Investigation of the dynamic of an expanding laser plume by a shadowgraphic technique

The new generation of EUV sources for lithography, based on a high current z-pinch, exploits the laser ablation of a Sn target for triggering of a discharge and delivery of working material. The dynamic of the Sn plume expansion during the first 120 ns, which strongly affects the later behavior of z-pinch was investigated by a shadowgraphic technique. Radiation in a spectral range from 18 to 20 nm was used for detection of shadow images of the Sn plume because 20 nm radiation is absorbed by the inner shells of neutrals and first ions. Thus, the probing beam is efficiently absorbed by the species most important in the formation and evolution of z-pinch. Images of the Sn plume were detected at 22 ns, 55 ns, 88 ns and 120 ns delays after the plume ignition. The technique enabled the observation of the dynamic of Sn species expansion within a range of 2 mm from the target surface. A software for the processing the detected images was developed. The estimated total number of ablated Sn neutrals and first ions varied from ∼ 2–4 × 1014 for intensities of the focused ablating pulse in the range 8 × 108–1010 W/cm2. The experimentally detected dynamic of Sn plume expansion and total number of ablated Sn species coincide reasonably with simulated data.