1. Field of the Invention
This invention relates to an apparatus and a method for pulling up single crystals, said apparatus of which is suitable for installation on a single-crystal manufacturing apparatus utilizing the CZ (Czochralski) method, especially suitable for the process of pulling up heavy single crystals.
2. Description of the Related Art
Conventionally, single crystal silicon is produced by the CZ method. In the CZ method, polycrystalline silicon is deposited into a quartz crucible of a single-crystal manufacturing apparatus, then the polycrystalline silicon is heated to melt by way of heaters disposed around the quartz crucible. Subsequently, a seed crystal installed within a seed holder is dipped into the melt. Afterward, the seed holder and the quartz crucible are respectively driven to rotate in the same or reverse directions. At the same time, the seed holder is pulled up to grow a single crystal with predetermined diameter and length.
In the seed crystal, dislocation will arise due to the heat shock occurring at the time of dipping it into melt. In order to stop propagation of this dislocation from the seed crystal to single crystal to be grown, a reduced portion with diameter from 3 to 5 mm is grown beneath the seed crystal so as to release the dislocation to the surface of thereduced portion. Making certain that the propagation of dislocation has been stopped, a shoulder is formed and the single crystal is enlarged until it reaches a predetermined diameter. Then, the pulling up operation is shifted to that of forming the body of the single crystal.
In recent years, with the enlargement of single crystals, the weight of an individual single crystal has increased, and the robustness of the reduced portion is reaching a practical limit. To solve the problem, as shown in FIG. 11, an enlarged portion 2b and a necked portion 2c with a diameter smaller than that of the enlarged portion 2b are formed between the reduced portion 2a and the shoulder 2d when the propagation of dislocation has been stopped by way of the reduction process. Since a buttress supports most of the weight of the single crystal 2, breakage of the reduced portion 2a can be prevented. Furthermore, dropping of the single crystal 2 can also be prevented, even if the reduced portion 2a breaks. For example, in the single-crystal pulling up apparatus disclosed in J P-B (Japanese Patent Publication) 5-65477, a clamp arm capable of being driven to open or close is engaged with the necked portion of a single crystal so as to suspend the single crystal. Furthermore, in the single-crystal pulling up apparatus disclosed in J P JP-B 7-515, a plurality of claws retained at a predetermined angle are provided at the lower end of an engaging holder capable of being driven to ascend or descend. The claws are engaged with the necked portion of a single crystal so as to suspend the single crystal. In addition, in the single-crystal pulling up apparatus disclosed in J P-B 7-103000, a plurality of claws engaging with the necked portion of a single crystal are provided. The single crystal is suspended by utilizing a plurality of holding levers capable of being driven to open or to close by their ascending or descending movement and a ring used for preventing opening of the holding levers.
However, in the single-crystal pulling up apparatus disclosed in J P-B 5-65477, the clamp arm wiil be driven to open when the weight of the single crystal increases to the point where there exists a danger of dropping of the single crystal during pulling up operation. In the single-crystal pulling up apparatuses disclosed in J P-B 7-515 and J P-B 7-103000, dropping of the single crystal being pulled up will not happen. However, the seed holders, the reduced portions, and the necked portions are embraced by a clamping means installed and suspended above them, therefore the holders or holding levers are inevitably long. Furthermore, the holders or holding levers have to be kept stationary at a location above the seed crystal during the formation of the reduced portion. Due to the foregoing reasons, the pulling up stroke of single crystals is shortened. Accordingly, the length of single crystals along their longitudinal axis is restrained and the clamping means becomes bulky.