Spectroscopy of highly charged ions for extreme ultraviolet lithography

Abstract After almost three decades of research, commercial sources for extreme ultraviolet lithography (EUVL) at 13.5 nm are now being manufactured and EUVL is expected to be introduced finally in 2018 or 2019. The source is based on laser produced plasmas (LPPs) from tin micro-droplets using double pulse irradiation in a pre-pulse/main pulse configuration. The currently accepted configuration uses a CO 2 laser main pulse to minimize self-absorption in the transitions responsible for the emission at 13.5 nm. The conversion efficiency (CE) of laser to in-band EUV energy in commercial systems is currently around 4% and values up to 6% have recently been observed. However the possibility of attaining significantly higher figures has been predicted by plasma modelling and research is still ongoing to increase the CE. The present paper describes the physical processes that occur in Sn LPPs and possible routes to attaining higher CE values.