| Summary: | 碩士 === 國立交通大學 === 電子研究所 === 99 === Low-temperature polycrystalline silicon (LTPS) thin film transistors (TFTs) have been widely used in active matrix flat panel displays (AMFPDs), since they feature in high field-effect mobility and low power consumption. Moreover, the peripheral driver circuits, controller ICs and functional circuits can be integrated into glass substrates by utilizing LTPS-TFTs to achieve the goal of low cost, high reliability and System-on-Panel (SOP). We can foresee that LTPS-TFTs have great potential in the realization of 3-D ICs in the near future.
In tradition, reducing the defect states via enlarging poly-Si grain size is an intrinsic approach to single crystal Si material, which leads to the silicon-on-insulator-like (SOI-like) device performance. There are several technologies to enlarge the grain size, including solid phase crystallization (SPC) metal induced crystallization (MIC), and excimer laser irradiation crystallization (ELC). These methods could be concluded that the a-Si thin films are recrystallized into polycrystalline silicon thin film by applying additional energy. However, these technologies had some drawbacks as below: First, the SPC TFTs suffer a lot of intra-granular defects, which results in a bad performance and the long annealing time will limit the throughput to fabricate poly-Si thin film. Second, the metal induced crystallization (MIC) technology suffers from metal contamination incorporated into poly-Si thin film and the metal contamination will result in poor TFT performance. Third, although the excimer laser irradiation crystallization (ELC) technology is an useful technology to enlarge the grain size, there are still some disadvantages such as narrow laser process window for SLG and poor grain-size uniformity. In this thesis, therefore, we proposed a method, which is so called Diode-Pumped Solid-State (DPSS) Continuous Wave Laser-Crystallization (CLC). With the benefits of this longitudinal growth crystallization method, the poly-Si TFTs with high field-effect mobility have been fabricated.
At the first part, a new and simple CW laser crystallization is proposed to produce large longitudinally grown grains in the center region of the laser beam crystallized poly-Si via controlling the laser powers and the laser scanning speeds. However there are still small grains and polygonal grains in the edge and the transition region, which were SPC and PMG to SLG-like respectively, in spite of enlarging the laser powers or reducing the laser scanning speeds due to the Gaussian laser energy distribution. The small and polygonal grains in the channel region will deteriorate the performances of the TFTs. Therefore, multi-scan (overlapping method) scheme was proposed to achieve large area of longitudinal crystallization. According to the experimental results, the directionally longitudinal grains with tens of micrometer, flat surface morphology, and excellent crystallinity were achieved without damages to the quartz substrates.
In the second part, the electrical characteristics of the continuous wave laser crystallized polycrystalline silicon thin-film transistors were also studied, . In the SPC regime, the equivalent field-effect mobility, subthreshold swing, and threshold voltage were 19.4cm2/V-s, 2.72V/decade, and 7.93V, respectively. In PMG to SLG-like regime, the equivalent field-effect mobility, subthreshold swing, and threshold voltage were 86.9cm2/V-s, 1.41V/decade, and 0.296V, respectively. In longitudinal growth regime, high performance CLC poly-Si TFTs with equivalent field-effect mobility, subthreshold swing , and threshold voltage exceeding 281cm2/V-s, 0.753 V/decade, and -1.17V, respectively for n-channel devices have been fabricated without any hydrogenation treatment. It reflects that the larger grains with better crystallinity will possess the better electrical characteristics. Moreover such simple CW laser crystallization technique is promising for the applications in the future 3D- ICs.
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