Kayex Silicon Crystal Growing Process
Czochralski Crystal Growth Process
A semi-metal such as silicon is melted in a crucible with an appropriate
amount of dopant, either donor or acceptor. The melting occurs and
is maintained in a partial pressure atmosphere of inert gas, usually
argon, at slightly above the melting point. The heat source is a
graphite resistance heater that surrounds the crucible and is contained
within an insulation layer inside of the furnace.
A seed crystal of the desired orientation is lowered into the liquid
and the temperature is reduced slightly until the liquid begins
to crystallize onto the seed. The seed crystal is then drawn up,
slowly at first and then more quickly, until a long, thin crystal
has formed on the end. This is known as the "neck".
Once the neck has been grown at a sufficient poll rate and diameter
to assure that it is free of dislocations (imperfections of the
crystal structure), the temperature is reduced further and the growth
rate is reduced to allow the crystal to become larger in diameter.
This is known as the "crown".
As the diameter of the crystal approaches the desired body diameter,
the growth rate is again increased and temperature is adjusted to
create a smooth rounded transition between the crown and body. This
is known as the "shoulder".
The crystal diameter is controlled during body growth by varying
the lift rate of the seed and the temperature of the melt. During
this period the crucible is also raised at a rate required to maintain
the position of the liquid surface in relation to the heated area
of the chamber. The seed and crucible are rotated in opposite directions
and at various rates to control the properties of the crystal. The
growth rate is dependent on crystal type, diameter, hot zone configuration
and various other factors. For silicon, typical body diameters now
range from 100 to 300 mm and typical growth rates vary from 40 to
85 mm/hr.
After the melt has been depleted the crystal diameter is again
reduced in a gradual cone shaped taper known as the "tail".
The crystal diameter must be reduced to a very small point to reduce
the thermal shock when the solid loses contact with the liquid.
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