Low-temperature polycrystalline silicon (LTPS) is polycrystalline silicon that has been synthesized at relatively low temperatures (~650 °C and lower) compared to in traditional methods (above 900 °C). LTPS is important for display industries, since the use of large glass panels prohibits exposure to deformative high temperatures. More specifically, the use of polycrystalline silicon in thin-film transistors (LTPS-TFT) has high potential for large-scale production of electronic devices like flat panel LCD displays or image sensors.
Polycrystalline silicon (p-Si) is a pure and conductive form of the element composed of many crystallites, or grains of highly ordered crystal lattice. In 1984, studies showed that amorphous silicon (a-Si) is an excellent precursor for forming p-Si films with stable structures and low surface roughness. Silicon film is synthesized by low-pressure chemical vapor deposition (LPCVD) to minimize surface roughness. First, amorphous silicon is deposited at 560–640 °C. Then it is thermally annealed (recrystallized) at 950–1000 °C. Starting with the amorphous film, rather than directly depositing crystals, produces a product with a superior structure and a desired smoothness. In 1988, researchers discovered that further lowering temperature during annealing, together with advanced plasma-enhanced chemical vapor deposition (PECVD),could facilitate even higher degrees of conductivity. These techniques have profoundly impacted the microelectronics, photovoltaic, and display enhancement industries.
LTPS is needed for:
1. Circuits on glass – integrated drivers and scanners and multiplexer – reduces use of external IC and connectors to panel
2. Much smaller TFT ⇒ larger aperture ratio – mobile applications
3.Much more stable than a-Si under high current loading (OLED needs current to drive)
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