Abstract:
Oven for non-metal melting, in particular silicon melting, with a housing enclosing an interior, at least one mould arranged in the interior for receiving a non-metal melt, at least one electrical heating device enclosing, at least partially, the at least one mould for influencing the temperature of the non-metal melt, and a power supply device connected in an electrically conductive manner to the at least one heating device for providing the heating device with a time-variable current I(t), wherein the current I(t) has a frequency of 0.1 Hz to 1000 Hz and the current I(t) is of a magnitude sufficient for setting a predetermined temperature of the non-metal melt, the currents in the plurality, where necessary, of heaters having a defined phase position in respect of one another.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to an oven for non-metal melting, a method for operating said oven and non-metal blocks produced by the method. 
   2. Background Art 
   A crystal growing unit is known from DE 103 49 339 A1. This has a round mould which is encompassed by a cylindrical coil mantle. This oven generates silicon worthy of improvement in a number of respects. 
   SUMMARY OF THE INVENTION 
   The problem of the invention is to create an oven for non-metal melting which can generate non-metal blocks which are as suitable as possible for further processing. 
   The object is solved by an oven for non-metal melting with a housing enclosing an interior, at least one mould arranged in the interior for receiving a non-metal melt, at least one electrical heating device enclosing at least partially the at least one mould for influencing the temperature of the non-metal melt, and a power supply device coupled in an electrically conductive manner to the at least one heating device for supplying the heating device with a time-variable current I(t), wherein the current I(t) has a frequency of 0.1 Hz to 1000 Hz and the current I(t) is of a magnitude suitable for setting a predetermined temperature of the non-metal melt. The object is also solved by a method for at least one of melting non-metals and solidifying non-metal melts comprising the following steps: providing an oven according to any one of the preceding claims, applying a time-variable current I(t), generating a time-variable magnetic field in the non-metal melt. The crux of the invention consists in using the lines which in any case are present in an electrical heating device to generate a time-variable magnetic field in the non-metal melt. To do this, a time-variable current must be applied to the lines. Convections arise in the non-metal melt by means of the time-variable magnetic field which even out the distribution of foreign atoms. 
   Additional features and details of the invention result from the description of a plurality of embodiments by reference to the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a view of an oven according to a first embodiment, 
       FIG. 2  shows a section according to the line II-II in  FIG. 1 , 
       FIG. 3  shows a section according to the line III-III in  FIG. 1 , 
       FIG. 4  shows a section according to the line IV-IV in  FIG. 1 , 
       FIG. 5  shows a view of an oven according to a second embodiment, 
       FIG. 6  shows a section according to the line VI-VI in  FIG. 5 , 
       FIG. 7  shows a section according to the line VII-VII in  FIG. 5 , 
       FIG. 8  shows a heating device according to a third embodiment, 
       FIG. 9  shows a heating device according to a fourth embodiment, 
       FIG. 10  shows a heating device according to a fifth embodiment, 
       FIG. 11  shows a heating device according to a sixth embodiment, 
       FIG. 12  shows a heating device according to a seventh embodiment, and 
       FIG. 13  shows a heating device according to an eighth embodiment. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIGS. 1 to 4 , a first embodiment of the invention is described below. An oven  1  serves for the melting and/or targeted solidification of non-metals, and silicon in particular. This means that raw silicon melted in the oven  1  as well as silicon which is already melted outside the oven can be cooled in a controlled manner. The oven  1  has a substantially cuboidal housing  2  which is a steel boiler in design. Housing  2  is a pressure vessel which is evacuated during operation of oven  1  in order to prevent impurities of the silicon melt  24  resulting from oxygen and other gases. The housing  2  encloses a housing interior  3 . A cuboidal graphite insulation  4  is arranged in the interior  3 . Inside the graphite insulation  4  there is arranged a support  6  supported on the floor  5  of the insulation  4 . Said support  6  has a horizontal supporting plate  7  as well as side walls  8  projecting downwards therefrom supported on the floor  5 . At least one cuboidal mould  9  which is rectangular in cross-section is supported on the supporting plate  7 , said mould  9  having a mould floor  10  as well as four side walls  11  in parallel pairs extending upwards from the floor  10 . A plurality of moulds  9 , for example two, four, six or eight moulds, may be arranged in the oven  1 . The advantage of moulds  9  which are rectangular in cross-section is that a plurality of moulds can be arranged adjacent to one another, thus saving space, and more effectively than is possible when using round moulds, for example. Depending on the operating state of the oven  1 , the mould  9  is filled with silicon to be melted, already melted silicon  24  or solidified silicon melt. The term “mould” denotes both a container designed for one use, which subsequently destroys itself or is destroyed; it also denotes a container which may be used several times, frequently also referred to as a crucible. 
   The oven  1  has an electric heating device  12  consisting of an overhead heating device  13  disposed above the mould  9 , a side heating device  14  encompassing the mould  9  on the circumferential face and a floor heating device  15  disposed below the mould  9 , not all the devices  13 ,  14 ,  15  needing to be present simultaneously. The heating device  12  encompasses the mould  9  at least partially, i.e. it is arranged at least above it and/or below it and/or laterally to the mould  9 . The devices  13 ,  14  and  15  are connected to a power supply device  16 , shown only in  FIG. 1  and only shown schematically therein, via electrical feed lines  17 . The overhead heating device  13  has two mutually separate lines  18 ,  19  which are led from outside through a side wall  20  of the graphite insulation  4  and are led outside again through the opposing side wall  21  of the graphite insulation  4 . The lines  18 ,  19  are connected to the power supply device  16  at both ends, being electrically conductive. When “lines” are mentioned in the patent application, this refers to those which are suitable for carrying the corresponding heating currents. As these currents can amount to several thousand amperes, these lines as a rule comprise solid strips or rods which preferably consist of a highly electrically conductive material. The actual heating lines preferably contain carbon and/or molybdenum and/or tungsten. The feed portions in the cooler region may contain copper and/or aluminium and/or carbon-based materials. The lines  18 ,  19  each have feed portions  22  running through the side walls  20 ,  21  as well as interposed looped portions  23 . The looped portions  23  are arranged mirror-symmetrically to one another. The loops of the looped portions  23  run horizontally. The floor heating device  15  arranged below the mould  9  running through the support  6  in the present case is similar in design to the overhead heating device  13 . 
   The side heating device  14  has two superposed line loops  25 ,  26  encompassing the mould  9  on the circumferential face. 
   The loops  25 ,  26  substantially follow the rectangular outer contour of the mould  9  and to this extent, apart from the feed portions  22 , are substantially rectangular. The feed portions of the floor heating device  15  or over-head heating device  13  led through the graphite insulation  4  on the one hand, and of the side heating device  14  are displaced at 90° from one another with respect to a vertical axis, as shown in  FIG. 2 . 
   The manner in which the oven is operated is described below. The mould  9  is filled with silicon. The interior of the oven  1  is evacuated. The interior can also be filled with an inert gas, for example argon. The power supply device  16  supplies the heating device  12  with electrical current I(t). The time-variable current I(t) may preferably consist of a direct current component I DC  and an alternating current component I AC (t), so that the following applies: I(t)=I DC +I AC (t). The alternating current component I AC (t) may comprise a normal sinusoidal alternating current. It is also possible for there to be other time-variable currents, for example sawtooth or rectangular current. The alternating current component I AC (t) has a frequency of 0.1 Hz to 1000 Hz, in particular 1 to 500 Hz, in particular 10 to 300 Hz, in particular 75 Hz to 250 Hz. It is also possible to operate at approx. 50 Hz. The alternating current portion I AC (t) lies approximately between 100 and 5000 ampere-turns. The direct current portion I DC  may lie between 0 and 5000 ampere-turns. The current portions are referred to in units of “ampere-turn”, this actually being a unit of the magnetomotive force generated by a current of 1 ampere in a single conductor loop. In the case of a plurality of conductor loops, the current is multiplied by the number of turns. Specifying the “ampere-turns” is more meaningful than specifying the currents in the individual loops because ultimately the number of conductor loops—in the case of the side heating device  14 , for example—may be freely selected. The various heating devices  13 ,  14  and  15  can all be operated in phase or with a corresponding phase shift, in particular of 60° or 120°. Travelling fields can also be generated with the various heating devices  13 ,  14  and  15 . 
   In the present embodiment, the phase shift amounts to 0° between the two loops  25 ,  26 . The phase shift of the current through the floor heating device  15  and overhead heating device  13  on the one hand and the side heating device  14  on the other hand amounts to +60°. The actual frequency used is 50 Hz. The phase shift  4  between a comparison current I V (t) and a reference current I B (t) is defined as follows: assuming the reference current can be represented as I B (t)=I B0  sin(2 πft), then the comparison current has a phase shift φ, where it can be represented as I V (t)=I V0  sin (2 πft+2πφ/360°). Here, f represents the frequency and φ the phase shift. 
   It is shown below by reference to an illustration how the alternating current portion I AC (t) enhances the quality of the polycrystalline silicon (mc-Si) blocks. 
   
     
       
             
             
             
             
             
           
         
             
                 
             
           
           
             
               Proportion of AC 
               0% 
               50% 
               50% 
               100% 
             
             
               in the total current 
             
             
               in % 
             
             
               Frequency of the 
               — 
               8 Hz 
               17 Hz 
               50 Hz 
             
             
               AC in Hz 
             
             
               Amperage 
               800 
               1000-1100 
               1000-1100 
               800-900 
             
             
               through overhead 
             
             
               heating device in A 
             
             
               Total amperage 
               800 
               1500-1600 
               1500-1600 
               1000-1100 
             
             
               through side heat- 
             
             
               ing device in am- 
             
             
               pere-turns 
             
             
               Amperage 
                0 
               130-270 
               130-270 
                0-200 
             
             
               through floor 
             
             
               heating device in A 
             
             
               Indication of the 
                1 
               2 
               2 
               3 
             
             
               yield of good 
             
             
               material from the 
             
             
               mc-Si blocks (the 
             
             
               higher the figure 
             
             
               the greater the 
             
             
               yield) 
             
             
                 
             
           
        
       
     
   
   By applying a time-variable current to the heating device  12 , time-variable magnetic fields are generated in the silicon melt  24  which lead to increased convection of the melt  24 . By this means it is possible to achieve a more homogeneous mixing of the melt  24  and therefore reduced inclusions of foreign atoms in the polycrystalline silicon. The heating device  12  may also have lines for heating purposes—hot during operation—for heating the melt, for example through direct current, and additional lines—cold during operation—for generating the travelling magnetic field. In this case, the electrical heating and generation of the magnetic fields would be decoupled from one another. 
   A second embodiment of the invention is described below with reference to  FIGS. 5 to 7 . Identically constructed parts are assigned the same reference symbols as in the first embodiment, to whose description reference is made here. Parts of differing construction but with identical functions are assigned the same reference symbols with an appended a. The substantial difference from the first embodiment lies in the fact that the mould  9  on the circumferential face is encompassed by three superposed loops  25   a ,  26   a ,  27   a  of rectangular cross-section, which are all closed apart from the feed portions  22   a  and form the side heating device  14   a . The floor heating device is not included. Above the mould  9  there is an overhead heating device  13   a  which consists of a line consisting of a feed portion  22   a , a looped portion  23   a  and an opposing feed portion  22   a , the portions  22   a  being led through the walls of the graphite insulation  4 . The loops of the looping portion  23   a  run horizontally and therefore parallel to the surface of the silicon melt  24 . The heating devices  13   a ,  14   a  are operated with an alternating current at a frequency of 50 Hz, although other frequencies are also possible. The phase shift of the heating currents, the heating current and the relevant yields are shown in the following table. 
   
     
       
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
           
         
             
                 
             
             
               Case 
               1 
               2 
               3 
               4 
               5 
               6 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               Phase shift of 
                 
                 
                 
                 
                 
                 
             
             
               heater current currents 
             
             
               Overhead heating device 
               +120 
               −60 
               −60 
               +120 
               +60 
               +240 
             
             
               13a 
             
             
               Top line loop 25a 
               +120 
               −60 
               +120 
               −60 
               +240 
               +240 
             
             
               Middle line loop 26a 
               +60 
               +60 
               +60 
               +60 
               +120 
               +120 
             
             
               Lower line loop 27a 
               0 
               0 
               0 
               0 
               0 
               0 
             
             
               Heater current in A 
             
             
               Overhead heating device 
               700 
               775 
               575 
               700 
               700 
               700 
             
             
               13a 
             
             
               Top line loop 25a 
               175 
               150 
               150 
               125 
               125 
               125 
             
             
               Middle line loop 26a 
               125 
               225 
               150 
               175 
               125 
               125 
             
             
               Lower line loop 27a 
               150 
               325 
               300 
               250 
               250 
               250 
             
             
               Indication of the yield of good 
               3 
               2 
               2 
               1 
               2 
               3 
             
             
               material from the mc-Si 
             
             
               blocks (the higher the number 
             
             
               the higher the yield) 
             
             
                 
             
           
        
       
     
   
   It is shown that the highest yield is obtained when a travelling magnetic field, in particular a current with a phase shift of +60° or +120°, is applied to the loops  25   a ,  26   a  and  27   a  and when the overhead heating device  13   a  is operated in phase with the current in the upper line loop  25   a.    
   Referring to  FIG. 8 , a third embodiment of the invention is described below. Identical parts are assigned the same reference symbols as in the first embodiment. Parts that are different in construction, but have identical functions are assigned the same reference symbols but with an appended b. By way of example,  FIG. 8  shows the structure of one of the heating devices  13   b ,  14   b  and/or  15   b  consisting of three heating rods  28  aligned parallel to one another, which are led through opposing side walls of the graphite insulation  4 . The heating rods  28  are preferably supplied with currents which are phase-shifted in such a way as to create a travelling magnetic field. Phase shifts of +60° or +120° are preferable. The arrangement according to  FIG. 8  can be disposed at the four side walls  11  of the mould  9  above and/or below it. More than or fewer than three heating rods  28  arranged adjacent to one another may also be used. In addition, the number of heating rods  28  on the various sides of the mould  9  does not have to be identical, on the circumferential face in particular on the one hand as well as, on the other hand, above it and below it. 
   Referring to  FIG. 9 , a fourth embodiment of the invention is described below. Identical parts are assigned the same reference symbols as in the first embodiment. Parts that are different in construction, but have identical functions are assigned the same reference symbols but with an appended c. The substantial difference compared with the embodiment according to  FIG. 8  is that a spiral heating line  29  consisting of feed portions  22   c  and a spiral portion  30  is provided. The spiral portion  30  has rectangular sides of reducing lengths which run parallel to the walls of the graphite insulation  4 . One of the feed portions  22   c  is connected to the middle of the spiral and is led behind the spiral portions  30  to the outside. The arrangement shown in  FIG. 9  may be disposed on the circumferential faces of the mould  9  and/or above it and/or below it. 
   Referring to  FIG. 10 , a fifth embodiment of the invention is described below. Identical parts are assigned the same reference symbols as in the first embodiment. Parts that are different in construction, but have identical functions are assigned the same reference symbols but with an appended d. The embodiment according to  FIG. 10  shows an overhead and/or side and/or floor heating geometry corresponding to the first embodiment. Also only one looped portion or three looped portions or even more looped portions may be arranged adjacent to one another. 
   Referring to  FIG. 11 , a sixth embodiment of the invention is described below. Identical parts are assigned the same reference symbols as in the first embodiment. Parts that are different in construction, but have identical functions are assigned the same reference symbols but with an appended e.  FIG. 11  shows possible floor and/or side and/or overhead heating geometries. The heating line as in  FIG. 10  has mutually parallel feed portions  22   e  to which are connected mirror-symmetrically looped portions  23   e  which at the end to the left in  FIG. 11  are interconnected by means of a connection portion  31 . Thus,  FIG. 11  forms only one electrical circuit, whereas  FIG. 10  forms two electrical circuits. 
   Referring to  FIG. 12 , a seventh embodiment of the invention is described below. Identical parts are assigned the same reference symbols as in the first embodiment. Parts that are different in construction, but have identical functions are assigned the same reference symbols but with an appended f.  FIG. 12  shows a side heating geometry which has already been described in the second embodiment. Thus, a line loop  25   f  is provided encompassing the mould  9  with substantially rectangular shape and mutually parallel feed portions  22   f.    
   Referring to  FIG. 13 , an eighth embodiment of the invention is described below. Identical parts are assigned the same reference symbols as in the first embodiment. Parts that are different in construction, but have identical functions are assigned the same reference symbols but with an appended g. The side heating geometry corresponds substantially to the first embodiment, according to which two lines encompass the mould  9  in a bow shape. Mutually parallel feed portions  22   g  are provided in each case which merge into rectangular bow-shaped line loops  25   g  and  26   g  respectively. 
   The heating device geometries shown in the previously described embodiments may substantially be combined freely with one another, for example a floor or overhead heating device according to  FIG. 11  and a side heating device according to  FIG. 12  may be provided. In addition to this, in general floor and overhead heating devices may also differ from one another or, as in the second embodiment, may in part be missing. In addition, several differing heating lines, for example as in  FIGS. 12 and 13 , may be provided superposed which together form the side heating device.