Explain the processes and write equations for production of

Explain the processes and write equations for production of electronic grade silicon from metallurgical grade silicon.

Solution

Electronic grade silicon is one of the purest form of silicon.It is approximately 99.99999999% purest form of silicon. It is purifies from Metallurgical grade silicon which contains iron and aluminium as impurities. EGS(Electronic grade silicon) can not produced directly .EGS can be produced after Metallurgical Grade Silicon. So to produce EGS we will produce first MGS . A number of methods have beenused to produce high-purity 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 metallugical grade silicon (MG-Si). It is 98% pure and is used extensively in the metallurgical industry.

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 circuite fabrication and there are proposals for alternative processes to creat a \"solar-grade\" silicon.