1D and 2D Monte Carlo Simulations of Pulsed Laser Ablation of Si/Ga Into Ar/O 2 with a Substrate
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evolves, and no analytical solution to the problem of an expanding source plume with a background gas and substrate is available. It is therefore desirable to develop a reliable method of modeling the problem computationally. We present in this work some initial results from 1D and 2D axisymmetric simulations, using the Direct Simulation Monte Carlo (DSMC) method developed by Bird [1], where we study the ablation of a target material into a background gas in the presence of a substrate. Simulations done in ID reveal some important characteristics of the plume and background gas interaction, including the evolution of the shock wave and contact front. In both ID and 2D a'i'symmetric simulations, the time-integrated flux of source particles onto the substrate appears nearly constant for very low background gas pressure (< 1 mTorr), and then drops abruptly near some critical pressure. This rapid drop in timeintegrated particle flux correlates well with the observed growth rate of Si0 2 films as a function of background pressure [2]. The time-integrated flux of target particles onto the substrate is shown to depend on target particle mass by comparing simulations expanding Si into Ar, Ga into Ar, and Ga + Si into Ar. Simulations done using a 2D axisymmetric cell grid offer insight into the radial dependence of gas properties during the plume expansion and upon contact with the substrate. Regions of high temperature in the simulations are consistent with emission spectroscopy images of PLD plumes[3], and images of silicon density are qualitatively similar to results 367 Mat. Res. Soc. Symp. Proc. Vol. 389 01995 Materials Research Society
obtained by laser induced fluorescence (LIF) on Si expanding into vacuum [4]. The results offer quantitative estimates of density and temperature, which strongly suggest that additional effects such as ionization and chemistry may be important in some PLD applications. THE DSMC METHOD The DSMC method for simulating rarified gas dynamics is described in detail by Bird [1]. The method is equivalent to solving the Boltzmann equation, and therefore includes the effects of viscosity, diffusion, and heat transport. It has been applied successfully to non-equilibrium flow, high Mach number flow, and rarefied flows where the scale length is comparable to the mean free path so that neither the collisionless particle approximation nor the fluid model applies. The Variable Hard Sphere (VHS) method can be included to ensure that collision cross sections, if applied in the continuum limit, would result in flow
viscosities calculated from standard kinetic-theory expressions. Polyatomic molecules such as 02 can also be realistically included in DSMC simulations by employing the LarsonBornake method of translational energy exchange with internal degrees of freedom during collisions. The complexity of the gas-dynamics in a PLD plume suggests that simulations based on the DSMC method may contribute significantly to a better understanding of reactive PLD. SIMULATION PARAMETERS In the ID simulation pres
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