Radiation transport and scaling of optical depth in Nd:YAG laser-produced microdroplet-tin plasma

Publication date
DOI http://dx.doi.org/10.1063/1.5117504
Reference R. Schupp, F. Torretti, R.A. Meijer, M. Bayraktar, J. Sheil, J. Scheers, D. Kurilovich, A. Bayerle, A.A. Schafgans, M. Purvis, K.S.E. Eikema, S. Witte, W.M.G. Ubachs, R. Hoekstra and O.O. Versolato, Radiation transport and scaling of optical depth in Nd:YAG laser-produced microdroplet-tin plasma, Appl.Phys.Lett. 115, 12: 124101: 1-6 (2019)
Group EUV Plasma Processes

Experimental scaling relations of the optical depth are presented for the emission spectra of a tin-droplet-based, 1-lm-laser-producedplasma source of extreme-ultraviolet (EUV) light. The observed changes in the complex spectral emission of the plasma over a wide range ofdroplet diameters (16–65lm) and laser pulse durations (5–25 ns) are accurately captured in a scaling relation featuring the optical depth ofthe plasma as a single, pertinent parameter. The scans were performed at a constant laser intensity of 1.41011W/cm2, which maximizesthe emission in a 2% bandwidth around 13.5 nm relative to the total spectral energy, the bandwidth relevant for industrial EUV lithography.Using a one-dimensional radiation transport model, the relative optical depth of the plasma is found to linearly increase with the droplet sizewith a slope that increases with the laser pulse duration. For small droplets and short laser pulses, the fraction of light emitted in the 2%bandwidth around 13.5 nm relative to the total spectral energy is shown to reach high values of more than 14%, which may enable conversionefficiencies of Nd:YAG laser light into—industrially—useful EUV radiation rivaling those of current state-of-the-art CO2-laser-drivensources.