Energy 2.0

Polycrystalline silicon

Posted on: July 8, 2008

Polycrystalline silicon (or polysilicon, poly-Si, or simply poly in context) is a material consisting of multiple small silicon crystals.

Polycrystalline silicon can be as much as 99.9999999% pure. Silicon is most often companioned with oxygen to form sand. When the oxygen is stripped from the silicon, crude polycrystalline silicon remains. Ultra-pure poly is used in the semiconductor industry, starting from poly rods that are five to eight feet in length.

Semiconductor grade (also solar grade) poly is converted to “single crystal” silicon – meaning that the randomly associated atoms of silicon in “polycrystalline silicon” are converted to large “single” crystals of silicon. Single crystal silicon is used to manufacture 99% of all electronic devices. The devices are used in watches, refrigerators, microwaves, televisions, radios, communications equipment such as cell phones, and controls for cars, ships, aircraft, missiles, and atomic weapons.

In microelectronic industry (semiconductor industry), poly is used both at the macro-scale and micro-scale (component) level.

At the macro scale, polysilicon is used as a raw material entering a process in which single crystals are grown ( Czochralski process, Bridgeman technique).

At the component level, polysilicon has long been used as the conducting gate material in MOSFET and CMOS processing technologies. For these technologies it is deposited using low-pressure chemical-vapour deposition (LPCVD) reactors at high temperatures and is usually heavily N or P-doped.

More recently, intrinsic and doped polysilicon is being used in large-area electronics as the active and/or doped layers in thin-film transistors. Although it can be deposited by LPCVD, plasma-enhanced chemical vapour deposition (PECVD), or solid-phase crystallization (SPC) of amorphous silicon in certain processing regimes, these processes still require relatively high temperatures of at least 300°C. These temperatures make deposition of polysilicon possible for glass substrates but not for plastic substrates. The drive to deposit Polycrystalline silicon or poly-Si on plastic substrates is powered by the desire to be able to manufacture digital displays on flexible screens. Therefore, a relatively new technique called laser crystallization has been devised to crystallize a precursor amorphous silicon (a-Si) material on a plastic substrate without melting or damaging the plastic. Short, high-intensity ultraviolet laser pulses are used to heat the deposited a-Si material to above the melting point of silicon, without melting the entire substrate. The molten silicon will then crystallize as it cools. By precisely controlling the temperature gradients, researchers have been able to grow very large grains, of up to hundreds of micrometers in size in the extreme case, although grain sizes of 10 nanometres to 1 micrometre are also common. In order to create devices on polysilicon over large-areas however, a crystal grain size smaller than the device feature size is needed for homogeneity of the devices.

Another method to produce poly-Si at low temperatures is metal-induced crystallization where an amorphous-Si thin film can be crystallized at temperatures as low as 150C if annealed while in contact of another metal film such as aluminum, gold, or silver

The main advantage of polysilicon over a-Si is that the mobility of the charge carriers can be orders of magnitude larger and the material also shows greater stability under electric field and light-induced stress. This allows more complex, high-speed circuity to be created on the glass substrate along with the a-Si devices, which are still needed for their low-leakage characteristics.

When polysilicon and a-Si devices are used in the same process this is called hybrid processing. A complete polysilicon active layer process is also used in some cases where a small pixel size is required, such as in projection displays.

Polycrystalline silicon (used to produce silicon monocrystals by Czochralski process)

 

A polycrystalline silicon rod made by the Siemens process

Polysilicon is a key component for integrated circuit and central processing unit manufacturers such as AMD and Intel, however it is also a key component of solar panel construction. The photovoltaic solar industry is growing rapidly but is likely going to be very limited in 2006-2008 due to severe shortages and allocations of the polysilicon material. Currently in 2006, 30% of the world’s supply of polysilicon is being used for production of renewable electricity solar power panels.

Major Polysilicon manufacturers include Hemlock Semiconductor, Wacker Chemie, REC, Tokuyama, MEMC, Mitsubishi (Japan and America) and Sumitomo-Titanium, as well as several small sites in China and CIS. The first 7 companies cover over 75% of the worldwide production capacity of polysilicon.

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