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 den sity 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 targ et and to relate particulate production to target surface damage. An a nalysis of laser ablation deposited aluminum films on silicon substrat es was used to determine the density of ablated particulate greater th an 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 sh ots. 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. D ye laser optical scattering was also used to measure, in real time, th e velocity of the particulate and the relative particulate density in the laser-ablation plume versus target damage. Results indicated a rap id rise in the production of particulate as target damage was increase d up to 3000 laser pulses; after this number of shots the density of p articulate in the laser ablation plume saturated. A peak in the scatte red light for each stage of target damage occurred 40 mu s after the i nitial KrF laser pulse, translating to a velocity of about 100 m/s for the smaller particulate (< 1 mu m diameter). The later scattered sign al, 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 p roduction is related to the formation of laser ablation-induced cones on the damaged targets. (C) 1996 American Institute of Physics.