JP3621344B2 - Method for purifying trichlorosilane - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for purifying trichlorosilane used in the production of polycrystalline silicon.
[0002]
[Prior art]
High-purity polycrystalline silicon, which is a raw material for producing a silicon single crystal, which is a material for semiconductor devices, is manufactured by a vapor phase growth method called a Siemens method. This manufacturing process will be described with reference to FIG.
[0003]
First, trichlorosilane (TCS: SiHCl ₃ ) is manufactured in a conversion furnace using metal silicon, silicon tetrachloride (STC: SiCl ₄ ), and hydrogen gas as raw materials. The produced trichlorosilane is sent to a virgin distillation system, where it is passed through a plurality of distillation columns to be purified into high-purity trichlorosilane. This high-purity trichlorosilane is supplied to a reduction furnace together with high-purity trichlorosilane and hydrogen purified by an unreacted distillation system described later.
[0004]
In the reducing furnace, polycrystalline silicon is vapor-deposited on the heated seed surface by a reduction reaction using trichlorosilane and hydrogen as raw materials. Along with this reaction, exhaust gas composed of unreacted trichlorosilane and hydrogen and silicon tetrachloride as a reaction product is discharged from the reduction furnace. This exhaust gas is sent to the hydrogen recovery process. In this step, the exhaust gas is cooled to −10 ° C. or lower, and trichlorosilane and silicon tetrachloride in the exhaust gas are liquefied and separated from hydrogen. The hydrogen recovered in this step is supplied as a raw material gas to the reduction furnace.
[0005]
On the other hand, the chlorosilane liquid mainly composed of liquefied trichlorosilane and silicon tetrachloride is sent to an unreacted distillation system. In the unreacted distillation system, the chlorosilane liquid is passed through a distillation column to separate trichlorosilane from silicon tetrachloride and impurities. High-purity trichlorosilane taken out from the top of the column is supplied as a raw material gas to the reduction furnace. Silicon tetrachloride containing impurities taken out from the bottom of the column is sent to the conversion step described above. In some cases, high-purity silicon tetrachloride is taken out as a product by side-cutting between the tower top and the tower bottom.
[0006]
[Problems to be solved by the invention]
In such a manufacturing process of polycrystalline silicon, a very high quality in which impurities such as PCl ₃ are eliminated as much as possible is required for high-purity trichlorosilane sent from an unreacted distillation column to a reduction furnace. For this reason, although the quality of trichlorosilane is controlled, since the required impurity concentration is low, the amount of impurities cannot be accurately measured by a normal chemical analysis method.
[0007]
Therefore, trichlorosilane purified in a distillation column is periodically sent to a small reduction furnace called a test furnace to actually produce a sample sample of polycrystalline silicon, and a silicon single piece produced from the sample product by the FZ method. The quality of trichlorosilane is inspected and controlled by measuring the specific resistance of the crystal.
[0008]
The quality required for trichlorosilane as a raw material for producing polycrystalline silicon used for the production of a silicon single crystal is, for example, N-type 3000 Ωcm or more in terms of the specific resistance of the sampled silicon single crystal. However, when the present inventors continuously conducted a long-term quality survey on trichlorosilane purified in an unreacted distillation column, the resistivity of the silicon single crystal produced as a sample was N-type 1500 Ωcm. It has become clear that the quality of trichlorosilane may be degraded to a level that is lower than.
[0009]
Although the quality degradation of the trichlorosilane occurs irregularly and may not occur for a relatively long period of time, it goes without saying that the adverse effect on the quality of the polycrystalline silicon produced using the trichlorosilane as a raw material cannot be ignored. As for the cause, there is a possibility that contamination occurs in the unreacted distillation system, but the causal relationship with the causal phenomenon is not clear.
[0010]
The object of the present invention is to prevent the deterioration of the quality of trichlorosilane purified by an unreacted distillation system in the production process of polycrystalline silicon, to stably supply high quality trichlorosilane, and to stabilize the quality of polycrystalline silicon. Another object of the present invention is to provide a method for purifying trichlorosilane that enables easy production.
[0011]
[Means for Solving the Problems]
By the way, in the production of polycrystalline silicon by vapor phase growth, a plurality of reduction furnaces are usually used in parallel. On the other hand, a trichlorosilane production system (virgin distillation system) and a recovery system (unreacted distillation system) are usually shared among a plurality of reduction furnaces. As a result of detailed investigations on the cycle of the phenomenon in which the quality of trichlorosilane purified by an unreacted distillation system deteriorates, the present inventors have found that the operation cycle of the reduction furnace is related to the quality deterioration of trichlorosilane. I found out. More specifically, the composition of the chlorosilane liquid supplied to the unreacted distillation column is affected by the operation cycles of a plurality of reduction furnaces, and this causes the quality of trichlorosilane to deteriorate.
[0012]
That is, in the unreacted distillation system, as shown in FIG. 2, a chlorosilane liquid mainly composed of trichlorosilane and silicon tetrachloride is supplied to the distillation column 10. The chlorosilane liquid supplied to the distillation column 10 is heated by the evaporator 11 at the bottom of the column. The trichlorosilane having a low boiling point is discharged as a vapor from the top of the column and is liquefied by the condenser 12 to become a high purity trichlorosilane liquid. A part of this trichlorosilane liquid is refluxed to the top of the column for temperature control in the column, and the rest is taken out as a purified product. The ratio (R / D) of the reflux liquid amount R to the purified product take-out liquid amount D is called the reflux ratio and affects the degree of purification of trichlorosilane.
[0013]
On the other hand, silicon tetrachloride having a high boiling point is extracted from the bottom of the column together with impurities such as PCl ₃ . Further, high-purity silicon tetrachloride may be extracted as a product by side-cutting between the tower bottom and the tower top. By the way, the boiling point of trichlorosilane under atmospheric pressure is about 32 ° C., the boiling point of silicon tetrachloride is about 58 ° C., and the boiling point of PCl ₃ is about 76 ° C.
[0014]
On the other hand, in a plurality of reduction furnaces connected to the unreacted distillation system, polycrystalline silicon is produced over several 10 to 200 hours. During this reduction reaction, the concentration of silicon tetrachloride in the exhaust gas discharged from each reduction furnace varies with the progress of the reaction. FIG. 3 shows changes with time in the exhaust gas composition (excluding hydrogen) in the reduction furnace. The following reduction furnace exhaust gas composition is a numerical value excluding hydrogen in the exhaust gas.
[0015]
The initial stage of the reaction is a time zone in which the flow rate of trichlorosilane is increased from the start of the reaction. Since the seed is thin and the reaction area is small, most of the trichlorosilane supplied to the reduction furnace is discharged as exhaust gas without being reacted. The concentration of silicon tetrachloride in the exhaust gas is as low as 5 to 30%.
[0016]
The middle period of the reaction is a time period in which the flow rate of trichlorosilane stops increasing and becomes a constant flow rate. Since the surface area of the seed increases, a relatively large amount of trichlorosilane supplied to the reduction furnace contributes to the reduction reaction. The concentration of silicon tetrachloride in the exhaust gas is as high as 20 to 60%.
[0017]
The end of the reaction is a time period from the middle of the reaction to the end of the reaction. Since the surface area of the seed is larger than that in the middle of the reaction, a relatively large amount of trichlorosilane supplied to the reduction furnace contributes to the reduction reaction as in the middle of the reaction. The concentration of silicon tetrachloride in the exhaust gas is 20 to 50%.
[0018]
Multiple reduction furnaces share an unreacted distillation system, but due to the production status of polycrystalline silicon, raw material gas supply capacity, etc., they are not operated synchronously and are operated in different phases. It is normal. For this reason, the composition of the exhaust gas toward the unreacted distillation system fluctuates in a complicated manner. As a result, the concentration of silicon tetrachloride in the chlorosilane liquid supplied to the distillation column 10 also fluctuates greatly and complicatedly within a range of 5 to 60%. However, according to a detailed investigation by the present inventors, when the concentration of silicon tetrachloride in the chlorosilane solution supplied to the distillation column 10 is low, the quality of trichlorosilane purified in the distillation column 10 is low. It became clear that it fell.
[0019]
That is, the supply flow rate and heating amount of the chlorosilane liquid in the distillation column 10 are basically constant. For this reason, when the composition of the chlorosilane liquid is constant, the reflux ratio R / D of the trichlorosilane liquid changes only depending on the temperature control in the column, and the amount of change is slight. However, in practice, the composition of the chlorosilane liquid varies under the influence of the operating conditions in the plurality of reduction furnaces described above. As a result, the silicon tetrachloride concentration in the chlorosilane solution may shift to a low level. In this case, the trichlorosilane concentration relatively increases and the amount of the purified trichlorosilane solution increases. If it does so, the taking-out liquid amount D of a trichlorosilane liquid will increase, and reflux ratio R / D will fall. This reflux ratio R / D affects the degree of purification of trichlorosilane, and the degree of purification decreases as the reflux ratio R / D decreases. For this reason, the quality of the trichlorosilane liquid refine | purified with the distillation tower 10 will fall.
[0020]
As a result of the investigation by the present inventors, in order to prevent the quality deterioration of the trichlorosilane liquid, the concentration of silicon tetrachloride in the chlorosilane liquid supplied to the distillation column 10 is set to 30% or more over the substantially entire period of the refining operation. It has been found that it is effective to maintain the above.
[0021]
The method for purifying trichlorosilane of the present invention has been made based on such knowledge, and trichlorosilane and silicon tetrachloride obtained from exhaust gas discharged from a reduction furnace accompanying the production of polycrystalline silicon by the vapor phase growth method. In a method for purifying trichlorosilane by separating a trichlorosilane by passing a chlorosilane liquid containing as a main component through a distillation tower, by controlling the silicon tetrachloride concentration in the chlorosilane liquid supplied to the distillation tower to 30% or more, The degradation of the quality of trichlorosilane purified in the distillation column is prevented, and stable production of high-quality trichlorosilane and, consequently, high-quality polycrystalline silicon is made possible.
[0022]
Specifically, duplication at the initial stage of the reaction is avoided so that a plurality of reduction furnaces sharing the distillation tower can maintain a silicon tetrachloride concentration of 30% or more in the chlorosilane liquid supplied to the distillation tower. Operate in a pattern. Alternatively, the silicon tetrachloride concentration in the chlorosilane solution supplied to the distillation column is measured continuously or intermittently, and when the measured silicon tetrachloride concentration is reduced to less than 30%, the side cut from the distillation column is performed. Is added to the chlorosilane solution.
[0023]
In the present invention, the concentration of silicon tetrachloride in the chlorosilane liquid flowing into the distillation column is preferably maintained at 30% or more over the entire period of the refining operation. For a short time (specifically, up to about 10 hours). This is because it usually takes a long time to change the composition in the distillation column. Therefore, if the silicon tetrachloride concentration decreases for a short time, the silicon tetrachloride concentration recovers and returns to the normal composition in the tower before the effect of quality deterioration due to composition change appears, so there is no significant quality deterioration. Does not occur.
[0024]
The present invention also does not exclude that the trichlorosilane contains dichlorosilane (SiH ₂ Cl ₂ ). This is because dichlorosilane is also used as a raw material for the reduction reaction in the same manner as trichlorosilane, and its mixture does not cause any problem in operation.
[0025]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 4 is a configuration diagram of a trichlorosilane purification facility for explaining an embodiment of the present invention.
[0026]
In an embodiment of the present invention, polycrystalline silicon is produced by a plurality of reduction furnaces 1, 1. The exhaust gas discharged from the plurality of reduction furnaces 1, 1... Is sent to the hydrogen recovery process 2 through a common pipe. The exhaust gas contains unreacted trichlorosilane and hydrogen, and reaction products such as silicon tetrachloride. The hydrogen recovery step 2 separates hydrogen by cooling the exhaust gas to −10 ° C. or lower and liquefying substances other than hydrogen. The liquid separated from hydrogen is a chlorosilane liquid mainly composed of trichlorosilane and silicon tetrachloride.
[0027]
This chlorosilane liquid is sent to the unreacted distillation system, and is supplied to the distillation column 10 through the tank 3 here. The distillation tower 10 heats the chlorosilane liquid with an evaporator 11 at the bottom of the tower and separates it into trichlorosilane and silicon tetrachloride. The trichlorosilane having a low boiling point is discharged as a vapor from the top of the column and is liquefied by the condenser 12 to become a high purity trichlorosilane liquid. A part of this trichlorosilane liquid is refluxed to the top of the column for temperature control in the column, and the rest is taken out as a product.
[0028]
On the other hand, silicon tetrachloride having a high boiling point is extracted from the bottom of the column together with impurities such as PCl ₃ . Further, high-purity silicon tetrachloride can be extracted as a product by side-cutting between the tower bottom and the tower top.
[0029]
Here, the concentration of silicon tetrachloride in the exhaust gas discharged from each reduction furnace 1 is 5 to 30% at the beginning of the reaction, and 20 to 60% at the middle of the reaction and at the end of the reaction. If the reaction initial stage overlaps in all of the plurality of reduction furnaces 1, 1..., The silicon tetrachloride concentration in the chlorosilane liquid supplied to the distillation column 10 is less than 30% even though there is the tank 3. The quality of the trichlorosilane liquid produced in the process is reduced.
[0030]
Therefore, in the present embodiment, the initial reaction is not overlapped in two or more reduction furnaces 1, 1 · so that the silicon tetrachloride concentration in the chlorosilane liquid supplied to the distillation column 10 is maintained at 30% or more. A plurality of reduction furnaces 1, 1,. That is, when one reducing furnace 1 is in the initial stage of reaction, a plurality of reducing furnaces 1, 1,... Are operated in a pattern in which one or more of the other reducing furnaces 1, 1,.
[0031]
Thereby, silicon tetrachloride exhausted from the reduction furnace 1 at the initial stage of the reaction, and silicon tetrachloride exhausted from the one or more reduction furnaces 1 in the middle or final stage of the reaction. As a result, the silicon tetrachloride concentration in the chlorosilane liquid supplied to the distillation column 10 is maintained at 30% or more. Thus, quality degradation of the trichlorosilane liquid produced in the distillation column 10 is avoided.
[0032]
In addition, the chlorosilane liquid supplied to the distillation column 10 passes through the tank 3 in advance and temporarily stays therein, so that the concentration fluctuation is somewhat relaxed. For this reason, if about 90% or more of the initial stage of the reaction overlaps with the middle stage of the reaction or the end of the reaction, the silicon tetrachloride concentration in the chlorosilane solution supplied to the distillation column 10 is maintained at 30% or more. Further, as described above, there is no problem even if a chlorosilane liquid having a silicon tetrachloride concentration of less than 30% temporarily flows into the distillation column 10.
[0033]
In another embodiment of the present invention, a measuring device 4 that measures the concentration of silicon tetrachloride in the chlorosilane solution is provided in a pipe that supplies the chlorosilane solution to the distillation column 10. The measuring device 4 is installed between the tank 3 and the distillation tower 10. A pipe for guiding the silicon tetrachloride liquid taken out from the distillation column 10 by side cut to the upstream side of the tank 3 is provided, and the flow rate control valve 5 provided in the pipe is controlled by the output of the measuring device 4.
[0034]
Specifically, when the measured silicon tetrachloride concentration is 30% or more, the flow control valve 5 is kept closed. When the measured silicon tetrachloride concentration is less than 30%, the flow control valve 5 is opened, and the silicon tetrachloride solution concentration in the chlorosilane solution in the tank 3 is increased. Thereby, the silicon tetrachloride density | concentration in the chlorosilane liquid supplied to the distillation tower 10 is maintained at 30% or more, and the quality fall of the trichlorosilane liquid produced | generated in the distillation tower 10 is avoided.
[0035]
In addition, since the chlorosilane liquid supplied to the distillation tower 10 passes through the tank 3 in advance and temporarily stays there, the measured value of the silicon tetrachloride concentration is 30% or more in about 90% or more of the whole reaction period. If maintained, the concentration of silicon tetrachloride in the chlorosilane solution supplied to the distillation column 10 is maintained at 30% or more. Further, as described above, there is no problem even if a chlorosilane liquid having a silicon tetrachloride concentration of less than 30% temporarily flows into the distillation column 10.
[0036]
FIG. 5 is a graph showing the results of investigating the relationship between the concentration of silicon tetrachloride in the chlorosilane solution supplied to the distillation column and the quality of the trichlorosilane solution produced in the distillation column.
[0037]
The quality of the trichlorosilane solution is represented by the specific resistance of a silicon single crystal produced by sending trichlorosilane purified by a distillation column to a test furnace to produce polycrystalline silicon and further producing the polycrystalline silicon from the polycrystalline silicon by the FZ method. The quality of the trichlorosilane liquid is preferably N-type 3000 Ωcm or more in this specific resistance.
[0038]
As can be seen from FIG. 5, as the concentration of silicon tetrachloride in the chlorosilane liquid supplied to the distillation tower increases, the quality of the trichlorosilane liquid purified by the distillation tower improves, and the silicon tetrachloride is 30% or more. The quality of N-type 3000 Ωcm or higher is ensured.
[0039]
Prior to the practice of the present invention, the situation in which the quality of trichlorosilane purified by a distillation column was lower than the N-type 1500 Ωcm in terms of the specific resistance occurred for a period of 12% over 6 months. However, by implementing the present invention, such quality degradation was reduced to 1%.
[0040]
【The invention's effect】
As described above, the method for purifying trichlorosilane according to the present invention reduces the quality of trichlorosilane purified in the distillation column by controlling the silicon tetrachloride concentration in the chlorosilane solution supplied to the distillation column to 30% or more. And high quality trichlorosilane can be produced. Thereby, the quality of the polycrystalline silicon manufactured using this trichlorosilane as a raw material can be stabilized.
[Brief description of the drawings]
FIG. 1 is a production flow diagram of polycrystalline silicon by a vapor phase growth method.
FIG. 2 is a configuration diagram of a distillation column used in an unreacted distillation system.
FIG. 3 is a graph showing the transition of the reaction in the reduction furnace as the exhaust gas composition changes with time.
FIG. 4 is a configuration diagram of a trichlorosilane purification facility for explaining an embodiment of the present invention.
FIG. 5 is a graph showing the relationship between the concentration of silicon tetrachloride in the chlorosilane solution supplied to the distillation column and the quality of the trichlorosilane solution produced in the distillation column.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Reduction furnace 2 Hydrogen recovery process 3 Tank 4 Measuring device 5 Flow control valve 10 Reduction furnace 11 Evaporator 12 Condenser

Claims (3)

Trichlorosilane which separates trichlorosilane by passing chlorosilane liquid mainly composed of trichlorosilane and silicon tetrachloride obtained from exhaust gas discharged from the reduction furnace in the vapor phase growth method through the distillation tower. The method for purifying trichlorosilane, characterized in that the concentration of silicon tetrachloride in the chlorosilane liquid supplied to the distillation column is controlled to 30% or more. A plurality of reduction furnaces sharing the distillation column are operated in a pattern that avoids duplication in the initial reaction so that the silicon tetrachloride concentration in the chlorosilane liquid supplied to the distillation column is maintained at 30% or more. The method for purifying trichlorosilane according to claim 1. The silicon tetrachloride concentration in the chlorosilane liquid supplied to the distillation column is measured continuously or intermittently, and when the measured silicon tetrachloride concentration is reduced to less than 30%, the silicon tetrachloride concentration is taken out from the distillation column by side cut. The method for purifying trichlorosilane according to claim 1, wherein the silicon tetrachloride is added to the chlorosilane solution.

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