Abstract
Poly-Si crystallization mechanism is examined by conducting numerical simulations, combining the thermal diffusion equation with a rigorous coupled wave analysis method. The ripples at the boundary of poly-Si grains are modeled as a grating surface structure. Under laser beam irradiation, the melting front profiles are accurately analyzed by including surface diffraction, polarization of the laser, and laser energy density. For two different lasers, XeCl excimer laser (λ = 308 nm) and Yb:YAG solid state laser (λ= 343 nm), the energy density range at which poly-Si grains are gradually ordered was determined. Furthermore, the energy density window of the Yb:YAG laser is found to be four times larger than that of XeCl laser. On the other hand, the Yb:YAG laser may produce amorphous-Si phase after completing the crystallization process. It is suggested that this amorphous-Si phase could be avoided, if a double pulse laser is used.
Highlights
In low temperature poly silicon (LTPS) process, high mobility can be obtained by forming large grain, allowing low power thin film transistor (TFT) devices to be achieved.[2,3,4,5]
Most LTPS process are conducted with 308 nm XeCl excimer laser that form uniform grains of about 300 nm.[5]
After crystallization, ripples are formed at the positions where poly-Si grains meet that are known to disturb the electron or hole carriers flow through the TFT channel.[2,3,4]
Summary
Laser annealing is widely used in Si based device as an activation process for ultrashallow junction and for display or solar cell device as a low temperature poly silicon (LTPS) process, converting amorphous-Si (a-Si) to poly-Si.[1,2,3,4,5] In LTPS process, high mobility can be obtained by forming large grain, allowing low power thin film transistor (TFT) devices to be achieved.[2,3,4,5] Currently, most LTPS process are conducted with 308 nm XeCl excimer laser that form uniform grains of about 300 nm.[5]. Subsequent laser pulse during the LTPS process are scattered by the ripples on the surface which produces an optical interference between the superposition of incident wave and the reflected electromagnetic waves. After multiple laser irradiations on the Si surface with irregular ripples, the structures with maximum positive feedback will survive. This grating modes competition results in the growth of the regular structures with period of Eq (2).[6,7,8] in previous reports,[6,7,8] authors considered only electromagnetic field perturbation caused by surface roughness. If a laser pulse duration is too short, a-Si may form due to the high cooling rate during the crystallization process. We propose that this problem could be mitigated by using double laser pulses
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