JP5096219B2 - Polycrystalline silicon manufacturing method - Google Patents

Description

  The present invention relates to a method for producing high-purity polycrystalline silicon used as a raw material for producing a silicon single crystal.

High-purity polycrystalline silicon used for the production of silicon single crystals and the like is industrially produced by a method called the Siemens method. In the production of polycrystalline silicon by the Siemens method, a mixed gas of SiHCl ₃ and hydrogen is supplied into the reaction furnace while energizing and heating a large number of silicon core rods set in a reaction furnace called a bell jar. In the reaction furnace, polycrystalline silicon is deposited on the surface of the silicon core rod by thermal decomposition of SiHCl ₃ , and the core rod grows to produce a polycrystalline silicon rod having a predetermined diameter as a product. The silicon core rods are formed in an inverted U shape with two wires as a set for energization heating, and are set on one set of electrodes.

SiHCl ₃ used as a raw material gas for the production of polycrystalline silicon by the Siemens method is produced by the following method, for example, and used for the production of polycrystalline silicon. That is, the production of polycrystalline silicon and the production of SiHCl _{3 as} a raw material are generally performed cyclically in one facility. A conventional example of such a manufacturing flow is shown in FIG.

In the manufacturing flow shown in FIG. 4, SiCl ₄ gas, hydrogen gas, and metallic silicon are supplied to the SiHCl ₃ generation step. In the SiHCl ₃ generation step, SiHCl ₃ is generated by reacting them in a fluidized furnace. Since this reaction is an equilibrium reaction, other silicon chlorides such as unreacted SiCl ₄ are discharged together with SiHCl ₃ from the fluidized furnace.

The silicon chloride removed from the SiHCl ₃ production process is sent to the rectification process. In the rectification process, a plurality of distillation columns connected in series in multiple stages are used to purify SiHCl ₃ . In this process, not only the separation of SiHCl ₃ and SiCl ₄ , but also the separation and removal of other silicon chlorides such as SiH ₂ Cl ₂ (dichlorosilane) from SiHCl ₃ , further harmful phosphorus, boron, carbon, etc. Removal is also performed. Among the impurities, carbon is particularly problematic and exists as a chloride such as CH ₃ SiHCl ₂ (methyldichlorosilane) and CH ₃ SiCl ₃ (methyltrichlorosilane). Other impurities are thought to be present in the form of chloride.

High-purity SiHCl ₃ obtained in the rectification process is sent as a raw material gas to the polycrystalline silicon manufacturing process. SiCl ₄ that occurs with the purification and separation of SiHCl ₃ is returned to the SiHCl ₃ generating step.

In the polycrystalline silicon manufacturing process, as described above, a mixed gas of SiHCl ₃ and hydrogen is supplied into a reaction furnace called a bell jar to generate polycrystalline silicon. Along with this, unreacted SiHCl ₃ and hydrogen, SiCl ₄ as a by-product, and the like are discharged from the reactor. The gas discharged from the reaction furnace is sent to the hydrogen recovery process, where the hydrogen gas is separated and removed. The separated hydrogen gas is sent to the SiHCl ₃ production process and the polycrystalline silicon manufacturing process, and the remaining SiHCl ₃ and SiCl ₄ are sent to the distillation process.

One of the important issues required for such a polycrystalline silicon manufacturing process is to lower the impurity concentration in the manufactured polycrystalline silicon. Among impurities, carbon that has a large adverse effect on semiconductor devices is required. It is important to remove. In order to solve this problem, it is necessary to reduce the amount of carbon in the high-purity SiHCl ₃ supplied to the polycrystalline silicon production process. For this reason, in the rectification process, a plurality of distillation columns connected in series in multiple stages are required. Thus, the distillation purification of SiHCl ₃ is repeated.

Impurities that are difficult to remove in the rectification step are carbon chlorides such as CH ₃ SiHCl ₂ and CH ₃ SiCl _{3 that} have a boiling point close to that of SiHCl ₃ . In particular, the boiling point of CH ₃ SiHCl ₂ is extremely close to that of SiHCl ₃ (the former boiling point is about 40 ° C. and the latter boiling point is about 32 ° C.), which is difficult to remove. It is no exaggeration to say that this carbon chloride is removed because a large amount of energy is consumed using a multi-stage distillation column. Incidentally, it is said that the main source of carbon contamination is metallic silicon.

However, the high-boiling impurities removed in the production flow here, particularly the SiHCl ₃ rectification process, are a kind of closed cycle, and therefore the carbon impurities in the impurities in particular have a span of several days or tens of days. There is a tendency to accumulate over time. If impurities accumulate, the operation burden must be increased in the rectification process, which further increases the energy cost in the rectification process.

Several measures for avoiding such an increase in burden in the rectification process have been proposed. One of them is the removal of carbon in hydrogen circulating in the production flow presented by Patent Document 1, and the other is the addition of a carbon removal device to the rectification process presented by Patent Document 2. It is. In the countermeasure proposed by patent document 1, the carbon content in hydrogen is removed using activated carbon in a hydrogen recovery process. Further, in the countermeasure presented by Patent Document 2, the carbon content in SiHCl ₃ is separated and removed by silica gel at the most downstream side of the distillation step.

Japanese Patent Laid-Open No. 11-49509 JP 2004-149351 A

  However, in any of the measures, there is a limit to the carbon removal ability. In addition, the cost of consumables such as activated carbon and silica gel is required separately. As a result, the avoidance of the increase in energy cost in the rectification process is offset, and the economic merit cannot be expected sufficiently.

The object of the present invention is to reduce the deterioration of product quality by economically and efficiently avoiding and mitigating the accumulation of carbon impurities over time in SiHCl ₃ which is a raw material for the production of high purity polycrystalline silicon. An object of the present invention is to provide a polycrystalline silicon manufacturing method that can be avoided at a cost.

  In order to achieve the above object, the present inventor paid attention to the rectification step in the production flow of high-purity polycrystalline silicon, in particular, the function of the distillation column in the former part, and to solve the problem here I found that there is a means.

That is, in the rectification step, as described above, the silicon chloride discharged from the upstream SiHCl ₃ generation step is processed in turn by a plurality of stages of distillation columns connected in series to obtain high-purity SiHCl. _{Get three} . The functions of the distillation columns at each stage are not the same, and the functions are particularly different between the first distillation tower and the second and subsequent distillation towers.

The function of the first distillation column is to separate silicon chloride (mainly a mixture of produced SiHCl ₃ and unreacted SiCl ₄ ) sent from the previous SiHCl ₃ production process into SiHCl ₃ and SiCl _4. It is to be. More specifically, SiHCl ₃ used for producing polycrystalline silicon is extracted from the top of the column, and SiCl ₄ having a higher boiling point is extracted from the bottom of the column. Then, in order to extract the maximum amount of SiHCl ₃ used for the production of polycrystalline silicon and supply it to the downstream side, as a result, only the SiCl ₄ is extracted from the bottom of the column. Driven. As a result, SiHCl ₃ extracted from the top of the column contains impurities such as other silicon chlorides and carbon chlorides, which are removed stepwise by the treatment by the second and subsequent distillation columns. This is the conventional idea for the rectification process.

However, in the conventional rectification process to prioritize productivity of such SiHCl _3, by the second and subsequent stages distillation column is removed during the purification of SiHCl ₃ impurities is returned to the distillation column of the first stage The rectification process is circulated and is not discharged out of the system. As a result, carbon chloride accumulates in the rectification process as the operation progresses. When carbon chloride accumulates in the rectification step, the purification capacity in the second and subsequent distillation towers is increased so that the carbon chloride does not flow out to the downstream polycrystalline silicon production step. Even in the second and subsequent distillation towers, chlorides, which are impurities having a boiling point lower than that of SiHCl ₃ , are discharged out of the system by the vent gas. Therefore, in the rectification process, some of the chlorides except for SiCl ₄ and SiHCl ₃ are used. Things will be contained within a closed cycle, which is the biggest reason for the increased burden in the rectification process.

In contrast, in the upstream side of the SiHCl ₃ generation step, a high temperature or high pressure, or SiCl ₄ at a high temperature and a high pressure state, SiHCl ₃ is produced by the reaction of hydrogen and silicon metal. A compound having a boiling point lower than that of SiHCl ₃ is discharged out of the system as exhaust gas by depressurization. On the other hand, a compound having a boiling point higher than that of SiCl ₄ is discharged out of the system through a high boiling point impurity treatment apparatus. That is, the SiHCl ₃ generation process is an open system, and SiCl ₄ that is a raw material for generating SiHCl ₃ is processed in an open cycle.

Even if the carbon chloride contained in the rectification step is returned to the open SiHCl ₃ production step, the carbon chloride has a boiling point close to that of SiHCl ₃ as described above, so it is not discharged out of the system. , generated again with SiHCl _3, since going back to the rectification process, return of carbon chlorides from the rectification step to SiHCl ₃ generating step was considered meaningless.

However, from a long-term investigation by the present inventor, unlike the conventional idea, the carbon compound returned from the rectification process to the SiHCl ₃ production process changes in form as part of the SiHCl ₃ production reaction, and the boiling point becomes SiHCl _3. A compound with a considerably lower boiling point and a compound with a boiling point much higher than SiCl ₄ is changed, and the former is discharged out of the system as exhaust gas by depressurization, and the latter is discharged out of the system through a high boiling point impurity treatment apparatus. found. Therefore, if a small amount of SiHCl ₃ is returned together with SiCl ₄ from the rectification process to the SiHCl ₃ production process, carbon chloride having a boiling point close to that of SiHCl ₃ can be discharged from the rectification process, and carbon chloride in the rectification process It became clear that accumulation could be prevented.

The polycrystalline silicon production method of the present invention has been completed on the basis of such knowledge, and sends silicon chloride A containing SiHCl ₃ and SiCl ₄ extracted from the SiHCl ₃ production step to the first rectification step, The low-boiling silicon chloride B mainly composed of SiHCl ₃ and the high-boiling silicon chloride C mainly composed of SiCl ₄ are separated into silicon chloride B mainly composed of SiHCl _3. the after sending enhanced SiHCl ₃ purity to the second rectification step, whereas the high purity SiHCl ₃ is supplied as a raw material into the polycrystalline silicon manufacturing process, silicon chloride separated as impurities in the second rectification step D back to the first fractionation step, for silicon chloride C mainly composed of SiCl _4, which in the polycrystalline silicon manufacturing method of returning to the SiHCl ₃ generation step, to SiHCl ₃ generating step The SiHCl ₃ concentration in the silicon chloride C to be returned is 0.1 mol% or more.

In the method for producing polycrystalline silicon according to the present invention, the silicon compound A (mainly a mixture of SiHCl ₃ and SiCl ₄ ) taken out from the SiHCl ₃ production step is sent to the first rectification step, and the low content mainly composed of SiHCl ₃ is obtained. It is separated into a boiling point silicon chloride B and a high boiling point silicon chloride C mainly composed of SiCl ₄ . The low boiling point silicon chloride B mainly composed of SiHCl ₃ is sent to the second rectification process, and after its purity is increased, it is sent to the polycrystalline silicon manufacturing process. The silicon chloride D separated mainly as high-boiling impurities in the second rectification step is returned to the first rectification step, and basically a closed cycle is formed. Silicon chloride C for the SiCl ₄ as a main component is returned to the SiHCl ₃ generating step an open system as yielding feedstock of SiHCl _3, it is processed by the open cycle.

The method for producing polycrystalline silicon according to the present invention mainly uses SiCl ₄ separated from low boiling point silicon chloride B mainly composed of SiHCl ₃ in the first rectification step in such a circulating polycrystalline silicon production flow. When returning the high boiling point silicon chloride C as a component to the preceding SiHCl ₃ production step, a small amount of low boiling point silicon chloride B mainly composed of SiHCl ₃ is mixed in the silicon chloride C. Is. Thereby, together with SiHCl ₃ which is the main component of silicon chloride B, impurities contained therein are also introduced into the open SiHCl ₃ generation step. As described above, the introduced impurities change in form as part of the SiHCl ₃ production reaction, and change into a compound whose boiling point is considerably lower than SiHCl ₃ and a compound whose boiling point is significantly higher than SiCl _4. For example, it is discharged out of the system as exhaust gas by depressurization, and the latter is discharged out of the system through, for example, a high boiling point impurity treatment apparatus.

As a result, a part of impurities such as carbon chloride, which has been treated in a closed loop with the rectification of SiHCl ₃ until now, is discharged out of the system through an open SiHCl ₃ generation process, and the rectification process Accumulation of impurities in this, and thereby an increase in operating costs in the rectification process are avoided. Specifically, when carbon chloride accumulates in the rectification process, this operation reverses the tendency of carbon chloride accumulation to a decreasing trend, and the amount of carbon chloride accumulation in the rectification process rapidly increases. Reduces operating costs. If the accumulation of carbon chloride is eliminated, the operation is stopped and the operation is performed under the original conditions.

As matter of concern, reduction of SiHCl ₃ generation efficiency at SiHCl ₃ generation step due to the fact that part of the SiHCl ₃ generated by the SiHCl ₃ generation process is returned again to the SiHCl ₃ generating step, and a first rectification step There is a decrease in the amount of high-purity SiHCl ₃ supplied due to a decrease in the amount of SiHCl ₃ sent to the second rectification process, but the operation of mixing SiHCl ₃ in silicon chloride C is as described above. However, it may be performed in a limited manner when carbon chloride accumulates, and is not always necessary. Further, the amount of SiHCl ₃ mixed in silicon chloride C may be small. For these reasons, the adverse effects of the mixing operation are negligible.

When SiHCl ₃ is mixed into silicon chloride C, the concentration of SiHCl ₃ in silicon chloride C needs to be 0.1 mol% or more, preferably 0.3 mol% or more. Since the impurity carbon chloride-based in question is typically a high boiling point than SiHCl _3, even a slight SiHCl ₃ concentrations of this order, a sufficient amount of problems impurities is returned to the SiHCl ₃ generating step, 0.3 moles %, The impurity return effect begins to saturate. The concentration of SiHCl ₃ in silicon chloride C, 0.1 mol%, is the turning point for the accumulation and discharge of problematic impurities in the rectification process.

The upper limit of the SiHCl ₃ concentration in the silicon chloride C when mixing SiHCl ₃ into the silicon chloride C is not particularly necessary from the viewpoint of eliminating the retention of impurities, but if the concentration is high, the production efficiency of the entire process decreases. To do. Further, as described above, the impurity return effect begins to saturate at 0.3 mol%. From this viewpoint, the upper limit of the SiHCl ₃ concentration in the silicon chloride C is better, specifically, 5 mol% or less is desirable, 1 mol% or less is more desirable, and 0.5 mol% or less is most desirable.

Incidentally, when the operation of mixing SiHCl ₃ into silicon chloride C is performed intermittently, the concentration of SiHCl ₃ in silicon chloride C during normal operation when the operation is not performed should be small in terms of reaction efficiency. Therefore, it is desirable to control it to less than 0.1 mol% as in the prior art, and 0.05 mol% or less is particularly desirable.

The SiHCl ₃ generation step is not particularly limited as long as the by-product or unreacted SiCl ₄ is discharged together with the generated SiHCl ₃ , as long as this requirement is satisfied. A typical SiHCl ₃ generation step is based on a method of generating SiHCl ₃ by introducing the raw material containing SiCl ₄ gas, hydrogen gas and metal silicon described above into a flow reactor, and in this case, the generated SiHCl _{3 is} generated. At the same time, unreacted SiCl ₄ is discharged. Moreover, it is preferable to use what was isolate | separated and refine | purified from the waste gas of a polycrystalline silicon manufacturing process as the hydrogen gas. Other methods include a method of generating SiHCl ₃ from metal silicon and hydrochloric acid, and a method called clean conversion in which SiHCl ₃ is generated from SiCl ₄ gas and hydrogen gas without using metal silicon. In the former method, SiCl ₄ is by-produced. In the latter method, unreacted SiCl ₄ is discharged together with SiHCl ₃ . In this method, since no metallic silicon is used, the accumulation of carbon chloride is small, but it is not at all. In this respect, the present invention is also effective for the latter method.

As described above, the timing of mixing SiHCl ₃ into silicon chloride C does not preclude continuous operation throughout the entire operation, but is not necessary. The accumulation level of impurities, particularly carbon chloride, in the second rectification step is investigated, and mixing is started when it reaches a predetermined level, and mixing is stopped when the accumulation returns to the elimination level. For example, it may be performed about several tens of days once every several years or ten years. In order to prevent accumulation, mixing may be performed once every several months or once a year. Regardless of the timing of mixing, if the mixing period is converted into the number of days per year, it is preferably 1 to 50 days / year, more preferably 5 to 20 days / year. If it is mixed for 100 days in the 10th year, it means 10 days / year. If SiHCl ₃ is mixed in silicon chloride C more than necessary, the operation efficiency is lowered, and the shortage leads to an increase in operating cost in the second rectification step.

Polycrystalline silicon manufacturing method of the present invention, downstream of the SiHCl ₃ generating step, a silicon chloride A from SiHCl ₃ generating step, a silicon chloride B low boiling mainly containing SiHCl _3, the SiCl ₄ In the first rectification step of separating into high-boiling silicon chloride C as the main component, a small amount of SiHCl ₃ is mixed into silicon chloride C to be returned to the upstream SiHCl ₃ production step. Accumulation of impurities in the rectification process, particularly carbon chloride, which is a problem in the polycrystalline silicon production process, can be prevented, and the operating cost in the second rectification process can be reduced. Since this is performed by the operation in the first rectification process, the equipment cost is unnecessary, and since a small amount of SiHCl ₃ mixed into the silicon chloride C is sufficient, the adverse effect on the production efficiency can be suppressed to an extremely low level. it can.

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a production flow diagram of a method for producing polycrystalline silicon showing an embodiment of the present invention, and FIG. 2 is an explanatory view of an operation of mixing SiHCl ₃ into SiCl ₄ in the first rectification step.

In the present embodiment, high-performance solar panels, high-purity polycrystalline silicon that is used in the production of silicon single crystal or the like is manufactured by Siemens process, and more particularly, as shown in FIG. 1, SiHCl ₃ generation step, The polycrystalline silicon is manufactured by a circulation system including the first rectifying step, the second rectifying step, the polycrystalline silicon manufacturing step, and the hydrogen recovery step.

In the SiHCl ₃ generation step, metal silicon (metallurgical grade: 98 wt% ≦), SiCl ₄ gas and hydrogen gas are supplied to the fluidized furnace, and SiHCl ₃ is generated by the reaction shown in Chemical Formula 1. At this time, unreacted SiCl ₄ gas is also discharged from the fluidized furnace together with SiHCl ₃ gas. As described above, the silicon chloride A contains impurities such as phosphorus, boron, and carbon mainly introduced from metallic silicon. As a result, silicon chloride A mainly composed of a mixture of SiHCl ₃ and SiCl ₄ is taken out from the SiHCl ₃ generation step.

In the SiHCl ₃ production step, the compound having a low boiling point is discharged out of the system as exhaust gas. Conversely, a compound having a high boiling point is discharged out of the system through a high boiling point impurity treatment apparatus or the like. Therefore, these compounds do not accumulate in the SiHCl ₃ production process. In this respect, the SiHCl ₃ generation process is an open system.

The gaseous silicon chloride A derived as a product from the SiHCl ₃ production process is liquefied by a condenser and then sent to the first rectification process. In the first rectification step, liquid silicon chloride A is separated into SiHCl ₃ and SiCl ₄ . This separation operation will be described in detail with reference to FIG.

The greatest purpose of the distillation column (hereinafter referred to as distillation column 2) in the first rectification step is to separate and extract the maximum amount of SiHCl ₃ from silicon chloride A. For this reason, chloride having a boiling point higher than that of SiCl ₄ is present, so that the boiling point of the distillation column 2 is higher than the boiling point of SiCl ₄ , and conversely, chloride having a boiling point lower than that of SiHCl ₃ is present at the top of the column. Therefore, the operation is performed under temperature conditions slightly lower than the boiling point of SiHCl ₃ . As a result, the low boiling point SiHCl ₃ is withdrawn from the top of the column, and the maximum amount of high boiling point SiCl ₄ is withdrawn from the bottom of the column. The circulation of the liquid at the top of the column is the same as in a general distillation column.

More specifically, in the distillation column 2, the introduced chloride is heated from the bottom of the column, and the temperature decreases from the bottom to the top. At a specific temperature T level between the tower bottom and the tower top, the composition ratio X: Y of SiHCl ₃ and SiCl ₄ is determined according to the temperature T. Normally, due to the operating conditions described above, the distribution of SiHCl ₃ and SiCl ₄ in the column is shown by a solid line. As a result, the top liquid contains SiHCl ₃ in silicon chloride A and a small amount of low-boiling impurities. Contains silicon chloride B. On the other hand, the column bottom liquid becomes silicon chloride C containing SiCl ₄ in silicon chloride A and some high-boiling impurities.

Since the silicon chloride C extracted as the bottom liquid from the distillation column 2 in the first rectification step contains a large amount of SiCl ₄ , it is returned to the preceding SiHCl ₃ production step as a production raw material. On the other hand, the silicon chloride B extracted from the distillation column 2 of the first rectification step as the top liquid is sent to the second rectification step to remove impurities contained in a small amount, and high purity SiHCl ₃ and Is done.

In the second rectification step, a plurality of distillation towers 3 connected in series are used in order to increase the purity of SiHCl ₃ . By passing the silicon chloride B through the plurality of distillation columns 3 in order, the purity of SiHCl ₃ in the silicon chloride B is sequentially increased. Thus, high-purity SiHCl ₃ having a purity of, for example, 10 N (99.99999999%) or higher is generated in the second rectification step. The produced high purity SiHCl ₃ is sent to the polycrystalline silicon manufacturing process. Various impurities including carbon chloride which are removed during the purification process of SiHCl _{3 and} are not discharged out of the system are returned to the first rectification step. Among impurities removed in the process of distilling and purifying SiHCl ₃ , chloride which is an impurity having a boiling point lower than that of SiHCl ₃ is discharged out of the system as a vent gas. Other impurities are removed from the first rectification step and the first rectification step. In the rectification process, a closed cycle is formed and sequentially accumulated.

In the polycrystalline silicon manufacturing process, high-purity polycrystalline silicon used for manufacturing a silicon single crystal or the like is manufactured by a method called a Siemens method. In the production of polycrystalline silicon by the Siemens method, a mixed gas of high-purity SiHCl ₃ and hydrogen sent from the second rectification step is used.

Unreacted SiHCl ₃ , by-products SiCl _4, hydrogen, and the like are discharged from the polycrystalline silicon manufacturing process. This exhaust gas is sent to the hydrogen recovery process. In the hydrogen recovery process, hydrogen in the exhaust gas introduced from the polycrystalline silicon manufacturing process is separated. The separated hydrogen is supplied as a raw material gas to the polycrystalline silicon manufacturing process or further to the SiHCl ₃ generation process. On the other hand, the separated silicon chloride E is returned to the distillation step including the first rectification step and the second rectification step and is recycled.

In such a polycrystalline silicon manufacturing method, a part of the impurities removed by the distillation purification operation of SiHCl _{3 in} the second rectification step is returned to the first rectification step, and the refinement is performed as the operation continues. It accumulates in the distillation process and increases. In the second rectification process, if the impurities increase, the operation capacity is increased and SiHCl ₃ having a predetermined purity is continuously supplied to the polycrystalline silicon manufacturing process. For this reason, the operation cost in a 2nd rectification process increases with progress of operation.

In the present embodiment, the carbon chloride concentration is monitored in the rectification process in order to solve the problem of the increase in operating cost. For example, the carbon concentration in the bottom liquid of the first distillation column 3 in the second rectification step is monitored. When the carbon chloride concentration in the rectification process exceeds the allowable value, SiHCl ₃ is forced into silicon chloride C (mainly SiCl ₄ ) returned from the first rectification process to the SiHCl ₃ production process. Mixed in. Specifically, the distillation column 2 is operated under the condition that the SiHCl ₃ concentration in the silicon chloride C sent to the SiHCl ₃ production step is 0.1 mol% or more, preferably 0.3 mol% or more. This driving operation will be described with reference to FIG.

As described above, under normal operating conditions in the distillation column 2, the concentration distributions of SiHCl ₃ and SiCl ₄ in the column are shown by solid lines. Specifically, since the distillation tower 2 is heated at the bottom of the tower, the temperature in the tower decreases as it goes from the bottom to the top. The ratio (X: Y) of SiHCl ₃ and SiCl ₄ at the level of the specific temperature T is constant. Under normal operating conditions, SiCl ₄ which is the bottom liquid of the distillation column 2 does not contain SiHCl _3, but when the temperature at point A in the column is lowered, for example, the concentration distribution of SiHCl ₃ and SiCl ₄ is Shift as shown by the dashed line. As a result, SiHCl ₃ is mixed into silicon chloride C, which is the tower bottom liquid, and the SiHCl ₃ concentration in silicon chloride C increases.

As a simpler method, the amount of silicon chloride A supplied to the distillation column 2, that is, the feed amount is increased. The amount of silicon chloride B sent from the top of the distillation column 2 to the second rectification step is constant, whereas the amount of silicon chloride C returned from the bottom of the distillation column 2 to the SiHCl ₃ production step is Since the feed amount increases as the feed amount increases, the amount of SiHCl ₃ in the distillation column 2 increases as the feed amount increases, and the concentration distribution of SiHCl ₃ and SiCl ₄ is a solid line as in the case of the temperature operation described above. As a result, the concentration of SiHCl ₃ in silicon chloride C, which is the bottom liquid, increases. As another method, the reflux amount of the top liquid in the distillation column 2 is increased. In this case as well, the amount of SiHCl ₃ in the distillation column 2 increases, and the concentration distribution of SiHCl ₃ and SiCl ₄ from the solid line. By shifting to the broken line, the concentration of SiHCl ₃ in silicon chloride C increases.

  Needless to say, these methods may be used in combination.

When SiHCl ₃ is mixed into silicon chloride C by such a method, carbon chloride having a slightly higher boiling point than SiHCl ₃ is also mixed into silicon chloride C and sent to the SiHCl ₃ generation step on the upstream side. The SiHCl ₃ generation step is an open system that discharges a compound having a boiling point higher than SiCl ₄ and a compound having a lower boiling point than SiHCl ₃ as described above, and the carbon chloride returned to the SiHCl ₃ generation step is as described above. As a part of the SiHCl ₃ formation reaction, the form changes, the compound has a boiling point considerably lower than SiHCl ₃ , and the boiling point changes to a compound much higher than SiCl ₄ , and the compound having a boiling point lower than SiHCl ₃ goes out of the system as exhaust gas. Discharged. On the contrary, a compound having a boiling point higher than that of SiCl ₄ is discharged out of the system through a high boiling point impurity treatment apparatus or the like. As a result, the carbon impurity concentration in the rectification step gradually decreases.

When the carbon chloride concentration in the rectification process is restored, the operating conditions in the distillation column 2 are restored, and mixing of SiHCl ₃ into the silicon chloride C is stopped. Thereby, the amount of SiHCl ₃ sent to the second rectification process is also recovered.

  By repeating this during the operation period, an increase in operating cost due to accumulation of impurities in the second rectification step is avoided.

  Next, the effectiveness of the present invention will be clarified with specific examples.

  Semiconductor grade polycrystalline silicon was produced according to the polycrystalline silicon production flow shown in FIG. During the production period, the carbon concentration in the bottom liquid of the first distillation column in the second rectification step was measured to monitor the accumulation of carbon impurities. The carbon concentration, which was initially 5-7 ppm, became 8-10 ppm after 1 year, and 10-12 ppm after 2 years.

  The amount of carbon impurities accumulated in the rectification step is preferably 10 ppm or less, more preferably 1 ppm or less, expressed as the carbon concentration in the bottom liquid of the first distillation column in the second rectification step. When this accumulation amount increases, the operating cost in the second rectification process increases.

In this example, when the carbon concentration reaches 15 ppm or more, the operating condition of the distillation column 2 in the first rectification process is determined by increasing the amount (feed amount) of silicon chloride A supplied to the distillation column 2. 2, the concentration of SiHCl ₃ in silicon chloride C returned from the distillation column 2 to the SiHCl ₃ production step was increased from less than 0.1 mol% to 0.3 mol%. The change over time in the carbon concentration after the start of this operation is shown by a solid line in FIG. The horizontal axis indicates the number of days elapsed, and the vertical axis indicates the carbon concentration in the bottom liquid of the first distillation column in the second rectification step, and the carbon concentration when the operating conditions are changed as 100.

  For reference, the change over time in the carbon concentration when the operation is continued without changing the operating conditions of the distillation column in the first rectification step is shown by a broken line in FIG. Gradually, the carbon concentration in silicon chloride C continues to increase.

On the other hand, when the concentration of SiHCl ₃ in silicon chloride C is changed from less than 0.1 mol% to 0.3 mol%, the amount of carbon impurities accumulated in the second rectification step is the point at which the operation starts. Therefore, the increasing trend reverses to a sharp decreasing trend, halves in about 20 days, becomes 1/10 in about 50 days, and almost returns to the original level in about 70 days. In this example, this state was continued for about half a year. As a result, the carbon concentration in the bottom liquid of the first distillation column in the second rectification process is 1/100 or less at the start of the operation, and the energy intensity in the entire rectification process is 70% or less than the conventional unit. Could be reduced to

In addition, the change over time in the carbon concentration when the SiHCl ₃ concentration in the silicon chloride C returned from the first rectification step to the SiHCl ₃ production step is increased from less than 0.1 mol% to 0.5 mol%. This is indicated by a one-dot chain line in FIG. In this case, the carbon concentration is halved in about 10 days, becomes 1/10 in about 40 days, and returns to the original level around 60 days. As a result of continuing this state for about three months, the carbon concentration in the bottom liquid of the first distillation column in the second rectification step is 1/100 or less at the start of operation, and the energy in the entire rectification step is reduced. The basic unit could be reduced to 70% or less.

In addition, the change over time in the carbon concentration when the SiHCl ₃ concentration in the silicon chloride C returned from the first rectification step to the SiHCl ₃ production step is increased from less than 0.1 mol% to 0.15 mol%. This is indicated by a two-dot chain line in FIG. In this case, the carbon concentration is halved in about 50 days, becomes 1/10 in about 150 days, and returns to the original level around 200 days. As a result of continuing this state for about one year, the carbon concentration in the bottom liquid of the first distillation column in the second rectification step is 1/100 or less at the start of the operation, and the energy source in the entire rectification step is The unit could be reduced to 70% or less than the conventional unit.

When the SiHCl ₃ concentration in the silicon chloride C returned from the first rectification process to the SiHCl ₃ production process is as low as about 0.1 to 0.2 mol, the decrease in carbon impurities is slow, The concentration of SiHCl _{3 in the} case of carrying out this operation regularly for preventing the accumulation of substances is sufficient to be 0.1 to 0.2 mol.

It is a flowchart of the polycrystalline silicon manufacturing method which shows one Embodiment of this invention. Is an illustration of SiHCl ₃ mixed operation to SiCl ₄ in the first rectification step. It is a graph which shows the decreasing tendency of the carbon impurity amount from mixing operation start. It is a flowchart of the conventional polycrystalline silicon manufacturing method.

Explanation of symbols

1 High boiling point impurity treatment equipment 2, 3 Distillation tower

Claims (5)

The silicon chloride A containing SiHCl ₃ and SiCl ₄ taken out from the SiHCl ₃ production step is sent to the first rectification step, and low boiling point silicon chloride B mainly composed of SiHCl ₃ and SiCl ₄ as the main component. Separating into high boiling silicon chloride C,
The silicon chloride B to the SiHCl ₃ as a main component, which after enhanced SiHCl ₃ Purity sent to the second rectification step, while supplying a raw material for producing the high purity SiHCl ₃ into the polycrystalline silicon manufacturing process The silicon chloride D separated as an impurity in the second rectification step is returned to the first rectification step,
For silicon chloride C mainly composed of SiCl ₄ , in the polycrystalline silicon manufacturing method for returning this to the SiHCl ₃ production step,
A method for producing polycrystalline silicon, wherein the concentration of SiHCl ₃ in silicon chloride C returned to the SiHCl ₃ production step is 0.1 mol% or more. 2. The method for producing polycrystalline silicon according to claim 1, wherein the SiHCl ₃ concentration in the silicon chloride C returned to the SiHCl ₃ production step is changed from a normal concentration of less than 0.1 mol% to 0.1 mol% during the operation period. The polycrystalline silicon manufacturing method which reverses the accumulation tendency of carbon chloride in the 2nd rectification process to a decreasing tendency by making it high above. _{3. The} method for producing polycrystalline silicon according to claim 1, wherein the SiHCl ₃ generation step is an open system in which a compound having a lower boiling point than SiHCl ₃ and a compound having a higher boiling point than SiCl ₄ are discharged out of the system. Production method. _{4. The} method for producing polycrystalline silicon according to claim 1, wherein in the SiHCl ₃ generation step, a raw material containing SiCl ₄ gas, hydrogen gas and metal silicon is introduced into a fluidized reactor to generate SiHCl _3. Crystalline silicon manufacturing method.   5. The method for producing polycrystalline silicon according to claim 4, wherein the hydrogen gas is hydrogen gas separated and purified from the exhaust gas in the polycrystalline silicon production process.

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