WO2011120598A1

WO2011120598A1 - Method for production of semiconductor grade silicon ingots, reusable crucibles and method for manufacturing them

Info

Publication number: WO2011120598A1
Application number: PCT/EP2010/065465
Authority: WO
Inventors: Kai Johansen; Stein Julsrud; Tyke Laurence Naas; Gero Wolfgang Neuroth
Original assignee: Rec Wafer Norway As
Priority date: 2010-03-30
Filing date: 2010-10-14
Publication date: 2011-10-06
Also published as: CN102859049B; WO2011120598A8; CN102859049A

Abstract

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, wherein the method is characterised in that the silicon ingots is produced in reusable crucibles made of carbon fibre-reinforced silicon carbide composite which has a coefficient of thermal expansion of less than 4- 10^-6 K^-1 at temperatures above 400 °C and less than 3- 10^-6 K^-1 at temperatures below 400 °C, and a thermal conductivity of at least 5 W/mK at temperatures from 25 °C to 1500 °C.

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 multi- crystalline 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 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 and/or recombination active oxygen complexes after annealing of the formed silicon. 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 - 100 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 have 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 phase 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 1^■ 10¹⁷ - 6· 10¹⁷/cm³ and 1^■ 10¹⁷ - 6· 10¹⁷/cm³ 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 has 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 and allow the silicon to infiltrate the green C/C-body (often termed Liquid Silicon Infiltration, LSI in the literature) such that at least some of the silicon 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

US 6 030 913 and EP 0 915 070.

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. The avoidance of SiO-formation in the hot zone alleviates the problem of oxygen induced LID affecting charge-carrier lifetimes. 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, S1₃N₄.

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- 10^"6 K ¹ at temperatures above 400 °C and less than 3 - 10^"6 K^"1 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 Table 1 The coefficient of thermal expansion of crystalline silicon as a function of temperature [2]

Temperature Coefficient o thermal expansion

[K] [10^~6 K^"1]

300 2.616

500 3.614

700 4.016

900 4.185

1 100 4.323

1300 4.442

1600 4.612

10 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 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 30 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.

A further advantage of C-C/SiC composites is that they may form reusable crucibles which in the case of CZ-process for growth of mo no -crystal line silicon crystals allows semi-continuous pulling by multiple usage of the crucible.

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 silicon,

- placing the feed of semiconductor/solar grade silicon in a crucible made of carbon fibre-reinforced silicon carbide composite which has a coefficient of thermal expansion of less than 4T 0^"6 K^"1 at temperatures above 400 °C and less than

3T0^"6 K^"1 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 in a hot zone which is sealed towards the ambient atmosphere of a melting and solidification furnace containing an inert atmosphere, and - heating the carbon fibre-reinforced silicon carbide composite crucible with silicon feed up to a temperature above 1414 °C to melt the silicon 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 mo no -crystal line 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 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 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 S1₃N₄ and thus bind the paste into a solid and wear resistant SiC-Si- S1₃N₄ release coating. If the coating is thin, all Si in the paste may be reacted such that a SiC- S1₃N₄ coating is formed. The coating is able to withstand several rounds of melting and

solidification of silicon before need of replacement/reapplyment. The release coat may advantageously be combined with a layer of conventional S1₃N₄ 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 silicon,

2) providing a crucible made of carbon fibre-reinforced silicon carbide composite which has a coefficient of thermal expansion of less than 4- 10^"6 K^"1 at temperatures above 400 °C and less than 3 · 10^"6 K^"1 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 in the coated crucible,

5) placing the crucible including feed of semiconductor/solar grade silicon 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 up to a temperature above 1414 °C to melt the silicon 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, 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 S1₃N₄ slip coating, either directly onto the inner walls of the crucible or onto the SiC-Si- S1₃N₄ 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 heating). 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- 10^"6 K^"1 at temperatures above 400 °C and less than 3 - 10^"6 K^"1 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 composite with thermal conductivity in the range from 25 to 35 W/mK as the bottom of the crucible, and employ carbon fibre-reinforced silicon carbide composite with thermal conductivity in the range from 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- 10^"6 K^"1 at temperatures above 400 °C and less than 3 - 10^"6 K^"1 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- 10^"6 K^"1 at temperatures above 400 °C and less than 3 - 10^"6 K^"1 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- S1₃N₄ 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 multi- crystalline 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 2007/148986. 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.

Example embodiments of the invention

The invention will be described in further detail by way of example embodiments of the first and second aspect of the invention. These examples should not be considered as a limitation of the general inventive idea of employing reusable C- C\SiC-crucibles with tailored properties for production of solar grade silicon feedstock. Example embodiment 1

This example embodiment is related to production of multi-crystalline material for solar cells.

A square C-C\SiC crucible of internal dimensions 690x690x420 mm, with 8 mm wall thickness, was manufactured by liquid silicon infiltration of a crucible made of a carbon/graphite fibre composite material, in accordance with a standard procedure (See e.g. Heidenreich 2007, [3]). The average coefficient of thermal expansion of the material after fabrication in the temperature range 30 - 1500 °C was 3 - 10^"6 K^_1. The crucible was coated with slurry of 33 wt% S1₃N₄ in de-ionized water, and afterwards cured following a temperature program ramping from room temperature to 1000 °C in 4 hours followed by 2 hours at top temperature of about 1200 °C.

The process was conducted in a vacuum furnace under argon atmosphere. Without undue exposure to ambient and in a controlled atmosphere room, the crucible was charged with 275 kg of PV grade silicon. Without undue delay the crucible was placed in the chamber of a crystallization furnace for the production of multi- crystalline material for solar cells. The crystallization furnace had before the test been baked out at 1500 °C for one hour to minimize the amount of adsorbed humidity and condensed oxides from previous runs. The run was conducted under standard conditions only modified to take into account the thermal properties of the crucible.

The CO content in the atmosphere was monitored during the run, and it was observed that the CO content was significantly lower that under runs with standard crucibles. The maximum content observed was less than 500 ppm while a typical run showed between 200 and 3000 ppm.

After the run the ingot could be slid out of the crucible using a turning device. No sticking to the crucible was observable. After demolding the ingot was cut with higher than normal yield. Some of the material was analyzed for carbon and oxygen, and showed very low values (<1 ppma of C and O). Several wafers from the ingot were processed further into solar cells. The average solar efficiency of the cells was higher than the average solar efficiency of cells made from standard production material.

Then the crucible was vacuum cleaned, recoated and reused according to the same procedure presented above and used for a second production round of producing multi-crystalline material for solar cells. The obtained results were similar to the results obtained in the first round described above.

Example embodiment 2

This example embodiment is related to production of mono-crystalline material for solar cells by the Czochralski-method.

An axially symmetric C-C\SiC crucible of 22" diameter (0i = 535 mm), with 1 1 mm wall thickness, was manufactured by liquid silicon infiltration of a crucible made of a carbon/graphite fibre composite material, in accordance with a standard procedure (See e.g. [3]). The average coefficient of thermal expansion of the material after fabrication in the temperature range 30 - 1500°C was

The crucible was charged in a controlled clean room with 120 kg of PV grade silicon. Without undue delay the crucible was placed in the chamber of an industrial CZ puller for the production of mo no -crystal line ingot material for solar cells. The CZ puller had before the test been baked out at 1500 °C for one hour to minimize the amount of adsorbed humidity and condensed oxides from previous runs. The run was conducted under standard conditions only modified to take into account the thermal properties of the crucible.

The CO content in the atmosphere was monitored during the run, and it was observed that the CO content was significantly lower that under runs with standard crucibles. The maximum content observed was less than 500 ppm while a typical run showed between 200 and 3000 ppm. The pot scrap (remaining silicon melt) in the crucible was removed using a graphite felt sponge.

After the run the ingot was demounted from the puller and test slugs have been taken in regular distances. Some of the material was analyzed for carbon and oxygen, and showed very low values (<1 ppma of C and O). Several wafers from the ingot were processed further into solar cells. The average solar efficiency of the cells was higher than the average solar efficiency of cells made from standard production material.

The crucible was reused according to the same procedure. The results were similar to the results obtained in the first round described above

The solar grade material obtained has comparable properties as FZ Silicon material when it comes to very low oxygen values which is used in semiconductor industry mainly for power device fabrication. Therefore the usage of C-C\SiC as crucible material for CZ crystal pulling is applicable as well for mo no -crystal line Silicon ingots for semiconductor purpose when it comes to low oxygen values as e.g. for electronic power devices.

References

1. XIN-BO et al, "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.

3. Heidenreich, Bernhard, "Carbon Fibre Reinforced SiC Materials Based on Melt Infiltration ", 6th International Conference on High Temperature Ceramic Matrix Composites, 2007, New Delhi, India.

Claims

1. Method for production of semiconductor or solar grade silicon ingots, wherein the method comprises

- providing a feed of semiconductor or solar grade silicon,

- placing the feed of semiconductor/solar grade silicon in a crucible made of carbon fibre-reinforced silicon carbide composite which has a coefficient of thermal expansion of less than 4- 10^"6 K ¹ at temperatures above 400 °C and less than 3 · 10^"6 K ¹ 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 in a hot zone which is sealed towards the ambient atmosphere of a melting and solidification furnace containing an inert atmosphere, and

- heating the carbon fibre-reinforced silicon carbide composite crucible with silicon feed up to a temperature above 1414 °C to melt the silicon 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 S1₃N₄-SKP coating.

3. Method for production of semiconductor grade silicon ingots, wherein the method comprises

1) providing a feed of semiconductor or solar grade silicon,

2) providing a crucible made of carbon fibre-reinforced silicon carbide composite which has a coefficient of thermal expansion of less than 4- 10^"6 K ¹ at temperatures above 400 °C and less than 3 - 10^"6 K ¹ 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,

5) placing the crucible including feed of semiconductor/solar grade silicon 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 up to a temperature above 1414 °C to melt the silicon feed, and perform 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 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, 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 mono- crystalline 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- 10^"6 K ¹ at temperatures above 400 °C and less than 3 - 10^"6 K ¹ 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 production of

semiconductor or solar grade silicon ingots, wherein the method comprises:

- 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- S1₃N₄ binding and sealing the elements of the crucible together.

1 1. 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 1 1 , wherein the bottom plate element and wall elements are formed with a thickness in the range from 1 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.