Polycrystalline silicon

Polycrystalline silicon or polysilicon has long been used as the conducting gate material in MOSFET and CMOS processing technologies. For these technologies it is deposited using 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 and the active and/or doped layers in thin-film transistors. Although it can be deposited by low-pressure chemical-vapour deposition (LPCVD), plasma-enhanced chemical vapour deposition (PECVD), or solid-phase crystallinzation (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. Instead, a relatively new technique called laser crystallization can be used to crystallize a precursor amorphous silicon (a-Si) material. 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 microns in size. 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.

The main advantage of polysilicon over a-Si is that the mobility 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.

External links:

FlexICs, a company which produces polysilicon on low-temperature, flexible substrates

See also: nanocrystalline silicon