Edge-defined Film-fed Growth (EFG) is a promising technology for production of silicon wafers with minimum loss of silicon material for photovoltaic applications. The growth of the hollow tubes by the EFG method is a very high temperature gradient along the direction of tube growth at the melt-solid interface. As this high temperature gradient is necessary to sustain the high speed of silicon growth through dissipating latent heat at the melt-solid interface, it generates substantial amount of thermal stresses that are associated with defect generation and deformation of the structure. With increase of the tube diameter and reduction of wall thickness, the large thermal stresses lead to significant buckling of the tube.
In this study, the buckling behavior of the growing silicon tube is studied experimentally by monitoring deflection of two pairs of opposite faces of an octagonal tube during the growth. Four capacitance sensors were placed on a fixed plane normal to the translation of the tube to measure the varying thickness of the air between the sensor and the tube surface throughout the tube growth. The recorded data reveal the buckling behavior of tubes for several growth runs. Despite the different growth conditions in the EFG systems, similar patterns of buckling behavior have been observed in octagonal silicon tubes with face width of 12.5 cm. A typical buckling pattern observed can be divided into three distinctively different stages, namely seed holder-influenced stage, tube growth stage, and a random fluctuation stage. Each stage has different amplitude of deflection and covers different length of the tube. More interestingly, the recorded surface deflection reveals the displacement of the center of the tube, suggesting off-centered growth and possible instability of the meniscus. It is considered that these mechanisms may contribute to the special buckling pattern observed during growth.
