Five stages in Polysilicon processing
Polysilicon is a critical material in the production of solar cells, integrated circuits, and other electronic devices. The production of high-purity polysilicon requires several stages of purification to remove impurities and achieve the desired level of purity. In this article, we will discuss the five stages of polysilicon purification, including the process description and results.
- Silicon Purification
The first stage of polysilicon purification is silicon purification. Metallurgical Grade Silicon (MeG-Si) is obtained by carbothermic reduction of silica in submerged electric arc furnaces. The boron content in MeG-Si is typically in the range of 10-40 ppmw, phosphorus 10-50 ppmw, and metals (Al, Ca, Fe, Ti, etc.) 1500-2000 ppmw. MeG-Si is not pure enough for use in solar cells, and therefore it needs to be purified further.
The second stage is hydrogenation. In this stage, the MeG-Si is crushed and then mixed with hydrogen gas at high temperature and pressure. This process removes the impurities, including boron and phosphorus, by forming gaseous compounds that can be removed from the silicon. The resulting product is called Polysilicon Feedstock (PF).
- Trichlorosilane (TCS) Production
In the third stage, the PF is converted into Trichlorosilane (TCS) through a chemical reaction with hydrogen chloride (HCl) gas. The TCS is a liquid that can be easily purified by fractional distillation to remove impurities. The purified TCS is then used as a feedstock for the next stage.
- Deposition and Purification
In the fourth stage, the TCS is vaporized and then decomposed into silicon and hydrogen gas in a reactor. The silicon is deposited onto a rod, which is rotated slowly to ensure even deposition. The impurities are left behind in the gas phase and are removed through a series of purification steps, including fractional distillation and gas chromatography. This stage is repeated multiple times to achieve the desired level of purity.
- Rod Production
The final stage is the production of polysilicon rods. The purified silicon is melted and then cast into a cylindrical shape. The rods are then sliced into wafers and used in the production of solar cells and other electronic devices.
The five-stage polysilicon purification process is highly effective in removing impurities and achieving high purity levels. The resulting polysilicon has a purity level of 99.9999999%, which is essential for the production of high-quality solar cells and other electronic devices.
In conclusion, the five stages of polysilicon purification, including silicon purification, hydrogenation, TCS production, deposition and purification, and rod production, are critical in achieving high-purity polysilicon for use in the photovoltaic industry and other electronic devices. The development of these purification procedures has enabled the production of cheaper and less polluting polysilicon, making solar energy more accessible and affordable for all.