X-Ray Conversion Efficiency In Laser-Produced Plasmas. Application To X-Ray Lithography

Laser produced plasmas have shown to be extremely bright sources of soft X-ray emission with many applications in research and industry. They can provide useful X-ray radiation from 0.1 keV to 5 keV. However, the conversion efficiency in a given X-ray energy range depends on the laser and target parameters. Previous studies have shown the influence of target atomic number and laser intensity at 1.06 μm and 0.26 μm on X-ray conversion efficiency. In this paper, we present the evolution of the conversion efficiency as a function of different parameters such as laser wavelength, pulse duration and pulse shape. Experimental results are compared with theoretical calculations obtained with an hydrodynamic code including radiation transport. We have studied separately the X-ray emission-in the subkeV (0.1-0.75 keV) and in the keV (0.75-2 keV) range. For example, the subkeV emission is relevant to X-ray microscopy since it falls within the so-called "water window" which corresponds to wavelengths between the oxygen and carbon K-edges (23-44 A). For X-ray lithography, keV X-rays are more appropriate than subkeV X-rays since they allow greater transmission through mask substrates. Moreover, they produce less diffraction effect on proximity printing.