Determination of Flow - Velocity in Silicon Melt
during an Industrial Czochralski Process
In the industrial growth of Silicon single crystals by the Czochralski process melt sizes of up to 300 kg are used. Due to the low Prandtl-number of liquid Silicon and the strong influence of buoyancy convection, the flow in the Czochralski melts is usually 3-dimensional, time dependent and tends to become more and more turbulent with increasing melt sizes.
Fluctuations of the temperature and of the concentration of dissolved oxygen are detrimental to the quality of the devices made from the single crystal. However, the optimization of the process parameters like crucible- and crystal rotation rates or magnetic fields suffers from the lack of measurement techniques for the melt flow velocity.
At the high temperature of molten silicon (typically 1700 - 1750K) most mechanical approaches for velocity measurement are not applicable, while tracer methods in combination with X-ray cameras cannot be applied to the melt sizes of actual interest. The determination of flow velocity by the correlation of temperature signals from multiple thermocouple arrangements was successfully used for the analysis of baroclinic instabilities in Czochralski melts. The development of an optical fiber thermometer with high spatial and time resolution for the application under industrial silicon Czochralski conditions will be shown. Experimental results of flow evaluation in industrial size and laboratory scale silicon Czochralski melts will be shown. The combination of 3 or more temperature sensors for the 2- and 3-dimensional detection of the melt flow vector is dicussed.