The Effect of Film Thickness and Pulse Duration Variation in Excimer Laser Crystallization of Thin Si Films
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Irradiation with low energy densities below the threshold for surface melting will have a negligible effect on microstructural evolution. At the other extreme, irradiation with energy densities above the threshold required to completely melt the film down to the substrate leaves only a chemically inert liquid-silica interface. In the absence of an underlying liquid-crystalline interface, solidification must occur through either the nucleation of crystal embryos in the undercooled liquid, or, in the case of near-complete melting- through lateral growth from pre-existing seeds. Such transformations cannot be rigorously modeled by one-dimensional numerical simulation, but requires a multidimensional model. In typical sample configurations the quench, nucleation, and growth rates will result in a very fine to small grain polycrystalline microstructure [2,5]. In the partial melting regime, the maximum extent of the melt lies within the silicon film, as determined by the total absorbed energy, pulse duration, enthalpy associated with melting, and rate of heat conduction into the substrate. In this case, residual unmelted material provides a solid-liquid interface from which rapid regrowth can occur upon cooling. This can be considered the regime in which conventional excimer laser crystallization processes [11] typically operate, and is accurately characterized by a one-dimensional numerical model. Numerical Model Although analytical expressions for temperature evolution exist for cases of pulsed laser melting of some bulk materials, complications that include a temperature dependent thermal conductivity, heat capacity and refractive index, non-equilibrium melting and solidification, as well as multilayer films of varied composition- can only be treated through numerical simulation. Finite differences methods have been applied with much success in pulsed laser annealing of silicon substrates [12]. The model used in this investigation is derived from the one-dimensional numerical code originally developed [7] for studying pulsed laser annealing of silicon surfaces. Specific features of the model include the following: (1) Temperature dependent absorption and reflection are determined from published optical properties for crystal silicon at a wavelength of 308 nm. (2) Interfacial motion is based on a linearized interface response function of 25.0 and 6.7 cm/(sec 'K) respectively, for melting and solidification. (3) The temporal profile of the pulse is gaussian, with a full width at half maximum duration ranging from 10 to 200 ns. (4) The silicon film is modeled as crystalline. In an actual crystallization process, this is encountered after the as-deposited amorphous layer has been crystallized by an initial pulse. This simplification avoids a number of complicated melt-mediated transformation scenarios, which may in turn depend on the microstructural and compositional details of the as-deposited amorphous film. Processing window thresholds were determined by repeated simulations at varying pulse energies while monitoring
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