WO2012143350A1

WO2012143350A1 - Crucible assembly

Info

Publication number: WO2012143350A1
Application number: PCT/EP2012/056993
Authority: WO
Inventors: Martin Cadek; Johann Daimer; Bodo Frings; Hartwig Rauleder
Original assignee: Sgl Carbon Se
Priority date: 2011-04-19
Filing date: 2012-04-17
Publication date: 2012-10-26
Also published as: DE102011007708A1

Abstract

The invention relates to a crucible assembly for melting and crystallizing a substance, in particular a metal, such as silicon for example, comprising a crucible (15) for receiving a melt, an outer support frame (11) for holding the crucible (15), and a graphite-containing foil (13), which is arranged at least in some regions between the outer wall (18) of the crucible (15) and the support frame (11).

Description

crucible assembly

The present invention relates to a crucible assembly for melting and crystallizing a substance, in particular a metal, such as silicon, as well as their use for melting and crystallizing preferably silicon.

In the production of crystalline silicon, as required for example for the semiconductor industry or photovoltaics, special crucibles are used to receive and crystallize the silicon melt. In current processes, molten silicon is crystallized in a directionally open manner in an open-topped crucible by controlled heat removal, the crystallization proceeding from the bottom of the crucible to the top. The crucible is held by an outer support frame in the correct position. During the melting and solidification process, the crucible including the associated support frame is usually arranged in a crystallization furnace. In order to prevent the inclusion of unwanted impurities in the resulting crystalline silicon block, the inner wall of such a crucible in contact with the silicon melt must have a high degree of purity.

Since one-piece crucible with increasing size are more sensitive to thermal stresses, crucibles of the aforementioned type can also be made by joining several items. Corresponding crucibles of several joined graphite plates are e.g. known from US 2006/0219162 A1.

A problem with the type of silicon production described is the ever-present risk of leakage of molten silicon from the Horny pickup cavity of crucible. Such leakage may occur, for example, as a result of cracks in the crucible wall which usually form during the melting or solidification process, because silicon undergoes an increase in volume of about 1 1% during solidification, as a result of which enormous forces are exerted on the crucible wall. In a crucible composed of several individual parts, molten silicon can escape in particular at the joints. Since molten silicon is highly reactive, it can in the event of leakage, the support frame and components of the crystallization furnace, such as its isolation, attack and damage, resulting in longer downtime of the associated manufacturing plant by itself. Furthermore, leaked silicon on contact with the support frame absorbs impurities from the material of the support frame. If the escaped silicon is still in contact with the silicon in the crucible, such impurities can migrate back through diffusion into the interior of the crucible and thus contaminate the silicon melt to be crystallized. In this regard, it should be noted that there is a considerable amount of diffusion at the relatively high temperatures at which the silicon melt is in the crucible. The contaminants carried back into the crucible by the support frame progressively inwardly advance from the peripheral regions of the silicon melt.

These difficulties have led, in some applications, to the need to cut and discard a comparatively thick periphery of the final crystallized silicon ingot or cake, thereby increasing material usage and manufacturing costs.

It is therefore an object of the present invention to provide a crucible-type crucible assembly in which the infiltration of contaminants by cracks into the crucible upon crystallization of a molten metal and, in particular, a silicon melt in the crucible, damage to the crucible the crucible-holding frame and damage to other components of the furnace in which the crucible is arranged, is reliably prevented.

According to the invention, this object is achieved by a crucible arrangement having the features of claim 1 and in particular by a crucible arrangement for melting and crystallizing a metal, in particular silicon, wherein the crucible arrangement comprises:

a crucible for receiving a melt,

- An outer support frame for holding the crucible and

- A graphite-containing film, which is at least partially disposed between the outer wall of the crucible and the support frame.

According to the invention, the crucible arrangement thus comprises a graphite-containing film, which is arranged at least in regions between the outer wall of the crucible and the support frame. The film acts as a flexible protective cover for the crucible, which absorbs and retains molten metal emerging from the crucible. The foil prevents, in particular, the passage of molten metal leaving the crucible into contact with the support frame or with parts of the associated crystallization furnace. As a result, damage to the components of the furnace and in particular the furnace insulation and thus downtime of the manufacturing plant can be avoided. Since the film is flexible and does not break even at thermal stresses, it can be used several times. The support frame can basically be used several times.

A further advantage of the crucible arrangement according to the invention results from the fact that molten metal emerging from the crucible reacts with the graphite contained in the foil to form a metal carbide, whereby a barrier layer is formed. In the case of silicon as a molten material, the barrier layer is silicon and silicon carbide and is typically less than 1 mm thick. This barrier layer acts as a diffusion barrier, ie it impedes the diffusion-dependent The passage of contaminants and thus blocks the migration of impurities, such as silicon carbide, in the crucible during the melting and crystallization process. In this way, impurities of the silicon block produced can be reliably prevented or at least considerably reduced, whereby in particular a cutting off of the edge regions of the solidified silicon block can be dispensed with. A graphite foil is further characterized by a relatively high thermal conductivity and therefore contributes to a more uniform heating and cooling of the crucible. Thus, the crucible assembly according to the present invention reduces both the cost of materials and downtime in the crystallization of silicon, and thus, overall, enables more economical production of crystalline silicon.

Preferably, the film is made entirely of graphite. In this way, impurities of the silicon melt are avoided by other film materials. In principle, all common Graphitsorten can be used for the production of the film. In order to adapt the film to the outer shape of the crucible, preforming of the film can be carried out by means of a tool which is known in principle.

Regardless of whether the film is entirely graphite or contains graphite among other ingredients, the graphite is preferably expanded graphite.

In a further development of the inventive idea, it is proposed that the film consists of very pure graphite, preferably with an ash content of less than 500 ppm, namely preferably graphite, which has been purified by a halogen treatment. A graphite material purified by a halogen treatment has a sufficiently high degree of purity to prevent contamination of melt leaving the crucible by the film itself, such as contamination with oxygen. In principle, however, it is also possible although less preferred to use an unpurified graphite foil.

The halogen treatment may for example be such that the graphite foil is charged with a halogen or with a halogen mixture and is left at a predetermined cleaning temperature of, for example, 2300 to 3000 ° C for a certain period of time. Contaminants present in the graphite react with the halogen (s) at the elevated temperature and volatilize, which is why the remaining graphite material has a particularly high degree of purity. The halogens can be comparatively deep in the described cleaning process, i. up to 30 cm, penetrate into the graphite layers, so that the film is reliably cleaned in its entire volume.

A measure of the purity of the graphite is the ash content. Commercially available material types have an ash value according to DIN 51903 of four percent to 0.15 percent or from 40,000 to 1,500 ppm. According to a preferred embodiment of the invention, the ash content of the graphite foil material is below 500 ppm, which can be obtained by the described cleaning of the graphite foil material.

According to a preferred embodiment of the present invention, the film is integrally formed. This ensures a secure collection of leaking melt through the film, as there are no seams or transitions as potential leaks.

Furthermore, it is preferred that the film is held by the support frame. That is, the support frame can hold both the crucible and the film, so that no attachment of the film to the crucible wall is necessary. In a further development of the inventive concept, it is proposed that the one-piece film is folded in the form of several overlapping film layers. As a result, a further improved protective effect can be achieved. In order to avoid breakage of the soft graphite material in this case, the film can be folded along fold areas with rounded corners.

According to a further advantageous embodiment of the present invention, the outer wall of the crucible is completely enveloped by the film at least up to the filling level of the melt. Thereby, the area between the support frame and the crucible is completely lined by the film, so that any emerging from the crucible melt is reliably collected regardless of the exit location and therefore can not reach the support frame or can absorb impurities there.

The film may be at least partially spaced from the outer wall of the crucible. This has the advantage that any deformations of the crucible due to thermal expansion and contractions do not affect the film. Rather, the film envelops the crucible wall continuously.

Basically, the present invention is not limited in the thickness of the film. However, good results are obtained especially when the single-layer film has a thickness between 0.2 mm and 5 mm, and preferably between 0.5 mm and 2 mm. This ensures sufficient stability, taking into account the required flexibility of the film.

Further, it is preferable that the film has a density of 0.7 g / cm ³ to 1.0 g / cm ³ . This is generally sufficient in terms of a reliable sealing effect. In principle, a comparatively low density and thus a higher compressibility can contribute to compensate for a bulge of the crucible during the melting or crystallization process. This prevents Dangerous stresses in the support frame and thus increases its life or the number of feasible melting / crystallization cycles before replacement of the support frame is required.

In accordance with one embodiment of the present invention, expanded graphite is placed in a die of at least two parts and compacted and molded at a pressure of at least 10 MPa and at most 100 MPa. The compacted and molded part is removed from the mold and placed in a mold of the same geometry of the forming surfaces of temperature resistant material and cleaned at a temperature above 2000 ° C in an atmosphere containing a halogen or in a halogen containing gas. The thus obtained compacted, formed and cleaned expanded graphite part is taken out of the mold and used as the film according to the invention. The ash value of the film thus produced is less than 500 ppm.

The film may be provided at least in regions with a coating comprising silicon nitride. Such a coating increases the durability of the film and also provides an additional diffusion barrier. Furthermore, a silicon nitride coating acts as a release agent, which allows easier removal of the film from the outer wall of the crucible after completion of the crystallization process. Due to the easy removability, the film is reusable, whereby the efficiency of the manufacturing process is improved.

According to a further preferred embodiment of the present invention, the crucible is assembled from a plurality of, in particular plate-shaped, individual parts. As a result, the size of the crucible and thus the size of the producible silicon block in wide ranges to the requirements of the respective Application to be customized. In particular, such a Fügebau way allows easy extension of an existing crucible. In one-piece crucibles - which are conventionally manufactured, for example, in slip casting using appropriate molds - this is associated with a significant overhead, since for the production of a larger or differently shaped crucible only a corresponding mold must be created. Furthermore, relatively flat-shaped crucible can be easily produced by the Fügebau way. If an excessively large expansion or deformation of the crystallization material contained in the crucible occurs in a crucible composed of individual parts, the crucible wall at the joints will crack in a defined manner, which in terms of process technology is advantageous in comparison with uncontrolled cracking.

Since, in the aforementioned embodiment, the melt exits, if at all, defined at the joints, it is proposed in a development of the invention that in this embodiment the film covers at least all joints to which the individual parts are joined. Depending on the application, it is thus possible to save on film material in that the film is dimensioned and arranged in such a way that only the joints are protected.

With respect to the manner of connection of the individual parts in the aforementioned embodiment, the present invention is not limited. For example, the items may be joined together in particular by means of at least one connecting element made of temperature-resistant material. As connecting elements, for example, pins, bolts, screws, clamps, dowels, Lamellos or the like may be provided. Alternatively or additionally, form-fitting interlocking connection sections can be formed directly on the individual parts, such as combinations of tongue and groove or dovetail connections. Come as a temperature-resistant material in particular quartz, graphite, carbon fiber reinforced composites (CFC), silicon carbide (SiC), silicon nitride (S13N4), zirconium oxides and metals such as titanium, tungsten or molybdenum. Before joining the individual parts they can be brought to the correct dimensions by mechanical processing steps such as grinding, milling, drilling, honing and the like.

The crucible is cuboid in accordance with another embodiment of the present invention. This is particularly advantageous in a crucible production in joint construction, as a cuboid crucible can be easily assembled from prefabricated panels. If the application so requires, other crucible shapes can be realized by e.g. in a known manner, two complementary half-forms are joined together.

In principle, the crucible may be made of any material known to those skilled in the art, such as graphite, silica, or a mixture of silica with one or more other materials, such as silicon nitride. Good results are obtained, in particular, when the crucible is made of silicon dioxide and preferably of high-purity silicon dioxide. By high-purity silica is meant in this connection silicon dioxide with a SiO 2 content of at least 99%. Preferably, in the high purity silica, the amounts of impurities do not exceed those amounts described in claim 1 1 of WO 2010/037702 Al or in claim 12 of WO 2010/037705 Al, the disclosures of which are hereby incorporated by reference. A crucible of such high purity silica is sufficiently resistant to molten silicon and prevents unwanted impurities from entering the melt. As starting materials for the silica to be provided for the production of crucibles, it is possible in particular to use natural quartz, pyrogenic or precipitated silicic acid or fused silica in an electric arc furnace. According to one embodiment of the invention are from Cast the mentioned starting materials silica shaped body, which are optionally sintered. In this way, in particular low-stress silicon dioxide plates for the assembly of a crucible base body can be manufactured.

In a further development of the inventive concept, it is proposed to coat an inner wall of the crucible with a sizing, which contains in particular silicon nitride. Such a size can form a smoothing coating of the inner wall and in particular act as a sealant and / or as a release agent. In particular, a silicon nitride size may provide additional protection against leaking silicon. In addition, adhesion of the resulting silicon crystal to the crucible wall can be prevented. It is up to the particular application, whether the coating of the inner wall of the crucible with the sizing takes place before any assembly of the crucible of several individual parts or just before the use of the crucible for melting and crystallization. Conveniently, a silicon nitride-containing sizing is applied as a spreadable mass of a silicon nitride-containing powder distributed in a binder on the inner wall of the crucible, for example by brushing or knife coating, and subsequently baked. In the case of a crucible assembled from several individual parts, the burning-in can be carried out immediately prior to assembly or even before the individual parts are sintered, depending on the application. The silicon nitride-containing layer could also be applied to the finished sintered items.

With a crucible made of assembled individual parts, the sizing can only be applied to the joints in order to save material. However, should the application require it, the entire inner wall of the crucible may also be coated with the size. According to a further preferred embodiment of the present invention, the support frame is made of graphite. A graphite rack has sufficient thermal resistance to carry out the melting and crystallization process in a crystallization oven, and is also resistant to possible contact with molten silicon.

In the crucible arrangement according to the invention, the crucible may have a size (BxLxH) of at least 400 mm χ 400 mm χ 250 mm, preferably of at least 600 mm x 600 mm χ 350 mm and particularly preferably of at least 700 mm χ 700 mm x 380 mm. The size specifications refer to the outer dimensions of the length, the width and the height of a rectangular crucible. In the case of a crucible with non-cuboid external dimensions, the details given refer to the smallest cuboid circumscribing the crucible. Such large crucibles can traditionally only be produced with unacceptably high costs and also have a relatively high risk of failure.

Due to the leakage protection according to the invention by means of the graphite-containing film - optionally combined with a joining construction as described above - however, in spite of the high probability of breakage, crucibles can be used which exceed the minimum sizes mentioned. This allows a considerable process and efficiency optimization in the production of materials, such as silicon, by melting and crystallization in crucibles.

Another object of the present invention is the use of a previously described crucible arrangement for melting and crystallizing metals, and preferably of silicon.

Hereinafter, the present invention will be described purely by way of example with reference to advantageous embodiments and with reference to the accompanying drawings. Showing:

1 is a side sectional view of a crucible assembly according to the invention comprising a crucible, a support frame and a film,

2 is a perspective view of two parts to be joined together of a support frame for the crucible arrangement according to FIG. 1,

Fig. 3 is a plan view of the items according to FIG. 2, which shows these in the assembled state, and

4 shows a film to be folded for the crucible arrangement according to FIG. 1.

According to FIG. 1, a crucible arrangement according to the invention is constructed as a layer composite and comprises, viewed from the outside inwards, a support frame 1 1 made of graphite, a flexible graphite foil 13, a crucible 15 made of silicon dioxide and a crucible coating 17 in the form of a silicon nitride sizing.

The crucible 15 is cuboid and has an upper opening 23. The inner wall 16 of the crucible 15, which defines a receiving cavity for a silicon melt, is completely covered by the crucible coating 17. The outer wall 18 of the crucible 15 is completely enveloped by the graphite foil 13, which is arranged between the crucible 15 and the support frame 1 1 and in turn is completely enveloped by the support frame 1 1. The support frame 1 1 thus forms a holder for both the crucible 15 and the graphite foil 13th

The graphite foil 13 is made entirely of expanded graphite, which is purified by a halogen treatment and has a correspondingly high degree of purity. The graphite foil preferably has a uniform thickness. between 0.5 mm and 2 mm. A silicon nitride coating, not shown in FIG. 1, covers the entire graphite foil 13 in a particular embodiment.

As can not be seen in FIG. 1, but can be seen from FIGS. 2 and 3, the support frame 1 1 is made of several parts. Specifically, a bottom plate 19 is connected by means of a threaded pin connection 25 with a plurality of side plates 21 - of which only one is shown in FIGS. 2 and 3 - connected.

Similarly, the crucible 15 shown in FIG. 1 is also assembled from a plurality of plate-shaped components made of high-purity silica by casting and subsequent sintering and joined together using high-temperature-resistant bonding agents.

According to FIG. 4, the graphite foil 13 is manufactured in one piece and is folded along predetermined fold lines 30 into a cuboid mold surrounding the crucible 15 and open on one side. In this case, in addition to a bottom region 31 and four lateral regions 33, respective overlapping regions 35 located between the lateral regions 33 are produced, which are each folded as additional film layers under or over one of the adjacent lateral regions 33.

To produce crystalline silicon, the crucible assembly of Fig. 1 is placed in a crystallization furnace and silicon present in the crucible 15 is melted and subsequently solidified, as is well known in the art. Should it come to a leakage of molten silicon from the crucible 15, for example, by a resulting crack or a joint gap, the outflowing melt is safely collected by the graphite foil 13 and thus can not penetrate to the support frame 1 1. By a reaction of the leaked silicon with the graphite foil 13 This results in the formation of a barrier layer of silicon carbide (SiC), which prevents back diffusion of impurities in the crucible 15. After completion of the crystallization process, the crystallized silicon is removed from the crucible 15 and the usually cracked crucible 15 is replaced by a new one. The graphite foil 13, on the other hand, can be reused for the next production process. However, if the graphite foil 13 is excessively attacked and flakes, for example, it is also replaced. Such replacement is usually required after about 5 manufacturing operations. Also, the support frame 1 1 can be reused in principle.

By the crucible 15 protective graphite foil 13 impurities of the resulting silicon crystal can be substantially reduced, whereby a subsequent removal of contaminated edge areas can be omitted. Furthermore, damage to the support frame 1 1 or the crystallization furnace are avoided by the highly reactive melt. The economics of silicon production through the process of melting and directional solidification can thus be significantly improved.

LIST OF REFERENCE NUMBERS

support stand

graphite foil

crucible

inner wall

pan coating

outer wall

baseplate

side plate

upper opening

Set screw connection

fold line

floor area

page range

overlap area

Claims

claims

A crucible assembly for melting and crystallizing a metal, in particular silicon, comprising:

a crucible (15) for receiving a melt,

an outer support frame (11) for holding the crucible (15) and a graphite-containing film (13) which at least partially between the outer wall (18) of the crucible (15) and the support frame

(11) is arranged.

2. crucible arrangement according to claim 1,

characterized in that

the film (13) is made entirely of graphite.

3. crucible arrangement according to claim 1 or 2,

characterized in that

the film (13) contains expanded graphite or consists entirely of expanded graphite.

4. crucible arrangement according to at least one of the preceding claims, characterized in that

graphite containing or consisting of graphite (13) is graphite purified by halogen treatment.

5. crucible arrangement according to at least one of the preceding claims, characterized in that

the film (13) is integrally formed.

6. crucible arrangement according to at least one of the preceding claims, characterized in that

the film (13) is held by the support frame (11).

7. crucible arrangement according to at least one of the preceding claims,

characterized in that

the outer wall (18) of the crucible (15) is completely enveloped by the film (13) at least up to the filling level of the melt.

8. crucible arrangement according to at least one of the preceding claims,

characterized in that

the film (13) is single-layered and has a thickness between 0.2 mm and 5 mm and preferably between 0.5 mm and 2 mm.

9. crucible arrangement according to at least one of the preceding claims,

characterized in that

the film (13) is at least partially provided with a coating which preferably contains silicon nitride.

10. crucible arrangement according to at least one of the preceding claims,

characterized in that

the crucible (15) is assembled from a plurality of, in particular plate-shaped, individual parts.

11. crucible arrangement according to claim 10,

characterized in that

the film (13) covers all joints, to which the individual parts are joined.

12. crucible arrangement according to at least one of the preceding claims, characterized in that

the crucible (15) consists of silicon dioxide and preferably of highly pure silicon dioxide.

13. crucible arrangement according to at least one of the preceding claims, characterized in that

the inner wall (16) of the crucible (15) is at least partially coated with a size (17) which preferably contains silicon nitride.

14. crucible arrangement according to at least one of the preceding claims, characterized in that

the crucible (15) has a size (BxLxH) of at least 400 mm χ 400 mm χ 250 mm, preferably of at least 600 mm χ 600 mm χ 350 mm and particularly preferably of at least 700 mm χ 700 mm χ 380 mm.

15. Use of a crucible arrangement according to one of the preceding claims for melting and crystallizing metals, and preferably of silicon.