Siemens and Monosilane Deposition Processes

In principle, the diameter of a silicon rod produced after the Siemens process (see part I) is limited. The endothermic deposition from SiHCl3 takes place at a temperature of the rod surface of 1,100°C and is established by an axial electric current. As the hottest region, the core region of the rod has the highest conductivity, which causes self-bunching of the heating current. If the rod diameter exceeds 160-180 mm, the silicon melting temperature can be reached in the core. When silicon melts, the electric conductivity jumps by a factor of about 30. Therefore, most of the current will be concentrated in the molten core. If that melt solidifies, the rod will brake by the specific volume expansion of ca. 8%. 

Rods of FZ quality have to be deposited with a low rate, not only to reduce that current bunching but also to achieve a dense structure and a smooth surface. The deposition conditions must be very pure to exclude SiC, SiC 2 and other inclusions, which would not melt in the FZ process because the dwell time in the molten zone is too short, in contrast to the CZ method, where the melt can be overheated in the crucible for some time before the growth starts. 

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