Interpreting laser ablation using cross sections

Abstract

Laser etching occurs by several processes depending on the surface and on the laser's power density (P), wavelength and pulse length (∼10 ns here). Important channels for vaporizing monolayers are thermal vaporization, photochemical dissociation and plasma breakdown. Because the plasma-vapor interaction can be viewed as a partial local equilibrium, collisional or reaction cross sections provide a detailed understanding of which reactions will be in equilibrium on a nanosecond time scale. Furthermore, laser ablation is more predictable the nearer the system is to equilibrium rather than acting as separate sub-systems. One important cross section is inverse bremsstrahlung, IB, which controls the local energy deposition of laser energy, and is promoted at P ≳ 109 W/cm2 and > 1019 ions (and electrons)/cm3. (Inverse bremsstrahlung is the scattering (dephasing away from pure dielectric response) of free electrons as they respond to the laser's electric field. The result is a loss component being added to the dielectric response.) In the vapor phase the above range of densities furnish an IB cross section of ≳ 10-19 cm2 in the UV. Alternately this cross section may be produced by a dense, ≳ 3 × 1020 cm-3, cloud of neutral species just over the surface. Using the above cross section one sees that a 10 J/cm2 (10 ns at 109W/cm2) pulse will supply ∼ 5 eV to each free electron; hence further ionization is rapid. While multiphoton ionization furnishes a finite number of free electrons to trigger the IB their density is not sufficient to represent a dominant energy pathway in the case of Cu. Ionization energy is primarily transferred to the plasma cloud by three body recombination as hot atoms are a direct product. This process operates on a nanosecond time scale because of the near atmospheric densities. In the case of copper, we have also shown that the breakup of diatomics provides subsidiary evidence for the concept of IB furnishing an average of > 2 eV/ electron; (the original 5 eV/electron is diminished by the above coupling into the nuclear motion). Less energy than this would not have dissociated the diatomics at the observed rate. These results connect smoothly with fusion results at P 1011 W/cm2 and thus form one conceptual area. Furthermore, the present densities and cross sections are essentially material independent and hence of broad applicability. © 1993.