The Investigation of High Performance TFT by Thin Beam Directional X-stallization Method
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0910-A15-04
The Investigation of High Performance TFT by Thin Beam Directional X-stallization Method Chihwei Chao1, Jiatien Peng1, CW Cheng1, Chunghung Chen1, Brandon A. Turk2, and Bernd Burfeindt2 1
RPA, AU Optronics, No.1, Lising 2nd Rd., City, Hsinchu, Taiwan, 300, Taiwan
2
TCZ GmbH, San Diego, California, 92127, American Samoa
ABSTRACT Thin-beam directional X’tallization (TDX) is a low temperature substrate compatible crystallization method that can form directionally solidified poly-si films directly on glass substrates. Without using a mask, extremely long uniformly-spaced poly-Si grains can be formed by TDX. In this paper, we have investigated the influence of laser energy and scan pitch on the properties of the resulting poly-Si films. Grain size and surface morphology of TDX-processed poly-Si films were observed by SEM and AFM. The AFM results demonstrate that, due to the lateral growth phenomenon, TDX poly-Si films are much smoother than films produced by the current industry standard method, Excimer Laser Annealing (ELA). The breakdown of the TDX method is also discussed in this paper. When the laser scan pitch is larger than the lateral growth length that is characteristic of a given poly-Si film and beam property combination, a discontinuous poly-Si film is formed with vertical protrusions and grain boundaries occurring perpendicular to the scan direction. Poly-Si films with vertical protrusions and grain boundaries perpendicular to the direction of current flow will degrade the resulting thin-film transistor (TFT) performance. INTRODUCTION ELA is the most common method currently used in the mass production of low-temperature poly-Si (LTPS) TFTs. The grain-size in ELA poly-Si is limited (~300nm) and the orientation of grain boundaries is random. TFT performance is degraded when the poly-Si grain boundaries lie perpendicular to the direction of current flow in the device active channel area.1.2. Some circuits can not be integrated directly onto the panel because of the inferior TFT performance. Lateral crystal growth techniques can potentially form materials that are devoid of perpendicular lying grain boundaries, thereby improving TFT performance. Several crystallization methods can form laterally crystallized poly-Si films such as sequential
lateral solidification (SLS)3,4, continuous wave laser crystallization (CLC)5,6, and phase modulated excimer laser annealing (PMELA)7,8. Compared with SLS and PMELA method, TDX precisely controls the laser beam spatial profile to form laterally-grown poly-Si grains without the use of a mask. CLC methods use continuous wave solid state lasers to form lateral poly-Si grains, but the power of solid state lasers is currently too low to enable crystallization of an entire a-Si coated glass substrate at one time, and suffers from low throughput. In the TDX method, the width of short-axis laser beam profile is narrower than that used in the conventional excimer-based laser method. The narrow top-hat shaped beam is formed without using a mask, and is ideally suited
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