MX2008009002A - Crucible for the treatment of molten silicon - Google Patents

Abstract

The invention relates to a crucible for the treatment of molten silicon comprising a basic body with a bottom surface and lateral walls defining an inner volume. According to the invention, the basic body comprises at least 65%by weight of silicon carbide, from 12 to 30%by weight of a constituent selected from silicon oxide or nitride. Moreover, the basic body comprises at least one silicon oxide and/or nitride coating, at least on the surfaces defining the inner volume of the crucible contrarily to the state of the art crucibles, such a crucible can be used several times without any visible degradation of its physical integrity.

Description

CRISOL FOR THE TREATMENT OF CAST SILICON DESCRIPTIVE MEMORY The present invention relates to a crucible for the treatment of molten silicon, to the manufacture of said crucible and to the use of said crucible for the treatment of molten silicon. Currently, the demand for high purity silicon has increased significantly. The applications of high purity silicon in the generation of photovoltaic energy are highly widespread. But successive energy crises have strengthened this need. The purpose of the present application is a container that is used for the treatment of molten silicon. Said treatment may consist of the crystallization of silicon, either by dà © reccional solidification or by the extraction of a crystal from a molten bath. The treatment may also consist of a metallurgical treatment which is intended for the production of very high purity silicon or one of its alloys. It can also consist of a metallurgical treatment of alloys or minerals that aims to eliminate certain impurities. For this type of applications, it is well known to use quartz crucibles or with a base of other materials that consist essentially of silicon dioxide (see, for example, DE-C-962868). In fact, as the main constituent of the crucible is silicon in the form of one of its oxides, the risk of contamination by other chemical compounds is greatly reduced. However, quartz crucibles have the great disadvantage that they are attacked by molten silicon, with the consequence that the solidifying silicon ingot tends to adhere to the walls of quartz crucible. Since quartz and silicon have different coefficients of thermal expansion, very important mechanical stresses can be generated either within the ingot, resulting in crystallization defects, or in the walls of the crucible, resulting in the cracking of the crucible. Also, the silicon ingot, once solidified, adheres strongly to the walls of the crucible and it is practically impossible to extract the ingot without destroying the crucible or at least seriously damaging it. It is also known that quartz and certain silica derivatives undergo crystallographic phase changes during their thermal cycle. These crystallographic phase changes induce very high mechanical stresses within the walls of the crucible. In addition, they can also induce density changes, and therefore thermal conductivity within the wall, resulting in problems of loss of homogeneity of the transmission or removal of energy to or from silicon. So far this crucial problem has not found a solution that can be carried out at an industrial level. In addition, at the temperatures of use, the quartz undergoes geometric changes. These geometrical changes are relatively difficult to handle, since in the treatment furnace in which the container containing the molten silicon, the amount of silicon that will be treated must be heated in a completely controlled manner. Any deformation of the container wall induces a loss of homogeneity in the transmission or extraction of energy, to or from the silicon, which adds to the loss of geometry of the silicon ingot during crystallization. This problem has been partially remedied by reinforcing the outer walls of the quartz crucible with carbon plates, more particularly with graphite plates. Said carbon plates, and more particularly the graphite plates, are widely used in all kinds of processes that are carried out at a high temperature, due to their excellent resistance to thermal stresses over long periods. For example, graphite crucibles have been used to receive a germanium bath during the course of a crystal extraction process according to the Czochraiski method. Nevertheless, until now it has not been possible to use said graphite crucibles for the treatment of silicon, since the bath of molten silicon, at a high temperature, attacks the graphite walls and forms silicon carbide, whose presence is incompatible with the purity that is required. According to the technique currently used, as indicated above, the different procedures for the treatment of silicon at high temperature, are carried out in quartz crucibles or other silicon-based materials whose walls are reinforced with plates of silicon. carbon, more particularly graphite.

This technique is not free of problems either. It is well known that gaseous phases, in the vicinity of molten silicon bath, have an influence on the formation of a balance between silicon vapor, which escapes from the molten silicon bath, and the prevailing carbon monoxide atmosphere. in the oven. Reactions are also observed in the carbon or graphite as in the silicon bath, resulting in a change in physical and mechanical properties. Starting from the same concept that consists in avoiding the introduction of other constituents besides silicon, the use of silicon nitride crucibles has also been proposed in the state of the art. Therefore WO-A1 -2004 / 016835 discloses a crucible which consists essentially of silicon nitride. Even though some of the properties of this crucible are satisfactory, its price makes its use currently unreal. In addition, it has been reported that these crucibles are also sensitive to deformation at high temperatures. The applicant has proposed an objective of providing a container for the treatment of molten silicon, which would not present the drawbacks seen in the prior art. In particular, it would be preferable that the crucible be used a number of times without any significant degradation of its physical integrity. In addition, the thermal conductivity properties of the crucible in question should not change during the course of its use; in other words, that the material is not sensitive anymore either to deformation or crystallographic phase changes. Eventually it is necessary that the crucible is not a source of silicon contamination. The applicant has established that these objectives and others are achieved with a crucible as claimed in claim 1. Said crucible comprises a basic body with a bottom surface and side walls that define an interior volume, and which is constituted mainly ( at least 65% by weight of the material) of silicon carbide. It is really surprising that the crucible for the treatment of molten silicon could be manufactured from a material that is mainly composed of silicon carbide. Actually, until now, those skilled in the art have always tried to avoid the presence of silicon carbide, which is perceived as a problem in any process for the treatment of molten silicon. On the contrary, the Applicant has shown that a crucible comprising a basic body that consists mainly of silicon carbide, does not have the disadvantages that are observed with conventional crucibles. In particular, the fact that the main component of the basic body consists of silicon carbide, showing a well defined crystallographic phase which is subject to phase transition at the temperatures of treatment of the molten silicon, allows to eliminate the problems of the loss of homogeneity of the transfer / extraction of energy that are observed with conventional crucibles. Also, silicon carbide has no plastic phases at these temperatures and, therefore, is not subject to deformation.

Thanks to these excellent properties, said crucible can be reused a significant number of times, while conventional crucibles must be replaced after each use. It is very surprising that the solution to this problem comes precisely from the use of a material that has been considered until now as a source of problems. The material forming the basic body of the crucible also comprises from 12 to 30% by weight of one or more selected constituents oxide or silicon nitride. The rest of the material forming the basic body can comprise up to 13% by weight of one or several other constituents such as binders (chemical, hydraulic or other), agents that regulate the fluidity of the composition before shaping and curing, etc. The selected constituent of silicon oxide or nitride can be introduced into the composition used to form the basic body, or it can be introduced in the form of metallic silicon that will be oxidized or nitrated during the curing of the crucible. The curing conditions (nitration or oxidation atmosphere) will be selected according to the desired composition. It should be noted that the silicon oxide can also have an effect on the fluidity of the composition before shaping and curing, as well as a binding effect, in particular when this compound is introduced in the amorphous pyrogenic silica form. In this case, obviously, it is taken into account only once (in 12 to 30% by weight of one or more selected constituents of oxide and / or silicon nitride).

Other viscosity regulating agents can also be introduced to modify the hot properties of the crucible. The addition of fine particles of reactive alumina (grain size less than or equal to 200 μm) is particularly advantageous for its effect of modifying the flowability during its shaping, as well as its binding effect after curing. Other binders that could be used comprise, for example, organic resins (leaving a carbonaceous residue after curing), magnesia and aluminate and / or calcium silicate. According to an advantageous embodiment, the agglutination is generated by the in situ formation of a nitride or silicon oxide agglutination. Said agglutination is easily obtained by regulating the curing conditions of the article, and, in particular, the curing atmosphere of the article. It has also been established that it is necessary to provide the interior walls with a coating of the silicon nitride type as described, for example, in WO-A1 -2004053207 or in European Patent Application 05447224.6, of the silicon oxide type or of a combination thereof as described, for example, in European patent application 05076520 or in WO-A-1 -2005 / 106084. Generally, an oxide-type coating is used for the crystallization of silicon as a monocrystal and of the nitride type for the polycrystalline crystallization of silicon. It will be noted that the coating can be produced during the curing of a crude crucible comprising silicon (for example a cured in a nitrating atmosphere will produce a surface coating of silicon nitride, while a cure in an oxidizing atmosphere will produce a surface coating of the silicon oxide type). According to the invention, the basic body is bonded. As described above, the binder can be a hydraulic binder (eg silicate or calcium aluminate), thereby forming a cement-type composition, a chemical binder (eg, magnesium silicate) or a binder of the free type. of cement (for example gels, orthosilicate, etc.) or also an agglutination produced by reactive agglutination (carbon agglutination, nitrant cure, etc.). Advantageously, the silicon carbide will be used in accordance with a well-defined particle size distribution. In particular it is preferable that the coarser grain fraction is made of silicon carbide in order to provide a silicon carbide matrix in order to provide a silicon carbide matrix composed of large grains, where more grains are present. fine nitride or silicon oxide. The majority of the silicon carbide will thus be constituted of grains having a particle size of more than 200 μm while silicon oxide and preferably silicon nitride and / or metal silicon grains will be introduced in the form of grains having a particle size of less than 10 μm. The following examples illustrate various embodiments of the invention. The following table 1 provides several examples of materials according to the invention, which constitute the basic body for the treatment of fused silica. In this table, the first column indicates the nature of the constituents, columns 2 to 13 indicate the percentages by weight of the different constituents. Examples A1, A2, C1, C2, E1 and E2 illustrate several variants of hydraulic binders. Examples A to F illustrate different variants of chemical or reactive agglutination. The crucibles were prepared from these materials and their internal walls have been coated with a nitride or silicon oxide coating. The crystallization of the same amount of silicon was carried out in each of these crucibles. It was observed that none of these crucibles was damaged during the crystallization of the silicon, so that they could be reused immediately in an additional crystallization operation without requiring any repair step.

TABLE 1 A1 A2 A B C1 C2 C D E1 E2 E F Silicon Carbide 1-3 mm 25 25 25 25 20 20 20 20 25 25 25 22 0.2-1 mm 20 20 20 20 30 30 30 25 25 25 25 25 < .025 mm 30 30 30 30 30 30 30 30 20 20 20 20 Total silicon carbide 75 75 75 75 80 80 80 75 70 70 70 67 Silicon nie 17 15 17 10 15 8 18 25 10 25 17 Silicon oxide 5 8 5 5 4 12 6 10 Total nie and oxide 22 23 22 15 15 12 18 12 25 16 25 27 Alumina 1 3 2 11 Calcium aluminate 2 5 3 3 Magnesium oxide 1 2 2 1 Carbon 2 4 3 Total of others 3 2 3 10 5 8 2 13 5 14 5 6 Total 100 100 100 100 100 100 100 100 100 100 100 100 100

Claims (10)

NOVELTY OF THE INVENTION CLAIMS

1. - A crucible for the treatment of molten silicon comprising a basic body with a bottom surface and side walls defining an interior volume, the basic body comprising: at least 65% by weight of silicon carbide; from 12 to 30% by weight of a constituent selected from silicon oxide or nitride, the basic body also comprises at least one oxide and / or silicon nitride coating, at least on the surfaces defining the interior volume of the crucible .

2. The crucible according to claim 1, further characterized in that the basic body also comprises up to 13% by weight of one (or more) optional constituent (s) selected from carbon, magnesium oxide, aluminum oxide, silicate and / or calcium aluminate.

3. The crucible according to claim 1 or 2, further characterized in that a layer of silicon oxide is present between the surface coating and the wall of the surfaces that define the interior volume of the crucible.

4. The crucible according to any of claims 1 to 3, further characterized in that a layer of silicon oxide is present in the walls of the basic body, on the side opposite the side that defines the interior volume.

5. - The crucible according to any of claims 1 to 4, further characterized in that at least 50% by weight of the silicon carbide grains have a particle size of more than 200 μm.

6. The crucible according to any of claims 1 to 5, further characterized in that the oxide or silicon nitride grains have a particle size of less than 10 μm.

7. A process for the manufacture of a crucible for the treatment of molten silicon, comprising a basic body with a bottom surface and side walls that define an interior volume, the basic body comprises at least 65% by weight of carbide of silicon, from 12 to 30% by weight of a constituent selected from silicon oxide or nitride and also comprises at least one oxide and / or silicon nitride coating, at least on the surfaces defining the interior volume of the crucible , comprising the following steps: a) forming the basic body with a bottom surface and side walls defining an interior volume; b) drying the basic body; c) cure the basic body; and d) forming a silicon oxide and / or nitride coating on at least the surfaces defining the interior volume of the crucible.

8. The method according to claim 7, further characterized in that steps c) and d) are carried out simultaneously by curing the basic body in an oxidizing or nitrating atmosphere.

9. - The method according to claim 7, further characterized in that step d) of forming the coating comprises applying the coating before the step of curing the basic body.

10. The use of a crucible comprising a basic body with a bottom surface and side walls that define an interior volume, the basic body comprises at least 65% by weight of silicon carbide, from 12 to 30% by weight of a constituent selected from silicon oxide or nitride and also comprises at least one oxide and / or silicon nitride coating, at least on the surfaces defining the interior volume of the crucible, for the treatment of molten silicon.