Poly-Si Thin Film Transistors Fabricated By Employing Selective Si Ion-Implantation And Excimer Laser Annealing
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Poly-Si Thin Film Transistors Fabricated By Employing Selective Si IonImplantation And Excimer Laser Annealing Min-Cheol Lee, Jae-Hong Jeon, Jin-Woo Park, and Min-Koo Han School of Electrical Engineering, Seoul National University, Seoul, 151-742, KOREA
ABSTRACT A new excimer laser annealing method is proposed in order to produce the poly-Si film with low defect density and large grain, by combining the selective Si ionimplantation and excimer laser annealing. Selective Si ion-implantation is employed to form artificial nucleation seeds in a-Si film prior to excimer laser annealing in order to increase the nucleation probability. The grain boundary location in poly-Si film has been controlled through implantation mask, and the grain size around micrometer order is obtained without any other process. TEM result shows that grain boundary is controlled according to mask pattern and the crystallinity of the poly-Si film is improved. INTRODUCTION Polycrystalline silicon thin film transistors (poly-Si TFTs) fabricated by excimer laser annealing (ELA) are currently utilized in the active matrix liquid crystal display (AMLCD) due to their high field-effect mobility and low thermal budget [1,2]. It is well known that the electrical characteristics of poly-Si TFTs are critically dependent on the grain size and the defect density of poly-Si film. Excimer laser recrystallization of a-Si film may be the most effective method to fabricate poly-Si film with low defect density when poly-Si TFT is fabricated by low temperature process. Another problem of polySi film annealed by excimer laser beam is that grain size varies very sensitively with ELA density and grain location is not uniform because nucleation takes place randomly. Various studies have been paid to increase the grain size and control the grain boundary in specific location [3,4]. Si ion-implantation has been reported as a method to increase the nucleation probability [5]. High-density and high-energy Si ion-implantation is utilized in order to
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Figure.1 the method of selective Si ion implantation and excimer laser annealing increase the nucleation probability artificially during ELA. In this paper, we have proposed a novel method to prepare poly-Si film with good quality employing selective Si ion-implantation and ELA. By the selective Si ionimplantation, the difference of the nucleation probability in the Si implanted region and in non-implanted region is introduced. This difference induces the lateral growth of large grain in near-complete melting regime during excimer laser annealing. Through TEM image, we obtained the enlarged grains and specifically controlled grain boundaries in the poly-Si film that is prepared by the proposed laser annealing method. EXPERIMENT We prepared an 800Å thick a-Si film by PECVD on oxidized silicon wafer and glass wafer. To carry out selective Si ion-implantation, photo-resist layer is patterned on the a-Si film as shown in Figure.1. Si ion-implantation is performed at 100KeV and ion dose is 1x1016atoms/cm2. After selective
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