Localization of the solid/liquid interface during directional solidification of silicon by a pulse-echo ultrasonic technique

An ultrasonic system is suited to detect the solid/liquid interface in a directional solidification process of silicon. Such processes are used to produce large multicrystalline ingots from which silicon wafers for low cost solar cells for terrestrial power applications are sliced. Several ultrasonic techniques are discussed in this paper. A pulse-echo system has been built and attached to a commercial type solidification machine. The pulses of a piezoelectric transducer are coupled into the melt via a transmission rod made of an appropiate material that withstands the temperature and the reactivity of liquid silicon. Several ceramic rods have been considered, SiC and Al2O3 were found to be suited best. Clear echoes from the liquid/solid interface have been detected. These signals allowed to calculate the depth of the melt and the solidification velocity. For comparison the depth was measured with an Al2O3 sounding rod. Good agreement was found.