Studies of laser produced plasma from foam targets for future nanolithography devices and soft X-ray sources*

Foam targets are expected to be more efficient candidates than solid targets for laser produced plasma (LPP) for extreme ultraviolet (EUV) and soft x-ray (SXR) radiation sources due to the expected plasma conditions that can be optimized regarding plasma opacities, volumetric heating effects, and the produced ions debris characteristics. Comparing LPP studies between Ni and Ti targets with different densities indicate that the ions in the foam target plasma have a relatively uniform distribution compared to solid targets. The average ion energy and the peak of the time-of-flight (TOF) signal decrease as the target density decreases, especially at higher laser intensities while the total charge values between foam and solid target plasma are comparable indicating a more volumetric absorption of laser energy for foam targets. Having lower ions debris energy and flux are very important to protect the expensive EUV optical collector system. The results of the mass ablation rate are consistent with the Faraday Cup (FC) signals and show more intense plasma shielding for solid target plasma. The SXR spectrum and the conversion efficiency of laser to photon energy at the required wavelength for nanolithography show similar values for solid and foam Ti targets. Similar results are expected for Sn foam target, the current leading candidate for nanolithography. These experimental results provide more insights for the laser interaction with foam targets.