Multi-crystalline silicon ingots growth with an innovative induction heating directional solidification furnace

Abstract

Multi-crystalline silicon ingots produced using directional solidification systems (DSS) represent the best way to obtain high quality crystalline silicon at low prices and with high throughputs. The DSS technology is widespread among PV silicon ingot producers and hundreds of furnaces are manufactured worldwide every year. The present challenge for crystal growers is to increase the maximum ingot mass in order to benefit from scale economy profits and to reduce energy consumption. This reason has pushed some companies to develop new DSS furnaces able to grow ingots up to 650 kg. As a matter of fact, the increase in size defines new challenges in design and process optimization due of the intensification of radial thermal instabilities and consequently of buoyancy driven flows in the melt [1]. These phenomena are related to heat and mass transfer during the solidification process and are important to control both the liquid/solid interface shape and the impurities distribution in the crystal [2,3,4,5,6]. The main features and the characteristic design of the hot-zone in the iDSS (induction-DSS) furnace are taken into account, especially in comparison with the standard DSSs ones. The reduction in thickness of the insulation boards and the smaller size of the hot-zone itself, together with the selective lateral induction coil system, all lead to an optimal control of the thermal instabilities into the silicon melt, increasing the ingot quality. In fact, the lateral induction coil system equipped with independent turns connections can be used to force selectively — at different vertical positions — the most suitable thermal condition. In this way one is able to compensate the radiative thermal losses and create a “virtual” adiabatic wall, producing a perfectly planar solidification front or modeling the radial thermal gradient in order to obtain the desired solidification front shapes. The results from a set of electro-magnetic, thermal and fluid-dynamic simulation are presented; the first data collected from the real scale prototype are shown.