Long-Pulse Duration Excimer Laser Processing in the Fabrication of High Performance Polysilicon TFTs for Large Area Elec
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LONG-PULSE DURATION EXCIMER LASER PROCESSING IN THE FABRICATION OF HIGH PERFORMANCE POLYSILICON TFTs FOR LARGE AREA ELECTRONICS E. Fogarassy1, B. Prévot1, S. de Unamuno1, C. Prat 2, D. Zahorski 2, Y. Helen 3, T. Mohammed-Brahim 3 1 Laboratoire CNRS-PHASE, BP 20, 67037 Strasbourg, France. 2 SOPRA S.A., 26 rue Pierre Joigneaux, 92270 Bois Colombes, France 3 G.M.V. Université de Rennes I, 35042 Rennes, France ABSTRACT In this work, was investigated both numerically and experimentally, the excimer laser processing of a-Si films deposited on SiO2-coated glass substrates, using the very large area (~ 20 cm2) and long pulse duration (200 ns) excimer source from SOPRA Company. Experiments were carried out in air or in neutral atmosphere, using both the single- and multishot mode. From the microstructural analysis of the laser irradiated area the formation of a large-grained material through the so-called SLG regime was evidenced. In addition, the application of a multi-shot process was demonstrated to be very efficient to prepare uniform polysilicon layers with enlarged grain sizes (up to 1.5 µm after 20 shots). Finally, poly-Si TFTs prepared in the optimized conditions (multi-shot, neutral ambience) exibited field effect mobilities up to 235 cm2/V.s (for N-type) and 84 cm2/V.s (for P-type), with fairly uniform device characteristics over large area and excellent stability under electrical stress. INTRODUCTION Excimer laser crystallization of amorphous silicon (a-Si) on cheap glass appears very promising to prepare at low temperature high quality polysilicon TFTs for flat panel display applications[1-5]. The rapid deposition of laser energy density on a nanosecond time scale into the near surface region of the a-Si layer leads to its melting and regrowth into polysilicon, while keeping the substrate at low temperature. The final quality of the device is strongly dependent on the phase transformation mechanisms which have to be controlled extremely carefully in order to achieve polycristals of maximum grain size and uniformity. From the microstructural analysis of the laser-induced crystallization process by using a XeCl excimer source of 30 ns duration, Im et al [6] have identified, in 1993, three main regimes : The first one, dealing with the low-energy density (or fluence) range, corresponds to fluences that lead to a partial melting of the Si film. This regime is characterized by an increase in the grain size with the laser fluence and is believed to involve an explosive crystallization process [7] followed by partial melting and regrowth. The second regime corresponds to the highest fluences, exceeding the threshold for complete melting of the Si layer. In this situation, the formation of fine-grained polysilicon was found to be nearly independent of the laser energy density and related to a homogeneous nucleation process from a supercooled melt [8]. A third region, occuring between the two previous ones, has been identified for laser fluences corresponding to the full-melt threshold of the a-Si film. In this regime, ra
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