Influence of crucible properties and Si3N4-coating composition on the oxygen concentration in multi-crystalline silicon ingots

In this work, the influence of the silica ceramic crucible type and the properties of the Si3N4-coating on the oxygen contamination of directionally solidified silicon ingots was investigated by G0 and G1 crystallization experiments. It was found that crucibles with rough inner surfaces with values of Rq > 18 µm lead to higher oxygen contamination levels in the silicon melt of more than one order of magnitude in contrast to the ones with smooth inner surfaces. The use of colloidal SiO2 particles in the nm-scale within the Si3N4-coating, which generate the required non-wetting behavior towards the silicon melt, will also result in higher oxygen concentrations, while its degree is proportional to the used SiO2/Si3N4 ratio. Thick Si3N4-coatings in the range of 150-350 µm reduce or even suppress the influence of the crucible type on the oxygen contamination and the coating itself becomes the new main oxygen source. Higher argon flow velocities throughout the crystallization can be used as one effective influencing parameter to reduce the oxygen contamination of the melt. However, the effect might be compensated by certain crucible types. From these results conclusions will be drawn, how to achieve the lowest possible oxygen contamination during direction solidification of silicon ingots in the industrial scale.