Zone refining is a technique companies can use in the production of highly pure crystals for components like transistors. In this method, impurities in an ingot, or mass of metal, are forced to one end or another through a series of melting and cooling phases. This leaves behind a chunk of mostly pure material available for use in seed crystals and other components. Creation of this technique dates to the 20th century, as one of a number of methods invented by semiconductor manufacturers and similar companies to meet the need for very pure materials.
In the process of zone refining, a technician takes an ingot of material and feeds it very slowly through a tube with heating elements. The heating elements heat entire cross sections all the way through to the melting point, creating a zone in the middle of the solid ingot that is in a liquid state. At the boundary between solid and liquid, impurities precipitate out on the atomic level. They may not be visible, but their presence could cause serious problems with products made using that ingot, which makes refining a critical part of the preparation.
Some impurities tend to reduce the melting point, while others raise it. The impurities will all cluster around one end of the ingot. Through multiple passes, the technician can force the impurities out, leaving a mostly clean ingot with some contamination at a far end. This end can be removed to make a refined and very pure ingot. It should have a very stable crystalline structure, as no impurities disrupt the lattice of connections between individual atoms.
This technique requires some specialized equipment and a high degree of control. The technician needs the right temperature for a given component to prevent problems with the zone refining. It is also necessary to carefully control the speed during zone refining as the ingot moves repeatedly through the equipment. Purity in the larger working environment is critical as well, because technicians do not want to introduce new materials to their purified ingots by handling them in contaminated spaces.
After zone refining is complete, the technician can carefully handle and package the purified ingot for use in another facility or transport to a different area. Very careful controls are necessary throughout the process, including repeat checks on the purity and quality of the material. This reduces the amount of waste in the production of semiconductors and similar components and limits the chance that an impurity will travel downstream and cause a cascade of problems.