Morphological Characterization of Polysilicon Films Laser-Annealed in Argon Ambient

摘要

Laser crystallization of amorphous silicon is one of the most interesting ways to obtain good quality polysilicon films in order to fabricate Thin Films Transistors (TFT) on glass substrates. This technique induces large grains but low uniformity and high surface roughness are also mentioned. Furthermore, the choice of annealing ambient can significantly improve the grain size since the presence of gas in the annealing chamber plays a part on the melt duration. The surface roughness of silicon layers crystallized in argon ambient using the Very large Excimer Laser (VEL), built at SOPRA, is studied. The structures of the resulting polysilicon films were investigated by Atomic Force Microscopy (AFM). Using this technique, surface roughness is found to increase with laser fluence until reaching complete melting of the film. We show that, when using only one shot, high surface roughness occurs in a very narrow window, corresponding to the well-known super lateral growth phenomenon. Above this peak fluence, the surface roughness drops. This narrow range of laser fluence constitutes a drawback for the laser crystallization. Indeed, a small variation around this energy density results in a lack of uniformity of the grain size. We have found that to improve this uniformity, using five shots induces complete melting of the film to occur in a larger fluence regime. Using this crystallization technique, TFTs are fabricated with a low temperature process (<600°C). They exhibit good electrical properties such as a field effect mobility around 180cm~2/V.s with high reproducibility. Moreover, field effect mobility and surface roughness seem to have the same behavior.