Ni-Metal Induced Lateral Crystallization of Amorphous Silicon-Growth Mechanism and LTPS TFTs Device Performance

• Main Authors: Chih-Yuan Hou, 侯智元
• Other Authors: YewChung Sermon Wu
• Format: Others
• Language: en_US
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/20854110303949352237

博士 === 國立交通大學 === 材料科學與工程系所 === 94 === In this thesis, Ni-metal induced lateral crystallization (NILC) of amorphous silicon (α-Si) has been studied. The influence of tensile stress on the growth mechanism of NILC is investigated. Furthermore, we fabricate the LTPS TFTs by Ni-metal imprint-induced crystallization method. Combine NILC and excimer laser annealing (ELA) method to produce high-performance LTPS TFTs. Moreover, in order to solve this issue of NILC poly-Si film, we develop an effective gettering method to reduce the Ni-metal impurity contamination of the NILC poly-Si films. Initially, three stages have been identified in the NILC process: (1) the formation of NiSi2 precipitates, (2) the nucleation of crystalline Si (c-Si) on NiSi2 precipitates, and (3) the subsequent migration of NiSi2 precipitates and growth of c-Si. It has been reported that the incubation time could be reduced by tensile stress. However, the detail has still not been clarified. In this study, a simple bending fixture was used to investigate the effects of tensile stress on the growth of NILC. Base on the results of this study, it was found that tensile stress did not enhance NiSi2 formation and c-Si nucleation stages, but enhanced the c-Si growth stage. It was also found that compressive stress did not change NILC rate. The Ni-metal imprint-induced crystallization method exhibited many superior characteristic over traditional NILC method. In this study, for the LTPS TFTs fabricated using <111> and <112> needle grains have been investigated. They were fabricated by traditional NILC and Ni–metal imprint-induced crystallization method. It is found that the performance of 112-TFT was far superior to that of 111-TFT. The device transfer characteristics of 112-TFT include 2.6-fold-higher field-effect mobility (μFE), 4-fold-higher on/off current ratio (ION/OFF), and 2.4-fold-lower leakage current (IOFF) compared with those of the 111-TFT. The improved performance and good uniformity LTPS TFTs have been fabricated using Ni-metal imprint-induced crystallization method. However, the polycrystalline silicon film contained many intra-grain defects with some un-crystallized regions between poly-Si needle grains. These defects degrade the transfer characteristics of TFT devices, including the field effect mobility (μFE) and the leakage current. In this study, combine the excimer laser crystallization method to reduce the defect density. To compare the performance of IMPRINT and IMPRINT-ELA TFT, upon increasing the laser energy to 345 mJ/cm2, the grain size increased from 50 to 250 nm and the performance of IMPRINT-ELA-TFT was found to be far superior to that of IMPRINT-TFT due to larger grains and fewer intra-grain defects of the IMPRINT-ELA poly-Si film than that of the IMPRINT poly-Si film. The mobility of the IMPRINT-ELA-TFT was 413 cm2/Vs, which was 31.7 times higher than that of the IMPRINT-TFT. The on/off current ratio of the IMPRINT-ELA-TFT was 4.24 ×106, which was 2 orders magnitude higher than that of the IMPRINT-TFT. Finally, develop two gettering methods to reduce the Ni contamination within the NILC poly-Si film. First, using α-Si films with a thickness of 100 nm as a Ni-gettering layer, silicon-nitride (SiNx) films with a thickness of 30 nm as the etching stop layers and annealed at 550°C for 90 h to reduce the Ni-metal impurity within the NILC poly-Si film. Second, an α-Si-coated wafers used as Ni-gettering substrates then bonding the gettering substrate and NILC poly-Si film together. The Ni-metal impurity within the NILC poly-Si film was greatly reduced.