Aligned polycrystalline silicon array thin film by XeCI excimer laser annealing for AMOLED displays

Low temperature polycrystalline silicon (LTPS) thin film transistors (TFTs) are demanded to fabricate high performance liquid crystal displays (LCD) and organic light-emitting diode displays (OLED). The mobility of poly-Si TFT can be two orders of magnitude higher than that of amorphous Si (a-Si) TFT. Excimer laser annealing has been studied to be the most promising technology to meet the stringent requirement in high speed operation. The process parameters were identified as a-Si thickness, laser energy density, overlap ratio, annealing atmosphere and pre-clean condition. The a-Si layer of 40-50 nm was deposited by plasma enhanced chemical vapor deposition (PECVD). The XeCl excimer laser was irradiated on the a-Si film at room temperature under N₂ or N ₂/O₂ environment. The energy density ranged 250-400 mJ/cm², and the overlap ratio was 95-99%. The highly aligned poly-Si array thin film could be obtained. The grain size has been about 0.31×0.33 μm², and the regular arrangement in poly-Si grains was discussed. In addition, the PMOS TFT has been fabricated from the aligned poly-Si array. The mobility was as high as 100 cm²/Vs and the sub-threshold swing was around 0.24 V/dec. The threshold voltage was -1.25 V and the on/off current ratio was about 10⁶.