Abstract:
A method as well as an apparatus for manufacturing a tube according to the EFG-method. To manufacture tubes with a desired even wall thickness, it is proposed to draw the tube from a melt whose temperature can be controllably adjusted section by section.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention concerns a method for manufacturing a crystalline tube from a material such as silicon by drawing the tube from a melt created from the melting down of the material introduced to a crucible by means of a heater, where the melt penetrates a capillary slot molding the geometry of the tube and projects beyond this slot with a meniscus with a height h, which overflows into a seed crystal corresponding to the geometry of the tube or a lower peripheral area of a drawn section of the tube being manufactured. The invention also concerns an apparatus for drawing a tube out of a melt comprising a crucible with a capillary slot penetrated by the melt and molding the geometry of the tube, where the capillary slot is penetrated by the melt with a meniscus of a height h, and at least a heater associated with the crucible as well as a drawing device drawing the tube. 
         [0002]    A corresponding method is also known as the EFG (edge-defined-film-fed-growth) method, with which polygonal, in particular octagonal tubes are drawn from a melt such as a silicon melt. Edge distances are typically 125 mm. Discs with edges measuring 100×100 mm or greater are cut out of the corresponding sides with a laser. 
         [0003]    Apparatuses with which the EFG-method is performed have long been known and are comprehensively described. In this respect, reference is made to EP-B-0 369 574 or U.S. Pat. No. 6,562,132 as well as the literature cited in these documents. In this context, U.S. Pat. No. 6,562,132 governs the feeding of silicon particles to be melted and the electromagnetic field of induction coils for the heating of the melt. 
         [0004]    DE-T-691 24 441 discloses a system for regulating an apparatus for crystal growing. In order to draw tubes with uniform wall thickness, it is proposed to use the actual weight of the growing crystalline body as a variable to feed the required amount of material into the melt. 
         [0005]    U.S. Pat. No. 4,544,528 describes a device for drawing tubes according to the EFG-method. The device is distinguished through the presence of heat shields at the area of the transition between the melt and the solidified tube. Internal and external after-heaters are installed in the area of the solidified tube. 
         [0006]    DE-A-23-25 104 concerns crystal-drawing methods for tubes and fibers. To keep the outer diameter of the tube within the prescribed limits, the height of the meniscus is measured, while temperature is regulated as a function of this height. 
         [0007]    To grow monocrystals, RU-C-2 222 646 and RU-C-2 230 839 propose using a crucible containing a melt, where arranged in the crucible is a mold with canals that are penetrated by the melt. The canals terminate in the upper area of the mold at a seed crystal. For the melting process, a heating device is provided that consists of U-shaped plates bent to fit the shape of the crucible. The plates are arranged in sections to create a radially running isothermal state. 
         [0008]    The drawing of tubes according to the EFG-method is described in “A 3D Dynamic Stress Module for the Growth of Hollow Silicon Polygons”, NL.Z.: BEHNKEN, H.; SEIDL, A.; FRANKE: Journal of Crystal Growth, 2005, Vol. 275, Page e375-e380. 
       SUMMARY OF THE INVENTION 
       [0009]    The goal of the present invention is to further develop a method and an apparatus of the type initially named so that tubes can be manufactured that feature a desired uniform wall thickness, i.e. exhibit a narrow thickness distribution. In particular, the average amount of material used to create the tube should be minimized. In particular during the manufacture of wafers made of silicon, thicknesses should be attainable that allow a high filling degree and thus a high degree of effectiveness for a solar power cell. 
         [0010]    According to the invention, the goal is essentially attained through a method of the type initially named in such a way that the temperatures in adjacent areas of the melt are adjusted through regulation independently of one another. 
         [0011]    In particular, it is provided that the temperature of the melt in the individual areas is regulated as a function of the height h of the meniscus of the melt flowing out of the respective area into the capillary slot and/or as a function of the wall thickness t of the tube section drawn from the area. 
         [0012]    Deviating from the state of the art, a regulating process is proposed on the basis of which reproducible tubes with a narrow thickness distribution can be manufactured with wall thickness being minimized so that the average amount of silicon used can be minimized particularly in the manufacture of silicon rods for the production of wafers. The tube is then drawn from a melt in which the temperature is controllably adjusted by section and segment. 
         [0013]    According to the invention, the temperature of the melt can be regulated so that in the meniscus, namely in the transition between the melt and the solidified tube section, a temperature prevails that corresponds to the melting point of the material. In the case of silicon being used as the material in particular, the temperature is equal to 1,412° C.±2° C., where the temperature is held constant within 0.1° C. to 2° C. 
         [0014]    So that it is not necessary to directly measure the temperature in the meniscus, the parameters of height h of the meniscus or thickness t of the tube in the respective area or above can be used as variables to set the temperatures in the area as a function of the measured values. At the same time, the drawing speed can be modified accordingly, while in principle both the drawing speed and the temperature should be regulated. 
         [0015]    Tube thickness is preferably measured at the peripheral area of the drawing device. 
         [0016]    Temperatures in the individual areas are independently regulated by having a separate heating element, preferably a resistance heating element, assigned to each area, where the heating elements are preferably connected to one another in a star circuit. 
         [0017]    However, it is also possible to have a single heating element assigned to all areas. In such case, the heating element used is an induction heating element, that is, the crucible is surrounded by an induction coil. However, to allow the temperature to be adjusted in the individual areas corresponding to the regulation to be undertaken, the magnetic field is individually varied in each area. For this purpose, ferrite elements that influence the magnetic field are provided. Each ferrite element assigned to an area can be radially displaced, for example, by means of a motor. It is thus possible through simple measures to regulate the temperature of the melt in such a way that the required constant temperature prevails in the meniscus. 
         [0018]    It is envisioned that the drawing speed should be adjusted to a value between 7 mm/min to 24 mm/min with a tolerance of 1 mm/min, where the preferred drawing speed is between 12 mm/min and 15 mm/min. 
         [0019]    If the tube is of polygonal geometry, then an area is assigned to each side of the polygon, that is, the temperature of each side is regulated independently. Thus in an octagon, eight heating elements or eight ferrite elements independently adjustable from one another are provided when an induction heating element is used. In a dodecagon, accordingly, twelve heating elements or ferrite elements are present to facilitate the regulation of temperature. 
         [0020]    If a resistance heating element is used, it should preferably be made of graphite. Metallic resistance heating elements with which the required temperatures for adjusting the melt temperatures can be generated are also an option. 
         [0021]    Because of construction factors, the temperature of the melt is frequently not directly measured. Instead, the temperature of the wall of the molten bath is ascertained, for example, with a pyrometer. However, it is also possible to directly ascertain the melt temperature by means of a thermocouple, for example. 
         [0022]    An optical sensor is used to measure the height h of the meniscus, that is, the transition between the solid and liquid phase. In particular, a CCD-camera with image processing is provided. For this purpose, viewing windows can be present in the shell of the molten bath to allow the height h of the meniscus to be ascertained in each area. 
         [0023]    The wall thickness of the tube section drawn from the area can be measured interferometrically, for example, with an IR-interferometer. The respective measuring devices for measuring wall thickness and the height of the meniscus are connected to a regulator, via which the heating of the individual areas is then regulated. The drawing speed can also be prescribed by the regulator. 
         [0024]    Furthermore, the material to be melted down can be fed into the crucible in such a way that the amount supplied to each area corresponds to the amount which was drawn from the melt. For this purpose, the tube and the material feed are each connected to a load cell to facilitate the required regulation. 
         [0025]    Based on the tenet of the invention, it is possible, in contrast to the previously known EFG-method for manufacturing tubes, to commercially manufacture tubes with a circumference of more than 1 m, in particular 1.50 m or greater and a length of more than 5.50 m, preferably greater than 6 m, whereby wall thickness exhibits a narrow thickness distribution. In particular, it is possible to manufacture tubes with wall thicknesses between 100 μm and 300 μm with a tolerance between 8 to 12%. If the drawn tube is made of silicon, silicon wafers of desired sizes, particularly with edges up to 156 mm (6 inches) in length, wall thicknesses below 350 μm, in particular below 290 μm, can thus be manufactured so that there result a high filling factor, a high short circuit current thickness and a high open-circuit voltage of a polycrystalline silicon solar cell manufactured from the wafer, values which at least on a commercial scale are not possible to achieve according to the state of the art at a degree of reproducibility made possible through the invention. 
         [0026]    The method of the type initially named is distinguished particularly by the fact that the temperature of the melt—preferably in adjacent areas independent of one another—is regulated in such a way that the temperature of the meniscus is kept at a constant or nearly constant value over the entire length of the slot and/or the temperatures of the adjacent areas of the crucible are regulated independently from one another as a function of the height h of the meniscus of the melt flowing out of the area into the capillary slot and/or as a function of the wall thickness t of the tube section drawn from the area and/or the drawing speed of the tube section drawn from the melt is regulated as a function of the height h of the meniscus and/or the wall thickness t of the tube section. 
         [0027]    An arrangement of the type initially named is distinguished by the fact that the temperatures of the individual areas of the crucible or melt, in particular those bordering one another, can be individually regulated by means of one or more heaters. In particular, it is provided that a resistance heating element is assigned to each area, where the resistance heating elements can be connected to one another in a star circuit. 
         [0028]    However, it is also possible to assign an induction heating element—such as coil—to all areas, where a ferrite element influencing the magnetic filed is assigned to each area, which can be adjusted independently of one another. In particular, each ferrite element can be adjusted by means of a motor. 
         [0029]    In a further development it is envisioned that assigned to each area is a first measuring device measuring the height h of the meniscus, where the device employed is a CCD camera with a connected image-processing unit. 
         [0030]    The sensors for measuring wall thickness t should be arranged at a sufficient distance from the melt, preferably in the upper peripheral area of the drawing machine or its housing. 
         [0031]    In an improvement, the invention provides that assigned to each area is a second measuring device measuring the wall thickness t of the tube section drawn from the area, where the measuring device is an interferometer such as an IR-interferometer. 
         [0032]    The first and/or second measuring device and the heaters regulating the temperatures of the areas are connected via a control unit, which should also be connected to the drawing device. 
         [0033]    A tube manufactured according to the inventive method is distinguished particularly by the fact that the tube features a circumference U, where U≧100 cm, in particular U≧150 cm, a length L, where L≧550 cm, in particular L≧600 cm, and a wall thickness t, where 100 μm≧t≧500 μm. The tube is preferably of dodecagonal geometry. 
         [0034]    The preferred wall thicknesses of the tube lie between 250 μm and 350 μm at a thickness tolerance between 20% and 30% and between 100 μm and 240 μm at a thickness tolerance between 8% and 12%. 
         [0035]    When resistance heating elements are used as the heaters regulating the temperature in the individual areas, it is envisioned in an improvement of the invention, that the arrangement is surrounded by a metal housing—an option not possible when an induction heater is used. The top of the metallic housing features an aperture corresponding to the geometry of the tube for the purpose of conducting the tube. The resistance heating elements are preferably made of graphite. 
         [0036]    The sensors for measuring wall thickness are preferably located in the top area. 
         [0037]    For measuring temperature, temperature sensors such as pyrometers or thermocouples are provided, whereby the pyrometer is used to measure the base wall of the crucible, in order to allow conclusions to be drawn on melting point temperature. 
         [0038]    The crucible is of ring-shaped geometry, whereby the outer diameter is 5% to 15% greater than the longest diagonal of the tube. 
         [0039]    The capillary slot is connected to the crucible, i.e. the melt present therein, via a plurality of small holes or slits. If the slot is of polygonal geometry, then each side of the polygon is associated with one of the areas, whose temperatures can be regulated independently. 
         [0040]    For feeding the material to be melted down to the crucible, a feeding device leads through the base of the apparatus to then distribute the material evenly among the individual areas of the crucible via a reversing device. 
         [0041]    Above the crucible, the tube drawn from the flux is surrounded by insulation elements to facilitate a systematic cooling-off. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0042]    Additional details, benefits and features of the invention are found not only in the claims or the features that can be derived from them individually and/or in combination, but also in the following description of the preferred embodiment illustrated in the drawing. 
           [0043]    Shown are: 
           [0044]      FIG. 1  A principal illustration of a first embodiment of an apparatus for drawing a tube out of a melt, 
           [0045]      FIG. 2  A detail of the apparatus shown in  FIG. 1 , 
           [0046]      FIG. 3  An equivalent circuit diagram, 
           [0047]      FIG. 4  A principal illustration of a second embodiment of an apparatus for drawing a tube from a melt, 
           [0048]      FIG. 5  A detail of the apparatus illustrated in  FIG. 4  and 
           [0049]      FIG. 6  A principal illustration for ascertaining variables. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0050]    The figures, which in principle use the same reference numbers for the same elements, illustrate the configurations and their respective details with which the tubes are drawn according to the EFG-method. However, this invention is not limited to tubes of polygonal geometry. The tubes can also be of circular cross-section. 
         [0051]    As  FIG. 1  illustrates, the configuration  10  features a housing  12  that constitutes a receptacle, which accepts a base insulation  14 , in which a ring-shaped crucible  16  is arranged. In the crucible  16  a material  20  supplied via feed device  18  is melted down. In the embodiments illustrated in  FIG. 1  and  FIG. 2  this is accomplished namely by means of resistance heating elements  22 ,  24 ,  26 ,  28 , through which the adjacent areas of the crucible  16  can be heated independently. A corresponding breakdown is illustrated in  FIG. 5  and labeled with the reference numbers  17 ,  19 ,  21 ,  23 ,  25 ,  27 ,  29 ,  31 . 
         [0052]    The material fed into the crucible  16 , can be of spherical, polygonal or powder-formed geometry and is introduced via an opening  30  penetrating the base of the housing  12  and the base insulation  14  by means of a blowing device  32 . A conical element  34  resembling an umbrella reverses the direction of movement, so that the material forming a granulate on the basis thereof is directed along the surface  36  of a conical element in the direction of the crucible  16 , i.e. its ring-shaped intake  40 . As a result, there is an equal distribution of the granulate across the entire circumference of the ring-shaped intake  40 . 
         [0053]    As  FIG. 6  illustrates, running in the outer area of the crucible  16  is a slot  42  producing a capillary effect, which is connected to the silicon melt  41  via slits or holes so that the melt  41  flows into the slot  42  and, owing to the capillary effect, exits this on the top-side, i.e. on the upper edge, and forms a meniscus  44 . When a tube  46  is drawn, apex of the meniscus  44  solidifies, so that the tube  46  or the solidified sections can be raised by means of a drawing device  48  in the direction of the arrow  50 . For the purpose of precisely coordinating the amount of the granulate  20  fed to the crucible  60  with the amount of the drawn tube section, the drawing device  48  is connected to a load cell  52 , the measurement values of which are fed to a control unit  54 . 
         [0054]    The material to be melted down is fed from a receiver  33  to the blowing device  32  via a dosing device  35  ( FIG. 4 ). The dosing device  35  is connected to the control unit  54  via a load cell  37 , so that precisely the amount of material to be melted, which is drawn from the melt, i.e. the capillary slot  42 , is fed to the crucible  16  via the blowing device  32 . This amount is ascertained by means of the load cell  52 . 
         [0055]    The drawing device  48  comprises a seed crystal holder  49 , which at the start of the drawing process is of the same geometry of the seed crystal corresponding to the tube to be drawn, where the seed crystal comes into contact with the meniscus  44 . 
         [0056]    Furthermore, above the crucible the tube  46  is surrounded by an insulation  58  and radiation shields  56  to allow the controlled cooling of the tube  46 . 
         [0057]    In order to regulate the drawing process, whereby it should be ensured, that the tube  46  features a reproducible wall thickness at a narrow thickness distribution, whereby the wall thickness is simultaneously adjusted in such a way, that the amount of the material such as silicon is minimized, the invention provides, that in the crucible  16  the temperature of the melt is individually regulated in the adjacent areas  17 ,  19 ,  21 ,  23 ,  25 ,  27 ,  29 ,  31 , and whereby in a tube of polygonal geometry each area corresponds to a side of the polygon. If, on the other hand, the tube has a circular cross-section, the areas individually temperature regulated are adapted to a width of the tube in which plates are cut from the tube to be used, for example, as wafers for manufacturing solar cells. 
         [0058]    To facilitate reproducibly manufacturing wall thickness with a narrow thickness distribution, the melting point temperature of the material  20 , 1,412° C. in the case of silicon, must prevail in the meniscus  44 , namely in the transition between solid and liquid phase, and remain constant, fluctuating less than 2° C., in particular, between 0.1° C. and 2° C. 
         [0059]    To facilitate measurement in a simple manner, the temperature of the meniscus  44  is not ascertained directly. Instead, in the variant illustrated in  FIG. 1 , the temperature of the base of the crucible  16  is measured by means of pyrometers  60 ,  61 , which penetrate the resistance heating units  22 ,  24 ,  26 ,  28 . Temperature regulation is provided by means of a control unit  54 , which is connected to the resistance heating units  22 ,  24 ,  26 ,  28  via the regulatable supply terminals  62 ,  64 ,  66 ,  68 . The measured heights h of the meniscus  44  in the individual areas  17 ,  19 ,  21 ,  23 ,  25 ,  27 ,  29 ,  31  are fed to the control unit  54  to facilitate regulating the temperature in the areas  17 ,  19 ,  21 ,  23 ,  25 ,  27 ,  29 ,  31  as a function of these heights. The heights h are measured with optical sensors  70 ,  71 . In particular, a CCD-camera with connected image processing is used as sensor. If the heights of the meniscus  44  change, the temperature of the heating element  22 ,  24 ,  26 ,  28  and thereby the melt  41  are changed to restore the required height h of the meniscus  44 , where the wall thickness t of the tube  46  is a direct function of height (h=f(t)). 
         [0060]    According to the invention, the melt temperature and thereby the temperature of the meniscus  44  are thus regulated in the solid-liquid phase transition based on the ascertained height h of the meniscus  44 . 
         [0061]    As a supplemental or alternative option, it is also possible to regulate by measuring the thickness t of the tube  46 . For this purpose, interferometers  72 ,  74  are provided on the upper edge of the housing  12  as  FIG. 1  illustrates, where an interferometer  72 ,  74  is respectively assigned to an area  17 ,  19 ,  21 ,  23 ,  25 ,  27 ,  29 ,  31 , just as the sensors  70 ,  71  for measuring the height h of the meniscus  44 . If, for example, the tube  46  features a dodecagonal geometry, then each of the 12 sides is respectively assigned to an optical sensor  70  or  71  and an interferometer  72  or  74 . As  FIG. 5  illustrates, in an octagonal tube  46 , eight areas  17 ,  19 ,  21 ,  23 ,  25 ,  27 ,  29 ,  31  can be regulated, each of which is assigned to a surface  92 ,  94  of the tube  46 . 
         [0062]      FIG. 2  illustrates a section of the resistance heating element, which is assigned to the crucible  16 . Thus in  FIG. 2  two heating elements  26 ,  28  are illustrated, where each heating element  26 ,  28  independently regulates the temperature in an area of the crucible. In principle, one heating element  26 ,  28  is assigned to one side of the tube  46 , provided that the tube is of polygonal geometry. If a dodecagonal tube is drawn, then 12 heating elements are provided, which are connected in a star circuit as illustrated in the equivalent circuit diagram in  FIG. 3 . The outer connections  66 ,  68  are then connected to the control unit  54 , while the inner connections  69  are connected to one another and then grounded. 
         [0063]    In the embodiment illustrated in  FIG. 1 , the housing  12  is made of steel and is water-cooled. To allow the meniscus  44  to be optically captured, windows  76 ,  78  are left in the housing  12  corresponding to the areas  17 ,  19 ,  21 ,  23 ,  25 ,  27 ,  29 ,  31  which are to be temperature-regulated. The variant in  FIG. 4  differs from that in  FIG. 1  in that the areas  17 ,  19 ,  21 ,  23 ,  25 ,  27 ,  29 ,  31  to be independently temperature-regulated by regulation are not heated by resistance heating elements, but rather through an induction heater. For this purpose, the arrangement  80  features a housing  82  or receptacles made of double-plated glass and surrounded by an induction coil  84 . To heat the crucible  16  and thus the melt  41  present therein, a susceptor  86  generating the heat is arranged below the ring-shaped crucible. The susceptor  86  is made of graphite. In this example the susceptor  86  is surrounded by an insulation  87  in the usual way, which extends along the base and side wall of the housing  82  made of a glass double jacket. The arrangement  80  otherwise features the same basic construction as illustrated in  FIG. 1 . 
         [0064]    To allow the areas  17 ,  19 ,  21 ,  23 ,  25 ,  27 ,  29 ,  31  of the melt  41  to be individually temperature-regulated to a desired extent corresponding to the tent of the invention in a regulating process, there is assigned to each area  17 ,  19 ,  21 ,  23 ,  25 ,  27 ,  29 ,  31  a displaceable element  88 ,  90  made of a ferritic material, via which the magnetic field of the induction coil  84  is influenced so that the desired heating of the melt  41  occurs in such a way, that the meniscus  44  exiting the slot  42  exhibits the desired temperature, 1,412° C. for silicon, at a high degree of stability. 
         [0065]    The temperature of the melt  41  is indirectly measured according to the models in conjunction with  FIG. 1 , namely by ascertaining the temperature of the crucible  16 . This is performed using a pyrometer  60 , where a pyrometer  60  is assigned to each area  17 ,  19 ,  21 ,  23 ,  25 ,  27 ,  29 ,  31  which is to be independently temperature-regulated. The height h of the meniscus  44  and the thickness  5  of the wall thickness of the tube  46  are both measured in the manner previously described. 
         [0066]    From  FIG. 5  it should be clear, that assigned to the induction coil  84  corresponding to the areas  17 ,  19 ,  21 ,  23 ,  25 ,  27 ,  29 ,  31  to be individually temperature-regulated are adjustable ferrite elements  88 ,  90 , where the number of the ferrite elements  88 ,  90  corresponds to the number of sides  92 ,  94  of the tube  46 . The ferrite elements  88 ,  90  can be adjusted, for example, radially in relation to the tube  46  by means of servo-motors.