WO2013160236A1

WO2013160236A1 - Silicon nitride containing crucible and a method of producing the silicon nitride containing crucible

Info

Publication number: WO2013160236A1
Application number: PCT/EP2013/058274
Authority: WO
Inventors: Haarvard SOERHEIM; Arve Solheim
Original assignee: Saint-Gobain Ceramic Materials A. S.
Priority date: 2012-04-24
Filing date: 2013-04-22
Publication date: 2013-10-31

Abstract

The invention is thus a ceramic crucible for the crystallization of polycrystalline silicon, which comprises: a base body (1) with an inner bottom surface and sidewalls with an inner height h, the inner bottom and the inner side walls having a first zone coating (2) in a first zone (II) up to a height h' containing silicon nitride and/or silicon oxynitride and the inner side walls having a second zone coating (3) in a second zone (III) above the first zone coating (2) with a height h'' containing silicon nitride or silicon oxynitride, wherein the oxygen content of the second zone coating (3) is higher than the oxygen content of the first zone coating (2).

Description

Silicon Nitride Containing Crucible and a Method of Producing the Silicon Nitride Containing Crucible

The invention relates to a reusable silicon nitride containing crucible with a coating for molten material like silicon that is solidified in the crucible and then removed as solidified silicon ingot. The invention further relates to a method of manufacturing of a reusable silicon nitride containing crucible.

W01998/035075 A1 discloses a crucible made of fused silica. Molten silicon reacts with the fused silica crucible to form silicon monoxide and oxygen. Oxygen contaminates the molten silicon. Silicon monoxide is volatile at the oven temperature and reacts with graphite in the furnace to form silicon carbide and carbon monoxide. Carbon monoxide reacts with the molten silicon to form more volatile silicon monoxide and silicon carbide.

WO 98/35075 discloses a crucible made of silica, graphite or ceramics and equipped with a protective film comprising silicon nitride. The protective film consists of silicon nitride powder with powder particles featuring an oxygen content of 0.3 weight % to 5 weight %, whereas the ratio of particle length to particle diameter is < 10. Preferred embodiments of the crucible hold a limited content of alkali metals, alkaline earth metals, fluoride, chloride, carbon and of iron, chrome, cobalt, nickel, tungsten and titanium containing particles in addition to the maximum oxygen content of 5 weight %.

Further, the chemical reaction between fused silica and silicon leads to adhesion and a sticking of silicon to the crucible. The sticking and the difference in thermal expansion coefficients cause stress and cracking of the solidified silicon ingot during the cooling process.

JP-59-162199 A discloses a crucible using silicon nitride and manufacture thereof. The crucible can be manufactured by pressing silicon powder. Afterwards it is heated in an inert atmosphere in a first step. Thereafter, in a second step the nitridation is carried out at higher temperature. The crucible has a density of 85% of the theoretical density for silicon nitride. The silicon showed a high degree of adherence to the crucible due to the wetting. A higher degree of cracking of the solidified silicon ingot even breakage of the crucibles occurred when releasing the solidified silicon ingot. WO 2004/016835 A1 discloses a silicon nitride crucible for use in connection with directional solidification and pulling of silicon single crystals. The mould crucible consists of Si₃N₄ having a total open porosity between 40 and 60% by volume and where more than 50% of the pores in the surface of the mould parts have a size which is larger than the means size of the Si₃N₄ particles. This crucible shows no tendency of being wetted by liquid silicon and allows a smooth release of the silicon ingot. WO 2004/016835 A1 also discloses a crucible which is coated on its inside walls with Si₃N₄ powder. The solidified silicon ingot was not affixed to the crucible.

WO 2004/074212 A1 discloses a Si₃N₄ surface coating on ceramic components. There is suggested to impregnate the crucible with a slip comprising silicon particles having a size lower than 200 μηη. Then the impregnated ceramic body is burned under nitrogen atmosphere whereby the silicon is converted into Si₃N₄.

WO 201 1/098319 A1 discloses a crucible for photovoltaic. There is a selection of reaction bonding Si₃N₄ coating disclosed for solving the problem of low ability sintering of Si₃N₄.

WO 2004/053207 A1 discloses a vessel for holding silicon. It is suggested to use a mixture of metallic silicon, silica and Si₃N₄ powder and to apply it to the surface by thermal spraying. This coating gets less impregnated by the bath and leads to lower bath contamination. It is an expensive solution and requires tools, which are usually not available in silicon melting plants.

WO 2006/002779 A1 discloses a process for chills with an anti-adhesive coating. A formulation comprising Si₃N₄ powder, an inorganic binder for example pure silica binder having a specific area up to 300m²/g and a solvent is suggested. This formulation can be deposited on the surface of the crucible by conventional coating methods.

WO 2007/039310 A1 proposes a coating suspension, which comprises Si₃N₄ particles and a low temperature mineral binder to avoid the oxidation of Si₃N₄ during the sintering of the coating and a fast increase of oxygen level within the coating. The total oxygen content of the coating has a range from 5 % by weight to 15 % by weight.

EP 2058289 A1 discloses a coating suspension, which comprises Si₃N₄ particles and a Si0₂-based high temperature binder. The coating may be heat treated under air in a temperature range from 300°C - 1300°C without oxidizing extensively the Si₃N₄ particles. However a reducing firing treatment is preferred in this process. US 4,099,924 discloses a crucible comprising a silicon oxynitride coating deposited by chemical vapor deposition (CVD). The coating is obtained by heating the crucible to a temperature of 800°C to 1500°C in the presence of ammonia, nitrogen oxide (nitric oxide or nitrous oxide) and silane or silicon tetra chloride, whereas hydrogen is used as carrier gas. Such a deposition of the coating by CVD is expensive and difficult to handle due to the mandatory safety precautions.

Multilayer coatings for crucibles are disclosed in DE 10 2008 031766 A1 or EP 1899508 B1. These coatings are difficult to process on an industrial scale.

WO 201 1/0786923 A1 discloses a reusable silicon nitride containing crucible for crystallizing silicon. The crucible comprises boron or boron containing compound in a concentration of < 19 ppmw and phosphorous or phosphorous containing compound in a concentration of < 3.7 ppmw. It is also disclosed therein that a crucible is coated with a silicon nitride powder so that the crucible is never in direct contact with silicon.

Coatings for reusable ceramic crucibles known in the state of the art have several shortcomings. The crucibles get wet by molten silicon and the molten silicon, which is solidified in the crucibles sticks to the walls of the crucibles. A chemical reaction between the molten silicon and Si₃N₄ of the crucible in particularly above the liquid level occurs. The oxygen content is too high and the contamination of the silicon ingot is the consequence. The cracking of the silicon ingot after cooling needs to be minimized by a release layer. The undesired wetting of the crucible by molten silicon especially on the top of the crucible and even in large crucibles is a common problem. It is well known that a silicon nitride coating needs certain oxygen content in order to be non wetting towards liquid silicon. Crucibles containing a certain amount of oxygen to avoid a sticking of the silicon are already known, but all of these crucibles cause a rather high contamination of the silicon ingot with oxygen.

Considering coatings for crucibles known in the state of the art there is a need for an appropriate coating for reusable ceramic crucibles, in particular silicon nitride crucibles and more particularly reaction bonded silicon nitride (RBSN) crucibles, which is able to avoid a sticking of silicon and to minimize the contamination of the silicon ingot with oxygen at the same time. The object of the present invention is to provide a coating formulation for reusable silicon nitride crucibles and economic and ecological process for coating the silicon nitride crucibles.

The solution of the object of the present invention is a reusable silicon nitride containing crucible with a coating for use in connection with directional solidification of molten silicon and pulling of silicon ingots comprising

a base body with an inner bottom surface and sidewalls with an inner height h, the inner bottom and the inner side walls having a first zone coating in a first zone up to a height h' containing silicon nitride or silicon oxynitride and

the inner side walls having a second zone coating in a second zone above the first zone coating with a height h" containing silicon nitride or silicon oxynitride and

the oxygen content of the second zone coating being higher than the oxygen content of the first zone coating. All specifications concerning the oxygen content are measured in weight percent.

The second zone coating could also contain a mixture of silicon nitride and silicon oxynitride.

The oxygen content of the first zone coating is comprised between 0.5 weight % and 15 weight %, more preferably between 1 weight % and 5 weight %, more preferably between 1 .2 weight % and 1.5 weight %.

The oxygen content of the second zone coating is comprised between 1 % and 50 %, preferably between 1 % and 20%, more preferably between 5 % and 15 %, even more preferably between 10 weight % and 15 weight %.

These oxygen contents of the first zone coating and the second zone coating are chosen within the given ranges to guarantee an optimum performance of the coating. The minimization of the oxygen content of the first coating leads to a low and acceptable contamination of the ingot, while the higher oxygen content of the second zone coating avoids sticking of the silicon melt. Especially an oxygen content of the second zone coating between 10 weight % and 15 weight % is highly advantageous as such a coating is easy to realize concerning its production and provides enough oxygen to avoid sticking. The oxygen content of the base body is less than 3 %, in preference less than 2 %. A low oxygen content of the base body prevents the contamination of the silicon ingot by the crucible itself .

Thus the coated crucible according to the invention reduces the oxygen content of the ingot most effectively and at the same time prevents adhesion of the silicon ingot and can be reused several times.

The oxygen content of the first zone coating and the second zone coating may be varied by the thickness of the coatings as a thicker coating is able to provide more oxygen than a thinner coating.

Preferably the second zone coating comprises a mixture of 50 weight % to 95 weight % containing Si₃N₄ and of 5 weight % to 50 weight % containing silica.

An advantageous embodiment of the invention is a silicon nitride containing crucible, wherein the second zone coating contains 75 weight % to 85 weight % silicon nitride and 15 weight % to 25 weight % silica. A very good result is achieved with the preferred second zone coating.

The open porosity of the base body is between 15 % and 60 % by volume, in preference from 20 % to 40 % by volume. The base body has an average pore size from 0.1

micrometers to 10 micrometers, preferably from 0.2 micrometers to 5 micrometers. By use of a porous material the insulating property of the crucible and the release of thermo mechanical stress is improved, which enhances the lifetime and reusability of the crucible. This was confirmed by experiments with crucibles according to the invention having a wall thickness between 10 mm to 25mm in particular a wall thickness of about 15 mm. All open porosities are measured in volume percent and all pore sizes are measured in micrometers. The average pore size is measured by mercury porosimetry.

The ceramic crucible is non oxide one, preferably a silicon nitride and more preferably RBSN type of crucible.

Preferably the first zone coating of the silicon nitride containing crucible contains Si₃N₄ powder. The coating according to the invention prevents the chemical reaction between the molten silicon and Si₃N₄ of the crucible in particularly upward the liquid level of silicon. The oxygen content is lower and the contamination of the silicon ingot is limited. The cracking of the silicon ingot after cooling is minimized. The wetting of the crucible by molten silicon, especially on the top of the crucible is avoided. This is very useful for large crucibles especially but not exclusively with dimensions like 690 mm x 690 mm x 450 mm and 840 mm x 840 mm x 450 mm. The reusability of crucibles is improved because the wetting of the coating by molten silicon is avoided and the crucibles can be easily removed and easily re- coated for the next smelting cycles. The coating has a strong mechanical resistance which avoids peeling and flake off problems. The coating is economical and can be applied fast on smelting plants.

The inner surface areas of the crucibles correspond to coating layers deposited on the inner surface of the crucible. The first zone coating is the lower coating, which is applied in the lower zone (first zone) with a height h' from the bottom of the crucible until about the level of the molten or liquid silicon surface. The second zone coating is the upper coating, which is applied in the upper or upward zone (second zone) with a height h" above about the level of the molten or liquid silicon. In a preferred embodiment the height of the first zone h' and the height of the second zone h" add up to the total height h.

Alternatively the crucible can be divided in more than two zones, whereas the total height is given as the sum of the heights of the different zones.

In another preferred embodiment the crucible is divided into three zones, whereas h' is the height of the first zone from the bottom of the crucible, h" is the height of the second zone and h'" is the height of the third zone. The heights h', h" and h'" add up to the total height h. Preferably the oxygen content within the third zone (h'") is lower than the oxygen content of the second zone (h").

An advantageous embodiment of the invention is a three zone crucible, in which no coating is applied within the third zone. Thus the costs for manufacturing of crucibles can be reduced as no coating has to be applied within the third zone. Furthermore the oxygen contamination of the ingot by the coating is reduced as there is less possibility of oxygen contamination of the ingot by oxygen from the coating. In another embodiment the first zone coating is applied within the third zone of a three zone crucible.

The silicon level rises continuously during crystallization. The increase of volume during solidification is defined as Δ. To ensure that the silicon level is within the second zone coating during the whole crystallization process a top safety factor (a) and a bottom safety factor (β) are added. Hence the height of the second zone h" is given by h" = Δ + α + β. This design ensures that the level of silicon is in contact to the second zone coating with its higher oxygen level during the whole process.

Preferably the sum of the top safety factor (a) and the bottom safety factor (β) is less than 30 % of the height of the second zone coating (h"), more preferably less than 20 % of h".

Preferably the top safety factor (a) varies between 5 % to 10 % of the height of the second zone coating (h"), while the bottom safety factor (β) is between 5 % to 20 % of h".

In a preferred embodiment of a three zone coated crucible the height h' of the first zone is preferably comprised between 20 % of the total height h and 80 % of h, more preferably between 30 % of h and 70 % of h. The height h" of the second zone is preferably higher than the sum of the overlap (V), the top safety factor (a) and the bottom safety factor (β). The height h'" of the third zone is preferably equal to h'" = h - h' - h" and h'" is typically higher than 5 % of the total height h, but not more than 40 % of h and more preferably comprised between 10% of h and 30% of h.

The difference of composition is preferably a difference of the oxygen level between the two zones. It is preferred that the oxygen content of the upper or second zone is higher. The formulations of the coatings are different prior to the deposition. Silica may be added to the second coating formulation if this additive is not present in the first coating or at a higher level. The silica content in the second coating may be sufficiently higher in the formulation of the second coating so as to insure a difference of oxygen level.

An advantageous embodiment of the invention is a silicon nitride containing crucible, wherein the second zone coating contains silicon oxynitride, pre oxidized silicon nitride or mixtures thereof. The formulation may differ by the addition of silicon oxynitride with the chemical formula SiO_xN_y. While in amorphous forms its composition can continuously vary between Si0₂ and Si₃N₄. One type of oxynitride is in particular Si_xON_y with x>0 and y>0, and more particularly Si₂ON₂ is known as an intermediate crystalline phase. Silicon oxynitride is present only within the second coating formulation or at a higher level in this formulation compared to this one of the first coating. An advantageous embodiment of the invention is a silicon nitride containing crucible, wherein the first zone coating contains Si₃N₄ powder. The advantage is that a solidified silicon ingot is not affixed to the crucible.

An advantageous embodiment of the invention is a silicon nitride containing crucible, wherein height h' is about equal to the height of molten silicon surface and height h" is about equal to the height above the molten silicon surface inside of the crucible. The heights h' and h" vary from the level of the molten silicon surface by up to 1 % - 5%. The first and second zone coating overlap by up to 5% to 10 %.

The first zone coating and the second zone coating have an average thickness of less than 1000 micrometers and preferably between 200 micrometers and 500 micrometers. An embodiment of the invention holds a second zone coating with a higher thickness compared to the thickness of the first zone coating. In this embodiment the average thickness of the first zone coating is preferably 200 micrometers to 500 micrometers, even more preferably between 200 micrometers and 300 micrometers and the average thickness of the second zone coating is preferably between 200 micrometers and 500 micrometers, even more preferably between 300 micrometers and 500 micrometers. The wall thickness of the crucible has to be adjusted to the thickness of the coatings to yield a continuous inner surface and to avoid an offset between the first coating and the second coating. Thus in the second zone the wall thickness of the crucible has to be thinner than the wall thickness of the crucible in the first zone exactly by the differential amount of the second zone coating thickness and the first zone coating thickness.

In a preferred embodiment of the invention the first zone coating and the second zone coating show an offset γ as the coatings overlap and the second zone coating has got a higher thickness than the first zone coating. Hence, a recess is formed at a height h', which could hinder the demolding. To avoid demolding problems the second zone coating contains a predetermined breaking point for ingot removal. This predetermined breaking point is a weak zone wherein a crack can occure to help ingot removal.

Preferably the offset γ is preferably lower than 500 micrometers in order to prevent ingot removal problems and it is preferably between 100 micrometers and 300 micrometers. The thickness of the coatings depends on the amount of material applied during the manufacturing process.

Another solution of the object of the present invention is a process of manufacturing a silicon nitride containing crucible comprising

-applying a formulation containing Si₃N₄ on the inner bottom and the inner side walls on the first zone up to a height h',

-applying a mixture of 50 weight % to 95 weight % containing Si₃N₄ and of 5 weight % to 50 weight % containing silica on the inner side walls of the second zone with a height h" and -heat treating the coated Si₃N₄ containing crucible in oxidizing atmosphere, in preference in air.

The process according to the present invention requires that the coated silicon nitride containing crucible is heat treated at a temperature of up to 900 °C, preferably less than 600 °C, more preferably 80 °C to 120 °C.

The process according to the present invention wherein the second zone coating formulation contains 75 weight % to 85 weight % Si₃N₄ and 15 weight % to 25 weight % silica.

The process according to the present invention wherein the second zone coating mixture contains silicon oxynitride, pre oxidized silicon or mixtures thereof.

Another solution of the object of the present invention is to pre oxidize silicon nitride powder before introducing in the coating formulation to be applied to zones, the lower zone contains less oxygen compared to the upward zone. According to this solution the coated crucible does not need to be fired because the coating is already oxidized and has good non wetting properties. It is only necessary to dry the coating at low temperatures for example 100°C. The silicon nitride powder is treated under oxidizing atmosphere, preferably air, at normal pressure and at a temperature of 800 °C to 1400 °C, resulting in oxidized silicon nitride powder. The silicon nitride powder has an average particle size of 0.3 micrometers to 30 micrometers, preferably 0.5 micrometers to 5 micrometers. The nitride crucible will not be oxidized during the drying. This solution will have very much lower oxygen source compared to silica crucibles and silicon nitride crucibles with firing cycle. In a preferred embodiment of the invention the second zone formulation contains a preoxidized powder with a higher level of oxidation while the first zone formulation contains a preoxidized powder with a lower oxidation level. These different oxidation levels can be achieved by firing the Si₃N₄ powder for the second zone formulation at higher temperatures, preferably 1000 °C to 1400 °C, and firing Si₃N₄ powder for the first zone formulation at lower temperatures, preferably 800 °C to 1000 °C.

An advantageous embodiment of the invention is a silicon nitride containing crucible with the inner bottom and the inner side walls having a first zone coating up to a height h' containing Si₃N₄ and the inner side walls having a second zone above the first zone coating with a height h", the second zone consisting of locally oxidized coating which contains silicon nitride or silicon oxynitride coating or mixtures of thereof.

Another solution is also to combine the two previous approaches coating formulation and heat treatment procedure, as for example the difference of composition may come also from the size of Si₃N₄ particles. In particular the use of finer powder in the formulation of the second zone, which is more sensitive to oxidizing treatment and more prone to oxidizing, will lead to higher oxygen content. In this embodiment the average particle size of the silicon nitride powder for the first zone is between 0.3 micrometers and 30 micrometers, preferably between 0.5 micrometers and 5 micrometers, while the second zone coating has an average particle size from 0.01 micrometers to 10 micrometer, preferably from 0.05 micrometers to 5 micrometers. The average particle size of the second zone formulation is preferably between 10 % and 50 % of the average particle size of the first zone formulation. The oxidizing of the second zone coating is achieved by firing the crucible at a temperature from 900 °C to 1400 °C, preferably from 1000 °C to 1200 °C in oxidizing atmosphere, preferably in an air atmosphere.

Another solution of the present invention is a process of manufacturing a silicon nitride containing crucible, comprising

-applying a formulation containing Si₃N₄ the on the inner bottom and the inner side walls up to a height h and

-oxidizing selectively the second zone coating with a height h" of the inner side walls.

It is common to use a deflocculant when making the coating slurry. A process for spraying is the common method. This way a specific particle size distribution for the Si₃N₄ particles can be achieved.

The reusable silicon nitride containing crucible can be used for crystallization polycrystalline silicon and for silicon ingot production. Further advantages and details of the present invention can be taken from the description of several exemplary embodiments with reference to the drawings.

Figure 1 shows a cross sectional view of a two zone coated crucible.

Figure 2 shows a cross sectional view of a two zone coated crucible, wherein the crucible is designed in order that the thickness of the coating in the upper second zone is higher than the thickness of the coating in the lower first zone.

Figure 3 shows a cross sectional view of a two zone coated crucible with two partially overlapping coatings during silicon solidification.

Figure 4 shows a cross sectional view of a three zone coated crucible with partially overlapping coatings.

Figure 5 shows a cross sectional view of a three zone crucible with two completely overlapping coatings, which form a recess.

Figure 6 shows a cross sectional view of a three zone crucible with two completely overlapping coatings.

Figure 7 shows a cross sectional view of a three zone crucible with two coatings without overlap.

Figure 8 shows a cross sectional view of a three zone crucible with two partially overlapping coatings, which form a recess.

Figure 9 shows a cross sectional view of a three zone crucible with three coatings without overlap.

Figure 1 depicts a silicon nitride containing crucible with a first zone (II) and a second zone (III) and a two zone coating according to the present invention. The base body (1 ) has an inner bottom surface and sidewalls with an inner height h. The inner bottom and the inner side walls have a first zone coating (2) up to a height h', which contains Si₃N₄. The inner side walls have a second zone coating (3) above the first zone coating (2) with a height h", which contains a mixture of 70 weight % to 90 weight % containing Si₃N₄ and of 10 weight % to 30 weight % containing silica.

Figure 2 depicts a silicon nitride containing crucible with a first zone (II) and a second zone (III) and a two zone coating according to the present invention. The base body (1 ) has an inner bottom surface and sidewalls with an inner height h. The inner bottom and the inner side walls have a first zone coating (2) up to a height h', which contains Si₃N₄, and a second zone coating (3) up to a height h" above the first zone coating (2). The thickness of the second zone coating (3) is higher than the thickness of the first zone coating (2).

Figure 3 depicts a silicon nitride containing crucible with a first zone (II) and a second zone

(III) and a two zone coating according to the present invention. The base body (1 ) has an inner bottom surface and sidewalls with an inner height h. The inner bottom and the inner side walls have a first zone coating (2) up to a height h', which contains Si₃N₄, and a second zone coating (3) up to a height h" above the first zone coating (2). The first zone coating (2) and the second zone coating (3) overlap within an overlapping area (V). The overlapping area (V) is located within the second zone (III) as the top layer within the overlapping area (V) is the second zone coating (3). The second zone coating (3) shows an offset γ against the first zone coating (2). Thus the second zone coating (3) forms a recess at the height of h', where the first zone (II) and the second zone (III) meet. In figure 3A) the silicon nitride containing crucible is filled with liquid silicon (5), while figure 3B) depicts the start of the ingot crystallization, in which a phase of solid silicon (6) has emerged at the bottom of the crucible, and figure 3C) shows the solid ingot. The silicon level rises continuously during crystallization. The increase of volume during solidification (Δ) and a top safety factor (a) and a bottom safety factor (β) add up to the height of the second zone (h"). This design ensures that the level of silicon is within the height h" of the second zone during the whole process. The increase of volume (Δ), the top safety factor (a) and the bottom safety factor (β) apply in an analogous way for all embodiments shown.

Figure 4 depicts a silicon nitride containing crucible with a first zone (II), a second zone (III) and a third zone (IV) and a zone coating according to the present invention. The base body (1 ) has an inner bottom surface and sidewalls with an inner height h. The inner bottom and the inner side walls have a first zone coating (2) up to a height h', which contains Si₃N₄, and a second zone coating (3) up to a height h" above the first zone coating (2). In the third zone

(IV) with a height h'" no coating is applied. The first zone coating (2) and the second zone coating (3) overlap within an overlapping area (V). The overlapping area (V) is located within the second zone (III) as the top layer within the overlapping area (V) is the second zone coating (3). The increase of volume during solidification (Δ) and a top safety factor (a) and a bottom safety factor (β) add up to the height of the second zone (h"). This design ensures that the level of silicon is within the height h" of the second zone during the whole process.

Figure 5 depicts a silicon nitride containing crucible with a first zone (II), a second zone (III) and a third zone (IV) and a zone coating according to the present invention. The base body (1 ) has an inner bottom surface and sidewalls with an inner height h. The inner bottom and the inner side walls have a first zone coating (2) up to a height h', which contains Si₃N₄, and a second zone coating (3) up to a height h" above the first zone coating (2). In the third zone (IV) with a height h'" no coating is applied. The first zone coating (2) and the second zone coating (3) overlap within an overlapping area (V), which equals the height of the second zone h". The second zone coating (3) shows an offset γ against the first zone coating (2). Thus the second zone coating (3) forms a recess at the height of h', where the first zone (II) and the second zone (III) meet. The increase of volume during solidification (Δ) and a top safety factor (a) and a bottom safety factor (β) add up to the height of the second zone (h"). This design ensures that the level of silicon is within the height h" of the second zone during the whole process.

Figure 6 depicts a silicon nitride containing crucible with a first zone (II), a second zone (III) and a third zone (IV) and a zone coating according to the present invention. The base body (1 ) has an inner bottom surface and sidewalls with an inner height h. The inner bottom and the inner side walls have a first zone coating (2) up to a height h', which contains Si₃N₄, and a second zone coating (3) up to a height h" above the first zone coating (2). In the third zone (IV) with a height h'" no coating is applied. The first zone coating (2) and the second zone coating (3) overlap within an overlapping area (V), which equals the height of the second zone h". The second zone coating (3) and the first zone coating (2) form a flush surface without any offset between these coatings. The increase of volume during solidification (Δ) and a top safety factor (a) and a bottom safety factor (β) add up to the height of the second zone (h"). This design ensures that the level of silicon is within the height h" of the second zone during the whole process.

Figure 7 depicts a silicon nitride containing crucible with a first zone (II), a second zone (III) and a third zone (IV) and a zone coating according to the present invention. The base body (1 ) has an inner bottom surface and sidewalls with an inner height h. The inner bottom and the inner side walls have a first zone coating (2) up to a height h', which contains Si₃N₄, and a second zone coating (3) up to a height h" above the first zone coating (2). In the third zone (IV) within a height h'" no coating is applied. The first zone coating (2) and the second zone coating (3) do not overlap and form a flush surface without any offset between these coatings. The increase of volume during solidification (Δ) and a top safety factor (a) and a bottom safety factor (β) add up to the height of the second zone (h"). This design ensures that the level of silicon is within the height h" of the second zone during the whole process.

Figure 8 depicts a silicon nitride containing crucible with a first zone (II), a second zone (III) and a third zone (IV) and a zone coating according to the present invention. The base body (1 ) has an inner bottom surface and sidewalls with an inner height h. The inner bottom and the inner side walls have a first zone coating (2) up to a height h', which contains Si₃N₄, and a second zone coating (3) up to a height h" above the first zone coating (2). In the third zone

(IV) with a height h'" no coating is applied. The first zone coating (2) and the second zone coating (3) overlap within an overlapping area (V). The second zone coating (3) shows an offset Y against the first zone coating (2). Thus the second zone coating (3) forms a recess at the height of h', where the first zone (II) and the second zone (III) meet. The overlapping area (V) is located within the second zone (III) as the top layer within the overlapping area

(V) is the second zone coating (3). The second zone coating (3) forms a recess at the height of h', where the first zone (II) and the second zone (III) meet. The increase of volume during solidification (Δ) and a top safety factor (a) and a bottom safety factor (β) add up to the height of the second zone (h"). This design ensures that the level of silicon is within the height h" of the second zone during the whole process. The second zone coating (3) comprises a predetermined breaking point for ingot removal (7), which ensures that the ingot can be removed easily.

Figure 9 depicts a silicon nitride containing crucible with a first zone (II), a second zone (III) and a third zone (IV) and a zone coating according to the present invention. The base body (1 ) has an inner bottom surface and sidewalls with an inner height h. The inner bottom and the inner side walls have a first zone coating (2) up to a height h', which contains Si₃N₄, and a second zone coating (3) up to a height h" above the first zone coating (2). In the third zone (IV) within a height h'" a third zone coating (4) containing Si₃N₄ is applied. The first zone coating (2), the second zone coating (3) and the third zone coating (4) do not overlap and are applied flush without any offset. The increase of volume during solidification (Δ) and a top safety factor (a) and a bottom safety factor (β) add up to the height of the second zone (h"). This design ensures that the level of silicon is within the height h" of the second zone during the whole process. Examples

It has been found that the sticking problems, which remained mainly on the top of the crucibles, were related to a specific wetting issue. The oxygen content in the two zones was equal before use. The oxygen content has been analyzed in the coating above and below the silicon liquid surface after use. The oxygen content in the coating above liquid surface was about 20% lower compared to coating below the silicon liquid surface. This was the proof that coating above the liquid silicon surface needs higher oxygen content compared to below the surface. It has been proven that a solution of two zone coating with higher oxygen content in the upper zone above the melted silicon surface solved the problem.

Crucibles with an outer diameter of 280 mm, an inner diameter of 265 mm, an external height of 220 mm and an internal height of 200 mm were used in example 1 and in all comparative examples. The crucibles were manufactured by filling silicon powder of a particle size below 75 micrometers (SILGRAIN, Elkem ASA) into a mould and compacting the crucible by vibration. Afterwards the crucible was nitrided at a temperature of 1 105 °C to 1380 °C in a vertical tube furnace until a conversion of silicon to Si₃N₄ of 97 % of the theoretical conversion was achieved.

The oxygen content of the first zone coating, the second zone coating and the crucible was measured by thermal evolution using a gas analyzer (TC 436, LECO Instruments).

The oxygen content of the silicon ingot after crystallization of the silicon was determined by measuring the interstitial oxygen in the silicon ingot using an infrared spectrometer (FTIR) according to the standard SEMI MF1 188-1 107. For a comparison of the interstitial oxygen contents in example 1 and the comparative examples a sample was taken from the bottom and the center of the silicon ingot, respectively. The average oxygen contents of these samples are compared in table 1.

Example 1

The first zone (II) was in the range up to 100 mm and the second zone (III) was in the range of 90 mm to 200 mm and there was a 10 mm overlap. A silicon nitride crucible was coated with a two zone coating. The first zone (II) below the silicon liquid (h') was coated with pure Si₃N₄. The second zone (III) above the silicon liquid surface was coated with a mixture of 20% silica and 80% silicon nitride. The coated crucible was fired at 900°C in air for 4 hours. A silicon ingot was grown in the crucible. There was no sticking on the coating.

The addition of silica to the coating above the silicon liquid surface worked and a non wetting coating was achieved. The coated crucible was fired only at 900°C. The addition of silica gave a non wetting coating. This crucible contains less oxygen compared to that one that was fired at 1 100°C in comparative example 2.

Comparative Example 2

The area of the inner bottom surface and sidewalls with an inner height h was coated with pure Si₃N₄ powder. The coated crucible was fired at 1 100°C in air for 4 hours. A silicon ingot was grown in the crucible. There was no sticking to the coating and the silicon ingot could be easily removed from the crucible. The crucible contained more oxygen compared to the one according to the example 1 .

Comparative Example 3

The area of the inner bottom surface and sidewalls with an inner height h was coated with pure S13N4 powder. The coated crucible was fired at 900°C in air for 4 hours. A silicon ingot was grown in the crucible. Silicon was sticking to the coating and crucible and the ingot could not be removed from the crucible. The sticking of silicon was only in the area of liquid silicon surface level and upwards.

Comparative Example 4

The area of the inner bottom surface and sidewalls with an inner height h was coated with pure S13N4 powder. The coated crucible was only dried at room temperature. A silicon ingot was grown in the crucible. Silicon was sticking to the coating and crucible and the ingot could not be removed from the crucible. The sticking of silicon was in the whole area of the crucible.

More detailed comparable facts between the crucibles of the example and comparative example become obvious from table 1 . Table 1

Example Example 1 Comparative Comparative Comparative

Example 2 Example 3 Example 4

Crucible RBSN RBSN RBSN RBSN

Upward Coating h" = 1 10 mm h = 200 mm h = 200 mm h = 200 mm formulation (second 20 weight % 100 weight % 100 weight % 100 weight % zone coating) Silica S13N4 S13N4 S13N4

80 weight %

Si₃N₄

Downward Coating h' = 100 mm

formulation (first 100 weight %

zone coating) S13N4

Firing for 4 hours 900°C /air 1 100°C /air 900°C /air None. Dried at room temperature

Sticking No No Yes. Only at Yes. Sticking liquid silicon both at liquid surface level silicon and upward. surface level and beneath.

Oxygen content in First zone: 1 .2 7.7 %wt 1 .2 %wt 1 .0 %wt coating after firing %wt.

Second zone :

12 %wt

Oxygen content of 1 .22 %wt 3.4 %wt 1 .22 %wt 1 .22 %wt crucible after firing

Oxygen Not relevant Not relevant contamination -18% Ref =100% because of because of ppm level at 0.1 sticking. sticking. from ingot solid

distance after

crystallization The comparison of the results of the example 1 with the comparative example 2 shows that the oxygen contamination of the silicon ingot is lowered by 18 % in the two zone coated crucible according to the present invention. The lower contamination of oxygen yields a silicon crystal with higher quality.

The silicon nitride containing crucible with zone coating is able to avoid sticking of silicon and to reduce the oxygen contamination of the silicon ingot at once. Furthermore the reusability of the crucible according to the invention is improved as the crucible easily can be cleaned and recoated. This result is unexpected and surprising.

Reference numbers

1 base body

2 first zone coating

3 second zone coating

4 third zone coating

5 liquid silicon

6 solid silicon

7 predetermined breaking point for ingot removal

II first zone

III second zone

IV third zone

V overlapping area h' height of the first zone

h" height of the second zone

h'" height of the third zone

Δ increase of volume during solidification

a top safety factor

β bottom safety factor

Y offset between second zone coating (3) and first zone coating (2)

Claims

1 . A silicon nitride containing crucible with a coating comprising:

a base body (1 ) with an inner bottom surface and sidewalls with an inner height h, the inner bottom and the inner side walls having a first zone coating (2) in a first zone (II) up to a height h' containing silicon nitride or silicon oxynitride and

the inner side walls having a second zone coating (3) in a second zone (III) above the first zone coating (2) with a height h" containing silicon nitride and/or silicon oxynitride, wherein

the oxygen content of the second zone coating (3) is higher than the oxygen content of the first zone coating (2),

the oxygen content of the first zone coating (2) is comprised between 0.5 weight % and 15 weight %,

the oxygen content of the second zone coating (3) is comprised between 1 weight % and 50 weight % and

the oxygen content of the base body (1 ) is less than 3 weight %.

2. The silicon nitride containing crucible according to claim 1 , wherein the oxygen

content of the first zone coating (2) is comprised between 1 weight % and 5 weight %, preferably between 1 .2 weight % and 1 .5 weight %.

3. The silicon nitride containing crucible according to one of the previous claims,

wherein the oxygen content of the second zone coating (3) is comprised between 1 weight % and 20 weight %, preferably between 5 weight % and 15 weight %, even more preferably between 10 weight % and 15 weight %.

4. The silicon nitride containing crucible according to one of the previous claims,

wherein the oxygen content of the base body (1 ) is less than 2 weight %.

5. The silicon nitride containing crucible according to one of the previous claims,

wherein the open porosity of the base body (1 ) is between 15 volume % to 60 volume % and the average pore size is between 0.1 micrometers to 10 micrometers.

6. The silicon nitride containing crucible according to one of the previous claims,

wherein the first zone coating (2) contains Si₃N₄ powder.

7. The silicon nitride containing crucible according to one of the previous claims, wherein the second zone coating (3) contains silicon oxynitride, pre oxidized silicon nitride or mixtures thereof.

8. The silicon nitride containing crucible according to one of the previous claims,

wherein the second zone coating (3) contains 50 weight % to 95 weight % Si₃N₄ and 5 weight % to 50 weight % silica.

9. The silicon nitride containing crucible according to one of the previous claims,

wherein the heights h' and h" vary from the level of the molten silicon surface by up to 1 % to 5 % and the first zone coating (2) and the second zone coating (3) overlap by up to 5 % to 10 % forming an overlap (V).

10. The silicon nitride containing crucible according to one of the previous claims,

wherein the inner side walls have a third zone (IV) above the second zone (III) with a height h'".

1 1 . The silicon nitride containing crucible according to one of the previous claims,

wherein the second zone coating (3) shows an offset γ towards the first zone coating (2) forming a recess at the height h'.

12. The silicon nitride containing crucible according to one of the previous claims with a coating comprising:

the inner bottom and the inner side walls having a first zone coating (2) up to a height h' containing silicon nitride or silicon oxynitride and

the inner side walls having a second zone (III) above the first zone coating (2) with a height h" having a second zone coating (3) containing silicon nitride or silicon oxynitride, wherein the second zone coating (3) has a higher thickness compared to the first zone coating (2) in at least one section of the second zone (III).

13. A process of manufacturing a silicon nitride containing crucible according to one of the claims 1 to 12 comprising:

-applying a formulation containing Si₃N₄ on the inner bottom and the inner side walls of the first zone (II) up to a height h',

-applying a formulation containing 50 weight % to 95 weight % containing Si₃N₄ and 5 weight % to 50 weight % containing silica on the inner side walls of the second zone (III) with a height h" and

-heat treating the silicon nitride containing crucible at a temperature of up to 900 °C preferably less than 600 °C, more preferably between 80 °C and 120 °C, in oxidizing atmosphere, in preference in air.

14. A process of manufacturing a silicon nitride containing crucible according to one of the claims 1 to 12, comprising

-applying a formulation containing preoxidized silicon nitride powder on the inner bottom and the side walls of the first zone (II) up to a hight h',

-applying a formulation containing preoxidized silicon nitride powder on the the inner side walls of the second zone (III) up to a hight h", wherein

the oxygen content of the preoxidized silicon nitride powder applied in the second zone (III) is higher than the oxygen content of the preoxidized silicon nitride powder of the first zone (II) and

-heat treating the silicon nitride containing crucible, preferably at a temperature of up to 900°C, preferably less than 600°C, more preferably between 80 to 120°C.

15. Process according to claim 14, wherein the preoxidized silicon nitride powder of the first zone formulation is oxidized at a temperature between 800 °C and 1000 °C and the preoxidized silicon nitride powder of the second zone formulation is oxidized at a temperature between 1000 °C and 1400 °C.

16. A process of manufacturing a silicon nitride containing crucible according to one of the claims 1 to 12, comprising:

- applying a formulation containing silicon nitride powder with an average particle size from 0.3 micrometers to 30 micrometers, preferably from 0.5 micrometers to 5 micrometers on the inner bottom and the inner side walls up to a height h'

- applying a formulation containing silicon nitride powder with an average particle size from 0.01 μηη to 10 μηη, preferably from 0.05 μηη to 5 μηη on the inner side walls of the second zone (III) up to a height h" and

- heat treating the silicon nitride containing crucible at a temperature from 900 °C to 1400 °C, preferably from 1000 °C to 1200 °C, in oxidizing atmosphere, preferably in an air atmosphere.

17. Use of a crucible according to one of the claims 1 to 12 for crystallization of polycrystalline silicon and for silicon ingot production.