| Summary: | 博士 === 國立交通大學 === 材料科學與工程系 === 90 === In this study, the crystallization of a-Si with semi-Gaussian excimer laser was investigated. After the single-shot excimer laser process, the poly-Si region showed grains with a wide range of sizes corresponding to the Gaussian distributed laser energy. From the view of laser energy, three crystallization regimes were found on the ELA a-Si films: (1) partial-melting, (2) near-complete-melting and (3) complete-melting regimes. Large super-lateral-grain-growth (SLG) grains were observed in the near-complete-melting regime. The grain size of poly-Si film using multiple shot laser annealing was constrained by the Gaussein distributed laser energy. The large grains were suppressed due to the small grains formed in the first shot.
In addition, the influence of substrate temperature on the properties of polysilicon films prepared by excimer laser annealing was studied. As the substrate temperature was elevated, the maximum crystallinity and grain size increased, while the laser energy needed to obtain the maximum crystallinity of polysilicon films decreased. The elevated substrate temperature also changed the surface roughness of polysilicon films. In the partially melting regime, the surface roughness increased with laser energy and substrate temperature. The surface roughness dropped pronouncedly before reaching the super-lateral-grain-growth regime. Further increasing energy to homogeneous nucleation regime did not change much of the surface roughness.
Furthermore, the influence of laser energy on the properties of excimer-laser-annealed (ELA) amorphous silicon (a-Si) and as-deposited polycrystalline silicon (poly-Si) films has been studied too. For the ELA poly-Si films, in the low energy region, the crystallinity decreased with the energy. After reaching the minimum, it increased to the maximum, and then dropped down. No SLG grains were found in the near-complete-melting regime. The largest grains were observed in the partial-melting regime. The largest grain size (100 nm) of ELA poly-Si was less than that of ELA a-Si (130 nm).
Finally, the effects of energy on the microstructure of amorphous silicon (a-Si) films annealed by two-step laser process were systematically investigated. For the low-crystallinity / small-grain films, which were formed after the first low-energy laser crystallization, the grain size decreased and then increased with the energy of second laser annealing. In contrast, for the high-crystallinity films, i.e. SLG-grain films, the grain size monotonously decreased with second laser energy increased. Two-step laser annealed poly-Si films revealed that fine grains were formed and extruded at the grain boundary after the second high-energy laser annealing. High performance poly-Si TFTs can be fabricated from the poly-Si films crystallized by low-energy annealing followed by second high-energy laser annealing. When the results were compared, the poly-Si TFT using the poly-Si film crystallized by single high-energy laser annealing showed poorer mobility and subthreshold swing.
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