Numerical investigation of laser pre-pulse effects on tin microdroplet targets and extreme ultraviolet emission

Laser-produced plasma sources based on tin microdroplets are key enablers of extreme ultraviolet (EUV) lithography, where the efficiency of EUV generation critically depends on laser–target energy coupling and plasma radiation characteristics. In this paper, we employ a hybrid numerical approach to investigate how nanosecond laser pre-pulses pre-deform tin droplet targets and thereby influence subsequent EUV emission driven by CO2 main pulses. The simulations reproduce experimentally observed propulsion and expansion dynamics, enabling quantitative analysis of kinetic energy partitioning and target geometry evolution as functions of laser pulse energy and focal spot diameter. We show that laser beam diameter and pulse energy govern the partition of kinetic energy between center-of-mass motion and lateral expansion, which directly determines the size, density, and curvature of the pre-pulsed target presented to the main pulse. Main pulse irradiation of these targets is further simulated to assess the influence of target morphology on EUV radiation in the 13.5 nm band. The results demonstrate that thinner and radially expanded targets enhance in-band EUV emission intensity, thereby yielding higher conversion efficiency. These findings provide physical insight into optimizing laser pre-pulse conditions for improved EUV source performance.