Investigation of the ion spectrum from 1 µm and 10 µm laser produced plasmas for extreme ultraviolet lithography

The ion energy and charge state spectrum far away from a laser produced plasma source is investigated with an electrostatic ion analyzer (EIA) probe. The effects of the wavelength of the pump laser are observed by using both a Nd:YAG laser with 1 µm wavelength, and a CO 2 laser with 10.6 µm wavelength to irradiate a planar Sn target. For both pump lasers, the additional laser parameters are set to achieve a high conversion efficiency of laser energy to extreme ultraviolet (EUV) x-rays in a 2% bandwidth centered about 13.5 nm, which is a figure of merit for the EUV lithography application. The laser irradiance to achieve this conversion efficiency is approximately 10 12 W/cm 2 for the Nd:YAG laser, and 10 11 W/cm 2 for the CO 2 laser. It was observed that the CO 2 laser generates ions at higher charge states far away from the laser plasma interaction compared to the Nd:YAG laser, even though the CO 2 laser is operated at a lower irradiance. In addition to the measurement of the ion spectrum at two laser wavelengths, the details of the custom built EIA probe are discussed, including the method used to calibrate the response of the probe to Sn ions with energies from approximately 1 to 10 keV.