Patent Application
20050188918
The invention relates to a device for producing crystal rods of a defined cross-section and columnar polycrystalline structure by floating-zone continuous crystallization (CCC=crucible-fed continuous crystallization), provided with at least a crucible filled with crystal material with a central output for transporting the content of the crucible to a growing crystal rod positioned under the crucible, with the central output immersed in the melt meniscus on the upper surface of the crystal rod, means for the continuous controllable feeding of the crucible with solid crystal material and means for the simultaneous feeding of the melt energy and setting of the crystallization front.
In accordance with the prior art, the crystallization of polycrystalline block silicon from silicon granulate takes place in a temporal sequence of melting the raw material and the subsequent crystallization of the melt or by simultaneous melting and crystallizing at a direct thermal interaction.
Thus, 11th E.C. Photovoltaic Solar Energy Conference, 12-16 Oct. 1992, Montreux, Switzerland, pp. 1070-1073, describes an arrangement for producing polycrystalline block silicon by electromagnetic continuous casting (EMC—Electromagnetic Casting). The melt is enclosed by a tubular round or square induction coil (the principle of the cold reflector) provides for melting of the fed pieces of silicon and, at the same time, for the crystallization of the crystal rod which is continuously pulled out from below. The high frequency field of the induction coil generates electromagnetic forces which keep the melt away from the wall of the crucible and form the block silicon. The heater enclosing the silicon crystal rod and the support of the melt by magnetic forces are characteristic of this arrangement.
However, owing to the lateral heat infusion the phase boundary is strongly parabolically bent which leads to extreme thermo-mechanical stresses in the polycrystalline rod. Also, the majority of the crystals is not axially (columnar) oriented which reduces the effectiveness of solar cells made of this material. This effect increases with increasing drawing speed and larger rod cross-sections. Hence, for technically relevant cross-sections the drawing speed is limited to 0.8 to 1.2 mm/min.
In the FZ-method described in DE 195 38 020A1 the melt is heated by a resistance heater and the required energy infusion into the crystallization front is carried out by an induction heating coil, which is preferably structured as a plate-shaped flat coil with a central inner opening. This technically complex heating arrangement consisting of two separate heating means serves the controlled post-charging of silicon granulate as well as the prevention of undercooling of the crystal rod in the production of rod-shaped silicon mono-crystals of large diameters.
The proceedings of the 16th European Photovoltaic Solar Energy Conference, 1 to 5 May 200, Glasgow, UK, pp. 1616-1619, describe a system for producing crystal rods by inductive top-heated continuous crystallization (ITCC). The arrangement is provided with a funnel to which silicon granulate is fed and the tube of which terminate closely above the surface of the melt at the upper end of the crystal rod. Solid raw material pre-heated by a heat lamp is fed to the funnel by a feed conduit. The material drops through the funnel, which is surrounded by a flat annular inductor, onto the melt on the silicon crystal rod where the inductive heat causes the material flowing there to be melted. 1 to 2 cm below the liquid/solid phase the cylindrical polycrystalline silicon rod crystallizes while rotating about its axis. While this arrangement makes possible a phase boundary which is bent less and correspondingly columnarly oriented crystallites at a growth rate of at most 1 to 1.5 mm/min because the heating energy for melting a large quantity of raw material simultaneously lowers the undercooling of the melt and, therefore, the rate of crystallization. However, by this arrangement, only crystal rods of circular cross-section can be realized.
In the status report 1996 Potovoltaik presented y the project supporter Biology, Energy, Ecology by the (German) Federal Ministry of Education, Science, Research and Technology-Research Center Juelich GmbH, on the occasion of the status seminar Photovoltaik 1996 at Bad Breisig (Germany) from 23 to 25 Apr. 1996, 5-1 to 5-11, there is a report about crucible-fed continuous crystallization of silicon as well. It discloses an arrangement with a surface lamp heater for the crucible-fed continuous crystallization of silicon rods of round cross-section. The heat energy of the lamps is directed towards the upper end surface of the silicon rod; further lamps are disposed at the periphery and serve to heat the melt and the post-heating of the rod following re-crystallization. A frame of non-melting material is arranged above the silicon rod; it is partially submerged in the melt and acts as a shape-imparting element. By this arrangement, crystal rods of square cross-section may also be produced. The setting of a defined heating power poses problems, however, since at too high a heating power, the height of the free melt between the silicon rod and the frame increases, and liquid silicon may escape. At too low a heating power, the silicon rod may grown into the frame. To avoid the disadvantages of feeding cold granulate, a flat tub of graphite was arranged above the frame for dispensing, by way of a central bore, the melt provided for a liquid after-charge, into the shape-imparting frame. As may be seen, the setting of a defined/optimal heating power is difficult and was here realized by complex means, i.e. focused lamp radiation and an additional reflector. As has already been mentioned supra, two optical heaters were applied, one being used for melting the raw materials, the other for controlling the crystallization of the silicon rod to be produced.
Another possibility for a crucible-fed continuous crystallization of silicon is described in the publication mentioned above and relates to a float-zone crystal drawing arrangement. Above an induction, within a recipient there is provided a storage bunker for receiving the silicon granulate for the port-charging. By way of a quartz funnel through openings in the inductor the granulate was guided to the surface of the silicon rod where it is almost completely melted. Several silicon grains did, however, escape to the edge of the rod where they nucleated. In order to achieve complete melting of the granulate, it was fed through a quartz tube protruding through the inductor and touching the melt on the silicon rod. While this results in a good columnar grain structure, melting of the material as well as crystallization in this arrangement take place below the frame. The energy distribution of the heating arrangement limits the melting rate as well as rate of crystallization. No shape-imparting frame was used in the second-mentioned arrangement.
It is an object of the invention to provide an arrangement for the production of crystal rods of defined cross-section and columnar polycrystalline structure which by using technically less complex heating means, compared to the prior art, nevertheless provides for a high rate of crystallization at a stable phase boundary and, at the same time, allows for an adjustability of the heating power in accordance with the actual application.
In accordance with the invention the object is accomplished by arrangement, closely above the growing crystal rod, a frame which touches the melt, that the means for the simultaneous feeding of melting energy and the setting of the crystallizing front on the growing crystal rod is an induction coil with an opening, the induction coil being vertically moveable as regards its distance from the crucible and/or crystallization front and structured such that above the induction coil the raw material is melted in the crucible and that below the induction coil the melted material transported through the central tubular output conduit extending through the opening of the induction coil to the surface of the silicon rod crystallizes and that a temperature field is generated which is adjusted to the desired cross-section of the crystal rod.
In the arrangement in accordance with the invention, the melt which is fed by way of the central cylindrical output conduit through the opening in the induction coil directly downwardly onto the crystal rod is produced within the crucible. Not only is the melting of the material thus thermally substantially disconnected from the crystallization of the rod, but it also allows for a greater degree of freedom as regards the structuring of the inductor. The latter ensures the presence of a closed melt meniscus and of a temperature field which in combination with the guiding frame touching the meniscus imposes upon the non-rotating rod a defined, preferably, square cross-section. By longitudinally moving the crystal rod in a downward direction, the growth phase boundary is generated a few mm below the frame, and in correspondence with the substantially axial heat current the orientation of the crystallites is predominantly columnar. The heating inductor arranged between the crucible and the crystal is to be rendered by the person skilled in the art to conform to the parameters or the crystal rod to be drawn (shaping of the temperature field for the desired cross-section of the rod, effective transmission of power).
By varying the distance of the induction coil from the melting crucible and/or from the crystallization front, the arrangement in accordance with the invention makes possible a thermal disconnection of the melting process of the granulate from the crystallization process of the melt; this leads to higher rates of crystallization at a stable phase boundary for the continuously growing polycrystalline rod. It is thus possible even during the production process to set the division of the total power between melting and crystallization power and, hence, the position of the phase boundary.
In one embodiment the guiding frame is formed of a resistant material and is of a defined geometric configuration.
In a further embodiment an additional radiation heat source is arranged above the crucible for a more rapid melting of the raw material; in this manner the admission of heat to the crucible can be set independently of the crystal rod. This may be necessary, for instance, for making available the required amount of heat when working under protective gas.
In another preferred embodiment the wall of the bottom of the crucible at the connection to the cylindrical output conduit is raised. This is to prevent a complete emptying of the crucible and, therefore, its destruction.
Radiation protection shields are arranged at the level of the crystallization front around the crystal rod for insulation and for ensuring planar phase boundaries.
In a further embodiment, the planar induction coil is structured such that the phase boundary does not touch the guide frame. The coil is of square or rectangular configuration.
The invention will be explained in greater detail in the following embodiment with reference to the drawings, in which:
The schematic top view in
| Number | Date | Country | Kind |
|---|---|---|---|
| 102 20 964.2 | May 2002 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/DE03/01515 | 5/6/2003 | WO | 11/18/2004 |
