DE102013214800A1 - Device for insulating and sealing electrode holders in CVD reactors - Google Patents

Abstract

The present invention relates to a device for insulating and sealing electrode holders in CVD reactors, comprising an electrode suitable for receiving a filament rod on an electrode holder of an electrically conductive material, which is mounted in a recess of a bottom plate, wherein between the electrode holder and bottom plate electrically insulating Ring of a material having a specific thermal conductivity at room temperature of 0.2-200 W / mK, a continuous temperature resistance greater than or equal to 300 ° C and a specific electrical resistance at room temperature of greater than 10 ^ 9 Ωcm provided, the electrically insulating Ring comprises grooves in which two O-rings of an elastomeric material are fixed, and a method for producing polycrystalline silicon.

Description

The invention relates to a device for the insulation and sealing of electrode holders in a reactor for the deposition of polycrystalline silicon and to a method for the production of polycrystalline silicon by means of such a device.

High purity silicon is usually produced by means of the Siemens process. In this case, a reaction gas containing hydrogen and one or more silicon-containing components in the reactor, equipped with the heated by the direct passage of current carrier bodies, initiated at which deposits Si in solid form. The silicon-containing compounds used are preferably silane (SiH ₄ ), monochlorosilane (SiH ₃ Cl), dichlorosilane (SiH ₂ Cl ₂ ), trichlorosilane (SiHCl ₃ ), tetrachlorosilane (SiCl ₄ ) or mixtures thereof.

Each support body usually consists of two thin filament rods and a bridge, which usually connects adjacent rods at their free ends. Most commonly, the filament rods are made of monocrystalline or polycrystalline silicon, less frequently metals or alloys or carbon are used. The filament rods are mounted vertically in the electrodes located at the bottom of the reactor via which the connection to the electrode holder and power supply takes place. High-purity polysilicon deposits on the heated filament rods and the horizontal bridge, causing their diameter to increase over time. After the desired diameter is reached, the process is terminated.

The silicon rods are held in the CVD reactor by special electrodes, which are usually made of graphite. In each case two filament rods with different voltage polarity at the electrode holders are connected at the other end of the thin rod with a bridge to a closed circuit. Electrical energy is supplied to the heating of the thin rods via the electrodes and their electrode holders. The diameter of the thin rods increases. At the same time, the electrode, beginning at its tip, grows into the rod base of the silicon rods.

After reaching a desired nominal diameter of the silicon rods of the deposition process is terminated, the silicon rods cooled and removed.

Of particular importance here is the protection of the electrode holder guided through the base plate. For this purpose, the use of Elektrodendichtungsschutzkörpern has been proposed, with particular importance of the arrangement and the shape of the electrode seal protection body and the material used.

There is an annular body between the head of the electrode holder projecting into the deposition unit and the bottom plate. This usually has two functions: a seal of the implementation of the electrode holder and an electrical insulation of the electrode holder to the bottom plate out.

Due to the high gas space temperature in the CVD reactor, thermal protection of a hydrocarbon-based sealing body is necessary. Inadequate thermal protection means premature wear of the sealing body by fouling of the sealing body, thermally induced flow of the sealing body, leakage of the reactor, below the minimum distance between the electrode holder and bottom plate and earth leakage to charred sealing bodies. Earth leakage or leaks have a failure of the separation plant and thus a termination of the deposition process result. This causes a reduced yield and higher costs.

Out US 20110305604 A1 It is known to shield the seals of the electrodes by means of protective rings made of quartz against thermal stress. The reactor bottom has a special design. The reactor bottom comprises a first region and a second region. The first region is formed by a plate facing the interior of the reactor and an intermediate plate carrying the nozzles. The second region of the reactor bottom is formed by the intermediate plate and a bottom plate carrying the supply terminals for the filaments. In the first region thus formed, the cooling water is passed, so as to cool the reactor bottom. The filaments themselves sit in a graphite adapter. This graphite adapter engages in a graphite clamping ring, which itself interacts with the plate via a quartz ring. The cooling water connections for the filaments can be designed in the form of quick couplings.

WO 2011116990 A1 describes an electrode holder with a Quarzabdeckring. The process chamber unit consists of a contact and clamping unit, a base element, a Quarzabdeckscheibe and a Quarzabdeckring. The contact and clamping unit consists of several, relatively movable contact elements, which form a receiving space for a thin silicon rod. The contact and clamping unit can be inserted into a corresponding receiving space of the base element, which narrows when introduced into the basic element of the receiving space for the thin silicon rod, and this is thereby securely clamped and electrically contacted. The The base member also has a lower socket for receiving a contact tip of the lead-through unit. The Quarzabdeckscheibe has central openings for passing the contact tip of the feedthrough unit. The quartz cover ring is dimensioned such that it can at least partially radially surround a region of the lead-through unit lying within a process chamber of a CVD reactor.

Since quartz has a low thermal conductivity, however, these components become so hot in deposition conditions that grow on their surface a thin silicon layer with high temperature. Under these conditions, the silicon layer is electrically conductive, resulting in a ground fault.

WO 2011092276 A1 describes an electrode holder in which the sealing element between the electrode holder and bottom plate is protected by a rotating ceramic ring from temperature influences. Several electrodes are mounted in a bottom of the reactor. The electrodes carry filament rods, which sit in an electrode body and via which the power is supplied to the electrodes or filament rods. The electrode body itself is mechanically biased with several elastic elements towards the top of the bottom of the reactor. Between the top of the bottom of the reactor and a parallel to the top of the bottom ring of the electrode body, a radially encircling sealing element is used. The sealing element itself is shielded in the region between the top of the bottom of the reactor and the parallel ring of the electrode body of a ceramic ring. The sealing element is made of PTFE and at the same time assumes the sealing and insulating function. The ceramic ring serves as a heat shield for the sealing ring. However, a thermal load of PTFE above 250 ° C leads to smearing / cracking on the seal surface as well as to the flow of the sealing body. As a result, the distance between the head of the electrode holder and bottom plate is below a minimum distance and leads to electrical flashovers / ground faults of the electrode holder to the bottom plate. The fouling / cracking also releases carbon compounds, which lead to contamination of the deposited silicon rods by incorporation of carbon.

US 20130011581 A1 discloses a device for protecting electrode supports in CVD reactors, comprising an electrode suitable for receiving a filament rod on an electrode holder of an electrically conductive material, which is mounted in a recess of a bottom plate, wherein a gap between the electrode holder and bottom plate is sealed with a sealing material and the sealing material is protected by a monolithic or multi-part construction, annularly arranged around the electrodes protective body, the protective body increases in the direction of the electrode holder at least in sections in its height. In this case, geometric bodies are provided in concentric arrangement around the electrode holder, the height of which decreases with increasing distance from the electrode holder. It can also be a one-piece body. This serves for the thermal protection of the sealing and insulating body of the electrode holder as well as a flow modification at the rod base of the deposited polysilicon rods, which positively influences the rate of inversion.

According to the devices WO 2011092276 A1 and after US 20130011581 A1 It may come between the electrode holder and bottom plate to a ground fault due to silicon splinters that flake due to thermal stresses due to the high feed throughput of the silicon rods, fall between the electrode holder and ceramic ring / protective body and establish an electrically conductive connection between the electrode holder and bottom plate. Short circuits mean an abrupt end to the process due to power failure to heat the bars. The rods can not be deposited to the intended final diameter. With thinner bars, the plant capacity is lower, which causes considerable costs.

CN 202193621 U discloses an apparatus in which two ceramic rings are provided between the head of the electrode holder and the bottom plate with a flat graphite gasket therebetween. However, there is no sealing function between the ceramic ring and the head of the electrode holder and between the ceramic ring and the bottom plate. Leaks in the reactor are the result.

CN 101565184 A discloses a zirconia ceramic (ZrO2) insulating ring between the head of the electrode support and the bottom plate. The insulating ring is sunk in the bottom plate. Therefore, an additional quartz ring is required for insulation between the head of the electrode holder and the bottom plate. The seal is made by means of two flat graphite gaskets between the head of the electrode holder and insulating ring and between the bottom plate and insulating ring. As an additional seal, an O-ring is used on the electrode feedthrough below the bottom plate.

CN 102616783 A discloses a ceramic insulating ring between the head of the electrode support and the bottom plate. The seal is made by means of two metal-embedded graphite flat seals above and below the insulating ring to the head of the electrode holder or to the bottom plate.

In the two last-mentioned documents, the problem is that a high surface pressure for the graphite flat gasket is necessary for sealing. Since ceramic is brittle and has a low flexural strength, high demands are placed on the flatness of the sealing surfaces of the bottom plate and head of the electrode holder. For the smallest unevenness, which can hardly be avoided in practice, the ceramic rings are broken due to the high surface pressure. Leaks in the reactor are the result.

US 2010058988 A1 provides a fixation of the electrode holder in the bottom plate by a conical PTFE sealing and insulating element. The conical PTFE sealing element is pressed on its upper side via a collar (cross-sectional widening) on the electrode holder. In addition, an O-ring is provided in each case between the sealing element and the electrode leadthrough through the base plate and between the sealing element and the shaft of the electrode holder. By pressing the conical sealing element, the removal of the electrode holder is difficult. By flowing the PTFE sealing body, the minimum distance between electrode holder and bottom plate can be undershot. Electric flashovers / ground faults are the result.

From this problem, the task of the invention resulted.

It should be an insulating / sealing body are found for sealing and insulation between the electrode holder and bottom plate with sufficient temperature resistance to smearing and cracking and high dimensional stability to avoid setting behavior of the insulating / sealing body.

The invention provides to separate sealing and insulating body or to divide sealing and insulating functions on two components, wherein an insulating ring for the electrical insulation and a sealing member are provided for the seal.

This makes it possible to choose different materials for insulating ring and sealing part, which are better suited for the respective functions of the two components.

The insulating ring should be high temperature resistant and dimensionally stable, while a sealing function is not necessary. Due to the higher dimensional stability higher insulating rings can be used. The greater distance between the electrode holder and bottom plate allows the application of higher electrical voltage. This has the advantage that several pairs of rods can be connected in series and thus investment costs in the power supply of the reactor can be saved.

The sealing function is taken over by the sealing part, namely by two O-rings made of an elastomeric material. These are preferably arranged in thermally protected position so that they are not exposed to high temperature stress. You therefore only have to seal.

Since all parts, in particular the insulating ring, come into contact with the reaction atmosphere, they should additionally be chemically resistant in an HCl / chlorosilane atmosphere.

The object is achieved by a device for insulating and sealing electrode holders in CVD reactors, comprising an electrode suitable for receiving a filament rod on an electrode holder made of an electrically conductive material which is mounted in a recess of a bottom plate, wherein between the electrode holder and bottom plate electrically insulating ring made of a material having a specific thermal conductivity at room temperature of 0.2-200 W / mK, a continuous temperature resistance greater than or equal to 300 ° C and a specific electrical resistance at room temperature of greater than 10 ^ 9 Ωcm is provided, wherein the electric includes insulating ring grooves in which two O-rings are made of an elastomeric material.

Preferred embodiments of the device can be taken from the appended claims and the following description.

The object of the invention is also achieved by a process for producing polycrystalline silicon, comprising introducing a reaction gas comprising a silicon-containing component and hydrogen into a CVD reactor containing at least one filament rod, which is based on a device according to the invention or on a device according to one of the preferred Embodiments is located, which is powered by the electrode and thus heated by direct current passage to a temperature at which polycrystalline silicon deposited on the filament rod.

The invention will be described below also with reference to the 1 to 3 explained.

Brief description of the figures

1 shows a schematic representation of the mounted insulating ring.

2 shows a schematic representation of the mounted insulating ring.

3 shows a schematic representation of the insulating ring.

LIST OF REFERENCE NUMBERS

1

electrode holder

2

insulating ring

3

baseplate

4

O-ring

5

Cooling floor plate

6

Inlet cooling electrode holder

7

Cooling electrode holder

8th

insulating sleeve

9

Groove for O-ring

a

Distance groove from inside diameter

b

overall width

H

Height insulating ring

c

Got over

D_E

Outer diameter electrode holder

D_R

Outer diameter insulating ring

Between electrode holder 1 and bottom plate 3 are insulating ring 2 and o-rings 4 ,

The bottom plate 3 is provided with a through hole, with an insulating sleeve 8th is lined and through the one electrode holder 1 passed through and fitted. baseplate 3 and electrode holder 1 be through cooling 5 respectively. 7 cooled. 6 shows the inlet for cooling 7 the electrode holder 1 ,

Sealing is done by O-rings 4 , not by flat gaskets. The insulating rings 2 have at the top and bottom each have a groove 9 , These are the O-rings 4 ,

The outer diameter of the electrode holder D_E may be flush or protruding relative to the outer diameter of the insulating ring D_R. Preferably, the electrode holder is overhanging.

1 shows an embodiment without supernatant.

2 shows an embodiment with supernatant c.

The supernatant c should be 0-8 · h, where h corresponds to the height of the insulating ring. Particularly preferred is a supernatant of 0-4 · h.

The grooves 9 are located at a distance a of 10-40% of the total width b of the insulating ring for electrode feedthrough, cf. 3 ,

As a result, the O-rings are sufficiently far away from the reactor-facing side of the insulating ring. This is advantageous since the thermal load on the O-rings is low. The O-rings are thus cooled particularly well, from the cooling medium in the bottom plate, the head of the electrode holder and the passage of the electrode through the bottom plate. Due to the good cooling, the O-rings almost approach the temperature of the cooling medium and are thus not thermally damaged.

A low thermal conductivity of the insulating ring favors the low temperature load of the O-rings. On the other hand, due to the low thermal conductivity of the insulating ring, its surface temperature towards the reactor side becomes higher. If the thermal conductivity is too low, the permissible surface temperature can be exceeded, which can lead to thermal damage to the insulating ring due to stewing and cracking. The choice of material with matching specific thermal conductivity of the insulator is of great importance for its trouble-free operation.

By using the O-rings a small pressing force is sufficient to seal the reactor from the atmosphere. As a low pressing force values <= 5 kN are understood. This has the advantage that the insulating ring is mechanically stressed little.

The inner O-rings are better protected against thermal influences from the reaction space (hot reaction gas, heat radiation) compared with a flat gasket or other types of gaskets.

Compared to a one-piece sealing and insulating ring, the material properties of multi-part designs can be better adapted to the respective requirements of the sealing and insulating function.

The insulating ring does not require sealing material properties.

The specific thermal conductivity at room temperature of the insulating ring is in the range of 0.2-200 W / mK, preferably 0.2-50 W / mK, particularly preferably 0.2-5 W / mK.

The specific electrical resistance of the insulating ring at room temperature is greater than 10 ^ 9 Ωcm, preferably greater than 10 ^ 11 Ωcm, more preferably 10 ^ 13 Ωcm.

To compensate for unevenness on the bearing surfaces of the bottom plate and the head of the electrode holder, the insulating ring should have a minimum bending strength. The bending strength of the insulating ring should be greater than (determined after ISO 178 for plastics and DIN EN 843 for ceramics) 120 MPa, preferably greater than 200 MPa.

In addition, in the case of ceramics, K _1C values (fracture toughness after DIN CEN / TS 14425 ) greater than 3 MPa m ^ 0.5 is preferred.

Suitable materials for the insulating ring are therefore: polyetheretherketone (PEEK), preferably PEEK with greater than 20% glass fiber content; Polyimide (PI); Polybenzimidazole (PBI); Polyamide-imide (PAI); Alumina (Al 2 O 3); Silicon nitride (Si3N4); Boron nitride (BN); Zirconia stabilized with yttria (ZrO2-Y2O3) or zirconia stabilized with magnesia (ZrO2-MgO), aluminum nitride (AlN).

By the present invention one is not bound to PTFE and can the o. G. Materials with higher dimensional stability and temperature resistance used.

The temperature resistance in continuous operation of PTFE is 250 ° C.

In contrast, PEEK, PI, PBI and PAI have a temperature resistance in continuous operation of 300 ° C.

The ceramic materials have a temperature resistance in continuous operation of greater than 1000 ° C and a higher dimensional stability than hydrocarbon-based materials, in particular PTFE.

Suitable O-rings are commercially available O-rings, which ensure a sealing function and are chemically resistant in an HCl / chlorosilane atmosphere.

Suitable examples are O-rings of fluoroelastomers (FPM, after ISO 1629 ), Perfluoroelastomers (FFKM, ASTM D-1418 ) and silicone elastomers (MVQ, ISO 1629 ).

Examples

In a Siemens separator reactor polycrystalline silicon rods were cut with a diameter between 160 and 230 mm. Several versions of insulating rings were tested. The parameters of the deposition process were the same for all experiments. The experiments differed only in the design of the insulating rings. The deposition temperature was in the batch process between 1000 ° C and 1100 ° C. During the deposition process, a feed consisting of one or more chlorine-containing silane compounds of the formula SiH _n Cl _4-n (where n = 0 to 4) and hydrogen as the carrier gas was added.

Comparative example

CVD reactor with PTFE insulating ring:

In this embodiment according to the prior art, the insulating ring of PTFE takes over sealing and insulating function. Due to the low dimensional stability, the height of the insulating ring is limited to 6 mm when new.

Due to the high thermal load during operation and the required pressing force of 30-40 kN to ensure the sealing function of the insulating ring, the height of the insulating ring was reduced to a minimum of 4 mm within 3 months. As a result, the service life is limited to 3 months. Due to the thermal load of the hot reaction gas both the sealing of the bottom plate and the electrical insulation by thermal cracking and setting of the sealing body was no longer given. After this time, therefore, a complex exchange of all insulating was necessary. Repair work resulted in a significant capacity loss.

example 1

CVD reactor with insulating ring made of PEEK with 30% glass fiber content:

In this version, the sealing and insulating function is divided into two components. The insulating ring made of PEEK is used for electrical insulation between the electrode holder and the base plate. The insulating ring is 8 mm high when new. The sealing function is carried out by 2 O-rings to the head of the electrode holder and to the bottom plate. The use of O-rings requires a pressing force of 0.7 kN. Due to the low surface pressure and the higher dimensional stability, the setting behavior of the insulating ring compared to a PTFE ring was significantly lower. After 6 months, the height compared to new condition was 0.5 mm below. Due to the higher temperature resistance to PTFE, the reactor-facing side of the insulating ring was less thermally attacked. The service life increased to 6 months.

Example 2

CVD reactor with aluminum oxide (Al2O3) insulating ring:

In this version, the sealing and insulating function is divided into two components. The insulating ring made of Al2O3 is used for electrical insulation between the electrode holder and the bottom plate. The insulating ring is 8 mm high when new. The supernatant of the electrode holder c was 10 mm. The sealing function is carried out by 2 O-rings to the head of the electrode holder and to the bottom plate. The use of O-rings requires a pressing force of 0.7 kN. Al2O3 has no setting behavior as a ceramic component. Due to the low surface pressure of the insulating ring made of ceramic is not broken. After 12 months, the insulating ring was replaced in the course of recurring maintenance cycles. By the very high temperature resistance and the significantly higher specific thermal conductivity of 30 W / mK at room temperature compared to PTFE, the reactor facing side of the insulating ring and the O-rings were not thermally attacked. The service life increased to 12 months.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 20110305604 A1 [0009]
• WO 2011116990 A1 [0010]
• WO 2011092276 A1 [0012, 0014]
• US 20130011581 A1 [0013, 0014]
• CN 202193621 U [0015]
• CN 101565184A [0016]
• CN 102616783 A [0017]
• US 2010058988 A1 [0019]

Cited non-patent literature

• ISO 178 [0050]
• DIN EN 843 [0050]
• DIN CEN / TS 14425 [0051]
• ISO 1629 [0058]
• ASTM D-1418 [0058]
• ISO 1629 [0058]

Claims (10)

A device for insulating and sealing electrode holders in CVD reactors, comprising an electrode suitable for receiving a filament rod on an electrode holder made of an electrically conductive material, which is mounted in a recess of a bottom plate, wherein between the electrode holder and bottom plate an electrically insulating ring made of a material with a specific thermal conductivity at room temperature of 0.2-200 W / mK, a continuous temperature resistance of greater than or equal to 300 ° C and a specific electrical resistance at room temperature greater than 10 ^ 9 Ωcm is provided, wherein the electrically insulating ring comprises grooves, in where two O-rings are fixed from an elastomeric material. The device of claim 1, wherein the specific thermal conductivity of the electrically insulating ring at room temperature is 0.2-50 W / mK. Device according to claim 2, wherein the specific thermal conductivity of the electrically insulating ring at room temperature is 0.2-5 W / mK. Device according to one of claims 1 to 3, wherein the electrical resistivity of the electrically insulating ring at room temperature is greater than 10 ^ 11 Ωcm. Device according to one of claims 1 to 4, wherein the electrically insulating ring has a minimum bending strength of 120 MPa. The device of any one of claims 1 to 5, wherein the material of the electrically insulating ring is selected from the group consisting of polyetheretherketone, polyimide, polybenzimidazole, polyamideimide, alumina, silicon nitride, boron nitride, yttria stabilized zirconia, magnesia stabilized zirconia, and aluminum nitride. Device according to one of claims 1 to 6, wherein the O-rings consist of fluoroelastomers, of perfluoroelastomers or of silicone elastomers. Method according to one of claims 1 to 7, wherein a radial projection c of the electrode holder relative to the electrically insulating ring is greater than or equal to 0 and less than or equal to eight times a height h of the electrically insulating ring. Device according to one of claims 1 to 8, wherein the grooves on the electrically insulating ring for receiving the O-rings of the recess of the bottom plate are spaced such that the distance from the recess is 10-40% of a total width of the electrically insulating ring. A process for the production of polycrystalline silicon, comprising introducing a reaction gas containing a silicon-containing component and hydrogen into a CVD reactor comprising at least one filament rod located on a device according to one of claims 1 to 9, which supplies power via the electrode and the so that it is heated by direct passage of current to a temperature at which polycrystalline silicon is deposited on the filament rod.

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