ZONE MELTING

DEFINITION Zone refining is a method of purifying crystals, in which a narrow region of a crystal is molten, and this molten zone is moved along the crystal (in practice, the crystal is pulled through the heater).

•  Zone refining was developed by William Gardner Pfann in Bell Labs as a method to prepare high purity materials for manufacturing transistors.

PRINCIPLE• For solute/solvent systems where partition

coefficient k is low, the impurity atoms will diffuse to the liquid region at the solid/liquid boundary.

• Thus, by passing a crystal  through a thin section of furnace very slowly, such that only a small region of the crystal is molten at any time, the impurities will be segregated at the end of the crystal.

• Because of the lack of impurities in the leftover regions which solidify, the boule can grow as a perfect single crystal if a seed crystal is placed at the base to initiate a chosen direction of crystal growth. When high purity is required, such as in semiconductor industry, the impure end of the boule is cut off, and the refining is repeated.

MATHEMATICAL EXPRESSION

ZONE LEVELING

● Growing large single crystals having high lattice perfection and containing an essentially uniform distribution of one or more desired impurities.

● Used for making doped semiconductors, junction diodes.

FLOATING ZONE METHOD

• 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

• If properly done, the newly crystallizing material could be obtained as a single crystal. Again, it was the Siemens AG that pioneered the use of FZ crystal growth for the production of high-quality Si single crystals.

• Since the melt never comes into contact with anything but vacuum (or inert gases), there is no incorporation of impurities that the melt picks up by dissolving the crucible material as in the CZ crystal growth method. This is especially true for oxygen, which can not be avoided in CZ crystal growth. FZ crystals therefore are always used when very low oxygen concentrations are important.

• The problem of FZ crystal growth is clear, however, if one looks at the drawing: How do we keep the liquid Si from just collapsing? If it would only be held in place bysurface tension, the maximum diameter of crystals possible in this way would be about 20 mm - not very useful. There are, fortunately, other stabilizing mechanisms, and drawing the liquid zone through a "hole" - as indicated - also helps. Still, for large diameter crystals the difficulties grow rapidly and FZ crystal growth is rarely (if at all) used for diameters larger than 150 mm

CZOCHRALSKI (CZ) SINGLE SILICON

CRYSTAL GROWTH

Parameters:●temperature gradient● rate of pulling ●speed of rotation

APPLICATIONS

● Upto 2m in length and 40cm diameter.● Boules are cut into wafers.● Production of the large-scale integrated circuit

chips used in computers, TVs, mobile phones and all types of electronic equipments.

Thank You