Improvement of the growth stability for large diameter Si-float zone crystals by controlling the gas flow

Abstract

For the growth of large float zone (FZ) silicon crystals, currently limited to 200 mm diameter, a high process stability is necessary. A major drawback are arc discharges at the inductor, when the increasing diameter of the growing crystal requires higher RF voltages. An arc discharge mostly interrupts the dislocation‐free growth of the crystal. Increasing the gas pressure or adding some nitrogen to the growth atmosphere increases its electrical strengths. For reduced working frequencies the inductor voltage is lowered, too. However, the usability of these measures is limited. The aim of this work was to investigate another concept to reduce the probability of arcing. Numerical calculations of the convective gas flow in a typical FZ puller revealed a high gas temperature, being close to the silicon melting point temperature in the vicinity of the melt surface. The gas temperature near the inductor and especially its influence on the risk of arcing, were investigated. Several experiments confirm a significantly higher breakdown voltage at lower temperature in comparable conditions by controlling the gas flow. This was performed using a thin quartz plate covering at least the whole lower inductor surface including the main slit where the electric field is strong. Therefore, the temperature will be lowered due to the separation of the hot gas flow from the critical slit region. It could been shown, that the risk of arcing could be noticeable reduced for 6 inch crystals grown under comparable conditions The results will be supported by global numerical simulations showing the flow field and the temperature distribution of the gas. The protection of the critical region by changing the gas flow appears as a new potent measure to prevent arcing and to enable larger crystals.