Excimer Laser Recrystallization of Selectively Floating a-Si Active Layer for Large-Grained Poly-Si Film
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Excimer Laser Recrystallization of Selectively Floating a-Si Active Layer for Large-Grained Poly-Si Film Cheon-Hong Kim, Juhn-Suk Yoo, In-Hyuk Song and Min-Koo Han School of Electrical Engineering, Seoul National University, San 56-1 Shinlim-dong, Kwanak-gu, Seoul 151-742, Korea ABSTRACT We report a new excimer laser annealing method by employing selectively floating a-Si active layer structure in order to increase the grain size of poly-Si film. The floating a-Si region blocks the heat conduction into the underlying substrate due to high thermal-insulating property of an air so that the lateral temperature gradient is successfully induced by the proposed simple air-gap structure. Our experimental results show that large grains were grown in the lateral direction from the edge to the center of the floating active region. The large grains exceeding 4 m were successfully obtained with only one laser irradiation.
INTRODUCTION Polycrystalline silicon thin film transistors (poly-Si TFTs) have been investigated for active matrix liquid crystal displays (AMLCDs) [1]. Excimer laser recrystallization of amorphous Si (a-Si) is a promising method for fabrication of high-quality poly-Si TFTs without any thermal budget on glass substrate [2-5]. The electrical characteristics of poly-Si TFT such as mobility and threshold voltage are closely related to grain size of poly-Si channel. However, the grain size of poly-Si film recrystallized by conventional laser annealing is not large enough (typically less than 1 m) for practical device and circuit applications [2]. In order to increase the grain size of poly-Si film, various techniques have been reported by many researchers [3-5]. It is well known that a lateral grain growth mechanism is responsible for a large grain and the lateral temperature gradient in a melted a-Si film would induce the lateral grain growth [2]. An effective way to obtain the lateral temperature gradient is the modulation of incident laser energy density through a beam slit that produces a locally molten zone in a-Si film [4]. However, this method requires an exact control of the stage movement. It has been already reported that excimer laser annealing of thin a-Si film on SiO2 membrane structure enlarged the grain size [5]. It is important that a bridge structure was employed in order to selectively block the vertical heat conduction. However, the membrane structure may be rather difficult for practical TFT applications because the underlying substrate should be etched out and the membrane would not be immune to a mechanical stress. A6.11.1
We propose a new excimer laser annealing method in order to increase the grain size of poly-Si film by employing selectively floating a-Si active layer structure. Although Si melt adherent to the substrate cools off rapidly, the vertical cooling rate in the floating active region is significantly reduced because the floating a-Si region is separated from the substrate by the air-gap. The considerable lateral temperature gradient is induced so that very large grains may b
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