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 (n_e), electron temperature (T_e), and radiative energy transfer as well as by macroscopic flows. However, plasma flow-velocity fields (v_flow) 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) v_flow 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 n_e, T_e, 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 n_e, T_e, Z, and v_flow indicate that the plasma inflows maintain the EUV source at a temperature suitable (25 eV < T_e < 40 eV) for EUV light emission at a high density (n_e > 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.