The journey of silicon starts with mining and refining quartzite, a rock containing quartz. The refining process involves mixing powdered quartzite with carbon and heating it in arc furnaces, resulting in metallurgical-grade silicon with a purity of up to 99%. This silicon is used for industrial purposes and metal alloys.
To achieve the necessary purity for semiconductor applications, further purification is required. Metallurgical-grade silicon powder is reacted with hot gaseous hydrochloric acid, producing trichlorosilane gas. Trichlorosilane is purified by fractional distillation.
Polycrystalline silicon (polysilicon) is then produced using the Siemens process. Trichlorosilane gas is used in a chemical vapor deposition chamber where high-purity silicon filaments are heated along with gaseous trichlorosilane and hydrogen. The gas decomposes, leaving silicon that accumulates on rods. Polysilicon produced through this process can have a purity level of 99.99999% ("seven nines" or 7N) or higher, and it is used in photovoltaic cells and semiconductor devices.
Alternatively, the Fluidized Bed Reactor (FBR) method can be employed, using either trichlorosilane or monosilane as feedstock. In an FBR reactor, powdered silicon is mixed with silane gas, and heated hydrogen gas is injected to maintain fluidization. This process leads to the accumulation of polycrystalline silicon beads. FBR offers power savings and continuous production but faces challenges in managing fluid dynamics.