This invention relates to a pulling rod which is used to pull a single-crystal boule from a melt according to the Czochralski method or the liquid encapsulated Czochralski method.
In accordance with the Czochralski method, as illustrated in FIG. 1, the surface of molten semiconductor material 1 is covered with molten B.sub.2 O.sub.3 material (not shown) as required (in the case of the liquid encapsulated Czochralski method), and a seed crystal 3 secured to a pulling rod 2 is inserted into the molten material 1. After merging well with the molten material 1, the seed crystal 3 is pulled upwardly while being rotated, thereby to pull a single-crystal boule 4.
In general, when a single-crystal boule is pulled as described above, the heat flow pattern in the boule is as shown in FIG. 1. The single crystal grows at the equilibrium point of the heat of solidification heat flow Q.sub.1 of the molten material 1, the heat flow Q.sub.2 at the interface between solid and liquid, the radiated heat flow Q.sub.3 from the surface of the boule 4, the heat flow Q.sub.4 of cooling through gas convection, and the heat flow Q.sub.5 through the rod 2.
In order to reduce the number of defects in crystal, a method may be employed in which the boule is pulled under a small temperature gradient. However, that method is disadvantageous in that, since the heat flows Q.sub.3, Q.sub.4 and Q.sub.5 are decreased, it is difficult to increase the diameter of the boule, and accordingly it is difficult to produce large single-crystal boules.
In order to overcome the above-described drawbacks, a method of producing large single-crystal boules by forcibly cooling the pulling rod 2 and a method of producing large single-crystal boules by increasing the heat flow Q.sub.5 have been proposed in the art (see Japanese Laid-Open Patent Application No. 158196/1980, and A. J. Singh, J. Cryst. Growth, 51, 635 (1981)).
Pulling rods for practicing these methods are as shown in FIGS. 2A and 2B. In the case of FIG. 2A, the pulling rod 5 has cooling fins 7. More specifically, the pulling rod 5 is made of a material such as molybdenum having a high thermal conductivity with the cooling fins 7 being provided at the upper end portion of the rod. A seed crystal 3 is secured to the lower end of the pulling rod 5 with a seed holder 6. Unfortunately, a large amount of heat flows into the rod through its side, and therefore the cooling effect of the seed part is low.
The pulling rod shown in FIG. 2B is of a dual-pipe type. Cooling gas or liquid introduced into the inner pipe 9 from above passes into the outer pipe 10 from the lower end of the inner pipe. The cooling gas or liquid thus moves upwardly in the outer pipe and is then discharged therefrom. A seed crystal 3 is secured to the rod as in the case of FIG. 2A. However, it has been found that when the cooling-type pulling rod made of dual pipes is used in a puller (such as a puller having a multi-stage heater such as an after-heater) which has a small temperature gradient, the pulling rod 8 or the cooling gas or liquid is exposed to the atmosphere at high temperature, and therefore the cooling effect through the seed crystal 3 is insufficient, and accordingly it is difficult to produce large single-crystal boules.