HOW IS POLYSILICON PRODUCED?
The process of producing polysilicon begins with quartz or sand, which is refined into metallurgical grade silicon, or MGS. MGS is then purified by various chemical processes. These can be divided into silane-based and trichlorosilane (SiHCl3)-based processes depending on the gas that is used in the process. There are two technologies for producing polysilicon from silicon gases: the Siemens reactor method and the fluidized bed reactor, or FBR method. In the Siemens reactor process, the silane or trichlorosilane gas is introduced into a thermal decomposition furnace (reactor) with high temperature polysilicon rods inside a cooled bell jar. The silicon contained in the gas will deposit on the heated rods, which gradually grow until the desired diameter has been reached. In the FBR process, silane or trichlorosilane gas is introduced into a tube-like reactor in which small polysilicon granules are suspended in the gas stream, referred to as the fluidized bed. The silicon contained in the gas deposits on the surface of the hot granules in the bed until the desired diameter has been reached. The end product is in the form of rods or chunks of polysilicon. The technology in the Siemens reactor was developed in the late 1950’s, is widely implemented, accounting for a majority of the polysilicon production today, and currently produces a higher purity of material.
HOW IS POLYSILICON USED?
Polysilicon is utilized by the integrated circuit industry to make semiconductors for computer chips and other electronic devices.
Polysilicon is utilized by the solar industry in the form of solar cells. Once polysilicon is processed into an ingot form, wafers are sliced and treated adding contacts to create semiconducting solar cells. Solar cells are connected or grouped to form solar modules which are then integrated into photovoltaic systems. Click here to go to Photovoltaic FAQ.
HOW PURE IS POLYSILICON?
The key difference between electronic-grade and solar grade polysilicon is the purity requirement. The purity requirement for solar-grade polysilicon is typically 99.9999%-99.999999% pure, while electronic grade polysilicon tends to be at least 99.9999999% pure.
WHAT IS THE SIEMENS PROCESS?
The Siemens process utilizes trichlorosilane (or silane) gas as a starting material and converts to polysilicon through chemical vapor deposition (CVD).
IN WHAT FORM IS POLYSILICON PURCHASED?
Polysilicon is available in the form of chunks, rods or granules. Additionally, leftover materials including pot scrap and ingot tops are sold.
WHY IS THE PURITY LEVEL IMPORTANT FOR USE IN SOLAR AND THE INTEGRATED CIRCUIT INDUSTRY?
Polysilicon is formed by a crystalline matrix which allows electrons to travel freely. Purity drives the ability of electrons to move freely and therefore affects efficiency in relation to the conversion of photons in the solar process. Purity is especially important within the integrated circuit industry as the ability of electrons to move freely is a key component of semiconductor performance.
WHAT PROCESS WILL HOKU MATERIALS USE TO MAKE POLYSILICON?
Hoku Scientific will utilize the Siemens reactor process to produce polysilicon. The Siemens reactor process has a history of proven reliability and durability.
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