What Is Zone Melting?

Zone melting is a purification process for crystals, especially those used in semiconductors, that uses heat to push impurities from the top of a crystal to the bottom of a crystal. When the crystal is heated, the impurities are pulled to the bottom of the crystal and the bottom is cut off, leaving behind a purified specimen. There are two ways of performing zone melting, with the heat zone either stationary or moving; both arrive at similar results. Zone refining is the process of performing the melting procedure several times, until the crystal is completely purified.

To start zone melting, a crystal is placed into a cylindrical crucible, with a seed crystal poking out from the top. The seed crystal is the main portion that needs to be purified, though some of the other crystal will be purified as well. A furnace is then prepared, with a narrow heat zone near the center, and the crystal is slowly subjected to the heat zone.

There are two main methods for performing a zone melting process. In the Bridgman method, the hot zone moves against the crucible while the crucible is stationary. The Stockbarger method is the opposite, with the crucible moving through a heat zone. While complete opposites, both do the same thing.

When the crystal is subjected to the heat zone, it becomes molten. This allows impurities in the crystal to move freely, and they tend to move toward the colder portion that is not being passed through the heat zone. To allow all the impurities to move, the crystal is heated very slowly, until the purities are drawn into the base of the crystal. The bottom portion of the crystal is sawed off, leaving behind a purified crystal.

To ensure that zone melting is successful, a process called zone refining is used. This is the same as zone melting, but the crystal is purified several times. By doing this several times to the same crystal, workers can be sure the crystal is entirely free of impurities.

This purification process is done primarily to crystals that are used as semiconductors, but it also can be used in other applications. Semiconductors are pieces used in engineering that have conductivity between a conductor and insulator. If impurities remain in the crystal, they would interfere with the flow of electricity, making the crystal either too conductive or not conductive enough.