Crystal- and Zone Pulling
Optimum structures with long-time stable generators
The artificial creation of crystals is vital to many industries. They are created by using of one these three processes: zone floating or zone melting, vertical pulling (Bridgman Stockbarger Procedure) or crucible pulling (Czochralski Process). Zone floating permits the creation of ultra-pure single crystals. For this reason, this process has prevailed over the Czochralski pulling process, particularly in semiconductor production.
Zone Floating Process
During the zone floating process, a vertical polycrystalline bar is melted on one end. The small, melted area is permitted to move slowly from one end of the bar to the other end. Monocrystalline silicon is created during solidification. The major advantage of this process: Impurities accumulate in the melted area and can be flushed out of the crystal.
Vertical Pulling Process (Bridgman Stockbarger Process)
During the vertical pulling process (Bridgman Stockbarger Process), the polycrystalline base material is heated in a closed crucible and then cooled slowly in small amounts. A graphite susceptor is used to transfer the heat. The crystal cools and hardens when the crucible is removed out of the high frequency field. As an alternative, this can be achieved by reducing the generator's power. The Bridgman-Stockbarger-method was developed mainly to produce semiconductors, but it is also used to produce alkaline halide and fluoride crystals.
Crucible Pulling Process (Czochralski Process)
In the case of the crucible pulling process (Czochralski process), the material to be processed is first melted in a crucible. A single crystalline seed crystal is brought close to the surface of the bath and then into contact with it. As a result of capillary action, the liquid rises up along the seed crystal, forms a meniscus and creates a triple liquid - solid - gas boundary layer.
The seed crystal is then pulled up slowly to the top to form a single crystal. Its growth is achieved by keeping the crystal rotating, so that the bath is homogenized and the shape of the boundary layer of solid material and liquid is kept under control.
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