Effects of laser-ablation target damage on particulate production investigated by laser scattering with deposited thin film and target analysis

Experiments have been carried out to correlate ablated particulate density and size to the number of KrF excimer laser (248 nm, 40 ns, < 1.2 J) pulses incident on a single location of a pure solid aluminum target and to relate particulate production to target surface damage. An analysis of laser ablation deposited aluminum films on silicon substrates was used to determine the density of ablated particulate greater than 0.5 mu m in diameter. For an undamaged target, the laser deposited particulate density was on the order of 8.6 x 10(5) cm(-2) per 1000 shots. A damaged target (following 1000 laser pulses) produced a density on the order of 1.6 x 10(6) cm(-2) per 1000 shots on the substrate. Dye laser optical scattering was also used to measure, in real time, the velocity of the particulate and the relative particulate density in the laser-ablation plume versus target damage. Results indicated a rapid rise in the production of particulate as target damage was increased up to 3000 laser pulses; after this number of shots the density of particulate in the laser ablation plume saturated. A peak in the scattered light for each stage of target damage occurred 40 mu s after the initial KrF laser pulse, translating to a velocity of about 100 m/s for the smaller particulate (< 1 mu m diameter). The later scattered signal, around 160 mu s was apparently due to the larger particulate (5-15 mu m), traveling at a velocity of approximately 25 m/s. Particulate production is related to the formation of laser ablation-induced cones on the damaged targets. (C) 1996 American Institute of Physics.