Production of Extreme-Purity Aluminum and Silicon by Fractional Crystallization Processing

Large scale fractional crystallization is used commercially at Alcoa to produce extreme purity Al (99.999 + % Al). The primary market is sputtering targets used to make interconnects for integrated circuits. For some applications, the impurities uranium and Th are reduced to < 1 ppbw to avoid "soft errors" associated with alpha particle emission. The crystallization process achieves segregation coefficients which are close to theoretical at normal yields, and this, coupled with the scale of the units, allows practical production of this material. The Si purification process involves crystallization of Si from molten Al alloys containing approx 30% Si. The crystallites from this process are further treated to remove residual Al and an extreme purity ingot is obtained. This material is considered suitable for single crystal or ribbon type photovoltaic cells and for certain IC applications, including highly doped substrates used for epitaxial growth. In production of both extreme purity Al and Si, impurities are rejected to the remaining melt as the crystals form and some separation is achieved by draining this downgraded melt from the unit. Purification of this downgrade by crystallization has also been demonstrated for both systems and is important for achieving high recoveries. 4 ref.--AA(US).