U.S. patent application number 13/145063 was filed with the patent office on 2011-11-10 for silica container and method for producing the same.
This patent application is currently assigned to SHIN-ETSU QUARTZ PRODUCTS CO., LTD.. Invention is credited to Tomomi Usui, Shigeru Yamagata.
Application Number | 20110272322 13/145063 |
Document ID | / |
Family ID | 44145271 |
Filed Date | 2011-11-10 |
United States Patent
Application |
20110272322 |
Kind Code |
A1 |
Yamagata; Shigeru ; et
al. |
November 10, 2011 |
SILICA CONTAINER AND METHOD FOR PRODUCING THE SAME
Abstract
The present invention is a method for producing a silica
container having a substrate containing gaseous bubbles in its
outer peripheral part and an inner layer comprised of a transparent
silica glass formed on an inner surface of the substrate, wherein a
powdered raw material for forming a substrate containing Li, Na,
and K with the total concentration of 50 or less ppm by weight and
a powdered raw material for forming an inner layer containing Ca,
Sr, and Ba with the total concentration of 50 to 2000 ppm by weight
are prepared; a preliminarily molded substrate is formed in a
frame; a preliminarily molded inner layer is formed on an inner
surface of the preliminarily molded substrate; and the
preliminarily molded substrate and molded inner layer are heated
from inside thereof by a discharge-heat melting method under a gas
atmosphere containing a hydrogen gas or a helium gas or a gas
mixture thereof with the ratio of more than 10% by volume thereby
making an outer peripheral part of the preliminarily molded
substrate to a sintered body and an inner peripheral part of the
preliminarily molded substrate and the preliminarily molded inner
layer to a fused glass body. With this, a method for producing a
silica container, producible with a low cost and having a high
durability and dimensional precision, and a container of this sort
can be provided.
Inventors: |
Yamagata; Shigeru; (Tokyo,
JP) ; Usui; Tomomi; (Tokyo, JP) |
Assignee: |
SHIN-ETSU QUARTZ PRODUCTS CO.,
LTD.
Tokyo
JP
|
Family ID: |
44145271 |
Appl. No.: |
13/145063 |
Filed: |
October 19, 2010 |
PCT Filed: |
October 19, 2010 |
PCT NO: |
PCT/JP2010/006179 |
371 Date: |
July 18, 2011 |
Current U.S.
Class: |
206/524.6 ;
65/17.3 |
Current CPC
Class: |
C03C 3/06 20130101; C03B
2201/54 20130101; C30B 35/002 20130101; C03B 19/095 20130101; C03C
2201/54 20130101; C30B 15/10 20130101; Y02P 40/57 20151101; B65D
13/02 20130101; C30B 29/06 20130101 |
Class at
Publication: |
206/524.6 ;
65/17.3 |
International
Class: |
B65D 85/00 20060101
B65D085/00; C03B 19/06 20060101 C03B019/06; C03B 19/09 20060101
C03B019/09; C03B 20/00 20060101 C03B020/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 10, 2009 |
JP |
2009-280417 |
Claims
1-9. (canceled)
10. A method for producing a silica container arranged with a
substrate, having a rotational symmetry, comprised of mainly a
silica, and containing gaseous bubbles at least in its peripheral
part, and an inner layer, formed on an inner surface of the
substrate and comprised of a transparent silica glass; wherein the
process comprises at least: a step of preparing a powdered silica
having particle diameter of 10 to 1000 .mu.m and containing Li, Na,
and K with the total concentration of 50 or less ppm by weight as a
powdered raw material for forming the substrate, a step of
preparing a powdered silica having particle diameter of 10 to 1000
.mu.m and containing at least one of Ca, Sr, and Ba with the total
concentration of 50 to 2000 ppm by weight as a powdered raw
material for forming the inner layer, a step of forming a
preliminarily molded substrate, wherein the powdered raw material
for forming the substrate is fed into a frame and then
preliminarily molded to an intended shape with rotating the frame,
a step of forming a preliminarily molded inner layer, wherein the
powdered raw material for forming the inner layer is fed onto an
inner surface of the preliminarily molded substrate and then
preliminarily molded to an intended shape in accordance with an
inner surface of the preliminarily molded substrate, and a step of
forming the substrate and the inner layer, wherein the
preliminarily molded substrate and the preliminarily molded inner
layer are heated from inside of the preliminarily molded substrate
and inner layer by a discharge-heat melting method under a gas
atmosphere containing a hydrogen gas or a helium gas or a gas
mixture thereof with the ratio of more than 10% by volume thereby
making an outer peripheral part of the preliminarily molded
substrate to a sintered body while an inner peripheral part of the
preliminarily molded substrate and the preliminarily molded inner
layer to a fused glass body.
11. A method for producing a silica container arranged with a
substrate, having a rotational symmetry, comprised of mainly a
silica, and containing gaseous bubbles at least in its peripheral
part, and an inner layer, formed on an inner surface of the
substrate and comprised of a transparent silica glass; wherein the
process comprises at least: a step of preparing a powdered silica
having particle diameter of 10 to 1000 .mu.m and containing Li, Na,
and K with the total concentration of 50 or less ppm by weight as a
powdered raw material for forming the substrate, a step of
preparing a powdered silica having particle diameter of 10 to 1000
.mu.m and containing at least one of Ca, Sr, and Ba with the total
concentration of 50 to 2000 ppm by weight as a powdered raw
material for forming the inner layer, a step of forming a
preliminarily molded substrate, wherein the powdered raw material
for forming the substrate is fed into a frame and then
preliminarily molded to an intended shape with rotating the frame,
a step of forming the substrate, wherein the preliminarily molded
substrate is heated from inside of the preliminarily molded
substrate by a discharge-heat melting method thereby making an
outer peripheral part of the preliminarily molded substrate to a
sintered body while an inner peripheral part of the preliminarily
molded substrate to a fused glass body, and a step of forming the
inner layer on an inner surface of the substrate, wherein the
powdered raw material for forming the inner layer is spread from
inside of the substrate with heating at high temperature from its
inside by a discharge-heat melting method under a gas atmosphere
containing a hydrogen gas or a helium gas or a gas mixture thereof
with the ratio of more than 10% by volume.
12. The method for producing a silica container according to claim
10, wherein the discharge-heat melting step is conducted with
aspirating from outside of the preliminarily molded substrate
through the frame.
13. The method for producing a silica container according to claim
11, wherein at least one of the discharge-heat melting steps is
conducted with aspirating from outside of the substrate or the
preliminarily molded substrate through the frame.
14. The method for producing a silica container according to claim
10, wherein the powdered raw material for forming the inner layer
is made to contain Ba with the concentration of 100 to 1000 ppm by
weight and Al with the concentration of 10 to 100 ppm by
weight.
15. The method for producing a silica container according to claim
11, wherein the powdered raw material for forming the inner layer
is made to contain Ba with the concentration of 100 to 1000 ppm by
weight and Al with the concentration of 10 to 100 ppm by
weight.
16. The method for producing a silica container according to claim
12, wherein the powdered raw material for forming the inner layer
is made to contain Ba with the concentration of 100 to 1000 ppm by
weight and Al with the concentration of 10 to 100 ppm by
weight.
17. The method for producing a silica container according to claim
13, wherein the powdered raw material for forming the inner layer
is made to contain Ba with the concentration of 100 to 1000 ppm by
weight and Al with the concentration of 10 to 100 ppm by
weight.
18. The method for producing a silica container according to claim
10, wherein a dew-point temperature of the gas atmosphere
containing a hydrogen gas or a helium gas or a gas mixture thereof
be set between 15.degree. C. and -15.degree. C. and controlled
within .+-.2.degree. C. of the set dew-point temperature.
19. The method for producing a silica container according to claim
11, wherein a dew-point temperature of the gas atmosphere
containing a hydrogen gas or a helium gas or a gas mixture thereof
be set between 15.degree. C. and -15.degree. C. and controlled
within .+-.2.degree. C. of the set dew-point temperature.
20. The method for producing a silica container according to claim
12, wherein a dew-point temperature of the gas atmosphere
containing a hydrogen gas or a helium gas or a gas mixture thereof
be set between 15.degree. C. and -15.degree. C. and controlled
within .+-.2.degree. C. of the set dew-point temperature.
21. The method for producing a silica container according to claim
13, wherein a dew-point temperature of the gas atmosphere
containing a hydrogen gas or a helium gas or a gas mixture thereof
be set between 15.degree. C. and -15.degree. C. and controlled
within .+-.2.degree. C. of the set dew-point temperature.
22. The method for producing a silica container according to claim
10, wherein, in the gas atmosphere containing a hydrogen gas or a
helium gas or a gas mixture thereof, the ratio of a hydrogen gas or
a helium gas or a gas mixture thereof is made to 100% by
volume.
23. The method for producing a silica container according to claim
11, wherein, in the gas atmosphere containing a hydrogen gas or a
helium gas or a gas mixture thereof, the ratio of a hydrogen gas or
a helium gas or a gas mixture thereof is made to 100% by
volume.
24. The method for producing a silica container according to claim
12, wherein, in the gas atmosphere containing a hydrogen gas or a
helium gas or a gas mixture thereof, the ratio of a hydrogen gas or
a helium gas or a gas mixture thereof is made to 100% by
volume.
25. The method for producing a silica container according to claim
13, wherein, in the gas atmosphere containing a hydrogen gas or a
helium gas or a gas mixture thereof, the ratio of a hydrogen gas or
a helium gas or a gas mixture thereof is made to 100% by
volume.
26. A silica container arranged with a substrate, having a
rotational symmetry, comprised of mainly a silica, containing
gaseous bubbles in its peripheral part, and having a transparent
silica glass in its inner peripheral part, and an inner layer,
formed on an inner surface of the substrate and comprised of a
transparent silica glass; wherein the substrate contains Li, Na,
and K with the total concentration of 50 or less ppm by weight and
shows a linear light transmittance of 91.8 to 93.2% at a light
wavelength of 600 nm for a sample having 10 mm thickness cut-out
from the inner peripheral part and finished with both surfaces
being parallel and optically polished, and the inner layer contains
Li, Na, and K with the total concentration of 100 or less ppb by
weight and at least one of Ca, Sr, and Ba with the total
concentration of 50 to 2000 ppm by weight and shows a linear light
transmittance of 91.8 to 93.2% at a light wavelength of 600 nm for
a sample having 10 mm thickness cut-out from the inner layer and
finished with both surfaces being parallel and optically polished,
and amount of water molecules released from a sample cut-out from
the inner layer upon heating under vacuum at 1000.degree. C. is
less than 2.times.10.sup.17 molecules/g.
27. The silica container according to claim 26, wherein the inner
layer is made to contain Ba with the concentration of 100 to 1000
ppm by weight and Al with the concentration of 10 to 100 ppm by
weight.
28. The silica container according to claim 26, wherein the inner
layer is made to contain OH groups with the concentration of 1 to
50 ppm by weight, Li, Na, and K with each concentration of 20 or
less ppb by weight, and Ti, Cr, Mn, Fe, Ni, Cu, Zn, Zr, Mo, and W
with each concentration of 10 or less ppb by weight.
29. The silica container according to claim 27, wherein the inner
layer is made to contain OH groups with the concentration of 1 to
50 ppm by weight, Li, Na, and K with each concentration of 20 or
less ppb by weight, and Ti, Cr, Mn, Fe, Ni, Cu, Zn, Zr, Mo, and W
with each concentration of 10 or less ppb by weight.
Description
TECHNICAL FIELD
[0001] The present invention relates to a silica container
comprised of mainly silica and to a method for producing it, in
particular, to a silica container of a low cost, a high dimensional
precision, and a high heat resistance and to a method for producing
it.
BACKGROUND ART
[0002] A silica glass is used for a lens, a prism and a photomask
of a photolithography instrument in manufacturing of a large-scale
integrated circuit (LSI), for a TFT substrate used for a display,
for a tube of a lamp, for a window material, for a reflection
plate, for a cleaning container in a semiconductor industry, for a
container for melting of a silicon semiconductor, and so forth.
However, an expensive compound such as silicon tetrachloride must
be used as a raw material for these silica glasses; on top of that,
melting temperature and processing temperature of a silica glass is
extraordinary high, as high as about 2000.degree. C., thereby
leading to a high energy consumption and a high cost. Accordingly,
from the past, a method for producing a silica glass by using a
relatively inexpensive, powdered raw material has been
considered.
[0003] For example, in Patent Document 1, a method (slip casting
method), wherein at least two different kinds of silica glass
particles, for example, silica glass fine particles and silica
glass granules are mixed to obtain a water-containing suspension
solution, which is then press molded and sintered at high
temperature to obtain a silica-containing composite body, is
disclosed. In Patent Document 2, a method, wherein a mixed solution
(slurry) containing silica glass particles having the size of 100
.mu.m or less and silica glass granules having the size of 100
.mu.m or more is prepared, then the slurry is cast into a molding
frame, dried, and then sintered to obtain an opaque silica glass
composite material, is disclosed. In these conventional slip
casting methods, however, shrinkage of a molded article in a drying
process and a sintering process is so significant that a thick
silica glass article with a high dimensional precision could not be
obtained.
[0004] Accordingly, there are problems in each method for producing
a silica glass article as mentioned above. Therefore, as a method
for producing a silica crucible for manufacturing of a single
crystal silicon used for LSI (for a device), such production
methods as those disclosed in Patent Document 3 and Patent Document
4 are being used still today. In these methods, after a powdered,
ultra-highly purified natural quartz or a powdered synthetic
cristobalite is fed into a rotating frame and then molded, carbon
electrodes are inserted from the top and then electrically charged,
thereby causing arc discharge to raise the atmospheric temperature
to a temperature range for melting of the powdered quartz
(temperature is estimated in the range from about 1800 to about
2100.degree. C.) so that the powdered raw quartz may be melted and
sintered.
[0005] In the methods such as those mentioned above, however, there
has been a problem of a high cost because a powdered raw material
quartz with high purity is used. In addition, because various kinds
of impure gases are dissolved in a produced silica crucible, the
gases are released and then incorporated into a silicon single
crystal as gaseous bubbles thereby causing such problems as defects
called a void and a pinhole when it is used as a silica crucible
for growing of a silicon single crystal; and thus this has been
causing problems in production cost as well as quality of the
silicon crystal. In addition, there has been a big problem in
durability of the silica crucible because of low etching resistance
to a silicon melt at the time of pulling up of a single crystal
silicon.
[0006] A method to improve the etching resistance to a silicon melt
in a silica crucible for pulling up of a single crystal is shown in
Patent Document 5. In Patent Document 5, an effect of applying a
crystallization accelerator on an inner surface of a silica glass
crucible is shown. As the crystallization accelerator, Mg, Sr, Ca,
and Ba, which are alkaline earth metal elements belonging to the 2a
group, and Al, which is the element belonging to the 3b group, are
shown. However, a silica glass crucible as shown in Patent Document
5 was not the one having a transparent silica glass layer
completely free from gaseous bubbles in an inner surface part of
the crucible, but the one containing micro gaseous bubbles and
inhomogeneously undissolved particles of various doped elements.
Accordingly, there have been problems frequently that a pulled-up
silicon single crystal contains silica fine particles as foreign
substances and has defects such as a void and a pinhole. In
addition, there appeared a problem of deformation of a crucible
inner surface caused by large expansion of micro gaseous bubbles
present inside the crucible during pulling up of a silicon single
crystal.
[0007] A method to reduce gaseous bubbles in a silica glass in an
inner surface of a silica crucible used for pulling up of a single
crystal so that bubble expansion of the silica crucible in use may
be suppressed is described in Patent Document 6. In Patent Document
6, it is disclosed that a silica crucible inner surface having few
gaseous bubbles can be obtained if a powdered raw material for the
silica crucible is made to contain hydrogen molecules with the
concentration of 5.times.10.sup.17 to 3.times.10.sup.19
molecules/g. However, with this method, although amount of gaseous
bubbles in the silica crucible inner surface could be reduced, an
etching resistance to a silicon melt could not be improved by
crystallizing the silica crucible inner surface to cristobalite. In
addition, there has been a problem of a poor storage property of a
hydrogen-containing powdered raw material because hydrogen
molecules doped in the powdered raw material are gradually released
to outside during storage of the powdered raw material.
[0008] A method for reducing growth of gaseous bubbles during the
time that a silica crucible for silicon pulling up is in use is
described in Patent Document 7. In this document, a method is
disclosed wherein inside of a molded container is made an
atmosphere of a helium gas or a hydrogen gas while degassing from
its outside by aspiration during arc discharge melting by carbon
electrodes in manufacturing of a crucible. With this method,
however, even though amount of gaseous bubbles in the silica glass
inner surface layer could be reduced, the OH group concentration in
the silica crucible could not be reduced to a certain controlled
level, nor was possible to improve durability and heat resistance
of the silica crucible by finely crystallizing the inner surface to
cristobalite during the time of using the crucible.
[0009] In Patent Document 8, a method for reducing amount of
gaseous bubbles contained in a silica crucible used for pulling up
of a silicon crystal is disclosed. In the Document, it is disclosed
that any of a hydrogen gas and a helium gas or both is supplied to
a powder article of the container during heating at the time of
crucible production.
[0010] In Patent Document 9, it is disclosed that, during heating
at the time of crucible production, an arc melting is started and
continued after a helium gas or an argon gas is supplied to a
powder article of the container; and before termination of the arc
melting, supply of a helium gas or an argon gas is stopped or the
amount thereof is reduced, while supply of a hydrogen gas is
started.
[0011] In these methods, however, in similar to the foregoing, even
though gaseous bubbles inside the silica crucible could be reduced,
durability and heat resistance of the crucible could not be
improved by protecting the inner surface by finely crystallizing
the inner surface to cristobalite during the time of using the
silica crucible.
CITATION LIST
Patent Document
[0012] Patent Document 1: Japanese Patent Application Laid-Open
Publication No. 2002-362932 [0013] Patent Document 2: Japanese
Patent Application Laid-Open Publication No. 2004-131380 [0014]
Patent Document 3: Japanese Examined Patent Publication No.
H04-22861 [0015] Patent Document 4: Japanese Examined Patent
Publication No. H07-29871 [0016] Patent Document 5: Japanese Patent
Application Laid-Open Publication No. H08-2932 [0017] Patent
Document 6: Japanese Patent Application Laid-Open Publication No.
2007-326780 [0018] Patent Document 7: Japanese Patent Application
Laid-Open Publication No. H08-268727 [0019] Patent Document 8:
Japanese Patent Application Laid-Open Publication No. H09-20586
[0020] Patent Document 9: Japanese Patent Application Laid-Open
Publication No. 2000-344536
SUMMARY OF THE INVENTION
Technical Problem to be Solved by the Invention
[0021] The present invention was made in view of the problems as
mentioned above, and has an object to provide; a method for
producing a silica container, comprised of mainly a silica, having
a high dimensional precision and heat resistance, and producible
with a low cost; and the silica container of this sort.
Solution to Problem
[0022] The present invention was made to solve the problems as
mentioned above and provides a method for producing a silica
container arranged with a substrate, having a rotational symmetry,
comprised of mainly a silica, and containing gaseous bubbles at
least in its peripheral part, and an inner layer, formed on an
inner surface of the substrate and comprised of a transparent
silica glass; wherein the process comprises at least:
[0023] a step of preparing a powdered silica having particle
diameter of 10 to 1000 .mu.m and containing Li, Na, and K with the
total concentration of 50 or less ppm by weight as a powdered raw
material for forming the substrate,
[0024] a step of preparing a powdered silica having particle
diameter of 10 to 1000 .mu.m and containing at least one of Ca, Sr,
and Ba with the total concentration of 50 to 2000 ppm by weight as
a powdered raw material for forming the inner layer,
[0025] a step of forming a preliminarily molded substrate, wherein
the powdered raw material for forming the substrate is fed into a
frame and then preliminarily molded to an intended shape with
rotating the frame,
[0026] a step of forming a preliminarily molded inner layer,
wherein the powdered raw material for forming the inner layer is
fed onto an inner surface of the preliminarily molded substrate and
then preliminarily molded to an intended shape in accordance with
an inner surface of the preliminarily molded substrate, and
[0027] a step of forming the substrate and the inner layer, wherein
the preliminarily molded substrate and the preliminarily molded
inner layer are heated from inside of the preliminarily molded
substrate and inner layer by a discharge-heat melting method under
a gas atmosphere containing a hydrogen gas or a helium gas or a gas
mixture thereof with the ratio of more than 10% by volume thereby
making an outer peripheral part of the preliminarily molded
substrate to a sintered body while an inner peripheral part of the
preliminarily molded substrate and the preliminarily molded inner
layer to a fused glass body.
[0028] According to the method for producing a powdered silica as
mentioned above, a high inhibiting effect of impurity diffusion, a
high durability, and the like during the time that the silica
container thus produced is used at high temperature can be
obtained; and in addition, generation of gaseous bubbles in an
inner wall of the silica container can be effectively
suppressed.
[0029] The present invention further provides a method for
producing a silica container arranged with a substrate, having a
rotational symmetry, comprised of mainly a silica, and containing
gaseous bubbles at least in its peripheral part, and an inner
layer, formed on an inner surface of the substrate and comprised of
a transparent silica glass; wherein the process comprises at
least:
[0030] a step of preparing a powdered silica having particle
diameter of 10 to 1000 .mu.m and containing Li, Na, and K with the
total concentration of 50 or less ppm by weight as a powdered raw
material for forming the substrate,
[0031] a step of preparing a powdered silica having particle
diameter of 10 to 1000 .mu.m and containing at least one of Ca, Sr,
and Ba with the total concentration of 50 to 2000 ppm by weight as
a powdered raw material for forming the inner layer,
[0032] a step of forming a preliminarily molded substrate, wherein
the powdered raw material for forming the substrate is fed into a
frame and then preliminarily molded to an intended shape with
rotating the frame,
[0033] a step of forming the substrate, wherein the preliminarily
molded substrate is heated from inside of the preliminarily molded
substrate by a discharge-heat melting method thereby making an
outer peripheral part of the preliminarily molded substrate to a
sintered body while an inner peripheral part of the preliminarily
molded substrate to a fused glass body, and
[0034] a step of forming the inner layer on an inner surface of the
substrate, wherein the powdered raw material for forming the inner
layer is spread from inside of the substrate with heating at high
temperature from its inside by a discharge-heat melting method
under a gas atmosphere containing a hydrogen gas or a helium gas or
a gas mixture thereof with the ratio of more than 10% by
volume.
[0035] Even according to the method for producing a powdered silica
as mentioned above, a high inhibiting effect of impurity diffusion,
a high durability, and the like during the time that the silica
container thus produced is used at high temperature can be
obtained; and in addition, generation of gaseous bubbles in an
inner wall of the silica container can be effectively
suppressed.
[0036] According to the method for producing a silica container by
the present invention, at least one of the discharge-heat melting
steps may be conducted with aspirating from outside of the
substrate or the preliminarily molded substrate through the
frame.
[0037] Accordingly, in the method for producing a silica container
by the present invention, at least one of the discharge-heat
melting steps can be conducted with aspirating from outside of the
substrate or the preliminarily molded substrate through the frame
so that a dissolved gas in the thus produced silica container may
be reduced further effectively.
[0038] In addition, it is preferable that the powdered raw material
for forming the inner layer be made to contain Ba with the
concentration of 100 to 1000 ppm by weight and Al with the
concentration of 10 to 100 ppm by weight.
[0039] When the powdered raw material for forming the inner layer
is made to contain Ba with the concentration of 100 to 1000 ppm by
weight and Al with the concentration of 10 to 100 ppm by weight,
the inner layer can be made to a silica glass layer having further
high light transmittance and containing extremely low amount of
gaseous bubbles.
[0040] In addition, it is preferable that a dew-point temperature
of the gas atmosphere containing a hydrogen gas or a helium gas or
a gas mixture thereof be set between 15.degree. C. and -15.degree.
C. and controlled within .+-.2.degree. C. of the set dew-point
temperature.
[0041] Accordingly, when the dew-point temperature of the gas
atmosphere is set and controlled as mentioned above, amount of OH
group and amount of water (H.sub.2O) contained in the silica
container can be reduced to intended values in spite of low
cost.
[0042] In addition, it is preferable that, in the gas atmosphere
containing a hydrogen gas or a helium gas or a gas mixture thereof,
the ratio of a hydrogen gas or a helium gas or a gas mixture
thereof be made to 100% by volume.
[0043] Accordingly, in the gas atmosphere containing a hydrogen gas
or a helium gas or a gas mixture thereof, when the ratio of a
hydrogen gas or a helium gas or a gas mixture thereof is made to
100% by volume, generation of gaseous bubbles in an inner wall of
the silica container can be suppressed further effectively.
[0044] In addition, the present invention provides a silica
container, a silica container arranged with a substrate, having a
rotational symmetry, comprised of mainly a silica, containing
gaseous bubbles in its peripheral part, and having a transparent
silica glass in its inner peripheral part, and an inner layer,
formed on an inner surface of the substrate and comprised of a
transparent silica glass; wherein
[0045] the substrate contains Li, Na, and K with the total
concentration of 50 or less ppm by weight and shows a linear light
transmittance of 91.8 to 93.2% at a light wavelength of 600 nm for
a sample having 10 mm thickness cut-out from the inner peripheral
part and finished with both surfaces being parallel and optically
polished, and
[0046] the inner layer contains Li, Na, and K with the total
concentration of 100 or less ppb by weight and at least one of Ca,
Sr, and Ba with the total concentration of 50 to 2000 ppm by weight
and shows a linear light transmittance of 91.8 to 93.2% at a light
wavelength of 600 nm for a sample having 10 mm thickness cut-out
from the inner layer and finished with both surfaces being parallel
and optically polished, and amount of water molecules released from
a sample cut-out from the inner layer upon heating under vacuum at
1000.degree. C. is less than 2.times.10.sup.17 molecules/g.
[0047] The silica container as mentioned above can be given in the
container inner wall a high inhibiting effect of impurity
diffusion, a high durability, and the like during its use at high
temperature, in spite of a low cost silica container having
adequate temperature uniformity; and in addition, generation of
gaseous bubbles in the container inner wall can be effectively
suppressed. As a result, a harmful effect to a material
accommodated in the silica container due to gaseous bubbles
generated in the container inner wall can be suppressed. Meanwhile,
the light transmittance reflects amount of gaseous bubbles in a
glass and uniform solubility of a doped element.
[0048] In this case, it is preferable that the inner layer be made
to contain Ba with the concentration of 100 to 1000 ppm by weight
and Al with the concentration of 10 to 100 ppm by weight.
[0049] When the inner layer is made to contain Ba with the
concentration of 100 to 1000 ppm by weight and Al with the
concentration of 10 to 100 ppm by weight, the inner layer can be
made to a silica glass layer having further high light
transmittance and containing extremely low amount of gaseous
bubbles.
[0050] In addition, it is preferable that the inner layer be made
to contain OH groups with the concentration of 1 to 50 ppm by
weight, Li, Na, and K with each concentration of 20 or less ppb by
weight, and Ti, Cr, Mn, Fe, Ni, Cu, Zn, Zr, Mo, and W with each
concentration of 10 or less ppb by weight.
[0051] When the inner layer is made to contain OH groups and
respective metals with the concentrations as mentioned above,
impurity contamination to a material accommodated in the produced
silica can be prevented further effectively.
Advantageous Effects of the Invention
[0052] According to the method for producing a silica container of
the present invention, a high inhibiting effect of impurity
diffusion, a high durability, and the like during the time that the
produced silica container is used at high temperature can be
obtained; and in addition, generation of gaseous bubbles in an
inner wall of the silica container can be effectively
suppressed.
[0053] In addition, the silica container according to the present
invention can be given in the silica inner wall a high inhibiting
effect of impurity diffusion, a high durability, and the like
during its use at high temperature, in spite of a low cost silica
container having adequate uniformity of temperature; and in
addition, generation of gaseous bubbles in the silica inner wall
can be effectively suppressed. As a result, a harmful effect to a
material accommodated in the silica container due to gaseous
bubbles generated in the silica container inner wall can be
suppressed.
BRIEF DESCRIPTION OF DRAWINGS
[0054] FIG. 1 is a flow chart showing outline of one example of the
method for producing a silica container according to the present
invention.
[0055] FIG. 2 is a flow chart showing outline of another example of
the method for producing a silica container of the present
invention.
[0056] FIG. 3 is a flow chart showing outline of one example of the
step of preparing a powdered raw material for forming the inner
layer according to the present invention.
[0057] FIG. 4 is a schematic cross section view showing one example
of the silica container according to the present invention.
[0058] FIG. 5 is a schematic cross section view showing one example
of the frame usable in the method of a silica container according
to the present invention.
[0059] FIG. 6 is a schematic cross section view showing another
example of the frame usable in the method of a silica container
according to the present invention.
[0060] FIG. 7 is a schematic cross section view schematically
showing one example of the step of forming the preliminarily molded
substrate in the method for producing a silica container according
to the present invention.
[0061] FIG. 8 is a schematic cross section view schematically
showing one example of the step of forming the preliminarily molded
inner layer on an inner surface of the preliminarily molded
substrate in the method for producing a silica container according
to the present invention.
[0062] FIG. 9 is a schematic cross section view schematically
showing one example of the step of discharge-heating of the
preliminarily molded substrate and the preliminarily molded inner
layer simultaneously in the method for producing a silica container
according to the present invention.
[0063] FIG. 10 is a schematic cross section view schematically
showing a part of one example of the step of forming the substrate
(before discharge-heat melting) in the method for producing a
silica container according to the present invention.
[0064] FIG. 11 is a schematic cross section view schematically
showing a part of one example of the step of forming the substrate
(during discharge-heat melting) in the method for producing a
silica container according to the present invention.
[0065] FIG. 12 is a schematic cross section view schematically
showing one example of the step of forming the inner layer on an
inner surface of the substrate in the method for producing a silica
container according to the present invention.
DESCRIPTION OF EMBODIMENTS
[0066] As mentioned above, in a conventional method for producing a
silica container, there have been problems in dimensional precision
and cost.
[0067] In addition, a silica container produced by a conventional
method for producing a silica container had a problem such as, a
harmful effect of gaseous bubbles to a material accommodated
therein, for example, incorporation of gaseous bubbles into a
silicon single crystal in a silica crucible for growing of a
silicon single crystal.
[0068] The inventors carried out investigation in view of the
problems as mentioned above and found the following problems to be
solved.
[0069] Firstly, a silica container such as a crucible and a boat
for melting of a metal silicon and for production of a silicon
single crystal or a polycrystalline silicon requires thermal
uniformity inside the container under atmosphere of a high heating
temperature. Because of this, the first problem to be solved is to
make the silica container at least a two-layer structure, wherein
an outside part of the container is made to a porous, white and
opaque silica glass while an inside part of the container is made
to a thick, colorless and transparent silica glass containing
substantially no gaseous bubbles.
[0070] The second problem to be solved is to give a function to
inhibit diffusion of an impure substance (impurity-shielding
function). This is to suppress a harmful contamination effect to a
material accommodated in a silica container due to an impure
substance contained in the silica container.
[0071] For example, if an impure metal element contained in the
silica container, for example, not only alkaline metal elements
such as Li, Na, and K, but also Ti, Cr, Mn, Fe, Ni, Cu, Zn, Zr, Mo,
W, and the like are incorporated into a silicon crystal during
production of silicon crystals, it causes decrease in the incident
photon-to-current conversion efficiency especially in a silicon
device for solar use. Accordingly, in order to inhibit diffusion of
an impure substance contained in the silica container into a
silicon melt, inner surface of the silica container is made finely
crystallized (made to a glass ceramics) so that a function to
inhibit diffusion of an impure substance may be given. In addition,
in view of quality of the finely crystallized part of the inner
surface of the silica container having dimensionally fine and
precise individual crystals, a crystallized layer is made of
cristobalite and the like having fine texture.
[0072] The third problem is to give an etching resistance by finely
crystallizing inner surface of the silica container with
cristobalite and the like having fine texture.
[0073] For example, if a component (SiO.sub.2) itself of the silica
container is dissolved into a silicon melt during production of a
silicon single crystal production thereby incorporating an oxygen
element into the silicon crystal, there appears a problem such as,
for example, to cause decrease in the incident photon-to-current
conversion efficiency in a silicon device for solar use.
Accordingly, in similar to the above, the inner surface of the
container is made to have characteristics not to be dissolved
easily into a silicon melt (i.e., having an etching resistance to a
silicon melt), that is, to make the inner surface of the container
finely crystallized by cristobalite and the like having fine
texture.
[0074] In the case that at least one element of alkaline earth
metal elements Ca, Sr, and Ba is doped non-uniformly as a
crystallization accelerator in the inner surface layer of the
silica container and the inner surface layer contains fine gaseous
bubbles, a gas contained therein is released from the gaseous
bubbles and eluted into a silicon melt during production of a
silicon crystal, thereby causing structural defects called a
pinhole and a void by incorporation of gas bubbles into the silicon
crystal. Accordingly, the fourth problem is to give a thick glass
layer not containing gaseous bubbles in the inner surface layer of
the silica container while containing an alkaline earth metal
element uniformly dissolved thereby making a completely colorless
and transparent glass having a high light transmittance.
[0075] As mentioned above, in the present invention, it was
necessary to simultaneously solve these four technical problems
with a lower cost as compare with a silica container such as a
crucible, produced by a conventional method using an expensive,
high purity powdered silica raw material, for pulling up of a high
purity single crystal silicon; accordingly, this is the fifth
problem to be solved.
[0076] Hereinbelow, the present invention will be explained in
detail with referring to the figures, but the present invention is
not limited to them. In particular, in what follows, a silica
container (a solar-grade crucible) applicable as a container for
melting of a metal silicon used as a material for a solar cell (a
solar photovoltaic power generation, or a solar power generation)
as well as a production method thereof will be mainly explained as
one suitable example of application of the present invention; but
the present invention is not limited to this and can be applied
widely to a general silica container comprised of mainly a silica
and used at high temperature.
[0077] In FIG. 4, a schematic cross section view of one example of
the silica container according to the present invention is
shown.
[0078] The silica container 71 according to the present invention
has a rotational symmetry, and its basic structure is comprised of
the substrate 51 and the inner layer 56.
[0079] The substrate 51 has a rotational symmetry and is comprised
of mainly a silica. The substrate 51 contains gaseous bubbles in
the substrate's outer peripheral part 51a. Namely, the substrate's
outer peripheral part has a porous, white and opaque layer part.
The substrate's inner peripheral part 51b contains a transparent
silica glass.
[0080] The inner layer 56 is formed on the inner surface of the
substrate 51 and is comprised of a transparent silica glass.
[0081] In the present invention, in addition to the foregoing, the
substrate 51 contains Li, Na, and K with the total concentration of
50 or less ppm by weight.
[0082] In addition, the inner layer 56 contains at least one of Ca,
Sr, and Ba with the total concentration of 50 to 2000 ppm by weight
and shows a linear light transmittance of preferably 91.8 to 93.2%,
or more preferably 92.4 to 93.2%, at a light wavelength of 600 nm
for a sample having 10 mm thickness with both surfaces being
parallel and optically polished. Further, amount of water molecules
released from a sample cut-out from the inner layer 56 is less than
2.times.10.sup.17 molecules/g, or preferably less than
1.times.10.sup.17 molecules/g upon heating at 1000.degree. C. under
vacuum.
[0083] In addition, in the silica container according to the
present invention, a linear light transmittance of the substrate 51
is also 91.8 to 93.2% at a light wavelength of 600 nm for a sample
having 10 mm thickness cut-out from the inner peripheral part 51b
and finished with both surfaces being parallel and optically
polished.
[0084] Meanwhile, as far as the container of the present invention
has at least the substrate 51 and the inner layer 56, the silica
container may further contain a layer other than these layers.
[0085] The silica container 71 having a composition as mentioned
above can have an adequate temperature uniformity with low cost. In
other words, in the silica container, when at least the substrate's
outer peripheral part 51a is made to a porous non-transparent
silica body and at least the inner layer 56 is made to a thick
transparent silica glass body not substantially containing gaseous
bubbles, temperature uniformity inside the silica container 71
during the time that the silica container 71 is used at high
temperature can be improved.
[0086] In addition, when the silica container 71 is used at high
temperature between 1400 and 1600.degree. C., if the inner layer 56
is made to contain at least one of Ca, Sr, and Ba, especially Ba as
mentioned above, a surface part of the silica glass can be
recrystallized by cristobalite and the like; and as a result,
elution by diffusion of an alkaline metal element such as Na, K,
and Li contained in the substrate 51 of the silica container 71 can
be prevented, and in addition, etching of an inner surface of the
silica container 71 by a material accommodated therein, such as a
metal silicon melt, which is treated in the silica container 71,
can be reduced. Ba is preferable, also because it is not easily
incorporated into a produced silicon single crystal.
[0087] In addition, according to the present invention, generation
of gaseous bubbles in the inner layer 56 and the inner peripheral
part 51b of the substrate can be effectively suppressed. As a
result, a harmful effect to a material accommodated in the silica
container, due to generation of gaseous bubbles in an inner wall of
the silica container 71, can be suppressed.
[0088] Meanwhile, if gaseous bubbles in the inner layer 56 is
adequately suppressed and an alkaline earth metal element such as
Ba is uniformly dissolved, a light transmittance at a light
wavelength of 600 nm for the sample having 10 mm thickness cut out
from the inner layer 56 and finished with both surfaces being
parallel and optically polished becomes 91.8 to 93.2%, as mentioned
above. If the gaseous bubbles are further reduced and the alkaline
earth metal element is uniformly dissolved, the light transmittance
becomes 92.4 to 93.2%. Among the values, the upper limit value
93.2% is theoretically the maximum value in the silica glass. In
addition, the present invention can provide the silica container 71
showing 91.8 to 93.2% of a linear light transmittance also in the
substrate 51 at a light wavelength of 600 nm for the sample having
10 mm thickness cut-out from the inner peripheral part 51b and
finished with both surfaces being parallel and optically
polished.
[0089] Meanwhile, a length of the sides other than one side having
the length of 10 mm in the sample having 10 mm thickness cut-out
from each layer and finished with both surfaces being parallel and
optically polished is not particularly limited as far as the linear
transmittance can be measured. For example, a linear transmittance
can be measured for a 2-mm.times.2-mm.times.10-mm sample.
[0090] If the inner layer 56 is made to contain Al with the
concentration of 10 to 100 ppm by weight, not only a further
enhanced inhibition effect of impurity diffusion can be given but
also the alkaline earth metal element such as Ba can be dissolved
further uniformly. Accordingly, generation of gaseous bubbles in an
inner wall of the silica container can be suppressed further
effectively.
[0091] Details of a mechanism for Al to prevent migration and
diffusion of an impure metal element in the silica glass from
occurring is not known; but it is assumed that, because of
difference in the coordination number by displacing a Si atom with
an Al atom, a positive ion (cation) of an impure alkaline metal
element such as Li.sup.+, Na.sup.+, and K.sup.+ is adsorbed and its
diffusion is inhibited in order to keep the electric charge balance
within a silica glass network.
[0092] It is assumed that displacement of a Si atom with an Al atom
has an effect to also immobilize a positive ion of an alkaline
earth metal element such as Ba.sup.2+ in order to keep the electric
charge balance so that the element such as Ba can be dissolved
further uniformly; and because of this, gaseous bubbles in the
silica glass can be suppressed as well.
[0093] An aim for the inner layer 56 not to make contain fine
gaseous bubbles may be accomplished by a procedure that a powdered
raw material for forming the inner layer 56 (powdered silica) is
made to contain in advance an element such as Ca, Sr, and Ba to
accelerate crystallization, and the atmospheric gas is made to
contain, just before the melting treatment of the powdered raw
material, a hydrogen gas, or a helium gas, or a gas mixture thereof
with the ratio of more than 10% by volume (hereinafter, this
atmosphere is sometimes simply abbreviated as
"hydrogen/helium-containing atmosphere").
[0094] When the powdered silica raw material incorporated in
advance with the foregoing crystallization accelerator is
heat-melted under the hydrogen/helium-containing atmosphere, a
silica glass layer with substantially no gaseous bubbles and with
the crystallization accelerator being dissolved uniformly can be
produced. That the crystallization accelerator is uniformly
dissolved (doped) and the silica glass layer contains substantially
no gaseous bubbles means that, by a visual examination, there are
no gaseous bubbles observed and the layer can be seen colorless and
transparent; and specifically, it means that, as mentioned above, a
linear transmittance at a light wavelength of 600 nm for a sample
having 10 mm thickness and finished with both surfaces being
parallel and optically polished is 91.8 to 93.2%, or preferably
92.4 to 93.2%.
[0095] Namely, to form a transparent silica glass by heat-melting
of a powdered silica containing at least one element of Ca, Sr, and
Ba with the total concentration of 50 to 2000 ppm by weight under
the hydrogen/helium-containing atmosphere, or to form a transparent
silica glass by heat-melting of a powdered silica containing
preferably Ba with the concentration of 100 to 1000 ppm by weight
and Al with the concentration of 10 to 100 ppm by weight under the
hydrogen/helium-containing atmosphere has not been previously
described in a literature, but was figured out and demonstrated for
the first time by the inventors.
[0096] Under the conditions as mentioned above, although an inner
layer with no gaseous bubbles can be obtained if total amount of
alkaline earth metal elements of Ca, Sr, and Ba is less than 50 ppm
by weight, recrystallization of the inner surface is difficult to
take place during the time that the silica container is used at
high temperature; while if the amount is more than 2000 ppm by
weight, it is difficult to dissolve these alkaline earth metal
elements into the inner layer uniformly and without forming gaseous
bubbles because the concentration is too high. In addition, in the
case that Ba is only one element contained therein among the
alkaline metal elements, Ba can be dissolved uniformly and without
forming gaseous bubbles if the Ba concentration is in the range
between 100 and 1000 ppm by weight; and in addition,
recrystallization of cristobalite can take place uniformly in the
inner surface during the time that the silica container is used at
high temperature, so that it is preferable. Especially, when Ba is
contained with the concentration of 100 to 1000 ppm by weight and
Al with the concentration of 10 to 100 ppm by weight, the foregoing
effect can be improved. When a powdered silica raw material
concurrently containing both Ba and Al with the amount as mentioned
above is heat-melted under the hydrogen/helium-containing
atmosphere, a silica glass layer having an extremely high light
transmittance and containing no gaseous bubbles can be
obtained.
[0097] Meanwhile, the mixing ratio of a hydrogen gas or a helium
gas in the hydrogen/helium-containing atmosphere is made more than
10% by volume as mentioned above. When other gas is mixed, an inert
gas such as a nitrogen gas and a rare gas is preferable; but a
hydrogen gas and a helium gas with the total concentration of 100%
by volume is more preferable.
[0098] In the method for producing a silica container of the
present invention, it is important that a crystallization
accelerator such as Ba is doped with a uniform concentration and
without incorporating gaseous bubbles into a silica glass after
melting. Detailed mechanism is not clear, but it is assume that a
hydrogen molecule (H.sub.2) reacts with an oxygen molecule
(O.sub.2) having a large molecular diameter to form a water
molecule (H.sub.2O) having a relatively small molecular diameter,
which can be diffused and released easily to outside a silica
glass, so that generation of gaseous bubbles may be prevented.
Amount of the water molecule contained therein must be made less
than 2.times.10.sup.17 molecules/g as the amount of steam released
at 1000.degree. C. under vacuum.
[0099] In addition, a hydrogen molecule itself has a small
molecular diameter so that its diffusion rate in a silica glass is
fast; and thus it does not cause formation of gaseous bubbles even
if it remains in the silica glass.
[0100] Detailed mechanism of an effect of a helium gas on reduction
of gaseous bubbles in a silica glass is not clear either; but it
may be assumed that a molecular diameter of a helium molecule
(namely, a helium atom) is further smaller than that of a hydrogen
molecule so that diffusion and release of gases contained in a
silica glass to outside thereof may be made easier thereby
preventing generation of gaseous bubbles.
[0101] A helium molecule has a further smaller molecular diameter
than a hydrogen molecule so that its diffusion rate in a silica
glass is fast; and thus it does not cause formation of gaseous
bubbles even if it remains in the silica glass.
[0102] In addition, to dissolve a crystallization accelerator such
as Ba uniformly into a silica glass is important in order to form
silica fine crystals abundantly and uniformly on surface part of
the silica glass during the time that a silica container is used at
high temperature. Although a detailed mechanism is not clear, in a
silica glass treated with heat-melting under an atmosphere
containing a hydrogen gas with the amount more than 10% by volume,
a growth rate of a crystal such as cristobalite tends to be slower.
Accordingly, if a silica container is prepared by using a powdered
silica that is containing Ba and the like and treated with
heat-melting under an atmosphere containing a hydrogen gas with the
amount more than 10% by volume, a fine and tight recrystallized
layer can be formed during the time that the silica container is
used. As the reason for this, it is assumed that the silica glass
that is treated with heat-melting under an atmosphere containing a
hydrogen gas with the amount more than 10% by volume contains some
sort of a defect related to an oxygen deficit so that this
structural defect may slow down appropriately the growth rate of
crystals such as cristobalite. Accordingly, in order to form a
recrystallized layer having fine texture on an inner surface of the
silica container, it is preferable that the powdered silica raw
material be made to contain a crystallization accelerator such as
Ba, and this powdered raw material be made to a melted glass under
an atmosphere containing a hydrogen gas with the amount more than
10% by volume.
[0103] In addition, as mentioned above, the present invention can
provide the silica container 71 having a linear transmittance of
the substrate 51 being also 91.8 to 93.2% at a light wavelength of
600 nm for a sample having 10 mm thickness cut-out from the inner
peripheral part 51b and finished with both surfaces being parallel
and optically polished. As seen above, a colorless and transparent
silica glass layer not substantially containing gaseous bubbles can
be formed also in the inner peripheral part 51b of the substrate 51
so that the silica container can withstand a long-time use under
conditions to increase in etching amount of the container inner
wall thereby increase in etching amount of the inner layer 56 as
well by the operation for many hours such as, for example,
continuous pulling up (multipulling) of a silicon single
crystal.
[0104] However, in order to secure thermal uniformity inside the
container, it is necessary to leave a layer containing gaseous
bubbles in the substrate's outer peripheral part 51a; and thus, it
is preferable that the thickness of the colorless and transparent
silica glass layer not substantially containing gaseous bubbles be
about less than half of the thickness of the substrate 51 (about 5
mm in the case that the substrate 51 has thickness of 10 mm).
[0105] Meanwhile, it is preferable that the inner layer 56 of the
silica container 71 contain OH groups with the amount of 1 to 50
ppm by weight, Li, Na, and K with each concentration of 20 or less
ppb by weight, and Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Zr, Mo, and W
with each concentration of 10 or less by ppb. This is because, if
the OH group concentration and concentration of each metal are
those as shown above, an impurity contamination to a material
accommodated in the silica container 71 can be reduced further
effectively. However, when the amount of OH group exceeds 50 ppm by
weight, heat resistance of the silica container is decreased; and
thus it is not preferable.
[0106] Hereinbelow, the method for producing a silica container of
the present invention that can produce the silica container 71 as
mentioned above will be explained further specifically. In
particular, a method for producing a silica container (solar-grade
crucible) producible with a low production cost, usable as a
container for melting of a metal silicon (Si) used as a material
for a solar photovoltaic power generation device and the like as
well as for pulling up of a single crystal, will be explained as
the example.
[0107] A schematic diagram of one example of a method for producing
the silica container 71 according to the present invention (first
embodiment) is shown in FIG. 1.
[0108] Firstly, as shown in FIG. 1 (1), the powdered raw material
11 for forming the substrate and the powdered raw material 12 for
forming the inner layer, each being a powdered silica, are
prepared.
[0109] Among them, the powdered raw material 11 for forming the
substrate is the one that will become a main composition material
of the substrate 51 in the silica container 71 of the present
invention (refer to FIG. 4).
[0110] This powdered raw material 11 for forming the substrate can
be obtained, for example as described below, by crushing a mass of
silica and then classifying the powders thereby obtained; though
the method is not limited to it.
[0111] Firstly, a mass of natural silica (naturally produced berg
crystal, quartz, silica, silica stone, opal stone, and so forth)
having diameter of about 5 to about 50 mm is heated at 600 to
1000.degree. C. for about 1 to about 10 hours under an air
atmosphere. Then, the mass of natural silica thus treated is poured
into water to be cooled down quickly, separated, and then dried.
With these treatments, subsequent crushing by a crusher or the like
and classification of the obtained powders can be carried out
easily; but crushing treatment may be carried out without
conducting the foregoing heating and quick cooling treatments.
[0112] Then, the mass of the natural silica is crushed by a crusher
or the like, and then classified to particles having diameter of 10
to 1000 .mu.m, or preferably 50 to 500 .mu.m, to obtain a powdered
natural silica.
[0113] Thereafter, the powdered natural silica thus obtained is
heated at 700 to 1100.degree. C. for about 1 to about 100 hours in
a rotary kiln made of a silica glass tube having an inclination
angle, inside of which is made to an atmosphere containing a
hydrogen chloride gas (HCl) or a chlorine gas (Cl.sub.2) for
high-purification treatment. However, for the use not requiring a
high purity, this high-purification treatment can be omitted to
proceed to the subsequent steps.
[0114] The powdered raw material 11 for forming the substrate
obtained after the foregoing steps is of a crystalline silica; but
depending on the use purpose of the silica container, an amorphous
silica glass scrap may also be used as the powdered raw material 11
for forming the substrate.
[0115] Diameter of the powdered raw material 11 for forming the
substrate is preferably 10 to 1000 .mu.m, or more preferably 50 to
500 .mu.m, as mentioned above.
[0116] Silica purity of the powdered raw material 11 for forming
the substrate is preferably 99.99% or higher by weight, or more
preferably 99.999% or higher by weight. In particular, total
concentration of Li, Na, and K is made 50 or less ppm by weight.
Further, according to the method for producing a silica container
of the present invention, even if silica purity of the powdered raw
material 11 for forming the substrate is made relatively low, such
as, 99.999% or lower by weight, in the silica container prepared
therefrom, impurity contamination to a material accommodated
therein can be adequately avoided. Accordingly, the silica
container can be produced with a lower cost as compared with
conventional methods.
[0117] Meanwhile, the powdered raw material 11 for forming the
substrate may be made to further contain Al with the concentration
of preferably 10 to 500 ppm by weight.
[0118] Al can be contained in the powdered silica by feeding the
powdered silica into an aqueous or an alcohol solution of an Al
salt such as a nitrate salt, an acetate salt, a carbonate salt, or
a chloride for soaking, and then by drying.
[0119] On the other hand, the powdered raw material 12 for forming
the inner layer is the one that will become a main composition
material of the inner layer 56 in the silica container 71 of the
present invention (refer to FIG. 4). As the powdered raw material
12 for forming the inner layer, a powdered silica having particle
diameter of 10 to 1000 .mu.m and containing at least one of Ca, Sr,
and Ba with the total concentration of 50 to 2000 ppm by weight is
prepared.
[0120] Outline of one example of a method for producing the
powdered raw material 12 for forming the inner layer as mentioned
above is shown in FIG. 3.
[0121] Firstly, as shown in FIG. 3 (1), a base material that is
powders having particle diameter of 10 to 1000 .mu.m and comprised
of a silica is prepared.
[0122] An illustrative example of the powdered raw material for
forming the inner layer for the silica container includes a
powdered, highly purified natural quartz, a powdered natural berg
crystal, a powdered synthetic cristobalite, and a powdered
synthetic silica glass. To reduce gaseous bubbles in a transparent
layer, a powdered crystalline silica is preferable; and to obtain a
transparent layer of highly purity, synthetic powders are
preferable. Particle diameter is preferably 100 to 500 .mu.m.
Purity is preferably 99.9999% or higher by weight as the silica
component (SiO.sub.2); and total concentration of the alkaline
metal elements Li, Na, and K is 100 or less ppb by weight, wherein
each concentration of the elements is preferably 20 or less ppb by
weight, or more preferably 10 or less ppb by weight. Content of
each of Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Mo, and W is preferably 10
or less ppb by weight, or more preferably 5 or less ppb by
weight.
[0123] Then, as shown in FIG. 3 (2), an alkaline earth metal
element is added to the powdered silica as the base material.
[0124] Specifically, the powdered silica is made to contain at
least one or more of calcium (Ca), strontium (Sr), and barium (Ba),
or preferably Ba. The method for addition may be as follows: a
chloride, an acetate salt, a nitrate salt, or a carbonate salt of
an alkaline earth metal element to be dissolved into water or an
alcohol is selected, and then an aqueous solution or an alcohol
solution of the selected compound is prepared, and then the
powdered silica raw material is soaked into the solution thus
prepared; and then, after drying the resulting mixture, the powders
added with a specific element can be obtained.
[0125] According to the procedure shown above, the powdered raw
material 12 for forming the inner layer can be produced, as shown
in FIG. 3 (3).
[0126] Then, as shown in FIG. 1 (2), the powdered raw material 11
for forming the substrate thus prepared is fed into a frame having
a rotational symmetry for molding the powdered raw material 11 for
forming the substrate.
[0127] In FIG. 5, a cross section view showing an outline of an
evacuable frame is illustrated, as one example of the frame to
preliminarily mold the powdered raw material 11 for forming the
substrate. The evacuable frame 101 is made of a material such as,
for example, graphite, and has a rotational symmetry. In the inner
wall 102 of the evacuable frame 101, the aspiration holes 103 are
arranged splittingly. The aspiration holes 103 are connected to the
aspiration path 104. The rotation axis 106 to rotate the evacuable
frame 101 is also arranged with the aspiration path 105, through
which aspiration can be done. Meanwhile, the holes 103 are
preferably provided with a porous filter (not shown).
[0128] However, in such a case that discharge-heating under
aspiration is not necessary, the frame 101', such as the one shown
in FIG. 6, may be used instead of the evacuable frame 101 shown in
FIG. 5. This frame 101' is made of a material such as, for example,
graphite, and has a rotational symmetry. The rotation axis 106' to
rotate the frame 101' is arranged; but holes and the like are not
particularly arranged on the inner wall 102'.
[0129] The powdered raw material 11 for forming the substrate is
fed into the inner wall 102 of the evacuable frame 101 to
preliminarily mold the powdered raw material 11 for forming the
substrate to a prescribed shape in accordance with the inner wall
102 of the evacuable frame 101, thereby giving the preliminarily
molded substrate 41 (refer to FIG. 7).
[0130] Specifically, the powdered raw material 11 for forming the
substrate is fed gradually into the inner wall 102 of the evacuable
frame 101 from a powdered raw material hopper (not shown) with
rotating the evacuable frame 101 thereby molding to a shape of the
container by utilizing a centrifugal force. Alternatively,
thickness of the preliminarily molded substrate 41 may be
controlled to the prescribed value by contacting a plate-like inner
frame (not shown) to the rotating powders from inside.
[0131] A feeding method of the powdered raw material 11 for forming
the substrate into the evacuable frame 101 is not particularly
limited; for example, a hopper equipped with an agitation screw and
a measuring feeder may be used. In this case, the powdered raw
material 11 for forming the substrate filled in the hopper is fed
with agitating by the agitation screw while controlling the feeding
amount by the measuring feeder.
[0132] Then, as shown in FIG. 1 (3), the powdered raw material 12
for forming the inner layer is fed onto an inner surface of the
preliminarily molded substrate 41 with rotating the evacuable frame
101 thereby forming the preliminarily molded inner layer 46 by
preliminarily molding to the prescribed shape in accordance with
the inner surface of the preliminarily molded substrate 41.
[0133] Basically, procedures similar to those in the case of
feeding of the powdered raw material 11 for forming the substrate,
as described above, are followed. Namely, the powdered raw material
12 for forming the inner layer is fed gradually onto the inner
surface of the preliminarily molded substrate 41 from a powdered
raw material hopper with rotating the evacuable frame 101 thereby
molding to a shape of the container by utilizing a centrifugal
force (refer to FIG. 8).
[0134] Then, as shown in FIG. 1 (4), the substrate 51 and the inner
layer 56 are formed by a discharge-heat melting method.
[0135] In the case that the heat-melting is conducted under
aspiration, specifically, as shown in FIG. 9, the preliminarily
molded substrate 41 and the preliminarily molded inner layer 46 are
degassed by aspiration from the peripheral side of the
preliminarily molded substrate 41 through the aspiration holes 103
formed in the evacuable frame 101, with simultaneously heating from
inside of the preliminarily molded substrate 41 and the
preliminarily molded inner layer 46 by a discharge-heat melting
method. With this, the substrate 51 and the inner layer 56, having
made the peripheral part of the preliminarily molded substrate 41a
sintered body and having made the inner part of the preliminarily
molded substrate 41 and the preliminarily molded inner layer 46 a
fused glass body, are formed.
[0136] On the other hand, as shown in FIG. 6, in the case that the
frame 101' without especially conducting aspiration is used, the
substrate 51 and the inner layer 56 are formed by heating at high
temperature from inside the preliminarily molded substrate 41 and
the preliminarily molded inner layer 46 by a discharge-heat melting
method without especially conducting aspiration.
[0137] Hereinbelow, the embodiment that the substrate 51 and the
inner layer 56 are formed by using the evacuable frame 101 under
aspiration will be explained mainly; but in the case of under
normal pressure, not conducting aspiration, the substrate 51 and
the inner layer 56 can be formed similarly, except for conducting
aspiration.
[0138] The equipment for forming the substrate 51 and the inner
layer 56 is comprised of, in addition to the rotatable and
evacuable frame 101 having a rotational axis symmetry as mentioned
above, the rotation motor (not shown), the carbon electrodes 212
which are the heat source of the discharge-heat melting (sometimes
called arc melting or arc discharge melting), the electric wirings
212a, the high voltage electricity source unit 211, the cap 213,
and so forth. In addition, structural components to control an
atmospheric gas to be supplied from inside the preliminarily molded
inner layer 46 such as, for example, the gas-supplying cylinders
411 and 412, the gas mixture-supplying pipe 420, the dehumidifying
equipment 430, the dew-point temperature meter 440, and so forth,
may be arranged. From the gas-supplying cylinders 411 and 412, such
gases as, for example, a hydrogen gas, a helium gas, and a nitrogen
gas are supplied.
[0139] For example, in such a case that a gas containing 100% of a
hydrogen gas is used as the atmospheric gas, only one gas-supplying
cylinder may be used. Alternatively, an atmospheric gas, containing
a hydrogen gas, or a helium gas, or a'gas mixture thereof with the
ratio of more than 10% by volume (hydrogen/helium-containing
atmosphere), may be prepared in advance by mixing them so that the
atmospheric gas thus prepared may be supplied from a single gas
cylinder.
[0140] It is preferable that the step of forming the substrate 51
and the inner layer 56 by a discharge-heat melting method be
conducted under an atmosphere with setting a dew-point temperature
in the range between 15.degree. C. and -15.degree. C. and with
controlling the temperature within .+-.2.degree. C. of the set
dew-point temperature. With this, amount of water contained in the
inner layer 56 and concentration of OH group that is bonded to a
silica glass network contained in the inner layer 56 can be
controlled at a certain value. Amount of OH group can be decreased
with lowering the dew-point temperature; wherein it is preferable
that concentration of OH group be 1 to 50 ppm by weight, as
described above. However, a preferable dew-point temperature can be
set depending on the use of the silica container.
[0141] For example, melting and sintering of the preliminarily
molded substrate 41 and the preliminarily molded inner layer 46 are
carried out by the procedures as follows: at first, before start of
the electricity charge between the carbon electrodes 212, supply of
an atmospheric gas, whose temperature is made below the set
dew-point temperature by dehumidification, containing a hydrogen
gas, or a helium gas, or a gas mixture thereof with the ratio of
more than 10% by volume (hydrogen/helium-containing atmosphere), is
started from inside the preliminarily molded substrate 41 and the
preliminarily molded inner layer 46. Specifically, as shown in FIG.
9, for example, a hydrogen gas in the gas-supplying cylinder 411
and an inert gas other than a hydrogen gas (for example, nitrogen
(N.sub.2), argon (Ar), and helium (He)) in the gas-supplying
cylinder 412 are mixed and supplied from inside the preliminarily
molded substrate 41 and the preliminarily molded inner layer 46
through the gas mixture-supplying pipe 420. Meanwhile, outlined
arrows shown by the reference number 510 show the flow direction of
the gas mixture.
[0142] The dew-point temperature can be set by an appropriate
dehumidifying equipment and the like; and to measure the dew-point
temperature, an appropriate dew-point temperature meter and the
like can be used. In FIG. 9, an embodiment that the dehumidifying
equipment 430 and the dew-point temperature meter 440 are
integrated to the gas mixture-supplying pipe 420 is shown, but the
embodiment is not limited to this; any embodiment enabling to make
the dew-point temperature of the gas mixture within a prescribed
range by dehumidification and the like can be used.
[0143] At this time, a gas in the evacuable frame 101 is preferably
ventilated simultaneously, as mentioned above. The ventilation can
be done by escaping the atmospheric gas in the evacuable frame 101
to outside, for example, through a space in the cap 213. Meanwhile,
outlined arrows shown by the reference number 520 show the flow
direction of the atmospheric gas by ventilation.
[0144] Then, under the condition of controlling the atmosphere as
mentioned above, a vacuum pump for degassing (not shown) is started
thereby aspirating the preliminarily molded substrate 41 from its
outer side through the aspiration holes 103 and the aspiration
paths 104 and 105 and at the same time charging of electricity
between the carbon electrodes 212 is started with rotating the
evacuable frame 101, containing the preliminarily molded substrate
41 and the preliminarily molded inner layer 46, at a certain
constant rate.
[0145] When the arc discharge between the carbon electrodes 212 is
started (shown by the reference number 220), temperature of the
inner surface part of the preliminarily molded substrate 41 and the
preliminarily molded inner layer 46 reaches melting region of the
powdered silica (estimated temperature of about 1800 to about
2000.degree. C.) thereby melting is started from the most surface
layer. When the most surface layer is melted, degree of vacuum by
aspiration with the vacuum pump for degassing increases (pressure
is dropped rapidly), whereby the change to a fused silica glass
layer progresses from inside to outside with degassing a dissolved
gas contained in the powdered raw material 11 for forming the
substrate and in the powdered raw material 12 for forming the inner
layer. The timing of aspiration is important; strong aspiration
should not be made before the inner surface layer inside the
container is changed to a glass. The reason for this resides in
that, if strong aspiration is made from the beginning, impure fine
particles contained in an atmospheric gas is adhered and
accumulated onto the inner surface part of the preliminarily molded
articles by a filtering effect. Accordingly, it is preferable that
degree of vacuum be not so high at the beginning, and aspiration is
intensified gradually as the inner surface changes to a melted
glass.
[0146] Heating by electric charge and aspiration by the vacuum pump
are continued until about half of the entire thickness of the inner
layer and the substrate is melted from inside so that the inner
layer 56 may be changed to a transparent silica glass, and the
inner peripheral side 51b of the substrate may be changed to a part
comprised of a transparent to semitransparent layer, while the
outer peripheral part 51a (about half of outside remained) of the
substrate 51 becomes a sintered, white and opaque silica (opaque
layer). Degree of vacuum is preferably 10.sup.4 Pa or lower, or
more preferably 10.sup.3 Pa or lower.
[0147] With this, the silica container 71 of the present invention,
as shown in FIG. 4, can be made.
[0148] Meanwhile, the inner layer 56 may be made comprised of a
plurality of transparent silica glass layers having different
purities and additives by further conducting, once or a plurality
of times, the step of the inner layer formation in the second
embodiment, as described later.
[0149] In FIG. 2, an outline of another example (second embodiment)
of the method for producing the silica container 71 according to
the present invention is shown.
[0150] Firstly, as shown in FIG. 2 (1), the powdered raw material
11 for forming the substrate and the powdered raw material 12 for
forming the inner layer, each being a powdered silica, are
prepared.
[0151] This step can be carried out in a manner similar to that of
the first embodiment as mentioned above.
[0152] Then, as shown in FIG. 2 (2), the powdered raw material 11
for forming the substrate is fed to the frame having a rotational
symmetry for molding.
[0153] This step also can be carried out in a manner similar to
that of the first embodiment as mentioned above. However, in such a
case that discharge-heating under aspiration is not necessary, the
frame 101' shown in FIG. 6 may be used other than the evacuable
frame 101 shown in FIG. 5 and FIG. 7, similarly to the case of the
first embodiment.
[0154] Then, as shown in FIG. 2 (3), the substrate 51 is formed by
a discharge-heat melting method.
[0155] Specifically, as shown in FIG. 10 and FIG. 11, the
preliminarily molded substrate 41 is degassed by aspiration from
the outer peripheral side of the preliminarily molded substrate 41
through the aspiration holes 103 formed in the evacuable frame 101,
with simultaneous heating from inside of the preliminarily molded
substrate by a discharge-heat melting method. With this, the
substrate 51, having the outer peripheral part of the preliminarily
molded substrate 41 made a sintered body and having the inner part
of the preliminarily molded substrate 41 made a fused glass body,
is formed.
[0156] On the other hand, as shown in FIG. 6, in the case that the
frame 101' without especially conducting aspiration is used, the
substrate 51 is formed by heating at high temperature from inside
the preliminarily molded substrate 41 by a discharge-heat melting
method without especially conducting aspiration.
[0157] Hereinbelow, the embodiment that the substrate 51 is formed
by using the evacuable frame 101 under aspiration will be explained
mainly; but in the case of under normal pressure, not conducting
aspiration, the substrate 51 can be formed similarly, except for
conducting aspiration.
[0158] The equipment for forming the substrate 51 is comprised of,
as shown in FIG. 10 and FIG. 11, in addition to the foregoing
rotatable and evacuable frame 101 (or may be the frame 101') having
a rotational axis symmetry, the rotation motor (not shown), the
carbon electrodes 212 which are the heat source of the
discharge-heat melting (sometimes called arc melting or arc
discharge melting), the electric wirings 212a, the high voltage
electricity source unit 211, the cap 213, and so forth. In
addition, structural components to control an atmospheric gas to be
charged from inside the preliminarily molded substrate such as, for
example, the gas-supplying cylinders 411 and 412, the gas
mixture-supplying pipe 420, the dehumidifying equipment 430, the
dew-point temperature meter 440, and so forth, may be arranged.
[0159] For example, melting and sintering of the preliminarily
molded substrate 41 are conducted by the procedures as follows: at
first, before start of the electricity charge between the carbon
electrodes 212, supply of a hydrogen/helium-containing atmosphere
whose temperature is made below the prescribed dew-point
temperature by dehumidification, is started from inside the
preliminarily molded substrate 41. Specifically, as shown in FIG.
10, for example, a hydrogen gas in the gas-supplying cylinder 411
and an inert gas other than a hydrogen gas (for example, nitrogen
(N.sub.2), argon (Ar), and helium (He)) in the inert gas-supplying
cylinder 412 are mixed and supplied from inside the preliminarily
molded substrate 41 through the gas mixture-supplying pipe 420.
Meanwhile, outlined arrows shown by the reference number 510 show
the flow direction of the gas mixture.
[0160] The dew-point temperature can be set by an appropriate
dehumidifying equipment and the like; and to measure the dew-point
temperature, an appropriate dew-point temperature meter and the
like can be used. In FIG. 10 and FIG. 11, an embodiment that the
dehumidifying equipment 430 and the dew-point temperature meter 440
are integrated to the gas mixture-supplying pipe 420 is shown, but
the embodiment is not limited to this; any embodiment enabling to
make the dew-point temperature of the gas mixture within a
prescribed range by dehumidification and the like can be used.
[0161] At this time, a gas in the evacuable frame 101 is preferably
ventilated simultaneously, as mentioned above. The ventilation can
be done by escaping the atmospheric gas in the evacuable frame 101
to outside, for example, through a space in the cap 213. Meanwhile,
outlined arrows shown by the reference number 520 show the flow
direction of the atmospheric gas by ventilation.
[0162] Then, under the condition of controlling the atmosphere as
mentioned above, a vacuum pump for degassing (not shown) is started
thereby aspirating the preliminarily molded substrate 41 from its
outer side through the aspiration holes 103 and the aspiration
paths 104 and 105 and at the same time charging of electricity
between the carbon electrodes 212 is started with rotating the
evacuable frame 101 containing the preliminarily molded substrate
41 at a certain constant rate.
[0163] When the arc discharge between the carbon electrodes 212 is
started (shown by the reference number 220), temperature of the
inner surface part of the preliminarily molded substrate 41 reaches
melting region of the powdered silica (estimated temperature of
about 1800 to about 2000.degree. C.) thereby melting is started
from the most surface layer. When the most surface layer is melted,
degree of vacuum by aspiration with the vacuum pump for degassing
increases (pressure is dropped rapidly), whereby the change to a
fused silica glass layer progresses from inside to outside with
degassing a dissolved gas contained in the powdered raw material 11
for forming the substrate. The timing of aspiration is important;
strong aspiration should not be made before the inner surface layer
inside the container is changed to a glass. The reason for this
resides in that, if strong aspiration is made from the beginning,
impure fine particles contained in an atmospheric gas is adhered
and accumulated onto the inner surface part of the preliminarily
molded articles by a filtering effect. Accordingly, it is
preferable that degree of vacuum be not so high at the beginning,
and aspiration is intensified gradually as the inner surface
changes to a melted glass.
[0164] Heating by electric charge and aspiration by the vacuum pump
are continued until about half of the entire thickness of the
substrate is melted from inside so that the inner peripheral side
51b of the substrate may be changed to a part comprised of a
transparent to semitransparent layer, while the outer peripheral
part 51a (about half of outside remained) of the substrate 51 may
become a sintered, white and opaque silica (opaque layer). Degree
of vacuum is made preferably 10.sup.4 Pa or lower, or more
preferably 10.sup.3 Pa or lower.
[0165] Then, as shown in FIG. 2 (4), the inner layer 56 is formed
on an inner surface of the substrate 51 with heating at high
temperature from its inside by a discharge-heat melting method,
while the powdered silica raw material for forming the inner layer
(the powdered raw material 12 for forming the inner layer) is
spread from inside of the substrate 51.
[0166] Meanwhile, the inner layer 56 may be made comprised of a
plurality of transparent silica glass layers having different
purities and additives by repeating this step.
[0167] The method for forming the inner layer 56 will be explained
with referring to FIG. 12.
[0168] Similarly to the previous step, the equipment for forming
the inner layer 56 on the inner surface of the substrate 51 is
comprised of, the rotatable and evacuable frame 101 arranged with
the substrate 51 having a rotational axis symmetry, the rotation
motor (not shown), the powdered raw material's hopper 303
containing the powdered raw material 12 for forming the inner layer
for forming the inner layer 56, the agitation screw 304, the
measuring feeder 305, the carbon electrodes 212 which are the heat
source of the discharge-heat melting, the electric wirings 212a,
the high voltage electricity source unit 211, the cap 213, and so
forth. Similarly to the previous step, in the case that the
atmospheric gas is controlled, the gas-supplying cylinders 411 and
412, the gas mixture-supplying pipe 420, the dehumidifying
equipment 430, the dew-point temperature meter 440, and so forth,
may be arranged further.
[0169] The inner layer 56 is formed as follows: firstly, the
evacuable frame 101 is set at the prescribed rotation speed, and
then high voltage is loaded gradually from the high voltage
electricity source unit 211 and at the same time the powdered raw
material 12 for forming the inner layer for forming the inner layer
56 (high purity powdered silica) is spread gradually from top of
the substrate 51 from the raw material's hopper 303. At this time,
the electric discharge has been started between the carbon
electrodes 212 so that inside the substrate 51 is in the
temperature range of melting of the powdered silica (estimated
temperature of about 1800 to about 2000.degree. C.); and with this,
the spread powdered raw material 12 for forming the inner layer
becomes melted silica particles thereby attaching to the inner
surface of the substrate 51. A mechanism is employed such that the
carbon electrodes 212 arranged in the upper opening site of the
substrate 51, a feeding port of the powdered raw material, and the
cap 213 may change their positions relative to the substrate 51 to
a certain degree; and by changing these positions, the inner layer
56 can be formed on the entire inner surface of the substrate 51
with a uniform thickness.
[0170] It is preferable that the step of forming the inner layer 56
by this discharge-heat melting method be conducted under an
atmosphere with setting a dew-point temperature in the range
between 15.degree. C. and -15.degree. C. and with controlling the
temperature within .+-.2.degree. C. of the set dew-point
temperature. With this, amount of water contained in the inner
layer 56 and concentration of OH group that is bonded to a silica
glass network contained in the inner layer 56 can be controlled at
a certain value. Amount of OH group can be decreased with lowering
the dew-point temperature, wherein it is preferable that
concentration of OH group be 1 to 50 ppm by weight as described
above. However, a preferable dew-point temperature can be set
depending on the use of the silica container.
[0171] Specifically, as shown in FIG. 12, a hydrogen gas in the
gas-supplying cylinder 411 and an inert gas other than a hydrogen
gas (for example, nitrogen, argon, and helium) in the gas-supplying
cylinder 412 can be mixed and supplied from inside the substrate 51
through the gas mixture-supplying pipe 420. Meanwhile, outlined
arrows shown by the reference number 510 show the flow direction of
the gas mixture. At this time, the gases in the evacuable frame 101
can be ventilated simultaneously, as mentioned above. The
ventilation can be done, for example, by escaping the gases of the
atmosphere inside the evacuable frame 101 to outside through a
space in the cap 213. Meanwhile, outlined arrows shown by the
reference number 520 show the flow direction of the gas mixture by
ventilation.
[0172] By conducting the foregoing steps, the silica container 71
according to the present invention as mentioned above and shown in
FIG. 4 can be produced.
EXAMPLES
[0173] Hereinbelow, the present invention will be explained more
specifically by showing Examples and Comparative Examples of the
present invention; but the present invention is not limited to
them.
Example 1
[0174] According to the method for producing a silica container of
the present invention shown in FIG. 1 (the first embodiment), the
silica container was produced, as described below.
[0175] Firstly, a powdered natural quartz having purity of 99.999%
by weight and particle diameter of 50 to 500 was prepared as the
powdered raw material 11 for forming the substrate.
[0176] The powdered raw material 12 for forming the inner layer was
prepared according to the procedures as shown in FIG. 3.
Specifically, at first, a powdered natural quartz having purity of
99.999% by weight and particle diameter of 50 to 500 .mu.m was
prepared (FIG. 3 (1)). Then, the powdered natural quartz thus
prepared was soaked in an aqueous ethyl alcohol containing barium
nitrate with a prescribed concentration, and then dried by heating
in a clean oven at 200.degree. C. for 50 hours (FIG. 3 (2)) to
obtain the powdered raw material 12 for forming the inner layer
(FIG. 3 (3)).
[0177] Then, the powdered raw material 11 for forming the substrate
and the powdered raw material 12 for forming the inner layer were
preliminarily molded in the frame 101 as shown in FIG. 5 by
integral molding with the procedures as follows. Firstly, the
powdered raw material 11 for forming the substrate was fed to the
inner wall 102 of the rotating, evacuable frame 101, which is made
of graphite with a column-like shape and has the aspiration holes
103 formed in the inner wall 102, with the thickness being
controlled at a prescribed value (refer to FIG. 7); and then the
powdered raw material 12 for forming the inner layer was fed to
form the preliminarily molded inner layer 46 on the inner surface
layer of the preliminarily molded substrate 41 (refer to FIG.
8).
[0178] Then, an atmosphere inside the preliminarily molded
substrate 41 and the preliminarily molded inner layer 46 was
displaced with a mixed gas atmosphere comprised of 30% by volume of
H.sub.2 and 70% by volume of He. Then, the preliminarily molded
substrate 41 and the preliminarily molded inner layer 46 were
sintered and fused by a discharge-heat melting method using carbon
electrodes (arc discharge heating) with gradually degassing both
preliminarily molded articles 41 and 46 by aspiration from outside
of the frame 102 by using a vacuum pump while controlling the
dew-point temperature at 10.+-.2.degree. C., namely in the range
between 8.degree. C. and 12.degree. C. (refer to FIG. 9).
Example 2
[0179] The silica container 71 was produced in a manner similar to
that of Example 1, except that the powdered raw material 12 for
forming the inner layer was doped with Ba, the concentration being
made approximately twice the amount in Example 1.
Example 3
[0180] The silica container 71 was produced in a manner similar to
that of Example 1, except that the powdered raw material 12 for
forming the inner layer was doped with Ba, the concentration being
made approximately four times of the amount in Example 1, and with
Al at the same time.
Example 4
[0181] The silica container 71 was produced in a manner similar to
that of Example 1, except that the powdered raw material 12 for
forming the inner layer was doped with Ba, the concentration being
made approximately eight times of the amount in Example 1, and with
Al at the same time.
Example 5
[0182] The silica container 71 was produced in a manner similar to
that of Example 2, except that the atmosphere during the arc
discharge-heating of both preliminarily molded articles under
aspiration was changed to 100% by volume of H.sub.2.
Example 6
[0183] The silica container 71 was produced in a manner similar to
that of Example 2, except that the atmosphere during the arc
discharge-heating of both of the preliminarily molded articles
under aspiration was changed to 50% by volume of H.sub.2 and 50% by
volume of N.sub.2.
Example 7
[0184] According to the method for producing a silica container
shown in FIG. 2 (the second embodiment), the silica container 71
was produced.
[0185] Firstly, a powdered natural quartz having purity of 99.999%
by weight and particle diameter of 50 to 500 .mu.m was prepared as
the powdered raw material 11 for forming the substrate.
[0186] The powdered raw material 12 for forming the inner layer was
prepared according to the procedures as shown in FIG. 3.
Specifically, at first, a powdered natural quartz having purity of
99.999% by weight and particle diameter of 50 to 500 .mu.m was
prepared (FIG. 3 (1)). Then, the powdered natural quartz thus
prepared was soaked in an aqueous ethyl alcohol containing barium
nitrate with a prescribed concentration, and then dried by heating
in a clean oven at 200.degree. C. for 50 hours (FIG. 3 (2)) to
obtain the powdered raw material 12 for forming the inner layer
(FIG. 3 (3)).
[0187] Then, the powdered raw material 11 for forming the substrate
was preliminarily molded in the frame 101 as shown in FIG. 5 and
with the procedure as follows. Namely, the powdered raw material 11
for forming the substrate was fed to the inner wall 102 of the
rotating, evacuable frame 101, which is made of graphite with a
column-like shape and has the aspiration holes 103 formed in the
inner wall 102, with the thickness being controlled at a prescribed
value (refer to FIG. 7).
[0188] Then, an atmosphere inside the preliminarily molded
substrate 41 was displaced with a mixed gas atmosphere comprised of
30% by volume of H.sub.2 and 70% by volume of He and the dew-point
temperature was controlled at 10.+-.2.degree. C. Under this
condition, the preliminarily molded substrate 41 was sintered and
fused with an arc discharge heating under aspiration to form the
substrate 51.
[0189] Then, under a mixed gas atmosphere comprised of 50% by
volume of H.sub.2 and 50% by volume of He and with controlling the
dew-point temperature at 10.+-.2.degree. C., the inner layer 56 was
formed by heating with an arc discharge-heating under normal
pressure with spreading the powdered raw material 12 for forming
the inner layer from top of the frame 101.
[0190] In this way, the silica container 71 was produced.
Example 8
[0191] The silica container 71 was produced in a manner similar to
that of Example 7, except that the atmosphere during formation of
the substrate was changed to 30% by volume of H.sub.2 and 70% by
volume of N.sub.2.
Example 9
[0192] The silica container 71 was produced in a manner similar to
that of Example 2, except that the atmosphere during the arc
discharge-heating of both of the preliminarily molded articles
under aspiration was changed to 15% by volume of H.sub.2 and 85% by
volume of N.sub.2.
Example 10
[0193] The silica container 71 was produced in a manner similar to
that of Example 2, except that the atmosphere during the arc
discharge-heating of both of the preliminarily molded articles
under aspiration was changed to 15% by volume of He and 85% by
volume of N.sub.2.
Comparative Example 1
[0194] A high purity powdered quartz having particle diameter of 50
to 500 .mu.m and purity of 99.999% by weight and a powdered
cristobalite having particle diameter of 50 to 300 .mu.m and purity
of 99.9999% by weight were prepared as the powdered raw material
for forming the substrate and the powdered raw material for forming
the inner layer, respectively. The preliminarily molded substrate
and inner layer were formed in an air without particular humidity
control, and then an arc discharge-heating was conducted under
aspiration for melting.
Comparative Example 2
[0195] According to mostly a conventional method, a silica
container (a silica crucible) was prepared as follows.
[0196] A high purity powdered quartz having particle diameter of 50
to 500 .mu.m and purity of 99.9999% by weight and a powdered
cristobalite having particle diameter of 50 to 300 .mu.m and purity
of 99.9999% by weight were prepared as the powdered raw material
for forming the substrate and the powdered raw material for forming
the inner layer, respectively. The substrate was formed by the
arc-discharge heating under normal pressure in an air without
particular humidity control, and the inner layer was formed by
melting with the arc-discharge heating under normal pressure in the
same air as the foregoing, with spreading the powdered raw material
from upper part of the frame.
Comparative Example 3
[0197] The silica container was produced in a manner similar to
that of Comparative Example 1, except that the powdered raw
material for forming the inner layer doped with high concentration
of Ba, i.e., 3000 ppm by weight of Ba was used.
Comparative Example 4
[0198] The silica container was produced in a manner similar to
that of Comparative Example 2, except that a low-purity powdered
raw material for forming the substrate with the purity of 99.99% by
weight, and a high-purity powdered synthetic cristobalite doped
with 100 ppm by weight of Ba as the powdered raw material for
forming the inner layer were used.
Comparative Example 5
[0199] The silica container 71 was produced in a manner similar to
that of Example 2, except that the atmosphere during the arc
discharge-heating of both of the preliminarily molded articles
under aspiration was changed to 5% by volume of H.sub.2 and 95% by
volume of N.sub.2.
Comparative Example 6
[0200] The silica container 71 was produced in a manner similar to
that of Example 2, except that the atmosphere during the arc
discharge-heating of both of the preliminarily molded articles
under aspiration was changed to 5% by volume of He and 95% by
volume of N.sub.2.
Evaluation Methods in Examples and Comparative Examples
[0201] In each Example and Comparative Example, measurements of
physical properties and property evaluation as to the powdered raw
material and the atmospheric gas used, and the silica container
produced, were carried out as follows.
[Method for Measuring Particle Diameter of Each Powdered Raw
Material]
[0202] Two-dimensional shape observation and area measurement of
each powdered raw material were carried out with an optical
microscope or an electron microscope. Then, the diameter was
obtained by calculation of the obtained area value with the
assumption that shape of the particle is a true circle. This
technique was repeated statistically to obtain the range of
particle diameter (99% or more by weight of particles are included
in this range).
[Measurement of the Dew-Point Temperature]
[0203] Measurement was done with a dew-point temperature meter.
[0204] Meanwhile, the measurement in each Example was done by the
dew-point temperature meter 440 arranged in the gas
mixture-supplying pipe 420, as mentioned above.
[Analysis of the Impure Metal Element Concentration]
[0205] When an impure metal element concentration is relatively low
(i.e., the glass is of high purity), ICP-AES (Inductively Coupled
Plasma-Atomic Emission Spectroscopy) or ICP-MS (Inductively Coupled
Plasma-Mass Spectroscopy) was used, and when an impure metal
element concentration is relatively high (i.e., the glass is of low
purity), AAS (Atomic Absorption Spectroscopy) was used.
[Thickness Measurement of Each Layer]
[0206] The container cross section at the half point of total
height of the side wall of the silica container (corresponding to
the height of 200 mm) was measured by a scale to obtain thickness
of the substrate and the inner layer.
[Measurement of OH Group Concentration]
[0207] Each sample was obtained by cutting out a transparent part
of the substrate and the inner layer, respectively, and then
polishing it. The measurement was done for each sample with an
infrared absorption spectroscopy. Conversion to the OH group
concentration was done according to the following literature:
[0208] Dodd, D. M. and Fraser, D. B., (1966), "Optical
determination of OH in fused silica", Journal of Applied Physics,
vol. 37, p. 3911.
[Measurement of Release Amount of a Steam Gas]
[0209] The gas amount released from a granular silica glass sample
with the particle diameter controlled in the range from 100 .mu.m
to 1 mm upon heating at 1000.degree. C. under vacuum was measured
by a mass spectrometry instrument. Details of the measurement were
according to the following literature. The amount was expressed by
the released molecules per unit mass (water molecules/glass gram)
with the assumption that all of water, H.sub.2O molecules contained
therein were released.
[0210] Nasu, S., et al., (1990), "Gas release of various kinds of
vitreous silica", Journal of Illuminating Engineering Institute of
Japan, vol. 74, No. 9, pp 595 to 600.
[Measurement of Light Transmittance]
[0211] A glass sample with the size of about 5.times.5 mm
(thickness of about 11 mm) was cut out from the inner layer to
obtain a sample having 10-mm thickness finished with both surfaces
being parallel and optically polished (surface precision:
1/20.lamda., wavelength: 633 nm). Then, the linear light
transmittance (the value called optical transmission, obtained by
subtracting reflection at the sample surface, back-side reflection
of inside the sample, and absorption of the sample itself from the
incident light which was taken as 100%) of this glass sample was
measured at a wavelength of 600 nm by using a visible light
transmittance measurement instrument having a mercury lamp as its
light source. Maximum value of the theoretical transmittance is
93.2%.
[0212] The incident light is scattered by micro gaseous bubbles,
microparticles, clusters, and the like contained in a glass sample;
and thus, the value of light transmittance is effective for
judgment that various elements are dissolved in a silica glass
uniformly and without gaseous bubbles.
[Evaluation of Continuous Pulling Up of a Silicon Single Crystal
(Multipulling)]
[0213] A metal polysilicon with purity of 99.9999999% by weight was
fed into a produced silica container; thereafter, the temperature
was raised to form a silicon melt, and then pulling up of a single
crystal silicon was repeated for three times (multipulling). The
evaluation was made as the success rate of single crystal growth.
The pulling up conditions were: atmosphere of an argon (Ar) gas
100% with the pressure inside the CZ equipment being 10.sup.3 Pa,
the pulling up rate of 1 mm/minute, rotation rate of 10 rpm, and
the size of the silicon single crystal being 150 mm in diameter and
150 mm in length. Operation time for one batch was set at about 12
hours. Classification of evaluation based on the success rate of
single crystal growth for repetition of three times was made as
follows:
[0214] success of three times: good
[0215] success of two times: fair
[0216] success of one time: poor
[Evaluation of Voids and Pinholes]
[0217] In the foregoing multipulling of the silicon single crystal,
ten of each silicon wafer having the size of 150 mm diameter and
200 .mu.m thickness and polished on the both sides were prepared
from an arbitrary portion of the second silicon single crystal
after multipulling of each silicon single crystal. Then, voids and
pinholes present on both sides of each silicon wafer were counted;
average void numbers and pinhole numbers per unit area (m.sup.2)
were obtained by a statistic numerical treatment.
[0218] average number of voids and pinholes is less than 1/m.sup.2:
good
[0219] average number of voids and pinholes is in the range from 1
to 2/m.sup.2: fair
[0220] average number of voids and pinholes is 3/m.sup.2 or more:
poor
[Evaluation of Etching Resistance of the Silica Container]
[0221] A sample was cut out from the side wall of the silica
container after three multipullings of a silicon single crystal, in
the part lower than the level of the silicon melt. The sample was
made for the size of the inner wall surface of the silica container
to be set to 100 mm.times.100 mm with full thickness in the
thickness direction. Then, the etched amount in the inner wall of
the inner layer was obtained by measuring the sample's cross
section by a scale. [0222] etched thickness of inner layer is less
than 3 mm: good [0223] etched thickness of inner layer is in the
range from 3 mm to less than 5 mm: fair [0224] etched thickness of
inner layer is 5 mm or more: poor
[Evaluation of Gaseous Bubble Expansion in the Transparent Silica
Glass Layer of the Container Side Wall]
[0225] A sample was cut out from the side wall of the silica
container after three multipullings of a silicon single crystal, in
the part lower than the level of the silicon melt. The sample was
made for the size of the inner wall surface of the silica container
to be set to 100 mm.times.100 mm with full thickness in the
thickness direction. Then, gaseous bubbles in the inner layer were
observed by a stereoscopic microscope for relative evaluation of
the gaseous bubble expansion. Comparative Example 2 was used as the
standard of a conventional level.
TABLE-US-00001 almost no expansion observed good slight expansion
observed fair same level of expansion as conventional observation
poor
[Evaluation of (Relative) Production Cost of the Silica
Container]
[0226] The production cost of the silica container was evaluated.
In particular, costs associated with silica raw materials, a
melting energy, and the like were summed up for the relative
evaluation. The cost by a conventional method was based on
Comparative Example 2. [0227] low cost (less than 50% relative to
cost of the conventional method): good [0228] moderate cost (50 to
less than 100% relative to cost of the conventional method): fair
[0229] high cost (cost of the conventional method was taken as
100%): poor
[0230] Production conditions, measured physical properties, and
evaluation results of each silica container produced in Examples 1
to 10 and Comparative Examples 1 to 6 are summarized in the
following Tables 1 to 8.
TABLE-US-00002 TABLE 1 Example No. Example 1 Example 2 Powdered
substrate's raw material Powdered natural silica Powdered natural
silica Particle diameter: 50 to 500 .mu.m Particle diameter: 50 to
500 .mu.m Purity: 99.999% by weight Purity: 99.999% by weight
Powdered Powdered base material Powdered natural silica Powdered
natural silica inner- Particle diameter: 50 to 500 .mu.m Particle
diameter: 50 to 500 .mu.m layer's Purity: 99.999% by weight Purity:
99.999% by weight raw Doping concentration of Ba: 110 Ba: 200
material alkaline earth metal element (ppm by weight) Atmosphere,
temperature, Air, 200.degree. C., 50 hours Air, 200.degree. C., 50
hours and time of heating and drying treatment Order of preliminary
molding and Preliminary molding of substrate Preliminary molding of
substrate heating of each layer and inner layer, followed by and
inner layer, followed by simultaneous heating simultaneous heating
Preliminary molding of substrate Rotation molding within frame
Rotation molding within frame Preliminary molding of inner layer
Rotation molding within frame Rotation molding within frame Melting
and sintering method of Arc discharge heating under Arc discharge
heating under substrate aspiration aspiration Atmosphere during
melting and H.sub.2: 30% by volume, H.sub.2: 30% by volume,
sintering of substrate He: 70% by volume He: 70% by volume Melting
method of inner layer Arc discharge heating under Arc discharge
heating under aspiration aspiration Atmosphere of inner layer
melting H.sub.2: 30% by volume, H.sub.2: 30% by volume, He: 70% by
volume He: 70% by volume Physical Outer diameter/height/ Outer
diameter 450/height 400/ Outer diameter 450/height 400/ properties
thickness (mm) thickness 10 thickness 10 of Color tone Outside:
white opaque Outside: white opaque substrate Inside: transparent
Inside: transparent OH group concentration 30 30 (ppm by weight)
Alkaline metal Li: 1, Na: 5, K: 1 Li: 1, Na: 5, K: 1 concentration
(ppm by weight) Light transmittance (%) at 91.9 91.9 wavelength of
600 nm (Inner peripheral part) Physical Thickness (mm) 5 5
properties Color tone Colorless and transparent Colorless and
transparent of inner OH group concentration 20 20 layer (ppm by
weight) Al Concentration 3 3 (ppm by weight) Alkaline earth metal
Ba: 100 Ba: 180 concentration (ppm by weight) Alkaline metal Li: 1,
Na: 3, K: 1 Li: 1, Na: 4, K: 1 concentration (ppb by weight)
Release amount of water <1 .times. 10.sup.17 <1 .times.
10.sup.17 molecules (molecules/g) Light transmittance (%) at 92.9
92.8 wavelength of 600 nm Evaluation Single crystal multipulling
Fair Good Void/pinhole of single Good Good crystal Container
etching Fair Good resistance Expansion of container Good Good
gaseous bubbles Production cost of Fair Fair container
TABLE-US-00003 TABLE 2 Example No. Example 3 Example 4 Powdered
substrate's raw material Powdered natural silica Powdered natural
silica Particle diameter: 50 to 500 .mu.m Particle diameter: 50 to
500 .mu.m Purity: 99.999% by weight Purity: 99.999% by weight
Powdered Powdered base material Powdered natural silica Powdered
natural silica inner- Particle diameter: 50 to 500 .mu.m Particle
diameter: 50 to 500 .mu.m layer's Purity: 99.999% by weight Purity:
99.999% by weight raw Doping concentration of Ba: 400 Ba: 800
material alkaline earth metal element (ppm by weight) Atmosphere,
temperature, Air, 200.degree. C., 50 hours Air, 200.degree. C., 50
hours and time of heating and drying treatment Order of preliminary
molding and Preliminary molding of substrate Preliminary molding of
substrate heating of each layer and inner layer, followed by and
inner layer, followed by simultaneous heating simultaneous heating
Preliminary molding of substrate Rotation molding within frame
Rotation molding within frame Preliminary molding of inner layer
Rotation molding within frame Rotation molding within frame Melting
and sintering method of Arc discharge heating under Arc discharge
heating under substrate aspiration aspiration Atmosphere during
melting and H.sub.2: 30% by volume, H.sub.2: 30% by volume,
sintering of substrate He: 70% by volume He: 70% by volume Melting
method of inner layer Arc discharge heating under Arc discharge
heating under aspiration aspiration Atmosphere of inner layer
melting H.sub.2: 30% by volume, H.sub.2: 30% by volume, He: 70% by
volume He: 70% by volume Physical Outer diameter/height/ Outer
diameter 450/height 400/ Outer diameter 450/height 400/ properties
thickness (mm) thickness 10 thickness 10 of Color tone Outside:
white opaque Outside: white opaque substrate Inside: transparent
Inside: transparent OH group concentration 20 35 (ppm by weight)
Alkaline metal Li: 1, Na: 5, K: 1 Li: 1, Na: 5, K: 1 concentration
(ppm by weight) Light transmittance (%) at 91.9 91.8 wavelength of
600 nm (Inner peripheral part) Physical Thickness (mm) 5 5
properties Color tone Colorless and transparent Colorless and
transparent of inner OH group concentration 15 25 layer (ppm by
weight) Al Concentration 30 80 (ppm by weight) Alkaline earth metal
Ba: 370 Ba: 740 concentration (ppm by weight) Alkaline metal Li: 1,
Na: 2, K: 1 Li: 1, Na: 3, K: 2 concentration (ppb by weight)
Release amount of water <1 .times. 10.sup.17 <1 .times.
10.sup.17 molecules (molecules/g) Light transmittance (%) at 92.5
91.8 wavelength of 600 nm Evaluation Single crystal multipulling
Good Good Void/pinhole of single Good Fair crystal Container
etching Good Good resistance Expansion of container Good Good
gaseous bubbles Production cost of Fair Fair container
TABLE-US-00004 TABLE 3 Example No. Example 5 Example 6 Powdered
substrate's raw material Powdered natural silica Powdered natural
silica Particle diameter: 50 to 500 .mu.m Particle diameter: 50 to
500 .mu.m Purity: 99.999% by weight Purity: 99.999% by weight
Powdered Powdered base material Powdered natural silica Powdered
natural silica inner- Particle diameter: 50 to 500 .mu.m Particle
diameter: 50 to 500 .mu.m layer's Purity: 99.999% by weight Purity:
99.999% by weight raw Doping concentration of Ba: 200 Ba: 200
material alkaline earth metal element (ppm by weight) Atmosphere,
temperature, Air, 200.degree. C., 50 hours Air, 200.degree. C., 50
hours and time of heating and drying treatment Order of preliminary
molding and Preliminary molding of substrate Preliminary molding of
substrate heating of each layer and inner layer, followed by and
inner layer, followed by simultaneous heating simultaneous heating
Preliminary molding of substrate Rotation molding within frame
Rotation molding within frame Preliminary molding of inner layer
Rotation molding within frame Rotation molding within frame Melting
and sintering method of Arc discharge heating under Arc discharge
heating under substrate aspiration aspiration Atmosphere during
melting and H.sub.2: 100% by volume, H.sub.2: 50% by volume,
sintering of substrate N.sub.2: 50% by volume Melting method of
inner layer Arc discharge heating under Arc discharge heating under
aspiration aspiration Atmosphere of inner layer melting H.sub.2:
100% by volume, H.sub.2: 50% by volume, N.sub.2: 50% by volume
Physical Outer diameter/height/ Outer diameter 450/height 400/
Outer diameter 450/height 400/ properties thickness (mm) thickness
10 thickness 10 of Color tone Outside: white opaque Outside: white
opaque substrate Inside: transparent Inside: transparent OH group
concentration 10 15 (ppm by weight) Alkaline metal Li: 1, Na: 5, K:
1 Li: 1, Na: 5, K: 1 concentration (ppm by weight) Light
transmittance (%) at 92.0 91.8 wavelength of 600 nm (Inner
peripheral part) Physical Thickness (mm) 5 5 properties Color tone
Colorless and transparent Colorless and transparent of inner OH
group concentration 5 10 layer (ppm by weight) Al Concentration 3 3
(ppm by weight) Alkaline earth metal Ba: 180 Ba: 180 concentration
(ppm by weight) Alkaline metal Li: 1, Na: 2, K: 1 Li: 1, Na: 4, K:
2 concentration (ppb by weight) Release amount of water <1
.times. 10.sup.17 <1 .times. 10.sup.17 molecules (molecules/g)
Light transmittance (%) at 93.1 92.0 wavelength of 600 nm
Evaluation Single crystal multipulling Good Good Void/pinhole of
single Good Good crystal Container etching Good Good resistance
Expansion of container Good Fair gaseous bubbles Production cost of
Fair Fair container
TABLE-US-00005 TABLE 4 Example No. Example 7 Example 8 Powdered
substrate's raw material Powdered natural silica Powdered natural
silica Particle diameter: 50 to 500 .mu.m Particle diameter: 50 to
500 .mu.m Purity: 99.999% by weight Purity: 99.999% by weight
Powdered Powdered base material Powdered natural silica Powdered
natural silica inner- Particle diameter: 50 to 500 .mu.m Particle
diameter: 50 to 500 .mu.m layer's Purity: 99.999% by weight Purity:
99.999% by weight raw Doping concentration of Ba: 200 Ba: 200
material alkaline earth metal element (ppm by weight) Atmosphere,
temperature, Air, 200.degree. C., 50 hours Air, 200.degree. C., 50
hours and time of heating and drying treatment Order of preliminary
molding and Preliminary molding of substrate Preliminary molding of
substrate heating of each layer and melting and sintering of and
melting and sintering of substrate, followed by spreading
substrate, followed by spreading of powdered inner-layer's raw of
powdered inner-layer's raw material and heating material and
heating Preliminary molding of substrate Rotation molding within
frame Rotation molding within frame Preliminary molding of inner
layer None None Melting and sintering method of Arc discharge
heating under Arc discharge heating under substrate aspiration
aspiration Atmosphere during melting and H.sub.2: 30% by volume,
H.sub.2: 30% by volume, sintering of substrate He: 70% by volume
N.sub.2: 70% by volume Melting method of inner layer Spreading of
powdered raw material Spreading of powdered raw material and arc
discharge heating under and arc discharge heating under normal
pressure normal pressure Atmosphere of inner layer melting H.sub.2:
50% by volume, H.sub.2: 50% by volume, He: 50% by volume He: 50% by
volume Physical Outer diameter/height/ Outer diameter 450/height
400/ Outer diameter 450/height 400/ properties thickness (mm)
thickness 10 thickness 10 of Color tone Outside: white opaque
Outside: white opaque substrate Inside: transparent Inside:
transparent OH group concentration 13 15 (ppm by weight) Alkaline
metal concentration Li: 1, Na: 3, K: 2 Li: 1, Na: 4, K: 1 (ppm by
weight) Light transmittance (%) at 92.0 92.0 wavelength of 600 nm
(Inner peripheral part) Physical Thickness (mm) 5 5 properties
Color tone Colorless and transparent Colorless and transparent of
inner OH group concentration 10 10 layer (ppm by weight) Al
Concentration 3 3 (ppm by weight) Alkaline earth metal Ba: 180 Ba:
180 concentration (ppm by weight) Alkaline metal concentration Li:
<1, Na: <1, K: <1 Li: <1, Na: <1, K: <1 (ppb by
weight) Release amount of water <1 .times. 10.sup.17 <1
.times. 10.sup.17 molecules (molecules/g) Light transmittance (%)
at 92.8 91.9 wavelength of 600 nm Evaluation Single crystal
multipulling Good Good Void/pinhole of single Good Good crystal
Container etching Good Good resistance Expansion of container Good
Fair gaseous bubbles Production cost of Fair Fair container
TABLE-US-00006 TABLE 5 Example No. Example 9 Example 10 Powdered
substrate's raw material Powdered natural silica Powdered natural
silica Particle diameter: 50 to 500 .mu.m Particle diameter: 50 to
500 .mu.m Purity: 99.999% by weight Purity: 99.999% by weight
Powdered Powdered base material Powdered natural silica Powdered
natural silica inner- Particle diameter: 50 to 500 .mu.m Particle
diameter: 50 to 500 .mu.m layer's Purity: 99.999% by weight Purity:
99.999% by weight raw Doping concentration of Ba: 200 Ba: 200
material alkaline earth metal element (ppm by weight) Atmosphere,
temperature, Air, 200.degree. C., 50 hours Air, 200.degree. C., 50
hours and time of heating and drying treatment Order of preliminary
molding and Preliminary molding of substrate Preliminary molding of
substrate heating of each layer and inner layer, followed by and
inner layer, followed by simultaneous heating simultaneous heating
Preliminary molding of substrate Rotation molding within frame
Rotation molding within frame Preliminary molding of inner layer
Rotation molding within frame Rotation molding within frame Melting
and sintering method of Arc discharge heating under Arc discharge
heating under substrate aspiration aspiration Atmosphere during
melting and H.sub.2: 15% by volume, He: 15% by volume, sintering of
substrate N.sub.2: 85% by volume N.sub.2: 85% by volume Melting
method of inner layer Arc discharge heating under Arc discharge
heating under aspiration aspiration Atmosphere of inner layer
melting H.sub.2: 15% by volume, He: 15% by volume, N.sub.2: 85% by
volume N.sub.2: 85% by volume Physical Outer diameter/height/ Outer
diameter 450/height 400/ Outer diameter 450/height 400/ properties
thickness (mm) thickness 10 thickness 10 of Color tone Outside:
white opaque Outside: white opaque substrate Inside: transparent
Inside: transparent OH group concentration 20 30 (ppm by weight)
Alkaline metal Li: 1, Na: 5, K: 1 Li: 1, Na: 5, K: 1 concentration
(ppm by weight) Light transmittance (%) at 91.9 91.9 wavelength of
600 nm (Inner peripheral part) Physical Thickness (mm) 5 5
properties Color tone Colorless and transparent Colorless and
transparent of inner OH group concentration 5 10 layer (ppm by
weight) Al Concentration 10 10 (ppm by weight) Alkaline earth metal
Ba: 180 Ba: 180 concentration (ppm by weight) Alkaline metal Li: 1,
Na: 2, K: 1 Li: 1, Na: 4, K: 2 concentration (ppb by weight)
Release amount of water <1 .times. 10.sup.17 <1 .times.
10.sup.17 molecules (molecules/g) Light transmittance (%) at 92.0
91.8 wavelength of 600 nm Evaluation Single crystal multipulling
Good Fair Void/pinhole of single Good Good crystal Container
etching Good Good resistance Expansion of container Fair Fair
gaseous bubbles Production cost of Fair Fair container
TABLE-US-00007 TABLE 6 Example No. Comparative Example 1
Comparative Example 2 Powdered substrate's raw material Powdered
natural silica Powdered natural silica Particle diameter: 50 to 500
.mu.m Particle diameter: 50 to 500 .mu.m Purity: 99.999% by weight
Purity: 99.999% by weight Powdered Powdered base material Powdered
synthetic cristobalite Powdered synthetic cristobalite inner-
Particle diameter: 50 to 300 .mu.m Particle diameter: 50 to 300
.mu.m layer's Purity: 99.9999% by weight Purity: 99.9999% by weight
raw Doping concentration of None None material alkaline earth metal
element (ppm by weight) Atmosphere, temperature, None None and time
of heating and drying treatment Order of preliminary molding and
Preliminary molding of substrate Preliminary molding of substrate
heating of each layer and inner layer, followed by and inner layer,
followed by simultaneous heating simultaneous heating Preliminary
molding of substrate Rotation molding within frame Rotation molding
within frame Preliminary molding of inner layer Rotation molding
within frame None Melting and sintering method of Arc discharge
heating under Arc discharge heating under normal substrate
aspiration pressure Atmosphere during melting and Air Air sintering
of substrate Melting method of inner layer Arc discharge heating
under Spreading of powdered raw material aspiration and arc
discharge heating under normal pressure Atmosphere of inner layer
melting Air Air Physical Outer diameter/height/ Outer diameter
450/height 400/ Outer diameter 450/height 400/ properties thickness
(mm) thickness 10 thickness 10 of Color tone Outside: white opaque
Outside: white opaque substrate Inside: transparent Inside: white
opaque OH group concentration 80 130 (ppm by weight) Alkaline metal
Li: 1, Na: 3, K: 1 Li: <0.1, Na: <0.1, K: <0.1
concentration (ppm by weight) Light transmittance (%) at 91.5 75.2
wavelength of 600 nm (Inner peripheral part) Physical Thickness
(mm) 5 5 properties Color tone Colorless and transparent Colorless
and transparent of inner OH group concentration 120 180 layer (ppm
by weight) Al Concentration 3 3 (ppm by weight) Alkaline earth
metal Ba: <1 Ba: <1 concentration (ppm by weight) Alkaline
metal Li: <1, Na: <1, K: <1 Li: <1, Na: <1, K: <1
concentration (ppb by weight) Release amount of water 1 .times.
10.sup.17 3 .times. 10.sup.17 molecules (molecules/g) Light
transmittance (%) at 92.5 91.6 wavelength of 600 nm Evaluation
Single crystal multipulling Fair Fair Void/pinhole of single Fair
Fair crystal Container etching Poor Poor resistance Expansion of
container Fair Poor gaseous bubbles Production cost of Fair Poor
container
TABLE-US-00008 TABLE 7 Example No. Comparative Example 3
Comparative Example 4 Powdered substrate's raw material Powdered
natural silica Powdered natural silica Particle diameter: 50 to 500
.mu.m Particle diameter: 50 to 500 .mu.m Purity: 99.999% by weight
Purity: 99.999% by weight Powdered Powdered base material Powdered
synthetic cristobalite Powdered synthetic cristobalite inner-
Particle diameter: 50 to 300 .mu.m Particle diameter: 50 to 300
.mu.m layer's Purity: 99.9999% by weight Purity: 99.9999% by weight
raw Doping concentration of Ba: 3000 Ba: 100 material alkaline
earth metal element (ppm by weight) Atmosphere, temperature, Air,
200.degree. C., 100 hours Air, 200.degree. C., 50 hours and time of
heating and drying treatment Order of preliminary molding and
Preliminary molding of substrate Preliminary molding of substrate
heating of each layer and inner layer, followed by and melting and
sintering of simultaneous heating substrate, followed by spreading
of powdered inner-layer's raw material and heating Preliminary
molding of substrate Rotation molding within frame Rotation molding
within frame Preliminary molding of inner layer Rotation molding
within frame None Melting and sintering method of Arc discharge
heating under Arc discharge heating under normal substrate
aspiration pressure Atmosphere during melting and Air Air sintering
of substrate Melting method of inner layer Arc discharge heating
under Spreading of powdered raw material aspiration and arc
discharge heating under normal pressure Atmosphere of inner layer
melting Air Air Physical Outer diameter/height/ Outer diameter
450/height 400/ Outer diameter 450/height 400/ properties thickness
(mm) thickness 10 thickness 10 of Color tone Outside: white opaque
Outside: white opaque substrate Inside: transparent Inside: white
opaque OH group concentration 90 70 (ppm by weight) Alkaline metal
Li: 1, Na: 2, K: 1 Li: 8, Na: 56, K: 10 concentration (ppm by
weight) Light transmittance (%) at 90.1 73.7 wavelength of 600 nm
(Inner peripheral part) Physical Thickness (mm) 5 5 properties
Color tone White and semitransparent White and semitransparent of
inner OH group concentration 130 120 layer (ppm by weight) Al
Concentration 3 3 (ppm by weight) Alkaline earth metal Ba: 2500 Ba:
80 concentration (ppm by weight) Alkaline metal Li: <1, Na:
<1, K: <1 Li: <1, Na: <1, K: <1 concentration (ppb
by weight) Release amount of water 1 .times. 10.sup.17 3 .times.
10.sup.17 molecules (molecules/g) Light transmittance (%) at 87.4
89.8 wavelength of 600 nm Evaluation Single crystal multipulling
Poor Poor Void/pinhole of single Poor Poor crystal Container
etching Good Good resistance Expansion of container Fair Fair
gaseous bubbles Production cost of Fair Good container
TABLE-US-00009 TABLE 8 Example No. Comparative Example 5
Comparative Example 6 Powdered substrate's raw material Powdered
natural silica Powdered natural silica Particle diameter: 50 to 500
.mu.m Particle diameter: 50 to 500 .mu.m Purity: 99.999% by weight
Purity: 99.999% by weight Powdered Powdered base material Powdered
natural silica Powdered natural silica inner- Particle diameter: 50
to 500 .mu.m Particle diameter: 50 to 500 .mu.m layer's Purity:
99.999% by weight Purity: 99.999% by weight raw Doping
concentration of Ba: 200 Ba: 200 material alkaline earth metal
element (ppm by weight) Atmosphere, temperature, Air, 200.degree.
C., 50 hours Air, 200.degree. C., 50 hours and time of heating and
drying treatment Order of preliminary molding and Preliminary
molding of substrate Preliminary molding of substrate heating of
each layer and inner layer, followed by and inner layer, followed
by simultaneous heating simultaneous heating Preliminary molding of
substrate Rotation molding within frame Rotation molding within
frame Preliminary molding of inner layer Rotation molding within
frame Rotation molding within frame Melting and sintering method of
Arc discharge heating under Arc discharge heating under substrate
aspiration aspiration Atmosphere during melting and H.sub.2: 5% by
volume, N.sub.2: 95% by volume He: 5% by volume, N.sub.2: 95% by
volume sintering of substrate Melting method of inner layer Arc
discharge heating under Arc discharge heating under aspiration
aspiration Atmosphere of inner layer melting H.sub.2: 5% by volume,
N.sub.2: 95% by volume He: 5% by volume, N.sub.2: 95% by volume
Physical Outer diameter/height/ Outer diameter 450/height 400/
Outer diameter 450/height 400/ properties thickness (mm) thickness
10 thickness 10 of Color tone Outside: white opaque Outside: white
opaque substrate Inside: transparent Inside: transparent OH group
concentration 50 60 (ppm by weight) Alkaline metal Li: 1, Na: 5, K:
1 Li: 1, Na: 5, K: 1 concentration (ppm by weight) Light
transmittance (%) at 91.6 91.6 wavelength of 600 nm (Inner
peripheral part) Physical Thickness (mm) 5 5 properties Color tone
Colorless and transparent Colorless and transparent of inner OH
group concentration 30 40 layer (ppm by weight) Al Concentration 3
3 (ppm by weight) Alkaline earth metal Ba: 180 Ba: 180
concentration (ppm by weight) Alkaline metal Li: 1, Na: 2, K: 1 Li:
1, Na: 4, K: 2 concentration (ppb by weight) Release amount of
water <1 .times. 10.sup.17 <1 .times. 10.sup.17 molecules
(molecules/g) Light transmittance (%) at 91.6 91.5 wavelength of
600 nm Evaluation Single crystal multipulling Fair Fair
Void/pinhole of single Fair Poor crystal Container etching Fair
Fair resistance Expansion of container Poor Fair gaseous bubbles
Production cost of Fair Fair container
[0231] As can be seen in Tables 1 to 8, in Examples 1 to 10 that
are in accord with the method for producing a silica container of
the present invention, the silica containers giving the results in
pulling up of a single crystal no way inferior to conventional
silica containers of Comparative Examples 1 and 2 could be
obtained, in spite of the silica containers produced with a low
cost and a higher productivity. In addition, the etching resistance
to a silicon melt could be remarkably improved as compared with a
conventional silica container of Comparative Example 2.
[0232] It was found that less numbers of voids and pinholes were
formed in a silicon single crystal produced by using a silica
container of Examples 1 to 10 as compared with Comparative Examples
1 to 6.
[0233] It must be noted here that the present invention is not
limited to the embodiments as described above. The foregoing
embodiments are mere examples; any form having substantially the
same composition as the technical concept described in claims of
the present invention and showing similar effects is included in
the technical scope of the present invention.
* * * * *