WO2012048905A1

WO2012048905A1 - Crucible for silicon and crucible arrangement

Info

Publication number: WO2012048905A1
Application number: PCT/EP2011/005210
Authority: WO
Inventors: Stephan Hussy; Oleksandr Prokopenko; Christian Hoess
Original assignee: Centrotherm Sitec Gmbh
Priority date: 2010-10-15
Filing date: 2011-10-17
Publication date: 2012-04-19
Also published as: DE102010048602A1

Abstract

A crucible for silicon is disclosed, comprising a bottom and at least eight outer side wall sections, which confine a receiving space having an octagonal cross-section. Furthermore, a crucible arrangement for silicon is disclosed, which comprises the above described crucible and an inner side wall element centered with respect to the receiving space, such as to form an annular space, together with the outer side wall sections. Additionally, an optional partition unit for a crucible for silicon is also de- scribed, wherein the partition unit comprises at least one partition element fitted to the shape of the receiving space/annular space, in order to separate the receiving space/annular space into at least two compartments.

Description

Crucible for Silicon and Crucible Arrangement

The present invention relates to a crucible for silicon and a crucible arrangement for silicon.

In the art of semiconductors and solar cells, it is known to produce polycrystalline silicon ingots by melting high purity silicon material in a crucible. DE 199 34 940 C2 describes an example of a corresponding arrangement used for this purpose. The arrangement generally consists of an isolated box containing heating elements, a crucible and a recharching unit. Bottom heaters, lateral heaters, and top heaters provided respectively below, laterally and above the crucible are known as heating elements.

DE 10 2010 031 819, which was not published before the filing date of this applica- tion, shows a similar arrangement, comprising so-called diagonal heaters in addition to the heating elements mentioned above, wherein the diagonal heaters are located diagonally and laterally above the crucible, in order to provide for a heating profile similar to the heating profile of a top-heater. During production of a silicon ingot, the crucible is first loaded while the isolated box is open, and thereafter, the granulated silicon is melted in the crucible by means of the heating elements, when the isolated box is closed. After an optional recharching of additional silicon material by means of a corresponding recharching unit, the molten silicon is cooled down in a controlled manner, in order to achieve a directional solidification of the molten material from the bottom to the top.

During solidification, the phase boundary between the molten part and the solidified part of the silicon material should be as flat as possible, which is achieved by controlling the temperature profile in the molten/solidified parts of the material. A completely flat phase boundary usually cannot be achieved, and the side of the smelt or molten silicon facing the solidified silicon usually has a concave shape. Therefore, solidification from the bottom to the top is completed first in the middle of the crucible and thereafter, solidification continues slowly radially outward until the corners of the cru- cible are reached. Depending on the curvature of the phase boundary, solidification may take several hours.

As is known in the art, quartz crucibles comprising e.g. a silicon nitride coating are used as crucibles. Usually, these crucibles have a square receiving space for the silicon material, when viewed from the top. Thus, a silicon ingot formed in such a crucible has a generally square configuration in the top view. For further processing, this silicon ingot is subsequently cut to form smaller blocks, which are also referred to as columns or bricks, which have a cross sectional area or base area of e.g. 156 x 156mm. Typically the circumferential area of the silicon ingot is first cut off in order to produce well defined and straight outer surfaces of the columns, which are subsequently cut. Thereafter, wafers (thin slices) may be cut from these columns, wherein the wafers may be further processed in order to finally become solar cells. In such crucibles, the size of the crucible is frequently defined as generation Y, wherein Y is an integer. The size of a generation Y crucible is chosen in such a way that Y x Y columns having a predefined size may be cut from the produced silicon ingot. Crucibles of generation five, which are presently used, allow for cutting out 5 x 5 columns of the type mentioned above.

Figure 8 shows a top view of a crucible 1 of the prior art, which comprises a generally square cross sectional area, wherein a silicon ingot 3 is formed therein. As may be seen in Figure 8, the corners of the crucible are slightly rounded, in order to avoid stresses in the material of the crucible in this region. The rounded edges are formed in such a way that the silicon ingot comprises corners of 90° after removing the usual edge cut-off 4 of the silicon ingot. The silicon crucible shown in Figure 8 has a size of generation five, i.e. from a silicon ingot formed in the crucible 1 , 5 x 5 columns of the above-mentioned type may be cut out, as is shown by squares in Figure 8, wherein each of the squares represents one column.

In this process, columns of differing qualities are created, since columns located near the circumference may have an inferior quality, e.g. caused by diffusion of (metallic) impurities from the crucible wall, when compared with columns in the middle. Such impurities may locally reduce the lifetime of charge carriers, which causes a lower efficiency of solar cells made from the corresponding columns.

As can be seen in Figure 8, three kinds of columns are created in a square crucible, i.e. a first type of column 6 (shown in cross-shading), which is separated from a side wall of the crucible 1 by at least one column, a second type of column 7, which has one lateral edge located adjacent to a side wall of the crucible 1 , and another type of column 8, wherein two of the side walls thereof are located adjacent to side walls of the crucible 1. The second and third types (columns 7, 8) are shown in normal shad- ing. Thus, nine columns 6 of the first type, i.e. having the highest quality, twelve columns 7 of the second type having medium quality, and four columns 8 of the third type having the lowest quality may be formed from a generation five crucible.

Usually, the different columns 6 to 8 are treated differently in the subsequent treat- ment for producing solar cells, which makes the process substantially more complicated. Therefore, some manufacturers usually discard columns of the third type, in order to simplify the subsequent processes, thus producing only medium and high quality solar cells, respectively. Furthermore, there is a trend in the industry toward larger crucibles, which leads e.g. to crucibles of generation six and higher. However, with increasing sizes of crucibles, there is a problem in that the curvature of the phase boundary becomes higher and is more difficult to control, inter alia because of the distance between the lateral heaters and the center of the crucible.

Starting from the prior art mentioned above, the problem to be solved by the present invention is to provide a crucible, a crucible arrangement, and a partition unit, for a crucible for silicon respectively, which overcome at least one of the problems mentioned above.

This problem is solved by a crucible according to claim 1 and a crucible arrangement according to claim 9. Further embodiments of the invention are shown in the corresponding dependent claims. In particular, a crucible for silicon is provided, which comprises at least eight outer side wall sections, which confine a receiving space having an octagonal cross sectional area, wherein at least the surfaces of the crucible which face the receiving space have a layer including silicon nitride. By using a receiving space having multi- pie corners, in particular eight corners, the ratio of the distance from the center of the crucible to the next side wall relative to the distance from the center of the crucible to a corner thereof becomes larger when compared to a square receiving space. This means that the corners of the crucible are closer to the center of the crucible, whereby the time of the whole solidification process may be reduced. Furthermore, by use of the octagonal shape, no columns of type three are produced, i.e. no columns having two side walls adjacent to side wall sections of the crucible are produced. A silicon nitride surface inhibits wetting with molten silicon, which prevents inter alia breaking of the crucible caused by different coefficients of expansion of the crucible and the silicon ingot. Furthermore, the crucible may be reused with such a silicon nitride layer.

At the place where the columns of type three would normally be formed in a square crucible, now in the crucible mentioned above, a solidified silicon material block is formed which is not sufficient for a column having the desired cross sectional area. However, it is possible to produce columns having a different cross sectional area, e.g. having a cross sectional area of 125 x 125 mm, which may be forwarded to subsequent processing. Of course, the cross sectional areas of the columns mentioned above are only examples, which are currently used in practice. It should be noted that columns having 200 x 200 mm are also presently known.

Preferably, each of the side wall sections is straight, and opposing side wall sections are preferably arranged parallel to each other. By this arrangement, a corresponding silicon ingot may be formed, which allows for cutting columns having the predetermined cross sectional area with only small cut-offs. In an alternative embodiment, the side wall sections are formed by straight and curved side wall sections, which are alternately arranged, wherein opposing straight side wall sections are preferably parallel to each other. Providing curved side wall sections may be advantageous, in order to allow for cutting columns having differing cross sectional areas in the area of the curved side wall sections. By this means, the cut-offs, i.e. the fractions of solidi- fied silicon material not cut into columns, may be reduced, but with the result that columns having different cross sectional areas need to be subsequently processed.

Preferably, the ratio of the radius of the circumcircle of the crucible to the radius of the incircle of the crucible is less than 1.30 and larger than 1 .05, most preferably larger than 1.10. In this context, the circumcircle of the crucible is the smallest circle that completely encloses the melting space. The incircle of the crucible is meant to be the largest circle which completely fits into the melting space, as seen in a cross sectional view of the crucible perpendicular to its heights. This ratio would be about 1.4142 in a square crucible and would be 1.0 for a circular crucible.

According to one embodiment of the invention, at least one partition element is provided within the receiving space, the partition element dividing the silicon receiving space into at least two separate compartments. Preferably, the at least one partition element divides the receiving space into four identical compartments. Dividing the receiving space into separate compartments has the advantage, in particular for larger crucibles, that stresses inside the silicon ingots being formed may be reduced. Cracks possibly formed in one silicon ingot would be confined to the respective compartment, which may increase the through-put of a crystallization apparatus.

Preferably, the surfaces of the partition element facing the receiving space are made of a layer containing silicon nitride, or the whole elements are made of silicon nitride. Such a surface inhibits wetting with molten silicon, which prevents inter alia breaking of the partition element caused by different coefficients of expansion of the partition element and the silicon ingot. Furthermore, the partition element may be reused with such a silicon nitride layer.

In one embodiment of the invention, the crucible is formed from a plurality of plate elements, in particular from silicon nitride, which substantially decreases the manu- facturing costs for such a crucible compared to a single piece crucible. When using silicon nitride plate elements, these plate elements may be used several times, since the plate elements are not wetted by the molten silicon. In particular, a crucible formed of a plurality of plate elements may also be detached in parts from the silicon ingot after the silicon ingot is formed, in order to release the silicon ingot. According to the invention, a crucible arrangement for silicon is provided, which comprises a crucible of the above type and an inner side wall element centrally located in the receiving space, in order to form an annular space together with the at least one outer side wall. Such a crucible arrangement is particularly adapted for crucibles of higher generations, such as generation seven and higher, in which formation of a flat phase boundary during solidification becomes increasingly difficult. The inner side wall element may also provide for heating/cooling of silicon material in the crucible from the center. In this way, the curvature of the phase boundary between the molten silicon and the solidified silicon ingot may be better controlled in a simple manner. The at least one inner side wall element preferably comprises a cavity for housing a heating element and/or a cooling element. The inner side wall element may have the form of e.g. a tube, which comprises an end wall adjacent to the bottom of the crucible, to allow for insertion of a heating element and/or a cooling element from above. Such a heating and/or cooling element may be fixed e.g. to a lid of a crystallization apparatus, and may be inserted into the side wall element during the closing operation of the lid. The inner side wall element may be formed as a separate element from the bottom and the outer side wall of the crucible. When compared with a single-piece construction, this construction has the advantage that stresses within the crucible may be reduced. Such a crucible arrangement may be advantageously employed in combination with a crucible of the type mentioned above. The bottom of the crucible may also comprise a central opening through which e.g. a tube-like inner side wall element of the crucible may extend or onto which the inner side wall element may be placed in such a way that the inner side wall element blocks the open- ing in the bottom, such that molten material may not be discharged through the opening in the bottom. The tube-like inner side wall element may also comprise openings at one end or at both ends of the tube for inserting a heating element from one side or from both sides. Thus, it is also conceivable that e.g. a part of the bottom heater extends into this inner side wall element, as long as an opening is provided at the end of the side wall element facing the bottom of the crucible.

A partition unit comprising at least one partition element, which is matched to the shape of the annular receiving space may be provided to divide the annular receiving space into at least two compartments. Such a partition element may inhibit stresses within the silicon ingot being formed independently from the shape of the crucible, as described above. Furthermore, the overall size of a silicon ingot may be reduced, which facilitates handling of silicon ingots solidified in separate compartments, in particular with larger crucibles, such as crucibles of generation eight. The at least one partition element would divide a silicon ingot in advance into at least two and preferably four identical blocks, wherein cutting of the blocks would not be necessary. Preferably, the at least one partition element should comprise a coating including silicon nitride, or should be made of a material including silicon nitride, in order to avoid wetting of the partition element with molten silicon and thus adhesion of a sili- con ingot to the partition elements.

Further embodiments of the invention may be derived from the following detailed description of preferred embodiments, which refers to the figures; in the figures: Fig. 1 is a schematic top view of a crucible and a solidified silicon ingot formed therein, which is divided into different sections;

Fig. 2 is a schematic top view of an alternative crucible and a silicon ingot

formed therein;

Fig. 3 is a schematic top view of a crucible according to Fig. 2, wherein a parti- tion element is inserted into the crucible;

Fig. 4 is a schematic top view of a crucible of another embodiment of the invention and of a silicon ingot formed therein;

Fig. 5 is a schematic top view of a crucible according to another embodiment of the invention and a silicon ingot formed therein;

Fig. 6 is a schematic top view of a crucible arrangement according to an embodiment of the present invention;

Fig. 7 is a schematic top view of a crucible according to another embodiment of the present invention, and

Fig. 8 is a schematic top view of a prior art crucible and a silicon ingot formed therein.

As was mentioned above, Figure 8 shows a schematic top view of a generally square crucible 1 , known in the prior art, and a silicon ingot 3 formed therein. The silicon ingot 3 is divided into different sections, as was mentioned above. The silicon ingot completely fills the inside of the crucible 1 and contacts the side edges thereof. An edge region 4 of the silicon ingot 3, which is shown as a white area, is cut-off or clipping material. The silicon ingot is usually cut-off after manufacturing at its edge region 4, where the greatest amount of impurities is located. Thereafter, the silicon ingot 3 is cut into a number of equal square columns 6 to 8 having e.g. a cross sectional area of 156 x 156 mm, wherein the columns may also have another cross sectional area. These columns 6 to 8 are also shown as square areas in Figure 8. Each of these square areas has a hatching, wherein three different types of hatching are shown. These different types symbolize the different types of columns. Thus, the cross-hatching shows that these are the so-called center columns, which are sufficiently spaced from the side walls of the crucible 1 so that impurities coming from the side walls of the crucible, such as metallic impurities, do not occur in these columns 6, or do occur only in a small amount.

Furthermore, columns 7 exist, having one of their side edges located directly adjacent to a side wall of the crucible 1 . Only the area 4, i.e. the cut-off, is located between the side wall of the crucible 1 and one side of a corresponding column 7.

These columns 7 are shown with a first hatching. Furthermore, columns 8 exist in the corner regions of the crucible 1 . These columns have two of their side edges located directly adjacent to one side wall of the crucible 1 . These columns 8 are shown in a second, more dense, hatching. The different hatchings for columns of types 6 and 7 are also used in the drawings of Figures 1 to 7, which show different crucibles 1 according to the invention, which do not have a square form in cross-section. The same reference signs as were used in Figure 8 are used in these Figures for the same or identical elements.

Figure 1 shows a schematic top view of a crucible 1 according to the invention and a silicon ingot 3 formed therein, which is divided into different regions. The white region 4 shows a cut-off region, the regions in cross-hatching show columns 6 of the inner type, and the hatched regions show columns 7 of the type having one side wall adjacent to a side wall of the crucible 1 . The crucible 1 comprises a bottom (not shown in detail) as well as a surrounding side wall, which is divided into eight side wall sections 1 1 to 18. Each of the side wall sections 1 1 to 18 forms a straight side wall section, and these side wall sections are arranged in such a way with respect to each other that the side wall sections 1 1 to 18 form an octagonal receiving space for silicon material. Thus, an octagonal form is defined for a silicon ingot 3 formed in the crucible 1 .

Furthermore, a square crucible 1 ' known from the prior art is indicated, which has the same width as the crucible 1 of the invention. The side wall sections 1 1 , 13, 15, and 17 extend parallel to the side walls of the prior art crucible 1 ', which is indicated by dashed lines. In particular, the side wall sections 1 1 and 15 extend parallel to each other and at an angle of 90° to the side wall sections 13 and 17. The side wall sections 1 1 , 13, 15, and 17 all have the same length in this embodiment. Each of the side wall sections 12, 14, 16, and 18 extends inclined with respect to the adjacent side wall sections 1 1 , 13, 15, and 17. Each of the side wall sections 12, 14, 16, and 18 has the same length. The side wall sections 12 and 16 extend parallel to each other and at a right angle with respect to the side wall sections 14 and 18, which are themselves parallel to each other. Compared to the prior art crucible 1 ', the side wall sections 12, 14, 16, and 18 generally cut off the corners of the square crucible 1 '. The crucible 1 according to Figure 1 is generally a crucible of generation five, which means that both the height and the width may be cut into five columns. Compared to a normal crucible 1 ' of generation five, four of the columns are omitted, which are usually formed in the corners of a square crucible 1 ', i.e. corresponding to the columns 8, according to Figure 8. In this arrangement, more cut-off or waste is created adjacent to the side wall sections 12, 14, 16, and 18, as is shown by the white areas. However, no columns 8 of the type having two sides generally adjacent to the side walls of the crucible are formed. Furthermore, the time required for complete solidification of the silicon ingot 3 is substantially reduced due to the octagonal configuration, as is indicated by the dashed circles in Figure 1 . The representation of the circle is based on the fact that a curved phase boundary between the molten material in the crucible 1 and the solidified silicon ingot below is radially symmetrical to the center of the crucible 1. The dashed circle A shows the situation in which the phase boundary reaches the side wall sections 1 1 , 13, 15, and 17 of the crucible 1 for the first time. The dashed circle A may also be referred to as an incircle, wherein the incircle is the largest circle which fits completely into the receiving space of the crucible in a cross sectional view. The dashed circle B shows the situation in which the silicon ingot 3 is completely solidified in the crucible 1 ; however, solidification clearly does not take place outside the crucible. The circle B may also be referred to as the circumcircle, wherein the circumcircle is the smallest circle to completely encircle the melting space of the crucible in the cross sectional view. The dashed circle C shows the situation in which the silicon in- got 3 would have been completely solidified in the crucible 1 ', (the circumcircle of a square crucible) wherein, of course, also in this case, solidification does not take place outside the crucible.

Therefore, the circles show the distance between the center of the crucible 1 and the nearest corner sections 1 1 , 13, 15, and 17 (circle A), the distance between the center of the crucible 1 and the corners between the adjacent side wall sections (circle B) and the distance between the center of prior art crucible V and the corner regions thereof (circle C). These distances are related to the time required for solidification of a silicon ingot. As may be seen, the circle C has a much larger radius than the circle B, which implies that much more time would be required to completely solidify molten silicon in the prior art crucible 1 ', when compared to the crucible 1 according to Figure 1 (based on the same solidification rate in both crucibles).

While the ratio of the radius of circle C to the radius of circle A (radius of circle C/radius of circle A) is about 1.41 , the ratio of circle A to circle B is about 1 .2. According to the invention, the ratio of the distance between the center of the crucible and a corner of the crucible to the distance between the center of the crucible and the next side wall shall be less than 1 .35, in particular less than 1 .3 and preferably larger than 1 .05. The exact ratio depends on the generation of the crucible, wherein the starting point is that at least the four column regions are cut off, which are located at the furthest distance from the center of the crucible.

Figure 2 shows a schematic top view of a crucible 1 and a silicon ingot 3 formed therein, wherein the silicon ingot 3 is divided into separate regions, i.e. cut-offs 4 (white area), square regions for columns 6 of the inner type (cross-hatched) as well as square regions for columns 7 of the type wherein one side is adjacent to a side wall of the crucible 1 . The crucible 1 has a bottom not shown in detail and one circumferential side wall having side wall sections 1 1 to 18. The crucible 1 is a crucible of generation six. This means that a silicon ingot 3 may be produced in the crucible, wherein six columns having a predetermined cross sectional area may be cut out in the width and the length direction. However, from the silicon ingot 3 formed in the crucible 1 , only 32 columns may be produced and not 36 columns, as would be the case for a square crucible, since the columns in the corner regions are cut off by the side wall regions 12, 14, 16, and 18. Of these 32 columns, 16 columns 6 are of the inner type, and 16 columns 7 are of the type having one side adjacent to a side wall section of the crucible 1 . As will be obvious to the person skilled in the art, the ratio of a distance from the center of the crucible to the next side wall and from the center of the crucible to one corner of the crucible would be about 1 to .2 in this embodiment.

Figure 3 shows a schematic top view of a crucible 1 , as described above with reference to Figure 2. Again, a silicon ingot 3 which is divided into several parts is shown inside the crucible 1 .

Additionally, a partition unit 20 is provided inside the crucible 1 , wherein the partition unit 20 is located inside the receiving space of the crucible 1. As shown in Figure 3, the partition unit consists of four plate elements 21 -24, which meet at a central point 25 and which fit together at the central point. The central point 25 of the partition unit 20 coincides with the central point of the crucible 1 . The wall element 21 extends, starting from a central point of the side wall section 1 1 , to the central point 25 of the partition unit 20. Equally, the wall elements 22, 23, and 24 extend from a corresponding central point of the side wall sections 13, 15, and 17, respectively, to the central point 25 of the partition unit 20. Each of the wall elements 21 , 22, 23, and 24 is sized in such a way that the wall elements touch and seal the bottom of the crucible 1 on the one hand and an inner region of the side wall elements 1 1 on the other hand. It is also possible that a small gap remains between the side wall sections 1 1 , 13, 15, and 17, respectively, and the corresponding wall elements 21 , 22, 23, and 24. However, this gap should be so small that no molten silicon enters into the gap. This may be achieved by an appropriate material, e.g. a material not wettable by the molten silicon. The wall elements 21 , 22, 23, and 24 are made of silicon nitride in the preferred embodiment. However, the wall elements may also be made of another material not wettable by the molten silicon, wherein this material does not introduce substantial impurities into adjacently located molten silicon.

Even though the wall elements 21 , 22, 23, and 24 have been described as separate elements joined in a middle region or at a central point 25, it is also possible to form the wall elements as a single piece wall element or of only two wall elements. These two wall elements may comprise e.g. meshing or interlocking grooves, in order to form a corresponding cross, as is shown in Figure 3. The partition unit 20 divides the receiving space of the crucible 1 into four identical compartments, whereby generally four separated silicon ingots 3 are formed inside the crucible. Thus, stresses inside the silicon material during the directional solidification may be reduced. Defects, such as cracks, inside a silicon ingot 3 are limited to one of the corresponding compartments in which the defect occurrs. Thus, the overall through-put may be increased. Furthermore, handling of the four silicon ingots after the directional solidification is simplified, since the single ingots or blocks are much smaller than the whole ingot, shown in Figure 2. The use of a partition unit 20 is particularly suitable for crucibles of higher generations, as will be obvious to the person skilled in the art.

Figure 4 shows a top view of another crucible 1 according to the present invention, having a silicon ingot 3 formed therein. Again, the crucible 1 has eight side wall sec- tions 1 1 -18, wherein the odd numbered side wall sections 1 1 , 13, 15, and 17 generally form a square base shape, and wherein the even numbered side wall sections 12, 14, 16, and 18 cut off the corners of the square base shape, such that a generally octagonal receiving space for the silicon ingot 3 is formed. It will be noted that the crucible 1 is a crucible of generation seven, wherein again the four columns formed at a location most distant from the center point of the crucible 1 do not have a square form, due to the inclined side wall sections 12, 14, 16, and 18. With this shape of the crucible, the ratio of the distance between the center point of the crucible 1 and one of the next side walls, such as the side walls 1 1 , 13, 15, and 17, to the distance between the center point of the crucible and one corner thereof, is approximately 1 to 1.23. A silicon ingot 3, which may be divided into twenty-five columns 6 and twenty columns 7, may be formed in such a crucible.

Figure 5 shows a schematic top view of an alternative crucible 1 of generation seven. In this embodiment, the side wall sections 11 to 18 form an equal-sided octagon. Thus, the silicon ingot 3 formed inside the silicon crucible 1 is cut off or clipped more than in the embodiment according to Figure 4. A silicon ingot formed in a crucible 1 according to Figure 5 may thus be divided into twenty five columns 6 and twelve columns 7. Additionally, the amount of cut-off or waste is about twice as much as in the previous embodiment. However, in this embodiment, the ratio of the distance from the center point of the crucible 1 to the respective side wall sections 1 to 18, to the distance between the center point of the crucible 1 and an edge region is approximately 1 to 1.09. Thus, a substantial reduction of the solidification time of molten silicon inside the crucible 1 compared to a crucible 1 according to Figure 4 results (given that the solidification rates are the same in both crucibles 1). Such a reduction in process time may justify the greater amount of cut-off. Furthermore, the directional solidification of molten silicon in the crucible 1 may be better controlled due to the symmetrical side wall sections. Figure 6 shows a schematic top view of another alternative crucible 1 of generation 7, which generally has the same base shape as the crucible 1 according to Figure 4. Again, the crucible 1 has side wall sections 11 to 18, which are also arranged in the same manner as in Figure 4. However, additionally, an inner side wall element 30 having the form of a tube is centrally positioned in the crucible. In the preferred embodiment, the side wall element 30 is an element distinct from the crucible 1 having the form of a tube closed at one end, wherein the element may be seated on the bottom of the crucible 1. Inside the side wall element 30, a cavity 32 is formed, into which a heating and/or cooling unit may be inserted from above in accordance with a preferred embodiment. The side wall element 30 may also be formed integrally with the crucible and may e.g. comprise a cavity open to the bottom side, in which a heating and/or cooling unit may be inserted from below. The side wall element 30 is sized in such a way that it has a substantially smaller cross sectional area compared to the cross sectional area of a corresponding column, as is indicated in Figure 6. The side wall element 30 preferably consists of a material not wetted by molten silicon, such as silicon nitride. Furthermore, the mate- rial should be chosen in such a way that no substantial impurities are introduced into the molten silicon.

However, directly adjacent to the side wall element 30, silicon material is formed during directional solidification, which corresponds to the silicon material of the columns 7, i.e. this silicon material does not have as high a quality as the material of columns 6. However, the side wall element 30 is arranged in such a way and at such a distance from an adjacent and complete column region that one of the columns 6, i.e. a column of the highest quality, may be formed. This is achieved due to the size and the corresponding centered location of the side wall element 30.

The crucible arrangement according to Figure 6 may be particularly advantageously used in combination with a crystallization apparatus comprising a lid which may be opened upwards, wherein a heating and/or cooling unit is connected to the lid and may be inserted into the cavity 32 of the side wall element 30 during closing opera- tion.

Such an arrangement allows for much better control of the temperature distribution within the molten silicon during the directional solidification, since central heating/cooling is now possible.

Thus, the inner side wall element 30 together with the outer side wall sections 11 to 18 form a ring-shaped receiving space, which allows for a completely novel solidification profile for the molten silicon in the receiving space when compared with receiving spaces that are not ring-shaped.

Figure 7 shows still another embodiment of a crucible 1 of generation seven, similar to the crucible 1 according to Figure 5. In this embodiment, the crucible 1 comprises straight side wall sections 11 , 13, 15, and 17, which occur alternately with curved side wall sections 12, 14, 16, and 18. Each of the side wall sections 11 , 13, 15, and 17 has the same configuration as in Figure 5. The curved side wall sections 12, 14, 16, and 18 are configured in such way that the solidification time for a silicon ingot 3 in the crucible 1 is not increased or is not substantially increased. By means of the curved side wall sections 12, 14, 16, and 18, the receiving space for silicon material is larger when compared with the embodiment of Figure 5. This arrangement allows for cutting out eight columns 6' besides cutting out twenty-five columns 6 and twelve columns 7. The columns 6' are smaller than the columns 6 or 7. However, the columns 6' have the highest quality, since none of their side edges is directly adjacent to a side wall section of the crucible 1. As will be obvious to the person skilled in the art, depending on the appropriate size of such columns, it may be determined whether a corresponding curvature of the side wall sections 12, 14, 16, and 18 is necessary, and also the size of the curvature may be determined.

Depending on the size of the corresponding columns, corresponding cutting out of a column 6' is also possible with straight side wall sections 12, 14, 16, and 18.

During operation of the crucibles 1 described above, these crucibles will first be positioned in the receiving spaces of a crystallization apparatus comprising corresponding heating and/or cooling elements, and usually the crucibles will be filled with sili- con material. In the embodiment according to Figure 3, the partition unit 20 is appropriately positioned prior to filling the crucible 1 and thereafter, the individual compartments of the crucible 1 may be filled with silicon material. In the embodiment of Figure 6, the side wall element 30 will be positioned prior to filling the crucible 1 and thereafter, the ring-shaped receiving space of the crucible 1 formed in this way will be filled.

After filling or loading, the crystallization apparatus is closed. In the embodiment according to Figure 6, e.g. a heating and/or cooling unit of the side wall element 30 is inserted. Thereafter, the silicon material is melted in the crucible 1 , and additional silicon material may be charged into the crucible 1 , in order to achieve a desired filling level of the silicon material in the crucible 1.

Finally, the molten silicon thus formed in the crucible 1 is cooled in a controlled manner, in order to achieve a directional solidification thereof from the bottom to the top inside the crucible 1 , as is known in the art. In particular, the embodiment according to Figure 6 allows for a novel process management, since the heating and/or cooling unit housed in the cavity 32 may influence the solidification process from the center of the crucible 1. This embodiment is particularly suitable for crucibles 1 of higher generations, in which a corresponding temperature control from the sides becomes increasingly difficult.

After completing the directional solidification, the silicon ingot 3 is further cooled down inside the crystallization apparatus, until the silicon ingot 3 reaches a tempera- ture for handling. Thereafter, the silicon ingot 3 may be removed from the crystallization apparatus and may be further processed in known manner.

The crucibles 1 of the embodiments described above may be e.g. single piece crucibles made from quartz, which comprise a coating including silicon nitride, at least on their inner regions. Alternatively, it is also possible that the crucibles may be composed from plate elements having a coating including silicon nitride or are composed from plate elements made of silicon nitride. Such a plate construction is particularly suited for reusable crucibles which may be dismantled and reassembled. Such a plate construction also allows for replacing individual wall elements in case a wall element is damaged.

The invention has been discussed above referring to preferred embodiments of the inventions; however, the invention is not limited to the embodiments directly shown therein.

Claims

A crucible for silicon having a bottom and at least eight external side wall sections confining a receiving space, which has an octagonal cross sectional area, wherein at least the surfaces of the crucible which face the receiving space, have a layer including silicon nitride.

The crucible according to claim 1 , wherein each of the side wall sections is straight, and wherein opposing side wall sections are preferably parallel to each other.

The crucible according to claim 1 , wherein the side wall sections consist of straight and curved side wall sections, which are arranged alternately, and wherein opposing straight side wall sections are preferably parallel with respect to each other.

The crucible according to one of the preceding claims, wherein the ratio of the radius of the circumcircle of the crucible to the radius of the incircle of the crucible is between 1.05 and 1.30.

The crucible according to one of the preceding claims, wherein at least one partition element is provided inside the receiving space, wherein the partition element divides the silicon receiving space into at least two separate compartments.

The crucible according to claim 5, wherein the at least one partition element separates the receiving space into four identical compartments.

The crucible according to claim 5 or 6, wherein the surfaces of the at least one partition element which face the receiving space have a layer including silicon nitride.

8. The crucible according to one of the preceding claims, wherein the crucible is formed of a plurality of plate elements, which are optionally in particular made from silicon nitride.

A crucible arrangement for silicon, which comprises a crucible as set forth in any one of the preceding claims, and

an inner side wall element positioned centrally in the receiving space so as to form an annular space together with the external side wall sections.

The crucible arrangement according to claim 9, wherein the at least one inner side wall element comprises a cavity for housing a heating element and/or a cooling element.

The crucible arrangement according to claim 9 or 10, wherein the inner side wall element is tube-shaped and comprises an end wall adjacent to the bottom of the crucible.

The crucible arrangement according to one of claims 9 to 11 , wherein the inner side wall element is formed as an element separate from the bottom and from the outer side wall of the crucible.

The crucible arrangement according to one of claims 9 to 12, wherein the crucible arrangement further comprises a partition unit comprising at least one partition element fitted to the shape of the annular space, so as to divide the annular space into at least two compartments.

The crucible arrangement according to claim 13, wherein the at least one partition element is adapted to divide the annular space into at least four identical compartments.

The crucible arrangement according to claim 13 or 14, wherein the at least one partition element comprises a layer including silicon nitride or is made of a material including silicon nitride.