Silicon is one of the essential materials in creating various technologies today. Since quartz makes up silicon, it's used to manufacture semiconductors and other materials. It's necessary to process raw silicon into a usable material to be further processed by silicon manufacturing companies.
The first step is to remove oxygen from quartz because it reacts quickly with silicon, even at room temperature, and is present as silicon dioxide (SiO2). This is carried out in furnaces using carbon at temperatures just above silicon's melting point (1414 °C). The formation of silicon carbide by the reaction of silicon and carbon is stopped by iron.
Since carbon monoxide is gaseous at these temperatures, separating it from molten silicon is simple. The raw silicon is still very polluted, though. Up to 5% contains impurities, like boron, phosphorus, aluminum, and iron. Additional procedures are required to get rid of these substances.
A lot of impurities are filtered out by distillation using a trichlorosilane process. At about 300 °C, the raw silicon and hydrogen chloride HCl react to produce gaseous hydrogen H2 and trichlorosilane SiHCl3.
Higher temperatures are required for the contaminants to transfer in the gaseous state and react with the chlorine. This enables the trichlorosilane to be separated. The only elements that cannot be filtered out using this method are carbon, phosphorus, and boron because they have similar condensation temperatures.
It is possible to reverse the trichlorosilane process and obtain polycrystalline silicon after it has been thoroughly purified. This is accomplished by adding hydrogen inside a quartz chamber that contains thin silicon rods, which is done at a temperature of about 1100 °C.
The silicon reflects on the silicon rods, which develop into bars with a diameter of more than 300 mm. The Czochralski process could already crystallize this polysilicon into a single unit, but the purity level required for semiconductor production still needs to be increased.
Using a cleaning process, the purity is once more increased. Due to the higher solubility of contaminants in the liquid phase and the surface tension of silicon, a high-frequency coil is used to melt the silicon bars, which causes contaminations to collect at the bottom.
This process can be repeated numerous times to reduce silicon's impurity content further, allowing it to be used to create single crystals. All functions are carried out in a vacuum environment to prevent further contamination.
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