Excimer Laser Recrystallization of a-Si Employing Aluminum Masking Window
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Excimer Laser Recrystallization of a-Si Employing Aluminum Masking Window Jae-Hong Jeon, Min-Cheol Lee, Sang-Hoon Jung and Min-Koo Han School of Electrical Engineering, Seoul National University Shinlim-dong, Kwanak-ku, Seoul, 151-742, KOREA ABSTRACT A new excimer laser recrystallization method of amorphous silicon is proposed to increase the grain size and control the grain boundary locations in polycrystalline silicon films. The proposed method is based on the lateral grain growth which occurs at the interface between molten and unmolten regions. To obtain selectively molten regions, the proposed method employs aluminum patterns on amorphous silicon. The aluminum patterns act as the beam shield during the laser irradiation as well as the lateral heat sink during the solidification period. The high reflectance of aluminum at the wavelength of XeCl excimer laser offers stable beam shielding property, and the high thermal conductivity enhances the lateral heat flow by the quick draining of laterally propagated heat. TEM observation has revealed that the well arranged large grains were successfully obtained. INTRODUCTION Excimer laser annealed (ELA) polycrystalline silicon (poly-Si) films have attracted a lot of attention for high performance thin film transistors (TFTs) in active matrix- liquid crystal displays (AM-LCDs) [1,2]. It is well known that ELA provides excellent electrical device performance and allows low temperature process which enables the utilization of glass or plastic substrates [3,4]. High quality of ELA poly-Si film originates from low in- grain defect density. However in conventional ELA poly-Si films, the grain size is rather small, below a few hundred nanometers, producing high density of the grain boundary. This is because the nucleation rate at the beginning of solidification is very high due to the quick quenching rate. The resultant high density of the grain boundary generates a large leakage current and deteriorates the long-term stability of TFTs. In conventional excimer laser annealing, there exists a certain energy density level which produces abnormally large grains due to the lateral grain growth [5]. However, this energy level exists in a very narrow range, so the reproducibility in grain size cannot be ensured when there is some energy fluctuation in the excimer laser system. Considerable efforts have been made on the lateral grain growth phenomena in order to increase the grain size of ELA poly-Si films [5,6,7]. The lateral grain growth occurs when the lateral temperature gradient in molten amorphous silicon (a-Si) is more dominant than the vertical one. An effective way to obtain the lateral temperature gradient is the modulation of incident laser energy density through the proper optical instrument or a beam slit which produces a locally molten zone in a-Si film [6,7]. However, these methods require an elaborated alignment of laser optics or the exact control of the stage movement. The purpose of our work is to report a new excimer laser annealing method which induces the lateral
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