What is LTPS?
LTPS (Low Temperature Poly-silicon) is a Si-based material that consists of numerous crystalline silicon clusters of 0.3 up to several microns. In the semiconductor manufacturing industries, poly-silicon is usually formed by solid phase crystallization (SPC) which involves annealing at a temperature above 900°C. However, since the deformation temperature of glass is only 650°C, the SPC method is not suitable for the flat panel display manufacturing industries. As a result, the LTPS technology is a poly-silicone technology dedicated to applications for flat panel displays.
Illustrations Structures of silicon and mobility schematic
Advantages of AUO LTPS Technology
The process of LTPS is far more complex than a-Si. On the other hand, the carrier mobility in LTPS TFT is 100 times (>100 cm2/V‧s) higher than that in a-Si TFT. In addition, LTPS allows CMOS processes directly on the glass substrate. The following are several aspects that p-Si is superior to a-Si:
- Slim border: Conventional amorphous silicon display structure requires two or three edges on the frame to accommodate the driver IC, which makes slim border difficult. On the other hand, LTPS can directly integrate the drive circuits onto the glass substrate, which facilitates a slim border and high picture quality for the panel. In addition, the integration circuit of LTPS requires fewer number of external signal connections, which reduces the module components by 40% and effectively lowers the cost.
- Compact module: Since part of the drive circuit can be fabricated on the glass substrate, the PCB circuit can be relatively simple and saves more PCB area.
- High aperture ratio and high resolution: High mobility means smaller geometry of transistors are capable of providing sufficient charging power and higher capacitance than conventional amorphous silicon, which translates to a larger effective area transmitted by light. Take the 2-inch QVGA by AUO as an example, since its aperture ratio is as high as 58%, the number of backlight LEDs can be reduced, enabling cost reduction and energy saving, which is suitable to applications for cell phones or mobile devices.
- Vehicle for OLED: Organic Light-Emitting Device (OLED) display has advantages such as fast response time, light weight, low power, and wide viewing angle. Compared to a conventional LCD, the drive method of OLED is a special current drive structure. In addition, the compensation circuit designed to overcome gray scale and achieve panel uniformity requires using two to six TFTs in one pixel. With the high-density layout characteristic of LTPS, an OLED display panel with high brightness and high picture quality can be easier to achieve. Meanwhile, the lifetime of OLED can be extended. High mobility means more current can be provided to the OLED device, which is more suitable to be used as the substrate of active OLED display.