| Summary: | 博士 === 國立交通大學 === 材料科學與工程學系 === 100 === Ni-metal-induced crystallization (MIC) of amorphous Si (α-Si) has been widely employed to fabricate low-temperature polycrystalline silicon (LTPS) thin-film transistors (TFTs). However, the high leakage current is an issue of MIC TFTs because Ni impurities trapped inside the MIC poly-Si films. Therefore, the main purposes of this thesis are to reduce Ni residues, to improve electrical performance of MIC TFTs, and further to investigate the effects of Ni concentration on others of importantly electrical characteristics.
First, a chemical oxide filter layer was introduced into MIC processes to reduce the leakage current of MIC TFTs, which was simple and without extra expensive instrument. It just added a step of dipping α-Si coated sample into chemical solution before depositing the Ni film. It was found that Ni concentration was decreased successfully in MIC poly-Si films and the electrical performance of MIC TFTs with chemical oxide layer was significantly improved. Compared with conventional MIC TFTs, CF-MIC TFTs shows a 14.3-fold decrease in the minimum leakage current and a 17.3-fold increase in the on/off current ratio. This is because the chemical oxide layer can avoid Ni directly contact with α-Si, avoid excess of Ni atoms into α-Si layer and remove unreacted Ni easily from surface.
Furthermore, the Ni concentration effect on source/drain (S/D) series resistance was investigated by transmission line method. In addition to well known Ni effects on leakage current, however, the S/D series resistance of MIC TFTs might be changed with reduction of Ni concentration, which also influences the device performance (driving ability). Therefore, we attained a new finding for the relation between Ni concentration and resistance. As the results, the S/D series resistance and channel resistance were decreased with the reduction of Ni concentration in MIC poly-Si. Recently, the bias reliability and thermal stability became major concerns for AMOLED display applications especially when devices are operated under hot carrier condition and high temperature environment. In this study, the effect of Ni concentration on bias reliability and thermal stability were also investigated. It was found that the low Ni residues device presented high immunity against the hot-carrier stress and elevated temperature. These findings proved that reducing Ni concentration in MIC films was also beneficial for S/D series resistance, bias reliability and thermal stability.
Finally, a new manufacturing method for poly-Si TFTs using drive-in Ni induced crystallization (DIC) was proposed. In DIC, F+ implantation was used to drive Ni in the α-Si layer. It was found that the electrical performance (especially leakage current) and thermal stability of DIC TFTs were improved due to the reduction of Ni concentration and passivation of trap states near the SiO2/poly-Si interface. However, the on-state currents were nearly unchanged due to the channel damages/defects caused by ion implantation. Therefore, a cap oxide layer was introduced into DIC process (DICC) to reduce ion implant damages. Compared with that of MIC TFTs, the on/off current ratio (Ion/Ioff) of DICC TFTs was increased by a factor of 9.7 from 9.21×104 to 8.94×105. The minimum leakage current (Imin) of DICC TFTs was 4.06 pA/μm, which was much lower than that of the MIC TFTs (19.20 pA/μm). DICC TFTs also possess high immunity against the hot-carrier stress and thereby exhibit good reliability.
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