Surface Tension Driven Flow of Molten Silicon: Its Instability and the Effect of Oxygen

Abstract

Surface-tension-driven flow of molten silicon, which is one of mechanisms of heat and mass transfer during crystal growth, was investigated by using a liquid-bridge configuration under microgravity and on earth. Using microgravity is a convenient way to study surface-tension-driven flow, because buoyancy flow can be suppressed so that only surface-tension-driven flow can be distinguished. In the liquid-bridge configuration, which corresponds to floating-zone growth, flow instability and its three-dimensional structure were investigated through measurement of temperature-oscillation, flow visualization, optical pyrometry of the melt surface, observation of oscillation of the melt/crystal interface, and observation of surface oscillation by phase-shift interferometry. Azimuthal wave number m for instability structure depends on the aspect ratio of the bridge, Γ, which is defined as the ratio of height h to radius r.Surface-tension-driven flow was found to be affected by oxygen partial pressure of the ambient atmosphere, which corresponds to concentration of oxygen in Si melt. This is very important finding, because for the Czochralski growth system, oxygen dissolves into melt from a crucible wall made of SiO2. It was also found that surface tension and its temperature coefficient strongly depend on oxygen partial pressure. Oxygen partial pressure at a Si melt surface can be deduced experimentally and theoretically by measuring the oxygen partial pressure at the inlet of the gas flow system.A cellular pattern was observed at a surface of 20 cm deep Czochralski melt, whereas we found a hydrothermal wave at a surface of 8-mm-thick thin melt. Observed patterns are discussed in light of driving force of surface-tension-driven flow in the Czochralski melt.