Substrate Latent Heat Effects in the Calculations for Pulsed Laser Irradiated Thin Films
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SUBSTRATE LATENT HEAT EFFECTS IN THE CALCULATIONS FOR PULSED LASER IRRADIATED THIN FILMS J.J. CAO AND K. ROSE Center for Integrated Electronics, Rensselaer Polytechnic Institute, Troy, New York 12181 0. AINA, W. KATZ, AND J. NORTON General Electric, Corporate Research and Development, Schenectady, 12309 New York ABSTRACT We present a numerical model for the calculation of the temperature rise caused by pulsed laser irradiation of a thin film/substrate structure. This model includes phase changes in both the thin film and the substrate. The inclusion of phase changes results in more complex thermal behavior and significantly affects melt durations. This model was applied to the AuGe/GaAs system. Morphological observation using the scanning electron microscope and SIMS profiles provides experimental verification for the numerical calculations. INTRODUCTION Thermal models of laser beam annealing allow calculations of the space and time dependence of temperature in a material. These temperature distributions can be used to predict crystallization and atomic redistribution. The accuracy of the prediction depends on the sophistication of the model. Thermal models of all degrees of sophistication have been developed for the laser beam annealing of homoqeneous materials. These vary from the simple one-dimensional analytical model of Ready [1] to numerical models which include temperature dependent parameters, melting, melt boundary motion, and evaporation [2,3]. Although analytical models have been developed for bimaterial structures [4,5] no numerical calculations have been made for such systems. We have constructed a numerical model for bimaterial structures which takes into account the latent heat of melting of both the thin film and the substrate. This model has been applied to the laser annealing of AuGe films on GaAs substrates for ohmic contact formation. The temperature rise as a function of laser energy density as calculated by an analytical model will be compared to calculations by numerical models which take GaAs melting into consideration. The numerical model predicts a shorter melt duration than the analytical model. The numerical calculations were verified by SEM examination and SIMS profiling. Calculations of the expected outdiffusions obtained from the numerical model are shown to correlate well with values estimated from the SIMS profiles. The experimental techniques, SEM examinations of morphology, and SIMS profiles have been reported previously [6]. Hat. Res.
Soc. Symp. Proc. Vol. 13 (1983) oElsevier Science Publishing Co.,
Inc.
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NUMERICAL THERMAL CALCULATIONS The temperature distribution caused by a laser beam incident on If a sample can be obtained by solving the heat flow equation. all thermophysical parameters are constant, this is a linear differential equation with analytical solutions. However, if these parameters are temperature dependent, if there is a phase change (such as melting) or a moving boundary condition, the equation is Numerical techninon-linear and cannot be solved analytical
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