1. Field of the Invention
The invention relates to a method for pulling a single crystal composed of silicon with a section having a diameter that remains constant. The single crystal grows while a monocrystalline seed crystal is raised from a melt contained in a crucible, with a specific pulling rate vp. The aim of the method is to obtain a single crystal composed of silicon with a longest possible cylindrical section having a predefined desired diameter which is suitable for further processing to form semiconductor wafers. Temperature fluctuations in the melt alter the crystallization rate v with which the single crystal grows. If the pulling rate vp and the crystallization rate v do not match, the diameter of the single crystal varies. It is necessary, therefore, for deviations of the diameter from the desired diameter that are brought about by temperature fluctuations in the melt to be minimized by means of the diameter being regulated.
2. Background Art
The requirement for achieving a situation in which the diameter in the section having a diameter that remains constant deviates as little as possible from the desired diameter can be met satisfactorily, considered in isolation, by compensating for deviations from the desired diameter by regulating the pulling rate vp and/or the supply of heat to the melt by means of a heating source arranged around the crucible. It is significantly more difficult to meet the requirement if it is simultaneously demanded that the quotient v/G of the crystallization rate v and the axial temperature gradient G at the phase boundary between the growing single crystal and the melt be kept within a narrow range. This is regularly demanded because v/G is crucial with regard to whether vacancies or silicon interstitials dominate as intrinsic point defects in the single crystal. In the event of supersaturation, vacancies or silicon interstitials aggregate to form larger units and form defects such as FPDs (“flow pattern defects”) or Lpits (“large etch pits”). In general, the formation of such defects must be avoided, but this is only accomplished if v/G remains within the narrowest possible limits during the pulling of the section having a diameter that remains constant. The requirement to match the diameter in the section having a diameter that remains constant with respect to the desired diameter and the requirement to keep v/G within narrow limits lead to a conflict because, on the one hand, an adaptation of the pulling rate vp to an altered crystallization rate v caused by temperature fluctuations in the melt readily has the consequence that the narrow limits for v/G are deviated from and, on the other hand, a correction of the altered crystallization rate v by changing the supply of heat to the melt by means of a heating source arranged around the crucible brings about a deviation from the desired diameter. It is difficult, therefore, to regulate the diameter to a desired diameter without having to disregard the regulation of v/G, and vice versa.
EP 1 541 721 A1 describes measures for regulating the diameter and measures for regulating v/G, but they cannot resolve the above-mentioned conflict.