Laser-pulse sputtering of aluminum: Vaporization, boiling, superheating, and gas-dynamic effects

Abstract

We have developed a numerical method to describe laser-pulse sputtering of Al in a thermal regime. The irradiation consists of a single pulse of triangular form having a duration of 30 ns. The laser light is assumed to be absorbed according to a simple exponential mechanism. Heat transport in the Al is described by the heat Bow equation with boundary conditions for vaporization, with or without boiling. Vaporization rates are evaluated by the Clausius-Clapeyron law and the boiling mechanism (when boiling is assumed to be possible) is implemented as soon as the vapor pressure reaches 1 atm. A critical analysis of the time scales necessary for true boiling, as well as for superheating above the boiling temperature, is made in order to understand the relevance of these phenomena with respect to particle emission from the Al surface. Moreover, on the basis of the calculated vaporization rates, it is possible to distinguish between different gas-dynamic regimes. When the rate is less than 1 ML in 20 ns, the particles emerging from the surface do not achieve local thermal equilibrium, and therefore undergo free Bight describable by a modified Maxwellian. When the rate is 1 ML in 20 ns, a Knudsen layer forms, at the boundary of which particles achieve local thermal equilibrium and only subsequently undergo free Bight. Finally, when the rate is sufBciently greater than~1 ML in 20 ns, the gas dynamics of the particles leaving the Knudsen layer may be described with the gas-dynamic equations, if the density is high enough, or, otherwise, by the Boltzmann equation. Numerical results concerning the effectiveness of laser sputtering in producing craters in irradiated Al, as well as the main features of the gas dynamics (including recondensation or re6ection of the gas at the Al surface), are illustrated.

Atomic/Molecular-Level Simulations of Laser–Materials Interactions

“…The latter nonequilibrium kinetic description has been shown to be necessary for subnanosecond pulses, when a fast thermal energy flow to/from the liquid-solid interface creates conditions for significant overheating/undercooling of the interface [43,44]. The material removal from the target can be incorporated into continuum models in the form of surface or volumetric vaporization models, e.g., [45][46][47][48][49], whereas the expansion of the vaporized plume is commonly described by solving gas dynamics equations, e.g., [45][46][47][48][49] or using the Direct Simulation Monte Carlo (DSMC) technique, e.g., [50][51][52][53][54][55]. Hydrodynamic computational models based on multiphase equations-of-state have also been used for simulation of laser melting, spallation, and ablation [56][57][58][59][60][61][62].…”

Modelling of pulsed electron beam induced graphite ablation: Sublimation versus melting

“…In the case of fluid (vapor/liquid) phase, thermodynamics of the surface target can be defined by introducing the recession velocity in the one-dimensional heat conduction equation [14],…”

“…When vaporization becomes substantial at higher surface temperature, velocity due to surface recession during this stage can be calculated by Hertz-Knudsen equation [14],…”

Knudsen layer formation in laser induced thermal desorption

“…1-5 Besides, laser desorption is an essential technique for pulsed laser deposition used in thin film growth [6][7][8][9][10][11][12] and mass spectrometry of protein employing matrix-assisted laser desorption ionization. 13 When desorption flux is small, the velocity distribution of desorbed atoms is directly governed by the desorption mechanism.…”