Vapor Breakdown During ablation by Nanosecond Laser Pulses
- PDF / 346,795 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 78 Downloads / 188 Views
"**Engineering Physics and Mathematics Division, ORNL ABSTRACT Plasma generation through vapor breakdown during ablation of a Si target by nanosecond KrF laser pulses is modeled using 0-dimensional rate equations. Although there is some previous work on vapor breakdown by microsecond laser pulses, there have been no successful attempts reported on the same subject by nanosecond laser pulses. This work intends to fill the gap. A kinetic model is developed considering the following factors: (1) temperatures of both electrons and heavy-body particles (ions, neutrals, and excited states of neutrals), (2) absorption mechanisms of the laser energy such as inverse bremstrahlung (IB) processes and photoionization of excited states, (3) ionization acceleration mechanisms that include electron-impact excitation of ground state neutrals, electron-impact ionization of excited states of neutrals, photoionization of excited states of neutrals, and all necessary reverse processes. The rates of various processes considered are calculated using a second order predictor-corrector numerical scheme. The rate equations are solved for five quantities, namely, densities of electrons, neutrals, and excited states of neutrals, and the temperatures of electrons and heavy-body particles. The total breakdown times (sum of evaporation time and vapor breakdown time) at different energy fluences are then calculated. The results are compared with experimental observations of Si target ablation using a KrF laser. I. Introduction One of the most promising techniques for laser materials processing is pulsed laser deposition (PLD) of thin films. The advantages of PLD compared to other techniques include, novel epitaxial and low temperature growth of homogeneous and heterogeneous films by utilizing energetic species, stoichiometric ablation of constituent species of the target, and growth of metastable phases layer-by-layer under nonequilibrium ablation conditions. While experimentalists are trying to find optimal conditions for thin-film growth by PLD, a systematic study in modeling of various physical processes during deposition is needed to assist their effort. For computational modeling of the complicated processes such as occur during PLD, one faces the challenge not only to better understand the fundamentals of the processes, but also to utilize the most appropriate computational techniques. Modeling of vapor breakdown due to the interactions between vapor and incoming laser beam during ablation is such a challenge. Understanding this phenomenon is extremely important in assessing the final state of the plume after the pulse and subsequent plume expansion and transport toward the substrate. There have been no successful attempts reported to model vapor breakdown under irradiation with nanosecond laser pulses, although some work has been done for microsecond laser pulses [1]. In this paper, we present preliminary results from theoretical modeling of vapor breakdown leading to plasma generation (fully ionized gas) through the interactions between
Data Loading...
