 80 to
1,000 .mu.g per 1 g of the carbon source. When the nitrogen source
is added in combination with the nonmetallic auxiliary sintering
agent in the step of sintering described above, the amount of the
nitrogen source is 200 to 2,000 .mu.g and preferably 1,500 to 2,000
.mu.g per 1 g of the nonmetallic auxiliary sintering agent.
[0029] The silicon carbide powder and the step of preparation of
the silicon carbide powder will be described in the following.
[0030] The silicon carbide powder described above may be a powder
of (.alpha.-type silicon carbide, .beta.-type silicon carbide,
amorphous silicon carbide, or silicon carbide which is a mixture of
these types. A powder of .beta.-type silicon carbide is
particularly preferably used. In the sintered silicon carbide of
the present invention, preferably 70% or more and further
preferably 80% or more of the total silicon carbide components is
.beta.-silicon carbide, and 100% of the total silicon carbide
components may be .beta.-silicon carbide. Therefore, preferably 60%
or more and further preferably 65% or more of the silicon carbide
powder used as raw material is .beta.-silicon carbide.
[0031] The grade of the powder of .beta.-type silicon carbide is
not particularly limited and, for example, a commercially available
powder of .beta.-type silicon carbide can be used. To obtain a
sintered silicon carbide having a high density, it is preferable
that silicon carbide powder has a small particle diameter. The
particle diameter is preferably about 0.01 to 10 .mu.m, and more
preferably about 0.05 to 1 .mu.m. When the particle diameter is
less than 0.01 .mu.m, handling in the steps of measuring and mixing
becomes difficult. When the particle diameter exceeds 10 .mu.m, the
specific surface area of the powder becomes small, i.e., the
contact surface area between the particles becomes small and
obtaining a sintered silicon carbide having a high density becomes
difficult. Therefore, such particle diameters are not
preferable.
[0032] As the preferable embodiment of the silicon carbide powder
described above, a silicon carbide powder having a particle
diameter of 0.05 to 1 .mu.m, a specific surface area of 5 m.sup.2/g
or more, a content of free carbon of 1% or less and a content of
oxygen of 1% or less is preferably used. The distribution of the
particle size of the silicon carbide powder is not particularly
limited. The distribution of the particle size may have two peaks
from the standpoint of increasing the packing density of particles
and from the standpoint of the reactivity of silicon carbide during
the preparation of the sintered silicon carbide.
[0033] To obtain a sintered silicon carbide having a high purity, a
silicon carbide powder having a high purity is used as the raw
material.
[0034] The silicon carbide powder having a high purity can be
obtained, for example, in accordance with a process comprising: a
step which comprises solidifying a mixture obtained by uniformly
mixing a silicon source containing at least one liquid silicon
compound, a carbon source containing at least one liquid organic
compound which generates carbon in the presence of heat, a
polymerization or crosslinking catalyst and, where desired, a
nitrogen source to obtain a solid material; and a step of sintering
the resulting solid material in a nonoxidizing atmosphere (these
steps occasionally referred to as the step of preparation of a
silicon carbide powder, hereinafter).
[0035] The silicon source containing the silicon compound described
above (occasionally referred to as the silicon source, hereinafter)
may be a combination of liquid silicon compounds and solid silicon
compounds. However, at least one compound selected from liquid
silicon compounds should be included. A mono-, di-, tri-, or
tetraalkoxysilane or a polymer of a tetraalkoxysilane can be used
as the liquid silicon compound. Among these alkoxysilanes,
tetraalkoxysilanes are preferable. Specific examples of the
tetraalkoxysilane include tetramethoxysilane, tetraethoxysilane,
tetrapropoxysilane, and tetrabutoxysilane. Tetraethoxysilane is
more preferable from the standpoint of handling. Examples of the
polymer of a tetraalkoxysilane include polymers having a low degree
of polymerization of about 2 to 15 (oligomers) and liquid polymers
of silicic acid having a higher degree of polymerization. Examples
of the solid silicon compound which is used in combination with the
liquid silicon compound include silicon oxides. In the present
invention, the silicon oxide includes, in addition to SiO, silica
sol (such as fluids containing extremely fine colloidal silica
which contains hydroxyl group and/or alkoxyl groups therein),
silicon dioxide (such as silica gel, fine silica, and powder of
quartz) and the like.
[0036] Among the silicon sources described above, oligomers of
tetraethoxysilane and mixtures of an oligomer of tetraethoxysilane
and a fine powder of silica are preferable from the standpoint of
better uniformity and handling. Substances having a high purity are
used as the silicon source. The original content of impurities in
the silicon source is preferably 20 ppm or less and more preferably
5 ppm or less.
[0037] The carbon source described above containing the organic
compound which generates carbon in the presence of heat
(occasionally referred to as the carbon source, hereinafter) may be
a liquid compound or a combination of a liquid compound and a solid
compound. As the carbon source, the organic compound which provide
a large amount of residual carbon and are polymerized or
crosslinked in the presence of a catalyst or heat are preferable.
Preferable examples of the organic compound include monomers and
prepolymers of resins such as phenol resins, furan resins,
polyimides, polyurethanes, and polyvinyl alcohol as well as liquid
materials such as cellulose, cane sugar, pitch, tar and the like.
Among these organic compounds, phenol resins of the resol type are
particularly preferable. The purity of the organic compound can
suitably be controlled or selected in accordance with the purpose.
When a silicon carbide powder having a particularly high purity is
required, it is preferable that an organic compound which does not
contain 5 ppm or more of any metal is used.
[0038] In the production of the silicon carbide powder having a
high purity described above, the ratio of carbon to silicon
(abbreviated hereinafter as C/Si ratio) is defined in accordance
with the result of elemental analysis of an intermediate product
obtained by carbonization of the mixture at 1,000.degree. C. From
the stoichiometry of the reaction, the content of free carbon in
the formed silicon carbide should be 0% when the C/Si ratio is 3.0.
However, in actuality, free carbon is formed at a lower C/Si ratio
because SiO gas formed simultaneously during the reaction is
removed. It is important that the mixing ratio be decided in
advance in a manner such that free carbon is not formed in the
silicon carbide powder in an amount which adversely affects the use
of the sintered silicon carbide. In general, the formation of free
carbon can be suppressed in baking at 1600.degree. C. or higher at
a pressure of about 1 atm when the C/Si ratio is 2.0 to 2.5. When
the C/Si ratio is 2.5 or more, the amount of free carbon markedly
increases. Therefore, the C/Si ratio of 2.0 to 2.5 is preferable.
However, a C/Si ratio of 2.5 or more may suitably be used depending
on the purpose of forming particles having objective particle size
because free carbon shows an effect of suppressing growth of
grains. When the pressure of the atmosphere in the baking is higher
or lower, a different value of the C/Si ratio may be suitable for
obtaining a silicon carbide having a high purity. Therefore, the
C/Si ratio is not necessarily limited to the above range of 2.0 to
2.5 in this case.
[0039] The effect of the free carbon obtained during the step of
sintering is much smaller than the effect of the carbon derived
from the nonmetallic auxiliary sintering agent covering the entire
surface of the silicon carbide powder, which will be described
later. Therefore, the effect of the free carbon is essentially
negligible.
[0040] A mixture of the silicon source described above and the
carbon source described above may be hardened into a solid mixture
to obtain a solid material in which the silicon source and the
carbon source are uniformly mixed together. Examples of the method
of hardening include crosslinking by heating, hardening in the
presence of a hardening catalyst, and methods using electronic
beams or irradiation. The hardening catalyst can be suitably
selected in accordance with the carbon source. When a phenol resin
or a furan resin is used as the carbon source, acids such as
toluenesulfonic acid, toluenecarboxylic acid, acetic acid, oxalic
acid, hydrochloric acid, sulfuric acid, and maleic acid, and amines
such as hexamine can be used.
[0041] If necessary, carbonization of the solid mixture is
conducted by heating the solid material in a nonoxidizing
atmosphere, such as an atmosphere of nitrogen or argon, at 800 to
1,000.degree. C. for 30 to 120 minutes.
[0042] Silicon carbide is formed by heating the solid mixture,
which has been carbonized by heating, in a nonoxidizing atmosphere,
such as an atmosphere of argon, at 1,350.degree. C. or higher and
2,000.degree. C. or lower. The temperature and the time of the
baking can suitably be selected in accordance with the desired
property of the sintered material, such as the particle diameter.
Baking at 1,600 to 1,900.degree. C. is preferable to achieve
effective formation of silicon carbide.
[0043] To achieve a still higher purity than the above powder
having a high purity, a further heat treatment is conducted at
2,000 to 2,100.degree. C. for 5 to 20 minutes during the above
baking.
[0044] As a process for producing a silicon carbide powder having a
particularly high purity, there can be used the process for
producing a material powder described in the specification of JP-A
Nos. 9-48605 as a process for producing single crystals . This
process comprises a step of forming a silicon carbide powder and an
after-treatment step. In the step of forming the silicon carbide
powder, a silicon source and a carbon source are uniformly mixed to
prepare a mixture, and the mixture is baked by heating in a
nonoxidizing atmosphere to obtain a silicon carbide powder. The
silicon source is at least one compound selected from high-purity
tetraalkoxysilanes and high-purity polymers of tetraalkoxysilanes,
and the carbon source is a high-purity organic compound which
generates carbon in the presence of heat. In the after-treatment
step, the silicon carbide powder is kept at a temperature of
1,700.degree. C. or higher and lower than 2,000.degree. C. and,
while the temperature is essentially kept in this range, the powder
is treated by heating at 2,000.degree. C. to 2,100.degree. C. for 5
to 20 minutes at least once. A high purity silicon carbide powder
containing 0.5 ppm or less of each impurity can be obtained in
accordance with this process comprising the above two steps.
[0045] The step of sintering described above will be described more
specifically in the following.
[0046] In the step of sintering, a mixture of the silicon carbide
powder, the nonmetallic auxiliary sintering agent and, where
desired, the nitrogen source (occasionally referred to as a mixture
of silicon carbide powder) is sintered by hot pressing at a
temperature of 2,000 to 2,400.degree. C. at a pressure of 300 to
700 kg/cm.sup.2 in a nonoxidizing atmosphere.
[0047] In the step of sintering, a substance which generates carbon
in the presence of heat is used as the nonmetallic auxiliary
sintering agent described above. Examples of the above substance
include organic compounds which generate carbon in the presence of
heat and powders of silicon carbide covered with these organic
compounds on the surface thereof (particle diameter: about 0.01 to
1 .mu.m). Among these substances, the former is preferable from the
standpoint of the effects achieved.
[0048] Specific examples of the organic compound which generates
carbon in the presence of heat include coal tar pitch containing a
large amount of residual carbon, pitch tar, phenol resins, furan
resins, epoxy resins, phenoxy resins, and various types of sugar,
such as monosaccharides such as glucose, oligosaccharides such as
sucrose, and polysaccharides such as cellulose and starch. Among
these substances, substances which are liquid at room temperature,
substances which are soluble into a solvent, or substances which
are softened or turned into a liquid by heating, for example,
substances which are thermoplastic or heat melting substances, are
preferable for achieving uniform mixing with the silicon carbide
powder. Phenol resins, particularly phenol resins of the resol
type, are more preferable because of the high strengths of molded
products.
[0049] The above organic compound which generates carbon in the
presence of heat functions as the auxiliary sintering agent
effectively, since inorganic carbon compounds such as carbon black
and graphite (which were generated from the organic compound which
generates carbon) are formed on the surface or in the vicinity of
the surface of silicon carbide in the presence of heat, and
therefore an oxide layer covered on the surface is efficiently
removed during the sintering.
[0050] It is preferable that the nonmetallic auxiliary sintering
agent described above is dissolved or dispersed in a solvent, and
the prepared solution or dispersion is used to prepare the mixture
of the silicon carbide powder and the nonmetallic auxiliary
sintering agent. The solvent is suitably selected in accordance
with the compound used as the nonmetallic auxiliary sintering
agent. Specifically, lower alcohols such as ethyl alcohol, an ethyl
ether, acetone or the like can be used for a phenol resin which is
preferable as the organic compound which generates carbon in the
presence of heat. It is also preferable that the used nonmetallic
auxiliary sintering agent and the used solvent contain small
amounts of impurities.
[0051] A considerably small amount of the nonmetallic auxiliary
sintering agent in the mixture leads to a low density of the
sintered material. On the other hand, a considerably large amount
tends to inhibit an increase in the density because of the increase
in free carbon contained in the sintered material. The amount of
the nonmetallic auxiliary sintering agent is generally 10% by
weight or less, preferably 2 to 5% by weight, although the amount
differs in accordance with the type of the nonmetallic auxiliary
sintering agent. This amount of the agent can be decided by
determining the amount of silica (silicon oxide) on the surface of
the silicon carbide powder by using hydrofluoric acid, followed by
stoichiometrically calculating the amount sufficient to reduce the
silica.
[0052] The silica in the amount determined above is reduced with
carbon derived from the nonmetallic auxiliary sintering agent
according to the following chemical reaction:
SiO.sub.2+3C.fwdarw.SiC+2CO
[0053] The amount of carbon to be added described above is
determined by taking the residual carbon ratio (the percentage of
the nonmetallic auxiliary sintering agent converted into carbon)
after thermal decomposition of the nonmetallic auxiliary sintering
agent into consideration.
[0054] In the sintered silicon carbide described above, it is
preferable that the total content of carbon atoms derived from
silicon carbide and carbon atoms derived from the nonmetallic
auxiliary sintering agent exceeds 30% by weight and is less than or
equal to 40% by weight. When the total content is 30% by weight or
less, relative amounts of impurities contained in the sintered
material increase. When the total content exceeds 40% by weight,
the density of the sintered material decreases, and thus properties
such as strengths and oxidation resistance of the sintered material
deteriorate due to the excessively large content of carbon.
[0055] To produce the sintered material of silicon carbide
described above, the silicon carbide powder and the nonmetallic
auxiliary sintering agent are uniformly mixed together. As
described above, a phenol resin used as the nonmetallic auxiliary
sintering agent is dissolved into a solvent, such as ethyl alcohol,
and then mixed with the silicon carbide powder sufficiently. In
case where the nitrogen source is added in accordance with desire,
the nitrogen source can be added in combination with the
nonmetallic auxiliary sintering agent.
[0056] The mixing described above can be conducted by a
conventional method, such as a method using a mixer or a planetary
ball mill. The mixing is conducted preferably for 10 to 30 hours,
more preferably for 16 to 24 hours. After sufficient mixing has
been achieved, the solvent is removed at a temperature suitable for
the solvent, for example at 50 to 60.degree. C. when ethyl alcohol
is used as described above, until the mixture is dried, and the
resulting mixture is sieved with a sieve to obtain a powder of the
mixture which is used as the raw material powder. From the
standpoint of achieving a high purity, the container and the balls
of the ball mill may be made of a synthetic resin which does not
contain metals. A granulating machine, such as a spray drier, may
be used for drying.
[0057] In the step of sintering described above, it is preferable
that the mixture of the powder or a molded product made of the
mixture of the powder obtained in the molding step described below
is placed in a mold and hot pressed at a temperature of 2,000 to
2,400.degree. C. at a pressure of 300 to 700 kgf/cm.sup.2 in a
nonoxidizing atmosphere.
[0058] As for the mold mentioned above, it is preferable from the
standpoint of the purity of the sintered material that a portion of
the mold or the entire mold is made of a material such as graphite,
or that Teflon sheets or the like are placed inside the mold, so
that the mixture of the powder or the molded product made of the
mixture of the powder do not directly contact metallic portions of
the mold.
[0059] In the present invention, the pressure of the hot pressing
can be in the range of 300 to 700 kgf/cm.sup.2. Particularly at
pressures of 400 kgf/cm.sup.2 or more, parts used for the hot
pressing such as dies and punches, should be parts having good
pressure resistance.
[0060] It is preferable that, before the hot pressing step is
conducted to prepare the sintered material, impurities are
sufficiently removed by heating under the following conditions and
complete carbonization of the nonmetallic auxiliary sintering agent
is achieved, and that hot pressing is then carried out under the
conditions described above.
[0061] It is preferable that the heating described above is
conducted by two-step heating in two steps, i.e., a first
heating-and temperature raising-step and a second heating-and
temperature raising-step as follows.
[0062] In the first heating-and temperature raising-step, a furnace
is evacuated and slowly heated from room temperature to 700.degree.
C. When it is difficult to control the temperature in the furnace,
the temperature may be increased continuously to 700.degree. C. in
a single step. However, it is preferable that, after the pressure
in the furnace is adjusted to 10.sup.-4 Torr, the temperature is
slowly increased from room temperature to 200.degree. C.,
maintained at this temperature, then increased slowly to
700.degree. C., and maintained at about 700.degree. C. In this
first heating-and temperature raising-step, absorbed water and the
organic solvent are removed, and carbonization proceeds through the
thermal decomposition of the nonmetallic auxiliary sintering agent.
The period of time over which the temperature is maintained at
about 200.degree. C. or 700.degree. C. is selected in a suitable
range which depends on the size of the sintered material. A
sufficient period of time can be determined on the basis of when a
decrease in the vacuum is reduced a certain extend. If the
temperature is raised before the sufficient period of the time has
elapsed, removal of impurities and carbonization of the carbon
source do not proceed sufficiently, and thus there is the
possibility that cracks and pores are formed in the molded
product.
[0063] In the first heating- and temperature raising-step, for
example, the following procedures are carried out for a sample of 5
to 10 g. After the pressure is adjusted to 10.sup.-4 Torr, the
temperature is slowly increased from room temperature to
200.degree. C., maintained at this temperature for about 30
minutes, and then slowly continued to be raised until the
temperature reaches 700.degree. C. The period of time required to
raise the temperature from room temperature to 700.degree. C. is
about 6 to 10 hours, preferably about 8 hours. It is preferable
that the temperature is maintained at about 700.degree. C. for 2 to
5 hours.
[0064] In the second heating-and temperature raising-step, the
temperature is further raised from 700.degree. C. to 1,500.degree.
C. in vacuo over 6 to 9 hours for the same sample as described
above. The temperature is maintained at 1,500.degree. C. for about
1 to 5 hours. It is believed that silicon dioxide and silicon oxide
are reduced in this step. To completely remove the oxygen bonded to
the silicon, it is important that this reduction reaction be
allowed to proceed completely. Therefore, it is important that the
temperature be maintained at 1,500.degree. C. for a period of time
sufficient to completely generate carbon monoxide which is formed
as a byproduct of the reduction reaction. In other words, the
temperature should be maintained at 1,500.degree. C. until the
decrease in the vacuum becomes small, and the pressure value
returns to the value observed before the start of the reduction,
i.e., the value at the time when the temperature was about
1,300.degree. C. By the reduction in the second heating-and
temperature raising-step, the silicon dioxide, which is attached to
the surface of the powdered silicon carbide and which adversely
affects the increasing of the density and which causes formation of
larger particles, can be removed. The gas containing SiO and/or CO,
which is generated in the reduction step, contains impurity
elements. This gas is continuously discharged and removed from the
reaction furnace by a vacuum pump. Therefore, preferably, the
temperature is sufficiently maintained at the above value from the
standpoint of producing a sintered material having a high
purity.
[0065] It is preferable that the hot pressing is conducted at a
high pressure after the above steps of heating have been completed.
When the temperature is raised above 1,500.degree. C., the
sintering starts. At this time, the pressure is increased up to
about 300 to 700 kgf/cm.sup.2 to suppress abnormal growth of
particles. Subsequently, an inert gas is introduced into the
furnace to achieve a nonoxidizing atmosphere. Nitrogen or argon gas
can be used as the inert gas, and argon gas is preferable because
argon gas is inert even at high temperatures.
[0066] To perform the hot pressing described above, the temperature
is raised to 2,000 to 2,400.degree. C. and the pressure is
increased to 300 to 700 kgf/cm.sup.2 after the atmosphere inside
the furnace has been converted into a nonoxidizing atmosphere. The
pressure for the hot pressing can be selected in accordance with
the particle diameter of the material powder. When the diameter of
the material powder is small, an excellent sintered material can be
obtained at a relatively small pressure for the hot pressing. The
heating from 1,500.degree. C. to the maximum temperature of 2,000
to 2,400.degree. C. is carried out over 2 to 4 hours. The sintering
reaction is accelerated at 1,850 to 1900.degree. C. The temperature
is maintained at the maximum temperature for 1 to 3 hours, and then
the sintering is completed.
[0067] A maximum temperature of 2,000.degree. C. to 2,400.degree.
C. leads to the production of a sintered material having a
sufficient density. When a maximum temperature is less than
2,000.degree. C., a sintered material can not have a sufficient
high density. When a maximum temperature is over than 2,400.degree.
C., the powder or the raw material used for the molding tend to
sublime or decompose. Therefore, such a maximum temperatures of
2,000.degree. C. to 2,400.degree. C. is preferable. A pressure of
500 kgf/cm.sup.2 to 700 kgf/cm.sup.2 leads to the production of a
sintered material having a sufficient density, and is preferable
from the standpoint of efficiency of production. When the pressure
is greater than 700 kgf/cm.sup.2, the pressure may cause fracturing
of the mold such as a mold made of graphite. When the pressure is
smaller than 500 kgf/cm.sup.2, the sintered silicone carbide can
not have sufficient high density.
[0068] In the hot pressing described above as well, it is
preferable that a graphite material having a high purity is used
for molds and for heat insulators for the heating furnace used in
the process, from the viewpoint of high purity of the obtained
sintered material. Therefore, it is preferable to use a graphite
material which has been treated to have a high purity in advance.
Specifically, it is preferable to use a graphite material which has
been sufficiently baked at 2,500.degree. C. or higher in advance,
and generates no or few impurities at the temperature of sintering.
The inert gas used in the process is preferably a gas of a high
purity grade which contains few impurities.
[0069] The sintered silicon carbide having excellent properties can
be obtained after the step of sintering described above. However, a
following molding step may be conducted before the step of
sintering to provide the finally obtained sintered material with a
higher density. The molding step is described hereinafter. The
molding step comprises placing a mixture of the silicon carbide
powder into a mold; and heating at a temperature in the range of 80
to 300.degree. C. for 5 to 60 minutes under pressure to prepare a
molded material of the mixture of the silicon carbide powder
(occasionally referred to as a molded material, hereinafter) in
advance. In this procedure, it is preferable that the mixture is
packed into the mold as densely as possible from the standpoint of
increasing the density of the finally obtained sintered material.
In this molding step, a bulky powder can be made compact before the
powder is packed for the hot pressing, and the production of a
molded product having a large thickness can be facilitated by
repeating the molding step.
[0070] A molded material made of the mixture of the silicon carbide
powder is obtained by pressing the material powder at a temperature
in the range of 80 to 300.degree. C. and preferably in the range of
120 to 140.degree. C. at a pressure in the range of 60 to 100
kgf/cm.sup.2 in accordance with the properties of the nonmetallic
auxiliary sintering agent so that the density of the packed
material powder becomes 1.5 g/cm.sup.3 or more and preferably 1.9
g/cm.sup.3 or more, followed by maintaining the material powder in
the compressed condition for 5 to 60 minutes and preferably for 20
to 40 minutes. The smaller the average diameter of the particles
is, the more difficult it is to provide a molded material with
higher density. It is preferable that a suitable method, such as
packing by vibration, is used to achieve a higher density when the
powder material is placed into the mold. Specifically, when a
powder has an average particle diameter of about 1 .mu.m, the
density is more preferably 1.8 g/cm.sup.3 or more, and when a
powder has an average particle diameter of about 0.5 .mu.m, the
density is more preferably 1.5 g/cm.sup.3 or more. When the density
is less than 1.8 g/cm.sup.3 in the former case or less than 1.5
g/cm.sup.3 in the latter case, it becomes difficult to provide the
finally obtained sintered material with a high density.
[0071] The molded material described above may be cut to a shape
fitting a hot pressing mold in advance before being used in the
step of sintering. The molded material, preferably the molded
material which has been covered with the nonmetallic auxiliary
sintering agent on the surface, is subjected to the step of
sintering, in which the molded material is placed in a mold and hot
pressed at a temperature of 2,000 to 2,400.degree. C. at a pressure
of 300 to 700 kgf/cm.sup.2 in a nonoxidizing atmosphere as
described above, to obtain a sintered silicon carbide having a high
density and a high purity. When the silicon carbide powder and/or
the combination of the silicon carbide powder and the nonmetallic
auxiliary sintering agent contains at least 500 ppm of the nitrogen
component, a sintered silicon carbide can be obtained after
sintering, which contains about 200 ppm of nitrogen as a uniform
solid solution and has a volume resistivity of 1 .OMEGA..cm or
less.
[0072] The sintered silicon carbide obtained in accordance with the
above method of production has a sufficient density, i.e., a
density of 2.9 g/cm.sup.3 or more. When a density is less than 2.9
g/cm.sup.3, physical properties, such as bending strength and
strength at break, and electric properties are lowered, and
undesired contaminations take place due to large particles.
Therefore, such a density of 2.9 g/cm.sup.3 or more is preferable.
The density of the sintered silicon carbide is more preferably 3.0
g/cm.sup.3 or more.
[0073] When the obtained sintered material is porous, the sintered
material has drawbacks in that heat resistance, oxidation
resistance, chemical resistance and mechanical strength are
inferior; that cleaning is difficult; that tiny cracks form and
tiny pieces of the material formed from the cracks become
contaminating substances; that gases can permeate therethrough; and
that application of the sintered silicon carbide is limited as a
result of these drawbacks.
[0074] The total content of impurities in the sintered silicon
carbide described above is 10 ppm or less, preferably 5 ppm or
less. However, the content of impurities, which is calculated by
chemical analysis, has importance merely as a reference. When the
sintered material is actually used in the fields, such as parts of
electronic device or semiconductor, the results of evaluation also
depend on the distribution of impurities, i.e., whether impurities
are distributed uniformly or unevenly. For example, persons skilled
in the art generally evaluate the degree of contamination with
impurities under prescribed heating conditions in accordance with
various methods using apparatuses actually used in production. In
accordance with the process described herein comprising carbonizing
the solid material obtained by uniformly mixing the liquid silicon
compound, the nonmetallic auxiliary sintering agent, and the
catalyst for polymerization or crosslinking by heating in a
nonoxidizing atmosphere and then sintering the obtained product in
a nonoxidizing atmosphere, the total content of impurity elements
in the sintered silicon carbide can be reduced to 10 ppm or less.
The silicon source and the nonmetallic auxiliary sintering agent,
which are used in the above step of preparation of the silicon
carbide powder and in the above step of preparation of the sintered
silicon carbide from the silicon carbide powder, and the inert gas,
which is used to form the nonoxidizing atmosphere, each preferably
contains each impurity element in an amount of 10 ppm or less and
more preferably 5 ppm or less. However, the amount contained of an
impurity is not limited to the above values as long as the amount
contained is in a range which allows sufficient purification in the
steps of heating and sintering. The impurity elements described
above substantially means elements other than Si, C, O, N, halogens
and rare gases.
[0075] The above sintered silicon carbide obtained by using the
nonmetallic auxiliary sintering agent has a high density of 2.9
g/cm.sup.3 or more and an advantageous sintered structure which
tends to become a polycrystalline semiconductor with excellent
electric conductivity. Since conductive electrons transfer between
crystals across the grain boundary, the junction of the grain
boundary phase and silicon carbide is also important for exhibition
of electric conductivity. Transferring properties of conductive
electrons are classified into two types, i.e., tunnel conduction
and thermally excited conduction.
[0076] When the sintered silicon carbide contains nitrogen, the
content of nitrogen is preferably 150 ppm or more and more
preferably 200 ppm or more. It is preferable that nitrogen is
contained as a solid solution from the standpoint of stability.
[0077] When a sintered silicon carbide contains 150 ppm or more of
nitrogen as a solid solution, the barrier of the space charge layer
formed at the grain boundary becomes about 0.15 eV or less, and an
excellent conduction can be exhibited. The sintered silicon carbide
has a volume resistivity of 1 .OMEGA..cm in this condition. When
the silicon carbide contains 200 ppm or more of nitrogen, the
barrier of the space charge layer at the grain boundary becomes
0.026 eV or less. Since this barrier can be overcome by thermal
excitation even at an ordinary temperature (300K), the thermally
excited conduction and the tunnel conduction take place.
[0078] It is generally known that volume resistivity of a
semiconductor first decreases (the NTC region) and then increases
(the PTC region) with an increase in the temperature. The smaller
the change in volume resistivity with temperature, the easier the
temperature control of the semiconductor used as an electric
heating element. As for the sintered silicon carbide, the larger
the content of nitrogen as a solid solution, the lower the
temperature of the boundary of the NTC region and the PTC region.
In other words, when the content of nitrogen is 150 ppm or more and
preferably 200 ppm or more as shown in the case of the sintered
silicon carbide of the present invention, the NTC region at lower
temperatures which exhibits the largest change in volume
resistivity with temperature becomes small. Therefore, the change
in volume resistivity with temperature decreases in a range from
room temperature to a high temperature.
[0079] The sintered silicon carbide obtained after the step of
producing the sintered silicon carbide described above is worked to
have a desired shape, treated by grinding and polishing and
thereafter, a step of cleaning is carried out. As the method of
working and the treatments of grinding and polishing, conventional
methods can be used. When nitrogen is introduced into the sintered
silicon carbide, working by electric discharge is preferable as the
method of working.
[0080] The method (the step) of cleaning a sintered silicon carbide
in a wet condition of the present invention will be described in
detail in the following.
[0081] When the step of cleaning a sintered silicon carbide of the
present invention is performed, the sintered silicon carbide having
the amount of an impurity present on the surface and in the
vicinity of the surface smaller than 1.0.times.10.sup.11
atoms/cm.sup.2, preferably smaller than 5.times.10.sup.10
atoms/cm.sup.2 and more preferably smaller than 1.0.times.10.sup.10
atoms/cm.sup.2 can be obtained easily.
[0082] The method for cleaning a sintered silicon carbide in a wet
condition of the present invention is applied not only to the
sintered silicon carbide described above but also to any sintered
silicon carbide having a high density and a high purity which can
be used for various parts of semiconductors and electronic parts.
For example, the method can be applied to silicon carbides sintered
by hot pressing using a nonmetallic auxiliary sintering agent and
the sintered silicon carbide described in the specification of
Japanese Patent Application No. 10-67565 applied by the present
applicant.
[0083] In the method for cleaning a sintered silicon carbide in a
wet condition of the present invention, since the cleaning liquids
are all composed of agents which are soluble in water or can be
rinsed with water, the method has an advantage in that no drying
process is required in the step of cleaning and, therefore, the
whole process can be simplified.
[0084] In the method for cleaning a sintered silicon carbide in a
wet condition of the present invention, the step of cleaning a
sintered silicon carbide described above comprises a step of
dipping a sintered silicon carbide (occasionally referred to as a
material to be cleaned, hereinafter) into a quasi-aqueous organic
solvent, a step of dipping the material to be cleaned into an
aqueous solution of an inorganic acid and a step of dipping the
material to be cleaned into pure water, successively. When these
steps are successively applied, organic substances on the surface
such as oil film, finger prints and waxes are removed with the
quasi-aqueous organic solvent, and next metal elements on the
surface and in the vicinity of the surface are removed with the
aqueous solution of an inorganic acid. It is preferable that a step
of dipping the material to be cleaned into an aqueous solution of
an ammonium compound is conducted between the above steps to
facilitate removal of the organic solvent used and particles.
[0085] The step of dipping into a quasi-aqueous organic solvent is
conducted to remove organic substances on the surface and in the
vicinity of the surface of the sintered silicon carbide.
[0086] The quasi-aqueous organic solvent means a solvent which is
soluble in water or a solvent which can be removed easily by
washing with water although the solvent itself is insoluble in
water. Examples of the quasi-aqueous organic solvent used in the
present invention include a solvent soluble in water, a solvent
obtained by partially introducing hydrophilic groups into a solvent
insoluble in water or a solvent obtained by adding surfactants to a
solvent insoluble in water in advance. Specific examples of the
quasi-aqueous organic solvent include petroleum hydrocarbons,
esters of organic acids, glycol ethers, mixed solvents of these
solvents, mixtures of the solvent and a surfactant(s) and mixtures
of the mixed solvents and a surfactant(s). Examples of the mixed
solvent and the mixture include mixed solvents of petroleum
hydrocarbons and esters of organic acid or glycol ethers, mixtures
of surfactants, petroleum hydrocarbons and esters of organic acids
or glycol ethers, mixtures of surfactants and petroleum
hydrocarbons, and mixtures of surfactants and esters of organic
acids.
[0087] Examples of the petroleum hydrocarbon include aliphatic
hydrocarbons such as naphthenes and hexane.
[0088] Examples of the ester of organic acid include esters of
fatty acids such as esters of methyl fatty acids, glycerol esters
and sorbitan esters.
[0089] Examples of the glycol ether include propylene glycol ether,
propylene glycol methyl ether and diethylene glycol dimethyl
ether.
[0090] The surfactant is not particularly limited as long as the
surfactant exhibits the desired effect. Preferable examples of the
surfactant include nonionic surfactants such as polyoxyethylene
methyl fatty acids, alkylamine oxides, polyoxyalkylene glycols and
addition products of ethylene oxide or propylene oxide to
alkylamines.
[0091] In the above step of dipping into a quasi-aqueous organic
solvent, the sintered silicon carbide is preferably dipped for 2 to
60 minutes, more preferably 10 to 30 minutes and most preferably 10
to 15 minutes, although the time period is different depending on
the amount and the type of the attached organic substances.
[0092] In the step of dipping into the quasi-aqueous organic
solvent, it is effective that the treatment is conducted under
heating at 50 to 70.degree. C. from the standpoint of enhancing the
ability to dissolve the attached organic substances.
[0093] The step of dipping into the aqueous solution of an
inorganic acid is conducted to remove metal impurities on the
surface and in the vicinity of the surface of the sintered silicon
carbide.
[0094] Examples of the aqueous solution of an inorganic acid
include an aqueous solution of hydrofluoric acid, an aqueous
solution of nitric acid, an aqueous solution of sulfuric acid, an
aqueous solution of hydrochloric acid, an aqueous solution of
hydrogen peroxide, an aqueous solution of ozone and aqueous
solutions of mixtures thereof. Examples of the aqueous solution of
the mixtures include an aqueous mixture of hydrofluoric acid and
nitric acid, aqueous solutions of mixtures of hydrofluoric acid,
nitric acid and sulfuric acid and aqueous solutions of mixtures of
hydrofluoric acid and hydrochloric acid.
[0095] The concentration of the aqueous solution of an inorganic
acid is preferably 0.3 to 68% by weight, more preferably 1 to 40%
by weight and most preferably 5 to 10% by weight. When the
concentration is less than 0.3% by weight, the effect of removing
metal impurities is occasionally insufficient. When the
concentration exceeds 68% by weight, the surface of the material to
be cleaned becomes occasionally rough.
[0096] To the above aqueous solution of an inorganic acid, a
nonionic surfactant may be added so that metal ions dissolved into
the solution are prevented from becoming attached to the material
to be cleaned again. Examples of the nonionic surfactant include
the same surfactants described above.
[0097] In the step of dipping into the aqueous solution of an
inorganic acid, the sintered silicon carbide is preferably dipped
for 5 to 120 minutes, more preferably 10 to 60 minutes and most
preferably 20 to 30 minutes.
[0098] The step of dipping into pure water is conducted to remove
components which are derived from the solvents and the aqueous
solutions used in the previous steps in the step of cleaning and
are left remaining on the surface and in the vicinity of the
surface of the sintered silicon carbide.
[0099] As the pure water, water having an impurity level of 100 ppt
or less and a specific resistance of 16 to 18 .OMEGA. is preferable
and water having an impurity level of less than 10 ppt is more
preferable.
[0100] In the step of dipping into pure water, it is preferable
that an overflow process is used so that the material to be cleaned
is always washed with fresh water.
[0101] The step of dipping into the aqueous solution of an ammonium
compound is conducted to remove the organic solvents used in
previous steps by the surface activity effect of the aqueous
solution of an ammonium compound, and to remove particles. The
organic solvents are considered to be left remaining on the surface
and in the vicinity of the surface of the sintered silicon carbide
in small amounts.
[0102] Examples of the aqueous solution of an ammonium compound
include aqueous solutions of addition polymers of ethylene oxide or
propylene oxide, such as alkylamine oxides or alkylamines, aqueous
solutions of quaternary ammonium salts such as tetraalkylammonium
halides (for example, tetramethylammonium halides) and
tetraalkylammonium perchlorate, aqueous ammonia, mixtures of these
aqueous solutions and an aqueous solution of hydrogen peroxide.
Among these aqueous solutions of an ammonium compound, the aqueous
solutions of quaternary ammonium salts such as tetraalkylammonium
halides or tetra-alkylammonium perchlorate, the aqueous ammonia and
mixtures of these aqueous solutions and an aqueous solution of
hydrogen peroxide are preferable.
[0103] An aqueous solution of an ammonium compound having a surface
tension of 25 to 35 dyne/cm is favorably used although the
preferable surface tension is different depending on the type of
the solution.
[0104] A single type or a combination of two or more types of the
aqueous solution of an ammonium compound may be used.
[0105] In the step of dipping into the aqueous solution of an
ammonium compound, the sintered silicon carbide is preferably
dipped for 5 to 120 minutes, more preferably 10 to 60 minutes and
most preferably 20 to 30 minutes.
[0106] In the method for cleaning a sintered silicon carbide in the
wet condition of the present invention, it is preferable that at
least one of the above steps is conducted while ultrasonic
vibration is applied to the aqueous solution so that dissolution of
impurities at the surface and in the vicinity of the surface is
promoted by the physical vibration applied to the material to be
cleaned. The ultrasonic vibration may be applied while the material
to be cleaned is vibrated or may be applied with sweeping of the
frequency of the ultrasonic vibration. Applying the ultrasonic
vibration in the step of dipping into the aqueous solution of an
inorganic acid is effective.
[0107] In the method for cleaning a sintered silicon carbide in a
wet condition of the present invention, the temperature of the
solvent or the aqueous solution in at least one of the above steps
is adjusted to be preferably 30.degree. C. or more, more preferably
40.degree. C. or more and most preferably 50.degree. C. or more to
enhance the ability of the solvent or the aqueous solution to
dissolve unpreferable impurities and attached substances. The
maximum value of the above temperature is at or lower than the
boiling point of the solvent or the aqueous solution used. Carrying
out this adjustment of the temperature in the step of dipping into
the quasi-aqueous organic solvent is effective.
[0108] In the method for cleaning a sintered silicon carbide in a
wet condition of the present invention, a step of dipping into
cleaning water may be conducted between the above steps. When the
step of dipping into cleaning water is conducted, for example,
after the step of dipping into the quasi-aqueous organic solvent,
contamination of the aqueous solution in the next step can be
prevented by simply washing and removing the solvent attached to
the material to be cleaned.
[0109] As the above cleaning water, pure water described above,
distilled water or ion-exchanged water can be used. Pure water
described above is preferable from the standpoint of preventing
contamination of the material to be cleaned by the cleaning water
in the step of dipping into cleaning water.
[0110] In the step of dipping into cleaning water described above,
the sintered silicon carbide is preferably dipped for 2 to 60
minutes, more preferably 5 to 30 minutes and most preferably 10 to
20 minutes.
[0111] In the step of dipping into cleaning water, an overflow
process may be used so that material to be cleaned is always washed
with fresh water.
[0112] It is preferable that the apparatuses and instruments used
in the step of cleaning a sintered silicon carbide described above
are made of polyvinyl chloride having excellent chemical resistance
and more preferably made of polyvinyl chloride treated for a high
purity. It is also preferable that the apparatus for generating
ultrasonic vibration and heaters are coated with Teflon on the
surface.
[0113] The sintered silicon carbide obtained in accordance with the
method for cleaning a sintered silicon carbide in a wet condition
of the present invention and the sintered silicon carbide of the
present invention obtained in accordance with the method comprising
the step of preparation of the sintered silicon carbide described
above and the step of cleaning the sintered silicon carbide
described above have a high density and only small amounts of
impurities on the surface and in the vicinity of the surface and
are advantageously used for various parts of semiconductors and
electronic parts. Examples of the parts of semiconductors include
parts which are required to have a high purity and to be free from
particles such as dummy wafers, heaters, electrodes for plasma
etching and targets in ion injection apparatuses.
EXAMPLES
[0114] The present invention will be described more specifically
with reference to examples in the following. However, the present
invention is not limited to the examples.
Example 1
[0115] (Preparation of a sintered silicon carbide)
[0116] A phenol resin of the resol type containing an amine (amount
of residual carbon after heat decomposition: 50%) in an amount of 6
g, and 94 g of a high purity n-type .beta.-silicon carbide powder,
which had an average particle diameter of 0.5 mm and one maximum
value in distribution of the particle size, were mixed into 50 g of
ethanol or acetone as the solvent using a wet type ball mill, and
the obtained mixture was dried and molded into a cylindrical shape
having a diameter of 20 mm and a thickness of 10 mm. Amounts of the
phenol resin and the amine contained in the molded material were 6%
by weight and 0.1% by weight, respectively. The molded material was
sintered at a pressure of 700 kgf/cm.sup.2 and at the temperature
of 2,300.degree. C. in an atmosphere of argon for 3 hours in
accordance with the hot press method to obtain a sintered silicon
carbide.
[0117] (Working of the sintered silicon carbide)
[0118] The obtained sintered silicon carbide was worked to prepare
a flat plate of 40 mm.times.40 mm.times.2 mmt. One face of the
formed plate was polished to form a rough surface and the other
face was polished to form a mirror finished surface. The sintered
silicon carbide obtained after the working showed a surface
cleanliness (the amount of attached impurities) of
1.times.10.sup.13 to 1.times.10.sup.16 atoms/cm.sup.2. The density
of the obtained sintered silicon carbide was 3.13 g/cm.sup.3 as
measured in accordance with the method of Japanese Industrial
Standard R1634. The total content of impurities was 3.5 ppm as
measured in accordance with the analysis using an ICP-MS or a like
apparatus. The total content of impurities did not include the
impurities on the surface and in the vicinity of the surface.
[0119] (Cleaning of the sintered silicon carbide)
[0120] The sintered silicon carbide obtained after the working was
dipped into a quasi-aqueous organic solvent (a mixed solvent
containing a petroleum hydrocarbon, an ester of an organic acid and
a nonionic surfactant; not diluted) at 50.degree. C. for 15 minutes
while ultrasonic vibration (100V-26.+-.2 kHz) was applied, rinsed
with water, dipped into an aqueous mixture of hydrofluoric acid and
nitric acid (38% hydrofluoric acid:68% nitric acid:water 1:1:20)
for 30 minutes and then dipped into pure water to obtain a sintered
silicon carbide of Example 1.
[0121] (Evaluation)
[0122] The surface cleanliness (the amounts of impurities) of the
obtained sintered silicon carbide of Example 1 was measured and
found to be 8.times.10.sup.9 to 1.times.10.sup.11 atoms/cm.sup.2.
The results are shown in Table 1 in more detail. In all of the
above treatments except the treatment of dipping into the
quasi-aqueous organic solvent, the temperature of the aqueous
solutions was kept at the room temperature. The surface cleanliness
was measured in accordance with the following method.
[0123] (Measurement of the surface cleanliness (the amounts of
impurities))
[0124] The surface cleanliness (the amounts of impurities) was
measured as follows: to obtain the amounts of light elements (B, Na
and Al), the surface of the sintered silicon carbide was washed
with an aqueous solution containing 1% each of hydrofluoric acid
and nitric acid to extract the impurities and the obtained aqueous
solution was analyzed using an ICP-MS (Inductively Coupled Plasma
Mass Spectrometer); and to obtain the amounts of other elements,
the sintered silicon carbide was dipped into pure water, dried and
then analyzed using a TXRF (Total Reflection X-Ray
Fluorescencemeter). In the analysis using the TXRF, a relative
sensitivity coefficient based on silicon was used. It was confirmed
in the analyses of K, Cr, Fe, Ni, Cu and Zn that the analysis using
an ICP-MS and the analysis using a TXRF gave approximately the same
results.
Example 2
[0125] The same procedures as those conducted in Example 1 were
conducted except that, in place of being cleaned in accordance with
the procedures conducted in Example 1, the sintered silicon carbide
obtained after the working was dipped into glycol ether for 20
minutes, dipped into an aqueous solution of a quaternary ammonium
salt for 30 minutes, dipped into an aqueous mixture of hydrofluoric
acid and nitric acid (38% hydrofluoric acid: 68% nitric
acid:water=1:1:20) for 30 minutes and then dipped into pure water
and a sintered silicon carbide of Example 2 was obtained.
[0126] (Evaluation)
[0127] The surface cleanliness of the obtained sintered silicon
carbide of Example 2 was measured and found to be 8.times.10.sup.9
to 9.times.10.sup.10 atoms/cm.sup.2. The results are shown in Table
1 in more detail. In the above treatments, the temperature of the
solvent and the aqueous solutions was kept at the room
temperature.
Example 3
[0128] The same procedures as those conducted in Example 1 were
conducted except that the ultrasonic vibration (100V-26.+-.2 kHz)
was applied in every step of cleaning the sintered silicon carbide
and a sintered silicon carbide of Example 3 was obtained.
[0129] (Evaluation)
[0130] The surface cleanliness of the obtained sintered silicon
carbide of Example 3 was measured and found to be 8.times.10.sup.9
to 9.times.10.sup.10 atoms/cm.sup.2. The results are shown in Table
1 in more detail.
Example 4
[0131] The same procedures as those conducted in Example 1 were
conducted except that the temperature of the solvent and the
aqueous solution was adjusted at 70.degree. C. in the step of
cleaning the sintered silicon carbide and a sintered silicon
carbide of Example 4 was obtained.
[0132] (Evaluation)
[0133] The surface cleanliness of the obtained sintered silicon
carbide of Example 4 was measured and found to be 8.times.10.sup.9
to less than 1.times.10.sup.11 atoms/cm.sup.2. The results are
shown in Table 1 in more detail.
Example 5
[0134] The same procedures as those conducted in Example 1 were
conducted except that, in place of being cleaned in accordance with
the procedures conducted in Example 1, the sintered silicon carbide
obtained after the working was dipped into a quasi-aqueous organic
solvent (a mixed solvent containing a petroleum hydrocarbon, an
ester of an organic acid and a nonionic surfactant; not diluted) at
50.degree. C. for 15 minutes while ultrasonic vibration
(100V-26.+-.2 kHz) was applied, dipped into an aqueous solution of
a quaternary ammonium salt for 30 minutes, dipped into an aqueous
solution of a mixture of hydrofluoric acid, nitric acid and
sulfuric acid (38% hydrofluoric acid:68% nitric acid:98% sulfuric
acid: water=1:1:1:20) for 30 minutes and then dipped into pure
water and a sintered silicon carbide of Example 5 was obtained. The
temperature of the aqueous solutions was kept at the room
temperature in the above treatments except for the treatment of
dipping into the quasi-aqueous organic solvent.
[0135] (Evaluation)
[0136] The surface cleanliness of the obtained sintered silicon
carbide of Example 5 was measured and found to be 4.times.10.sup.9
to less than 9.times.10.sup.10 atoms/cm.sup.2. The obtained values
of the analyses of Zn and Cr were the values at the lower limit of
the detectable range of TXRF. The results are shown in Table 1 in
more detail.
Comparative Example 1
[0137] The same procedures as those conducted in Example 1 were
conducted except that the sintered silicon carbide was not dipped
into the aqueous mixture of hydrofluoric acid and nitric acid in
the step of cleaning the sintered silicon carbide and a sintered
silicon carbide of Comparative Example 1 was obtained.
[0138] (Evaluation)
[0139] The surface cleanliness of the obtained sintered silicon
carbide of Comparative Example 1 was measured and found to be
1.times.10.sup.11 to less than 1.times.10.sup.15 atoms/cm.sup.2.
The results are shown in Table 1 in more detail.
Comparative Example 2
[0140] The same procedures as those conducted in Example 1 were
conducted except that the sintered silicon carbide was not dipped
into the quasi-aqueous organic solvent in the step of cleaning the
sintered silicon carbide and a sintered silicon carbide of
Comparative Example 2 was obtained.
[0141] (Evaluation)
[0142] The surface cleanliness of the obtained sintered silicon
carbide of Comparative Example 2 was measured and found to be
1.times.10.sup.10 to less than 1.times.10.sup.14 atoms/cm.sup.2.
The results are shown in Table 1 in more detail.
1 TABLE 1 Comparative Example Example Impurity not cleaned 1 2 3 4
5 1 2 B 1050 4.6 5.2 8.0 7.0 8.3 88.5 50.2 Na 7260 7.8 8.0 1.6 2.1
6.5 15.9 13.6 Al 1250 8.8 7.6 8.3 9.5 1.2 756.0 64.5 K 65800 0.8
0.8 0.8 0.8 0.4 12.2 11.0 Ca 14500 8.0 8.0 6.0 7.2 5.0 18.3 15.7 Cr
8450 3.0 2.6 0.8 1.2 1.0 1620.0 13.6 Fe 24320 8.1 9.0 1.3 2.1 9.0
9295.0 1260.0 Ni 67800 9.5 5.0 1.8 1.8 1.9 1740.0 42.5 Cu 148400
7.1 6.5 1.5 1.9 4.7 32650.0 324.0 Zn 2400 1.2 1.5 0.8 (*1) 0.6 20.5
6.5 Note: The unit of the numbers in the Table 1: .times. 10.sup.10
atoms/cm.sup.2 (*1): 8 .times. 10.sup.9 to 8 .times. 10.sup.10
[0143] The results of Examples 1 to 5 and Comparative Examples 1
and 2 show that the sintered silicon carbide of the present
invention had a surface cleanliness at a level smaller than
1.times.10.sup.11 atoms/cm.sup.2, a high density and a high purity.
A sintered silicon carbide having surface cleanliness of this level
can be applied to various parts of semiconductors and electronic
parts.
[0144] It is shown by the results in Table 1 that the sintered
silicon carbide which was treated by cleaning in a wet condition of
the present invention had a surface cleanliness smaller than
1.times.10.sup.1 atoms/cm.sup.2 and can be applied to various parts
of semiconductors and electronic parts. The results obtained in
Example 5 show a remarkable effect of the combined use of sulfuric
acid on the removal of aluminum.
[0145] As described above, the present invention can provide a
sintered silicon carbide having a high density, with small amounts
of organic and inorganic impurities present on the surface and in
the vicinity of the surface.
[0146] Moreover, the present invention can provide a method for
cleaning a sintered silicon carbide in a wet condition in
accordance with which organic and inorganic impurities present on
the surface and in the vicinity of the surface of the sintered
silicon carbide can be removed easily.
* * * * *