UNIT 3 • SAND TO POLYSILICON

Sand is silicon dioxide

SiO₂ → Si

Quartz sand and quartzite are almost pure silicon dioxide, SiO₂. To get silicon you must strip away the oxygen, then purify what is left far beyond anything found in nature.

Getting the oxygen off is easy. Getting everything else off is the hard part.

THE PIPELINE

Four steps to electronic-grade silicon

Each step raises the purity. A furnace makes rough silicon, a reactor turns it into a liquid compound, distillation cleans that liquid, and a second reactor deposits pure solid silicon from it.

Purification works by changing silicon's form so impurities can be left behind.

STEP 1 · THE FURNACE

Stripping the oxygen

SiO₂ + 2C → Si + 2CO

Sand and carbon (coal or coke) are heated in a submerged-arc furnace at roughly 1,900 °C or more. The carbon takes the oxygen away as carbon monoxide, leaving liquid metallurgical-grade silicon that is about 98 to 99 percent pure.

Good enough for aluminum alloys and silicones, nowhere near good enough for chips.

STEP 2 · A LIQUID FORM

Turning silicon into a liquid

Si + 3HCl → SiHCl₃ + H₂

Ground silicon reacts with hydrogen chloride gas at around 300 °C to form trichlorosilane, SiHCl₃, plus hydrogen. Trichlorosilane boils at about 32 °C, so it can be purified by fractional distillation, the same trick used to separate the components of oil.

Distillation separates impurities that silicon itself can't shed.

STEP 3 · THE SIEMENS REACTOR

Growing silicon from gas

SiHCl₃ + H₂ → Si + 3HCl

Purified trichlorosilane and hydrogen flow over thin silicon rods heated to about 1,100 °C. Silicon deposits on the rods, which grow into thick polycrystalline bars that are broken into chunks. Fluidized-bed reactors that use silane are an alternative route.

The result is polysilicon: ultra-pure, but still a jumble of tiny crystals.

HOW PURE IS PURE?

Purity, counted in nines

Metallurgical silicon 98–99% · about two nines

Solar-grade silicon about 99.9999% · six nines

Electronic-grade silicon 99.9999999% to 99.999999999% · nine to eleven nines

What it means Impurities measured in parts per billion or even per trillion

Parts-per-billion impurities can change how silicon conducts, so they must be kept below that.

UNIT 3 STUDY COMPLETE

🏖️ → ⚙️

Ready for the Fab Challenge?

You can follow sand through the furnace, trichlorosilane, and the Siemens reactor to polysilicon.