Abstract:
Method for producing a silicon ingot comprising the following steps: providing a container for receiving a silicon melt with a base wall extending perpendicular to an axial direction and side walls, providing at least one flat monocrystalline seed crystal with an axial orientation selected from the group of &lt;110&gt;, &lt;100&gt; and &lt;111&gt; orientation, arranging the at least one seed crystal on the base wall of the container and directional solidification of a silicon melt in the container (2) to form a silicon ingot proceeding from the at least one seed crystal, wherein the axial orientation of the at least one seed crystal predetermines an axial orientation for the silicon ingot and wherein the at least one seed crystal is configured on the base wall of the container in such a way that a twin formation is avoided in an edge region adjoining the side walls.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of German Patent Application, Serial No. 10 2012 203 524.9, filed Mar. 6, 2012, pursuant to 35 U.S.C. 119(a)-(d), the content of which is incorporated herein by reference in its entirety as if fully set forth herein. 
       FIELD OF THE INVENTION 
       [0002]    The invention relates to a method for producing silicon ingots. The invention also relates to seed crystals for use in the production of a silicon ingot. 
       BACKGROUND OF THE INVENTION 
       [0003]    The production of silicon ingots is an important step in the production of silicon wafers, in particular for photovoltaic applications. A method for producing silicon ingots is, for example, known from US 2010/0203350 A1. There is continuously a need to develop methods of this type. 
       SUMMARY OF THE INVENTION 
       [0004]    An object of the invention is to improve a method for producing silicon ingots. This object is achieved by a method for producing a silicon ingot comprising the steps of providing a container for receiving an Si melt with, a base wall extending perpendicular to an axial direction and side walls, providing at least one flat monocrystalline seed crystal with an axial orientation selected from the group of &lt;110&gt;, &lt;100&gt; and &lt;111&gt; orientation, arranging the at least one seed crystal on the base wall of the container and directional solidification of a silicon melt in the container to form a silicon ingot proceeding from the at least one seed crystal, wherein the axial orientation of the at least one seed crystal predetermines an axial orientation for the silicon ingot and wherein the at least one seed crystal is configured and/or arranged on the base wall of the container in such a way that a twin formation is avoided in an edge region adjoining the side walls and extending into an interior of the container. 
         [0005]    The core of the invention is to configure monocrystalline seed crystals and/or to arrange them on a crucible base such that a twin formation in an edge region adjoining the side walls of the crucible and extending into the interior of the crucible is avoided. According to the invention, it was recognized that the tendency to a twin formation depends on the orientation of seed crystals on the base of the crucible. There is a danger, in particular in the edge regions close to the crucible walls, in which seed crystals cannot terminate completely with the crucible, of so-called crystal twins being formed. These may have a disadvantageous effect on the crystalline structure of the silicon ingot to be produced. It was recognized according to the invention that the tendency to twin formation depends on the configuration and orientation of the seed crystals. The tendency to twin formation can be reduced or even completely avoided by a suitable configuration and/or arrangement of the seed crystals on the base wall of the container. 
         [0006]    According to the invention, it is provided, in particular, that seed crystals with a specific orientation in an axial direction be provided and the tendency to twin formation be reduced by a suitable rotation of these seed crystals about an axis oriented in the axial direction. It was recognized according to the invention that the tendency to twin formation, in particular in the edge region of the crucible, can be reduced in that the seed crystals are rotated in such a way that the projection of their &lt;111&gt; lattice plane normal into a horizontal plane has a predetermined orientation relative to the lateral, in particular the outer, peripheral seed crystal edges surrounding the mono region and/or to the side walls of the crucible. 
         [0007]    It is, in particular, advantageous if this projection onto at most 25%, in particular at most 12.5%, in particular 0% of the total length of the periphery of the entirety of the seed crystals is perpendicular to the respective closest seed crystal edge and/or side wall. 
         [0008]    According to one aspect of the invention, the seed crystals are therefore arranged on the crucible base in such a way that the length of all the portions of the outer periphery of the entirety of the seed crystals, in which a projection of a &lt;111&gt; lattice plane normal of the seed crystal belonging to this portion into the horizontal is perpendicular to the outer, peripheral seed crystal edge surrounding the mono region and/or side wall, is at most 25%, in particular at most 12.5%, in particular 0% of the total length of the periphery. 
         [0009]    The total length of the periphery corresponds here substantially to the inner periphery of the crucible. The tendency to twin formation can be reduced by an arrangement of this type of the seed crystals. 
         [0010]    According to a further aspect of the invention, the seed crystals are configured in such a way that they have at least one horizontal seed crystal cutting face limited by cutting edges located in the horizontal, all the cutting edges in each case enclosing an angle of at most 45°, in particular at most 20°, in particular at most 5°, ideally 0°, with a projection of a &lt;111&gt; lattice plane normal onto the corresponding horizontal seed crystal cutting face. 
         [0011]    The tendency to twin formation can also be reduced by this. 
         [0012]    All the seed crystals are preferably arranged in such a way that they have a lateral &lt;110&gt; orientation, which, with all the side walls thereof, encloses an angle in the range of 15° to 75°, in particular in the range of 30° to 60°, in particular of about 45°. A lateral orientation is to be taken to mean an orientation parallel to the crucible base here. 
         [0013]    All the seed crystals preferably have an axial orientation selected from the group of &lt;110&gt;, &lt;100&gt; and &lt;111&gt; orientations. 
         [0014]    All the seed crystals, when using an axial &lt;110&gt; orientation, preferably have a lateral &lt;110&gt; orientation, which, with all the outer cutting edges or side walls thereof, encloses an angle not equal to 0°, in particular equal to 45°, or equal to 90°. A lateral orientation is to be taken to mean here an orientation parallel to the crucible base. The angle is, in particular, in the range of 35° to 55°. 
         [0015]    All the seed crystals, when using an axial &lt;100&gt; orientation, have a lateral &lt;110&gt; orientation, which, with all the outer cutting edges or side walls thereof, encloses an angle not equal to 90°, preferably equal to 45°. The angle is, in particular, in the range of 35° to 55°. 
         [0016]    All the seed crystals, when using an axial &lt;111&gt; orientation, preferably have a lateral &lt;110&gt; orientation, which, with all the outer cutting edges or side walls thereof, encloses an angle not equal to 90°, preferably equal to 15°. The angle is, in particular, in the range of 10° to 20°. 
         [0017]    According to the invention, it may be provided that all the seed crystals have an identical lateral orientation. 
         [0018]    However, it may also be advantageous to arrange the seed crystals in such a way that at least two adjacent seed crystals have different lateral orientations. It may, in particular, be advantageous to rotate the seed crystals, which are arranged in the corners of the crucible, about an axis running perpendicular to the crucible base in such a way that they have a different lateral orientation to their adjacent seed crystals. 
         [0019]    It may in general be advantageous if at least one of the seed crystals, which is arranged adjacent to at least one of the side walls, has a different lateral orientation to at least one of the other seed crystals. 
         [0020]    All the seed crystals preferably have an identical axial orientation. As a result, the crystalline structure of the silicon ingot to be produced is improved. 
         [0021]    A further object of the invention is to improve a seed crystal for use in the production of a silicon ingot. 
         [0022]    This object is achieved by a seed crystal for use in the production of a silicon ingot comprising a monocrystalline silicon disc with lateral cutting faces, with an axial orientation selected from the group of &lt;110&gt;, &lt;100&gt; and &lt;111&gt; orientation, and with a lateral &lt;110&gt; orientation such that the &lt;110&gt; orientation ( 17 ) encloses an angle in the range of 15° to 75° with the lateral cutting faces. The advantages correspond to those described above. Using a seed crystal of this type, the tendency to twin formation, in particular, can be reduced, preferably completely avoided. 
         [0023]    Further features and details of the invention emerge from the description of a plurality of embodiments with the aid of the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  shows a schematic cross section through a crucible for producing silicon ingots with an arrangement of seed crystals on the base, 
           [0025]      FIG. 2  shows a schematic plan view of a conventional arrangement of seed crystals with an axial &lt;110&gt; orientation on the base of the crucible, 
           [0026]      FIGS. 3 to 5  show schematic plan views of arrangements according to the invention of seed crystals with axial &lt;110&gt; orientations on the base of the crucible, 
           [0027]      FIG. 6  shows a view according to  FIG. 2  of seed crystals with an axial &lt;100&gt; orientation, 
           [0028]      FIG. 7  shows a view according to  FIG. 6  with an arrangement according to the invention of seed crystals with an axial &lt;100&gt; orientation, 
           [0029]      FIG. 8  shows a view according to  FIG. 2  for seed crystals with an axial &lt;110&gt; orientation, 
           [0030]      FIG. 9  shows a view according to  FIG. 8  with an arrangement according to the invention of the seed crystals and 
           [0031]      FIG. 10  shows a schematic perspective part view of the crucible with the seed crystals according to  FIG. 2 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0032]    In a method for producing silicon ingots, a container  2  configured as a crucible or mould is firstly provided to receive a silicon melt  3 . 
         [0033]    The container  2  has a base  5  extending perpendicular to a longitudinal direction  4 , also called the axial direction and four side walls  6  extending at least component-wise in the longitudinal direction. The base  5  is square, i.e. the container  2  has a square cross section. It is, however, also possible to configure the container  2  with a cross section deviating from this, in particular with a round, in particular circular cross section. 
         [0034]    A plurality of seed crystals  7  are arranged on the base  5  of the container  2 . The seed crystals  7  are preferably made of a monocrystalline silicon crystal. In particular, they have no grain boundaries. In particular, they have a known crystalline structure with a known axial orientation and known lateral orientation. In this case, the axial orientation designates the orientation in the direction perpendicular to an upper and lower axial seed crystal cutting face  13 ,  14 . The lateral orientations designate the crystalline structure of the seed crystals  7  in the direction parallel to the seed crystal cutting faces  13 ,  14 . The axial direction corresponds to a longitudinal direction perpendicular to the base  5  of the container  2 . 
         [0035]    The seed crystal cutting faces  13 ,  14  are, in particular, configured parallel to one another. The seed crystals  7  are preferably configured as thin, cuboid platelets. They have, in particular, a thickness in the direction parallel to the seed crystal cutting faces  13 ,  14  of up to 5 cm. In the direction parallel to the seed crystal cutting faces  13 ,  14 , they may have dimensions of at least  10  cm, in particular at least  20  cm. The seed crystal cutting faces  13 ,  14 , in particular, have dimensions, which substantially, in each case, correspond to an integral multiple of the side lengths of wafers to be produced. 
         [0036]    The upper axial seed crystal cutting face  13  is limited by four upper cutting edges  15 . The lower axial seed crystal cutting face  14  is limited by four lower cutting edges  16 . 
         [0037]    The seed crystals  7  also have lateral cutting faces  18 . These are, in particular, oriented perpendicular to the axial seed crystal cutting faces  13 ,  14 . They are, in particular, arranged perpendicular to the base  5  of the container  2 . 
         [0038]    The seed crystals  7  are in each case separated from one another by seed crystal joints  9 . The seed crystal joints  9  are, in particular, linear. 
         [0039]    The seed crystals  7 , in the direction perpendicular to the longitudinal direction  4 , have a rectangular, in particular, a square cross section. 
         [0040]    The base  5  of the container  2 , apart from the seed crystal joints  9  and optionally an edge gap  12 , is preferably covered with seed crystals  7  covering the surface. For this purpose, 3×3, 4×4, 5×5 or 6×6 seed crystals  7  are provided, for example. In particular, 5×5 seed crystals  7  with square cross sections are provided. However, it is also possible to use 1×5 seed crystals  7  with a rectangular, strip-like cross section for arrangement on the base  5  of the containers  2 . In this case, the longer side of the seed crystals  7  preferably just corresponds to the extent of the base  5  perpendicular to the longitudinal direction  4  parallel to the side wall  6 . A different number and/or arrangement of the seed crystals  7  is also possible. 
         [0041]    The seed crystals  7  preferably have, in the direction perpendicular to the longitudinal direction  4 , a width B, which just corresponds to an integral multiple of a width of columns to be sawn later from the silicon ingot, optionally increased by corresponding saw intermediate spaces. The width B of the seed crystals  7  may, in particular, substantially just correspond to the later column width. This is to be taken to mean that it deviates by a maximum of 10%, in particular a maximum of 5%, from the width of the columns to be sawn later from the silicon ingot. Accordingly, the edge gaps  12  in the direction perpendicular to the longitudinal direction  4  may have dimensions corresponding to a thickness of the side rinds to be removed. 
         [0042]    After the arrangement of the seed crystals  7  on the base  5  of the container  2 , the silicon melt  3  is provided in the container  2 . For this purpose, pieced silicon may be arranged in the container  2  and melted. It is likewise possible to melt silicon in a separate container and pour it in liquid form, i.e. as a silicon melt  3 , into the container  2 . 
         [0043]    In both alternatives, it is ensured by corresponding temperature guidance that the seed crystals  7  only start to melt, i.e. partially melt, but do not completely melt. The seed crystals  7  are, in particular at most 70%, in particular at most 50%, in particular at most 30%, melted in the longitudinal direction  4 . 
         [0044]    The silicon melt  3  is then solidified in a directional manner. For details of the melting of the silicon and the solidification of the silicon melt  3 , reference is made to DE 10 2005 013 410 B4. 
         [0045]    After the solidification of the silicon melt  3  to form the silicon ingot, the latter is sawn into columns with cuts parallel to the longitudinal direction  4 . Side rinds in extension of the edge gaps  12  accumulate as waste here. Moreover, a base and a cap of the silicon ingot are removed as waste by cuts perpendicular to the longitudinal direction  4 . The silicon ingot is sawn, in particular, in such a way that the saw cuts are located precisely in an extension of a seed crystal joint  9  in the longitudinal direction  4 . As a result, the loss of high-quality silicon due to sawing is reduced. 
         [0046]    Different arrangements according to the invention of the seed crystals will be described below with the aid of the figures. 
         [0047]      FIG. 2  shows a conventional arrangement of the seed crystals  7  with an axial &lt;110&gt; orientation. In order to characterize the orientation of the seed crystals  7  with regard to their lateral orientation, in other words with regard to a rotation about an axis parallel to the longitudinal direction  4 , the direction of the lateral &lt;110&gt; orientation  17  is shown as a double arrow in  FIGS. 2 to 9 . 
         [0048]    In the arrangement of the seed crystals  7  shown in  FIG. 2 , a {111} lattice plane has a cutting line  19  with a plane extending horizontally, in other words parallel to the base  5  and therefore parallel to the upper axial seed crystal cutting face  13 . The cutting line  19  runs parallel to two opposing upper cutting edges  15  of the seed crystal  7 . In the arrangement of the seed crystals  7  shown in  FIG. 2 , it is, in each case, parallel to the right-hand and left-hand side wall  6  of the container  2 . The projection of corresponding &lt;111&gt; lattice plane normal  21  (cf.  FIG. 10 ) into a horizontal plane is therefore perpendicular to the right-hand and left-hand upper cutting edge  15  or the side wall  6  of the container. This leads to an increased probability of twin formation, the twins being able to grow into the silicon ingot parallel to the projection  20  of the &lt;111&gt; lattice plane normal onto a horizontal plane, parallel to the {111} lattice plane. 
         [0049]    In general, the &lt;111&gt; lattice plane normal  21  encloses an angle δ with the lateral cutting faces  18  or the side wall  6 . The angle δ depends on the axial orientation of the seed crystals  7 . The smaller this angle, the further the twin growth extends along the {111} lattice plane in the direction of the ingot centre. 
         [0050]    In general, the projection  20  of the &lt;111&gt; lattice plane normal into a horizontal plane encloses an angle β with the upper cutting edges  15  or the side wall  6  or their cutting lines with the horizontal plane. If the projection of the &lt;111&gt; lattice plane normal  20  is perpendicular to the upper cutting edge  15  or the side wall  6  (β=90°), the probably of twin formation is greatest. In the parallel case, the probability is least (β=0°). 
         [0051]    In seed crystals  7  with an axial &lt;110&gt; orientation, the {111} lattice planes are oriented parallel to the lateral &lt;110&gt; orientation  17 . Accordingly, the cutting lines  19  are oriented parallel to the lateral &lt;110&gt; orientation. As emerges from  FIG. 2 , the sum of the length of the cutting edges  15  arranged at the edge, which run parallel to a {111} lattice plane, in this arrangement, is precisely half as great as the sum of the length of all the upper cutting edges  15  arranged at the edge. Accordingly, the sum of the length of the peripheral portions of an outer periphery of the entirety of all the seed crystals  7  in a horizontal plane, in which the projection of the &lt;111&gt; lattice plane normal into this horizontal plane is perpendicular to the closest side wall  6 , is precisely half as great as the total length of this outer periphery. 
         [0052]    This situation can be improved in that some of the seed crystals  7  arranged at the edge, the {111} lattice planes of which run parallel to the upper cutting edge  15  and parallel to the side wall  6 , are rotated by 90° in such a way that the corresponding {111} lattice plane is perpendicular to the upper cutting edge  15  or the side wall  6 . By rotating some of the seed crystals  7  arranged at the edge by 90°, the position of the horizontal cutting line  19  of the {111} lattice plane with the lateral cutting face  18  of the seed crystals  7  can be displaced into the block interior. The sum of the length of the peripheral portions, in which the projection of the &lt;111&gt; lattice plane normal into the horizontal plane is perpendicular to the closest upper cutting edge  15 , which is parallel to the side wall  6 , can be reduced by a rotation of this type. In the arrangement according to  FIG. 3 , it is only still 12.5% of the total length of the periphery. 
         [0053]    In general, the seed crystals are arranged in such a way that the sum of the length of the peripheral portions, in which the projection of the &lt;111&gt; lattice plane normal into the horizontal plane perpendicular to the closest upper cutting edge  15 , which is parallel to the side wall  6 , is at most 25%, in particular at most 12.5%, in particular 0% of the total length of the periphery. 
         [0054]    The seed crystals  7  can preferably be arranged in such a way that the ratio of the sum of the length of all the upper cutting edges  15 , which are arranged at the edge and run parallel to a {111} lattice plane of the respective seed crystal  7 , to the sum of the length of all the upper cutting edges  15  arranged at the edge is at most 1:4, in particular at most 1:6, in particular at most 1:8. 
         [0055]    In this arrangement, adjacent seed crystals  7  having different lateral orientations therefore exist. In particular, one of the seed crystals  7 , which are arranged adjacent to at least one of the side walls  6 , has a different lateral orientation to at least one of the other seed crystals  7 . 
         [0056]    All the seed crystals  7  preferably have an identical axial orientation. 
         [0057]    The above-mentioned ratio can be further reduced in that the seed crystals  7 , which are arranged in the connection region of two side walls  6  of the container  2 , i.e. in the corners of the container  2 , are arranged rotated by 45° about an axis parallel to the longitudinal direction  4  relative to the adjacent seed crystals  7  (see  FIG. 4 ). The lateral &lt;110&gt; direction  17  encloses an angle here of 45° with the lateral side faces  18 . The lateral &lt;110&gt; direction  17  in particular encloses an angle of 45° with the side walls  6  of the container  2 . 
         [0058]    An arrangement of the seed crystals  7  has proven particularly advantageous, in which their lateral &lt;110&gt; orientation encloses an angle of 45° with all their cutting edges, in particular with all their lateral side faces  18 , in particular with all the side walls  6  of the container  2  (see  FIG. 5 ). In general, the angle, which the lateral &lt;110&gt; orientation encloses with the cutting edges  15  and/or the side walls  6 , may be in the range of 15° to 75°. 
         [0059]    A configuration of the seed crystal  7 , in which the upper cutting edges  15  in each case enclose an angle of at least 5°, in particular at least 10°, in particular at least 15°, in particular at least 30°, preferably 45°, with the cutting lines  19 , is advantageous. 
         [0060]    Also advantageous is a configuration of the seed crystal  7 , in which the projections of a &lt;111&gt; lattice plane normal into a horizontal plane in each case enclose an angle of at most 45°, in particular at most 30°, in particular at most 15°, in particular at most 10°, preferably 0°, with the lateral side faces  18  of the respective seed crystal  7 , in particular with outer seed crystal edges on the reverse side. 
         [0061]    Advantageously, all the seed crystals  7  have an identical lateral orientation. 
         [0062]      FIG. 6  shows the conventional arrangement of seed crystals  7  with an axial &lt;100&gt; orientation and the course of their lateral &lt;110&gt; orientation  17 . In this arrangement, a twin formation with a growth direction projected onto a horizontal plane can occur in the entire edge region. 
         [0063]    In order to prevent this, it is provided according to the invention that the seed crystals  7 , as shown in  FIG. 7 , be configured in such a way that the lateral &lt;110&gt; orientations  17  enclose an angle of 45° with the cutting edges  15 ,  16 , in particular with the upper cutting edges  15 , in particular with the side walls  6 . As a result, the possibility of a twin formation is reduced and in the event of the formation thereof, the growth direction projected on a horizontal plane is oriented such that the twin formations influence the ingot centre less. 
         [0064]    As a result, the total length of the portions of the outer periphery of the entirety of all the seed crystals  7 , in which the projection of a &lt;111&gt; lattice plane normal into a horizontal plane runs perpendicular to the closest upper cutting edge  15 , in particular to the closest side wall  6 , is significantly reduced. 
         [0065]    In the case of seed crystals  7  with an axial &lt;111&gt; orientation, it is also not particularly advantageous to orient the cutting edges  15  parallel to the &lt;110&gt; direction  17  (see  FIG. 8 ). In an axial orientation of this type of the seed crystals  7 , three possible growth directions  20  are found for the undesired twin formation. In seed crystals  7  with an axial &lt;111&gt; orientation, an orientation thereof is advantageous such that their lateral &lt;110&gt; orientation  17  encloses an angle of 15° or 75° with one of the upper cutting edges  15  or with the side walls  6  (see  FIG. 9 ).