1. Field of the Invention
The present invention relates generally to a process for solidification of liquid material, and is directed more particularly to a process for solidification used in a crystal grown method of melt material with sufficient electrical conductivity.
2. Description of the Prior Art
In the art, when, for example, silicon is crystal-grown by the Czochralski method, since silicon melts at about 1420.degree. C. which exceed the critical Rayleigh number, thermal convective flow always occurs in the silicon melt. Therefore, the surface of the silicon melt is vibrated and hence the thickness of the boundary layer on the solid-liquid interface in the growing material is fluctuated. At this time, microscopic re-dissolution occurs in the grown crystal. Thus, point defects such as irregular segregation of impurities, swirl or the like appear in the crystal thus grown.
Also, the silicon melt reacts with the face of a crucible made of quartz (SiO.sub.2) due to the thermal convective flow of silicon melt, and hence oxygen, which is one of the components of the quartz crucible, is dissolved into the silicon melt. In general, the crystal made as above contains oxygen of about 10.sup.18 atoms/cm.sup.3. At a temperature where thermal oxidization is carried out in a process of manufacturing semiconductor devices, for example, 1000.degree. C., the solubility of oxygen into silicon is about 3.times.10.sup.17 atoms/cm.sup.3. Therefore, during the usual heating process, the oxygen is supersaturated, and hence precipitation of oxygen occurs. The materials thus precipitated become seeds to make grow dislocation loops, stacking faults and so on. Further, the precipitation of SiO.sub.2 causes warpage in a semiconductor wafer in the heating cycle during the manufacturing process thereof.
As described above, the silicon crystal manufactured by the Czochralski method presents problems such as microscopic fluctuation of impurity distribution, band-like segregation of oxygen, swirl, defects and so on. On the contrary, a silicon crystal manufactured by the floating zone method is less contaminated by a crucible material and the oxygen concentration thereof is less than 10.sup.16 atoms/cm.sup.3. In this case, few oxygen atoms are precipitated. However, when the oxygen concentration in a crystal becomes lower than 1.times.10.sup.17 atoms/cm.sup.3, another problem appears that the dislocation in the crystal is apt to be propagated leading to a trouble during the process to manufacture a semiconductor device.