Float-zone silicon is a high-purity alternative to crystals grown by the Czochralski process.

The basic idea in float zone (FZ) crystal growth is to move a liquid zone through the material. If properly seeded, a single crystal may result.

The production takes place under vacuum or in an inert gaseous atmosphere.

The process starts with a high-purity polycrystalline rod is held in a vertical position and is rotated.

With a radio frequency the rod is partially melted. The seed is brought up from below to make contact with the drop of melt formed at the tip of the poly rod.

As the molten zone is moved along the polysilicon rod, the molten silicon solidifies into a single Crystal and, simultaneously, the material is purified.

FZ crystals are doped by adding the doping gas phosphine (PH3) or diborane (B2H6) to the inert gas for n- and p-type, respectively.

Unlike CZ growth, the silicon molten Zone is not in contact with any substances except ambient gas, which may only contain doping gas.

Therefore FZ silicon can easily achieve much higher purity and higher resistivity.

After the heating coils move over the whole polysilicon rod, it converts to a single crystal silicon ingot.

Advantages:

1. When extremely high purity silicon is required the growth technique of choice is float-zone method.
2. There is no need for a crucible and so there is lower melt contamination, especially oxygen and carbon which cannot be avoided in CZ crystal growth.