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Single Crystal Growing for Wafer Production (Page 2 of 2)
Float Zone Crystal
Growth
The
float
zone (FZ) process is another method for growing single-crystal silicon.
It involves the passing of a
molten zone through a polysilicon rod that
approximately has the same dimensions as the final ingot. The
purity of an ingot produced by the FZ process is higher than that of an
ingot produced by the CZ process. As such, devices that require
ultrapure starting silicon substrates should use wafers produced using
the FZ method.
The FZ process consists of
the following steps.
1. A
polysilicon rod
is mounted vertically inside a chamber, which may be under vacuum or
filled with an inert gas.
2. A needle-eye
coil
that can run through the rod is activated to provide RF power to the
rod,
melting
a 2-cm long zone in the rod. This molten zone can be maintained in
stable liquid form by the coil.
3. The coil is then moved
through the rod, and the
molten zone
moves along with it.
4. The movement of the
molten zone through the entire length of the rod purifies the rod and
forms the near-perfect single crystal.
FZ growing equipment can
also use a stationary coil, coupled with a mechanism that can move the
silicon rod through it.
Fig. 2.
Examples of
After the single-crystal
silicon ingot has been manufactured, it undergoes a routine evaluation
of its resistivity, impurity content, crystal perfection, size and
weight. It is then
ground
using diamond wheels to make it a perfect cylinder that has the right
diameter. It then undergoes an
etching
process to remove the mechanical imperfections left by the grinding
process.
Fig. 3.
A Single-Crystal Silicon Rod
The cylindrical ingot is
then given one or more 'flats' by another round of
grinding.
The largest flat, called the
primary flat, is used
by automated wafer handling systems for alignment. Flats (primary
and secondary) are also used to
identify
the crystallographic
orientation and
conductivity
of the wafer.
The ingot is then
sawn
into thin wafer slices, each of which will be subjected to further
etching
and
polishing
until it is ready for use as substrates for VLSI fabrication. The
above process of silicon growing, grinding, shaping, sawing, etching,
and polishing to produce input wafers is known as
wafering.
Fig. 4.
An ingot slicer (left) and a wafer grinder/polisher (right)
See also:
Crystal
Defects; IC
Manufacturing;
Semiconductor Wafers; What is a semiconductor?
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©
2003-2005
www.SiliconFarEast.com.
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