Silicon dioxide (SiO2) is the most abundant mineral in the earth's crust. The manufacture of the hyperpure silicon for photovoltaics starts with locating a source of silicon dioxide in the form of silica or sand. Ideally the silica has low concetrations of iron, aluminium and other metals. The silica is reduced (oxygen removed) through a reation with carbon in the form of coal, carcoal and heating to 1500-2000 °C in an electrode arc furnace.
The resulting silicon is metallugical grade silicon (MG-Si). It is 98% pure and is used extensively in the metallugical industry.
Metallurgical silicon production and typical impurity levels. The fraction used for semiconductors is very small and the fraction used for photovoltaics is a small fraction of that.
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 fluidised bed reactor to form SiHCl3
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.
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 circuite fabrication and there are proposals for alternative processes to creat a "solar-grade" silicon.