below we will discuss how monocrystalline and multicrystalline silicon wafers can be produced. In the Figure we illustrate the production process of onocrystalline silicon wafers.
The lowest quality of silicon is the so-called metallurgical silicon, which is made from quartzite. Quartzite is a rock consisting of almost pure silicon dioxide (SiO2). For producing silicon the quartzite is molten in a submerged-electrode arc furnace by heating it up to around 1900°C, as illustrated in above Figure Then, the molten quartzite is mixed with carbon. As a carbon source, a mixture of coal, coke and wood chips is used. The carbon then starts reacting with the SiO2. Since the reactions are rather complex, we will not discuss them in detail here. The overall reaction how ever can be written as
As a result, carbon monoxide (CO) is formed, which will leave the furnace in the gas phase. In this way, the quartzite is purified from the silicon. After the reactions are finished, the molten silicon that was created during the process is drawn off the furnace and solidified. The purity of metallurgic silicon, shown as a powder in above Figure, is around 98% to 99%.
About 70% of the worldwide produced metallurgical silicon is used in the aluminium casting industry for make aluminium silicon alloys, which are used in automotive engine blocks. Around 30% are being used for make a variety of chemical products like silicones. Only around 1% of metallurgical silicon is used as a raw product for making electronic grade silicon.
The silicon material with the next higher level of purity is called polysilicon. It is made from a powder of metallurgical silicon in the Siemens process. In the process, the metallurgical silicon is brought into a reactor and exposed hydrogen chloride (HCl) at elevated temperatures in presence of a catalyst. The silicon reacts with the hydrogen chloride,
leading to the creation of trichlorosilane (HSiCl3). This is a molecule that contains one silicon atom, three chlorine atoms and one hydrogen atoms. Then, the trichlorosilane gas is cooled and liquified. Using distillation, impurities with boiling points higher or lower than HSiCl3 are removed. The purified trichlorosilane is evaporated again in another reactor and mixed with hydrogen gas. There, the trichlorosilane is decomposed at hot rods of highly purified Si, which are at a high temperature in between around 850°C and 1050°C. The Si atoms are deposited on the rod whereas the chlorine and hydrogen atoms are desorbed from the rod surface back in to the gas phase. As a result a pure silicon material is grown. This method of depositing silicon on the rod is one example of chemical vapour deposition (CVD). As the exhaust gas still contains chlorosilanes and hydrogen, these gasses are recycled and used again: Chlorosilane is liquified, distilled and reused. The hydrogen is cleaned and thereafter recycled back in to the reactor. The Siemens process consumes a lot of energy in production of silicon wafers.