The
method of obtaining single crystal Si for VLSI fabrication involves several
steps:
1.
Raw materials (e.g.
quartzite, a type of sand), is refined by a complex, multistage process which
produces electronic grade polysilicon (EGS).
2.
This polysilicon is
used to grow single crystal silicon by Czochralski (CZ) crystal growth or float
zone(FZ) growth.
Electronic-Grade Silicon
Electronic-grade
silicon (EGS), a polycrystalline material of high purity, is the raw material
for the preparation of single-crystal silicon. To obtain EGS requires a multi
step process. [2]
1.
First,
metallurgical-grade silicon (MGS) with purity up to 99% is produced in a
submerged-electrode arc furnace. The furnace is charged with quartzite, a
relatively pure form of sand, and carbon in the form of coal, coke, and wood
chips. In the furnace a number of reactions take place, the overall reaction
being
SiC + SiO2 ® Si(liquid) + SiO(gas) +
CO (gas)
2.
The next process step
is to pulverize the MGS mechanically and react it with anhydrous hydrogen
chloride to form trichlorosilane (SiHCl3) according to the reaction:
Si(solid) +
3HCl(gas) ® SiHCl3 (gas) + H2 (gas) + heat
At this point the purification process occurs.
Trichlorosilane(TCS) is a liquid at room temperature (boiling point 32ºC), as
are many of the unwanted chlorides. Purification is therefore done by
fractional distillation.
3.
EGS must have
99.999999999 % purity. EGS is prepared from the purified TCS in a chemical
vapor deposition (CVD) process. The chemical reaction is a hydrogen reduction
of trichlorosilane(TCS).
2SiHCl3(gas) +
2H2(gas) ® 2Si(solid) + 6HCl(gas)
An alternate process for the production of EGS that
is starting to receive commercial attention is the pyrolysis of silane. The
overall reaction is
SiH4(gas) + heat ® Si(solid) + 2H2(gas)
The advantages of producing EGS from silane are
potentially lower cost and less harmful byproducts.