BRIEF DESCRIPTION OF THE DRAWINGS
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.
Shown are:
FIG. 1 A principal illustration of a first embodiment of an apparatus for drawing a tube out of a melt,
FIG. 2 A detail of the apparatus shown in FIG. 1,
FIG. 3 An equivalent circuit diagram,
FIG. 4 A principal illustration of a second embodiment of an apparatus for drawing a tube from a melt,
FIG. 5 A detail of the apparatus illustrated in FIG. 4 and
FIG. 6 A principal illustration for ascertaining variables.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
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.
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.
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.
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.
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.
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.
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.
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.
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.
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)).
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.
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.
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.
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.
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.
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.
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.
Patent Application
20080053367
