Observation of plasma inflows in laser-produced Sn plasma and their contribution to extreme-ultraviolet light output enhancement.

Plasma dynamics are governed by electron density (ne), electron temperature (Te), and radiative energy transfer as well as by macroscopic flows. However, plasma flow-velocity fields (vflow) inside laser-produced plasmas (LPPs) have rarely been measured, owing to their small sizes (< 1 mm) and short lifetimes (< 100 ns). Herein, we report, for the first time, two-dimensional (2D) vflow measurements of Sn-LPPs ("double-pulse" scheme with a CO2 laser) for extreme-ultraviolet (EUV) light sources for semiconductor lithography using the collective Thomson scattering technique, which is typically used to measure ne, Te, and averaged ionic charge (Z) of plasmas. Inside the EUV source, we observed plasma inflow speed exceeding 104 m/s magnitudes toward a plasma central axis from its peripheral regions. The time-resolved 2D profiles of ne, Te, Z, and vflow indicate that the plasma inflows maintain the EUV source at a temperature suitable (25 eV < Te < 40 eV) for EUV light emission at a high density (ne > 3 × 1024 m-3) and for a relatively long time (> 10 ns), resulting increment of total EUV light emission. These results indicate that controlling the plasma flow can improve EUV light output and that there is potential to increase the EUV output further.

Time-resolved two-dimensional profiles of electron density and temperature of laser-produced tin plasmas for extreme-ultraviolet lithography light sources.

Time-resolved two-dimensional (2D) profiles of electron density (n e) and electron temperature (T e) of extreme ultraviolet (EUV) lithography light source plasmas were obtained from the ion components of collective Thomson scattering (CTS) spectra. The highest EUV conversion efficiency (CE) of 4% from double pulse lasers irradiating a Sn droplet was obtained by changing their delay time. The 2D-CTS results clarified that for the highest CE condition, a hollow-like density profile was formed, i.e., the high density region existed not on the central axis but in a part with a certain radius. The 2D profile of the in-band EUV emissivity (ηEUV) was theoretically calculated using the CTS results and atomic model (Hullac code), which reproduced a directly measured EUV image reasonably well. The CTS results strongly indicated the necessity of optimizing 2D plasma profiles to improve the CE in the future.