The hydrodynamics of excimer laser ablation processing of materials in vacuum and gases.

Abstract

Laser beam deflection, schlieren photography, and dye-laser-resonance-absorption photography (DLRAP), a species-specific diagnostic, were used to obtain the first temporally and two-dimensional spatially resolved characterization of the hydrodynamics of laser-ablated material from polymers and a metal: polymethylmethacrylate (PMMA), polyethyleneterephthalate (PET), polyimide, and aluminum. The ablation source was 248nm UV radiation from a KrF excimer laser at low to moderate fluence (20mJ/cm$\sp2$ to 20J/cm$\sp2$). Experiments were performed in vacuum and 50 to 760 Torr of air, N$\sb2$, Ar, and He. Below the ablation threshold, thermal waves and sound waves were generated by irradiating the material surface. Above the ablation threshold, the sound wave was observed to transform to a shock wave and the thermal wave gave rise to a reduced density cavity. Using DLRAP, it was demonstrated for the first time that the ablated material residues in this cavity. The cavity is therefore termed an ablation plume. Ablation plumes in air for polymers are larger than those in N$\sb2$ or Ar suggesting combustion is occurring. In vacuum, above the threshold, DLRAP and laser deflection measurements showed that neutral atoms were ablated, the leading edge of which travelled at 0.1 to 10cm/$\mu$s depending on the fluence and the material. DLRAP measurements permitted the diagnosis of the neutral atom ablation plume to several centimeters from the target. Laser deflection measurements at higher fluences (around 4J/cm$\sp2$) indicated that two mass components are ablated; at lower fluences (2J/cm$\sp2$ and below) measurements indicated that the expanding material or plume initially behaves like a reflected rarefaction wave and then becomes collisionless. This observation was confirmed by a numerical model. Laser deflection measurements were used for the first time to calculate the inventory of the gaseous species in the ablation plume. These results showed that a substantial portion of the ablated material is not in gaseous form and is probably particulate.