Excimer-Laser Crystallization of Si Films Via Bi-Directional Irradiation of Dual-Layer Films on Transparent Substrates
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JUNG H. YOON and JAMES S. IM Columbia University, Department of Chemical Engineering, Materials Science and Mining Engineering, New York, NY 10027 Abstract In this paper, we report on a new excimer-laser crystallization (ELC) method that is highly effective in extending the super-lateral growth (SLG) distance and which does not involve any preheating of the substrate. The technique utilizes bi-directional irradiation of a dual layer Si film stack (separated by an oxide layer) deposited on a quartz wafer. The top layer is irradiated with a projection system which transfers a mask image in order to produce grain-boundarylocation-controlled (GLC) regions, and the bottom layer, upon irradiation with a uniform beam, acts as a medium that favorably affects the thermal evolution of the top layer. The technique is effective and attractive in that the heating is The thermal spatially and temporally localized in an optimal manner. environment required for extending the SLG distance, as is induced by the melting and solidification of the bottom layer, is physically regulated by the melting temperature of Si, and the enthalpy difference between liquid and solid can be used to initially store and subsequently release heat. Using the method, we were able to attain GLC regions with widths up to 10 pm in 1000-A Si films without any substrate heating. We elaborate on the applicability of the method to various artificially controlled super-lateral growth (ACSLG) techniques, and discuss process optimization by means of varying the multilayer configuration. Introduction
The growing demand for crystalline-Si TFT devices with high performance and device uniformity has resulted in an increased focus on the Here, the microstructure of the active-channel portion of a device [1]. presence of a high density of randomly located high-angle grain boundaries is known to have detrimental effects on the resulting device characteristics. Recently, it has been shown that non-random microstructures can be obtained via ACSLG techniques in which lateral solidification is induced in a controlled manner [2,3,4]. The extent to which lateral growth can proceed is a key factor in all of the ACSLG processes, and is important in optimizing the resulting microstructures. Straightforward means of extending the lateral growth distances by increasing the Si film thickness or increasing the substrate temperature have previously been discussed [5,6]. However, overall process compatibility requirements, combined with the need for usage of inexpensive glass substrates make processing at elevated temperatures undesirable. As well, the use of thick Si films is not desired from a TFT device standpoint. In this paper, we report on an alternative and effective means of enhancing the lateral growth distance on thin Si films without any preheating of the substrate. The method entails bi-directional irradiation of dual Si layers on
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Mat. Res. Soc. Symp. Proc. Vol. 397 ©1996 Materials Research Society
transparent substrates, where one layer is crystallized for device
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