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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).
