Investigation on the interaction of long duration Nd:YAG laser pulse with Sn plasma for an EUV metrology source

Abstract

The effect of pulse duration of Nd:YAG laser with wavelength of 1.064 µm on the generation of 13.5 nm extreme ultraviolet (EUV) emission and ions was investigated. It was found that almost constant in-band (2 % bandwidth) conversion efficiency (CE) is obtained from Sn plasmas irradiated with Nd:YAG laser pulse with durations from 0.13 to 30 ns. It was also noted that Sn ions kinetic energy generated with a 30 ns laser pulse is much less than those with 0.13 and 7 ns laser pulses. The measurement on the narrow-band EUV imaging showed that EUV source size strongly depends on laser intensity instead of pulse duration and small EUV size is still possible with laser pulse duration as long as 30 ns. The key reason for the constant CE and the still small EUV source size obtained with long laser pulse duration comes from the small laser focal spot employed in the present experiment, i.e., 40 μm (FWHM). This research shows that an efficient and bright EUV source is feasible with a long pulse duration Nd:YAG laser. The lower peak intensity of EUV emission due to the long pulse duration makes the EUV source more suitable for EUV metrology.