Purification processes which produce solar-grade silicon via the Siemens process result in a higher purity product. However, the purification process itself is very energy intensive and hazardous.
Upgraded metallurgical grade (UMG) silicon is an alternative method of producing solar grade silicon by means of directional solidification. This process exploits the relatively low segregation coefficients of metals to remove impurities and purify the remaining silicon. The metallurgical grade silicon (MGS) used in this process is fabricated in the same manner as the MGS prepared for chemical purification [e.g. using fluidised bed reactor (FBR) and Siemens Process] via carbothermic reactions.
The production of UMG silicon is done using three distinct steps; 1) casting of the MGS in an ingot crucible to create UMG-1, The removal of further metallic impurities to create UMG-2.
The first step involves casting of MGS in an ingot crucible. The bottom of the crucible is lined with silicon seeds, with a purity that is either equal to, or greater than, the purity of solar-grade silicon. A heat source is then applied from the top of the crucible which begins to melt the MGS, such that the seed crystals are the last to melt. The heat source is then removed and the melt starts to cool. As the melt starts to cool, solidification usually begins at the bottom of the crucible forming a liquid-solid interface which moves slowly towards the top. Since metallic impurities are more likely to segregate into molten liquid than solids, these impurities migrate to the top of the ingot and removed as slag. The resulting silicon can be referred to as UMG-1
The second step is used to remove additional metallic impurities, such as phosphorus atoms. Phosphorus has a relatively high segregation coefficient of 0.8, which makes it very hard to remove from the melt. In this process, the UMG-1 is transferred to an electric arc furnace and a remelting process is performed in an argon ambient, to avoid high oxygen concentrations in the melt. Once again, the segregation of metallic impurities and phosphorus into the melt purifies the silicon to contain less than 500 ppm of impurities. The resulting silicon is known as UMG-2.
UMG-2 can then be transferred to a second electric arc furnace to be further purified under plasma. A radio frequency range of 0.1‑4 MHz is used to generate a voltage in an inductive torch to strike and sustain a plasma. The plasma usually contains a combination of chlorine, fluorine, argon, anhydrous hydrochloric acid or hydrofluoric acid. Steam and/or hydrogen gas can also be injected into the gas mixture. The first plasma stage includes an oxidizing treatment of the silicon to remove boron and/or carbon impurities, according to the reactions,
B + OH → BOH↑ or B + O + H → BOH↑(1)
C + O → CO↑ (2)
This plasma treatment effectively removes boron from the surface of the liquid melt. However, the introduction of oxygen (from steam) can be detrimental to the performance of solar cells. Thus, a second deoxygenating plasma treatment in an argon or argon-hydrogen plasma is used to promote the outgassing of oxygen in the form of silicon oxide gas,
Si + O → SiO↑ or SiO2 + Si → 2SiO (3)
The silicon which remains after the plasma treatment is solar grade silicon which can be used to cast multicrystalline or Czochralski UMG ingots.
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