Apparatus for manufacturing silicon single crystals

Abstract: A silicon single crystal manufacturing apparatus in which a partition member (11) formed with at least one small hole (13) through its lower part is arranged in a rotating quartz crucible so as to surround a large cylindrical silicon single crystal (5) which is rotated and pulled. The whole or part of the partition member (11) is made from cellular silica glass whose cell content (volume percentage) is between 0.01 and 15% or less than 0.01% but increased to 0.01 through 15% by the heat used for melting starting silicon material. Thus, the molten material contacting with the inside of the partition member is prevented from decreasing in temperature and solidification of the molten material from this portion is prevented.

Description: The present invention relates to an apparatus for manufacturing silicon single crystals by the Czochralski method. The silicon single crystal pulling process according to the Czochralski method has been used in the past and it has become a practically complete technique. However, where the specification is severe, the yield of usable wafers will be reduced to less than 50% due to the maldistribution of the dopant and oxygen. As an effective means of solving such problems, there has been known a method in which a silicon starting material is continuously fed into a double structure crucible and thereby maintaining the surface level of the molten material constant (Japanese Patent Publication No.40-10184). In particular, the manufacture of high-quality granular polycrystal silicon has recently become possible and it has been considered relatively easy to feed this granular silicon to the molten material continuously at a constant feed rate, thus leading to the publication, of some inventions and a paper (inventions of Japanese Laid-Open Patents No.58-130195 and No.63-95195 and Japanese Utility Model No.59-141578 and the paper Ann. Rev. Mater, Sci, 1987, Vol. 17, p273-278). The inventions disclosed in these publications, etc., are of the type employing a silica glass crucible of the double structure and, particularly as pointed out in the publication of Japanese Laid-Open Patent No.62-241889, there is a problem of solidification tending to occur where the inner surface of the inner crucible contacts the surface of the molten silicon, thus making it difficult to decrease the furnace temperature to the temperature required for the stable growth of a single crystal. If the pulling of a single crystal is effected by keeping the temperature of the molten silicon high so as to prevent the occurrence of solidification, not only is the rate of solidification decreased but also the occurrence of dislocation is increased thus making it difficult stably to stably manufacture a single crystal. Also, the invention disclosed in the publication of Japanese Laid-Open Patent No.61-36197 employs a crucible of the double structure as well as a heat insulating material arranged above the outer material melting section and a separate heater arranged at the bottom of the crucible so as to promote the melting of the starting material supplied. However, this invention also involves no contrivance for preventing the dissipation of heat from the contacting portion between the inner surface of the inner crucible and the molten silicon surface so as to prevent the occurrence of solidification. In addition, the heating by the heater at the bottom of the crucible decreases the temperature of another heater arranged along the side of the crucible thus promoting the occurrence of solidification. In the manufacturing method of the above-mentioned type using the partition or the inner crucible (hereinafter referred to as a partition), particularly in the method of manufacturing large-diameter (12 to 30cm) silicon single crystals, the heat dissipation from inside the partition tends to decrease the temperature of the molten material inside the partition, particularly the molten material contacting the partition. This is considered to be caused by the fact that the material of the partition is transparent silica glass having an emissivity considerably greater than that of the molten silicon and a great amount of heat is dissipated from the partition to the water-cooled cover above the crucible. Also, due to the crucible being of the double structure, the convection of the molten silicon is limited and the temperature inside the partition has a lesser tendency to rise. Moreover, while the temperature of the molten silicon inside the partition (crystal growing section) must be maintained just above the melting point of silicon in order to ensure the growth of a single crystal, due to the previously mentioned phenomena, there is caused a problem of solidification starting where the partition contacts the surface of the molten material inside the partition. The previously mentioned conventional techniques have not been able to prevent the occurrence of such solidification. The present invention has been made to overcome the foregoing deficiencies in the prior art, and it is an object of the present invention to provide an apparatus for manufacturing silicon single crystals, more particularly large cylindrical silicon single crystals of 12 to 30cm in diameter, into which a starting material in granular or lumpy form may continuously be fed into a crucible containing molten material and which is capable of preventing the occurrence of solidification where the partition immersed in the crucible contacts the surface of the molten material inside the partition. According to one aspect of the present invention there is provided a silicon single crystal manufacturing apparatus in which a rotatable quartz (1) crucible adapted to contain molten silicon material (4) is divided by a partition member (11) into a central zone for growing a single crystal of diameter 12 to 30cm and a melting zone, said partition(11) being formed with at least one small hole (13) therethrough such that molten material may move smoothly from the melting zone to the central zone, and is provided with feed means (14) continuously to feed starting silicon material to the melting zone outside said partition member (11) and with means for rotating and for pulling the silicon single crystal, characterised in that said partition member (11) is made from cellular silica glass having a cell (12) content (volume percentage) of not less than 0.01% and not greater than 15%. According to a second aspect of the present invention there is provided a silicon single crystal manufacturing apparatus in which a rotatable quartz (1) crucible adapted to contain molten silicon material (4) is divided by a partition member (11) into a central zone for growing a single crystal of diameter 12 to 30cm and a melting zone, said partition(11) being formed with at least one small hole (13) therethrough such that molten material may move smoothly from the melting zone to the central zone, and is provided with feed means (14) continuously to feed starting silicon material to the melting zone outside said partition member (11) and with means for rotating and for pulling the silicon single crystal, characterised in that said partition member (11) is made from cellular silica glass having a cell content (volume percentage) less than 0.01%, and wherein said cell content (volume percentage) is increased to not less than 0.01% and not greater than 15% during melting of starting silicon material preliminarily charged into said crucible. An embodiment of the present invention will now be more particularly described by way of example and with reference to the accompanying drawings, in which: FIGURE 1 is a longitudinal sectional view showing schematically an embodiment of the present invention; FIGURE 2 is a sectional view taken along the line I-I of Figure 1; FIGURE 3 is a side view showing an embodiment of the partition member; and FIGURES 4a and 4b are diagrams comparing the operations of the transparent silica glass and the cellular silica glass. Referring now to the drawings, the apparatus comprises a crucible 1, a graphite crucible 2, a pedestal 3, molten material 4, a silicon single crystal 5, a heater 6, a heat insulating material 7, a chamber 8, a partition member 11, cells 12, a small hole 13, a starting material feeder 14, temperature detectors 15 and 16, a heat insulating cover 17 and feed material 18. The construction of the partition member forming the principal part of the present invention will be described first. Referring to Figures 4a and 4b, there are illustrated schematic diagrams showing respectively the conditions when transparent silica glass 21a and cellular silica glass 11a are immersed in molten material. In the case of the transparent silica glass 21a shown in Figure 4a, it will be seen that the contact portion between the molten material 4 and the glass 21a is transparent and therefore the amount of heat dissipated from the molten material surface through the glass is comparatively large. The heat dissipation of the transparent silica glass 21a is in fact so large that the portion of the molten material 4 contacting the glass 21a is decreased in temperature and solidification tends to occur at this portion. In the case of the cellular silica glass 11a, the presence of cells 12 in the glass has the effect of scattering the dissipation of heat from the contact portion between the molten material 4 and the cellular silica glass 11a and thus the heat dissipated from the molten material surface through the glass 11a is decreased as compared with the transparent silica glass 21a. Also, cooling due to the heat transmission is reduced by the presence of the cells. As a result, the portion of the molten material 4 contacting the cellular silica glass 11a is not practically decreased in temperature and thus the molten material 4 is prevented from solidifying. Also, since the dissipation of heat from the molten material 4 through the glass is reduced as the secondary effect of the cellular silica glass 11a, there is the effect of decreasing temperature variations and wettability variations in the portion of the molten material 4 contacting the cellular silica glass 11a. Figure 1 is a sectional view showing schematically an embodiment of the present invention and Figure 2 is a sectional view taken along the line I-I of Figure 1. In the Figures, numeral 1 designates a silica crucible set in a graphite crucible 2, and the graphite crucible 2 is vertically movably and rotatably supported on a pedestal 3. Numeral 4 designates molten silicon material contained in the crucible 1, and a silicon single crystal 5 grown into a large cylindrical form of 12 to 30cm diameter is pulled from the molten silicon material 4. Numeral 6 designates a heater surrounding the graphite crucible 2, and 7 a hot-zone heat insulating material surrounding the heater 6. These component parts are basically the same with the component parts of the ordinary single crystal pulling apparatus according to the Czochralski method. Numeral 11 designates a partition member made from a cellular high-purity silica glass and arranged concentrically with the crucible 1, with at least one small hole 13 formed through its lower region, substantially below the cental portion in the height direction as shown by way of example in Figure 3. When a starting material is charged, the partition member 11 is set, along with the starting material, in the crucible 1 so that after the starting material has been melted, the partition member is immersed in the molten material 4 so as to surround the single crystal 5 and its upper portion is exposed above the molten material surface. Also, its lower edge portion is practically fused to the crucible 1 and therefore it is prevented from floating. As a result, the molten material 4 outside the partition member 11 moves to the inner side thereof only through the small hole 13. It is to be noted that the partition member 11 may be preliminarily fused to the crucible 1. Numeral 9 designates an opening formed in a chamber 8 in correspondence to the molten material surface outside the partition member 11, and fixedly inserted into the opening 9 is a feeder 14 for feeding a starting material in granular or lumpy form. The forward end of the feeder 14 is in opposition to the molten material surface outside the partition member 11. The feeder 14 is connected to a starting material feed chamber (not shown) arranged outside the chamber 8 thereby continuously feeding the granular or lumpy starting material. Numerals 15 and 16 designate temperature detectors such as radiation thermometers arranged in the upper part of the chamber 8 so that the temperature detector 15 measures the temperature of the molten material surface outside the partition member 11 and the other temperature detector 16 measures the temperature of the molten material inside thereof. Numeral 17 designates a heat insulating cover which in this embodiment is provided to enhance further the heat insulating effect on the partition member 11 although the cellular silica glass partition member 11 itself has a function of preventing the occurrence of solidification. In accordance with the present invention, due to the fact that the partition member 11 is made from cellular silica glass, the heat dissipation from the contact portion between the inner surface of the partition member 11 and the molten silicon material 4 is decreased to prevent solidification of the molten material at the partition member 11. This solidification preventing effect on the molten material 4 by the cellular silica glass is caused usually with a cell content (volume percentage) of 0.01% or over. In the case of cellular silica glass having a cell content (volume percentage) of less than 0.01%, however, the solidification preventing effect on molten material can be similarly obtained if new cells are formed by the heat for melting the silicon material or if the cell content (volume percentage) is increased to greater than 0.01%, due to the expansion of the already existing cells. On the other hand, in the case of cellular silica glass with a cell content of greater than 15%, the probability of impeding the growing of a single crystal due to stripping off of the glass is increased. In view of these results, the growing of a silicon single crystal is effected by using cellular silica glass having a cell content (volume percentage) of preferably between 0.01 and 15%. As will be seen from the foregoing description, a silicon single crystal manufacturing apparatus has a rotable crucible for containing molten silicon material and which is divided by a partition member formed with at least one small hole therethrough such that the partition member surrounds a large cylindrical silicon single crystal of 12 to 30cm in diameter which may be pulled while being rotated and the molten silicon material is movable smoothly through the partition as the silicon single crystal is grown inside the partition member while starting silicon material may be fed continuously to the outer side of the partition member, the whole part of the partition member being made from cellular silica glass so that the dissipation of heat from the molten material near the partition member is reduced and the molten material is prevented from solidifying where it contacts the partition member, thereby pulling a sound silicon single crystal. Thus, the working of the present invention has great effects that the yield is improved due to the uniform quality in the pulling direction of a silicon single crystal, with improved productivity.

Claims: A silicon single crystal manufacturing apparatus in which a rotatable quartz (1) crucible adapted to contain molten silicon material (4) is divided by a partition member (11) into a central zone for growing a single crystal of diameter 12 to 30cm and a melting zone, said partition(11) being formed with at least one small hole (13) therethrough such that molten material may move smoothly from the melting zone to the central zone, and is provided with feed means (14) continuously to feed starting silicon material to the melting zone outside said partition member (11) and with means for rotating and for pulling the silicon single crystal, characterised in that said partition member (11) is made from cellular silica glass having a cell (12) content (volume percentage) of not less than 0.01% and not greater than 15%. A silicon single crystal manufacturing apparatus in which a rotatable quartz (1) crucible adapted to contain molten silicon material (4) is divided by a partition member (11) into a central zone for growing a single crystal of diameter 12 to 30cm and a melting zone, said partition(11) being formed with at least one small hole (13) therethrough such that molten material may move smoothly from the melting zone to the central zone, and is provided with feed means (14) continuously to feed starting silicon material to the melting zone outside said partition member (11) and with means for rotating and for pulling the silicon single crystal, characterised in that said partition member (11) is made from cellular silica glass having a cell content (volume percentage) less than 0.01%, and wherein said cell content (volume percentage) is increased to not less than 0.01% and not greater than 15% during melting of starting silicon material preliminarily charged into said crucible.

Inventors: KAMIO HIROSHI, SHIMA YOSHINOBU, KAMIO, HIROSHI, SHIMA, YOSHINOBU
Applicants: NIPPON KOKAN KK, NKK CORPORATION
Application year: 1989
Main classification: C30B15
IPC codes: C30B15, H01L21, C30B29