GB2479165A

GB2479165A - Reusable crucible

Info

Publication number: GB2479165A
Application number: GB1005399A
Authority: GB
Inventors: Stein Julsrud; Tyke Laurence Naas; Kai Johansen; Oivind Gjerstad
Original assignee: REC Wafer Norway AS
Current assignee: REC Wafer Norway AS
Priority date: 2009-10-14
Filing date: 2010-03-30
Publication date: 2011-10-05
Also published as: GB201005399D0

Abstract

A reusable crucible comprises a carbon fibre-reinforced silicon carbide composite which has a coefficient of thermal expansion of less than 4.10-6K-1at temperatures above 400°C and less than 3.10-6K-1at temperatures below 400°C, and a thermal conductivity of at least 5 W/mK at temperatures from 25°C to 1500°C. A method of producing silicon ingots comprising placing a feed of semiconductor or solar grade material into the crucible, placing the crucible and feed material in the sealed hot zone of a melting and solidification furnace containing an inert atmosphere, heating the crucible and feed material to above 1414°C and performing a directional solidification of the melted feed material to form an ingot. A method of production of the crucible is also provided for.

Description

Method for production of semiconductor grade silicon ingots, reusable crucibles and method for manufacturing them This invention relates to a method for production of ingots of semiconductor grade silicon, including solar grade silicon, to reusable crucibles employed in the method and a method for manufacturing the reusable crucibles.

Background

The world supplies of fossil oil are expected to be gradually exhausted in the coming decades. This means that our main energy source for the last century will have to be replaced within a few decades, both to cover the present energy consumption and the coming increase in the global energy demand.

In addition, many concerns are raised that the use of fossil energy increases the earth's greenhouse effect to an extent that may turn dangerous. Thus, the present consumption of fossil fuels should preferably be replaced by energy sources/carriers that are renewable and sustainable for our climate and environment.

One such energy source is solar light, which irradiates the earth with vastly more energy than the present day consumption, including any foreseeable increase in human energy consumption. One way of harvesting this source is by employing photovoltaic solar cells. However, solar cell electricity has to this date been too expensive to be competitive. This needs to change if the huge potential of the solar cell electricity is to be realised.

The cost of electricity from a solar panel is a function of the energy conversion efficiency and the production costs of the solar panel. Both the production cost of solar cells and the energy efficiency should be improved.

The dominating process route for silicon based solar panels of multi-crystalline wafers is presently by sawing multi-crystalline ingots into blocks and then further to wafers. The multicrystalline ingots are formed by directional solidification using the Bridgman method or related techniques. A main challenge in the ingot fabrication is to maintain the purity of the silicon raw material and to obtain a sufficient control of the temperature gradients during the directional solidification of the ingots in order to obtain satisfactory crystal quality.

The problem with contamination is strongly connected to the crucible material since the crucible is in direct contact (or indirect contact through a release coating) with the molten silicon. Build-up of carbon in the silicon metal may lead to formation of SiC crystals, especially in the uppermost region of the ingot. These SiC crystals are known to short-cut pn-junctions of the semiconductor cell, leading to drastically reduced cell efficiencies. Build up of interstitial oxygen may lead to oxygen precipitates andlor recombination active oxygen complexes after annealing of the formed silicon metal. The material of the crucibles should therefore be chemically inert towards molten silicon and withstand the high temperatures up to about 1500 °C for relatively long periods.

The crucible material is also important for achieving an optimal control of the temperature since the heat extraction during solidification of the ingot in these production methods is obtained by maintaining a lower temperature in the area below the crucible support, creating a heat sink for the heat of crystallization and transported heat from the upper part of the furnace through the silicon melt, silicon crystals, crucible bottom and support plate. The upper part of the furnace consists of the volume above the support plate, including the crucible or crucibles with contents. The heat transport through the crucible bottom and further through the support plate is dominated by thermal conduction according to Fourier's law of heat conduction.

In present day industrial production based on the Bridgman method, the crucibles usually stand on a graphite platform of dimensions sufficient to carry the load of the filled crucibles. The necessary thickness for mechanical stability will be in the range 3-10 cm. The thermal conductivity of isotropic graphite is in the range 50 -W/mK.

The heat flux from the heat of crystallization of the silicon, the heat transported from the top heater to the bottom heater through the ingot and crucible and the heat stored in the materials in the hot zone should ideally be vertically oriented, i.e., have no lateral component. However, in current practice, the various known furnace designs are all characterized by lateral transport of heat. This gives rise to thermal stresses and generates dislocations in the crystallized silicon.

Prior art

Silicon dioxide (fused silica), Si02, is presently the preferred material for crucible and mould applications due to availability in high purity form. When employed for directional solidification methods, the silica is wetted by the molten silicon, leading to a strong adherence between the ingot and the crucible. During cooling of the ingot, the strong adherence leads to cracking of the ingot due to build-up of mechanical tension resulting from the higher coefficient of thermal expansion of the silicon as compared to silica. During the furnace process, the silica material in the crucible is transformed from a glassy to a partly crystalline phase. During cooling, the crystalline Si02 undergoes a phase transition that causes cracking. For this reason, the silica crucibles may only be used once. This gives a significant contribution to the production cost of the ingots.

Further, the thermal conductivity of the fused silica material from which the crucible is made is around 1-2 W/mK. The crucible walls and bottom will typically Ifave a thickness in the range of 1-3 cm. Thus, in the configuration presently employed by the industry, the crucible bottom is the dominating thermal resistance.

With typical crucible bottom thickness of about 2 cm and support plate thickness 5 cm, 90 -98% of the total temperature difference is localised across the crucible bottom. Thus the attainable rate of heat removal is limited by the great thermal resistance of the silica crucible. Also, any attempt to vary the heat flux locally, e.g. in the lateral direction will be hampered by the very limited possibility to control the heat flux.

The use of silicon oxide crucibles also entails a problem of contamination of the silicon ingot, since the reaction products of Si and Si02 is gaseous SiO, which may subsequently escape the molten metal and react with graphite in the hot zone forming CO gas. The CO gas readily enters the silicon melt and thus introduces carbon and oxygen into the silicon. That is, the use of a crucible of oxide-containing materials may cause a sequence of reactions leading to introduction of both carbon and oxygen in the solid silicon. Typical values associated with the Bridgman method is interstitial oxygen levels of ii'-610'7/cm2 and ii'-6iO'7/cm2 of substitutional carbon. Frequently, carbon is present above the saturation level as precipitates of SiC.

It has therefore been attempts to find other materials for crucibles that may be reused as crucible or mould for directional solidification of semiconductor grade silicon. Such a crucible needs to be made of a material that is sufficiently pure and chemically inert towards the molten silicon to allow high-purity ingots being formed, and which has a thermal expansion that does not lead to the strong mechanical tensions between ingot and crucible during cooling.

One proposed solution to these problems with silica crucibles have been using crucibles made of graphite. This material will, however, react with liquid silicon and contaminate the melt with carbon as well as stick to the ingot. Coating the inner surface of the crucible with various protection layers has been suggested to avoid this, but no coating has so far proved effective in an industrial setting.

Another candidate has been crucibles made from essentially silicon carbide. A problem with these has been that the linear thermal expansion is higher than the linear thermal expansion of silicon, thus leading to the ingot being stuck in the crucible after cooling or the crucible cracking, preventing reuse.

Also, from WO 2007/148986 it is suggested to make reusable crucibles from plate elements made of RBSN (Reaction Bonded Silicon Nitride) and NBSN (Nitride Bonded Silicon Nitride). The crucible is mounted by sealing wall elements and a bottom element together by use of silicon particle containing paste which is nitrided to form solid silicon nitride by heating in a nitrogen atmosphere. Alternatively, the RBSN-or NBSN-crucible may be formed in one piece. But neither of these have so far found industrial application, most likely due to problems with mechanical stability during the furnace process.

From [1] it is known that silicon carbide is an important structural ceramic, because of its many excellent properties, such as oxidation resistance, strength retention at high temperature, wear resistance and thermal shock resistance. However, like all ceramic materials, it is generally notch-sensitive and has a low toughness, resulting in relatively low reliability as structural components and thus limited applications.

Therefore, a number of investigations have focused on the improvement in the reliability, and continuous fibre reinforced SiC composites have been demonstrated to be the most effective approach to improving toughness.

A material known to have extreme resistance towards thermal shock and mechanical wear is ceramic composites comprising carbon fibre-reinforced silicon carbide ceramics (C-C/SiC or C/SiC composites). These materials have presently found use as friction linings in braking systems of automotive applications, in aero-space applications, as combustor chamber linings, in turbine blades, in jet engine nozzles, etc. From US 7 238 308 it is known that C-C/SiC or C/SiC composites may be produced by forming an intermediate body of carbon fibre-reinforced polymer (CRFP), heating the CRFP until the polymer is pyrolysed to form a porous green body of carbon fibre-reinforced carbon (C/C-body), contacting the green C/C-body with molten silicon metal and allow the metal to infiltrate the green C/C-body (often termed Liquid Silicon Infiltration, LSI in the literature) such that at least some of the silicon metal reacts with the carbon phase of the C/C-body and forms silicon carbide, and thus providing a carbon fibre-reinforced composite ceramic having a matrix comprising SiC, Si, and C. Similar techniques and materials are known from US6 030 913 andEPO 915 070.

EP 1 547 992 disclose a method for manufacturing C-C/SiC composites from a mixture of resin and carbon fibres which is pyrolysed to a green body directly without first hardening the resin. Then the green body is infiltrated with silicon to form the C-C/SiC composite. By varying the relative amounts of the ingredients, it is possible to produce composites with tailored amounts of C/C and C/SiC, and thus make composites with different thermal conductivities.

Objective of the invention The main objective of the invention is to provide a method for manufacturing high-purity ingots of semiconductor/solar grade silicon employing reusable crucibles.

A further objective is to provide reusable crucibles for production of high-purity ingots of semiconductor/solar grade silicon.

A further objective of the invention is to provide a method for manufacturing the reusable crucibles.

The objective of the invention may be realised by the features as set forth in the description of the invention below, and/or in the appended patent claims.

Description of the invention

The invention is based on the realisation that C-C/SiC composites may be employed to form reusable crucibles for manufacturing semiconductor/solar grade silicon ingots.

C-C/SiC composites have no oxygen-containing components such that the problem with formation of gaseous SiO in the hot zone of the furnace and subsequent migration of CO into the silicon melt is significantly alleviated. It is also relatively easy to control the level of other detrimental impurities in C-C/SiC composites to acceptable levels for use as crucibles for the manufacture of semiconductor/solar grade silicon ingots. The effect of employing a crucible with no oxygen-containing components (except for inevitable impurities) may be enhanced by employing a hot zone of the melting/solidification furnace made of materials without oxygen-containing compounds. Examples of such materials are carbon and/or graphite materials carbon and/or graphite materials as insulating and structural load carrying elements and electric insulating elements made of silicon nitride, Si3N4.

Another advantage with C-C/SiC composites is that they may be manufactured with a tailored coefficient of thermal expansion such that the crucibles may be given a lower thermal expansion than the silicon ingot, and thus avoid the problem of a higher shrinkage of the crucible than the solid ingot during cooling, resulting in breakage of the crucible or the ingot. According to [2], the coefficient of thermal expansion of crystalline silicon is as shown in Table 1, and the melting point of crystalline silicon is 1414 °C. Thus, in practice, the coefficient of thermal expansion of the C-C/SiC composite to be used in crucibles for production of crystalline silicon ingots should be less than 4.106 K' at temperatures above 400 °C and less than 3* 106 K' at temperatures below 400 °C.

A further advantage is that C-C/SiC composites also may be given tailored anisotropic thermal conductivities providing C-C/SiC composites with thermal conductivities typically in the range from 10 W/mK up to about 35 W/mK at room temperature, and from about 10 W/mK up to about 25 W/mK at 1600 °C For present day direct solidification furnaces, including those based on the Bridgman method, the thermal resistance across the graphite support carrying the crucible is typically in the order from 0.002 to 0.0003 m2K/W (thickness typically from about 3 to about cm and thermal conductivity in the order of 50 to 100 W/mK). For a crucible with bottom thickness of 1-3 cm, this implies that the thermal conductivity of the Table 1 The coefficient of thermal expansion of crystalline silicon as a function of temperature [2] Temperature Coefficient o thermal expansion EKI 11o6 K'] 300 2.616 500 3.614 700 4.016 900 4.185 1100 4.323 1300 4.442 1600 4.612 crucible material should be at least about 5 W/mK or higher. As mentioned above, the C-C/SiC composite crucible may obtain thermal conductivities from 10 up to 35 W/mK. C-C/SiC composites will thus have a thermal conductivity of a factor of 5 to higher than the presently employed Si02-crucibles, and thus significantly alleviate the problem with the bottom of the crucible dominating the thermal resistance during the directional solidification of the ingot. Moreover, due to the more advantageous mechanical properties of the C-C/SiC composites, it is possible to manufacture a mechanically stable crucible with a thinner bottom than the presently used Si02-crucibles.

Thus, in a first aspect of the invention there is provided a method for production of semiconductor grade silicon ingots, wherein the method comprises -providing a feed of semiconductor or solar grade metal, -placing the feed of semiconductor/solar grade metal in a crucible made of carbon fibre-reinforced silicon carbide composite which has a coefficient of thermal expansion of less than 4.j�6 K' at temperatures above 400 °C and less than 3 10 K1 at temperatures below 400 °C, and a thermal conductivity of at least 5 W/mK at temperatures from 25 °C to 1500 °C, -placing the carbon fibre-reinforced silicon carbide composite crucible with silicon feed metal in, a towards the ambient atmosphere, sealed hot zone of a melting and solidification furnace containing an inert atmosphere, and -heating the carbon fibre-reinforced silicon carbide composite crucible with silicon feed metal up to a temperature above 1414 °C to melt the metal feed, and -performing a directional solidification of the melted silicon to form a semiconductor/solar grade silicon ingot.

The method according to the first aspect of the invention may be employed for any known process including for crystallising semiconductor grade silicon ingots, including solar grade silicon ingots, such as the Bridgman process or related direct solidification methods, the block-casting process, and the CZ-process for growth of monocrystalline silicon crystals.

The term "inert atmosphere" as used herein means an atmosphere which is essentially chemically inert towards the materials of the melting and solidification furnace, crucible, and the silicon metal phase, both in the solid and liquid state. The term as used herein includes any gas pressure of the inert atmosphere, including vacuum. Argon is an example of suitable inert gases, other examples includes the other noble gases. Also, any gas known to be chemically inert towards silicon and carbon at temperatures from room temperature up to about 1700 °C may be employed as inert atmosphere.

The C-C/SiC composite crucible may advantageously be coated with a release coating before use to alleviate the release of the silicon ingot after melting and solidification. This may be obtained by spraying the surface of the crucible which is to be coated by a slurry containing silicon particles and silicon nitride particles, followed by heating the crucible including coating in a nitrogen atmosphere up to a temperature of about 1200 °C or higher. At these elevated temperatures, silicon particles of the paste in close proximity to carbon phases of the C-C/SiC composite crucible will react to form SiC and thus form a tight bonding to the crucible, while silicon particles of the paste which comes in contact with nitrogen gas is nitrided to form Si3N4 and thus bind the paste into a solid and wear resistant SiC-Si-Si3N4 release coating. If the coating is thin, all Si in the paste may be reacted such that a SiC-Si3N4 coating is formed. The coating is able to withstand several rounds of melting and solidification of silicon metal before need of replacement/reapplyment.

The release coat may advantageously be combined with a layer of conventional Si3N4 slip coating onto the SiC-Si-Si3N4 or SiC-Si3N4 coating.

Thus in a second aspect, the invention relates to a method for production of semiconductor grade silicon ingots, wherein the method comprises 1) providing a feed of semiconductor or solar grade metal, 2) providing a crucible made of carbon fibre-reinforced silicon carbide composite which has a coefficient of thermal expansion of less than 4.106 K' at temperatures above 400 °C and less than 310 K' at temperatures below 400 °C, and a thermal conductivity of at least 5 W/mK at temperatures from 25 to 1500 °C, 3) spraying at least the inner surface of the carbon fibre-reinforced silicon carbide composite crucible by a slurry containing silicon particles and silicon nitride particles, followed by heating the crucible including coating in a nitrogen atmosphere up to a temperature of about 1200 °C or higher, followed by cooling to room temperature to form a crucible with release coating, 4) placing the feed of semiconductor/solar grade silicon metal in the coated crucible, 5) placing the crucible including feed of semiconductor/solar grade metal in a hot zone compartment of a melting and solidification furnace, sealing off the hot zone compartment of the furnace towards the ambient atmosphere, and filling the hot zone compartment with an inert gas, 6) heating the carbon fibre-reinforced silicon carbide composite crucible with silicon feed metal up to a temperature above 1414 °C to melt the metal feed, and performing a directional solidification of the melted silicon to form a semiconductor/solar grade silicon ingot, 7) cooling the crucible including silicon ingot to a temperature below 200 °C and removing the silicon ingot, 8) controlling that the release coating is in functional order for a new cycle of melting and directional solidification of a feed of semiconductor/solar grade silicon metal, and 9) if the release coating is not in functional order, repeating steps 3) to 8); and if the coating is in functional order, repeating steps 4) to 8).

Both the first and second aspect of the invention may advantageously be combined with applying a layer of conventional Si3N4 slip coating, either directly onto the inner walls of the crucible or onto the SiC-Si-Si3N4 or SiC-Si3N4 coating.

It may be advantageous to apply C-C/SiC composites with a relatively high thermal conductivity at the bottom of the crucible and apply C-C/SiC composites with a low thermal conductivity compared to the bottom at the walls of the crucible. This will provide the possibility of employing an increased rate of crystallization combined with a more vertically oriented and linear heat flux in the crucible and silicon. The situation where heat is extracted in such a way that the temperature gradients are linear within one material layer with respect to vertical position can be termed a quasi steady state cooling (or beating). It is possible to maintain this situation over a much wider range of cooling (heating) rates using the present invention than in conventional methods employing silica crucibles. The thermal insulation effect of the walls may be enhanced by employing a layer of graphite or carbon felt outside the crucible walls.

In a third aspect, the invention relates to crucibles for production of semiconductor or solar grade silicon ingots, wherein the crucibles are made of carbon fibre-reinforced silicon carbide composite which has a coefficient of thermal expansion of less than 4.106 K' at temperatures above 400 °C and less than 3.106 K' at temperatures below 400 °C, and a thermal conductivity of at least 5 W/mK at temperatures from 25 °C to 1500 °C.

As mentioned, the crucible may advantageously be given a relatively high thermal conductivity across the bottom and a comparably lower thermal conductivity across the walls in order to alleviate problems with heat fluxes in lateral direction and with too high thermal resistance across the crucible. The crucible according to the invention may thus advantageously employ carbon fibre-reinforced silicon carbide composites with thermal conductivities in the range of 25 to 35 W/mK as the bottom of the crucible, and employ carbon fibre-reinforced silicon carbide composites with thermal conductivities in the range of about 10 to 15 W/mK as the wall elements of the crucible. This may be obtained by forming plate elements of the low thermal conductivity C-C/SiC composite for the wall and high thermal conductivity C-C/SiC composite for the bottom of the crucible, and mount the elements to form a crucible by employing the coating slurry containing silicon particles and silicon nitride particles to both coat the inner walls with a release coating and to bind the bottom and wall elements together. The bonding is obtained by the reaction of the silicon particles with nitrogen and with carbon phases of the C-C/SiC composite when heated to at least 1200 °C to form silicon nitride and silicon carbide, respectively. Thus the release coating and assembly of the crucible may be combined to one operation.

Thus in a fourth aspect, the present invention relates to a method for manufacturing reusable crucibles for production of semiconductor or solar grade silicon ingots, wherein the method comprises: -forming in a per se known manner a bottom plate element of carbon fibre-reinforced silicon carbide composite which has a coefficient of thermal expansion of less than 4.106 K' at temperatures above 400 °C and less than 3.106 K' at temperatures below 400 °C, and a thermal conductivity of at 25 -35 W/mK at temperatures from 25 °C to 1500 °C, -forming in a per se known manner one or more wall elements of carbon fibre-reinforced silicon carbide composite which has a coefficient of thermal expansion of less than 4.106 K' at temperatures above 400 °C and less than 3.106 K' at temperatures below 400 °C, and a thermal conductivity of 10 -15 W/mK at temperatures from 25 °C to 1500 °C, -applying a paste/slurry containing silicon particles and silicon nitride particles to at least the surfaces of the bottom plate element and the at least one wall element forming the inner walls of the crucible to be, and the joint surfaces of the bottom plate element and the at least one wall element, -assembling the bottom plate element and the at least one wall element to a green crucible, and -heating the green crucible in a nitrogen atmosphere up to a temperature of 1200 °C or higher to form a combined release coating and adhesive of SiC-Si-Si3N4 or SiC-Si3N4 binding and sealing the elements of the crucible together.

The dimensions and shape of the C-C/SiC composite bottom and wall elements may be any conceivable for present and future production of semiconductor or solar grade silicon ingots. In case of multicrystalline silicon made by the Bridgman method, the crucible may advantageously be given a square shape where the bottom element is formed as a square shape and wall elements are formed as plates with square or rectangular shape. In this case one may use four plate elements and one bottom element, as shown in WO 20071148986 In case of employing the CZ-method, the crucible may advantageously be cylindrical, which may be obtained by employing a circular bottom plate element and one tubular wall element. The thickness of the bottom and wall elements will usually be in the range from 1 to 3 cm. The bottom is typically 2 cm thick, while the wall elements are 1 -2 cm thick.

1. XIIN-BO et a!., "Carbon-fiber-reinforced silicon carbide composites", Journal of Materials Science Letters, 19 (2000) 417-419.

2. Robert Hull, ed., "Properties of crystalline silicon", INSPEC, London, 1999.

Claims

CLAIMS1, Method for production of semiconductor grade silicon ingots, wherein the method comprises: -providing a feed of semiconductor or solar grade metal, -placing the feed of semiconductor/solar grade metal in a crucible made of carbon fibre-reinforced silicon carbide composite which has a coefficient of thermal expansion of less than 4.106 K' at temperatures above 400 °C and less than 310 K' at temperatures below 400 °C, and a thermal conductivity of at least 5 W/rnK at temperatures from 25 °C to 1500 °C, -placing the carbon fibre-reinforced silicon carbide composite crucible with silicon feed metal in, a towards the ambient atmosphere, sealed hot zone of a melting and solidification furnace containing an inert atmosphere, and -heating the carbon fibre-reinforced silicon carbide composite crucible with silicon feed metal up to a temperature above 1414 °C to melt the metal feed, and -performing a directional solidification of the melted silicon to form a semiconductor/solar grade silicon ingot.
2. Method according to claim 1, wherein the method also comprises coating the inner walls of the crucible with a layer of Si3N4 slip coating.
3. Method for production of semiconductor grade silicon ingots, wherein the method comprises 1) providing a feed of semiconductor or solar grade metal, 2) providing a crucible made of carbon fibre-reinforced silicon carbide composite which has a coefficient of thermal expansion of less than 410 K' at temperatures above 400 °C and less than 3.106 K' at temperatures below 400 °C, and a thermal conductivity of at least 5 W/mK at temperatures from 25 to 1500 °C, 3) spraying at least the inner surface of the carbon fibre-reinforced silicon carbide composite crucible by a slurry containing silicon particles and silicon nitride particles, followed by heating the crucible including coating in a nitrogen atmosphere up to a temperature of about 1200 °C or higher, followed by cooling to room temperature to form a crucible with release coating, 4) placing the feed of semiconductor/solar grade silicon metal in the coated crucible, 5) placing the crucible including feed of semiconductor/solar grade metal in a hot zone compartment of a melting and solidification furnace, sealing off the hot zone compartment of the furnace towards the ambient atmosphere, and fill the hot zone compartment with an inert gas, 6) heating the carbon fibre-reinforced silicon carbide composite crucible with silicon feed metal up to a temperature above 1414 °C to melt the metal feed, and perform a directional solidification of the melted silicon to form a semiconductor1-solar grade silicon ingot, 7) cooling the crucible including silicon ingot to a temperature below 200 °C and remove the silicon ingot, 8) control that the release coating is in functional order for a new cycle of melting and directional solidification of a feed of semiconductor/solar grade silicon metal, and 9) if the release coating is not in functional order, repeat steps 3) to 8); and if the coating is in functional order, repeat steps 4) to 8).
4. Method according to claim 1 or 3, wherein the method is applied for production of polycrystalline silicon ingots according to the Bridgeman process or the block-casting process, or for production of monocrystalline silicon ingots according to the Czochralski process.
5. Method according to claim 3, wherein the method also comprises coating the inner walls of the crucible with a layer of Si3N4 slip coating directly onto the SiC-Si-Si3N4 or SiC-Si3N4 coating.
6. Crucible for production of semiconductor or solar grade silicon ingots, wherein the crucibles are made of carbon fibre-reinforced silicon carbide composite which has a coefficient of thermal expansion of less than 4.1 0 K' at temperatures above 400 °C and less than 3.10.6 W' at temperatures below 400 °C, and a thermal conductivity of at least 5 W/mK at temperatures from 25 °C to 1500 °C.
7. Crucible according to claim 6, wherein the thickness of the bottom and wall elements is in the range from 1 to 3 cm, preferably the bottom has thickness of 2 cm and the wall elements are 1 -2 cm thick.
8. Crucible according to claim 6 or 7, wherein the inner walls of the crucible is provided with a slip coating of one of the following; a layer of SiC-Si-Si3N4 or SiC-Si3N4; a layer of Si3N4; or a layer of SiC-Si-Si3N4 or SIC-Si3N4 followed by a layer of Si3N4.
9. Crucible according to claim 6, 7, or 8, wherein the carbon fibre-reinforced silicon carbide composite applied as the bottom of the crucible has a thermal conductivity of 25 -35 W/mK and that the carbon fibre-reinforced silicon carbide composite applied as the walls of the crucible has a thermal conductivity of 10 -15 W/mK.
10. Method for manufacturing reusable crucibles for manufacturing reusable crucibles for production of semiconductor or solar grade silicon ingots, wherein the method comprises: -forming in a per se known manner a bottom plate element of carbon fibre-reinforced silicon carbide composite which has a coefficient of thermal expansion of less than 4.106 K' at temperatures above 400 °C and less than 3.106 K1 at temperatures below 400 °C, and a thermal conductivity of at 25 -35 W/ml( at temperatures from 25 °C to 1500 °C, -forming in a per se known manner one or more wall elements of carbon fibre-reinforced silicon carbide composite which has a coefficient of thermal expansion of less than 4*10 K' at temperatures above 400 °C and less than 3.106 K1 at temperatures below 400 °C, and a thermal conductivity of 10 -15 W/mK at temperatures from 25 °C to 1500 °C, -applying a paste/slurry containing silicon particles and silicon nitride particles to at least the surfaces of the bottom plate element and the at least one wall element forming the inner walls of the crucible to be, and the joint surfaces of the bottom plate element and the at least one wall element, -assembling the bottom plate element and the at least one wall element to a green crucible, and -heating the green crucible in a nitrogen atmosphere up to a temperature of 1200 °C or higher to form a combined release coating and adhesive of SIC-Si-S13N4 or SiC-Si3N4 binding and sealing the elements of the crucible together.
11 Method according to claim 10, wherein the method further comprises coating the inner walls of the crucible with a layer of S13N4 slip coating in a per se known manner after the step of heating the green crucible in a nitrogen atmosphere.
12. Method according to claim 10 or 11, wherein the bottom plate element and wall elements are formed with a thickness in the range from I to 3 cm.
13. Method according to claim 12, wherein the bottom plate element is given a thickness of 2 cm and the wall elements of 1 -2 cm.