Zone melting is a process that purifies crystals, especially those used in semiconductors, by heating them and pushing impurities to the bottom. The crystal is slowly subjected to a heat zone and the impurities move towards the cooler part. Zone refining is used to ensure complete purification.
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. As the crystal is heated, impurities are drawn to the bottom of the crystal and the bottom is cut away, leaving a purified sample. There are two ways to do zone fusion, with the thermal zone still 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 in a cylindrical crucible, with a crystal seed peeking out of the top. The crystal seed is the main portion that needs to be purified, although some of the other crystal will also be purified. A furnace is then set up, 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 spot fusion process. In the Bridgman method, the hot spot moves against the crucible while the crucible is stationary. The Stockbarger method is the opposite, with the crucible moving through a heat zone. While they are complete opposites, they both do the same thing.
When the crystal is subjected to the heat zone, it becomes molten. This allows the impurities in the crystal to move freely and tend to move towards the cooler part that is not passed through the heat zone. To allow any impurities to move around, the crystal is heated very slowly, until the purees are drawn into the base of the crystal. The underside of the crystal is sawn off, leaving a purified crystal.
To ensure that zone fusion is successful, a process called zone refinement is used. This is the same as zone fusion, but the crystal is purified multiple times. By doing this multiple times on the same crystal, workers can be sure that the crystal is completely free of impurities.
This purification process is mainly performed on crystals used as semiconductors, but can be used in other applications as well. Semiconductors are pieces used in engineering that have conductivity between a conductor and an insulator. If impurities remain in the crystal, they will interfere with the flow of electricity, making the crystal either too conductive or not conductive enough.
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