How polysilicon is made

Electronics-grade polysilicon carries fewer than one impurity atom in a billion, grown onto glowing rods.

stations: 5
things upstream: 17
raw sources: 8
steps from the ground: 5

The short version

Polysilicon starts as metallurgical silicon that is turned into liquid trichlorosilane, distilled clean, then decomposed with hydrogen onto thin silicon rods heated to 1,150 °C. The rods thicken for about two days until they reach 120 mm across. They are then broken into chunks and sealed in bags filled with nitrogen or argon.

Turn silicon into a gas

Metallurgical silicon of about 98% purity is reacted with hydrogen chloride gas to make trichlorosilane, a volatile liquid that boils easily, with hydrogen released as a by-product. Because silicon is now part of a molecule, metals and dopants can be separated out.

Goes in
Metallurgical silicon Hydrochloric acid
Distill it clean

The trichlorosilane is distilled repeatedly, and transition metals and dopant impurities stay behind. Electronics grade ends up with impurity levels below one part per billion.

under 1 part per billion
Grow silicon on hot rods

Thin silicon rods stand inside a water-cooled reactor, and an electric current heats them to 1,150 °C. Trichlorosilane and hydrogen flow over them, and silicon settles on the hot surface until the rods reach 120 mm in diameter after about 46 hours.

1,150 °C for 46 hours

Goes in
Trichlorosilane Hydrogen
Harvest and break rods

Each batch is opened and the rods are pulled out and broken into chunks. The whole cycle takes about 60 hours, and plants keep backup power because an interrupted run is ruined.
Bag it under inert gas

Chunks are packed in clean rooms inside bags filled with nitrogen or argon, so nothing settles on the surface in transit.

Goes in
Nitrogen gas

End of the linePolysilicon Next it goes into silicon wafer.

Worth repeating

Rods from the process run two to three meters long before they are broken up.
Plants keep backup power for the reactors, because a batch runs about 60 hours and an interrupted one is ruined.
The Siemens company invented the process in 1954, and it still accounted for close to 75% of polysilicon output in 2005.