Laser-pulse sputtering of aluminium: gas-dynamic effects with recondensation and reflection conditions at the Knudsen layer

Abstract

Laser-pulse sputtering of Al is described in terms of a thermal model where vaporization rates are evaluated by the Clausius-Clapeyron law. 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 monolayer in 20 ns, the particles emerging from the surface do not achieve local thermal equilibrium and therefore undergo free flight describable by a modified Maxwellian. When the rate is ∼ 1 monolayer in 20 ns, a Knudsen layer forms, within which particles achieve local thermal equilibrium and only subsequently undergo free flight. Finally when the rate is sufficiently greater than ∼ 1 monolayer in 20 ns, the gas dynamics of the particles leaving the Knudsen layer may be described with the gas-dynamic equations. Boundary conditions at the limit of the Knudsen layer depend on whether recondensation or reflection occurs at the target surface. These boundary conditions are then coupled with the unsteady adiabatic expansion of the particles when the gas-dynamic equations are numerically integrated.