Crystallization of Amorphous Silicon Thin Films by Microwave Heating

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Crystallization of Amorphous Silicon Thin Films by Microwave Heating Tomohiko Nakamura1, Shinya Yoshidomi1, Masahiko Hasumi1, Toshiyuki Sameshima1, and Tomohisa Mizuno2 1 Tokyo University of Agriculture and Technology, Tokyo, 184-8588 Japan 2 Kanagawa University, Kanagawa, 259-1293 Japan ABSTRACT We report crystallization of amorphous silicon (a-Si) thin films and improvement of thin film transistors (TFTs) characteristics using 2.45 GHz microwave heating assisted with carbon powders. Undoped 50-nm-thick a-Si films were formed on quartz substrates and heated by microwave irradiation for 2, 3, and 4 min. Raman scattering spectra revealed that the crystalline volume ratio increased to 0.42 for the 4-min heated sample. The dark and photo electrical conductivities measured by Air mass 1.5 at 100 mW/cm2 were 2.6x10-6 and 5.2x10-6 S/cm in the case of 4-min microwave heating followed by 1.3x106-Pa-H2O vapor heat treatment at 260oC for 3 h. N channel polycrystalline silicon TFTs characteristics were improved by the combination of microwave heating with high-pressure H2O vapor heat treatment. The threshold voltage decreased from 5.3 to 4.2 V and the effective carrier mobility increased from 18 to 25 cm2/Vs. INTRODUCTION Crystallization of amorphous silicon thin films is an important processing technology for fabricating the thin film transistors (TFTs) with a high carrier mobility and low threshold voltage, which can be applied to switching and driving circuits in flat panel displays [1,2]. Laser crystallization using XeCl excimer laser has been widely used for crystallization of amorphous silicon (a-Si) thin films [3]. However, laser crystallization requires a complicated equipment and high operation cost. These problems give us a motivation of development of simple crystallization technique at a low cost. In this paper, we report a crystallization method using microwave heating with carbon powders. We report crystallization condition of a-Si films and their crystallographic and electrical properties. In addition, we discuss improvement in TFT characteristics by the present microwave heating. EXPERIMENT 50-nm-thick a-Si films were formed on quartz substrates with a diameter of 4 inch using low pressure chemical vapor deposition (LPCVD). The samples were divided into 4 quarter pieces. The samples were placed in a quartz vessel with an internal diameter of 2.4 inches. Carbon powders at 12 g were also put in the vessel to completely cover the sample, as shown in Fig. 1. The vessel was placed in a 2.45 GHz commercial microwave oven. Microwave irradiation was operated at 1000 W for 2, 3, and 4 min. Carbon powders effectively absorbed microwave power. They began to emit blight red light at 2 min and emitted blight red orange light at 4 min. According to the blackbody radiation theory, carbon powders were heated to about 1000oC for 4 min microwave irradiation. The samples were then heated with 1.3x106 Pa H2O vapor at 260oC for 3 h to reduce defect states of silicon films [4-6]. The samples were heated by heat conduction from carbo

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