Process and plant that decomposes monosilane (schmid silicon technology) national gas average 2012

High-purity silicon is generally produced in a multistage process starting from metallurgical silicon generally still comprising a relatively high proportion of impurities. To purify the metallurgical silicon it can, for example, be converted into a trihalogensilane such as trichlorosilane (SiHCl3) that is subsequently thermally decomposed to afford high-purity silicon. Such a procedure is known from DE 29 19 086 A1, for example. Alternatively, high-purity silicon may also be obtained by thermal decomposition of monosilane (SiH4) as is described in DE 33 11 650 A1, for example.

In recent years, obtaining highest-purity silicon by thermal decomposition of monosilane has increasingly come to the fore. Thus, for example, DE 10 2011 089 695 A1 and DE 10 2009 003 368 B3 disclose reactors into which monosilane may be injected and in which highly heated silicon rods on which the monosilane is decomposed are arranged. The silicon generated is deposited in metallic form on the surface of the silicon rods.

So that the deposition may be better controlled, it is customary to inject into reactors such as that described in DE 10 2009 003 368 B3, for example, a mixture of monosilane and a carrier gas such as hydrogen rather than pure monosilane. However, care must be taken to ensure that this gas mixture does not become excessively hot. Above a temperature of 400° C. there is a danger of decomposition of the monosilane occurring even in the gas phase which can result in intensified formation of undesired byproducts. To avoid this, the concentration of monosilane in the mixture is normally kept very low.

In practice, the temperature of the gas mixture inside a reactor is very difficult to control since large temperature gradients exist inside the reactor. These problems are further intensified when the amount of the gas mixture injected into the reactor is increased to deposit larger amounts of silicon. In modern reactors, the target throughput of monosilane-containing gas mixture may be more than 10000 Nm3 (standard cubic meters).

Such a high throughput can easily lead to turbulent flows inside the reactor that results in an undesirably efficient heat exchange between the gas mixture and the highly heated silicon rods occurring. This brings about the mentioned undesired increase in the temperature of the gas mixture. The silicon rods are also cooled. This results in increased energy consumption.

Such disadvantages have hitherto been accepted since turbulent flows are certainly also associated with positive effects. It is desired that silicon be deposited as evenly as possible on all regions of the mentioned highly heated silicon rods. Turbulizing the gas mixture injected into the reactors is regarded as conducive thereto.

It could therefore be helpful to provide a process for thermal decomposition of monosilane where even at a throughput of monosilane-containing gas mixture of more than 10000 Nm3 the known problems occur only to a comparatively small extent, if at all. SUMMARY

We provide a method that decomposes monosilane where a monosilane-containing gas stream is circulated in a circuit system including a reactor that decomposes the monosilane present in the gas stream, the method including (1) injecting a monosilane-containing gas stream into the reactor, (2) bringing the gas stream into contact with a heated surface inside the reactor at which surface a portion of the monosilane present in the gas stream is decomposed to deposit a solid silicon layer on the surface so that the concentration of the monosilane in the gas stream decreases, (3) discharging the gas stream from the reactor, (4) reprocessing the gas stream including at least partially compensating the decrease in the monosilane concentration resulting from the decomposition by addition of monosilane, and (5) reinjecting the reprocessed, monosilane-containing gas stream into the reactor according to step (1), wherein during deposition inside the circuit system an operating pressure of 2.5 bar to 10 bar (absolute) is established and the gas stream enters the reactor at a velocity of less than 7.5 m/s.

We also provide a plant that decomposes monosilane in accordance with the method including (1) a circuit system in which a monosilane-containing gas stream can be circulated at an operating pressure of 2.5 bar to 10 bar, (2) a reactor which is part of the circuit system and includes a device that injects the monosilane-containing gas stream into the reactor and an outlet that discharges the gas stream from the reactor, (3) a supply that provides a highly heated silicon surface inside the reactor, (4) a cooler that cools the gas stream discharged from the reactor, (5) a reprocessor that reprocesses the gas stream, including a device that increases the concentration of monosilane in the gas stream, and (6) a circulator of the gas stream at the operating pressure.

(1) In this step, a monosilane-containing gas stream is injected into the reactor. Similarly to known processes, the gas stream is preferably a gas mixture composed of a carrier gas and monosilane. The carrier gas used is particularly preferably hydrogen.

(2) In this step, the gas stream is brought into contact with a highly heated surface inside the reactor. A portion of the monosilane present in the gas stream decomposes at this surface. The concentration of monosilane in the gas stream thus decreases. As a result of the decomposition, a solid silicon layer is deposited on the highly heated surface. This layer is the desired product of the process.

(3) In this step, the gas stream is discharged from the reactor. The gas stream discharged from the reactor has a markedly lower concentration of monosilane than the gas stream injected into the reactor. The gas stream moreover has a markedly higher temperature than when it was injected into the reactor.

(4) In this step, the gas stream discharged from the reactor is reprocessed. The reprocessing process comprises in particular at least partially compensating, preferably fully compensating, the decrease in the monosilane concentration resulting from decomposition. This is effected by appropriate addition of monosilane. Reprocessing of the gas stream may moreover preferably comprise additional purification and cooling steps.