U.S. patent application number 14/822378 was filed with the patent office on 2015-12-03 for method for cleaning bell jar, method for producing polycrystalline silicon, and apparatus for drying bell jar.
This patent application is currently assigned to Shin-Etsu Chemical Co., Ltd.. The applicant listed for this patent is Shin-Etsu Chemical Co., Ltd.. Invention is credited to Yasushi KUROSAWA, Shinichi KUROTANI, Shigeyoshi NETSU, Kyoji OGURO.
Application Number | 20150345862 14/822378 |
Document ID | / |
Family ID | 45347822 |
Filed Date | 2015-12-03 |
United States Patent
Application |
20150345862 |
Kind Code |
A1 |
KUROSAWA; Yasushi ; et
al. |
December 3, 2015 |
METHOD FOR CLEANING BELL JAR, METHOD FOR PRODUCING POLYCRYSTALLINE
SILICON, AND APPARATUS FOR DRYING BELL JAR
Abstract
A bell jar includes a metallic bell jar (1), and a metallic base
plate (2) on which the bell jar (1) is placed, and packing (3)
seals an inside of a container. To the base plate (2), a pressure
gauge (4), a gas introduction line (5), and a gas discharge line
(6) are connected so as to allow monitoring of internal pressure of
the bell jar (1) and introduction and discharge of a gas. A vacuum
pump (7) is provided in a path of the gas discharge line (6), and
the vacuum pump (7) reduces internal pressure of the bell jar so as
to be lower than vapor pressure of water. The vacuum pump (7)
reduces the internal pressure of the bell jar so as to be lower
than vapor pressure of water, thereby efficiently removing
moisture, and completing drying of the bell jar in a short
time.
Inventors: |
KUROSAWA; Yasushi; (Niigata,
JP) ; OGURO; Kyoji; (Niigata, JP) ; KUROTANI;
Shinichi; (Niigata, JP) ; NETSU; Shigeyoshi;
(Niigata, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Shin-Etsu Chemical Co., Ltd. |
Tokyo |
|
JP |
|
|
Assignee: |
Shin-Etsu Chemical Co.,
Ltd.
Tokyo
JP
|
Family ID: |
45347822 |
Appl. No.: |
14/822378 |
Filed: |
August 10, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13704767 |
Dec 17, 2012 |
9126242 |
|
|
PCT/JP2011/001319 |
Mar 7, 2011 |
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14822378 |
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Current U.S.
Class: |
34/92 |
Current CPC
Class: |
B08B 3/04 20130101; C01B
33/035 20130101; C23C 16/4407 20130101; F28G 13/00 20130101; F26B
5/042 20130101; F26B 5/12 20130101; B08B 3/10 20130101 |
International
Class: |
F26B 5/04 20060101
F26B005/04; C23C 16/44 20060101 C23C016/44 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 16, 2010 |
JP |
2010-137646 |
Claims
1-8. (canceled)
9. An apparatus for drying a bell jar used for producing
polycrystalline silicon by the Siemens method, wherein the
apparatus can form an airtight space by providing the bell jar, and
includes a vacuum line for reducing air pressure in the airtight
space, and a dry air line for returning the air pressure in the
airtight space to normal pressure.
Description
TECHNICAL FIELD
[0001] The present invention relates to a cleaning technique of a
bell jar used for producing polycrystalline silicon, and more
particularly to a method and an apparatus capable of efficiently
removing moisture in an inner wall surface of the bell jar, which
may cause mixing of impurity into polycrystalline silicon.
BACKGROUND ART
[0002] High purity polycrystalline silicon is a material for a
single crystal silicon substrate for producing a semiconductor
device, or a material for producing a solar cell. Generally, high
purity polycrystalline silicon is produced batch-wise by a method
(Siemens method) for performing pyrolysis or hydrogen reduction of
a silicon-containing reaction gas as a source gas into high purity
silicon, and precipitating the high purity silicon on a thin
silicon filament rod. The silicon-containing reaction gas includes
gases such as monosilane, dichlorosilane, trichlorosilane, and
tetrachlorosilane, or a halogen gas generally represented by
SiH.sub.nX.sub.4-n (n=0,1,2,3;X.dbd.Br, I).
[0003] A general precipitation reaction container used for
producing high purity polycrystalline silicon is constituted by a
metallic bed plate (base plate) and a metallic bell jar placed on
the base plate, and an inside of the bell jar is a reaction space.
It is necessary that the precipitation reaction container can be
cooled and can seal a gas in the bell jar. This is because the
reaction gas described above is corrosive, and may cause ignition
or explosion by mixture with air.
[0004] If precipitation reaction of polycrystalline silicon is
performed in the precipitation reaction container, during a CVD
process, amorphous silicon dust is formed by a homogeneous
nucleation process, and silicon adheres to an inner surface of the
precipitation reaction container. The silicon dust contains a high
level contaminant, and is settled on polycrystalline silicon as a
product to cause surface defect or contamination (see Patent
Literature 1).
[0005] Since the precipitation reaction of polycrystalline silicon
described above is performed batch-wise, the inner surface of the
bell jar inevitably comes into contact with the atmosphere when the
polycrystalline silicon is taken out from the bell jar. The
silicon-containing reaction gas as a source gas and chlorosilanes
or halogen gases obtained as by-products by the precipitation
reaction remain on the inner surface of the bell jar after the
precipitation reaction of polycrystalline silicon. It is known that
such gases become highly corrosive when reacting with moisture in
the atmosphere.
[0006] The corrosive gas described above expresses and activates
hazardous substances (for example, boron, aluminum, phosphorus,
arsenic, antimony) that reduce quality of polycrystalline silicon,
from a structure member on the inner surface of the bell jar.
[0007] Such hazardous substances are taken into polycrystalline
silicon during a precipitation reaction process for a next batch to
reduce quality of the polycrystalline silicon (for example, see
Patent Literature 2).
[0008] From such circumstances, a precipitation bell jar is washed
using high purity water or a carbon dioxide pellet for each batch
or every few batches to clean the inner surface.
[0009] Meanwhile, generally, an automated washing apparatus is used
for a bell jar because of a large inner surface area or difficulty
in wiping in terms of structure. Patent Literatures 1 and 2
mentioned above and Patent Literature 3 disclose such a washing
apparatus and a washing method using the apparatus.
CITATION LIST
Patent Literature
[0010] Patent Literature 1: Japanese Patent Laid-Open No. 6-216036
[0011] Patent Literature 2: Japanese Patent Laid-Open No.
2008-37748 [0012] Patent Literature 3: Japanese Patent Laid-Open
No. 2009-196882
SUMMARY OF THE INVENTION
Technical Problem
[0013] A precipitation reactor (bell jar) for producing
polycrystalline silicon is opened for each batch to take out a
product therefrom. Then, washing is performed for each batch or
every few batches to clean an inner surface.
[0014] Amorphous silicon or silane chloride polymer or the like
adhering to the inner surface of the bell jar should be removed by
washing. However, it is known that such a substance reacts with
moisture to finally turn into a fine powdered substance, and it is
very difficult to completely remove moisture that has been taken
into the finely powdered substance.
[0015] The present inventors have studied and revealed that in a
conventional washing method of heating a bell jar using steam or
the like and simultaneously replacing an inside of the bell jar
with a high purity nitrogen gas or the like, it is difficult to
completely remove moisture in a short time, while a longer drying
time easily reduces quality of polycrystalline silicon to be
produced for a next batch.
[0016] The present invention is achieved in view of the problems of
the conventional bell jar cleaning technique, and has an object to
provide a technique of efficiently removing moisture from an inner
surface of a bell jar, completing cleaning of the bell jar in a
short time, and thus increasing cleanliness of an inner surface of
the bell jar to contribute to production of high purity
polycrystalline silicon.
Solution to Problem
[0017] To achieve the object, the present invention provides a
method for cleaning a bell jar used for producing polycrystalline
silicon by the Siemens method, including: a washing step using
water in the bell jar; and then a drying step of reducing pressure
so that an inside of the bell jar has pressure lower than vapor
pressure of water at an inner surface temperature to remove
moisture.
[0018] Preferably, the drying step is a drying step of using a
vacuum pump having vacuum reachability of 200 Pa or less, and
reducing pressure so that air pressure in the bell jar is 1000 Pa
or less.
[0019] The method for cleaning a bell jar according to the present
invention preferably includes, after the drying step, a step of
introducing a high purity inert gas with reduced moisture into the
bell jar to return internal pressure to atmospheric pressure.
[0020] The present invention provides a method for producing
polycrystalline silicon performed by repeating a plurality of times
a precipitation step of polycrystalline silicon by the Siemens
method, including, after completion of the precipitation step and
before a precipitation step for a next batch, a cleaning step of a
bell jar used for precipitating the polycrystalline silicon,
wherein the cleaning step of the bell jar includes a water washing
step of washing the bell jar using water, and a drying step
subsequent to the water washing step, the drying step is a step of,
after the water washing step, reducing pressure so that air
pressure in the bell jar is 1000 Pa or less using a vacuum pump
having vacuum reachability of 200 Pa or less, and thus reducing
pressure so that an inside of the bell jar has pressure lower than
vapor pressure of water at an inner surface temperature to remove
moisture, and a time from completion of the water washing step to
completion of the drying step is 1.2 hours or less.
[0021] Preferably, the cleaning step of the bell jar further
includes, after the drying step, a step of introducing a high
purity inert gas with reduced moisture into the bell jar to return
internal pressure to atmospheric pressure.
[0022] Also, preferably, the time from completion of the water
washing step to completion of the drying step is 0.8 hours or less.
More preferably, the time from completion of the water washing step
to completion of the drying step is 0.4 hours or less.
[0023] In the present invention, for example, the drying step is
completed after a lapse of five minutes after the air pressure in
the bell jar is 1000 Pa or less.
[0024] The present invention provides an apparatus for drying a
bell jar used for producing polycrystalline silicon by the Siemens
method, wherein the apparatus can form an airtight space by
providing the bell jar, and includes a vacuum line for reducing air
pressure in the airtight space, and a dry air line for returning
the air pressure in the airtight space to normal pressure.
Advantageous Effect of Invention
[0025] In the present invention, instead of a conventional method
of increasing a temperature of a surface of a bell jar to remove
moisture, pressure in the bell jar is reduced to a boiling point of
water or less to efficiently remove moisture. Thus, moisture can be
efficiently removed from the inner surface of the bell jar, and
cleaning of the bell jar can be completed in a short time. This
increases cleanliness of the inner surface of the bell jar and
significantly contributes to an increase in quality of high purity
polycrystalline silicon to be produced.
BRIEF DESCRIPTION OF DRAWINGS
[0026] FIG. 1 illustrates an exemplary configuration of a bell jar
drying apparatus according to the present invention.
[0027] FIG. 2 illustrates a result of checking a relationship
between an open time of a bell jar and electric resistivity of
polycrystalline silicon.
[0028] FIG. 3 illustrates a result of checking a relationship
between a time from completion of a water washing step of the bell
jar to assembling as a polycrystalline silicon producing reactor
for a next batch to generate a vacuum therein, and electric
resistivity of polycrystalline silicon.
[0029] FIG. 4 illustrates an exemplary configuration of a bell jar
drying apparatus used for steam drying.
DESCRIPTION OF EMBODIMENT
[0030] Now, with reference to the drawings, a bell jar cleaning
method and a bell jar drying apparatus according to the present
invention will be described.
[0031] FIG. 1 illustrates an exemplary configuration of the bell
jar drying apparatus according to the present invention. The bell
jar drying apparatus is an apparatus for drying a bell jar used for
producing polycrystalline silicon. The bell jar includes a metallic
bell jar 1, and a metallic bed plate (base plate) 2 on which the
bell jar 1 is placed, and packing denoted by reference numeral 3
seals an inside of a container. An inside of the bell jar 1 placed
on the base plate 2 is a space for precipitation reaction of
polycrystalline silicon.
[0032] To the base plate 2, a pressure gauge 4, a gas introduction
line 5, and a gas discharge line 6 are connected so as to allow
monitoring of internal pressure of the bell jar 1 and introduction
and discharge of a gas. A vacuum pump 7 is provided in a path of
the gas discharge line 6, and the vacuum pump 7 reduces internal
pressure of the bell jar so as to be lower than vapor pressure of
water.
[0033] Usually, an automatic valve or the like is placed on a
suction side of the vacuum pump 7 so as to prevent backflow of oil
in the vacuum pump 7 to the bell jar 1 during stop of the vacuum
pump 7. However, a phenomenon is known in which frequent
repetitions of operation and stop of the vacuum pump causes
backflow of oil through an inner surface of a pipe. Thus, the
vacuum pump 7 is desirably of a low contamination type such as a
dry vacuum pump. As an ability of the vacuum pump, a discharging
ability may be selected according to a size of a bell jar to be
used, and a vacuum pump having vacuum reachability of about 200 Pa
or less may be used.
[0034] In an aspect shown in FIG. 1, the bell jar itself used for
producing polycrystalline silicon constitutes a part of the drying
apparatus, but the present invention is not limited to the
aspect.
[0035] As described above, in the present invention, the vacuum
pump 7 reduces internal pressure of the bell jar so as to be lower
than vapor pressure of water, thereby efficiently removing
moisture, and completing drying of the bell jar in a short
time.
[0036] As described above, a trace quantity of amorphous silicon or
silane chloride polymer or the like adheres to the inner surface of
the bell jar used for the precipitation reaction of polycrystalline
silicon. The amorphous silicon or silane chloride polymer expresses
and activates hazardous substances with respect to quality of the
polysilicon described above from the surface of the bell jar in the
presence of moisture, thereby preventing cleaning of the bell jar.
Thus, to produce high purity polycrystalline silicon, after
completion of washing, moisture adhering to the inner surface of
the bell jar needs to be efficiently removed to prevent generation
of the contamination substance.
[0037] The present inventors have repeatedly studied, and decided
to use a method of reducing pressure in the bell jar to be a
boiling point of water or less to efficiently remove moisture
instead of a conventional method of increasing a temperature of a
surface of the bell jar to remove moisture.
[0038] An advantage of using this method is, first, a reduction in
drying time by efficient removal of moisture.
[0039] It is considered that if a cleaning operation time of the
bell jar is increased, a time when the inside of the bell jar is in
an open state is necessarily increased, which reduces cleanliness
of the inner wall of the bell jar. The reduction in the drying time
means a reduction in the cleaning operation time, and is thus
effective for increasing cleanliness of the inner wall of the bell
jar.
[0040] The present inventors have studied and found that after
cleaning of the bell jar after completion of the polycrystalline
silicon production step, cleanliness of the inner wall of the bell
jar in a stage for preparation of a next polycrystalline silicon
production step depends on a time from after washing of the bell
jar to completion of drying rather than a total open time of the
inside of the bell jar during this time. Thus, if a drying step
requires a long time as in a conventional drying method, the inner
wall of the bell jar cannot be maintained in a clean state, thereby
increasing the risk of reducing quality of polycrystalline silicon
in a next production step. Also in this respect, reducing a drying
time by efficient removal of moisture in the inner surface of the
bell jar is extremely effective for producing high purity
polycrystalline silicon.
[0041] The dry state of the bell jar can be monitored by a pressure
reduction gauge as the most convenient method. A more accurate dry
state can be determined by a degree of pressure reduction.
Specifically, the dry state can be determined by the degree of
pressure reduction being a value specific to a drying apparatus
including a vacuum pump and a bell jar by completion of
vaporization of moisture.
[0042] The dry state of the apparatus can be confirmed by
terminating pressure reduction drying of the bell jar in midstream,
returning to normal pressure with a dry gas, and then measuring a
dew point of the gas in the drying apparatus with the normal
pressure. Based on such data, for example, an operational standard
of the drying step for obtaining a dew point of -40.degree. C. or
less or -60.degree. C. or less may be prepared.
[0043] In the case of drying a large bell jar, particularly, a bell
jar having a capacity of 1 m.sup.3 or more, checking of the dry
state by releasing pressure reduction as described above is
difficult in practice. Then, the dry state is preferably monitored
while being in the pressure reduction state. In such a case, the
dry state is checked by the degree of pressure reduction, and for
example, a time after a lapse of five minutes after the degree of
pressure reduction is 1000 Pa or less is regarded as completion of
drying with the dew point being -40.degree. C. or less.
[0044] In addition to the advantage of reduction in the drying
time, there is an advantage of eliminating the need for steam for
heating. The size of the bell jar has been increased, and the
increase in the size increases a heat capacity of the bell jar
itself. Also, generally, the bell jar includes a jacket containing
cooling water or a heat medium therein, thereby further increasing
a total heat capacity for heating the bell jar.
[0045] If such a bell jar having a large heat capacity is to be
dried by steam heating, there are a need to increase a steam
temperature and also a need for large equipment. If steam is
directly introduced into the jacket, an extraction operation
thereafter is also required.
[0046] In contrast to this, in the case of using the method of
reducing pressure in the bell jar to a boiling point or less of
water to efficiently remove moisture as in the present invention,
the bell jar preferably has a large total heat capacity. This is
because even if vaporization of moisture takes heat, the
temperature of the inner surface of the bell jar is hard to
change.
[0047] An advantage of using the method of reducing pressure in the
bell jar to a boiling point or less of water to remove moisture is
also that an amount of high purity gas consumed in the drying
operation can be significantly saved. The conventional steam
heating method requires a large amount of high purity inert gas as
a carrier gas for discharging vaporized moisture to the outside of
the ball jar and a replacement gas for preventing re-adsorption of
moisture to the inside of the bell jar after removal of
moisture.
[0048] In contrast to this, by the method of bringing the inside of
the bell jar into the pressure reduction state to remove moisture,
there is no need to use a carrier gas to discharge moisture to the
outside of the bell jar, and further, for replacement of the inside
of the bell jar with an inert gas, an object is sufficiently
achieved only by using a high purity inert gas when the inside of
the bell jar is returned to atmospheric pressure after completion
of the drying operation.
[0049] Now, an increase in quality of polycrystalline silicon by a
reduction in time for the cleaning step according to the present
invention will be described.
[0050] Table 1 shows a result of checking, for 12 polycrystalline
silicon production batches, the open time of the bell jar and the
time from completion of the water washing step of the bell jar to
assembling as a polycrystalline silicon producing reactor for a
next batch, and electric resistivity of polycrystalline
silicon.
TABLE-US-00001 TABLE 1 OPEN TIME AFTER ELECTRIC TIME WATER WASHING
RESISTIVITY BATCH [TIME] [TIME] [.OMEGA. cm] 1 4.0 1.2 1633 2 7.5
0.8 2400 3 12.3 2.3 731 4 4.3 2.5 829 5 10.2 2.0 1414 6 7.0 1.5
1498 7 7.4 1.5 1473 8 9.5 1.7 1328 9 8.7 1.7 1750 10 8.8 1.7 894 11
4.9 1.5 985 12 3.1 0.4 2510
[0051] As shown in Table 1, when the time from completion of the
water washing step to completion of the drying step was 1.2 hours
or less, polycrystalline silicon having electric resistivity of
1500 .OMEGA.cm or more was obtained. When the time from completion
of the water washing step to completion of the drying step was 0.8
hours or less, polycrystalline silicon having electric resistivity
of 2000 .OMEGA.cm or more was obtained. Further, when the time from
completion of the water washing step to completion of the drying
step was 0.4 hours or less, polycrystalline silicon having electric
resistivity of 2500 .OMEGA.cm or more was obtained.
[0052] FIG. 2 illustrates a result of checking a relationship
between the open time of the bell jar and the electric resistivity
of the polycrystalline silicon. The open time of the bell jar
refers to a time when the bell jar is in the open state after
completion of the polycrystalline silicon production step until
start of a next polycrystalline silicon production step.
Specifically, a time after completion of the polycrystalline
silicon production step for a previous batch until the bell jar is
opened to take out the polycrystalline silicon, cleaning of the
bell jar (conveyance, water washing, conveyance, removal of
moisture in the inner surface, replacement of the inside of the
bell jar with a high purity inert gas) is performed, and assembling
as a reactor for the polycrystalline silicon production step for a
next batch is completed is the open time of the bell jar.
[0053] It can be read from FIG. 2 that electric resistivity of
polycrystalline silicon tends to be reduced with increasing open
time. The reduction in electric resistivity refers to an increase
in level of electrically active impurity taken into the
polycrystalline silicon, and it is found that high purity of the
polycrystalline silicon tends to be prevented with increasing open
time. Specifically, it can be read that the reduction in the open
time of the bell jar described above is effective for producing
high purity polycrystalline silicon.
[0054] FIG. 3 illustrates a result of checking a relationship
between a time from completion of water washing of the bell jar to
completion of drying, and electric resistivity of polycrystalline
silicon. The completion of the drying step is a time when a vacuum
is maintained for 10 minutes after the pressure gauge indicates a
fixed value during pressure reduction by the vacuum pump described
above.
[0055] It can be read from FIG. 3 that the time from completion of
water washing of the bell jar to completion of drying, and the
electric resistivity of the polycrystalline silicon can be
approximated by a line obtained by a least square method, and
electric resistivity of polycrystalline silicon is reduced with
time. Specifically, to control quality of the polycrystalline
silicon, it is more effective to reduce the time from completion of
the water washing step to completion of the drying step rather than
reduce the open time of the bell jar, and it is found that reducing
the time of the drying step using the vacuum pump is an extremely
effective method for production of the bell jar high purity
polycrystalline silicon.
EXAMPLES
[0056] Now, the cleaning technique according to the present
invention will be described using examples.
[0057] First, after completion of the precipitation step of the
polycrystalline silicon, the bell jar 1 is opened, moved to a
washing apparatus, and subjected to a washing operation by a normal
procedure. After completion of the washing operation, the bell jar
1 is placed on the base plate 2 by a crane or the like to assemble
a drying apparatus. In this state, the vacuum pump 7 is operated to
reduce pressure in the bell jar 1 to be vapor pressure of water or
less. The pressure reduction causes moisture adhering to the inner
surface of the bell jar 1 to be discharged to the outside of the
bell jar 1 in the washing step.
[0058] Set pressure in the pressure reduction needs to be set so
that the inside of the bell jar 1 has pressure lower than vapor
pressure of water at the inner surface temperature, but when a
vacuum pump having vacuum reachability of about 200 Pa or less is
used, a target dry state can be reached in a short time without
caring about the temperature.
[0059] When moisture or attachment on the inner surface of the bell
jar 1 vaporizes, heat of vaporization is taken from the bell jar 1
and the base plate 2, but the bell jar 1 and the base plate 2 have
a sufficiently large heat capacity, and a reduction in temperature
can be virtually ignored.
[0060] The moisture or attachment on the inner surface of the bell
jar 1 quickly vaporizes with reduction in pressure. When the
pressure gauge is used to check entry into a dry state, a time
after a lapse of preferably five minutes after the internal
pressure of the bell jar 1 reaches 1000 Pa or less can be regarded
as completion of drying, but in view of stability or the like of
the apparatus, pressure reduction is preferably maintained for
further five minutes or more. Behavior of the pressure gauge in
this duration can be observed to check that there is no abnormality
in a monitor system.
[0061] After completion of the drying step of holding the inside of
the bell jar 1 at predetermined pressure, operation of the vacuum
pump 7 is stopped, a high purity inert gas without moisture is
introduced into the bell jar 1 to turn internal pressure into
atmospheric pressure. The high purity inert gas is introduced in
order to prevent reentry of moisture into the bell jar 1, and is
preferably a gas having a dew point of -40.degree. C. or less. A
nitrogen gas is desirable as an inert gas.
[0062] It is preferable that the cleaned bell jar 1 and the base
plate 2 are assembled as quickly as possible as a polycrystalline
silicon producing reactor, and a standby state for production for a
next batch, that is, a state where cleanliness is maintained with
an inert gas such as hydrogen or nitrogen is preferably
entered.
[0063] Table 2 shows a result of checking a relationship between a
dry state and a pressure reduction maintaining time when a bell jar
having an inner volume of 3.5 m.sup.3 is dried by the method of the
present invention. Vacuum reachability in specification of the
vacuum pump used at this time was 20 Pa, but the degree of vacuum
of the inside after a lapse of 7 minutes was 1000 Pa or less, and
then the degree of vacuum of 1000 Pa or less was maintained. For
the dry state, a high purity nitrogen gas was introduced into a
chamber after completion of pressure reduction to return to
atmospheric pressure, and further a nitrogen gas (carrier gas)
having a flow rate of 200 NL/min was passed to measure a dew point.
Thus, a precipitation bell jar is washed using high purity water or
a carbon dioxide pellet for each batch or every few batches to
clean the inner surface.
TABLE-US-00002 TABLE 2 TIME (min) DEW POINT (.degree. C.) 5 -35 7
-42 10 -61 20 -65 30 -72 40 -72
[0064] As shown in Table 2, it was confirmed that a dew point of a
carrier gas when pressure reduction was released with the carrier
gas was lower than -40.degree. C. in a pressure reduction holding
time of 7 minutes, and reached -61.degree. C. in 10 minutes or
more, and a sufficient dry state was obtained.
[0065] In contrast to this, drying by steam heating requires a long
time to obtain a dry state equal to the above.
[0066] Table 3 shows a result of introducing a heat medium by steam
heating into a jacket of a bell jar having the same inner volume as
the above, heating and holding the bell jar at about 110.degree.
C., supplying a high purity nitrogen gas of 200 NUmin (dew point of
-72.degree. C.) into the bell jar, and evaluating a dew point of
the nitrogen gas (carrier gas) using a dew point meter.
[0067] FIG. 4 illustrates a configuration of a bell jar drying
system used in this measurement. In FIG. 4, reference numeral 8
denotes a jacket, reference numeral 9 denotes a heat medium
circulating path, reference numerals 10 and 11 denote a heat
exchanger and a heat medium circulating pump.
TABLE-US-00003 TABLE 3 TIME (hr) DEW POINT (.degree. C.) 1 >-30
2 >-30 3 -32 4 -41 5 -49 6 -55 7 -58 8 -61 9 -62 10 -64 12
-64
[0068] A time before a dew point of a carrier gas reaches
-60.degree. C. or less as a target of drying is 8 hours or more,
and requires a long time about 50 times of that of the present
invention.
INDUSTRIAL APPLICABILITY
[0069] According to the present invention, moisture is efficiently
removed from the inner surface of the bell jar, thereby reducing a
time for cleaning the bell jar. This provides a technique of
increasing cleanliness of the inner surface of the bell jar to
contribute to production of high purity polycrystalline
silicon.
REFERENCE SIGNS LIST
[0070] 1 bell jar [0071] 2 base plate [0072] 3 packing [0073] 4
pressure gauge [0074] 5 gas introduction line [0075] 6 gas
discharge line [0076] 7 vacuum pump [0077] 8 jacket [0078] 9 heat
medium circulating path [0079] 10 heat exchanger [0080] 11 heat
medium circulating pump
* * * * *