1. Field of the Invention
The present invention relates to a method of manufacturing a polycrystalline silicon rod, and more specifically to a method of manufacturing a rod which is used in producing a very high-purity polycrystalline silicon by a CVD method.
2. Description of the Related Art
While a silicon substrate which is a major substrate for manufacturing semiconductor devices is fabricated largely by cutting and grinding a single-crystalline silicon ingot which is grown by a Czochralski method (hereafter abbreviated as a “CZ method”), the growing of such a single-crystalline silicon ingot is performed by melting polycrystalline silicon chunks charged in a quartz crucible, dipping a single-crystalline seed (seed crystal) of silicon into the silicon melt through the liquid surface thereof, gradually pulling up and cooling the seed crystal while rotating it.
The polycrystalline silicon chunks used as the raw material for such crystal growth are commonly manufactured by a chemical vapor deposition method (hereafter abbreviated as a “CVD method”) and a typical of such method is “Siemens method”. Specifically, that is a method in which a mixed gas of high-purity trichlorosilane (SiHCl3: hereafter abbreviated as “TCS”) and hydrogen is brought into contact with a thin, rod-like silicon core wire (a seed) which is kept at a high temperature in a reaction furnace by electric heating so that polycrystalline silicon is deposited on the surface of the silicon core wire.
The silicon material used as the above described silicon core wire is typically one which is cut off from a polycrystalline silicon rod or a silicon rod manufactured by a pedestal pulling method, and which is an established method (see, for example, Japanese Patent Laid-Open No. 2005-112662).
On the other hand, metal impurities and dopant impurities in the polycrystalline silicon which is the raw material for crystal pulling may be taken into a silicon ingot which is solidified from the silicon melt to be single-crystallized, thereby causing a deterioration of the quality (purity) of the single-crystalline silicon to be grown and a deviation from the initially specified resistivity.
Moreover, in recent years, large diameter silicon ingots are produced by a CZ method making it necessary that not only polycrystalline silicon chunks are charged before starting the pulling up of single-crystalline silicon, but also polycrystalline silicon chunks are recharged into the quartz crucible during the crystal pulling up.
In this respect, since most of the impurities brought into the silicon melt by polycrystalline silicon have a segregation coefficient of less than 1, the concentrations in the silicon melt will increase along with the progress of the pulling up of the single-crystalline silicon. As the result of increasing impurity concentrations in the silicon melt, the purity of the single-crystalline silicon portion grown from the silicon melt is decreased. This may result in that the merit of growing a long single-crystalline silicon ingot by recharging is sacrificed.
Thus, for the objective of further increasing the diameter of silicon ingot, the purification of polycrystalline silicon exceeding the level of current state-of-the-art polycrystalline silicon will be desired; however, conventional manufacturing methods of polycrystalline silicon rods have their limits in the view point of removing impurities. One factor of such limitations is the difficulty in controlling the impurities caused by the above described silicon core wire. For example, prior to the process of cutting out a core wire from a polycrystalline silicon rod, heat treatment at a high temperature is carried out, and impurity contaminations are likely to take place in such high temperature heat treatment.
Specifically, since a polycrystalline silicon rod immediately after CVD growth has strain in its crystal, if the rod is cut as it is to obtain a polycrystalline silicon for core wire, the rod will be broken during the cutting process. In order to prevent such breakage, the polycrystalline silicon rod is in some cases processed by heat treatment at a high temperature to remove internal strain, and is likely to be subjected to impurity contaminations from heat treatment furnace materials and the furnace environment during the process. Although, as the countermeasure to avoid such impurity contamination, there is a method to remove internal strain by heating and gradually cooling the polycrystalline silicon rod in the CVD reactor after growing the polycrystalline silicon therein, such method will increase the time period to occupy the CVD reactor thereby resulting in a decline in productivity.
For a polycrystalline silicon rod used in a floating zone method (hereafter abbreviated as an “FZ method”), which is, along with the CZ method, known as a growing method for single-crystalline silicon, improvement of so called “one-pass rate” has become a crucial issue as the diameter of a crystal to be grown increases. However, when polycrystalline silicon is used as the core wire to carry out a CVD process, the grain boundary size near the interface between the core wire and the polycrystalline silicon which grows on the surface of the core wire increases and, as the result of that, the single-crystallization rate in the FZ method declines leading to declines in yield and productivity.
It is noted that although there is a method in which a silicon rod produced by the pedestal pulling method is used as the core wire material, a silicon rod obtained by the pedestal pulling method is only partially single-crystallized and the entire silicon rod does not have a fixed crystal axis orientation. Therefore, even if crystal growth by an FZ method is carried out with the polycrystalline silicon obtained by using such a silicon rod as the core wire, the one-pass rate will remain to be low.