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HOME -> TECHNOLOGY -> TFT-LCD INTRODUCTION -> LTPS



Polysilicon is a Silicon-based material, which contains numerous Si grains with sizes ranging from 0.1 to several um. In semiconductor manufacturing, polysilicon is usually prepared by LPCVD (Low Pressure Chemical Vapor Deposition) and then annealed above 900°C, i.e. so called SPC (Solid Phase Crystallization) method. Obviously, the same way could not be applied to FPD industry since the strain temperature of glass is only about 650°C. Therefore, Low Temperature Polysilicon (LTPS) technology is the novel technology specific for FPD application.



Currently there are several approaches in the preparation of LTPS film on glass or plastic substrate:

Metal Induced Crystallization (MIC): It is a kind of SPC method but, compared to conventional SPC, it can achieve polysilicon under lower temperature (about 500°C - 600°C). This is because a thin metal layer is coated before crystallization anneal. The metal element is the key in lowering the activation energy of crystallization.

Cat-CVD: This method directly deposits poly-film without further thermal anneal. The deposition temperature has reportedly gone as low as 300°C. The growth mechanism involves the catalytic cracking reaction of SiH4-H2 mixture.

Laser anneal: This is the most popular method used currently. Excimer laser is used as an energy source to heat and melt a-Si with low Hydrogen content. It is later recrystallized as poly-film.


Why use LTPS?

The preparation of LTPS film is apparently more complicated than a-Si. However, LTPS TFT has 100 times higher mobility than a-Si TFT and can carry out CMOS process on the glass substrate. Some significant advantages for p-Si over a-Si are shown below:

Capability for integrating driving circuit on glass substrate means slimmer peripheral dimension and cheaper cost.

Higher aperture ratio: Higher mobility means that we can achieve the pixel charging time by smaller-size TFT, so that contribute more pixel area for light transmission.

Vehicle for OLED: Higher mobility means ample current supply required by OLED device driving.

Module's compactness: Less PCB area required due to driver integrated on Glass

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