Thin film transistors (TFTs) are used in active matrix displays and other electronic devices. A TFT includes an active channel of semiconductor material, with a source and a drain electrode connected to the active channel. An insulated gate is then used to control the current flowing through the active channel by applying an electric field.
Most TFTs are made with amorphous silicon thin film as the material for the active channel. Polycrystalline material, including low temperature polycrystalline silicon (LTPS), is also used for high performance TFTs (i.e., TFTs having high mobility, steep subthreshold swing, and large on/off ratio). High performance TFTs are needed for active matrix organic light emitting diodes (AMOLEDs) as well as for high resolution liquid crystal displays (LCDs). High performance TFTs are also needed for system-on-glass applications.
There are three major LTPS technologies for producing LTPS: (1) solid phase crystallization (SPC), (2) excimer laser crystallization (ELA) or flash lamp annealing, and (3) metal induced crystallization (MIC) and its related variations. SPC is the simplest and the most direct approach to obtain polycrystalline silicon (poly-Si) film with low cost, but it takes a relatively long time and SPC TFTs have relatively poor performance. ELA can be used to produce relatively high performance TFTs, but is very expensive. MIC gives relatively moderate performance TFTs, but there are contamination issues associated with the MIC process. None of these methods can meet the requirements of high performance and low cost.
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