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Production and Purification of Electronic Grade Silicon: From Quartzite to Poly-Silicon, Assignments of Electrical and Electronics Engineering

An overview of the production process of electronic grade silicon (egs) from quartzite, focusing on the crystalline structure of silicon and the four-step process to obtain egs with impurity levels below a few parts per billion (ppb).

Typology: Assignments

Pre 2010

Uploaded on 09/02/2009

koofers-user-xra
koofers-user-xra 🇺🇸

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1
1) Production of Electronic Grade Silicon
1.1) Crystalline Structure of Silicon
Crystalline silicon takes the diamond lattice unit cell, which is based on the
face
centered cubic (fcc) lattice
face
-
centered
cubic
(fcc)
lattice
.
To count the number of atoms in the fcc unit cell imagine a cage
that is the same size and displace it up, back and to the right
Question: How many silicon atoms are completely contained in the cage?
Answer: 4
1.1) Crystalline Structure of Silicon (cont)
The silicon (i.e. diamond) lattice unit cell consists of two interpenetrating
fcc lattices. Study figure 2.6 on p 16 of the text book.
The unit cell will therefore contain 8 atoms. Assuming that the length of the
edge of the unit cell is 5.43Å you can calculate that each cubic centimeter of
silicon contains 5 x 1022 atoms (see next week’s homework assignment).
Compare this to the typical p- and n-type doping concentrations of 1015-1020
cm-3. In very pure silicon we therefore require the initial impurity levels to be
less than 1 in 10
8
i e a few parts per billion (ppb)
less
than
1
in
10
i
.
e
.
a
few
parts
per
billion
(ppb)
.
pf3

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1) Production of Electronic Grade Silicon

1.1) Crystalline Structure of Silicon

Crystalline silicon takes the diamond lattice unit cell, which is based on the face centered cubic (fcc) latticeface-centered cubic (fcc) lattice.

To count the number of atoms in the fcc unit cell imagine a cage that is the same size and displace it up, back and to the right Question : How many silicon atoms are completely contained in the cage? Answer: 4

1.1) Crystalline Structure of Silicon (cont)

The silicon (i.e. diamond) lattice unit cell consists of two interpenetrating fcc lattices. Study figure 2.6 on p 16 of the text book.

The unit cell will therefore contain 8 atoms. Assuming that the length of the edge of the unit cell is 5.43Å you can calculate that each cubic centimeter of silicon contains 5 x 10 22 atoms (see next week’s homework assignment).

Compare this to the typical p- and n-type doping concentrations of 10^15 -10^20 cm-3. In very pure silicon we therefore require the initial impurity levels to be less than 1 in 10less than 1 in 10 i.e. a few parts per billion (ppb). 8 i e a few parts per billion (ppb)

1.2) Manufacture of Electronic Grade Silicon

Starting from washed quartzite (sand), electronic grade poly-crystalline silicon (poly-Si) with impurity levels less than a few ppb is produced by a four step process.p p

i) Quartzite to metallurgical grade silicon (mgs): The reaction takes place in an electric arc furnace at temperatures in excess of 2000° C. The liquid silicon is drawn off with a purity of 98%. The main impurities are aluminium, boron and iron.

e.g. SiO 2 + 2C → Si + 2 CO + Al 2 O 3 and B 2 O 3

impurities

Coal, coke, wood will act as p-type dopants if not removed

1.2) Manufacture of Electronic Grade Silicon (cont)

ii) MGS to trichlorosilane: MGS powder is treated with hydrochloric acid (HCL) gas in the presenceMGS powder is treated with hydrochloric acid (HCL) gas in the presence of a catalyst.

e.g. Si + 3HCl → SiHCl 3 + H 2

300º C

Impurities present in the MGS (Al 2 O 3 etc) are turned into chlorides eg AlCl 3 , which are readily separated during the distillation in step (iii).