Excimer Laser Annealed Poly-Si TFT Technologies

ABSTRACTThis paper describes the excimer laser annealed (ELA) poly-Si TFT technologies in terms of excimer laser annealing of Si films, the leakage current, and the TFT stability. A laser energy density and a shot dependencies of TFT characteristics was analyzed by TEM, SEM, and Raman. The mobility increases with increasing not only the energy density but also the shot density. The mobility increase with the energy density is due to the grain size enlargement. On the other hand, the mobility increase up to 10 to 20 shots is due to a decrease of defects, including small grains, grain boundaries and defects inside grains. The contribution of grain-growth is small. The ELA TFT has a micro-offset structure to reduce the leakage current. Moreover, we have proposed a dynamic leakage current reduction structure. The combination of these technologies provides a sufficiently small leakage current for AMLCDs. The stability of the gate insulator was analyzed. The TFT shows negative threshold voltage shift under gate bias stress. This is due to water penetration and the subsequent field activated chemical reaction in the gate insulator. The dissociation of Si-OH bonds with hydrogen-bonded water was a fundamental contributor. The shift was suppressed sufficiently by hydrogen passivation. Obtained ELA TFTs;s have mobilities of over 100 cm2/Vsec, threshold voltages of less than 3 V, effective leakage currents of less than 10−13 A, and are stable more than 10 years.