System for producing polycrystalline silicon, apparatus for producing polycrystalline silicon, and process for producing polycrystalline silicon

Abstract

The present invention provides a technique by which heat can be efficiently recovered from a coolant used to cool a reactor, and contamination with dopant impurities from an inner wall of a reactor when polycrystalline silicon is deposited within the reactor can be reduced to produce high-purity polycrystalline silicon. With the use of hot water 15 having a temperature higher than a standard boiling point as a coolant fed to the reactor 10, the temperature of the reactor inner wall is kept at a temperature of not more than 370 C. Additionally, the pressure of the hot water 15 to be recovered is reduced by a pressure control section provided in a coolant tank 20 to generate steam. Thereby, a part of the hot water is taken out as steam to the outside, and reused as a heating source for another application.

Claims

1. A process for producing polycrystalline silicon, comprising generating steam during growth of polycrystalline silicon while keeping a temperature of an inner wall surface of a reactor at not more than 370 C., wherein the inner wall surface of the reactor which contacts a process gas comprises a steel type comprising an alloy for which a value of a relational expression in mass content percentage among chromium, nickel, and silicon, [Cr]+[Ni]1.5 [Si], is not less than 40%, wherein water is removed and returned to said reactor via a coolant circulation path, which comprises a first pressure control section, a second pressure control section, and a coolant tank, wherein said steam is generated by feeding hot water, having a temperature higher than a standard boiling point, to said reactor, then vaporizing a portion of said hot water, where the pressure of said hot water is reduced so that a portion of said hot water itself is flashed into said steam, wherein the pressure of water discharged from said reactor is controlled by said first pressure control section and the pressure in said coolant tank is controlled by said second pressure control section, and said hot water is flashed to generate steam and to cool the hot water simultaneously by reducing the pressure of the hot water in the first pressure control section, wherein said first pressure control section comprises a first pressure indicator controller and a first pressure control valve configured for reducing the pressure of said hot water, and said second pressure control section comprises a second pressure indicator controller and a second pressure control valve configured for controlling pressure within said coolant tank.

2. The process of claim 1, wherein the value [Cr]+[Ni]1.5 [Si] is not less than 60%.

3. The process of claim 1, wherein [Cr] is in a range of 14.6 to 25.2% by mass, [Ni] is in a range of 19.6 to 77.5% by mass, and [Si] is in a range of 0.3 to 0.6% by mass.

4. The process of claim 1, wherein the temperature of said hot water fed into said reactor is less than 200 C.

5. The process of claim 1, wherein the polycrystalline silicon produced has a total amount of dopant impurities of not more than 100atomic ppt.

6. A process for producing polycrystalline silicon, comprising producing polycrystalline silicon in a reactor which is cooled by feeding hot water having a temperature higher than a standard boiling point to said reactor, wherein said hot water discharged from said reactor is flashed to generate steam, wherein water is removed and returned to said reactor via a coolant circulation path, which comprises a first pressure control section, a second pressure control section, and a coolant tank, where the pressure of said hot water is reduced so that a portion said hot water itself is flashed into said steam, wherein the pressure of water discharged from said reactor is controlled by said first pressure control section and the pressure in said coolant tank is controlled by said second pressure control section, and said hot water is flashed to generate steam and to cool the hot water simultaneously by reducing the pressure of the hot water in the first pressure control section, wherein said first pressure control section comprises a first pressure indicator controller and a first pressure control valve configured for reducing the pressure of said hot water, and said second pressure control section comprising a second pressure indicator controller and a second pressure control valve configured for controlling pressure within said coolant tank.

7. The process of claim 6, wherein the temperature of said hot water fed into said reactor is less than 200 C.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a drawing illustrating an example of a configuration of a polycrystalline silicon producing system according to the present invention.

(2) FIG. 2 is a sectional view illustrating a structure (an inner wall, an outer wall, and a coolant flow passage) of a wall portion of a reactor for producing polycrystalline silicon according to the present invention.

(3) FIG. 3 is a drawing showing dependency of corrosiveness of a CrNiSi alloy material on a composition (mass content (% by mass): R=[Cr]+[Ni]1.5 [Si]).

(4) FIG. 4 is a drawing showing a relationship between a temperature of the inner wall surface on a coolant outlet end side immediately before a polycrystalline silicon deposition step is completed and a concentration of dopant impurities taken into polycrystalline silicon in a reactor having an inner wall comprising a CrNiSi alloy material, wherein the CrNiSi alloy material is SUS310S and Hastelloy C.

DESCRIPTION OF EMBODIMENT

(5) Hereinafter, with reference to the drawings, an embodiment according to the present invention will be described.

(6) FIG. 1 is a drawing illustrating an example of a configuration of a system for producing polycrystalline silicon according to the present invention. The drawing shows a polycrystalline silicon producing system 100 in which polycrystalline silicon is deposited by the Siemens method.

(7) A reactor 10 is provided on a baseplate 1. Within the reactor 10, an approximately U-shaped silicon core 5 is set, and both ends of the silicon core 5 are connected to electrodes 2a and 2b, respectively, to be electrically conductive. A raw material gas such as trichlorosilane gas for depositing polycrystalline silicon and a process gas such as nitrogen gas and hydrogen gas are fed from a gas nozzle 3 into the reactor 10. By vapor deposition, polycrystalline silicon 6 is deposited on the surface of the silicon core 5 heated by feeding the current from the electrodes 2a and 2b. The gas within the reactor 10 is discharged from a vent 4. In the present invention, by adjusting a flow rate of a coolant (hot water) 15 described later, the temperature of the innermost surface of the reactor is controlled at a temperature of less than 400 C.

(8) A coolant tank 20 stores hot water 15 as a coolant. The hot water 15 is fed by a hot water feeding pump 21 provided in a coolant circulation path 24a from the coolant tank 20 to the lower portion of the reactor 10. The hot water 15 is passed through a coolant flow passage 13 (described later) provided in the reactor 10, and discharged from the upper portion of the reactor 10.

(9) The pressure of the hot water 15 discharged from the upper portion of the reactor 10 is detected by a first pressure control section, namely, a pressure indicator controller PIC 22 provided in the coolant circulation path 24b. A degree of opening of a control valve 23 is controlled to prevent boiling of the hot water as the coolant in the coolant flow passage. After the hot water is passed through the pressure valve, the hot water is flashed, and part of the hot water becomes steam. The hot water 15 is recovered to the coolant tank 20 while the hot water 15 is cooled.

(10) The pressure within the coolant tank 20 increased along with the generation of the steam is detected by a second pressure control section, namely, a pressure indicator controller PIC 31. The steam is recovered through a control valve 32. Namely, part of the coolant recovered to the coolant tank 20 can be taken out as steam, and reused as a heating source for another application. The temperature of the hot water fed to the reactor 10 is uniquely manageable by the second pressure control section. Alternatively, the temperature of the hot water can be managed by direct detection of the temperature of the coolant, instead of the above pressure control mechanism.

(11) The temperature of the innermost surface of the reactor can be calculated from the temperature of the hot water serving as the above temperature-controlled coolant (the pressure of the coolant tank 20), the amount of the hot water to be circulated, the amount of the steam taken out, the amount of the energy to be applied to perform the production reaction of the polycrystalline silicon 6 in the reactor 10, the heat conductivity based on the structure and material of the reactor 10, and calculation of the heat balance using an arrangement or the like of the silicon core 5 arranged for production of the polycrystalline silicon 6. When these reaction conditions are determined, the target temperature of the innermost surface of the reactor can be controlled at not more than 400 C., preferably not more than 370 C., by controlling the pressure within the coolant tank and the amount of the hot water to be circulated as described above.

(12) Alternatively, the temperature of the outlet of the coolant flow passage is measured and the amount of heat-removal energy is determined based on the temperature in the coolant tank and the amount of the hot water to be circulated so that an estimated value of the temperature of the innermost surface of the reactor can be determined by a heat conductivity based on the structure and material of the reactor.

(13) In the present invention, for the reason below, the hot water having a temperature higher than the standard boiling point (100 C.) is used as the coolant fed to the reactor 10.

(14) In the prior art, use of a variety of silicone oils and hydrocarbon oils as the coolant for the reactor 10 is known. These coolants have a relatively low boundary film heat transfer coefficient. For this reason, when a polycrystalline silicon rod is grown by the Siemens method or the like, the temperature of a silicone oil or a hydrocarbon oil as the coolant needs to be reduced to approximately 100 C. and the coolant needs to be fed to the reactor 10 in order to keep the temperature of the inner wall surface of the reactor 10, for example, not more than 370 C. immediately before the reaction is completed, in which the diameter of the polycrystalline silicon rod is increased.

(15) In such a case, the temperature of the coolant discharged from the reactor 10 is approximately 130 C. By heat exchange between the coolant having the temperature above and water, the temperature of the coolant needs to be reduced to approximately 100 C. and the coolant needs to be circulated and used. Additionally, the steam needs to be generated for heat recovery. Unfortunately, the coolant having the temperature of approximately 130 C. has difficulties to efficiently generate the steam.

(16) In contrast, water can have an extremely large boundary film heat transfer coefficient and thus is effective as the coolant. If the temperature of the water to be fed is not more than 100 C., the temperature of the water discharged from the reactor 10 is approximately 120 C. at most. Additionally, the temperature of the water needs to be decreased to not more than 100 C. for the purpose of circulation use. Accordingly, water is not practical from the viewpoint of recovery of the steam.

(17) Then, in the present invention, the hot water having a temperature more than the standard boiling point of water (i.e., 100 C.) is used as the coolant. According to the examination by the present inventors, in the case where polycrystalline silicon is grown by the Siemens method, when the hot water of 125 C., for example, is fed to the reactor 10 immediately before the reaction to increase the diameter of the polycrystalline silicon rod is completed, the temperature of the hot water discharged from the reactor 10 is 141 C. At this time, the temperature of the inner surface of the reactor is cooled to approximately 231 C. on the hot water outlet end side in which the temperature is highest, and the temperature of the inner wall surface of the reactor 10 can be sufficiently kept at a temperature of not more than 370 C. Moreover, if the temperature of the hot water discharged from the reactor 10 is 141 C., when the pressure of the hot water is controlled (reduced) and the hot water is flashed, the hot water itself becomes steam. The heat exchange between the coolant and the steam generator is unnecessary, and the heat can be efficiently recovered.

(18) If the hot water having a temperature of not less than 200 C. is used for the reactor 10, the vapor pressure when the hot water is discharged from the reactor 10 is as high as not less than 2 MPaG. For this reason, practically, the temperature of the hot water is desirably more than 100 C. and less than 200 C.

(19) In the case where the hot water having a temperature more than the standard boiling point is used as the coolant, boiling of the hot water may cause problems such as unstable flow of the hot water in the coolant flow passage. Accordingly, the pressure of the hot water is preferably controlled by the pressure exceeding the vapor pressure at the boundary film temperature to prevent boiling of the hot water on a heat removing surface boundary film in the reactor 10. The boiling is prevented by the first pressure control valve 23. Further, the pressure indicator controller PIC 31 that detects the pressure within the coolant tank 20 and the control valve 32 controls the pressure within the coolant tank 20. Thereby, the temperature of the hot water in the coolant tank 20 is controlled.

(20) A level controller LIC 41 detects the height of the solution level of the hot water 15 in the coolant tank 20, and controls the degree of opening of a control valve 42 to feed an amount of pure water equivalent to or slightly larger than the amount of the hot water 15 lost by the recovery of the steam. The hot water 15 in the coolant tank 20 is circulated through the hot water feeding pump 21 to the reactor 10.

(21) FIG. 2 is a sectional view illustrating the structure of the wall portion of the reactor 10 according to the present invention. The coolant flow passage 13 for circulating the hot water 15 as the coolant is, for example, spirally provided on an outer side of an inner wall 11, namely, between the inner wall 11 on the inner side of the reactor and an outer wall 12 on the outer side of the reactor. The hot water 15 is fed from the lower portion of the reactor 10 and discharged from the top of the reactor 10.

(22) The inner wall 11 has a two-layer structure: an anticorrosive layer 11a comprising a highly anticorrosive alloy material is provided on the inner side of the reactor contacting a corrosive process gas, and a heat conductive layer 11b for efficiently conducting the heat within the reactor 10 from the inner wall surface to the coolant flow passage 13 is provided on the outer side of the reactor (outer-wall side).

(23) The heat conductive layer 11b comprises an alloy material having a heat conductivity higher than that of the alloy material used in the anticorrosive layer 11a. For example, the heat conductive layer 11b comprises a material such as SB steel (carbon steel for boilers and pressure containers) and SGV steel (carbon steel for mid and normal temperature pressure containers). The heat conductive layer 11b is not limited to those comprising a single steel material, and may be those comprising a clad steel material having several kinds of metals applied to each other.

(24) For the reason described later, the alloy material used for the anticorrosive layer is an alloy material having a composition for which a value R, defined by R=[Cr]+[Ni]1.5 [Si], is not less than 40%, wherein [Cr] is a mass content (% by mass) of chromium (Cr), [Ni] is a mass content (% by mass) of nickel (Ni), and [Si] is a mass content (% by mass) of silicon (Si). Preferably, an alloy material having a value R of not less than 60% is selected.

(25) The following describes a corrosiveness test which is the background that leads to the selection of the alloy material having the above composition.

(26) The corrosiveness test was performed as follows: as a sample, a variety of alloy materials were cut into a test piece having a length of 30 mm, a width of 25 mm, and a thickness of 2 mm; the weight of the test piece was precisely weighed, and hung in a highly heated portion in a quartz reactor prepared as a test reactor; an exhaust gas discharged from the polycrystalline silicon reactor was introduced into the quartz reactor; and after a predetermined time at a predetermined temperature had passed, the weight of the test piece was weighed to determine change in the weight.

(27) As a first condition selected, the temperature was 200 C. and the time was 9 days. As the second condition selected, the temperature was 300 C. and the time was 9 days. Under these first and second conditions, the corrosiveness test was performed.

(28) First, an exhaust gas within the quartz reactor is replaced by nitrogen. Further, nitrogen is replaced by hydrogen. Part of the gas discharged from a Siemens type polycrystalline silicon reactor is introduced into the quartz reactor in which a test piece is hung in a highly heated portion. The exhaust gas discharged from the polycrystalline silicon reactor is a mixed gas containing H.sub.2, HCl, and SiHnCl.sub.4-n (n=0 to 3) as principal components.

(29) After the corrosiveness test was completed, the exhaust gas introduced into the quartz reactor was changed to hydrogen to cool the quartz reactor. Next, hydrogen was replaced with nitrogen, and the reactor was opened to the air. Then, the test piece was taken out from the quartz reactor, washed by water, and dried. The weight of the test piece was precisely weighed. As a result, under the first and second conditions, change in the weight was hardly found in any of the test pieces comprising the respective alloy materials. Namely, it was confirmed that at a temperature in the range of not less than 200 C. and not more than 300 C., the alloy materials that constitute the inner wall surface are hardly corroded.

(30) Then, in order to accelerate corrosion of the alloy material, a temperature at 400 C. and a time of 19 days were selected as a third condition, and a temperature of 500 C. and a time of 19 days as a fourth condition. Again, the corrosiveness test was performed in the same manner as above except the temperature and the time.

(31) Table 1 and FIG. 3 show the results of the corrosiveness test performed under the third condition (temperature of 400 C., time: 19 days). Table 1 shows a specific composition of the alloy materials (kinds of steel) and change in the weight after the corrosiveness test. FIG. 3 is a graph of the result wherein the abscissa represents an alloy composition (R=[Cr]+[Ni]1.5 [Si]) and the ordinate represents change in the weight after the corrosiveness test.

(32) NAR is a registered trademark of Sumitomo Metal Industries, Ltd. Incoloy and Inconel are registered trademarks of Inco Limited. Hastelloy is a registered trademark of Haynes-Stellite Company. Carpenter is a registered trademark of Carpenter Technology Corporation.

(33) TABLE-US-00001 TABLE 1 [Cr] + [Ni] Change in Alloy material [Cr] [Ni] [Si] 1.5[Si] weight (steel type) (%) (%) (%) (%) (mg/cm.sup.2) NAR-SN-1 17.2 13.9 4.3 24.7 4.05 SUS-305B 19.2 13.0 3.5 27.0 3.01 SUS-310S 25.2 19.6 0.5 44.1 2.87 Incoloy 800 20.5 31.3 0.3 51.4 2.78 Hastelloy C 15.0 57 0.4 71.4 1.25 NAR-25-50MTi 24.9 49.8 0.6 73.8 1.07 Inconel 600 14.6 77.5 0.37 92.7 1.53

(34) As for a steel type in which the value of the relational expression in mass content (% by mass) among chromium, nickel, and silicon, R=[Cr]+[Ni]1.5 [Si], is less than 40%, as apparent from Table 1 and FIG. 3, when this steel type is used as a material for the inner wall of the reactor, corrosion thereof is easily promoted in deposition of polycrystalline silicon within the reactor.

(35) Namely, as the material for the anticorrosive layer provided in the inner wall of the reactor for producing polycrystalline silicon, the alloy material preferably has a value R of not less than 40%, and more preferably a value R of not less than 60%. In the corrosiveness test on the fourth condition (temperature of 500 C., time: 19 days), a more marked change in the weight was found in compared with the corrosiveness test on the third condition.

EXAMPLE 1

(36) Based on the examination above, reactors were produced using a steel type that satisfies the condition of the value R of not less than 40%, i.e., SUS310S (value R: 41 to 46%) or Hastelloy C (value R: not less than 62%) as the anticorrosive layer for the inner wall. Using these reactors, polycrystalline silicon was actually deposited. Dependency of the concentration of dopant impurities in the obtained polycrystalline silicon rod on the temperature of the inner wall surface was examined.

(37) While hydrogen gas and trichlorosilane gas as the principal raw material were fed from the gas nozzle 3 into each reactor 10 having the anticorrosive layer for the inner wall which anticorrosive layer comprised the steel type SUS310S or Hastelloy C, growth of a polycrystalline silicon rod was performed at a temperature of not less than 1000 C. and not more than 1100 C. to obtain a polycrystalline silicon rod having a diameter of 120 to 130 mm.

(38) FIG. 4 is a drawing showing a relationship between the temperature of the inner wall surface (the abscissa) on the coolant outlet end side immediately before the polycrystalline silicon deposition step is completed and the concentration of dopant impurities taken into polycrystalline silicon in the respective reactors having the inner wall comprising a CrNiSi alloy material, i.e., SUS310S or Hastelloy C. The total amount of the dopants shown by the ordinate is the total amount of the dopants obtained by a photoluminescence analysis, and specifically, the sum of the contents of phosphorus, arsenic, boron, and aluminum.

(39) Because of a practical demand in control of the resistivity during growth of the CZ single-crystal or FZ single-crystal used for the semiconductor, the total amount of the dopants in polycrystalline silicon is desirably not more than 100 atomic ppt. As shown in FIG. 4, in the case where SUS310S is used for the inner wall surface, by keeping the temperature of the inner wall surface at not more than 220 C., the total amount of the dopants of not more than 100 atomic ppt can be obtained in polycrystalline silicon. In the case where Hastelloy C is used for the inner wall surface, by keeping the temperature of the inner wall surface not more than 370 C., the total amount of the dopants not more than 100 atomic ppt can be obtained in polycrystalline silicon.

(40) Table 2 shows examples of alloy materials for which a value R, defined by R=[Cr]+[Ni]1.5 [Si], is not less than 40% and which are preferable as the material for the anticorrosive layer provided in the inner wall of the reactor for producing polycrystalline silicon.

(41) TABLE-US-00002 TABLE 2 Alloy material [Cr] [Ni] [Si] [Cr] + [Ni] 1.5[Si] (steel type) (%) (%) (%) (%) SUS310S 24-26 19-22 1.5 41-46 Carpenter 20 19-22 28-30 1 45-50 Incoloy 800 18-23 28-35 0.5 45-57 Hastelloy C 14-17 50 1 62 Inconel 600 14-17 72 0.5 85

(42) Using the reactor according to the present invention, the deposition reaction of polycrystalline silicon is performed. The procedure is mainly as follows. First, the silicon core 5 is connected to the electrodes 2, and the reactor 10 is closely placed on the baseplate 1. Nitrogen gas is fed from the gas nozzle 3 to replace the air within the reactor 10 with nitrogen. The air and nitrogen within the reactor 10 are discharged from the vent 4.

(43) After replacement of the air within the reactor 10 with a nitrogen atmosphere is started, the hot water 15 is fed to the coolant flow passage 13, and heating of the inside of the reactor 10 is started. After replacement of the air within the reactor 10 with a nitrogen atmosphere is completed, hydrogen gas is fed from the gas nozzle 3 instead of nitrogen gas to provide a hydrogen atmosphere within the reactor 10.

(44) Next, using a heater not shown, the silicon core 5 is preheated to the temperature of not less than 250 C. to provide the conductivity such that the current efficiently flows. Subsequently, the current is supplied from the electrodes 2 to the silicon core 5 to heat the silicon core 5 to the temperature of not less than 900 C. Further, along with hydrogen gas, trichlorosilane gas is fed as a raw material gas, vapor deposition is performed on the silicon core 5 at the temperature in the range of not less than 900 C. and not more than 1200 C. to obtain the polycrystalline silicon 6. The unreacted gas and the by-product gas are discharged from the vent 4.

(45) During a period of time from the start of heating the silicon core 5 to the step of the deposition reaction of the polycrystalline silicon 6 (or cooling of the polycrystalline silicon rod after the deposition reaction step is completed), the hot water 15 is fed as the coolant to cool the reactor 10. At least during the deposition reaction of the polycrystalline silicon 6, a sufficient amount of the coolant is determined by calculation of the heat balance such that by keeping the pressure within the coolant tank 20 at 0.15 MPaG by a second pressure control section including the pressure indicator controller 31 and the pressure control valve 32, the temperature of the hot water 15 fed to the reactor 10 is kept at 125 to 127 C., the temperature of the inner wall surface of the reactor 10 is kept at the temperature of not more than 370 C., and the temperature of the hot water outlet in the reactor does not exceed 200 C.; and the obtained hot water is fed to the reactor by the hot water feeding pump 21.

(46) The temperature of the hot water 15 fed as the coolant that cools the reactor 10 is set in the range of more than 100 C. exceeding the standard boiling point and less than 200 C., as described above. In order to prevent boiling in the heat removing surface boundary film of the heat conductive layer 11b, the pressure of the hot water 15 is controlled at the pressure exceeding the vapor pressure at the boundary film temperature by the pressure control valve 23 of the first pressure control section.

(47) The hot water 15 having a controlled pressure is fed by the hot water feeding pump 21 from the lower portion of the reactor 10. The hot water 15 is passed through the coolant flow passage 13 contacting the heat conductive layer 11b to cool the inner wall 11, and heated by the heat conductive layer 11b to increase the temperature of the hot water, and discharged from the upper portion of the reactor 10.

(48) After the polycrystalline silicon 6 is grown to have a desired diameter, the raw material gas and the current fed to the polycrystalline silicon 6 are stopped in this order, and the temperature of the inside of the reactor 10 is reduced. After the temperature of the inside of the reactor 10 is sufficiently reduced, the hot water 15 is switched to cool water, and the reactor 10 is cooled to the temperature around room temperature. Finally, the atmosphere within the reactor 10 is replaced from hydrogen to nitrogen, the reactor 10 is opened to the air, and the grown polycrystalline silicon 6 is extracted.

EXAMPLE 2

(49) In a reactor 10 having an inner wall 11 including an anticorrosive layer 11a made of Hastelloy C and a heat conductive layer 11b of SB (carbon steel for boilers and pressure containers) and having two coolant flow passages 13, while the hot water 15 of 125 C. to 127 C. was fed to the two coolant flow passages 13 at 72 m.sup.3/hr, the pressure of the hot water was controlled by the pressure indicator controller PIC 22 at 0.5 MPaG.

(50) During the control of the pressure, while a silicon core 5 of a 7-mm square was electrically conducted and heated to approximately 1060 C. in a hydrogen atmosphere, trichlorosilane gas as the principal raw material was fed for a growth time of approximately 80 hours to obtain a polycrystalline silicon 6 having a diameter of approximately 120 mm.

(51) The temperature of the hot water 15 discharged from the reactor 10 was 129 C. when electrical conduction was started, and 141 C. when the reaction was completed. Table 3 shows the result of analyzing the heat transfer when electrical conduction of the reactor was started and when the reaction was completed. The calculated surface temperature of the anticorrosive layer 11a on the hot water outlet end side was 231 C. at the maximum.

(52) TABLE-US-00003 TABLE 3 Hot water inlet end side of reactor Hot water outlet end side of reactor Surface temperature Surface temperature Surface temperature Surface temperature of heat conductive of anticorrosive of heat conductive of anticorrosive layer on flow layer on reactor layer on flow layer on reactor passage side ( C.) inner side ( C.) passage side ( C.) inner side ( C.) Start of 129 139 131 141 electrical conduction End of 140 218 152 231 reaction

(53) The pressure of the hot water 15 discharged from the reactor 10 is kept at 0.5 MPaG (boiling temperature of the hot water of 158 C.) by the pressure indicator controller PIC 22. Accordingly, the hot water 15 does not boil within the coolant flow passage 13 (because the boiling temperature of the hot water exceeds the surface temperature on the flow passage side of the heat conductive layer of 152 C.). Moreover, by keeping the pressure within the coolant tank 20 at 0.15 MPaG by the pressure indicator controller 31, the temperature of the hot water 15 fed to the reactor 10 can be stably kept at 125 to 127 C., and the steam can be taken out from the control valve 32. The steam could be recovered at 0.4 ton/hr at an initial stage of the reaction and at 3.5 ton/hr at a final stage of the reaction. This heat recovery is equivalent to approximately 65% of the amount of electricity fed to the reactor 10 by applying current thereto.

INDUSTRIAL APPLICABILITY

(54) In the present invention, hot water itself used as a coolant is reused as steam. Accordingly, heat can be efficiently recovered from the coolant used to cool the reactor.

(55) Further, as a material for the anticorrosive layer provided on the inner side of the inner wall of the reactor, an alloy material having a composition in which a value R, defined by R=[Cr]+[Ni]1.5 [Si], is not less than 40% is used. Consequently, it is possible to provide a technique to obtain high-purity polycrystalline silicon by reducing contamination with dopant impurities from the inner wall of the reactor when polycrystalline silicon is deposited within the reactor, while maintaining the reactor at a relatively high temperature by using hot water as the coolant to generate steam.

REFERENCE SIGNS LIST

(56) 1 Baseplate 2a, 2b Electrode 3 Gas nozzle 4 Vent 5 Silicon core 6 Polycrystalline silicon 10 Reactor (reaction vessel) 11 Inner wall 11a Anticorrosive layer 11b Heat conductive layer 12 Outer wall 13 Coolant flow passage 15 Hot water 20 Coolant tank 21 Hot water feeding pump 22 Pressure indicator controller 32 Control valve 24a, 24b Coolant circulation path 31 Pressure indicator controller 32 Control valve 41 Level controller 42 Control valve 100 Polycrystalline silicon producing system