From Quartz Sand to Electronic Grade Silicon

Silicon in the Universe and on Earth

Hydrogen and helium dominate the visible matter of the universe, the mass  fraction of silicon is less than 0.1 %. This element mainly arises from the fusion of two oxygen nuclei (2 ^₁₆O -> ^₂₈Si + 4 He) at temperatures > 10_⁹ K inside stars with more than eight solar masses.
The entire planet Earth contains approx. 17 % silicon, the third most abundant
element after iron and oxygen, closely followed by magnesium.
In the earth’s iron-based core, silicon (» 7 mass %) is the second most abundant element.
The earth’s approx. 40 km thick crust contains silicon (in the form of silicates and SiO^₂) with a mass fraction of 28 % as the second most abundant element after oxygen.
Quartz = crystalline SiO^₂ is the raw material for silicon production.

Metallurgical-Grade Silicon Production

Quartz sand is reduced with carbon in an electric arc furnace at temperatures > 1900°C to metallurgical grade silicon (> 98 % pure). The major part of the world production (2008: approx. 6 million tons) is used for manufacturing alloys with aluminium and steel, and as raw material for polysiloxane production.
In 2010, Si wafer production consumed approx. 200 kilotons purified silicon. Approx. 90 % of this volume was used for mono- and polycrystalline solar cells, the remaining 10 % (corresponding to a wafer area of > 5 km^₂) went into
the semiconductor industry.

Purification of Silicon

The impurity concentration in metallurgical-grade silicon is many orders of magnitude too high for an application in photovoltaics and microelectronics, thus the silicon has to be purified.
For this reason, silicon intended for wafer production is converted into trichlorosilane gas (HSiCl_³) at » 300°C using hydrochloric acid via Si + 3 HCl -> HSiCl_³ + H_² which already removes many impurities which don’t form volatile chlorine compounds at the applied process temperature.
Trichlorosilane (boiling point 32°C) mixed with other gaseous chlorine compounds undergoes multiple distillation thereby improving the purity up to 99,9999999 %. („9N“) and is subsequently thermally decomposed to polycrystalline silicon.

The polycrystalline silicon formation is performed in the so-called Siemens-Process (fig. right): The purified trichlorosilane mixed in hydrogen is thermally decomposed on the surface of a heated (approx. 1100°C) silicon rod via
HSiCl_³ + H_² -> Si + 3 HCl to polycrystalline silicon and HCl, which corresponds
to the reverse reaction of the trichlorosilane formation. This “electronic-grade“ (purity concentration < 10_¹⁴ cm_^-3) polysilicon is the raw material for
silicon single crystals which are grown by two different processes as described in the following sections.