Single shot excimer laser crystallization and LPCVD silicon TFTs

One of the most interesting ways to develop good quality polysilicon films in order to fabricate thin film transistors (TFTs) on low cost glass substrates is by laser crystallization of amorphous silicon (a-Si). Crystallization technique using pulsed-ECL is characterized by films with high electrical properties but low uniformity. In this way, technology using single shot ECL with very large excimer laser (VEL) may be very promising. It is used here to crystallize undoped amorphous silicon films deposited by LPCVD which constitute the active layer of n-type thin film transistors. The performances of these TFTs are studied versus the laser fluences and the number of shots at fixed fluence. An optimum fluence is highlighted from the behaviours of the subthreshold slope S and the threshold voltage V-T Of these TFTs. The optimum occurs at lower fluence when the number of shots increases. The behaviour of the field effect mobility shows a maximum. This maximum occurs at higher fluence than that of the S and V-T optimum. Then at low fluence, the effect of the shot number is more important on S and V-T parameters, which are more linked to the insulator-silicon interface, than on the mobility. The difference is explained from the energy actually absorbed at the film surface during the laser annealing. (C) 1999 Elsevier Science S.A. All rights reserved.