A single crystal used as a substrate of semiconductor devices, for example, a silicon single crystal, is mainly produced by Czochralski method (hereafter abbreviated to CZ method).
When producing a single crystal by CZ method, for example, a single crystal production apparatus 10 as shown FIG. 2 is used to produce the single crystal. This single crystal production apparatus 10 has a member for containing and melting a raw material polycrystal such as silicon, heat insulation members to insulate heat, and etc. They are installed in a main chamber 11. A pulling chamber 12 extending upwardly is continuously provided from a ceiling portion of the main chamber 11, and a mechanism for pulling a single crystal 13 by a wire 14 (not shown) is provided above it.
In the main chamber 11, a quartz crucible 16 for containing a melted raw material melt 15 and a graphite crucible 17 supporting the quartz crucible 16 are provided, and these crucibles 16 and 17 are supported by a shaft 18 so that they can be rotated and moved upwardly or downwardly by a driving mechanism (not shown). To compensate for decline of the melt level of the raw material melt 15 caused by pulling of the single crystal 13, the driving mechanism for the crucibles 16 and 17 is designed to rise the crucibles 16 and 17 as much as the melt level declines.
And, a graphite heater 19 for melting the raw material is provided so as to surround the crucibles 16 and 17. A heat insulating member 20 is provided outside the graphite heater 19 so as to surround it in order to prevent that the heat from the graphite heater 19 is directly radiated on the main chamber 11.
Moreover, a graphite cylinder 23 is provided above the crucibles, and a heat insulating material 24 is provided on the outside of the lower end of the graphite cylinder 23 so as to oppose to the raw material melt 15 so that the heat radiation from the melt surface is intercepted and the temperature of the raw material melt surface is kept.
A raw material lump is put in the quartz crucible 16 provided in the single crystal production apparatus as described above, the crucible 16 is heated by the graphite heater 19 as described above to melt the raw material lump in the quartz crucible 16. A seed crystal 22 fixed by a seed holder 21 connected with the lower end of the wire 14 is immersed into the raw material melt 15 melted from the raw material lump as described above. Thereafter, the single crystal 13 having a desired diameter and quality is grown under the seed crystal 22 by rotating and pulling the seed crystal 22. In this case, after bringing the seed crystal 22 into contact with the raw material melt 15, so-called necking, once forming the neck portion by narrowing the diameter to about 3 mm, is performed, and then, a dislocation-free crystal is pulled by spreading to a desired diameter.
The silicon single crystal produced by the CZ method as described above is mainly used for producing a semiconductor devices. In these years, with higher integration of semiconductor devices, an element becomes finer. The problem of Grown-in crystal defects introduced during the crystal growth has become more significant as an element has become finer.
Hereafter, the Grown-in crystal defects will be described (see, FIG. 4).
In the silicon single crystal, if the crystal growth rate is relatively high, Grown-in defects such as FPD (Flow Pattern Defect) considered to be originated from voids aggregated of void-type point defects, are present at high density over the region in a crystal radial direction. The region where the defects are present is called V (Vacancy) region. Moreover, if the growth rate is lower, OSF (Oxidation Induced Stacking Fault) region is generated as in a ring shape from the periphery of the crystal with lowering of the growth rate. And defects such as LSEPD (Large Secco Etch Pit Defect) and LFPD (Large Flow Pattern Defect) considered to be originated from dislocation loops aggregated of interstitial silicons in the outside of the ring are present at low density, and the region where the defects are present is called I (Interstitial) region. With further lowering of the growth rate, OSF ring is contracted to disappear in the center of a wafer, and the whole plane becomes I region.
In these years, in the outside of the OSF ring between V region and I region, a region has been discovered where neither such as FPD originated from voids nor such as LSEPD and LFPD originated from interstitial silicons are present. The region is called N (Neutral) region. Moreover, the N region is further categorized as follows. There are Nv region (a region where more voids are present) next to the outside of the OSF ring and Ni region (a region where more interstitial silicons are present) next to I region. In the Nv region, amount of precipitated oxygen is rich when thermal oxidation treatment is performed, and in the Ni region, amount of precipitated oxygen is little.
The Grown-in defects are considered to be determined the introduced amount by the parameter of F/G which is ratio of the pulling rate (F) and an average value of a temperature gradient in the crystal along a pulling axis from the melting point of silicon to 1400° C. (G) (See, for example, V. V. Voronkov, Journal of Crystal Growth, 59 (1982) 625-643). Namely, if the pulling rate and the temperature gradient are controlled so that F/G is constant, a single crystal can be pulled in a desired defect region or in a desired defect-free region (See, for example, Japanese Patent Laid-open (Kokai) Publication No. 2000-178099).
Therefore, it is conventionally necessary to pull a single crystal by controlling the pulling rate etc. for N region so as to obtain a single crystal of defect-free region. It is difficult to control the rate because the single crystal of N region can be grown in a range of a relatively limited pulling rate. Thus, productivity and yield of the single crystal become low. Accordingly, a method of expanding a pulling rate range where a defect-free region can be obtained has been required so as to produce it more easily.