Our Technology

At Hoku Materials, we use a modified Siemens process to produce polysilicon through chemical vapor deposition (CVD).  Hoku’s plant will utilize trichlorosilane gas as a starting material for conversion to polysilicon through CVD.

The modified Siemens process results in more efficient utilization of trichlorosilane (TCS), requires less electricity, and is also more environmentally responsible with a substantial reduction in the pollutants produced through the original Siemens reactor process.  The process includes four distinct steps that are all part of a closed loop vent gas recycling process that uses the by-products of polysilicon production to regenerate TCS - an approach that is both energy and economically efficient.

Hydrochlorination 

The hydrochlorination process recycles the by-product of the polysilicon deposition process, silicon tetrachloride (STC) by combining it with hydrogen gas to produce trichlorosilane (TCS).  The HCl that is a by-product of this process is further recycled to a fluid bed reactor (FBR). The fluid bed reactor is designed to accommodate a reaction between milled metallurgical grade silicon (MG-Si) and anhydrous hydrogen chloride (HCl). This reaction produces trichlorosilane (TCS).

Distillation 

The process removes all impurities from the trichlorosilane, resulting in a high purity feed stock, utilized in the deposition reactors to produce polysilicon.

Polysilicon deposition 

The purified TCS that results from the distillation process is mixed with hydrogen and vaporized into a gas.  The resulting gas is then released into the reactor where heated silicon seed rods are contained inside the bell jar of the reactor.  The silicon in the gas is deposited on the heated rods, which gradually grow until the desired diameter is reached.  Once the rods are cooled, the polysilicon is harvested and broken into chunks.

Vent gas recovery

The process of manufacturing polysilicon generates a gas comprised primarily of hydrogen, chlorosilanes, and hydrogen chloride. The vent gas is recovered using a low temperature absorption method.  The recovery system separates the vent stream into component gases that are readily recycled for reuse.  The modified Siemens reactor recovers and utilizes the vent gas resulting in production of an even higher quality of polysilicon and reduces the need for storage and disposal of by-products.