Method and apparatus for recharging of silicon granules in Czochralski single crystal growing

Abstract: The invention provides an improvement in a recharger system consisting of a feeder and a feed conduit for recharging polycrystalline silicon granules into the crucible after a run of growing a single crystal silicon rod by the Czochralski method so as to prepare for the next run of crystal growing. The improvement comprises controlling the amount of holdup of the silicon granules in the feed conduit by means of a sensor provided on the feed conduit so as to ensure smooth and high-rate feed of the silicon granules in such a way that the feed rate of the silicon granules from the feeder to the feed conduit and/or the descending velocity of the crucible can be controlled by the signals generated in the sensor corresponding to the amount of the holdup in the feed conduit.

Description: BACKGROUND OF THE INVENTION The present invention relates to a method and apparatus for recharging of polycrystalline silicon granules into the crucible in a Czochralski single crystal growing process, hereinafter referred to as the CZ process, especially, of semiconductor silicon. As is well known, most of the semiconductor-grade silicon single crystals are manufactured by the so-called CZ process in which a single crystal rod of silicon is grown from a melt of silicon contained in a fused silica glass crucible on the lower end of a seed crystal which is pulled up at a controlled rate. This CZ process is a typical batch-wise process and it is traditional that, when most of the silicon melt has been converted into the single crystal rod, the process is discontinued and a next run is started anew. Such a process is of course not quite satisfactory in respect of the productivity even by setting aside the problem that the expensive fused silica glass crucible cracks when it is cooled down. In this regard, it is recently practiced that, when a single crystal rod of silicon has been grown and the amount of the silicon melt remaining in the crucible has been decreased below a certain level, the crucible is recharged with polycrystalline silicon granules through a feed conduit installed above the crucible without cooling the crucible in such an amount as to enable starting of another run of single crystal growing. Further, a continuous recharging method is known in which supply of silicon granules is conducted continuously while a silicon single crystal is still under growing at such a controlled rate of, for example, 0.3 to 1.0 g/second as to compensate the decrease in the amount of the melt in the crucible. This method provides a possibility of greatly reducing the manufacturing costs of silicon single crystals. The above mentioned continuous recharging method has several problems. For example, falling of the silicon granules on to the surface of the silicon melt in the crucible sometimes causes splashing of the melt or vibration of the melt surface to greatly disturb the single crystal growing process so that the silicon single crystal as grown, if it could ever be obtained, would suffer from occurrence of so many dislocations to the contrary to the object of cost reduction. This problem due to disturbance of the melt surface by the falling silicon granules can of course be avoided by using a double-walled crucible in which the melt surface to which the silicon granules fall is isolated from the melt surface for the single crystal growing. This method, however, is not practicable due to the expensiveness of such a crucible of special structure. EP 0 315 156 discloses an apparatus for growing crystals according to this continuous recharging method. In this application the problems mentioned above have been solved by providing a feed pipe specially designed for lowering the falling speed of the material to the crucible. Semiconductor Silicon Crystal Technology by F. Shimura, pages 178-179 (1989) teaches a multiple CZ growth method for the reduction of the manufacturing costs of semiconductor silicon single crystals by recharging of silicon granules in the conventional batch-wise CZ process. In this method, a single crystal rod after completion of growth is removed from above the crucible and polycrystalline silicon in the form of a rod is introduced, without decreasing the temperature, into the silicon melt remaining in the crucible to be melted there before start of another run of the CZ process so that fused silica glass crucible, which otherwise can be used only for a single run of the CZ process due to cracking caused by the temperature decrease, can be re-used repeatedly for several runs greatly decreasing the manufacturing costs in respect of the improvement in the productivity and saving of the cost for the expensive fused silica glass crucibles. An alternative method of recharging is disclosed in Japanese Patent Kokai 62-260791 according to which the temperature of the melt remaining in the crucible after completion of a run of the CZ process for growing of a single crystal rod is slightly decreased so as to form a solidified crust on the melt surface on to which silicon granules are introduced through a feed conduit opening above the crucible. In the above described recharging methods in general, it is a matter of course that the time taken for recharging of polycrystalline silicon should desirably be as short as possible in order to accomplish maximum improvement in the productivity of the CZ process so that the feed rate of the polycrystalline silicon for recharge must be high enough provided that no damages are caused thereby in the fused silica glass crucible. In this regard, the polycrystalline silicon used for the recharging is preferably in the form of a rod or block so as to enable the polycrystalline silicon introduced at one time or within a short period of time. In the multiple CZ growth method taught by F. Shimura mentioned above, the polycrystalline silicon for recharge is in the form of a rod or block which is hung on the lower end of the pulling-up shaft or wire and introduced into the crucible by lowering the shaft or wire. It is natural in this case that the polycrystalline silicon recharge can be performed only after complete removal of the already grown-up single crystal from above the crucible and can never be performed concurrently with the removal of the single crystal rod so that the contribution thereby to the improvement in the productivity is limited. In contrast thereto, the method taught in Japanese Patent Kokai 62-260891 can be performed concurrently with removal of the grown-up silicon single crystal rod so as to accomplish considerable improvement in the productivity. This method, however, has some disadvantages. For example, the recharging of silicon granules can be performed only after crust formation on the surface of the silicon melt remaining in the crucible so that a length of time is unavoidably taken for cooling of the melt in the crucible. Further, formation of a crust on the surface of the melt causes a mechanical damage on the inner surface of the crucible coming into contact with the crust so that the desired advantage by the cost saving for the expensive fused silica glass crucibles cannot be fully obtained. The above mentioned problems relative to the recharging of silicon granules could be solved by the improvement in the feeding method thereof into the crucible. When the feeder system for recharge is not provided with a mechanism for the control of the feed rate of silicon granules, it is frequently the case that splashing of the silicon melt or bouncing of the granules is caused by falling of the silicon granules on to the surface of the melt. This problem of course can be at least partly solved by the use of a tapered feed conduit having a narrowed opening above the melt surface. The use of such a tapered feed conduit naturally has another problem that the smooth falling of the silicon granules is more or less disturbed so that the feed rate of the granules can not always be high enough and can be controlled only with difficulties. SUMMARY OF THE INVENTION The present invention accordingly has an object to provide an improvement in the method and apparatus for recharging of polycrystalline silicon granules into the crucible for the CZ process, by which the silicon granules can be introduced directly and smoothly at an adequately controlled rate to complete the recharging within a minimum length of time so that the productivity of the CZ process can be greatly improved. Thus, the present invention provides an improvement, in a method as set forth in claim 1 of the Czochralski single crystal growing of silicon for recharging of granules of polycrystalline silicon supplied by a feeder means into a feed conduit and falling through the lower open end of the feed conduit on to the surface of a melt of silicon contained in a crucible provided with a means for ascending and descending, a gap space being kept between the lower open end of the feed conduit and the surface of the melt, the improvement comprising detecting and controlling the holdup of the granules of silicon held inside of the feed conduit in such a way that, preferably, the space between the lower open end of the feed conduit and the surface of the melt is filled by a heap of the silicon granules. The above mentioned holdup of the silicon granules in the feed conduit can be controlled by adjusting the feed rate of the granules from the feeder to the feed conduit or by adjusting the descending velocity of the crucible while the adjustment of the feed rate of the silicon granules or the descending velocity of the crucible can be effected by means of the signals generated in a sensor means provided on the feed conduit to detect the holdup of the granules in the feed conduit. The improvement in the apparatus as set forth in claim 5, by which the above described improved method of the invention can be accomplished, comprises, in an apparatus used in the Czochralski single crystal growing of silicon for recharging of granules of polycrystalline silicon supplied by a feeder means into a feed conduit and falling through the feed conduit on to the surface of the melt of silicon contained in a crucible provided with a means for ascending and descending, a gap space being kept between the lower end of the feed conduit and the surface of the melt, providing the feed conduit with a sensor means to generate signals corresponding to the holdup of the silicon granules in the feed conduit and utilizing the signals for the adjustment of the feed rate of the silicon granules from the feeder to the feed conduit or the descending velocity of the crucible. BRIEF DESCRIPTION OF THE DRAWING Figure 1 is a schematic vertical cross sectional view of a CZ single crystal growing apparatus provided with the feeder system for recharging of silicon granules according to the invention. Figure 2 is a schematic vertical cross sectional view of a CZ single crystal growing apparatus provided with another embodiment of the feeder system for recharging of silicon granules according to the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following, the method and apparatus according to the invention are described in detail by making reference to the accompanying drawing. Figure 1 is a schematic vertical cross sectional view of a CZ single crystal growing apparatus provided with the feeder system of silicon granules according to the invention which is under the step of recharging after removal of the grown-up single crystal rod of silicon prepared in the preceding run. As is conventional, the fused silica glass crucible 1 containing a melt of silicon 2 is supported on the rotatable shaft 8 and enclosed inside of the furnace body 3 of the single crystal growing apparatus by the CZ process. It is essential in the apparatus according to the invention that the height of the crucible 1 is adjustable by ascending or descending the shaft 8. As being supported by the furnace body 3, a feeder system for silicon granules consisting of a feeder 6 and a feed conduit 10 is installed, of which the lower end of the feed conduit 10 opens near to the surface of the silicon melt 2. Granules of silicon 4 supplied into the feed conduit 10 from the feeder 6 at a controlled rate are introduced into the melt of silicon 2 in the crucible 1 and melted there to increase the volume of the silicon melt 2 to be prepared for the next run by recharging. The feed conduit 10 is provided with a sensor means 5 which serves to detect the holdup of the silicon granules 4 in the feed conduit 10. The feed conduit 10 is made from a refractory material such as fused silica glass and can be in the form of a straight pipe although it can be in the form of a tapered pipe having a downwardly decreasing diameter. The dimensions of the feed conduit 10 should be selected by taking into consideration various factors including the desired feed rate of the silicon granules 4 into the silicon melt 2, particle size of the silicon granules 4 and so on. It is usual that the lower open end of the feed conduit 10 has an inner diameter in the range from 10 to 50 mm. The silicon granules 4 supplied from the feeder 6 are held within the feed conduit 10 as a holdup which is under monitoring by the sensor means 5 in order to ensure smooth falling thereof on to the silicon melt 2. The working principle of the sensor means 5 is not particularly limitative including, typically, an optical level gauge which detects the uppermost level 7 of the granule holdup. When the uppermost part 7 of the granule holdup 4 reaching the level defined by the sensor means 5 is detected by the sensor means 5, the signals generated in the sensor means 5 are supplied to the feeder 6 so as to decrease the feed rate of the silicon granules therefrom so that the uppermost level of the granule holdup is always kept not to exceed the predetermined height 7. When a stationary state is established in the holdup of the silicon granules 4 within the feed conduit 10 in the above described manner, very smooth introduction of the granules 4 into the silicon melt 2 is ensured even at a relatively high feed rate of, for example, 50 to 70 g per second assuming that the silicon granules have a diameter of few millimeters. No troubles are encountered in the feed conduit 10 due to weld-agglomeration of the granules 4 since the granules 4 are stationarily under downward movement. The position at which the sensor means 5 is mounted on to the feed conduit 10 is not particularly limitative although, in Figure 1, it is usually just below the feeder 6. It is of course that the holdup of silicon granules 4 in the feed conduit 10 can be controlled by adjusting the position of the sensor means 5 along the feed conduit 10. When the holdup is increased, the falling rate of the silicon granules 4 is decreased due to the increase in the frictional resistance between the inner walls of the feed conduit 10 and the silicon granules 4 or vice versa. Particularly suitable sensor means 5 include optical pyrometers, image sensors and the like. Figure 2 illustrate an alternative type of the sensor means which is a CCD camera 15 mounted above the feed conduit 10. The signal generated in the sensor means 5 corresponding to the height 7 of the granule holdup 4 in the feed conduit 10 is sent to the feeder 6 and serves to control the feed rate of the silicon granules therefrom to the feed conduit 10 so that the amount of the granule holdup can be controlled and maintained always at an appropriate level. The feeder 6 serves to control the feed rate of the silicon granules discharged out of a granule holder (not shown in the figure) into the feed conduit 10. Though not particularly limitative, the feeder 6 can be a vibration feeder which works under varied intensity of vibration to control the feed rate of granules. It is of course that other types of feeders can be used including those having an adjustable valve which is closed or opened by the signals from the sensor means 5. As is described above the recharging apparatus of the invention serves to feed the silicon granules to the melt of silicon in the crucible at a controlled rate so as to keep the holdup of the silicon granules not to exceed an appropriate level. While the silicon granules falling on to the surface of the melt are melted there to increase the volume of the silicon melt in the crucible, the feed rate of the silicon granules is controlled to be somewhat larger than the velocity at which the silicon granules on the melt surface are melted so that it is a consequence that the silicon granules before melting are accumulated below and around the lower end of the feed conduit to form a heap which naturally disturbs smooth feeding of further silicon granules on to the melt surface. This difficulty can be overcome by gradually lowering the crucible by descending the shaft 8 so that feeding of the silicon granules can be continued without being disturbed by the heap of granules below and around the lower open end of the feed conduit. This descending movement of the crucible can be controlled also by the signals coming from the sensor means to detect the holdup of the silicon granules in the feed conduit and maintain an appropriate amount of the holdup. In an example to start the recharging process after a run of crystal growing from a fused silica glass crucible of 18 inch diameter to obtain a single crystal silicon rod which had been removed from above the crucible, the gap distance between the lower open end of the feed conduit, which had a length of 110 cm, inner diameter of 36 mm and outer diameter of 40 mm, and the surface of the melt remaining in the crucible, which should be as small as possible in order to ensure rapid establishment of the granule holdup within the feed conduit, was set at 5 to 10 mm in consideration of various factors including the form and diameter of the lower open end of the feed conduit, size of the crucible and so on. When this distance is too small, there is a risk that the lower end of the feed conduit is eventually contacted with the melt of silicon while, when the distance is too large, the silicon granules falling out of the lower open end of the feed conduit unavoidably cause splashing of the silicon melt if not to mention the disadvantageously long time taken before establishment of the granule holdup within the feed conduit. As the holdup of the silicon granules was established by the initiatory feed to grow in the feed conduit and, when the amount of holdup reached a certain predetermined level, the sensor means generated a signal which served to control the feed rate of the silicon granules by the feeder so as to keep the holdup not to exceed the appropriate level. While maintaining the holdup of the silicon granules in the feed conduit not to exceed an appropriate level and maintaining the temperature of the silicon melt in the crucible at a temperature suitable for melting of the silicon granules, the crucible was gently rotated at a low velocity of 2 revolutions/minutes so that the silicon granules falling on to the melt surface formed a heap in contact with the lower end of the feed conduit and further grew in an annular form by the accumulation of the additional silicon granules discharged out of the feed conduit because the feed rate of the silicon granules was somewhat larger than the velocity of melting of the granules into the melt of silicon in the crucible. As the silicon granules heaped up on the melt surface to such a height that further feed of the granules out of the feed conduit could no longer be continued, the top level of the granule holdup in the feed conduit was detected by the sensor means generating a signal by which the downward movement of the crucible was started to increase the gap distance between the lower open end of the feed conduit and the melt surface so as to enable further feeding of the silicon granules out of the feed conduit. The descending velocity of the crucible in this case was selected in the range from 5 to 30 mm/minute, although the velocity should be selected adequately depending on the condition of the granule heaps and other factors such as the dimensions of the feed conduit and crucible. By undertaking the above described procedure adequately, the velocity of recharging could be as large as 50 g/second or even larger so that the time taken for recharging before starting of the next run of single crystal growing could be greatly decreased.

Claims: A method of the Czochralski single crystal growing of silicon with recharging of granules of polycrystalline silicon supplied by a feeder into a feed conduit and falling through the feed conduit on to the surface of the melt of silicon contained in a crucible provided with a means for ascending and descending, a gap space being kept between the lower open end of the feed conduit and the surface of the melt, characterized by detecting and controlling the amount of holdup of the granules of silicon held inside of the feed conduit. The method as claimed in claim 1 whereby the holdup of the granules of silicon in the feed conduit is controlled in such a way as to fill the gap space between the lower open end of the feed conduit and the surface of the melt in the crucible by a heap of the silicon granules. The method as claimed in claim 1 whereby the holdup of the granules of silicon in the feed conduit is controlled by adjusting the feed rate of the silicon granules from the feeder to the feed conduit or by adjusting the descending velocity of the crucible. The method as claimed in claim 3 whereby adjustment of the feed rate of the silicon granules from the feeder to the feed conduit or the descending velocity of the crucible is effected by means of the signals generated in a sensor means provided on the feed conduit to detect the amount of holdup of the silicon granules in the feed conduit. An apparatus for recharging of polycrystalline silicon granules into a crucible capable of descending and ascending and containing a melt of silicon through a feeder system consisting of a feeder and a feed conduit through which the silicon granules fall on to the surface of the melt of silicon in the crucible, characterized by providing the feed conduit with a sensor means which detects and controls the amount of the holdup of the silicon granules in the feed conduit. The apparatus as claimed in claim 5 whereby the sensor means generates a signal corresponding to the amount of the holdup of the silicon granule, by means of which signal the feed rate of silicon granules from the feeder to the feed conduit or the descending velocity of the crucible is adjusted.

Inventors: HARADA ISAMU, NAGAI NAOKI, ODA MICHIAKI, OHTSUKA SEIICHIRO, HARADA, ISAMU, NAGAI, NAOKI, ODA, MICHIAKI, OHTSUKA, SEIICHIRO
Applicants: SHINETSU HANDOTAI KK, SHIN-ETSU HANDOTAI COMPANY LIMITED
Application year: 1995
Main classification: C30B15
IPC codes: C30B15, C30B29