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Chapter: Physics - Crystal Physics

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Czochralski technique

The czochralski methods is crystal pulling technique from the melt. The process is based on a liquid – solid phase transition driven by a send crystal in contact with the melt.

Melt growth:

Melt growth is the process of crystallization by fusion and re solidification of the starting materials

The growth from melt is further sub-grouped into various techniques. The mani techniques are

a)     Czochralski technique

b)    Bridgmen Technique

c)     Verneuil method

d)    Zone melting technique

The major practical factors is to be considered during growth crystals from melt are 

1)    Velatiltiy

2)    Chemical resistivity

3)    Melting point



Czochralski technique

The czochralski methods is crystal pulling technique from the melt. The process is based on a liquid – solid phase transition driven by a send crystal in contact with the melt.

Basically, the seed is to be considered as a heat sink, by which the latent heat of solidification escapes and as a nucleation center; the solidified fraction at the surface of the seed will reproduce its single-crystal structure.

By raising the seed slowly, a crystal is ‘pulled’ from the melt. Decreasing the melt temperature makes the crystal diameter increasing and vice versa.

 

Description and working

Large single crystals of Si (for IC fabrication) are grown by the Czochralski method. It involves growing a single – crystal ingot from the melt, using solidification on a seed crystal as illustrated in the figure.


Molten Si is held in a quartz(Crystalline SiO2) crucible in a graphite susceptor. It is heated by radio frequency induction coil (RF heating).

A small dislocation – free crystal called a seed is moved down to touch the melt and then slowly pulled out of melt.

A crystal grows by solidifying on the seed crystal. The seed is rotated during the pulling stage, to obtain a cylindrical ingot.

 

ADVANTAGES

Growth from free surface

Growth of large oriented crystals

Convenient chemical composition

Control of atmosphere

Limitations

High vapor pressure materials

Liquid phase encapsulation

Possible contamination of the melt by the crucible

No reproductivety of the crystal shape.


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