In situ ultrasonic interface tracking for photovoltaic silicon directional solidification

Abstract

This paper presents an experimental validation of an in situ ultrasound technique for tracking the solid–liquid interface during directional solidification of photovoltaic silicon. Ultrasound bursts are introduced from the top into the melt via the tip of a carbon glass waveguide directly plunged into the liquid. Several runs of solidification have been conducted using the same waveguide thus demonstrating its reusability, its mechanical resilience and its appropriate acoustical and chemical compatibility with the melt. We present the times of flight analysis to track the location of the solid–liquid interface. As part of the signal crosses the solidifying ingot, we also measure cs, the average celerity of sound waves in the solid. Since cs exhibits a significant dependance on crystal orientation, we consider the possibility to use the echoes to extract some information on the crystalline orientation in the ingot. By quantifying the uncertainties and their dependence with the duration of the experiment in particular, we outline the conditions necessary to detect accurately the average orientation in the solid.