Reactor for the separation of silicon from a process gas

Abstract

Reactor for the deposition of silicon from a process gas, comprising a double-walled reactor bell (12) made of stainless steel mounted on a base plate (48), the bell having an outer wall (16) and an inner wall (18) forming an intermediate space (20) supplied with cooling medium, wherein current-carrying support rods (30) for the crystallization of pure silicon from the vapor phase are mounted on the base plate (48) within a process chamber (28) enclosed by the reactor bell (12), characterized in that a double-walled inner cylinder (32) is arranged in the process chamber (28) at a distance (42) from the inner wall (18) of the reactor bell (12) and at a distance above the base plate (48), the annular space of which within the double wall of the inner cylinder (32) is supplied with a cooling medium and the inner surface of which, facing into the process chamber (28), bears an inert coating (34) which preferably consists of silver.

Description

[0001] The invention relates to a reactor for the separation of silicon from a process gas, comprising a double-walled reactor bell made of stainless steel mounted on a base plate, with an outer wall and an inner wall forming an intermediate space supplied with cooling medium, wherein current-carrying support rods for the crystallization of pure silicon from the vapor phase are mounted on the base plate within a process space enclosed by the reactor bell.

[0002] Polysilicon is the basic material for semiconductors and solar cells. A purity of 99.999999% is required, corresponding to a residual content of 1 × 10⁻⁹. This purity cannot be achieved by reduction from the purest quartz, but is instead obtained by converting the metallurgically produced silicon into trichlorosilane (TCS) and distilling it. The metallic silicon must then be recovered in polycrystalline form by chemical vapor deposition on glowing rods of ultrapure silicon, known as slim rods, at a temperature of approximately 1000°C. This vapor phase crystallization is a crucial purification step in the manufacturing process, making it essential that the temperature distribution across the rod surface is as homogeneous as possible to ensure consistent and defined crystal growth rates.If, for example, it gets too hot at the top of the rods, then the polycrystalline growth transitions into so-called popcorn growth, which absorbs significantly more foreign atoms.

[0003] The three chlorine atoms separated per molecule during deposition at the polycrystalline surface combine with three neighboring TCS molecules to form silicon tetrachloride (STC). At the deposition temperature of TCS, this is not reduced at the polycrystalline surface and, with increasing concentration, slows down the deposition process.

[0004] Therefore, the STC must be removed from the gas space (process chamber). This is usually done by replacing all the gas, whereby the proportion of TCS contained in the exhaust gas is still about 80% of the TCS contained in the reactant gas.

[0005] From DE 28 54 707 A1 and US 4 173 944 A of Wacker-Chemitronic, it is known that coating the inner wall of the bell housing with silver protects the inner walls of Siemens-type reactors from corrosion while simultaneously saving heating energy. However, this requires a subsequent and therefore complex coating of the existing bell housing in reactors already in operation.

[0006] The invention is based on the objective of designing a reactor of the type outlined above in such a way that, in order to reduce power loss and corrosion, a subsequent and therefore costly finishing of the reactor inner surface with highly reflective coatings inert to the process atmosphere, for example silver or gold, hereinafter referred to as coating, is not necessary, so that the stainless steel bell wall can be protected from corrosion, e.g. by siliconization and pitting corrosion by chlorine, with a relatively simple measure.

[0007] To solve this problem, according to the characterizing feature of claim 1, an installation component, preferably in the form of a double-walled inner cylinder, is arranged in the process chamber of the reactor bell, which, due to the high pressure, is made of relatively thick stainless steel. This inner cylinder is also made of stainless steel and has an inert coating, preferably of silver or gold, on its inner surface facing the process chamber. A cooling medium, for example water or thermal oil, flows through the annular space within the double wall of the inner cylinder.

[0008] The inner cylinder is positioned at a distance above the base plate and from the inner wall of the reactor bell, creating a relatively narrow, annular gap between the inner cylinder and the reactor bell. This gap results in a lower temperature of the process gas flowing through it, as the cooling surface area more than doubles. With appropriate temperature control of the cooling water in the double-walled bell, a surface temperature in the lower part can be achieved between the boiling point of TCS at operating pressure and that of STC, leading to the condensation of STC. This condensate can be collected in a trough or similar structure and discharged from the reactor in liquid form.

[0009] The coating of the inner cylinder with silver or other highly reflective metals that are inert to the process atmosphere, as provided for in the invention, results in significantly lower manufacturing costs compared to the coating of the bell wall due to the simple geometry and requires less effort for maintenance and repair after possible damage.

[0010] The coating on the inner wall of the inner cylinder reflects the visible radiation from the support rods inwards, contributing to energy savings because less electricity is required to heat the support rods. Furthermore, the coating on the inside of the inner cylinder protects the outer bell of the reactor, keeping the temperature there below 250°C and thus preventing any reaction between the TCS and the nickel contained in the stainless steel. At temperatures above 250°C, nitrogen silicide is formed, causing the nickel to turn black, which results in increased light absorption and a build-up effect.

[0011] The coating of the inner cylinder with a reflectivity of more than 90% for wavelengths between 400 nm and 900 nm, emitted by the glowing silicon, means that the reflected visible waves are hardly attenuated, but rather reabsorbed by the rods upon re-entry. This reduces the power required for heat generation inside the rods. As a result, the energy required by the reactor for deposition is significantly reduced.

[0012] The manufacturing process can proceed as follows: first, two thin-walled stainless steel plates are spot-welded together, and the resulting plate is coated on one side. It is then rolled into a cylinder, whereupon, for example, using the pillow plating process by Omega Thermo Products (see, e.g., US 2011 / 0 100 212 A1), high pressure is built up between the two plates, leading to cavity formation. However, electroplating the coating onto an already rolled cylinder is also significantly more economical due to the smaller electrolyte volumes and simpler bath design. The cylinder is then installed as an inner cylinder (so-called CVD intensifier) ​​in the existing reactor bell. This simplifies and reduces the cost of repairs.

[0013] According to another feature of the invention, the inner cylinder is suspended on an intermediate flange on the reactor lid by means of pipes which also serve for the supply and removal of the cooling water.

[0014] It is particularly advantageous if the support rods adjacent to the inside of the inner cylinder are arranged at a distance from the inside that is greater than the maximum thickness of the silicon layer growing on the support rods. This prevents contact between the two components and thus a short circuit.

[0015] Further features and advantages of the invention will become apparent from the following description of an exemplary embodiment, which is illustrated in the drawing. In this drawing: Fig. 1 a longitudinal section through a reactor according to the invention and Fig. 2 a cross-section through the reactor of the Fig. 1, where the support rods are visible.

[0016] The reactor 10 shown in the figures has an outer, double-walled reactor bell 12. This is bolted to a base plate 48 via a flange 14 and has a cylindrical outer wall 16 and a cylindrical inner wall 18, which form an intermediate space 20 through which a cooling medium flows. The reactor bell 12 is closed by a domed roof 22 and has windows 52 for visual monitoring of the ongoing process.

[0017] The inner wall 18, made of stainless steel (e.g., V4), is designed with a relatively thick wall because the cooling water in the space 20 of the reactor 10 is subject to a pressure of at least 2.7 bar. This pressure is caused by the water temperature being, for example, at least 130°C, in order to prevent the condensation of the process gases at a process gas pressure of, for example, 600 kPa within the reactor 10. However, the thickness of the inner wall 18 leads to higher temperatures on the inside, which in turn cause corrosion.

[0018] Through a connection nozzle 24 in the lower area of ​​the outer wall 16, the cooling medium, for example water, is directed into the space 20 between the outer wall 16 and the inner wall 18 and discharged again in the upper area via a discharge nozzle 26.

[0019] Out of Fig. Figure 2 shows that on the base plate 48 of the reactor type used as an example, tubular support electrodes 30 for receiving the thin rods are attached in a known manner within a process chamber 28 enclosed by the reactor bell 12. The already grown rods 30, shown in cross-section, are connected in a known manner at their upper ends in a U-shape and connected in pairs to a power supply, so that they reach temperatures of over 1000°C and silicon is deposited on them, which is deposited from the process gas TCS at the high temperature. After the desired growth of the deposited Si layer, the pure silicon can be harvested.

[0020] According to the invention, a double-walled inner cylinder 32 (“CVD Intensifier”) is arranged in the process chamber 28 at a distance 42 from the inner wall 18 of the reactor bell 12, the inner surface of which carries the inert coating 34, preferably silver. At its upper end, as Fig. Figure 1 shows that the inner cylinder 32 is attached via pipes 36 to an additional flange 44 between the roof 22 and the cover flange. The cooling medium is also supplied to and discharged from the annular space 40 within the double wall of the inner cylinder 32 via these pipes 36 and the additional flange 44, as indicated by the two arrows.

[0021] In the roof 22 a discharge nozzle 38 is formed, which leads to a purge gas outlet 50.

[0022] In Fig.2 can be seen that the inner wall 34 of the inner cylinder 32, which is coated with silver, has a distance to the adjacent support rods 30 that is greater than the maximum expected thickness of the silicon layer growing on the support rods 30, so that it cannot come into contact with the coating 34 of the inner cylinder 32.

[0023] The invention makes it possible to separate a portion of the STC from the process gas TCS in reactor 10 due to its higher boiling point. This is achieved by condensing the STC from the gas flowing between the outer wall of the inner cylinder 32 and the inner wall 18 of the reactor bell 12. The temperature of the flowing gas is lower there because the cooling surface area is almost doubled. Furthermore, with appropriate temperature control of the cooling water in the space 20 of the bell wall, a surface temperature between the boiling point of TCS at operating pressure and that of STC can be reached in the lower part, causing the latter to condense. The STC condensate can then be collected and discharged in liquid form from the reactor 10, for example, in a collecting device 46 designed as a trough.

Claims

[1] Reactor for the separation of silicon from a process gas, comprising a double-walled reactor bell (12) made of stainless steel mounted on a base plate (48) with an outer wall (16) and an inner wall (18) forming an intermediate space (20) supplied with cooling medium, wherein current-carrying support rods (30) for the crystallization of pure silicon from the vapor phase are mounted on the base plate (48) within a process space (28) enclosed by the reactor bell (12), characterized by , that in the process chamber (28) at a distance (42) from the inner wall (18) of the reactor bell (12) and at a distance above the base plate (48) a double-walled inner cylinder (32) is arranged, the annular space of which within the double wall of the inner cylinder (32) is permeated by a cooling medium and the inner surface of which, facing into the process chamber (28), bears an inert coating (34) which preferably consists of silver. [2] Reactor according to claim 1, characterized by, that the inner wall of the inner cylinder (32) consists of a thin-walled stainless steel plate which carries the coating (34) and is rolled cylindrically after coating. [3] Reactor according to any one of the preceding claims, characterized by , that the inner cylinder (32) is connected via pipes (36) to an intermediate flange (44) for the supply and discharge of the cooling medium. [4] Reactor according to any one of the preceding claims, characterized by , that a device (46) for collecting and diverting liquid STC is attached to the inner wall (18) of the reactor bell (12). [5] Reactor according to any one of the preceding claims, characterized by , that the support rods (30) adjacent to the inside of the inner cylinder (32) are arranged at a distance to the inside bearing the coating (34) which is greater than the maximum thickness of the silicon layer growing on the support rods (30).

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