(254d) The Impurities Removal In Polysilicon

Polysilicon, as a base material, is widely used in the electronic information and photovoltaic industry. The Japan earthquake, happened on March 11, and the subsequent nuclear leakage caused a rise in polysilicon price. An American market research organization reported that about 25% of the global silicon was suspended due to the earthquake. At the same time, nuclear leakage made it understandable that solar energy has found favor in clean energy. Our work aims at the purification of polysilicon in Modified Simens Process and the methods adopted to remove boron and phosphorus impurities affect the product quality most, which are important problems hamper the industry’s development.

At present, Modified Simens Process which accounts for about 85% of the total output is the best way to produce polysilicon. The feedstock trichlorosilane(TCS) used here requires that the purity is above 99.999%, the content of boron and phosphorus elements is no more than 10ppb, polymers 10ppm, total metal elements is less than 10ppb. It is of great importance to control the content of impurities. Here 26 metallic impurities, 5 non-metallic impurities, polysilanes, organosilane compounds et al. were summarized.

New ways to removal boron and phosphorus impurities contain: 1) Reactive distillation; 2) Distillation – Absorption; 3) Wet nitrogen bubble method; 4) Adsorptive distillation. Our work is to exploit new purification technology, integrate the existing technological process and improve purification efficiency. Energy saving and environmental protection are also discussed. Reactive distillation and adsorptive distillation are two methods we use.

The key thing in reactive distillation is the choice of reactant. Reactant suitable for industrial application should contain these features: 1) It will react with boron or phosphorus compounds to form stable complex, quickly and efficiently. 2) The boiling point of it and its complex must be higher than the components of chlorosilanes, only in this way will the excess reactant and the complex formed be removed easily. 3) It will not react with the component of chlorosilanes, unless there is a special need. 4) It never reacts with components of the reactor, vessel, column and their accessory devices to introduce new impurities into the chlorosilane system. 5) The price and amount should be suitable for large-scale production. Some kinds of reactants are recommended.

Accordingly, choosing the adsorbents in adsorptive distillation, we should regard physical and chemical properties of the components and impurities in the system, e.g. boiling point, polarity. The adsorbent should be of the following characters. 1) It should have bigger adsorption amounts and adsorb impurities effectively at appropriate temperature and pressure. 2) It will not release impurities into the system.3) It has a uniform particle size and large surface area. 4) Regeneration is easy. Activated charcoal, silica gel and some special adsorbents are good choices.

We also proposed two technological processes using the two methods mentioned above to reduce the boron and phosphorus in polysilicon. The simulation result is good.