Refining Silicon

Source: Semantic Scholar

Metallurgical Grade Silicon

Metallurgical grade silicon (MG-Si) is produced by removing oxygen from silica through a reaction with carbon in an electrode arc furnace. This process results in silicon that is 98% pure and is widely used in the metallurgical industry. Another form of silicon produced in large quantities is ferrosilicon, which is made in the presence of iron. Ferrosilicon finds extensive use in metals manufacturing, with 80% of silicon production in 2013 being in the form of ferrosilicon.

Metallurgical silicon is produced at a large scale and at a low cost. It is not suitable for electronic applications due to high levels of contamination with carbon, alkali-earth and transition metals, as well as boron and phosphorus impurities.

Electronic Grade Silicon

A portion of metallurgical grade silicon is further refined to produce electronic grade silicon for the semiconductor industry. The refinement process involves reacting powdered MG-Si with anhydrous HCl to form SiHCl3. This compound is then purified through distillation to remove impurities, resulting in a material with electrically active impurities of less than 1 ppba.

The purified SiHCl3 is then reacted with hydrogen at high temperatures to produce a very pure form of silicon. This reaction takes place in vacuum chambers, and the silicon is deposited onto thin polysilicon rods to create high-purity polysilicon rods.

These rods are broken up to serve as feedstock for the crystallization process, which is energy-intensive. While semiconductor grade silicon is crucial for electronics, solar cells can tolerate higher impurity levels, leading to the development of processes for producing “solar-grade” silicon.

Source: MDPI

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