Special high-temperature furnaces first heat the purified silicon to around 1450º Celsius. Then a rod (1), the so-called seed crystal, is lowered down into the molten mass, which is held in a crucible (2) made of quartz. (3) The rod and crucible rotate in opposite directions. As soon as the seed is immersed in the liquid silicon, the rod, which continues rotating the entire time, is slowly pulled out of the molten liquid. Since the surface temperature of the liquid silicon is just a little bit above silicon’s melting point, the liquid silicon deposits itself on the seed crystal and then solidifies, taking on the same crystalline structure as the seed crystal in the solidification process—a monocrystal (4) up to 60 cm in diameter and a length of more than 2 meters begins to grow.
The stable temperature of the molten silicon, the speed of rotation and the speed at which the rod is pulled upward set the crystal’s diameter and length. Its quality is significantly determined by the purity and continuity of the the material of the installed graphite components. SGL Carbon manufactures complete "hot zone" systems from high purity graphite as well as fiber-based composites, if needed also with surface treatments. In close collaboration with our partners, we design, simulate and produce complex equipment according to the individual requirements of the specific production technologies and processes of our customers. The components include heat- and corrosion-resistant crucibles (5), heaters (6), heat shields (7) and insulating components (8) made of graphite. Graphite is the material of choice because it has no problem resisting extreme temperatures and chemical processes. Since it’s highly pure, it also prevents contamination of the molten silicon, thereby providing the best conditions for the highest quality.
Growing these monocrystal ingots is one of the most important steps in the production process. It must result in a lattice structure that is as perfect as possible, thereby laying the foundation for the quality and performance of the chips produced from it.
Dr. Timo Taetz, Director Product Management EIS, Business Unit Graphite Solutions at SGL Carbon