Patent Application
20110129404
The present invention relates to an apparatus and process for the production of polycrystalline silicon for photovoltaic use.
In particular, the invention relates to an apparatus in which the loading of the material containing silicon for purification and the extraction of the finished ingots are carried out without any need to switch off the furnace of the apparatus.
The typical processes of the thermal cycle for the crystallisation of polycrystalline silicon for photovoltaic use generally involve the following stages:
(i) loading the crucible, generally made of sintered silica, at room temperature, with the silicon feedstock to be crystallised;
(ii) positioning the crucible in the furnace, gradually increasing the temperature above the silicon melting temperature, typically around 1400-1500° C. in inert atmosphere, generally argon;
(iii) proceeding with the cycle following a thermal curve suitable for accomplishing the directional crystallisation of the silicon, possibly carrying out one or more annealing stages;
(iv) allowing the solidification of the smelted material by cooling it in the furnace, again in inert atmosphere;
(v) extracting the crucible from the furnace, generally when a temperature of the order of 200° C. is reached, bringing it down to room temperature and removing the thus obtained silicon solid.
Many furnaces and apparatus are known in the art for carrying out the above-mentioned thermal cycle and for obtaining silicon-based polycrystalline materials for photovoltaic use. In particular, a furnace is described in patent EP 0 186 249 whose crucible containing the silicon feedstock to be re-smelted and re-crystallised, is placed on a cooled pedestal which, when moved vertically, transfers it into the upper part of the furnace to an area which is heated in an inert gas atmosphere at a temperature above the silicon melting temperature.
Subsequently, at the end of the smelting, the temperature is gradually lowered (by reducing the electrical power output delivered) and, as a result of the joint effect of the cooling of the pedestal, the smelted material starts to crystallise from the bottom of the crucible upwards. On completing the crystallisation thermal cycle, the furnace is cooled to 200° C., and then purged of the inert gas therein contained and opened for extraction of the silicon ingot and for loading other material to be crystallised. This operation of cooling down to a temperature of 200° C. is necessary in case of premature opening of the furnace, the graphitic component of the heating part would be exposed to the air and, in the presence of oxygen, would undergo serious deterioration phenomena.
The apparatus thus described also presents other drawbacks, the most important of which are:
The heating components of the furnace are subjected to very wide thermal cycles, ranging from the melting temperature of approximately 1500° C. to the furnace opening temperature of approximately 200° C. and vice versa, which subject the components to considerable high wear, thus reducing its average working life;
In addition, cooling to 200° C. and then reheating in the subsequent cycle, starting from 200° C. rather than from higher temperatures causes inevitable, substantial energy losses.
An improvement in the apparatus is described in patent EP 1 867 759, which, however, does not solve the problems outlined above.
There was therefore the need to reduce production costs, particularly in terms of reducing the times of introduction of the crucible into the furnace and extraction from it.
An apparatus that overcomes the above-mentioned drawbacks has now been produced and constitutes an object of the present invention.
The apparatus, according to the present invention, is described in the Claims and in the attached figures.
The apparatus is characterised in that the operations of loading the material to be crystallised and unloading the finished ingots take place without needing to open the furnace to the atmosphere, enabling the graphite components to be left at temperatures well above 200° C., which results in a drastic reduction of the thermal cycle excursion, a gain in terms of process times, a reduction in energy consumption and, additionally, the ability to obtain an end product which is less subject to pollution phenomena and thus substantially purer.
In the apparatus according to the invention, the final cooling of the ingot down to a temperature of approximately 200° C. takes place in an area separate from the furnace. Therefore, the cooling of the ingot can take place in parallel with the loading of a new ingot into the furnace and the time required for said cooling is not added to the total time of the production cycle.
Another object of the invention is the crystallisation process carried out in the apparatus according to the invention.
Additional objects of the invention will be evident from the detailed description of the invention.
The figures represent a complete, repeatable cycle of the various process stages that can be realised in the apparatus according to the invention.
The apparatus for the preparation of silicon-based polycrystalline materials according to the present invention is characterised in that it comprises multiple chambers, preferably three (1, 2, 3), delimited by curved and/or flat side walls, formed in such a way that a cooling fluid circulates inside them, and arranged longitudinally one after the other and equipped with:
With particular reference to the attached figures, which illustrate a preferred embodiment of the invention, the apparatus comprises a first chamber (1) and a third chamber (3), each bound by side walls (1′) and (3′), respectively, designed in such a way that a cooling fluid circulates inside them. The first is a so-called “loading” and preheating chamber, equipped with an opening to the outside and with another opening to the second chamber or “hot” chamber. The third is a so-called “unloading” and cooling chamber, equipped in turn with an opening to the “hot” chamber and another opening to the outside.
All the chambers (1, 2, 3) are vacuum sealed and are equipped, on the openings, with means for ensuring air-tightness (8), for example air-tightness bulkheads.
Chamber (2), interposed longitudinally between the first and third chambers, communicating with them via the openings and capable of being insulated by means of the air-tightness means (8), is conformed such as to have a central body, generically cylindrical, with its axis orthogonal to the longitudinal axis of the apparatus connected up to the first and third chambers via the longitudinal connecting walls (2′). Said central body is equipped with cylindrical walls (2″), with an upper cover (2′″) and a lower cover (2″″), both of which can be opened to permit easy maintenance, the lower cover (2″″) being additionally equipped with a central hole for passage of the heat-stable pedestal (5) for raising or lowering the crucible (6). The crucible (6) is positioned on the pedestal (5) to be transferred vertically into the “hot” chamber and housed in the smelting area (4). The “hot” chamber is generally made with stainless steel walls within which a cooling fluid circulates. The actual silicon smelting area (4) is placed in the upper part of the “hot” chamber. Said area (4) is insulated with refractory material and heated by means of graphite resistors. As can be seen in the figures, the crucible (6) is placed on the thermostated pedestal (5). The vertical excursion of the pedestal is such as to carry the crucible (6) into the smelting area (4). As shown in the figure, the right-hand side of the hot chamber (2) is connected, via the air-tightness means (8), to the loading chamber (1), while the left-hand side is connected to the cooling and unloading chamber (3). The loading and unloading chambers typically have a volume similar to that of the crucible (6), while the “hot” chamber (2) has at least twice the volume of the crucible.
By means of the guides (7) the crucible (6) is transferred from the outside to chamber (1), then to the second chamber (2), then to chamber (3) and is then transferred to outside, passing through the openings that make the various chambers to communicate, by opening and closing the air-tightness means (8).
With the set-up of the chambers illustrated in
(a) The loading chamber (1) is opened to the outside and the air-tightness means (8) hermetically seal the opening that connects chamber (1) to chamber (2). First moving means position the crucible (6) on the guides (7), and additional moving means transfer it into chamber (1), after which the air-tightness means (8) hermetically seal off access to the outside. Access to the hot chamber (2) is still closed. The air is extracted from the loading chamber (1) by means of vacuum pumps; on reaching the desired vacuum, typically around 10−2 bar, an inert gas, generally argon, is introduced to create an inert atmosphere, typically at a pressure of 0.1-0.3 bar;
(b) The air-tightness means (8) are opened to permit access and transportation of the crucible (6) from the loading chamber (1) to the hot chamber (2); further moving and guide means position the crucible (6) on the pedestal (5), which is in the fully lowered position;
(c) The air-tightness means (8) hermetically seal the hot chamber (2); lifting means raise the pedestal (5) to bring the crucible (6) into the smelting area (4). Heating means raise the temperature inside the furnace in order to smelt and then crystallise the silicon according to the thermal profile and the conditions required for the smelting and crystallisation process. On completion of these operations, lowering means lower the pedestal (5) to bring the crucible (6) containing the ingot of crystallised silicon back to the level of the moving and guide means suitable for transferring said crucible from chamber (2) to the cooling chamber (3), the atmosphere of which has previously been rendered similar (in terms of temperature and inert gas) to that of chamber (2) by means of the heating means, the pumps and the air-tightness means (8). After this, the air-tightness means (8) are opened, thus permitting communication between the two chambers, and the moving and guide means transfer the ingot (6) into chamber (3), which, at the end of the operation, is insulated by means of the air-tightness devices (8) and the crucible (6) is left to cool;
(d) meanwhile, with the same implementation modalities, a new crucible loaded with silicon feedstock to be crystallised is brought from the outside into the loading chamber (1) and then transferred, as previously described, into chamber (2) to be subjected to the smelting and crystallisation cycle. During this period of time, of the order of tens of hours, the preceding ingot, placed in the cooling chamber (3), will have had time to cool down completely to room temperature and can therefore be unloaded to the outside;
(e) the air-tightness means (8) are then opened and the guide and moving means unload the crucible (6) containing the now cold ingot to the outside; chamber (3) is closed again by means of the air-tightness means (8), emptied of air by means of vacuum pumps, and filled with inert gas (argon) to recreate the milieu of the hot chamber (2).
At this point, chamber (3) is ready to receive another crucible from the hot chamber (2) and thus continue the cycle.
As can be easily inferred from the aforesaid operations, the furnace is never opened to the outside and its internal milieu is always maintained inert thanks to the presence of the air-tightness means (8) which insulate it from the external environment and connect it to chambers (1) and (3) only when the latter have been brought to the same temperature and inert gas milieu conditions. This makes it possible to limit possible sources of pollution and to obtain silicon of a high grade of purity for photovoltaic use. In addition, process times are shortened, generally by about twenty hours, corresponding substantially to the time necessary for cooling the crystallised ingot, the cooling no longer being done in the furnace but in chamber (3) adjacent to it.
The following example is to be regarded as illustrative and not limitative of the scope of the invention.
An apparatus produced as illustrated in the figures and described above is used. A crucible (6) containing silicon of solar purity (98%) is placed in the loading chamber (1). After using pumps to produce a vacuum of the order of 10−4 millibar, the chamber is filled with argon and brought to a pressure of 0.3 bar. Access to chamber (2) is opened and the crucible (6) is transferred onto the thermostated pedestal (5). Access to the hot chamber (2) is closed again by means of the air-tight bulkhead (8) and the pedestal (5) travels vertically bringing the crucible (6) into the smelting area (4). The crucible (6) is heated to a temperature of 1500° C. with the result that the silicon it contains melts. Since a cooling fluid circulates in the pedestal (5) a temperature gradient is created inside the crucible (6) along its vertical axis. When all the silicon is smelted, the temperature in the smelting zone (4) is reduced by 0.5° C. per hour so that, as a result of the combined effect of this temperature reduction and of the cooling of the pedestal, the crystallisation process of the silicon contained in the crucible (6) starts from the base and proceeds upwards. At the same time as the temperature reduction, during the crystallisation phase, the pedestal (5) is lowered at a rate equal approximately to the crystallisation rate (from 3 to 30 mm/hr). By doing this, the spatial position of the separation surface between molten silicon and solid crystalline silicon is maintained constant. On completing the crystallisation process, the pedestal (5) is rapidly lowered into the bottom part of the hot chamber (2) and finally transferred into the cooling and unloading chamber (3).
After closing access to the cooling chamber (3), the hot chamber (2) is ready to accept another crucible from chamber (1) containing the silicon to be smelted and crystallised according to the modalities described above. Meanwhile, the crucible placed inside the cooling chamber (3) is cooled in approximately 20 hours and can be unloaded to the outside. The cooling chamber (3) is thus opened and the ingot is unloaded. The cooling chamber (3) is then closed again. In it, with the aid of the air-tightness bulkheads (8), a vacuum is produced and, on reaching a value of approximately 10−4 bar, the chamber is filled with argon to a pressure of 0.3 bar. At this point it is ready to receive a new ingot from the hot chamber (2) and the cycle proceeds in a semicontinuous manner.
The polycrystalline silicon obtained with the system according to the invention is of excellent quality for photovoltaic use; the mean lifetime of the minority carriers measured in it is greater than 2 microseconds with a mean value of around 5 microseconds (SEMI MF28 method). Therefore, the material is well within the specifications required of the manufacturers of photovoltaic cells which prescribe that the lifetime should be greater than 2 microseconds.
| Number | Date | Country | Kind |
|---|---|---|---|
| RM2008A000316 | Jun 2008 | IT | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP09/57093 | 6/9/2009 | WO | 00 | 2/15/2011 |
