Modeling the thermal-to-plasma transitions for Cu photoablation

Excimer laser ablation of metals starts as a thermal process in the ∼1-J/cm2fluence range and makes a rapid transition near 5 J/cm2to a highly ionized plume (for 10-ns pulses). The 1-5-J/cm2range is of particular interest because it overlaps the irradiance range used to fabricate high- temperature superconductors, diamondlike carbon films, and conducting Cu films. Covered here are analyses aimed at a quantitative evaluation of the transition using a previously described model. The model is based primarily on the thermal (diffusivity and vapor pressure) properties of copper, along with electron heating by inverse bremsstrahlung due to electrons scattering off both neutrals and ions. The analyses provide a good fit and insight into previously obtained laser-induced fluorescence results for Cu0, Cu+, and Cu2. Also, the surface shadowing and plasma heating beginning in the 5-J/cm2(500-MW/cm2) region are clearly illustrated.