SILICON PURITY CONTROLLED UNDER ELECTROMAGENTIC LEVITATION (SPYCE)– INFLUENCES ON UNDERCOOLING M. Beaudhuin 1* , K. Zaidat 1 , T. Duffar 1 , M. Lemiti 2 1 SIMAP EPM – CNRS, 1340 rue de la piscine, F-38402, Saint Martin d’Hères Cedex, France 2 INL, UMR-CNRS 5270, INSA de Lyon, Bat. 502, 20 Av. Albert Einstein, F-69621, Villeurbanne Cedex, France *Corresponding author: mickael.beaudhuin@simap.grenoble-inp.fr , Tel: +33476 825 258, Fax: +33476 825 211 Abstract The rapid evolution of photovoltaic Si production induced a shortage of high purity silicon raw material. The use of lowest purity silicon has a strong effect on the casting conditions and ingot structure and properties. During solidification, solute rejection at the growth interface leads to an increase of the impurities concentration in the liquid phase and then to the precipitation of silicon nitride (Si 3 N 4 ) and silicon carbide (SiC). As a consequence, the grain structure of the ingot changes from columnar to small grains, also known as grits. A new electromagnetic levitation set up is presented which has been developed in order to measure the undercooling versus impurity concentration. The impurity concentration in the levitated Si drop is controlled by the partial pressure of ammoniac or hydrocarbon gas. The concentration of nitrogen and carbon dissolved will be compared with theoretical predictions. As nucleation is a random phenomenon, statistical measurements are presented, from samples which showed numerous heating/melting and cooling/solidification phases. The effect of impurities on the undercooling of silicon droplet is discussed. Keywords: Photovoltaic silicon, Electromagnetic levitation, Impurity, Undercooling, Partial pressure Introduction Multicrystalline silicon is intensively used in photovoltaic industry. Due to a rapid increase of silicon demand, the main source of pure Si raw material decreased. Previously, the microelectronic scraps, with impurities in the ppb(a) – ppt(a) range, were used to produce cells, now silicon with impurities in the ppb(a) – ppm(a) range becomes a typical feed material [8]. This permits to decrease the total production cost but leads to less “perfect” cells. Changes in the morphological multi-crystalline structure have been observed through the solidification process. Instead of columnar grains which permit converting a maximum of photons to electron-hole pairs some areas are composed with small grains, also known as “grits” [9]. These small grains are characterised by a large concentration of carbon and small precipitates of silicon carbide were observed [12].These areas induce a decrease of the cell efficiency but also cause the breaking of wire saws. Precipitation of silicon carbide occurs if a given supersaturation of carbon is reached. Knowing this value it would be possible to predict when a classical, preferred, columnar structure will change to an undesired, equiaxed, grit structure. However there is no way to measure the supersaturation of a given impurity during the solidification process. It has been observed since years that an increase of impurity concentration provokes a decrease of undercooling. This means that impurities became nucleation sites which diminished the barrier energy to create the first silicon nuclei. To study this phenomenon it is necessary to conceive an experimental set up which could certify that the only heterogeneous nucleation sites come from impurities which has been intentionally and precisely injected. In that way a new electromagnetic levitation set up has been developed which permits to control the concentration of impurities by controlling the partial pressure of an ammoniac or hydrocarbon gas. Experimental setup The experimental setup, named SPYCE, is composed of a chamber which is evacuated to 10 -8 mbar with a turbo molecular pump and an ionic pump (see Fig.1). It is filled with a mixture of 6N pure Argon and 6N pure Helium gases. Undoped silicon of 6N purity was used as source material. Lump of silicon about 2.8g are directly positioned in the experimental set up. In order to remove impurities on the surface, a chemical treatment with a solution of 20%NaOH at 80°C is done beforehand while 30min. As silicon resistivity is too high at ambient temperature up to 1000°C to be electromagnetically levitated, it is