Refining Silicon

Silicon dioxide (SiO2) is the most abundant mineral in the earth's crust.  The manufacture of the hyperpure silicon for photovoltaics occurs in two stages. The oxygen is removed to produce metallurgical grade silicon. It is further refined to produce semiconductor grade silicon. An intermediate grade with impurity levels between metallurgical silicon and semiconductor grade silicon is often termed solar grade silicon.

Metallurgical Grade Silicon

Silica is the dioxide form of silicon (SiO2) and occurs naturally in the form of quartz. While beach sand is also largely quartz, the most common raw material for electronic grade is high purity quartz rock.  Ideally the silica has low concentrations of iron, aluminum and other metals. The silica is reduced (oxygen removed) through a reaction with carbon in the form of coal, charcoal and heating to 1500-2000 °C in an electrode arc furnace.

SiO2 + C → Si + CO2

The resulting silicon is metallurgical grade silicon (MG-Si). It is 98% pure and is used extensively in the metallurgical industry.

An even greater production of silicon is in the form of ferrosilicon that is manufactured using a similar process to that described above but is done in the presence of iron. Ferrosilicon is used extensively in metals manufacturing. In 2013 the total production of silicon was 7.6 million tonnes and 80 % of that was in the form of ferrosilicon.1

percentange of metallurgical grade silicon for solar production

Of the 1.8 million tonnes of metallurgical silicon produced in 2010, 12 % was for the production of silicon solar cells.2

Metallurgical (MG) silicon is produced at the rate of millions of tons/year at a low economic cost of few $/kg and an energy cost of 14–16 kWh/kg. As such, it is 98–99% pure, with a major contamination of carbon, alkali-earth and transition metals, and hundreds of ppmw of B and P.3

The transition metals in the silicon result in deep levels in the bandgap and the high recombination activity make metallurgical grade silicon unsuitable for use in electronics. In addition, boron and phosphorus dopant impurities are much too high in concentration (>50–100 ppmw) to allow suitable compensation procedures 4

The impurity levels in metallurgical grade silicon5 vary widely due to process variation and the source raw materials of silica and carbon

Electronic Grade Silicon

A small amount of the metallurgical grade silicon is further refined for the semiconductor industry. Powdered MG-Si is reacted with anhydrous HCl at 300 °C in a fluidized bed reactor to form SiHCl3

Si + 3HCl → SiHCl3 + H2

During this reaction impurities such as Fe, Al, and B react to form their halides (e.g. FeCl3, AlCl3, and
BCl3). The SiHCl3 has a low boiling point of 31.8 °C and distillation is used to purify the SiHCl3 from the
impurity halides. The resulting SiHCl3 now has electrically active impurities(such as Al, P, B, Fe, Cu or Au) of less than 1 ppba.

Finally, the pure SiHCl3 is reacted with hydrogen at 1100°C for ~200 – 300 hours to produce a very pure form of silicon.

SiHCl3 + H2 →Si + 3 HCl

The reaction takes place inside large vacuum chambers and the silicon is deposited onto thin polysilicon rods (small grain size silicon) to produce high-purity polysilicon rods of diameter 150-200mm. The process was first developed by Siemens in the 60's and is often referred to as the Siemens process.

The resulting rods of semiconductor grade silicon are broken up to form the feedstock for the crystallisation process. The production of semiconductor grade silicon requires a lot of energy. Solar cells can tolerate higher levels of impurity than integrated circuit fabrication and there are proposals for alternative processes to create a "solar-grade" silicon.