U.S. patent application number 12/977739 was filed with the patent office on 2011-06-30 for methods for reducing the deposition of silicon on reactor walls using peripheral silicon tetrachloride.
This patent application is currently assigned to MEMC ELECTRONIC MATERIALS, INC.. Invention is credited to Henry F. Erk.
Application Number | 20110158888 12/977739 |
Document ID | / |
Family ID | 43827215 |
Filed Date | 2011-06-30 |
United States Patent
Application |
20110158888 |
Kind Code |
A1 |
Erk; Henry F. |
June 30, 2011 |
METHODS FOR REDUCING THE DEPOSITION OF SILICON ON REACTOR WALLS
USING PERIPHERAL SILICON TETRACHLORIDE
Abstract
Fluidized bed reactor systems and distributors are disclosed as
well as processes for producing polycrystalline silicon from a
thermally decomposable silicon compound such as trichlorosilane.
The processes generally involve reduction of silicon deposits on
reactor walls during polycrystalline silicon production by use of a
silicon tetrahalide.
Inventors: |
Erk; Henry F.; (St. Louis,
MO) |
Assignee: |
MEMC ELECTRONIC MATERIALS,
INC.
St. Peters
MO
|
Family ID: |
43827215 |
Appl. No.: |
12/977739 |
Filed: |
December 23, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61290692 |
Dec 29, 2009 |
|
|
|
Current U.S.
Class: |
423/349 |
Current CPC
Class: |
F27B 15/10 20130101;
B01J 8/1827 20130101; C01B 33/031 20130101; B01J 8/44 20130101;
C01B 33/1071 20130101; C01B 33/03 20130101; F23C 10/20 20130101;
C01B 33/10773 20130101 |
Class at
Publication: |
423/349 |
International
Class: |
C01B 33/02 20060101
C01B033/02 |
Claims
1. A process for producing polycrystalline silicon product in a
reactor comprising a reaction chamber having at least one reaction
chamber wall, the process comprising: directing silicon tetrahalide
to the reaction chamber wall and directing a thermally decomposable
silicon compound inward of the silicon tetrahalide, wherein the
thermally decomposable compound contacts silicon particles to cause
silicon to deposit onto the silicon particles and increase in
size.
2. The process as set forth in claim 1 wherein the reactor
comprises a distributor for evenly distributing gas into the
reaction chamber, the distributor comprising a plurality of
distribution openings which provide fluid communication between one
or more sources of gas and the reaction chamber, the plurality of
distribution openings comprising at least one peripheral opening
and at least one interior opening, the process comprising: feeding
silicon tetrahalide and the thermally decomposable silicon compound
from one or more sources of gas though the distribution openings of
the distributor and into the reaction chamber, wherein the
concentration of silicon tetrahalide fed through the peripheral
openings exceeds the concentration of silicon tetrahalide in the
gas fed through the interior openings to reduce the amount of
silicon deposited on the reactor wall.
3. The process as set forth in claim 1 wherein the thermally
decomposable silicon compound is a trihalosilane selected from the
group consisting of trichlorosilane, tribromosilane,
trifluorosilane and triiodosilane.
4. The process as set forth in claim 3 wherein the trihalosilane is
trichlorosilane.
5. The process as set forth in claim 1 wherein silicon tetrahalide
enters the reaction chamber at a temperature of at least about
1000.degree. C.
6. The process as set forth in claim 2 wherein the concentration of
silicon tetrahalide by volume in the gas fed through the peripheral
openings is at least about 60%.
7. The process as set forth in claim 2 wherein the gas fed through
the peripheral openings comprises an amount of the thermally
decomposable silicon compound.
8. The process as set forth in claim 7 wherein the concentration of
the thermally decomposable silicon compound by volume in the gas
fed through the peripheral openings is from about 1% to about
50%.
9. The process as set forth in claim 2 wherein the gas fed through
the peripheral openings consists essentially of silicon
tetrahalide.
10. The process as set forth in claim 2 wherein the concentration
of the thermally decomposable compounds by volume in the gas fed
through the interior openings is at least about 55%.
11. The process as set forth in claim 2 wherein the gas fed through
the interior openings comprises hydrogen.
12. The process as set forth in claim 11 wherein the concentration
of hydrogen by volume in the gas fed through the interior openings
is at least about 20%.
13. The process as set forth in claim 11 wherein the molar ratio of
hydrogen to thermally decomposable compounds in the gas fed through
the interior openings is from about 1:1 to about 7:1.
14. The process as set forth in claim 1 wherein hydrogen is
directed into the reaction chamber, the plurality of distribution
openings comprising at least one central opening, the process
comprising feeding hydrogen from a source of gas through the
distribution openings of the distributor into the reaction chamber
at a temperature of at least about 1000.degree. C., wherein the
concentration of hydrogen in the gas fed through the central
openings exceeds the concentration of hydrogen in the gas fed
through the interior openings and exceeds the concentration of
hydrogen fed through the peripheral openings.
15. The process as set forth in claim 1 wherein the silicon
tetrahalide is selected from the group consisting of silicon
tetrachloride, silicon tetrabromide, silicon tetrafluoride and
silicon tetraiodide.
16. The process as set forth in claim 1 wherein the silicon
tetrahalide is silicon tetrachloride.
17. A process for producing polycrystalline silicon, the process
comprising: introducing trihalosilane into a reaction chamber
having a reaction chamber wall and containing silicon particles,
the trihalosilane thermally decomposing in the reaction chamber to
deposit an amount of silicon on the silicon particles, wherein an
amount of trihalosilane is converted to silicon tetrahalide;
discharging the silicon tetrahalide from the reaction chamber; and
introducing a portion of the discharged silicon tetrahalide into
the reaction chamber near the reaction chamber wall.
18. The process as set forth in claim 17 wherein the silicon
tetrahalide etches silicon deposits on the reaction chamber
wall.
19. The process as set forth in claim 18 wherein the silicon
tetrahalide reacts with silicon deposits to form
hexahalodisilane.
20. The process as set forth in claim 17 wherein the entire portion
of silicon tetrahalide discharged from the reaction chamber is
recycled back to the reaction chamber.
21. The process as set forth in claim 17 comprising: reacting a
source of silicon with hydrogen halide acid to produce
trihalosilane, wherein an amount of by-product silicon tetrahalide
is produced; and introducing the trihalosilane and silicon
tetrahalide produced as a by-product of the reaction of silicon and
hydrogen halide acid into the reaction chamber.
22. The process as set forth in claim 21 wherein the silicon
tetrahalide produced as a by-product of the reaction of silicon and
hydrogen halide acid etches silicon deposits on the reaction
chamber wall.
23. The process as set forth in claim 21 wherein the entire portion
of silicon tetrahalide produced as a by-product of the reaction of
silicon and hydrogen halide acid is introduced into the reaction
chamber
24. The process as set forth in claim 21 wherein the amount of
silicon tetrahalide produced from trihalosilane in the reaction
chamber and the amount of silicon tetrahalide produced as a
by-product of the reaction of silicon and hydrogen halide acid is
substantially the same as the amount of silicon trihalosilane
consumed in the reaction chamber by reaction with silicon.
25. The process as set forth in claim 21 wherein the hydrogen
halide acid is hydrochloric acid.
26. The process as set forth in claim 17 wherein: a spent gas is
discharged from the reaction chamber, the spent gas comprising
silicon tetrahalide, hydrogen halide acid, any unreacted
trihalosilane and any unreacted hydrogen; hydrogen halide acid is
separated from the spent gas; and silicon tetrahalide, any
unreacted trihalosilane and any unreacted hydrogen discharged from
the reaction chamber are introduced into the reaction chamber.
27. The process as set forth in claim 17 wherein the reaction
chamber comprises a reaction chamber wall and wherein at least a
portion of the silicon tetrahalide introduced into the reaction
chamber is directed to the reaction chamber wall.
28. The process as set forth in claim 17 wherein the trihalosilane
is selected from the group consisting of trichlorosilane,
tribromosilane, trifluorosilane and triiodosilane.
29. The process as set forth in claim 17 wherein the trihalosilane
is trichlorosilane.
30. The process as set forth in claim 17 wherein the silicon
tetrahalide is selected from the group consisting of silicon
tetrachloride, silicon tetrabromide, silicon tetrafluoride and
silicon tetraiodide.
31. The process as set forth in claim 17 wherein the silicon
tetrahalide is silicon tetrachloride.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Patent Application No. 61/290,692, filed Dec. 29, 2009, which is
incorporated herein by reference it its entirety.
BACKGROUND
[0002] This disclosure relates to fluidized bed reactor systems and
to the production of polycrystalline silicon from a thermally
decomposable silicon compound such as, for example, trichlorosilane
and, particularly, to methods that involve reduction of silicon
deposits on reactor walls during polycrystalline silicon
production.
[0003] Fluidized bed reactors are used to carry out multiphase
reactions. In typical fluidized bed reactor systems a fluid is
passed through a bed of granular material such as a catalyst or
growing product particles. The flow of fluid causes the bed of
granular material to become fluidized in the reactor.
[0004] Polycrystalline silicon is a vital raw material used to
produce many commercial products including, for example, integrated
circuits and photovoltaic (i.e., solar) cells. Polycrystalline
silicon is often produced by a chemical vapor deposition mechanism
in which silicon is deposited from a thermally decomposable silicon
compound onto silicon particles in a fluidized bed reactor. The
seed particles continuously grow in size until they exit the
reactor as polycrystalline silicon product (i.e., "granular"
polycrystalline silicon). Suitable decomposable silicon compounds
include, for example, silane and halosilanes such as
trichlorosilane.
[0005] In many fluidized bed reactor systems and especially in
systems where material from the fluid phase chemically decomposes
to form solid material such as in polycrystalline silicon
production systems, solids may deposit onto the walls of the
reactor. The wall deposits often alter the reactor geometry which
can decrease reactor performance. Further, portions of the wall
deposits can dislodge from the reactor wall and fall to the reactor
bottom. Often the reactor system must be shut down to remove the
dislodged deposits. To prevent an untimely reactor shut down, the
deposits must be periodically etched from the reactor wall and the
reactor must be cleaned thereby reducing the productivity of the
reactor. The etching operations may cause stress to the reactor
system due to thermal shock or differences in thermal expansion or
contraction which may result in cracking of the reactor walls which
requires the unit to be rebuilt. These problems are particularly
acute in fluidized bed reactor systems used in the production of
polycrystalline silicon.
[0006] Thus a continuing need exists for reactor systems and
methods for producing polycrystalline silicon which limit or reduce
the amount of deposits on the reactor walls.
SUMMARY
[0007] One aspect of the present disclosure is directed to a
process for producing polycrystalline silicon product in a reactor
including a reaction chamber. The reaction chamber has at least one
reaction chamber wall. Silicon tetrahalide is directed to the
reaction chamber wall and a thermally decomposable silicon compound
is directed inward of the silicon tetrahalide. The thermally
decomposable compound contacts silicon particles to cause silicon
to deposit onto the silicon particles and increase in size.
[0008] In another aspect, a process for producing polycrystalline
silicon includes introducing trihalosilane into a reaction chamber
containing silicon particles. The trihalosilane thermally
decomposes in the reaction chamber to deposit an amount of silicon
on the silicon particles. An amount of trihalosilane is converted
to silicon tetrahalide. The silicon tetrahalide is discharged from
the reaction chamber. A portion of the discharged silicon
tetrahalide is introduced into the reaction chamber.
[0009] Various refinements exist of the features noted in relation
to the above-mentioned aspects of the present disclosure. Further
features may also be incorporated in the above-mentioned aspects of
the present disclosure as well. These refinements and additional
features may exist individually or in any combination. For
instance, various features discussed below in relation to any of
the illustrated embodiments of the present disclosure may be
incorporated into any of the above-described aspects of the present
disclosure, alone or in any combination.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a schematic of the flows entering and exiting a
first embodiment of a fluidized bed reactor system;
[0011] FIG. 2 is a longitudinal section of one embodiment of a gas
distribution unit;
[0012] FIG. 3 is a second longitudinal section the gas distribution
unit;
[0013] FIG. 4 is a bottom view of the distributor of the gas
distribution unit;
[0014] FIG. 5 is a top view of the distributor of the gas
distribution unit;
[0015] FIG. 6 is a perspective longitudinal section of the gas
distribution unit;
[0016] FIG. 7 is a schematic of the flows entering and exiting a
second embodiment of a fluidized bed reactor system;
[0017] FIG. 8 is a longitudinal section of a second embodiment of a
gas distribution unit;
[0018] FIG. 9 is a second longitudinal section of the gas
distribution unit;
[0019] FIG. 10 is a bottom view of the distributor of the gas
distribution unit;
[0020] FIG. 11 is a top view of the distributor of the gas
distribution unit;
[0021] FIG. 12 is a perspective longitudinal section of the gas
distribution unit;
[0022] FIG. 13 is a schematic depiction of reaction mechanisms that
take place in a granular polycrystalline silicon reactor
system;
[0023] FIG. 14 is a graph of the triaxial molar equilibrium of a
silicon, chlorine and hydrogen gas system; and
[0024] FIG. 15 is a schematic of the flows in a system for
preparing polycrystalline silicon.
[0025] Corresponding reference characters indicate corresponding
parts throughout the drawings.
DETAILED DESCRIPTION
[0026] The fluidized bed reactor systems and gas distribution units
described herein are suitable for distributing a first gas, a
second gas and, in some embodiments, a third gas to a fluidized bed
reactor and for depositing a compound from one of the gases on the
surface of fluidized particles. The reactor systems and
distribution units are especially well suited for reducing the rate
of deposition of thermally decomposable compounds (e.g., deposition
of silicon from trichlorosilane) on the walls of the reactor. The
distributors of the reactor systems are configured to direct the
thermally decomposable compounds to the interior portion of the
reactor and away from the reactor wall to prevent deposition of
material (e.g., such as silicon) on the reactor wall. In some
embodiments, the distributors are configured to direct a third gas
(e.g., hydrogen) that may be at an elevated temperature relative to
the thermally decomposable compound into the reactor at a point
different than the thermally decomposable compounds to allow the
third gas to transfer heat to the thermally decomposable compounds
away from the distributor and lower portions of the reactor. The
systems may be used to produce polycrystalline silicon from a
thermally decomposable silicon compound as described below under
the heading "Process for Producing Polycrystalline Silicon."
I. Fluidized Bed Reactor System
[0027] Referring now to FIG. 1, a fluidized bed reactor constructed
in accordance with embodiments of the present disclosure is
generally designated as 1. The reactor system 1 includes a reaction
chamber 10 and a gas distribution unit 2. A source of a first gas 5
and a source of second gas 7 are introduced into the distribution
unit 2 to evenly distribute the respective gases into the inlet of
the reaction chamber 10. The distribution unit 2 helps evenly
distribute reactive gases throughout the reaction chamber 10 to
maximize contact between the fluid and the fluidized particles in
the chamber.
[0028] As used herein, "first gas" is a gas with a different
composition than the "second gas" and vice versa. The first gas and
second gas can compose a plurality of gaseous compounds as long as
the mass composition or molar composition of at least one of the
compounds in the first gas is different than the composition of
that compound in the second gas.
[0029] A product withdrawal tube 12 extends through the gas
distribution unit 2. Product particles can be withdrawn from the
tube 12 and transported to product storage 15. Spent gas 16 exits
the reactor chamber 10 and can be introduced into further
processing units 18.
[0030] The gas distribution unit 2 is shown in greater detail in
FIG. 2. The gas distribution unit 2 is suitable for distributing a
first gas and a second gas to a fluidized bed reactor and is
especially well suited for distributing a carrier gas and a
thermally decomposable gas to a fluidized bed reactor.
[0031] The gas distribution unit 2 includes an inlet block 21,
distributor 25 and taper liner 28. An outer ring 37 and a
concentric interior ring 39 are located between the inlet block 21
and distributor 25. A first gas plenum 32 is defined between the
outer ring 37 and the interior ring 39. The product recovery tube
12 is concentric to the outer ring 37 and interior ring 39. The
tube 12 extends below the inlet block 21. A second gas plenum 34 is
defined between the interior ring 39 and the tube 12.
[0032] The taper liner 28 defines a liner chamber 45. The liner
chamber 45 opens into the cylindrical section of the reaction
chamber (not shown) and tapers outward in diameter from the
distributor 25 to the cylindrical section of reaction chamber.
Because particles and incoming gases come into contact in the liner
chamber 45 and because a majority of the system reactions may take
place in the liner chamber, the liner chamber is considered part of
the reaction chamber. For purposes of the application, "reaction
chamber" as used herein includes the liner chamber 45.
[0033] A series of peripheral distribution openings 42 and interior
distribution openings 44 are located within the distributor 25. As
used herein, "peripheral distribution openings" or "peripheral
openings" refer to distributor openings that are generally near the
outer wall(s) of the reaction chamber in relation to the interior
openings and "interior distribution openings" or "interior
openings" refer to distributor openings that are generally interior
to the peripheral openings. The peripheral openings 42 are in fluid
communication with the first gas plenum 32 and the reaction chamber
10. The interior openings 44 are in fluid communication with the
second gas plenum 34 and the reaction chamber 10. Generally, the
peripheral openings are not in fluid communication with the second
gas and the interior openings are not in fluid communication with
the first gas.
[0034] When the first gas does not contribute (or contributes to a
lesser extent than the second gas) to deposition of material on the
particles (e.g., when the first gas is hydrogen or an inert gas
such as a noble gas), configuring the peripheral openings to be in
fluid communication with the first gas causes a larger
concentration of the first gas to be present at the walls of the
reaction chamber as compared to the interior space. This causes
less material to deposit from the second gas onto the reactor walls
as compared to a configuration where the first and second gases are
evenly distributed through the distributor 25. In some embodiments,
the first gas may include materials that etch deposits on the walls
of the reactor (e.g., as in polycrystalline silicon production when
silicon tetrahalide is used as an etching gas as described
below).
[0035] In some embodiments, a portion of the peripheral openings
are in fluid communication with the second gas and a portion of the
interior openings are in fluid communication with the first gas. In
these embodiments, generally the percentage of the peripheral
openings in fluid communication with the first gas is greater than
the percentage of interior openings in fluid communication with the
first gas. This configuration also causes a larger concentration of
first gas (e.g., a carrier gas such as hydrogen, an inert or an
etching gas such as silicon tetrachloride) to be present at the
walls of the reaction chamber as compared to the interior space of
the reaction chamber.
[0036] The interior openings 44 and peripheral openings 42 include
a channel portion 60, a throttle portion 62 and flare out portion
64. The flare out portion 64 opens into a cone 66. The throttle
portion 62 helps to provide resistance to flow and allows the gas
to be evenly distributed through each opening 42, 44 and into the
inlet of the reaction chamber 10. The cones 66 help distribute the
gas from the openings 42, 44 into the reaction chamber 10. The
cones 66 are generally hexagonal in shape (FIG. 5).
[0037] Another longitudinal section of the gas distribution unit 2
is illustrated in FIG. 3 with several other features of the unit
shown. A first gas inlet tube 50 extends through the inlet block 21
and is in fluid communication with the first gas plenum 32 and a
source of first gas (not shown). A second gas inlet tube 52 extends
through the inlet block 21 and is in fluid communication with the
second gas plenum 34 and a source of second gas (not shown).
[0038] Cooling channels 55 are located in the distributor 25. Fluid
(e.g., air or cooling liquid) is circulated through the cooling
channels 55 to cool the distributor below the temperature at which
material thermally decomposes from the first or second gases. The
cooling channels 55 prevent material from depositing on the
interior distributor openings 44. The distributor 25 may optionally
include cooling channels (not shown) near the peripheral
distributor openings 42 (FIG. 2). In this regard, it should be
understood that arrangements of cooling channels other than as
shown and described may be used without limitation and, in some
embodiments, cooling channels are not formed in the distributor
25.
[0039] The bottom view of the distributor 25 is illustrated in FIG.
4 and the top view of the distributor 25 is illustrated in FIG. 5.
As can be seen from FIG. 4, the peripheral openings 42 are spaced
from the interior openings 44 at the bottom of the distributor. The
peripheral openings 42 angle toward the interior openings 44 from
the bottom to the top of the distributor 25. As can be seen from
FIG. 5, the peripheral openings 42 are adjacent to the interior
openings 44 at the top of the distributor 25.
[0040] A second embodiment of a fluidized bed reactor and
distributor is shown in FIGS. 7-12. The distribution unit 102 of
the reactor shown in FIGS. 7-12 generally allows a third gas to be
introduced into the reaction chamber to allow more flexibility in
processing and fluidized bed reactor operations. In this regard, it
should be noted that parts or features of the reactor and
distribution unit shown in FIGS. 7-12 that are analogous to those
of the reactor system 1 and distribution unit 2 shown in FIGS. 1-6
are designated by the corresponding reference numeral of FIGS. 1-6
plus "100" (e.g., part 2 becomes part 102). It should be understood
that reference is made to the description of the reactor system 1
and distribution unit 2 shown in FIGS. 1-6 above when any
corresponding part in FIGS. 7-12 is not independently described
below for all relevant and consistent purposes. Further, it should
be understood that as the reaction system of FIGS. 1-6 may be
referred to herein as a "two-gas" system and the reactor system of
FIGS. 7-12 may be referred to herein as a "three-gas" system, the
terms "two-gas" and "three gas" refer to distinct mixtures of gases
rather than distinct compounds (e.g., the two-gas system includes
use of two distinctly different gases and more than two gaseous
compounds may be used).
[0041] Referring now to FIG. 7, a source of third gas 109 is
introduced into the distribution unit 102 with the source of first
gas 105 and the source of second gas 107. Generally, the "third
gas" is a gas with a different composition than the "first gas" and
"second gas" and vice versa. The first, second and third gases may
contain a plurality of gaseous compounds as long as the mass
composition or molar composition of at least one of the compounds
in the first gas is different than the composition of that compound
in the second gas and third gas and as long as the mass composition
or molar composition of at least one of the compounds of the second
gas is different than the composition of that compound in the first
gas and third gas.
[0042] Referring now to FIG. 8, an outer ring 137, concentric
interior ring 139 and concentric central ring 127 are located
between the inlet block 121 and distributor 125. A first gas plenum
132 is defined between the outer ring 137 and the interior ring 139
and a second gas plenum 134 is defined between the interior ring
139 and the central ring 127. A third gas plenum 122 is defined
between the central ring 127 and the concentric product recovery
tube 112 that extends below the inlet block 121.
[0043] A series of peripheral distribution openings 142, interior
distribution openings 144 and central distribution openings 146 are
located within the distributor 125. As used herein, "central
distribution openings" or "central openings" refer to distributor
openings that are generally interior to the interior openings. The
peripheral openings 142 are in fluid communication with the first
gas plenum 132 and the reaction chamber 110. The interior openings
144 are in fluid communication with the second gas plenum 134 and
the reaction chamber 110. The central openings 146 are in fluid
communication with the third gas plenum 122 and the reaction
chamber 110. Generally, the peripheral openings 142 are not in
fluid communication with the second gas or third gas, the interior
openings 144 are not in fluid communication with the first gas or
third gas and the central openings are not in fluid communication
with the first gas or second gas.
[0044] In embodiments wherein the first or second gas is reactive
with the third gas, configuring the central openings to be in fluid
communication with the third gas allows the reaction to be delayed
until the third gas is well mixed with the first gas or second gas
which is typically downstream of the distributor 125 and liner
chamber 145. This may effectively limit deposition of material
(e.g., silicon) onto the distributor 125 or taper liner 128. This
configuration also allows at least one of the first, second or
third gases to be introduced into the reaction chamber 110 at a
temperature different than the other gases to provide further
processing flexibility. For instance in the polycrystalline silicon
process described below, the second gas (e.g., trichlorosilane) may
be introduced at a temperature much less the first gas (e.g.,
silicon tetrachloride) and third gas (hydrogen). This configuration
delays the second gas from reaching its deposition temperature
until it has passed downstream from the distributor 125 and liner
chamber 145 to the reaction chamber 110. By heating one or more of
the gases to a temperature much higher than the other reacting gas,
it is possible to minimize or possibly even eliminate application
of extraneous heat applied to the reaction chamber walls.
[0045] In some embodiments, a portion of the peripheral openings
are in fluid communication with the second gas and/or third gas, a
portion of the interior openings are in fluid communication with
the first gas and/or third gas and/or a portion of the central
openings are in fluid communication with the first gas and/or
second gas. Similar to the two-gas system described above, in these
embodiments, generally the percentage of the peripheral openings in
fluid communication with the first gas is greater than the
percentage of interior openings in fluid communication with the
first gas and the percentage of central openings in fluid
communication with the first gas. Similarly, the percentage of the
interior openings in fluid communication with the second gas is
greater than the percentage of peripheral openings and central
openings in fluid communication with the second gas and the
percentage of central openings in fluid communication with the
third gas is greater than the percentage of peripheral openings and
interior openings in fluid communication with the third gas.
[0046] Referring now to FIG. 9, a first gas inlet tube 150 extends
through the inlet block 121 and is in fluid communication with the
first gas plenum 132 and a source of first gas (not shown). A
second gas inlet tube 152 extends through the inlet block 121 and
is in fluid communication with the second gas plenum 134 and a
source of second gas (not shown). A third gas inlet tube 157
extends through the inlet block 121 and is in fluid communication
with the third gas plenum 122. Cooling channels 155, 156 in the
distributor 125 may be arranged such that one or more of the first,
second and third gases are cooled. Central cooling channels 155 may
be used to cool the third gas supplied to the third gas plenum 122
as the gas passes through the central distribution openings 146 and
interior cooling channels 156 may be used to cool the second gas
supplied from the second gas plenum 134 as it passes through the
interior distributor openings 144. In one or more embodiments, the
central cooling channels 155 and/or interior cooling channels 156
may actually act as heating channels to heat one or more of the
second or third gases by circulating hot gas or liquid through the
channels depending on the particular reactions that occur in the
reaction chamber. The distributor 125 may optionally include
further cooling channels (not shown) to cool the first gas as it
passed through the peripheral distributor openings 142 (FIG.
8).
[0047] In this regard, while the distribution unit 102 of the
reactor shown in FIGS. 7-12 is configured for distribution of three
gases into the reactor, the distribution unit 102 may be modified
to supply more than three distinct gases to the reactor by use of
additional gas plenums and distribution openings. Such modified
distribution units are considered to be within the scope of the
present disclosure.
II. Process for Producing Polycrystalline Silicon
[0048] In accordance with embodiments of the present disclosure,
polycrystalline silicon may be produced by thermal decomposition of
one or more silicon-containing compounds (synonymously "thermally
decomposable silicon compounds") in a fluidized bed reactor. The
process of the present disclosure includes introducing a feed gas
including a gaseous silicon compound capable of being thermally
decomposed into a reactor. The feed gas is heated in the reaction
chamber to cause at least a portion of the silicon in the silicon
compound to deposit, by chemical vapor deposition, onto the silicon
particles in the reaction chamber, thereby growing the silicon
particles into larger particles typically referred to as granular
polysilicon. A variety of reactions may take place in the reaction
chamber. The reaction mechanisms believed to occur in a
trichlorosilane fluidized bed reactor systems are generally
illustrated in FIG. 13. In this regard, it should be noted that
these mechanisms in no way limit embodiments of the present
disclosure as they do not constitute the entire set of reactions
which might take place in the reactor system.
[0049] With reference to FIG. 13, in a trihalosilane system and, in
particular, a trichlorosilane system, silicon heterogeneously
deposits onto the growing silicon particles (1). Trichlorosilane
may also decompose to produce silicon vapor (3) which can
homogenously nucleate to form undesirable silicon dust
(synonymously silicon "fines" or "powder") (4) and which can
deposit on the growing silicon particle (6). The silicon fines can
grow in size by deposition of silicon from trichlorosilane (2) or
from any silicon vapor (5). The fines can combine to form larger
fines (7). Silicon fines can agglomerate with growing silicon
particles (8). The agglomeration is caused by bombardment of the
fines and particles. Without being bound to a particular theory, it
is believed that once the fines contact the particles they
agglomerate due to the applicable molecular forces.
[0050] Deposition of silicon from halosilanes (e.g.,
trichlorosilane) is believed to occur by one or more of reaction
(i) or reaction (ii) shown below,
SiHX.sub.3+H.sub.2.fwdarw.Si+3HX (i),
4SiHX.sub.3.fwdarw.Si+3SiX.sub.4+2H.sub.2 (ii),
wherein X is a halogen such as chlorine. It should be noted that
while reaction (ii) is a thermal decomposition reaction and
reaction (i) may be characterized as a hydrogen reduction reaction,
both reactions (i) and (ii) are generally referenced herein as
involving a "thermal decomposition" of trihalosilane.
[0051] In accordance with fluidized bed reactor operations for
producing polycrystalline silicon granular product, polycrystalline
silicon seed particles may be added to the reaction chamber to
initiate deposition of silicon. The particle sizes of the seed
particles may be from about 50 .mu.m to about 800 .mu.m and are
more typically from about 250 .mu.m to about 600 .mu.m. Two types
of silicon seed particles are commonly used to initiate silicon
deposition. Silicon seed particles provided by grinding or breaking
product particles collected from the reactor to a typical particle
size from about 250 .mu.m to about 350 .mu.m may be used.
Alternatively or in addition, small polycrystalline particles
gathered with and separated from the granular polycrystalline
product having a particle size of from about 500 .mu.m to about 600
.mu.m may be used as seed particles.
[0052] A. Feed Gases Containing One or More Thermally Decomposable
Silicon Compounds
[0053] The process for producing polycrystalline silicon will now
be described with reference to the fluidized bed reactor system 1
illustrated in FIG. 1. A gas containing one or more thermally
decomposable compounds 7 (e.g., trihalosilane) and a gas containing
silicon tetrahalide 5 are fed from their respective source to the
reactor system 1. Thermally decomposable silicon compounds include
compounds generally capable of being thermally decomposed to
produce silicon. Additional products may be produced from the
decomposition process, without departing from the scope of the
present disclosure, as long as the thermally decomposable compound
provides a source of silicon to grow the polysilicon particles to
form polysilicon granules. Thermally decomposable silicon compound
gases include all gases containing silicon, that can be
heterogeneously deposited by chemical vapor deposition, such as
silicon tetrahydride (commonly referred to as silane), any of the
trihalosilanes and any of the dihalosilanes. Suitable halogens
which may be used in such silicon halides include chlorine,
bromine, fluorine and iodine. In some embodiments, one or more
trihalosilanes selected from trichlorosilane, tribromosilane,
trifluorosilane and triiodosilane are used as the thermally
decomposable silicon compound
[0054] In this regard it should be understood that, as used herein,
"thermally decomposable silicon compounds" do not include silicon
tetrahalides (e.g., silicon tetrachloride) as silicon tetrahalide
is introduced into the reaction chamber to etch silicon deposits on
the reactor walls. Accordingly, a thermally decomposable silicon
compound other than silicon tetrahalide is fed to the reactor along
with an amount of silicon tetrahalide. In some embodiments,
trihalosilane (e.g., trichlorosilane) is introduced into the
reactor as the thermally decomposable silicon compound.
[0055] The thermally decomposable compound may be introduced into
the reactor without dilution or the gas may be diluted with a
carrier gas such as hydrogen, argon, helium or combinations
thereof. When a trihalosilane compound is used as a thermally
decomposable compound, hydrogen may be used as a carrier gas as
hydrogen gas reacts with trihalosilane in the decomposition
reaction as shown in reaction (i) above. In various embodiments of
the present disclosure, the concentration of the thermally
decomposable compound (e.g., or of trihalosilane when trihalosilane
is used as the thermally decomposable compound) by volume in the
gases fed to the reactor (e.g., as through the interior openings of
the distributor) is at least about 5%, at least about 20%, at least
about 40%, at least about 55%, at least about 70%, at least about
95%, at least about 99% or from about 40% to about 100%, from about
55% to about 95% or from about 70% to about 85%. Further, the
concentration of carrier gas (e.g., hydrogen) by volume in this gas
(e.g., the gas fed through the interior openings of the
distributor) may be at least about 1%, at least about 5%, at least
about 20%, at least about 40%, at least about 60%, at least about
80% or from about 1% to about 90%, from about 20% to about 90% or
from about 40% to about 90%. The molar ratio of carrier gas (e.g.,
hydrogen) to thermally decomposable compound (e.g., or of
trihalosilane when trihalosilane is used as the thermally
decomposable compound) may be from about 1:1 to about 7:1, from
about 2:1 to about 4:1 or from about 2:1 to about 3:1.
[0056] The thermally decomposable silicon compound may be added to
the reactor below temperatures at which the compound decomposes to
prevent deposition of silicon onto the distributor and distributor
openings. The temperature of the thermally decomposable silicon may
vary and depend on the available heat from other gases introduced
into the reactor and the amount of extraneous heat applied through
the reactor walls. In general, the thermally decomposable compound
(e.g., trichlorosilane) may be added to the reactor at a
temperature of less than about 1100.degree. C., less than about
1000.degree. C., less than about 900.degree. C., less than about
800.degree. C., less than about 600.degree. C., less than about
400.degree. C., less than about 200.degree. C. or from about room
temperature (e.g., 20.degree. C.) to about 1100.degree. C., from
about room temperature (e.g., 20.degree. C.) to about 800.degree.
C., from about room temperature (e.g., 20.degree. C.) to about
600.degree. C., from about room temperature (e.g., 20.degree. C.)
to about 400.degree. C. or from about room temperature (e.g.,
20.degree. C.) to about 200.degree. C. In the case of
trichlorosilane, trichlorosilane may be added from about room
temperature (e.g., 20.degree. C.) to about 400.degree. C. or from
about room temperature (e.g., 20.degree. C.) to about 300.degree.
C. As explained below, the thermally decomposable compound may be
heated upon addition to the reaction chamber.
[0057] B. Reaction Chamber and Reaction Conditions
[0058] The process of the present disclosure may carry out the
reaction in a single fluidized bed reactor or may incorporate one
or more fluidized bed reactors configured in series or in parallel.
The fluidized bed reactors may be operated in a continuous manner
in which feed and product are continually introduced and withdrawn
from the reactor or in a batch process without departing from the
scope of the present disclosure.
[0059] The reaction chamber is typically a fluidized bed in which
silicon particles are suspended by an upward flow of the fluidizing
gas in the reactor. Fluidized bed reactors provide high mass
transfer and heat transfer rates between growing silicon particles
and the gas phase which enhances the deposition rate of silicon
onto the particles. The fluidized bed reactor is generally a
cylindrical vertical vessel; however, it should be understood that
any configuration that is acceptable to fluidized bed operations
may be utilized. The particular dimensions of the reactor will be
primarily dependent upon system design factors that may vary from
system to system such as the desired system output, heat transfer
efficiencies, system fluid dynamics and the like, without departing
from the scope of the present disclosure.
[0060] Extraneous heat may be applied to the walls of the reaction
chamber to cause the temperature of the thermally decomposable gas
to increase to the point at which the gas decomposes. Methods for
heating include, for example, capacitive heating, induction coils
and electrical resistance elements. In certain embodiments, heat is
not applied to the reaction chamber as one or more of the feed
gases may be heated before addition to the reaction chamber.
[0061] Referring now to FIG. 3, silicon tetrahalide (e.g. silicon
tetrachloride) is fed through the first gas inlet tube 50 and
proceeds to the first gas plenum 32. From the first gas plenum 32
silicon tetrahalide passes through a series of peripheral
distribution openings 42 (FIG. 2) and into the liner chamber 45 and
reaction chamber 10 (FIG. 1). Suitable silicon tetrahalides
include, for example, silicon tetrachloride, silicon tetrabromide,
silicon tetrafluoride and silicon tetraiodide. Other gases may be
fed with the silicon tetrahalide into the first gas plenum 32
without departing from the scope of the present disclosure.
[0062] The thermally decomposable compound (e.g., trihalosilane
such as trichlorosilane) is fed through the second gas inlet tube
52 and proceeds to the second gas plenum 34. A gas other than the
thermally decomposable compound (e.g., an amount of carrier gas
such as hydrogen) may be fed with the thermally decomposable
compound through the second gas inlet tube 52 without departing
from the scope of the present disclosure. From the second gas
plenum 34 the thermally decomposable compound passes through a
series of interior distribution openings 44 and into the liner
chamber 45 and reaction chamber 10 (FIG. 1). Because silicon
tetrahalide enters the reaction chamber 10 near the reaction
chamber wall (and enters the liner chamber 45 near the taper liner
28), relatively small amounts of thermally decomposable compounds
contact the reaction chamber wall. This arrangement prevents
undesirable build-up of silicon on the reactor wall.
[0063] Furthermore, it has been found that silicon tetrahalides
such as silicon tetrachloride may actually etch silicon deposits
that have already formed on the reactor wall. In the case of
silicon tetrachloride, at temperatures above about 1000.degree. C.
it is believed that silicon tetrachloride predominantly reacts with
silicon (e.g., silicon deposits on the reactor wall) according to
reaction (iii) shown below:
6SiCl.sub.4+2Si.fwdarw.4Si.sub.2Cl.sub.6 (iii).
[0064] The equilibrium triaxial chart for silicon, chlorine and
hydrogen vapor is shown in FIG. 14. It is generally believed that
because silicon tetrachloride is added to the reactor along the
reactor walls, silicon tetrachloride (and any carrier gas for
silicon tetrachloride) is largely the only gas that contacts the
walls of the reactor and hydrogen is not present at the reactor
walls even when added with trichlorosilane. In this regard, FIG. 14
may be viewed as if hydrogen-containing compounds are absent. As
can be seen from FIG. 14, as the reactor temperature increases,
smaller vapor-phase molecular ratios of silicon to chlorine are
favored (i.e., a higher vapor-phase amount of silicon is favored)
which allows the silicon deposits to be etched. In this manner,
silicon tetrachloride may behave as an etching gas rather than a
deposition gas at relatively higher temperatures.
[0065] In this regard, reaction (iii) should not be viewed in a
limiting sense as other reactions may occur and reaction (iii) may
occur at temperatures other than as described. In general, it
should be understood that silicon tetrachloride acts as an etchant
in the absence of hydrogen so as to etch silicon deposits on the
reactor walls at temperatures above about 1000.degree. C. In one or
more embodiments in which silicon tetrachloride is selected for
use, silicon tetrachloride may enter the reaction chamber at a
temperature of at least about 1000.degree. C., at least about
1100.degree. C., at least about 1200.degree. C., at least about
1300.degree. C. or at least about 1400.degree. C. Generally, the
silicon tetrachloride is introduced at a temperature that does not
cause the reactor walls to be overheated or that heats the
fluidized silicon to or above its melting temperature of
1414.degree. C. In one or more embodiments, the temperature at
which silicon tetrachloride is added to the reaction chamber is
from about 1000.degree. C. to about 1600.degree. C., from about
1000.degree. C. to about 1500.degree. C., from about 1100.degree.
C. to about 1500.degree. C. or from about 1200.degree. C. to about
1400.degree. C. It this regard, the temperature ranges recited
above may also be used for silicon tetrahalides other than silicon
tetrachloride.
[0066] The gas introduced into the reaction chamber through the
peripheral openings may contain an amount of gaseous compounds
other than silicon tetrahalide compounds. Generally, however the
silicon tetrahalide contains little or even substantially no amount
of hydrogen as hydrogen may promote deposition of silicon on the
reactor walls. Other compounds that may be introduced with silicon
tetrahalide gas include thermally decomposable silicon compounds
(e.g., trichlorosilane) or compounds that are inert relative to
thermally decomposable silicon compounds and silicon tetrahalides
such as argon and nitrogen. In one or more of these embodiments,
the concentration of silicon tetrahalide by volume in the gas fed
through the peripheral openings is at least about 1%, at least
about 5%, at least about 20%, at least about 40%, at least about
60%, at least about 80%, at least about 95%, at least about 99% or
from about 1% to about 100%, from about 20% to about 100% or from
about 40% to about 100%. The concentration of the thermally
decomposable silicon compounds (other than silicon tetrahalide
which may deposit an amount of silicon upon contact with hydrogen)
by volume in the gas fed through the peripheral openings may be
less than about 50%, less than about 30%, less than about 5% or
from about 1% to about 50% or from about 5% to about 30%. In some
embodiments, the gas fed through the peripheral openings of the
distributor does not contain gases other than silicon tetrahalide
or contains only minor parts of other gases (e.g., less than about
1% or less than about 0.1% by volume).
[0067] In this regard, it should be understood that it may be
desirable to add an amount of hydrogen as a carrier gas for the
thermally decomposable compound distributed through the central
distribution openings. If silicon tetrahalide is introduced into
the reactor at a very high temperature and if there are no silicon
deposits on the reactor wall, silicon tetrahalide may etch the
reactor lining, particularly if the lining is composed of silicon
(e.g., CVD silicon). Increasing amounts of hydrogen cause silicon
tetrahalides to act more as a deposition gas and less as an etching
gas, thereby protecting the reactor liner. As stated above, the
amount of hydrogen by volume in the gas fed through the central
distribution openings may be at least about 1%, at least about 5%,
at least about 20%, at least about 40%, at least about 60%, at
least about 80% or from about 1% to about 90%, from about 20% to
about 90% or from about 40% to about 90%.
[0068] In various embodiments of the present disclosure, the
concentration of the thermally decomposable compound (e.g., or of
trihalosilane when trihalosilane is used as the thermally
decomposable compound) by volume in the gases fed to the reactor
(e.g., as through the interior openings of the distributor) is at
least about 5%, at least about 20%, at least about 40%, at least
about 55%, at least about 70%, at least about 95%, at least about
99% or from about 40% to about 100%, from about 55% to about 95% or
from about 70% to about 85%. Further, the concentration of carrier
gas (e.g., hydrogen) by volume in this gas (e.g., the gas fed
through the interior openings of the distributor) may be at least
about 1%, at least about 5%, at least about 20%, at least about
40%, at least about 60%, at least about 80% or from about 1% to
about 90%, from about 20% to about 90% or from about 40% to about
90%. The molar ratio of carrier gas (e.g., hydrogen) to thermally
decomposable compound (e.g., or of trihalosilane when trihalosilane
is used as the thermally decomposable compound) may be from about
1:1 to about 5:1 or from about 2:1 to about 3:1.
[0069] According to another embodiment of the present disclosure,
the silicon tetrahalide 5 fed through the peripheral distribution
openings 42 may contain an amount of the thermally decomposable
compounds and/or the thermally decomposable gas 7 fed through the
interior distribution openings 44 may contain an amount of silicon
tetrahalide. In these embodiments the concentration of silicon
tetrahalide in the gas fed through the peripheral openings exceeds
the concentration of silicon tetrahalide in the gas fed through the
interior openings to reduce the amount of silicon which deposits on
the reactor wall.
[0070] Referring now to FIGS. 7-12, in various embodiments, a
fluidized bed reactor system that allows three distinct gas streams
to be introduced into the reaction chamber may be used for
production of polycrystalline silicon. This design allows hydrogen
to be introduced into the reaction chamber inward of the thermally
decomposable silicon compounds. Similar to the reactor system of
FIGS. 1-6, a first gas containing one or more silicon tetrahalides
is fed through the first gas inlet tube 150 and proceeds to the
first gas plenum 132 and through a series of peripheral
distribution openings 142 and into the liner chamber 145 and
reaction chamber 110. A second gas containing a thermally
decomposable compound (e.g., a trihalosilane such as
trichlorosilane) is fed through the second gas inlet tube 152 and
proceeds to the second gas plenum 134 and a series of interior
distribution openings 144 and into the liner chamber 145 and
reaction chamber 110. A third gas containing hydrogen may be fed
through the third gas inlet tube 157 and proceeds to a third gas
plenum 122 and a series of central distribution openings 146 and
into the liner chamber 145 and reaction chamber 110. Generally the
central openings 146 are closer to the centerpoint of the
distributor 125 than the interior openings 144 and the peripheral
openings 142 and the interior openings are generally closer to the
centerpoint than the peripheral openings. The concentration of
hydrogen in the gas fed through the central openings 146 generally
exceeds the concentration of hydrogen in the gas fed through the
interior openings 144 and the concentration of gas fed through the
peripheral openings 142.
[0071] The hydrogen gas introduced through the central distribution
openings 146 may be at a temperature above the thermally
decomposable silicon compound introduced through the interior
openings 144. In this regard, the hydrogen gas may be used to heat
the thermally decomposable silicon compound as the gases mix and
pass vertically through the reaction chamber and the amount of
extraneous heat that must be applied through the walls of the
reaction chamber is reduced or may even be eliminated.
[0072] In one or more embodiments, the hydrogen gas may be
introduced into the reaction chamber at a temperature of at least
about 1000.degree. C., at least about 1100.degree. C., at least
about 1200.degree. C., at least about 1300.degree. C., at least
about 1400.degree. C., from about 1000.degree. C. to about
1500.degree. C., from about 1100.degree. C. to about 1500.degree.
C. or from about 1200.degree. C. to about 1500.degree. C.
Additionally or alternatively, silicon tetrahalide may be added at
a temperature above the thermally decomposable silicon compounds.
For instance, the silicon tetrahalide may be added within the range
of temperatures described above for a two-gas system. By
introducing the thermally decomposable gas at a temperature below
the hydrogen gas and/or the silicon tetrahalide gas, the thermally
decomposable compounds may be maintained at a temperature below
which the compounds decompose as the gas passes through the
distributor 125 and taper liner 128. For instance, the thermally
decomposable compound may be added to the reaction chamber at a
temperature of less than about 1100.degree. C., less than about
1000.degree. C., less than about 900.degree. C., less than about
800.degree. C., less than about 600.degree. C., less than about
400.degree. C., less than about 200.degree. C. or from about room
temperature (e.g., 20.degree. C.) to about 1100.degree. C., from
about room temperature (e.g., 20.degree. C.) to about 800.degree.
C., from about room temperature (e.g., 20.degree. C.) to about
600.degree. C., from about room temperature (e.g., 20.degree. C.)
to about 400.degree. C. or from about room temperature (e.g.,
20.degree. C.) to about 300.degree. C. In the case of
trichlorosilane, trichlorosilane may be added from about room
temperature (e.g., 20.degree. C.) to about 400.degree. C. or from
about room temperature (e.g., 20.degree. C.) to about 300.degree.
C.
[0073] During decomposition, by-product hydrogen halide acid (e.g.,
HCl) is produced which may be used to generate further
trihalosilane feed gas. In embodiments where silane is used as a
thermally decomposable compounds, hydrogen is produced that may be
recycled for use as a carrier gas for additional quantities of
thermally decomposable feed gas in the operation of the reactor
system.
[0074] During operation of the reactor system (either the two-gas
system or three-gas system described above), the gas velocity
through the reaction zone is maintained above the minimum
fluidization velocity of the silicon particles. The gas velocity
through the reactor is generally maintained at a velocity of from
about one to about eight times the minimum fluidization velocity
necessary to fluidize the particles within the fluidized bed. In
some embodiments, the gas velocity is from about two to about five
times, and in at least one embodiment is about four times the
minimum fluidization velocity necessary to fluidize the particles
within the fluidized bed. The minimum fluidization velocity varies
depending on the properties of the gas and particles involved. The
minimum fluidization velocity may be determined by conventional
means (see, for example, p. 17-4 of Perry's Chemical Engineers'
Handbook, 7th. Ed., incorporated herein by reference for all
relevant and consistent purposes).
[0075] The minimum fluidization conditions may be calculated for
conditions as they exist near the distribution unit. Using these
conditions, which include temperatures that are normally cooler
than the rest of the reactor, it is possible to ensure the minimum
fluidization in the entire bed. Although the present disclosure is
not limited to specific minimum fluidization velocities, minimum
fluidization velocities useful in the present disclosure may range
from about 0.7 cm/sec to about 350 cm/sec or even from about 6
cm/sec to about 150 cm/sec.
[0076] Gas velocities higher than the minimum fluidization flow
rate are often desired to achieve higher productivities. As the gas
velocity increases beyond the minimum fluidization velocity the
excess gas forms bubbles, increasing the bed voidage. The bed can
be viewed to consist of bubbles and "emulsion" containing gas in
contact with silicon particles. The quality of the emulsion is
quite similar to the quality of the bed at the minimum fluidization
condition. The local voidage in the emulsion is close to the
minimum fluidization bed voidage. Hence, bubbles are generated by
the gas introduced in excess of what is required to achieve the
minimum fluidization. As the ratio of actual gas velocity divided
by the minimum velocity increases, the bubble formation
intensifies. At a very high ratio, large slugs of gas are formed in
the bed. As the bed voidage increases with the total gas flow rate,
the contact between solids and gases becomes less effective. For a
given volume of the bed, the surface area of solids in contact with
reacting gases decreases with increasing bed voidage. Thus, for the
given bed length, the conversion of thermally decomposable gas
decreases. Conversion may also decrease with decreased gas
residence time through the reaction chamber. In addition, different
undesired reactions can take place at higher rates producing more
fines.
[0077] In certain embodiments, extraneous heat is added to the
reaction chamber such that the temperature in the reactor is
maintained within the decomposition temperature range of the
thermally decomposable compound and the melting point temperature
of silicon. The temperature of the reactor may be maintained at
above about 200.degree. C., above about 700.degree. C., above about
900.degree. C. or even above about 1100.degree. C. (e.g., from
about 200.degree. C. to about 1400.degree. C., from about
900.degree. C. to about 1100.degree. C. or from above about
1000.degree. C. to about 1100.degree. C.). In this regard, it
should be understood that the recited temperature ranges are
suitable for any halogen chosen for use in the reactor; however,
lower temperatures may be used if bromine is used (e.g.,
temperatures less than about 900.degree. C.) and even lower
temperatures may be used if iodine is chosen for use. The heat that
is used to maintain the reaction zone at such temperatures may be
provided by conventional heating systems such as electrical
resistance heaters disposed on the exterior of the reactor vessel
wall. In some embodiments, the pressure in the reactor is from
about 1.25 atm to about 2.25 atm as measured at the top of the
bed.
[0078] C. Systems for Producing Polycrystalline Silicon
[0079] The two-gas fluidized bed reactor system of FIGS. 1-6 or the
three-gas system of FIGS. 7-12 may be incorporated into a system
for producing polycrystalline silicon from a silicon source (e.g.,
metallurgical grade silicon). An exemplary system is shown in FIG.
15. The system may be a substantially closed-loop system with
regard to silicon tetrahalide in that the system does not require
that silicon tetrahalide by-product be thermally converted to
trihalosilane as in conventional methods.
[0080] According to the processing scheme of FIG. 15, trihalosilane
(e.g., trichlorosilane) is introduced into the reaction chamber of
a fluidized bed reactor to deposit silicon on growing silicon
particles. An amount of silicon tetrahalide (e.g., silicon
tetrachloride) is produced as a by-product and is discharged from
the fluidized bed reactor. Silicon tetrahalide that is discharged
from the fluidized bed reactor may be reintroduced to the reaction
chamber (e.g., near the reaction chamber wall such as by
introduction into peripheral openings of the distributor as
described above) to etch silicon deposits in the reaction chamber.
In one or more embodiments, the entire portion of silicon
tetrahalide discharged from the reaction chamber is recycled back
to the reaction chamber.
[0081] Additionally or alternatively and as shown in FIG. 15, a
source of silicon may react with hydrogen halide (e.g., HCl) to
produce trihalosilane, wherein an amount of by-product silicon
tetrahalide is produced. This silicon tetrahalide (and in some
embodiments the entire portion of this silicon tetrahalide) may
also be added to the fluidized bed reactor to etch silicon deposits
on the reaction chamber wall. In this regard, the system may be a
substantially closed-loop system with regard to silicon tetrahalide
as the amount of silicon tetrahalide produced from trihalosilane in
the reaction chamber and the amount of silicon tetrahalide produced
as a by-product of the reaction of silicon and hydrogen halide is
substantially the same as the amount of silicon trihalosilane
consumed in the reaction chamber by reaction with silicon.
[0082] With reference to FIG. 15, a source of silicon 209 may be
reacted with an acid 211 (e.g., hydrochloric acid which is
typically anhydrous hydrochloric acid) in an acid digester 220.
Sources of silicon include sand (i.e., SiO.sub.2), quartz, flint,
diatomite, mineral silicates, metallurgical grade silicon (i.e.,
polycrystalline silicon), fused silica, fluorosilicates and
mixtures thereof. Generally, the source of silicon is metallurgical
grade silicon. Other materials (sand, quartz, flint, diatomite and
the like) can be converted to metallurgical grade silicon according
to a carbothermic reduction reaction. The reaction product 213
contains both trihalosilane and silicon tetrahalide. The reaction
product 213 is typically a liquid at room temperature. Conventional
processes typically include thermal conversion of the silicon
tetrahalide to trihalosilane (e.g., conversion of silicon
tetrahalide to silicon and hydrogen in further production of
trichlorosilane or reaction of silicon tetrahalide with hydrogen
and a source of silicon to produce trihalosilane). In contrast, in
embodiments of the present disclosure, trihalosilane and silicon
tetrahalide may be fed to the fluidized bed reactor. Optionally,
trihalosilane and silicon tetrahalide may be fed to the fluidized
bed reactor separately. For instance, silicon tetrahalide may be
separated from the trihalosilane in a distillation column 230.
Silicon tetrahalide may be fed to the fluidized bed reactor 240 by
feeding the silicon tetrahalide 233 through peripheral distribution
openings as described above such that silicon deposits on the walls
of the reaction chamber may be etched. The trihalosilane 235 may be
fed through interior openings with an amount of hydrogen 237 as
described above in a two-gas system or with hydrogen being fed
through central openings as described in a three-gas system.
[0083] Granular polycrystalline silicon 249 may be withdrawn from
the fluidized bed reactor 240 and conveyed to product storage 242.
Spent gas 244 may contain hydrogen halide acid (e.g., HCl) produced
as a by-product of the trihalosilane decomposition process, as well
as unreacted hydrogen, unreacted trihalosilane, silicon tetrahalide
and small amounts of other by-product gases. The silicon
tetrahalide may include silicon tetrahalide that was introduced
into the fluidized bed reactor as an etching gas but did not react
with silicon and silicon tetrahalide produced as a by-product of
the thermal decomposition of trihalosilane as in reaction (ii)
above.
[0084] The spent gas 244 may include an amount of silicon dust
particulate that is carried out of the reactor. Typically, the size
of the silicon dust particulate is less than about 50 .mu.m and, in
some embodiments, is less than about 5 .mu.m. In contrast, granular
polycrystalline product typically has a particle size of about 600
.mu.m to about 2000 .mu.m and more typically from about 800 .mu.m
to about 1200 .mu.m or from about 900 .mu.m to about 1000 .mu.m.
The silicon dust is separated from the spent gas 244 that exits the
reactor in a solid-gas separation device 250 such as, for example,
bag-filtration, cyclone separation or liquid scrubbers. Recovered
silicon dust 246 may be sold for industrial use (e.g., as in a
silicon charge for pulling single crystal silicon by the Cz method)
or may be recycled back into the reactor and/or fed into one or
more additional fluidized bed reactors wherein the silicon dust is
at least partially scavenged by silicon particles.
[0085] The dust-depleted spent gas 245 may be cooled and condensed
in one or more condensers 260 to remove hydrogen and hydrogen
halide acid (collectively designated as "262"). The hydrogen and
hydrogen halide acid 262 may be subjected to further processing 285
which may include introducing the gases into a wet scrubber to
separate the acid from hydrogen gas. Any hydrogen may be recycled
back to the fluidized bed reactor and the acid may be reacted with
a source of silicon in the acid digester 220.
[0086] The remaining condensate 265 may be fed to a distillation
column 270 wherein silicon tetrahalide 273 is separated from
trihalosilane 275, both of which may be recycled back to the
fluidized bed reactor 240. This is in contrast to conventional
systems in which the silicon tetrahalide is thermally converted to
trihalosilane.
[0087] Thus it can be seen that the system illustrated in FIG. 15
is a substantially closed loop system with regard to silicon
tetrahalide. In this regard, silicon tetrahalide generated as part
of trihalosilane production and silicon tetrahalide generated as a
by-product gas in the fluidized bed reactor may both be consumed in
the fluidized bed reactor such that no net silicon tetrahalide need
be thermally converted to trihalosilane.
[0088] It should be understood that the system illustrated in FIG.
15 should not be viewed in a limiting sense as systems contemplated
by the present disclosure include those with additional fluidized
bed reactors, distillation columns and condensers including systems
which use these units in series or parallel. Additional fluidized
bed reactors, when present, may be operated according to the
present disclosure or may be operated in more conventional means
without departing from the scope of the present disclosure.
Further, additional known processing steps may be included such as
further separation and/or purification steps which may be readily
determined by one of ordinary skill in the art. In one or more
embodiments, the system of FIG. 15 may include one or more
halogenation reactors to convert silicon tetrahalide to
trihalosilane by reacting silicon tetrahalide with a source of
silicon and hydrogen and may include further distillation columns
to separate the reaction product gas into its constituent
gases.
[0089] Furthermore, it should be understood that while the methods
of the present disclosure have been described with regard to a
fluidized bed reactor in which granular polysilicon is produced,
the methods are also suitable in processing systems in which
polysilicon rods are produced by chemical vapor deposition
according to methods known in the art as the Siemens process. For
instance, in the chemical vapor deposition process of the Siemens
method, silicon tetrahalide may be directed to the reaction chamber
wall and a thermally decomposable silicon compound directed inward
of the silicon tetrahalide. Further, embodiments of the distributor
and/or reactor systems described herein and shown in any of FIGS.
1-15 may be used in the Siemens process, particularly in
embodiments wherein the walls of the Siemens reactor are
insulated.
[0090] When introducing elements of the present disclosure or the
embodiments(s) thereof, the articles "a", "an", "the" and "said"
are intended to mean that there are one or more of the elements.
The terms "comprising", "including" and "having" are intended to be
inclusive and mean that there may be additional elements other than
the listed elements.
[0091] In view of the above, it will be seen that the several
objects of the disclosure are achieved and other advantageous
results attained.
[0092] As various changes could be made in the above methods
without departing from the scope of the disclosure, it is intended
that all matter contained in the above description and shown in the
accompanying figures shall be interpreted as illustrative and not
in a limiting sense.
* * * * *