3. INTRODUCTION
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.
This method was first used for purification (zone
melting), taking advantage of the small segregation
coefficients of many impurities. The impurities
contained in the feed material would then prefer to
remain in the melt and thus could be swept to the end
of the feed stock.
4. Three generic methods of producing (single) crystalline
material from a melt may be distinguished, depending on
whether the crystal is pulled' out of the melt, or
crystallization takes place in a crucible container, or is
container-less, occurring within a solid rod of material.
Float zone technique is mainly used for smalized wafers. The
float zone technique is used for producing specialty wafers
that have low oxygen impurity concentration.
5. Float zone technique
• Float Zone (FZ) produces smaller wafers
• Start with polycrystalline Si rod
• Touch rod to seed crystal
• Heat with moving Radio Frequency (RF) coil
• Melts rode near coil
• Move melt front from crystal to end and back
• The RF coil starts from the fused region, containing the seed, and
travels up.
• When the molten region solidifies, it has the same orientation as the
seed.
• The furnace is filled with an inert gas like argon to reduce gaseous
impurities.
• Leaves single crystal rod behind
6.
7. The Process
In a chamber with a controlled atmosphere, a clean polycrystalline
Silicon rod (source rod) is positioned above a high-frequency
induction coil. When the high- frequency current is introduced into
the Silicon rod very close to the surface, the bottom end of the rod
starts to melt in a non-contact process. The rod is rotated slowly to
ensure that it melts as homogeneously as possible. The melted zone
is brought into contact with a thin monocrystalline seed crystal that is
located under the coil and moved upward through a small hole in the
middle of the coil. Once the seed crystal has been brought into
contact with liquid bottom end of the source rod, the seed crystal is
moved slowly downward while also rotating. A monocrystalline
crystal now starts to grow on the seed crystal, following its crystal
structure and orientation. The liquid zone is maintained as the source
rod is fed in slowly from above and starts to melt, while the single
crystal continues to grow on the seed crystal.
8. ADVANTAGES AND USES
The Float Zone process -also known as "zone melting" is ideal for
producing high-purity monocrystalline Silicon such as that found in
components used in high-performance electronics, microsystem
technology and semiconductor technology.
The Polysilicon industry, solar industry and R&D sector also benefit
from this product. The advantage of the FZ process over processes
involving Quartz crucibles is that the crystals are exposed to
significantly less Oxygen contamination. There are no additional
costs for the crucibles, which can be used only once.
