Thermal model of nanosecond pulsed laser ablation: Analysis of energy and mass transfer

Abstract

A thermal model of nanosecond laser ablation considering kinetics of surface evaporation is proposed. Equations concerning heat transfer in the target and associated gas dynamics are coupled by mass and energy balances at the surface and Knudsen layer conditions. Rigorous analysis of gas-dynamics related to condensation at the target surface is introduced in this model. Laser energy absorbed by the target is partly spent for evaporation and partly dissipated in the target by thermal conduction. The sum of thermal and kinetic energies of the gas phase is, usually, less than the energy of evaporation. The fraction of energy lost for target heating increases with decrease in laser fluence and attains 100% at the ablation threshold. The dependence of ablated depth on fluence is, thus, determined by energy partition between the solid and gas phases. The gas-dynamic flow accompanying ablation consists of a layer of compressed high-temperature vapor adjacent to the target that expands and pushes the ambient gas from the surface to generate a strong shock wave. Ablation of Al and Au by laser with 193 nm wavelength, 12 ns full width at half maximum (FWHM) pulses, and 5.3 J cm2 incident fluence and that of Au by laser with 266 nm wavelength, 6 ns FWHM, and 3.5 J cm2 incident fluence is analyzed utilizing the present thermal model. It is concluded that optical breakdown does not occur at the considered conditions. The present model can be applied when the target surface temperature is less than the critical temperature. In case of nanosecond laser ablation of metals this, normally, restricts the value of absorbed fluence by the maximum of several J cm2. © 2005 American Institute of Physics.