Modeling of plasma dynamics during pulsed electron beam ablation of graphite.

Abstract

Recent advances in the field of plasma nanofabrication suggest that plasma-based technologies may replace many of the conventional chemical and thermal routes in the synthesis of nanomaterials (with at least one dimension below 100 nm) and thin films. In contrast to the conventional processing routes, where only neutral species are involved, a plasma is made up of energetic species including ions, electrons, and excited molecules in addition to neutrals. Due to the highly energetic nature of interactions among these species and with other surfaces (substrates), a plasma allows for the formation of materials at higher rates even though their concentrations might be low as compared with those of neutral species in non-plasma based methods. While the mechanisms of the various interactions in a plasma are undoubtedly complex and require a fundamental understanding, they offer new opportunities for material nanofabrication. Pulsed electron beam ablation (PEBA) has recently emerged as a novel and promising technique for high quality thin films growth. Pulsed electron beam film deposition consists of many physical processes including target material heating, target ablation, plasma plume expansion, and film growth on a substrate. Electron beam ablation is a complex process, which comprises heating, phase change, and removal of a fine fraction from the target surface. Ablation strongly affects the space distribution, composition, mass transfer processes, which in turn has a critical bearing on the structure, stoichiometry and properties of thin films. Plasma plume expansion into an ambient gas is a fundamental issue in PEBA as the quality of thin films deposited onto the substrate depends on the composition, energy and density of particles ejected from the target. A one-dimensional heat conduction model is presented to investigate the heating and ablation of a graphite target upon interaction with a polyenergetic electron beam. The effect of electron beam efficiency, power density, accelerating voltage, and Knudsen layer just above the target surface during ablation are taken into account in the model. Phase transition induced during ablation is considered through the temperature dependent thermodynamic properties of graphite. The temperature distribution, surface receding velocity, melting depth, ablation depth, and ablated mass per unit area are numerically simulated. Upon ablation, plasma expansion, induced by interaction of a nanosecond electron beam pulse (~100 ns) with a graphite target in an argon atmosphere at reduced pressure, was investigated by solving gas-dynamics equations. The spatiotemporal profiles of the temperature, pressure, velocity, and density of the plasma plume are numerically simulated for a beam efficiency of 0.6 and accelerating voltage of 15 kV. Each model is validated by comparing some of the obtained simulation results with experimental data available in the literature.