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PolySilicon
PolySilicon
Polycrystalline silicon, also called polysilicon or poly-Si, is a high purity, polycrystalline form of silicon, used as a raw material by the solar photovoltaic and electronics industry.
Polysilicon is produced from metallurgical grade silicon by a chemical purification process, called the Siemens process.
This process involves distillation of volatile silicon compounds, and their decomposition into silicon at high temperatures. An emerging, alternative process of refinement uses a fluidized bed reactor.
The photovoltaic industry also produces upgraded metallurgical-grade silicon (UMG-Si), using metallurgical instead of chemical purification processes. When produced for the electronics industry, polysilicon contains impurity levels of less than one part per billion (ppb), while polycrystalline solar grade silicon (SoG-Si) is generally less pure.
MG-Si
All silicon metal shares a common origin: Quartz (SiO2). Quartz is reduced to silicon (Si) in a high temperature melting process using carbon and high temperatures in an electric arc furnace.
For silicon metal to be used in solar and semiconductor applications it must be extremely pure. For solar applications, the metal must be 99.9999% Si, while semiconductor applications require 99.999999999% purity. No quartz product can be purified above 99.999% SiO2, which means silicon metal must go through successive purification processes before it can be used.
The most common method of silicon metal production is through a metallurgical process. This process begins with a lump of quartz of medium to low purity mixed with carbon, which then goes through several cleaning steps to be processed into higher grades. This silicon metal can then be used for silicon crystals, which are sliced and used as wafers for the semiconductor and solar industry. Silicon metal processing often involves slag refining, which is made possible using high purity quartz.