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Posted on 14 November 2019

Silicon Production

raw silicon

 

 

 

 

 

 

 

Depending on its purity, silicon can be used for many different applications in mettallurgie, photovoltaics, and in the production of semiconductors. It is therefore logical to classify silicon according to the amount of impurities contained in the silicon. Some of the most common types include raw silicon (98.99% purity), solar silicon (99.99% purity), and semiconductor silicon (impurities < 1 ppb). At the industrial level, basic silicon can be produced through the reduction of silicon dioxide using carbon in an electric arc furnace set at 2000°C. The chemical equation for this process is:

SiO2 + 2C ---> Si + 2 CO

This artificial raw silicon can be produced in the amount of millions of ton and is pure enough to be used in metallurgy, for example, as an alloying component for tinplate, steel (for corrosion prevention), and aluminium.

Production of Polycrystalline High-Purity Silicon

For application in the production of photovoltaics, raw silicon requires more purification. In order to do this, the Siemens procedure is applied whereby gaseous hydrogen chloride is combined with the raw silicon at 1100°C to form  trichlorosilane, given by the following chemical equation:

Si + 3 HCl ---> H2 + HSiCl3

After a long distillation cleaning process and applying the reversing the equation above, the trichlorosilane decomposes at high temperature and exposure to hydrogen into purified silicon rods. Elemental silicon then grows on the thin silicon rods. Polycristalline silicon produced in this way has a purity of over 99.99% and is used for the production of solar cells. The silicon rod can also be doped during this process by introducing a doping agent in gas form.

Figure 1. Polycrystalline Silicon

Polycrystalline, course grained pure silicon is acquired by pouring the melted silicon into a cast and cooling very slowly. The silicon is then cut into discs which are used, for example, in the production of solar cells.

Production of Pure Single-Crystal Silicon by the Czochralski Process (CZ-Silicon)

Monocrystalline silicon is needed in semiconductor elements since, unlike polycrystalline silicon, it contains no crystal borders. In order to obtain the purest monocrystalline semiconductor silicon, the silicon that is produced using the Siemens process is melted in quartz crucibles. A monocrystaline seed crystal is dipped into the hot melt, then drawn out again slowly while rotating so that the monocrystalline silicon grows on the seed crystal and the impurities remain in the hot melt. This method is refered to as the Czochralski process after the Polish chemist who developed it. Due to contact of the liquid silicon with the quartz crucibles and the use of graphite crucibles, CZ-silicon contains a relatively high amount of oxygen and carbon.

Production of Pure Single-Crystal Silicon by the Float Zone Process (FZ-Silicon)

An alternative to CZ-Silicon is float zone silicon. In the the float zone process, a molten region, or melt zone, is created by electric induction heating and is passed along the silicon rod. Impurities tend to stay in the molten region so that the resolidified silicon is left with fewer impurities.

The resolidified silicon is monocrystaline. Purification can be increased by performing multiple passes of the melt zone. Some impurities, however, are more difficult to remove than others. This is due to the different solubilities of the impurities in the silicon.  Boron, for example, is more difficult to remove than Phosphorus, so the resulting silicon produced by the float zone process tends to be p-type. If n-type silicon is required, this effect must be compensated by the addition of Phosphorus.

Comparison Between CZ-Silicon And FZ-Silicon

CZ-Silicon

Most semiconductor construction elements are produced using the CZ-Silicon. Large silicon crystals can be produced using this method. Since they contain a high amount of carbon and oxygen due to the crucibles used in the process, the maximum specific electrical resistance of this silicon is generally less than 50 Ωcm. Therefore, these crystals are seldom used in the production of conductors. They are mainly used as substrates for epiwafers. An epiwafer is a wafer of semiconducting material made by the growth of  crystals of one mineral on the crystal face of another mineral, such that the crystalline substrates of both minerals have the same structural orientation, also refered to as epitaxial growth.

FZ-Silicon

FZ-silicon is pricier than CZ-Silicon and is of higher quality than the CZ-Silicon since it does not come into contact with any crucibles during production. A speciific resistance greater than 1000 Ωcm can be reached. FZ-silicon is mainly used for power components in which the entire volume of the wafer is required. A maximum bar diameter of 20 cm is currently available.

 

For more information, please read:

Basic Principles of Electricity and Physics of Semiconductors

What is a Semiconductor?

Semiconductor Doping

 

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One Response

  1. avatar Altangerel,Mongolia says:

    Hi. I need same of the silicon,that can be used for the animal in to  the rumemant to keep a few mounthses. Do you have the same one.

    Best regard's D.Altangerel 

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